@article{li_zhi_weed_broome_knappe_duckworth_2024, title={Commercial compost amendments inhibit the bioavailability and plant uptake of per- and polyfluoroalkyl substances in soil-porewater-lettuce systems}, volume={186}, ISSN={["1873-6750"]}, DOI={10.1016/j.envint.2024.108615}, abstractNote={Compost is widely used in agriculture as fertilizer while providing a practical option for solid municipal waste disposal. However, compost may also contain per- and polyfluoroalkyl substances (PFAS), potentially impacting soils and leading to PFAS entry into food chains and ultimately human exposure risks via dietary intake. This study examined how compost affects the bioavailability and uptake of eight PFAS (two ethers, three fluorotelomer sulfonates, and three perfluorosulfonates) by lettuce (Lactuca sativa) grown in commercial organic compost-amended, PFAS spiked soils. After 50 days of greenhouse experiment, PFAS uptake by lettuce decreased (by up to 90.5 %) with the increasing compost amendment ratios (0-20 %, w/w), consistent with their decreased porewater concentrations (by 30.7-86.3 %) in compost-amended soils. Decreased bioavailability of PFAS was evidenced by the increased in-situ soil-porewater distribution coefficients (Kd) (by factors of 1.5-7.0) with increasing compost additions. Significant negative (or positive) correlations (R2 ≥ 0.55) were observed between plant bioaccumulation (or Kd) and soil organic carbon content, suggesting that compost amendment inhibited plant uptake of PFAS mainly by increasing soil organic carbon and enhancing PFAS sorption. However, short-chain PFAS alternatives (e.g., perfluoro-2-methoxyacetic acid (PFMOAA)) were effectively translocated to shoots with translocation factors > 2.9, increasing their risks of contamination in leafy vegetables. Our findings underscore the necessity for comprehensive risk assessment of compost-borne PFAS when using commercial compost products in agricultural lands.}, journal={ENVIRONMENT INTERNATIONAL}, author={Li, Yuanbo and Zhi, Yue and Weed, Rebecca and Broome, Stephen W. and Knappe, Detlef R. U. and Duckworth, Owen W.}, year={2024}, month={Apr} }
 @article{ferreira_queiroz_boim_duckworth_otero_bernardino_ferreira_2024, title={Contrasting plant-induced changes in heavy metals dynamics: Implications for phytoremediation strategies in estuarine wetlands}, volume={279}, ISSN={["1090-2414"]}, DOI={10.1016/j.ecoenv.2024.116416}, abstractNote={Wetland plants play a crucial role in regulating soil geochemistry, influencing heavy metal (HM) speciation, bioavailability, and uptake, thus impacting phytoremediation potential. We hypothesized that variations in HM biogeochemistry within estuarine soils are controlled by distinct estuarine plant species. We evaluated the soils (pH, redox potential, rhizosphere pH, HM total concentration, and geochemical fractionation), plant parts (shoot and root), and iron plaques of three plants growing in an estuary affected by Fe-rich mine tailings. Though the integration of multiple plant and soil analysis, this work emphasizes the importance of considering geochemical pools of HM for predicting their fate. Apart from the predominance of HM associated with Fe oxides, Typha domingensis accumulated the highest Cr and Ni contents in their shoots (> 100 mg kg−1). In contrast, Hibiscus tiliaceus accumulated more Cu and Pb in their roots (> 50 mg kg−1). The differences in rhizosphere soil conditions and root bioturbation explained the different potentials between the plants by altering the soil dynamics and HM's bioavailability, ultimately affecting their uptake. This study suggests that Eleocharis acutangula is not suitable for phytoextraction or phytostabilization, whereas Typha domingensis shows potential for Cr and Ni phytoextraction. In addition, we first showed Hibiscus tiliaceus as a promising wood species for Cu and Pb phytostabilization.}, journal={ECOTOXICOLOGY AND ENVIRONMENTAL SAFETY}, author={Ferreira, Amanda Duim and Queiroz, Hermano Melo and Boim, Alexys G. Friol and Duckworth, Owen W. and Otero, Xose L. and Bernardino, Angelo Fraga and Ferreira, Tiago Osorio}, year={2024}, month={Jul} }
 @article{hu_wang_williams_dwyer_gatiboni_duckworth_vengosh_2024, title={Evidence for the accumulation of toxic metal(loid)s in agricultural soils impacted from long-term application of phosphate fertilizer}, volume={907}, ISSN={["1879-1026"]}, DOI={10.1016/j.scitotenv.2023.167863}, abstractNote={Phosphate fertilizers may contain elevated concentrations of toxic metals and metalloids and therefore, their excessive application can result in the accumulation of both phosphorus (P) and metal(loid)s in agricultural soils. This study aims to investigate the occurrence, distribution, and potential plant-availability of metal(loid)s originating from phosphate fertilizer in a long-term experimental field at the Tidewater Research Station in North Carolina, where topsoil (10–20 cm deep) and subsoil (up to 150 cm deep) samples were collected from five plots with consistent and individually different application rates of P-fertilizer since 1966. We conducted systematic analyses of P and metal(loid)s in bulk soils, in the plant available fraction, and in four sequentially extracted soil fractions (exchangeable, reducible, oxidizable, and residual). The results show that P content in topsoils were directly associated with the rate of P-fertilizer application (ρ = 1, p < 0.05). Furthermore, P concentrations were highly correlated with concentrations of Cd, U, Cr, V, and As in the bulk topsoil (ρ > 0.58, p < 0.05), as well as the potential plant-available fraction (ρ > 0.67, p < 0.01), indicating the accumulation of the fertilizer-derived toxic metal(loid)s in the topsoil. Significant correlations (p < 0.001) of metal(loid)s concentrations between the bulk soil and the potential plant-available fraction raises the possibility that P-fertilizer application could increase the accumulation of toxic metal(loid)s in plants, which could increase human exposure. Results from sequential leaching experiments revealed that large portions of the trace elements, in particular Cd, occur in the soluble (exchangeable and reducing) fractions of topsoil with higher P-fertilizer input, whereas the levels of redox-sensitive elements (As, V, U, Cr) were higher in the reducible and oxidizable fractions of the soils. Overall, the data presented in this study demonstrate the effect of long-term P-fertilizer application on the occurrence and accumulation of a wide range of toxic metal(loid)s in agricultural topsoil.}, journal={SCIENCE OF THE TOTAL ENVIRONMENT}, author={Hu, Jun and Wang, Zhen and Williams, Gordon D. Z. and Dwyer, Gary S. and Gatiboni, Luke and Duckworth, Owen W. and Vengosh, Avner}, year={2024}, month={Jan} }
 @article{baker_schunk_scholz_merck_muenich_westerhoff_elser_duckworth_gatiboni_islam_et al._2024, title={Global-to-Local Dependencies in Phosphorus Mass Flows and Markets: Pathways to Improving System Resiliency in Response to Exogenous Shocks}, volume={5}, ISSN={["2328-8930"]}, url={http://dx.doi.org/10.1021/acs.estlett.4c00208}, DOI={10.1021/acs.estlett.4c00208}, abstractNote={Uneven global distribution of phosphate rock deposits and the supply chains to transport phosphorus (P) make P fertilizers vulnerable to exogenous shocks, including commodity market shocks; extreme weather events or natural disasters; and geopolitical instability, such as trade disputes, disruption of shipping routes, and war. Understanding bidirectional risk transmission (global-to-local and local-to-global) in P supply and consumption chains is thus essential. Ignoring P system interdependencies and associated risks could have major impacts on critical infrastructure operations and increase the vulnerability of global food systems. We highlight recent unanticipated events and cascading effects that have impacted P markets globally. We discuss the need to account for exogenous shocks in local assessments of P flows, policies, and infrastructure design choices. We also provide examples of how accounting for undervalued global risks to the P industry can hasten the transition to a sustainable P future. For example, leveraging internal P recycling loops, improving plant P use efficiency, and utilizing legacy soil P all enhance system resiliency in the face of exogenous shocks and long-term anticipated threats. Strategies applied at the local level, which are embedded within national and global policy systems, can have global-scale impacts in derisking the P supply chain.}, journal={ENVIRONMENTAL SCIENCE & TECHNOLOGY LETTERS}, author={Baker, Justin and Schunk, Nathan and Scholz, Matt and Merck, Ashton and Muenich, Rebecca Logsdon and Westerhoff, Paul and Elser, James J. and Duckworth, Owen W. and Gatiboni, Luke and Islam, Minhazul and et al.}, year={2024}, month={May} }
 @article{french_chukwuma_linshitz_namba_duckworth_cubeta_baars_2024, title={Inactivation of siderophore iron-chelating moieties by the fungal wheat root symbiont <i>Pyrenophora biseptata</i>}, volume={1}, ISSN={["1758-2229"]}, url={https://doi.org/10.1111/1758-2229.13234}, DOI={10.1111/1758-2229.13234}, abstractNote={Abstract}, journal={ENVIRONMENTAL MICROBIOLOGY REPORTS}, author={French, Katie S. and Chukwuma, Emmanuel and Linshitz, Ilan and Namba, Kosuke and Duckworth, Owen W. and Cubeta, Marc A. and Baars, Oliver}, year={2024}, month={Jan} }
 @article{ferreira_duckworth_queiroz_nobrega_barcellos_bernardino_otero_ferreira_2024, title={Seasonal drives on potentially toxic elements dynamics in a tropical estuary impacted by mine tailings}, volume={474}, ISSN={["1873-3336"]}, DOI={10.1016/j.jhazmat.2024.134592}, abstractNote={This study investigates the impact of seasonality on estuarine soil geochemistry, focusing on redox-sensitive elements, particularly Fe, in a tropical estuary affected by Fe-rich mine tailings. We analyzed soil samples for variations in particle size, pH, redox potential (Eh), and the content of Fe, Mn, Cr, Cu, Ni, and Pb. Additionally, sequential extraction was employed to understand the fate of these elements. Results revealed dynamic changes in the soil geochemical environment, transitioning between near-neutral and suboxic/anoxic conditions in the wet season and slightly acidic to suboxic/oxic conditions in the dry season. During the wet season, fine particle deposition (83%) rich in Fe (50 g kg-1), primarily comprising crystalline Fe oxides, occurred significantly. Conversely, short-range ordered Fe oxides dominated during the dry season. Over consecutive wet/dry seasons, substantial losses of Fe (-55%), Mn (-41%), and other potentially toxic elements (Cr: -44%, Cu: -31%, Ni: -25%, Pb: -9%) were observed. Despite lower pseudo-total PTE contents, exchangeable PTEs associated with carbonate content increased over time (Cu: +188%, Ni: +557%, Pb: +99%). Modeling indicated climatic variables and short-range oxides substantially influenced PTE bioavailability, emphasizing the ephemeral Fe oxide control during the wet season and heightened ecological and health risks during the dry seasons.}, journal={JOURNAL OF HAZARDOUS MATERIALS}, author={Ferreira, Amanda Duim and Duckworth, Owen W. and Queiroz, Hermano Melo and Nobrega, Gabriel Nuto and Barcellos, Diego and Bernardino, Angelo Fraga and Otero, Xose L. and Ferreira, Tiago Osorio}, year={2024}, month={Aug} }
 @article{hill_williams_wang_hu_el-hasan_duckworth_schnug_bol_singh_vengosh_2024, title={Tracing the Environmental Effects of Mineral Fertilizer Application with Trace Elements and Strontium Isotope Variations}, volume={5}, ISSN={["2328-8930"]}, DOI={10.1021/acs.estlett.4c00170}, abstractNote={Fertilizer utilization is critical for food security. This study examines the occurrence of trace elements (TEs) and Sr isotope (87Sr/86Sr) variations in phosphate rocks and mineral fertilizers from a sample collection representative of major phosphate producing countries. We show high concentrations of several TEs in phosphate rocks (n = 76) and their selective enrichment in phosphate fertilizers (n = 40) of specific origin. Consistent with the concentrations in parent phosphate rocks, phosphate fertilizers from the U.S. and Middle East have substantially higher concentrations of U, Cd, Cr, V, and Mo than those in fertilizers from China and India. Yet, fertilizers from China and India generally have higher concentrations of As. The 87Sr/86Sr in phosphate fertilizers directly mimic the composition of their source phosphate rocks, with distinctive higher ratios in fertilizers from China and India (0.70955–0.71939) relative to phosphate fertilizers from U.S. and Middle East (0.70748–0.70888). Potash fertilizers have less Sr and TEs and higher 87Sr/86Sr (0.72017–0.79016), causing higher 87Sr/86Sr in mixed NPK-fertilizers. Selective extraction (Mehlich III) of soils from an experimental agricultural site shows relative enrichment of potentially plant-available P, Sr, and TEs in topsoil, which is associated with Sr isotope variation toward the 87Sr/86Sr of the local utilized phosphate fertilizer.}, journal={ENVIRONMENTAL SCIENCE & TECHNOLOGY LETTERS}, author={Hill, Robert C. and Williams, Gordon D. Z. and Wang, Zhen and Hu, Jun and El-Hasan, Tayel and Duckworth, Owen W. and Schnug, Ewald and Bol, Roland and Singh, Anjali and Vengosh, Avner}, year={2024}, month={May} }
 @article{doydora_baars_cubeta_duckworth_urbieta_castro_2024, title={Using Manganese Oxidizing Fungi to Recover Metals from Electronic Waste}, volume={14}, ISSN={["2075-163X"]}, url={https://www.mdpi.com/2075-163X/14/1/111}, DOI={10.3390/min14010111}, abstractNote={Discarded electronic materials (e-waste) contain economically valuable metals that can be hazardous to people and the environment. Current e-waste recycling approaches involve either energy-intensive smelting or bioleaching processes that capture metals in their dissolved forms. Our study aimed to use Mn oxidizing fungi for recovering metals from e-waste that could potentially transform recycled metals directly into solid forms. We hypothesized that Mn oxidizing fungi can extract metals through chelation by siderophores and subsequent metal (or metal-chelate) adsorption to Mn oxides produced by fungi. Pure cultures of the three fungal species examined were grown on solidified Leptothrix medium with or without ground lithium ion batteries and incubated under ambient room temperature. The results showed Mn and Co were recovered at the highest concentrations of 8.45% and 1.75%, respectively, when grown with Paraconiothyrium brasiliensis, whereas the greatest concentration of Cu was extracted by Paraphaeosphaeria sporulosa at 20.6% per weight of e-waste-derived metals. Although metal-siderophore complexes were detected in the fungal growth medium, metal speciation data suggested that these complexes only occurred with Fe. This observation suggests that reactions other than complexation with siderophores likely solubilized e-waste metals. Elemental mapping, particularly of P. brasiliensis structures, showed a close association between Mn and Co, suggesting potential adsorption or (co)precipitation of these two metals near fungal mycelium. These findings provide experimental evidence for the potential use of Mn oxidizing fungi in recycling and transforming e-waste metals into solid biominerals. However, optimizing fungal growth conditions with e-waste is needed to improve the efficiency of metal recovery.}, number={1}, journal={MINERALS}, author={Doydora, Sarah A. and Baars, Oliver and Cubeta, Marc A. and Duckworth, Owen W. and Urbieta, Maria Sofia and Castro, Laura}, year={2024}, month={Jan} }
 @article{mclamore_duckworth_boyer_marshall_call_bhadha_guzman_2023, title={Perspective: Phosphorus monitoring must be rooted in sustainability frameworks spanning material scale to human scale}, volume={19}, ISSN={["2589-9147"]}, DOI={10.1016/j.wroa.2023.100168}, abstractNote={Phosphorus (P) is a finite resource, and its environmental fate and transport is complex. With fertilizer prices expected to remain high for years and disruption to supply chains, there is a pressing need to recover and reuse P (primarily as fertilizer). Whether recovery is to occur from urban systems (e.g., human urine), agricultural soil (e.g., legacy P), or from contaminated surface waters, quantification of P in various forms is vital. Monitoring systems with embedded near real time decision support, so called cyber physical systems, are likely to play a major role in the management of P throughout agro-ecosystems. Data on P flow(s) connects the environmental, economic, and social pillars of the triple bottom line (TBL) sustainabilty framework. Emerging monitoring systems must account for complex interactions in the sample, and interface with a dynamic decision support system that considers adaptive dynamics to societal needs. It is known from decades of study that P is ubiquitous, yet without quantitative tools for studying the dynamic nature of P in the environment, the details may remain elusive. If new monitoring systems (including CPS and mobile sensors) are informed by sustainability frameworks, data-informed decision making may foster resource recovery and environmental stewardship from technology users to policymakers.}, journal={WATER RESEARCH X}, author={McLamore, Eric and Duckworth, Owen and Boyer, Treavor H. and Marshall, Anna-Maria and Call, Douglas F. and Bhadha, Jehangir H. and Guzman, Sandra}, year={2023}, month={May} }
 @article{soares_duckworth_styblo_cable_alleoni_2023, title={Pyrolysis temperature and biochar redox activity on arsenic availability and speciation in a sediment}, volume={460}, ISSN={["1873-3336"]}, DOI={10.1016/j.jhazmat.2023.132308}, abstractNote={Biochar is widely used for water and soil remediation in part because of its local availability and low production cost. However, its effectiveness depends on physicochemical properties related to its feedstock and pyrolysis temperature, as well as the environmental conditions of its use site. Furthermore, biochar is susceptible to natural aging caused by changes in soil or sediment moisture, which can alter its redox properties and interactions with contaminants such as arsenic (As). In this study, we investigated the effect of pyrolysis temperature and biochar application on the release and transformations of As in contaminated sediments subjected to redox fluctuations. Biochar application and pyrolysis temperature played an important role in As species availability, As methylation, and dissolved organic carbon concentration. Furthermore, successive flooding cycles that induced reductive conditions in sediments increased the As content in the solution by up to seven times. In the solid phase, the application of biochar and the flooding cycle altered the spatial distribution and speciation of carbon, iron (Fe) and As. In general, the application of biochar decreased the reduction of Fe(III) and As(V) after the first cycle of flooding. Our results demonstrate that the flooding cycle plays an important role in the reoxidation of biochar to the point of enhancing the immobilization of As.}, journal={JOURNAL OF HAZARDOUS MATERIALS}, author={Soares, Matheus B. and Duckworth, Owen W. and Styblo, Miroslav and Cable, Peter H. and Alleoni, Luis R. F.}, year={2023}, month={Oct} }
 @article{soares_duckworth_alleoni_2023, title={The role of dissolved pyrogenic carbon from biochar in the sorption of As (V) in biogenic iron (oxyhydr)oxides}, volume={865}, ISSN={["1879-1026"]}, DOI={10.1016/j.scitotenv.2022.161286}, abstractNote={Water contamination by arsenic (As) affects millions of people around the world, making techniques to immobilize or remove this contaminant a pressing societal need. Biochar and iron (oxyhydr)oxides [in particular, biogenic iron (oxyhydr)oxides (BIOS)] offer the possibility of stabilizing As in remediation systems. However, little is known about the potential antagonism in As sorption generated by the dissolved organic carbon (DOC) from biochar, or whether DOC affects how As(V) interacts with BIOS. For this reason, our objectives were to evaluate the i) As(V) sorption potential in BIOS when there is presence of DOC from pyrolyzed biochars at different temperatures; and ii) identify whether the presence of DOC alters the surface complexes formed by As(V) sorbed in the BIOS. We conducted As(V) sorption experiments with BIOS at circumneutral pH conditions and in the presence of DOC from sugarcane (Saccharum officinarum) straw biochar at pyrolyzed 350 (BC350) and 750 °C (BC750). The As(V) content was quantified by inductively coupled plasma mass spectrometry, and the BIOS structure and As(V) sorption mechanisms were investigated by X-ray absorption spectroscopy. In addition, the organic moieties comprising the DOC from biochars were investigated by attenuated total reflectance Fourier transform infrared spectroscopy. The addition of DOC did not change the biomineral structure or As(V) oxidation state. The presence of DOC, however, reduced by 25 % the sorption of As(V), with BC350 being responsible for the greatest reduction in As(V) sorption capacity. Structural modeling revealed As(V) predominantly formed binuclear bidentate surface complexes on BIOS. The presence of DOC did not change the binding mechanism of As(V) in BIOS, suggesting that the reduction of As(V) sorption to BIOS was due to site blocking. Our results bring insights into the fate of As(V) in surface waters and provide a basis for understanding the competitive sorption of As(V) in environments with biochar application.}, journal={SCIENCE OF THE TOTAL ENVIRONMENT}, author={Soares, Matheus B. and Duckworth, Owen W. and Alleoni, Luis R. F.}, year={2023}, month={Mar} }
 @article{duckworth_polizzotto_thompson_2022, title={Bringing soil chemistry to environmental health science to tackle soil contaminants}, volume={10}, ISSN={["2296-665X"]}, DOI={10.3389/fenvs.2022.981607}, abstractNote={With an estimated five million sites worldwide, soil contamination is a global-scale threat to environmental and human health. Humans continuously interact with soil, both directly and indirectly, making soils potentially significant sources of exposure to contaminants. Soil chemists are thus a potentially dynamic part of a collaborative cohort attacking environmental health science problems, yet collaborations between soil chemists and environmental heath scientists remain infrequent. In this commentary, we discuss the unique properties of soils that influence contaminants, as well as ways that soil chemists can contribute to environmental health research. Additionally, we describe barriers to, and needs for, the integration of soil chemistry expertise in environmental health science research with a focus on the future.}, journal={FRONTIERS IN ENVIRONMENTAL SCIENCE}, author={Duckworth, Owen W. W. and Polizzotto, Matthew L. L. and Thompson, Aaron}, year={2022}, month={Sep} }
 @article{zhi_paterson_call_jones_hesterberg_duckworth_poitras_knappe_2022, title={Mechanisms of orthophosphate removal from water by lanthanum carbonate and other lanthanum-containing materials}, volume={820}, ISSN={["1879-1026"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85123356200&partnerID=MN8TOARS}, DOI={10.1016/j.scitotenv.2022.153153}, abstractNote={Removing phosphorus (P) from water and wastewater is essential for preventing eutrophication and protecting environmental quality. Lanthanum [La(III)]-containing materials can effectively and selectively remove orthophosphate (PO4) from aqueous systems, but there remains a need to better understand the underlying mechanism of PO4 removal. Our objectives were to 1) identify the mechanism of PO4 removal by La-containing materials and 2) evaluate the ability of a new material, La2(CO3)3(s), to remove PO4 from different aqueous matrices, including municipal wastewater. We determined the dominant mechanism of PO4 removal by comparing geochemical simulations with equilibrium data from batch experiments and analyzing reaction products by X-ray diffraction and scanning transmission electron microscopy with energy dispersive spectroscopy. Geochemical simulations of aqueous systems containing PO4 and La-containing materials predicted that PO4 removal occurs via precipitation of poorly soluble LaPO4(s). Results from batch experiments agreed with those obtained from geochemical simulations, and mineralogical characterization of the reaction products were consistent with PO4 removal occurring primarily by precipitation of LaPO4(s). Between pH 1.5 and 12.9, La2(CO3)3(s) selectively removed PO4 over other anions from different aqueous matrices, including treated wastewater. However, the rate of PO4 removal decreased with increasing solution pH. In comparison to other solids, such as La(OH)3(s), La2(CO3)3(s) exhibits a relatively low solubility, particularly under slightly acidic conditions. Consequently, release of La3+ into the environment can be minimized when La2(CO3)3(s) is deployed for PO4 sequestration.}, journal={SCIENCE OF THE TOTAL ENVIRONMENT}, author={Zhi, Yue and Paterson, Alisa R. and Call, Douglas F. and Jones, Jacob L. and Hesterberg, Dean and Duckworth, Owen W. and Poitras, Eric P. and Knappe, Detlef R. U.}, year={2022}, month={May} }
 @misc{peel_balogun_bowers_miller_obeidy_polizzotto_tashnia_vinson_duckworth_2022, title={Towards Understanding Factors Affecting Arsenic, Chromium, and Vanadium Mobility in the Subsurface}, volume={14}, ISSN={["2073-4441"]}, DOI={10.3390/w14223687}, abstractNote={Arsenic (As), chromium (Cr), and vanadium (V) are naturally occurring, redox-active elements that can become human health hazards when they are released from aquifer substrates into groundwater that may be used as domestic or irrigation source. As such, there is a need to develop incisive conceptual and quantitative models of the geochemistry and transport of potentially hazardous elements to assess risk and facilitate interventions. However, understanding the complexity and heterogeneous subsurface environment requires knowledge of solid-phase minerals, hydrologic movement, aerobic and anaerobic environments, microbial interactions, and complicated chemical kinetics. Here, we examine the relevant geochemical and hydrological information about the release and transport of potentially hazardous geogenic contaminants, specifically As, Cr, and V, as well as the potential challenges in developing a robust understanding of their behavior in the subsurface. We explore the development of geochemical models, illustrate how they can be utilized, and describe the gaps in knowledge that exist in translating subsurface conditions into numerical models, as well as provide an outlook on future research needs and developments.}, number={22}, journal={WATER}, author={Peel, Hannah R. and Balogun, Fatai O. and Bowers, Christopher A. and Miller, Cass T. and Obeidy, Chelsea S. and Polizzotto, Matthew L. and Tashnia, Sadeya U. and Vinson, David S. and Duckworth, Owen W.}, year={2022}, month={Nov} }
 @article{zhi_call_grieger_duckworth_jones_knappe_2021, title={Influence of natural organic matter and pH on phosphate removal by and filterable lanthanum release from lanthanum-modified bentonite}, volume={202}, ISSN={0043-1354}, url={http://dx.doi.org/10.1016/j.watres.2021.117399}, DOI={10.1016/j.watres.2021.117399}, abstractNote={Lanthanum modified bentonite (LMB) has been applied to eutrophic lakes to reduce phosphorus (P) concentrations in the water column and mitigate P release from sediments. Previous experiments suggest that natural organic matter (NOM) can interfere with phosphate (PO4)-binding to LMB and exacerbate lanthanum (La)-release from bentonite. This evidence served as motivation for this study to systematically determine the effects of NOM, solution pH, and bentonite as a La carrier on P removal. We conducted both geochemical modeling and controlled-laboratory batch kinetic experiments to understand the pH-dependent impacts of humic and fulvic acids on PO4-binding to LMB and La release from LMB. The role of bentonite was studied by comparing PO4 removal obtained by LMB and La3+ (added as LaCl3 salt to represent the La-containing component of LMB). Our results from both geochemical modeling and batch experiments indicate that the PO4-binding ability of LMB is decreased in the presence of NOM, and the decrease is more pronounced at pH 8.5 than at 6. At the highest evaluated NOM concentration (28 mg C L−1), PO4-removal by La3+ was substantially lower than that by LMB, implying that bentonite clay in LMB shielded La from interactions with NOM, while still allowing PO4 capture by La. Finally, the presence of NOM promoted La-release from LMB, and the amount of La released depended on solution pH and both the type (i.e., fulvic/humic acid ratio) and concentration of NOM. Overall, these results provide an important basis for management of P in lakes and eutrophication control that relies on LMB applications.}, journal={Water Research}, publisher={Elsevier BV}, author={Zhi, Yue and Call, Douglas F. and Grieger, Khara D. and Duckworth, Owen W. and Jones, Jacob L. and Knappe, Detlef R.U.}, year={2021}, month={Sep}, pages={117399} }
 @article{doydora_baars_harrington_duckworth_2021, title={Salicylate coordination in metal-protochelin complexes}, volume={11}, ISSN={["1572-8773"]}, DOI={10.1007/s10534-021-00352-7}, abstractNote={Molybdenum (Mo) is an essential trace element for bacteria that is utilized in myriad metalloenzymes that directly couple to the biogeochemical cycling of nitrogen, sulfur, and carbon. In particular, Mo is found in the most common nitrogenase enzyme, and the scarcity and low bioavailability of Mo in soil may be a critical factor that contributes to the limitation of nitrogen fixation in forests and agroenvironments. To overcome this scarcity, microbes produce exudates that specifically chelate scarce metals, promoting their solubilization and uptake. Here, we have determined the structure and stability constants of Mo bound by protochelin, a siderophore produced by bacteria under Mo-depleted conditions. Spectrophotometric titration spectra indicated a coordination shift from a catecholate to salicylate binding mode for Mo VI -protochelin (Mo-Proto) complexes at pH < 5. pKa values obtained from analysis of titrations were 4.8 ± 0.3 for Mo VI O 2 H 3 Proto -  and 3.3 ± 0.1 for Mo VI O 2 H 4 Proto. The occurrence of negatively charged Mo-Proto complexes at pH 6 was also confirmed by mass spectrometry. K-edge Extended X-ray absorption fine structure spectroscopy confirmed the change in Mo coordination at low pH, and structural fitting provides insights into the physical architecture of complexes at neutral and acidic pH. These findings suggest that Mo can be chelated by protochelin across a wide environmental pH range, with a coordination shift occurring at pH < 5. This chelation and associated coordination shift may impact biological availability and mineral surface retention of Mo under acidic conditions.}, journal={BIOMETALS}, author={Doydora, Sarah A. and Baars, Oliver and Harrington, James M. and Duckworth, Owen W.}, year={2021}, month={Nov} }
 @article{whitaker_austin_holden_jones_michel_peak_thompson_duckworth_2021, title={The structure of natural biogenic iron (oxyhydr)oxides formed in circumneutral pH environments}, volume={308}, ISSN={["1872-9533"]}, DOI={10.1016/j.gca.2021.05.059}, abstractNote={Biogenic iron (Fe) (oxyhydr)oxides (BIOS) partially control the cycling of organic matter, nutrients, and pollutants in soils and water via sorption and redox reactions. Although recent studies have shown that the structure of BIOS resembles that of two-line ferrihydrite (2LFh), we lack detailed knowledge of the BIOS local coordination environment and structure required to understand the drivers of BIOS reactivity in redox active environments. Therefore, we used a combination of microscopy, scattering, and spectroscopic methods to elucidate the structure of BIOS sampled from a groundwater seep in North Carolina and compare them to 2LFh. We also simulated the effects of wet-dry cycles by varying sample preparation (e.g., freezing, flash freezing with freeze drying, freezing with freeze drying and oven drying). In general, the results show that both the long- and short-range ordering in BIOS are structurally distinct and notably more disordered than 2LFh. Our structure analysis, which utilized Fe K-edge X-ray absorption spectroscopy, Mössbauer spectroscopy, X-ray diffraction, and pair distribution function analyses, showed that the BIOS samples were more poorly ordered than 2LFh and intimately mixed with organic matter. Furthermore, pair distribution function analyses resulted in coherent scattering domains for the BIOS samples ranging from 12–18 Å, smaller than those of 2LFh (21–27 Å), consistent with reduced ordering. Additionally, Fe L-edge XAS indicated that the local coordination environment of 2LFh samples consisted of minor amounts of tetrahedral Fe(III), whereas BIOS were dominated by octahedral Fe(III), consistent with depletion of the sites due to small domain size and incorporation of impurities (e.g., organic C, Al, Si, P). Within sample sets, the frozen freeze dried and oven dried sample preparation increased the crystallinity of the 2LFh samples when compared to the frozen treatment, whereas the BIOS samples remained more poorly crystalline under all sample preparations. This research shows that BIOS formed in circumneutral pH waters are poorly ordered and more environmentally stable than 2LFh.}, journal={GEOCHIMICA ET COSMOCHIMICA ACTA}, author={Whitaker, Andrew H. and Austin, Robert E. and Holden, Kathryn L. and Jones, Jacob L. and Michel, F. Marc and Peak, Derek and Thompson, Aaron and Duckworth, Owen W.}, year={2021}, month={Sep}, pages={237–255} }
 @article{doydora_gatiboni_grieger_hesterberg_jones_mclamore_peters_sozzani_van den broeck_duckworth_2020, title={Accessing Legacy Phosphorus in Soils}, volume={4}, ISSN={2571-8789}, url={http://dx.doi.org/10.3390/soilsystems4040074}, DOI={10.3390/soilsystems4040074}, abstractNote={Repeated applications of phosphorus (P) fertilizers result in the buildup of P in soil (commonly known as legacy P), a large fraction of which is not immediately available for plant use. Long-term applications and accumulations of soil P is an inefficient use of dwindling P supplies and can result in nutrient runoff, often leading to eutrophication of water bodies. Although soil legacy P is problematic in some regards, it conversely may serve as a source of P for crop use and could potentially decrease dependence on external P fertilizer inputs. This paper reviews the (1) current knowledge on the occurrence and bioaccessibility of different chemical forms of P in soil, (2) legacy P transformations with mineral and organic fertilizer applications in relation to their potential bioaccessibility, and (3) approaches and associated challenges for accessing native soil P that could be used to harness soil legacy P for crop production. We highlight how the occurrence and potential bioaccessibility of different forms of soil inorganic and organic P vary depending on soil properties, such as soil pH and organic matter content. We also found that accumulation of inorganic legacy P forms changes more than organic P species with fertilizer applications and cessations. We also discuss progress and challenges with current approaches for accessing native soil P that could be used for accessing legacy P, including natural and genetically modified plant-based strategies, the use of P-solubilizing microorganisms, and immobilized organic P-hydrolyzing enzymes. It is foreseeable that accessing legacy P will require multidisciplinary approaches to address these limitations.}, number={4}, journal={Soil Systems}, publisher={MDPI AG}, author={Doydora, Sarah and Gatiboni, Luciano and Grieger, Khara and Hesterberg, Dean and Jones, Jacob L. and McLamore, Eric S. and Peters, Rachel and Sozzani, Rosangela and Van den Broeck, Lisa and Duckworth, Owen W.}, year={2020}, month={Dec}, pages={74} }
 @misc{zhi_zhang_hjorth_baun_duckworth_call_knappe_jones_grieger_2020, title={Emerging lanthanum (III)-containing materials for phosphate removal from water: A review towards future developments}, volume={145}, ISSN={["1873-6750"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85090951095&partnerID=MN8TOARS}, DOI={10.1016/j.envint.2020.106115}, abstractNote={The last two decades have seen a rise in the development of lanthanum (III)-containing materials (LM) for controlling phosphate in the aquatic environment. >70 papers have been published on this topic in the peer-reviewed literature, but mechanisms of phosphate removal by LM as well as potential environmental impacts of LM remain unclear. In this review, we summarize peer-reviewed scientific articles on the development and use of 80 different types of LM in terms of prospective benefits, potential ecological impacts, and research needs. We find that the main benefits of LM for phosphate removal are their ability to strongly bind phosphate under diverse environmental conditions (e.g., over a wide pH range, in the presence of diverse aqueous constituents). The maximum phosphate uptake capacity of LM correlates primarily with the La content of LM, whereas reaction kinetics are influenced by LM formulation and ambient environmental conditions (e.g., pH, presence of co-existing ions, ligands, organic matter). Increased La solubilization can occur under some environmental conditions, including at moderately acidic pH values (i.e., < 4.5–5.6), highly saline conditions, and in the presence of organic matter. At the same time, dissolved La will likely undergo hydrolysis, bind to organic matter, and combine with phosphate to precipitate rhabdophane (LaPO4·H2O), all of which reduce the bioavailability of La in aquatic environments. Overall, LM use presents a low risk of adverse effects in water with pH > 7 and moderate-to-high bicarbonate alkalinity, although caution should be applied when considering LM use in aquatic systems with acidic pH values and low bicarbonate alkalinity. Moving forward, we recommend additional research dedicated to understanding La release from LM under diverse environmental conditions as well as long-term exposures on ecological organisms, particularly primary producers and benthic organisms. Further, site-specific monitoring could be useful for evaluating potential impacts of LM on both biotic and abiotic systems post-application.}, journal={ENVIRONMENT INTERNATIONAL}, author={Zhi, Yue and Zhang, Chuhui and Hjorth, Rune and Baun, Anders and Duckworth, Owen W. and Call, Douglas F. and Knappe, Detlef R. U. and Jones, Jacob L. and Grieger, Khara}, year={2020}, month={Dec} }
 @article{rai_fisher_duckworth_baars_2020, title={Extraction and Detection of Structurally Diverse Siderophores in Soil}, volume={11}, ISSN={["1664-302X"]}, DOI={10.3389/fmicb.2020.581508}, abstractNote={Although the biochemistry of bacterial and fungal siderophores has been intensively studied in laboratory cultures, their distribution and impacts on nutrient cycling and microbial communities in soils remain poorly understood. The detection of siderophores in soil is an analytical challenge because of the complexity of the soil matrix and their structural diversity. Liquid chromatography-mass spectrometry (LC-MS) is a suitable method for the sensitive analysis of siderophores in complex samples; however, siderophore extraction into liquid phases for analysis by LC-MS is problematic because of their adsorption to soil particles and organic matter. To determine extraction efficiencies of structurally diverse siderophores, spike-recovery experiments were set up with standards representing the three main siderophore classes: the hydroxamate desferrioxamine B (DFOB), the α-hydroxycarboxylate rhizoferrin, and the catecholate protochelin. Previously used solvent extractions with water or methanol recovered only a small fraction (< 35%) of siderophores, including < 5% for rhizoferrin and protochelin. We designed combinatorial chemical extractions (22 total solutions) to target siderophores associated with different soil components. A combination of calcium chloride and ascorbate achieved high and, for some soils, quantitative extraction of DFOB and rhizoferrin. Protochelin analysis was complicated by potential fast oxidation and interactions with colloidal soil components. Using the optimized extraction method, we detected α-hydroxycarboxylate type siderophores (viz. rhizoferrin, vibrioferrin, and aerobactin) in soil for the first time. Concentrations reached 461 pmol g–1, exceeding previously reported concentrations of siderophores in soil and suggesting a yet unrecognized importance of α-hydroxycarboxylate siderophores for biological interactions and biogeochemical processes in soil.}, journal={FRONTIERS IN MICROBIOLOGY}, author={Rai, Vineeta and Fisher, Nathaniel and Duckworth, Owen W. and Baars, Oliver}, year={2020}, month={Sep} }
 @article{roper_duckworth_grossman_israel_2020, title={Rhizobium leguminosarum strain combination effects on nodulation and biological nitrogen fixation with Vicia villosa}, volume={156}, ISSN={["1873-0272"]}, DOI={10.1016/j.apsoil.2020.103703}, abstractNote={Biological nitrogen fixation (BNF) resulting from symbiosis between legumes and rhizobia helps improve soil N fertility. Inoculated soils with specific strains of rhizobia may increase potential BNF in legumes, but the efficacy of these rhizobia in promoting BNF may be limited by competition from resident rhizobia already present in soils. We evaluated the ability of four strains to nodulate and increase BNF in Vicia villosa (hairy vetch) as individual and combined inoculants in a laboratory experiment. Plants were inoculated with a single rhizobia strain or equal ratios of 2–4 strains and grown under controlled conditions. After 46 d, entire plant shoots were harvested and analyzed for biomass, N content, nodule number, and nodule mass. Nodule occupancy was assessed using DNA fingerprinting of characteristic rhizobia genes. Negative-N and positive-N control plants averaged 6.3 and 83.2 mg total shoot N, respectively. Average total shoot N of inoculated vetch treatments was between that of uninoculated control treatments. Nodule number, total nodule mass, and BNF efficiency (total shoot N mass per total nodule mass) did not significantly differ between individual strains. Neither BNF efficiency nor nodule number were indicative of competitive ability for nodule occupancy. The rhizobia did not display any consistent synergistic or antagonistic effect on BNF when combined in inoculants and nodule occupancy did not favor any specific strain. Because vetch inoculated with local Rlv strains produced similar amounts of N as uninoculated plants fertilized with N, our work suggests that rhizobia sourced from local soils may produce sufficient BNF with vetch.}, journal={APPLIED SOIL ECOLOGY}, author={Roper, Wayne R. and Duckworth, Owen W. and Grossman, Julie M. and Israel, Daniel W.}, year={2020}, month={Dec} }
 @article{sowers_wani_coward_fischel_betts_douglas_duckworth_sparks_2020, title={Spatially Resolved Organomineral Interactions across a Permafrost Chronosequence}, volume={54}, ISSN={["1520-5851"]}, DOI={10.1021/acs.est.9b06558}, abstractNote={Permafrost contains a large (1,700 Pg C) terrestrial pool of organic matter (OM) that is susceptible to degradation as global temperatures increase. Of particular importance is syngenetic Yedoma permafrost containing high organic matter content. Reactive iron phases promote stabilizing interactions between OM and soil minerals and this stabilization may be of increasing importance in permafrost as the thawed surface region ("active layer") deepens. However, there is limited understanding of Fe and other soil mineral phase associations with OM carbon (C) moieties in permafrost soils. To elucidate the elemental associations involved in organomineral complexation within permafrost systems, soil cores spanning a Pleistocene permafrost chronosequence (19,000, 27,000, and 36,000 years old) were collected from an underground tunnel near Fairbanks, Alaska. Subsamples were analyzed via STXM-NEXAFS spectroscopy at the nano to micro scale. Amino acid-rich moieties decreased in abundance across the chronosequence. Strong correlations between C and Fe with discrete Fe(III) or Fe(II) regions selectively associated with specific OM moieties were observed. Additionally, Ca co-associated with C through potential cation bridging mechanisms. Results indicate Fe(III), Fe(II), and mixed valence phases associated with OM throughout diverse permafrost environments, suggesting organomineral complexation is crucial to predict C stability as permafrost systems warm.}, number={5}, journal={ENVIRONMENTAL SCIENCE & TECHNOLOGY}, author={Sowers, Tyler D. and Wani, Rucha P. and Coward, Elizabeth K. and Fischel, Matthew H. H. and Betts, Aaron R. and Douglas, Thomas A. and Duckworth, Owen W. and Sparks, Donald L.}, year={2020}, month={Mar}, pages={2951–2960} }
 @article{gillispie_matteson_duckworth_neumann_phen_polizzotto_2019, title={Chemical variability of sediment and groundwater in a Pleistocene aquifer of Cambodia: Implications for arsenic pollution potential}, volume={245}, ISSN={["1872-9533"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85057339109&partnerID=MN8TOARS}, DOI={10.1016/j.gca.2018.11.008}, abstractNote={Low-arsenic (As) groundwater from Pleistocene aquifers is vulnerable to future geogenic and allogenic arsenic pollution in South and Southeast Asia, threatening the millions who use it as a “safe source” of drinking and irrigation water. The abundance and chemical reactivity of iron and manganese oxides within these aquifer sediments control the occurrence and mobility of arsenic. In the present study, sediment samples varying in As, Fe, and Mn content were obtained from a Pleistocene aquifer in the Kandal Province of Cambodia. Laboratory and spectroscopic characterization of the sediment combined with groundwater analyses revealed that the availability and abundance of sedimentary As varied across a Pleistocene aquifer from the pore to field scales. Concentrations of sediment As (0.47–7 μg/g) correlated more strongly with Fe (R2 > 0.66) than with Mn (R2 > 0.35) concentrations in sediment well cuttings and tended to peak between 10 and 15 m. Chemical extractions and X-ray adsorption spectroscopy indicated the majority of As was strongly adsorbed to aquifer sediments or coprecipitated in oxides in the form of As(V) but that As(III) could be found in sediment microenvironments across the aquifer. Groundwater chemistry and Mn mineralogy indicated that the Pleistocene aquifer was suboxic, with average dissolved oxygen of 1.9 mg/L (±0.9 mg/L), redox potential of 0.155 V (±0.097 V), and abundant Mn(III/IV) oxide minerals. According to our results, allogenic As transport and geogenic As release will likely be dictated by localized geochemical processes that vary over a range of scales. Collectively, the specific Fe and Mn mineralogy and content within aquifers will ultimately govern As pollution potential, so understanding their multi-scale distributions and variability is essential for better predicting future risks to well-water quality in currently low-As aquifers.}, journal={GEOCHIMICA ET COSMOCHIMICA ACTA}, author={Gillispie, Elizabeth C. and Matteson, Audrey R. and Duckworth, Owen W. and Neumann, Rebecca B. and Phen, Nuon and Polizzotto, Matthew L.}, year={2019}, month={Jan}, pages={441–458} }
 @article{field_whitaker_henson_duckworth_2019, title={Sorption of copper and phosphate to diverse biogenic iron (oxyhydr) oxide deposits}, volume={697}, ISSN={["1879-1026"]}, DOI={10.1016/j.scitotenv.2019.134111}, abstractNote={Iron (Fe) transformations partially control the biogeochemical cycling of biologically and environmentally important elements, such as carbon (C), nitrogen (N), phosphorus (P), and trace metals. In marine and freshwater environments, iron oxidizing bacteria commonly promote the oxidation of ferrous iron (Fe(II)) at circumneutral oxic-anoxic interfaces, resulting in the formation of mineral-organic composites known as biogenic Fe(III) (oxyhydr)oxides (BIOS). Previous studies have examined the microbial ecology, composition, morphology, and sorption reactivity of BIOS. However, a broad survey of BIOS properties and sorption reactivity is lacking. To further explore these relationships, this study utilized X-ray absorption spectroscopy (XAS) to characterize the Fe mineral species, acid digestions and elemental analysis to determine composition, Brunauer-Emmett-Teller (BET) analysis to measure specific surface area, and copper (Cu) and phosphorus (P) adsorption experiments at concentrations designed to measure maximum sorption to evaluate reactivity of BIOS samples collected in lakes and streams of the North Carolina Piedmont. Sample composition varied widely, with Fe and C content ranging from 6.3 to 34% and 3.4-13%, respectively. XAS spectra were best fit with 42-100% poorly crystalline Fe (oxyhydr)oxides, with the remainder composed of crystalline Fe minerals and organic complexes. On a sorbent mass basis, Cu and P sorption varied by a factor of two and 15, respectively. Regression analyses reveal interrelationships between physicochemical properties, and suggest that differences in P binding are driven by sorption to Fe(III) (oxyhydr)oxide surfaces. In total, results suggest that the physical and chemical characteristics of organic and Fe(III) (oxyhydr)oxide phases in BIOS interplay to control the sorption of solutes, and thus influence nutrient and contaminant cycling in soil and natural waters.}, journal={SCIENCE OF THE TOTAL ENVIRONMENT}, author={Field, Hannah R. and Whitaker, Andrew H. and Henson, Joshua A. and Duckworth, Owen W.}, year={2019}, month={Dec} }
 @article{cr(vi) uptake and reduction by biogenic iron (oxyhydr)oxides_2018, volume={20}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85050571845&partnerID=MN8TOARS}, DOI={10.1039/c8em00149a}, abstractNote={Biogenic iron (oxyhydr)oxides adsorb dissolved Cr(vi), as well as promote its reduction to less mobile and toxic Cr(iii)viaa Fe(ii) mediated process.}, number={7}, journal={Environmental Science: Processes and Impacts}, year={2018}, pages={1056–1068} }
 @article{whitaker_duckworth_2018, title={Cu, Pb, and Zn Sorption to Biogenic Iron (Oxyhydr)Oxides Formed in Circumneutral Environments}, volume={2}, ISSN={["2571-8789"]}, DOI={10.3390/soilsystems2020018}, abstractNote={The transportation and immobilization of potentially toxic metals in near-surface environments may be partially controlled by sorption processes at the solid-water interface. Myriad studies have shown that iron (oxyhydr)oxides have large sorption capacities and form strong surface complexes with metal ions. Biogenic iron (oxyhydr)oxides (BIOS) form at redox gradients where dissolved ferrous iron encounters oxygenated conditions, allowing bacteria to outcompete abiotic Fe oxidation. This process produces biominerals with distinct surface and structural properties (incorporation of cell-derived organic matter, poor crystallinity, and small particle sizes) that may alter their metal-binding affinity and sorption processes. To better understand metal binding by BIOS, Cu, Pb, and Zn, sorption rate and isotherm studies were conducted with synthetic two-line ferrihydrite and BIOS. Additionally, X-ray absorption spectroscopy and total scattering were used to elucidate the BIOS mineral structure and metal ion surface structures. On a mass normalization basis, BIOS sorbed approximately 8, 4, and 2 times more Cu, Pb, and Zn, respectively, than 2LFh over similar dissolved concentrations. Spectroscopic analyses revealed poorly crystalline structures and small coherent scattering domain sizes for BIOS. Additionally, extended X-ray absorption fine-structure spectroscopy revealed Cu, Pb, and Zn sorbed to BIOS via inner-sphere complexes, similar to 2LFh. These results suggest that, in metal contaminated environments, BIOS are more efficient in metal binding than their synthetic counterparts.}, number={2}, journal={SOIL SYSTEMS}, author={Whitaker, Andrew H. and Duckworth, Owen W.}, year={2018}, month={Jun} }
 @article{duckworth_markarian_parker_harrington_2017, title={A two-column flash chromatography approach to pyoverdin production from Pseudomonas putida GB1}, volume={135}, ISSN={["1872-8359"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85012261822&partnerID=MN8TOARS}, DOI={10.1016/j.mimet.2017.01.019}, abstractNote={Our knowledge of the biological and environmental reactivity of siderophores is limited by the difficulty and cost of obtaining reasonable quantities by purification or synthesis. In this note, we describe a modified procedure for the low-cost, mg-scale purification of pyoverdin-type siderophores using a dual-flash chromatography (reverse-phase absorption and size exclusion) approach.}, journal={JOURNAL OF MICROBIOLOGICAL METHODS}, author={Duckworth, Owen W. and Markarian, Dawn S. and Parker, Dorothy L. and Harrington, James M.}, year={2017}, month={Apr}, pages={11–13} }
 @article{almaraz_whitaker_andrews_duckworth_2017, title={Assessing Biomineral Formation by Iron-oxidizing Bacteria in a Circumneutral Creek}, volume={160}, ISSN={["1936-704X"]}, DOI={10.1111/j.1936-704x.2017.03240.x}, abstractNote={Abstract}, number={1}, journal={JOURNAL OF CONTEMPORARY WATER RESEARCH & EDUCATION}, author={Almaraz, Nohemi and Whitaker, Andrew H. and Andrews, Megan Y. and Duckworth, Owen W.}, year={2017}, month={Apr}, pages={60–71} }
 @article{rivera_hesterberg_kaur_duckworth_2017, title={Chemical Speciation of Potentially Toxic Trace Metals in Coal Fly Ash Associated with the Kingston Fly Ash Spill}, volume={31}, ISSN={["1520-5029"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85029897922&partnerID=MN8TOARS}, DOI={10.1021/acs.energyfuels.7b00020}, abstractNote={Coal ash released into the environment may release toxic trace elements into water, sediments, and soils. The objective of this study was to characterize the chemical speciation of As, Se, Cu, Zn, Cr, and U in coal fly ash samples related to the 2008 Kingston ash spill. Three ash samples were analyzed using X-ray absorption spectroscopy (XAS) to determine oxidation states or dominant species of trace elements, which were previously found to range in concentration from 8 to 20 mg kg–1. Linear combination fitting (LCF) of X-ray absorption near edge structure (XANES) spectra from ash samples indicated that both reduced and oxidized forms of the trace elements were present in the fly ash samples. We used the mineralogical composition of the fly ash to select the most relevant standards for LCF fitting of XANES spectra, which included metal-doped glasses, trace elements sorbed to iron oxy(hydroxides), and pure mineral phases for each element. Arsenic K-edge XANES spectra were best fit as oxidized As(V) (95–100...}, number={9}, journal={ENERGY & FUELS}, author={Rivera, Nelson and Hesterberg, Dean and Kaur, Navdeep and Duckworth, Owen W.}, year={2017}, month={Sep}, pages={9652–9659} }
 @article{robarge_duckworth_osmond_smyth_river_2017, title={Commentary on "A possible trade-off between clean air and clean water" by Smith et al. (2017)}, volume={72}, ISSN={["1941-3300"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85033378332&partnerID=MN8TOARS}, DOI={10.2489/jswc.72.6.121a}, abstractNote={Authors of the recent feature article by Smith et al., which was published in the A Section of the July/August 2017 issue of the Journal of Soil and Water Conservation (A Section articles are not peer-reviewed while articles in the Research Section of the journal are peer-reviewed), have conducted water quality research in the Western Lake Erie Basin (WLEB) and demonstrated multiple potential causes of increased soluble phosphorous (SP) loading, including agricultural practice changes such as increased no-till, tile drainage, surface application of fall fertilizer, and weather. In their article, “A possible trade-off between clean air and clean water,” these authors propose an additional cause: the connection between the success of the Clean Air Act in improving air quality in the United States and increases in SP loading that have contributed to harmful algal blooms in the WLEB. Although we agree that scientists must always be vigilant for pernicious consequences of well-meaning actions, we believe that there are flaws in the study design and data interpretation that undermine the conclusion of article. Specifically, what we see as flaws in the interpretation of the data presented in table 1 and figures 2 and 3 are described below. Furthermore, the authors do…}, number={6}, journal={JOURNAL OF SOIL AND WATER CONSERVATION}, author={Robarge, Wayne and Duckworth, Owen and Osmond, Deanna and Smyth, Jot and River, Mark}, year={2017}, pages={121A–122A} }
 @article{duckworth_rivera_gardner_andrews_santelli_polizzotto_2017, title={Morphology, structure, and metal binding mechanisms of biogenic manganese oxides in a superfund site treatment system}, volume={19}, ISSN={["2050-7895"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85010739340&partnerID=MN8TOARS}, DOI={10.1039/c6em00525j}, abstractNote={Manganese oxides, which may be biogenically produced in both pristine and contaminated environments, have a large affinity for many trace metals.}, number={1}, journal={ENVIRONMENTAL SCIENCE-PROCESSES & IMPACTS}, author={Duckworth, O. W. and Rivera, N. A. and Gardner, T. G. and Andrews, M. Y. and Santelli, C. M. and Polizzotto, M. L.}, year={2017}, month={Jan}, pages={50–58} }
 @article{scheibener_rivera_hesterberg_duckworth_buchwalter_2017, title={Periphyton uptake and trophic transfer of coal fly-ash-derived trace elements}, volume={36}, ISSN={["1552-8618"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85024902922&partnerID=MN8TOARS}, DOI={10.1002/etc.3864}, abstractNote={Abstract}, number={11}, journal={ENVIRONMENTAL TOXICOLOGY AND CHEMISTRY}, author={Scheibener, Shane A. and Rivera, Nelson A. and Hesterberg, Dean and Duckworth, Owen W. and Buchwalter, David B.}, year={2017}, month={Nov}, pages={2991–2996} }
 @article{duckworth_andrews_cubeta_grunden_ojiambo_2017, title={Revisiting Graduate Student Training to Address Agricultural and Environmental Societal Challenges}, volume={2}, ISSN={2471-9625}, url={http://dx.doi.org/10.2134/ael2017.06.0019}, DOI={10.2134/ael2017.06.0019}, abstractNote={Core Ideas
Society is faced with daunting environmental and agricultural challenges.
There is a pressing need for multidisciplinary teams of collaborative scientists.
Novel graduate educational models may be needed to train students to address grand challenges.
An example of illustrating the model through microbiome science of plants and soil is presented.
}, number={1}, journal={Agricultural & Environmental Letters}, publisher={Wiley}, author={Duckworth, Owen W. and Andrews, Megan Y. and Cubeta, Marc A. and Grunden, Amy M. and Ojiambo, Peter S.}, year={2017}, pages={170019} }
 @article{saad_sun_chen_borkiewicz_zhu_duckworth_tang_2017, title={Siderophore and Organic Acid Promoted Dissolution and Transformation of Cr(III)-Fe(III)-(oxy)hydroxides}, volume={51}, ISSN={["1520-5851"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85018405588&partnerID=MN8TOARS}, DOI={10.1021/acs.est.6b05408}, abstractNote={The role of microbial activities on the transformation of chromium (Cr) remediation products has generally been overlooked. This study investigated the stability of Cr(III)-Fe(III)-(oxy)hydroxides, common Cr(VI) remediation products, with a range of compositions in the presence of common microbial exudates, siderophores and small organic acids. In the presence of a representative siderophore, desferrioxamine B (DFOB), iron (Fe) was released at higher rates and to greater extents relative to Cr from all solid phases. The presence of oxalate alone caused the release of Cr, but not of Fe, from all solid phases. In the presence of both DFOB and oxalate, oxalate acted synergistically with DFOB to increase the Fe, but not the Cr, release rate. Upon reaction with DFOB or DFOB + oxalate, the remaining solids became enriched in Cr relative to Fe. Such incongruent dissolution led to solid phases with different compositions and increased solubility relative to the initial solid phases. Thus, the presence of microbial exudates can promote the release of Cr(III) from remediation products via both ligand complexation and increased solid solubility. Understanding the potential reaction kinetics and pathways of Cr(VI) remediation products in the presence of microbial activities is necessary to assess their long-term stability.}, number={6}, journal={ENVIRONMENTAL SCIENCE & TECHNOLOGY}, author={Saad, Emily M. and Sun, Jingying and Chen, Shuo and Borkiewicz, Olaf J. and Zhu, Mengqiang and Duckworth, Owen W. and Tang, Yuanzhi}, year={2017}, month={Mar}, pages={3223–3232} }
 @article{sowers_harrington_polizzotto_duckworth_2017, title={Sorption of arsenic to biogenic iron (oxyhydr)oxides produced in circumneutral environments}, volume={198}, ISSN={["1872-9533"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85007364750&partnerID=MN8TOARS}, DOI={10.1016/j.gca.2016.10.049}, abstractNote={Arsenic (As) is a widespread and problematic pollutant that can be derived from natural or anthropogenic sources. Iron (oxyhydr)oxides readily sorb As and thus play critical roles in As cycling in terrestrial environments; however, little is known about the affinity and mechanism of As sorption by biogenic iron (oxyhydr)oxides formed in circumneutral environments. To investigate this, we conducted sorption isotherm and kinetics experiments to compare As(V) and As(III) sorption to synthetic 2-line ferrihydrite and iron biominerals harvested from the hyporheic zone of an uncontaminated creek. Inductively coupled plasma mass spectrometry (ICP-MS) was used to quantify both As(V) and As(III), and X-ray absorption spectroscopy (XAS) was utilized to obtain As and Fe K-edge spectra for As(V) and As(III) sorbed to environmentally collected and laboratory produced Fe(III) minerals. All environmental Fe(III) biominerals were determined to be structurally similar to 2-line ferrihydrite. However, environmental Fe(III) biominerals have a surface area normalized affinity for As(V) and for As(III) that is greater than or equivalent to synthetic 2-line ferrihydrite. Whereas the extent of sorption was similar for As(III) on all minerals, As(V) sorption to environmental Fe(III) biominerals was approximately three times higher than what was observed for synthetic 2-line ferrihydrite. Structural modeling of EXAFS spectra revealed that the same surface complexation structure was formed by As(V) and by As(III) on environmental Fe(III) biominerals and ferrihydrite. These results suggest that, despite similarities in binding mechanisms, Fe(III) biominerals may be more reactive sorbents that synthetic surrogates often used to model environmental reactivity.}, journal={GEOCHIMICA ET COSMOCHIMICA ACTA}, author={Sowers, Tyler D. and Harrington, James M. and Polizzotto, Matthew L. and Duckworth, Owen W.}, year={2017}, month={Feb}, pages={194–207} }
 @article{andrews_duckworth_2016, title={A universal assay for the detection of siderophore activity in natural waters}, volume={29}, ISSN={["1572-8773"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84994285887&partnerID=MN8TOARS}, DOI={10.1007/s10534-016-9979-4}, abstractNote={Siderophores, a family of biogenic metal chelating agents, play critical roles in the biogeochemical cycling of Fe and other metals by facilitating their solubilization and uptake in circumneutral to alkaline oxic environments. However, because of their small concentrations (ca. nM) and large number of molecular structures, siderophore detection and quantification in environmental samples requires specialized equipment and expertise, and often requires pre-concentration of samples, which may introduce significant bias. The "universal" CAS assay, which was originally designed for use in bacterial cultures, quantifies the iron chelating function of a pool of siderophores but only at concentrations (>2 µM) well above the concentrations estimated to be present in marine, freshwater, and soil samples. In this manuscript, we present a high sensitivity modification of this universal assay (HS-CAS) suitable for detecting and quantifying siderophore activity in the nM concentration range, allowing for direct quantitation of siderophore reactivity in transparent aqueous samples.}, number={6}, journal={BIOMETALS}, author={Andrews, Megan Y. and Duckworth, Owen}, year={2016}, month={Dec}, pages={1085–1095} }
 @article{andrews_santelli_duckworth_2016, title={Digital image quantification of siderophores on agar plates}, volume={6}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84957059723&partnerID=MN8TOARS}, DOI={10.1016/j.dib.2016.01.054}, abstractNote={This article presents visual image data and detailed methodology for the use of a new method for quantifying the exudation of siderophores during fungal growth. The data include images showing time series for calibration, fungal exudation, and negative controls, as well as replication accuracy information. In addition, we provide detailed protocols for making CAS assay layer plates, the digital analysis protocol for determining area of color change, and discuss growth media that do and do not work with the layer plate method. The results of these data, their interpretation, and further discussion can be found in Andrews et al., 2016 [1].}, journal={Data in Brief}, author={Andrews, M.Y. and Santelli, C.M. and Duckworth, O.W.}, year={2016}, pages={890–898} }
 @article{andrews_santelli_duckworth_2016, title={Layer plate CAS assay for the quantitation of siderophore production and determination of exudation patterns for fungi}, volume={121}, ISSN={["1872-8359"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84953386448&partnerID=MN8TOARS}, DOI={10.1016/j.mimet.2015.12.012}, abstractNote={The chrome azurol S (CAS) assay measures the chelating activity of siderophores, but its application (especially to fungi) is limited by toxicity issues. In this note, we describe a modified version of the CAS assay that is suitable for quantifying siderophore exudation for microorganisms, including fungi.}, journal={JOURNAL OF MICROBIOLOGICAL METHODS}, author={Andrews, Megan Y. and Santelli, Cara M. and Duckworth, Owen W.}, year={2016}, month={Feb}, pages={41–43} }
 @article{gillispie_austin_rivera_bolich_duckworth_bradley_amoozegar_hesterberg_polizzotto_2016, title={Soil Weathering as an Engine for Manganese Contamination of Well Water}, volume={50}, ISSN={["1520-5851"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84988464669&partnerID=MN8TOARS}, DOI={10.1021/acs.est.6b01686}, abstractNote={Manganese (Mn) contamination of well water is recognized as an environmental health concern. In the southeastern Piedmont region of the United States, well water Mn concentrations can be >2 orders of magnitude above health limits, but the specific sources and causes of elevated Mn in groundwater are generally unknown. Here, using field, laboratory, spectroscopic, and geospatial analyses, we propose that natural pedogenetic and hydrogeochemical processes couple to export Mn from the near-surface to fractured-bedrock aquifers within the Piedmont. Dissolved Mn concentrations are greatest just below the water table and decrease with depth. Solid-phase concentration, chemical extraction, and X-ray absorption spectroscopy data show that secondary Mn oxides accumulate near the water table within the chemically weathering saprolite, whereas less-reactive, primary Mn-bearing minerals dominate Mn speciation within the physically weathered transition zone and bedrock. Mass-balance calculations indicate soil weathering has depleted over 40% of the original solid-phase Mn from the near-surface, and hydrologic gradients provide a driving force for downward delivery of Mn. Overall, we estimate that >1 million people in the southeastern Piedmont consume well water containing Mn at concentrations exceeding recommended standards, and collectively, these results suggest that integrated soil-bedrock-system analyses are needed to predict and manage Mn in drinking-water wells.}, number={18}, journal={ENVIRONMENTAL SCIENCE & TECHNOLOGY}, author={Gillispie, Elizabeth C. and Austin, Robert E. and Rivera, Nelson A. and Bolich, Rick and Duckworth, Owen W. and Bradley, Phil and Amoozegar, Aziz and Hesterberg, Dean and Polizzotto, Matthew L.}, year={2016}, month={Sep}, pages={9963–9971} }
 @article{droz_dumas_duckworth_pena_2015, title={A Comparison of the Sorption Reactivity of Bacteriogenic and Mycogenic Mn Oxide Nanoparticles}, volume={49}, ISSN={["1520-5851"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84926434041&partnerID=MN8TOARS}, DOI={10.1021/es5048528}, abstractNote={Biogenic MnO2 minerals affect metal fate and transport in natural and engineered systems by strongly sorbing metals ions. The ability to produce MnO2 is widely dispersed in the microbial tree of life, leading to potential differences in the minerals produced by different organisms. In this study, we compare the structure and reactivity of biogenic Mn oxides produced by the biofilm-forming bacterium Pseudomonas putida GB-1 and the white-rot fungus Coprinellus sp. The rate of Mn(II) oxidation, and thus biomineral production, was 45 times lower for Coprinellus sp. (5.1 × 10(-2) mM d(-1)) than for P. putida (2.32 mM d(-1)). Both organisms produced predominantly Mn(IV) oxides with hexagonal-sheet symmetry, low sheet stacking, small particle size, and Mn(II/III) in the interlayer. However, we found that mycogenic MnO2 could support a significantly lower quantity of Ni sorbed via inner-sphere coordination at vacancy sites than the bacteriogenic MnO2: 0.09 versus 0.14 mol Ni mol(-1) Mn. In addition, 50-100% of the adsorbed Ni partitioned to the MnO2, which accounts for less than 20% of the sorbent on a mass basis. The vacancy content, which appears to increase with the kinetics of MnO2 precipitation, exerts significant control on biomineral reactivity.}, number={7}, journal={ENVIRONMENTAL SCIENCE & TECHNOLOGY}, author={Droz, Boris and Dumas, Naomi and Duckworth, Owen W. and Pena, Jasquelin}, year={2015}, month={Apr}, pages={4200–4208} }
 @article{rivera_kaur_hesterberg_ward_austin_duckworth_2015, title={Chemical Composition, Speciation, and Elemental Associations in Coal Fly Ash Samples Related to the Kingston Ash Spill}, volume={29}, ISSN={["1520-5029"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84923296129&partnerID=MN8TOARS}, DOI={10.1021/ef501258m}, abstractNote={Environmental impacts of potentially toxic trace elements from coal fly ash are controlled in part by the mineralogy of the ash matrix and the chemical speciation of the trace elements. Our objective was to characterize the chemical and mineralogical composition of fly ash samples that are pertinent to the 2008 release of coal ash from a containment area at the Tennessee Valley Authority (TVA) Kingston fossil plant, which left 4 to 500 t of trace elements in adjoining river systems. Three fly ash samples were analyzed for elemental composition by digestion or neutron activation analysis, mineralogy and macroelement speciation by conventional and synchrotron-based X-ray diffraction (XRD and SXRD) and X-ray absorption spectroscopy (XAS), and for spatial associations of elements by electron probe microanalysis (EPMA). Ash samples were mainly composed of Si (20–27% w/w), Al (10–14% w/w), Fe (4–6% w/w), and Ca (4–6% w/w). Concentrations of selected trace elements ranged from 8 to 1480 mg kg–1, with the followi...}, number={2}, journal={ENERGY & FUELS}, author={Rivera, Nelson and Kaur, Navdeep and Hesterberg, Dean and Ward, Colin R. and Austin, Robert E. and Duckworth, Owen W.}, year={2015}, month={Feb}, pages={954–967} }
 @article{kraemer_duckworth_harrington_schenkeveld_2015, title={Metallophores and Trace Metal Biogeochemistry}, volume={21}, ISSN={["1573-1421"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84931006609&partnerID=MN8TOARS}, DOI={10.1007/s10498-014-9246-7}, number={2-4}, journal={AQUATIC GEOCHEMISTRY}, author={Kraemer, Stephan M. and Duckworth, Owen W. and Harrington, James M. and Schenkeveld, Walter D. C.}, year={2015}, month={Jul}, pages={159–195} }
 @article{gillispie_sowers_duckworth_polizzotto_2015, title={Soil Pollution Due to Irrigation with Arsenic-Contaminated Groundwater: Current State of Science}, volume={1}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85039931557&partnerID=MN8TOARS}, DOI={10.1007/s40726-015-0001-5}, abstractNote={Food with elevated arsenic concentrations is becoming widely recognized as a global threat to human health. This review describes the current state of knowledge of soil pollution derived from irrigation with arsenic-contaminated groundwater, highlighting processes controlling arsenic cycling in soils and resulting arsenic impacts on crop and human health. Irrigation practices utilized for both flooded and upland crops have the potential to load arsenic to soils, with a host of environmental and anthropogenic factors ultimately determining the fate of arsenic. Continual use of contaminated groundwater for irrigation may result in soils with concentrations sufficient to create dangerous arsenic concentrations in the edible portions of crops. Recent advances in low-cost water and soil management options show promise for mitigating arsenic impacts of polluted soils. Better understanding of arsenic transfer from soil to crops and the controls on long-term soil arsenic accumulation is needed to establish effective arsenic mitigation strategies within vulnerable agronomic systems.}, number={1}, journal={Current Pollution Reports}, author={Gillispie, E.C. and Sowers, T.D. and Duckworth, O.W. and Polizzotto, M.L.}, year={2015} }
 @article{harrington_duckworth_haselwandter_2015, title={The fate of siderophores: antagonistic environmental interactions in exudate-mediated micronutrient uptake}, volume={28}, ISSN={["1572-8773"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84939962296&partnerID=MN8TOARS}, DOI={10.1007/s10534-015-9821-4}, abstractNote={Organisms acquire metals from the environment by releasing small molecules that solubilize and promote their specific uptake. The best known example of this nutrient uptake strategy is the exudation of siderophores, which are a structurally-diverse class of molecules that are traditionally viewed as being integral to iron uptake. Siderophores have been proposed to act through a variety of processes, but their effectiveness can be mitigated by a variety of chemical and physical processes of both biotic and abiotic origin. Processes that occur at the surface of minerals can degrade or sequester siderophores, preventing them from fulfilling their function of returning metals to the organism. In addition, biotic processes including enzymatic degradation of the siderophore and piracy of the metal or of the siderophore complex also disrupt iron uptake. Some organisms have adapted their nutrient acquisition strategies to address these potential pitfalls, producing multiple siderophores and other exudates that take advantage of varying kinetic and thermodynamic factors to allow the continued uptake of metals. A complete understanding of the factors that contribute to metal uptake in nature will require a concerted effort to study processes identified in laboratory systems in the context of more complicated environmental systems.}, number={3}, journal={BIOMETALS}, author={Harrington, James M. and Duckworth, Owen W. and Haselwandter, Kurt}, year={2015}, month={Jun}, pages={461–472} }
 @article{kang_mclaughlin_amoozegar_heitman_duckworth_2015, title={Transport of dissolved polyacrylamide through a clay loam soil}, volume={243}, ISSN={["1872-6259"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84920432092&partnerID=MN8TOARS}, DOI={10.1016/j.geoderma.2014.12.022}, abstractNote={Polyacrylamide (PAM) is becoming a widely used soil conditioning and erosion control agent, and a better understanding of its transport is required to improve its use. In this study vertical PAM transport through a clay loam soil was investigated using thin soil columns (7.62-cm diameter × 2-cm thick) under saturated condition. The columns received a water-soluble, anionic PAM solution (16 Mg mol− 1 with 50 mol% charge density) under pulse and step (continuous) inputs using a constant-head method. The pulse input was 500 mg L− 1 PAM solution applied for 0.6 pore volume (PV), after which the input was switched to deionized (DI) water for 25 PVs. The step input was 25 mg L− 1 PAM solution applied continuously for 129 PVs. Saturated hydraulic conductivity (Ksat) was measured prior to PAM application and was monitored during PAM and DI water leaching. Leachate samples were collected frequently with time from each column and analyzed for the dissolved PAM concentration. The PAM applications reduced Ksat to 1% of the initial Ksat (4 cm h− 1) under the pulse input and to 0.3% of the initial Ksat under the step input. Transport of PAM was best-fitted with a two-region (dual-porosity) model. The fitted retardation factor (R) was more than two-fold greater for the step input (R = 2695) than for the pulse input (R = 1242). The results from transport modeling and pore size distribution analysis suggested that viscous PAM solution contributes to a mechanical entrapment of the PAM molecules, clogging most water-conducting pores smaller than 225–274 μm in diameter. Under saturated condition, either the pulse or step input of dissolved PAM could reduce seepage with limited mobility in the soil profile.}, journal={GEODERMA}, author={Kang, Jihoon and McLaughlin, Richard A. and Amoozegar, Aziz and Heitman, Joshua L. and Duckworth, Owen W.}, year={2015}, month={Apr}, pages={108–114} }
 @article{akafia_harrington_bargar_duckworth_2014, title={Metal oxyhydroxide dissolution as promoted by structurally diverse siderophores and oxalate}, volume={141}, ISSN={["1872-9533"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84904861957&partnerID=MN8TOARS}, DOI={10.1016/j.gca.2014.06.024}, abstractNote={Siderophores, a class of biogenic ligands with high affinities for Fe(III), promote the dissolution of metal ions from sparingly soluble mineral phases. However, most geochemical studies have focused on quantifying the reactivity of DFOB, a model trishydroxamate siderophore. This study utilized three different siderophores, desferrioxamine B, rhizoferrin, and protochelin, with structures that contain the most commonly observed binding moieties of microbial siderophores to examine the siderophore-promoted dissolution rates of FeOOH, CoOOH, and MnOOH in the absence and presence of the ubiquitous low molecular mass organic acid oxalate by utilizing batch dissolution experiments at pH = 5–9. Metal-siderophore complex and total dissolved metal concentrations were monitored for durations of one hour to fourteen days, depending on the metal oxyhydroxide identity and solution pH. The results demonstrate that MnOOH and CoOOH generally dissolve more quickly in the presence of siderophores than FeOOH. Whereas FeOOH dissolved exclusively by a ligand-promoted dissolution mechanism, MnOOH and CoOOH dissolved predominantly by a reductive dissolution mechanism under most experimental conditions. For FeOOH, siderophore-promoted dissolution rates trended with the stability constant of the corresponding aqueous Fe(III) complex. In the presence of oxalate, measured siderophore-promoted dissolution rates were found to increase, decrease, or remain unchanged as compared to the observed rates in single-ligand systems, depending on the pH of the system, the siderophore present, and the identity of the metal oxyhydroxide. Increases in observed dissolution rates in the presence of oxalate were generally greater for FeOOH than for MnOOH or CoOOH. These results elucidate potential dissolution mechanisms of both ferric and non-ferric oxyhydroxide minerals by siderophores in the environment, and may provide further insights into the biological strategies of metal acquisition facilitated by coordinated exudation of low molecular weight organic acids and siderophores.}, journal={GEOCHIMICA ET COSMOCHIMICA ACTA}, author={Akafia, Martin M. and Harrington, James M. and Bargar, John R. and Duckworth, Owen W.}, year={2014}, month={Sep}, pages={258–269} }
 @article{duckworth_akafia_andrews_bargar_2014, title={Siderophore-promoted dissolution of chromium from hydroxide minerals}, volume={16}, ISSN={["2050-7895"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84901660425&partnerID=MN8TOARS}, DOI={10.1039/c3em00717k}, abstractNote={Biomolecules have significant impacts on the fate and transport of contaminant metals in soils and natural waters. Siderophores, Fe(iii)-binding agents that are exuded by microbes and plants, may form strong complexes with and promote the dissolution of contaminant metal ions, such as Co(iii), U(iv), or Pu(iv). Although aqueous Cr(iii)-siderophore complexes have been recognized in the laboratory setting for almost 40 years, few studies have explored interactions of siderophores with Cr-bearing minerals or considered their impacts on environmental chemistry. To better understand the possible effects of siderophores on chromium mobility, we conducted a series of dissolution experiments to quantify the dissolution rates of Cr(iii)(OH)3 in the presence of hydroxamate, catecholate, and α-hydroxycarboxylate siderophores over a range of environmentally relevant pH values. At pH = 5, dissolution rates in the presence of siderophores are similar to control experiments, suggesting a predominantly proton-promoted dissolution mechanism. At pH = 8, the sorption of the siderophores desferrioxamine B and rhizoferrin can be modeled by using Langmuir isotherms. The dissolution rates for these siderophores are proportional to the surface concentrations of sorbed siderophore, and extended X-ray absorption fine structure spectra of dissolution products indicates the formation of Cr(iii)HDFOB(+) and Cr(iii)rhizoferrin(3-) complexes, suggesting a ligand-promoted dissolution mechanism at alkaline pH. Because siderophores promote Cr(iii)(OH)3 dissolution at rates similar in magnitude to those of iron hydroxides and the resulting Cr(iii)-siderophore complexes may be persistent in solution, siderophores could potentially contribute to the mobilization of Cr in soils and sediments where it is abundant due to geological or anthropogenic sources.}, number={6}, journal={ENVIRONMENTAL SCIENCE-PROCESSES & IMPACTS}, author={Duckworth, Owen W. and Akafia, Martin M. and Andrews, Megan Y. and Bargar, John R.}, year={2014}, pages={1348–1359} }
 @article{abit_vepraskas_duckworth_amoozegar_2013, title={Dissolution of phosphorus into pore-water flowing through an organic soil}, volume={197}, ISSN={["1872-6259"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84873020656&partnerID=MN8TOARS}, DOI={10.1016/j.geoderma.2012.12.012}, abstractNote={Understanding dissolution and transport of phosphorus (P) from organic soils in agricultural fields and restored wetlands is critical for devising management strategies to mitigate P losses. The objective of this study was to evaluate the dissolution of P in water flowing through the vadose zone-shallow ground water continuum of an organic soil. Three 90 cm × 50 cm × 8 cm flow cells were uniformly packed with organic soil material from a Ponzer muck (Terric Haplosaprists) collected from an area that had been in agricultural production for at least 30 years. The packed flow cells were instrumented with soil solution samplers and platinum-tipped redox electrodes at approximately 5 cm below a simulated water table (WT), and in the capillary fringe (CF) at approximately 5 and 20 cm above the WT to collect soil solution and monitor the reduction potential (Eh), respectively. Distilled water was continuously supplied at constant rates of 1.2, 2.4, and 3.6 L d− 1 to one end, and drained at the other end of the flow cells while maintaining a WT at 12 cm above the bottom of the midpoint of each flow cell. Phosphorus concentration in the outflow solution was consistently above the USEPA water quality criteria of 0.1 mg L− 1. Changes in pore-water velocity did not alter the amounts of P leached within the time frame of the experiment. Dissolved phosphorus concentrations at both 5 cm below and 5 cm above the WT were significantly higher than at 20 cm above the WT. This observation indicated that P that leached out of the flow cells was not only from the saturated zone but also from the lower part of the CF. These results suggest that controlling the height of the WT to limit saturation of P enriched surficial soils, while considering the contribution of the CF, may be an effective management tool for limiting P export from restored wetland sited on former agricultural fields.}, journal={GEODERMA}, author={Abit, Sergio M. and Vepraskas, Michael J. and Duckworth, Owen W. and Amoozegar, Aziz}, year={2013}, month={Apr}, pages={51–58} }
 @article{kruft_harrington_duckworth_jarzecki_2013, title={Quantum mechanical investigation of aqueous desferrioxamine B metal complexes: Trends in structure, binding, and infrared spectroscopy}, volume={129}, ISSN={["1873-3344"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84887497559&partnerID=MN8TOARS}, DOI={10.1016/j.jinorgbio.2013.08.008}, abstractNote={A systematic density functional theory study supported by extended X-ray absorption fine structure (EXAFS) and infrared spectroscopic data was conducted to elucidate how structure and vibrational spectra of aqueous desferrioxamine B (DFOB) metal complexes vary with the metal ion identity. Structural parameters derived from EXAFS analyses and trends in metal binding constants are well reproduced and validated by the applied computational model. Vibrational mode analysis guides determination and recognition of crucial structure- and metal-sensitive infrared marker bands. The key marker bands, CO and CN stretching modes, dominate the infrared spectra in the 1400–1650 cm− 1 region. The modes are sensitive to the stability and size of the metal core (first coordination shell) and indicative of its deformation from the octahedral symmetry. The results shed light on the fundamental structural and electronic factors that control metal binding by siderophores, and drive their potentially rich and largely unexplored interactions with trace metals.}, journal={JOURNAL OF INORGANIC BIOCHEMISTRY}, author={Kruft, Bonnie I. and Harrington, James M. and Duckworth, Owen W. and Jarzecki, Andrzej A.}, year={2013}, month={Dec}, pages={150–161} }
 @article{kang_sowers_duckworth_amoozegar_heitman_mclaughlin_2013, title={Turbidimetric Determination of Anionic Polyacrylamide in Low Carbon Soil Extracts}, volume={42}, ISSN={["1537-2537"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84887582502&partnerID=MN8TOARS}, DOI={10.2134/jeq2013.07.0279}, abstractNote={Concerns over runoff water quality from agricultural lands and construction sites have led to the development of improved erosion control practices, including application of polyacrylamide (PAM). We developed a quick and reliable method for quantifying PAM in soil extracts at low carbon content by using a turbidimetric reagent, Hyamine 1622. Three high-molecular weight anionic PAMs differing in charge density (7, 20, and 50 mol%) and five water matrices, deionized (DI) water and extracts from four different soils, were used to construct PAM calibration curves by reacting PAM solutions with hyamine and measuring turbidity development from the PAM-hyamine complex. The PAM calibration curve with DI water showed a strong linear relationship ( = 0.99), and the sensitivity (slope) of calibration curves increased with increasing PAM charge density with a detection limit of 0.4 to 0.9 mg L. Identical tests with soil extracts showed the sensitivity of the hyamine method was dependent on the properties of the soil extract, primarily organic carbon concentration. Although the method was effective in mineral soils, the highest charge density PAM yielded a more reliable linear relationship ( > 0.97) and lowest detection limit (0.3 to 1.2 mg L), compared with those of the lower charge density PAMs (0.7 to 23 mg L). Our results suggest that the hyamine test could be an efficient method for quantifying PAM in environmental soil water samples as long as the organic carbon in the sample is low, such as in subsurface soil material often exposed at construction sites.}, number={6}, journal={JOURNAL OF ENVIRONMENTAL QUALITY}, author={Kang, Jihoon and Sowers, Tyler D. and Duckworth, Owen W. and Amoozegar, Aziz and Heitman, Joshua L. and McLaughlin, Richard A.}, year={2013}, pages={1902–1907} }
 @article{park_kim_kim_duckworth_komarneni_2012, title={Hydronium-Promoted Equilibrium Mechanism for the Alkali Metal Cation Exchange Reaction in Na-4-Mica}, volume={116}, ISSN={["1932-7447"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84865974086&partnerID=MN8TOARS}, DOI={10.1021/jp300759n}, abstractNote={A general stoichiometric cation exchange mechanism does not fully explain the unique exchange behaviors of Na-4-mica, the most highly charged swelling 2:1 phyllosilicate. Herein, we present for the first time a novel model of hydronium-promoted equilibrium for the cation exchange reaction of highly charged swelling 2:1 phyllosilicates with alkali metal cations. The exchange reaction of Na-4-mica with alkali metal cations revealed that adsorbed cations undergo further exchange with hydronium ions to result in the unprecedented back-release and position rearrangement of adsorbed cations. During the exchange reaction, hydronium ions play a key role as not competitive but stabilizing cation. Stabilization is accomplished through replacing half of the adsorbed cations and consequently relocating their position. Therefore, this hydronium-promoted equilibrium mechanism would provide a novel intellectual framework not only for understanding cation exchange reactions of various highly charged phyllosilicates but a...}, number={35}, journal={JOURNAL OF PHYSICAL CHEMISTRY C}, author={Park, Man and Kim, Hong-Jin and Kim, Kwang Seop and Duckworth, Owen W. and Komarneni, Sridhar}, year={2012}, month={Sep}, pages={18678–18683} }
 @article{wu_osmond_graves_burchell_duckworth_2012, title={Relationships Between Nitrogen Transformation Rates and Gene Abundance in a Riparian Buffer Soil}, volume={50}, ISSN={0364-152X 1432-1009}, url={http://dx.doi.org/10.1007/s00267-012-9929-z}, DOI={10.1007/s00267-012-9929-z}, abstractNote={Denitrification is a critical biogeochemical process that results in the conversion of nitrate to volatile products, and thus is a major route of nitrogen loss from terrestrial environments. Riparian buffers are an important management tool that is widely utilized to protect water from non-point source pollution. However, riparian buffers vary in their nitrate removal effectiveness, and thus there is a need for mechanistic studies to explore nitrate dynamics in buffer soils. The objectives of this study were to examine the influence of specific types of soluble organic matter on nitrate loss and nitrous oxide production rates, and to elucidate the relationships between these rates and the abundances of functional genes in a riparian buffer soil. Continuous-flow soil column experiments were performed to investigate the effect of three types of soluble organic matter (citric acid, alginic acid, and Suwannee River dissolved organic carbon) on rates of nitrate loss and nitrous oxide production. We found that nitrate loss rates increased as citric acid concentrations increased; however, rates of nitrate loss were weakly affected or not affected by the addition of the other types of organic matter. In all experiments, rates of nitrous oxide production mirrored nitrate loss rates. In addition, quantitative polymerase chain reaction (qPCR) was utilized to quantify the number of genes known to encode enzymes that catalyze nitrite reduction (i.e., nirS and nirK) in soil that was collected at the conclusion of column experiments. Nitrate loss and nitrous oxide production rates trended with copy numbers of both nir and 16s rDNA genes. The results suggest that low-molecular mass organic species are more effective at promoting nitrogen transformations than large biopolymers or humic substances, and also help to link genetic potential to chemical reactivity.}, number={5}, journal={Environmental Management}, publisher={Springer Science and Business Media LLC}, author={Wu, Lin and Osmond, Deanna L. and Graves, Alexandria K. and Burchell, Michael R. and Duckworth, Owen W.}, year={2012}, month={Aug}, pages={861–874} }
 @article{harrington_parker_bargar_jarzecki_tebo_sposito_duckworth_2012, title={Structural dependence of Mn complexation by siderophores: Donor group dependence on complex stability and reactivity}, volume={88}, ISSN={["1872-9533"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84861685080&partnerID=MN8TOARS}, DOI={10.1016/j.gca.2012.04.006}, abstractNote={Siderophores traditionally have been viewed as solely being involved in the biogeochemical cycling of Fe(III). This paradigm, however, ignores the diverse roles siderophores may play in the cycling of other trace metals, such as Mn, Co, Mo, and V. Recent work has shown that siderophores form complexes with high stability constants with Mn(III), which are in some cases higher than that of the corresponding Fe(III) complex. Herein, we report on a structural analysis of the dissolved Fe(III)- and Mn(III)–siderophore complexes of rhizoferrin and two pyoverdin-type siderophores using X-ray spectroscopic techniques. Additionally, the stability constants of the Mn(III)–pyoverdinPaA and Mn(III)–rhizoferrin complexes have been quantified as log β111 = 47.5 ± 0.3 and log β110 = 29.8 ± 0.3, respectively. Comparisons of thermodynamic stability and solution structures of Fe(III)- and Mn(III)-complexes with a variety of siderophores demonstrate the relationship between donor group identity, siderophore structure, and strength of complex formation. Rhizoferrin and two mixed-moiety pyoverdins bind with a higher affinity for Mn(III) than Fe(III), possibly because of binding moiety composition which makes them better able to accommodate Jahn–Teller distortion. In contrast, Fe(III) forms complexes of higher relative stability with siderophores that contain hydroxamate and catecholate moieties, more rigid donor groups that form five-membered chelate rings.}, journal={GEOCHIMICA ET COSMOCHIMICA ACTA}, author={Harrington, James M. and Parker, Dorothy L. and Bargar, John R. and Jarzecki, Andrzej A. and Tebo, Bradley M. and Sposito, Garrison and Duckworth, Owen W.}, year={2012}, month={Jul}, pages={106–119} }
 @article{harrington_bargar_jarzecki_roberts_sombers_duckworth_2012, title={Trace metal complexation by the triscatecholate siderophore protochelin: structure and stability}, volume={25}, ISSN={["1572-8773"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84861693339&partnerID=MN8TOARS}, DOI={10.1007/s10534-011-9513-7}, abstractNote={Although siderophores are generally viewed as biological iron uptake agents, recent evidence has shown that they may play significant roles in the biogeochemical cycling and biological uptake of other metals. One such siderophore that is produced by A. vinelandii is the triscatecholate protochelin. In this study, we probe the solution chemistry of protochelin and its complexes with environmentally relevant trace metals to better understand its effect on metal uptake and cycling. Protochelin exhibits low solubility below pH 7.5 and degrades gradually in solution. Electrochemical measurements of protochelin and metal-protochelin complexes reveal a ligand half-wave potential of 200 mV. The Fe(III)Proto(3-) complex exhibits a salicylate shift in coordination mode at circumneutral to acidic pH. Coordination of Mn(II) by protochelin above pH 8.0 promotes gradual air oxidation of the metal center to Mn(III), which accelerates at higher pH values. The Mn(III)Proto(3-) complex was found to have a stability constant of log β(110) = 41.6. Structural parameters derived from spectroscopic measurements and quantum mechanical calculations provide insights into the stability of the Fe(III)Proto(3-), Fe(III)H(3)Proto, and Mn(III)Proto(3-) complexes. Complexation of Co(II) by protochelin results in redox cycling of Co, accompanied by accelerated degradation of the ligand at all solution pH values. These results are discussed in terms of the role of catecholate siderophores in environmental trace metal cycling and intracellular metal release.}, number={2}, journal={BIOMETALS}, author={Harrington, James M. and Bargar, John R. and Jarzecki, Andrzej A. and Roberts, James G. and Sombers, Leslie A. and Duckworth, Owen W.}, year={2012}, month={Apr}, pages={393–412} }
 @article{kim_duckworth_strathmann_2010, title={Reactions of aqueous iron-DFOB (desferrioxamine B) complexes with flavin mononucleotide in the absence of strong iron(II) chelators}, volume={74}, ISSN={["1872-9533"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-75149144851&partnerID=MN8TOARS}, DOI={10.1016/j.gca.2009.12.020}, abstractNote={The mechanisms controlling microbial uptake of FeIII–siderophore complexes and subsequent release of the metal for cellular use have been extensively studied in recent years. Reduction of the FeIII center is believed to be necessary to labilize the coordinated Fe and facilitate exchange with cellular ligands. Previous studies report reduction of FeIII–DFOB by various reducing agents in solutions containing FeII-chelating colorimetric agents for monitoring reaction progress, but the importance of these findings is unclear because the colorimetric agents themselves stabilize and enhance the reactions being monitored. This study examines the reduction of FeIII complexes with DFOB (desferrioxamine B), a trihydroxamate siderophore, by the fully reduced hydroquinone form of flavin mononucleotide (FMNHQ) in the absence of strong FeII-chelating agents, and Fe redox cycling in solutions containing DFOB and oxidized and reduced FMN species. Experimental results demonstrate that the rate and extent of FeIII–DFOB reduction is strongly dependent on pH and FMNHQ concentration. At pH ⩾ 5, incomplete FeIII reduction is observed due to two processes that re-oxidize FeII, namely, the autodecomposition of FeII–DFOB complexes (FeII oxidation is coupled with reduction of a protonated hydroxamate moiety) and reaction of FeII–DFOB complexes with the fully oxidized flavin mononucleotide product (FMNOX). Chemical speciation-dependent kinetic models for the forward reduction process and both reverse FeII oxidation processes are developed, and coupling kinetic models for all three Fe redox processes leads to successful predictions of steady-state FeII concentrations observed over a range of pH conditions in the presence of excess FMNHQ and FMNOX. The observed redox reactions are also in agreement with thermodynamic constraints imposed by the combination of FeIII/FeII and FMNOX/FMNHQ redox couples. Quantitative comparison between kinetic trends and changing Fe speciation reveals that FMN species react predominantly with diprotonated FeIII–DFOB and FeII–DFOB complexes, where protonation of one hydroxamate group opens up two Fe coordination positions. This finding suggests that ternary complex formation (FMN–Fe–DFOB) facilitates inner-sphere electron transfer reactions between the flavin and Fe center.}, number={5}, journal={GEOCHIMICA ET COSMOCHIMICA ACTA}, author={Kim, Dongwook and Duckworth, Owen W. and Strathmann, Timothy J.}, year={2010}, month={Mar}, pages={1513–1529} }
 @article{bi_hesterberg_duckworth_2010, title={Siderophore-promoted dissolution of cobalt from hydroxide minerals}, volume={74}, ISSN={["1872-9533"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-77951296385&partnerID=MN8TOARS}, DOI={10.1016/j.gca.2010.02.028}, abstractNote={Recent research has revealed that siderophores, a class of biogenic ligands with high affinities for Fe(III), can also strongly complex Co(III), an element essential to the normal metabolic function of microbes and animals. This study was conducted to quantify the rates and identify the products and mechanisms of the siderophore-promoted dissolution of Co from synthetic Co-bearing minerals. The dissolution reactions of heterogenite (CoOOH) and four Co-substituted goethites (Co-FeOOH) containing different Co concentrations were investigated in the presence of a trihydroxamate siderophore, desferrioxamine B (DFOB), using batch and flow-through experiments. Results showed that DFOB-promoted dissolution of Co from Co-bearing minerals may occur via pH-dependent ligand-promoted or reductive dissolution mechanisms. For heterogenite, ligand-promoted dissolution was the dominant pathway at neutral to alkaline pH, while production of dissolved Co(II) for pH <6. It was not possible from our data to decouple the separate contributions of homogenous and heterogeneous reduction reactions to the aqueous Co(II) pool. Cobalt substitution in Co-substituted goethite, possibly caused by distortion of goethite structure and increased lattice strain, resulted in enhanced total dissolution rates of both Co and Fe. The DFOB-promoted dissolution rates of Co-bearing minerals, coupled with the high affinity of Co(III) for DFOB, suggest that siderophores may be effective for increasing Co solubility, and thus possibly Co bioavailability. The results also suggest that siderophores may contribute to the mobilization of radioactive 60Co from Co-bearing mineral phases through mineral weathering and dissolution processes.}, number={10}, journal={GEOCHIMICA ET COSMOCHIMICA ACTA}, author={Bi, Yuqiang and Hesterberg, Dean L. and Duckworth, Owen W.}, year={2010}, month={May}, pages={2915–2925} }
 @article{saal_duckworth_2010, title={Synergistic Dissolution of Manganese Oxides as Promoted by Siderophores and Small Organic Acids}, volume={74}, ISSN={["1435-0661"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-78249245759&partnerID=MN8TOARS}, DOI={10.2136/sssaj2009.0465}, abstractNote={Recent studies have revealed that siderophores, biogenic chelating agents that strongly complex Fe(III) and other hard metals, may function in concert with low‐molecular‐mass organic acids (LMMOAs) to facilitate Fe (hydr)oxide dissolution via synergistic reactions. The siderophore desferrioxamine B (DFOB) may also participate in a number of chemical reactions at Mn (hydr)oxide surfaces. The goal of this study was to determine the rates and products of δ‐MnO2, a layer type Mn(IV) oxide, dissolution as promoted by LMMOA–DFOB mixtures. As with Fe (hydr)oxides, the rate of DFOB‐promoted dissolution of δ‐MnO2 is strongly influenced by the presence of the LMMOAs citrate and oxalate. In the presence of DFOB, citrate increases the dissolution rate relative to the sum of dissolution rates from corresponding single‐ligand systems by promoting both Mn(III)HDFOB+ complex formation and Mn(II) production in a pH‐dependent mechanism. In contrast, oxalate–DFOB mixtures produce predominantly Mn(II), with rates enhanced up to threefold from the sum of dissolution rates in single‐ligand systems at acidic pH values. We investigated possible mechanisms to describe this synergistic dissolution processes by building on observations of single‐ligand systems. These results suggest that biological exudation of LMMOAs in conjunction with siderophores may allow the selection of dissolution products, including regulation of the formation of potentially reactive aqueous Mn(III) complexes.}, number={6}, journal={SOIL SCIENCE SOCIETY OF AMERICA JOURNAL}, author={Saal, Lauren B. and Duckworth, Owen W.}, year={2010}, pages={2021–2031} }
 @article{duckworth_holmstroem_pena_sposito_2009, title={Biogeochemistry of iron oxidation in a circumneutral freshwater habitat}, volume={260}, ISSN={["0009-2541"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-60749134694&partnerID=MN8TOARS}, DOI={10.1016/j.chemgeo.2008.08.027}, abstractNote={Iron(II) oxidation in natural waters at circumneutral pH, often regarded as an abiotic process, is frequently biologically mediated at iron-rich redox gradients. West Berry Creek, a small circumneutral tributary that flows through a mixed coniferous forest in Big Basin State Park, California, contains localized iron (hydr)oxide precipitates at points along its course where anoxic groundwater meets oxygenated creek water. These mixing zones establish redox gradients that may be exploited by microbes forming microbial mats that are intimately associated with iron (hydr)oxide precipitates. Water sampling revealed strong correlations between the concentrations of aqueous inorganic species, suggesting a rock-weathering source for most of these solutes. Liquid chromatography–electrospray ionization–mass spectrometry techniques detected significant concentrations of organic exudates, including low molecular mass organic acids and siderophores, indicating that active biogeochemical cycling of iron is occurring in the creek. X-ray diffraction and elemental analysis showed the precipitates to be amorphous, or possibly poorly crystalline, iron-rich minerals. Clone libraries developed from 16S rDNA sequences extracted from microbial mat communities associated with the precipitates revealed the presence of microorganisms related to the neutrophilic iron oxidizing bacteria Gallionella and Sideroxydans. Sequences from these libraries also indicated the presence of significant populations of organisms related to bacteria in the genera Aquaspirillum, Pseudomonas, Sphingomonas, and Nitrospira. These geosymbiotic systems appear to be significant not only for the biogeochemical cycling of iron in the creek, but also for the cycling of organic species, inorganic nutrients, and trace metals.}, number={3-4}, journal={CHEMICAL GEOLOGY}, author={Duckworth, Owen W. and Holmstroem, Sara J. M. and Pena, Jasquelin and Sposito, Garrison}, year={2009}, month={Mar}, pages={149–158} }
 @article{duckworth_bargar_sposito_2009, title={Coupled biogeochemical cycling of iron and manganese as mediated by microbial siderophores}, volume={22}, ISSN={["1572-8773"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-67651242008&partnerID=MN8TOARS}, DOI={10.1007/s10534-009-9220-9}, abstractNote={Siderophores, biogenic chelating agents that facilitate Fe(III) uptake through the formation of strong complexes, also form strong complexes with Mn(III) and exhibit high reactivity with Mn (hydr)oxides, suggesting a pathway by which Mn may disrupt Fe uptake. In this review, we evaluate the major biogeochemical mechanisms by which Fe and Mn may interact through reactions with microbial siderophores: competition for a limited pool of siderophores, sorption of siderophores and metal-siderophore complexes to mineral surfaces, and competitive metal-siderophore complex formation through parallel mineral dissolution pathways. This rich interweaving of chemical processes gives rise to an intricate tapestry of interactions, particularly in respect to the biogeochemical cycling of Fe and Mn in marine ecosystems.}, number={4}, journal={BIOMETALS}, author={Duckworth, Owen W. and Bargar, John R. and Sposito, Garrison}, year={2009}, month={Aug}, pages={605–613} }
 @article{kim_duckworth_strathmann_2009, title={Hydroxamate siderophore-promoted reactions between iron(II) and nitroaromatic groundwater contaminants}, volume={73}, ISSN={["1872-9533"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-59649115651&partnerID=MN8TOARS}, DOI={10.1016/j.gca.2008.11.039}, abstractNote={Recent studies show that ferrous iron (FeII), which is often abundant in anaerobic soil and groundwater, is capable of abiotically reducing many subsurface contaminants. However, studies also demonstrate that FeII redox reactivity in geochemical systems is heavily dependent upon metal speciation. This contribution examines the influence of hydroxamate ligands, including the trihydroxamate siderophore desferrioxamine B (DFOB), on FeII reactions with nitroaromatic groundwater contaminants (NACs). Experimental results demonstrate that ring-substituted NACs are reduced to the corresponding aniline products in aqueous solutions containing FeII complexes with DFOB and two monohydroxamate ligands (acetohydroxamic acid and salicylhydroxamic acid). Reaction rates are heavily dependent upon solution conditions and the identities of both the FeII–complexing hydroxamate ligand and the target NAC. Trends in the observed pseudo-first-order rate constants for reduction of 4-chloronitrobenzene (kobs, s−1) are quantitatively linked to the formation of FeII species with standard one-electron reduction potentials, EH0 (FeIII/FeII), below −0.3 V. Linear free energy relationships correlate reaction rates with the EH0 (FeIII/FeII) values of different electron-donating FeII complexes and with the apparent one-electron reduction potentials of different electron-accepting NACs, EH1′(ArNO2). Experiments describing a redox auto-decomposition mechanism for FeII–DFOB complexes that occurs at neutral pH and has implications for the stability of hydroxamate siderophores in anaerobic environments are also presented. Results from this study indicate that hydroxamates and other FeIII-stabilizing organic ligands can form highly redox-active FeII complexes that may contribute to the natural attenuation and remediation of subsurface contaminants.}, number={5}, journal={GEOCHIMICA ET COSMOCHIMICA ACTA}, author={Kim, Dongwook and Duckworth, Owen W. and Strathmann, Timothy J.}, year={2009}, month={Mar}, pages={1297–1311} }
 @article{duckworth_bargar_sposito_2009, title={Quantitative Structure-Activity Relationships for Aqueous Metal-Siderophore Complexes}, volume={43}, ISSN={["1520-5851"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-60949097462&partnerID=MN8TOARS}, DOI={10.1021/es802044y}, abstractNote={Siderophores, biogenic chelating agents that facilitate the solubilization and uptake of ferric iron, form stable complexes with a wide range of nutrient and contaminant metals and thus may profoundly affect their fate, transport, and biogeochemical cycling. To understand more comprehensively the factors that control the stability and reactivity, as well as the potential for microbial uptake, of metal-siderophore complexes, we probed the structures of complexes formed between the trihydroxamate siderophore desferrioxamine B (DFOB) and Cu(II), Ga(III), Mn(II), Ni(II), and Zn(II) in solution by using extended X-ray absorption fine structure (EXAFS) spectroscopy. We find that all metals studied are dominantly in octahedral coordination, with significant Jahn-Teller distortion of the Cu(II)HDFOB(0) complex. Additionally, log-transformed complex stability constants correlate not only with the charge-normalized interatomic distances within the complex, affirming and expanding existing predictive relationships, but also with the Debye-Waller parameter of the first coordination shell. The derived structure-activity relationships not only quantitatively relate the measured physical architecture of aqueous complexes to their observed stability but also allow for the prediction of siderophore-metal stability constants.}, number={2}, journal={ENVIRONMENTAL SCIENCE & TECHNOLOGY}, author={Duckworth, Owen W. and Bargar, John R. and Sposito, Garrison}, year={2009}, month={Jan}, pages={343–349} }
 @article{duckworth_bargar_jarzecki_oyerinde_spiro_sposito_2009, title={The exceptionally stable cobalt(III)-desferrioxamine B complex}, volume={113}, ISSN={["1872-7581"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-60949105860&partnerID=MN8TOARS}, DOI={10.1016/j.marchem.2009.01.003}, abstractNote={The biogeochemistry of trivalent iron, manganese, and cobalt in the oceans is dominated by soluble complexes formed with high-affinity organic ligands that are believed to be microbial siderophores or similar biogenic chelating agents. Desferrioxamine B (DFOB), a trihydroxamate siderophore found in both terrestrial and marine environments, has served as a useful model for a large class of microbial siderophores in studies of 1:1 complexes formed with trivalent iron and manganese. However, no data exist concerning DFOB complexes with Co(III), which we hypothesize should be as strong as those with Fe(III) and Mn(III) if the current picture of the ocean biogeochemistry of the three trivalent metals is accurate. We investigated the complexation reaction between DFOB and Co(III) in aqueous solution at seawater pH using base and redox titrations, and then characterized the resulting 1:1 complex Co(III)HDFOB+ using X-ray absorption, resonance Raman spectroscopy, and quantum mechanical structural optimizations. We found that the complex stability constant for Co(III)HDFOB+ (log K [Co(III)HDFOB+] = 37.5 ± 0.4) is in fact five and seven orders of magnitude larger than that for Fe(III)HDFOB+ (log K[Fe(III)HDFOB+] = 32.02) and Mn(III)HDFOB+ (log K[Mn(III)HDFOB+] = 29.9), respectively. Spectroscopic data and the supporting theoretical structural optimizations elucidated the molecular basis for this exceptional stability. Although not definitive, our results nevertheless are consistent with the evolution of siderophores as a response by bacteria to oxygenation, not only because of sharply decreasing concentrations of Fe(III), but also of Co(III).}, number={1-2}, journal={MARINE CHEMISTRY}, author={Duckworth, Owen W. and Bargar, John R. and Jarzecki, Andrzej A. and Oyerinde, Oyeyemi and Spiro, Thomas G. and Sposito, Garrison}, year={2009}, month={Jan}, pages={114–122} }
 @article{smits_herrmann_duane_duckworth_bonneville_benning_lundström_2009, title={The fungal-mineral interface: Challenges and considerations of micro-analytical developments}, volume={23}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-77954861346&partnerID=MN8TOARS}, DOI={10.1016/j.fbr.2009.11.001}, abstractNote={Over recent years, the role of fungi, especially mycorrhizal fungi, in the weathering of rock-forming minerals has been increasingly recognised. Much of our understanding of the effects of fungi on mineral weathering is based on macroscopic studies. However, the ability of fungi to translocate materials, including organic acids and siderophores, to specific areas of a mineral surface leads to significant spatial heterogeneity in the weathering process. Thus, geomycologists are confronted with unique challenges of how to comprehend and quantify such a high degree of diversity and complicated arrays of interactions. Recent advances in experimental and analytical techniques have increased our ability to probe the fungal–mineral interface at the resolution necessary to decouple significant biogeochemical processes. Modern microscopy, spectroscopy, mass spectrometry, wet chemistry, and scattering techniques allow for the selective extraction of physical, chemical, and structural data at the micro- to nano-scale. These techniques offer exciting possibilities to study fungal–mineral interactions at the scale of individual hyphae. In this review, we give an overview of some of these techniques with their characteristics, advantages and limitations, and how they can be used to further our understanding of biotic mineral weathering.}, number={4}, journal={Fungal Biology Reviews}, author={Smits, M.M. and Herrmann, A.M. and Duane, M. and Duckworth, O.W. and Bonneville, S. and Benning, L.G. and Lundström, U.}, year={2009}, pages={122–131} }
 @article{duckworth_bargar_sposito_2008, title={Sorption of ferric iron from ferrioxamine B to synthetic and biogenic layer type manganese oxides}, volume={72}, ISSN={["1872-9533"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-45849086926&partnerID=MN8TOARS}, DOI={10.1016/j.gca.2008.04.026}, abstractNote={Siderophores are biogenic chelating agents produced in terrestrial and marine environments that increase the bioavailability of ferric iron. Recent work has suggested that both aqueous and solid-phase Mn(III) may affect siderophore-mediated iron transport, but scant information appears to be available about the potential roles of layer type manganese oxides, which are relatively abundant in soils and the oligotrophic marine water column. To probe the effects of layer type manganese oxides on the stability of aqueous Fe–siderophore complexes, we studied the sorption of ferrioxamine B [Fe(III)HDFOB+, an Fe(III) chelate of the trihydroxamate siderophore desferrioxamine B (DFOB)] to two synthetic birnessites [layer type Mn(III,IV) oxides] and a biogenic birnessite produced by Pseudomonas putida GB-1. We found that all of these predominantly Mn(IV) oxides greatly reduced the aqueous concentration of Fe(III)HDFOB+ at pH 8. Analysis of Fe K-edge EXAFS spectra indicated that a dominant fraction of Fe(III) associated with the Mn(IV) oxides is not complexed by DFOB as in solution, but instead Fe(III) is specifically adsorbed to the mineral structure at multiple sites, thus indicating that the Mn(IV) oxides displaced Fe(III) from the siderophore complex. These results indicate that layer type manganese oxides, including biogenic minerals, may sequester iron from soluble ferric complexes. We conclude that the sorption of iron–siderophore complexes may play a significant role in the bioavailability and biogeochemical cycling of iron in marine and terrestrial environments.}, number={14}, journal={GEOCHIMICA ET COSMOCHIMICA ACTA}, author={Duckworth, Owen W. and Bargar, John R. and Sposito, Garrison}, year={2008}, month={Jul}, pages={3371–3380} }
 @article{peña_duckworth_bargar_sposito_2007, title={Dissolution of hausmannite (Mn<inf>3</inf>O<inf>4</inf>) in the presence of the trihydroxamate siderophore desferrioxamine B}, volume={71}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-36049052050&partnerID=MN8TOARS}, DOI={10.1016/j.gca.2007.03.043}, abstractNote={That microbial siderophores may be mediators of Mn(III) biogeochemistry is suggested by recent studies showing that these well known Fe(III)-chelating ligands form very stable Mn(III) aqueous complexes. In this study, we examine the influence of desferrioxamine B (DFOB), a trihydroxamate siderophore, on the dissolution of hausmannite, a mixed valence Mn(II, III) oxide found in soils and freshwater sediments. Batch dissolution experiments were conducted both in the absence (pH 4–9) and in the presence of 100 μM DFOB (pH 5–9). In the absence of the ligand, there is a sharp decrease in the extent of proton-promoted dissolution above pH 5 and no appreciable dissolution above pH 8. The resulting aqueous Mn2+ activities were in good agreement with previous studies, indirectly supporting the accepted two-step mechanism involving the formation of manganite and reprecipitation of hausmannite. Desferrioxamine B enhanced hausmannite dissolution over the entire pH range investigated, both via the formation of a Mn(III) complex and through surface-catalyzed reductive dissolution. Above pH 8, non-reductive ligand-promoted dissolution dominated, whereas below pH 8, dissolution was non-stoichiometric with respect to DFOB. Concurrent proton-promoted, ligand-promoted, reductive, and induced dissolution was observed, with Mn release by either reductive or induced dissolution increasing linearly with decreasing pH. The fast kinetics of the DFOB-promoted dissolution of hausmannite, as compared to iron oxides, suggest that the siderophore-promoted dissolution of Mn(III)-bearing minerals may compete with the siderophore-promoted dissolution of Fe(III)-bearing minerals.}, number={23}, journal={Geochimica et Cosmochimica Acta}, author={Peña, J. and Duckworth, O.W. and Bargar, J.R. and Sposito, G.}, year={2007}, pages={5661–5671} }
 @article{duckworth_sposito_2007, title={Siderophore-promoted dissolution of synthetic and biogenic layer-type Mn oxides}, volume={242}, ISSN={["1872-6836"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-34347325125&partnerID=MN8TOARS}, DOI={10.1016/j.chemgeo.2007.05.007}, abstractNote={Siderophores are biogenic chelating agents exuded in terrestrial and marine environments to increase the bioavailablity of ferric iron. Recent work suggests that both solid and aqueous manganese may affect the aqueous speciation of siderophores and thus siderophore-mediated iron transport. Although the interaction of the trihydroxamate siderophore desferrioxamine B (DFOB) with several lower-valence manganese oxides has been studied, the effects of siderophores on Mn(III,IV) oxide dissolution are unknown. To remedy this situation, we measured the dissolution rates of two synthetic layer-type Mn(IV) oxides and a biogenic oxide produced by a model organism, Pseudomonas putida GB-1. For pH 5–7, we find that all minerals studied dissolve by traditional reductive (R1) dissolution, yielding Mn(II); for pH 7–9, dissolution yields Mn(III)–siderophore complexes, either by selective ligand-promoted dissolution of structural Mn(III) or by reduction of >Mn(IV) to >Mn(III) followed by complexation and solubilization of Mn(III) by DFOB. Because reductive dissolution results in siderophore oxidation, manganese oxide dissolution at acidic pH may provide a significant abiotic sink for siderophores in natural waters. At alkaline pH, Mn(III)–siderophore complexes produced may profoundly affect the aqueous speciation of siderophores as well as provide a source of reactive Mn(III) complexes.}, number={3-4}, journal={CHEMICAL GEOLOGY}, author={Duckworth, Owen W. and Sposito, Garrison}, year={2007}, month={Aug}, pages={497–508} }
 @article{chelating ligand alters the microscopic mechanism of mineral dissolution_2005, volume={127}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-17744378596&partnerID=MN8TOARS}, DOI={10.1021/ja042737k}, abstractNote={Ethylenediamine tetraacetate (EDTA)-mediated calcite dissolution occurs via a different process than water-promoted dissolution. Near-atomic-scale observations in EDTA solutions demonstrate that, after penetration through a critical pit depth barrier, step velocity increases linearly with pit depth for EDTA-promoted dissolution. The parallel processes of water-dominated dissolution at point defects and ligand-dominated dissolution at linear defects are clearly observable in real-time atomic force micrographs. EDTA and water initiate and propagate dissolution steps with pit-depth-dependent and -independent step velocities, respectively. The linear defects are susceptible to continuously increasing step velocities, but the point defects are not. The findings update the conceptual framework of the microscopic mechanism of mineral dissolution.}, number={16}, journal={Journal of the American Chemical Society}, year={2005}, pages={5744–5745} }
 @article{duckworth_sposito_2005, title={Siderophore-Manganese(III) interactions II. Manganite dissolution promoted by desferrioxamine B}, volume={39}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-23844504565&partnerID=MN8TOARS}, DOI={10.1021/es050276c}, abstractNote={Recent laboratory and field studies suggest that Mn(III) forms persistent aqueous complexes with high-affinity ligands. Aqueous Mn(III) species thus may play a significant but largely unexplored role in biogeochemical processes. One formation mechanism for these species is the dissolution of Mn(III)-bearing minerals. To investigate this mechanism, we measured the steady-state dissolution rates of manganite (γ-MnOOH) in the presence of desferrioxamine B (DFOB), a common trihydroxamate siderophore. We find that DFOB dissolves manganite by both reductive and nonreductive reaction pathways. For pH > 6.5, a nonreductive ligand-promoted reaction is the dominant dissolution pathway, with a steady-state dissolution rate proportional to the surface concentration of DFOB. In the absence of reductants, the aqueous Mn(III)HDFOB+ complex resulting from dissolution is stable for at least several weeks at circumneutral to alkaline pH and at 25 °C. For pH < 6.5, Mn2+ is the dominant aqueous species resulting from mangani...}, number={16}, journal={Environmental Science and Technology}, author={Duckworth, O.W. and Sposito, G.}, year={2005}, pages={6045–6051} }
 @article{duckworth_sposito_2005, title={Siderophore-manganese (III) interactions. I. Air-oxidation of manganese(II) promoted by desferrioxamine B}, volume={39}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-23844512468&partnerID=MN8TOARS}, DOI={10.1021/es050275k}, abstractNote={Recent studies suggest that aqueous Mn(III) complexes, particularly those with non-carboxylated ligands such as microbial siderophores, may be stable in soil and aquatic environments. In this paper, we determine the stability constants for Mn(II) and Mn(III) complexes with the common trihydroxamate siderophore, desferrioxamine B (DFOB). Base and redox titrations were conducted to determine DFOB conditional protonation constants and conditional stability constants for 1:1 DFOB complexes with Mn(II) and Mn(III). The conditional protonation constants agree well with literature values. We determined stability constants for three Mn(II)−DFOB species and one Mn(III)−DFOB species at 25 °C in 0.1 M NaCl. The Mn(III) HDFOB+ complex can be formed readily by air-oxidation of Mn(II)−DFOB. This reaction exhibits pseudo first-order kinetics with a rate coefficient that can be characterized as the product of oxygen concentration with a linear combination of the concentrations of the three Mn(II)−DFOB complexes. The seco...}, number={16}, journal={Environmental Science and Technology}, author={Duckworth, O.W. and Sposito, G.}, year={2005}, pages={6037–6044} }
 @article{duckworth_martin_2004, title={Dissolution rates and pit morphologies of rhombohedral carbonate minerals}, volume={89}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-1942499445&partnerID=MN8TOARS}, number={4}, journal={American Mineralogist}, author={Duckworth, O.W. and Martin, S.T.}, year={2004}, pages={554–563} }
 @article{effects of the biologically produced polymer alginic acid on macroscopic and microscopic calcite dissolution rates_2004, volume={38}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-2542420918&partnerID=MN8TOARS}, DOI={10.1021/es035299a}, abstractNote={Dissolution of carbonate minerals has significant environmental effects. Microorganisms affect carbonate dissolution rates by producing extracellular metabolites, including complex polysaccharides such as alginic acid. Using a combined atomic force microscopy (AFM)/flowthrough reactor apparatus, we investigated the effects of alginic acid on calcite dissolution. Macroscopic dissolution rates, derived from the aqueous metal ion concentrations, are 10(-5.5) mol m(-2) s(-1) for 5 < pH < 12 in the absence of alginic acid compared to 10(-4.8) mol m(-2) s(-1) in its presence. The AFM images demonstrate that alginic acid preferentially attacks the obtuse steps of dissolution pits on the calcite surface. In pure water, the obtuse and acute steps retreat at similar rates, and the pits are nearly isotropic except under highly acidic conditions. In alginic acid, the acute step retreat rate is nearly unchanged in comparison to water, whereas the obtuse step retreat rate increases with decreasing pH values. As a result, the pits remain rhombohedral but propagate faster in the obtuse direction. To explain these observations, we propose that alginic acid preferentially forms dissolution active surface complexes with calcium atoms on the obtuse step, which results in anisotropic ligand-promoted dissolution.}, number={11}, journal={Environmental Science and Technology}, year={2004}, pages={3040–3046} }
 @article{duckworth_cygan_martin_2004, title={Linear free energy relationships between dissolution rates and molecular modeling energies of rhombohedral carbonates}, volume={20}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-1842741155&partnerID=MN8TOARS}, DOI={10.1021/la035348x}, abstractNote={Bulk and surface energies are calculated for endmembers of the isostructural rhombohedral carbonate mineral family, including Ca, Cd, Co, Fe, Mg, Mn, Ni, and Zn compositions. The calculations for the bulk agree with the densities, bond distances, bond angles, and lattice enthalpies reported in the literature. The calculated energies also correlate with measured dissolution rates: the lattice energies show a log-linear relationship to the macroscopic dissolution rates at circumneutral pH. Moreover, the energies of ion pairs translated along surface steps are calculated and found to predict experimentally observed microscopic step retreat velocities. Finally, pit formation excess energies decrease with increasing pit size, which is consistent with the nonlinear dissolution kinetics hypothesized for the initial stages of pit formation.}, number={7}, journal={Langmuir}, author={Duckworth, O.W. and Cygan, R.T. and Martin, S.T.}, year={2004}, pages={2938–2946} }
 @article{duckworth_martin_2004, title={Role of molecular oxygen in the dissolution of siderite and rhodochrosite}, volume={68}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0742322779&partnerID=MN8TOARS}, DOI={10.1016/S0016-7037(00)00464-2}, number={3}, journal={Geochimica et Cosmochimica Acta}, author={Duckworth, O.W. and Martin, S.T.}, year={2004}, pages={607–621} }
 @article{duckworth_martin_2004, title={Role of molecular oxygen in the dissolution of siderite and rhodochrosite}, volume={68}, ISSN={0016-7037}, url={http://dx.doi.org/10.1016/S0016-7037(03)00464-2}, DOI={10.1016/S0016-7037(03)00464-2}, abstractNote={The dissolution of siderite (FeCO3) and rhodochrosite (MnCO3) under oxic and anoxic conditions is investigated at 298 K. The anoxic dissolution rate of siderite is 10−8.65 mol m−2 s−1 for 5.5 < pH < 12 and increases as [H+]0.75 for pH < 5.5. The pH dependence is consistent with parallel proton-promoted and water hydrolysis dissolution pathways. Atomic force microscopy (AFM) reveals a change in pit morphology from rhombohedral pits for pH > 4 to pits elongated at one vertex for pH < 4. Under oxic conditions the dissolution rate decreases to below the detection limit of 10−10 mol m−2 s−1 for 6.0 < pH < 10.3, and hillock precipitation preferential to steps is observed in concurrent AFM micrographs. X-ray photoelectron spectroscopy (XPS) and thermodynamic analysis identify the precipitate as ferrihydrite. At pH > 10.3, the oxic dissolution rate is as high as 10−7.5 mol m−2 s−1, which is greater than under the corresponding anoxic conditions. A fast electron transfer reaction between solution O2 or [Fe3+(OH)4]− species and surficial >FeII hydroxyl groups is hypothesized to explain the dissolution kinetics. AFM micrographs do not show precipitation under these conditions. Anoxic dissolution of rhodochrosite is physically observed as rhombohedral pit expansion for 3.7 < pH < 10.3 and is chemically explained by parallel proton- and water-promoted pathways. The dissolution rate law is 10−4.93[H+] + 10−8.45 mol m−2 s−1. For 5.8 < pH < 7.7 under oxic conditions, the AFM micrographs show a tabular precipitate growing by preferential expansion along the a-axis, though the macroscopic dissolution rate is apparently unaffected. For pH > 7.7 under oxic conditions, the dissolution rate decreases from 10−8.45 to 10−9.0 mol m−2 s−1. Flattened hillock precipitates grow across the entire surface without apparent morphological influence by the underlying rhodochrosite surface. XPS spectra and thermodynamic calculations implicate the precipitate as bixbyite for 5.8 < pH < 7.7 and MnOOH (possibly feitnkechtite) for pH >7.7.}, number={3}, journal={Geochimica et Cosmochimica Acta}, publisher={Elsevier BV}, author={Duckworth, Owen W and Martin, Scot T}, year={2004}, month={Feb}, pages={607–621} }
 @article{duckworth_martin_2003, title={Connections between surface complexation and geometric models of mineral dissolution investigated for rhodochrosite}, volume={67}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0038210267&partnerID=MN8TOARS}, DOI={10.1016/S0016-7037(02)01305-4}, abstractNote={Mineral dissolution rates have been rationalized in the literature by surface complexation models (SCM) and morphological and geometric models (GM), and reconciliation of these conceptually different yet separately highly successful models is an important goal. In the current work, morphological alterations of the surface are observed in real time at the microscopic level by atomic force microscopy (AFM) while dissolution rates are simultaneously measured at the macroscopic level by utilizing the AFM fluid cell as a classic flow-through reactor. Rhodochrosite dissolution is studied from pH = 2 to 11 at 298 K, and quantitative agreement is found between the dissolution rates determined from microscopic and macroscopic observations. Application of a SCM model for the interpretation of the kinetic data indicates that the surface concentration of >CO3H regulates dissolution for pH < 7 while the surface concentration of >MnOH2+ regulates dissolution for pH > 7. A GM model explains well the microscopic observations, from which it is apparent that dissolution occurs at steps associated with anisotropic pit expansion. On the basis of the observations, we combine the SCM and GM models to propose a step-site surface complexation model (SSCM), in which the dissolution rates are quantitatively related to the surface chemical speciation of steps. The governing SSCM equation is as follows: R = χ1/2(kco + kca)[>CO3H] + χ1/2(kmo + kma)[>MnOH2+ ], where R is the dissolution rate (mol m−2 s−1), 2χ1/2 is the fraction of surface sites located at steps, [>CO3H] and [>MnOH2+ ] are surface concentrations (mol m−2), and kco, kca, kmo, and kma are the respective dissolution rate coefficients (s−1) for the >CO3H and the >MnOH2+ surface species on obtuse and acute steps. We find kco = 2.7 s−1, kca = 2.1 × 10−1 s−1, kmo = 4.1 × 10−2 s−1, kma = 3.7 × 10−2 s−1, and χ1/2 = 0.015 ± 0.005. The rate coefficients quantify the net result of complex surface step processes, including double-kink initiation and single-kink propagation. We propose that the SSCM model may have general applicability for dissolution far from equilibrium of flat mineral surfaces of ionic crystals, at least those that dissolve by step retreat.}, number={10}, journal={Geochimica et Cosmochimica Acta}, author={Duckworth, O.W. and Martin, S.T.}, year={2003}, pages={1787–1801} }
 @article{martin_schlenker_chelf_duckworth_2001, title={Structure-activity relationships of mineral dusts as heterogeneous nuclei for ammonium sulfate crystallization from supersaturated aqueous solutions}, volume={35}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0035870205&partnerID=MN8TOARS}, DOI={10.1021/es001535v}, abstractNote={Mineral inclusions, present in aqueous atmospheric salt droplets, regulate crystallization when relative humidity decreases by providing a surface for heterogeneous nucleation and thus reducing the critical supersaturation. Although laboratory studies have quantified these processes to some extent, the diverse atmospheric mineralogy presents more chemical systems than practically feasible for direct study. Structure--activity relationships are necessary. To that end, in the present work the interactions of ammonium sulfate with corundum, hematite, mullite, rutile, anatase, and baddeleyite were studied by diffuse reflectance fourier transform infrared spectroscopy (DRIFTS) and by epitaxial modeling. The spectroscopic results show that shifts in sulfate peak positions due to chemisorption are not a correlative indicator of the efficacy of heterogeneous nucleation. In contrast, epitaxial modeling results of unreconstructed surfaces explain the sequence of critical supersaturations for constant particle size. If validated by further work, this computer modeling method would provide an important structure--activity tool for the estimation of heterogeneous nucleation properties of the atmospheric mineralogy.}, number={8}, journal={Environmental Science and Technology}, author={Martin, S.T. and Schlenker, J. and Chelf, J.H. and Duckworth, O.W.}, year={2001}, pages={1624–1629} }
 @article{duckworth_martin_2001, title={Surface complexation and dissolution of hematite by C<inf>1</inf>-C<inf>6</inf> dicarboxylic acids at pH = 5.0}, volume={65}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0035664317&partnerID=MN8TOARS}, DOI={10.1016/S0016-7037(01)00696-2}, abstractNote={Adsorption of organic ligands to the surfaces of minerals is a ubiquitous environmental process that often regulates interfacial aqueous chemistry. In the current work, the infrared spectra of dicarboxylate ligands adsorbed to hematite are collected by attenuated total reflectance (ATR) spectroscopy. For each ligand, spectra are recorded at several concentrations at pH = 5.0. Each series of spectra is analyzed by singular value decomposition constrained to a Langmuir adsorption surface model. Oxalate, malonate, and glutarate form bidentate surface complexation structures, whereas succinate and adipate form monodentate structures. The absence of a linear trend in the qualitative form of the binding structure (e.g., bidentate for n = 0, 1, and 3 and monodentate for n = 2 and 4 where n is the length of the carbon chain between carboxylate groups) is attributed to the variation of strain energies for the geometries of possible surface complexation structures. For the bidentate ligands, a linear relationship between the Langmuir binding constant and the second acidity constant is demonstrated. The ligand-promoted dissolution rates at pH = 5.0 are also determined through batch reactor experiments. For the bidentate surface complexes, the dissolution rate at monolayer ligand surface coverage slows in the order oxalate, glutarate, to malonate. Linear relationships are found between the ligand-promoted dissolution rate constants and both the Langmuir binding constants and the second acidity constants. In contrast, succinate and adipate form monodentate surface structures that dissolve slowly, if at all. In this manner, a connection is established between the macroscopic dissolution rate and the microscopic surface complexation structures.}, number={23}, journal={Geochimica et Cosmochimica Acta}, author={Duckworth, O.W. and Martin, S.T.}, year={2001}, pages={4289–4301} }