@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{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{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 Iron oxidizing bacteria and environmental conditions influence the formation of iron biominerals in aquatic environments. This 10‐week Research Experience for Undergraduates (REU) study focused on elucidating how water chemistry and iron oxidizing bacteria affect the formation of iron oxides in creeks with circumneutral pH. Two locations, each with multiple microenvironments containing abundant iron oxide deposits, were studied. Water chemistry was assessed via both in situ and laboratory analysis over a 5‐week period, revealing correlations between aqueous components that indicate differing groundwater sources may feed nearby discharge points. Microscopy of iron oxide deposits reveals morphologies consistent with the presence of iron oxidizing bacteria. Although efforts to isolate iron oxidizing bacteria did not produce pure cultures, 16S ribosomal DNA analysis also suggests the presence of iron oxidizing bacteria in these sites. Taken together, these results show the diversity of iron oxide forming microenvironments in spatially collocated sites, which may result in unique formations of iron oxide structures, microbial communities, and aqueous chemical cycling.}, 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{whitaker_penn_warren_2016, title={Surface application of a saline-sodic oil & gas drilling waste to winter wheat (Triticum aestivum L.)}, volume={274}, ISSN={["1872-6259"]}, DOI={10.1016/j.geoderma.2016.03.024}, abstractNote={Increased oil and gas drilling has resulted in large quantities of water-base “mud” (WBM) that requires disposal. Land application of WBM to agricultural land is a common disposal technique that presents agronomic and environmental challenges since the material is rich in total soluble salts (TSS). The objective of this study was to determine the impact of WBM application rate on salt accumulation and leaching in the soil, and the impact of application timing and rate on wheat (Triticum aestivum L.) production. A field study was conducted where WBM was applied once, at varying times (Oct., Dec., Jan., Feb., and March) at a 0.66 × (4480 kg TSS ha− 1) rate and 1.0 × (6720 kg TSS ha− 1). Soil samples were taken at 0, 30, and 90 days after application and on August 28th (post-harvest) for evaluating electrical conductivity (EC), pH, and sodium adsorption ratio (SAR). WBM rates had no effect on soil pH. In general, increasing WBM application rates increased soil EC beyond non-amended soils at 0, 30 and 90 days after application. However, this was highly dependent on the amount of precipitation received between WBM applications and sampling. By August, soil EC had decreased below 4 mS cm− 1 at the 0–15 cm depth. However, soil SAR increased at every sampling date after application. WBM application date and rate had no significant effect on wheat (T. aestivum L.) yield. Yield loss and salt accumulation can be minimized if WBM is applied at less than 6720 kg ha− 1 with proper timing relative to rainfall and crop growth patterns.}, journal={GEODERMA}, author={Whitaker, Andrew and Penn, Chad and Warren, Jason}, year={2016}, month={Jul}, pages={97–103} }
 @article{penn_whitaker_warren_2014, title={Surface Application of Oil-Base Drilling Mud Mixed with Gypsum, Limestone, and Caliche}, volume={106}, ISSN={["1435-0645"]}, DOI={10.2134/agronj14.0184}, abstractNote={The current increase in oil and gas drilling activity has resulted in increased production of oil‐base “mud” (OBM) in need of disposal. Land application of OBM to agricultural land is a common disposal technique that presents agronomic and environmental challenges since the material is rich in petroleum hydrocarbons. The objective of this study was to determine the effect of mixing OBM with bulking materials on hydrocarbon degradation and forage production after land application of the mixtures. An OBM was collected from Oklahoma and characterized for environmentally relevant properties such as total petroleum hydrocarbons (TPH) and trace metals. The OBM was bulked with either limestone, gypsum, or caliche, at a ratio of 3:1 or 1.5:1 bulking material/OBM. All mixtures were surface applied at equal TPH loading rates (8625 kg ha−1) and soil samples taken at 7, 45, 60, and 170 d after application for evaluation of TPH, electrical conductivity (EC), pH, and sodium absorption ratio (SAR). After 7 d >50% of applied TPH degraded, which resulted in soil concentrations less than thresholds recommended for residential neighborhoods. By Day 170, 99% of applied TPH degraded. There was mostly no difference in TPH degradation as a function of type and amount of bulking agents used with OBM. Application of OBM did not significantly decrease forage yield compared to unamended control. Therefore, use of caliche, limestone, or gypsum bulked with OBM at a 1.5:1 ratio (bulking agent/OBM) would suffice for achieving acceptable TPH degradation when mixtures are surface applied and non‐incorporated.}, number={5}, journal={AGRONOMY JOURNAL}, author={Penn, Chad J. and Whitaker, Andrew H. and Warren, Jason G.}, year={2014}, pages={1859–1866} }