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Crop and Soil Sciences Calendar

Soil Science PhD Exit Seminar

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April 9, 2018 @ 1:00 pm - 2:00 pm

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Andrew Whitaker, PhD Student, Soil Science, NCSU, will present an exit seminar titled:

“Structure and reactivity of biogenic iron (oxyhydr)oxides”
under the direction of Dr. Owen Duckworth

 

Abstract

Bacteriogenic iron (oxyhydr)oxides (BIOS) form in suboxic circumneutral pH environments where dissolved Fe(II) meets oxygenated conditions (e.g., springs, wetlands, rhizospheres). Recent studies have shown that the structure of BIOS resembles that of synthetic two-line ferrihydrite (2LFh); however, BIOS have distinct surface and structural properties from those of synthetic analogues arising from the incorporation of cell-derived organic matter and other impurities (e.g., Al, P, Si), enhanced structural disorder, and small particle sizes. Although these structural differences are often invoked to explain enhanced BIOS sorption of anions and cations, few studies provide an in-depth view of BIOS structure, local coordination environment, and reactivity compared to 2LFh using a multi-pronged approach.

To address these knowledge gaps, the goals of this dissertation were to: (i) elucidate the structure and local coordination environment of BIOS under simulated wet/dry cycles (e.g. freezing, drying) compared to 2LFh; (ii) determine surface excess values and surface binding mechanisms for Cu, Pb, and Zn to BIOS and 2LFh; and (iii) investigate potential differences in reactivity between BIOS and 2LFh in the presence of the redox active contaminant Cr(VI). We utilized macroscale analyses (e.g. wet chemistry, sorption kinetics, sorption isotherms) and molecular scale analyses (e.g. spectroscopy, X-ray total scattering) to probe holistically structure and reactivity of BIOS. In terms of structure, results showed that the compositions of BIOS varied, although they were predominantly Fe (~20–50%) and C (~2–17%). The local coordination of BIOS consisted of ~100% octahedral Fe(III), whereas 2LFh also contained tetrahedral Fe(III).When compared to 2LFh, BIOS were more poorly ordered, resistant to structural changes induced by wet/dry cycles, and had smaller coherent scattering domain sizes (12–18 Å), which was attributed to the incorporation of cellular derived organic matter and other impurities (e.g. Si, and P).

BIOS were also distinct from 2LFh in terms of redox and sorption reactivity at circumneutral pH. BIOS sorbed approximately 11, 5, and 2.5 times more Cu, Pb, and Zn, respectively, than 2LFh over similar dissolved metal concentrations when normalized to specific surface area. However, Cu, Pb, and Zn were sorbed to the surfaces of BIOS and 2LFh via similar inner-sphere complexes, suggesting metals predominantly sorb to iron (oxyhydr)oxide surfaces in the BIOS, not organic matter. When normalized to specific surface area, Cr(VI) surface excess values were ~2.5 times higher for BIOS than 2LFh. Interestingly, after a 14 d incubation, approximately 50% of the sorbed Cr(VI) was reduced to Cr(III) due to the presence of Fe(II) and cellular derived organic matter within the BIOS. These results suggest that the structure, local coordination environment, and reactivity of BIOS are substantially different than 2LFh and synthetic minerals may not be the best options to model the structure and reactivity of natural minerals that form in complex environments. Furthermore, their poor ordering and resistance to structural changes suggests that BIOS formed in circumneutral pH environments may be effective scavengers of contaminants and nutrients over extended periods of time.

 

Details


Date
April 9, 2018
Time
1:00 pm - 2:00 pm
Event Category:
Event Tags
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Location
2223 Williams Hall (McKimmon Room)
101 Derieux Place
Raleigh, NC 27695

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