Structural Incorporation of Sorbed Molybdate during Iron(II)-Induced Transformation of Ferrihydrite and Goethite under Advective Flow Conditions
Date
2020-06-08
Authors
Schoepfer, Valerie
Qin, Kaixuan
Robertson, Jared M.
Das, Soumya
Lindsay, Matthew B. J.
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American Chemical Society (ACS) Publications
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Abstract
Aqueous Fe(II) can induce recrystallization of ferrihydrite and goethite [α‑FeOOH] to their more crystalline or molecularly homogenous counterparts. Despite common association with these and other Fe(III) (oxyhydr)oxides, relationships between Fe(II)-induced transformation and Mo mobility remain poorly constrained. We conducted laboratory column experiments to examine repartitioning of sorbed Mo during Fe(II)-induced transformation of ferrihydrite and goethite under advective flow conditions. We first pumped (~0.25 L d−1) artificial groundwater containing 0.1 mM MoO42− and buffered to pH 6.5 through columns packed with ferrihydrite- and goethite-coated sand until > 90 % Mo breakthrough was observed. Extended X-ray absorption fine structure (EXAFS) spectroscopy shows that initial MoO42− attenuation resulted from inner sphere complexation of MoO4 tetrahedra at ferrihydrite and goethite surfaces. We then pumped Mo-free anoxic artificial groundwater containing 0.2 mM or 2.0 mM Fe(II) through the columns until effluent Mo concentrations remained < 0.005 mM. Raman spectroscopy shows that Fe(II) introduction induced transformation of both ferrihydrite and goethite to lepidocrocite. Additionally, Fe(II) introduction mobilized 4 to 34 % of sorbed Mo and total mass release was greater for (i) ferrihydrite compared to goethite columns and (ii) low Fe(II) compared to high Fe(II) influent. Effluent pH decreased to ~5.8 for columns receiving the high Fe(II) influent and returned to pH 6.5 after 5 to 10 pore volumes. EXAFS spectroscopy indicates that structural incorporation of MoO6 octahedra into neoformed phases contributes to Mo retention during Fe(II) induced transformation. Our results offer new insight into Mo repartitioning during Fe(II)-induced transformation of Fe(III) (oxyhydr)oxides and, more generally, controls on Mo mobility in geohydrologic systems.
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Copyright © 2020 American Chemical Society
Keywords
molybdenum, ferrihydrite, goethite, ferrous iron, recrystallization, lepidocrocite
Citation
Schoepfer, V.A., Qin, K., Robertson, J. M., Das, S. & Lindsay, M.B.J. (2020). Structural Incorporation of Sorbed Molybdate during Iron(II)-Induced Transformation of Ferrihydrite and Goethite under Advective Flow Conditions. ACS Earth and Space Chemistry, 4(7): 1114–1126. https://doi.org/10.1021/acsearthspacechem.0c00099
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Citation
Schoepfer, V.A., Qin, K., Robertson, J. M., Das, S. & Lindsay, M.B.J. (2020). Structural Incorporation of Sorbed Molybdate during Iron(II)-Induced Transformation of Ferrihydrite and Goethite under Advective Flow Conditions. ACS Earth and Space Chemistry, 4(7): 1114–1126. https://doi.org/10.1021/acsearthspacechem.0c00099
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DOI
10.1021/acsearthspacechem.0c00099