Schoepfer, ValerieLindsay, Matthew B. J.2022-10-262022-10-262022-08-29Schoepfer, V.A, & Lindsay, M.B.J. (2022). Repartitioning of co-precipitated Mo(VI) during Fe(II) and S(-II) driven ferrihydrite transformation. Chemical Geology, 610. https://doi.org/10.1016/j.chemgeo.2022.121075https://hdl.handle.net/10388/14273Copyright © 2022 Elsevier B.V. All rights reserved.Molybdenum is an essential element for most biological systems. Biosynthesis of Mo-enzymes that support global biogeochemical cycles of N, C, and S depends upon bioavailable molybdate (MoO42−). Interactions with Fe(III) (oxyhydr)oxides can limit Mo bioavailability in aerobic environments, while redox-driven mineral transformations can enhance Mo sequestration. Here, we examine Mo repartitioning during Fe(II) and S(-II) driven ferrihydrite transformation under anaerobic conditions. We reacted Mo(VI) co-precipitated 2-line ferrihydrite with Fe(II)(aq) or S( II)(aq) and monitored geochemical and mineralogical changes over time. Inductively coupled plasma–mass spectrometry (ICP–MS) and synchrotron powder X-ray diffraction (XRD) results revealed rapid Fe(II)(aq) and S(-II)(aq) uptake and limited Mo release during extensive ferrihydrite transformation to goethite [α-FeOOH] and lepidocrocite [γ-FeOOH] in the Fe(II) and S( II) experiments. Transmission electron microscopy–selected area electron diffraction (TEM-SAED), coupled with Mo LIII-edge and K-edge X-ray absorption near edge structure (XANES) spectroscopy, revealed partial Mo(VI) reduction and precipitation of kamiokite [Fe2MoIV3O8] and sidwillite [MoVIO3‧2H2O] in both experiments. Extended X-ray absorption fine structure (EXAFS) spectroscopy at the Mo K-edge revealed Mo-O, Mo-Fe and Mo-Mo bonding consistent with kamiokite precipitation in both Fe(II) and S(-II) experiments, and the absence of Mo-S bonding in the S(-II) experiments. Similar Mo(VI) repartitioning pathways during Fe(II) and S( II) driven ferrihydrite transformation suggests that: (i) Fe(II) served as the electron donor for ferrihydrite transformation in both experiments, with Fe(II)(aq) likely produced in situ via ferrihydrite sulfidation in the S( II) experiments; and (ii) co-precipitation inhibited Mo release and limited subsequent interactions S(-II)(aq) during ferrihydrite transformation. Overall, our findings indicate that initial association with FeOx phases can strongly influence Mo sequestration pathways in anaerobic environments.enAttribution-NonCommercial-NoDerivs 2.5 CanadaMo(VI) reductionferrihydritesidwillitekamiokiteferruginoussulfidicArticle10.1016/j.chemgeo.2022.121075