Schoepfer, Valerielum, jullietaLindsay, Matthew B. J.2022-10-252022-10-252021-08-04Schoepfer, V.A., Lum, J.E. & Lindsay, M.B.J. (2021). Molybdenum(VI) sequestration mechanisms during iron(II)-induced ferrihydrite transformation. ACS Earth and Space Chemistry, 5(8): 2094–2104. https://doi.org/10.1021/acsearthspacechem.1c00152https://hdl.handle.net/10388/14266Copyright © 2021 American Chemical SocietyAdsorption and coprecipitation reactions with Fe(III) (oxyhydr)oxides contribute to Mo(VI) attenuation within geohydrologic systems. Redox transitions within these systems can promote transformation of metastable phases, including ferrihydrite, and repartitioning of associated Mo(VI). Recent studies show that Mo(VI) coordination shifts from tetrahedral to octahedral during Fe(II)-induced ferrihydrite transformation. However, effects of initial conditions including solution pH, the Mo(VI) uptake mechanism, and Mo(VI) loading on repartitioning are not known. We performed batch experiments using ferrihydrite suspensions prepared with adsorbed or coprecipitated Mo(VI) (0, 25, and 100 μmol g–1) at two initial pH values (pH0; 5.0 and 6.5). We catalyzed ferrihydrite transformation under anoxic conditions by adding Fe(II)(aq) (0.5 mM) and monitored pH, [Mo]T, and [Fe]T over time. After 168 h, we collected reacted solids for analysis by powder X-ray diffraction (XRD), transmission electron microscopy-selected area electron diffraction (TEM-SAED), and Mo K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy. XRD data indicate that bulk ferrihydrite transformation was limited in all but the pH0 6.5 coprecipitated Mo(VI) experiments. The TEM-SAED results reveal that nanoscale lepidocrocite and goethite formed at ferrihydrite surfaces in all experiments, whereas nanoscale bamfordite [FeMo2O6(OH)3·H2O] crystallites were observed in pH0 6.5 experiments. EXAFS models reveal changes in Mo(VI) coordination and bonding consistent with bamfordite precipitation combined with structural incorporation into neoformed goethite and lepidocrocite. Our results improve the understanding of Mo(VI) retention pathways in geohydrologic systems.enmolybdenumferrihydritebamforditelepidocrocitegoethiteArticle10.1021/acsearthspacechem.1c00152