Adsorption 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.

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