Molybdenum(VI) Sequestration Mechanisms During Iron(II)-Induced Ferrihydrite Transformation

Abstract

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.

Ferrihydrite transformations in flooded paddy soils: rates, pathways, and product spatial distributions

“…At pH below the point of zero charge of ferrihydrite, Fe(II)-catalysed transformation is hindered due to competitive sorption with H + . 37,54 However, in the acidic pH range, the dissolution of ferrihydrite by H + occurs more slowly than Fe(II)-catalysed transformation and, therefore, may not be observed in the timeframe of this experiment. 68,90 In the present study, ferrihydrite was fastest to transform in soils with low pH in the rst week of the study, but this trend was also explained by Fe(II) concentration and the relative pH of each soil aer week two does not correspond to the rate of ferrihydrite transformation.…”

“…35,37,40,41 The rates and products depend strongly on the ratio of Fe(II) to ferrihydrite. 21,37,[42][43][44][45][46] Moreover, previous studies have measured effects on the rates and products of Fe(II)-catalysed ferrihydrite transformation caused by diverse dissolved or sorbed metal ions, 20,23,47,48 structurally incorporated cations, 9,17,19,23,49,50 dissolved, sorbed or structurally incorporated inorganic anions, 19,21,22,[51][52][53][54] co-precipitated or sorbed organic compounds, 17,18,[55][56][57] cultivated bacteria, 56,58,59 as well as varying pH, 34,37,60 temperature, 36,51,60 and ferrihydrite-to-solution ratio. 44 These studies suggest that soil components may reduce electron ow from sorbed Fe(II), for example, by competition with Fe(II) for ferrihydrite surface sorption sites, or that soil components may be toxic to soil microbes, reducing the rate of mineral transformation.…”

Probing Atomic-Scale Processes at the Ferrihydrite-Water Interface With Reactive Molecular Dynamics

“…Past studies of charged interfaces in water experiencing adsorption events have commonly relied on empirical studies, supported by various spectroscopic techniques and surface complexation models (SCM), while some recent studies have used more advanced methodologies 7, [14][15][16][17][18][19] . SCM account for surface charge along with solute-surface adsorption complex equilibrium constants to fit a model of surface complexes to batch adsorption experiments data 20,21 .…”

Probing atomic-scale processes at the ferrihydrite-water interface with reactive molecular dynamics

“…Past studies of charged interfaces in water experiencing adsorption events have commonly relied on empirical studies, supported by various spectroscopic techniques and Surface complexation models (SCM), while some recent studies have used more advanced methodologies [7,14,15,16,17,18,19]. SCM account for surface charge along with solute-surface adsorption complex equilibrium constants to fit a model of surface complexes to batch adsorption experiments data [20,21].…”