(2,7) The solubility, and thus mobility, of V is highly dependent on its oxidation state, with solubility increasing with oxidation state at circumneutral pH.
Vanadium is a redox active metal present in the +3, +4, and +5 oxidation states in terrestrial environments. (19,23−25) Such sediment weathering has resulted in V mobilization to aquifer pore spaces resulting in well water contamination throughout California. (15−22) Weathering processes drive the release of V from these mineral phases into the groundwater-sediment matrix leading to elevated pore water concentrations, as well as remove V through adsorption processes. (7,14) This is most relevant to regions with aquifers developing on parent material rich in Fe III and Al III (hydr)oxides due to the high weight percentages of V III and V IV substitution that can occur in these minerals. (13) However, with few exceptions, the source of mobile V in subsurface environments appears to be dominated by weathering processes. (7,12−14) Recent estimates suggest that anthropogenic emissions of V into the biosphere now exceed V emissions from geologic processes. (10,11) Increases in steel demand and the extraction and combustion of fossil fuels have drastically increased the mobilization of V from the Earth’s crust over the past century. (5,6) Vanadium has demonstrable toxicity at exposures as low as 1.2–80 μg/L for sensitive aquatic species, (5−7) and elevated concentrations have been shown to alter microbial community structure (8,9) and reduce crop yields. Geogenic and anthropogenic emission of vanadium (V) into the biosphere poses an increasing threat to water quality, (1−3) human health, (4) and sensitive ecological systems. Our results demonstrate that V(V) adsorption is exergonic for a variety of surfaces with differing amounts of terminal −OH groups and metal–O bond saturations, implicating the conjunctive role of varied mineral surfaces in controlling the mobility and fate of V(V) in terrestrial and aquatic systems. Inner-sphere V(V) complexes were detected on all phases, with mononuclear V(V) species dominating the adsorbed species distribution. Here, we utilize extended X-ray absorption fine structure (EXAFS) and thermodynamic calculations to compare the surface complexation of V(V) by the common Fe and Mn mineral phases ferrihydrite, hematite, goethite, birnessite, and pyrolusite at pH 7. Iron (Fe) and manganese (Mn) (oxy)hydroxides represent key mineral phases in the cycling of V(V) at the solid–solution interface, and yet, fundamental descriptions of these surface-processes are not available.
In oxic systems, V is present as an oxyanion with a +5 formal charge on the V center, typically described as H xVO 4 (3– x)–, but also here as V(V). Anthropogenic emissions of vanadium (V) into terrestrial and aquatic surface systems now match those of geogenic processes, and yet, the geochemistry of vanadium is poorly described in comparison to other comparable contaminants like arsenic.
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