Adsorption of (Poly)vanadate onto Ferrihydrite and Hematite: An In Situ ATR–FTIR Study

Vanadium (V) geochemistry offers insight into Earth’s global biogeochemical cycles over geologic time. Additionally, increasing anthropogenic release of this redox-sensitive metal has led to elevated V concentrations in soils, sediments, and waters. Although Fe (oxyhydr)oxides are important sinks for aqueous V in soils and sediments, our understanding of adsorption mechanisms is currently limited to mononuclear species (i.e., HxVO4(3–x)–). Here, we use in situ attenuated total reflectance–Fourier transform infrared spectroscopy to examine the sorption mechanisms for (poly)vanadate attenuation by ferrihydrite and hematite from pH 3 to 6. Adsorption isotherms illustrate the low affinity of polyvanadate species for ferrihydrite surfaces compared to that for hematite. Mononuclear V species (i.e., [HxVO4](3–x)– and VO2+) were present at all experimental conditions. At low surface loadings and pH values of 5 and 6, H2VO4– adsorption onto ferrihydrite and hematite surfaces results in the formation of inner-sphere complexes. At [V]T above 250 μM, adsorbed polynuclear V species in this study include H2V2O72– and V4O124–, whereas, HV10O286–, H3V10O285–, and NaHV10O284– are the predominant adsorbed species at pH values of 3 and 4 and elevated [V]T’s. Surface polymers were identified on hematite at all experimental pH values, whereas polymeric adsorption onto ferrihydrite was limited to pH values of 3 and 4. Results also suggest that hematite is a more suitable substrate for polymer complexation than ferrihydrite. Our results demonstrate the pH- and concentration-dependent removal of (poly)vanadate species by Fe(III) (oxyhydr)oxides, which has implications for understanding V mobility, behavior, and fate in the environment.