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 mu M, 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.