Manganese-Oxide Transformation of Vanadium in Redox-Fluctuating Soil Systems.

Vanadium is a ubiquitous redox active transition metal found in soil systems, which can have adverse effects on the neurological development of children when ingested. Human exposure to vanadium can occur via consumption of contaminated groundwater containing V that is mobilized from soils and sediments under redox-fluctuating conditions. Past studies suggest that V is predominantly present in the more mobile V(V) form under oxidizing conditions; however, specific geochemical processes contributing to vanadium redox cycling and subsequent change in mobility into groundwater systems is not well understood. Past studies have demonstrated that permanganate is a strong oxidant of V, rapidly oxidizing V(IV) to V(V) upon contact. Therefore, we hypothesize that naturally-occurring manganese (Mn) oxides, such as birnessite, could be a potential oxidant of V in soils and sediment systems. To understand the fate and transport of V, a Donnan reactor that simulates diffusion-limited environments within soils was used to examine V(IV) oxidation and subsequent sorption of V(V) on Mn oxides. Vanadium sorption and oxidation was monitored over time by measuring the concentration of aqueous V using ICP-OES in conjunction with V speciation determined by kinetic spectrophotometry. X-ray absorption spectroscopy (EXAFS and XANES analysis) was used to determine changes in Mn oxide structure and speciation of adsorbed V over time. The oxidation of V resulting from contact with birnessite, with subsequent reductive dissolution of the oxidant, will affirm the potential role of V transformation from V(IV) to V(V) by Mn oxides. The results of this study demonstrate that the presence of naturally-occurring Mn oxides in soils and sediments can impact the fate and transport of V in natural systems and their role in controlling V mobilization into water resources.