Apparent Relationship Between Soil Vanadium and Arsenic Bioaccessibility Across Many Soils: Are There Geochemical Reasons?.

Rapid and inexpensive assessment of arsenic (As) toxicity from contaminated soil is a crucial part of contaminated site planning and soil remediation efforts. In vitro bioaccessibility (IVBA) extractions can estimate the As systemic uptake after contaminated soil ingestion and be a tool for risk assessment but more commonly, total elemental concentrations are the data available. So, a predictive model of As IVBA was developed using total elemental concentrations in a dataset of 115 soils. Interestingly, vanadium (V) was second only to iron (Fe) as a significant negative correlant for As IVBA. Arsenic toxicity is determined in part by its chemical speciation and the soil mineralogy but to the best knowledge of the authors, no studies have been published considering As and V geochemistry together in soils. Our objective was to characterize and investigate the geochemical relationship between As and V. In soil, both elements are influenced by iron (Fe), manganese (Mn), and/or aluminum (Al) oxides and hydroxides, so we hypothesized that their selective dissolution would also release both elements. We also hypothesized that both V and As would be in more recalcitrant pools shown via selective extraction methods. Three soils were selected for characterization and speciation. Selected soils had total V of (~100 mg kg-1) but dissimilar soil properties (e.g. pH, elemental totals) and As contamination sources (e.g. mining and pesticide). Soils were selectively extracted and supernatant measured on ICP-OES as well as characterization by bulk-XANES, Fe57 Mossbauer and micro-XRF mapping for element co-location. Results showed that As and V were associated with metal oxides and hydroxides to varying degrees in each soil. Both As and V showed co-localization with Fe in all soils in XRF maps, but not with each other which was also reflected in the bulk-XANES. Reductive dissolution released variable amounts of Fe, Mn, As and V in each soil-- including one soil, where Fe, As and V were almost exclusively released by dissolution of hematite (>90% of total As, V, and Fe). Further details of results will be discussed. Our data suggest that soil vanadium may be indirectly related to As geochemistry via soil mineralogy and explain its importance in predictive models of As IVBA.