Influence of soil geochemical and physical properties on chromium(VI) sorption and bioaccessibility

The Department of Defense (DoD) is faced with the daunting task of possible remediation of numerous soil-Cr(VI) contaminated sites throughout the continental U.S. The primary risk driver at these sites is hand-to-mouth ingestion of contaminated soil by children. In the following study we investigate the impact of soil geochemical and physical properties on the sorption and bioaccessibility of Cr(VI) in a vast array of soils relevant to neighboring DoD sites. For the 35 soils used in this study, A-horizon soils typically sorbed significantly more Cr(VI) relative to B-horizon soils. Multiple linear regression analysis suggested that Cr(VI) sorption increased with increasing soil total organic C (TOC) and decreasing soil pH. The bioaccessibility of total Cr (CrT) and Cr(VI) on the soils decreased with increasing soil TOC content. As the soil TOC content approached 0.4%, the bioaccessibility of soil bound Cr systematically decreased from approximately 65 to 10%. As the soil TOC content increased from 0.4 to 4%, the bioaccessibility of Cr(VI) and CrT remained relatively constant at approximately 4% and 10%, respectively. X-ray absorption near edge structure (XANES) spectroscopy suggested that Cr(VI) reduction to Cr(III) was prevalent and that the redox transformation of Cr(VI) increased with increasing soil TOC. XANES confirmed that nearly all bioaccessible soil Cr was the Cr(VI) moiety. Multiple linear regression analysis suggested that the bioaccessibility of Cr(VI) and its reduced counterpart Cr(III), decreased with increasing soil TOC and increasing soil pH. This is consistent with the observation that the reduction reaction and formation of Cr(III) increased with increasing soil TOC and that Cr(III) was significantly less bioaccessible relative to Cr(VI). The model was found to adequately describe CrT bioaccessibility in soils from DoD facilities where Cr(VI) contaminated sites were present. The results of this study illustrate the importance of soil properties on Cr(VI) sorption and bioassessability and help define what soil types have the greatest risk associated with Cr(VI) exposure.