PNNL

Abstract

Hyperalkaline (pH ~ 14), high temperature (> 100 °C), high ionic strength (>5 mol L-1) waste fluids (WF) contaminated with Cr(VI) have accidentally leaked from storage tanks at the Hanford site, WA, into the underlying sediments. Previous laboratory studies conducted in extreme alkaline geosystems have shown that Cr(VI) was abiotically reduced to Cr(III) by aqueous Fe(II) released from dissolving soil minerals. However, these studies were conducted in the absence of other electron acceptors, such as O2, that may be present in inherently oxidized vadose zones and may compete for Fe(II) electrons. In addition, Cr(VI) adsorption could become an important attenuation mechanism in WF altered sediments since sorbents such as cancrinite, sodalite and Fe oxides were formed in appreciable amounts in these geosystems. The objectives of this study were to determine the potential for Cr(VI) sorption in the vadose zone sediments of the hyperalkaline plume and to estimate Cr(VI) attenuation rates in the presence of O2. Results from batch and column experiments conducted at 50 °C with simulated WF, and results of microprobe elemental mapping and µ–XANES analyses performed on post-treatment sediments, confirmed that the main attenuation mechanism was Cr(VI) reduction to less mobile Cr(III). Cr(VI) transport through mineralogically altered sediment was slightly retarded, suggesting limited adsorption. Oxygen that was periodically introduced into the otherwise closed geosystem competed effectively with Cr(VI) for the available electrons only at low base concentrations, i.e., 1 mol L-1. Localized reduced zones were created in the sediment when intensive dissolution of soil minerals occurred in the presence of high base concentrations, i.e., 4 mol L-1.