CR(VI) fate in mineralogically altered sediments by hyperalkaline waste fluids.

摘要

Hyperalkaline (pH~14), high temperature (>100 degrees C), high ionic strength (>5 mol L-1) waste fluids (WFs) contaminated with Cr(VI) have accidentally leaked from storage tanks at the Hanford Site in Washington into the underlying sediments. Previous laboratory studies conducted under such extreme conditions 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 O< sub>2 that may be present, although in limited amounts, in inherently oxidized vadose zones, and may compete for Fe(II) electrons. In addition, Cr(VI) adsorption can become an important attenuation mechanism in WF-altered sediments because sorbents such as cancrinite, sodalite, and Fe oxides were formed in appreciable amounts in these geosystems. The objectives of this study were to estimate Cr(VI) attenuation via reduction in the presence of limited amounts of O< sub>2 and to determine the potential for Cr(VI) sorption in the vadose zone sediments of the hyperalkaline plume. Results from batch and column experiments conducted at 50 degrees C with simulated WF and results of microprobe elemental mapping and micro-X-ray absorption near edge structure analyses performed on posttreatment sediments confirmed that the main attenuation mechanism was Cr(VI) reduction to less mobile Cr(III). Oxygen that was periodically introduced into the otherwise closed geosystem competed effectively with Cr(VI) for the available electrons only at low base concentrations, that is, 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, that is, 4 mol L-1, confirming that contaminant Cr mobility may be significantly reduced even in the presence of O< sub>2.