Copyright @ Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited. CR(VI) FATE IN MINERALOGICALLY ALTERED SEDIMENTS BY HYPERALKALINE WASTE FLUIDS Nikolla P. Qafoku 1 , Calvin C. Ainsworth 1 , and Steve M. Heald 2 Hyperalkaline (pH ~14), high temperature (9100-C), high ionic strength (95 mol L j 1 ) waste fluids (WFs) contaminated with Cr(VI) have ac- cidentally 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 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 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-C with simulated WF and results of microprobe elemental mapping and micro–X-ray absorp- tion 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 j1 . 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 j1 , confirming that contaminant Cr mobility may be significantly reduced even in the presence of O 2 . (Soil Science 2007;172:598–613) Key words: Redox reactions, Cr mobility, Cr transport, Hanford sediment, alkaline solutions, saline solutions. H IGH temperature, alkaline, and saline radioactive waste fluids (WFs) rich in contaminants such as hexavalent Cr [Cr(VI)] have leaked from the underground single-shell waste tanks at the Hanford Site in Washington. These alkaline liquids had a high concentration of Cr(VI) (up to 0.413 mol L j1 ) ( Jones et al., 2000), which is considered highly toxic to plants and animals (Abbasi and Soni, 1984; Paschin et al., 1983) and a human carcinogen (Ono, 1988). In addition, the contaminated Hanford Site is close to the Columbia River, a significant water resource in the northwestern United States. It is possible that the highly mobile Cr(VI) will be transported to the groundwater and the river if attenuation mechanisms are not operating in the vadose zone and aquifer sediments. Recent studies have shown that Cr(VI) reduction to less mobile Cr(III) occurred when biotite (He et al., 2005), magnetite (He and Traina, 2005), and Hanford sediments (Zachara et al., 2004; Qafoku et al., 2003a) were contacted with simulated WF. Approximately 42% of the Cr(VI) originally present in the WF was immobilized mostly as Cr(III), although 598 0038-075X/07/17208-598–613 August 2007 Soil Science Vol. 172, No. 8 Copyright * 2007 by Lippincott Williams & Wilkins, Inc. Printed in U.S.A. 1 Pacific Northwest National Laboratory, P.O. Box 999, MSIN: K3-61, Richland, WA 99352. Nikolla P. Qafoku is corresponding author. E-mail: nik. qafoku@pnl.gov 2 Argonne National Laboratory, Argonne, IL 60439. Received Dec. 11, 2006; accepted Apr. 3, 2007. DOI: 10.1097/SS.0b013e31806db5b2