Lawrence Berkeley National Laboratory

In-situ reduction of toxic Cr(V1) to less hazardous Cr(II1) is becoming a popular strategy for remediating contaminated soils. However, the long term stability of reduced Cr remains to be understood, especially given the common presence of MnfIIIJV) oxides that reoxidize Cr(II1). This 4.6 year laboratory study tracked Cr and Mn redox transformations in soils contaminated with Cr(V1) which were then treated with different amounts of organic carbon (OC). Changes in Cr and Mn oxidation states within soils were directly and nondestructively measured using micro X-ray absorption near edge structure spectroscopy. Chromate reduction was roughly lst-order, and the extent of reduction was enhanced with higher OC additions. However, significant Cr(||1) reoxidation occurred in soils exposed to the highest Cr(V1) concentrations (2,560 mg kg"'). Transient Cr(II1) reoxidation up to 420 mg kg1 was measured at 1.1 years after OC treatment, followed by further reduction. Chromate concentrations increased by 220 mg kgm1a t the end of the study (4.6 years) in one soil. The causal role that Mn oxidation state had in reoxidizing Cr was supported by trends in Mn K-edge energies. These results provide strong evidence for longterm dependence of soil Cr oxidation states on balances between OC availability and Mn redox status.