Oxidation‐reduction chemistry of chromium: Relevance to the regulation and remediation of chromate‐contaminated soils

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Abstract
Developing health‐protective clean‐up standards and remediation strategies for chromium‐contaminated soils based on the hexavalent forms of this heavy metal is a complex and controversial issue, because certain forms of Cr(III) can oxidize to Cr(VI) and Cr(VI) can be reduced to Cr(III) under diverse soil conditions. The extent of oxidation of Cr(III) in soils amended with wastes is based on four interacting parameters: (1) solubility and form of Cr(III) related to oxidation (waste oxidation potential, or WOP), (2) reactive soil Mn(III,IV) (hydr)oxide levels (soil oxidation potential for Cr(III), or SOP), (3) soil potential for Cr(VI) reduction (soil reduction potential, or SRP), and (4) soil‐waste pH as a modifier of the first three parameters (pH modification value, or PMV). Each of these four parameters can be quantified with laboratory tests and ranked numerically; the sum of which is the Potential Chromium Oxidation Score (PCOS) for assessing the relative hazard of a waste‐soil combination. The PCOS value is proposed as the basis for setting acceptable maximum limits for Cr(III)‐containing wastes in particular soil environments to maintain Cr(VI) levels at or below health‐based limits. Adjusting and controlling the PCOS parameters will allow practitioners to design remediation strategies and regulators to establish Cr clean‐up standards under soil conditions that minimize Cr(III) oxidation and maximize Cr(VI) reduction. Effective remediation of Cr(VI)‐Contaminated soils by reduction depends on three principles: (1) reduction of Cr(VI) to forms of Cr(III) that are inert toward re‐ oxidation, (2) the absence of undesirable reaction products (e.g., the oxidized forms of certain reducing agents), and (3) establishment or maintenance of soil pH and Eh conditions that favor reduction of Cr(VI) and disfavor the oxidation of Cr(III). This article focuses on the importance of understanding the waste forms of Cr and the redox‐related properties of Cr‐contaminated soils in the design of remedial goals to protect human health and ecological function.