Fate and Stability of Cr Following Reduction by Microbially Generated Fe(II)

A suite of industrial activities has lead to widespread Cr contamination within soils and natural waters. Although Cr is an essential element for humans, the hexavalent form is toxic, mutagenic, and carcinogenic (National Research Council, 1974). As such, the widespread presence of Cr in the environment poses a serious threat to human and animal welfare. The toxicity of Cr, however, is a function of oxidation state. Hexavalent Cr, which typically exists as the oxyanion chromate (CrO4), has a high solubility in soils and groundwater and, as a consequence, tends to be mobile in the environment. In contrast, the reduced form of chromium, Cr(III), has a limited hydroxide solubility and forms strong complexes with soil minerals (Sass and Rai, 1987). While trivalent Cr is relatively innocuous and immobile, hexavalent Cr is actively transported into cells by the sulfate transport system where it is capable of causing lesions in DNA as well as indirectly generating oxygen radicals. Accordingly, reduction of Cr(VI) to Cr(III) is an important means by which the deleterious effects of this toxin are mitigated. This general process forms the fundamental basis of a large number of technologies currently being tested for remediation of chromium-contaminated soils.