Chromium Transformations in Natural Environments: The Role of Biological and Abiological Processes in Chromium(VI) Reduction

Chromium is a redox-dynamic element that has many industrial uses. As a consequence, it is often introduced at elevated levels into the surface environment through human activity. Additionally, ultramafic rocks such as serpentinite are commonly enriched in chromium, and thus can also lead to appreciable levels of this element within soils and waters. In the trivalent state, it poses little hazard to biological activity, but, unfortunately, in the hexavalent state it is very toxic to living matter. One must therefore assess the oxidation state of Cr in a given system and determine the potential for transformation between valence states. The objective of this paper to is review and provide new insight on reduction reactions of Cr(VI) within natural environments. A number of aerobic and anaerobic bacteria demonstrate the enzymatic ability to reduce Cr(VI) to Cr(III); two species can even grow using Cr(VI) as the terminal electron acceptor in respiration. The ability to reduce chromium in itself is not evidence that the process will take place at appreciable levels in natural environments, however. Reduced materials such as ferrous iron or hydrogen sulfide may compete with biological pathways in the reduction of Cr(VI). On the basis of measured reaction rates and derived rate expressions, we demonstrate that biological pathways are not likely to contribute to the reduction of chromate in anaerobic systems. Ferrous iron will dominate the reduction of chromate at pH values greater than ～ 5.5, whereas hydrogen sulfide will dominate at pH values below this value. In contrast, bacteria may be the principal means by which Cr(VI) is converted to Cr(III) in aerobic environments. Thus, the process by which Cr(VI) is reduced will depend primarily on the aeration status of the system, and secondarily on pH and the concentrations of specific reduced phases.