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 con sequence, it is often introduced at elevated levels into the surface enviro nment through human activity. Additionally, ultramafic rocks such as serpen tinite are commonly enriched in chromium, and thus can also lead to appreci able 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 as sess the oxidation state of Cr in a given system and determine the potentia l 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 demonstrat e the enzymatic ability to reduce Cr(VI) to Cr(III); two species can even g row using Cr(VI) as the terminal electron acceptor in respiration. The abil ity to reduce chromium in itself is not evidence that the process will take place at appreciable levels in natural environments, however. Reduced mate rials such as ferrous iron or hydrogen sulfide may compete with biological pathways in the reduction of Cr(VI). On the basis of measured reaction rate s 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 similar to 5.5, whereas hydrogen sulfide will dominate at pH values b elow 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 b y which Cr(VI) is reduced will depend primarily on the aeration status of t he system, and secondarily on pH and the concentrations of specific reduced phases.