Environmental contamination by toxic hexavalent chromium [Cr(VI)] due to diverse anthropogenic activities has increased extensively in the recent past, which demands the development of ecofriendly technologies to remediate contaminated sites. Since remediation of toxic chromium by physico-chemical techniques are quite expensive and generates large quantities of solid waste for disposal, bioremediation by chromate reducing bacteria has been recognized as the most cost-effective green technology. Microbial reduction of Cr(VI) not only mitigate toxic hexavalent chromium, but also leads to the physical separation of nontoxic trivalent chromium [Cr(III)] at neutral pH following precipitation. Reductions of chromate during bacterial growth, by whole cells and by cell-free extracts have been reported by a variety of bacterial strains.

They have also been exploited under immobilized conditions using several different matrices and also in the form of biofilms. The possible applications of immobilized cells, cell-free extracts, and biofilms in Cr(VI) removal from contaminated effluents using different bioreactor systems have also been assessed successfully under laboratory as well as in in situ conditions. The main aim of this review is to provide a thorough overview of the existing systems of Cr(VI) reduction utilizing immobilized cells, crude enzymes, and biofilms in different types of bioreactors and their effectiveness in environmental Cr(VI) bioremediation.