Integrated mechanisms revealing detoxification and attenuation of Cr(VI) in contaminated soil and aquatic systems - A critical review on bioremediation strategies

Chromium (Cr) as Trivalent: Cr3+ and Hexavalent: Cr6+ forms in nature is a known toxic heavy element and commonly located as a metal pollutant in mine and industrial sites. Cr-speciation and Cr-cytotoxicity are important factors that cause cancerous problems in humans and reduce plant growth and productivity. Its soil-plant shift is accomplished by carriers of essential ions. The harmful nature of Cr toxicity has attracted more attention because of the entry of Cr into the food chain. Sustainable restoration of soil and water quality is therefore highly crucial to avoid chromium pollution. Hexavalent chromium in mine soil, mine waste water and effluents of chromium processing industries create serious hazards to living organisms, and therefore, there is a need for the situation to be addressed through detoxification and attenuation of chromium by advanced, integrated, sustainable and eco-friendly technology. Biodegradation and bioremediation approaches are cost-effective and eco-friendly to detoxify and attenuate contaminated chromium. Therefore, the detailed gamut of Cr bioavailability from microbes to higher plants is indispensable. This review enlists the modes of biodegradation, biodetoxification, and bioremediation approaches through microbial and phytoremediation studies from contaminated soil and water. The roles of phyco and myco-biosorption methods for the remediation of Cr are described. The roles of tolerant species, i.e., hyperaccumulators, metal-resistant species and hydrophytes for Cr-bioremoval are reviewed. The use of microbes in enzymatic reduction, rhizospheric remediation and integrated chelate-based remediation techniques for Cr removal, including the applications of green remediation techniques and genetic engineering, has been emphasized.