Bioremediation of chromium solutions and chromium containing wastewaters

Abstract

Cr(VI) represents a serious threat to human health, living resources and ecological system as it is persistent, carcinogenic and toxic, whereas, Cr(III), another stable oxidation state of Cr, is less toxic and can be readily precipitated out of solution. The conventional methods of Cr(VI) removal from wastewaters comprise of chemical reduction followed by chemical precipitation. However, these methods utilize large amounts of chemicals and generate toxic sludge. This necessitates the need for devising an eco-technological strategy that would use the untapped potential of the biological world for remediation of Cr(VI) containing wastewaters. Among several viable approaches, biotransformation of Cr(VI) to relatively non-toxic Cr(III) by chromium resistant bacteria offers an economical- and environment-friendly option for its detoxification. Various studies on use of Cr(VI) tolerant viable bacterial isolates for treatment of Cr(VI) containing solutions and wastewater have been reported. Therefore, a detailed account of mechanisms and processes involved in bioreduction of Cr(VI) from solutions and wastewaters by bacterial isolates are the focus of this review article in addition to a discussion on toxicity of Cr(VI) on bacterial strains and various factors affecting Cr(VI) bioreduction.

Recent bioreduction of hexavalent chromium in wastewater treatment: A review

“…The conventional processes are complex due to the different intensive sub-processes and use of large amounts of chemicals and generation of toxic sludges. [13,42,43] Carbothermal reduction of Cr(VI) was conducted by adding carbon at temperature ranging from 1000 to 1400 o C in a TGA apparatus with particle size limitation of the Cr(VI) contaminated soil sample [27]. To achieve a complete reduction 15% carbon was added to soil, however, evaluation of CO2 emission was ignored during the process.…”

“…This has stimulated the interest in microorganisms that can use Cr(VI) as an electron acceptor. The biological detoxification involves different inner and outer cellular reactions such as direct reduction by chromium reducing bacteria, biosorption, and phytoremediation, and indirect reduction by application of different electron shuttles, bioremediation, and bioaccumulation [13,46]. The ascorbate with glutathione and cysteine produced during the cellular metabolism is responsible for more than 95% of Cr(VI) reduction [49,50].…”

“…Since the ground state electronic configuration of chromium is 'high spin' [Ar]4s 1 3d 5 , it shows varying oxidation states such as -2, -1, 0, +1, +2, +3, +4, +5 and +6 with an associated wide range of chemical and physical properties. [13] It can form acidic, alkaline or amphoteric oxides according to the oxidation state. The most stable chromium occurs when the oxidation states are +3 (trivalent, Cr(III)) and +6 (hexavalent, Cr(VI)).…”

“…[8] Chromium metal is a steely-grey colour, has a metallic luster, is hard and brittle, resists tarnishing, and has high melting and boiling points (1,907 o C and 2,671 o C respectively). [13,14] Chromium metal is passivated by oxidation reactions and this forms a thin protective surface layer which prevents the diffusion of oxygen onto the underlying metal.…”

Progress Towards Bioelectrochemical Remediation of Hexavalent Chromium

“…The pH, with its effects on the surface properties of the cells, including cell surface hydrophobicity, net surface electrostatic charge, and biofilm structure, may also heavily affect complex biological and electrochemical reactions at the biocathode. Variation in pH may also affect enzymatic activity, and produced Cr(III) precipitation or bio-adsorption [105]. In Huang et al [64,65], 50 mg/L initial Cr(VI) concentration inhibited the catalytic activity of electrochemical bacteria in the biocathode, whereas, at a 20 mg/L Cr(VI) concentration, chromium reduction efficiencies increased (+27.3%) and decreased (−21%) in acidic (pH = 5) and alkaline catholyte (pH = 8), with respect to neutral pH.…”