Bacterial Reduction of Cr(VI): Operational Challenges and Feasibility

Abstract

Purpose of Review Hexavalent chromium, Cr(VI), and trivalent chromium, Cr(III), are two chromium compounds with practical importance due to their high occurrence and solubility in the environment. Current Cr(VI) treatment techniques involve chemical reduction of Cr(VI) to Cr(III), which posed serious threat to workers and environment notably from long exposure and toxic fumes.Recent Findings Numerous reports have demonstrated the feasibility of using biological processes for the treatment of Cr(VI) industrial effluents by either pure culture or a consortium of Cr(VI)-reducing bacteria, with various degrees of success. Among issues to be considered include high cost of nutrient for the bacteria, low Cr resistant-reducing ability of environmental isolates, difficulty in scaling up finding in the laboratory to pilot scale and on-site application as well as the understanding on the dynamic underlying mechanisms for bacterial Cr(VI) reduction. Summary This review highlights cytotoxicity and genotoxicity properties of Cr(VI), which form the biggest motivation for continuous development in the field of Cr(VI) treatment technologies, latest finding in aerobic and anaerobic bacterial reduction of Cr(VI), operational challenges for bacterial Cr(VI) reduction, and some examples for laboratory-scale and pilot-scale evaluation of free and immobilized (biofilm) cells of Cr(VI) resistant-reducing bacteria.

Killing Two Birds with One Stone: Biomineralized Bacteria Tolerate Adverse Environments and Absorb Hexavalent Chromium

“… 16 Mass transport effects induced by the outer membrane of Gram-negative bacteria and the thin cell wall of peptidoglycan containing reductase promote the biosorption of heavy metals by Gram-negative bacteria. 15 , 16 We used the microfluidic method for the encapsulation of E. coli and Bacillus subtilis , treating aqueous solutions of 5, 25, and 50 mg/L hexavalent chromium. We found that 5 mg/L of hexavalent chromium was absorbed at a higher rate than the others.…”

“… 16 A variety of microorganisms have been shown to be able to reduce Cr(VI) directly or indirectly, and their metabolites are also capable of acting as reducing agents, such as organic acids, amino acids, sulfides, and divalent iron. 2 , 15 , 16 Intracellular and membrane-bound Cr(VI) reduction is usually mediated by flavin-dependent reductase and hydrogenase, respectively, while extracellular and periplasmic Cr(VI) reduction is accomplished by cytochromes. 2 Intracellular reduction of Cr(VI) is the common pathway, and the influencing factors are NADH, flavin proteins, and other heme proteins.…”

“…Cr(VI) is reduced by chromate reductase and the reduction product is translocated out of the cell via plasmids ( Figure S8 ). 16 The enzymatic reduction of Cr(VI) by E. coli is dependent on oxygen-insensitive NADPH nitroreductase, which was encoded by the nfsA gene. 14 However, the different electron transfer mechanisms of these bacterial reductases need to be further investigated.…”

“… 4 , 11 − 13 Organisms including bacteria, fungi, yeast, algae, and plants have shown the ability to remediate. 2 , 14 − 16 However, the poor resistance of bacteria to contaminants is a bottleneck for biotechnology application. 3 , 13 Hexavalent chromium has good cell membrane permeability and readily enters bacteria, where it subsequently undergoes reduction reactions and generates free radicals (e.g., reactive oxygen species), leading to bacterial death.…”

“…Biosorption is a promising technology for the removal of hexavalent chromium from wastewater, which is economical, efficient, and environmentally friendly. ,− Organisms including bacteria, fungi, yeast, algae, and plants have shown the ability to remediate. ,− However, the poor resistance of bacteria to contaminants is a bottleneck for biotechnology application. , Hexavalent chromium has good cell membrane permeability and readily enters bacteria, where it subsequently undergoes reduction reactions and generates free radicals (e.g., reactive oxygen species), leading to bacterial death. , Bacteria with resistance to hexavalent chromium are usually isolated from contaminated soil and water for further use in the biosorption. , The isolation and culture of specific bacteria is complex and difficult. , Therefore, we hope that, by improving the tolerance of common bacteria to extreme environments, common bacteria can also be applied to biosorption. In recent years, researchers found that bacteria could be modified by inorganic nanoparticles, polymers, and metal–organic frameworks (MOFs) to enhance their resistance to harsh environments. − MOFs are synthesized by cross-linking metal ions/clusters with organic linkers.…”