ABSTRACT
Chromium (Cr) is a transition element that is widely used in industries associated with electroplating, leather tanning, cement, dyeing, canning, wood preservatives, paint and pigments, textile, and metal processing. The toxicity of chromium is dependent on its oxidation state. Highly soluble and readily bioavailable Cr(VI) is approximately 1,000 times more toxic than Cr(III) which is less soluble under neutral pH and unable to cross cell membranes, thereby making it less toxic. Cr(VI), which is a potent mutagen, carcinogen, and teratogen, has deleterious effects on the health of animals and plants. Conventional physical and chemical methods for chromium removal include reduction, precipitation, adsorption, ion exchange, reverse osmosis, and electrodialysis. These methods are not only expensive but also have several limitations like inefficiency for complete metal removal, high reagent consumption, high energy requirements, and the generation of toxic sludge. Therefore, biological techniques have gained wide attention in recent decades to develop an environmentally-benign, more effective, and rapid method of bioremediation. This chapter presents an elaborate account of various heavy metal resistant/tolerant microbes like Ochrobactrum sp., Pseudomonas sp., Bacillus sp., Arthrobacter sp., Deinococcus sp., Enterobacter sp., Agrobacterium sp., Escherichia coli, Microbacterium sp. Shewanella sp., Desulfovibrio sp., Thermus sp., Thiobacillus thioparus, and others that have adopted several mechanisms for the bioaccumulation, bioconversion, and detoxification of chromium. Further, the cellular mechanism behind the microbial bioremediation of toxic chromium is presented. The development of microbe-based technologies using immobilized cells and genetically modified microbes in future can present potential alternative strategies to the conventional techniques of chromium removal and recovery.
