Mechanism of hexavalent chromium detoxification by microorganisms and bioremediation application potential: A review
Introduction
The anthropogenic inputs of chromium have increased rapidly since the industrial revolution (Nriagu and Pacyna, 1988; Ayres, 1992). Chromium is extensively used in electroplating (as chromeplating), resistant alloys (e.g., stainless steel), leather tanneries and dye productions (Bailar, 1997; United States Environmental Protection Agency, 1998; Ryan et al., 2002). Chromium exists in a wide range of valency states from −4 to +6, with the hexavalent species (Cr6+) predominant in natural aquifers and its trivalent counterpart (Cr3+) prevailing in the municipal wastewater rich in organics (Fukai, 1967; Jan and Young, 1978). Apart from its toxicity (discussed in Section 2), Cr6+ is also highly soluble and thus mobile and biologically available in the ecosystems. In contrast, Cr3+ displays a high affinity for organics resulting in the formation of complexes that precipitate as amorphous hydroxide (Palmer and Wittbrodt, 1991; Sawyer et al., 1994). Because of its persistence in the environment, anthropogenic release of Cr6+ is a matter of concern (Barlett, 1991; Katz and Salem, 1994).
Section snippets
Toxicity of hexavalent chromium
Chromium is an essential trace element for living organisms (Bailar, 1997). However, a slight elevation in the level of Cr6+ elicits environmental and health problems because of its high toxicity (Petrilli and Flora, 1977; Sharma et al., 1995), mutagenicity (Nishioka, 1975) and carcinogenicity (Venitt and Levy, 1974). Almost every regulatory agency has listed Cr6+ as a priority toxic chemical for control, with the maximum allowable level in drinking water of 50–100 μg l−1 (Tchobanoglous and
Microbial detoxification of hexavalent chromium
Traditionally, physico-chemical processes are used to reduce Cr6+ concentrations to levels that comply with statutory standards. Most commonly used processes include reduction–precipitation, ion exchange and reverse osmosis. However, the costs to set up the required equipment and to operate these processes are prohibitively high for large-scale treatment (Mahajan, 1985; Bhide et al., 1996; Beleza et al., 2001). The cell membrane is nearly impermeable to Cr3+ and thus Cr3+ has only approx.
Bioremediation limitations and potentials
The application of microorganisms to detoxifying metals has been tested in a number of systems, but the viability and metabolic activity of cells are still the major limiting factors affecting the detoxification efficiency of the cellular biomass and enzymes involved. The immobilization of microorganisms on surfaces in treatment systems may increase the biomass loading and hence the rate of metal transformation. For example, the immobilization of cells in the agarose–alginate gel slightly
Acknowledgments
We thank Miss Catty Chan for the computer graphic production. The preparation of this manuscript was supported by The University of Hong Kong and an 863 Project (no. 2002AA601160).
References (97)
- et al.
Essential residues in the chromate transporter ChrA of Pseudomonas aeruginosa
FEMS Microbiology Letters
(2004) - et al.
Kinetics of chromium (V) formation and reduction in fronds of the duckweed Spirodela polyrhiza—a low frequency EPR study
Journal of Inorganic Biochemistry
(2000) - et al.
Interactions of chromium with microorganisms and plants
FEMS Microbiology Reviews
(2001) - et al.
Plasmid chromate resistance and chromate reduction
Plasmid
(1992) - et al.
Reduction of chromate (CrO42−) by an enrichment consortium and an isolate of marine sulphate-reducing bacteria
Chemosphere
(2003) Potential hazards of hexavalent chromium in our drinking water
Toxicology and Applied Pharmacology
(2003)- et al.
Hexavalent chromium uptake by sensitive and tolerant mutants of Schizosaccharomyces pombe
FEMS Microbiology Letters
(1999) - et al.
Optimal operation of bioreactor system developed for the treatment of chromate wastewater using Enterobacter cloacae HO-1
Water Science and Technology
(1996) - et al.
Kinetics of chromium (VI) reduction by a type strain Shewanella alga under different growth conditions
Environmental Pollution
(2001) Effect of chromium (CrVI) on the growth rate of denitrifying bacteria
Water Research
(1994)