Zahid Hasan:
Rapid industrialization throughout the world has resulted in enormous environmental pollution. Among them, heavy metal pollution is one of the most important environmental concerns. Untreated effluents generated from industries, municipal areas and agricultural sites which contains different types of toxic pollutants continuously polluted terrestrial and aquatic environments. These waste waters contain a variety of metals such as Ag, As, Au, Cd, Co, Cr, Cu, Hg, Ni, Pb, Pd, Pt, Rd, Sn, Th, U and Zn, which can produce very harmful effects on human health when they entered into human body through different ways. As we know, heavy metals are very toxic at very little concentration and there is no easy way to let them out from the body.

However, conventionally several physico-chemical methods including precipitation, neutralization, ion exchange, membrane separation, electro-dialysis, activated carbon adsorption technique etc. are used for the removal of these pollutants. Though they can be effective at reducing wide range of contaminants but at the same time have several drawbacks. These methods are complex, uneconomical, produce hazardous by-products, lack of public acceptance and in many cases, practically not feasible due to the range of contamination. Therefore, various strategies are being developed and further research is currently underway to develop means of sustaining the environment. Bioremediation is an emerging process of using in situ or ex situ microorganisms to clean-up a contaminated site. Bioremediation can be defined as “the application of biological processes for the removal of hazardous pollutants present in the environment”. It was reported that the biological processes for treating toxic effluents are better than chemical and physical methods in terms of their efficiency and economy. They are eco-friendly, easy to operate and do not produce secondary pollutants. Bioremediation uses biological agents, mainly micro-organisms, e.g. yeast, fungi or bacteria to clean up contaminated soil and water. Bioremediation can be used at the site of contamination (in situ) or on contamination removed from the original site (ex situ). In bioremediation processes, microorganisms use the contaminants as nutrient or energy sources. These process use naturally occurring micro-organisms to degrade or detoxify components hazardous to human health and environment and transform them into non-toxic compounds.

In bioremediation process, micro-organisms enzymatically attack the pollutants and convert them to harmless products i.e. carbon dioxide or methane, water and biomass. Bioremediation can be carried out both in aerobic and anaerobic condition. But aerobic conditions proved to be faster compared to that of anaerobic condition. Important parameters for bioremediation process are- i) nature of pollutants, ii) temperature, pH, salinity, electrical conductivity, redox- potential, nutritional status and microbial diversity of the site and iii) microorganism used in the treatment process. Natural microorganisms, either indigenous or introduced, are used for bioremediation. They can vary, depending on the nature of pollutants and should be selected carefully as they only survive within a limited range of contaminants. The first patent for a biological remediation agent was registered in 1974, being a strain of Pseudomonas putida that was able to degrade petroleum.

Several microorganisms have been reported by the scientists for their application in remediation of heavy metals as well as other toxic compounds. Different fungal species including Aspergillus niger, Aspergillus oryzae, Phanerochaete chrysogenum, Penicillium spinulosum, Saccharomyces cerevisiae, Rhizopus nigricans, Rhizopus arrhizus and Mucor rouxii have been found to absorb different metal ions such as Cu, Cd, Cr, Ni, Zn, Pb, U and Hg in their biomass from contaminated sites.

Bacterial species include Bacillus subtilis, Bacillus cereus, Escherichia coli, Pseudomonas putida, Acinetobacter calcoaceticus, Rhodococcus sp., Stenotrophomonas maltophilia, Rhodococcus rubber, Methylosinus trichosporium etc. which are found effective in remediation of Cr, Cd, Fe, Ni, Zn, Cu as well as other toxic compounds including hydrocarbons, phenolic compounds, polyethylene, synthetic dyes etc. There are also reports on different species of actinomycetes, mycorrhizae and algae for their application in bioremediation process.

The microorganisms which are used in bioremediation process, are generally collected from contaminated sites, because prolonged exposure of microbes to elevated concentration of pollutants, developed resistance in them. These resistant microbial species can be effectively used in bioremediation process. However, bioremediation technologies have been successfully employed in the field and are gaining more importance with increased acceptance of eco-friendly remediation solutions. This process can be considered as a replacement of an existing pollutants remediation technology or can also be used as a polishing unit for an existing treatment facility.