Isolation, characterization and tolerance survey of bacterial strains to cadmium in soils receiving Hamadan industrial parks wastewater treatment plant effluent

Biosorption is a relatively new clean up method used for the removal of heavy metals from the environment. Entry of heavy metals into the environment may result in change in population structure and further alteration of resistance mechanism(s) in exposed microorganisms. The first question in this study sought to determine the threshold of bacterial resistance to cadmium (Cd). The second question was to identify indigenous bacterial species capable of removing the Cd from the soil and consequently to achieve a more efficient biological treatment for industrial effluents. Natural topsoil samples were obtained from three industrial parks in Hamadan, Iran. The concentration of Cd in soil samples was measured and thence the bacterial cultures were prepared.Following the screening process, the minimum inhibitory concentration [MIC] and minimum bactericidal concentration [MBC] were determined in order to find the threshold of bacterial tolerance. Indeed, the adsorption capacity of bacterial strains came down in favor of the identification of resistant bacteria. Respectively, 42, 4 and 4 bacterial strains were identified for cultures having average cadmium concentrations of 0.78, 500 and 750 mg/L. This experiment did also detect that Tcd2 and Tcd4 showed the highest MIC with 1250 mg Cd/L. Moreover, TCd2, belonging to the family of Enterobacteriaceae and Enterobacter genus, demonstrated the maximum cadmium removal efficiency (30.2%). The test was successful as it was able to identify the indigenous cadmium tolerant bacteria with high adsorption capacity. This finding has important implications for developing special biological wastewater treatment such as a biofilter in the cadmium-releasing industries.