Biosorption of nickel by two bacterial species: Bacillus subtilis 117S and Pseudomonas cepacia 120S was studied. The maximum uptake of nickel was achieved at 234.4 μg Ni2 +  ml−1 by P. cepacia 120S (living and dead biomass) and at 117.2 and 351.6 μg Ni2 +  ml−1 by living and dead biomass of B. subtilis 117S. The increase in biomass concentration has shown an increase in the nickel uptake. The nickel removal increased significantly during contact time from 1 to 8 h then remained constant until 24 h where the equilibrium occurred. Biosorption efficiency of nickel increased with increasing pH from 2 to 7 for living and dead biomass of P. cepacia 120S and B. subtilis 117S. Temperature had an important role in nickel biosorption by both species. The nickel removal by living biomass was significantly disturbed after pretreatment of bacterial biomass with sodium azide, mercuric chloride and formaldehyde. Esterification of carboxyl groups, methylation of amino groups and extraction of lipid fraction of biomass by acetone and benzene significantly reduced the biosorption capacity of nickel. Repeated biosorption and desorption operations exhibited that the biosorption capacity of bacterial biomass regenerated with HNO3 and NaOH as desorbing medium increased significantly in cycle 4 for P. cepacia 120S and B. subtilis 117S. In case of regeneration with HNO3 and distilled water the biosorption capacity increased significantly in cycle 4 for B. subtilis 117S and did not differ significantly from cycle 1 to cycle 4 for P. cepacia 120S. The biosorption capacity of living and dead biomass of B. subtilis 117S and dead biomass of P cepacia 120S (155.5 as compared to 175.6 and 169.8 mg Ni2 +  g−1) was higher than that of sludge, tea and saw dust (148.4, 52.7 and 44.6 mg Ni2 +  g−1).

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