Cr6+ bioremediation efficiency of Oscillatoria laete-virens (Crouan & Crouan) Gomont and Oscillatoria trichoides Szafer: kinetics and equilibrium study

Two species of cyanobacteria, Oscillatoria laete-virens (Crouan & Crouan) Gomont and Oscillatoria trichoides Szafer, were isolated from a polluted environment and studied for their Cr6+ removal efficiency from aqueous solutions. The parameters studied included the solution pH, contact time, initial concentration of Cr6+ and culture density. Living biomass is more efficient than dead biomass in Cr6+ removal. Removal by living biomass involves bioreduction and biosorption. Below pH 3.1, bioreduction is favored and biosorption is dominant at higher pH. The highest removal through biosorption for living biomass was achieved between pH 5 and 5.9 and for dead biomass at pH 2. The maximum removal was on the tenth day of exposure for both the species. Cr6+ removal increased from 0.2 to 0.4 g L-1 of culture biomass with a decrease with further increase in biomass. Increased Cr6+ concentration decreases growth of both the species over time. Both species tolerate a concentration as high as 30 mg L-1 Cr6+. There was no evidence of bioreduction in the case of dead biomass. Living biomass of O. laete-virens followed both Langmuir and Freundlich models with maximum sorptive capacity (q (max)) of 21.88 mg g(-1). The results of dead biomass were well fitted only to Langmuir isotherm. O. trichoides living biomass did not follow either of the isotherms, but removed the metal to a maximum extent of 38.7mg g(-1). The removal was better described by Freundlich isotherm in case of dead biomass. The pseudo-first-order model describes the kinetics better than the pseudo-second-order model in the case of living biomass. Participation of carboxylic, carbonyl, and amino groups in Cr6+ removal was confirmed by FTIR analysis. Both species seem to be promising biosorbents for Cr6+.