Cobalt(II) bioaccumulation and distribution in Rhodopseudomonas palustris

Bioaccumulation by growing cells is a potential technique for the removal of heavy metals from wastewater. Cobalt bioaccumulation by Rhodopseudomonas palustris via different metabolic pathways was investigated for various pH levels, temperatures, co-cations and initial Co2+ concentrations. The distribution of cobalt and its chemical nature in R. palustris were examined by cell compartmentalization and X-ray diffraction analysis. Our results indicated that biomass and bioaccumulation capacity were superior under aerobic conditions in the dark at pH 6.5 and 30 degrees C. Cobalt removal efficiency and bioaccumulation capacity were, respectively, 84.9% and 287.18 mg/g for cultures with an initial Co2+ concentration of 160 mg/L. A pseudo-second-order kinetic model fits the removal process well ( R-2 = 0.9508-0.9913). Co2+ removal efficiency was not significantly influenced by Cu2+, Zn2+, Cd2+, Mn2+ or Fe2+ but was influenced by Ni2+. Cobalt was distributed between the cell surface, periplasmic space, membrane and cytoplasm. The periplasmic space and membrane accounted for 57.37% and 27.95% of Co2+ uptake, respectively. X-ray diffraction analysis showed that cobalt ions were predominantly deposited in the cell as cobalt phosphate octahydrate. These results demonstrate that R. palustris could be used to remove cobalt from industrial wastewater; it would therefore be beneficial to further study the mechanism of cobalt bioaccumulation in this organism.