Zinc biosorption by the purple non-sulfur bacterium Rhodobacter capsulatus

This paper presents the first report providing information on the zinc (Zn) biosorption potentialities of the purple non-sulfur bacterium Rhodobacter capsulatus. The effects of various biological, physical, and chemical parameters on Zn biosorption were studied in both the wild-type strain B10 and a strain, RC220, lacking the endogenous plasmid. At an initial Zn concentration of 10 mg.L-1, the Zn biosorption capacity at pH 7 for bacterial biomass grown in synthetic medium containing lactate as carbon source was 17 and 16 mg Zn.(g dry mass)(-1) for strains B10 and RC220, respectively. Equilibrium was achieved in a contact time of 30-120 min, depending on the initial Zn concentration. Zn sorption by live biomass was modelled, at equilibrium, according to the Redlich-Peterson and Langmuir isotherms, in the range of 1-600 mg Zn.L-1. The wild-type strain showed a maximal Zn uptake capacity (Q(m)) of 164 +/- 8 mg.(g dry mass)(-1) and an equilibrium constant (K-ads) of 0.017 +/- 0.00085 L.(mg Zn)(-1), compared with values of 73.9 mg.(g dry mass)(-1) and 0.361 L.mg(-1) for the strain lacking the endogenous plasmid. The Q(m) value observed for R. capsulatus B10 is one of the highest reported in the literature, suggesting that this strain may be useful for Zn bioremediation. The lower Q(m) value and higher equilibrium constant observed for strain RC220 suggest that the endogenous plasmid confers an enhanced biosorption capacity in this bacterium, although no genetic determinants for Zn resistance appear to be located on the plasmid, and possible explanations for this are discussed.