Microbial Pb(II) precipitation : minimum inhibitory concentration and precipitate identity

Abstract

The biohydrometallurgical processing of metals have generated significant interest due to the potential for bioleaching and biological electrowinning processes to replace current energy intensive, costly, and environmentally contaminating smelting and hydrometallurgical processes. An ongoing research project aims to identify, study, and refine a Pb(II) bioremediation and biorecovery process for industrial application. Previous work has successfully demonstrated Pb(II) bioprecipitation of 86.5% of a 1,000 ppm Pb(II) initial concentration batch experiment within 22 days. The objectives of the current study were: (1) to determine the minimum inhibitory concentration (MIC) at which a specific industrial consortium would cease to grow and/or precipitate Pb(II) from solution, (2) to determine the identity of the precipitate formed. The consortium was obtained from a borehole at an automotive battery recycling plant in Gauteng province South Africa. The MIC was studied using inoculated nutrient and simulated (reduced NaCl) agar plates. Pb(II) concentrations from 500 ppm to 200,000 ppm were tested in the nutrient agar, and Pb(II) concentrations of 50,000 ppm to the solubility limit of Pb(NO3)2 of 310,000 ppm Pb were tested in the simulated agar. The results from the MIC study showed that the industrially obtained consortium was able to grow and precipitate Pb(II) at concentrations up to approximately 30,000 ppm. MIC values for the reduced NaCl and no NaCl runs of 34,914±5,995 ppm and 27,164±5,728 ppm, respectively. The results from the nutrient agar showed no evidence of inhibition, likely a result of decreased effective Pb(II) as a result of PbCl2 precipitation. The XPS analysis of the metallic ring on the surface of the nutrient agar indicated the presence of PbS and elemental Pb, in the ratio 0.818:0.182. These results confirm a Pb-reduction (Pb(II) to Pb0) capability is present in the consortium. From the results, it can be concluded that the industrial consortium has the ability to grow and precipitate Pb(II) in significantly high concentrations. In addition, a biological Pb(II) reduction to elemental lead capability was confirmed, potentially providing a replacement for the electrowinning step in traditional hydrometallurgical processing of Pb(II).