Biorecovery of palladium as nanoparticles by Enterococcus faecalis and its catalysis for chromate reduction

Precious metal Palladium (Pd) is a scarce natural resource with increasing industrial demand. The use of microorganisms for biorecovery of Pd is a promising approach because it mimics natural biogeochemical cycles. In this study, we investigated the biorecovery of Pd-0 nanoparticles (Pd-0-NPs) by Enterococcus faecalis (E. faecalis) with sodium formate as electron donor. X-ray diffraction analyses exhibited that the process could successfully reduce Pd2+ and generate Pd-0-NPs. Transmission electron microscope observation showed that the Pd-0-NPs were associated at the membrane and inside of the cell. Under the optimum conditions of temperature (40 degrees C), sodium formate concentration (25 mM), biomass (1.2 g L-1), and 210 mg L-1 Pd2+ (pH 3.0-3.5) were completely reduced within 48 h. The percentage of the Pd-0-NPs with an average size lower than 10 nm increased from 91.5% to 99.6% as the biomass increased from 1.2 to 3.6 g L-1, respectively. The catalytic property of the Pd-0-NPs treated by ultrasonication was higher than chemically prepared Pd-0 It showed that 6.3 mg Pd-0-NPs could catalyze the complete reduction of 5.0 mu mol Cr6+ in 12 h. All of these findings suggest that it is potentially feasible to recover Pd-0-NPs with catalytic activity from industrial wastes using E. faecalis. (C) 2015 Elsevier B.V. All rights reserved.