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 Pd 2+ 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   °C), sodium formate concentration (25   mM), biomass (1.2   g   L −1 ), and 210   mg   L −1 Pd 2+ (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   μmol Cr 6+ 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 .