From trash to treasure: Microbial conversion of palladium contaminants into valuable Pd nanoparticles by Bacillus thuringiensis Y9

Microbial reduction is a sustainable approach for both environmental remediation and recycling of precious metals from waste. Bacteria-mediated reduction of palladium ions (Pd 2+ ) in wastewater under mild conditions is such a strategy. However, external electron donors are usually required for palladium bioreduction. Herein, a novel gram-positive bacterium, Bacillus thuringiensis Y9 ( B. thuringiensis Y9), was found to reduce Pd 2+ to palladium nanoparticles (Pd-NPs) with endogenous electron donors. Transcriptomic analyses revealed 443 and 439 differentially expressed genes response to palladium treatment under anaerobic and aerobic conditions , respectively. It was found the genes encoding NADH-quinone oxidoreductase, dehydrogenases , cytochrome c reductase, cytochrome c oxidase , quinone cycle and ribE in B. thuringiensis Y9 have strong positive relationship to palladium reduction. The removal efficiencies of palladium by B. thuringiensis Y9 are as high as 93 mg g −1 under anaerobic conditions and 60 mg g −1 under aerobic conditions. Hydrogenase also plays vital role in the bioreduction process. The maximum Pd removal ratio of 99.18% was obtained with the primary influencing factors using response surface methodology (RSM). Our study provides a novel and sustainable strategy for Pd recovery from wastewater. The transcriptome data are valuable for understanding the global bioreduction mechanism of the microbial conversion of palladium.