Pathway Analysis for Biocrystallisation and Biodeposition of Pd(II) and Pt(II) Metals by Sulfate-Reducing Bacteria

Abstract

Despite limited availability of platinum group metals such as palladium and platinum, there is an increasing demand to use them especially as catalysts for fossil fuel free energy sources, such as electric cars and hydrogen fuel cells. Despite this increase in demand, conventional recovery of these metals from wastewater and solid waste streams is still not practiced, and the release of large amounts of metals tends to upset the delicate balance of biodiversity in sensitive ecosystems. Therefore, with the effect of climate change being apparent research focus has shifted to non-carbon energy systems such as biotechnology. Microbial recovery of platinum group metals is emerging as a clean alternative bioremediation processes as compared to the traditional physical and chemical recovery processes, and Sulfate-Reducing Bacteria have drawn a great deal of attention because they have proven to have excellent metal reaction properties for platinum group metals such as palladium and platinum. However, to effectively reduce palladium and platinum to their elemental form a clear understanding of the following is needed; the particle physics, how the organisms interact with the metals under certain environmental conditions as well as the limitations posed by the metal’s occurrence in chelated states on the adsorption and uptake by living organisms. Therefore, the aim of the study was to investigate the use of Sulfate-Reducing Bacteria and Desulfovibrio desulfuricans DSM642 in the bioreduction, biodeposition and biocrystallisation of palladium and platinum. Sulfate-Reducing Bacteria were isolated from sludge from a wastewater treatment plant in the North west, South Africa, and Desulfovibrio desulfuricans DSM642 was purchased from Deutsche Sammlung von Mikroorganismen und Zellkulturen (DSMZ) in Germany. Batch experiments were conducted at different palladium and platinum concentrations from 356.3 mg/L to 1928 mg/L for palladium and 20 mg/L to 140 mg/L for platinum. The experiments were conducted at an optimum pH of 4, a temperature of 30°C under 120 rpm shaking under in a dark room under oxygen free nitrogen to achieve anaerobic conditions. After cell preparation, cells were harvested and challenged with different concentrations of Pd(NH3)4Cl2 and Platinum Standard solution. Removal of the metals by the cells happened at the expanse of formate as an electron donor for 6 and 7 hours for palladium and platinum respectively. After incubation a maximum of 96 % and 99 % of palladium was removed and a maximum of 59% and 56% of platinum was removed by Sulfate-Reducing Bacteria and Desulfovibrio desulfuricans respectively. TEM analysis revealed black oblique deposits on the cell wall of both treatments, which revealed the biomineralisation processes happened on the cell membrane. Palladium deposits were confirmed by X-Ray Diffraction (XRD) to be elemental palladium nanoparticles with a maximum crystal size of 16.9 nm, confirming bioreduction and statistical analysis of the data proved that both treatments have the potential to bioremediate palladium and platinum contaminated environments.