Bioreduction of aqueous palladium ions by Shewanella algae under atmospheric conditions

Abstract

Biohydrometallurgy utilizing microorganisms capable of reducing precious metal ions to zero-valent precious metals is attracting attention as an environmentally benign process, but challenges in terms of recovery costs have hindered its practical application. In this study, we focused on addressing one of the challenges in biohydrometallurgy through the exploration of anaerobic conditions for bioreduction under the atmosphere. Interestingly, we revealed that Shewanella algae cells possess the capability to catalyze formate oxidation and reduce dissolved oxygen by pre-mixing the cell suspension with low concentrations of cytotoxic formate under atmospheric conditions. These findings indicate that bioreduction conditions can be attained without the need for anaerobic manipulation. Furthermore, we demonstrated that a 50% aqua regia leachate containing Pd(II) can be bioreduced under atmospheric condition when neutralized to pH 7 or higher. Finally, the Pd concentration following a 1-h bioreduction was determined to be 7.2 wt% for wet cells and 33 wt% for dry cells, meaning the remarkable ability of bioreduction to achieve significant Pd concentration.