Palladium bionanoparticles production from acidic Pd(II) solutions and spent catalyst leachate using acidophilic Fe(III)-reducing bacteria

The acidophilic, Fe(III)-reducing heterotrophic bacteria Acidocella aromatica PFBC^T and Acidiphilium cryptum SJH were utilized to produce palladium (Pd) bionanoparticles via a simple 1-step microbiological reaction. Monosaccharide (or intracellular NADH)-dependent reactions lead to visualization of intra/extra-cellular enzymatic Pd(0) nucleation. Formic acid-dependent reactions proceeded via the first slow Pd(0) nucleation phase and the following autocatalytic Pd(II) reduction phase regardless of the presence or viability of the cells. However, use of active cells (with full enzymatic and membrane protein activities) at low formic acid concentration (5 mM) was critical to allow sufficient time for Pd(II) biosorption and the following enzymatic Pd(0) nucleation, which consequently enabled production of fine, dense and well-dispersed Pd(0) bionanoparticles. Differences of the resultant Pd(0) nanoparticles in size, density and localization between the two bacteria under each condition tested suggested different activity and location of enzymes and membrane “Pd(II) trafficking” proteins responsible for Pd(0) nucleation. Despite the inhibitory effect of leaching lixiviant and dissolved metal ions, Pd(0) bionanoparticles were effectively formed by active Ac. aromatica cells from both acidic synthetic Pd(II) solutions and from the actual spent catalyst leachates at equivalent 18–19 nm median size with comparable catalytic activity.