BACKGROUND: Desulfovibrio spp. biofabricate metallic nanoparticles (e.g. Bio-Pd) which catalyse the reduction of Cr(VI) to Cr(III) and dehalogenate polychlorinated biphenyls (PCBs). Desulfovibrio spp. are anaerobic and produce H2S, a potent catalyst poison, whereas Escherichia coli can be pre-grown aerobically to high density, has well defined molecular tools, and also makes catalytically-active Bio-Pd. The first aim was to compare Bio-Pd catalysts made by Desulfovibrio spp. and E. coli using suspended and immobilized catalysts. The second aim was to evaluate the potential for Bio-Pd-mediated dehalogenation of PCBs in used transformer oils, which preclude recovery and re-use. RESULTS: Catalysis via Bio-PdD.desulfuricans and Bio-PdE.coli was compared at a mass loading of Pd:biomass of 1:3 via reduction of Cr(VI) in aqueous solution (immobilized catalyst) and hydrogenolytic release of Cl- from PCBs and used transformer oil (catalyst suspensions). In both cases Bio-PdD.desulfuricans outperformed Bio-PdE.coli by similar to 3.5-fold, attributable to a similar to 3.5-fold difference in their Pd-nanoparticle surface areas determined by magnetic measurements (Bio-PdD.desulfuricans) and by chemisorption analysis (Bio-PdE.coli). Small Pd particles were confirmed on D. desulfuricans and fewer, larger ones on E. coli via electron microscopy. Bio-PdD.desulfuricans-mediated chloride release from used transformer oil (5.6 +/- 0.8 mu g mL-1) was comparable with that observed using several PCB reference materials. CONCLUSIONS: At a loading of 1:3 Pd:biomass Bio-PdD.desulfuricans is 3.5-fold more active than Bio-PdE.coli, attributable to the relative catalyst surface areas reflected in the smaller nanoparticle sizes of the former. This study also shows the potential of Bio-PdD.desulfuricans to remediate used transformer oil. Copyright (c) 2012 Society of Chemical Industry