BACKGROUNDThe aim of this study was to investigate the feasibility of utilizing a continuous-flow anaerobic granular sludge bed (UASB) reactor for the removal of palladium (II) from aqueous streams and the recovery of this precious metal as biogenic Pd(0). Anaerobic methanogenic granular sludge has been recently shown to reduce Pd(II) in the presence of a suitable electron donor leading to the formation of biomass-associated Pd(0). RESULTSTwo laboratory-scale UASB reactors were fed with ethanol or acetate, in order to compare their performance in terms of Pd removal and reduction. Pd(II) was added in two periods of increasing concentration, 5mg Pd(II) L-1 and then 15mg Pd(II) L-1. Extensive removal of Pd (up to 99%), was attained in both reactors. X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDS) analysis confirmed significant differences in the content of Pd(0) in the biomass, depending on the electron donor used. The main mechanisms of Pd(II) removal when acetate and ethanol were used as electron donor were biosorption and reduction to Pd(0) by biogenic H-2 released as fermentation product, respectively. CONCLUSIONSThe results obtained demonstrate that it is possible to apply a continuous process for Pd reduction and recovery using a methanogenic consortium. (c) 2015 Society of Chemical Industry