Under certain conditions bacteria can act as a good biosorbent for different toxic heavy metals. However, no study on this aspect has been reported in case of acidophilic, heterotrophic, Gram-negative Acidiphilium strains, which are mostly resistant to several heavy metals. FTIR, SEM, TEM along with sorption experiments using bacterial cells of Acidiphilium symbioticum H8 were conducted to establish the mechanism of Cd2+ ion sorption. The anionic functional groups present in the cell envelop were the components primarily responsible for the metal-binding capability of the bacterium. Sorption experiments further confirmed that 248.62 mg of cadmium was adsorbed per gram biomass at pH 6.0. The process can better be explained by Langmuir-Freundlich dual isotherm model. Blocking of the functional groups by chemical modification suggested that the binding of cadmium on the biomass occurs through electrostatic reaction and complex formation. Accumulation of cadmium on the cell envelop was supported by fine structure study. (c) 2011 Elsevier Ltd. All rights reserved.