BACKGROUNDIn this work, the ability of biosorption and bioaccumulation of Zn (II) ions by Klebsiella sp. 3S1 has been investigated. This bacterium had already demonstrated its ability to remove Pb and Ag ions by different mechanisms. The mechanisms involved in the process have been studied using four techniques: FTIR, SEM-EDX, TEM-EDX and confocal microscopy. Kinetic and equilibrium studies have also been performed. RESULTSThe main operating variables were optimized by a Rotatable Central Composite Design (RCCD) that yielded the following values: 0.36 g L-1 biomass, pH 5.20 and 34 degrees C temperature, although the equilibrium data were obtained at a temperature of 25 degrees C, more in line with the environmental conditions. The model that best fitted the experimental kinetic data turned out to be the pseudo-first-order one. Maximum capacity of biosorption under the Langmuir model was 48.4 mg g(-1). Finally, the presence of two general mechanisms in the biosorption process was confirmed: cytoplasmic bioaccumulation and complex surface adsorption. CONCLUSIONSKlebsiella sp. 3S1 can be an effective and inexpensive microorganism to remove heavy metals from contaminated effluents, which along with its ubiquity and great ability to form biofilms, make it a promising biosorbent for use in biofilters. (c) 2017 Society of Chemical Industry