Abstract The sorption kinetics of ammonium onto three types of zeolite, natural zeolite, natural zeolite covered by biofilm and ammonium-bearing zeolite covered by biofilm, at two particle sizes were studied. The pseudo-first order and pseudo-second order models were fitted to the results by a non-linear method. The batch sorption model, based on a pseudo-second order mechanism, was applied to predict the rate constant of sorption, the equilibrium capacity and the initial sorption rate. Ion exchange between NH 4 + and cations in the zeolite increased with decreasing particle size of the zeolite. Biofilm covered on the zeolite did not affect the ion exchange for the smaller particle size but decreased the ion exchange capacity by 22% for the larger particle size. In addition, bioregeneration should be considered for the recovery of ion exchange capacity of the bio-zeolite made from ammonium-bearing zeolite by the microorganisms in the biofilm, 78.0 and 63.9% regeneration for the smaller and larger particle size of zeolite, respectively
