Antibiotics are potential pollutants being responsible for disturbing the wastewater treatment processes and the microbial ecology of surface waters. The potential use of dried Rhizopus arrhizus and activated sludge as a substitute for powdered activated carbon for removal of penicillin G, one of the most widely used antibiotics, from aqueous solution was examined. The biosorption/adsorption of penicillin G on the three sorbents was investigated in a batch system by considering the decomposition of penicillin G as a function of pH, temperature and initial penicillin G concentration. The decomposition of penicillin G was strongly dependent on the aqueous phase pH and temperature and followed a first-order decomposition kinetics. Adsorption studies were performed under mild conditions of pH and temperature in order to minimise decomposition of penicillin G. Maximum sorption was observed at the initial pH value of 6.0 and at 35degreesC, and the equilibrium uptake increased with increasing initial penicillin G concentration up to 1000 mg l(-1) for all penicillin G-sorbent systems. Penicillin G uptake capacity was determined as 330.0 mg g(-1) for activated sludge, 459.0 mg g(-1) for R. arrhizus and 375.0 mg g(-1) for activated carbon at these conditions. The suitability of the Freundlich, Langmuir, Redlich-Peterson and Koble-Corrigan adsorption models to the equilibrium data was also investigated at various temperatures for all three sorbents. For all penicillin G-sorbent systems, the adsorption kinetics followed a pseudo-second-order and a saturation type kinetic model rather than a pseudo-first-order kinetic model at all temperatures studied. The activation energy of adsorption was determined from the Arrhenius equation using saturation type kinetic constants for each sorbate-sorbent system. Using the thermodynamic equilibrium coefficients obtained at different temperatures, the thermodynamic constants of each sorption process (DeltaGdegrees, DeltaHdegrees and DeltaSdegrees) were also evaluated. (C) 2004 Elsevier Ltd. All rights reserved.