The removal of Zn(II) from aqueous solution by different adsorbents was investigated. Clarified sludge (a steel industry waste material), rice husk ash, neem bark and a chemical adsorbent activated alumina were used for the adsorption studies. The influence of pH, contact time, initial metal concentration, adsorbent nature and concentration on the selectivity and sensitivity of the removal process was investigated. The adsorption of Zn(11) increased with increased concentration of the adsorbents and reached maximum uptake at 10 g/L and pH between 5 and 7. The equilibrium time was achieved after 1 h for clarified sludge, 3 h for rice husk ash and 4 h for activated alumina and neem bark, respectively. The adsorption process was found to follow a first-order rate mechanism and rate constant was evaluated at 30 degrees C. The rate constant was highest in case of clarified sludge (6.90 x 10(-2) min(-1)) and the activated alumina gave the lowest value (1.86 x 10(-2) min(-1)). Langmuir and Freundlich adsorption isotherms fit well in the experimental data and their constants were evaluated. The thermodynamic equilibrium constant and the Gibbs free energy were calculated for each system. The adsorption capacity (q(max)) calculated from Langmuir isotherm and the values of Gibbs free energy obtained showed that clarified sludge has the largest capacity and affinity for the removal of Zn(II) compared to the other adsorbents used in the study. (c) 2006 Elsevier B.V. All rights reserved.