In order to remove and recover copper, zinc, cadmium, and chromium from the wastewater treatment sludge generated by an electroplating process, the heavy metal extraction kinetics was studied in a batch reactor using two different extraction agents (nitric and citric acid) at constant agitation speed (150 rpm) and solid to liquid ratio (10 g/L), but varying acid concentrations (0.02-0.10N), temperatures (25-85 degrees C in nitric acid solution, 25-95 degrees C in citric acid solution), and sludge particle sizes. The shrinking-core model and empirical kinetic model were adopted to analyze the experimental data. Although both models could fit the experimental kinetic data well, the obtained parameters of the shrinking-core model did not show reasonable trends varying with the experimental variables while the empirical model parameters showed significant trends. The experimental and modeling results showed that the metal extraction rates increased with acid concentration, temperature, but decreased with increasing particle size. Nitric acid was found to be more effective than citric acid to extract the heavy metals from the sludge. The extraction activation energies obtained in this study suggested that both a physical diffusion process and a chemical reaction process might play important roles in the overall extraction process. (c) 2005 Elsevier B.V. All rights reserved.