The immobilization of lead ions by a vermicompost with calcite added was evaluated by adsorption isotherms and the results were explained on basis of the pH dependent surface charge and by IR spectroscopy. The results showed maximum adsorption values between 113.6 mg g(-1) (33 degrees C) and 123.5 mg g(-1) (50 degrees C). The point of zero net charge (PZC) was 7.5 +/- 0.1, indicating the presence of a positive surface charge at the pH of batch experiments. The differences in the IR spectra at pH 3.8 and 7.0 in the region from 1800 to 1300 cm(-1), were interpreted on the basis of the carboxyl acid ionization, that reduced the band intensity around 1725 cm(-1), producing signals at 1550 cm(-1) and 1390 cm(-1) of carboxylate groups. Similar changes were detected at pH 3.8 when Pb(2+) was present suggesting that the ion complexation takes place by a cationic exchange equilibrium, between the protons and Pb(2+) ions. (C) 2008 Elsevier Ltd. All rights reserved.