Modification by chemical agents affected the sorption capacities of sludge from municipal wastewater treatment plants (WWTPs). We tried to characterize changes in sorption capacity of dried anaerobic sludge (DANS) after chemical modification by formaldehyde, methanol, and hydrochloric acid. Modified sludges were characterized by FT-IR spectroscopy, cation exchange capacity, and electrothermal atomic absorption spectrometry (ETAAS) analysis. Maximum sorption capacities Q_max at pH 6.0 for Co²⁺ ions calculated from Langmuir isotherm model were 175±5 μmol/g for unmodified DANS, 170±2 μmol/g for CH₂O-modified DANS, 12±0.2 μmol/g for CH₃OH-modified DANS, and 15±0.3 μmol/g for HCl-modified DANS. Our paper established the crucial role of carboxyl functional groups against amino and hydroxyl groups in sorption process of cobalt ions.