Chitosan (Ch) was chemically modified with ethylenesulfide (Es) under solvent-free conditions to give (ChEs), displaying a high content of thiol groups due to opening of the three member cyclic reagent. Elemental analysis showed a decrease in nitrogen content. This result indicated the incorporation of two ethylenesulfide molecules for each unit of the polymeric structure of the precursor biopolymer. Infrared spectroscopy, thermogravimetry, and (13)C NMR in the solid state demonstrated the effectiveness of the reaction, with signals at 30 ppm for ChEs due to the change in the methylene group environment. Divalent metal uptake by chemically modified biopolymer gave the order Cu>Ni>Co>Zn, reflecting the corresponding acidity of these cations in bonding to the sulfur and the basic nitrogen atoms available on the pendant chains. The equilibrium data were fitted to Freundlich, Temkin, and Langmuir models. The maximum monolayer adsorption capacity for the cations was found to be 1.54+/-0.02, 1.25+/-0.03, 1.13+/-0.01, and 0.83+/-0.03 mmol g(-1), respectively. The Langmuir model best explained the cation-sulfur bond interactions at the solid-liquid interface. The thermodynamics for these interactions gave exothermic enthalpic values of -43.02+/-0.03, -28.72+/-0.02, -26.27+/-0.04, and -17.32+/-0.02 kJ mol(-1), respectively. The spontaneity of the systems is given by negative Gibbs free energies of -31.2+/-0.1, -32.7+/-0.1, -31.7+/-0.1, and -32.2+/-0.1 kJ mol(-1), respectively, in spite of the unfavorable negative entropic values of -39+/-1, -13+/-1, -18+/-1, and -49+/-1 J K(-1)mol(-1) due to solvent ordering in the course of complexation. This newly synthesized biopolymer is presented as a chemically useful material for cation removal from aqueous solution.