A thermodynamic investigation on chitosan–divalent cation interactions

The α-form of the biopolymer chitin, with particle sizes in the 80–200 mesh range was deacetylated under hot alkaline conditions to yield chitosan. These biopolymers were characterized by nitrogen elemental analyses, infrared spectroscopy, X-ray diffractometry and carbon-13 NMR. The degree of deacetylation found was 80.5%, which was determined via infrared spectroscopy. Chitosan suspended in water adsorbs the divalent cations cobalt, nickel, copper and zinc, as represented by batch process isotherms, in which the number of moles adsorbed as a function of cation concentration gave a defined isotherm for each cation, indicating saturation of the available nitrogen centers on the organic polymeric chain by cations in an acid–base interaction. The cation–biopolymer interaction was quantitatively followed through calorimetric titration, whose values enabled the calculation of the thermodynamic values. The exothermic enthalpy, the negative Gibbs free energy and the positive entropic values are in agreement with a favorable thermodynamic condition for cation–chitosan amine group interaction. Based on negative enthalpic values, −26.66±0.05, −27.58±0.03, −39.05±0.01 and −27.71±0.04 kJ mol−1, the sequence expresses the Irving–Williams series, CoZn.