Chitosan–cyanuric chloride intermediary as a source to incorporate molecules—Thermodynamic data of copper/biopolymer interactions

The reaction of a chitosan–cyanuric chloride (ChC) intermediate with ethylenediamine (d) and diethylenetriamine (t) molecules yielded the new biopolymers ChCd and ChCt, which were characterized by elemental analysis, thermogravimetry, X-ray diffractometry, scanning electron microscopy and infrared and C13 nuclear magnetic resonance spectroscopies. The precursor chitosan (Ch) and all derivatives adsorb copper from aqueous solution at 298 ± 1 K, determined using a batchwise procedure. The results were fitted to a modified Langmuir equation. The ability to adsorb copper is dependent on the availability of the basic nitrogen atoms attached to the pendant biopolymer chains in the order ChCt > ChCd > ChC > Ch, as given by the values 2.84 ± 0.03, 2.62 ± 0.05, 2.55 ± 0.04 and 2.09 ± 0.03 mol g−1, respectively. The same interactive process was also followed through calorimetric titration at 298.15 ± 0.20 K. The net thermal effects were also adjusted to a modified Langmuir equation to give the thermodynamic data at the solid/liquid interface. The exothermic enthalpic values were −28.98 ± 0.05, −32.77 ± 0.04, −60.60 ± 0.03 and −56.41 ± 0.05 kJ mol−1 for the biopolymers Ch, ChC, ChCd and ChCt, respectively. The spontaneity of the systems is shown by the negative ΔG values, −21.1 ± 0.1, −22.1 ± 0.1, −22.1 ± 0.1 and 23.4 ± 0.1 kJ mol−1 for the same sequence. The negative entropic values −26 ± 1, −36 ± 1, −129 ± 1 and −111 ± 1 J mol−1 K−1 indicate an ordering of solvent as complexation occurred. The thermodynamic data demonstrate the capability of these biopolymers for cation removal from aqueous solutions, being new derivative biomaterials that may act as useful agents to renew an ecosystem.