Experimental and DFT Study of the Monocomponent and Binary Adsorption of Heavy Metals in Chitosan Hydrogels at Different Temperatures.

High levels of global water pollution have driven the development of effective strategies for removing priority contaminants; this is a worldwide issue that calls for innovation in traditional methodologies using affordable materials with high removal capacities. Experimental and DFT studies were combined to elucidate the adsorption, selectivity, and competitive mechanisms of Hg2+, Ni2+, and Cu2+ onto a chitosan-based hydrogel, linking experimental behavior with molecular-level interactions. FTIR and XRD analysis allowed the determination of the hydrogel composition, the verification of the cross-linking of the polymer with the cross-linking agent, and the detection of heavy metals analyzed on the adsorbent. After the characterization, the isotherms were experimentally obtained in single and binary systems at 298.15, 303.15, and 313.15 K and pH 4. In binary experiments, antagonistic adsorption occurred due to competitive binding of heavy metals to the hydrogel's active sites. The highest adsorption capacities achieved from individual solutions were 0.723, 0.187, and 0.420 mmol/g at 313.15 K for Hg2+, Ni2+, and Cu2+, respectively, and those for binary solutions were 0.633, 0.080, and 0.238 mmol/g at 313.15 K for Hg2+, Ni2+, and Cu2+, respectively. Adsorption kinetics in both single- and binary systems were well fitted by a pseudo-second-order model, consistent with a chemisorption-controlled mechanism. The adsorption process was endothermic and spontaneous, and it was determined that the adsorption was attributed to primary amine and hydroxyl groups available on the surfaces of the cross-linked hydrogels. DFT calculations revealed preferential interactions of Hg2+, Ni2+, and Cu2+ with specific hydrogel functional groups, supporting the experimental adsorption selectivity.