Study of chemical, kinetic, and theoretical sorption properties of activated carbons obtained from agroindustrial origin, comparison of anionic and cationic model molecules: Part II—ZnCl₂ activation

This study corresponds to the second part of a research in which activated carbons derived from agroindustrial residues activated with phosphoric acid were used. These activated carbons were in this study, activated with zinc chloride, and were characterized to understand the interactions during the surface adsorption process, for which molecules of opposite ionic origin were used. The results obtained are also compared with the relevant results with Part 1 of this study, in order to understand the effect of the activating agent. Agroindustrial residues of orange peel, coffee grounds, sugarcane bagasse, agave bagasse, cedar sawdust, coconut shell, and citrus residue were used to prepare zinc chloride activated carbons. Activated carbons were studied in elemental composition, surface morphology, functional groups, and affinity predominance, using the proper techniques. Adsorption capacities and kinetics with two molecules of opposite origin were also studied. Finally, the study was completed with a theoretical approach to understand the interaction of molecules with the functional groups of the surface of activated carbons. The results revealed that when compared to H₃PO₄, ZnCl₂-activated carbons have a higher ratio of acidic oxygenated functional groups that are associated with carbonyl, carboxyl, phenolic, and lactone surface groups. The predominant affinity towards anions was recognized. Additionally, the sorption process of two model molecules of opposite ionic origin and at pH 7 increases the sorption since the point of zero charge of materials are close to neutral; the molecules will not be protonated or deprotonated causing favorable interactions with adsorbates. The Density Functional Theory identified the predominant interactions π-π*, hydrogen bonds, and covalent bonds in the adsorption process that increases the sorption capacities. The activated carbons with the highest ionic adsorption capacity were coconut shell, orange peel and agave bagasse with both molecules, around 37 mg/g, due to the chemical and developed surface that suggests an easy access.