A chitosan-lignin biocomposite adsorbent for RO16 dye and Cr(VI) heavy metal removal from aqueous solutions: new interpretations via experiments and statistical physics analysis

A biocomposite composed of chitosan and lignin was synthesized for the removal of dyes and metals from aqueous solutions. The structural and surface properties of the adsorbent were characterized using FT-IR spectroscopy, SEM micrograph, X-ray diffraction, nitrogen adsorption-desorption isotherms, BJH pore size distribution, and zeta potential evolution. This study also presented a physicochemical investigation of the adsorption mechanism of reactive orange 16 (RO16) dye and hexavalent chromium (Cr(VI)) ions on chitosan-lignin biocomposite, using both experimental adsorption data and theoretical modeling based on statistical physics theory to elucidate the underlying interactions. An advanced statistical physics adsorption model, namely heterogeneous monolayer model with two functional groups (HMLM2FG), was employed to simulate the adsorption behavior, indicating that RO16 and Cr(VI) interacted with two distinct functional groups on the chitosan + 50% lignin surface. This model enabled detailed stereographic and energetic studies of the tested adsorption systems. Hence, stereographic analysis revealed that the studied adsorbent functional groups preferentially capture the attached species with n > 1 at specific temperatures, suggesting a multi-ionic mechanism with significant aggregation. The total maximum adsorbed quantities of chitosan-lignin adsorbent, determined by the proposed model HMLM2FG, were found to be 59.43-79.76 mg/g for RO16 and 52.06-72.61 mg/g for Cr(VI). Chitosan + 50% lignin demonstrated then greater efficiency in removing RO16 and Cr(VI) from aqueous solutions, showing an exothermic adsorption process characterized by adsorption energies ranging from 4.88 to 16.97 kJ/mol. These energy values were consistent with physisorption mechanisms. Overall, this study combined experimental findings with a theoretical approach to offer a novel microscopic and macroscopic analysis of the adsorption behavior of two major industrial pollutants on chitosan-lignin biocomposite.Clinical trial number: Not applicable.