Eco-friendly synthesis of biosorbent based in chitosan-activated carbon/zinc oxide nanoparticle beads for efficiency reduction of cadmium ions in wastewater

This study aims to synthesize a new bio-adsorbent through the dual valorization of agricultural and marine waste (date pits and shrimp shells) for the purification of cadmium-contaminated water. The bio-composite is synthesized by combining the properties of chitosan with activated carbon and zinc oxide nanoparticles (CS-AC/ZnO). The optimum ratio between AC/ZnO and CS is 1/2 (w/w) to achieve maximum removal. Successful synthesis was confirmed by the analysis of the composite using FTIR, XRD, SEM, BET, and EDX techniques. The specific surface area of the CS-AC/ZnO was 20.54 m² g⁻¹, with a removal efficiency of 82%, which was higher than other adsorbents tested in this study. Batch adsorption studies were carried out to investigate operational factors as well as the kinetics, isotherms, and thermodynamics of the adsorption mechanism. The adsorption of Cd²⁺ was spontaneous and exothermic. Experiments demonstrate the preferential elimination of Cd²⁺ in the presence of interfering ions such as Na⁺, K⁺, and Mg²⁺. Non-linear modeling was used to analyze the adsorption isotherm and kinetics. The Freundlich and Redlich-Peterson isotherms, together with the pseudo-second-order kinetic model, were the best suited to the experimental data. The highest adsorption capacity determined by Langmuir was found to be 18.63 mg g⁻¹. Remarkably, the adsorbent maintained high performance over 5 regeneration cycles, with a slight reduction in efficiency from 83.76 to 80.18%.