Interfacial Adsorption Interactions of Dyes and Chitosan/Activated Carbon@Curcumin Derivatives in Single-Component and Binary Solutions

The remediation of wastewaters contaminated with dyes (discharged mainly from industry) is very important for preserving environmental quality and human health. In this study, a new composite chitosan (CS)-based adsorbent combined with activated carbon (AC) and curcumin (Cur) (abbreviated hereafter as CS/AC@Cur) in three different ratios (12.5%, 25%, and 50%) was synthesized for the removal of anionic [reactive black 5 (RB5)] and cationic [methylene blue (MB)] dyes in single-component or binary systems. The synthesized materials were completely characterized through Fourier transform infrared spectroscopy, scanning electron microscopy, Brunauer-Emmett-Teller analysis, and X-ray diffraction. Specifically, a decrease in the surface area of CS/AC@ was observed with the addition of curcumin, from 163 to 18 m²/g, for all CS/AC@Cur derivatives. In terms of the adsorption results, the optimal derivative for the removal of both RB5 and MB was found to be CS/AC@Cur50%, providing 93% removal at pH 2.0 ± 0.1 for RB5 and 54% removal at the optimum pH of 9.0 ± 0.1 due to electrostatic attractions. The Elovich and pseudo-second-order kinetic model, with a correlation coefficient R² of >0.98, better tailored the results, indicating that adsorption was controlled by chemisorption. In addition, the Sips (Langmuir-Freundlich) isotherm model fitted better to the results, with calculated capacities of 338 and 307 mg/g for RB5 and MB, respectively. The thermodynamic analysis showed a spontaneous and endothermic procedure, with chemisorption as the main mechanism. Reuse experiments showed that the removal efficiency was retained at high levels, while stability studies revealed that the adsorbent retains its structural integrity, even at extreme pH values. Finally, the adsorption of RB5 and MB in a mixed solution was investigated, providing a competitive effect between the anionic and cationic dyes.