Guanyl Thiourea/Chitosan@Cellulose Composite for Cadmium and Congo Red Adsorption: Experimental, Theoretical, and Fixed-Bed Column Evaluation

Abstract

In view of serious concerns of increasing toxic heavy metals and synthetic dyes in water, a novel, sustainable adsorbent using guanyl thiourea (GTU)-cross-linked chitosan (CS) integrated with cellulose (CE) (GTU/CS@CE) has been developed, which showed effective removal of cadmium ions (Cd2+) and anionic dye Congo red (CR). The synthesized adsorbent was comprehensively characterized before and after adsorption using XPS, FT-IR, and SEM-EDS, confirming the successful uptake of Cd2+ and the CR dye and providing insights into the adsorption mechanism. Density functional theory calculations were further performed to elucidate the adsorption mechanism at the molecular level. The effects of initial pH, adsorbent dosage, initial Cd2+ and CR dye concentrations, contact time, temperature, and coexisting ions on the adsorption performance for Cd2+ and CR dye were systematically evaluated. Under optimized experimental conditions, GTU/CS@CE exhibited remarkable adsorption efficiencies of 99.10% for Cd2+ (20 mg L–1 Cd2+ solution, pH 7, 1 g L–1 adsorbent dosage at 298 K) and 90.32% for CR dye (100 mg L–1 CR dye concentration, pH 5, 1g L–1 dosage at 318 K). The adsorption kinetics of Cd2+ ions were well described by the pseudo-second-order model, whereas CR dye adsorption followed the pseudo-first-order model. The equilibrium data were best fitted by the Freundlich isotherm model for Cd2+ and CR dye, suggesting multilayer adsorption on a heterogeneous surface. Thermodynamic studies revealed that Cd2+ adsorption was spontaneous and exothermic, whereas CR dye adsorption was spontaneous and endothermic. Furthermore, the synthesized adsorbent exhibited reusability, even after five consecutive regeneration cycles for Cd2+ and the CR dye. Fixed-bed column studies highlighted the potential for continuous wastewater treatment applications. Collectively, these findings establish GTU/CS@CE as a robust, efficient, and sustainable adsorbent for the removal of Cd2+ and CR dye from aqueous media.