EDC/NHS-crosslinked carboxymethyl xylan/chitosan hydrogel as a sustainable adsorbent for the removal of heavy metals

Valorization of sugarcane bagasse xylan, an agricultural byproduct, for sustainable material design. • CXY–CS hydrogel fabricated via EDC/NHS-mediated covalent crosslinking between carboxymethyl xylan and chitosan. • The polysaccharide-based hydrogel effectively adsorbs three heavy metal ions: Cr(VI), Cu(II), and Zn(II). • A unique reduction-coordination mechanism is revealed for Cr(VI) removal, distinguishing it from typical cation adsorption. The development of high-performance adsorbents from agricultural waste is hindered by the limited solubility and the absence of functional groups in native polymers. To address these challenges, this study fabricated a novel hydrogel by crosslinking carboxymethylated xylan (CXY) with chitosan (CS) via an environmentally benign amidation reaction. A series of systematic batch adsorption experiments was performed at low concentrations (0–20 mg L −1 ) to investigate the adsorption kinetics and thermodynamics. A series of high-concentration tests (500 mg L −1 ) was conducted to ascertain the maximum adsorption capacities and cyclic regeneration performance for Cu 2+ , Zn 2+ , and Cr(VI). The underlying mechanism was elucidated through X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations. The adsorption process exhibited conformance with the pseudo-second-order kinetic model and Langmuir isotherm, thereby indicating monolayer chemisorption. The thermodynamic parameters confirmed the spontaneity and exothermic nature of the process. In conditions of high concentration (500 mg L −1 ), the hydrogel exhibited noteworthy maximum adsorption capacities (Q max ) of 129.23, 123.24, and 126.19 mg g −1 for Cu 2+ , Zn 2+ , and Cr(VI), respectively, demonstrating over 85% adsorption efficiency following five regeneration cycles. Mechanistic analysis revealed a synergistic “reduction-coordination” pathway, while the highly toxic Cr(VI) undergoes unique surface reduction to less hazardous Cr(III). The integration of experimental and theoretical approaches in this study provides a systematic clarification of the adsorption performance and the dual-function “reduction-coordination” mechanism of the hydrogel toward multiple heavy metals. The study provides a robust scientific foundation for the development of sustainable, waste-derived materials for the remediation of complex heavy metal pollution. Download: Download high-res image (317KB) Download: Download full-size image