Synthesis of lignocellulose-based composite hydrogel as a novel biosorbent for Cu²⁺ removal

A lignocellulose-based composite hydrogel, as a novel biosorbent, was prepared for Cu²⁺ removal from wastewater. TEMPO-oxidized cellulose nanofibrils (TOCN) were dispersed in a 7 wt% NaOH/12 wt% urea aqueous solution at room temperature. Meanwhile, the dissolved cellulose was obtained in the same system at subzero temperature. The composite hydrogels were prepared by blending the dissolved cellulose solution, TOCN dispersion, and alkali lignin solution in an NaOH/urea aqueous solution. The composite hydrogel exhibits excellent adsorption capacity for heavy metals, which can be attributed to the synergistic effects of physical adsorption (porous 3D structure) and chemical adsorption (active sites: carboxyl and phenolic groups). The maximum amount of adsorbed Cu²⁺ onto composite hydrogel can reach 541 mg/g, which was achieved after 45 min. The adsorption behavior is well-described by the pseudo-second-order kinetics and the Freundlich model (R² > 0.999). Furthermore, the composite hydrogel exhibits high-strength properties, indicating that the presence of TOCN and lignin contributes to mechanical improvements.