Waste-derived-lignin based hydrogel as a reusable adsorbent for efficient removal of dyes from aqueous solution

Wastewater contamination by synthetic dyes poses a significant environmental challenge due to their persistence, toxicity, and resistance to degradation. In this study, a lignin based hydrogel was synthesized and appraised as an efficient and sustainable adsorbent for removal of crystal violet (CV) and rhodamine B (RhB) dyes from aqueous solution. Prior to synthesis, lignin was extracted from black liquor, a waste-by-product of pulp and paper processing, and incorporated in a poly(vinyl alcohol) (PVA) gel matrix via a one-pot crosslinking reaction with epichlorohydrin as the crosslinking agent. The resulting lignin/PVA hydrogel was characterized by using Fourier-transform infrared spectroscopy, thermogravimetric analysis, scanning electron microscopy coupled with energy-dispersive spectroscopy, and tensile testing, confirming a porous structure with abundant functional groups. Batch adsorption experiments were conducted by varying the initial dye concentration, contact time, pH, and adsorbent dosage. The lignin/PVA hydrogel showed removal efficiencies of 86 and 69% for CV and RhB dye, respectively, with maximum adsorption capacities of 250.9 mg g^-1 for CV and 19.4 mg g^-1 for RhB. Isotherm analysis justified that adsorption followed the Freundlich isotherm model, suggesting multilayer adsorption on a heterogeneous surface, while kinetic studies revealed a pseudo-second-order nature. Thermodynamic analysis confirmed that the adsorption process is spontaneous and endothermic. The adsorption mechanism is governed by a combination of electrostatic interactions, hydrogen bonding, and π-π interactions. The hydrogel also demonstrated worthy reusability, maintaining performance over five adsorption-desorption cycles. Therefore, the lignin/PVA hydrogel could be a cost-effective, sustainable, and reusable adsorbent for dye removal from wastewater.