Lignin-based hydrogels for efficient dye removal via synergistic effect of multiple interactions

Dye removal from wastewater via adsorption constitutes an active area of research in effluent treatment. For this purpose, superadsorbent hydrogels have emerged as a promising tool; however, large adsorption capacities for various dyes are still required. As a novel adsorbent, lignin is attracting attention owing to its abundant sources and environmental friendliness; unfortunately, its hydrophobicity limits its practical application. Herein, lignin was directly esterified using methacrylate anhydride, itaconic anhydride, or maleic anhydride to produce three lignin esters (LEs) with different hydrophilic properties. These LEs were subsequently used to copolymerize with acrylic acid, yielding three types of hydrogels. Because of high water-absorbing capacity of hydrogels, the highest adsorption capacity reported to date for methylene blue was obtained (2445 mg g−1) using lignin-based adsorbents. The type of LEs, the reactant concentrations and ratios, and the pH of the soaking solution affected the properties and hence the adsorption capacities of the hydrogels. The adsorption process was fitted with a pseudo-second-order model and the Langmuir adsorption isotherm model. This study reveals the synergistic adsorptive action of lignin and confirms the potential of LE–acrylic acid hydrogels for removing methylene blue from sewage.