Aminated Phenolated Lignin for Effective Anionic Dye Removal for Water Remediation

Abstract

Lignin, a renewable biopolymer sourced from plant cell walls, is gaining attention due to its extensive availability from natural resources, native functional groups, low cost, and biodegradability in various applications. In recent years, lignin and its derivatives have been utilized as adsorbents, flocculants, and sterilants in a broad range of applications, including wastewater treatment and sustainable packaging. The growing global demand for clean water—driven by rapid industrialization, urban expansion, and agricultural intensification—has made effective wastewater treatment a pressing environmental priority. In this effort, a dual-functionalization strategy to transform raw lignin into a high-performance adsorbent for the removal of hazardous anionic dyes from wastewater was attempted. Through sequential phenolation and amination via a Mannich reaction—enhancing phenolic hydroxyl groups and introducing nitrogen-rich amine functionalities, respectively—aminated phenolated lignin (Am-PL) was synthesized with nitrogen contents up to 9.6 at%. After each modification, chemical, thermal, and morphological properties of lignin were analyzed. Adsorption capacity and kinetics of Am-PL were investigated for two anionic dyes, Congo red (CR) and methyl orange (MO), as a function of pH and contact time. Am-PL exhibited strong affinity toward CR and MO, achieving maximum adsorption capacities of ca. 53 mg.g^− 1 and 18 mg.g^− 1, with removal efficiencies of 96% and 81%, respectively, under alkaline conditions after 96 h. Am-PL followed pseudo-second-order adsorption kinetics for both aqueous dyes examined. This study demonstrates a green and scalable route to valorize lignin into a next-generation bio-adsorbent, offering a promising solution for sustainable wastewater remediation.