Covalently Linked Poplar and Ayous Lignin-Based Hydrogels: Sustainable Materials for Water Remediation

Lignin-based materials are attracting increasing attention for their effectiveness in treating polluted water. In this study, special emphasis is placed on hydrogels modified with lignin extracted from Poplar and Ayous (Alpi Spa wood waste) which were developed for the removal of organic dyes, using methylene blue as a model pollutant. The two types of wood differ notably in their characteristics. Poplar is a medium-density hardwood with a relatively low lignin content, higher water repellence, and good ability to be shaped and glued. Ayous is a tropical wood; its lignin has a different chemical makeup, offering greater resistance to microbial degradation. However, Ayous is more prone to moisture absorption and less durable outdoors if untreated. The lignins covalently incorporated into polyvinyl alcohol (PVA)-based hydrogel via direct esterification with polyamide epichlorohydrin (PAE) were investigated. This chemical crosslinking strategy significantly reduced lignin leaching. The resulting hydrogels were thoroughly characterized by ATR-FTIR spectroscopy, thermogravimetric analysis, and scanning electron microscopy to elucidate their structure and morphology. Batch adsorption experiments revealed that hydrogels incorporating lignin from Ayous hardwood waste achieved remarkable methylene blue removal efficiency (up to 88%), a substantial improvement over both hydrogels incorporating commercial lignin (64%) and unmodified hydrogel (9%). By integrating lignin's adsorption capabilities with the porosity and recoverability of the hydrogel matrix, the Ayous lignin-modified hydrogels present a highly promising and sustainable solution for eliminating organic pollutants from wastewater. This innovative approach not only enhances pollutant removal but also promotes environmental sustainability and aligns with circular economy principles.