Unveiling the nature of lignin’s interaction with molecules: a mechanistic understanding of adsorption of complex organic molecules

The valorization of lignin into advanced materials for water and soil remediation is experiencing a surge in demand. However, it is imperative that material research and manufacturing are sustainable to prevent exacerbating environmental issues. Meeting these requirements necessitates a deeper understanding of the role of lignin’s functional groups in attracting targeted species. This research delves into the interaction mechanisms between lignin and organic molecules, using the adsorption of the cationic dye Methylene Blue (MB+) as a case study. Herein, we aim to quantitatively estimate the contribution of different interaction types in the overall adsorption process. While carbonyl groups were found to have no significant role in attraction, carboxylic groups (–COOH) exhibited significantly lower adsorption compared to hydroxyl groups (–OH). Through alternately blocking aliphatic and phenolic –OH groups, we determined that 61% of the adsorption occurred through H-bonding and 38% via electrostatic interactions. Performance studies of modified lignin, along with spectroscopic methods (XPS, FTIR) confirmed the negligible role of π-π interactions in adsorption. This study offers fundamental insights into the mechanistic aspects of MB adsorption on lignin, laying the groundwork for potential modifications to enhance the performance of lignin-based adsorbents. The findings underscore the importance of hydroxyl groups and provide a roadmap for future studies examining the influence of steric factors and interactions with other organic molecules.