Preparation of a Lignin-Based Adsorbent from Xylitol Residue for the Capture of Metal Ion in Aqueous Solutions

Lignin with rich functional groups and high mechanical strength has been employed for the capture of different pollutants under complex environmental conditions. The capture performance can be further improved by chemical modifications such as grafting, cross-linking, and copolymerization. Herein, high-purity lignin was first extracted from xylitol residue using a strategy of ionic liquid extraction followed by acid hydrolysis. Then the prepared macrocyclic compounds, such as crown ethers and columnar aromatics, were grafted onto lignin via Mitsunobu reactions, obtaining Lig-CE12, Lig-CE14, and Lig-PL[5]. These lignin-macrocycle complexes were applied for the selective capture of metal ions. Lig-CE12 and Lig-CE14 showed higher selectivity for the capture of sodium and lithium ions, respectively, and Lig-PL[5] had better capture ability for Co²⁺ and Mn²⁺ in aqueous solutions. Compared with lignin and macrocycles alone, the combination of lignin and macrocycles resulted in a greatly improved capture performance for metal ions. These adsorbents derived from lignin could bring more environmental benefits due to its stability, biocompatibility, and ease of separation and recovery.