Reusable porous amphoteric lignin for water desalination

With the increasing world population and demand for desalinated water, feasible alternatives are urgently needed to address the global water crisis. In this work, porous amphoteric lignin, i.e., carboxymethylated and aminated lignin (C-CM-AL) was synthesized via grafting carboxymethyl and tertiary amine groups to kraft lignin and crosslinking the modified lignin with poly(ethylene glycol) diglycidyl ether, and the product was used for desalination purposes. The produced C-CM-AL polymer was found to possess the highest adsorption capacity of 1.25, 0.97, 0.71, and 0.62 mmol/g for KCl, NaCl, CaCl2, and MgCl2, respectively. Notably, C-CM-AL settled faster after adsorbing salts in solutions. In addition, by increasing the temperature, the produced polymer can release the adsorbed salts, which makes it a reusable adsorbent. The reusability analysis revealed that the C-CM-AL polymer maintained 70–80% of its adsorption capacity (i.e., cleaning potential) after the 15th round of reusing the polymer. The XPS results revealed that the overall adsorption capacity of the polymer would remain unchanged. In the binary saline systems, C-CM-AL adsorbed more K+ than Na+ in KCl/NaCl, and more Na+ in both NaCl/CaCl2 and NaCl/MgCl2 solutions. Overall, the produced lignin-based polymer can be used as a highly efficient and environmentally friendly amphoteric adsorbent for cleaning saline water.