Facile preparation of cross-linked lignin for efficient adsorption of dyes and heavy metal ions

In this article, we report a simple and environmentally friendly method to produce cross-linked lignin as efficient adsorbents for organic dyes and heavy metal ions. The cross-linked lignin is simply prepared by freeze-drying of lignin followed by low-temperature annealing. It is found that freeze-drying can provide increased surface areas while abundant functional groups of lignin can be retained at lower annealing temperatures like 300 °C, which give the annealed freeze-dried lignin high adsorption capacities for model pollutants, rhodamine B and Cu²⁺ ions, which reach 156.4 mg/g and 290 mg/g, respectively. Freeze-drying and low-temperature annealing also leads to much higher product yields than carbonization at high temperatures, greatly improving the quantity of adsorbents produced. Furthermore, the morphology of the freeze-dried lignin facilitates the evaporation of small molecules produced in cross-linking reactions, promoting the formation of cross-linking lignin to avoid secondary pollution caused by leaching of contaminated molecules. Thus, this is a promising approach to sustainable low-cost adsorbents for water treatment.