Designed synthesis of demethylated-hydroxymethylated lignin-based adsorbent for removal of Cr(VI) ions from wastewater

Lignin-based adsorbents for the removal of Cr(VI) ions have attracted intensive attention due to the advantages of renewability, biodegradablity, low cost and environmental friendliness. However, there are still a lot of challenges such as poor adsorption capacity, low lignin content in adsorbents, and harsh preparation conditions. Here, a tandem hydroxymethylation-demethylation method is proposed for preparing an excellent lignin-based Cr(VI) adsorbent (DHKL), which features with high lignin content (>85 wt%) and high hydroxyl content (up to 6.26 mmol/g) under milder conditions. The prepared DHKL exhibits an adsorption capacity reaching up to 1040.9 mg/g and can maintain this capacity even in the presence of other metal ions in the solution. Model analyses indicate that chemisorption occurring in a monolayer is the main process, which is spontaneous and endothermic. Structural changes of DHKL before and after adsorption indicated that Cr(VI) ions are mainly reduced to Cr(III) ions by hydroxyl groups with some of the absorbed Cr ions dispersed into the inner part of DHKL. Based on these results, the detailed possible adsorption mechanism is deduced, providing guidance for designing, producing and utilizing lignin-based adsorbents.