Full-Lignin-Based adsorbent for removal of Cr(VI) from waste water

• The lignin content of the adsorbent is more than 98%. • The adsorption capacity of Cr(VI) ions is up to 703.6 mg/g at 30°C. • The removal efficiency of the adsorbent for low concentration Cr(VI) ions is up to 99.9%. • The reduction of Cr(VI) to Cr(III) by phenolic hydroxyl drives the adsorption. The development of full-lignin-based adsorbents with high adsorption capacity for the removal of Cr(VI) ions from wastewater has intensively attracted attention. However, it is still a challenge up to now due to the low reactivity and complexity of lignin structure. Herein, a high hydroxyl content demethylated alkaline lignin (DAL) was prepared using AlCl 3 as the modification reagent. With this DAL as the full-lignin-based adsorbent, the adsorption capacity for Cr(VI) was up to 703.6 mg/g at 30°C. Moreover, when the concentration of Cr(VI) ions was below 100 mg/L, the removal efficiency was high up to 99.9% by only using 10 mg adsorbent in 50 mL solution. On the basis of the isotherm, kinetic, and thermodynamic model analyses, it could be concluded that the chemisorption occurred in monolayer on the surface of the adsorbent, and the adsorption process was endothermic and spontaneous. The characterizations of the spent adsorbents showed that the adsorption might be driven by the reduction of negatively charged Cr 2 O 7 2- or HCrO 4 - ions to positively charged Cr 3+ , which could generate strong electrostatic interaction with the oxygen containing groups in lignin. Simultaneously, the toxicity of chromium ions was significantly decreased during the reduction. This work provides a new way to realize high adsorption capacity of Cr(VI) ions by a full-lignin-based adsorbent, and accelerates the high-value utilization of lignin.