Utilization of aminated lignin as an adsorbent to remove cationic and anionic dyes from aqueous solutions

Lignin is a type of biomass that is generally found in nature and has a wide range of applications as a functional material. A novel sorbent, aminated lignin (AL), was synthesized in a one-step method to improve its adsorption capabilities. The AL was characterized through Brunauer–Emmett–Teller surface area analysis, thermogravimetric analysis, scanning electron microscopy, and zeta potential analysis. AL was small in size and had a clear obvious porous structure with a surface area of 47.1 m2·g−1, nearly ten times larger than that of kraft lignin, which was 4.75 m2·g−1. AL was utilized for the adsorption of cationic dyes (crystal violet, CV) and anionic dyes (Congo red, CR) from aqueous solutions. The maximum CV and CR adsorption of AL at 25 °C were 97.09 and 129.87 mg·g−1, respectively. Five factors affecting adsorption including adsorption time, pH of dyes, initial dye concentration, AL dosage, and adsorption temperature were investigated. The fitting results of the two dyes revealed that the adsorption process followed pseudo-second-order kinetics and the Langmuir isotherm equation. This study demonstrates that AL can be used as a low-cost adsorbent for both cationic and anionic dyes from wastewater.