Lignin is a green, renewable macromolecular polymer and the most abundant aromatic biopolymer in nature. However, most of the lignin were direct used a fuel for power generation, which could be regarded as a waste of organic matter and lead to environmental pollutions. Therefore, utilizing lignin or lignin-derived products as high-end materials to unlock its full potential is essential. Lignin amination, which could graft more adsorption sites onto lignin, was regarded as an effective method to improve adsorption capacity. Therefore, in this work, lignin was aminated and used for the adsorption of silver ions and dyes in aqueous solution.
Firstly, lignin was aminated in a green and facile method. Chemical characterization indicated the amino groups were grafted onto lignin macromolecules successfully. The optimized aminated lignin (AL) showed 4.37 % of nitrogen content. Furthermore, the optimized AL’s adsorption of Ag(Ⅰ) ion in aqueous solution was explored. Adsorption experiments showed that Ag(Ⅰ) ion could be completely removed at original pH value. The saturated adsorption capacity of silver ions onto the AL was up to 20.75 mg g-1. The adsorption isotherms and kinetics were well described by the Langmuir and pseudo-second-order equations. Ag(Ⅰ) could be adsorbed and reduced to metallic Ag(0) by AL, thereby forming AgNPs on the surface of AL. Importantly, the obtained Ag@AL complex, was proven to be an effective antibacterial agent against gram-positive (Bacillus cereus, Staphylococcus aureus) and gram-negative (Salmonella enterica) bacteria.
Secondly, the optimized AL was further used as adsorbent for dye adsorption. 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 applied for the adsorption of cationic dye (crystal violet, CV) and anionic dye (Congo red, CR) from aqueous solution. The maximum CV and CR adsorption of AL at 25 ℃ was 97.09 and 129.87 mg·g-1, respectively. Five factors affecting the adsorption including adsorption time, pH of dyes, initial dye concentration, AL dosage and adsorption temperature were discussed. The fitting results of two dyes revealed that the adsorption process followed pseudo-second-order kinetics, and the Langmuir isotherm equation. This study reveals that AL can be used as low-cost adsorbent for both cationic and anionic dyes from wastewater.