Modification of sulfomethylated alkali lignin catalyzed by horseradish peroxidase

Abstract

Alkali lignin (AL) is a by-product from alkali pulping and sulfomethylated alkali lignin (SL) with excellent water solubility was obtained from AL by sulfomethylation. A novel method to improve the sulfonic group content and molecular weight of SL by means of horseradish peroxidase/H₂O₂ catalysis was investigated. The structural changes of SL during HRP modification were characterized by gel permeation chromatography (GPC), UV-Visible spectroscopy, infrared spectroscopy (IR), hydrogen nuclear magnetic resonance spectrometry (¹H-NMR) and headspace gas chromatography (HS-GC). The results showed that M_w and the sulfonation degree of SL by HRP modification increased remarkably by over 20-fold and 40%, respectively. In addition, during HRP modification, the carboxyl group in SL increased, while the phenolic and methoxyl groups decreased with the increase of M_w. The water solubility of SL improved and the methoxyl group decreased as the sulfonation degree of SL increased to improve SL's reactivity during HRP modification, and HRP modification could facilitate the radical sulfonation reaction of SL. Therefore, the sulfomethylation and HRP incubation in lignin modification had a synergistic effect. Besides, the adsorption properties of SL were significantly enhanced by the HRP modification.