Structural Changes of Alkali Lignin under Ozone Treatment and Effect of Ozone-Oxidized Alkali Lignin on Cellulose Digestibility

Junli Ren,Lihong Zhao,Hongyuan Wang,Beihai He

doi:10.3390/pr10030559

Abstract

In this study, the structural changes of alkali lignin induced by ozonation were investigated, and the effect of ozone-treated alkali lignin and its mechanism on Avicel enzymatic hydrolysis was examined. The physicochemical properties of alkali lignin were analyzed by FTIR, 1H-13C HSQC NMR, and GPC. It was revealed that ozone pretreatment increased the content of carboxyl and/or aldehyde groups and the negative zeta potential of alkali lignin, which enhanced the electrostatic repulsion between alkali lignin and cellulase; The S/G ratio was reduced, indicating the hydrophobic interaction was diminished. The Langmuir adsorption isotherm showed that the cellulase binding strength of ozone pretreated alkali lignin (OL-pH3, OL-pH7, and OL-pH12 were 16.67, 13.87, and 44.05 mL/g, respectively) was significantly lower than that of alkali lignin (161.29 mL/g). The 72 h hydrolysis yields of Avicel added with OL-pH3, OL-pH7, and OL-pH12 were 55.4%, 58.6%, and 54.9% respectively, which were 2.6–6.3% higher than that of Avicel added with AL (52.3%). This research aimed to reduce the non-productive adsorption between cellulase and lignin by investigating the structural changes of lignin caused by ozone treatment. For the first time, we discovered that ozone-treated alkali lignin has a further promotion effect on the enzymatic digestion of cellulose, providing a green and feasible pretreatment process for the enzymatic hydrolysis of lignocellulose and aiding in the more efficient utilization of biomass.

Highlights

In recent years, lignocellulose biomass has attracted increasing attention because of its advantages of low price, easy availability, and abundant reserves [1]
The biorefinery of lignocellulose via enzymatic hydrolysis aims to convert cellulose and hemicellulose into monosaccharides and oligosaccharides, which can be transformed into a variety of chemical products that are regarded as potential substitutes for plastics and fossil fuels, whose utilization would assist in alleviating the food issue [2]
Lignin typically affects the enzymatic hydrolysis of lignocellulose in three ways: steric hindrance, comprising physical barriers generated by lignin wrapping around cellulose and hemicellulose surfaces and limiting cellulase accessibility to cellulose; the induction of non-productive enzyme adsorption by ineffective lignin adsorption on cellulase, which can significantly lower enzymatic digestion efficiency; and the inhibition by lignin of cellulase activity [5–7].Thereinto, the non-productive adsorption of lignin with cellulase plays a leading role

Summary

Introduction

Lignocellulose biomass has attracted increasing attention because of its advantages of low price, easy availability, and abundant reserves [1]. The biorefinery of lignocellulose via enzymatic hydrolysis aims to convert cellulose and hemicellulose into monosaccharides and oligosaccharides, which can be transformed into a variety of chemical products that are regarded as potential substitutes for plastics and fossil fuels, whose utilization would assist in alleviating the food issue [2]. Lignin typically affects the enzymatic hydrolysis of lignocellulose in three ways: steric hindrance, comprising physical barriers generated by lignin wrapping around cellulose and hemicellulose surfaces and limiting cellulase accessibility to cellulose; the induction of non-productive enzyme adsorption by ineffective lignin adsorption on cellulase, which can significantly lower enzymatic digestion efficiency; and the inhibition by lignin of cellulase activity [5–7].Thereinto, the non-productive adsorption of lignin with cellulase plays a leading role. It was reported that milled wood lignin, when treated by the Fenton reaction system, has a significant promotion effect on the enzymatic digestion of cellulose, which is contrary to our traditional view that lignin plays a negative role in the enzymatic system [9]

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