Comparative study on chemical pretreatment methods for improving enzymatic digestibility of crofton weed stem

Abstract

In order to utilize and control the invasive weed, crofton weed (Eupatorium adenophorum Spreng), a potential pathway was proposed by using it as a feedstock for production of fermentable sugars. Three chemical pretreatment methods were used for improving enzymatic saccharification of the weed stem. Mild H₂SO₄ pretreatment could obtain a relatively high yield of sugars in the pretreatment (32.89%, based on initial holocellulose), however, it led to only a slight enhancement of enzymatic digestibility. NaOH pretreatment could obtain a higher enzymatic conversion ratio of cellulose compared with H₂SO₄ pretreatment. Peracetic acid (PAA) pretreatment seemed to be the most effective for improving enzymatic saccharification of the weed stem in the three chemical pretreatment methods under the same conditions. The conversion ratio of cellulose in the sample pretreated by PAA under the “optimal” condition was increased to 50% by cellulase loading of 80 FPU/g cellulose for 72 h incubation. A number of empirical quadratic models were successfully developed according to the experimental data to predict the yield of sugar and degree of delignification.

Introduction

Crofton weed (Eupatorium adenophorum Spreng) is a tufty, semi-shrubby, perennial herbaceous plant of about 1–2 m in height. It is native to Central America, mainly Mexico, although it is now distributed widely in the United States of America, Australia, New Zealand and many countries in Southeast Asia. This exotic weed prefers a warm and wet environment, but its ability to adapt to different environmental conditions is very strong. It can grow in environments with temperature ranges from 5 °C to 42 °C on dry and barren hills, and can even grow in cracks in stone walls and houses (Sun, 2003). Since the 1950s, this exotic plant has spread rapidly across southwest China, damaging native ecosystems and causing great economic losses (Yu et al., 2005). Many farmlands, pasture fields and forests have been destroyed by the weed. Much attention has been paid to management of this weed and several methods have been developed to control it, including manual, chemical and biological control, however, no obvious progress is made. The weed is still spreading from southwest to northeast at a rate of 10–60 km per year in China (Lu et al., 2005).

Compared with simple control processes, utilization of the weed gives more benefits. According to the published literatures, the weed can be used as bio-pesticide (Wang et al., 2002, Liu et al., 2004, Kaushal et al., 2001), organic fertilizer and feedstuff, feedstock for production of marsh gas (Patrabansh and Madan, 2000). We determined the glycan content in the stem of this weed and found that it was similar to that of some agricultural wastes, such as corn stover, rice straw and wheat straw. Therefore, in this article, it was proposed to use the weed as a biomass resource to produce fermentable sugars for further production of biofuel such as ethanol, instead of just simply controlling it.

There are two common ways to convert lignocellulosic biomass to sugars, namely acid hydrolysis and enzymatic hydrolysis. Acid hydrolysis has been developed for a long time for direct saccharification. It is also a pretreatment method for improving the enzymatic digestibility of biomass. Enzymatic saccharification is a promising way because of the advantages such as low energy input, less pollution, no side products and so on, but the low enzymatic accessibility of the native cellulose is a key problem for biomass-to-ethanol processes. Therefore, pretreatment is an essential element in the bioconversion of lignocellulosic substrates. The objective of this article is thus to investigate the effect of different chemical pretreatment processes on the enzymatic digestibility of the weed stem, in order to obtain a high enzymatic conversion ratio of cellulose-to-sugar and preliminarily study the possibility of utilization of the weed for production of fermentable sugars.