Application of Fenton pretreatment on the degradation of rice straw by mixed culture of Phanerochaete chrysosporium and Aspergillus niger

Highlights

• Fenton pretreatment effectively destroyed the resistant structure of rice straw.

• The recalcitrant lignin was effectively degraded by 15.01%.

• The activities of ligninolytic enzymes were enormously enhanced after pretreatment.

• GC–MS analysis gave further insight into the degradation of Fenton pretreatment.

Abstract

The rice straw (RS) is a kind of recalcitrant lignocellulosic material. The Fenton reaction has been widely used for the degradation of organic compounds and toxic chemicals. Therefore, applying it to pretreatment straw seems to have good potential. This study was to investigate the degradation of the pretreated RS by the mixed solid-state culture with Phanerochaete chrysosporium and Aspergillus nigert. The results showed that the degree of delignifition was 15.01% by using an optimized Fenton reagent (6 g/L of FeSO₄·7H₂O and 15% concentration of H₂O₂) at a solids loading of 5% (w/v) and a moderate temperature for 15 h. The analysis of the Fenton pretreated RS by Scanning electron microscope (SEM) and Fourier transform infrared spectroscopy (FTIR) showed significant changes in physicochemical structure, favoring the subsequently susceptibility of ligninolytic enzymes. The volatile organic compounds collected from the mixed solid-state culture implied the degradation degree of the lignocellulose to some extent. In conclusion, Fenton pretreatment could effectively disrupt the recalcitrant structure of RS and accelerate its biodegradation, which has high potential application in the energy conversion and utilization of lignocellulose.

Introduction

The increasing demand for energy and the reduction of global fossil fuels, together with climate changes have attracted extensive attention. These frustrating challenges have aroused our interest in non-conventional energy sources (Sindhu et al., 2016). Rice straw (RS), as one of the most abundant agricultural wastes, there is around 200 million tons produced in China and a large number of them are simply disposed by burning in the fields (Ranjan and Moholkar, 2013). The utilization of RS in an economically feasible way would not only prevent environmental pollution but produce chemicals and biofuels.

RS is mainly composed of cellulose and hemicellulose surrounded by lignin sheath. The three main components formed a tightly packed, tough and complex, water-insoluble structure that is resistant to depolymerization by microbial and chemical attack (Chen et al., 2014, Kumar et al., 2008). Both cellulose and hemicellulose could be converted to fuels by enzymatic hydrolysis and fermentation (Blanch et al., 2011). Lignin with non-carbohydrates polymer structure is a complex aromatic derivative composed of phenylpropanoid units, which acts as a physical barrier to prevent the hydrolysis of lignocellulose (Ranjan and Moholkar, 2013). Chandler and Jewell showed that one percent lignin decreased organic matter digestion by about 3% (Chandler and Jewell, 1980). Therefore, the degradation of lignin is the key step to the lignocellulose transformation (Huang et al., 2010). In addition to the physical and chemical rigidity and recalcitrance, there are many other factors that restrict the digestibility of RS, such as lignin content, crystallinity of cellulose and particle size (Hendriks and Zeeman, 2009). Studies have shown that the highly recalcitrant nature of lignocellulose material limits the access of lignocellulolytic enzymes to penetrate its interior to carry out the hydrolysis of the glycosidic bonds (Allardyce et al., 2010).

Biological treatments are considered as more environmental friendly than physical and chemical methods in lignocellulose degradation due to its low energy consumption, mild reaction conditions and simple procedures and equipment (Ye and Cheng, 2002). Basidiomycete strains are the most efficient lignocellulose degraders among fungi (Sánchez, 2009). Phanerochaete chrysosporium (P. chrysosporium) is one of the typical basidiomycete white-rot fungi, which has prominent ability to degrade lignin (Chen et al., 2016, Yu et al., 2011, Zhang et al., 2014). Aspergillus niger (A. niger) is a well-known fungus for producing cellulases. Mixed culture of lignocellulolytic fungi can upregulate the secretion of ligninolytic enzymes and improve the degradation of lignin and cellulose (Chia et al., 2007, Gregorio et al., 2006). Therefore, the mixed culture of P. chrysosporium and A. niger was expected to achieve better degradation effect and reducing sugar yields. However, there are a few researches about improving the mixed culture by pretreatment of RS. To facilitate the hydrolysis of lignin degrading enzymes, pretreatment method is necessary in removing lignin and reducing crystallinity of cellulose as well as increasing material porosity. Various pretreatment methods have been investigated such as steam explosion, ammonia fiber explosion, alkali treatment, acid hydrolysis, hot water treatment, super-critical carbon dioxide explosion, extrusion pretreatment, and hydrodynamic cavitation (Al-Zuhair et al., 2013, Hilares et al., 2016). Although these pretreatment methods have been extensively studied, they still have some drawbacks such as high energy consumption and formation of toxic byproducts on enzymes and microorganisms (Younghoon et al., 2013). A more effective and economically feasible pretreatment method is still needed to make full use of potential lignocellulosic materials (Wyman et al., 2005). Fenton pretreatment has been widely developed and applied due to its simplicity, availability and low cost (Deng, 2007). In general, a classical Fenton process is that hydrogen peroxide produces high activity hydroxyl radical under the catalysis of ferrous ion. The Fenton reagent can increase the accessibility of the enzyme by oxidizing and disrupting the lignocellulosic structure (Neyens and Baeyens, 2003). The cellulose substrates was treated with 0.5 mM concentration of Fe²⁺ and 2% concentration of H₂O₂ and reacted for 48 h could provide the highest enzyme activity (Jain and Vigneshwaran, 2012). The combination of Fenton pretreatment and biological treatment by mixed fungi would be an acceptable way for RS degradation and reuse.

The present study is focused on the integrated process of Fenton pretreatment and biological treatment to determine the efficiency of the Fenton pretreatment for the delignification. Meanwhile, the subsequent digestibility of RS was evaluated by the ligninolytic enzymes in the mixed culture of P. chrysosporium and A. niger. The morphological structure changes of RS were investigated by SEM and FTIR. Meanwhile, the composition changes of VOCs from solid-state cultivation were analyzed to study the effect of Fenton pretreatment on the degradation of RS by mixed culture of P. chrysosporium and A. niger.