Elsevier

Bioresource Technology

Volume 102, Issue 17, September 2011, Pages 7953-7958
Bioresource Technology

Microscopic structure and properties changes of cassava stillage residue pretreated by mechanical activation

https://doi.org/10.1016/j.biortech.2011.05.067Get rights and content

Abstract

This study has focused on the pretreatment of cassava stillage residue (CSR) by mechanical activation (MA) using a self-designed stirring ball mill. The changes in surface morphology, functional groups and crystalline structure of pretreated CSR were examined by using scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction (XRD) under reasonable conditions. The results showed that MA could significantly damage the crystal structure of CSR, resulting in the variation of surface morphology, the increase of amorphous region ratio and hydrogen bond energy, and the decrease in crystallinity and crystalline size. But no new functional groups generated during milling, and the crystal type of cellulose in CSR still belonged to cellulose I after MA.

Highlights

► Mechanical activation significantly changed the properties of cassava stillage residue. ► Mechanical activation caused the increase of amorphization and hydrogen bond energy. ► Mechanical activation caused the decrease in crystallinity index and crystalline size. ► The crystal type of cellulose was not altered by mechanical activation. ► No new functional groups generated during milling.

Introduction

With the increasing shortage of fossil fuels and air pollution from the combustion of petroleum products, developing renewable and clean energy has become the world-wide effort. Currently, increased attention has been focused on bioethanol as one of the promising alternative energy sources (Yang et al., 2010). Bioethanol is mainly produced from bioenergy crops and lignocellulosic biomass, such as wheat, corn, grain, cassava, molasses and plant fibers (Zhang et al., 2010a). Cassava, a shrubby tropical plant grown for its large, tuberous, starchy roots, is an important cash crop in tropical countries because it has the remarkable capacity to adapt to various agro-ecological conditions, such as marginal lands where other agricultural crops cannot grow well (Pandey et al., 2000). As the advantages of large distribution, relative cheap, high yield and starch content, cassava becomes a good feedstock for bioethanol production at the present time (Leng et al., 2008). In addition, the cassava agro-industrial residues with relatively high starch and lignocellulose contents, such as its bagasse, stalk and rhizome, are also the materials for the production of bioethanol (Fang et al., 2010).

Cassava stillage residue (CSR) is the solid waste generated in the still bottoms following fermentation and distillation during the production process of cassava-based bioethanol. Due to the low nutrition and high lignocellulosic components of this residue, CSR is not suitable to be treated as the distillers grains (DG) which are obtained during the distilling step of dry-grind ethanol production using other crops (in particular corn) (Cookman and Glatz, 2009). Recently, some of the CSR is used as cattle feed, packaging materials and boiler fuel, and the remaining CSR is discard as waste. Every year, more than 3.6 million tons of stillage is generated from the production of 3 million tons of cassava bioethanol in China, and CSR is the main component of stillage. The decay of so much solid residue will cause serious environmental pollution, so it is a significant issue to dispose the CSR efficiently. At the present time, there has been an increasing trend towards more efficient utilization of lignocellulosic biomass, which is regarded as abundant, inexpensive, and readily available natural organic resources (Zhang et al., 2010b). Consequently, various technologies are being pursued for the production of high added value products from lignocellulosic materials (Liu et al., 2008, Sun and Cheng, 2002). The main component of CSR is lignocellulose, which is mainly composed of cellulose, hemicellulose and lignin. These three components of lignocellulose are associated with each other to form a cellulose–hemicellulose–lignin complex. The cellulose in CSR consists of both regions with crystalline structure and amorphous structure, and the microfibril bundles of cellulose are weakly bound through hydrogen bonding. Lignin gives the CSR structural support, impermeability, and resistance against oxidative stress and microbial attack. Hemicellulose serves as a connection between the cellulose and lignin fibers and gives more rigidity to the whole cellulose–hemicellulose–lignin network (Hendriks and Zeeman, 2009). The highly-ordered and recalcitrant structure of lignocellulose makes it resists assault of other reagents and then restricts its efficient utilization. In order to enhance the chemical reactivity of lignocellulose and make it more accessible to other reagents, the pretreatment of lignocellulosic materials is essential to break the lignin seal, disrupt the crystalline structure and increase the amorphization of cellulose (Mosier et al., 2005). Currently, many methods have been studied for the pretreatment of lignocellulosic biomass, including biological pretreatments, physical pretreatments (comminution and extrusion), chemical pretreatments (acid pretreatment, alkaline pretreatment, organosolv pretreatment, ozonolysis and ionic liquids pretreatment), physicochemical pretreatments (hot-compressed water pretreatment, steam explosion, ammonia fiber explosion, wet oxidation, carbon dioxide pretreatment, microwave pretreatment, and ultrasound pretreatment), and their combination pretreatments (Alvira et al., 2010, Fang et al., 2010, Hendriks and Zeeman, 2009). Every method has its own advantages and disadvantages, and it is necessary to adopt a suitable pretreatment technology based on the properties of different lignocellulosic biomass and objective product, the subsequent processing steps and production area.

Mechanical activation (MA), usually carried out by high-energy milling, refers to the use of mechanical actions to change the crystalline structures and physicochemical properties of the solids (Baláž et al., 2005). The size reduction and structural disorder (evaluated by solid amorphization) of the solids during mechanical milling are accompanied by chemical bonds distorting and bond length extending due to the imposed stress. When the imposed stress is beyond the chemical bonding energy, bond rupture occurs. This produces activated radicals and functional groups which may easily react with other reagents (Zhang et al., 2008). In addition, a part of mechanical energy could be converted into internal energy of the milled solids during MA and thus enhanced the chemical reactivity of solids (Rossberg et al., 1982). MA was originally applied to the pretreatment of minerals, and currently it has been widely used in various fields, such as extractive metallurgy, crystal engineering, nanomatrix composites, agriculture, pharmacy, waste disposal, organic material synthesis, and so on (Baláž, 2003, Guo et al., 2010, Tongamp et al., 2006, Zyryanov et al., 2009). It is well known that the main disadvantage of MA is high energy requirement (Zhu et al., 2010). But compared with other methods, MA pretreatment is considered to be a relatively simple and environmentally friendly procedure attributing to the operations without the use of solvents, intermediate fusion, etc., especially for the effective production of high added value products (Baláž and Dutková, 2009). In our previous studies, MA has been successfully used for the pretreatment of sugarcane bagasse (SCB), cassava starch and maize starch. The results showed that MA could significantly change the crystalline structures and physicochemical properties of SCB and starch and thus enhance their chemical reactivity (Huang et al., 2007, Huang et al., 2008, Huang et al., 2009). Hopefully, MA is feasible for the pretreatment of CSR. Furthermore, to know the lignocellulosic structure characteristics is an important topic for the reactivity of lignocellulosic biomass, which has significant impact on the potential applications of lignocellulosic biomass for the production of high added value products, such as high quality natural fibers/thermoplastics composites, cellulose derivatives, etc. Therefore, the aim of this study was to investigate the changes in crystal structure and physicochemical properties of CSR during MA pretreatment, and the surface morphology, physicochemical and conformational properties and crystalline structure were measured by scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction (XRD) in detail.

Section snippets

Materials

CSR was obtained from a local bioethanol plant (Nanning, China). The sun-dried CSR was comminuted and screened to prepare 40–80 mesh size (0.18–0.38 mm) particles. The comminuted CSR was further oven-dried at 80 ± 3 °C for 24 h. The major components of CSR were crude fiber, crude protein, crude fat and ash, and the content of crude fiber was 35.5 wt.%.

Mechanical activation pretreatment

MA pretreatment of CSR was performed in a customized stirring mill driven by a commercial available drill press equipped with a speed-tuned motor (

Morphology modification

Based on observation of the SEM images of nonactivated and activated CSR samples in Fig. 1, it is found that the surface morphology of CSR was significantly changed during the processing of MA. It can be observed in Fig. 1a1 and a2 that the unmilled CSR consists of compact particles with different sizes, and the fiber structure of CSR is made up of cellulose microfibrils bound together by lignin matrix and hemicellulose, forming compact and larger fiber bundles with smooth flat surfaces. In

Conclusions

MA pretreatment of CSR was conducted through the self-designed stirring ball mill in this study. The SEM, FTIR and XRD analysis demonstrated that MA could significantly destroyed the crystal structure of CSR, resulting in the increase of amorphization and the decrease in crystallinity and crystalline size. After milled for 2.0 h, the D002, infrared TCI and CrI of CSR decreased from 2.378 nm, 0.238 and 44.2% to 1.258 nm, 0.187 and 19.5%, respectively, and the hydrogen bond energy increased by 3.9 × 10

Acknowledgements

This work was financially supported by Guangxi Graduate Student Educational Innovation Foundation of China (No. 105931003022), Nanning Science and Technology key project of Guangxi, China (No. 201002021A) and National Natural Science Foundation of China (No. 51064002).

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