Elsevier

Journal of Molecular Liquids

Volume 254, 15 March 2018, Pages 55-63
Journal of Molecular Liquids

Optimization of sugar recovery efficiency using microwave assisted alkaline pretreatment of cassava stem using response surface methodology and its structural characterization

https://doi.org/10.1016/j.molliq.2018.01.091Get rights and content

Highlights

  • The present study investigated the effect of pretreatment with alkaline sodium hydroxide (NaOH).

  • Alkaline pretreatment of cassava stem using response surface methodology is effective for bioethanol production.

  • FTIR and SEM provide the physiochemical changes in cassava stem after pretreatment.

  • Optimal factors were time 116.4 s, NaOH concentration 3.21% (w/v), S/L ratio 62.07 g/ml and microwave intensity 719.86 Hz.

Abstract

Cassava stem is one of the prominent lignocellulosic wastes and has potential as a feedstock for fermentable sugar production. In this study, response surface methodology (RSM) with Box-Behnken design (BBD) was employed to investigate optimum conditions for microwave assisted alkaline pretreatment of cassava stem. Effect of four variables such as reaction time (60–120 s), NaOH concentration (2–4% w/v), solid to liquid ratio (1:25–1:75 g/ml), and microwave frequency (360–720 Hz) were evaluated to improve the sugar recovery. The quadratic model indicated that, reaction time of 116.4 s, NaOH concentration of 3.21% (w/v), substrate to liquid ratio of 1:62.07 g/ml and microwave frequency of 719.86 Hz was found to be optimum and obtained a maximum yield of 43.60 μg/ml of reducing sugar and 91.71 μg/ml of xylose. Under this condition, the cellulose content of cassava stem was increased from 33.27% to 52.34%, while the hemicellulose and lignin content was decreased from 32.30% to 27.15% and 27.15% 14.59%, respectively. Moreover, to evaluate the effectiveness of the pretreatment, Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD) and scanning electron microscope (SEM) analysis were employed on the untreated and pretreated cassava stem. These results suggest that the microwave assisted alkaline NaOH pretreatment (MAASHP) influences the fermentable sugar production significantly and further it can be utilized effectively for bioethanol production.

Introduction

Lignocellulosic materials and agricultural residues are the most attractive and abundant renewable biomass source available in nature. These materials constitute three main polymers namely cellulose, hemicellulose and lignin [1,2]. Cassava (Manihot esculenta Crantz) is an everlasting woody shrub belongs to the family Euphorbiaceae grows well in tropical and subtropical areas and is generally regarded as a third largest carbohydrate source [3]. It is mainly used ad food (48%) and feed (34%), feedstock (18%) for biofuels and biochemical [4,5]. Globally, cassava stem waste have today reached about 6.7 Pg; 60% of this is available for biofuel production, yielding ca. 64.8 EJ/of energy (given the net heat value of 16.2 MJ/kg). This reported value equals the annual energy use of about 840 million people based on the global average of energy use per capita [6]. A detailed phytochemistry report and a case study on above ground carbon stocks of cassava were found elsewhere in the world. Previous studies reported that non-edible parts of cassava especially stem can be feasibly utilized for fermentable sugars and bioethanol production [7,8].

The major advantage of using non-edible part is neglecting food versus fuel conflict and non-compete with the food supply. To increase the yield of fermentable sugars, an efficient pretreatment method should be incorporated into the lignocellulosic conversion process. Various researchers have reported that pretreatment methods are able to change the efficient lignocellulosic conversion and enhance the biofuel production [7,9,10]. Among the methods, alkaline pretreatment was shown to be more effective and advantageous since it operates at lower temperatures [2] utilizing readily available low-cost chemicals such as sodium hydroxide (NaOH), calcium hydroxide (Ca(OH)2), potassium hydroxide (KOH) and ammonia. However, long retention time and neutralizing pretreated slurry fetch drawbacks to this method. Application of microwaves on pretreatment of lignocellulosic has been considered as an alternative to conventional methods [11,12]. Microwave, a high-frequency electromagnetic radiation offers uniform and rapid heating leading to improved digestibility and disruption of recalcitrant structures in biomass [13].

Various researchers on several different lignocellulosic feedstocks such as switch grass, wheat straw, water hyacinth, banana waste, sugarcane bagasse, catalpa sawdust, are canut husk [1,3,7,[14], [15], [16], [17], [18]], and so on. Overview of fundamentals, recent advances in microwave assisted pretreatment methods and the importance of microwave reactor development was also available in the literature [11,19].

The present study involves the optimization of MAASHP using cassava stem powder as raw material to increase sugar recovery using response surface methodology. This study aimed to develop the model using cassava stem powder as output and reaction time, NaOH concentration, solid to liquid ratio and microwave frequency as input. The parameters influencing pretreatment conditions were statistically optimized to enhance sugar production. FTIR, XRD and SEM were investigated for determination of functional group modifications, structure of cassava stem and cellulose crystallinity.

Section snippets

Sample collection and preparation

Lignocellulosic biomass is a substrate for the production of bioethanol was cassava stem from a local farm in Namakkal District (latitude 11.378476; longitude 77.894493), Tamil Nadu, India and the collected material was stored at room temperature for their further analysis. The procured sample was initially shredded into small pieces and then followed by grinding using mixer grinder (Preethi Trio, India). The resultant was sieved using a standard mesh to get fine particles (≤1 mm) and

Optimization of MAASHP

In the present work, the relationship between the yield of reducing sugars and four process variables (reaction time, NaOH concentration, solid to liquid ratio, and microwave frequency) was developed by using RSM. The results at each point based on the experimental Box-Behnken design template are presented in Table 1. The experimental and predicted values of TRS and xylose yield are shown in Table 2. Design Expert 10.0.6 software was employed to determine and evaluate the coefficients of the

Conclusion

The application of response surface methodology on the effect of alkaline NaOH pretreatment on cassava stem powder under microwave conditions was investigated. The optimal process parameters for maximum TRS and xylose yield (43.6% and 91.56% respectively) was found to be: reaction time of 116.4 s, NaOH concentration of 3.21%, solid to liquid ratio of 1:62.07, and a microwave frequency of 719.86 Hz. An XRD study indicates the increase in values of crystallinity index signifying the effect of

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