*4.1.3. Microwave irradiation*

electron. The electron is emitted from an electron beam gun and accelerated by accelerator (**Figure 5**). In this pretreatment process, the electron energy can be controlled and modulated by varying the irradiation dose. The high-energy electrons emitted travel into biomass and biofiber component and transfer the energy within the materials. The heating process initiates chemical and thermal reaction in the biomass including cellulose depolymerization, and production of carbonyl group, resulting from the oxidation of the biomass. Crosslinking of biomass component has also been reported to occur when the biomass is exposed to irradiation beam [39]. Also, reduction of the biomass mechanical strength has been observed from the biomass exposed to electron beam. This could be due to the disruption of hydrogen bond

Recently various research groups have studied the potential of electron beam radiation on various type of biomass including tropical biomass and biofiber such as bamboo, rice straw, oil palm, fruit bunch, and kenaf [30, 41, 42]. Overall, most of the EBI pretreatment indicated that a significant cellulose degradation was observed after the process [39]. Moreover, the study also showed that this pretreatment has enhanced enzymatic saccharification and reduce sugar production from biomass [31, 41]. A study on EBI pretreatment of hybrid grass biomass indicated that the pretreatment could enhance 59% of glucose yield from the biomass com‐ pared to untreated sample. This is similar to a study by Bak et al. [43] who reported that EBI pretreatment on rice straw could increase enzyme digestibility and energy during the pre‐

Similar to other pretreatment process, EBI pretreatment process could be influenced by several factors. EBI dosage is one of the factors that play a major role in the EBI pretreatment of biomass process [39, 42]. A study on the EBI pretreatment of bamboo chips at various EBI dosage range 0.5–50 kGy, indicated that significant cellulose degradation was attained from the pretreat‐ ment dosage between 0–50 kGy. Furthermore, the study showed no significant changes on the hemicellulose content. This indicates that EBI pretreatment process is a selective process and

the degradation level can be controlled by the EBI dosage [39].

between cellulose chains making it less crystalline and more amorphous [40].

**Figure 5.** Experimental set-up for electron beam irradiation.

treatment process.

338 Radiation Effects in Materials

Microwave is electromagnetic waves between the frequency range of 0.3–300 GHz, and most of the microwave systems used for industrial and domestic purposes range between 0.9 GHz to 2.45 GHz [44]. Microwave radiation is a radiating wave movement and takes a straight-line path type of energy. This radiation do not require any medium to travel through and could penetrate non-metal materials such as plastic and glass. Microwaves can affect the material thermally and non-thermally. Thermally, microwaves heat the material by the interaction of the molecules of material with electromagnetic field produced by microwave energy (**Figure 6**). Non-thermally, microwaves affect and interact with the polar molecules and ions in the materials causing physical, chemical, and biological reactions [45].

**Figure 6.** Conventional and microwave heating mechanisms of biomolecule.

Many studies on the potential of microwave pretreatment towards various types of biomass and biofiber digestibilities such as switchgrass, sweet sorghum bagasse and mischantus have been reported [46–48]. Generally, the microwave pretreatment can be carried out through three different approaches:


Mechanical pretreatment is used to reduce biomass particle size and provide more surface area for further microwave pretreatment. Pretreatment through combination of microwave and chemical approaches will generally involve either acid or alkaline as catalyst. In this process, alkaline is used to swell the biomass structure and remove lignin component from the biomass [49]. On the other hand, acid catalyst used in this process will convert hemicellulose and cellulose component into a small monomer sugar such as glucose and xylose, which is the main platform for biofuel production [49]. In contrast to the combination of microwave and steam explosion approach, the high pressure and temperature radically disrupts the lignocellulosic biomass structure and provide better assess for hydrolytic enzyme to degrade cellulose and hemicellulose.

Study on microwaves pretreatment on biomass such as palm biomass has been widely reported [50–52]. Akhbar et al. [53] compared the microwave assisted chemical pretreatment of empty fruit bunches (EFB) with conventional method and found higher lignin removal of up to 72% using microwave assisted chemical treatment. The presence of chemical such as alkaline or acid in microwave pretreatment could assist fractionation of the biomass and biofiber.

The microwave pretreatment has also been applied on other types of palm biomass. Lai and Idris [54] in their study on the microwave pretreatment of oil palm trunk (OPT) and frond (OPF), found that this pretreatment was able to disrupt the OPT and OPF. In this study, the biomass was pretreated at 700 W at 80°C for 60 min, and approximately 41.6% and 64.42% of cellulose was released from the OPT and OPF respectively. They also suggested that pretreat‐ ment at this condition is more effective in extracting hemicellulose and cellulose component compared to lignin in both OPT and OPF. In their other study on the determination of optimum condition for lignin extraction from OPT indicated that the highest lignin reduction (22.38%) was attained when the pretreatment was performed at 100°C for 80 min at 900 W. This study is in agreement to the conclusion that microwave pretreatment is significantly influenced by the temperature, reaction time, and microwave power [52].

Apart from oil palm biomass, several studies on the microwave pretreatment on other biomass such as kenaf, sago pith, sago bark waste, banana trunk, and mischantus have also been reported elsewhere [55, 56]. Study by Ooi et al. [57] on the microwave alkali-assisted pretreat‐ ment of kenaf pulp showed that the pretreatment at 50°C is the suitable temperature to convert crystalline cellulose to amorphous form, and produce higher sugar yield compared to untreated sample. In another study on microwave pretreatment of sago pith, a starch-based crop that contain substantial amount of starch and fiber, indicated that direct heating of sago pith in water by microwave treatment can swell and gelatinize the starch, resulting to a more amorphous and more susceptible fiber for subsequent enzyme reaction [55]. In a study on microwave chemical assisted pretreatment of miscanthus under different temperature range of 130–200°C, found that the suitable condition for miscanthus pretreatment is at 180°C for 20 min [58]. This study concluded that temperature plays an important role in microwave pretreatment process. Pretreatment at high temperature increases biomass solubility, shorten the pretreatment reaction period, and reduce recalcitrant characteristic of the biomass. However, the pretreatment process at high temperature also produced a substantial amount of inhibitor that is harmful to the subsequent enzymatic saccharification and fermentation.

### *4.1.4. Ultrasonication*

Another irradiation pretreatment that is widely used to pretreat biomass and biofiber for biofuel production is ultrasonication. This process can be performed either using probe-type ultrasonication or an ultrasonic bath. In this process, ultrasonic waves can be generated via piezoelectric or magnetostrictive transducers in the frequency range of 20–1000 kHz, in which the waves induced provide pressure difference in the medium. The pressure wave that travels through the liquid medium has high pressure (compression) and low pressure (rarefaction) regions. The rarefaction of the cycle can stretch the liquid molecules apart and create cavities also known as bubbles. As the wave cycles through the liquid, the bubbles expand and contract with the rarefaction and compression of the wave, respectively, drawing more liquid mole‐ cules into the bubbles as they grow. The bubbles that either continue to expand and then float to the surface, are subjected to coalescence due to the forces or collapse during compression of the wave (**Figure 7**). This collapse is almost adiabatic and can result in localized temperatures of around 5000 K and pressures of 1000 atm [59]. The collapse results in the formation of radicals through dissociation of the molecules within and around the bubbles, luminescence due to excited molecules formed losing energy, and microjets shooting out of the bubbles of speeds in the realms of hundreds of km per hour.

**Figure 7.** Ultrasonication pretreatment of biomass and biofiber mechanisms.

biomass structure and provide better assess for hydrolytic enzyme to degrade cellulose and

Study on microwaves pretreatment on biomass such as palm biomass has been widely reported [50–52]. Akhbar et al. [53] compared the microwave assisted chemical pretreatment of empty fruit bunches (EFB) with conventional method and found higher lignin removal of up to 72% using microwave assisted chemical treatment. The presence of chemical such as alkaline or acid in microwave pretreatment could assist fractionation of the biomass and biofiber.

The microwave pretreatment has also been applied on other types of palm biomass. Lai and Idris [54] in their study on the microwave pretreatment of oil palm trunk (OPT) and frond (OPF), found that this pretreatment was able to disrupt the OPT and OPF. In this study, the biomass was pretreated at 700 W at 80°C for 60 min, and approximately 41.6% and 64.42% of cellulose was released from the OPT and OPF respectively. They also suggested that pretreat‐ ment at this condition is more effective in extracting hemicellulose and cellulose component compared to lignin in both OPT and OPF. In their other study on the determination of optimum condition for lignin extraction from OPT indicated that the highest lignin reduction (22.38%) was attained when the pretreatment was performed at 100°C for 80 min at 900 W. This study is in agreement to the conclusion that microwave pretreatment is significantly influenced by

Apart from oil palm biomass, several studies on the microwave pretreatment on other biomass such as kenaf, sago pith, sago bark waste, banana trunk, and mischantus have also been reported elsewhere [55, 56]. Study by Ooi et al. [57] on the microwave alkali-assisted pretreat‐ ment of kenaf pulp showed that the pretreatment at 50°C is the suitable temperature to convert crystalline cellulose to amorphous form, and produce higher sugar yield compared to untreated sample. In another study on microwave pretreatment of sago pith, a starch-based crop that contain substantial amount of starch and fiber, indicated that direct heating of sago pith in water by microwave treatment can swell and gelatinize the starch, resulting to a more amorphous and more susceptible fiber for subsequent enzyme reaction [55]. In a study on microwave chemical assisted pretreatment of miscanthus under different temperature range of 130–200°C, found that the suitable condition for miscanthus pretreatment is at 180°C for 20 min [58]. This study concluded that temperature plays an important role in microwave pretreatment process. Pretreatment at high temperature increases biomass solubility, shorten the pretreatment reaction period, and reduce recalcitrant characteristic of the biomass. However, the pretreatment process at high temperature also produced a substantial amount of inhibitor that is harmful to the subsequent enzymatic saccharification and fermentation.

Another irradiation pretreatment that is widely used to pretreat biomass and biofiber for biofuel production is ultrasonication. This process can be performed either using probe-type ultrasonication or an ultrasonic bath. In this process, ultrasonic waves can be generated via piezoelectric or magnetostrictive transducers in the frequency range of 20–1000 kHz, in which the waves induced provide pressure difference in the medium. The pressure wave that travels through the liquid medium has high pressure (compression) and low pressure (rarefaction)

the temperature, reaction time, and microwave power [52].

hemicellulose.

340 Radiation Effects in Materials

*4.1.4. Ultrasonication*

Ultrasonic pretreatment has been performed on a great variety of lignocellulosic biomass and biofiber including kenaf powder, kenaf bast fiber, corn meal, and corn stover [60–63]. This approach has also been performed on tropical biomass such as EFB and kenaf fiber. Most of the study concluded that ultrasonication pretreatment is capable to enhance conversion of biomass to biofuel. A study on ultrasonic pretreatment of EFB at low temperature indicated that this pretreatment could assist the acid hydrolysis performed at low temperature and pressure [64]. The study showed that xylose production from the pretreated EFB was two times higher than that of un-pretreated sample when the pretreatment was performed at 100°C for 40 min. Similar to the study on the ultrasonic pretreatment of kenaf powder in ionic liquid indicated that higher reducing sugar was attained from pretreated sample [60]. In this study, a significant change on the hemicellulose content was observed in the pretreated biomass.
