**5. Irradiation effect on biomass and biofiber**

The main goal of the pretreatment process in liquid biofuel production is to modify the surface morphology structure and properties aiming to improve digestibility in the subsequent enzymatic saccharification. The pretreatment has also been reported to affect the chemical composition of the biomass (**Table 2**). A significant reduction of lignin and hemicellulose were observed from the EFB after it went through the EBI pretreatment process.


**Table 2.** Chemical composition content of oil palm empty fruit bunches (OPEFB) untreated and gamma irradiated OPEFB. (Adapted from Kristiani et al. [36]).

#### **5.1 Surface morphology and chemical structure**

The changes in chemical composition and surface morphology structure of biomass and biofiber are the main obvious effect observed from the irradiation pretreatment process. Typically, biomass or fiber with high crystallinity may consist a significant amount of crystal‐ linity cellulose, and appear to be relatively smooth. The biomass with high degree of crystal‐ linity indicates that it has high tensile strength properties [65].

Various investigations on the effect of irradiation pretreatment on tropical biomass including EFB, kenaf, rubberwood, and bamboo have been reported [29, 42, 64]. The study agreed that the pretreatment applied on these biomass have a significant effect on the biomass structure and chemical properties. For instance, a study on irradiation pretreatment of EFB at 300 kGy indicated a significant change in the surface morphology before and after pretreatment process [36]. The study found that the EFB, which is solid, intact, rough, and rigid structure becomes brittle and flaky after irradiated with gamma ray. Similar observation has been reported on the irradiation pretreatment of rubberwood. Darji et al. [66] compared the rubberwood structure before and after pretreatment and found that most of the fibrous in the rubberwood lost and disappeared after the pretreatment process.

The surface morphology structure change after pretreatment could be attributed to the irradiation process that is able to break the intermolecular hydrogen bond, resulting to the decrease of intra and intermolecular order in cellulose. Furthermore, under high energy and pressure, the cellulose macromolecule will undergo scission and increase the fragment with low degree of polymerization [67]. On the other hand, irradiation has also been reported to influence the biomass pore size. Brunauer–Emmett–Teller (BET) analysis on the kenaf core and cellulose, indicated that a significant increase of pore size was observed for both materials after irradiation pretreatment process [42, 68]. High pore size is a very important characteristic that could provide easy access for subsequent process prior to biofuel production.

Most of the studies reported that the change of the biomass surface morphology is correlated to the chemical structure in the biomass. A change on the degree of crystallinity was found to change surface morphology [69]. Typically, X-ray diffraction (XRD) analysis is applied to evaluate the effect of irradiation on biomass crystallinity. Chen et al. [70] reported that the major diffraction peak for cellulose crystallography can be identified for 2θ ranging between 22° and 23° as a primary peak, whereas a secondary peak is in the range of 16° to 18°. As reported by Liu et al. [28], the I002 peak intensity (the maximum intensity of the 002 lattice diffraction) represents the primary peak and is classified as the diffraction intensity of crystalline regions, whereas the secondary peak represents the diffraction intensity of the amorphous zone. XRD analysis of irradiated cellulose at different irradiation dosage between 10–100 kGy indicated that increase of dosage could reduce crystallinity index and crystallite size [68]. In another study on irradiation of OPTT and OPF, it was found that an obvious peak reduction on the primary and second peak, indicates the transformation of cellulose molecular hydrogen bond due to rapid heating during the irradiation pretreatment process [54].

Apart from XRD analysis, the effect of irradiation pretreatment can also be evaluated by Fourier transform infrared spectroscopy (FT-IR). This method is widely used to determine the chemical structure changes after pretreatment of various types of biomass [71, 72]. FT-IR analysis on the irradiated oil palm trunk and oil palm frond indicated that radiation has affected the intensity of all bands in the IR spectra [51, 54]. Obvious changes were observed at absorbance between 3500–3200 cm−1, 2840–2690 cm−1, 1740–1720 cm−1, 1500–1450cm−1, 1300– 1000 cm−1, 1315–1318 cm−1, and 900–898 cm−1. These bands represent a specific chemical structure in biomass as summarized in **Table 3**.


**Table 3.** FT-IR band assignment in biomass [54].

composition of the biomass (**Table 2**). A significant reduction of lignin and hemicellulose were

observed from the EFB after it went through the EBI pretreatment process.

OPEFB untreated 35.94 30.41 20.70 OPEFB-C 10.32 77.5 6.83 OPEFB-CI 100 kGy 8.23 68.87 14.27 OPEFB-CI 200 kGy 5.86 71.96 15.20 OPEFB-CI 300 kGy 7.01 65.64 13.94 OPEFB-CI 400 kGy 7.65 64.92 13.39 OPEFB-CI 500 kGy 7.86 63.81 13.45

OPEFB. (Adapted from Kristiani et al. [36]).

342 Radiation Effects in Materials

**5.1 Surface morphology and chemical structure**

linity indicates that it has high tensile strength properties [65].

lost and disappeared after the pretreatment process.

**Sample Total lignin (%) Cellulose (%) Hemicellulose (%)**

**Table 2.** Chemical composition content of oil palm empty fruit bunches (OPEFB) untreated and gamma irradiated

The changes in chemical composition and surface morphology structure of biomass and biofiber are the main obvious effect observed from the irradiation pretreatment process. Typically, biomass or fiber with high crystallinity may consist a significant amount of crystal‐ linity cellulose, and appear to be relatively smooth. The biomass with high degree of crystal‐

Various investigations on the effect of irradiation pretreatment on tropical biomass including EFB, kenaf, rubberwood, and bamboo have been reported [29, 42, 64]. The study agreed that the pretreatment applied on these biomass have a significant effect on the biomass structure and chemical properties. For instance, a study on irradiation pretreatment of EFB at 300 kGy indicated a significant change in the surface morphology before and after pretreatment process [36]. The study found that the EFB, which is solid, intact, rough, and rigid structure becomes brittle and flaky after irradiated with gamma ray. Similar observation has been reported on the irradiation pretreatment of rubberwood. Darji et al. [66] compared the rubberwood structure before and after pretreatment and found that most of the fibrous in the rubberwood

The surface morphology structure change after pretreatment could be attributed to the irradiation process that is able to break the intermolecular hydrogen bond, resulting to the decrease of intra and intermolecular order in cellulose. Furthermore, under high energy and pressure, the cellulose macromolecule will undergo scission and increase the fragment with low degree of polymerization [67]. On the other hand, irradiation has also been reported to influence the biomass pore size. Brunauer–Emmett–Teller (BET) analysis on the kenaf core and cellulose, indicated that a significant increase of pore size was observed for both materials after irradiation pretreatment process [42, 68]. High pore size is a very important characteristic that

could provide easy access for subsequent process prior to biofuel production.

Normally, the structure of lignin consists of guaiacyl propane units (G) and syringyl propane units (S) containing one and two methoxy groups. It is known that the presence of guaiacyl propane could restrict the swelling of biomass [73, 74]. These chemical structures can be identified by FT-IR spectra with frequencies in the region of 1509, 1464, and 1422 cm−1. Reduction of spectra in this region indicated that most of the lignin in OPT and OPF have been removed from the biomass during the pretreatment process. Removal of lignin in the biomass after irradiation gives a better access for enzyme to attack cellulose and hemicellulose.

The FT-IR analysis of the irradiation pretreatment on biomass also indicated that significant changes on absorbance was observed at 1732 cm−1 and 3300 cm−1, attributed to the vibration of hydrogen bonded OH-group. Liu et al. [75] reported a shifting and reduction of band 2899 cm −1, indicating to the disruption of biomass resulting from the C-H shifting vibration. The study also found that high-energy irradiation pretreatment could interrupt and destroy the intramolecular and inter molecular hydrogen bond in the cellulose. The degradation of cellulose generated carbonyl group could be determined at band at 1603 cm−1. Apart of this region, the shifting of band region between 1164 cm−1, 1112 cm−1, and 1058 cm−1 attributed to the vibration of C-O-C of cellulose.
