Chemical Characterisation
The primary constituents of biomass include cellulose (~ 50%), hemicellulose (~ 25%), lignin (~ 25) (Kaur et al., 2016). The proportion of these components varies with the type of plant species and the geographical location of their growth (Casey 1980). A high proportion of lignin is undesirable for pulp and paper. The initial chemical composition in control was found to be: 37% cellulose (estimated as glucose), 22.8% hemicellulose (including 20.1% xylose and 2.7% arabinose) and 6.2 ± 0.12% ash content. The values are comparable to the reported cellulose, hemicellulosic of wheat straw as 35.10% and 25.60%, respectively (Chandra et al., 2012).
Hydrothermal treatment of raw material was performed by placing it in a reactor surrounded by an aqueous medium at an elevated temperature and pressure. According to Liu et al., 2012 the temperature below 100 ◦C has no hydrolytic effect on the material, and at above 220 ◦C, cellulose degradation occurs. Thus, the present study was planned in a temperature ranges between 120 to 180 ◦C for 0 min and 120 min reaction time. After conducting a preliminary optimization experiment, the material to water ratio (1:10) has finalized according to the apparent and bulk density (Fasake and Dashora, 2021c).
Table 1
chemical characterisation of hydrolysed sample and raw material
Reaction Temp. (◦C)
|
Reaction time (Min)
|
Yielda (%)
|
Glucose
(%)
|
Xylose
(%)
|
Arabinose
(%)
|
ASL
(%)
|
AIL
(%)
|
Crystallinity (CrI%)
|
Ash (%)
|
Untreated (Raw sample)
|
100
|
37
|
20.1
|
2.7
|
1.3
|
24.7
|
41
|
6.2 ± 0.12
|
120
|
0
|
93 ± 1.38
|
38.9
|
20.7
|
3.4
|
0.91
|
25.63
|
39
|
6.0 ± 0.21
|
|
120
|
90 ± 1.15
|
38.9
|
21.0
|
3.3
|
0.85
|
28.30
|
37
|
6.1 ± 0.16
|
140
|
0
|
88 ± 1.44
|
42.3
|
22.2
|
3.5
|
0.95
|
27.18
|
47
|
5.8 ± 0.11
|
|
120
|
82 ± 1.32
|
42.0
|
22.5
|
2.9
|
0.81
|
27.21
|
38
|
7.2 ± 0.25
|
160
|
0
|
87 ± 0.98
|
42.8
|
23.2
|
3.0
|
0.86
|
28.92
|
35
|
7.0 ± 0.28
|
|
120
|
68 ± 1.11
|
51.6
|
17.8
|
0.9
|
0.94
|
30.93
|
46
|
6.3 ± 0.14
|
180
|
0
|
67 ± 1.61
|
60.3
|
5.2
|
0.0
|
1.01
|
32.71
|
49
|
6.4 ± 0.22
|
|
120
|
62 ± 1.36
|
60.2
|
5.2
|
0.0
|
1.22
|
38.08
|
48
|
7.6 ± 0.17
|
aWeight % based on the starting materials, AIL: Acid insoluble lignin; ASL: Acid soluble lignin.
Table 1 shows the effect of hydrothermal treatment on the chemical characteristics of raw material. The maximum yield of 93% was obtained at the temperature of 1200C with no holding time. It was found to decrease with a further increase in temperature or contact time. The increase in contact time did not help in improving yield. The raw sample exhibited glucose, xylose, arabinose as 37%, 20.1%, and 2.7%, respectively. No regular pattern on the effect of temperature on xylose and arabinose content was observed.
In contrast, long incubation times noticeably changed the chemical, morphological parameters and yields. The glucose and xylose contents were constantly varied up to 140°C for 2 hr incubation time and then abruptly changed when temperatures higher than 160°C were used. The increase in temperature and contact time resulted in the lowering of xylose and arabinose content. It might be because of the removal of extractives and hemicellulose fraction by hydrothermal. The treatment of biomass to a higher temperature (1800C) resulted in low yield (33–38%) and glucose content (60%).
Compared to increasing the reaction temperature, increasing incubation time is more efficient in booming cellulose bioconversion. Similar observations were reported by Yu et al. (2010). As shown in Table 1, the glucose value at 120 ◦C for 2 hr and 140 ◦C for 0 hr were almost equal, indicates both the incubation time and temperature had shown a positive effect on hydrothermally treated material under the relatively low temperature (120 ◦C to 140 ◦C). However, the increase in cellulose biodegradation can be attributed to the destruction of fibre cell walls due to solubilizing or dissolving lignin, breaking the bond of lignin-carbohydrate connection (LCC), and hydrolyzing the amount of hemicellulosic moieties (Mosier et al. 2005). When the temperature and incubation period was increased, the retained glucose percentage in the material also increased, accompanied by the simultaneous degradation of the hemicellulose carbohydrates (Xylose and arabinose) content. A relative increase in lignin content accompanied the process.
Moreover, the relative increase in cellulose decreased amorphous hemicelluloses, and part of lignin resulted in a gradual rise in crystallinity index (CrI). At 120°C, the crystallinity index of the sample and control is almost the same. At any particular temperature with an increase in treatment time, CrI% decreased. Unusual behaviour at 160°C was observed at 160°C, where CrI% decreased with the temperature rise.
Cellulose is a polysaccharide made up of D-glucose units, joined by β-1-4-glucosidic bonds. There are three free hydroxyl groups present in each anhydroglucose unit (AGU) connected by intermolecular hydrogen bonds, leading to forming two domains of high ordered arrangement (crystalline) and random arrangement (amorphous region). The availability of enzymes, solvents and chemicals to the hydroxyl group and overall reactivity of cellulose is altered. The degree of crystallinity indicates the reactivity level of cellulose present in the compounds (Ferreira et al., 2014). The rise of CrI % value shows that a higher amount of cellulose is available for reaction with increasing temperature and treatment time.
Raw dung material has a porous structure that includes lumens and pits, as shown in Fig. 3. When dung fibres were immersed in the water, initially, the fibre surface was socked. Later, due to penetration, fibre became wet. The capillaries are helpful to fill the void space and the amorphous regions of the cell wall. The sorption of water into the fibre is a complex process. In pulp preparation, water diffuses into the amorphous regions of the cellulose matrix and breaks inter-molecular hydrogen bonds between cellulose surfaces. Swelling fibres increases the volume, while the surface area does not expand (Botkova et al., 2013). This swelling effect increases the inter-molecular distance of the cellulose chains. It also facilitates the diffusion of sugars and oligomers of hemicelluloses to the aqueous medium (Rowell, 2016). The ash content was observed between 6 to 8% in controlled and all hydrothermally treated samples and which is equal and less than the other non-wood materials, such as Bagasse (8.02%), rice straw (20.02%), corn stover (7.82%) etc. Generally, the lower ash content indicates maximum pulp yield with good quality of the paper. Chemicals play a crucial role during the production process of the paper. The soda and kraft process is the most commonly used method for treating wood and non-wood related materials. Making paper with the help of chemical is not an economically feasible process for cattle dung fibre. The lignocellulosic fodder material has processed into cattle’s rumen system using bacteria, microbes and get a semi-digested material in the form of fresh dung. One kg of fresh cattle dung gives 10–12% raw fibre depending on their diet and other factors reported by Fasake and Dashora 2020.
Yield plays a vital role in any processing industry. The chemical method reported resulted in about 50% yield, which means that about half of the dung fibre material was lost during the digestion process at 140°C for 2 hr with only 1% NaOH. In the present studies, the hydrothermal treatment resulted in a yield of 82% at the same temperature and time. Though slightly higher cellulose and lower lignin content were reported in chemically treated samples, composite hydrothermal treated samples are also suitable in the making of paper.
Scanning Electron Microscopy (SEM)
SEM is used to study surface characteristics and change in morphology of fibres after treatment. The SEM images of control and treated fibres are shown in Fig. The external surface exhibited a rough structure, and uneven surface morphology with heavy deposition may be of hemicelluloses, lignin, wax, pectin and inorganic components. It can be seen from Figure 4 that the surface of the control sample was smooth, light coloured and non-porous. In comparison, the treated samples were broken, dark-coloured. With increasing temperature and treatment time, the colour intensity and cracks also increased. The surface of the untreated sample exhibited a porous structure, unlike the non-porous system of control. By visual observations, it was found that as the temperature increased, the treated fibres smelled of burnt sugar, which might be due to Maillard Browning products in C5 sugar degradation and fermentation of peptide amino groups (Liavoga et al., 2007). The lignin component could be melted and amalgamating in the form of tiny granular and spherical droplets on the fibre's surface. However, some flattened disks and irregular droplets could be found due to the reshaping process and nonuniform distribution of lignin in cell walls and the simple aggregation of the hydrophobic lignin in a hydrophilic environment (Donohoe et al., 2008).
FTIR
Spectra of the sample treated at different temperatures and time were recorded, as shown in Figure 5. Two dominant features appear in the region between 500-2000 cm-1 and 2200-3700 cm-1. IR spectra between 3000 to 3700 cm−1 corresponds to cellulose hydrogen bonds. C-H stretching occurs between 2800–3000 cm−1. C-H bending, C-H scissoring, and C-H2 wagging in cellulose is reported between 1300 and 1400 cm−1. The region below 1300 cm−1 shows changes in C-O-C bonding pattern, asymmetric stretching from glycosidic linkages, C-O stretching, C-O-C deformation.
The region between 600 to 1500 cm
−1 corresponds to cellulose and hemicelluloses. The sharp peaks in this region show the change in the chemical structure of cellulose and hemicellulose on heating. Characteristic bands at 1510, 1595, 1740 and 1770 cm
−1 are related to the change in lignin content of the pulp on heating to 120
0C for 0 hours. Further heating beyond 120
0C for more and more period brings on a significant shift in lignin structure. The studies suggest heating for 120°C for 0 hours is the most optimum temperature to remove lignin and concentrate the pulp to have higher cellulosic content. Findings are similar to the observations reported in the chemical analysis shown in Table 1. The unusual behaviour of fibres reported for CrI% has observed in FTIR also where peaks crossed each other in the region of 4000-3000 cm
-1.