Yield and composition
Table 1 summarizes the yields and compositional data for the five lignin samples. Amount of lignin extracted from the five sources was considerably different. While banyan provided the highest lignin content of about 23.77 %, cornhusk had the lowest lignin content. Bagasse, cornhusks and banyan roots had lignin content of 18.52, 9.14 and 23.77%, respectively. Based on the EDAX analysis, the chemical composition of lignin was found to be predominantly carbon (50-55%) and O (45-50%), except for rice straw. In rice straw, there was about 15% of silica and, consequently, only about 25% carbon (Nylese et al. 2015).
However, further study using ICP-MS study showed presence of several metals and the most abundant are reported in Table 1. The complete ICP-MS data, including metals present in trace amount, have been reported in the supporting information (Table 1S). Calcium was the most prominent among the different metals. It was highest in bagasse and cornhusk and lowest in rice straw. Magnesium ranged from 0.7 to 2.2 mg/g and, again, it was highest in bagasse and lowest in rice straw. Sodium and barium were present more consistently and varied from 0.2 to 0.5 mg/g and 0.01 to 0.02 mg/g, respectively. Among the heavy metals, iron is the most abundant with the highest amount in rice straw (Table 1). Content of Mn was significantly higher in lignin from straw rice than in the other samples. Unlike other metals, chromium was lowest in sugarcane but highest in cornhusk (Table 1S). The amount of trace elements should mainly be dependent on the conditions during growth. Bamboo and banyan were grown naturally whereas bagasse, rice and corn were cultivated. Extent and use of pesticides and composition of the water should also contribute to the variations in the presence of trace elements.
Table 1: Yields and compositional data for the five lignin samples
Sources
|
Life Span
|
Lignin (%)
|
EDX
|
ICP-MS(a)
metals (mg/g)
|
C %
|
O %
|
Ca
|
Mg
|
Al
|
Na
|
K
|
Bagasse (S1)
Bamboo (S2)
Banyan (S3)
Cornhusk (S4)
Rice straw (S5)
|
6-9 months
Unlimited
Unlimited
4-6 months
3-6 months
|
18.52
19.51
23.77
9.14
17.71
|
64.41
63.01
59.92
67.37
58.33
|
34.57
35.49
38.48
31.67
31.74
|
4.7
1.8
2.9
3.3
1.4
|
2.2
1.0
1.4
1.3
0.7
|
0.1
0.08
0.1
0.18
0.66
|
0.5
0.3
0.3
0.2
0.2
|
0.2
0.1
0.18
0.11
0.16
|
- The complete composition of metals in lignin has been reported in the supporting information.
XPS showed that the functional groups on the surface of the lignins were similar (Figure1). A single peak was observed at energy of 37.5 eV. Strongest peak was produced by rice straw and the least intense peak was by bagasse. Compositional analysis showed that the lignin from the five sources had considerable variation, particularly in terms of the trace elements.
Based on the FTIR analysis lignin from all the five sources had similar chemical composition (Figure2). The broad peak between 3410-3460 cm-1 is due to the hydroxyl groups present in the phenolic and aliphatic structures in lignin (Boeriu et al. 2004). Another set of peaks around the 2917 and 2847 cm-1 region are due to the CH stretching and related to fatty acids reasonably present in lignin. Considerably strong peaks are seen at the 1750-1720 cm-1 wavelength, which are from the carbonyl/carboxyl groups. Although the intensity of the peaks at 1515 and 1426 cm-1 varies significantly between the lignin samples, these peaks should be from the C-H vibration and aromatic rings.
Crystalline structure
Lignin has relatively poor crystalline structure and produces a single prominent diffraction peak at about 22°, similar to that seen in cellulose. Lignin from rice straw has a relatively weaker X-ray diffraction pattern compared to the other four samples. This slight shift in the peak of lignin from rice straw was probably due to the changes in the composition (Figure3). It has been suggested that the amount of guaiacyl (G), syringyl (S), and p-hydroxyphenyl (H) units affects the structure and properties of lignin (Vanholme et al. 2010). Although the X-ray structure of lignin suggests that it is considerably amorphous, the high durability against natural degradation and recalcitrance of lignin to various chemicals and treatments should be due to the complex chemical structure.
Thermal degradation behavior
Degradation curves of the lignin samples reveals loss of water at about 100 °C. Main degradation of lignin occurs between 300-400 °C. Broader and smaller peak for the rice straw lignin suggests that the lignin structure could be less cross linked or may contain lower amounts of the C-C interlinked bonds (Hussin et al. 2014). However, no correlation was observed between the age of the plant and thermal degradation of lignin. Lignin from banyan roots had similar degradation rates compared to cornhusk which has considerably shorter life (Figure4). Thermal resistance of the lignins should be adequate for processing into various products.
Morphology
Lignin in bagasse showed a layered structure whereas bamboo lignin was porous as seen from the TEM images (Figure 5). Particulate matter was observed on the surface of all the samples. At 10k magnification, bagasse lignin showed a relatively smooth and non-particulate surface. However, higher magnifications showed a layered structure for bagasse and banyan whereas the other three samples had particles on their surfaces. The size of the particles also appears to be different in each type of lignin. Micro-sized particulate aggregates were also observed when lignin and cellulose were simultaneously extracted from jute fibers (Ahuja et al. 2018). Similar variations in morphology have been reported by other researchers. For example, the distribution of lignin in several plants such as beech and spruce wood has been found to have a lamellar pattern and follow the direction of the cellulose microfibrillar structure (Fromm et al. 2003). Such lamellar pattern was not observed in any of the samples. However, a network structure was revealed in the cornhusk lignin sample whereas rice straw lignin showed aggregates of particles and the bagasse sample did not show any particular shape. The effect of these morphological differences on the properties of lignin and the reason for the different arrangements will be investigated further.