The Structural and Thermal Characteristics of Musa paradisiaca L. Lignin for Carbon Footprint Reduction Applications

Abstract

The need for the use of suitable natural alternative materials to oil-derived carbon-based materials, largely because of carbon IV oxide emissions and the attendant global health and environmental impact, has led to the discovery of lignin, a biomass-derived material, as a precursor for carbon fibre (CF) manufacture and as a reinforcement for biologically derived polymers like polylactide (PLA) with a variety of biomedical and industrial applications. This study investigated the thermal, structural, and compositional properties of lignin extracted from the pseudostem of Musa paradisiaca L. (the plantain tree). Dried and milled plantain pseudostem was pretreated using diethyl ether. Lignin was extracted from the untreated and pretreated pseudostem samples using 5M HCl for 1 h at 200 °C and 250 °C (acid hydrolysis). The results revealed that lignin obtained from pretreated pseudostem at 200 °C and 250 °C possesses superior thermal stability, as shown by the thermogram, with a DTG_max of 429.97 °C and 442.62 °C in contrast to 397.22 °C and 382.53 °C for lignin from untreated pseudostem due to the removal of volatile impurities and unwanted constituents after pretreatment. The FTIR spectrum of the extracted lignin samples shows similar absorption bands, like 1703.4 cm⁻¹ (C=O–conjugated carbonyl group), 1606–1602 cm⁻¹ (C=C stretching–aromatic compounds, benzene ring), 1315 cm⁻¹ (C-O stretching–syringyl units), and 1200.2 cm⁻¹ (C-H stretching, guaiacyl units), with the pretreated biomass having higher transmittance (%) values, indicating increased purity after pretreatment. The results presented above showed that lignin has been successfully extracted and can serve as a potential precursor for the production of carbon fibre, thereby reducing dependence on fossil-fuel-based precursors, with a reduction in carbon dioxide emission pollution.

1. Introduction

Carbon fibres have been used in several industries, such as aerospace, aircraft, automobile, and medical equipment, due to their high strength, modulus, and light weight [1]. Carbon pollution as a result of effluent from automobiles is on the increase due to the heavy weight of the materials being used. Domestic vehicle fuel consumption can be lowered by ensuring a reduction in the weight of the vehicle. Carbon fibre can be used to actualize this aim. The weight of the vehicle can be efficiently reduced by the replacement of the heavyweight metals with lightweight resin-fibre composites and cost-effective materials such as carbon fibre. The addition of lightweight materials into automobiles can reduce the structural steel used in vehicles by approximately two-thirds, with enhanced fuel efficiency and a drastic reduction in gas emissions [2]. Lignin alone can produce enough fibre to replace half of the steel in all domestic transport vehicles [3].

The most common precursor for carbon fibre production is polyacrylonitrile (PAN), which accounts for about 90% of carbon fibre production [4]. Other sources of carbon fibre production include pitch and viscous fibre, which are all fossil-fuel-based, and their use will result in carbon dioxide emissions. The dependence on fossil-fuel-based materials as a percussor in carbon fibre production has been a cornerstone for different industries for a long time. However, over-dependence on fossil fuels has come under serious scrutiny due to its significant contribution to carbon emissions and subsequent environmental pollution. From 1995 to 2015, greenhouse gas emissions from material production increased by 120%, with 11 billion tons of CO₂ equivalent emitted in 2015 [5]. The global community battles with the need for the reduction in carbon pollution and its attendant effects such as global warming and the greenhouse effect. Thus, an urgent need to shift from fossil-fuel-based materials to sustainable and environmentally benign sources of carbon fibre precursors arises. The utilization of bio-based precursors for carbon fibre production presents a promising avenue towards mitigating environmental concerns that are associated with carbon emission. The use of biomass-derived lignin as a precursor for carbon fibre manufacture and reinforcement for biologically derived polymers like polylactide (PLA) has gained interest and research focus. Lignin is a three-dimensional, highly cross-linked macromolecule with phenols of coniferyl, sinapyl, and p-coumaryl alcohols [6] and the second most abundant biopolymer in nature, accounting for ~30% of plants. It is linked to other structural components of the cell wall, cellulose, and hemicelluloses via covalent linkages, forming a lignin–carbohydrate complex (LCC) [7]. Lignin sources include softwoods, hardwoods, barks, and plants such as corn stalks, sugar or bamboo canes, ferns, flax fibre, banana/plantain trees, wheat straw, pine straw, jute, hemp, and cotton. Lignin is one of the top industrial waste products and is produced in large quantities from lignocellulose biomass as an agricultural by-product of wood processing, such as pulp and paper production [8]. The world’s annual lignin production varies between 40 and 50 million tons. Kraft lignin accounts for about 85% of the world’s annual production, while lignosulfonates are a prominent commercially available lignin source with a yield of about 1 million tons [9]. Lignin thus has important hydroxyl groups, which play a crucial role in its physiochemical properties, such as water absorption and reaction with other materials. Lignin, being natural, renewable, biodegradable, biocompatible, and possessing good physical and chemical properties, finds application in areas like reinforcement for polymers, carbon fibre precursors, emulsifiers, dyes, synthetic flooring, sequestering, binding, thermosets, dispersal agents, paints, and fuels.

Various methods are employed for lignin extraction, including chemical, enzymatic, and biological approaches. Chemical methods involve the use of solvents and alkalis to break down and dissolve lignin, while enzymatic processes leverage on enzymes to selectively degrade lignin bonds. The biological method often utilizes fungi or bacteria to metabolize lignin. Efficient lignin extraction not only improves the quality of lignin but also allows for the utilization of extracted lignin as a valuable feedstock for the production of renewable chemicals and bio-based materials, contributing to sustainable and environmentally friendly industrial processes. In recent times, Jingjing [10] isolated lignin from softwood and hardwood using Kraft and Klason isolation methods, with the yields slightly increasing with decreasing pH values. The yields in softwood were higher than in hardwood, and the lignin obtained from the different isolation methods showed different functional groups and dissimilar chemical structures. Watkins et al. [6] extracted lignin from non-wood cellulosic biomass, namely, wheat straw, pine straw, alfalfa, kenaf, and flax fibre, using formic acid treatment followed by peroxy-formic acid treatment. The alfalfa lignin was found to have the highest lignin yield, while that from wheat straw showed superior thermal stability, which was followed closely by lignin from flax fibre. In a similar study, Ramirez and Enríquez [11] utilized the lignin of black liquor generated from paper production process (Kraft lignin) via banana stems for the adsorption of lead ions (Pb²⁺) with a recovery efficiency of 82%. Earlier, Oliveira et al. [12] had extracted dioxane lignin samples from two fractions of banana plants, namely, Dwarf Cavendish floral stalk (DLFS) and rachis (DLR). The DLFS showed almost twice the abundance of syringyl (H) and guaiacyl (G) units and almost half the abundance of syringaresinol (S) units when compared to DLR. Both lignin samples are structurally associated with suberin-like components in cell wall tissues. Recently, Geies et al. [13] isolated lignin from sweet sorghum, rice straw, and sugarcane bagasse using an in situ sodium hydroxide–sodium bisulphate technique. The morphology of the extracted lignin samples consisted of a sponge-like structure, except for sugarcane bagasse lignin, which had a rock-like structure. Sugarcane bagasse lignin was the most thermally stable; however, all the lignin samples exhibited a low percentage of biochar, and less than 10% was not volatilized. Li et al. [14] fabricated lignin from eight sorghum samples with diverse characteristics into carbon fibres (CFs). XRD and Raman’s spectroscopy revealed that a higher lignin uniformity enhanced the CFs’ microstructures, and this was found to define CFs’ mechanical performance with a new structure–property relationship. In a much earlier study, Lallave et al. [15] had electrospun a solution of lignin, ethanol, and glycerol in a coaxial or triaxial setup to obtain fibres that were thermostabilized by heating at a rate of 0.25 °C/min to 200 °C, which was maintained isothermally for 24 h and followed by carbonization at 900 °C. The diameters of the produced fibres were found to range from 400 nm to 2 mm.

While it is recognized that several studies have been conducted on the extraction of lignin from different parts of the banana plant (Musa acuminate), little work has been found in the literature on lignin extraction from plantain plant pseudostem (Musa paradisiaca L.) and its characterizations. Thus, this work presents the extraction and characterization of Musa paradisiaca L. pseudostem lignin as a possible precursor for the manufacturing of lignin carbon fibres and polymer matrix composite fibre mats, which reduces dependence on fossil fuels and achieves a reduction in the heavyweight materials used in an automobile, thus reducing carbon pollution.

2. Methodology

2.1. Raw Materials

The plantain pseudostem was obtained from a plantain plantation at the University of Lagos, Lagos, Nigeria. The pseudostem was sliced/cut into circular shapes. The circular shape pseudostem (trunk) was washed in water to remove superficial dirt. It was then cut into smaller pieces and sun-dried for 8 days at an average daily temperature of 33 °C. The dried sample was milled to 150 μm using a ball milling machine at the Federal Institute of Industrial Research Oshodi (FIRO), Lagos, Nigeria.

2.2. Di-ethyl Ether PreTreatment

About 500 g of the as-milled plantain pseudostem sample was mixed with a 30% v/v diethyl ether solution (3466 mL), prepared by adding 800 mL diethyl ether into 2666 mL of distilled water. The solution was stirred at 30 min intervals for 4 h, and the mixture was decanted. The residue was washed repeatedly for two weeks with distilled water until a stable pH of 4.85 was attained. The filtered sample was dried for 12 h over two days to expel moisture using a laboratory dry oven at 110 °C. On completion, 360 g of Di-ethyl Ether pretreated sample was recovered.

2.3. Lignin Extraction

About 20 g of lignin was extracted from 80 g of the pretreated and untreated samples by acid hydrolysis using a 5M HCl solution at 200 °C on a magnetic stirrer hot plate for 1 h and allowed to cool to room temperature. The procedure was repeated for both the pretreated and untreated samples with 5M HCl at 250 °C for 1 h, producing a yield of ~21 g. The extracted lignin samples were decanted, repeatedly washed with distilled water to remove the residual acid solution, filtered, and dried in the laboratory dry oven at 110 °C for 6 h to expel all moisture contents.

2.4. Functional Group Determination Using Fourier Transform Infrared Spectroscopy (FTIR)

An Agilent Cary 630 spectrometer (USA) was used to determine the functional groups which were present in samples. Its operation is based on the interaction of infrared radiation (4000–500 nm wavelength) with the sample, as each molecule absorbs energy characteristic of its specific intramolecular bonds.

2.5. Analysis of Phases and Crystal Structure Using X-ray Diffraction (XRD)

The EPYERN diffractometer model XRD-600 at the National Geosciences Research Laboratory, Kaduna, Nigeria, was used to analyse the phases and crystal structure present in the samples. The instrument exposes the sample to incoming X-rays, and the intensity and scattering angles of the X-rays that exit the sample are measured. The sample is finely ground and homogenized before analysis. The crystallinity index was calculated from the height ratio in the diffractogram using Equation (1).

$$\mathrm{Crystallinity}=\frac{IntensityoftheCrystallinePeak}{(IntensityoftheCrystallinepeak+IntensityoftheAmorphpouspeak)}\times 100\%$$

(1)

2.6. Thermal Behaviours of the Samples Using Differential Scanning Calorimetry (DSC)

A Schimadzu DSC-50 (Kyoto, Japan) instrument at the National Geosciences Research Laboratory, Kaduna, Nigeria, was used to characterize the thermal behaviour of the samples. The device measures the temperatures of the sample and that of a reference material while adjusting the sample’s temperature. The calorific value is then calculated using this temperature difference.

2.7. Thermal Stability Using Thermogravimetric Analysis (TGA)

The Thermogravimetric Analyzer TGA Q500 instrument at the National Geosciences Research Laboratory, Kaduna, Nigeria, was used to characterize the thermal stability of the samples. The thermogravimetric analyser measures the amount and rate of weight change in the sample as it volatilizes or decomposes under controlled temperature, time, and atmospheric conditions.

2.8. Elemental Composition Determination Using X-ray Fluorescence (XRF)

An XRF instrument at Umaru Musa Yar’adua University, Katsina, Nigeria, was used to determine the elemental composition of the samples. The XRF analyser determines the chemistry of a sample by detecting the fluorescence (or secondary) X-ray that the sample emits when it is activated by a primary X-ray source.

3. Results and Discussion

3.1. Sample Functional Group Determination

The FTIR spectra for both the untreated and pretreated plantain pseudostem share similar absorption band characteristics of specific functional groups, bonds, and compounds (Figure 1). However, the pretreated plantain pseudostem shows a higher transmittance (%), which is an indication of the removal of impurities, as more infrared radiation can pass through the sample after pretreatment with diethyl ether. The major peaks and their corresponding functional groups/compounds observed in the untreated and pretreated plantain pseudostem are 3298–3269 cm⁻¹ (O-H stretching–hydroxyl group), which is similar to the peak identified by [16] and 2920–2907 cm⁻¹ (C-H stretching–methoxyl group). The peak at 1728 cm⁻¹ is attributed to a C=O–Carbonyl group, 623–1588 cm⁻¹ (C=C stretching—aromatic compounds, benzene ring), 1315–1309 cm⁻¹ (C-O stretching–syringyl units), 1200 cm⁻¹ (C-H stretching, guaiacyl units), and 1022–1000 cm⁻¹ (C-O deformation, secondary alcohols, and aliphatic esters). These functional groups have also been identified and reported by Watkin [6] and Cecci [17] as being present in banana pseudostem fibres. The FTIR spectra of the lignin samples also showed very similar absorption bands (Figure 2, Figure 3, Figure 4 and Figure 5). In all lignin samples, the following absorption peaks and their corresponding functional groups or compounds were observed: 3336–3291 cm⁻¹ (O-H stretching–hydroxyl group), 2903–2899 cm⁻¹ (C-H stretching–methoxyl group), 1703.4 cm⁻¹ (C=O–conjugated carbonyl group), 1606–1602 cm⁻¹ (C=C stretching–aromatic compounds, benzene ring), 1315 cm⁻¹ (C-O stretching–syringyl units), 1200.2 cm⁻¹ (C-H stretching, guaiacyl units), and 1054–1028 cm⁻¹ (C-O deformation–secondary alcohols and aliphatic esters). These observed functional groups for the lignin samples were also identified earlier and reported by [6,13,18] for lignin extracted from different biomass sources.

3.2. Analysis of Phases and Crystal Structures That Were Present in the Study Samples

The XRD pattern (Figure 6) shows that both the untreated and pretreated plantain pseudostem have similar structures, characterized by broad humps of hemicellulose and lignin and a few peaks at 2θ = 14.9, 22.2, and 30.1. Vardhini et al. [19] had earlier reported similar 2θ peak values of ~22.5 and ~15 for treated and untreated banana fibres. Pereira et al. [20] also reported 2θ peak values of ~21, ~33, and ~47 for different parts of the banana. Darmawan et al. [21] assigned a peak of around 22.6 for three different lignocellulosic biomasses to the crystal structure of cellulose. The crystallinity index of the untreated and pretreated plantain pseudostem was calculated as 66.36% and 66.52% using Equation (1). [22] had earlier reported the degree of crystallinity of cellulose in some varieties of biomass to range from 43% to 65%. These values, however, deviate from the 35.3% crystallinity index reported later by Pereira [20] for the outer sheaths of the banana pseudostem. The XRD patterns (Figure 7 and Figure 8) of lignin from both untreated and pretreated pseudostem samples likewise share similar characteristics of broad humps (amorphous phases of hemicellulose and lignin) and a few similar intensity peaks at 2θ = 16 and 22.3 (cellulose phase) and minor peaks at 26.8 and 34.8, which indicates that a cellulose phase exists within the broad region. Wang et al. [23] had earlier in their study assigned the peak at 22.6 to the cellulose crystalline plane. There was no significant effect of the pretreatment and lignin extraction temperature on the crystallographic nature of the lignin samples obtained. The degree of crystallinity of the lignin samples was calculated using Equation (1) with lignin from untreated pseudostem (5M, 200 °C, 1 h) = 67%; lignin from pretreated pseudostem (5M, 200 °C, 1 h) = 68.80%; lignin from untreated pseudostem (5M, 250 °C, 1 h) = 70.36%; and lignin from pretreated pseudostem (5M, 250 °C, 1 h) = 70.57%.

3.3. Thermal Behaviours of the Sample Using Differential Scanning Calorimetry (DSC)

The DSC curve for the untreated plantain (Figure 9) pseudostem shows a glass transition temperature (T_g) of 65.08 °C in contrast to that of the pretreated plantain pseudostem, which is not identifiable from the curve either due to a low T_g or due to an increase in crystallinity of the lignin sample. This temperature is important in determining the service temperature requirements, thermal and mechanical shock requirements, flexibility requirements, and adhesive, cohesive, and shear strength properties. The pretreatment also promoted an increase in the melting temperature of the plantain pseudostem from 91.03 °C to 98.79 °C. [20], which is attributed to a similar endothermic peak at ~100 °C to water evaporation from the different fractions of the banana fibres. The DSC curves for the lignin samples (Figure 10 and Figure 11) show an increased glass transition temperature (T_g) for the lignin samples from pretreated pseudostem, with the T_g increasing slightly (101.59 °C) over the untreated biomass lignin (97.91 °C). There is an observed increase in the T_g and T_m of lignin from pretreated pseudostem with an increase in lignin extraction temperature, while a decrease in T_m of lignin from untreated plantain pseudostem with an increasing extraction temperature is observed. [24] reported a T_g value of 108 °C for Alcell lignin obtained from ethanol pulping of hardwood, while [25] reported 148 °C and 137 °C T_g for indulin AT and alkali lignin, respectively. The pretreatment improved the melting temperature (T_m) of the lignin from the pretreated pseudostem (98.79–267.12 °C) due to the removal of hemicelluloses, LCC, and extractives.

3.4. Thermal Stability of the Study Samples

The TGA and DTG curves (Figure 12 and Figure 13) of the untreated and pretreated plantain pseudostem are characteristics of a pyrolysis reaction involving depolymerization, decarboxylation, dehydration, and cracking in the samples [26]. The TGA shows a three-step decomposition process [27], as shown in

Table 1. Thermal degradation temperature ranges for lignin samples.

S/N	Lignin Sample	1st Stage (°C)	2nd Stage (°C)	3rd Stage (°C)	DTGmax (°C)
1.	Untreated plantain pseudostem	64.96–284.18	284.18–462.69	462.69–800	386.18
2.	Pretreated plantain pseudostem	59.54–306.02	306.02–464.86	464.86–800	384.14
3.	Lignin from untreated pseudostem (5M, 200 °C, 1 h)	65.53–318.77	318.77–477.32	477.32–800	397.22
4.	Lignin from pretreated pseudostem (5M, 200 °C, 1 h)	60.10–320.65	320.65–533.75	533.75–800	429.97
5.	Lignin from untreated pseudostem (5M, 250 °C, 1 h)	58.31–304.24	304.24–477.32	477.32–800	382.53
6.	Lignin from pretreated pseudostem (5M, 250 °C, 1 h)	58.31–313.34	313.34–535.54	535.54–800	442.62

, commencing after an initial weight loss due to the evaporation of structural moisture and volatile matter. Decomposition starts at 284.18 °C and 306.02 °C and ends at 462.46 °C and 484.46 °C with a calculated weight loss of 82.33% and 84.09% at the end of decomposition for the untreated and pretreated pseudostem, respectively. The DTG curves also provide the maximum decomposition temperature DTG_max as 386.18 °C and 384.4 °C for the untreated and pretreated pseudostem. Abdullah et al. [26] reported the onset and end set temperature of pyrolysis as 150–430 °C and with a DTG_max of 325 °C for banana pseudostem, while Pereira et al. [20] earlier gave an onset and end set pyrolysis temperature of 247.80–459 °C and a DTG_max of 280 °C for banana pseudostem. The TGA and DTG curves of the lignin from untreated and pretreated pseudostem also showed three-stage endothermic degradation with similar characteristics as the untreated and pretreated plantain pseudostem (Figure 14 and Figure 15), which agrees with results reported by Amit et al. [25] for indulin AT and alkali-treated lignin. A drying stage preceded the first stage of degradation, with the breaking down of side chains and the release of volatiles like CO, CO₂, and CH₄ [28]. The second stage involves a rapid pyrolytic degradation of the lignin samples, while the third stage involves the decomposition of some aromatic rings [27]. Lignin samples from pretreated pseudostem (5M, 200 °C, 1 h; 5M, 250 °C, 1 h) show a wider temperature range between the start and end of decomposition compared to their untreated pseudostem counterparts. Lignin from untreated pseudostem (5M, 250 °C, 1 h), however, exhibited the highest weight loss of 85.80%, which is closely followed by the lignin from pretreated pseudostem samples due to the removal of unwanted extracts and inorganic compounds. Both lignin samples from pretreated pseudostem samples show significantly higher DTG_max of 429.87 °C and 442.62 °C (Table 1), indicating better thermal stability, while the DTG_max of the lignin samples from untreated pseudostem was 397.12 °C and 382.53 °C. The DTG_max values are seen to increase with an increasing lignin extraction temperature for lignin from pretreated pseudostem, but a decreased DTG_max value is observed for lignin from untreated plantain pseudostem. These are an improvement over results from earlier studies by Watkins et al. [6] that reported a DTG_max between 331.87 °C and 336.11 °C for lignin extracted from four different biomasses and Amit et al. [25] with DTG_max of 395 °C and 355 °C and close to that of Geies et al. [13], with DTG_max of 494.76 °C, 450.66 °C, and 464.33 °C for lignin extracted from sugarcane bagasse, sweet sorghum, and rice straw, respectively.

3.5. Samples’ Elemental Composition Determination

Table 2. X-ray fluorescence elemental composition of untreated plantain pseudostem.

S/N
1	Element	Fe	Cu	Ni	Zn	Al	Mg	Na	S	P	Ca	K	Mn	Rb	Sr
2	Concentration (%)	0.079	0.003	0.009	0.009	0.030	0.015	0.011	0.066	0.080	0.676	4.435	0.131	0.004	0.003
		Br	Cl	Cr	V	Mo	W	Bi	Ba	Pb	Sn	Si	As	Nb	Ta	Ag
		0.001	0.324	0.001	0.00	0.00	0.038	0.100	0.001	0.674	0.010	[0.18]	0.00	0.056	0.007	0.00

,

Table 3. X-ray fluorescence elemental composition of pretreated plantain pseudostem.

S/N
1	Element	Fe	Cu	Ni	Zn	Al	Mg	Na	S	P	Ca	K	Mn	Rb	Sr
2	Concentration (%)	0.206	0.002	0.036	0.010	0.079	0.023	0.206	0.125	0.069	2.577	0.150	0.168	0.002	0.006
		Br	Cl	Cr	V	Mo	W	Bi	Ba	Pb	Sn	Si	As	Nb	Ta	Ag
		0.005	0.502	0.002	−0.007	0.00	0.058	0.143	1.318	0.748	0.00	0.00	−0.002	0.117	0.020	0.00

,

Table 4. X-ray fluorescence elemental compositional analysis of lignin from untreated pseudostem (5M, 200 °C, 1 h).

S/N
1	Element	Fe2O3	SiO2	Al2O3	MgO	P2O5	SO3	TiO2	MnO	CaO	K2O	CuO	ZnO	Cr2O3	V2O5
2	Concentration (%)	0.396	11.507	1.987	0.88	0.254	0.357	0.107	0.107	1.180	0.785	0.006	0.017	0.017	0.003
		As2O3	PbO	Rb2O	Ga2O3	NiO	Cl	ZrO2	Ta2O5	WO3	Br	CeO2	ThO2	Y2O3	Nb2O5	I	BaO
		0.006	0.062	0.003	0.002	0.002	0.272	−0.020	0.005	0.011	0.006	0	0	0.002	0.001	0	0.020
		Ag2O	SnO2	U3O8	Bi2O3	GeO2	Cs2O	Sb2O3	La2O3	CdO	Eu2O3	Gd2O3	Lu2O3	Co3O4
		0.001	0	0	0	0.001	0	0	0	0	0	0	0	0

,

Table 5. X-ray fluorescence elemental compositional analysis of lignin from pretreated pseudostem (5M, 200 °C, 1 h).

S/N
1	Element	Fe2O3	SiO2	Al2O3	MgO	P2O5	SO3	TiO2	MnO	CaO	K2O	CuO	ZnO	Cr2O3	V2O5
2	Concentration (%)	0.243	10.132	2.122	0	0.361	0.414	0.092	0.056	1.022	0.941	0.004	0.006	0.014	0.002
		As2O3	PbO	Rb2O	Ga2O3	NiO	Cl	ZrO2	Ta2O5	WO3	Br	CeO2	ThO2	Y2O3	Nb2O5	I	BaO
		0.001	0.010	0.002	0.001	0.001	0.259	−0.03	0.002	0.001	0.002	0.274	0	0	−0.030	0.001	0.020
		Ag2O	SnO2	U3O8	Bi2O3	GeO2	Cs2O	Sb2O3	La2O3	CdO	Eu2O3	Gd2O3	Lu2O3	Co3O4
		0.001	0.222	0	0.035	0.001	0	0	0	0	0	0	0	0

,

Table 6. X-ray fluorescence elemental compositional analysis of lignin from untreated pseudostem (5M, 250 °C, 1 h).

S/N
1	Element	Fe2O3	SiO2	Al2O3	MgO	P2O5	SO3	TiO2	MnO	CaO	K2O	CuO	ZnO	Cr2O3	V2O5
2	Concentration (%)	0.409	12.072	2.205	0.37	0.106	0.281	0.095	0.077	0.682	0.694	0.003	0.008	0.032	0.001
		As2O3	PbO	Rb2O	Ga2O3	NiO	Cl	ZrO2	Ta2O5	WO3	Br	CeO2	ThO2	Y2O3	Nb2O5	I	BaO
		0.002	0.010	0.002	0.001	0.001	0.488	−0.03	0.001	0.001	0.004	0	0	0	−0.030	0.001	0.020
		Ag2O	SnO2	U3O8	Bi2O3	GeO2	Cs2O	Sb2O3	La2O3	CdO	Eu2O3	Gd2O3	Lu2O3	Co3O4
		0.001	0	0	0.035	0.001	0	0	0	0	0	0	0	0

and

Table 7. X-ray fluorescence elemental compositional analysis of lignin from pretreated pseudostem (5M, 200 °C, 1 h).

S/N
1	Element	Fe2O3	SiO2	Al2O3	MgO	P2O5	SO3	TiO2	MnO	CaO	K2O	CuO	ZnO	Cr2O3	V2O5
2	Concentration (%)	0.597	11.069	3.005	0	1.276	1.052	0.120	0.131	0.673	2.078	0.007	0.025	0.020	0.004
		As2O3	PbO	Rb2O	Ga2O3	NiO	Cl	ZrO2	Ta2O5	WO3	Br	CeO2	ThO2	Y2O3	Nb2O5	I	BaO
		0.007	0.099	0.002	0.003	0.001	0.762	−0.02	0.003	0.011	0.001	0.277	0	0.002	−0.030	0	0.024
		Ag2O	SnO2	U3O8	Bi2O3	GeO2	Cs2O	Sb2O3	La2O3	CdO	Eu2O3	Gd2O3	Lu2O3	Co3O4
		0.001	0.209	0	0	0	0	0	0	0	0	0	0	0

show the X-ray fluorescence elemental compositional results for (a) untreated plantain pseudostem, (b) pretreated plantain pseudostem, (c) lignin from untreated pseudostem (5M, 200 °C, 1 h), (d) lignin from pretreated pseudostem (5M, 200 °C, 1 h), (e) lignin from untreated pseudostem (5M, 250 °C, 1 h), and (f) lignin from pretreated pseudostem (5M, 200 °C, 1 h). There is an observed increase in the concentrations of Fe, Na, and Ca, as well as a decrease in the concentration of K after the pretreatment process. The concentrations of elements and compounds are observed to be consistently present in all lignin samples, only slightly varying in concentration between the lignin from untreated and pretreated pseudostem samples.

3.6. Comparison of the Properties of Lignin from Plantain Pseudostem and Fossil-Based Carbon Fibre Precursors

Table 8. Lignin properties from plantain pseudostem compared with those from fossil-based precursors.

Scheme	Property	Polyacrylonitrile (Pan)	Pseudostem Lignin
1.	Availability	Fossil-based [29]	Agrowaste-derived
2.	Thermal Stability	Decomposes between 296 and 434 °C. [30]	Decomposes between 313.34 and 535.54 °C
3.	Melting Temperature	Above 300 °C [31]	Around 225–267 °C
4.	Crystal Structure	Interconnected carbon chain with a rigid structure [32]	Mostly amorphous
5.	Chemical Structure	Synthetic polymer with a linear chain [33]	Natural polymer with a complex structure-based source

shows the property comparison between lignin from plantain pseudostem and those from fossil-based precursors.

4. Conclusions

This study investigated the properties of lignin extracted from plantain pseudostem, with a focus on the effects of pretreatment on its thermal, structural, and compositional characteristics. The results indicated that pretreatment using diethyl ether had no significant effect on the XRD pattern of the obtained lignin but increased the glass transition and melting temperatures of the lignin extracted from plantain pseudostem. Furthermore, the functional group analysis revealed the functional group of lignin, which is similar to the one reported by earlier studies. The thermal stability and purity of the lignin obtained from pretreated pseudostem using diethyl ether were superior to the untreated ones. These findings suggest that lignin extracted from plantain pseudostem could be used as a non-fossil-based precursor for the manufacture of lignin carbon fibre and as a reinforcement in polymer matrix composites for carbon dioxide emission reduction.