Characterization, Compositional and Thermogravimetric Analyses of Lignocellulosic Fibers of Syzygium Cumini (L.) Leaf Litter: Investigating the Potential for Biofuel Production

Collection and identification of plant materials

Fallen leaves of Syzygium cumini were gathered and collected from Maiduguri metropolis in January 2024 for this study. Prior to the study, fresh leaves of the plant were taken for identification and authentication at the Department of Biology, Nile University of Nigeria, Abuja, Nigeria. A voucher specimen labelled NUN.H.0005 was prepared and kept in the Herbarium for future reference.

Preparation of raw and lignocellulose fibers of Syzygium cumini

The fallen leaves of Syzygium cumini (5 kg) were thoroughly rinsed under running tap water to remove dirt and other debris before oven drying at 105 °C for 7 h. The resulting product was then subjected to mechanical grinding to obtain raw Syzygium cumini (RSC) fiber [21]. To obtain lignocellulose fiber, RSC fiber (1.5 kg) was subjected to alkali pretreatment by immersing it in 20 L of 0.5 M NaOH (Kermel, 96.00%) for 24 h [22]. This process was done to dewax and render biomass free of all extractives. The resulting product was washed and neutralized with 2% glacial acetic acid to obtain lignocellulose Syzygium cumini (LSC) fiber [23].

Determination of lignocellulose content

The lignocellulose composition, including hemicellulose, lignin, and cellulose, was determined according to standard methods. The resulting samples were weighed and calculated in percentage to determine the yield of each component.

Proximate analysis

The proximate analysis of RSC and LSC fibers was done in triplicates according to standard methods described in AOAC [27]. The nutrients evaluated were moisture, ash, lipids, nitrogen, fiber, proteins and carbohydrates.

Ultimate analysis

The concentrations of carbon (C), hydrogen (H), nitrogen (N), and sulfur (S) in RSC and LSC fiber samples were determined using LECO CHNS 2000 Elemental Analyzer (Leco Corporation, USA). Briefly, the sample (2 g) was weighed in a platinum crucible and placed in the Leibig-Pregle chamber containing 2 M magnesium perchlorate (Mg(ClO₄)₂, Kermel, 98.00%) and 2 M sodium hydroxide (NaOH, Kermel, 96.00%). The sample was burnt to produce carbon dioxide (CO₂) and water (H₂O), which were absorbed by sodium hydroxide and magnesium perchlorate, respectively. The display gave the elemental composition of C, H, N, and S while oxygen (O) was obtained by difference.

Determination of pH value

To determine the pH, 2 g of RSC fiber was soaked in 20 ml of deionized water and the initial pH was recorded using a pH meter (Biobase, China). The same process was done at each reaction stage [9].

Thermogravimetric analysis

Thermograms of RSC and LSC fibers were recorded using a DTG-60H differential thermal analyzer (Shimadzu Corporation, Japan). RSC fiber (25 mg) and LSC fiber (12 mg) were placed separately in a crucible. The experiment was conducted under inert conditions with a constant flow of nitrogen gas at a rate of 100 mL/min. The samples were heated at a rate of 15 °C/min in the temperature range of 20 °C to 600 °C. The resulting thermogravimetric data were compiled into a thermogravimetric curve.

Fourier transform-infrared (FTIR) spectroscopic analysis

Characterization of RSC and LSC fibers was done using a Shimadzu 8400 FTIR spectrometer to determine the structure of the functional groups present in the samples. The unit has an interferometer having a Globar SiC element as the IR source and the time dependence of the IR intensity is measured with a pyroelectric deuterated triglycine sulfate (DTGS) detector. Potassium bromide (250 mg) was added to the sample (1 mg) followed by homogenization using a mortar and pestle and moulded into pellets. The pelleted sample was placed in the sample holder of the instrument to obtain the spectra data. The spectra were displayed as absorbance vs frequency in wavenumbers ranging from 500 to 4000 cm⁻¹ with 32 scans in each case at a resolution of 4 cm⁻¹. The functional groups present were identified using IR correlation charts.

Statistical analysis

Data obtained were analyzed using SPSS  10 with results expressed as mean ± standard deviation (SD) for three observations. Statistical evaluations were done using design expert (Design-Expert^® (ver. 11, Stat-Ease Inc., Minneapolis, USA) in one-way analysis of variance (ANOVA) followed by Tukey’s post hoc test for multiple comparison. Differences between means were considered statistically significant at p < 0.05.

Lignocellulose composition of Syzygium cumini leaves

The concentrations of hemicellulose, lignin, and cellulose in LSC fiber are presented in Table 1. The results indicated that LSC has a significantly high alpha cellulose content of 50.52%, compared to lignin and hemicellulose content (p < 0.05). This finding agrees with that of Bratishko et al. [28], which reported high cellulose content in fallen maple and linden leaves. This is an indication that biomass resources of this nature, including fallen leaves of Syzygium cumini, have a higher potential for biofuel production. The concentration of lignin at 31.53% indicates high fiber content, which is a good source of phenolic compounds. Thermal decomposition of cellulose leads to reformation of D-glucose monomers into furans, paraffinic, esters, and alkyl-alcohols [29]. The decomposition of lignin and hemicellulose leads to the formation of aromatic and phenolic compounds [10]. Findings from this study affirm the economic and industrial viability of LSC fibers for use as biofuel. Isolation and modification of cellulose are constantly being investigated for applications in several industries, such as renewable and sustainable energy, environment, pharmaceuticals, and nanotechnology. The use of an appropriate concentration of alkali or acid to hydrolyze RSC fiber is an important factor in avoiding the dissolution of excessive sugar molecules from lignocellulose fibers [30]. In this study, the extraction of volatile compounds rendered LSC fiber much lighter than RSC fiber, thereby occupying a larger surface area than its weight. However, this may require further verification with larger quantities of samples.

Proximate composition of RSC and LSC fibers

Alkaline pretreatment considerably lowered the concentrations of proteins, lipids and other extractives in LSC fiber, as presented in Table 2. Given the objective of this study to determine the yield of energy compounds, it is imperative that carbohydrates and fiber remain intact. Results show that the concentrations of carbohydrates and fiber are not significantly different and relatively higher than other parameters in both RSC and LSC fibers. Carbohydrates and fiber represent the concentration of total lignocellulose content [13]. The fiber content of 20%–25% is also an indication of the potency of both samples for biofuel production. Carbohydrates and fiber are the major determinants of approximate biofuel yield to be expected from thermal or chemical decomposition of LSC fibers. The relatively high fiber reported is not surprising, given the fibrous nature of Syzygium cumini leaves. The moisture contents of RSC and LSC fibers are not significantly different, and they are compared favorably with fir tree, a common biomass used in Europe, as reported by McKendry [31] and Basu [32]. In a study by Siddique [33], walnut shells, almond shells, babul tree barks, and neem tree barks were used as biomass based on their low moisture content (3.08%–5.69%) and nitrogen content (0.19%–0.88%). Their results are in agreement with our findings, where extraction of RSC with NaOH reduced nitrogen content from 0.54% to 0.19%, which subsequently lowered the protein content from 3.33% to 1.23% in LSC. The lower the nitrogen content, the higher the quality of biofuel produced [34].

Elemental composition of RSC and LSC fibers

The elemental composition of RSC and LSC fibers are presented in Table 3. The carbon and hydrogen concentrations recorded for both samples are higher than the values reported for most biomass wastes [35]. The levels of these two elements indicate the heating value of fiber. Higher carbon and hydrogen contents improve the heating value of samples as opposed to higher oxygen value which is an indicator of high calorific value of the fibers [36]. The oxygen content in the fiber samples (15–16%) is relatively low compared to the value reported for the biomass sample [35]. This is a positive affirmation of the viability of the feedstock for biofuel production.

Thermogravimetric and differential thermal profiles of RSC and LSC fibers

The thermogravimetric and differential thermal profiles of RSC and LSC fibers are presented in Figure 1. The curves clearly depict the decomposition temperatures of both RSC and LSC fibers with respect to a time of 60 mins at 10 °C/min heating rate. However, based on the thermogravimetric data obtained for RSC, the temperature range required to yield moisture content (50 °C – 200 °C) led to 28% weight loss. This implies a higher volatile content than moisture. This finding agrees with the study of Rashidi et al. [37], where Pistacia terebinthus oil was employed as a biomass resource to produce biodiesel. The authors reported that weight loss occurred in the range of 250 °C – 450 °C, leading to the removal of volatile organic compounds including microcrystalline cellulose and stearic acid. This resulted in the loss of 31% of its original weight before reaching stability, which aligns with the findings in the present study.

The weight loss percentage of each temperature peak was calculated with respect to the corresponding initial mass of each sample (Table 4). The decomposition temperature and time gave an idea of carbonization temperature and time range. The weight loss recorded for RSC at 42.71 °C could be attributed to excessive light volatile compounds, including amino acids, and oxides of nitrogen and sulfur. This finding aligns with the study of Onokwai et al. [26] which explored the potential of seven lignocellulose biomass samples, including rice husk and sugarcane straw as sources of biofuel. The authors reported that nitrogen content negatively influenced pyrolysis yield due to low HHV. In our study, light volatile compounds were not extracted from RSC fibers. Therefore, an instant weight drop was expected, as observed in Figure 1a. Such a phenomenon is not depicted on the LSC curve due to reduced weight loss at lower temperature, which accounted for the steady mass recorded between 0–20 mins until it reached 250 °C. This may be attributed to pretreatment with alkali, which dewaxed the sample rendering it free of light volatile compounds. It is expected that the weight loss recorded at 250 °C represents the mass of hemicellulose as it fell within the range of 220 °C – 315 °C while lignin and cellulose decomposition occurs between 315 °C – 400 °C [38]. However, the highest decomposition temperature of RSC fiber which was 597.12 °C at 59 min, is significantly higher than that of LSC fiber which was 466.71 °C at 45 mins. The differential thermal values for both RSC and LSC revealed that the decomposition reactions were exothermic since the curve was above 0 (𝛥T >0).

Temperature profiles provide an anticipated data range and reaction kinetics for thermal degradation of biomass in processes such as gasification, pyrolysis, thermal liquefaction and carbonization [39]. During pyrolysis of RSC fiber, light volatile gases and liquids should be expected at low temperatures of 50 °C – 200 °C, whereas the heavier oils are expected from 200 °C – 550 °C. However, for LSC fibers, pyrolysis fielded heavier fractions of oils from 250 °C – 450 °C.

FTIR spectra of RSC and LSC fibers

The FTIR spectra of RSC and LSC fibers (Figure 2) exhibited more than five distinct peaks, which confirmed the presence of many organic functional groups by forming complex compounds. The spectral peaks at 1023.2 for RSC and 1026.9 for LSC, indicate the presence of an alkyl amine group, cyclic alkene or OH group, depending on the peak intensities at 3250–3670 as observed in Table 5. The broad band seen on RSC spectra is that of OH group when accompanied by peaks at 1600–1300, 1200–1000. The peak at 1606.5 indicates unsaturated C=C bond or aromatics, while the peaks at 2914.8 and 2922.8 indicate the presence of aliphatic compounds, such as asymmetric or symmetric methylene stretch. Previous studies using FTIR have also reported the presence of OH groups and C–O bonds in waste calcium carbonate and magnesium oxide tablets used as catalysts in transesterification reactions to produce biofuel from waste cooking oil. This suggests that LSC may be used as a catalyst to accelerate the production of biofuel from other biomass resources [40, 41].

The peak at 1712.7 is attributed C=O of ketones, esters or aldehydes. The second sharp peak at 3500 indicates an OH belonging to alcoholic or phenolic compounds. The peak at 1159.2 suggests the presence of carboxyl and tertiary alcohol, thus supporting the OH peak at 3500. These variations may be attributed to degradation of RSC by NaOH which confirmed the disintegration of the lignin, hemicellulose and cellulose.

pH value of RSC and LSC fibers

The pH values recorded for RSC and LSC fibers are presented in Table 6. RSC fiber immersed in deionized water had a low pH of 4.3 after 24 h. This is an indication that hydrolysis of organic salts has occurred. However, after alkaline hydrolysis and thorough washing, the pH was 10.3, while the neutralized LSC using 2% acetic acid, resulted in a pH of 7.3. Generally, low pH greatly influences sample properties, including causing hydrolysis of hemicellulose to monomeric sugars while keeping the cellulose and lignin intact without the formation of toxic compounds. High pH results in the dissolution of lignin, while some hemicellulose remains solid [9].