Analysis and Characterization of Lignocellulosic Biomass Extracted from Selected Agricultural Wastes

Abstract

Agricultural wastes are a viable source of lignocellulosic biomass for biofuel production. Precise biomass characterization is required to assess the new potential lignocellulose biosource. This study assesses the characterization and compositional analysis of three agricultural wastes (melon husk, moringa pod, and mango endocarp) obtained from Sheda Village, Federal Capital Territory of Abuja, Nigeria. Gravimetric method was used to assess the samples’ lignocellulose biomass composition and they were further characterized using FTIR. The findings indicated that hemicellulose content ranged from 19.38 to 27.74%, with melon husk having the highest concentration. The highest concentration of cellulose was found in the mango endocarp (45.84%). Melon husk possesses the highest lignin content (29.87%). FTIR spectroscopic examination revealed a broad spectrum around 3422.85 cm−1, which represented bonded -OH groups. A peak observed at around 1637 cm−1 is attributed to the stretching of C=C bonds in conjugated carboxylic acids. Peaks, obtained at 1205.72, 1204.50, and 1206.24 cm−1, reveal the vibrations of the aliphatic chains, ∙CH2- and ∙CH3, which constitute the fundamental structure of cellulose material. The findings demonstrate that the samples contain a sizable amount of lignocellulosic biomass. Therefore, wastes generated from agricultural wastes may be useful in the production of biofuel.

1. Introduction

The European Union must reduce its overall greenhouse gas emissions by 8% from 1990 levels between 2008 and 2012 [1]. This can be made possible by substituting common fossil fuels with low-carbon fuels for energy generation. Wastes of various origins, such as industrial or agricultural wastes, are usually disposed via incineration. This act can be replaced by utilizing them as sources of energy. The cost of landfill, environmental pollution/degradation, and the diminishing space available for landfilling trash, particularly in densely populated places, has become a source of concern lately. This in turn has become major driver for waste-to-energy and it is receiving more and more attention. Due to emerging issues from lack of land for landfills, CO₂ and CH₄ emissions, potential groundwater pollution, and the need for more recycling and reuse of trash, policy and regulation of waste disposal have been a front-burner issue in many nations. As stated in the EU legislation that is now being prepared, landfilling of garbage that can serve as a source of renewable energy will be prohibited. Organic materials derived from plants are commonly referred to biomass. The chemical energy in biomass can be extracted when it undergoes combustion and it is transformed into heat or electricity. Green biomass refers to biomass resources that are sustainably managed as they have become renewable energy sources and do not contribute to global warming. Sustainable management of plants has helped in reabsorbing carbon dioxide released during the combustion of other biomass thus leaving no net carbon dioxide emissions in the atmosphere. Both valuable compounds and energy (biofuel) can be obtained from biomass [2]. Fossil fuels, in addition to increasing the amount of carbon, sulfur, nitrogen, and other air pollutants by thousands of tons, also cause environmental hazards. These adverse effects serve as a pointer to the need to find creative and affordable alternative fuels. A reliable alternative consists of renewable energy sources. Many researchers across the globe have been conducting extensive studies on these sources to examine their uses [3, 4].

Renewable fuels are processed from biomass, which consists of organic waste products obtained from sources such as industry, forestry, and agriculture. Agro-based biomass is mostly biomass obtained from considerable organic wastes that are left in fields after crops have been harvested. Although some of these wastes are used as animal feed, the majority lie on the fields and are usually burnt or left as a nuisance in the environment. Crop wastes, such as wheat and paddy straw, have been utilized as raw materials in biomass-based power plants for the production of electricity. Biomass such as melon seed husk, mango endocarp, and moringa pod are just a few of the abundant agricultural by-products obtainable in an agrarian nation like Nigeria. Worthy of note is that Nigeria ranks as Number 10 among the world’s top producers of mangoes [5]. About 35 to 60% of the mango fruit are being wasted after processing [6]. Also, more than a million tons of mango seeds are produced yearly as waste and are currently not adding value to the nation’s economy [7]. A typical moringa tree (15–20 feet tall) can produce thousands of seed pods, yielding an incalculable number of moringa seeds [8]. The majority of agricultural wastes lack commercially viable conversion technology as they are in most cases set ablaze on the field. A significance of agricultural wastes is that they serve as one of the alternatives to grains for the generation of ethanol or gasoline without compromising the security of the food supply. Owing to their quantity and renewability, agricultural wastes are a major raw material in the commercial manufacture of bioethanol [9]. The lignocellulose content of agricultural wastes has been acknowledged as a significant and promising source of biofuels and other products of immense value [10]. The effect of global warming can be significantly alleviated by reducing greenhouse gas emissions through the manufacture of biofuels using lignocellulosic materials [11]. The majority of the global biomass supply comes from lignocelluloses. Lignocellulose can be obtained in significant quantities from municipal solid wastes, crop leftovers, forest residues, and specific energy crops [12, 13]. The components of plants known as extractives are those found outside of the cell wall; they are made up of low or medium molecular weight materials and may be readily extracted using particular solvents such as acetone, toluene, alcohol, and water [14, 15, 16]. Studies have shown that lignocelluloses are converted to biofuel at a greater rate when cellulose’s crystallinity is reduced significantly [17]. Four main steps are involved in the synthesis of biofuel (e.g., ethanol) from lignocellulose biomass: These are pretreatment, hydrolysis, fermentation, and product purification. Monomeric sugars are produced when carbohydrate polymers undergo hydrolysis, and they (monomeric sugars) are then fermented to produce ethanol. To make it simpler for the enzyme to convert carbohydrate polymers into fermentable sugars, the pretreatment step lyses the lignin seal and disrupts the crystalline portion of the cellulose [12]. The amount of ethanol output (L/mg) depends on the precise assessment of the lignocellulose content in the biomass [18]. A major factor that determines how effectively the processes can convert lignocellulosic biomass into ethanol is the composition of the lignocellulose. To the best of the authors’ knowledge, little has been done to determine the lignocellulose biomass content of mango endocarp, moringa pods, and melon husks. This study is therefore aimed at characterizing and analyzing the lignocellulose biomass of these selected agricultural wastes in order to determine their morphological, chemical, and compositional characteristics.

2. Materials and method

The materials, mango endocarp, melon husk, and moringa seed pod were sourced for at Kwali Area Council, an agrarian community of the Federal Capital Territory of Nigeria. Each of the biomass materials was subjected to compositional analysis using the gravimetric method reported [19].

2.1 Extractives

About 2.5 g of each of the dried samples was extracted with 150 mL acetone using a Soxhlet extractor setup. After extraction, the samples were air-dried at room temperature. The extracted samples were put in a convection oven at 105°C to achieve a constant weight. The %(w/w) of the extractives content was evaluated as the difference in weight between the raw extractive-laden biomass and extractive-free biomass [20, 21, 22].

2.2 Hemicellulose

About 150 mL of 0.5 M NaOH was added to 1 g of extractives-free dried biomass in a 250-ml Erlenmeyer flask. The mixture was boiled for 3.5 h, allowed to cool, filtered, and washed with distilled water until a neutral pH was obtained. The residue was dried to a constant weight at 105°C in a convection oven. The difference between the sample weight before and after this treatment is the hemicellulose content (%w/w) of dry biomass [13, 20, 21, 22].

2.3 Lignin

About 3 mL of 72% H₂SO₄ was added to 0.3 g of dried extractive-free samples. The mixture was kept at room temperature for 2 h while carefully shaking at intervals of 30 minutes to allow for initial hydrolysis. The second step of hydrolysis was carried out by adding 84 mL of distilled water and the resulting mixture was placed in an autoclave for 1 h at 121°C. The slurry was then cooled at room temperature. Hydrolysates were filtered through a vacuum using a filtering crucible. The residue is dried at 105°C and then incinerated in a muffle furnace at 575°C. The lignin content was calculated as the summation of acid-insoluble lignin (AIL) and acid-soluble lignin (ASL). The acid-soluble lignin was quantified by UV-visible spectroscopy at 320 nm, and the acid-insoluble lignin was determined by gravimetric analysis [23].

$$\text{Lignin}=ASL+AIL$$E1

2.4 Cellulose

The cellulose content (%w/w) was calculated by difference.

2.5 Fourier transform infrared (FT-IR) spectroscopy

Nicolet IS 5 Thermo Fisher Scientific, USA FTIR spectrophotometer was used to analyze the surface chemistry of the agricultural waste samples. The dried waste samples were analyzed by scanning between wavelengths 400 and 4000 cm^–1. The distinctive functional groups on the surface of the melon seed shell, moringa seed pod, and mango seed endocarp can be observed from the samples’ FTIR spectra.

3. Result and discussion

3.1 Composition of lignocellulose biomass

Figure 1 displays the lignocellulose biomass content of the selected agricultural wastes. Among them, mango endocarp was shown to possess the highest amount of cellulose (45.84%) while hemicellulose quantity was the highest in moringa pod. Generally, the amounts of lignin in the samples do not differ noticeably. As indicated in Figure 2, the main components of lignocellulosic biomass feedstock are cellulose, hemicelluloses, lignin, extractives, and ash. The results of the investigation align with findings from previous works, as indicated in Table 1.

Figure 1.

Analysis of the raw lignocellulose content of the samples (%).

Figure 2.

Composition of lignocellulosic biomass [26].

	Cellulose	Hemicellulose	Lignin	Ash	Extractives	References
Sugarcane bagasse	35–49	16–33	11–25	4–9	2	[19]
Siam weed	4.8 ± 0.9	40.2 ± 2.3	29.9 ± 0.7	23.2 ± 5.3	0.9 ± 3.1	[19]
Rice straw	32–47	19–27	5–24	12.4		[24]

Table 1.

Reported compositional analysis of raw lignocelluloses of sugarcane bagasse, Siam weed, and Rice straw (%w/w).

The availability of various lignocellulose components of biomass and the ease of their usage in the production of biofuel is mostly determined by their quantity in the source. Among, the lignocellulose biomass, the cellulose and hemicellulose components are the most abundant renewable organic resource [25, 26]. The high cellulose content found in the mango seed endocarp and the considerable hemicellulose content of the moringa seed pod from the present investigation are an indication of their potential to be useful in the production of biofuel. In the cell walls of plants, the bonds holding cellulose, hemicellulose, and lignin together differ. Molecules of cellulose and hemicellulose, or lignin are generally held together by hydrogen bonds. In addition to the hydrogen bond, lignin and hemicellulose also chemically bind together, causing the lignin to always contain a tiny amount of carbohydrates when it is separated from natural lignocelluloses [25]. This interaction influences the release of simple sugars during hydrolysis. In this study, the proportion of hemicellulose to lignin ratio of the mango endocarp sample was the lowest (0.67), followed by that of the melon husk (0.93) and the moringa pod (1.77). Therefore, the degree of hydrolysis of these agricultural wastes may be in order: melon seed husk, mango endocarp, and moringa pod.

3.2 Fourier transform infrared (FT-IR) spectroscopy

Functional groups in complicated chemical mixtures are usually identified and compared using Fourier transform infrared spectroscopy. Figures 3–5 show the Fourier transform infrared spectra of the agricultural waste samples components. The characteristic prominent peaks for samples are provided in Table 2.

Figure 3.

FT-IR spectrum for mango endocarp.

Figure 4.

Melon husk FT-IR Spectrum.

Figure 5.

FT-IR spectrum for Moringa pod.

Functional groups	Mango endocarp	Melon husk	Moringa pod
O∙H stretch	3422.99 cm^−1	3422.66 cm^−1	3422.85 cm^−1
C∙C stretch	1637.50 cm^−1	1636.99 cm^−1	1637.20 cm^−1
∙CH2- and ∙CH3- vibration	1205.72 cm^−1	1204.50 cm^−1	1206.24 cm^−1
C∙O∙R (ester)	1056.15 cm^−1	1035.80 cm^−1	1055.86 cm^−1

Table 2.

FT-IR of acid hydrolysis of extractive-free mango endocarp, melon husk, and moringa seed pod.

The FTIR spectroscopy examination revealed spectra at 3422.99, 3422.66, and 3422.85 cm⁻¹ for mango seed endocarp, melon shell, and moringa seed pod, respectively. These spectra signify the presence of bonded ∙OH groups, as illustrated in the peaks in Figures 3–5 and Table 2. Conjugated carboxylic acids undergo a C∙C stretching at the peak of about 1637 cm⁻¹. At 1205.72, 1204.50, and 1206.24 cm⁻¹, the vibrations of the aliphatic chains, ∙CH₂- and ∙CH₃-, which constitute the fundamental structure of cellulose material, are observed. Peaks at 1056.15, 1035.80, and 1055.86 cm–1 may be attributed to the vibration of C∙O∙R or C∙O∙R (alcohols or esters) [27, 28]. The observation in this study is in agreement with that reported by [29, 30]. These bands are therefore common to those observed in cellulose, hemicelluloses, and lignin FT-IR spectra [31, 32].

4. Conclusion

The amount and quality of lignocellulosic biomass, particularly cellulose and hemicellulose, obtained from the selected agro wastes from this study is an indication that such and even more agricultural and food residues could be useful raw materials in the production of biofuel. Thus, rather than constituting an environmental nuisance, agricultural wastes (such as melon husk, mango seed endocarp, and moringa seed pod) may serve as alternate and affordable natural source of raw material in biofuel production.

Acknowledgments

The authors wish to acknowledge Sheda Science and Technology Complex (SHESTCO) for the use of the laboratory facilities at their Chemistry Advanced Research Centre (CARC).

Conflict of interest

The authors here declare that there is no known conflict of interest.