Statement of Novelty

Groundnut shells are lignocellulose biomass that has been discovered to have little engineering application, and they have become a pollutant to the environment. This economical substrate is organic material that can produce renewable and green energy through anaerobic digestion. But their composition is a major obstacle that makes the process uneconomical due to long retention time and lesser methane yield. Therefore, pretreatment before anaerobic digestion is required to lower the resistance to methanogenic bacteria during anaerobic digestion. This study has presented the influence of five different pretreatment methods on groundnut shells' structural arrangement and methane yield. The most suitable among the methods in terms of retention time and cumulative methane yield was reported in this study. It is envisaged that this investigation will catch the attention of policymakers in environmental management and renewable energies, researchers, investors, and organizations when published because no researcher has been able to compare these pretreatment methods in a single study. Previous reports on groundnut shells' pretreatment studied some of these pretreatments as an individual method. This study has proved that the methane yield of groundnut shells can be enhanced with the selected pretreatment methods but at varying percentage.

Introduction

The recent energy crisis, due to the growing differences between demand and supply and the rise in the price of fossil fuel in the international market, is threatening the economies of many countries. Also, environmental management and appropriate use of available resources has become a significant area of concern in the world over the last decades [1] These challenges can be addressed through renewable energy generation from renewable origins. Biomass from agriculture and forestry residues, animal and food waste, energy crops, sewage sludge, and the organic portion of municipal solid waste can be used for energy generation [2]. Lignocellulose materials are considered abundant and renewable feedstocks for biofuel (biomethane, bioethanol, biohydrogen) generation. The global lignocellulose materials released to the environment per annum have been reported to be approximately 120 × 109 tons, equivalent to 2.2 × 1021 J, more than 300 times the present world energy demand [3]. Crop residues produced during the harvesting and processing of agricultural products have been identified as a considerable source of lignocellulose feedstock for biogas. It does not have other usages in most cases. In some developing countries, residues from agricultural activities are left on the field and grow pathogens, contribute to greenhouse gas emissions, or burnt off and lost their potential with air pollution [4] These lignocellulose residues can be converted to energy through two major promising routes, which are biological (anaerobic digestion to liberate biogas) and thermos-chemical (biomass gasification to produce gas fuel) [5]. Biogas liberation through anaerobic digestion has gained better attention globally and remains an acceptable and promising means of meeting future energy demands.

Anaerobic digestion technology for biogas production has existed for a very long time, and it is simple and readily available for application at both household and commercial levels. The process consists of four stages that are biological and chemical in nature, and they are hydrolysis, acidogenesis, acetogenesis, and methanogenesis [6]. During hydrolysis, the biomass’s big organic polymers of carbohydrates, proteins, and fats are reduced into smaller molecules of simple sugars, fatty acids, and amino acids. It is an important stage of anaerobic digestion, where complex organic matter is hydrolyzed into soluble molecules by the fermentative bacteria’s catalytic action, which determines the process’s success. Hydrolysis products such as hydrogen and acetate can be utilized by methanogens in the subsequent stages of anaerobic digestion. The remaining feedstock that is still relatively big needs to be reduced further during the acidogenesis stage to be useful for methane production. The second stage of anaerobic digestion is acidogenesis; at this stage, acidogenic bacteria reduce the big molecules of the feedstock and organic products from the hydrolysis stage further. The acidogenic bacteria generate an acidic environment in the digester when producing CO2, H2, NH3, H2S, organic acids, shorter volatile acids, and some other by-products in traces. Acetic, butyric, and propionic acids are some of the major acids released at this stage of anaerobic digestion. During acetogenesis stage, acetate, a derivative of acetic acid, is produced from carbon and energy sources by the activities of acetogenic bacteria. Some of the products of acidogenesis are catabolized into acetic acid, H2, and CO2 by the microorganism; the feedstock is further reduced to a point where methanogens can be used it for methane production. The final and last stage of anaerobic digestion is methanogenesis. At this point, methanogens release methane from the end products of the acidogenesis stage and other intermediate products released during the hydrolysis and acidogenesis stages. Methane production during the methanogenesis stage can follow two paths, either the use of acetic acid or CO2, the two major products of the first three stages of anaerobic digestion. CO2 can produce methane and water, while the principal mechanism to release methane in methanogenesis is the path that utilizes acetic acid. In this stage, methane and carbon dioxide are the two major products of anaerobic digestion [6].

Biogas is a mixture of mainly methane and carbon dioxide, with other gases like ammonia, hydrogen sulphide, carbon monoxide, and water vapor in traces with a calorific value of about 4700 Kcal/m3 [7]. The composition of biogas depends on the type of feedstock used and the process parameters employed. Biogas can be used for cooking, heating, and electricity generation and can also be purified and used as fuel for internal combustion engines or injected into the electricity grid. Using lignocellulose materials as biogas feedstock has been experimented with by various research, either as a co-digestion or mono-digestion, and remarkable success was recorded [8,9,10,11]. Despite the ability of the lignocellulose materials to produce biogas, their intrinsic arrangement makes them resilient to enzymatic degradation during anaerobic digestion [12]. Due to these recalcitrant properties of the lignocellulose feedstocks, pretreatment is needed before anaerobic digestion to improve the accessibility of the materials to anaerobic digestion microorganisms and enhance the biogas quantity and quality. Several pretreatment methods have been utilized to improve the digestion rate of lignocellulose feedstock and subsequently enhance the biogas yield [13, 14]. Lignocellulose feedstock pretreatment techniques have been categorized into different methods, which include physical, chemical, thermal, biological, physicochemical, nanoparticle additives, and combined pretreatment methods [15]. It was observed that the influence of these pretreatment methods is not universal on lignocellulose materials. It depends on the microstructural arrangement of the feedstock and treatment conditions [15]. When thermal and particle size reduction pretreatment was investigated on wheat and barley straw, it was observed that particle size reduction improves methane yield by more than 80%. In comparison, thermal pretreatment increases the methane yield by more than 60% [16]. The methane yield of corn straw pretreated with microwave irradiation before anaerobic digestion was improved by 73.08% [17]. Corn stover was pretreated with thermal alkali and steam explosion before anaerobic digestion, and it was reported that the methane yield was enhanced by 40.0 and 56.4% for steam and thermal explosion, respectively [18].

Groundnut is a prominent leguminous plant in semiarid equatorial and tropical countries. It was reported that about 53 638 932 tons were produced in 2020, whereby China, India, Nigeria, the USA, and Sudan are the leading producers with 34, 19, 8, 5, and 5%, respectively, with some other notable countries [19]. The main product of groundnut in most places is oil because of its excellent protein content. Other groundnut harvesting and processing residues, such as shells and cakes, are left on the farm or not efficiently used [20]. It has been observed that 65–75% of the groundnut pod is seed while the remaining 25–35% is the covering layer regarded as the shells [21]. Groundnut shells are lignocellulose because they contain high percentages of cellulose, hemicellulose, and lignin content [22]. The vast quantity of groundnut shells is readily available as residue after harvesting and processing, which are left unattended to and pollute the environment or burnt off with the inherent value lost. Managing this residue has become a significant challenge in some developing countries, like other wastes. This necessitated investigating the strength and engineering use of this valuable material. Groundnut shells are organic and can be processed into various bio-products like biogas, bioethanol, briquettes, biochar, char, and nano-sheet [23,24,25]. To produce renewable, green, and cost-efficient energy, the application of groundnut shells as biogas feedstock has been experimented with and adjudged to be an economical and viable option [22]. Due to its lignocellulose nature, several pretreatment techniques have been experimented with on this substrate. It was discovered that pretreatment techniques significantly influence groundnut shells’ microstructure and methane yield. Different pretreatment methods such as alkali, thermal, nanoparticle additives, particle size reduction, and acidic pretreatment have been experimented with on groundnut shells [12, 26, 27]. It was discovered that all these techniques significantly influence its microstructure and improve the methane yield of the process. Groundnut shells were pretreated with organosolv method using butanol as the solvent, and it was observed that the lignin content of 48.67% was eliminated [28]. During biohydrogen production from groundnut shells, thermal pretreatment was applied at different temperatures and times, and it was reported that the total reducing sugar was improved by 73.6% [23]. About a 3.8% increase in ethanol was recorded when Bacillus anthracis was used to pretreat groundnut shells before biohydrogen production [25], and high-pressure CO2 hydrothermal pretreatment before glucose production released the optimum yield of 80.7% [29]. All these pretreatment methods on groundnut shells produce different effects on the substrate. Still, to our knowledge, no literature has been able to compare these pretreatment methods and present the best methods for the anaerobic digestion of this economic and viable biogas feedstock.

Therefore, this study aims to study the influence of five different pretreatment techniques on groundnut shells’ microstructure and methane yield using the optimum pretreatment conditions reported in our previous experiment. The optimum conditions for thermal, alkali, acid, nano additives, and combined (mechanical + nano additive) pretreatments were examined on groundnut shells, and the influence on microstructure and methane yield was observed. The effect on the morphological structure was examined with scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier transform infrared (FTIR) and presented the most efficient method on the morphological structure of groundnut shells. The pretreated substrates were also digested in a batch digester at mesophilic temperature, and the methods with optimum methane yield were identified.

Materials and Methods

Feedstock and Inoculum Collection

Groundnut shells used for this experiment were sourced locally from a nearby farm after processing their groundnut. The inoculum was prepared from anaerobic co-digestion of kitchen waste and cow dung. The digestion took place at ambient temperature for 60 days. The substrate and inoculum were stored at 4 °C in a well-ventilated environment before laboratory analysis, substrate pretreatment, and anaerobic digestion. The substrate and inoculum were examined in the laboratory for cellulose, hemicellulose, lignin, volatile solid, total solid, ash content, nitrogen, and sulphur concentration using the official standard of the Association of Official Analytical Chemists (AOAC) procedures [30].

Pretreatment Application

The pretreatment of groundnut shells before anaerobic digestion was carried out using the optimum conditions reported in our previous studies on groundnut shells as biogas feedstock and are discussed in the below sections.

Thermal Pretreatment

Autoclaving, a conventional thermal pretreatment, was adopted as the thermal pretreatment technique as prescribed by [31]. During pretreatment, dried groundnut shells were soaked in deionized water in a 500 ml flask using a solid: liquid ratio of 1: 10 for 5 min. The slurred groundnut shells were autoclaved at 100 °C for 30 min, as reported in the previous literature [32]. At the end of the exposure time, the mixture was cooled down to room temperature and then filtered. The filtered substrate was then oven dried at 45 °C for 48 h and, cooled to an ambient condition, then kept at 4 °C before laboratory analysis and anaerobic digestion.

Alkali Pretreatment

To enhance the accessibility of methanogenic bacteria, the groundnut shells were pretreated with alkaline pretreatment as previously reported [12] with a slight adjustment. Alkali pretreatment was carried out using sodium hydroxide (NaOH) in a batch mode. Groundnut shells were dipped in the NaOH at a concentration of 3% w/w using 1: 10 of w: v and autoclaved at 90 °C for 15 min, as previously reported [12]. At the end of the 15 min exposure time, the constituent was cooled to room temperature and filtered using (Grade 1, 55 mm diameter) to separate the solid from the liquid. The pretreated substrate was cleaned with running water and checked continuously with a pH meter until a neutral pH was achieved. The pretreated substrate was oven-dried at 100 °C for 4 h and then packed in a zip-lock plastic bag before being kept in a well-ventilated environment before laboratory analysis and anaerobic digestion.

Acidic Pretreatment

During the acidic pretreatment of groundnut shells, sulphuric acid (H2SO4) was used to alter the structural arrangement of the substrate due to its ability to lower the lignin and eliminate the hemicellulose content of lignocellulose feedstock to the barest minimum percentage [1, 33]. The pretreatment was carried out in humid steam in an autoclave using 0.5% v v−1 of H2SO4 concentration for 15 min under an autoclave temperature of 90 °C as reported in previous studies with slight adjustment [1, 33]. The substrate was soaked in the prepared solution using 1: 10 as a solid-to-liquid ratio. The pretreated substrate was separated from the liquid and washed with tap water until the neutral pH was reached before oven-dried at 100 °C for 4 h before storing it in a plastic bag in a well-ventilated environment before laboratory analysis and anaerobic digestion.

Particle Size Reduction

Mechanical pretreatment using particle size reduction was applied using the hammer mill (SSY 4000902, South Africa) to reduce the substrate size to 6 mm. The particle size was selected according to the previous study [34], and a screen size of 6 mm was selected. This was used for combined pretreatment, while other pretreatments used the raw size from the farm.

Microstructural Arrangement Analysis

The influence of the applied pretreatment techniques on the structural arrangement of groundnut shells was analyzed using different appropriate instruments. The effect of pretreatment methods on the microstructural arrangement was examined using scanning electron microscopy (SEM) (VEGA 3 TESCAN X-Max, Czech Republic). The process was replicated twice, and the images were picked at 400× magnification. The degree of cellulose crystallinity of the pretreated and untreated was also investigated with X-Ray diffraction (XRD) (D-8 Advance, Bruker, USA), using the ranges of 5 to 35 °C with scanning speed of 5 °C/min with an angle of diffraction 2θ. Numerical results from the XRD analysis were used to determine the crystallinity index of the pretreated and untreated groundnut shells using Eq. 1 [35]. Fourier transforms infrared (FTIR) (SHIMADZU—IRAfinity-1, Japan) was used to study the surface functional group of the pretreated and untreated groundnut shells and was observed between 4000 and 500 cm−1. The absorbent ratio of the pretreated to the untreated substrate was calculated from the FTIR results using Eq. 2 [1].

$${I}_{c}= \frac{{I}_{max}- {I}_{x}}{{I}_{max}} \times 100.$$
(1)

where: Ic = Crystallinity index, Imax = Maximum diffraction at peak position at 2θ = 22.23°, and Ix = The intensity at 2θ = 18°

$$Relative \,variation \left(\%\right)= \frac{{A}_{u }- {A}_{p}}{{A}_{u}} \times 100.$$
(2)

where: Au = Absorbance of the untreated substrate, Ap = Absorbance of the pretreated substrate.

Anaerobic Digestion

A laboratory-scale batch experiment following the European standard method VDI 4630 [36] was set up to investigate the influence of the pretreatment methods on groundnut shells’ methane yield. Automatic Methane Potential Test System II (AMPTS II) was employed as the digester, and the experiment was carried out at mesophilic temperature. Twelve (12) 500 ml digester bottles of the AMPTS II were loaded with 400 g of stable inoculum as prescribed by VDI 4630 [36]. The quantity of substrate was determined by Eq. 3 and calculated using the volatile solid (VS) of the substrate and inoculum. 2: 1 of the substrates to inoculum was adopted, and the digestion was carried out at mesophilic temperature (37 °C ± 2). The calculated amount of thermally, alkali, and acid-pretreated substrate were loaded in separate digesters filled with stable inoculum. For nano additive, the calculated amount of untreated groundnut shells were loaded in another digester, and 20 mg/l of Fe3O4 (< 50 nm) was added to the digester as reported [37]. Combined pretreatment using particle size reduction and nano additive was examined for methane yield optimization. The calculated quantity (using Eq. 3) of 6 mm particle size of groundnut shells was charged into a digester already filled with stabled inoculum, and 20 mg/l of Fe3O4 (50 < nm) was added. A control experiment was also set up whereby untreated groundnut shells were charged into a digester filled with the inoculum. This experiment was duplicated twice, as recommended by the previous studies [38]. Two digesters that contained only the recommended quantity of inoculum were run as a parallel experiment and used for yield correction. The gas yield from this blank digester was deducted from the biogas released by the digesters that contain both substrate and inoculum. The digester bottles were labeled as shown in Table 1. Before running the AMPTS II, the following information was inputted into the software. 10% concentration was inputted as carbon dioxide flush gas. The agitation time was set at 60 s, the off time was also 60 s, and the mixing speed adjustment was kept at 80% during digestion. Methane content was assumed to be 60% [32], and the digester headspace was maintained at 100 ml. To set up an anaerobic condition in the digester, the digesters were flushed with nitrogen gas to remove the traces of oxygen available within the digester. At the carbon dioxide removal unit, screw cap bottles of 100 ml were filled with NaOH (3 M NaOH) to remove the carbon dioxide in the gas released. Flexible pipes connected the carbon dioxide removal section containing 75 ml of NaOH to the digester bottles. Another flexible tube was used to connect the carbon dioxide unit to the third unit, where the volume of biomethane released was recorded. The gas yield was analyzed with gas chromatography attached to the system to ascertain the quantity of biomethane released. The experiment was stopped by day 30 when it was noticed that the gas released was less than 1% of the total yield.

$${M}_{s}=\frac{{M}_{i}{C}_{i}}{2{C}_{S}}\text{ }\text{ }\text{ }$$
(3)

where: Ms = Mass of the substrate (g), Mi = Mass of inoculums (g), Cs = concentration of substrate (%), and Ci = Concentration of inoculum (%) [36].

Table 1 Different pretreatment conditions for groundnut shells
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Theoretical Methane Yield (TMY)

The theoretical methane yield of the groundnut shell was determined with the Buswell equation. This employs the stoichiometry value obtained from elemental analysis of the substrate as presented in Eqs. 4 and 5 [39].

$${C}_{a}{H}_{x}{O}_{y}{N}_{z}+ \left(a- \frac{x}{4}- \frac{y}{2}- \frac{3z}{4}\right){H}_{2}O \to \left(\frac{a}{2}+ \frac{x}{8}- \frac{y}{4}- \frac{3z}{8}\right) C{H}_{4}+ \left(\frac{a}{2}- \frac{x}{8}+ \frac{y}{4}+ \frac{3z}{8}\right) C{O}_{2}+zN{H}_{3}$$
(4)
$$TMY\left( {\frac{{mLCH_{4} }}{{gVS}}} \right) = \frac{{22.4{\text{ }} \times 1000 \times \left( {a/2 + x/8 - y/4 - 3z/8} \right)}}{{12a + x + 16y + 14z}}$$
(5)

where a, x, y, and z are the stoichiometry ratio of carbon, hydrogen, oxygen, and nitrogen, respectively.

Results and Discussion

Feedstock Characterization

Physicochemical Properties of Substrate and Inoculum

The proximate and ultimate properties of untreated groundnut shells and inoculum were analyzed in the laboratory, and the results are presented in Table 2. The table shows the feedstock has 100 and 99.87% of total solid (TS) and volatile solid (VS), respectively. This result implies that the feedstock has a very high organic content that can release biogas and methane during anaerobic digestion. Compared to some other lignocellulose biogas feedstocks, rice straw (74.9 ± 0.2%), corn straw (75.05 ± 0.3%), and dairy manure (84.7 ± 2.4%) [40], the substrate has a better biogas potential. The table shows that the substrate consists of 37.30, 17.80, and 27.67% cellulose, hemicellulose, and lignin, respectively. It has been observed that feedstock with more than 75% lignocellulose content limits the hydrolysis stage, which will require lengthy retention time with poor yield and make the process uneconomical [41]. Since the lignocellulose content of this substrate is more than 75% (82.77%), it indicates that the substrate needs pretreatment for the process to be effective and economical. The organic content was determined to be C27.55H43.55O30.55 N using the elemental composition used to calculate the TMY using Eq. 5.

Where x = 43.55, a = 27.55, y = 30.55 and z = 1

$$= \frac{{22.4{\text{ }} \times 1000X\left( {27.55/2 - 43.55/8 - {\text{ }}30.55/4 - {\text{ }}3 \times 1/8} \right)}}{{12\left( {27.55} \right) + 43.55 + 16\left( {30.55} \right) + 14\left( 1 \right)}}$$
$$= \frac{\text{250,880}}{876.95 } =286.08$$

TMY = 286.08 mLCH4/gVSadded.

Table 2 Physicochemical properties of the substrate and inoculums
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Analysis of the Effect of Pretreatment Methods on the Structural Arrangement

Microstructural Analysis

The influence of pretreatment methods on the microstructure of groundnut shells was studied with Scanning Electron Microscopy (SEM). Figure 1 illustrates the SEM image of thermal, alkali, acid, mechanical pretreatments, and untreated substrate. The image from the untreated groundnut shells (Fig. 1E) presented a more compact and smooth surface with various fiber layers resisting bacteria during anaerobic digestion. Figure 1D shows the influence of particle size reduction, a mechanical pretreatment method, on the microstructural arrangement of groundnut shells. It can be noticed that there is not any significant difference in the structural arrangement of the untreated and particle size reduction. This indicates that particle size reduction does not alter the microstructural arrangement of groundnut shells. After other pretreatments, the cell walls of the groundnut shells were affected significantly. The initial compacted and smooth surfaces were broken, loosed, and separated, which can be observed from the morphological images shown after pretreatment (Fig. 1A–C). This result supports a previous report that pretreatment techniques significantly affect the morphological structure of lignocellulose [14].

Figure 1A illustrates the micro-arrangement of groundnut shells pretreated thermally, fractured, and disconnected with a divided surface fickled. Thermally pretreated substrate showed an agglomerate of flat-like small grains formed on the particles, indicating that the cellulose and hemicellulose were degraded significantly due to the thermal pretreatment [42]. After thermal pretreatment, a chiseled structure image of groundnut shells enhanced the feedstock’s available surface area and improve the microorganisms’ activities during anaerobic digestion. A similar result was reported when different thermal pretreatment techniques were used to pretreat rice straws [42]. The SEM image of alkali-pretreated groundnut shells (Fig. 1B) showed some partially limited loosening of the original fixed structure of the cell walls, with higher disruption and fiber exposure. The treatment showed some discrepant morphological arrangement fragmented, with the intracellular form showing irregular structure with different cracks and fibers separated apart. During pretreatment with alkali, the aromatic and aliphatic lignin arrangement interrupted the bonds between the cell wall components, enhanced the inner surface area, and reduced the polymerization level [43]. Acidic pretreatment was observed to lower the coarseness and defibrillation of the feedstock surface. The image in Fig. 1C was more swollen and indicated higher fiber fragments on its surface. This image revealed more space for microbial activities during anaerobic digestion. Acidic pretreatment was also reported to produce similar effects when sugar cane bagasse was examined with SEM after pretreatment [44]. Different pretreatment techniques on groundnut shells, as shown in the SEM images from this study, have demonstrated that pretreatment with different techniques has a varying influence on the same lignocellulose materials. It can be reported from the SEM images (Fig. 1) that acidic pretreatment produced a more porous structure, which indicates a large surface area, followed by thermal pretreatment and, lastly, alkaline pretreatment. Based on the level of available surface area as depicted in SEM images, acidic pretreatment is expected to produce the highest biogas yield, followed by thermal pretreatment and alkaline pretreatment with the most negligible yield, excluding the untreated substrate. Although, this degree of porosity may not translate into biogas success due to the release of inhibiting compounds during the pretreatment process, which can hinder methanogenesis [45]. This result agreed with the previous assertion that pretreatment techniques significantly affect the structural arrangement of lignocellulose materials, but it is not universal. Each technique’s effectiveness varies on the same feedstock, and success on a particular feedstock does not translate to success in another lignocellulose feedstock [15].

Fig. 1
figure 1

SEM images of A thermal, B alkali, C acid, D mechanical pretreatments, and E untreated groundnut shells

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Influence of Pretreatment on the Crystallinity of Groundnut Shells

Lignocellulose feedstocks are made up of crystalline and non-crystalline structures; hemicellulose and lignin are regarded as amorphous portions, while cellulose is considered crystalline [26, 46]. Because of the crystalline nature of lignocellulose feedstock, there is a tendency to have strong resistance to biodegradation compared to amorphous materials. The effectiveness of pretreatment methods on lignocellulose feedstock can be ascertained through the crystallinity of the feedstock [47]. The microstructure of cellulose in the feedstock was examined with X-ray diffraction (XRD), using its ability to diffract and form graphic patterns through crystalline cellulose [48]. Figure 2 presents the diffraction pattern of the influence of different pretreatment techniques on groundnut shells. The Figure shows a pattern with varying peak points different from the untreated substrate, indicating that pretreatment techniques influenced the crystallinity of groundnut shells, as reported in the previous study [18]. XRD results with similar patterns was observed between untreated and mechanically pretreated groundnut shells, while thermal and acid follow the same trend, but alkali pretreatment differs completely. The pattern changes were noticed at 2θ of 22.70° and 18.00° as a result of variation in crystallinity. The principal peak for all the samples was observed at 22.70° for 2θ. Figure 2 shows a sharp peak from the thermal and acid-pretreated groundnut shells followed by particle size reduction and untreated substrate shells, indicating a higher crystalline arrangement. Alkali pretreatment produced the least peak separated from others. The exact pattern change was recorded for thermal and acidic pretreatment with lower peak points than others. This result showed a lower crystallinity arrangement for groundnut shells when pretreated with thermal and acid. A secondary peak was also observed, which indicates the amorphous portion. Thermal and acidic pretreatment could affect the cellulose portion of the groundnut shells, which improved its crystallinity. It can be observed from Fig. 2 that particle size reduction and alkali pretreatments do not affect the crystalline structure much compared to thermal and acidic pretreatment. This shows that pretreatment with conventional heating and H2SO4 influenced only the crystalline and amorphous surface, and the crystalline portion was partially influenced and maintained its co-existence. The effectiveness of thermal pretreatment could be traced to higher hydroxylation of the crystalline fragments. Lignin and hemicellulose were hydrolyzed during thermal pretreatment, leading to further cellulose vulnerability. Previous studies reported similar results when lignocellulose feedstocks were pretreated and observed with the XRD spectrum [18, 49, 50]. It has been observed that smaller crystalline indicates higher polymerization, improved surface area, and porosity, which implies that there will be enough surface area for the methanogenic bacteria activities and subsequent improvement in the digestion rate of groundnut shells [51]. It was observed that smaller particles of lignocellulose materials improve the digestibility of the feedstock, improve biogas yield, and reduce the lag time of the process [52, 53].

Fig. 2
figure 2

XRD patterns of the effect of pretreatment methods on Arachis hypogea shells

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The crystallinity index (Ic) values for the selected pretreatment techniques are presented in Table 3. It can be inferred from the table that the Ic values were 61.09, 30.80, 48.48, 56.55, and 56.55% for thermal, alkali, acid, particle size reduction pretreatments, and untreated substrate, respectively. This indicates that thermal pretreatment enhanced the crystallinity of the groundnut shells by 7.98% compared to the untreated substrate. The Ic values recorded for alkali and acid pretreatment negatively influenced the crystallinity of the groundnut shells, while particle size reduction showed no significant impact. It can be reported that thermal pretreatment at 100 °C for 30 min had a synergetic effect of removing non-crystalline hemicellulose and lignin compared to other pretreatment techniques. The process reduces the lignin facing the region, making the crystal region clearer and increase the crystallinity index. Therefore, thermal pretreatment is expected to enhance the biogas yield of groundnut shells, provided the level of inhibitory compound released is lower. A pretreatment technique was previously reported to improve the Ic of lignocellulose materials in a similar study [44]. The Ic values recorded for alkali and acidic pretreatments are lower than that of the untreated substrate. This can be linked to the increased diffraction intensity produced in the amorphous portion and the reduction observed in the crystalline region. There is a partial reduction of amorphous contents like lignin and hemicellulose due to the lower concentration of chemicals and exposure time. Crystalline cellulose hinders microbial activities more than amorphous cellulose [54]. Several works of literature have examined the impact of the crystallinity index on the anaerobic digestion of lignocellulose feedstocks. There is no agreement on the role of the crystallinity index on enzymatic hydrolysis since other factors are equally or more vital. Enzymatic hydrolysis of pretreated lignocellulose feedstock was investigated, and it was discovered that Ic values do not relate directly to the enzymatic hydrolysis of the feedstock [54]. A similar study noticed a significant negative relationship between Ic values and enzymatic hydrolysis at higher enzyme loading [55]. Therefore, the influence of Ic on enzymatic hydrolysis needs to be investigated in the absence of enzymes that can serve as a hindrance factor.

Table 3 Crystallinity index of pretreated and untreated groundnut shells
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Influence of Pretreatment Methods on Functional Group of Groundnut Shells

The impact of different pretreatment techniques on the functional groups of groundnut shells before and after pretreatment was examined with FTIR spectra. The spectroscopy result and characterization are presented in Fig. 3 and Tables 4 and 5. The peak assignment for each functional group can be noticed in the spectra image, as observed in the previous study [56]. The spectra image indicated that the feedstock is cellulosic because all the bonds were observed between 3350 and 2900 cm−1, primarily cellulose regions [57]. The value of the absorbance ratio of the process determined the influences of pretreatment methods. It can be observed that the absorbance was enhanced after pretreatment techniques and significantly improved in the concentration of the feedstock after pretreatment, except for the particle size reduction. Wavenumbers 3350, 3402, 3402, and 3304 cm−1 were recorded for thermal, alkali, acid, and particle size reduction pretreatment techniques. The increase in cellulose strength was observed in all the pretreatments compared to an untreated substrate except for particle size reduction, which has the same value as the untreated substrate. This can be traced to the rise in the cellulosic O–H bonds, which also increased absorbance at these bands. This influence was also observed in other lignocellulose feedstock after pretreatment [14, 49]. Another alteration observed was the change in the structure of the cellulose after pretreatment due to the influence of elevated temperature and chemical addition.

Looking at the alteration in the lignin portion of the groundnut shells, bands around 1732, 2887, and 2372 cm−1 were observed. They were principally linked to C=O asymmetric bending straining of xylan, which aids the elimination of hemicellulose and lignin. It can be observed that alkali and acid pretreatment rigorously enhanced their peak because they were noticed to have been eliminated or flattened totally, which might be due to the strength of the H2SO4 and temperatures [42]. Alkali pretreatment was observed not to reduce the lignin content at this band due to their strong affinity with lignin. This supports previous reports that acid and thermal pretreatment influence the lignocellulose content of lignocellulose materials [1, 48]. The influence of acidic and thermal pretreatments of groundnut shells on the lignin contents led to the pseudo lignin like the ones observed by Kainthola et al. [42] and Siddhu et al. [18] when corn stover and rice straw were treated, respectively. The resultant effect of these compounds could result in a reduction in the total biogas released or a complete disruption of the process. The degree of vibration at 1635, 1652, 1652, 1646, and 1646 cm−1 for thermal, alkali, acid, and particle size reduction pretreatments and untreated substrate can be linked to the C=C straining of the aromatic ring and lignin removal. It can be observed that only the thermal pretreatment removed or redistributed the lignin. Phenolic lignin was detected at 1372, 1418, and 1623 cm−1 peaks for thermal, alkali, acid, and thermal pretreatment, respectively, which was lowered when the feedstock was pretreated with thermal and alkali pretreatments.

On the contrary, it was increased for acid, whereas there was no significant influence on particle size reduction, as shown in Tables 4 and 5. Thermal and alkali pretreatment on this lignin type is more desirable, whereas acid and particle size reduction did not eliminate this type of lignin from other groundnut shells after pretreatment. A similar range of values was also reported for this type of lignin (syringyl lignin) and was observed at the 1320 cm−1 band [1]. Another form of lignin observed in groundnut shells is acetyl lignin (C–O–C straining vibration). It was noticed at 1252, 1260, 1154, and 1223 cm−1 bands for thermal, alkali, acid, and particle size reduction pretreatments. The percentage was eliminated/lowered only when the feedstock was pretreated with acid and thermal pretreatments but more pronounced with acid, while it remained the same for particle size reduction. Acetyl lignin was increased when the substrate was treated with alkali, and this support what was previously observed when wheat straw was pretreated in a similar study [49].

Observing the effects in the cellulose and hemicellulose content of the groundnut shells, C–O straining vibration was observed at 1057, 1020, 1017, and 1029 cm−1 bands, with a very strong tied with hemicellulose. Acid and alkali pretreatment lower the cellulose and hemicellulose but are more visible in acid, while it increases during thermal pretreatment and remains the same for particle size reduction. As reported earlier, acid and alkali pretreatments remove substantial hemicellulose and cellulose with higher cellulose and hemicellulose reduction [48]. The –CH bending vibration in the plane of the amorphous cellulose was observed at 834, 889, 869, and 846 cm−1 bands for thermal, alkali, acid, and particle size reduction pretreatments. It can be inferred that the amorphous cellulose was lowered in thermal pretreatment, but it increased with alkali and acid but remained the same in particle size reduction. This could be due to the higher temperature in the thermal pretreatment, as higher temperature significantly affects the amorphous substances. This study has shown that different pretreatment techniques influence the functional groups of groundnut shells, as indicated in the FTIR spectra. The result also suggested that the cellulose intermolecular and extra-molecular bonds were degraded, which altered the microstructure of the groundnut shells and improved their accessibility to microorganisms. These alterations are expected to enhance digestion, improve biogas yield, and lower retention time [17]. Lower amorphous cellulose recorded from the alkali and acid-pretreated groundnut shells, as observed from the functional group results, shows that they will be more porous compared to thermally pretreated substrates. This indicates better surface area, which is expected to transform into a higher methane yield compared to the thermal pretreated substrate. This result agreed with what was reported in previous literature when the influence of pretreatment techniques on functional groups and biogas yield was investigated [18, 32].

Table 4 FTIR spectrum peak assignments of Arachis hypogea shells after pretreatment
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Fig. 3
figure 3

FTIR spectra showing the effect of pretreatment methods on Arachis hypogea shells

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Table 5 Wavelengths correlate to a particular functional group and their percentage relative variation to infrared spectroscopy
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Effect of Pretreatment Methods on Methane Yield

Effect of Pretreatment Methods on Daily Methane Yield

The daily methane yield of pretreated and untreated groundnut shells for 30 days retention period is presented in Fig. 4. It can be observed from the Figure that all the pretreatment methods enhanced methane yield compared to the untreated substrate. But the variation in methane yield and the day of optimum yield differs with pretreatment methods. Thermal pretreatment method (A) was observed to produce the highest daily yield of 54.92 ml CH4/g VSadded on day 2, followed by alkali pretreatment (B) with 16.67 ml CH4/g VSadded on day 8. Acid (C), nano additive (D), combined pretreatments (E), and untreated substrate (F) released the highest daily methane yield of 16.38, 13.24, 16.48, and 10.67 ml CH4/g VSadded, respectively. The results also showed that alkali and combined pretreatment methods released the daily optimum methane yields on day 8, while it was recorded on day 5 for acid and nano additive. These results aligned with previous reports that pretreatment before anaerobic digestion improves the methane yield of lignocellulose feedstocks [15, 58]. It can be noticed from the result that thermal pretreatment was the first to release the optimum and highest daily methane yield.

The ability of the thermal pretreatment to release the optimum daily methane yield at the early stage of the digestion could be traced to the strength of heat to remove or redistribute the lignin portion of the substrate and open the cellulose and hemicellulose to bacteria attack and reduce the retention time [59]. It can also be noticed that most of the methane yield from this treatment was released within the first 7 days of digestion, which was identified as the most suitable when lowering the retention time is essential. The ability of thermal pretreatment to solubilize the lignin content influences the delignification and hemicellulose polymerization and improves the digestion rate, which leads to a lower retention time [60]. Alkali, acid, nano additive, and combined pretreatment methods were also noticed to enhance the daily methane yield compared to the untreated substrate. Alkali and acid pretreatment showed a similar trend of daily yield, which can be attributed to the ability of chemicals to depolymerize the hemicellulose and improve the available surface area for microorganism activities. The major challenge with this method is the production of toxic materials that can inhibit the rate of methane release. Combining nanoparticles with particle size reduction released better daily methane yield (16.48 ml CH4/g VSadded) than a single treatment of nano additive (10.67 ml CH4/g VSadded). This is due to the improved surface area for Fe3O4 to pierce the substrate making it more available to the anaerobic microorganism. Adding metal traces to the anaerobic digestion process supplies essential nutrients that enhance microorganisms’ activities and stabilize the process [61, 62]. Nanoparticles of iron origin can reduce the hydrogen sulphide and increase the methane yield [37]. Combined particle size reduction and nano additive released higher daily methane yield compared to a single treatment of nano additive. This is due to the more accessible surface area resulting from particle size reduction, thus improving nanoparticle attachment areas on the substrate and enhance the performance of methanogenic bacteria [52]. When all these methods are compared, thermal pretreatment produces the highest daily methane yield, but the possibility of generating an inhibitory compound at a higher temperature is high [42]. The concentration of alkali and acid is also an important factor that controls the level of the inhibitory compound. Still, adding nanoparticles seems less likely to produce inhibitory materials when the appropriate quantity is used.

Fig. 4
figure 4

Daily methane yield of pretreated and untreated groundnut shells

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Effect of Pretreatment Methods on Cumulative Methane Yield

At the end of the 30 days retention time, the total methane yield of pretreated and untreated groundnut shells is illustrated in Fig. 5. It can be noticed from Fig. 5 that methane yield produced are 222.92, 214.00, 171.02, 140.99, 261.36, and 100.58 ml CH4/g VSadded for thermal (A), alkali (B), acid (C), nano additive (D), combined pretreatments (E), and untreated substrate (F), respectively. When compared with the theoretical methane yield (TMY) calculated (286.08 ml CH4/g VSadded), it can be observed that all the yields are below TMY value, which indicates that despite the pretreatment applied, the full potential of the feedstock was not utilized. This result shows that only 35.16% of the feedstock was available for methane production without pretreatment. Compared with the untreated substrate (control), the cumulative methane yield was improved by 121.63, 112.77, 70.03, 40.18, and 159.85% for thermal, alkali, acid, nano additive, and combined pretreatments. The result shows that all the pretreatment methods experimented with at the selected treatment conditions improve the cumulative methane yield of groundnut shells. The improvement in methane yield can be traced to the ability of the pretreatment methods to breakdown the lignin-polysaccharide bonds and open the feedstock for easy accessibility of the methanogenic bacteria to the cellulose and hemicellulose, leading to the effective digestion of groundnut shells [63]. This agrees with previous literature reports that pretreatment techniques can improve the total methane yield [15]. The methane yield of Safflower straw and rice straw was improved by 98.3 and 22.8% after the thermal pretreatment method [64, 65]. The methane yield of Napier grass and corn straw was also enhanced by 35 and 80.34%, respectively, when pretreated with hydrothermal and microwave irradiation [45, 66]. Nanoparticle addition was reported to improve the methane yield of lignocellulose feedstocks by 36.32 and 29.46% [67, 68]. Combined pretreatment has been adjudged to be more effective than individual treatment in different research, but the economy of the process remains a major concern [1, 15, 69]. All these findings agreed with the results of this study and further established the earlier literature on pretreatment before anaerobic digestion.

Thermal pretreatment improves the methane yield of groundnut shells by 121.63%, a significant improvement compared to the untreated substrate. It can also be observed that about 22.08% of the substrate was left unused or lost due to the inhibition produced during pretreatment. This can be linked to the arrangement of the substrate whereby the hemicellulose has a higher percentage of amorphous with a lower stable structure compared to cellulose, and the method enhance hemicellulose mostly [70, 71]. Thermal treatment of groundnut shells at higher temperatures can eliminate a certain percentage of hemicellulose, reducing methane yield. The substrate is composed of xylan, which digests faster and enhances the methane yield [72]. An increase in the temperature of the process has been observed to increase the amount of hydronium ions released, leading to hemicellulose hydrolysis. The process temperature dictates the cumulative sugar released that hydrolysates during pretreatment, affecting the total sugar left for hydrolysis during anaerobic digestion [73]. The full potential of the substrate can not be accessed because the acetyl groups released during hydrolysis reduce the hydrolysate pH [74]. Inhibitory compounds like furfural, acetic acids, and 5-hydroxymethylfurfural release is possible during thermal pretreatment, and the level is determined by the process temperature [64]. The cumulative methane yield recorded for alkaline pretreatment increased by 112.77%, lower than thermal pretreatment but higher than acid and nano additive pretreatments. It can be noticed that about 25.20% of the substrate was not converted to methane either due to unavailability or hindered by inhibitory compounds. Methane yield was improved due to the ability of the alkaline to improve the crystallinity structure and make it accessible to methanogenic bacteria. The pretreatment method reduces/eliminates the lignocellulose content like lignin that is difficult to digest and improves the methane yield. Alkali pretreatment was very effective in lignin elimination but had minimal effect on cellulose concentration. The crystallinity of the groundnut shells was also reduced, as shown in the crystallinity index result. The bond between lignin and carbohydrate was degraded, leading to the lignin structure interruption [2]. Acidic pretreatment produced the second least improvement of 70.03%. It shows that about 40.22% of the substrate potential was inaccessible despite the acidic pretreatment. The improvement in the methane yield can be linked to the strength of the acid used to reduce the hemicellulose content and break down essential bonds such as hydrogen and covalent bonds. The acid significantly affects van der Waals bonds, increases the solubilization rate of hemicellulose, and partially solubilizes the cellulose [26]. Acid can hydrolyze the xylose portion of groundnut shells to monosaccharides, improving the hemicellulose depolymerization, as observed in this study. The previous report observed that there is a loss of hemicellulose content and partial cellulose content, which can be the reason why the percentage increase in methane yield (70.03%) is not more than this [75].

When the five pretreatment methods were compared, single pretreatment of nano additive produced the slightest improvement. In this particular pretreatment method, the structural arrangement of the substrate was not altered, and the added nanoparticle was limited in piercing the substrate and making them available to the methanogenic bacteria. The methane yield was enhanced compared to the untreated substrate due to the strength of the Fe3O4 nanoparticle to generate some important enzymes and co-factors that have been observed to induce and stabilize the anaerobic digestion process [61, 62]. However, about 50.72% of the substrate was unavailable for methane production due to the recalcitrant characteristics of the groundnut shells. The nanoparticle of iron origin has been reported to have the capacity to improve methane yield released and reduce the percentage of hydrogen sulphide [76]. Fe2−/Fe3+ ion introduction to the process through Fe3O4 nanoparticles could enhance microorganisms’ growth and improve their methanogenic activities [77]. Physicochemical characteristics of Fe3O4 were reported to consist of magnetite and some goethite. The magnetite produces bioavailable ions (Fe2− and Fe3+) that have been observed as a crucial nutrient for methanogenic microbes’ power production [78]. Fe3O4 improves hydrogenotrophic methanogenesis by releasing electrons or hydrogen generation from iron corrosion, which increases the methane produced from carbon dioxide consumption, as shown in Eqs. 6, 7 and 8 [62, 79].

$${CO}_{2}+4{Fe}^{o}+8{H}^{+} \to {CH}_{4}+4{Fe}^{2+}+2{H}_{2}O$$
(6)
$${Fe}^{o}+2{H}_{2}O \to {Fe}^{2+}+ {H}_{2}+2O{H}^{-}$$
(7)
$${CO}_{2 }+ {4 H}_{2} \to C{H}_{4 }+2{H}_{2}O$$
(8)

Combined pretreatment of Fe3O4 nanoparticle additives and particle size reduction produced the highest cumulative methane yield (159.85% increase) compared to other pretreatment methods. It can be inferred from this pretreatment that only about 8.64% of the substrate is unavailable for methanogenic bacteria consumption. This is due to the combination of particle size reduction strength and Fe3O4 nanoparticle additives to enhance the activities of the methanogenic microbes. The particle size reduction can improve the lysis rate and the methane yield [22]. A kinetic model of the chemical reaction control can be used to study the process rate. When anaerobic digestion occurs at an equal rate, the digestion rate can be represented by Eq. 9 [80]. Particle size reduction determines the substrate surface area and is crucial during digestion. This agreed with what was reported when nanoparticle additives were combined with other pretreatment methods compared to single pretreatment of either pretreatment [15, 67, 78]. Particle size reduction improves the surface area, polymerizes the substrate, and lowers the final cellulose crystallinity [16]. Since the particle size reduction increased the surface area, the available space for Fe3O4 attachment increased and improved the hydrolysis and methanogenesis stages. Combined pretreatment methods are more efficient and enhance the methane yield of biogas feedstocks drastically compared to individual pretreatment, but the technique is more complicated than single pretreatment methods.

$$V= - \frac{dm}{dt}=kA{C}^{n}$$
(9)
Fig. 5
figure 5

Cumulative methane yield of pretreated and untreated groundnut shells

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Conclusion

This study has shown that all five pretreatment methods considered can alter the microstructure arrangement and enhance the methane yield of groundnut shells but at varying percentages. It can be observed from the result that thermal pretreatment can improve the crystallinity index by eliminating/redistributing the lignin content of the substrate, encouraging early methane yield, and reducing the retention time to the barest minimum. Chemical pretreatment with alkali and acid showed some improvement (112.77 and 70.03%) which is lower than the improvement recorded from thermal and combined pretreatment. This can be traced to the toxicity of the chemicals which inhibit the activities of the methanogenic bacteria. Nanoparticle additive using Fe3O4 shows its biostimulating effects on the performance of methanogenic microbes during digestion and enhances cumulative yields. The highest cumulative methane yield was produced from combined particle size reduction and nanoparticle additive due to the improved surface area for Fe ions attachment, increased methanogenic bacterial activities, ability to produce little/no inhibitory compounds and hydrogen sulphide reduction. Therefore, combining particle size reduction and nanoparticle additive using Fe3O4 is the most effective pretreatment method among the methods considered. The findings from this study can serve as baseline information for subsequent studies in lignocellulose pretreatment and can be investigated at the industrial scale.