1 Introduction

The extensive consumption of fossil fuels due to the current energy demand has been noticed as the primary source of the anthropogenic release of carbon dioxide, which is majorly responsible for climate change and global warming [1]. These environmental challenges require a long-term solution to sustainable growth. Green and renewable energy production from renewable sources has been identified as one of the most effective and efficient solutions. These renewable energies can be sourced from solar, geothermal, wind, hydropower, and biofuels like biogas, bioethanol, hydrogen, and bioethanol [2]. Biogas is a clean and renewable biofuel that can replace conventional energy sources threatening the ecology and environment and quickly depleting due to the high demand [3]. Biogas can be generated from various liquid and solid wastes through anaerobic digestion. Anaerobic digestion is the process whereby microorganisms break down various organic wastes without oxygen, which releases energy-rich gas that can be utilized for vehicle fuel, heat, and electric power. The residue from the anaerobic digestion process contains high organic nutrients that can be applied as plant fertilizer [4]. Biogas can occur under natural conditions and be produced in a designed and controlled environment to generate biogas at a commercial scale. Depending on the feedstock, it is a gas mixture with an energy content of between 19 and 26 MJ/m3. The main composition of biogas is methane (50–65%) and carbon dioxide (35–50%), with traces of other gases like nitrogen, hydrogen, hydrogen sulfide, ammonia, and water vapor [5]. These mixtures’ constituent depends on the feedstock’s nature and process parameters. Methane has been observed to have a higher heating value of 37.8 MJ/m3, and there is no significant energy traced to the carbon dioxide content. Biogas can be purified to remove carbon dioxide and other contaminants, and the purified gas can substitute natural gas [6]. Energy recovered from phytomass waste using two-fraction of anaerobic digestion was used as an alternative to fossil fuels [7]. The adoption and efficient application of biogas could assist in developing different regions, which can lead to improved social and economic cohesion in a country [8]. The cost of emissions on society can be reduced through the production and utilization of biogas, which will encourage investment in low-carbon and green industries and increase the capital flow [9]. The process can provide additional income from the residues, and the digestate is rich in nutrients that can be used in agriculture and as a source of additional income [10, 11].

Biogas can be generated from biomass sources such as agricultural and forest residues, energy crops, animal wastes, sewage sludge, municipal solid waste, and food waste [12]. Plants have a higher carbon capture ability, making them suitable feedstock for biogas production [13]. Lignocellulose biomass is one of the most abundant renewable feedstocks for biofuels, such as biomethane, bioethanol, biohydrogen, and biodiesel. Globally, lignocellulose biomass is released at the rate of approximately 120 × 109 tons per year, which is equivalent to 2.2 × 1021 Joule and is more than the present global energy need by 300 times [14]. Because of its availability and cost, energy recovery from lignocellulose biomass is crucial to sustainability. Due to the recalcitrant characteristics of lignocellulose biomass, there is resistant to cellulose hydrolysis microorganisms and lowers the conversion rate of organics to methane. Lignocellulose feedstocks consist of lignin, hemicellulose, and cellulose in their microstructural arrangement, and it varies depending on the structure, region, age, storage, etc. Cellulose is identified as the homogenous material that forms the backbone of the lignin carbohydrate complex, and hemicellulose is the intracellular material that includes the covalent bonds that strengthen the cell wall layer [15]. The lignin content of lignocellulose biomass acts as a glue that binds the cellulose and hemicellulose and strengthens the cell wall integrity [16]. This complex arrangement of the lignocellulose feedstocks is the major limitation to microorganisms and reduces the conversion rate to biogas [17]. To overcome this challenge, pretreatment to alter the structural arrangement of lignocellulose is required. Different pretreatment methods such as biological, chemical, mechanical, thermal, nanoparticle additives, and combined have been reported in the literature [18]. The focus of these pretreatment methods is to significantly breakdown the arrangement of lignocellulose feedstock to improve the surface area, eliminate/redistribute the lignin, and enhance the crystalline nature of the cellulose. These have been observed in different literature [19, 20]. It was noticed that the pretreatment method does not have the same effect on other lignocellulose feedstocks. The influence depends on the microstructural arrangement of individual feedstock [18]. To improve the effectiveness of pretreatment methods, combining two or more pretreatment methods has been experimented with and reported to be more effective than single pretreatment of each technique [21]. Biological pretreatment combined with a chemical during the treatment of willow saw dust was reported to improve the biogas yield by 49% [22]. Maize straw pretreated with 1% w/w NaOH and enzyme pretreatment released a 25% increase in methane yield [23]. Combined alkaline (banana peel and calcium hydroxide) and thermal (90 °C for 6 h) pretreatment of rice straw and corn stalk was observed to produce 39 and 47% lignin degradation. Biogas yield was enhanced by 62 and 66%, respectively, compared to the untreated substrates [24]. Corn cob was pretreated with alkali extrusion and enzymatic pretreatment before anaerobic digestion, and it was noticed that methane released was increased by 22.3% [25]. During the anaerobic digestion of greenhouse residue, the feedstock was pretreated with combined thermal and acidic pretreatment, and methane yield was improved by 24.4% [26]. Rice straw and coconut shells were treated with combined ultrasonication-alkali hydrolysis and liquid hot water before biogas production, and methane yield was reported to increase by 150 and 290%, respectively [27]. However, the combined pretreatment method is a complicated process, and the economy of the process is another major challenge.

The grass is considered a promising feedstock for biogas generation because of the low water intake compared to other crops and the fact that it can be grown on non-fertile soil and does not compete with the food supply. These grasses are lignocellulosic materials, and their complex arrangement hinders their efficiency during anaerobic digestion [6]. Xyris capensis is a rush-like perennial grass that grows in the creeping rhizome, with a roundish spike that is high upon a wiry leafless peduncle with yellow clustered flowers. It can be 60–300 mm tall with weak rhizomes, and grass-like leave of about 50–150 × 4 mm [28]. This grass is a widespread species that do not pose any threat and is regarded as the least concern (LC); thereby, there is no threat of extinction soon. Xyris capensis is not peculiar to South Africa, but it can be found in nearly all the provinces of South Africa other than the Northern Cape [29]. It extends from the Southwestern Cape to Tropical Africa, India, Malaysia, Madagascar, South America, and China. Xyris capensis stem’s potential has been assessed as beer strainers and for weaving mats, and the roots were used to prepare cold water infusion that young men occasionally used to induce vomiting when courting [29]. Xyris capensis is a lignocellulose material considering its structural composition. This material can be regarded as a potential feedstock for biogas production and serve as an energy source that can lower greenhouse gas emissions and reduce environmental challenges posed by the combustion of fossil fuels. But its potential as a biogas feedstock is yet to be investigated. Since it is a lignocellulose feedstock, it is recalcitrant and not accessible to methanogenic microbes during anaerobic digestion. Therefore, pretreatment is needed before anaerobic digestion to break down the cell walls that hinder organic matter from microorganism accessibility. This has necessitated the need to develop pretreatment methods that can break down the recalcitrant characteristics of Xyris capensis. Therefore, this study aims to study the influence of combined oxidative and Fe3O4 additives on the microstructural arrangement and methane yield of Xyris capensis. The effect of oxidative pretreatment on the microstructure was examined using scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier transforms infrared (FTIR). Oxidative pretreated feedstock as single pretreatment and in combination with Fe3O4 nano additive was subjected to an anaerobic digestion process to investigate the impact of pretreatment methods on methane yield. Findings from this study are expected to assist as a baseline for other studies on lignocellulose substrates in the quest for low-carbon energy sources and commercial applications.

2 Materials and method

2.1 Substrate collection

Xyris capensis grass was sourced locally, chopped into smaller sizes (2–4 mm), and kept in well-ventilated conditions in the laboratory at 4 °C for further use. The feedstock was then analyzed in the laboratory for lignin, cellulose, hemicellulose, total solids, ash content, volatile solids, carbon, nitrogen, and sulfur following the Association of Official Analytical Chemists (AOAC) standard procedure. The lignin, hemicellulose, and cellulose content of the feedstock were determined according to Van Soest procedure in a Fibra plus unit [31]. The feedstock was dried at 55 °C for 24 h and was treated with sodium lauryl sulfate and neutral detergent solution according to AOAC procedure [32]. After the pretreatment, the insoluble portion of the feedstock was the neutral detergent fiber (NDF). The feedstock was also treated with 20 g of cetyl trimethyl ammonium bromide in 1 N H2SO4 solution (acid detergent solution). The feedstock’s insoluble portion is the acid detergent fiber (ADF). To determine the lignin content, the acid detergent fiber was treated with 72% H2SO4 as recommended by AOAC, and the insoluble content after treatment was the acid detergent lignin (ADL). Therefore, the lignocellulose content of Xyris capensis was calculated thus: lignin = ADL, hemicellulose = NDF – ADF, and cellulose = NDF – (hemicellulose + lignin).

2.2 Inoculum

The stabled inoculum from an anaerobic digester where food waste and livestock manure were co-digested at a mesophilic temperature from a concluded experiment in our laboratory was collected and used for this study. The inoculum sample was also analyzed for total solids, ash content, volatile solids, and elemental compositions using the Association of Official Analytical Chemists (AOAC) standard procedure [32]. The inoculum was kept in the laboratory at room temperature for the experimental setup.

2.3 Theoretical methane yield (TMY)

The theoretical methane yield of the Xyris capensis was determined with the Buswell equation. The stoichiometry value recorded from the elemental analysis of the substrate was used, as shown in Eqs. 1 and 2 [33].

$${C}_{a}{H}_{x}{O}_{y}{N}_{z}+ \left(a- \frac{x}{4}- \frac{y}{2}- \frac{3z}{4}\right){H}_{2}O \to \left(\frac{a}{2}+ \frac{x}{8}- \frac{y}{4}- \frac{3z}{8}\right) C{H}_{4}+ \left(\frac{a}{2}- \frac{x}{8}+ \frac{y}{4}+ \frac{3z}{8}\right) C{O}_{2}+zN{H}_{3}$$
(1)
$$TMY \left(\frac{mLC{H}_{4}}{gVS}\right)= \frac{22.4\;X\;1000\;X\;\left({~}^{a}\!\left/ \!{~}_{2}\right.+ {~}^{x}\!\left/ \!{~}_{8}\right.- {~}^{y}\!\left/ \!{~}_{4}\right.- {~}^{3z}\!\left/ \!{~}_{8}\right.\right)}{12a+x+16y+14z}$$
(2)

2.4 Pretreatment

Oxidative pretreatment was carried out using Piranha solution, and the solution was prepared as reported by Shrivash et al. [34] with slight modification after it was discovered that the first trial burnt off the entire feedstocks. The low cost of the major acid (H2O2) used in this treatment method is one of the economic merits of the technique. Hence, it is an economical and effective method of recovering energy from Xyris capensis. Seventy-five grams of ice cube was put in a 500 mL beaker, and H2O2 (SIGMA-ALDRICH, 30% Assay) and H2SO4 (SIGMA-ALDRICH, ACS Reagent, 95.0–98.0%) procured for the experiment from Sigma-Aldrich (pty) Limited, Johannesburg, South Africa) were added, as shown in Table 1. The mixture was stirred continuously to form a homogenous solution. The chopped Xyris capensis was then soaked in the prepared solutions in a ratio of 1: 10 of solid to liquid. The beaker with its contents was placed on a magnetic stirrer for 2 h at 200 rpm set at 90 °C. After the treatment exposure time, 10% of NaOH was added to stop further substrate oxidation and warm distilled water was added as an anti-solvent to prevent further reaction. The pretreated Xyris capensis was filtered using filter paper and washed with tap water until a neutral pH of 7 was achieved. The pretreated substrate was then dried in an oven set at 60 °C for 6 h and kept in zip-lock bags for laboratory analysis and anaerobic digestion.

Table 1 Oxidative pretreatment conditions of Xyris capensis
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2.5 Structural characterization of the substrate

The influence of the applied pretreatment techniques on the structural arrangement of Xyris capensis 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 500 × magnification. The degree of cellulose crystallinity of the pretreated and untreated substrate 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 2Ө. Numerical results from the XRD analysis were used to determine the crystallinity index of the pretreated and untreated Xyris capensis using Eq. 3 [35]. Fourier transforms infrared (FTIR) (SHIMADZU – IRAfinity-1, Japan) was used to study the surface functional group of the pretreated and untreated Xyris capensis 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. 4 [36].

$${I}_{c}= \frac{{I}_{max}- {I}_{x}}{{I}_{max}}\;X\;100$$
(3)

where: Ic is the crystallinity index, Imax is the maximum diffraction at peak position at 2Ө = 22.23°, and Ix is the intensity at 2Ө = 18°.

$$\mathrm{Relative\;variation}\;\left(\%\right)= \frac{{A}_{u }- {A}_{p}}{{A}_{u}}\;X\;100$$
(4)

where: Au is the absorbance of the untreated substrate, and Ap is the absorbance of the pretreated substrate.

2.6 Anaerobic digestion

A laboratory-scale batch experiment following the European standard method VDI 4630 [37] was set up to investigate the influence of pretreatment methods on the methane yield of Xyris capensis. Automatic Methane Potential Test System II (AMPTS II) was utilized as the digester, and the experiment was carried out at mesophilic temperature. Twenty (20) 500 mL digester bottles of the AMPTS II were loaded with 400 g of stable inoculum as prescribed by VDI 4630 [37]. The quantity of substrate charged into the digester was determined by Eq. 5 and calculated using the substrate and inoculum’s volatile solid (VS). A total of 2: 1 of the substrates to inoculum was adopted, and the digestion was carried out at mesophilic temperature (37 °C ± 2). Fe3O4 (< 50 nm, 544,884—Merck (pty) Limited, Darmstadt, Germany) used for the experiment was procured from Sigma-Aldrich (pty) Limited, Johannesburg, South Africa. The calculated amounts of oxidative pretreated substrates were loaded in separate digesters filled with stable inoculum. For combined pretreatment, the calculated amounts of oxidative pretreated substrates were loaded in the digester and 20 mg/L of Fe3O4 (< 50 nm) nanoparticle was added. For another set of digesters, untreated Xyris capensis was loaded, and 20 mg/L of Fe3O4 (< 50 nm) was added to the digester, as reported in the previous study [38]. A control experiment was also set up whereby untreated Xyris capensis was charged into a digester filled with the inoculum. This experiment was duplicated twice, as recommended by the previous studies [39]. Two digesters that contained only the recommended quantity of inoculum were run as a parallel experiment and used for methane 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 2. Before running the AMPTS II, the following information was programmed into the software, 10% concentration 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% [40], 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 were connected from the carbon dioxide removal 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 27 when it was noticed that the gas released was less than 1% of the total yield.

$${M}_{s}= \frac{{M}_{i}{C}_{i}}{2{C}_{S}}$$
(5)

where: Ms is the mass of the substrate (g), Mi is the mass of inoculums (g), Cs is the concentration of substrate (%), and Ci is the concentration of inoculum (%) [37].

Table 2 Oxidative pretreatment conditions of Xyris capensis
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3 Results and discussion

3.1 Physicochemical properties of Xyris capensis and inoculum

The physicochemical characteristics of Xyris capensis are illustrated in Table 3. It can be observed from Table 3 that the total solids (TS) and volatile solids (VS) of the substrate are 85 and 95%, respectively. The result of the VS indicates a higher organic matter available for biogas production, which is a sign of promising potential feedstock. Compared with other lignocellulose feedstocks, groundnut shells 91% [21], rice straw (75 ± 0.2%), and corn straw (75 ± 0.3%) [41], it can be observed that the substrate has a higher potential for biogas production. It indicates a high buffering capacity for microorganisms during the anaerobic digestion of Xyris capensis. It has been observed that the biogas and methane yield from the anaerobic digestion process depend on the feedstock’s volatile solid [42]. Higher TS, conversely, can hinder sufficient compaction of the substrate inside the digester and promote anaerobic digestion at the digester outlet [43]. TS of 28–40% is often considered ideal for optimal anaerobic digestion. The TS of the feedstock and inoculum was higher than the required standard of 30 and 98% of substrate and inoculum; therefore, water was added to the substrate and inoculum to lower the TS to the acceptable standard [37].

Table 3 Physicochemical characteristics of Xyris capensis and inoculum
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3.2 Theoretical methane yield

The organic content was determined to be C31.45H48.91O26.18N using the elemental composition, and this was used to calculate the TMY using Eq. 2.

Where x = 48.91, a = 31.45, y = 26.18, and z = 1

$$= \frac{22.4\;X\;1000\;X\;\left({~}^{31.45}\!\left/ \!{~}_{2}\right.+ {~}^{48.91}\!\left/ \!{~}_{8}\right.- {~}^{26.18}\!\left/ \!{~}_{4}\right.- {~}^{3X1}\!\left/ \!{~}_{8}\right.\right)}{12(31.45)+48.91+16(26.18)+14(1)}$$
$$= \frac{\mathrm{333,98}4.00}{859.19 } =388.72$$
$$\mathrm{TMY}\;=\;388.72\;{\mathrm{mLCH}}_4/\;{\mathrm{gVS}}_{\mathrm{added}}$$

The theoretical methane yield of Xyris capensis was calculated to be 388.72 mL CH4/gVSadded, which is in between the values reported for some other lignocellulose feedstocks. TMY reported for switch grass (454 mLCH4/gVS), rice straw (460 mLCH4/gVS), corn stover (455 mLCH4/gVS), and rice husk (495 mLCH4/gVS) were higher compared to Xyris capensis [44]. But TMY of 286.08 mLCH4/gVSadded reported for Arachis hypogea shells [45] is lower compared to this result.

3.3 Effect of oxidative pretreatment on morphological arrangement of Xyris capensis

3.3.1 Effect of oxidative pretreatment on the microstructural structure of Xyris capensis

A scanning electron microscope (SEM) was used to study the influence of oxidative pretreatment at different concentrations on the microstructural arrangement of the Xyris capensis. The images from the SEM analysis are presented in Fig. 1, and it can be observed that oxidative pretreatment significantly affects the microstructure of Xyris capensis compared with the untreated substrate. This supports the previous study that observed that pretreatment can alter the microstructure of lignocellulose materials [19]. The untreated Xyris capensis (Fig. 1E) showed a compacted and fine bundle surface arrangement with several fiber layers that can restrict microbial activities during anaerobic digestion. After oxidative pretreatment, noticeable changes in the cell wall arrangement of untreated were observed, as shown in Fig. 1A–D. The cell walls were degraded, and the original fine and rigid arrangement was broken and separated. Defibrillation and coarseness of the substrate surface can be noticed to improve with the increase in the percentage of H2SO4. Noticeably, Fig. 1D showed higher swollen and fiber fragments on its surface. This can be linked to the ability of H2SO4 to break the cell walls and expose the hemicelluloses and celluloses of lignocellulose materials [46]. It is also important to note that due to the damage done to the structure of Xyris capensis, their internal tissues were exposed, making them easily accessible for microorganisms’ conversion. It can be observed that all the pretreatment conditions considered influence the microstructural arrangement of the feedstock compared to the untreated, as shown in Fig. 1. The improved porosity shown in the images of the pretreated substrate compared to the untreated substrate could be linked to partial lignin degradation and enhanced hemicellulose and cellulose availability. This aligned with a previous report on the influence of oxidative pretreatment on lignocellulose materials [47].

Fig. 1
figure 1

SEM images of A 100% H2O2, B 95% H2O2 and 5% H2SO4, C 85% H2O2 and 15% H2SO4, D 75% H2O2 and 25% H2SO4, and E untreated substrate

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3.3.2 Effect of oxidative pretreatment on the crystallinity of Xyris capensis

Xyris capensis is a lignocellulose feedstock made up of non-crystalline and crystalline constituents where lignin and hemicellulose are regarded as an amorphous material and cellulose as crystalline [48]. Because of the high crystalline nature of Xyris capensis, it is highly resistant to anaerobic microorganisms’ activities compared to the amorphous constituent. The level of crystallinity of the feedstock determines the effectiveness of the pretreatment on lignocellulose feedstock [49]. An X-ray diffraction machine was used to diffract the materials and form graphic patterns, and this was used to examine the crystallinity of feedstock [50]. The XRD spectra pattern for the pretreated and untreated Xyris capensis is presented in Fig. 2. It can be noticed from the figure that the untreated Xyris capensis shows a sharp peak which indicates a high crystalline percentage. The picture also shows another peak which is regarded as the secondary peak from the untreated Xyris capensis. It can be observed from the figure that there is no significant influence of oxidative pretreatment on the crystalline arrangement for treatments A, B, and C. For treatment D, the peak presented is below that of untreated Xyris capensis, which shows that an increase in the percentage of H2SO4 improves the influence on the crystallinity of the substrate. This indicates that using H2O2 had minimal effect on the substrate crystallinity, but the H2SO4 showed a significant effect. This can be linked to the acid’s strength to break the material’s structural arrangement. Hydrogen peroxide impacts the amorphism and crystalline surface and does not have a major influence on the crystalline arrangement; therefore, there is a minimal effect on the co-being of the feedstock. This influence could be traced to the level of hydroxylation of the crystalline regions. Lignin and hemicellulose were hydrolyzed during oxidative pretreatment, leading to subsequent cellulose exposure. The higher percentage of H2O2 enhances the breaking down of the hydrogen bonds in the lattice of Xyris capensis. It enhances the biodegradability of the feedstock and improved methane yield compared to the untreated feedstock.

Fig. 2
figure 2

XRD pattern for A 100% H2O2, B 95% H2O2 and 5% H2SO4, C 85% H2O2 and 15% H2SO4, D 75% H2O2 and 25% H2SO4, and E untreated Xyris capensis

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The crystallinity index (Ic) of oxidative pretreatment at different conditions and untreated Xyris capensis is presented in Table 4. It can be observed from the table that the crystallinity index increased with an increase in the H2SO4 percentage until a point whereby a further increase in H2SO4 led to a reduction in Ic below the untreated substrate. The least crystallinity index of 46% was recorded at the highest percentage of H2SO4 (treatment D), and treatment C produced the highest Ic (58%). It can be noticed that treatments B (57%) and E (57%) have an Ic that is very close, and treatment D produces the least Ic. This influence can be traced to the exposure of the substrate to the oxidation process, whereby the amorphous content has reached a point whereby it melts and lowers the crystalline structure. For other treatments (A–C), the treatment impacted only the crystalline region of the feedstock. This influence of pretreatment on the feedstock is in line with what was observed when XRD analysis was used on similar lignocellulose materials in previous studies [19, 51, 52]. Improvement in the crystallinity index of lignocellulose feedstock was observed during pretreatment [53].

Table 4 Cellulose crystallinity index of oxidative pretreated and untreated Xyris capensis
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3.3.3 Effect of oxidative pretreatment on the functional group of Xyris capensis

The influence of oxidative pretreatment on the functional group of Xyris capensis showing the arrays of the molecules made up of the untreated and pretreated Xyris capensis as investigated using FTIR spectra is illustrated in Fig. 3 and Tables 5 and 6. The figure shows that the feedstock is a cellulosic material because the bonds between 3370 and 3287 cm−1 are regarded as cellulose regions [54]. To ascertain the significance of the absorbance ratio on the pretreated feedstock, Wilcoxon’s rank-sum test was utilized to analyze the result of the absorbance ratio [55]. The investigation was carried out with the absorbance ratio of untreated feedstock (treatment E) as the benchmark and evaluated against the absorbance ratio of individual pretreated feedstock at a 95% (p < 0.05) confidence level. The absorbance ratio of pretreated feedstock was observed to be significantly different from the untreated feedstock when the p ≤ 0.05, but p > 0.05, there is no significant difference. Wilcoxon’s test result showed p-values of 0.05, 0.31, 0.50, and 0.07 for treatments A, B, C, and D, respectively. The absorbance of the substrate can be observed to be significantly affected by treatment A after oxidative pretreatment but not significant with treatments B, C, and D at 95%. The cellulose strength was reduced in all the oxidative pretreated substrates due to the reduction in cellulosic O–H bonds, which led to lower absorbance at the 3370 cm−1 band. This result agreed with what was reported in previous studies when lignocellulose feedstock was pretreated and examined under an FTIR machine [51, 56]. After pretreatment, the cellulose arrangement was broken down, and the degree of alteration depends on the percentage of H2SO4 used during oxidation.

Fig. 3
figure 3

FTIR spectra for A 100% H2O2, B 95% H2O2 and 5% H2SO4, C 85% H2O2 and 15% H2SO4, D 75% H2O2 and 25% H2SO4, and E untreated Xyris capensis

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Table 5 FTIR spectrum peak assignments of Xyris capensis after pretreatment
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Table 6 Wavelengths correlate to a particular functional group and their percentage relative variation to infrared spectroscopy
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The alteration in the lignin portion of Xyris capensis was found around 1698, 1718, 1732, 1701, and 1698 cm−1 for treatments A, B, C, D, and E, respectively, and is majorly linked to the chemical groups of lignin portion. It can be observed that oxidative pretreatment severely affected their peaks since they were observed to either be eliminated/flattened, which could be due to the strength of the acid used for pretreatment [57]. The influence was more significant when 85% of H2O2 was combined with 15% H2SO4. It can be inferred from the result that oxidative pretreatment could not alter the lignin when 100% of H2O2 was used. Pretreatment has no significant effect on lignin except for 1495 cm−1 (treatment E) due to their strong affinity with lignin content. This shows the strength of the H2SO4 to influence lignin content, as reported in the previous study [18]. The moderate effect of oxidative pretreatment on Xyris capensis was determined by the H2SO4 percentage on the lignin portion and produced pseudo lignin content, as reported in previous studies [53, 57]. This influence could lead to a reduction in the total biomethane released or a complete disruption of the process. Phenolic lignin is another lignin derivative observed at 1312 cm−1 peaks and lowered when the feedstock was pretreated with 100% H2O2. But other pretreatment conditions did not affect this lignin, as presented in Table 6. Syringyl lignin is another representation detected at the 1212 cm−1 band [36]. The effect of oxidative pretreatment was more pronounced in treatments A and B but moderate in treatments C and D. This implies that H2O2 has more influence on syringyl lignin than H2SO4. Another important lignin type discovered in Xyris capensis is acetyl lignin and found around 1029 cm−1. It can be inferred from Tables 5 and 6 that oxidative pretreatment increases this lignin on Xyris capensis. Treatments A and E experienced the same influence, and the effect is more significant in treatment C. This aligned with what was reported in the earlier study about the partial delignification of lignocellulose feedstock when pretreated with an oxidizing agent [58].

The result in Tables 5 and 6 and Fig. 3 shows that the hemicellulose of Xyris capensis was affected. This can be found at band 837 cm−1 and follows the same trend with acetyl lignin and have a solid interaction with hemicellulose. Treatments A and C show some level of hemicellulose reduction. Still, there is no significant influence on treatments B and D. This implies that improvement in hemicellulose solubilization is envisaged with treatments A and C with a reduction in hemicellulose, as observed in the previous study [50]. As recorded under FTIR, the functional group result has shown that different oxidative pretreatment conditions on Xyris capensis influence the structural arrangement differently. Findings from this study have shown that treatment with 75% H2O2 and 25% H2SO4 shows the most significant effects. This can be traced to the strength of H2SO4 to remove/redistribute the lignin and partial solubilization of the hemicellulose. The effect of oxidative pretreatment on the functional group observed in treatment D can be noticed to improve the subsequent biomethane yield when single pretreatment is considered. But when it was combined with nano additive (treatment H), the influence on biomethane yield was not optimum. This deviation in biomethane released compared to single pretreatment can be linked to the fast hydrolysis due to the unmasking of the lignin. Lignin removal/redistribution opened the feedstock and improved the surface area for Fe3O4 attachment, increasing the hydrolysis rate. The improved hydrolysis rate led to over-accumulation of the digester and altered the process’s volatile fatty acids (VFAs). Changes in the VFAs of the process altered the pH of the process. pH outside the recommended range (6–8) is harmful to methanogenic bacteria and reduces methane release [59, 60]. This result agreed with what was reported in previous literature when the influence of pretreatment techniques on functional groups and biogas yield was investigated [50].

3.4 Effect of pretreatment on biomethane yield of Xyris capensis

3.4.1 Effect of pretreatment on daily biomethane of Xyris capensis

The effects of single oxidative pretreatment at different conditions, Fe3O4 nano additive single pretreatment, and combined oxidative and 20 mg/L of Fe3O4 were examined on the biomethane yield of Xyris capensis, and the daily biomethane yield is presented in Fig. 4. It can be observed that the daily highest peak of biomethane differs in yield and occurs on different days, but it ranges between days 2 and 8. It can be inferred from the figure that the highest peak of 32.79 CH4/gVSadded was recorded from treatment E on day 7. The optimum daily biomethane yields of 32.62, 30.23, 14.93, 15.88, 12.37, 24.38, 20.39, 14.38, and 13.99 CH4/gVSadded were observed for treatments A, B, C, D, F, G, H, I, and J at 2, 7, 5, 10, 3, 2, 3, 4, and 8 days, respectively. It can be noticed that the untreated substrate released the least daily highest biomethane yield, which implies that all the pretreatment conditions improve the daily biomethane yield. It was also observed that combined pretreatment does not determine the optimum daily biomethane yield since the treatments with a combination of oxidative pretreatment and Fe3O4 nanoparticle additives were not wholly different from the single pretreatment of the oxidative pretreatment method. There is a slight difference between single oxidative pretreatment (treatment C) and single pretreatment of nanoparticle additive (treatment I). It can be observed that the pretreatment methods used improved the accessibility of the Xyris capensis. The results also show that most of the biomethane yield was released within the first ten days of the experiment, indicating a reduction in the retention period compared to the untreated substrate. This result agreed with previous studies that pretreatment methods improve the daily biomethane yield but to a varying degree [61, 62].

Fig. 4
figure 4

Effect of pretreatment methods on daily biomethane yield of Xyris capensis

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3.4.2 Effect of pretreatment on cumulative biomethane of Xyris capensis

The cumulative biomethane yield of Xyris capensis pretreated with oxidative, nanoparticle additive, and combined pretreatment after 27 days retention period is presented in Fig. 5. The biomethane yield of 185.97, 192.17, 195.72, 212.18, 208.50, 219.15, 278.59, 251.20, 216.41, and 143.21 CH4/gVSadded were recorded for treatments A, B, C, D, E, F, G, H, I, and J, respectively. The electrical potential estimate of the biomethane yield was calculated assuming that 1 m3 of methane is equivalent to 36 MJ, and with an electrical conversion efficiency of 35%. Therefore, 1 m3 of methane released will generate 10 kWh of electricity [63]. It was observed that 0.00186. 0.00192, 0.00196, 0.00212, 0.00209, 0.00219, 0.00279, 0.0025, 0.00216, and 0.0014 kWh electricity can be generated from treatments A, B, C, D, E, F, G, H, and I, respectively. Compared to the untreated substrate, it can be observed that the cumulative biomethane yields were improved by 30, 34, 37, 48, 46, 53, 95, 75, and 51% for treatments A, B, C, D, E, F, G, H, and I, respectively. The theoretical methane yield of the substrate was calculated to be 388.72 CH4/gVSadded, which indicates that none of the pretreatment conditions utilized the full potential of the feedstock. This study observed that only about 37% of the substrate’s potential was utilized for biomethane generation without pretreatment. This can be traced to the recalcitrant properties of the substrate that resist the activities of anaerobic microbes and limit the hydrolysis rate and subsequent reduction in biomethane yield [64]. Single pretreatment of oxidative pretreatment at different conditions improved the utilized capacity by 48–55%. In contrast, single pretreatment with Fe3O4 nanoparticle additives led to the utilization of the substrate’s 54% capacity, which is higher than all the conditions considered for mono oxidative pretreatments. This could be due to the nanoparticle’s ability to produce lesser inhibitory compounds than the oxidative pretreatment and its ability to release lower hydrogen sulfide [21]. When greenhouse waste was pretreated with a macerator at 75–95 °C hot, a methane yield of 410 ± 17.9 m3 tVS−1 was observed [65]. Steam explosion pretreatment at 80–90 °C on rapeseed straw released an optimum methane yield of 345.3 m3 tVS−1 [66]. The increase in biomethane yield from the Fe3O4 nanoparticle addition supported the earlier observation that Fe nanoparticles can improve biomethane yield [21]. The addition of Fe3O4 was observed to enhance the activities of microorganisms and increase the biomethane yield. Due to its fluid nature, this can be traced to its ability to regulate the process [67]. Fe2+/Fe3+ nanoparticle additives in the form of Fe3O4 during anaerobic digestion could serve as growth supplements for the microbes and influence their activities. Combined pretreatment was observed to improve the accessibility to the substrate, and the substrate utilization for biomethane yield was between 56 and 72%, which is satisfactory performance.

Fig. 5
figure 5

Effect of pretreatment methods on cumulative biomethane yield of Xyris capensis

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Single pretreatment of the oxidative method released its highest cumulative biomethane yield of 212.18 CH4/gVSadded when 75% H2O2 was combined with 25% H2SO4 (treatment D). The least biomethane yield was recorded when only H2O2 was used as the sole oxidizing agent. It can be noticed that the biomethane yield keeps increasing with the increase in the percentage of H2SO4. It was noticed, as previously stated, that when the H2O2 and H2SO4 were combined at 50: 50% as an oxidizing agent during the experiment, the entire substrate was burnt off. The influence of H2SO4 in this oxidizing agent can be linked to the strength of H2SO4 to remove or redistribute the lignin and hemicellulose portion of the substrate and make the cellulose available for biomethane release. This ability of H2SO4 to eliminate or redistribute lignin was reported when rice straw was pretreated with H2SO4 [68]. Single pretreatment of H2O2 on sorghum bicolor stalk enhances the biomethane yield by 65%, whereas H2SO4 pretreatment on the same feedstock improves the biomethane yield by 54.5%. Therefore, the combination of these two acids is more effective and economical, as observed in this study, since H2O2 is relatively cheaper than H2SO4. But the H2SO4 is required to improve the strength of the H2O2 and make it more efficient. Single pretreatment of 20 mg/L of Fe3O4 nanoparticle additives can be observed to improve the biomethane yield by 51%. This is a result of the ability of Fe3O4 to release some nutrients that support the growth of the methanogenic bacteria and subsequent increase in biomethane yield [35, 69].

Combined 20 mg/L Fe3O4 with the different oxidative pretreatment conditions enhances the biomethane yield of Xyris capensis. It can be observed from Fig. 5 that the biomethane yield was improved by 46, 53, 95, and 75% for treatment E, F, G, and H, respectively, compared to the control. It can also be noticed that 62, 65, 82, and 74% of the treatments E, F, G, and H were utilized during the anaerobic digestion. Compared with the single pretreatment treatments, it can be observed that the addition of Fe3O4 significantly improves the yield. The oxidative pretreatment removed the lignin portion of the substrate and opened up the hemicellulose and cellulose for nanoparticle attachment [70]. The major challenge of oxidative pretreatment is the release of inhibitory compounds at some pretreatment conditions [18]. Treatment G was pretreated with 85% H2O2 and 15% H2SO4, and the percentage of H2SO4 in this treatment condition could remove/redistribute the lignin content with minimal inhibitory compounds. This treatment released the highest biomethane yield, which can be traced to the more space opened for the attachment of Fe3O4 and minimal inhibitory compound that can hinder the methanogenic bacteria. Adding Fe3O4 improved the biomethane yield due to its ability to release some vital enzymes and co-factors that have been noticed to induce and stabilize the biogas production process [71, 72]. It has been noticed that nanoparticles of iron origin can enhance the biomethane release and lower the level of hydrogen sulfide [73]. Adding Fe2−/Fe3+ through Fe3O4 nanoparticles could improve the development of methanogenic bacteria and enhance their activities [38]. Physicochemical properties of Fe3O4 have been observed to have some goethite and magnetite. The magnetite releases bioavailable ions (Fe2− and Fe3+), which have been noticed to be a vital nutrient for microorganism’s power generation [74]. During iron corrosion through Fe3O4, hydrogen or an electron is produced that improves hydrogenotrophic methanogenesis and enhanced biomethane released from carbon dioxide intake, as presented in Eqs. 6–8 [72, 75]. A kinetic model of the chemical reaction control can be used to study the process rate.

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

This result agreed with what was reported when nanoparticle additives were combined with other pretreatment methods compared to single pretreatment of either pretreatment [74, 76]. 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. Biogas yield was improved by 56% when cassava peels were pretreated with combined enzyme and alkaline pretreatment [77]. The retention time was reduced significantly compared to mono pretreatment when some pretreatment methods were combined [78]. Methane yield was improved by 57% compared to single pretreatment when the steam explosion was combined with NaOH during the pretreatment of Miscanthus lutariorriparius pretreated before anaerobic digestion [79, 80]. In a similar study, the biogas yield of sugarcane bagasse was reported to increase by 92% when 1.0% acetic acid was combined with a steam explosion [80]. Nevertheless, it should be noted that the number of pretreatment methods applied determines the process’s economy. Higher pretreatment costs will make the process uneconomical and may not be able to compete with fossil fuels. Therefore, it is crucial to study the economy of each combined pretreatments methods and present the most effective pretreatment techniques that are economical and sustainable.

Compared with other studies, the variation in methane released could be traced to different factors such as purity of acid used, concentration, autoclave temperature, and the structural arrangement of the feedstock, although it is difficult to compare with other feedstock because of the difference in feedstock. It should also be observed that results are presented at the laboratory scale, while some have been investigated at the pilot and commercial scales. This method required equipment with high corrosion resistance due to the acid toxicity and the release of inhibitory compounds, which are the major limitations of the process. This method can be reproduced on other lignocellulose feedstocks, particularly energy grasses and other feedstocks that have an identical microstructural arrangement, like Xyris capensis. Enhancing methane generation from lignocellulose feedstocks will reduce the influence of international oil market prices on the macroeconomics of some countries, especially developing countries [81]. Pressure on nation’s foreign exchange can be reduced through the efficient application of methane produced from the abundant and cheap lignocellulose materials instead of total reliance on oil importation [82].

4 Conclusion

This study shows that oxidative pretreatment at various conditions significantly influences the microstructure, crystallinity, and functional groups of Xyris capensis. The highest cumulative biomethane yield of 212.18 mL CH4/gVSadded, representing a 48% increase, was observed for single pretreatment. Combined pretreatment with nanoparticle additive produces the optimum cumulative biomethane yield of 278.59 mL CH4 /gVSadded (95% increase). Unlike other chemicals, applying relatively cheap H2O2 is uncommon in lignocellulose pretreatment. It can be observed from this study that H2O2 can reduce the recalcitrant properties of lignocellulose and improve the biomethane yield. Adding 20 mg/L Fe3O4 (< 50 nm) to oxidative pretreated substrate enhances biomethane yield further. Therefore, this new novel combined pretreatment could be regarded as a probable lignocellulose disintegration technique and can be applied at the industrial scale.