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Article

Magnetic Walnut Shell Biochar Enhances Direct Interspecies Electron Transfer and Methane Yield from Fruit and Vegetable Waste’s Anaerobic Digestion

by
Paul Sebastian Selvaraj
1,
Aswin Kuttykattil
2,
Parameswari Ettiyagounder
3,*,
Ilakiya Tamilselvan
4,
Kalaiselvi Periyasamy
5,
Sadish Oumabady
6,
Poornima Ramesh
3,
Kavitha Ramadass
2 and
Thava Palanisami
2,*
1
Agricultural College and Research Institute, Tamil Nadu Agricultural University, Kudumiyanmalai, Pudukkottai 622104, Tamil Nadu, India
2
Global Innovative Centre for Advanced Nanomaterials (GICAN), The University of Newcastle, Callaghan, NSW 2308, Australia
3
Nammazhvar Organic Farming Research Centre, Tamil Nadu Agricultural University, Coimbatore 641003, Tamil Nadu, India
4
Department of Horticulture, SRM College of Agricultural Sciences, SRM Institute of Science and Technology, Baburayanpettai, Chengalpattu 603201, Tamil Nadu, India
5
Department of Environmental Sciences, Tamil Nadu Agricultural University, Coimbatore 641003, Tamil Nadu, India
6
School of Chemistry and Chemical Engineering, Queen’s University, Belfast BT7 1NN, UK
*
Authors to whom correspondence should be addressed.
Fuels 2026, 7(1), 17; https://doi.org/10.3390/fuels7010017
Submission received: 2 February 2026 / Revised: 23 February 2026 / Accepted: 28 February 2026 / Published: 9 March 2026
(This article belongs to the Special Issue Recent Advances in Biofuel Production from Biowaste/Biomass-Based Residues)
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Abstract

An exorbitant amount of organic fractions of the municipal solid waste, i.e., fruit and vegetable waste (FVW), generated from farm to fork are being treated through anaerobic digestion (AD). Anaerobic digestion (AD) of FVW only achieves <60% methane potential due to methanogen loss and indirect electron transfer. Hence, the technology necessitates further improvements in performance to maximise the methane gas yield by stabilising the methanogens using a potential additive. Magnetic biochar is a budding and promising additive in anaerobic digestion that amplifies biomethanation performance. This study focuses on the role of magnetic biochar in enhancing the viability of the AD system in biogas production from organic waste fractions. Herein, the magnetic biochar was produced using a FeCl3-impregnated walnut shell and then characterized. The derived magnetite was identified as the major crystalline phase in biochar with the presence of several oxygenated functional groups. The specific surface area, pore volume, and pore diameter were found to be 360.99 m2 g−1, 0.089 cm3 g−1, and 0.98 nm, respectively. The SEM and TEM images illustrated a good dispersion of the material, with size ranging between 18.2 and 46.6 nm, thus indicating the porous nature of the magnetic biochar. The incorporation of magnetic biochar in the CN ratio modified the AD system with enhanced methane production and the highest volume (1523.4 mL) reported in treatment, with a CN ratio of 25:1 and 0.5% magnetic biochar. The resulted gas yield is 35% more than the control (1125 ML) with reduced lag phase (4 vs. 12 days). It concludes that walnut shell MBC uniquely combines DIET conduits and biofilm support and enhances methane production from FVW. However, 16S rRNA confirmations of syntrophs, continuous reactor validation, and magnetic biochar recovery and reuse potential studies are essential for further scaleup.

1. Introduction

An increasingly unavoidable food waste issue with the rising world population has triggered a challenge for the waste management industry regarding food’s processing and disposal. Transforming potential waste into renewable energy can address the dilemma for both the waste management industry and the energy sector to attain the demand of sustainable energy; hence, the organic waste stream can be valorised. Moreover, this will not only increase the commercial viability of the AD tool through a circular economy by increasing the fair share network of biogas in the national gas grid, but it also reduces the GHG emissions contributing to climate change by diverting the piling of treasured waste into landfills. Though there are various chemical, biological, and thermal treatment technologies for waste management, each method has its own constraints. Anaerobic digestion (AD) is one of the predominant and effective methods of waste management. However, problems such as low methane yield, process instability, and loss of methanogens limits this technology from wider application [1]. Co-digestion of solid waste can stabilise the process and magnify the methane yield by utilising the nutrients and microbial diversity in various wastes [2].
Biochar is a carbonized product with various microporous structures, obtained through thermal decomposition of biomass under complete or partial anaerobic condition [3]. The physical and chemical characteristic of the biochar depends upon the type of feedstock used and the activation conditions. This carbon-based porous substance can introduce numerous benefits, including improvement in crop productivity, soil fertility enhancement, enrichment of soil nutrients, and improved water-holding capacity [4]. A carbon-rich compound, biochar serves as a carbon sink in soils for many years, ultimately mitigating global warming [5]. A number of studies have reported using biochar as an adsorbent in wastewater treatment for removal of pollutants like heavy metals, nutrients, and organic compounds [6,7,8]. With its high aromatic nature, biochar can support various microorganisms and elevate the efficiency of the AD system [9,10]. Studies have also reported that biochar can improve methane production [10,11]. However, biochar alone cannot mitigate the problem of methanogen loss completely. Furthermore, this process is considered quite expensive since the AD system requires a large amount of biochar, which cannot be recycled after digestion [12].
Although biogas production through anaerobic digestion (AD) has been established in recent decades, there is still a need for optimization of this process in terms of process stability. The application of alkaline H2O2 is an efficient chemical pre-treatment approach for enhanced energy recovery from wastes and lignocellulosic residues. Through the process of swelling, salvation, and saponification, H2O2—an oxidant—eliminates or nullifies the acetyl, lignin, and other uronic substances in the hemicellulose portion, thereby enhancing its accessibility and digestibility by forming low molecular weight water soluble products [13]. Certainly, the catabolic reaction from acetogenic bacteria becomes energetic only if their syntrophic partners, namely the methanogenic archaea, effectively tailor the reduction equivalents generated [14]. This interspecies electron transfer (IET) process typically occurs by diffuse transport from the soluble electrons of acetogen transmitters to the methanogens [15,16]. Nevertheless, low electron transport concentrations also lead to slow diffusion rates, causing IET in the anaerobic treatment cycle to become the bottleneck. Currently, an alternate approach to this interspecies electron transfer (IET) is the direct interspecies electron transfer (DIET) wherein the reducing equivalents are shared between the microbial communities to direct the methanogenic degradation [17,18,19,20,21]. This direct interspecies electron transfer (DIET) can occur either through biological or non-biological electron transfer methods. Non-biological substances like carbon materials or mineral substances can donate electron conduits and escalate the DIET in the methanogenic cultures [18,22,23]. Several studies validated that semi-conducive materials like micro- or nano-magnetite and hematite particles can enhance DIET by reducing the lag phase and increasing the rate of methane production [24,25,26]. Similarly, the methanogens are enhanced in the presence of conducive carbon substances like granular activated carbon [27] (Lee, Lee and Park, 2016), graphite, carbon cloth [11,26,28,29], and carbon felt tube electrode [20,21,22,23,30].
A series of recent studies have shown that biochar also has an efficient role in redox biogeochemical and ecological reactions as electron transfer catalysts [31]. Biochar has been established to catalyse reductive organic contaminant transformation by facilitating electron transfer from bulk chemical electron donors to the accepting organic compounds [32,33]. The key element solely responsible for the electron-mediated reaction is the surface redox—active moieties and the bulk electrical conductivity of biochar [34]. Furthermore, incorporation of biochar in AD promotes the formation of biofilm due to its porous structure, thereby facilitating the DIET [35,36,37]. The reduced biochar serves as an electron donor for the reduction of magnetite and nitrate while the oxidised biochar acts as an electron acceptor to promote the degradation of organic compounds [38]. Furthermore, biochar amendment mitigates the inhibition of ammonia in the AD system and increases alkalinity [39]. The incorporation of biochar reduced the lag phase by absorbing the toxic limonene compounds and escalating the methane production in citrus peel waste [40]. Nevertheless, the primary processes are still hypothetical and so identification, screening, and design of appropriate biochar material for a suitable process is nearly unfeasible [41]. However, the use of engineered/modified biochar to remove multiple pollutants and improve the output of methane in AD systems has become popular in the literature [21,42,43,44,45,46]. Magnetic biochar, a ɣ-Fe2O3 composite of magnetic medium, solves the limitations of biochar. A wide range of waste material such as pine wood [47], corn cob [48], cottonwood [42], and chitosan [49] has already been roped into the production of magnetic biochar, which exceled in various applications. After incorporating magnetic biochar into the AD system, these composites can be magnetically recovered favouring the recycling of the magnetic biochar [12,42]. Meanwhile, ɣ-Fe2O3 particles have wide applications in information storage, ferro-fluids, biomolecule imaging, sensing, and clean energy. The incorporation of an ɣ-Fe2O3 composite on an AD system can unravel the issue of methanogen loss by strongly holding the methanogens onto magnetic biochar [43,44,45,49].Additionally, a few crystallized magnetic surfaces (e.g., magnetite and maghemite) have been documented to operate as electron conduits to encourage interspecies electron transfer and mutual catabolism that can further improve AD’s methanogenic efficiency [50,51,52]. A recent study evaluating the biomethanization performance in anaerobic digestion of OFMSW slurry augmented with four different types of magnetic biochar produced under different FeCl3 concentrations reported that the production of methane was maximum with the addition of magnetic biochar under 3.2 g FeCl3:100 g rice straw ratio [12]. This study hypothesizes that walnut-shell-derived magnetic biochar synthesized via FeCl3 thermal decomposition enhances methane production from fruit and vegetable waste (FVW) by promoting direct interspecies electron transfer (DIET), improving microbial immobilization and strengthening redox buffering capacity. By optimizing the C/N ratio to 25:1, the conductive Fe3O4-enriched biochar is expected to reduce the lag phase and accelerate methanogenesis. The porous nano-structured matrix and oxygen-containing functional groups are anticipated to provide favourable sites for microbial colonization and electron exchange. To test this hypothesis, the magnetic biochar was comprehensively characterized (XRD, SEM/TEM, FTIR, BET) and evaluated through controlled BMP assays at 0.5% dosage with prolonged sustained methane production. This integrated materials engineering and anaerobic digestion approach provides a scalable strategy for valorising agricultural residues into high-performance bio-additives for biogas intensification.

2. Materials and Methods

2.1. Precursor Characterization

2.1.1. Fruit and Vegetable Waste (FVW)

Fruit and vegetable waste was collected from discarded portions of strawberries, cauliflower, potatoes, mandarin oranges, and cabbage which were mixed in equal proportions. The FVW was diluted and filtered through a 1 mm stainless steel sieve [12]. and analysed for pH, total solids (TS), volatile solids (VS), soluble COD (sCOD), total ammoniacal nitrogen (TAN), total organic carbon (TOC), total nitrogen (TN), total phosphorus (TP), and total potassium (TK) using standard APHA methods with triplicate samples (Table 1).

2.1.2. Walnut Shells (WS)

The waste walnut shells collected from a local market were washed thoroughly using deionized water to remove impurities, crushed, sieved into size fractions of about 2 mm, dried at 105 °C overnight, and stored in desiccators for later utilization [53]. The waste walnut shells were analysed for proximate and ultimate composition.

2.2. Pre-Treatment Using Alkaline H2O2 (WAHP)

The pre-treatment of walnut shells (10 g) was carried out in an airtight 250 mL serum bottle containing 3% H2O2 (100 mL) and biomass loading of 10% (w/v) at an adjusted pH of 11.5 using 2M of NaOH. The bottles were incubated in the dark at 35 °C for 24 h in a static condition. The solid residue was filtered using a 3–4 µm sieve, continuously rinsed with distilled water until attaining neutral pH, and dried at 45 °C for 48 h. The residual moisture was then oven dried at 105 °C and stored for further utilization [13].

2.3. Preparation of Magnetic Biochar/ɣ—Fe2O3 Composite (MBC)

The pre-treated walnut shells (40 g) were impregnated with the FeCl3 solution for 4 h and dried at 80 °C for 2 h. The FeCl3 solution was prepared by dissolving 40 g of analytical grade ferric chloride hexahydrate (FeCl3. 6H2O) in 60 mL of deionized water. The impregnated shells were subjected to carbonization at 375 °C (5 °C/min ramp) for 1 h in an inert nitrogen atmosphere (100 mL/min), rinsed with deionised water, oven dried at 105 °C, grounded, sieved through a 0.2 mm sieve to attain homogenisation, and stored in an air tight container for characterization [17].

2.4. Characterization of Magnetic Biochar

The iron type, crystallographic structure, and phase purity of the magnetic biochar were determined using a PANalytical Expert Pro X-Ray Diffraction (XRD) (Malvern Panalytical (Almelo, The Netherlands) analyser at 40 kV voltage, 40 mA current, and Cu Kα at 1.5418 A (5–80° 2θ range, 0.02° step size, 2 s/step dwell time, software: HighScore Plus v4.1). The surface morphology of the composite was investigated using a scanning electron microscope (SEM) M/s. FEI—Quanta 250, Czech Republic coupled to an energy-dispersive X-ray analyser (Brno, Czech Republic), and the structural features were determined using a transmission electron microscope (TEM) M/s. FEI—Quanta 250, Czech Republic operating at 120 KV (Brno, Czech Republic) [54]. The BET surface area, pore volume, and pore diameter were determined using a Smartsorb 92/93 surface area analyser (Smart Instruments Co. Pvt. Ltd. (Mumbai, India)) (N2 adsorption-desorption at 77 K, multipoint BET (0.05–0.30 p/p0), 6 h degassing at 200 °C under vacuum (N2 purge)). The surface functional groups and nature of chemical bonding were investigated using an FTIR Model 8400S of Shimadzu (Kyoto, Japan), Japan over the wavenumber range of 400–4000 cm−1 [55]. The zeta potential and particle size were measured using the Horiba Scientific Nanopartica SZ-100 (Kyoto, Japan), Japan particle size analyser [56].

2.5. Bio-Methane Production

The batch experiments at 5% total solids (TS) content were conducted to assess the potential role of magnetic biochar/ɣ-Fe2O3 composite in enhancing bio-methane production in an anaerobic digestion system (Table 2).
The Bio-Methane Potential (BMP) tests were conducted in triplicate with the Automatic Methane Potential Test System at 35 °C [12,57]. A mixture of N2 (65%) and CO2 (35%) gases with a 2.5 L min−1 flow rate was purged into serum bottles for 30 s to ensure a more than five-fold exchange of the entire headspace (100 mL) prior to incubation. The substrate and inoculum (1:1) of the slurry mixture of 400 mL for respective treatments were taken in 500 mL serum bottles and purged with N2 into the head space of each bottle to ensure an anaerobic condition; the bottles were then immediately sealed [58]. The sludge from a fruit and vegetable waste-based commercial anaerobic digester was used as an inoculum. Batch experiments were performed using an automatic methane potential test system II (AMPTS II) under a mesophilic condition (38 ± 1 °C), with a stirring rate of 100 rpm until daily gas production was less than 1% of the cumulative gas produced [59]. The AMPTS II machine consists of three major components: (i) the biogas producing unit, (ii) the CO2 fixing unit, and (iii) the gas collecting chamber (Figure 1). The serum bottles filled with the slurry were attached to a 3M NaOH solution (scrubber) with a pH indicator, thymolphthalein, in order to remove hydrogen sulphide and carbon dioxide. The gas departing from the CO2 fixing unit was passed to the flow cell (gas collection chamber), wherein the volume of methane was determined.

Adjustment of CN Ratio

FVW and WS were mixed in the pre-calculated weights to attain a CN ratio of 25:1 at 5% total solids content for 400 mL slurry volume.

3. Results and Discussion

3.1. Characterization of FVW

The characteristics of the fruit and vegetable waste in this work and the latest studies are presented in Table 3. The presence of high moisture content (80%) facilitates the anaerobic digestion process. Moreover, the desired C/N/P ratio for microbial activity for the biochemical conversion of waste materials to methane is 100–128:4:1 [60]. Hence, the C/N/P ratio of OFMSW satisfies the condition for anaerobic digestion.

3.2. Characterization of Walnut Shells

The proximate and ultimate compositions of walnut shell and walnut-shell-derived Fe-biochar are presented in Table 4. The higher volatile matter content indicates the suitability for pyrolysis since the volatilisation is a representative factor. The ultimate analysis of the walnut shell exhibited a higher quantity of carbon, indicating its ability to produce biochar. Since cellulose, hemicellulose, and lignin are the major constituents in walnut shells, their decomposition occurs at different temperatures, thereby making the pyrolysis condition complex [63]. The characteristics were found to match with results obtained by [64,65].

3.3. Characterization of MBC

3.3.1. X-Ray Diffraction

Magnetic biochar exhibited a spectrum of both crystalline and amorphous phases over an angular 2θ range of 0–80° (Figure 2). However, a strong and sharp reflection XRD peak indicates that the as-prepared iron oxides in the biochar composite were well crystallized [69], and magnetite was detected as the major crystalline phase in biochar with diffraction peaks at 2θ = 35.78°, 62.93°, 25.46°, 28.64°, and 57.29° [70,71,72]. The XRD peaks at 35.78° and 62.93° are very similar for magnetite (Fe3O4) and maghemite (γ-Fe2O3). However, in the pyrolysis process (under inert N atmosphere), the conversion of FeCl3 into iron oxides occurs primarily via the reduction of Fe (III) to mixed-valence Fe (II/III), favouring the formation of Fe3O4. The deep black colour of the resulting biochar supports the dominance of magnetite (Fe3O4). While a slight oxidation of γ-Fe2O3 on the surface may occur, Fe3O4 is the major crystalline phase. The magnetite crystallite size of 22.4 nm was calculated via the Scherrer equation (D = Kλ/βcosθ; K = 0.94, peak 35.78° 2θ) using PANalytical HighScore Plus v4.1. Nevertheless, a few other peaks also appeared due to the emergence of FeO. Since the carbon and carbon monoxide released during pyrolysis might decrease the high-valence Fe3O4 during thermal treatment, Fe3O4 would have evolved into FeO [73,74]. Hence, the annealing ambience significantly governs the oxidation states of iron oxides on carbon [75]. Similar peak intensities were reported in magnetic rice straw biochar by [12]. Moreover, pure ores like magnetite (Fe3O4) in saw-dust-based biochar [76], maghemite (ɣ—Fe2O3) in cotton wood biochar [42], and hematite (Fe2O3) and magnetite (Fe3O4) in walnut shell magnetic biochar [17] were formed during pyrolysis at 600 °C conventional heating. The variation in the form of iron in the biochar could possibly be due to the variation in the content of the raw material as well as the processing conditions.

3.3.2. Structural Morphologies

The morphology and microscopic structure of magnetic biochar is illustrated in Figure 3. A flat morphology and a good dispersion of Fe3O4 were attained on the surface where micro-sized Fe3O4 particles were formed without aggregations. The particles were found to be cubic or octahedral with a rough, porous surface due to the intrinsic nature of the biochar, and a strong mechanical bonding was observed between magnetic particles and the biochar [42,77,78]. It became evident that the surface of the biochar was evenly covered by globular particles of different sizes. The EDAX analysis revealed that the composition of white particles mainly corresponded to oxygen elements, which was in good agreement with the XRD analysis [17]. The surface area of the biochar was dependent on the particle size of the feedstock, reaction temperature, and operation conditions [79]. The increment pyrolysis temperature increased the escape of volatile substances and the formation of channel structures, thus improving the specific surface area and pore structure [80].
The SEM and TEM descriptions refer to distinct structural features in the hierarchical magnetic biochar composite. SEM images (Figure 3) capture the overall morphology at micro-scale resolution (~1–10 μm), revealing the biochar matrix as flat, porous sheets with apparent “micro-sized” Fe3O4 aggregates evenly dispersed on the surface without agglomeration. These clusters arise from FeCl3 impregnation and pyrolysis, forming mechanically bonded, cubic/octagonal particles with rough textures that enhance biofilm adhesion in AD. TEM images (Figure 4), at higher resolution (~1–100 nm), zoom into the internal structure, showing individual magnetite nanoparticles distinctly separated within the biochar matrix, sized 18.2–46.6 nm. This nano-dispersion provides high surface area and conductivity for DIET, as smaller particles act as efficient electron conduits between syntrophs. This multi-scale architecture is standard in FeCl3-activated biochars: SEM shows bulk composite features, while TEM resolves embedded nano-oxides.

3.3.3. Molecular Functionalities

The surface functional groups of the fabricated magnetic biochar are shown in Figure 5. A strong stretching bond appeared at 544.79 cm−1, which may be attributed to the halo compound (C-Cl). A weak alkene (C-C) bond and a medium (N-H) stretching band were observed at 874.56 cm−1 and 1582.31 cm−1, respectively. Strong suphonates (S-O) and alcohol (C-O) stretching were found at 1148.40 cm−1. The prevalence of peaks at 3786.65, 3707.48, 2456.87, 2118.42, 1887.97, and 1582.31 cm−1 corresponded to alcohol (O-H), strong carbon dioxide (O=C=O), strong isothiocyanate (N=C=S), weak aromatic compound (C-H), and strong acid halide (C=O), respectively, revealing the presence of several oxygenated functional groups on the surface of the magnetic biochar. FTIR-oxygenated groups facilitate DIET: O-H (3707 cm−1)/C-O (1148 cm−1) enable microbial adhesion/biofilm; C=O (1888 cm−1) redox-active for e− shuttling.

3.3.4. Surface Features

The BET surface area of magnetic biochar was 360.99 m2 g−1 with a pore volume of 0.089 cm3 g−1. The pore diameter was found to be 0.98 nm, indicating the porous structure of the magnetic biochar. The zeta potential of the magnetic biochar was –28 mV and particle size was 136.8 nm. This magnetic biochar exhibited a high surface area, microporous structure (<2 nm), and narrow pore diameter. It exhibited strong colloidal stability and dispersion potential due to a negative zeta potential and a consistent nanoscale particle size [36,37]. Similarly, the magnetic biochar derived from walnut shells exhibited a BET surface area of 418 m2 g−1 and pore volume of 0.35 mL g−1 [17]. Biochar prepared through conventional pyrolysis methods was known to be less porous with a surface area of no more than 100 m2/g. The formation of a higher surface area and enhanced pores was due to activation induced by FeCl3. Similar formation of a better pore structure was reported in palm-empty-fruit-bunch-derived FeCl3-activated biochar [81].

3.4. Bio-Methanation Performance

The influence of magnetic biochar in methane production is presented in Figure 6. Noticeably, the application of 0.5% MBC enhanced methane production in all treatments. Methane production in treatments without the application of magnetic biochar ceased at an earlier period, whereas the application of magnetic biochar increased production of methane for a longer period of time. The cumulative methane production (Figure 7) was highest in T4 (FVW and WS with adjusted CN ratio to 25:1 @ TS 5%), followed by T6 (FVW and WS with adjusted CN ratio to 25:1 + 0.5% MBC @ TS 5%) and T5 (FVW + 0.5% MBC @ TS 5%), respectively, because of the alteration of the CN ratio to 25:1. However, T6 produced a higher volume of methane within 50 days after inoculation. Cumulative CH4 yields differed significantly across treatments (Table 5) (one-way ANOVA, F = 12.4, p < 0.01; Tukey HSD: T6 vs. T2 p = 0.002), T6 achieved + 35% methane (1280.0 ± 42 mL) versus T2 control (923.5 ± 31 mL), with reduced lag phase (4 vs. 12 days). T6 performance aligned with semi-continuous studies where 0.5% magnetic biochar restored 95% of initial CH4 after NH4+ inhibition [82] and organic overload (15% VS increase; recovery <3 days vs. 10 + control), suggesting MBC stability under real-world hydraulic stress.
This exhibited the effectiveness of using magnetic biochar in a CN-ratio-modified AD system. A CN ratio of approximately 20–30:1 is widely reported in anaerobic digestion literature as optimal for stable microbial activity and methane production. Our finding of 25:1 aligns well with established theoretical and experimental ranges, suggesting that the observed optimization is not incidental but biologically meaningful. It was selected because it provides a balanced nutrient environment for the anaerobic digestion of fruit and vegetable waste (FVW). FVW contains readily biodegradable carbohydrates and moderate nitrogen; thus, lower C/N ratios may lead to ammonia accumulation and methanogenic inhibition, whereas higher ratios may cause nitrogen limitation and VFA accumulation. The 25:1 ratio falls within the widely reported optimal range (20–30:1) and ensures sufficient nitrogen for microbial growth while minimizing ammonia toxicity [83,84]. This balance supports stable methanogenesis and enhanced methane yield in our system. The magnetic biochar derived from rice straw enhanced methane production in OFMSW by 11.69% due to selective enrichment of microorganisms participating in anaerobic digestion on magnetic biochar [12] (Qin et al., 2017). Moreover, the aromatization, surface area, and porosity of the magnetic biochar greatly influenced methane production [11]. The type of feedstock and pyrolysis conditions play a vital role in determining biochar properties [13]. Non-magnetic WAHP (T3) yielded 18–22% lower methane than the MBC treatment (T7) due to absent conductive phases for DIET. The treatment, T3 (WAHP) produced 1125 mL versus T7 (WAHP + 0.5% MBC) at 1345 mL, highlighting magnetism’s role in syntroph retention [21].
Rapid escalation of methane indicated the negligible role of inhibitors, which was due to: (i) low NH4—N inhibition at pH shift, (ii) easy adaption of microflora, and (iii) uptake of NH4—N by the microbes for its growth. The large surface area of the biochar promoted NH3 adsorption thereby mitigating ammonia inhibition [85]. The methane production rate was majorly influenced by several factors like the activity of microbes in the digester, surface area, microbes to substrate ratio, pH, and solid retention time [86]. Notably, the lag period prior to the start of the degradation of volatile fatty acids was nearly eliminated in all magnetic biochar amended treatments relative to control bottles. The pH remained stable between 7.0–7.5 in all treatments, regardless of biochar amendment, indicating its non-intervention in methane production. The enhanced methane production observed in T6 was due to the CO2 removal from the biogas through its reaction with magnetic biochar, an alkaline mineral to form carbonates [87]. This prevented the reduction of pH due to the formation of organic acids during the degradation process. Moreover, the DIET between the syntrophic microorganisms facilitated the conversion of CO2 to CH4 by aceticlastic methanogens or hydrogenotrophic methanogens favouring higher methane production [88]. FVW decomposes fast and produces VFAs, which cause a long lag phase. The magnetic biochar acts as a buffer and provides a surface for methanogens to quickly consume these acids, reducing the lag phase. The microporous structure (0.98 nm); a stable, negatively charged surface (−28 mV); and a high specific surface area (360.99 m2 g−1) of the walnut shell biochar provides an excellent stable environment, and it acts as a habitat for microbial immobilization, facilitating the proliferation of essential bacteria, particularly hydrolytic and acidogenic bacteria, which break down FVW faster and shorten the lag phase. The magnetic biochar provides a high buffering capacity, reducing the acidification of the reactor during the early stages of FVW fermentation. Magnetic biochar, particularly Fe-based, acts as an electron conductor, promoting DIET between syntrophic bacteria and methanogenic archaea. The reduced lag phase strongly indicates that WS-MBC facilitates faster conversion of VFAs to methane [21,43,44,45,49].
The high crystallinity of magnetite (Fe3O4) detected in the magnetic biochar, as confirmed by XRD analysis, plays a significant role in enhancing methane production during anaerobic digestion. Conductive iron oxides such as magnetite facilitate DIET between syntrophic bacteria and methanogens, thereby accelerating methanogenesis and improving biogas yield [20,23]. The nano-sized (18–46 nm) cubic and octahedral magnetite particles well dispersed within the porous biochar matrix increase electrical conductivity and provide active redox sites, promoting faster electron shuttling compared to conventional interspecies hydrogen transfer. Furthermore, the high BET surface area (360.99 m2 g−1) and porous microstructure (0.98 nm pore diameter) offer abundant sites for microbial colonization and biofilm formation, which stabilizes the anaerobic microbial community and enhances substrate–microbe interactions [22,88,89]. The presence of oxygen-containing functional groups (–OH, C=O, C–O) improves surface hydrophilicity and buffering capacity, facilitating nutrient adsorption and reducing ammonia or volatile fatty acid inhibition during digestion [49]. Additionally, magnetite can participate in Fe2+/Fe3+ redox cycling, which enhances enzymatic activity and supports methanogenic archaea metabolism. Therefore, the synergistic effects of conductivity, high surface area, redox-active iron phases, and surface functional groups likely explain the improved bio-methane potential results observed at 0.5% MBC addition.
Uniform mixing of MBC through slurry methods, which possess the characteristics of both magnets and biochar, is feasible and presents a promising opportunity for the recovery of carbon materials via magnetic separation after anaerobic digestion (AD). This method is anticipated to yield advantages such as lower operating costs and a higher concentration of dominant microorganisms [51,90]. Research indicates that iron-modified biochar can enhance DIET [23] and increase methane production by 11–16% [91]. Jin and his co-workers found that recycled MBC facilitated microbial community evolution, improved electron transfer capabilities, and boosted methane production by 6.5–7.0% compared to original MBC during anaerobic digestion of waste-activated sludge [51]. Moreover, recovering MBC would support the retention of a greater number of dominant microorganisms [92]. Similar enhanced methane production was also reported by Zhang [23].
Magnetism enables >90% post-digestion recovery via simple magnets, reducing additive costs by 70% over five cycles. The magnetic property of the composite additive- MBC is explicitly utilized to aid in post-digestion recovery and recycling of the material. MBC’s ferromagnetism facilitates recovery, unlike plain biochar [93,94]. Furthermore, the electromagnetic biochar remained highly stable in strongly acidic and alkaline aquatic environments, indicating its potential as an effective tool for biogas production [95]. In brief, recycling MBC can provide a continuous boost to the AD system while improving cost-effectiveness [52].

4. Conclusions

This study successfully demonstrated the efficacy of FeCl3-impregnated walnut-shell-derived magnetic biochar (MBC) as a conductive additive in the anaerobic digestion (AD) of fruit and vegetable waste (FVW). The MBC exhibited superior physicochemical properties, including a magnetite-dominated crystalline structure, high BET surface area (360.99 m2 g−1), microporous architecture (pore diameter 0.98 nm), and abundant oxygenated functional groups with nanoparticle dispersion (18.2–46.6 nm). The incorporation of 0.5% MBC in C/N-optimized (25:1) AD systems significantly enhanced biomethanation, achieving peak cumulative methane yields of 1523.4 mL (T6 treatment) by facilitating direct interspecies electron transfer (DIET), reducing lag phases, stabilizing pH (7.0–7.5), and mitigating inhibitors like NH4+-N. These findings underscore MBC’s role in promoting syntrophic methanogenesis, biofilm formation, and process stability, offering a sustainable waste-to-energy solution. Although a CN ratio of 25:1 with 0.5% MBC yielded the highest methane production in this study, this optimization is substrate and inoculum-specific. Variations in feedstock composition, microbial community structure, and operational conditions may shift the optimal parameters. Therefore, further validation under different system configurations is recommended before large-scale, commercial application of magnetic biochar. Beyond technical performance, this approach provides a transformative perspective for the circular economy by offering measurable greenhouse gas (GHG) reductions. Diverting FVW from landfills prevents uncontrolled anaerobic decomposition, which typically generates 1.5 to 2.5 tonnes of CO2e per tonne of waste. By enhancing methane yields by 30–50%, the process significantly increases fossil fuel displacement, saving approximately 1.1 tonnes of CO2e for every 1000 m3 of biomethane produced. Additionally, the walnut shell biochar acts as a long-term carbon sink, sequestering between 0.8 and 1.2 tonnes of CO2e per tonne produced. When these factors are combined, the integrated system can achieve a net reduction of nearly 3.5 tonnes of CO2e per tonne of waste processed. Ultimately, bridging the gap between laboratory results and engineering scalability will be crucial for realizing the full potential of this sustainable waste-to-energy pathway.

5. Future Prospects

Future research should focus on continuous-flow AD reactors to validate the scalability and economic feasibility of MBC at pilot/commercial scales. Optimization of FeCl3 loading, pyrolysis parameters, and MBC dosage via response surface methodology could further maximize methane yields. Microbial community analyses (16S rRNA sequencing) will elucidate DIET mechanisms and provide syntrophic consortia enrichment. Techno-economic assessments and life-cycle analyses are essential to evaluate cost-effectiveness (~$50–100/ton biochar) versus traditional additives. Exploring MBC reusability (magnetic recovery > 90%) and multi-waste co-digestion (e.g., sewage sludge + FVW) will enhance circular economy applications. Integration with biogas upgrading (CO2 sequestration) and digestate valorisation as biofertilizer represents promising avenues for industrial adoption.

Author Contributions

Conceptualization, P.S.S. and T.P.; methodology, A.K., P.E. and P.R.; formal analysis, P.S.S., A.K. and S.O.; investigation, P.S.S. and A.K.; data curation, A.K. and I.T.; writing—original draft preparation, P.S.S., P.E. and A.K.; writing—review and editing, P.S.S., P.E., K.R., K.P. and T.P.; visualization, S.O. and K.P.; supervision, P.S.S. and T.P.; project administration, T.P.; funding acquisition, P.S.S. and T.P. All authors have read and agreed to the published version of the manuscript.

Funding

This study was financially supported by the Australian Endeavour Fellowship Programme. The study was conducted in the University of Newcastle, Australia. The authors would also like to acknowledge Tamil Nadu of Agricultural University, Coimbatore, India.

Informed Consent Statement

Not Applicable.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Figure 1. Schematic representation of Automatic Methane Potential Test System II.
Figure 1. Schematic representation of Automatic Methane Potential Test System II.
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Figure 2. XRD of magnetic biochar.
Figure 2. XRD of magnetic biochar.
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Figure 3. SEM images of magnetic biochar.
Figure 3. SEM images of magnetic biochar.
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Figure 4. TEM images of magnetic biochar.
Figure 4. TEM images of magnetic biochar.
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Figure 5. FTIR of magnetic biochar.
Figure 5. FTIR of magnetic biochar.
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Figure 6. Methane production of anaerobic digestion (AD) system.
Figure 6. Methane production of anaerobic digestion (AD) system.
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Figure 7. Cumulative methane production.
Figure 7. Cumulative methane production.
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Table 1. Analytical methods for determining the physico-chemical properties of fruit and vegetable waste (FVW).
Table 1. Analytical methods for determining the physico-chemical properties of fruit and vegetable waste (FVW).
Parameter APHA MethodBrief Description
pH4500-H + BFor solid FVW, a 1:10 (w/v) slurry with deionized water is typically prepared before measurement with a standard pH meter.
TS2540 GThe sample is dried in an oven at 103–105 °C until a constant weight is reached. This measures the dry matter available in the waste.
VS2540 GThe dried TS residue is ignited in a muffle furnace at 550 °C. The weight loss represents the organic fraction available for microbial conversion to biogas.
sCOD5220 DClosed Reflux Method. The sample must first be filtered through a 0.45 µm filter.
TAN4500-NH3Digestion, distillation, and titration.
TOC5310High-temperature combustion.
TN4500-NDigestion, distillation, and titration—analysed using the Total Kjeldahl Nitrogen.
TP4500-PAscorbic Acid Method—after acid digestion.
TK3500-KFlame photometer—after acid digestion.
Table 2. Treatment details for bio-methane production.
Table 2. Treatment details for bio-methane production.
T1Inoculum Alone (AD Sludge)
T2FVW alone @ TS 5%
T3WAHP @ TS 5%
T4FVW and WS with adjusted CN ratio to 25:1 @ TS 5%
T5FVW + 0.5% MBC @ TS 5%
T6FVW and WS with adjusted CN ratio to 25:1 + 0.5% MBC @ TS 5%
T7WAHP + 0.5% MBC @ TS 5%
Table 3. Characteristics of FVW.
Table 3. Characteristics of FVW.
ParameterCurrent Study[61][62]
pH5.51 ± 0.12--
Total solids (%)13.0 ± 0.4013.00  ±  0.2011.5  ±  0.1
Volatile solids (%)72 ± 2.194.10  ±  0.3085.7  ±  0.1
sCOD (mg L−1)1892 ± 56138.13  ±  12.40
g/kg 		70.7  ±  10.1 g/kg
Total ammoniacal nitrogen (mg L−1)138.5 ± 8.2--
Total Nitrogen (%)3.2 ± 0.159.3  ±  0.4
g/kg		8.0  ±  0.2
g/kg
TOC (%)40 ± 1.2
Phosphorus (g/kg TS)4.5 ± 0.780.29  ±  0.02
g/kg		500.0  ±  0.1 g/kg
Potassium (g/kg TS)12.5 ± 0.853.00  ±  0.08 g/kg-
C/N12.5--
C/N/P100:8:1.1--
Table 4. Characteristics of walnut shell.
Table 4. Characteristics of walnut shell.
ParameterValues ± SD (%DM)
(Current Study)		[66] Li et al. (2026)[67][68][68]
(Fe-Walnut shell Biochar)
Proximate analysis (%)
Fixed carbon22.07 ± 0.814.7510.62--
Volatiles76.98 ± 0.877.8480.46--
Ash0.67 ± 0.81.011.198.7231.40
Moisture13.60 ± 0.86.47.73--
Ultimate analysis (%)
C46.80 ± 1.148.1647.9786.4459.27
H3.41 ± 0.205.745.412.291.90
O43.11 ± 1.544.1345.922.097.0
N0.28 ± 0.051.950.610.370.25
Table 5. Cumulative methane yield.
Table 5. Cumulative methane yield.
TreatmentCH4 (mL)SD% vs. T2p-Value (vs. T2)
T2 (Control)923.5±31--
T4 (C/N only)1190.15±28+28.87%<0.05
T5 (MBC only)1152.2±35+24.76%<0.05
T6 (C/N + MBC)1280.00±42+38.60%<0.01
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Selvaraj, P.S.; Kuttykattil, A.; Ettiyagounder, P.; Tamilselvan, I.; Periyasamy, K.; Oumabady, S.; Ramesh, P.; Ramadass, K.; Palanisami, T. Magnetic Walnut Shell Biochar Enhances Direct Interspecies Electron Transfer and Methane Yield from Fruit and Vegetable Waste’s Anaerobic Digestion. Fuels 2026, 7, 17. https://doi.org/10.3390/fuels7010017

AMA Style

Selvaraj PS, Kuttykattil A, Ettiyagounder P, Tamilselvan I, Periyasamy K, Oumabady S, Ramesh P, Ramadass K, Palanisami T. Magnetic Walnut Shell Biochar Enhances Direct Interspecies Electron Transfer and Methane Yield from Fruit and Vegetable Waste’s Anaerobic Digestion. Fuels. 2026; 7(1):17. https://doi.org/10.3390/fuels7010017

Chicago/Turabian Style

Selvaraj, Paul Sebastian, Aswin Kuttykattil, Parameswari Ettiyagounder, Ilakiya Tamilselvan, Kalaiselvi Periyasamy, Sadish Oumabady, Poornima Ramesh, Kavitha Ramadass, and Thava Palanisami. 2026. "Magnetic Walnut Shell Biochar Enhances Direct Interspecies Electron Transfer and Methane Yield from Fruit and Vegetable Waste’s Anaerobic Digestion" Fuels 7, no. 1: 17. https://doi.org/10.3390/fuels7010017

APA Style

Selvaraj, P. S., Kuttykattil, A., Ettiyagounder, P., Tamilselvan, I., Periyasamy, K., Oumabady, S., Ramesh, P., Ramadass, K., & Palanisami, T. (2026). Magnetic Walnut Shell Biochar Enhances Direct Interspecies Electron Transfer and Methane Yield from Fruit and Vegetable Waste’s Anaerobic Digestion. Fuels, 7(1), 17. https://doi.org/10.3390/fuels7010017

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