Exploring magnetic nanomaterials with a focus on magnetic biochar in anaerobic digestion: from synthesis to application

Abstract

Anaerobic digestion technology, effective for sustainable waste management and renewable energy, but challenged by slow reaction rates and low biogas yields, could benefit from advancements in magnetic nanomaterials. This review explores the potential of magnetic nanomaterials, particularly magnetic biochar nanocomposites, to address these challenges by serving as electron conduits and providing essential iron. This review contributes a thorough overview of the application of magnetic nanoparticles loaded into biochar in anaerobic digestion and engages in a comprehensive discussion regarding the synthesis methods and characterization of various magnetic nanoparticles, elucidating their mechanisms of action in both the absence and presence of magnetic fields. Our review underscores the predominance of co-precipitation (53%) and commercially sourced nanoparticles (29%) as the main synthesis methods, with chemical reduction, pyrolysis, and green synthesis pathways less commonly utilized (8%, 5%, and 5%, respectively). Notably, pyrolysis is predominantly employed for synthesizing magnetic biochar nanocomposites, reflecting its prevalence in 100% of cases for this specific application. By offering a critical evaluation of the current state of knowledge and discussing the challenges and future directions for research in this field, this review can help researchers and practitioners better understand the potential of magnetic biochar nanocomposites for enhancing anaerobic digestion performance and ultimately advancing sustainable waste management and renewable energy production.

Graphical Abstract

Highlights

• Critical insights into using magnetic nanoparticles within biochar as a solution to sustainable waste management.

• Fe₃O₄, nZVI, and Fe₂O₃ are prevalent in enhancing anaerobic digestion processes.

• Insights offered on nanoparticle utilization, magnetic field impacts, and sustainable solutions integrating biochar.

1 Introduction

Magnetic nanoparticles have recently gained significant attention as sustainable tools for environmental operations (Tang and Lo 2013; Khoshnevisan et al. 2021). These nanoparticles, typically made from iron oxide or other magnetite materials, exhibit unique magnetic properties that make them useful in disparate applications (Li et al. 2021). With further research and development, these nanoparticles could play a pivotal part in environmental remediation and monitoring (Jiang et al. 2018), serving to create a more sustainable and greener planet for future generations. Unlike many other nano-additives that generate waste, magnetic nanoparticles can be easily separated and reused (Le et al. 2023), making them a more sustainable option for environmental applications. For instance, in biodiesel production processes, MnFe₂O₄@biochar nanocatalysts were successfully reused multiple times without significant loss of efficacy (Maleki et al. 2024). Similarly, studies have demonstrated the reuse of biochar-CoFe₂O₄-chitosan magnetic composites in wastewater remediation projects, where they have shown remarkable resilience and effectiveness in removing anionic and cationic dyes (Cheng et al. 2024). These instances underscore the potential of magnetic nanoparticles to contribute to sustainable environmental practices through their ability to be recycled and reused across various applications.

An area where magnetic nanoparticles show great potential for use is in the field of anaerobic digestion. This is a technology aimed at sustainable waste management and the generation of renewable energy (Barrena et al. 2022). The distinctive characteristics of these nanoparticles, including their exceptional surface area-to-volume ratio and robust magnetic activity, imply that they will be well-suited for incorporating additives into the anaerobic digestion process to augment biogas generation (Demirel 2016). Hence, harnessing the full capabilities of magnetic nanoparticles in anaerobic digestion has the potential to drive notable progress in sustainable waste management and to revolutionize anaerobic digestion technology by enabling more efficient and effective biogas production.

Magnetic nanoparticles are versatile agents that significantly enhance various aspects of the anaerobic digestion process. Figure 1a illustrates their five distinct roles, all advantageous for the successful anaerobic digestion of waste. Firstly, they can expedite the breakdown of organic fragments, resulting in more methane-rich biogas output (Abdelsalam and Samer 2019). Secondly, these nanoparticles aid in providing essential micronutrients (Liu et al. 2021). Moreover, they excel at immobilizing heavy metals and pathogens, ensuring a less hazardous end product (Hoffmann et al. 2022). Penultimately, magnetic nanoparticles also expedite organic matter degradation by facilitating direct interspecies electron transfer (DIET) (Pan et al. 2019a). This process is achieved by serving as electron conduits, which accelerates DIET amongst volatile fatty acid (VFA)-oxidizing bacteria and methanogens (Kumar et al. 2021). Finally, they contribute to precise process control and monitoring (Chen et al. 2018). An often-overlooked benefit is their role in separating digested sludge from the liquid phase, a critical factor in making anaerobic digestion more cost-effective and environmentally friendly.

Fig. 1

a) Magnetic nanoparticle applications in anaerobic digestion process and b) overview of commonly used nanoparticle synthesis methods. The word cloud associated with each category slice reflects the detailed analysis breakdown of the frequency of occurrence for each nanoparticle

There are many reviews on the use of nanoparticles in anaerobic digestion, including metallic and metal oxide nanoparticles (Yang et al. 2013; Baniamerian et al. 2019; Lee and Lee 2019). Nonetheless, there is a conspicuous void in the literature since no recent review papers have addressed the advanced class of magnetic nanoparticles, leaving the existing information outdated. Recent advancements in superparamagnetic nanoparticles offer new prospects for sustainable nanoparticle applications, emphasizing the importance of investigating diverse synthesis and characterization methods to advance research in anaerobic digestion systems. This paper aims to critically review the latest advancements in magnetic nanoparticles used for anaerobic digestion, with a focus on material engineering. Our specific objectives are as follows: (i) To illustrate the latest advancements in magnetic nanoparticles, including magnetic biochar nanocomposites, used for anaerobic digestion, (ii) To compare their properties and performance, (iii) To determine the potential of various types of magnetic fields to enhance anaerobic digestion, and (iv) To provide a critical connection between the synthesis methods and the characterization parameters of these nanomaterials. Our review encompasses existing literature and goes a step further in offering novel insights and discussions regarding the potential advantages and challenges of utilizing various types of magnetic fields to enhance anaerobic digestion in conjunction with these nanoparticles. By synthesizing the available information, our review provides a valuable resource for researchers, practitioners, and policymakers interested in advancing the field of anaerobic digestion.

2 Data collection method

An organized five-step Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) framework (Liberati et al. 2009) was employed to collect papers for the current study. In this context, the initial step was to create the core keywords of the present research by combining the keywords "magnetic" and "nano" in various ways. In light of this, the search string, as given in Table 1, was developed. This search string would return articles that mention "magnetic nanoparticles" or "magnetic nanomaterials" in either the title, keywords, or abstract fields, as well as "anaerobic digestion" in either the title, keywords, or abstract fields from the Web of Science (WOS) database. Thus, the search results included articles that discuss the use of magnetic nanoparticles or magnetic nanomaterials in anaerobic digestion processes or systems. Forty-eight items were returned exclusively to English-language papers that have undergone peer review. Titles and abstracts were evaluated in the third phase to choose papers relevant to the core subject of this review. Following the selection of articles, the whole text was read in-depth to complete the sample selection, resulting in 30 articles. In order to ensure comprehensive coverage of the selected papers, we employed a snowballing method to examine the references of the articles collected in the previous step. Following this process, a total of 46 publications were selected as essential papers for the purpose of review.

Table 1 Search strategy details for research data pool acquisition

3 Review of synthesis methods

To begin investigating magnetic nanoparticles' potential applications in anaerobic digestion, it is crucial to comprehend their origins and the various ways in which they can be modified. Our in-depth literature examination, as illustrated in Fig. 1b, revealed that co-precipitation emerged as the predominant method for nanoparticle synthesis, constituting approximately 53% of the studies. The next most common method, representing 29% of the surveyed studies, was the procurement of materials for anaerobic digestion research by commercial entities. Chemical reduction, pyrolysis, and green synthesis pathways, on the other hand, were less frequently employed approaches for magnetic nanoparticle synthesis, comprising 8%, 5%, and 5% of the cases, respectively. It is noteworthy that magnetic biochar nanocomposites were only synthesized through the pyrolysis method.

Furthermore, our word count analysis, depicted in Fig. 1b, offers a detailed breakdown of the frequency of occurrence of each nanoparticle alongside specific synthesis methods. According to our comprehensive analysis of the data, it was evident that Fe₃O₄ was the most frequently used magnetic material, followed by nano zero-valent iron (nZVI) and Fe₂O_3, reflecting their extensive exploration in this context. In the upcoming sections, we will provide in-depth insights into the materials highlighted in Fig. 1b, offering readers a comprehensive understanding of the techniques used in their synthesis and characterization. This section serves as an introduction to the diverse range of magnetic nanostructures, offering an overview of their various synthesis and characterization methodologies.

3.1 Co-precipitation method

The co-precipitation method involves the stoichiometric mixing of Fe³⁺ and Fe²⁺ salts, achieved by adding an alkaline solution (Besenhard et al. 2020). This process initiates the nucleation and rapid precipitation of hydrophilic nanoparticles at either room temperature or with moderate heating, as illustrated by reaction Eq. (1).

$${\text{Fe}}^{2+}+{2\text{Fe}}^{3+} + {8\text{OH}}^{-}\leftrightarrow \text{Fe}{(\text{OH})}_{2}+2\text{Fe}{(\text{OH})}_{3} \to {\text{Fe}}_{3}{\text{O}}_{4}+4{\text{H}}_{2}\text{O}$$

(1)

The analysis of the literature highlights the popularity of well-established co-precipitation methods for synthesizing magnetic nanoparticles, such as the one initially proposed by Kang et al. in 1996 (Kang et al. 1996) for synthesizing Fe₃O₄ nanoparticles within the research community (Yamada et al. 2015; Al Bkoor Alrawashdeh et al. 2022). This method hinges on the utilization of an aqueous solution with a carefully controlled Fe²⁺/Fe³⁺ molar ratio, usually around half, and a pH range spanning 11–12 (Wu et al. 2008). As summarized in Table 2, the majority of researchers noted that this approach yields uniform nanoparticles with an average diameter of less than 10 nm, showcasing a notably narrow size distribution, all achieved without the necessity of surfactants. Notably, Yang's research group (Yang et al. 2015b, a) effectively employed Kang’s method in their own work, exemplifying its widespread application. Furthermore, Amo-Duodu et al. (2023) shed light on the substantial surface area of 27 m² g⁻¹, a pore size of 1.5 nm, and a volume of 0.008 cm³ g⁻¹ that characterize Fe₃O₄ synthesized through this method (Amo‐Duodu et al. 2023).

Table 2 Summary of magnetic nanostructures synthesized for enhanced anaerobic digestion performance

On the other hand, another group reported the use of the co-precipitation method but observed significantly larger average diameters of 100–150 nm (Cruz Viggi et al. 2014). This problem highlights the sensitivity of the co-precipitation synthesis to the reaction conditions, which affects the quality of the output materials (Petcharoen and Sirivat 2012). To tackle this problem, scholars adopted a modified method where slight modifications were made to control the growth nucleation, using agents such as ironporphyrin (Zhou et al. 2015) or the addition of surfactants, including cetyltrimethyl ammonium bromide (CTAB) (Ali et al. 2017), tetramethylammonium hydroxide (TMAOH) (Casals et al. 2014), and carbamide (urea) (Ali et al. 2020). In the context of using TMAOH as a capping and stabilization agent, the Casals group reported the synthesis of Fe₃O₄ with a 7 nm mean diameter inspired by Massart’s method (Massart 1981) and a 20 nm mean diameter by adapting the procedure described in Nyirő-Kósa et al. (2009). According to Table 2, these techniques have demonstrated the ability to create nanoparticles of the desired size. Ali et al. (2017) synthesized Fe₃O₄ nanoparticles with CTAB, yielding sizes of 10–35 nm and an average height of 20 nm (Ali et al. 2017). On the other hand, Ali et al. (2020) employed urea as a stabilizing agent, resulting in nanoparticles with dimensions ranging from 0.25 to 2.5 nm in height and 9 to 15 nm in diameter, with an overall size spanning 18 to 24 nm (Ali et al. 2020). However, it is essential to note that while the co-precipitation method allows for size control through the use of capping agents (Zhou et al. 2012), it has the drawback of relying on toxic surfactants. This toxicity factor could significantly impact biological processes like methanogenesis. Furthermore, the synthesis method's incompatibility with green chemistry principles highlights the need for more environmentally friendly approaches in material production, especially when applied to biological processes.

By slightly altering the co-precipitation technique in alkaline environments, the MFe₂O₄ (M = Fe, Ni, Co) can be synthesized using monoisopropanolamine (MIPA) as both alkaline and capping agents (Pereira et al. 2012). The following can be used to illustrate the reaction:

$${\text{M}}^{2+}+{2\text{Fe}}^{3+} + {8\text{OH}}^{-}\to \text{M}{\text{Fe}}_{2}{\text{O}}_{4}+4{\text{H}}_{2}\text{O} (\text{M}={\text{Fe}}^{2+},{\text{Co}}^{2+},{\text{Ni}}^{2+})$$

(2)

In this context, the group led by Sliem reported the use of the aforementioned technique for synthesizing CoFe₂O₄ and NiFe₂O₄ (Sliem et al. 2022). They claimed to achieve particle sizes of 5.7 nm, 5 nm, and 3.5 nm for Fe₃O₄, CoFe₂O₄, and NiFe₂O₄, respectively, in a homogeneous spherical and slightly aggregated shape, thanks to the use of MIPA. They found that nanoparticles co-precipitated with MIPA were smaller than those with NaOH, NH₃, and TMAOH, suggesting that MIPA restricts nanoparticle growth during co-precipitation (Table 2). Saturation magnetization values of 56.9, 34.5, and 18 emu g⁻¹ were observed in the MIPA-prepared nanoparticles for Fe₃O₄, CoFe₂O₄, and NiFe₂O₄, respectively (Sliem et al. 2022). In another study, the co-precipitation method used by Amo‑Duodu et al. served as the primary approach for synthesizing NiFe₂O₄ (Amo-Duodu et al. 2022a), aluminium ferrite (AlFe₂O₄) and magnesium ferrite (MgFe₂O₄) (Amo-Duodu et al. 2022b). The introduction of Mg²⁺ and Al³⁺ ions caused a shift in the oxidation states of Fe₃O₄, subsequently leading to alterations in its physicochemical properties. Only 6% of Ni was found in NiFe₂O₄ by EDX analysis, despite the fact that NiFe₂O₄ was prepared experimentally using a 3:2:1 ratio of Fe³⁺, Fe²⁺, and Ni²⁺ (Amo-Duodu et al. 2022a). The dip-bands at 584 and 629 cm⁻¹ in NiFe₂O₄'s FTIR spectrum were likely caused by magnetite and maghemite oxidative states, while the absorption peak at 530 cm⁻¹ was due to Fe–O vibrations in its magnetite phase (Amo-Duodu et al. 2022a).

A strategy worth considering is the creation of hybrid nanocomposites, achieved by blending Fe₃O₄ nanoparticles with other materials. This approach allows for the combination of Fe₃O₄ with a variety of nanomaterials, including metal oxides like MnO₂, ZnO, and SiO₂, and metal nanoparticles such as Au and Ag, and even carbon-based nanomaterials like graphene and carbon nanotubes. Depending on the application, these combinations may produce materials with improved characteristics like improved magnetic behaviour, catalytic activity, or biocompatibility. For instance, as shown in Table 2, the co-precipitation method was used to create Fe₃O₄/MnO₂ nanocomposites with an average diameter of 7 nm, more considerable than Fe₃O₄ alone, and a reduced saturation magnetization of 0.503 emu g⁻¹ compared to 12.283 emu g⁻¹ for Fe₃O₄ (Attia et al. 2018). Biochar materials have been integrated with Fe₃O₄, as P. Li et al. described in their work published in 2022 (Li et al. 2022). This combination of biochar and Fe₃O₄ offers promising possibilities for various applications, building on the beneficial properties of both materials (Liu et al. 2022). Furthermore, Fe₃O₄ nanoparticles underwent silica coating via the Stober method, introducing Si–O–Si bonds and increasing particle size from 20 to 35 nm (Sadeghi et al. 2023). As detailed by Maurya et al. (2023), the researchers used post-coating of previously synthesized Fe₃O₄ with a hot tetraethyl orthosilicate (TEOS) mixture. After post-coating, the atomic composition included Fe 63.0%, O 23.6%, C 8.9%, and Si 3.7%, with slight decreases in Fe, O, and C compared to bare Fe₃O₄, while the crystalline structure remained unchanged. Minor weight loss (3%) occurred at 200–500 °C, and dynamic light scattering (DLS) showed a zeta potential of approximately − 43 eV, akin to the uncoated particles (Maurya et al. 2023). Additionally, Cu nanoparticles were prepared via ultrasonic-assisted co-precipitation to produce core–shell nanostructure of Cu@Fe₃O₄ (Hassan et al. 2022). However, it is crucial to note that the resulting nanoparticles may exhibit wide-size distributions and potential impurities, impacting their magnetic properties and stability. This consideration becomes particularly significant in evaluating their appropriateness in various applications, including anaerobic digestion processes.

Indeed, aligning the co-precipitation method with the principles of green chemistry is an intriguing approach. However, it is worth noting that this approach has only been adopted by a minority of researchers in this field, as observed in the work conducted by the Abdul Aziz group (Abdul Aziz et al. 2022). They synthesized Fe₃O₄ using coconut shell extract as a reducing agent. In addition to co-precipitation, they used hydrothermal synthesis at 130 °C for 150 min. The average nanoparticle diameter was 75 ± 6 nm, with a zeta potential of − 7.305 mV. The nanoparticles were spherical and rod-like, all nano-sized. However, the synthesis method mainly produced polydisperse scattering due to difficulties in producing monodisperse particles. Table 2 shows that the nanoparticles were ferromagnetic with saturation magnetization of 44.025 emu g⁻¹ (Abdul Aziz et al. 2022). The co-precipitation method yields magnetic nanomaterials with varying saturation magnetization values, offering versatility in magnetic property tailoring. However, its direct impact on biogas production efficiency remains unclear. Further research is needed to elucidate this relationship.

The co-precipitation method, widely utilized for synthesizing magnetic nanoparticles, is noted for its scalability and reproducibility. However, questions regarding its suitability for applications in anaerobic digestion arise due to potential environmental impacts and lack of environmental friendliness. Notably, the synthesis of magnetic biochar nanocomposites, a subset of magnetic nanoparticles, has been exclusively achieved through the pyrolysis method. This highlights a potential limitation of the co-precipitation method in integrating biochar into magnetic nanoparticle synthesis processes for anaerobic digestion applications.

3.2 Commercially sourced nanoparticles

In many research articles, it is observed that researchers often choose to directly purchase synthesized materials from various companies. Analysis of various commercial types of magnetic nanostructures and their occurrence is presented in Fig. 1b. The investigated materials include cobalt oxide (CoO) (Gran et al. 2022), nickel oxide (NiO) (Gran et al. 2022), and yttrium iron oxide (Y₃Fe₅O₁₂) (Patel et al. 2022), each occurring once in the literature. Additionally, the occurrence of other magnetic nanostructures, such as iron-nickel oxide (Fe₂NiO₄) and iron-nickel-zinc oxide (Fe₄NiO₄Zn) is also discussed, with each of them found in one research study (Chen et al. 2018). The most commonly encountered use of commercial magnetic nanostructure in the literature is Fe₃O₄ (Suanon et al. 2016; Chen et al. 2019; Hassanein et al. 2019; Gran et al. 2022; Patel et al. 2022), with four occurrences, followed by iron (Fe) nanoparticles (Hassanein et al. 2019; Abdelwahab et al. 2022, 2023) and nZVI (Su et al. 2015; Suanon et al. 2016), each appearing in three and two studies, respectively. Similarly, nickel (Ni) nanoparticles (Hassanein et al. 2019; Abdelwahab et al. 2023), cobalt (Co) nanoparticles (Hassanein et al. 2019; Abdelwahab et al. 2023), and Fe₂O₃ (Hsieh et al. 2016; Kökdemir Ünşar and Perendeci 2018) have been found in two research works.

The use of commercial materials highlighted above has gained popularity among researchers in anaerobic digestion. This method not only streamlines the research process but also conserves valuable time and effort. By adopting this established approach, researchers can allocate their resources towards advancing their specific research objectives rather than investing substantial time and resources into the intricacies of material synthesis. Moreover, this approach facilitates easier reproducibility for other researchers looking to replicate the studies, thereby promoting the dissemination of knowledge and the growth of the field. However, it is essential to acknowledge that many researchers may face constraints, whether in terms of personnel or infrastructure, which might hinder their ability to independently synthesize and thoroughly characterize these magnetic materials within their own laboratories.

Speaking of the crucial aspect of characterizations, it is noteworthy that, among the various studies, only a handful have undertaken the endeavour of re-evaluating the obtained materials through characterization. These notable efforts are minimal and encompass the use of SEM coupled by EDS for tracking nanoparticles (Su et al. 2015; Suanon et al. 2016; Hassanein et al. 2019) and only the use of SEM technique for Fe, Ni, and Co analysis (Abdelwahab et al. 2023). This attention to characterization reinforces the rigour and credibility of the research, contributing to a more comprehensive understanding of the synthesized magnetic materials' properties.

In a nutshell, commercially available synthesized materials often provide consistent and reliable quality, ensuring reproducibility in experiments with many other researchers who can afford to buy the materials from industrial providers. However, relying on purchased materials in research can have some drawbacks. One of the main concerns is the lack of control over the synthesis process, which may introduce variations in the material's properties and composition. Finally, researchers may miss out on the opportunity to tailor the synthesis parameters and customize the materials according to their specific research needs.

3.3 Chemical reduction

Among the notable techniques for magnetic nanoparticle synthesis, chemical reduction, typically involving borohydride (NaBH₄), holds a prominent place (Willard et al. 2004). This process is usually rapid and straightforward, but it often involves the use of non-environmentally friendly reducing agents, such as borohydride. However, some research groups have attempted to replace conventional reducing agents (such as sodium borohydride, citrate, and hydrazine) with plant-based reducers amidst growing sustainability concerns (Pati et al. 2014). Surprisingly, this method has seen limited traction in anaerobic digestion research, as evidenced in Fig. 1b. For instance, in a study by Abdelsalam et al. (2017), Fe nanoparticles were synthesized with a diameter of 7–9 nm via chemical reduction (Abdelsalam et al. 2017). The process involved adding NaBH₄ drop-wise to a ferric chloride (FeCl₃·6H₂O) and using cetyltrimethyl ammonium chloride (CTAC) as a capping agent to prevent nanoparticle agglomeration. The subsequent equation aptly captures this phenomenon:

$${\text{Fe}({\text{H}}_{2}\text{O})}_{6}^{3+}+ 3{\text{BH}}_{4}^{-} + 3{\text{H}}_{2}\text{O} \to \text{Fe}^{0} NPs + 3\text{B}{(\text{OH})}_{3} + 10.5{\text{H}}_{2}$$

(3)

Aside from its application in the production of Fe nanoparticles, the nZVI synthesis method utilized in the research by Barrena et al. (2021) involved the inclusion of NaBH₄ to a deoxygenated FeCl₃ solution, resulting in Fe⁰ precipitate (Barrena et al. 2021). Due to weak surface charges, nZVI of sizes 10 to 30 nm tended to agglomerate and form microscale aggregates (Barrena et al. 2021).

The combination of co-precipitation followed by chemical reduction results in the formation of Fe–Co–Cu trimetallic nanoparticles (TMNPs) proposed by Jadhav et al. (2022). After the co-precipitation of Fe, Co, and Cu cations had occurred, a 0.05 M NaBH₄ solution was added, and the mixture was stirred for TMNP development (Table 2). The analysis revealed that the metallic nanoparticles consisted of 15.46% Fe, 12.54% Co, and 25.96% Cu, coexisting within the phase presence of Fe₃O₄, CuO, and CoO structures. The average TMNP diameter measured around 50–100 nm, and TEM imaging showcased a 2D crystalline structure corresponding to clusters of the three metals, ranging in size from 3 to 6 nm (Jadhav et al. 2022).

Amidst growing sustainability concerns, there is a notable surge in research towards eco-friendly and green synthesis methods for nanoparticle production utilizing plant extracts, as mentioned previously. These biological materials, derived from plants, offer a cost-effective avenue for playing dual roles as both reduction and capping agents. A particular study investigated utilizing a non-toxic leaf extract from Azadirachta indica, which serves not only as a reducing agent but also as a capping and stabilizing agent (Singh et al. 2022). While XRD characterization was not conducted to unveil the exact structure of the iron oxide nanoparticles, TEM analysis revealed semi-spherical shapes averaging 52.5 nm. The nanoparticles exhibited a zeta potential of − 27.0 mV and displayed a UV–Vis peak at 432 nm, accompanied by a polydispersity index of 219 nm, indicative of mild aggregation (Singh et al. 2022). Considering the relatively large size and moderate aggregation, it can be concluded that further optimization is required for this green method to produce smaller particles suitable for successful use in anaerobic digestion.

3.4 Pyrolysis for the synthesis of magnetic biochar nanocomposites

Pyrolysis stands out as one of the prominent methods for synthesizing magnetic nanoparticles, offering versatility and efficiency in its approach, as shown in Fig. 1b. This method involves thermally decomposing metal precursors in the presence of carbon-rich material, usually agricultural biomass. Through this high-temperature process, the synthesis of magnetic nanoparticles occurs in parallel with the production of biochar, as demonstrated in Table 2 and the study by the Qin group (Qin et al. 2017).

Zhang et al. (2023) impregnated 100 g of biomass with 1 M FeCl₃ and pyrolyzed it at 600 °C for 1 h under a continuous nitrogen atmosphere to make magnetic biochar. Unfortunately, no results of characterizations were reported by the authors to enable us to fully understand the success of the synthesis method (Zhang et al. 2023). Another intriguing avenue is showcased by the method detailed in Qin's study, wherein magnetic biochar is derived from rice straw (Qin et al. 2017). Here, 100 g of rice straw with a diameter of less than 2 mm was immersed in an 800 mL FeCl₃ solution at varying ratios [0, 0.32 g (1F), 3.2 g (10F), 32 g (100F)]. The pre-treated rice straw was then subjected to carbonization at 500 °C for 2 h under a nitrogen environment. The sample designated as 100F showed magnetite by XRD and 10.5% (wt) iron by EDS. Consequently, magnetic biochar 100F exhibited an amorphous structure of iron oxide, attributed to its higher iron content and lower crystallized iron oxide content when compared to magnetic biochar 10F. This approach resulted in notable changes in several material characteristics (Table 2). Notably, an increase in both pore volume, reaching 0.12 cm³ g⁻¹, and specific surface area (BET), which reached 192 m² g⁻¹, was observed. These alterations in material properties are of particular interest due to their potential implications for improved anaerobic digestion application performance.

The significant advantage of this integrated method lies in its environmentally friendly nature, where both magnetic nanoparticles and biochar can be generated in a single step under a flow of inert gas, minimizing environmental impacts. Consequently, this approach proves highly appealing to researchers aiming to harness the combined benefits of magnetic nanoparticles and biochar in a more ecologically conscious manner. It is worth noting that there is a lack of consensus among researchers regarding certain factors, such as the ratios of biomass to iron, temperature ranges, flow rates, and holding times during the pyrolysis procedure. These variations have led to disparate results and a lack of coherence in the use of this method. Nonetheless, when the magnetic iron oxide-treated biomass is pyrolyzed, it does produce magnetic biochar that contains both crystalline and amorphous iron oxide phases.

3.5 Green synthesis pathway

Striving to establish a cost-effective and non-toxic avenue for producing magnetic nanoparticles, a biological synthesis (green) pathway centers on harnessing the potential of a recently discovered bacterium. In this context, as shown in Table 2, a biologically friendly method for producing maghemite (γ‑Fe₂O₃) nanoparticles has been developed, employing Bacillus subtilis SE05 strain isolated from the Red Sea (Abouelkheir et al. 2023). This eco-friendly synthesis involves inoculating FeSO₄·7H₂O with 1 g of cells from the isolated bacterium, followed by one-day incubation at 37 °C. The resulting bio-transformed solution changes color from brown to dark brown, indicating nanoparticle formation, which XRD analysis confirmed to have the crystalline cubic spinel structure of maghemite (Table 2). FTIR spectroscopy demonstrates the presence of organic compounds that can function as capping agents in the nanoparticles alongside Fe–O bonds. These spherical nanoparticles, averaging 7.68 nm in size, exhibited superparamagnetic behaviour, featuring the highest saturation magnetization reported thus far at 52 emu g⁻¹ among researchers (Table 2). EDX identifies the composition of the detected elements besides Fe and O, which originated from the cell culture medium (Abouelkheir et al. 2023). Despite their unique properties, such as high saturation magnetization, the impact of these bio-synthesized nanomaterials on biogas production efficiency remains understudied, posing a critical research gap. Further exploration into this relationship is vital for a comprehensive understanding of their potential applications in sustainable waste management and biogas production, particularly in the context of green synthesis methods.

4 Stand-alone magnetism in anaerobic digestion

Sustainable biogas generation is a key area where magnetic nanoparticles offer significant promise for the future. In this section, we explore their multifaceted impact, dedicating specific subsections to exploring the contributions of Fe₃O₄, nZVI, Fe₂O₃, and other metal nanoparticles. It should be noted that the effects of magnetic nanoparticles on anaerobic digestion can be challenging to generalize as they vary across different studies. Therefore, we refrain from specifying indicators for these effects and, instead, offer a comprehensive summary of research findings. We focus on their impact on anaerobic digestion, putative mechanisms that enhance efficiency, performance indicators, and optimal conditions. Table 3 provides an overview of studies investigating the effects of magnetic nanoparticles on anaerobic digestion performance. The order of Fe₃O₄, nZVI, Fe₂O₃, and other metal nanoparticles in each subsection is determined based on their frequency of appearance. Each subsection offers a comprehensive review of pertinent studies, providing insights into how these nanomaterials have evolved into valuable tools for optimizing anaerobic digestion processes.

Table 3 Summary of key findings on the impact of magnetic nanostructures on anaerobic digestion performance and their corresponding mechanism

4.1 Fe₃O₄

Abdelsalam et al.'s study highlights the utilization of magnetic nanoparticles, Fe₃O₄, to enhance anaerobic digestion efficiency in manure, resulting in a 1.7-fold boost in biogas production compared to the control (Abdelsalam et al. 2017), as shown in Table 3. This improvement was reported to be attributed to the introduction of trace metals in nanoparticle form, which facilitated the efficient supply of iron ions within the system, ensuring its sustained delivery to the bioreactor (Abdelsalam et al. 2017). The addition of the optimized amount of 20 mg L⁻¹ Fe₃O₄ reduced the time between the lag phase and the peak of biogas production. Improved methanogenic activity and iron ion dispersion in the bioreactor caused this bio-stimulating effect (Abdelsalam et al. 2017). However, a 2017 study by Ali et al. found that the Fe₃O₄ optimum concentration of nanoparticles is between 50 and 75 mg L⁻¹, and methane production dropped dramatically once the concentration reached 125 mg L⁻¹ (Ali et al. 2017). They also found that adding 75 mg L⁻¹ bio-compactable Fe₃O₄ nanoparticles to anaerobic digestion of organic municipal solid waste increased methane production by 5000 mL CH₄ (Ali et al. 2017). As given in Table 3, the nanoparticles were shown to stimulate the enzymatic activities of methanogenic bacteria through the effective distribution of iron ions in the solution (Ali et al. 2017). Other researchers, however, including the Z. Yang group, found an effect that was not dependent on dose, as shown in Table 3 (Yang et al. 2015b).

As presented in Table 3, Fe₃O₄ mixed with glutamic acid biostimulated Streptococcus equi cells to produce hyaluronic acid. In their research, Attia et al. (2018) developed a new method to produce hyaluronic acid using Fe₃O₄ nanoparticles and amino acids, particularly highlighting the role of Fe₃O₄/glutamic acid (GA) as stabilizers. This approach resulted in the highest dry weight of hyaluronic acid (0.435 g L⁻¹) with the addition of 20 mg L⁻¹ of Fe₃O₄/GA nanoparticles. Although this innovative method significantly enhanced hyaluronic acid production, the study did not explicitly explore the direct impact of this increased hyaluronic acid production on biogas generation, indicating a potential area for future research. Furthermore, adding Fe₃O₄ in anaerobic digestion demonstrated promising results in expediting methane production from acetate, signifying its potential application for enhancing methanogenic performance through an effective interspecies electron transfer (IET) mechanism (Yang et al. 2015b). This research notably identified the selective enrichment of Rhodocyclaceae-related species as the critical factor responsible for a specific mechanism of acetate degradation when exposed to Fe₃O₄ (Yang et al. 2015b). Further investigations into the effects of Fe₃O₄ revealed its capacity to sustain magnetite-promoted DIET in continuous anaerobic digestion processes (Baek et al. 2017). Baek et al. (2017) introduced a long-term magnetite recycling approach, which proved its effectiveness in preserving elevated DIET and methanogenic activities over an extended period exceeding two hundred fifty days (Baek et al. 2017).

Another significant observation relates to Fe₃O₄'s role in significantly boosting acetate and propionate methanogenesis within thermophilic methanogenic communities, resulting in augmented methanogenesis even under thermophilic conditions, as reported by Yamada et al. (2015). Additionally, their contribution to electric syntrophy between methanogenic archaea and organic acid-oxidizing bacteria is noteworthy, as depicted in Table 3 based on the findings of Yamada et al. (2015). Additionally, it was observed that Methanosaeta likely played a pivotal role as the primary methanogen set responsible for DIET-based methanogenesis (Baek et al. 2017). These combined findings underscore the potential of Fe₃O₄ nanoparticles in revolutionizing anaerobic digestion processes, not only enhancing methane production from acetate but also promoting interspecies electron transfer and electric syntrophy in a variety of operational conditions, from ambient to thermophilic, thus highlighting their valuable contribution to the field of sustainable waste management and renewable energy production.

The silica-coated Fe₃O₄ modified with ironporphyrin was reported to have a remarkable reusability for VSS, sCOD, and TOC reduction over five cycles, mainly attributed to robust chemical bonding, signifying substantial cost savings in practical applications (Zhou et al. 2015). The use of supported biomimetic catalysts displayed promising results in improving anaerobic digestion efficiency in near-neutral pH values (Zhou et al. 2015), which is a crucial part to follow for other researchers as iron oxides supply Fe²⁺ and Fe³⁺ relatively slowly compared to nZVI, primarily due to their insolubility at neutral pH levels.

4.2 nZVI

In the realm of anaerobic digestion systems, studies have frequently showcased the positive influence of nZVI. When nZVI is present, the oxidation–reduction potential (ORP) drops quickly, making conditions more favourable for the formation of VFAs like acetic and butyric acid (Wang et al. 2018). Moreover, the release of iron cations from nZVI promotes dominant hydrolysis-acidification microorganisms, particularly Clostridia (53.2%) and Methanosarcina (22.6%), thereby enhancing hydrogenotrophic methanogenesis. This enhancement, as explored by Yu et al. (2016), significantly amplifies methane production from carbon dioxide in anaerobic digestion systems treating sludge fermentation liquor (SFL). With the addition of nZVI, methane production reached 108.24 mL per gVS, a 46.1% increase compared to the no-ZVI assay. This increase can be attributed to nZVI's promotion of hydrolysis-acidification and the creation of a more favourable substrate environment for methanogens, as nZVI facilitated the release of biodegradable compounds without propionic acid accumulation. These findings highlight the potential of nZVI in enhancing the efficiency of anaerobic digestion processes, particularly in the degradation of complex substrates like SFL.

Methanogenesis is greatly aided by nZVI, which also promotes the generation of volatile fatty acids. As an electron donor, nZVI effectively converts CO₂ to CH₄ (Amen et al. 2018). Specifically, nZVI increased the significant number of microorganisms playing a role in hydrolysis-acidogenesis and methanogenesis at concentrations below 200 mg L⁻¹ (Pan et al. 2019b). This enrichment happened because free sulfate sources became immobilized, which in turn reduced the number of sulfate-reducing bacteria (SRB). SRB are known to compete with acetogens and methanogens in anaerobic digesters (Su et al. 2013). Notably, the introduction of nZVI into anaerobic digesters prompts several questions regarding its impact on the dynamics of VFA production and the subsequent fate of sulfide. Questions arise, such as whether nZVI selectively influences the microbial pathways leading to VFA production or if it alters the redox conditions in such a way that affects sulfide solubility and toxicity. The potential for nZVI to mitigate sulfide toxicity in anaerobic digestion systems also warrants further exploration to understand its role in reducing the inhibitory effects of sulfides on methanogens. These considerations underscore the need for further investigations to elucidate the specific mechanisms through which nZVI may influence VFA production and its ability to mitigate sulfide toxicity within anaerobic digestion systems.

nZVI corrosion, leading to in-situ hydrogen supply, has demonstrated substantial improvements in biogas upgrading efficiency (Zhao et al. 2023). Microbial analysis revealed a 53.72% relative abundance of Methanobacterium and Methanolinea at a concentration of 10 g L⁻¹ nZVI, nearly 1.5 times that of the control group. However, it is essential to consider the system as a whole. Both methane content and loosely bound extracellular polymeric substances (LB-EPS) were lower in the NaHCO₃ buffer system with 10 g L⁻¹ nZVI than in the control (Tang et al. 2022). This suggests that while nZVI corrosion positively influences microbial composition, its impact on other key parameters of anaerobic digestion, like methane production, may be more complex.

Suanon et al. (2016) investigated the alteration of heavy metal distribution within the anaerobic digestion of sewage sludge when iron nanoparticles were introduced, as presented in Table 3. Their study revealed that nZVI has a remarkable capacity not only to regulate heavy metal distribution but, when applied judiciously, to boost the production of biogas (Suanon et al. 2016). While several studies have focused on the successful recovery and small-scale implementation of nZVI for treating heavy metals in wastewater (Li et al. 2014b, a, 2017; Wang et al. 2015), its full-scale integration into anaerobic digestion processes remains a topic of ongoing research.

4.3 Fe₂O₃

Maghemite (γ-Fe₂O₃), hematite (α-Fe₂O₃), and iron (III) oxide (Fe₂O₃) are superparamagnetic materials recognized for their significance as essential nutrients in anaerobic processes. However, their insolubility at neutral pH levels leads to a slower release of Fe²⁺ and Fe³⁺ compared to nZVI, which might explain the significantly higher increase in methane production volume (387%) observed in nZVI-amended groups in a study by Pan et al. (2019a, b), which focused on enhancing tetracycline wastewater treatment, compared to the approximately 20% improvement achieved with nano-Fe₂O₃ in waste activated sludge as reported by Wang et al. (2016). It is important to note that these studies are not directly comparable due to their use of different substrates and conditions; Pan et al. investigated tetracycline (TC) wastewater with varying concentrations of TC and nZVI, while Wang et al. focused on the anaerobic digestion of waste activated sludge (WAS) with different nanoparticle types. Such variations in experimental design and substrate can significantly influence the outcomes and should be considered when interpreting the results.

Fe₂O₃ serves as an electron conduit in its insoluble state, facilitating electron transfer amongst organic-oxidizing bacterial and CO₂-reducing archaeal communities, mainly through DIET (Zhu et al. 2020). This high conductivity significantly impacts the methane production phase, positively affecting anaerobic conditions. Anaerobic treatment of benzoate-containing wastewater, for instance, was shown to increase methanogenic benzoate degradation rates by 25% when compared to iron-free controls. This effect was attributed to the stimulation of DIET-facilitated methanogenesis (Zhuang et al. 2015).

Despite the positive effects observed by many researchers, a study conducted by Kökdemir Ünşar and Perendeci in 2018 found that Fe₂O₃ had negative impacts on long-term anaerobic digestion. Their research, as detailed in Table 3, showed that Al₂O₃ had a positive influence, resulting in a 14.8% increase in methane production. In contrast, Fe₂O₃ was confirmed to inhibit methane production, leading to a significant 28.9% decrease (Kökdemir Ünşar and Perendeci 2018). This study emphasizes the diverse outcomes linked to Fe₂O₃ supplementation in anaerobic digestion processes. It suggests that as the accumulation of Fe₂O₃ nanoparticles within anaerobic microorganisms' cells increases, the concentration of Fe as a trace element likely surpasses a critical threshold, ultimately resulting in cell death.

Furthermore, the Hsieh group's study shows that Fe₂O₃ has significant effects on hydrogen production. Their findings showed that the addition of 50 mg L⁻¹ Fe₂O₃ nanoparticles significantly augmented hydrogen generation by 24.9% (Hsieh et al. 2016). This positive stimulation effect was most pronounced under these conditions. However, it is worth noting that the effect decreased with increasing metal concentration, and at concentrations of 400 and 800 mg L⁻¹, inhibition occurred. The rise in hydrogen output was postulated to have been caused by electron flow during gas production (Hsieh et al. 2016).

4.4 Metal nanoparticles

The addition of Fe, Ni, and Co in poultry litter anaerobic digestion demonstrated encouraging gains in enhancing methane and reducing hydrogen sulfide (H₂S) production. Zaidi et al. (2018) employed Ni nanoparticles to achieve a remarkable increment of over 25% in the production of biogas from microalgal biomass (Zaidi et al. 2018). The combination of nanoparticles at specific concentrations showed the highest impact, while higher nanoparticle concentrations were more effective in reducing H₂S production (Hassanein et al. 2019). Significantly, Hassanein et al. (2019) introduced Fe, Ni, and Co nanoparticles both individually and as mixtures into anaerobic digestion reactors containing poultry litter. As given in Table 3, their study reported that these nanoparticles had a notable impact on enhancing methane production rates, with optimal concentrations identified at 12 mg L⁻¹ of Ni, 5.4 mg L⁻¹ of Co, and 100 mg L⁻¹ of Fe nanoparticles (Hassanein et al. 2019).

Moreover, the utilization of nanoparticle mixtures demonstrated promise in reducing the production of H₂S. In a study by Lizama et al. (2019a, b), ultrasonic pretreatment's effects at varying dosages of Fe nanoparticles were explored in the context of sewage sludge anaerobic digestion. This led to a notable reduction in ORP values (to − 300 mV) and increased volatile fatty acid concentrations (up to 2000 mg L⁻¹), promoting hydrolysis, acidogenesis, and acetogenesis without causing acidification (Lizama et al. 2019a). These findings underscore the significant potential of metal nanoparticles, similar to their documented potential in soil for carbon stabilization and degradation (Song et al. 2022), in enhancing the efficiency of anaerobic digestion processes.

4.5 Others

Chen's group utilized Fe₂NiO₄ and Fe₄NiO₄Zn nanoparticles, acknowledging the former's beneficial effects, while the latter inhibited the anaerobic digestion activity of synthetic municipal wastewater (Chen et al. 2018). As detailed in Table 3, at a concentration level of 100 mg Ni²⁺ L⁻¹, Fe₄NiO₄Zn led to sustained inhibition of anaerobic digestion, in stark contrast to the behaviour of Fe₂NiO₄, confirmed by the resazurin reduction assay (Chen et al. 2018). This study underscores the significance of magnetic nanoparticle shape and structure in optimizing anaerobic digestion efficiency, providing valuable insights for future research in this field.

4.6 Magnetic biochar nanocomposites

According to research by Zhang et al. (2023), adding 25 mg g⁻¹ of magnetic biochar to the anaerobic digestion process significantly increased biogas output by promoting organic matter degradation and enriching methanogenic archaea (Zhang et al. 2023). Remarkably, the total absolute abundance of mobile genetic elements (MGEs) was enhanced, with the peak increase (77%) observed at a lower dosage of 12.5 mg g⁻¹. Furthermore, the microbial community was significantly impacted by magnetic biochar by the time anaerobic digestion was complete (Zhang et al. 2023). Another intriguing approach, as demonstrated by Qin et al. (2017), involves using rice straw-based magnetic biochar. The inclusion of magnetic biochar in the system for organic municipal solid waste improved methane generation by 11.69% when a 3.2 g FeCl₃:100 g rice-straw ratio was employed. However, a higher ratio led to a 38.34% reduction in the production of methane, primarily due to iron oxide competition for electrons. Interestingly, the magnetic biochar was able to adsorb 25% of the total methanogens, offering a potential solution to mitigate methanogen loss (Qin et al. 2017). These findings underscore the promising role of magnetic-loaded biochar in enhancing anaerobic digestion outcomes.

The study by Jiao et al. (2022) utilized a mixing ratio of 1:1 for combining nZVI and crop straw biochar powders, resulting in enhanced methane production (151.06 mL g⁻¹ VS) observed at a combined addition amount of 9%, representing a 20.73% increase over the control. However, an adverse effect was observed at addition levels of 12% and 15%. Importantly, this study demonstrated the nZVI recoverability by magnetic separation, potentially reducing the cost of additives (Jiao et al. 2022). Biochar-supported nZVI was applied to tackle challenges associated with low organic matter content and elevated levels of heavy metals. The study conducted by Zhang et al. (2019) demonstrated notable enhancements in methane content and cumulative methane yields, with increases of 29.56% and 115.39%, respectively, compared to the control. Additionally, the addition of nZVI-biochar improved process stability by enhancing the generation and degradation of intermediate organic acids. Furthermore, a positive effect was observed in stabilizing metals (Cu, Cd, Ni, Cr, and Zn) in the digestate, although inhibitory effects were noted at higher dosages (Zhang et al. 2019).

In the study conducted by Di et al. (2022), the utilization of nano-Fe₃O₄ particle-loaded biochar in anaerobic digestion processes of chicken manure significantly improved system stability and methane production efficiency, particularly under high ammonia nitrogen concentrations. Serving as an effective carrier for nano-Fe₃O₄ particles, biochar successfully addressed challenges related to agglomeration and loss. Notably, the highest methane yield was achieved at a nano-Fe₃O₄ biochar concentration of 15%, representing a substantial 62.61% increase compared to the control group. Moreover, the incorporation of nano-Fe₃O₄ particles onto biochar significantly mitigated the inhibition induced by high ammonia nitrogen concentrations in the anaerobic digestion system (Di et al. 2022). Building upon these findings, a subsequent study by Di et al. (2023) further explored the potential of nano-Fe₃O₄ biochar in alleviating inhibition caused by volatile fatty acids and ammonia during anaerobic digestion. Remarkably, this research reported a significant 139.0% increase in methane yield compared to the control group. Additionally, nano-Fe₃O₄ biochar was found to enhance the degradation of acetic acid via the syntrophic acetate oxidation pathway, thereby contributing to improved methane production efficiency in the digestion of chicken manure (Di et al. 2023). Similarly, micro-nano-sized magnetite-loaded biochar showed promising results in enhancing methane production during anaerobic digestion. By facilitating the DIET process, it increased daily methane yields by 157%. This enhancement is achieved by accelerating the consumption of short-chain fatty acids and improving electron transfer efficiency, ultimately leading to enhanced methanogenesis. The Fe²⁺/Fe³⁺ oxides present in magnetite-loaded biochar play a crucial role in promoting acetate production and accelerating VFA degradation through the selective enrichment of iron-reducing microorganisms via dissimilatory iron reduction (Chen et al. 2022).

These studies mentioned above effectively highlight the significant improvements in system stability and methane production efficiency achieved through the incorporation of Fe₃O₄ particle-loaded biochar. They address challenges related to high ammonia nitrogen concentrations and volatile fatty acids while enhancing the degradation of acetic acid via the syntrophic acetate oxidation pathway. Furthermore, they facilitate DIET and improve methanogenesis by accelerating short-chain fatty acid consumption and enhancing electron transfer efficiency, as summarized in Table 3.

5 Microbial community responses to magnetic biochar nanocomposites

Anaerobic microbes are known for their slow growth rate and limited capacity to multiply, often requiring supplements to enhance their metabolism. In this context, the incorporation of magnetic nanoparticles into biochar has emerged as a significant factor influencing the structure of anaerobic digestion microbiomes and enriching methanogenesis. In this section, we aim to highlight several studies and their significant findings regarding the impact of magnetic biochar nanocomposites on the microbial community and the proposed mechanisms underlying methanogenesis in anaerobic digestion systems.

One such study by Zhang et al. (2019) explored microbial community analysis following the addition of nZVI-biochar, revealing notable shifts in archaeal diversity and abundance. The inclusion of nZVI-biochar led to a significant increase in the Shannon diversity index and Chao1 richness index of archaea, indicating enhanced microbial diversity compared to control digesters. Particularly noteworthy was the dominance of Methanosaeta across all digesters at the genus level. Additionally, the relative abundance of hydrogenotrophic methanogens, including Methanobacterium and Methanospirillum, saw a substantial increase of 35.39% in nZVI-biochar amended digesters, ultimately resulting in elevated methane production levels (Zhang et al. 2019). These findings hold promise for the development of microbial management strategies aimed at bolstering the stability of anaerobic digestion processes.

The magnetite-loaded biochar demonstrated its efficacy in facilitating the methanogenesis process, likely through the establishment of DIET between iron-reducing bacteria Ruminococcaceae and methanogenic species Methanothrix and Methanosarcina (Chen et al. 2022). Furthermore, Chen et al. (2020) showcased the application of nano-Fe₃O₄ nanoparticles (100 g L⁻¹) to mitigate the stress induced by high salinity organic effluent. The analysis of microbial communities revealed an increase in the abundance of bacterial taxa known for strong biofilm production, including Pseudomonas, upon the introduction of magnetite (Chen et al. 2020). Similarly, the incorporation of green-prepared magnetic biochar (MBC) into anaerobic digestion systems was significantly reported to enhance methane production efficiency due to MBC's ability to promote critical stages of anaerobic digestion, including hydrolysis, acidification, and methanogenesis. Moreover, MBC triggers the secretion of electroactive substances and selectively enriches electroactive microorganisms, such as Clostridium and Methanosarcina, ultimately establishing DIET pathways, thus demonstrating its potential for advancing anaerobic digestion processes (Jin et al. 2023). Overall, the findings highlight the multifaceted impact of magnetic biochar nanocomposites on microbial communities and methanogenesis pathways, underscoring the importance of further research to harness their full potential for sustainable anaerobic digestion processes.

Similarly, the utilization of nano magnetite-loaded biochar in a batch anaerobic fermentation system has shown promise in enhancing methanogenic performance within propionate-degrading consortia (PDC). This enhancement is attributed to the promotion of direct interspecies electron transfer facilitated by nano magnetite-loaded biochar, particularly between syntrophic bacteria like Syntrophobacter and Thauera and their partners, such as Methanosaeta. This process is believed to be driven by the rapid extracellular electron release triggered by magnetite, with the external functional groups and intrinsic graphitic matrices of biochar serving as electron bridges for efficient electron transport (Wang et al. 2023). In the absence of biochar and solely using magnetic iron oxide nanoparticles, Faisal et al. (2020) explored the utilization of vegetable wastes in anaerobic digestion for methane production. Their study revealed that the addition of iron oxide nanoparticles significantly promoted key microbial communities. Specifically, the abundance of Syntrophomonas, Clostridium, and Methanosarcina increased by 143%, 124%, and 81.55%, respectively, upon the addition of 40 mg L⁻¹ iron oxide nanoparticles (Faisal et al. 2020). In conclusion, these studies shed light on the significant role of magnetic biochar nanocomposites in influencing microbial communities and enhancing methane production efficiency in anaerobic digestion systems, offering promising avenues for improving process stability and resource recovery.

Anaerobic microbes, known for their sensitivity to environmental factors, face persecution under high ammonia nitrogen concentrations. The inclusion of nano-Fe₃O₄ particles helped mitigate the inhibition caused by high ammonia nitrogen concentrations, fostering the enrichment of microorganisms. Moreover, the introduction of these additives further accelerated the hydrolysis and acidification rate of chicken manure, leading to a notable increase in the relative abundance of DIET-capable Chloroflexi. The study suggests that DIET-mediated syntrophic acetate oxidation methanogenesis serves as the primary mechanism for methane production at high ammonia–nitrogen concentrations (Di et al. 2022). Furthermore, nano-Fe₃O₄ biochar was found to enhance methane production by facilitating syntrophic acetate oxidation and electron transfer between microorganisms, as evidenced by the enrichment of unclassified Clostridiales and Methanosarcina (Di et al. 2023). In summary, the diverse microbial responses observed in anaerobic digestion systems treated with magnetic biochar nanocomposites underscore the importance of understanding microbial responses to magnetic biochar nanocomposites for optimizing anaerobic digestion processes.

In conclusion, the utilization of magnetic nanoparticles, either alone or in combination with biochar, presents exciting opportunities for enhancing microbial activity and methane production efficiency in anaerobic digestion systems, paving the way for more sustainable waste treatment and energy generation practices. These findings underscore the importance of understanding the complex interactions between magnetic biochar nanocomposites and microbial communities in anaerobic digestion systems. Thorough monitoring and assessment are imperative to comprehensively evaluate the long-term effects of magnetic nanoparticles on microbial adaptation and resilience within anaerobic digestion environments. Further research is needed to elucidate these mechanisms and optimize the utilization of magnetic biochar nanocomposites for enhanced methane production and process stability in anaerobic digestion systems.

6 Magnetism-enhanced anaerobic digestion under magnetic fields

Magnetism-enhanced anaerobic digestion offers an intriguing avenue for optimizing this process, taking advantage of magnetic nanoparticles' unique properties under magnetic fields. The impact of magnetic fields, both in conjunction with magnetic nanoparticles and in their absence, is a relatively unexplored area. This section examines recent studies to uncover insights regarding the interplay of magnetism and anaerobic digestion. A summarized overview of this interaction is presented in Fig. 2, highlighting key aspects such as heightened enzyme activity, augmented bacterial lysis, enhanced biogas and methane production, and stimulated growth of methanogenic archaea. These aspects will be further discussed in detail below.

Fig. 2

Impact of magnetic fields and magnetism on anaerobic digestion processes

These diverse studies collectively illustrate the advantageous impact of constant magnetic fields on various microbial processes, ranging from enhanced ethanol production by S. cerevisiae (Liu et al. 2009) to improved starch enzymatic hydrolysis rates facilitated by magnetic chitosan beads (Yang et al. 2010). This electromotive force was also reported to enrich electrotrophic methanogens, specifically Methanothrix, and exoelectrogens, exemplified by Exiguobacterium, when subjected to a dynamic magnetic field (Yang et al. 2023). Furthermore, the magnetic field encouraged the production of polyhydroxyalkanoates from short-chain fatty acids in the activated sludge (Chen and Li 2008). These findings underscore the need for further research into the influence of constant magnetic fields on anaerobic digestion. For instance, Dębowski et al. (2016) exposed a reactor to a 0.6 T intensity magnetic field during the anaerobic digestion of algal biomass. The results showed a substantial rise in the production of methane compared to the control reactor (Dębowski et al. 2016). However, prolonged exposure led to decreased methane content in biogas, potentially due to methanogens inhibition. Interestingly, optimizing retention times in the magnetic field (144–216 min day⁻¹) contributed to higher biogas yields (448.9–456.6 L kg⁻¹ VS) (Dębowski et al. 2016). Similarly, Zieliński et al. (2021) adopted a static magnetic field to a reactor during the anaerobic digestion of model dairy wastewater, causing a dramatic rise in methane output, achieving 373 mL g⁻¹ VS, while the control reactor only achieved 200 mL g⁻¹ VS. This enhancement was accompanied by increasing methane content to 58% from 49% in the control reactor, with Trichococcus sp. dominating the microbial community (Zieliński et al. 2021). Nevertheless, extended exposure led to decreased methane content in biogas, potentially due to inhibition. Balancing the magnetic field exposure is, therefore, crucial for maximizing anaerobic digestion benefits.

Amo‐Duodu et al. (2023) highlighted the remarkable impact of magnetic fields on the anaerobic digestion of wastewater and activated sludge, underscoring the potential for enhanced biogas production from municipal water works substrates. They employed a magnetized bioreactor and observed significant biogas and methane production increases. After 3 h of daily exposure to a 20 mT magnetic field, biogas yield reached 210 mL per day, with a methane content of 96.8%. Moreover, methane and biogas production both saw increases of 3% and 16%, respectively, while CO₂ content decreased to 3.2%. These findings underscore the enhanced performance of Fe₃O₄ when exposed to a magnetic field (Amo‐Duodu et al. 2023). Another study by Madondo et al. (2023) demonstrated the potential of nano-sized Fe₃O₄ and a static magnetic field in microbial fuel cells for bio-electrochemical methane production and contaminant removal using 1 L biochemical methane potential experiments. Their findings revealed that the highest biogas production achieved in the microbial fuel cell equipped with magnetite nanoparticles and a magnet digester was 545.2 mL g⁻¹ VS_fed, a significant improvement over the control's 117.7 mL g⁻¹ VS_fed (Madondo et al. 2023). The results of these studies have shown a notable boost in biogas output, highlighting the role of magnetic nanoparticles and magnetic fields in improving anaerobic digestion processes.

Dik et al. (2023) highlighted the potential of magnetic fields to enhance enzymatic reactions with Fe₃O₄ nanoparticles within the context of anaerobic digestion. They reported a substantial increase in the activity of L-asparaginase (L-ASNase) enzymes when exposed to magnetic fields, with a 3.2-fold increase at 10 Hz and a remarkable 4.3-fold increase at 20 mT. This enhancement under magnetic fields can contribute to more economically efficient enzyme applications in anaerobic digestion processes, where enzyme cost-effectiveness is pivotal (Dik et al. 2023). Furthermore, Usvaliev et al. (2023) found that rod-like Fe₃O₄ nanoparticles, activated by a 50 Hz, 68.5 mT low-frequency magnetic field, doubled the enzyme reaction rate for the lysis of Escherichia coli (E. coli) JM 109 and MH 1 strains. Although the initial research was focused on applications against gram-negative pathogens, these findings could also apply to the improved breakdown of similar cell structures within anaerobic digestion systems, enhancing overall digestion efficiency without the need for heating. Such advancements could be particularly influential in anaerobic digestion systems where the reduction of pathogenic bacteria is essential, and the enhancement of enzyme activity could lead to stronger biogas production (Usvaliev et al. 2023).

In conclusion, magnetic field applications hold significant potential for enhancing various facets of anaerobic digestion, as shown in Fig. 2, spanning from biogas and methane production to enzymatic reactions. This avenue promises to be instrumental in further advancing sustainable waste management processes.

7 Challenges and future directions

In the context of using magnetic nanostructures in anaerobic digestion, it is vital to consider the challenges and future prospects. Whilst these structures have shown promise in improving anaerobic digestion processes, there are still hurdles to overcome. These include concerns about the cost, scalability, long-term stability, and compatibility of these materials with existing systems.

While various synthesis methods demonstrate tunable magnetic properties, their specific impact on biogas production efficiency remains largely unexplored. Understanding the relationship between saturation magnetization and biogas production efficiency is essential for optimizing waste management practices. Future research should focus on investigating this relationship to advance our understanding of the role of magnetic nanomaterials in sustainable waste management.

The integration of magnetic nanoparticles within biochar represents a sustainable approach to waste management and renewable energy generation. By harnessing the combined benefits of magnetic nanoparticles and biochar, anaerobic digestion processes can be optimized for greater efficiency and environmental sustainability. However, it is important to note certain challenges and limitations in the application of magnetic biochar nanocomposites. Variations in synthesis methods, ratios of biomass to iron, and operational parameters during pyrolysis have led to disparate results and a lack of coherence in the use of this method. Furthermore, inhibitory effects have been observed at higher dosages of magnetic nanoparticles, highlighting the need for careful optimization to avoid adverse impacts on anaerobic digestion processes. Additionally, while magnetic biochar nanocomposites show promise in enhancing methane production, further research is needed to fully understand their long-term effects on microbial communities and ecosystem dynamics within anaerobic digestion systems.

Moreover, the application of external magnetic fields to activate magnetic nanoparticles within a steel anaerobic digester presents additional challenges. These may include ensuring that the external magnetic field strength is sufficient to activate the nanoparticles uniformly throughout the digester, particularly in regions with complex flow patterns or areas distant from the magnetic source. Compatibility with the steel infrastructure of the anaerobic digester is another concern, as interactions between the external magnetic field and the steel components could affect the effectiveness of nanoparticle activation or even interfere with the digester's operation. In addition to the challenges associated with external magnetic fields, the interaction of magnetic nanoparticles with the steel anaerobic digester itself presents essential considerations. The nanoparticles may adhere to the inner surface of the digester, potentially affecting its performance and efficiency. Practical limitations such as power requirements, field stability over time, and potential safety concerns for personnel working near the magnetic field must also be addressed. By recognizing and addressing these challenges, researchers can develop strategies to optimize the application of external magnetic fields for activating magnetic nanoparticles in steel anaerobic digesters, thereby enhancing the efficiency and effectiveness of the process.

The use of a classical risk assessment procedure is essential for evaluating the usage of magnetic nanoparticles, as understanding the environmental fate of these nanoparticles upon release from the digester is crucial for exposure assessment and addressing biosafety concerns. However, the current level of knowledge seems insufficient to reliably assess the risks associated with the use and fate of these nanoparticles in anaerobic digesters. Therefore, a fair assessment of nanoparticles requires evaluating both risks and benefits relative to current solutions.

Future research endeavours should be directed toward developing cost-effective and scalable methods for synthesizing and employing nanostructures in anaerobic digestion, with an emphasis on tailoring their properties for specific applications. To fully unlock the potential of magnetic nanostructures in anaerobic digestion applications, it is imperative to employ a diverse array of synthesis techniques tailored to meet specific requirements while adhering to the principles of green chemistry. Our comprehensive analysis has shed light on the efficacy of various synthesis methods, with up-and-coming prospects for biological methods employing bacteria or plant extract solutions. Additionally, the pyrolysis method demonstrates the potential to align with green chemistry principles, offering sustainable alternatives for researchers to explore.

As the pursuit of sustainability gains momentum in the scientific community, we anticipate that these environmentally friendly approaches, including the integration of biochar, will become increasingly prevalent in future research endeavours. The resulting magnetic biochar nanocomposites offer synergistic benefits, combining the magnetic properties of nanoparticles with the porous structure and carbon-rich composition of biochar. Such integrated approaches not only enhance waste valorization but also open new possibilities for applications in anaerobic digestion processes, soil remediation, and wastewater treatment. By embracing green chemistry and selecting synthesis techniques that harmonize with sustainable practices, researchers can further enhance the capabilities of magnetic nanostructures in anaerobic digestion, ushering in a more efficient and environmentally responsible era for waste management and renewable energy production.

The application of magnetic nanomaterials in anaerobic digestion not only addresses critical challenges in waste management and bioenergy production but also aligns strongly with key United Nations Sustainable Development Goals (SDGs), contributing to a more sustainable and equitable future. Specifically:

• SDG 6, Clean Water and Sanitation: Improved anaerobic digestion processes, facilitated by magnetic nanomaterials, reduce the environmental impact of waste disposal, promoting clean water and sanitation.

• SDG 7, Affordable and Clean Energy: Magnetic nanomaterials enhance energy production efficiency in anaerobic digestion, fostering sustainable and environmentally friendly energy practices while reducing reliance on fossil fuels.

• SDG 12, Responsible Consumption and Production: Magnetic nanomaterials reduce waste through efficient organic matter degradation in anaerobic digestion.

• SDG 13, Climate Action: The supported energy production practices help mitigate climate change.

Furthermore, future research in this domain should encompass an analysis of various reactor configurations, process control aspects, and the potential integration of artificial intelligence (AI) for controlling anaerobic digestion systems enhanced with magnetized nanoparticles. While reliable sampling methods are essential for micro-mechanistic analysis, understanding the broader operational dynamics, optimal reactor designs, and the integration of advanced control strategies, including AI applications, will contribute significantly to the advancement and practical implementation of magnetic nanostructures in anaerobic digestion processes. These multifaceted investigations aim to enhance overall system efficiency and offer a holistic understanding of the complex interactions between magnetic nanostructures and anaerobic digestion.

8 Conclusions

Our review has explored a wide array of synthesis methods for magnetic nanoparticles and magnetic biochar nanocomposites, emphasizing the strategic selection based on desired properties. These nanoparticles play a crucial role in boosting biogas yield and microbial communities, thereby offering promising avenues for advancing waste management and biogas production. Importantly, we have observed a growing trend towards the adoption of green synthesis methods in the field. This shift reflects an increasing awareness of sustainability concerns and a recognition of the importance of environmentally friendly approaches in nanoparticle synthesis. Integrating magnetic nanoparticles in magnetism-enhanced anaerobic digestion represents a paradigm shift in waste management, offering sustainable solutions for optimizing degradation processes and biogas production. Moreover, the application of magnetic biochar nanocomposites in anaerobic digestion systems presents significant potential for enhancing methane production, improving process stability, overcoming challenges such as high ammonia nitrogen concentrations and inhibition by volatile fatty acids, and facilitating the DIET process, ultimately leading to enhanced methane production efficiency. Furthermore, novel magnetic nanostructures hold immense potential for transforming sustainable waste management and energy production. By optimizing anaerobic digestion processes, these advancements in green technology could drive significant progress towards a more sustainable future.