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Review

Review on Mechanisms of Iron Accelerants and Their Effects on Anaerobic Digestion

by
Han Wang
,
Wanli Zhang
*,
Wanli Xing
and
Rundong Li
School of Energy and Environment, Shenyang Aerospace University, No. 37 Daoyi South Avenue, Shenyang 110136, China
*
Author to whom correspondence should be addressed.
Agriculture 2025, 15(7), 728; https://doi.org/10.3390/agriculture15070728
Submission received: 23 January 2025 / Revised: 11 March 2025 / Accepted: 24 March 2025 / Published: 28 March 2025
(This article belongs to the Special Issue Agricultural Biomass Generation and Utilization: Progress, Challenges and Prospects)
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Abstract

:
Anaerobic digestion is an important technology for energy recovery from organic waste. However, methanogenesis is restricted by some barriers, such as the low-speed bottleneck of interspecies electron transfer (IET), the low hydrogen partial pressure limitation, trace element deficiency, etc., resulting in poor system stability and low methane production. Recently, multiple iron accelerants have been employed to overcome the above challenges and have been proven effective in enhancing methanogenesis. This study reviews the effects of iron accelerants (Fe0, Fe3O4 and magnetite, Fe2O3 and hematite, iron salts and other iron accelerants) on anaerobic digestion in terms of methane production, process stability and the microbial community and elaborates the mechanisms of iron accelerants in mediating the direct interspecies electron transfer (DIET) of the syntrophic methanogenic community, strong reducibility promoting methanogenesis, provision of nutrient elements for microorganisms, etc. The potential engineering application of iron accelerants in anaerobic digestion and the current research advances regarding the environmental impacts and the recovery of iron accelerants are also summarized. Although iron accelerants exhibit positive effects on anaerobic digestion, most of the current research focuses on laboratory and small-scale investigations, and its large-scale engineering application should be further verified. Future research should focus on elucidating the mechanisms of iron accelerants for enhancing anaerobic digestion, developing diverse application methods for different types of anaerobic systems, optimizing large-scale engineering applications, and exploring the environmental impacts and high-efficiency recovery strategies of iron accelerants.

1. Introduction

Anaerobic digestion is a highly promising technology for organic waste treatment and resource utilization that can convert organic matter into methane with microorganisms [1]. However, recent studies have shown that the anaerobic digestion of organic waste is confronted with many challenges, such as volatile fatty acid (VFA) inhibition, a low methane recovery rate, and poor process stability [2]. One major factor is the mismatched growth rates between acidogens and methanogens, which lead to the imbalance between VFA production and consumption and the subsequent VFA accumulation and pH drop [3]. Another key factor is the low-speed bottleneck of the IET between syntrophic methanogenic microorganisms, which greatly limits VFA syntrophic oxidation and methane production [4]. Moreover, the high hydrogen partial pressure in the digester results in the thermodynamic barrier of syntrophic methanogenesis that impedes VFA conversion. Moreover, the deficiency in trace elements (Fe, Co, Mo, Ni, Zn, Cu, Mn, etc.) essential in the synthesis of multiple coenzymes and cofactors involved in the growth and metabolism of anaerobic microorganisms is an obstacle to anaerobic digestion, especially when the substrate is deficient in trace elements.
To overcome these challenges, multiple iron accelerants, such as zero-valent iron (Fe0), iron oxides (magnetite/Fe3O4, hematite/Fe2O3, etc.), iron-bearing minerals, and iron salts, are used for improving the anaerobic digestion process performance [5]. Firstly, owing to the high electrical conductivity, certain iron accelerants can mediate the DIET among the syntrophic methanogenic community. DIET bypasses the need for indirect electron carriers (hydrogen and formate) and enables high-efficiency electron exchanges, which enhances the IET efficiency, thereby accelerating the VFA conversion and improving the anaerobic system stability [6]. Moreover, the strong reducibility of Fe0 may decrease the oxidation–reduction potential (ORP) of anaerobic systems and participate in the CO2 reduction to CH4 reaction, as the direct electron donor promoting anaerobic methane production [7]. Moreover, iron accelerants release Fe2+/Fe3+ into anaerobic systems, which could complement the nutritional requirement of the iron element of anaerobic microorganisms and enhance the activities of some Fe-dependent metalloenzymes (hydrogenase, carbon monoxide dehydrogenase, etc.) [8,9]. Hydrogenase is indispensable in H2 production and consumption, and carbon monoxide dehydrogenase plays a vital role in the CO2 reduction into methane. The combined benefits of the application of multiple iron accelerants in anaerobic systems greatly promote organic degradation, accelerate methane production, and ensure process stability. In this context, this study reviews the mechanisms of iron accelerants and their effects on anaerobic digestion in terms of the IET of the syntrophic methanogenic community, strong reducibility promoting methanogenesis, and provision of trace nutrient elements for anaerobic microorganisms. The emphasis is focused on research advances regarding the influences of different iron accelerants (Fe0, Fe3O4 and magnetite, Fe2O3 and hematite, iron salts, and other iron accelerants) on methane production, process stability, and the microbial community. The potential engineering application and the future development direction of iron accelerants in anaerobic digestion are also proposed. This study expands the understanding of the pathways and mechanisms of iron accelerants affecting anaerobic digestion and provides practical guidance on iron accelerant application in enhancing the anaerobic digestion process performance.

2. Thermodynamic Barrier of Syntrophic Methanogenesis Involved in the Anaerobic Digestion

Anaerobic digestion is a complex biochemical process comprising four phases: hydrolysis, acidogenesis, acetogenesis, and methanogenesis. Under anaerobic conditions, organic matters are converted into methane and carbon dioxide by anaerobic microorganisms (Figure 1) [10]. VFA (acetate, propionate, butyrate, valerate, etc.) and alcohols (mainly ethanol) are the main intermediate products of hydrolysis and acidogenesis, and they should only be previously converted to acetate and H2/CO2 by acetogens and then could be utilized by methanogens. However, as shown in Table 1, VFA and alcohol oxidation via acetogenesis with the positive standard Gibbs free energy (ΔG0′) cannot occur spontaneously in thermodynamics [3,11]. To break this thermodynamic barrier, methanogens should efficiently consume acetate and H2, which reduces the hydrogen partial pressure in the anaerobic system and promotes the oxidation of VFA and alcohols [12,13]. Acidogenic bacteria and methanogens utilize the finite energy released from organic decomposition through the above collaborative mechanism, thereby breaking the thermodynamic bottleneck and converting VFA and alcohols to methane, which is known as “syntrophic methanogenesis”.

3. Pathways and Mechanisms of Iron Accelerants Affecting the Anaerobic Digestion

3.1. DIET of Syntrophic Methanogenic Microorganisms Mediated by Iron Accelerants

During syntrophic methanogenesis of organic matter, syntrophic bacteria oxidize VFA and alcohols, generating electrons. These electrons are transferred to methanogens via electron carriers, enabling methanogens to reduce CO2 to CH4. This IET is a key step in syntrophic methanogenesis (Figure 2) [14]. The traditional theory suggests that the syntrophic methanogenic community depends on H2 and formate as electron carriers of the indirect IET, which is termed interspecies hydrogen transfer (IHT) and interspecies formate transfer (IFT) (Figure 3) [15]. This process adheres to Fick’s diffusion law, where the electron transfer rate depends on several factors, such as the molecular diffusion coefficients of H2 and formate and the differential concentrations of hydrogen/formate-producing bacteria and hydrogen/formate-consuming bacteria [16]. However, due to the low solubility of H2 and the low diffusivity of formate, the pathway of IHT/IFT exhibits a low electron transfer efficiency, making it a key limiting step in syntrophic methanogenesis [16].
In recent years, many studies have reported a more efficient electron transfer pathway than the IET/IFT during syntrophic methanogenesis—DIET, in which electrons are directly transferred to electron acceptor microorganisms through conductive mediums [17]. As shown in Figure 4, these conductive mediums could be conductive pilis [18], cytochrome proteins [19], exogenetic conductive materials [20], etc. Particularly, when the anaerobic system lacks conductive pili and cytochrome proteins, thereby failing to establish a natural DIET pathway, the addition of exogenetic conductive materials could build an artificial direct electron transfer channel, thereby accelerating syntrophic methanogenesis and enhancing anaerobic system performance [21]. The common exogenetic conductive additives mainly include iron-based materials (magnetite, hematite, zero-valent iron, etc.) and carbon-based materials (activated carbon, biochar, carbon fiber, etc.). Owing to high conductivity, iron-based materials acting as conductive additives for mediating the DIET among the syntrophic methanogenic community have attracted more and more attention [22,23].

3.2. Strong Reducibility of Fe0 Promotes Anaerobic Methane Production

Fe0 exhibits a strong reducibility with an electrode potential of E0 (Fe2+/Fe)= −0.44 V. The positive effects of Fe0 on anaerobic digestion are exhibited in multiple aspects. Firstly, Fe0 can decrease the ORP of the anaerobic system, therefore contributing to maintaining a suitable growth environment for anaerobic microorganisms. Moreover, Fe2+ released from Fe0 could enhance the activities of certain Fe-dependent metalloenzymes [9,24,25]. Moreover, as the direct electron donor, Fe0 could substitute H2 in the CO2 reduction to CH4, as shown in Equation (1) [26]. Moreover, according to Equations (2) and (3), Fe0 corrosion produces H2, which increases the substrate of methanogenesis and enhances methane production [27]. Fe0 corrosion is a common electrochemical reaction in anaerobic systems containing zero-valent iron. Briefly, iron oxidation produces electrons and Fe2+ (Fe → Fe2+ + 2e−) on the iron surface, and then H+ accepts electrons to produce H2 (2H+ + 2e− → H2).
CO2 + Fe0 + 8H+ → CH4 + 4Fe2+ + 2H2O
Fe0 + 2H2O → Fe2+ + 2OH− + H2
CO2 + 4H2 → CH4 + 2H2O

3.3. Nutritional Requirement of Iron Element of Anaerobic Microorganisms

Fe element is a trace nutritional element essential for the growth and metabolism of multiple anaerobic microorganisms [28]. It serves not only as a fundamental component of microbial cells but also as an activator of microbial enzymes. Additionally, Fe could mitigate H2S toxicity by forming sulfide precipitates [3,29]. Figure 5 and Table 2 illustrate the roles and functions of the Fe element in various enzymes involved in anaerobic reactions and anaerobic microorganisms. In the CO2-reduction methanogenic pathway, Fe-dependent enzymes, such as formylmethanofuran dehydrogenase, F420-reducing hydrogenase, and coenzyme M methyltransferase, play critical roles [30,31]. Similarly, in the aceticlastic methanogenic pathway, Fe-dependent enzymes like carbon monoxide dehydrogenase/acetyl coenzyme A synthetase are essential in methanogenesis from acetate and homotypic acetogenesis [32]. Scherer et al. [33] detected the elemental composition of ten different methanogens and found that Fe was the most abundant trace metal element. In addition, sulfate-reducing bacteria might compete with methanogens for electrons and substrates (acetic acid and hydrogen), which negatively impacts methane production. However, Fe2+ could inhibit sulfate-reducing bacteria activity and precipitate with S2−, which alleviates the toxicity of H2S on methanogens [34]. Moreover, Fe3+ could accelerate organic decomposition through dissimilatory Fe(III) reduction, thereby increasing methane production [35,36].

4. Effects of Iron Accelerants on Anaerobic Digestion: Methane Production, Process Stability, and Microbial Community

4.1. Zero-Valent Iron (Fe0)

Fe0 is widely used as a stimulant in anaerobic systems treating various organic wastes (food waste, livestock manure, sewage sludge, etc.) and exhibits satisfactory strengthening effects on methane production performance (Table 3). The positive effects of Fe0 on anaerobic digestion are indicated in multiple respects: (1) it reduces the ORP of the anaerobic system, creating a favorable environment for methanogens [45]; (2) it produces H2 through Fe0 corrosion, increasing the substrate available for producing methane [26]; (3) it releases Fe2+ into the digester, complementing the nutritional requirement of Fe element of microorganisms [29]; (4) it mediates the DIET among the syntrophic methanogenic community, strengthening syntrophic methanogenesis; (5) it substitutes H2 during the CO2 reduction to CH4 as the direct electron donor [46]. Zheng et al. [47] indicated that 5 g/L of Fe0 greatly enhanced the methane yield (increased by 17.16%) of the anaerobic system of swine manure owing to additional electrons and substrates (H2) provided by Fe0 for methane production. Furthermore, Fe0 supplementation markedly promoted the growth of Clostridia and Anaerolineae with 83.9% and 120.0% higher relative abundances than the control. Clostridia mainly participate in butyric acid fermentation, homoacetogenesis, and syntrophic acetate oxidation during anaerobic digestion [48]. Abdelsalam et al. [49] added 5–20 mg/L Fe0 to an anaerobic digester treating fresh manure and observed that Fe0 stimulated the enrichment of methanogens, promoting the methane yield to 1.38–1.59 folds of the digester without Fe0 supplementation. Similarly, the remarkable stimulatory effects of Fe0 on anaerobic systems fed by food waste were also verified. Wang et al. [46] suggested that Fe0 supplementation with the dosage of 2 g/L induced a great increase in biogas and methane yield of food waste by 62.58% and 35.47% and proposed that Fe0 reduced ORP of the anaerobic system and increased the relative abundance of acetotrophic Methanosaeta from 0.94% to 2.13%. Kassab et al. [50] prepared nano Fe0 particles (110 nm) by reducing ferrous chloride with sodium borohydride and added them into an anaerobic system of food waste at the dosage of 20–60 mg/L. However, the nano Fe0 particles did not significantly enhance the process performance because the agglomeration of nano Fe0 particles in the digester weakened their effects. Moreover, influences of Fe0 on the anaerobic digestion of sludge have also been investigated in depth [7,8,51,52]. Amen et al. [51] indicated that 250 mg/L of Fe0 greatly enhanced the microbial activity of the anaerobic system of activated sludge and improved the biogas yield by 25.23%. Similarly, Li et al. [8] added 25–250 mg/g TS (total solid) of Fe0 (particle size of 150 μm) to the anaerobic system of activated sludge and found that the methane yield enhanced from 2.89 mL/g TS to 34.43 mL/g TS, reaching 12 folds of the control. They suggested that micro Fe0 particles intensified the CO2 reduction for methane production via mediating the DIET among the syntrophic methanogenic community. On the contrary, Suanon et al. [53] concluded that high-concentration nano Fe0 (up to 1% of the substrate weight) was toxic to anaerobic microorganisms, as indicated by a 29.7% decrease in methane yield of the anaerobic digestion of activated sludge. Thus, the dosage is the key factor affecting the influences of Fe0 on anaerobic digestion.

4.2. Fe3O4 and Magnetite

The main component of magnetite is Fe3O4, which is conductive, stable, superparamagnetic, and has a lower reducibility than Fe0. Table 4 summarizes the effects of Fe3O4 and magnetite on anaerobic digestion reported in the literature. Notably, the mechanisms of Fe3O4 and magnetite affecting anaerobic digestion are not exactly the same as that of Fe0 [55]. Ajayi-Banji and Rahman [56] found that VFA concentrations in the anaerobic system fed by dairy manure and corn straw declined by 80% with the addition of 20 mg/L nano Fe3O4 and proposed that Fe3O4 mediated the DIET among the syntrophic methanogenic community, therefore accelerating the VFA conversion and meanwhile significantly enhancing microbial diversity. Wang et al. [57] found that cytochrome c concentration in the anaerobic system supplementing magnetite was only 1/5 of that in the control trial and speculated that magnetite substituted cytochrome c for mediating the DIET between syntrophic Porphyromonadaceae, Syntrophaceae, and methanogens. Moreover, Fe3O4 and magnetite might provide Fe nutrient elements for the growth and metabolism of anaerobic microorganisms [49,58]. Hassanein et al. [58] proposed that Fe3O4 complemented the Fe element essential for microorganisms, resulting in the great enhancement of methane yield (by 27.5%) of the anaerobic digestion of manure. Additionally, the Fe2+ released from Fe3O4/magnetite mitigates the toxic effects of H2S on methanogens by precipitating with S2−. Farghali et al. [59] utilized waste Fe powder (containing 85% Fe3O4) to reduce H2S concentration in the anaerobic digester of dairy manure by 77.24%. However, the overdose of Fe3O4/magnetite may inhibit anaerobic digestion [60]. Suanon et al. [53] found that the methane yield of the anaerobic digestion of sludge decreased by 11.5% when the dosage of magnetite reached 1% of the substrate weight.

4.3. Fe2O3 and Hematite

The main component of hematite is Fe2O3, which is also a common additive of anaerobic digestion (Table 5). Tang et al. [63] investigated the effects of hematite on the phenol degradation in the anaerobic system of artificial wastewater and found that hematite promoted the growth of Fe(III)-reducing bacteria (Moorella, Trichococcus, Caloramator, Shewanella, and Enterococcus, etc.) and facilitated the conversion of the metabolic intermediates of phenol (such as VFA) to methane by improving electron transfer among the syntrophic methanogenic community. Hematite and Fe2O3 also could release iron ions into the anaerobic system, which stimulates the related biological enzyme activity and microbial metabolism [64]. Ambuchi et al. [65] explored the effects of Fe2O3 on the anaerobic digestion of sugar beet industrial wastewater and demonstrated that Fe2O3 stimulated the enrichment of Methanosaeta from 24.6% to 39.4%, resulting in an increased methane yield of 28.9%. Farghali et al. [59] found that H2S concentration in the anaerobic system of dairy manure was decreased by supplementing nano Fe2O3. However, excessive hematite/Fe2O3 may exhibit inhibitory effects on anaerobic microorganisms’ activities. Lu et al. [66] found that 350 mmol/L of Fe2O3 inhibited the anaerobic digestion of the swine manure and led to a 7.8% decline in methane yield, and the inhibitory effect became stronger with the increasing Fe2O3 dosage.

4.4. Iron Salts and Other Iron Accelerants

Iron salt supplementation is an important way to meet the iron requirements of anaerobic microorganisms. Yu et al. [68] found that 200 mg/L of FeCl3 shortened the lag phase of methanogenesis and enhanced the methane production rate of the anaerobic digestion of the activated sludge and suggested that FeCl3 stimulated the significant growth of Coprothermobacter and Methanosarcina (up to 18.7% and 63.2%, respectively). Qin et al. [69] also confirmed the positive effects of FeCl3 and FeCl2 on the anaerobic digestion of sewage sludge. Moreover, some literature has reported the influences of other iron accelerants (iron composite materials and mixtures of iron accelerants of different chemical speciations) on anaerobic digestion (Table 6). Xu et al. [70] loaded Fe2O3 onto the carbon cloth and added this iron–carbon composite material to the anaerobic system treating the sodium propionate. They found that Fe2O3-carbon cloth stimulated iron-reducing bacterium Clostridium growth, greatly alleviated propionate accumulation inhibition (degradation rate enhanced by 19.67%), and improved methane production by 15.4%. Their research also indicated that the promoting effects of Fe2O3-carbon cloth were superior to single-carbon cloth (methane yield increased by 10.8%). Shen et al. [71] demonstrated that Fe2O3-ceramsite enriched methanogens and enhanced methane production of the anaerobic digestion of the activated sludge. Zhu et al. [72] utilized hematite (α-Fe2O3) supported bentonite to enhance the methane yield of the anaerobic digestion of the food waste, reaching 3.3–12.3 folds of that in the control trial. They concluded that α-Fe2O3-bentonite increased the abundances of hydrolytic and acidifying bacteria as well as hydrogenotrophic methanogens. In short, composite materials of iron accelerants with carbon cloth, ceramsite, bentonite, and other materials can also significantly improve anaerobic digestion, which provides practical guidance on the application of advanced composite material in the field of anaerobic biotechnology.

5. Potential Engineering Application of Iron Accelerants in Anaerobic Digestion

In recent years, the positive effects of multiple iron accelerants on anaerobic digestion have been verified, but current related research mainly focuses on laboratory and small-scale trials, and the engineering application is still rare. Because the affecting factors of anaerobic digestion performance are too complicated, the influences of iron accelerants with different types, particle sizes, and dosages on anaerobic systems exhibit great differences under different operating conditions [74]. Moreover, the economic feasibility of iron accelerants compared to other additives (such as carbon-based materials) remains unclear. Only a few studies analyzed the feasibility of biogas engineering applications of iron accelerants. Wei et al. [30] took a wastewater treatment plant with a treatment capacity of 300,000 m3/d as an example and found that the anaerobic system supplemented by 33.3 g/L of scrap iron (8 mm × 3 mm × 0.5 mm) could save the operational cost of about 272,400 US $/yr and could cut down the carbon emission by 1660 t CO2/yr, compared to the conventional anaerobic system. Similarly, Farghali et al. [59] proposed that 1 g/L of scrap iron supplemented with the anaerobic digester with a treatment capacity of 1 m3 of manure could enhance the power production of biogas to 515.53 kWh, and the net benefit reached 42.04 US $. Although these previous studies suggest that the application of iron in biogas engineering is economically feasible, further investigation is still needed. Current research mainly focuses on exploring the optimal dosing strategy of iron accelerants and systematically evaluating their impacts on anaerobic digestion performance and ecological environment during long-term application. In the future, emphasis should be focused on developing high-efficiency, low-cost, and environmentally friendly iron accelerants and diverse application strategies suitable for different types of anaerobic systems and extending laboratory and small-scale investigations to large-scale engineering applications.

6. Environmental Impact and Recovery of Iron Accelerants

Although iron accelerants have been proven effective in enhancing anaerobic digestion, their environmental impacts and ecological risks are also important considerations, especially the potential toxicity of the digestate used as fertilizer and the risk of metal accumulation [75]. You et al. [76] explored the biological effects of nanoscale zero-valent iron on anaerobic microorganisms and found that nanoscale zero-valent iron supplementation in an anaerobic system led to ferroptosis-like death with hallmarks of iron-dependent lipid peroxidation and glutathione depletion during the first 12 days of exposure. Yang et al. [77] reviewed the environmental and human health risks of magnetic iron oxide nanoparticles present in aquatic, terrestrial, and atmospheric environments and proposed that magnetic iron oxide nanoparticles could deposit deep into the lungs, reaching alveoli through the respiratory system, and migrate into extrapulmonary organs via blood circulation. Qualhato et al. [78] carried out a genotoxic and mutagenic assessment of iron oxide nanoparticles in typical aquatic organisms (female guppies Poecilia reticulate) and found that iron oxide nanoparticles induced DNA damage after acute (3 and 7 days) and long-term exposure (14 and 21 days). On the contrary, some previous reports indicated that iron accelerants have minimal environmental impact, and the iron-containing digestate can provide trace elements for plants [75]. Ghafariyan et al. [79] found that magnetite nanoparticles greatly enhanced the chlorophyll content of soybean leaves in hydroponic conditions when they were used as nano-fertilizers. Furthermore, some scholars even suggested that multiple iron accelerants, such as nanoscale zero-valent iron, iron oxides, and biochar-supported nano zero-valent iron, could mitigate the risk of antibiotic resistance genes (ARGs) in organic waste under anaerobic digestion conditions [80,81]. Yu et al. [82] achieved a great reduction in ARGs by 38.35% and 21.77% in the two-phase anaerobic digestion of food waste by adding biochar-supported nano zero-valent iron. Thus, the fate of iron accelerants in biological treatment systems and the environment should be further explored.
Iron recovery and recycling is a significant approach to reducing the cost of iron accelerants and mitigating the health and ecological risks of iron in anaerobic systems and the environment [83]. Magnetic separation has been proven to be an effective method for iron accelerant recovery [84]. Owing to strong magnetic properties, iron accelerants can be easily separated from the digestate, allowing for recycling during the continuous anaerobic digestion process [85]. Hutchins and Downey [84] developed an in-line, water-cooled magnetic collection module with a high collection efficiency (regularly exceeding 98%) of nano-scale magnetite in wastewater. Li et al. [85] achieved a high recovery rate of Fe3O4 (89.37–91.57%) from the digestate via magnetic collection and investigated the effects of recycled Fe3O4 on anaerobic co-digestion of sewage sludge with corn straw. They found that recycled Fe3O4 maintained the predominant position of acetotrophic Methanosaeta and promoted the growth of Methanosarcina (from 3.85% to 6.28%), broadening the methanogenic pathways and resulting in the great increase in methane yield by 79.81%. Akar et al. [86] proposed that iron nanoparticle recovery and recycling in the field of anaerobic digestion could partly reduce reliance on costly laboratory-synthesized chemicals. Yusuf et al. [83] achieved the goal of nano zero-valent iron recovery and reuse and continuously enhanced methane production by iron-containing percolate recirculation during the high solid anaerobic digestion of fecal slag and food waste. Wu et al. [87] reviewed current research concerning the topic of iron salt use and recycling in municipal wastewater and sludge treatment and suggested that recovery and recycling of iron in the sludge could greatly reduce the treatment cost and total sludge volume. These previous reports confirmed the economic and technical feasibility of iron recovery and recycling and provided theoretical and practical guidance on integrating circular economy principles into anaerobic digestion systems. Nevertheless, an in-depth investigation concerning the long-term environmental impacts of iron accelerants should be carried out, and large-scale application of iron recovery from anaerobic digestion still needs to be further verified.

7. Conclusions

Iron accelerants (Fe0, Fe3O4 and magnetite, Fe2O3 and hematite, iron salts, and other iron accelerants) exhibit positive effects on anaerobic digestion via multiple mechanisms, including mediating the DIET among the syntrophic methanogenic community, strong reducibility promoting methanogenesis, provision of Fe nutrient elements for anaerobic microorganisms, iron corrosion resulting in hydrogen evolution to methane yield, inhibiting sulfate-reducing bacteria activity, accelerating organic decomposition via dissimilatory Fe(III) reduction, precipitating with S2−, and alleviating the toxicity of H2S on methanogens, etc. The advantages of iron accelerant supplementation on the anaerobic system have been verified, but the overdose or the inappropriate dosing strategy of iron accelerants might result in the risk of inhibiting methane production. Most of the current research focuses on laboratory and small-scale investigations, and its large-scale engineering application is still to be explored. In the future, more in-depth explorations in regard to the mechanisms of iron accelerants for enhancing anaerobic digestion, diverse iron accelerant application strategies suitable for different types of anaerobic systems, high-efficiency large-scale engineering applications of iron accelerants, their environmental impacts, as well as high-efficiency and cost-effective recovery strategies should be carried out.

Author Contributions

Conceptualization, H.W. and R.L.; methodology, H.W. and W.Z.; literature search and analysis, H.W. and W.Z.; writing—original draft, H.W. and W.X.; writing—review and editing, W.X., W.Z. and R.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (NO. 52476215), Natural Science Foundation of Liaoning Province (NOs. 2024JH3/10200047 and 2024-MS-139), and Fundamental Research Funds for the Universities of Liaoning Province (NO. LJ212410143033).

Institutional Review Board Statement

Not applicable.

Data Availability Statement

No new data were created or analyzed in this study.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Principle of anaerobic digestion [10].
Figure 1. Principle of anaerobic digestion [10].
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Figure 2. Microorganisms interspecies electron transfer during anaerobic digestion [15]: interspecies hydrogen transfer (IHT); interspecies formate transfer (IFT); direct interspecies electron transfer (DIET).
Figure 2. Microorganisms interspecies electron transfer during anaerobic digestion [15]: interspecies hydrogen transfer (IHT); interspecies formate transfer (IFT); direct interspecies electron transfer (DIET).
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Figure 3. Indirect IET via hydrogen/formate [10].
Figure 3. Indirect IET via hydrogen/formate [10].
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Figure 4. DIET pathways [10]: DIET via conductive pili (a); DIET via cytochrome proteins (b); DIET via exogenetic conductive materials (c).
Figure 4. DIET pathways [10]: DIET via conductive pili (a); DIET via cytochrome proteins (b); DIET via exogenetic conductive materials (c).
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Figure 5. Enzymes containing Fe element involved in anaerobic digestion [30]: acetyl coenzyme A synthetase/Carbon monoxide dehydrogenase (a); formylmethanofuran dehydrogenase (b); methyltransferase (c); methenyl-tetrahydromethanopterin cyclohydrolase (d); F420-reducing hydrogenase (e); methenyl-tetrahydromethanopterin coenzyme M methyltransferase (f); methyl-coenzyme M reductase (g). The energy value on the right side represents the Gibbs free energy (G0) of each biochemical reaction. The black arrows are the CO2 reduction methanogenesis pathway, the blue arrows are the acetate fermentation methanogenesis pathway, and the green arrows are the methyl cleavage methanogenic pathway.
Figure 5. Enzymes containing Fe element involved in anaerobic digestion [30]: acetyl coenzyme A synthetase/Carbon monoxide dehydrogenase (a); formylmethanofuran dehydrogenase (b); methyltransferase (c); methenyl-tetrahydromethanopterin cyclohydrolase (d); F420-reducing hydrogenase (e); methenyl-tetrahydromethanopterin coenzyme M methyltransferase (f); methyl-coenzyme M reductase (g). The energy value on the right side represents the Gibbs free energy (G0) of each biochemical reaction. The black arrows are the CO2 reduction methanogenesis pathway, the blue arrows are the acetate fermentation methanogenesis pathway, and the green arrows are the methyl cleavage methanogenic pathway.
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Table 1. Standard Gibbs free energy (25 °C, 1.013 × 105 Pa, pH = 7) of acetogenesis in anaerobic digestion.
Table 1. Standard Gibbs free energy (25 °C, 1.013 × 105 Pa, pH = 7) of acetogenesis in anaerobic digestion.
SubstrateAcetogenesis ReactionStandard Gibbs Free Energy ΔG0′ (kJ/mol)
PropionateCH3CH2COO− + 3H2O→CH3COO− + HCO3− + 3H2 + H++76.1
ButyrateCH3CH2CH2COO− + 2H2O→2CH3COO− + 2H2 + H++48.1
EthanolCH3CH2OH + H2O→CH3COO− + 2H2 + H++9.6
Table 2. Fe in enzymes of microbial conversions.
Table 2. Fe in enzymes of microbial conversions.
ElementEnzymeMicroorganismReference
FeCarbon monoxide dehydrogenaseMethanogen/Acetogen[37]
Superoxide DismutaseMethanogen[38]
HydrogenaseDesulfovibrio/Esherichia coli[39]
Acetyl coenzyme A synthetaseMoorella thermoacetica[40]
NO reductasePseudomonospore denitrifying bacteria[41]
Nitrite reductasePseudomonas stutzeri[42]
Nitrate reductaseParacoccus denitrificans[42]
Nitrogenase [43]
Methane monooxygenase [44]
Table 3. Effects of Fe0 on anaerobic digestion.
Table 3. Effects of Fe0 on anaerobic digestion.
SubstrateOperating ModeParticle Size of Fe0DosageInoculum Ratio (Substrate: Inoculum, on vs. Basis)Reaction TemperatureReaction TimeEffects on Anaerobic DigestionReference
Swine manureBatch500 nm5 g/L1:135 °C30 daysMethane yield increased by 17.16%; relative abundance of bacteria Bacteroidia and Gammaproteobacteria increased by 19.4% and 10.3%, respectively; relative abundance of archaea Methanothrix and Methanolinea increased by 19.8% and 16.2%, respectively.[47]
Fresh manureBatch9 nm5–20 mg/L-37 °C50 daysBiogas yield was 1.44–1.45 folds of that in the control trial; methane production was 1.38–1.59 folds of that in the control trial. The growth rate of gas production was proportional to the dosage of Fe0.[49]
Food wasteBatch40 nm2 g/L2:135 °C15 daysBiogas yield increased by 62.58%; methane yield increased by 35.47%.[46]
Food wasteBatch300–600 nm2–10 g/L1:137 °C65 daysMethane yield increased by 2.7–8.5%.[54]
Food wasteBatch110 nm20–60 mg/L1.5:135 °C23 daysMethane production did not show significant change.[50]
Artificial wastewaterBatch100 nm5 g/L-37 °C54 hMethane yield increased by 23.9%.[45]
Activated sludgeBatch150 μm25–250 mg/g TS-55 °C32 daysMethane yield reached 0.8–12 folds of that in the control trial.[8]
Activated sludgeBatch< 100 nm250 mg/L-37 °C14 daysBiogas yield increased by 25.23%.[51]
Activated sludgeBatch40–60 nm5–9 mg/g VS0.5:136 °C40 daysBiogas yield rose with the increasing concentration of Fe0; the greatest increment of biogas yield reached 135%.[52]
Activated sludgeBatch20 nm0.1%-37 °C17 daysThe concentration of H2S decreased by 98.0%; biogas yield increased by 30.4%; methane yield increased by 40.4%.[7]
Activated sludgeBatch50 nm1%-37 °C12 daysMethane yield decreased by 29.7%.[53]
Table 4. Effects of Fe3O4 and magnetite on anaerobic digestion.
Table 4. Effects of Fe3O4 and magnetite on anaerobic digestion.
Iron AccelerantSubstrateOperating ModeParticle Size of Fe3O4 and MagnetiteDosageInoculum Ratio (Substrate: Inoculum, on vs. Basis)Reaction TemperatureReaction TimeEffects on Anaerobic DigestionReference
Fe3O4Poultry litterBatch-15 mg/L1:135 °C79 daysMethane yield increased by 27.5%.[58]
Nano Fe3O4Dairy manure (55.7%) and
corn stover (7.2%)		Batch15–20 nm20 mg/L2.5:135 °C80 daysReaction time reduced by 27 days; methane yield increased.[56]
Nano Fe3O4Fresh manureBatch7 nm5–20 mg/L-37 °C50 daysBiogas yield rose to 1.63–1.66 folds of that in the control trial; methane yield reached 1.82–1.96 folds of that in the control trial.[49]
Nano Fe3O4Food wasteBatch29.5 nm25–80 mg/L1.5:135 °C28 daysMethane yield increased by 7–50.8%.[50]
Nano Fe3O4Activated sludgeBatch20–30 nm20–200 mg/L-36 °C12 daysMethane yield increased by 1.1–1.6 folds compared to the control.[55]
Nano Fe3O4Activated sludgeBatch12–18 nm and 50–100 nm40–250 mg/L1.5:137 °C25 days120 mg/L of Fe3O4 (12–18 nm) enhanced methane yield by 1.7 folds compared to the control; 250 mg/L of Fe3O4 (50–100 nm) increased methane yield by 1.4 folds compared to the control.[61]
Waste iron powder
(85% Fe3O4)		Dairy manureBatch<20 μm1000 mg/L1:338 °C30 daysH2S concentration decreased by 77.24%; methane yield increased by 56.89%.[59]
MagnetiteFood wasteBatch300–600 nm2–10 g/L1:137 °C65 daysMethane yield enhanced by 3.2–6%.[54]
MagnetiteHigh salinity food wasteSemi-continuous-2 g/L-35 °C108 daysMethane yield decreased by 18.9%.[62]
MagnetiteActivated sludgeBatch20 nm1%-37 °C12 daysMethane yield decreased by 11.5%.[53]
Table 5. Effects of Fe2O3 and hematite on anaerobic digestion.
Table 5. Effects of Fe2O3 and hematite on anaerobic digestion.
Iron AccelerantSubstrateOperating ModeParticle Size of Fe2O3 and HematiteDosageInoculum Ratio (Substrate: Inoculum, on vs. Basis)Reaction TemperatureReaction TimeEffects on Anaerobic DigestionReference
Fe2O3Beet sugar industrial wastewaterBatch20 nm750 mg/L-36 °C74 daysMethane yield increased by 28.9%; chemical oxygen demand concentration decreased by 21.8%.[65]
Red mud (45.46% of hematite)Activated sludgeBatch50–300 µm20 g/L-35 °C32 daysMethane yield enhanced by 35.52%.[67]
Fe2O3Food wasteBatch300–600 nm2–10 g/L1:137 °C65 daysMethane yield increased by 4.4–6.7%.[54]
Fe2O3Dairy manureBatch20–40 nm1000 mg/L-38 °C30 daysMethane yield rose by 21.11%; H2S concentration significantly decreased.[59]
Fe2O3Swine manureBatch30 nm75–350 mmol/L3:137 °C30 daysAnaerobic digestion reaction was enhanced by Fe2O3 in the first twelve days but then methane production was inhibited by Fe2O3; the inhibition level was proportional to the dosage of Fe2O3; methane yield decreased by 7.8%.[66]
Table 6. Effects of ferrous salt and other iron composite materials on anaerobic digestion.
Table 6. Effects of ferrous salt and other iron composite materials on anaerobic digestion.
Iron AccelerantSubstrateOperating ModeDosageInoculum Ratio (Substrate: Inoculum, on vs. Basis)Reaction TemperatureReaction TimeEffects on Anaerobic DigestionReference
FeCl3Activated sludgeBatch200 mg/L-55 °C48 daysBiogas yield increased by 79.6%.[68]
FeCl2Dewatered sludgeBatch100–1000 mg/L2:135 °C27 daysMethane yield enhanced by 6.4%.[69]
FeCl3Dewatered sludgeBatch100–800 mg/L2:135 °C27 daysMethane increased by 28.9%.[69]
Fe2O3-carbon clothSodium propionate
(30 mM)		Batch--37 °C24 daysMethane yield rose by 15.4%; propionate degradation rate increased by 19.67%.[70]
Fe2O3-ceramsiteActivated sludgeBatch10 g/L2:135 °C55 daysMethane yield increased to 1.4 folds of that in the control trial.[71]
Rusted iron (FeOOH-Fe2O3)Food waste and municipal sludgeBatch--35 °C36 daysMethane yield increased by 64.4%; the peak value of methane production rate enhanced by 12.2%.[73]
α-Fe2O3-bentoniteFood wasteBatch0.5–3.75 g/g VS2:137 °C45 daysBiogas yield increased by 60.6–232.1%; methane yield rose to 3.3–12.3 folds of that in the control trial.[72]
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Wang, H.; Zhang, W.; Xing, W.; Li, R. Review on Mechanisms of Iron Accelerants and Their Effects on Anaerobic Digestion. Agriculture 2025, 15, 728. https://doi.org/10.3390/agriculture15070728

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Wang H, Zhang W, Xing W, Li R. Review on Mechanisms of Iron Accelerants and Their Effects on Anaerobic Digestion. Agriculture. 2025; 15(7):728. https://doi.org/10.3390/agriculture15070728

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Wang, Han, Wanli Zhang, Wanli Xing, and Rundong Li. 2025. "Review on Mechanisms of Iron Accelerants and Their Effects on Anaerobic Digestion" Agriculture 15, no. 7: 728. https://doi.org/10.3390/agriculture15070728

APA Style

Wang, H., Zhang, W., Xing, W., & Li, R. (2025). Review on Mechanisms of Iron Accelerants and Their Effects on Anaerobic Digestion. Agriculture, 15(7), 728. https://doi.org/10.3390/agriculture15070728

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