Abstract
A high solid anaerobic incubation system was used to study the effect of dissimilatory iron reduction on the inhibition of methanogenesis in a landfill. Different iron sources including FeC6H5O7, Fe2(SO4)3, Fe2O3, and Fe0 were studied. The different iron sources significantly affected the methanogenesis process in the simulated landfill system. FeC6H5O7 and Fe2(SO4)3 inhibited methanogenesis but Fe0 and Fe2O3 increased it. The dissimilatory iron reduction with FeC6H5O7 as the iron source demonstrates an anaerobic mineralization process, which enhances the biodegradation but inhibits methanogenesis. The highest rate of reduction of CH4 production (51.9%) was obtained at a dosage rate of 16,000 mg·kg−1, which corresponded to a reduction of 0.86 g of CH4 per kg of organic matrix. Active inhibition by methanogens using both the hydrogenotrophic and acetoclastic pathways is considered to be the main mechanism underlying the reduction of CH4 production by dissimilatory iron reduction with FeC6H5O7 as the iron source. This is the first report on the effect of different iron forms on the reduction of CH4 production during landfilling with organic solid waste.
1. Introduction
Iron is the fourth most abundant element in the Earth’s crust and the most abundant of the metals that naturally undergoes redox changes [1]. Dissimilatory iron reduction involves the oxidation of a carbon source coupled with the reduction of ferric iron (Fe(III)) as the primary electron acceptor [2]. It has been termed the most important chemical change that takes place in the development of anaerobic soils and sediments [3]. Dissimilatory iron reduction thus has important geological and environmental significance. Many deep pristine aquifers have extensive anaerobic zones in which organic matter degradation is coupled to Fe(III) reduction [4, 5]. A model of iron geochemistry in Toolik Lake, Alaska, indicated that up to 50% of metabolism in the anoxic sediment was the result of Fe(III) reduction [6]. In paddy soil, Fe(III) reduction accounted for 35% to 65% of organic matter oxidation to carbon dioxide [7]. Microbial dissimilatory reduction of Fe(III) therefore greatly influences the biogeochemical cycles of carbon and metals in wetlands and aquatic ecosystems.
Landfill has been the main management route for municipal solid waste (MSW), and in many parts of the world, especially in developing countries, it still is. In China, more than 90% of the MSW is disposed of in landfills [8]. On the contrary, landfills are large iron reservoirs [9]. Iron concentrations in fresh MSW and degraded waste are up to 12,400 and 10,300 mg·kg−1, respectively [10], while that in the leachate can be 5,500 mg L−1 [11]. It was also found that iron concentrations in the leachate sediment varied between 13,500 and 285,000 mg·kg−1 [12, 13]. We therefore hypothesize that landfills provide a good opportunity for iron cycling coupled to organic mineralization. However, there is no research focusing on the role of iron reduction in the carbon biochemical cycle in landfills, which have a very different biochemical environment to that of wetlands, aquatic ecosystems, and rice fields.
In this study, a kind of high solid anaerobic condition was simulated to study the effect of dissimilatory iron reduction on CH4 reduction in a landfill. The purpose of the research was (1) to investigate the hypothesis that ferric iron reduction is of quantitative importance in the anaerobic mineralization process in landfills and (2) to distinguish the optimal iron source for suppression of CH4 generation in landfills.
2. Materials and Methods
2.1. Reagents and Materials
Protein, fat, and starch were chosen to simulate the fast biodegradable fractions of organic waste, and cellulose was chosen to simulate the slowly biodegradable fraction of municipal solid waste [14]. FeC6H5O7, Fe2(SO4)3, Fe2O3, and metallic iron powder (Fe0) (ACS-grade chemicals, Sigma-Aldrich) were chosen to represent different kinds of iron sources in landfill wastes, namely, organic-bound Fe, inorganic-bound Fe, oxidizable Fe, and free Fe, respectively.
Based on the measured iron content in an actual landfill site, the iron concentrations were set as 0 (control), 4,000 mg·kg−1, 8,000 mg·kg−1, 16,000 mg·kg−1, 24,000 mg·kg−1, 32,000 mg·kg−1, and 40,000 mg·kg−1 to simulate different concentrations of different iron sources within landfill waste. The different iron concentrations can also be considered to represent those present in a landfill during different phases of anaerobic digestion.
Detailed information regarding the composition of the mixed substance is presented in Tables 1 and 2. The seed sludge used was collected from the anaerobic process of the Gaobeidian wastewater treatment plant (Beijing, China). It had a total iron concentration of 373 ± 10.8 mg L−1, and the total solid concentration was 9.2 g L−1.
2.2. High Solid Anaerobic Incubation
The high solid anaerobic incubation was carried out in vials with a volume of 125 mL. In each vial, 12.5 g of mixed substance (Table 1) was added first and was subsequently inoculated with 5% (wt/wt) of seed anaerobic sludge. The moisture content of the matrix was adjusted to 75% by adding deionized water to the vials. Iron sources were then added into the matrix according to the experimental design [15]. Finally, the vials were sealed and O2 was expelled by purging N2 into the vials for 15 min. All vials were incubated at 35°C. During the incubation, the gas volume and the concentrations of H2, CO2, and CH4 in the headspace of each vials were determined. Each incubation and treatment was carried out in triplicate to ensure validity of the results. The graphical data are average values.
2.3. Destructive Testing
Based on the identification of the optimal iron source and concentration, which was investigated in the high solid anaerobic incubation described in Section 2.2, mechanisms of the dissimilatory iron reduction on CH4 reduction were further investigated by a destructive test. This test was also carried out in 125 mL vials. 12.5 g of mixed substance (Table 1) was added to the vials and inoculated with 5% (wt/wt) of seed anaerobic sludge. The moisture content of the matrix was adjusted to 75% with deionized water [15]. Then, FeC6H5O7 was added to each vial before they were sealed, and O2 was expelled by purging the vials with N2 for 15 min. All vials were incubated at 35°C for 60 days. Each incubation and treatment was carried out in triplicate to ensure validity of the results. Vials without FeC6H5O7 were used as controls.
With respect to the matrix sampling, samples were taken on days 1, 3, 6, 10, 15, 20, 30, 40, 50, and 60. Six vials (three parallels of the FeC6H5O7 treatment and three controls) were randomly sampled for analysis at each sampling date. The gas volume and the concentrations of H2, CO2, and CH4 in the headspace of each vial were determined. The volatile fatty acid (VFA) and Fe(II) concentrations, and pH of the sampled solid matrix, were also determined.
2.4. Analyses
The gas volume in each vial was determined using the vacuum dewatering method. Gases in the vials were also sampled using a syringe (1 mL) by the balance method. A gas chromatograph (Agilent 6890 N) with a thermal conductivity detector was used to measure CH4, H2, and CO2 concentrations in the gas samples.
In the destructive testing, vials were sampled to determine pH, VFAs, and Fe(II). For pH and VFA testing, the solid matrix was extracted by distilled water with a liquid-to-solid ratio of 10 : 1 [16], while for Fe(II) testing, the solid matrix was extracted by 1 M HCl at the same liquid-to-solid ratio [17]. All determinations used standard methods [18].
3. Results and Discussion
3.1. Effects of Iron Source on Biogas Production under Landfill Conditions
Gas production is an important indicator of the anaerobic biodegradation process. Figure 1 shows that the different iron sources including Fe0, Fe2O3, FeC6H5O7, and Fe2(SO4)3 obviously affect gas production in the anaerobic system. Among the four tested iron sources, FeC6H5O7 promoted gas production, presumably because of fast degradation of the mixed substance, especially in the initial stages of the incubation. After 15 days, the gas production rate gradually stabilized. In general, the promotion of gas production by FeC6H5O7 differed significantly among its dosage rates with the optimal dosage rate being 16,000 mg·kg−1, as evidenced by its final cumulative gas volume of 856.7 mL, which was almost twice that of the control (429.8 mL). Fe2O3 also promoted biodegradation of the substance at low dosage rates, but not at high dosage rates. The highest cumulative gas volume was 560.9 mL at the dosage rate of 16,000 mg·kg−1. Addition of Fe0 had negative effects on gas production to some extent. As Figure 1 shows, the highest cumulative gas volume in the treatment of Fe0 was 433.9 mL, which was not significantly different from that of the control (429.8 mL). In the Fe2(SO4)3 treatment, the gas production increased by 45.5% at the dosage rate of 800 mg·kg−1. However, there were obvious negative effects at other dosage rates, especially at rates higher than 24,000 mg·kg−1 when gas production was almost totally inhibited. In general, from the viewpoint of gas production, the order of the promoting effect of the different iron sources on the anaerobic biodegradation process was FeC6H5O7 > Fe2O3 > Fe0 > Fe2(SO4)3.
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The tested iron sources represent four different states of iron including an organic-bound state (FeC6H5O7), an inorganic-bound state (Fe2(SO4)3), an oxidized state (Fe2O3), and free iron (Fe0). The result confirms that the bioavailability of Fe(III) is not a key factor in the dissimilatory iron reduction because microbes can reduce solid electron acceptors on the cell surface by extracellular respiration [19]. Among the four tested iron states, organic-bound Fe such as FeC6H5O7 is the optimal state for dissimilatory iron reduction that promotes the anaerobic biodegradation process.
To verify this effect more accurately, the cumulative net gas volumes in the four tested systems with different iron sources were calculated usingwhere VNet gas refers to the real gas contributed by degradation of the matrix, VAccumulated gas is the determined value of the gas volume in the tested vials, and VCK is the gas volume produced in the control vial. Figure 2 shows that the cumulative net gas production was in the order FeC6H5O7 > Fe2O3 > Fe0 > Fe2(SO4)3. The Fe2(SO4)3 treatment showed significant inhibition, apparently caused by the adverse effects of SO42− on methanogens and other anaerobes. Therefore, consideration of the cumulative net production of gas also indicates that organic-bound Fe, FeC6H5O7, is the optimal tested form of iron for promoting the biodegradation of organic substances, and that the optimum dosage rate was 16,000 mg·kg−1, which represents the general level in landfill refuse.
3.2. Effects of Iron Source on CH4 Production and Reduction
During the formation of anaerobic conditions, molecular oxygen and nitrates disappear rapidly followed by the formation of manganous and ferrous ions. The Eh of the soil decreases from +0.6∼+0.5 V to −0.1∼−0.2 V and rapid production of ammonia and carbon dioxide occurs, while the accumulation of products of anaerobic metabolism, such as sulfides, organic acids, molecular hydrogen and methane, can hardly be detected, except either in organic matter-rich or active iron-poor soil. After the first step, production of sulfides, organic acids, molecular hydrogen, and methane occurs successively. Organic acids, which have temporarily accumulated in the soil, disappear again, and the amount of sulfides and methane increases continuously [20]. Therefore, theoretically, the production of CH4 will be inhibited at the stage when iron reduction is actively occurring. However, our results show that with respect to differences in the metabolic ability of the different iron sources, the inhibition of CH4 production differs.
Figure 3 shows that the CH4 concentration changed with different iron sources and concentrations. When the anaerobic system had Fe0 added to it, the CH4 concentration increased rapidly from day 15 to day 40 before slowing down after day 40. Also, the CH4 concentrations were always higher than in the control during that period. Thus, the introduction of Fe0 into the anaerobic system obviously promotes the methanogenesis process. When the iron source was replaced with Fe2O3, a similar trend in CH4 concentrations was observed from day 15 to day 40. However, a difference from the addition of Fe0 was that the increasing rate of CH4 concentration was moderated after day 40. Addition of both FeC6H5O7 and Fe2(SO4)3 led to their CH4 concentrations always remaining at a very low level (almost undetectable), which suggested that the methanogenesis process in those two systems was inhibited.
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The net generation of CH4 in the treatments with the different iron sources can be calculated bywhere VNet methane refers to the real methane contributed by degradation of the matrix, VAccumulated methane is the determined value of the methane volume in the tested vials, and VCK is the methane volume produced in the control vial. Figure 4 shows that the final net CH4 produced in the control treatment was 37.6 mL. With the introduction of Fe0, the CH4 volume increased with increasing dosage rate, with an increase of 88.5% when the dosage rate was 40,000 mg·kg−1. This is because only Fe(III), rather than Fe0, can be metabolized during the dissimilatory iron reduction process. Therefore, there was no inhibition effect of Fe0 on methanogenesis. Fe0 can also promote hydrogen production in an anaerobic system, which results in CH4 production [21]. When Fe2O3 was added to the anaerobic system, the CH4 production experienced a weak inhibition at low dosage rates but a strong promotion at high dosage rates, with an increase of 52.8% when the dosage rate was 24,000 mg·kg−1. Theoretically, Fe2O3 can by metabolized by dissimilatory iron reducers such as Geobacter and Pseudomonas [22]. However, its bioavailability is low. As was the case with Fe2(SO4)3, CH4 production was completely inhibited. However, the inhibition differed with the dosage rate. More specifically, the CH4 production was inhibited by the dissimilatory iron reduction process when the Fe2(SO4)3 dosage rate was lower than 8,000 mg·kg−1. When the dosage rate of Fe2(SO4)3 exceeded 8,000 mg·kg−1, its bioactivity was obviously impaired. When FeC6H5O7 was added, CH4 production almost always never happened during the experimental period. This suggests that FeC6H5O7 has better bioavailability when coupling iron reduction to organic mineralization. Thus, it is in some respects a kind of successful CH4 control process because of its ability to outcompete methanogens by enhancing the activity of iron reducers.
3.3. Effects of Iron Source on H2 Production
H2 is an important intermediate product of anaerobic biodegradation systems. Figure 5 shows the variation in H2 concentrations in the anaerobic biodegradation system with different iron sources. In the control treatment, the H2 concentration increased rapidly at the acidification and hydrogen production stages, and then sharply decreased because of consumption by hydrogenotrophic methanogens and other fermenting bacteria, before eventually remaining at a low concentration. When Fe0 was added to the anaerobic system, the H2 concentration always remained at a low level during the experimental period. This is suggested as being because of the reducing atmosphere caused by Fe0 in the anaerobic system, which promoted CH4 production. In this strong CH4 production process with Fe0, H2 would simultaneously be consumed by hydrogenotrophic methanogens. Therefore, H2 always remained in a state of balance between “production and consumption,” and its concentration was always at a low level. When the iron source was replaced by Fe2O3, the same phenomenon was also observed except that the peak value of the H2 concentration was lower and slower than with Fe0. At the acidification stage, part of the H2 is assumed to be consumed by the dissimilatory iron reduction process caused by the introduction of Fe2O3 into the anaerobic system. Then, the H2 concentration correspondingly decreases. Although Fe(II), the product of dissimilatory iron reduction, can promote the production of H2, this promotion gradually attenuated as the consumption of Fe(III) proceeded. Therefore, the balance of “production and consumption” of H2 was broken by the fast production and slow consumption that caused a peak value. Subsequently, when methanogenesis became predominant, the H2 production gradually weakened and its concentration decreased again. Addition of FeC6H5O7 and Fe2(SO4)3 led to changes in H2 concentrations that were similar to those observed when Fe2O3 was added. However, there was probably a different metabolic pathway when these forms of iron were added as H2 is unlikely to be consumed by methanogenesis because of the inhibition effect of FeC6H5O7 and Fe2(SO4)3 as mentioned above. On the contrary, H2 could be chemically consumed by or react with Fe(III), and thus H2 concentrations should also gradually reduce. This metabolic behavior significantly differs from that in anoxic paddy soil systems where H2 is mainly consumed by methanogenesis [7, 23, 24]. In our high solid incubation system, the dissimilatory iron reduction apparently controlled CH4 generation not by reducing the H2 use but by inhibiting the activity of methanogens, especially the hydrogenotrophic methanogens.
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3.4. Effects of Iron Source on CO2 Production
The variation in CO2 concentrations during the course of the high solid incubation with different iron sources is shown in Figure 6. In general, the CO2 production following addition of Fe0 and Fe2O3 was almost the same. It increased rapidly over the first 10 days and then gradually stabilized at a low level, which is probably attributable to the acidification and hydrogen production phases of the anaerobic system in the first part of the incubation period when CO2 and H2 are the main gas components. Subsequently, H2 is consumed but CO2 continues to be produced as biodegradation proceeds, leading to the observed decrease in the H2 concentration (Figure 5), while the CO2 concentration continues to increase until the methanogenesis stage.
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The FeC6H5O7 treatment also led to a rapid increase in the CO2 concentration over the first 10 days, especially at the dosage rate of 16,000 mg·kg−1. However, this rapid increase ceased after day 10. The main reason for this is considered to be the iron reducers outcompeting the methanogens and mineralizing the organic substances before methanogenesis. Thus, carbon in the organic substrate was mainly converted to CO2 rather than CH4.
CO2 is the final product of organic decomposition in the dissimilatory iron reduction process. Higher CO2 production in the treatments with added FeC6H5O7 confirmed that organic-bound Fe enhanced the dissimilatory iron reduction process and significantly inhibited CH4 production. Among the tested concentrations of FeC6H5O7, 16,000 mg·kg−1 was the optimal dosage rate.
3.5. Mechanism of the Inhibition of CH4 Production by FeC6H5O7
The destructive test showed that the concentration of Fe(II) in the treatment containing FeC6H5O7 at a dosage rate of 16,000 mg·kg−1 sharply increased in the initial stage and subsequently maintained a concentration of 8,300 mg·kg−1 (Figure 7(a)). The proportional reduction of Fe(III) to Fe(II) was about 51.9%, which was in accordance with that of the volatile fatty acids (VFAs, Figure 7(b)) measurements. In the treatment containing FeC6H5O7 at a dosage rate of 16,000 mg·kg−1, the VFAs (including acetic acid, propionic acid, butyrate, and valeric acid) accumulated and resulted in the pH decrease (below that of the control, Figure 7(c)). Obviously, the low pH, which restrains the activity of methanogens, is a reason for the suppression of CH4 production during dissimilatory iron reduction.
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As shown in equations (3) and (4), the methanogenesis process in anaerobic biodegradation proceeds by two pathways, the hydrogenotrophic pathway (equation (3)) and the acetoclastic pathway (equation (4))
In the dissimilatory iron reduction process, Fe(III) accepts electrons supplied by organic compounds. If the electrons come from the intermediate products of the methanogenesis process, namely, hydrogen or acetic acid, then reactions (5) and (6) will happen and the accepted methanogenesis pathways (equations (3) and (4)) will cease. Therefore, the inhibition effect on methanogenesis caused by dissimilatory iron reduction can be evaluated by considering the electron balance.
From equations (3) and (5), which represent the hydrogenotrophic pathway of the methanogenesis process, production of one mole of CH4 requires the consumption of four moles of H2. Correspondingly, the oxidation of one mole of H2 requires two moles of Fe(III) in the dissimilatory iron reduction process. Thus, the reduction of one mole of CH4 produced by the hydrogenotrophic pathway requires eight moles of Fe(III) reduction (to Fe(II)). Similarly, it can be calculated that the reduction of one mole of CH4 produced by the acetoclastic pathway requires the reduction of four moles of Fe(III) (to Fe(II)). If the acetoclastic pathway accounts for approximately two-thirds of the methanogenesis and the hydrogenotrophic pathway accounts for approximately one-third, it can be estimated by the weighing method that a reduction of one mole of CH4 produced in the anaerobic biodegradation process requires 5.33 moles of Fe(III) to be reduced, if dissimilatory iron reduction is assumed to be the only biogeochemical process involved in the inhibition of methanogenesis. Therefore, the CH4 reduction caused by dissimilatory iron reduction can be directly calculated by the swhere MCH4 is the CH4 (g·kg−1 organic matrix) reduction caused by dissimilatory iron reduction, MFe (mg·kg−1 organic matrix) is the iron content of the organic matrix, 56 (without units) is the molar weight of iron, 1000 (without units) is a correction factor for mg to g, and 16 (without units) is the molar weight of oxygen. If MFe is taken as 16,000 mg·kg−1, the corresponding CH4 reduction caused by dissimilatory iron reduction is 0.86 g·kg−1 organic matrix.
The CH4 emission from a landfill can be evaluated using the model provided by the Intergovernmental Panel on Climate Change [25],where MCH4 (kg) is the CH4 emission from degradation of refuse, MSW (kg) is the refuse received by the landfill site, DOC (%) is the biodegradable organic carbon content, and r (%) is the decomposition rate of carbon in the DOC. The suggested values of DOC and r by the IPCC are 15% and 77%, respectively. Therefore, it can be calculated that 77.0 g of CH4 will be discharged if 1.0 kg of refuse is fully decomposed. This in turn indicates that the contribution of CH4 reduction by dissimilatory iron reduction is 1.07%. In absolute terms, this does not seem to be a high contribution. However, it needs to be recognized because it is an endogenesis effect inside the landfill site.
The results of this study suggest that the iron reducer system has fraction and uses rate preferences. Therefore, in eutrophic conditions, the effect of dissimilatory iron reduction in a landfill on the organic mineralization rate and methanogenesis inhibition ratio should take into account the iron fraction and use rate rather than the total iron concentration. For dystrophic conditions, such as those present in irrigated paddy soils and wetlands, whether the dissimilatory iron reduction process affects organic mineralization requires further research.
4. Conclusions
Organic-bound (FeC6H5O7) and inorganic-bound (Fe2(SO4)3 iron forms can inhibit methanogenesis, while free (Fe0) and oxidized (Fe2O3) forms of iron have the opposite effect. Dissimilatory iron reduction with FeC6H5O7 as the iron source suggests a kind of anaerobic mineralization process that not only can promote biodegradation but also can reduce CH4 emissions in a landfill. The optimal dosage rate of FeC6H5O7 was 16,000 mg·kg−1, in which the Fe(II) remained at a concentration of 8,300 mg·kg−1, and the correspondingly highest reduction rate was 51.9%. The main mechanism of dissimilatory iron reduction with FeC6H5O7 as the iron source is the active inhibition of two kinds of methanogen (those using the hydrogenotrophic and acetoclastic pathways).
Data Availability
The data used to support the findings of this study are included within the article.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (21876165, 51778579, 51678531, and 51878617) and Natural Science Foundation of Zhejiang Province (LY18B070009).