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Can H2S affect the methane oxidation in a landfill?
Ecological Engineering 60 (2013) 438–444
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Can H2 S affect the methane oxidation in a landfill? Yu-Yang Long a , Yan Liao a,d , Kun Zhang a , Li-Fang Hu b , Cheng-Ran Fang c , Dong-Sheng Shen a,∗ a Zhejiang Provincial Key Laboratory of Solid Waste Treatment and Recycling, School of Environmental Science and Engineering, Zhejiang Gongshang University, Hangzhou 310012, China b College of Quality and Safety Engineering, China Jiliang University, Hangzhou 310018, China c School of Civil Engineering and Architecture, Zhejiang University of Science and Technology, Hangzhou 310023, China d Mississippi International Water (China) Co. Ltd., Hangzhou 310023, China
a r t i c l e
i n f o
Article history: Received 28 April 2013 Received in revised form 24 July 2013 Accepted 11 September 2013 Available online 13 October 2013 Keywords: H2 S Behavior CH4 Oxidation Landfill
a b s t r a c t The effects of H2 S on CH4 biological oxidation in landfill cover soil (LCS) and aged municipal solid waste (AMSW) at different CH4 concentrations were investigated. The CH4 biological oxidation rates of LCS and AMSW were found to be significantly affected by CH4 concentration, with maximum oxidized concentrations of 5% and 20% of CH4 , respectively, occurring within 20 d. These differences may have been due to different dominant methanotroph populations. The CH4 oxidation of LCS and AMSW was significantly inhibited by H2 S at low CH4 concentrations (5%), but not at high levels of CH4 (20% and 50%). One possible pathway of the effects of H2 S on CH4 oxidation was competitive inhibition. These findings indicate that AMSW, which could adapt to the complex LFG environment more easily in comparison to LCS, was more suitable for use as a landfill cover for CH4 emission mitigation. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The Intergovernmental Panel on Climate Change 2007 defined methane (CH4 ) as an important greenhouse gas with a global warming potential 25 times greater than carbon dioxide (IPCC, 2007). The current contribution of CH4 emission to global warming is estimated at 15%, and this value continues to escalate (Themelis and Ulloa, 2007). Landfills are considered the fourth largest anthropogenic source of CH4 , and global CH4 emission from landfill is estimated to reach 500–800 Mt CO2 -eqyr−1 (EPA, 2010). Moreover, it is estimated that by 2030 the emission of CH4 from landfill will be 1500 Mt CO2 -eqyr−1 (EPA, 2006). Accordingly, urgent mitigation to lower the total CH4 concentration in the atmosphere is needed. Biological mitigation does not produce secondary pollution and is low-cost when compared with other CH4 mitigation methods (Barlaz et al., 2004). A large number of materials including agricultural and horticultural soil, compost, sand, peat, and mechanically–biologically treated municipal solid waste residues have been reported to be effective at CH4 reduction (Einola et al., 2008; Pedersen et al., 2011; Zhang et al., 2013; Jugnia et al., 2008). Aged municipal solid waste (AMSW) has the potential to mitigate CH4 emission and has gained increasing interest because it is
∗ Corresponding author. Tel.: +86 571 88832369; fax: +86 571 88832369. E-mail address: [email protected] (D.-S. Shen). 0925-8574/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ecoleng.2013.09.006
cost-effective and easily obtained from landfill sites (Lou et al., 2011; Wang et al., 2011). In addition to CH4 and CO2 , odors in LFG such as H2 S present serious problems (EPA, 2006; Rasi et al., 2007; Wu et al., 2013; Faulwetter et al., 2009). Landfills, particularly those that co-dispose construction and demolition debris containing gypsum board, produce large amounts of H2 S. Indeed, the H2 S concentration in landfills is approximately 120,000 ppm (Kim, 2006; Yang et al., 2006). More importantly, the toxicity of H2 S is comparable to that of hydrogen cyanide (OSHA, 2012). Such a toxicity may result in eye damage, olfactory nerve paralysis, and pulmonary edema with the possibility of death at concentrations of approximately 50 ppm to 100 ppm, 100 ppm to 150 ppm, and 320 ppm to 530 ppm, respectively (Campagna et al., 2004). H2 S is considered immediately hazardous to humans and microorganisms owing to its potential to damage mitochondrial cytochrome oxidase, which is an enzyme that humans and many microorganisms produce (Cooper and Brown, 2008). H2 S has been reported to disrupt the oxidation of coalmine gas by methanotrophs (Yu, 2007); thus, methanotrophs in LFG environments may also be inhibited by long-term exposure to H2 S. Methanotrophs are key microorganisms for landfill CH4 mitigation that are naturally occurring in landfills. A number of hydrocarbons and sulfur compounds are known to have toxic effects on methanotrophs (Albanna et al., 2010; Knowles, 2005; ˛ Pawłowska and Stepniewski, 2006). Yu (2007) found that the CH4
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oxidation capacity of a mixed culture isolated from a paddy field would not be inhibited when the H2 S concentration was less than 660 ppm, and that the corresponding CH4 monooxygenase activity would be upregulated slightly. However, Lee (2011) found that the CH4 oxidation capacity of Methylocystis sp. isolated from landfill cover soil (LCS) was significantly inhibited by H2 S at a concentration of 200 ppm. In landfills, the concentration and load of H2 S are higher than those in paddy fields, especially in the cover layer, the main outlet of LFG. In addition, CH4 concentration decreases with increasing deposit age. However, the contribution of H2 S to LFG increases continuously, especially under subsequent reducing conditions following landfill closure. In terms of CH4 concentration and low CH4 flux, natural oxidation by methanotrophs in the cover layer is the only reduction method. Evidently, the cover layer may suffer high risk from H2 S. Moreover, CH4 concentration is an important factor that must be considered when selecting a landfill cover material ˛ 2006). Therefore, the effects of H2 S (Pawłowska and Stepniewski, on CH4 oxidation may also vary with CH4 concentrations. However, published studies have focused on low CH4 concentrations (≤5%), which only represent the CH4 concentration on the landfill cover surface, paddy field soil, and forest soil (Visscher and Cleemput, 2003). In an actual landfill cover layer, which has the highest level of CH4 oxidation in the landfill, the CH4 concentration is approximately 5–60% (Lou et al., 2011). Thus, the effect of H2 S on microbial CH4 oxidation in landfills remains unclear and consequently requires further study. In this study, the effects of H2 S on CH4 biological oxidation behavior in a landfill were investigated at different CH4 concentrations. Two types of materials, LCS and AMSW, were evaluated to obtain a suitable landfill cover material adapted to the complex LFG environment.
2. Materials and methods 2.1. Sampling and pretreatment LCS and AMSW samples were collected from Dawuao MSW landfill, which is located in a valley south of Shaoxing, Zhejiang Province, China (Liao et al., 2013). AMSW samples aged 8–15 years were located under LCS in the landfill area. At least 100 kg of LCS and AMSW samples were collected in terms of the extreme homogeneity. All samples were immediately sealed in an airtight plastic bag during the collection process before being transported to the laboratory. Large inert objects (such as stones, pieces of brick, concrete, and cinders) in the LCS and AMSW samples were removed, after which each sample was separated into 3 kg subsamples and sieved through a 4 mm sieve. Approximately 0.2 kg of each sample was used to determine the pH, moisture content, and the concentration of organic matter, total nitrogen, total phosphorus, ammonia nitrogen, and nitrate, respectively. All analyses were conducted within 24 h of sample collection. The remaining samples were used as acclimation materials.
2.2. Experimental methods 2.2.1. Experimental design Batch tests were designed to study the effects of H2 S on CH4 oxidation in LCS and AMSW, respectively. The CH4 oxidation capacity of LCS and AMSW samples with (first experimental set) and without H2 S (second experimental set) under three levels of CH4 concentration (5%, 20%, and 50%) was studied. The initial H2 S concentration
439
(500 ppm) used was based on its concentration in landfills reported by Anderson et al. (2010). Moreover, a control was also set. All the tests were conducted using 150 mL serum bottles, and a total of 10 g sieved LCS or AMSW sample were placed into each serum bottle, after which the moisture content was adjusted to 25% using distilled water (Park et al., 2004). For all experimental sets, all the serum bottles were tightly sealed with butyl rubber stoppers and sealed with metal caps. Moreover, a designated hole for gas sampling from the bottle’s headspace was drilled in the middle of the stopper. The gas sample was sampled by gas-tight needle and syringe. Moreover, a sufficient amount of toluene (0.1 mL per 10 g) was added to the control set to inactivate the microorganisms. To achieve initial CH4 concentrations of 5%, 20%, and 50%, 21, 42, and 84 mL of air were withdrawn from the sealed serum bottles using a syringe. To replace the released air, 7, 28, and 70 mL of CH4 (99.99% purity) were injected, respectively. Thereafter, 14 mL of O2 (99.9% purity) were added to maintain aerobic conditions during the experiment. Finally, 70 L of H2 S (99.999%) were injected to create a force atmosphere of 500 ppm for the first experimental set. Based on the process of air replenishment above, the pressure in each serum bottle was balanced with that of the atmosphere. All serum bottles were subsequently incubated in a 30 ◦ C thermostatic chamber. The CH4 concentration was regularly tested and analyzed during the incubation process, and decreases in pressure inside the serum bottles attributed to CH4 oxidation were balanced through air replenishment. The incubation process lasted for 42 d and all tests were conducted in triplicate. 2.2.2. CH4 oxidation dynamics CH4 oxidation behavior for the three tested concentrations under H2 S was investigated at 0, 20, and 35 d, respectively. Before each test, the gas in the serum bottles was replaced with air to simulate the atmosphere at the landfill cover surface using the method described above. For each experimental set, determination of the CH4 concentration was conducted at least at five time points (including at time 0). The test was completed when the CH4 concentration in all serum bottles was below 50 ppm. All samples were incubated under similar conditions to those described in Section 2.2.1. The CH4 oxidation dynamics processes followed the exponential function described below: C = Ae−t/b + c
(1) (mL L−1 );
where C is the concentration of CH4 in the serum bottle t is the incubation time (h); and A, b, and c are the constants for the exponential equation. The CH4 oxidation rate constant (K) was then calculated through the derivation of CH4 concentration against time. K values could be interpreted as CH4 oxidation rates, and therefore represent the CH4 oxidation capacity of material in a given treatment (Contin et al., 2012). 2.3. Analyses The pH values of all samples were measured at a solid-to-liquid ratio of 1:5 using a pH meter (Seven Multi, MCPRC, 2000). Moisture content was then measured after drying at 105 ◦ C for 2 h. Organic matter was analyzed using the dichromate titration oxidation and volumetric procedure. Total nitrogen, total phosphorus, ammonia nitrogen, and nitrate were determined using the ascorbic acid and ammonium molybdate blue method, semimicro-kjeldahl method, indophenol-blue colorimetric method, and ultraviolet spectrophotometry, respectively (Lu, 1999). The sulfate content of LCS and/or AMSW before and after incubation was determined using the extraction and ion chromatography method (Metrohm 882 IC, MAPRC, 2006). The CH4 concentration was determined using a
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gas chromatograph equipped with a flame ionization detector (GC 7890, Shanghai Tianmei Scientific Instruments Co., Ltd., China). The carrier gas was nitrogen and the column was a 5 A molecular packed column. The detector, injection port, and column temperatures were 120, 120, and 60 ◦ C, respectively, and the sample volume was 1 mL. 3. Results and discussion LCS had a particle size of 0.002–8 mm and a clay-like appearance, while AMSW consisted of 57% large inert objects (such as stones, pieces of brick, concrete, and cinders) and 43% fine-grained material (such as soil, grit, and wood chips). LCS and AMSW were all shredded and/or ground into approximately 4 mm sized pieces prior to testing. Both LCS and AMSW were suitable for the growth of methanotrophs based on their neutral pH and their moisture contents (24.8% and 21.99%, respectively) (Einola et al., 2007). The organic matter contents of the LCS and AMSW samples were 1.63% and 7.14%, respectively. 3.1. Effect of CH4 concentration on biological oxidation in LCS and AMSW The dynamics of the CH4 oxidation process in LCS and AMSW can be described using Eq. (1). The effects of CH4 concentration on CH4 oxidation in LCS and AMSW varied with incubation time. The CH4 oxidation rate constants of LCS and AMSW at the same incubation time and CH4 concentration were compared by analysis of variance using the statistical software SPSS 19.0 (Table 1). The results showed that the CH4 oxidation rate constants of LCS with 5% CH4 at three different incubation times were all significantly higher than those of AMSW. In contrast, when the CH4 concentrations were 20% and 50%, the CH4 oxidation rate constants of AMSW, except for that for the 20% CH4 concentration at days 0 and 35, were significantly higher than those of LCS. The best CH4 oxidation rate constant was obtained in AMSW at a 20% CH4 concentration, which may be attributed to the different locations of LCS and AMSW. LCS is located at the landfill surface, which is subject to long-term exposure to low CH4 concentrations, whereas AMSW is located in the inner layers of the landfill, where there are high CH4 concentrations. Therefore, the response of the methanotrophic communities in LCS and AMSW to CH4 concentration varied (Einola et al., 2007). These results are in accordance with those of previous studies. For example, He et al. (2008) found that Type I methanotrophs were dominant in clay soil, whereas Type II methanotrophs were abundant in waste soil, where the highest population was found. Moreover, CH4 oxidation activity was dominated by Type I methanotrophs during incubation with low CH4 , whereas Type
II methanotrophs primarily contributed to CH4 oxidation activity under high CH4 mixing ratios (Henckel et al., 2000). Thus, Type I and Type II methanotrophs may predominate in LCS and AMSW, respectively. The CH4 oxidation rate constants of LCS and AMSW initially increased with increasing CH4 concentration. Specifically, the CH4 oxidation rate constant of LCS was significantly higher than that of AMSW when the CH4 concentrations were 5% and 20%. In contrast, AMSW exhibited a higher CH4 oxidation rate constant than LCS at a CH4 concentration of 50%. However, these findings changed after 20 d of incubation. The CH4 oxidation rate constant of LCS was significantly higher than that of AMSW at 5% CH4 , but significantly lower at high levels of CH4 (20% and 50%). Moreover, the highest CH4 oxidation rate constant of LCS was obtained at 5% CH4 , whereas that of AMSW was determined at 20% CH4 . The corresponding CH4 oxidation rate constants were 10.0 and 7.4 times that at 0 d for LCS and AMSW, respectively. These results indicate that the CH4 oxidation capacities of LCS and AMSW were promoted by 20 d of incubation in a suitable environment with continuous CH4 supplementation. Moreover, the CH4 oxidation capacities of LCS and AMSW were significantly upregulated at their optimum CH4 concentrations (5% and 20%, respectively). These differences can be attributed to the different responses of LCS and AMSW to CH4 concentration. The findings at 35 d of incubation were similar to those at 20 d, but the most efficient CH4 oxidation capacity of AMSW was observed at 50% CH4 . In addition, the CH4 oxidation capacities of both LCS and AMSW were lower than at 20 d. Therefore, the best CH4 oxidation capacities of LCS and AMSW were obtained under 5% and 20% CH4 at 20 d, respectively. The corresponding CH4 oxidation rate constants were 1.648 and 1.751 mL L−1 h−1 g−1 dw, respectively. Considering the CH4 oxidation rate constants of LCS and AMSW at different concentrations, the CH4 oxidation rate constants of LCS at 20% and 50% CH4 and those of AMSW at 5% and 50% CH4 were found to be minimal. Additionally, comparison of the CH4 oxidation rate constants of LCS or AMSW at the same CH4 concentration at different incubation times by analysis of variance (Table 1) revealed no significant differences throughout the incubation process. These results indicate that the dominant population of methanotrophs may be unchanged by variations in CH4 concentration within 30 d, which is in accordance with the results of a study conducted by He et al. (2008). When the CH4 concentration was 70%, the dominant population of methanotrophs in waste soil and clay soil did not change during 120 d of incubation in landfill cover soil. Thus, LCS had better CH4 oxidation capacity than AMSW at low CH4 concentrations, but AMSW was better than LCS at high CH4 concentrations. Moreover, the CH4 oxidation capacity of AMSW may be more flexible under complex LFG environments.
Table 1 Differences in oxidation rate constants (mL L−1 h−1 g−1 dw) of LCS and AMSW with different treatment. Incubation time (d)
5%
20%
LCS
AMSW
50%
LCS
AMSW
LCS
AMSW
0
Without H2 S With H2 S
0.164 ± 0.018 0.237 ± 0.054 ␣
0.060 ± 0.005 0.065 ± 0.002 ␣
0.433 ± 0.063 0.507 ± 0.060 ␣
0.237 ± 0.003 0.236 ± 0.018 ␣
0.615 ± 0.158 0.483 ± 0.064 ␣
0.774 ± 0.063 I␣ 0.849 ± 0.032 ␣
20
Without H2 S With H2 S
1.648 ± 0.391 a␣ 1.701 ± 0.101 ␣
0.290 ± 0.052 b␣ 0.013 ± 0.005 
0.266 ± 0.007 A␣ 0.287 ± 0.033 ␣
1.751 ± 0.322 B␣ 2.335 ± 0.162 ␣
0.689 ± 0.010 I␣ 1.003 ± 0.117 ␣
1.199 ± 0.075 II␣ 1.070 ± 0.104 ␣
35
Without H2 S With H2 S
0.956 ± 0.127 a␣ 0.656 ± 0.206 
0.356 ± 0.114 b␣ 0.004 ± 0.001 
0.240 ± 0.006 A␣ 0.185 ± 0.001 
0.335 ± 0.028 B␣ 0.533 ± 0.035 
0.380 ± 0.022 I␣ 0.537 ± 0.055 ␣
0.984 ± 0.102 II␣ 0.781 ± 0.027 ␣
a␣
b␣
A␣
B␣
a and b refer to significant differences between LCS and AMSW at the same incubation time at 5% CH4 (p < 0.05). A and B refer to significant differences between LCS and AMSW at the same incubation time at 20% CH4 (p < 0.05). I and II refer to significant differences between LCS and AMSW at the same incubation time at 50% CH4 (p < 0.05). ␣ and  refer to significant differences in LCS or AMSW with and without H2 S (p < 0.05).
I␣
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3.2. Effect of H2 S on CH4 biological oxidation dynamics in LCS and AMSW As shown in Fig. 1, CH4 concentration decreased during the incubation process when H2 S was introduced, except for the control. These findings indicate that the CH4 content absorbed by LCS and AMSW was limited. Therefore, the reduction of CH4 was mainly caused by biological oxidation. Moreover, the CH4 oxidation rate constants of LCS or AMSW with and without H2 S at the same incubation time and CH4 concentration were compared by analysis of
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variance (Table 1). As the incubation time increased, the effect of H2 S on CH4 oxidation in both matrices became increasingly evident, suggesting that the CH4 oxidation capacity of LCS and AMSW was significantly affected by long-term exposure to H2 S. As shown in Fig. 1a and Table 1, the CH4 oxidation capacities of LCS at 5% CH4 were significantly inhibited by H2 S at 35 d. Complete CH4 oxidation of LCS without H2 S was observed within 5 h, whereas that for LCS with H2 S was observed within 14 h. Moreover, the CH4 oxidation rate constant of LCS without H2 S (KL 5 = 0.956 mL L−1 h−1 g−1 dw) was almost three times larger than
Fig. 1. Effect of H2 S on LCS and AMSW CH4 oxidation processes at different CH4 concentrations (5%, 20% and 50%). (a) LCS at 5% CH4 , (b) AMSW at 5% CH4 , (c) LCS at 20% CH4 , (d) AMSW at 20% CH4 , (e) LCS at 50% CH4 and (f) AMSW at 50% CH4 .
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that with H2 S (KL -5-H2 S = 0.66 mL L−1 h−1 g−1 dw). When the CH4 concentration increased to 20%, the degree of suppression of H2 S on LCS was also significant at 35 d (Table 1). As shown in Fig. 1c, the CH4 in serum bottles without H2 S was completely oxidized within 149.5 h, whereas the samples with H2 S needed 165 h after 35 d of incubation. When the CH4 concentration reached 50% (Fig. 1e), no significant difference was found between samples with and without H2 S (Table 1). These findings suggest that the inhibitory effect of H2 S on LCS is stronger at low CH4 concentrations. A similar phenomenon was observed in a previous study (Lee, 2011). Moreover, the CH4 oxidation rate of LCS at 5% CH4 without H2 S was significantly higher than that at high CH4 concentrations (Section 3.1). These findings indicate that the methanotrophs in LCS could hardly oxidize the high levels of CH4 . The aforementioned results may be attributed to the vulnerability of LCS at low CH4 concentration and the protective action of high CH4 levels (Borjesson, 2001). A similar trend was also observed in AMSW. The CH4 oxidation capacities of AMSW at 5% CH4 were significantly inhibited by H2 S after 20 d (Table 1), at which time only 0.49% CH4 was oxidized by AMSW with H2 S. However, CH4 was completely oxidized by AMSW without H2 S within 45 h (Fig. 1b). When the CH4 concentration increased to 20%, no inhibitory effect was exhibited by AMSW with H2 S on CH4 oxidation throughout the entire incubation process (Fig. 1d). These findings were also observed at 50% CH4 (Fig. 1f).
Thus, H2 S does not inhibit CH4 oxidation in AMSW at high levels of CH4 , indicating that high levels of CH4 may relieve the suppressive effect of H2 S on methanotrophs in AMSW. The inhibitory effect of H2 S on the CH4 oxidation process of LCS was less than that of AMSW at 5% CH4 . In contrast, the inhibitory effect of H2 S on the AMSW CH4 oxidation process was less evident than that of LCS at high CH4 concentrations. These results may be attributed to the different dominant bacterium groups in different materials and at different CH4 concentrations. Type II methanotrophs activated under high CH4 concentrations were more stable and abundant than Type I. Thus, Type II methanotrophs may have strong tolerance for H2 S. Luo et al. (2007) reported that the CH4 oxidizing mixed microbial consortium (MY9, containing Type II methanotrophs) could survive in an environment with pollutants such as phenol and dichloromethane. Consequently, AMSW was found to be more suitable for use as a landfill cover because of its high flexibility. 3.3. Possible pathway of effects of H2 S on CH4 oxidation process Based on the material properties, the pH, moisture, and ammonia nitrogen contents of LCS and AMSW were suitable for the growth of methanotrophs (Einola et al., 2007; Yang et al., 2011). Thus, the physicochemical properties of LCS and AMSW were not inhibitors of CH4 oxidation.
Fig. 2. Sulfate contents and increase in sulfate in landfill cover soil microcosms at different CH4 concentration (5%, 20% and 50%). (a) The sulfate contents of LCS and AMSW after 35 d of incubation with and without H2 S. (b) The increase in sulfate of LCS and AMSW at 35 d.
Y.-Y. Long et al. / Ecological Engineering 60 (2013) 438–444 8.5
AMSW-B AMSW-A AMSW-H 2S
8.0
7.5
7.5
7.0
7.0
pH
pH
8.5
LCS-B LCS-A LCS-H2S
8.0
443
6.5
6.5
6.0
6.0
5.5
5.5 0
10
20
30
40
50
60
0
10
20
30
40
50
60
CH4 concentration (%)
CH4 concentration (%)
Fig. 3. pH values of pre-incubated and post-incubated LCS and AMSW samples at different CH4 concentrations (5%, 20% and 50%). (LCS-B) LCS was not incubated. (LCS-A) LCS incubated without H2 S. (LCS-H2 S) LCS incubated with H2 S. (AMSW-B) AMSW was not incubated. (AMSW-A) AMSW incubated without H2 S. (AMSW-H2 S) AMSW incubated with H2 S.
To explore possible pathways of the effects of H2 S on the LCS and AMSW CH4 oxidation process, the sulfate contents of the two cover materials were measured. The sulfate contents of the original LCS and MWS were 45.6 ± 3.1 and 799.0 ± 13.1 mg kg−1 dw, respectively. In experimental sets without H2 S, the sulfate content of LCS remained almost the same after 35 d of incubation (Fig. 2). In contrast, the sulfate content of AMSW increased, and the maximum level (1289.0 ± 25.41 mg kg−1 dw) was observed at 20% CH4 . Overall, these findings indicate that some sulfur compounds that originated from AMSW might be significant contributors to this sulfate increase. In sets where H2 S was introduced, the sulfate content of LCS increased significantly, with a maximum (168.5 ± 5.3 mg kg−1 dw) 3.7 times higher than the original being observed at 5% CH4 . The increase in sulfate in the LCS incubated at 5% CH4 was 1229.2 ± 26.5 g. In addition, the CH4 oxidation capacity of LCS at 5% CH4 was significantly inhibited (Table 1), but no inhibitory effect was observed at high CH4 concentrations (50%). These results indicate that H2 S was converted into sulfate by microbial and/or physical-chemical reactions during incubation. Moreover, the H2 S oxidation process was harmful to CH4 oxidation at low CH4 concentrations. In AMSW with H2 S, the maximum sulfate content (1473.0 ± 24.7 mg kg−1 dw) was also observed at a CH4 concentration of 5%. The increase in sulfate in the AMSW incubated at 5% CH4 was 6740.8 ± 33.2 g; however, this increase was only 3433.3 ± 27.4 g and 1011.5 ± 17.2 g in AMSW incubated with 20% and 50% CH4 , respectively. Moreover, the CH4 oxidation capacity of AMSW at 5% CH4 was also significantly inhibited, but no inhibitory effect was observed at high CH4 concentrations (Table 1). These findings may be ascribed to the lower sulfate formation at higher CH4 concentrations and the high resistance of type II methanotrophs in AMSW for sulfate. As shown in Fig. 3, the pH values of LCS and AMSW incubated with H2 S decreased relative to the original LCS (7.49 ± 0.08) and AMSW (7.51 ± 0.02). The reduction in pH of both LCS and AMSW might be attributed to the production of H+ by hydrolysis during the oxidation of H2 S (He et al., 2012). However, differences in CH4 oxidation could not be ascribed to variations in pH because the pH values after incubation remained neutral. In conclusion, the inhibitory effect of H2 S on LCS and AMSW CH4 oxidation may be ascribed to competitive inhibition, which stemmed from competition with CH4 for the same enzyme (Burgess
et al., 2001). Moreover, a study conducted by Burgess et al. (2001) showed that the concentration of the inhibitor (CH3 SH) became less important when the CH4 concentration was very high and Type I methanotrophs were more affected than type II. Therefore, a similar result may also be obtained for H2 S. For this reason, the high level of CH4 may prevent its oxidation of LCS and AMSW from being inhibited by H2 S, and AMSW may have stronger resistance in H2 S because of dominant population-Type II methanotrophs. Moreover, the OM content of AMSW was significantly higher than that of LCS, indicating that the AMSW samples were more suitable for long-term incubation because AMSW contained more nutrients for the growth of microorganisms. 4. Conclusions The CH4 biological oxidation rates of LCS and AMSW were found to be significantly affected by CH4 concentration, with the maximum rates being obtained at 5% and 20% CH4 , respectively, within 20 d. The oxidation process of LCS and AMSW could be significantly inhibited by H2 S at low CH4 concentrations (5%), but a high level of CH4 (20% and 50%) could relieve the inhibitory effects of H2 S. A possible pathway of the effects of H2 S on CH4 oxidation was competitive inhibition. Acknowledgements This work was financially supported by National Natural Science Foundation of China (41071310, 41101453, 41101466, and 51108419), Research Plan of Department of Education of Zhejiang Province (Y201119953, and Y201122794), Funded project for youth researcher of Zhejiang Gongshang University (QY11-22). References Albanna, M., Warith, M., Fernandes, L., 2010. Kinetics of biological methane oxidation in the presence of non-methane organic compounds in landfill bio-covers. Waste Manage. 30 (2), 219–227. Anderson, R., Jambeck, J.R., McCarron, G.P., 2010. Modeling of Hydrogen Sulfide Generation from Landfills Beneficially Utilizing Processed Construction and Demolition Materials. Environmental Research and Education Foundation, Alexandria, VA, pp. 13–20. Barlaz, M.A., Green, R.B., Chanton, J.P., Goldsmith, C.D., Hater, G.R., 2004. Evaluation of a biologically active cover for mitigation of landfill gas emissions. Environ. Sci. Technol. 38 (18), 4891–4899.
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Borjesson, G., 2001. Inhibition of methane oxidation by volatile sulfur compounds (CH3 SH and CS2 ) in landfill cover soils. Waste Manage. Res. 19 (4), 314–319. Burgess, J.E., Parsons, S.A., Stuetz, R.M., 2001. Developments in odour control and waste gas treatment biotechnology a review. Biotechnol. Adv. 19, 35–63. Campagna, D., Kathman, S.J., Pierson, R., Inserra, S.G., Phifer, B.L., Middleton, D.C., Zarus, G.M., White, M.C., 2004. Ambient hydrogen sulfide, total reduced sulfur, and hospital visits for respiratory diseases in northeast Nebraska, 1998–2000. J. Expo. Anal. Environ. Epidemiol. 14 (2), 180–187. Contin, M., Goi, D., De Nobili, M., 2012. Land application of aerobic sewage sludge does not impair methane oxidation rates of soils. Sci. Total Environ. 441, 10–18. Cooper, C.E., Brown, G.C., 2008. The inhibition of mitochondrial cytochrome oxidase by the gases carbon monoxide, nitric oxide, hydrogen cyanide and hydrogen sulfide: chemical mechanism and physiological significance. J. Bioenergy Biomembr. 40 (5), 533–539. Einola, J.K.M., Kettunen, R.H., Rintala, J.A., 2007. Responses of methane oxidation to temperature and water content in cover soil of a boreal landfill. Soil Biol. Biochem. 39, 1156–1164. Einola, J.K.M., Karhu, A.E., Rintala, J.A., 2008. Mechanically-biologically treated municipal solid waste as a support medium for microbial methane oxidation to mitigate landfill greenhouse emissions. Waste Manage. 28 (1), 97–111. EPA, 2006. Methane: Sources and Emissions, Human-related sources. United States Environmental Protection Angency, Washington, DC, USA. EPA, 2010. Methane Emissions. United States Environmental Protection Agency, Washington, DC, USA. Faulwetter, J.L., Gagnon, V., Sundberg, C., Chazarenc, F., Burr, M.D., Brisson, J., Camper, A.K., Stein, O.R., 2009. Microbial processes influencing performance of treatment wetlands: a review. Ecol. Eng. 35 (6), 987–1004. He, R., Ruan, A., Jiang, C., Shen, D.s., 2008. Responses of oxidation rate and microbial communities to methane in simulated landfill cover soil microcosms. Bioresour. Technol. 99 (15), 7192–7199. He, R., Xia, F.F., Bai, Y., Wang, J., Shen, D.S., 2012. Mechanism of H2 S removal during landfill stabilization in waste biocover soil, an alterative landfill cover. J. Hazard. Mater. 217-218, 67–75. Henckel, T., Roslev, P., Conrad, R., 2000. Effects of O2 and CH4 on presence and activity of the indigenous methanotrophic community in rice field soil. Environ. Microbiol. 2 (6), 666–679. IPCC, 2007. Climate Change 2007. The Physical Science Basis. in: Contribution of Working Group I to the Fourth Assessment Report of the IPCC. Cambridge University Press, New York. Jugnia, L.B., Cabral, A.R., Greer, C.W., 2008. Biotic methane oxidation within an instrumented experimental landfill cover. Ecol. Eng. 33 (2), 102–109. Kim, K.H., 2006. Emissions of reduced sulfur compounds (RSC) as a landfill gas (LFG): a comparative study of young and old landfill facilities. Atmos. Environ. 40 (34), 6567–6578. Knowles, R., 2005. Denitrifiers associated with methanotrophs and their potential impact on the nitrogen cycle. Ecol. Eng. 24 (5), 441–446.
Lee, E.H., 2011. Characterization of methane oxidation by a methanotroph isolated from a landfill cover soil, South Korea. J. Microbiol. Biotechnol. 21 (7), 753–756. Liao, Y., Yang, Y.Q., Shen, D.S., Long, Y.Y., 2013. Effect of deposit age on adsorption and desorption behaviors of ammonia nitrogen on municipal solid waste. Environ. Sci. Pollut. Res. 20, 1546–1555. Lou, Z.Y., Wang, L., Zhao, Y.C., 2011. Consuming un-captured methane from landfill using aged refuse bio-cover. Bioresour. Technol. 102 (3), 2328–2332. Luo, M.F., Wu, H., Wang, L., Xing, X.H., 2007. Study on the structure and function of a stable methane oxidizing mixed microbial consortium. Acta Microbiol. Sin. 47 (1), 103–109 (in Chinese). Lu, R.K., 1999. Agriculture Chemical Analysis of Soil. China Agricultural Scientech Press, Beijing (in Chinese). MCPRC (The Ministry of Construction of People’s Republic of China), 2000. Municipal domestic Refuse-Determination of pH-Glass electrode method. Vol. CJ/T 99-1999 (in Chinese). OSHA, 2012. Hydrogen Sulfide (H2 S). U.S. Department of Labor, Occupational Safety and Health Administration, Washington, DC. Park, S., Brown, K., Thomas, J., 2004. The use of biofilters to reduce atmospheric methane emissions from landfills: Part I. Biofilter design. Water Air Soil Pollut. 155 (1), 63–85. ˛ Pawłowska, M., Stepniewski, W., 2006. An influence of methane concentration on the methanotrophic activity of a model landfill cover. Ecol. Eng. 26 (4), 392–395. Pedersen, G.B., Scheutz, C., Kjeldsen, P., 2011. Availability and properties of materials for the Fakse Landfill biocover. Waste Manage. 31 (5), 884–894. Rasi, S., Veijanen, A., Rintala, J., 2007. Trace compounds of biogas from different biogas production plants. Energy 32 (8), 1375–1380. Themelis, N.J., Ulloa, P.A., 2007. Methane generation in landfills. Renew. Energ. 32 (7), 1243–1257. Visscher, A.D., Cleemput, O.V., 2003. Induction of enhanced CH4 oxidation in soils: NH4 + inhibition patterns. Soil Biol. Biochem. 35 (7), 907–913. Wang, J., Xia, F.F., Bai, Y., Fang, C.R., Shen, D.S., He, R., 2011. Methane oxidation in landfill waste biocover soil: kinetics and sensitivity to ambient conditions. Waste Manage. 31 (5), 864–870. Wu, S.B., Kuschk, P., Wiessner, A., Müller, J., Saad, R.A.B., Dong, R.J., 2013. Sulphur transformations in constructed wetlands for wastewater treatment: a review. Ecol. Eng. 52, 278–289. Yang, K., Xu, Q., Townsend, T.G., Chadik, P., Bitton, G., Booth, M., 2006. Hydrogen sulfide generation in simulated construction and demolition debris landfills: impact of waste composition. J. Air Waste Manage. 56 (8), 1130–1138. Yang, N., Lu, F., He, P., Shao, L., 2011. Response of methanotrophs and methane oxidation on ammonium application in landfill soils. Appl. Environ. Microbiol. 92 (5), 1073–1082. Yu, H.X., 2007. Studies on Bio-oxidizing Methane in Coal Mines with Microbial Technology. Zhejiang University, Zhejiang, China (in Chinese). Zhang, H.H., Li, W.X., Cai, Z.C., Zhang, Y., Feng, Y.H., Zhang, H.L., He, F., 2013. Landfill CH4 oxidation and N2 O emissions by aged refuse: effects of wastewater NH4 + -N incubation, heavy metals and pH. Ecol. Eng. 53, 243–249.
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Can H2 S affect the methane oxidation in a landfill? Yu-Yang Long a , Yan Liao a,d , Kun Zhang a , Li-Fang Hu b , Cheng-Ran Fang c , Dong-Sheng Shen a,∗ a Zhejiang Provincial Key Laboratory of Solid Waste Treatment and Recycling, School of Environmental Science and Engineering, Zhejiang Gongshang University, Hangzhou 310012, China b College of Quality and Safety Engineering, China Jiliang University, Hangzhou 310018, China c School of Civil Engineering and Architecture, Zhejiang University of Science and Technology, Hangzhou 310023, China d Mississippi International Water (China) Co. Ltd., Hangzhou 310023, China
a r t i c l e
i n f o
Article history: Received 28 April 2013 Received in revised form 24 July 2013 Accepted 11 September 2013 Available online 13 October 2013 Keywords: H2 S Behavior CH4 Oxidation Landfill
a b s t r a c t The effects of H2 S on CH4 biological oxidation in landfill cover soil (LCS) and aged municipal solid waste (AMSW) at different CH4 concentrations were investigated. The CH4 biological oxidation rates of LCS and AMSW were found to be significantly affected by CH4 concentration, with maximum oxidized concentrations of 5% and 20% of CH4 , respectively, occurring within 20 d. These differences may have been due to different dominant methanotroph populations. The CH4 oxidation of LCS and AMSW was significantly inhibited by H2 S at low CH4 concentrations (5%), but not at high levels of CH4 (20% and 50%). One possible pathway of the effects of H2 S on CH4 oxidation was competitive inhibition. These findings indicate that AMSW, which could adapt to the complex LFG environment more easily in comparison to LCS, was more suitable for use as a landfill cover for CH4 emission mitigation. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The Intergovernmental Panel on Climate Change 2007 defined methane (CH4 ) as an important greenhouse gas with a global warming potential 25 times greater than carbon dioxide (IPCC, 2007). The current contribution of CH4 emission to global warming is estimated at 15%, and this value continues to escalate (Themelis and Ulloa, 2007). Landfills are considered the fourth largest anthropogenic source of CH4 , and global CH4 emission from landfill is estimated to reach 500–800 Mt CO2 -eqyr−1 (EPA, 2010). Moreover, it is estimated that by 2030 the emission of CH4 from landfill will be 1500 Mt CO2 -eqyr−1 (EPA, 2006). Accordingly, urgent mitigation to lower the total CH4 concentration in the atmosphere is needed. Biological mitigation does not produce secondary pollution and is low-cost when compared with other CH4 mitigation methods (Barlaz et al., 2004). A large number of materials including agricultural and horticultural soil, compost, sand, peat, and mechanically–biologically treated municipal solid waste residues have been reported to be effective at CH4 reduction (Einola et al., 2008; Pedersen et al., 2011; Zhang et al., 2013; Jugnia et al., 2008). Aged municipal solid waste (AMSW) has the potential to mitigate CH4 emission and has gained increasing interest because it is
∗ Corresponding author. Tel.: +86 571 88832369; fax: +86 571 88832369. E-mail address: [email protected] (D.-S. Shen). 0925-8574/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ecoleng.2013.09.006
cost-effective and easily obtained from landfill sites (Lou et al., 2011; Wang et al., 2011). In addition to CH4 and CO2 , odors in LFG such as H2 S present serious problems (EPA, 2006; Rasi et al., 2007; Wu et al., 2013; Faulwetter et al., 2009). Landfills, particularly those that co-dispose construction and demolition debris containing gypsum board, produce large amounts of H2 S. Indeed, the H2 S concentration in landfills is approximately 120,000 ppm (Kim, 2006; Yang et al., 2006). More importantly, the toxicity of H2 S is comparable to that of hydrogen cyanide (OSHA, 2012). Such a toxicity may result in eye damage, olfactory nerve paralysis, and pulmonary edema with the possibility of death at concentrations of approximately 50 ppm to 100 ppm, 100 ppm to 150 ppm, and 320 ppm to 530 ppm, respectively (Campagna et al., 2004). H2 S is considered immediately hazardous to humans and microorganisms owing to its potential to damage mitochondrial cytochrome oxidase, which is an enzyme that humans and many microorganisms produce (Cooper and Brown, 2008). H2 S has been reported to disrupt the oxidation of coalmine gas by methanotrophs (Yu, 2007); thus, methanotrophs in LFG environments may also be inhibited by long-term exposure to H2 S. Methanotrophs are key microorganisms for landfill CH4 mitigation that are naturally occurring in landfills. A number of hydrocarbons and sulfur compounds are known to have toxic effects on methanotrophs (Albanna et al., 2010; Knowles, 2005; ˛ Pawłowska and Stepniewski, 2006). Yu (2007) found that the CH4
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oxidation capacity of a mixed culture isolated from a paddy field would not be inhibited when the H2 S concentration was less than 660 ppm, and that the corresponding CH4 monooxygenase activity would be upregulated slightly. However, Lee (2011) found that the CH4 oxidation capacity of Methylocystis sp. isolated from landfill cover soil (LCS) was significantly inhibited by H2 S at a concentration of 200 ppm. In landfills, the concentration and load of H2 S are higher than those in paddy fields, especially in the cover layer, the main outlet of LFG. In addition, CH4 concentration decreases with increasing deposit age. However, the contribution of H2 S to LFG increases continuously, especially under subsequent reducing conditions following landfill closure. In terms of CH4 concentration and low CH4 flux, natural oxidation by methanotrophs in the cover layer is the only reduction method. Evidently, the cover layer may suffer high risk from H2 S. Moreover, CH4 concentration is an important factor that must be considered when selecting a landfill cover material ˛ 2006). Therefore, the effects of H2 S (Pawłowska and Stepniewski, on CH4 oxidation may also vary with CH4 concentrations. However, published studies have focused on low CH4 concentrations (≤5%), which only represent the CH4 concentration on the landfill cover surface, paddy field soil, and forest soil (Visscher and Cleemput, 2003). In an actual landfill cover layer, which has the highest level of CH4 oxidation in the landfill, the CH4 concentration is approximately 5–60% (Lou et al., 2011). Thus, the effect of H2 S on microbial CH4 oxidation in landfills remains unclear and consequently requires further study. In this study, the effects of H2 S on CH4 biological oxidation behavior in a landfill were investigated at different CH4 concentrations. Two types of materials, LCS and AMSW, were evaluated to obtain a suitable landfill cover material adapted to the complex LFG environment.
2. Materials and methods 2.1. Sampling and pretreatment LCS and AMSW samples were collected from Dawuao MSW landfill, which is located in a valley south of Shaoxing, Zhejiang Province, China (Liao et al., 2013). AMSW samples aged 8–15 years were located under LCS in the landfill area. At least 100 kg of LCS and AMSW samples were collected in terms of the extreme homogeneity. All samples were immediately sealed in an airtight plastic bag during the collection process before being transported to the laboratory. Large inert objects (such as stones, pieces of brick, concrete, and cinders) in the LCS and AMSW samples were removed, after which each sample was separated into 3 kg subsamples and sieved through a 4 mm sieve. Approximately 0.2 kg of each sample was used to determine the pH, moisture content, and the concentration of organic matter, total nitrogen, total phosphorus, ammonia nitrogen, and nitrate, respectively. All analyses were conducted within 24 h of sample collection. The remaining samples were used as acclimation materials.
2.2. Experimental methods 2.2.1. Experimental design Batch tests were designed to study the effects of H2 S on CH4 oxidation in LCS and AMSW, respectively. The CH4 oxidation capacity of LCS and AMSW samples with (first experimental set) and without H2 S (second experimental set) under three levels of CH4 concentration (5%, 20%, and 50%) was studied. The initial H2 S concentration
439
(500 ppm) used was based on its concentration in landfills reported by Anderson et al. (2010). Moreover, a control was also set. All the tests were conducted using 150 mL serum bottles, and a total of 10 g sieved LCS or AMSW sample were placed into each serum bottle, after which the moisture content was adjusted to 25% using distilled water (Park et al., 2004). For all experimental sets, all the serum bottles were tightly sealed with butyl rubber stoppers and sealed with metal caps. Moreover, a designated hole for gas sampling from the bottle’s headspace was drilled in the middle of the stopper. The gas sample was sampled by gas-tight needle and syringe. Moreover, a sufficient amount of toluene (0.1 mL per 10 g) was added to the control set to inactivate the microorganisms. To achieve initial CH4 concentrations of 5%, 20%, and 50%, 21, 42, and 84 mL of air were withdrawn from the sealed serum bottles using a syringe. To replace the released air, 7, 28, and 70 mL of CH4 (99.99% purity) were injected, respectively. Thereafter, 14 mL of O2 (99.9% purity) were added to maintain aerobic conditions during the experiment. Finally, 70 L of H2 S (99.999%) were injected to create a force atmosphere of 500 ppm for the first experimental set. Based on the process of air replenishment above, the pressure in each serum bottle was balanced with that of the atmosphere. All serum bottles were subsequently incubated in a 30 ◦ C thermostatic chamber. The CH4 concentration was regularly tested and analyzed during the incubation process, and decreases in pressure inside the serum bottles attributed to CH4 oxidation were balanced through air replenishment. The incubation process lasted for 42 d and all tests were conducted in triplicate. 2.2.2. CH4 oxidation dynamics CH4 oxidation behavior for the three tested concentrations under H2 S was investigated at 0, 20, and 35 d, respectively. Before each test, the gas in the serum bottles was replaced with air to simulate the atmosphere at the landfill cover surface using the method described above. For each experimental set, determination of the CH4 concentration was conducted at least at five time points (including at time 0). The test was completed when the CH4 concentration in all serum bottles was below 50 ppm. All samples were incubated under similar conditions to those described in Section 2.2.1. The CH4 oxidation dynamics processes followed the exponential function described below: C = Ae−t/b + c
(1) (mL L−1 );
where C is the concentration of CH4 in the serum bottle t is the incubation time (h); and A, b, and c are the constants for the exponential equation. The CH4 oxidation rate constant (K) was then calculated through the derivation of CH4 concentration against time. K values could be interpreted as CH4 oxidation rates, and therefore represent the CH4 oxidation capacity of material in a given treatment (Contin et al., 2012). 2.3. Analyses The pH values of all samples were measured at a solid-to-liquid ratio of 1:5 using a pH meter (Seven Multi, MCPRC, 2000). Moisture content was then measured after drying at 105 ◦ C for 2 h. Organic matter was analyzed using the dichromate titration oxidation and volumetric procedure. Total nitrogen, total phosphorus, ammonia nitrogen, and nitrate were determined using the ascorbic acid and ammonium molybdate blue method, semimicro-kjeldahl method, indophenol-blue colorimetric method, and ultraviolet spectrophotometry, respectively (Lu, 1999). The sulfate content of LCS and/or AMSW before and after incubation was determined using the extraction and ion chromatography method (Metrohm 882 IC, MAPRC, 2006). The CH4 concentration was determined using a
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gas chromatograph equipped with a flame ionization detector (GC 7890, Shanghai Tianmei Scientific Instruments Co., Ltd., China). The carrier gas was nitrogen and the column was a 5 A molecular packed column. The detector, injection port, and column temperatures were 120, 120, and 60 ◦ C, respectively, and the sample volume was 1 mL. 3. Results and discussion LCS had a particle size of 0.002–8 mm and a clay-like appearance, while AMSW consisted of 57% large inert objects (such as stones, pieces of brick, concrete, and cinders) and 43% fine-grained material (such as soil, grit, and wood chips). LCS and AMSW were all shredded and/or ground into approximately 4 mm sized pieces prior to testing. Both LCS and AMSW were suitable for the growth of methanotrophs based on their neutral pH and their moisture contents (24.8% and 21.99%, respectively) (Einola et al., 2007). The organic matter contents of the LCS and AMSW samples were 1.63% and 7.14%, respectively. 3.1. Effect of CH4 concentration on biological oxidation in LCS and AMSW The dynamics of the CH4 oxidation process in LCS and AMSW can be described using Eq. (1). The effects of CH4 concentration on CH4 oxidation in LCS and AMSW varied with incubation time. The CH4 oxidation rate constants of LCS and AMSW at the same incubation time and CH4 concentration were compared by analysis of variance using the statistical software SPSS 19.0 (Table 1). The results showed that the CH4 oxidation rate constants of LCS with 5% CH4 at three different incubation times were all significantly higher than those of AMSW. In contrast, when the CH4 concentrations were 20% and 50%, the CH4 oxidation rate constants of AMSW, except for that for the 20% CH4 concentration at days 0 and 35, were significantly higher than those of LCS. The best CH4 oxidation rate constant was obtained in AMSW at a 20% CH4 concentration, which may be attributed to the different locations of LCS and AMSW. LCS is located at the landfill surface, which is subject to long-term exposure to low CH4 concentrations, whereas AMSW is located in the inner layers of the landfill, where there are high CH4 concentrations. Therefore, the response of the methanotrophic communities in LCS and AMSW to CH4 concentration varied (Einola et al., 2007). These results are in accordance with those of previous studies. For example, He et al. (2008) found that Type I methanotrophs were dominant in clay soil, whereas Type II methanotrophs were abundant in waste soil, where the highest population was found. Moreover, CH4 oxidation activity was dominated by Type I methanotrophs during incubation with low CH4 , whereas Type
II methanotrophs primarily contributed to CH4 oxidation activity under high CH4 mixing ratios (Henckel et al., 2000). Thus, Type I and Type II methanotrophs may predominate in LCS and AMSW, respectively. The CH4 oxidation rate constants of LCS and AMSW initially increased with increasing CH4 concentration. Specifically, the CH4 oxidation rate constant of LCS was significantly higher than that of AMSW when the CH4 concentrations were 5% and 20%. In contrast, AMSW exhibited a higher CH4 oxidation rate constant than LCS at a CH4 concentration of 50%. However, these findings changed after 20 d of incubation. The CH4 oxidation rate constant of LCS was significantly higher than that of AMSW at 5% CH4 , but significantly lower at high levels of CH4 (20% and 50%). Moreover, the highest CH4 oxidation rate constant of LCS was obtained at 5% CH4 , whereas that of AMSW was determined at 20% CH4 . The corresponding CH4 oxidation rate constants were 10.0 and 7.4 times that at 0 d for LCS and AMSW, respectively. These results indicate that the CH4 oxidation capacities of LCS and AMSW were promoted by 20 d of incubation in a suitable environment with continuous CH4 supplementation. Moreover, the CH4 oxidation capacities of LCS and AMSW were significantly upregulated at their optimum CH4 concentrations (5% and 20%, respectively). These differences can be attributed to the different responses of LCS and AMSW to CH4 concentration. The findings at 35 d of incubation were similar to those at 20 d, but the most efficient CH4 oxidation capacity of AMSW was observed at 50% CH4 . In addition, the CH4 oxidation capacities of both LCS and AMSW were lower than at 20 d. Therefore, the best CH4 oxidation capacities of LCS and AMSW were obtained under 5% and 20% CH4 at 20 d, respectively. The corresponding CH4 oxidation rate constants were 1.648 and 1.751 mL L−1 h−1 g−1 dw, respectively. Considering the CH4 oxidation rate constants of LCS and AMSW at different concentrations, the CH4 oxidation rate constants of LCS at 20% and 50% CH4 and those of AMSW at 5% and 50% CH4 were found to be minimal. Additionally, comparison of the CH4 oxidation rate constants of LCS or AMSW at the same CH4 concentration at different incubation times by analysis of variance (Table 1) revealed no significant differences throughout the incubation process. These results indicate that the dominant population of methanotrophs may be unchanged by variations in CH4 concentration within 30 d, which is in accordance with the results of a study conducted by He et al. (2008). When the CH4 concentration was 70%, the dominant population of methanotrophs in waste soil and clay soil did not change during 120 d of incubation in landfill cover soil. Thus, LCS had better CH4 oxidation capacity than AMSW at low CH4 concentrations, but AMSW was better than LCS at high CH4 concentrations. Moreover, the CH4 oxidation capacity of AMSW may be more flexible under complex LFG environments.
Table 1 Differences in oxidation rate constants (mL L−1 h−1 g−1 dw) of LCS and AMSW with different treatment. Incubation time (d)
5%
20%
LCS
AMSW
50%
LCS
AMSW
LCS
AMSW
0
Without H2 S With H2 S
0.164 ± 0.018 0.237 ± 0.054 ␣
0.060 ± 0.005 0.065 ± 0.002 ␣
0.433 ± 0.063 0.507 ± 0.060 ␣
0.237 ± 0.003 0.236 ± 0.018 ␣
0.615 ± 0.158 0.483 ± 0.064 ␣
0.774 ± 0.063 I␣ 0.849 ± 0.032 ␣
20
Without H2 S With H2 S
1.648 ± 0.391 a␣ 1.701 ± 0.101 ␣
0.290 ± 0.052 b␣ 0.013 ± 0.005 
0.266 ± 0.007 A␣ 0.287 ± 0.033 ␣
1.751 ± 0.322 B␣ 2.335 ± 0.162 ␣
0.689 ± 0.010 I␣ 1.003 ± 0.117 ␣
1.199 ± 0.075 II␣ 1.070 ± 0.104 ␣
35
Without H2 S With H2 S
0.956 ± 0.127 a␣ 0.656 ± 0.206 
0.356 ± 0.114 b␣ 0.004 ± 0.001 
0.240 ± 0.006 A␣ 0.185 ± 0.001 
0.335 ± 0.028 B␣ 0.533 ± 0.035 
0.380 ± 0.022 I␣ 0.537 ± 0.055 ␣
0.984 ± 0.102 II␣ 0.781 ± 0.027 ␣
a␣
b␣
A␣
B␣
a and b refer to significant differences between LCS and AMSW at the same incubation time at 5% CH4 (p < 0.05). A and B refer to significant differences between LCS and AMSW at the same incubation time at 20% CH4 (p < 0.05). I and II refer to significant differences between LCS and AMSW at the same incubation time at 50% CH4 (p < 0.05). ␣ and  refer to significant differences in LCS or AMSW with and without H2 S (p < 0.05).
I␣
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3.2. Effect of H2 S on CH4 biological oxidation dynamics in LCS and AMSW As shown in Fig. 1, CH4 concentration decreased during the incubation process when H2 S was introduced, except for the control. These findings indicate that the CH4 content absorbed by LCS and AMSW was limited. Therefore, the reduction of CH4 was mainly caused by biological oxidation. Moreover, the CH4 oxidation rate constants of LCS or AMSW with and without H2 S at the same incubation time and CH4 concentration were compared by analysis of
441
variance (Table 1). As the incubation time increased, the effect of H2 S on CH4 oxidation in both matrices became increasingly evident, suggesting that the CH4 oxidation capacity of LCS and AMSW was significantly affected by long-term exposure to H2 S. As shown in Fig. 1a and Table 1, the CH4 oxidation capacities of LCS at 5% CH4 were significantly inhibited by H2 S at 35 d. Complete CH4 oxidation of LCS without H2 S was observed within 5 h, whereas that for LCS with H2 S was observed within 14 h. Moreover, the CH4 oxidation rate constant of LCS without H2 S (KL 5 = 0.956 mL L−1 h−1 g−1 dw) was almost three times larger than
Fig. 1. Effect of H2 S on LCS and AMSW CH4 oxidation processes at different CH4 concentrations (5%, 20% and 50%). (a) LCS at 5% CH4 , (b) AMSW at 5% CH4 , (c) LCS at 20% CH4 , (d) AMSW at 20% CH4 , (e) LCS at 50% CH4 and (f) AMSW at 50% CH4 .
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that with H2 S (KL -5-H2 S = 0.66 mL L−1 h−1 g−1 dw). When the CH4 concentration increased to 20%, the degree of suppression of H2 S on LCS was also significant at 35 d (Table 1). As shown in Fig. 1c, the CH4 in serum bottles without H2 S was completely oxidized within 149.5 h, whereas the samples with H2 S needed 165 h after 35 d of incubation. When the CH4 concentration reached 50% (Fig. 1e), no significant difference was found between samples with and without H2 S (Table 1). These findings suggest that the inhibitory effect of H2 S on LCS is stronger at low CH4 concentrations. A similar phenomenon was observed in a previous study (Lee, 2011). Moreover, the CH4 oxidation rate of LCS at 5% CH4 without H2 S was significantly higher than that at high CH4 concentrations (Section 3.1). These findings indicate that the methanotrophs in LCS could hardly oxidize the high levels of CH4 . The aforementioned results may be attributed to the vulnerability of LCS at low CH4 concentration and the protective action of high CH4 levels (Borjesson, 2001). A similar trend was also observed in AMSW. The CH4 oxidation capacities of AMSW at 5% CH4 were significantly inhibited by H2 S after 20 d (Table 1), at which time only 0.49% CH4 was oxidized by AMSW with H2 S. However, CH4 was completely oxidized by AMSW without H2 S within 45 h (Fig. 1b). When the CH4 concentration increased to 20%, no inhibitory effect was exhibited by AMSW with H2 S on CH4 oxidation throughout the entire incubation process (Fig. 1d). These findings were also observed at 50% CH4 (Fig. 1f).
Thus, H2 S does not inhibit CH4 oxidation in AMSW at high levels of CH4 , indicating that high levels of CH4 may relieve the suppressive effect of H2 S on methanotrophs in AMSW. The inhibitory effect of H2 S on the CH4 oxidation process of LCS was less than that of AMSW at 5% CH4 . In contrast, the inhibitory effect of H2 S on the AMSW CH4 oxidation process was less evident than that of LCS at high CH4 concentrations. These results may be attributed to the different dominant bacterium groups in different materials and at different CH4 concentrations. Type II methanotrophs activated under high CH4 concentrations were more stable and abundant than Type I. Thus, Type II methanotrophs may have strong tolerance for H2 S. Luo et al. (2007) reported that the CH4 oxidizing mixed microbial consortium (MY9, containing Type II methanotrophs) could survive in an environment with pollutants such as phenol and dichloromethane. Consequently, AMSW was found to be more suitable for use as a landfill cover because of its high flexibility. 3.3. Possible pathway of effects of H2 S on CH4 oxidation process Based on the material properties, the pH, moisture, and ammonia nitrogen contents of LCS and AMSW were suitable for the growth of methanotrophs (Einola et al., 2007; Yang et al., 2011). Thus, the physicochemical properties of LCS and AMSW were not inhibitors of CH4 oxidation.
Fig. 2. Sulfate contents and increase in sulfate in landfill cover soil microcosms at different CH4 concentration (5%, 20% and 50%). (a) The sulfate contents of LCS and AMSW after 35 d of incubation with and without H2 S. (b) The increase in sulfate of LCS and AMSW at 35 d.
Y.-Y. Long et al. / Ecological Engineering 60 (2013) 438–444 8.5
AMSW-B AMSW-A AMSW-H 2S
8.0
7.5
7.5
7.0
7.0
pH
pH
8.5
LCS-B LCS-A LCS-H2S
8.0
443
6.5
6.5
6.0
6.0
5.5
5.5 0
10
20
30
40
50
60
0
10
20
30
40
50
60
CH4 concentration (%)
CH4 concentration (%)
Fig. 3. pH values of pre-incubated and post-incubated LCS and AMSW samples at different CH4 concentrations (5%, 20% and 50%). (LCS-B) LCS was not incubated. (LCS-A) LCS incubated without H2 S. (LCS-H2 S) LCS incubated with H2 S. (AMSW-B) AMSW was not incubated. (AMSW-A) AMSW incubated without H2 S. (AMSW-H2 S) AMSW incubated with H2 S.
To explore possible pathways of the effects of H2 S on the LCS and AMSW CH4 oxidation process, the sulfate contents of the two cover materials were measured. The sulfate contents of the original LCS and MWS were 45.6 ± 3.1 and 799.0 ± 13.1 mg kg−1 dw, respectively. In experimental sets without H2 S, the sulfate content of LCS remained almost the same after 35 d of incubation (Fig. 2). In contrast, the sulfate content of AMSW increased, and the maximum level (1289.0 ± 25.41 mg kg−1 dw) was observed at 20% CH4 . Overall, these findings indicate that some sulfur compounds that originated from AMSW might be significant contributors to this sulfate increase. In sets where H2 S was introduced, the sulfate content of LCS increased significantly, with a maximum (168.5 ± 5.3 mg kg−1 dw) 3.7 times higher than the original being observed at 5% CH4 . The increase in sulfate in the LCS incubated at 5% CH4 was 1229.2 ± 26.5 g. In addition, the CH4 oxidation capacity of LCS at 5% CH4 was significantly inhibited (Table 1), but no inhibitory effect was observed at high CH4 concentrations (50%). These results indicate that H2 S was converted into sulfate by microbial and/or physical-chemical reactions during incubation. Moreover, the H2 S oxidation process was harmful to CH4 oxidation at low CH4 concentrations. In AMSW with H2 S, the maximum sulfate content (1473.0 ± 24.7 mg kg−1 dw) was also observed at a CH4 concentration of 5%. The increase in sulfate in the AMSW incubated at 5% CH4 was 6740.8 ± 33.2 g; however, this increase was only 3433.3 ± 27.4 g and 1011.5 ± 17.2 g in AMSW incubated with 20% and 50% CH4 , respectively. Moreover, the CH4 oxidation capacity of AMSW at 5% CH4 was also significantly inhibited, but no inhibitory effect was observed at high CH4 concentrations (Table 1). These findings may be ascribed to the lower sulfate formation at higher CH4 concentrations and the high resistance of type II methanotrophs in AMSW for sulfate. As shown in Fig. 3, the pH values of LCS and AMSW incubated with H2 S decreased relative to the original LCS (7.49 ± 0.08) and AMSW (7.51 ± 0.02). The reduction in pH of both LCS and AMSW might be attributed to the production of H+ by hydrolysis during the oxidation of H2 S (He et al., 2012). However, differences in CH4 oxidation could not be ascribed to variations in pH because the pH values after incubation remained neutral. In conclusion, the inhibitory effect of H2 S on LCS and AMSW CH4 oxidation may be ascribed to competitive inhibition, which stemmed from competition with CH4 for the same enzyme (Burgess
et al., 2001). Moreover, a study conducted by Burgess et al. (2001) showed that the concentration of the inhibitor (CH3 SH) became less important when the CH4 concentration was very high and Type I methanotrophs were more affected than type II. Therefore, a similar result may also be obtained for H2 S. For this reason, the high level of CH4 may prevent its oxidation of LCS and AMSW from being inhibited by H2 S, and AMSW may have stronger resistance in H2 S because of dominant population-Type II methanotrophs. Moreover, the OM content of AMSW was significantly higher than that of LCS, indicating that the AMSW samples were more suitable for long-term incubation because AMSW contained more nutrients for the growth of microorganisms. 4. Conclusions The CH4 biological oxidation rates of LCS and AMSW were found to be significantly affected by CH4 concentration, with the maximum rates being obtained at 5% and 20% CH4 , respectively, within 20 d. The oxidation process of LCS and AMSW could be significantly inhibited by H2 S at low CH4 concentrations (5%), but a high level of CH4 (20% and 50%) could relieve the inhibitory effects of H2 S. A possible pathway of the effects of H2 S on CH4 oxidation was competitive inhibition. Acknowledgements This work was financially supported by National Natural Science Foundation of China (41071310, 41101453, 41101466, and 51108419), Research Plan of Department of Education of Zhejiang Province (Y201119953, and Y201122794), Funded project for youth researcher of Zhejiang Gongshang University (QY11-22). References Albanna, M., Warith, M., Fernandes, L., 2010. Kinetics of biological methane oxidation in the presence of non-methane organic compounds in landfill bio-covers. Waste Manage. 30 (2), 219–227. Anderson, R., Jambeck, J.R., McCarron, G.P., 2010. Modeling of Hydrogen Sulfide Generation from Landfills Beneficially Utilizing Processed Construction and Demolition Materials. Environmental Research and Education Foundation, Alexandria, VA, pp. 13–20. Barlaz, M.A., Green, R.B., Chanton, J.P., Goldsmith, C.D., Hater, G.R., 2004. Evaluation of a biologically active cover for mitigation of landfill gas emissions. Environ. Sci. Technol. 38 (18), 4891–4899.
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Borjesson, G., 2001. Inhibition of methane oxidation by volatile sulfur compounds (CH3 SH and CS2 ) in landfill cover soils. Waste Manage. Res. 19 (4), 314–319. Burgess, J.E., Parsons, S.A., Stuetz, R.M., 2001. Developments in odour control and waste gas treatment biotechnology a review. Biotechnol. Adv. 19, 35–63. Campagna, D., Kathman, S.J., Pierson, R., Inserra, S.G., Phifer, B.L., Middleton, D.C., Zarus, G.M., White, M.C., 2004. Ambient hydrogen sulfide, total reduced sulfur, and hospital visits for respiratory diseases in northeast Nebraska, 1998–2000. J. Expo. Anal. Environ. Epidemiol. 14 (2), 180–187. Contin, M., Goi, D., De Nobili, M., 2012. Land application of aerobic sewage sludge does not impair methane oxidation rates of soils. Sci. Total Environ. 441, 10–18. Cooper, C.E., Brown, G.C., 2008. The inhibition of mitochondrial cytochrome oxidase by the gases carbon monoxide, nitric oxide, hydrogen cyanide and hydrogen sulfide: chemical mechanism and physiological significance. J. Bioenergy Biomembr. 40 (5), 533–539. Einola, J.K.M., Kettunen, R.H., Rintala, J.A., 2007. Responses of methane oxidation to temperature and water content in cover soil of a boreal landfill. Soil Biol. Biochem. 39, 1156–1164. Einola, J.K.M., Karhu, A.E., Rintala, J.A., 2008. Mechanically-biologically treated municipal solid waste as a support medium for microbial methane oxidation to mitigate landfill greenhouse emissions. Waste Manage. 28 (1), 97–111. EPA, 2006. Methane: Sources and Emissions, Human-related sources. United States Environmental Protection Angency, Washington, DC, USA. EPA, 2010. Methane Emissions. United States Environmental Protection Agency, Washington, DC, USA. Faulwetter, J.L., Gagnon, V., Sundberg, C., Chazarenc, F., Burr, M.D., Brisson, J., Camper, A.K., Stein, O.R., 2009. Microbial processes influencing performance of treatment wetlands: a review. Ecol. Eng. 35 (6), 987–1004. He, R., Ruan, A., Jiang, C., Shen, D.s., 2008. Responses of oxidation rate and microbial communities to methane in simulated landfill cover soil microcosms. Bioresour. Technol. 99 (15), 7192–7199. He, R., Xia, F.F., Bai, Y., Wang, J., Shen, D.S., 2012. Mechanism of H2 S removal during landfill stabilization in waste biocover soil, an alterative landfill cover. J. Hazard. Mater. 217-218, 67–75. Henckel, T., Roslev, P., Conrad, R., 2000. Effects of O2 and CH4 on presence and activity of the indigenous methanotrophic community in rice field soil. Environ. Microbiol. 2 (6), 666–679. IPCC, 2007. Climate Change 2007. The Physical Science Basis. in: Contribution of Working Group I to the Fourth Assessment Report of the IPCC. Cambridge University Press, New York. Jugnia, L.B., Cabral, A.R., Greer, C.W., 2008. Biotic methane oxidation within an instrumented experimental landfill cover. Ecol. Eng. 33 (2), 102–109. Kim, K.H., 2006. Emissions of reduced sulfur compounds (RSC) as a landfill gas (LFG): a comparative study of young and old landfill facilities. Atmos. Environ. 40 (34), 6567–6578. Knowles, R., 2005. Denitrifiers associated with methanotrophs and their potential impact on the nitrogen cycle. Ecol. Eng. 24 (5), 441–446.
Lee, E.H., 2011. Characterization of methane oxidation by a methanotroph isolated from a landfill cover soil, South Korea. J. Microbiol. Biotechnol. 21 (7), 753–756. Liao, Y., Yang, Y.Q., Shen, D.S., Long, Y.Y., 2013. Effect of deposit age on adsorption and desorption behaviors of ammonia nitrogen on municipal solid waste. Environ. Sci. Pollut. Res. 20, 1546–1555. Lou, Z.Y., Wang, L., Zhao, Y.C., 2011. Consuming un-captured methane from landfill using aged refuse bio-cover. Bioresour. Technol. 102 (3), 2328–2332. Luo, M.F., Wu, H., Wang, L., Xing, X.H., 2007. Study on the structure and function of a stable methane oxidizing mixed microbial consortium. Acta Microbiol. Sin. 47 (1), 103–109 (in Chinese). Lu, R.K., 1999. Agriculture Chemical Analysis of Soil. China Agricultural Scientech Press, Beijing (in Chinese). MCPRC (The Ministry of Construction of People’s Republic of China), 2000. Municipal domestic Refuse-Determination of pH-Glass electrode method. Vol. CJ/T 99-1999 (in Chinese). OSHA, 2012. Hydrogen Sulfide (H2 S). U.S. Department of Labor, Occupational Safety and Health Administration, Washington, DC. Park, S., Brown, K., Thomas, J., 2004. The use of biofilters to reduce atmospheric methane emissions from landfills: Part I. Biofilter design. Water Air Soil Pollut. 155 (1), 63–85. ˛ Pawłowska, M., Stepniewski, W., 2006. An influence of methane concentration on the methanotrophic activity of a model landfill cover. Ecol. Eng. 26 (4), 392–395. Pedersen, G.B., Scheutz, C., Kjeldsen, P., 2011. Availability and properties of materials for the Fakse Landfill biocover. Waste Manage. 31 (5), 884–894. Rasi, S., Veijanen, A., Rintala, J., 2007. Trace compounds of biogas from different biogas production plants. Energy 32 (8), 1375–1380. Themelis, N.J., Ulloa, P.A., 2007. Methane generation in landfills. Renew. Energ. 32 (7), 1243–1257. Visscher, A.D., Cleemput, O.V., 2003. Induction of enhanced CH4 oxidation in soils: NH4 + inhibition patterns. Soil Biol. Biochem. 35 (7), 907–913. Wang, J., Xia, F.F., Bai, Y., Fang, C.R., Shen, D.S., He, R., 2011. Methane oxidation in landfill waste biocover soil: kinetics and sensitivity to ambient conditions. Waste Manage. 31 (5), 864–870. Wu, S.B., Kuschk, P., Wiessner, A., Müller, J., Saad, R.A.B., Dong, R.J., 2013. Sulphur transformations in constructed wetlands for wastewater treatment: a review. Ecol. Eng. 52, 278–289. Yang, K., Xu, Q., Townsend, T.G., Chadik, P., Bitton, G., Booth, M., 2006. Hydrogen sulfide generation in simulated construction and demolition debris landfills: impact of waste composition. J. Air Waste Manage. 56 (8), 1130–1138. Yang, N., Lu, F., He, P., Shao, L., 2011. Response of methanotrophs and methane oxidation on ammonium application in landfill soils. Appl. Environ. Microbiol. 92 (5), 1073–1082. Yu, H.X., 2007. Studies on Bio-oxidizing Methane in Coal Mines with Microbial Technology. Zhejiang University, Zhejiang, China (in Chinese). Zhang, H.H., Li, W.X., Cai, Z.C., Zhang, Y., Feng, Y.H., Zhang, H.L., He, F., 2013. Landfill CH4 oxidation and N2 O emissions by aged refuse: effects of wastewater NH4 + -N incubation, heavy metals and pH. Ecol. Eng. 53, 243–249.