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Investigation of volatile methyl siloxanes in biogas and the ambient environment in a landfill
j o u r n a l o f e n v i r o n m e n t a l s c i e n c e s 9 1 ( 2 0 2 0 ) 5 4 e6 1
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Investigation of volatile methyl siloxanes in biogas and the ambient environment in a landfill Ning Wang 1,2,3,*, Li Tan 3, Lianke Xie 4, Yu Wang 5,*, Timothy Ellis 6 1
School of Environmental Science and Engineering, Shandong Key Laboratory of Water Pollution Control and Resource Reuse, Shandong University, Jinan, 266237, China 2 Shanghai Key Laboratory of Atmospheric Particle Pollution and Prevention (LAP3), Shanghai, 200433, China 3 School of Municipal and Environmental Engineering, Shandong Jianzhu University, Jinan, 250100, China 4 State Grid Shandong Electric Power Company, Electric Power Science Research Institute, Jinan, 250100, China 5 Beijing Key Laboratory of Water Resources & Environment Engineering, China University of Geosciences (Beijing), Beijing, 100083, China 6 Department of Civil, Construction, and Environmental Engineering, Iowa State University, Iowa, USA
article info
abstract
Article history:
Landfill biogas is a potential alternative for fossil fuel, but the containing impurities, vol-
Received 12 November 2019
atile methyl siloxanes (simplified as siloxanes), often cause serious problems in gas tur-
Received in revised form
bines when applied to generate electricity. In this research, a collecting and analyzing
6 January 2020
method based on solvent adsorption and purge and trap-gas chromatography-mass
Accepted 6 January 2020
spectrometry was established to determine the siloxanes in biogas from a landfill in Jinan,
Available online 25 January 2020
China, and adjacent ambient samples, such as soil, air, and leachate of the landfill. The results showed that, octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane
Keywords:
(D5) accounted for 63% of total siloxanes; and without considering D4 and D5, the order of
Biogas
detected siloxanes in concentration was found relating to Gibbs free energies of molecules,
Siloxane
namely that higher abundant siloxane (except for D4 and D5) usually had lower Gibbs free
Landfill
energy. Additionally, the mass ratio between D4 and octamethyltrisiloxane (L3) in the
Purge and trap GC-MS
biogas varied with different garbage age in landfills, possibly revealing the breaking-down of larger siloxane molecules with time. The samples, which were collected from environmental samples adjacent to the landfill, such as soil, water, and air, presented much higher siloxane level than urban or rural area away from landfills. The current H2S scrubber of the landfill biogas could decrease the total siloxanes from 10.7 to 5.75 mg/m3 due to Fe2O3 and a refrigerant drier in a purification system and cyclic siloxanes were more easily removed than linear ones. © 2020 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.
* Corresponding authors. E-mail addresses: [email protected] (N. Wang), [email protected] (Y. Wang). https://doi.org/10.1016/j.jes.2020.01.005 1001-0742/© 2020 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.
j o u r n a l o f e n v i r o n m e n t a l s c i e n c e s 9 1 ( 2 0 2 0 ) 5 4 e6 1
Introduction Landfill is one of major means to deal with municipal solid wastes, while one common byproduct of this process is landfill gas, mostly consisting of methane and carbon dioxide (Nam et al., 2013). Currently, most of the landfill gas is usually used as fuel of reciprocating internal combustion engines or directly burned in a flare (Xu et al., 2019). Obviously, the latter treatment would risk of the leakage of unburned gas containing methane, which is a kind of powerful greenhouse gas; while the former should be a relatively economical and environment-friendly option. However, once burned, the trace impurity gas, siloxane, would form silica depositing on the combustion chamber inner surface and abnormally increase working temperature of the engine, therefore reducing efficiency. Siloxanes mostly originate from personal-care consumer products, antifoaming agents, and polymeric silicone products (as precursors), which are a group of linear or cyclic compounds with repeating silica-oxygen atom sequence surrounded by methyl groups (Xu et al., 2018). Long term exposure of siloxane causes engine failure by increasing the roughness of the cylinder and immobilizing the piston rez and Egusquiza, 2015). To rings in engines (Alvarez-Fl o prolong the life of the engines, it was necessary to prevent siloxane entering the biogas (Dewil et al., 2006). Researchers have tested and reported some methods to remove siloxanes from biogas, including photo-catalyzed decomposition with TiO2 thin films (Sun et al., 2003) or TiO2 coated particles (Lamaa et al., 2014); adsorptive removal with activated carbon (Oshita et al., 2010), silica gel (Oshita et al., 2010), and zeolite (Oshita et al., 2010); separation with siliconeerubber membranes (Ajhar et al., 2012); and biodegradation (Accettola et al., 2008; Li et al., 2014). Although the methods were efficient, most of them were based on siloxane removal under laboratory conditions; and little research focused on evaluating or improving current H2S or CO2 scrubbers for siloxane removal on economical purpose, which could be essential for practical large-scale usage. Besides the siloxane removal, researchers also investigated the distribution and profile of siloxanes in different landfill gas in different countries. Gas chromatography-mass spectrometry (GCeMS) was agreed for the quantification of siloxanes due to the low detection limit and high accuracy (Marine et al., 2012). However, there was no consensus on sampling methods or analysis methods. For sampling methods, Tedlar bags are easily to use but faced wall loss of target compounds (Marine et al., 2012); sorbent tubes have storage stability but expensive; traditional solvent method is robust for collecting of biogas but time consuming, normally more than three hours (Wheless and Pierce, 2004). In this research, a modified method, based on solvent method combining purge and trap (P&T)-GC-MS, was established, aiming at shrinking the collecting time and obtaining robust determination of siloxanes in different matrix, such as biogas, soil, and leachate. Compared to regular GC-MS, P&TGC-MS was more sensitive and dramatically decreased the sampling time of the solvent adsorption method. Solving the time-consuming problem and taking the advantage of solvent method, the method is a more economic option than sorbent
55
tube collecting, and more reliable than Tedlar bag method for siloxane analysis in biogas. Meanwhile, this P&T-GC-MS system is feasible for liquid and solid samples without changing the analysis instrument settings. Furthermore, since most related literatures forced on the pollution level and profiles of siloxane only in biogas from landfill, we performed a systematic investigation including siloxanes in surrounding environmental matrix, influence by biogas purifier, etc.
1.
Materials and methods
1.1.
Sampling
The sampling sites located in the Jinan No. 2 municipal solid waste landfill and disposal center, which dealt with 0.7 million tons of domestic garbage every year for Jinan, the capital city in Shandong Province, China and owned five 700 kw generators using biogas. The dumping in the landfill was a mixture of residential garbage, construction residue, and incineration ash. As shown in Fig. 1, 27 gas samples were collected from 10 biogas-observing wells and the main pipe conveying biogas to generator. Biogas samples were collected with two seriallyconnected impingers which were ice-rinsed in a cooler. Siloxanes in the biogas were absorbed and enriched in 10 mL methanol (AR, Hengxing Inc., China) in each impinger at a flowrate of 0.2 L/min, regulated by an active sampler (Model 224-PCXR8, SKC Inc., USA), for one hour. After absorption, the solution was accurately adjusted to 10 mL with methanol and partially transferred into vials at 4 C for further instrumental analysis. Three surrounding air sample was obtained via the same method. Surrounding soil and leachate samples (three for each matrix) were collected in clean metal box and widenecked bottle respectively and also stored at 4 C.
Fig. 1 e Biogas sampling sites (wells) in the landfill area.
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1.2.
Instrument analysis
P&T-GC-MS applied in this research consisted of purge and trap device (P&T, Atomx Automated VOC Sample Preparation system, Teledyne Technologies Inc., USA) and gas chromatography-mass spectrometry (GC-MS, Trace™1300 þ ISQ, Thermo Fisher Scientific Inc., USA). The information for target siloxanes, such as quantization and characterized ions, retention time for GC-MS, etc., was listed in Table 1. The setting parameters were set as following:
Purge and Trap conditions Carrier gas Purge rate Purge duration preheating temperature for desorption Desorbing temperature Baking temperature Trapping temperature Sample inject volume GC-MS conditions Column IInjector temp Temperature program
Interface temperature Ion source temperature Scan mode Carrier gas
99.999% N2 40 mL/min 11 min 245 C 250 C for 2 min 330 C for 10 min ambient (<30 C) 10 mL sample solution in purge bottle filled with 40 mL water DB-5MS 40 C isothermal at 40 C for 2 min and programmed from 40 to 100 C at 20 C/min; isothermal at 100 C for 3 min and programmed from 100 to 200 C at 10 C/min; isothermal at 200 C for 5 min 250 C 250 C SIM 99.999% He
Forsoil samples, US Environmental Protection Agency method (EPA5030B and 5035) were referenced; 0.2 g soil were weighed and transferred into a 40-mL purge bottle with stir bar for purging and analysis.
1.3.
Calibration curve and QA&QC
The standard solution (100 mg/L, NEOCHEMA GmbH Co., Germany) containing 8 siloxanes was purchased and diluted into a series of solutions as external calibration curve,
including 0.2, 0.4, 0.6, 0.8, and 1.0 m/L in methanol (HPLC grade, Fisher, Inc., USA). Potential siloxane-containing personal products, such as cosmetics, were avoided to use by the analysts and silicon-containing cushion or seals materials were not allowed throughout the experiments. All glassware were cleaned with hexane and heated at 300 C overnight before employed. The filed blank and procedure blank test showed no detection of target chemicals, except for D4, D5, and D6, the amounts of which (0.03e0.09 mg/m3, 0.16e0.19 mg/m3, and 0.09e0.10 mg/m3,respectively as concentrations in biogas) were deducted from the final results for each instrument determination. The limit of detection (LOD) was 6.9e73.2 ng/L for liquid samples (including air, biogas, leachate in menthol), 7.7e50.5 ng/g for solid samples, which was triple times of S/N signal for 0.5 mg/L of standard solution. The recovery rates were 76.3%e113% for 0.5 mg/L of spike sample, 90.7%e122% for 1.0 mg/L, and 90.7%e116% for 2.0 mg/L. A repeatability test was carried out by determining five parallel 10 mL blank samples (level of siloxanes less than LOD), which was added a certain 0.5 mg/L of standard solution and the RSD for 8 siloxanes was 4.2%e9.5%. All the data referring to gas phase was adjusted to the values at standard conditions.
2.
Results and discussion
As show in Fig. 2, all eight specifies of target siloxane were detected in the biogas from landfill and the abundances of D4 and D5, accounting for approximately 63% of total siloxanes, were significantly larger than others. The results matched previous research, in which both siloxanes were present at the highest concentrations in biogas (Liu et al., 2014; Schweigkofler and Niessner, 2001; Wang et al., 2013a). All the siloxanes might mostly originate from evaporation or broken-up of polysiloxanes used as makeup ingredient or antifoaming additives in the anaerobic environment, and in which, the concentrations of linear siloxanes were normal higher or comparable to cyclic ones including D4 and D5 (Lu et al., 2011). It means that, in the commercial products, D4 and D5 are not significantly higher than other siloxanes, implying that the abundant D4 or D5 in the biogas could not all attribute to the evaporation from original sources. If considering the Gibb’s free energies of the siloxanes (Tansel and Surita, 2014), smaller molecules should be the more prevalent because of the lower energies barrier for molecule formation. Actually, the assumption was right for siloxanes only
Table 1 e Information of GC-MS analysis for siloxane standards. Component
1. 2. 3. 4. 5. 6. 7. 8.
Hexamethyldisiloxane Hexamethylcyclotrisiloxane Octamethyltrisiloxane Octamethylcyclotetrasiloxane Decamethyltetrasiloxane Decamethylcyclopentasiloxane Dodecamethylpentasiloxane Dodecamethylcyclohexasiloxane
Abbr.
Retention time (min)
R2
Qualitative mass to charge ratio (m/z)
Quantitative mass to charge ratio (m/z)
L2 D3 L3 D4 L4 D5 L5 D6
3.22 4.83 5.55 6.94 7.83 8.98 10.19 11.37
0.9903 0.9975 0.9986 0.9978 0.9931 0.9943 0.9915 0.9908
73, 147, 189, 149 96, 207, 208 73, 103, 221 281, 282 73, 207, 208 73, 267, 355 73, 147, 281 73, 341, 429
73 207 221 281 207 267 147 341
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Fig. 2 e Concentrations of siloxanes in biogas generated from the landfill.
if D4 and D5 were excluded. For example, the average concentration of L2 was larger than L3, then overwhelming L4 and L5 which were even below the LOD in some samples. The concentration of D3 was also found higher than D6 for the similar reason. The reason why D4 and D5 did not follow the rule could be that both siloxanes have more stable structure in molecule than others. Since the backbone of siloxane molecule is a chain of alternate O and Si atoms and the SieOeSi bond is relatively more flexible than OeSieO bond (Kim, 1991), once the O atoms locate inside a tetrahedral configuration, more energy is needed to bend the O atoms out. This could explain the high stability of D4 and D5, in which the O atoms faced insides the tetrahedral-like molecule and were protected from losing by the surrounding methyl groups (Tansel and Surita, 2014). Many previous studies support the structure of D4 and D5, both of which accounted for most siloxanes in landfill biogas. It is found that, excluding D4 and D5, other siloxanes decreased in concentration with the increasing molecular weight in different reports shown in Table 2. It is also found that the concentrations varied significantly from site to site, for example, D4 in Guangzhou, China was tenfold
€ skyla € , Finland. It could be attributed to more than that , in Jyva the variety of landfill age, waste profile, or even sampling method (Piechota et al., 2012; Ru¨cker and Ku¨mmerer, 2015; Wang et al., 2013b). Little systematic research was found in China and the current available data differed a lot for D3, D4, and total siloxane to our research, as shown in Table 2. The different garbage type may attribute to this since the landfill in this research contained lots of construction waste which could adsorb some siloxane or delaying the escaping, which needs further investigations. Nam et al. (Nam et al., 2013) has found that L3, L4, L5, and D6 formed when only L2, D4, and D5 passed an adsorption reactor loaded with activated carbon in lab test. Although the authors did not discuss the possibility of the catalysis of activated carbon, this may imply that besides the degrading of larger molecule siloxanes, small ones were able to polymerize over time (Soreanu et al., 2011); and the reaction might be reversible with the equilibrium decided by the Gibbs energy and molecular structure as mentioned above. In landfills, the biogas has to be purified to avoid CO2, moisture, and H2S before entering combustion chamber. This landfill employed Fe2O3 particles as H2S scrubber and dehydrated biogas with a refrigerant dryer. Although both of facilities were not designated to remove siloxane, it was noticed that the concentration decreased in the biogas in our research, as shown in Fig. 3. Based on the results of t-test (a ¼ 0.1), the concentrations of D4 (p ¼ 0.0598) and D5 (p ¼ 0.0674) were significantly decreased after traversing the scraping system, whereas other siloxanes were little influenced. The abnormal increase of L3 implies the possible breakdown of larger siloxanes passing the scrapper, which need more profound experiments and analysis on molecular level. After traversing the scrubber in this landfill, the ratio (0.08) between total linear siloxanes and cyclic ones was much lower than the value (0.24) before passing through the scrubber. It seemed that the cyclic ones were more easily trapped by the scrubbing process. The average remaining siloxane was total 5.75 mg/m3, larger than the suggested maximum values for microturbines (0.03 mg/m3), fuel cell
Table 2 e Concentrations of siloxanes in biogas of this research and other references. Concentrations of siloxanes (mg/m3) in landfill biogas
Sampling sites L2 This research € skyla € , Finland (La € ntela € et al., Jyva 2012) Istanbul, Turkey (Orhan and Berrin, 2012) Miami, USA (Tansel and Surita, 2014) Miami, USA (Surita and Tansel, 2015) Istanbul, Turkey (Orthan and Berrin, 2013) Guangzhou, China (Wang et al., 2001) China A (Takuwa et al., 2009) China B (Takuwa et al., 2009) Japan (Takuwa et al., 2009) Franki, Poland (Piechota et al., 2012)
D3
L3
D4
L4
D5
L5
D6
0.837 ± 0.151 0.874 ± 0.180 0.614 ± 0.089 4.159 ± 0.627 0.506 ± 0.104 2.896 ± 0.448 0.122 ± 0.041 0.659 ± 0.153 0.565 ± 0.154 0.129 ± 0.083 0.023 ± 0.007 1.083 ± 0.276 0.006 ± 0.004 1.135 ± 0.293 ND e <1.0
e
<0.5
4.9e6.8
<0.5
2.3e3.7
e
<1.0
0.680 ± 0.048 0.515 ± 0.007 0.097 ± 0.004 2.155 ± 1.135 ND
1.750 ± 0.750 ND
0.089 ± 0.008
1.35
0.39
0.12
5.78
0.04
2.98
ND
0.33
1.6 ± 0.1
<1.0
<0.5
5.0 ± 0.2
<0.5
2.9 ± 0.1
e
<1.0
e
3.6 ± 3.1
e
11.4 ± 5.5
e
e
e
Total Total Total Total
2.06e6.88 (L2, L3, L4, L5, D3, D4, D5, D6) 2.51e4.49 (L2, L3, L4, L5, D3, D4, D5, D6) 0.28e51.7 (L2, L3, L4, L5, D3, D4, D5, D6) 38.9 ± 0.1 (L2, D3, D4, D5, D6)
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Fig. 3 e Concentrations of siloxanes in biogas before and after the biogas purification system of the landfill.
engines (0.1 mg/m3) and gas turbines (0.7 mg/m3). However, it might be acceptable for internal combustion engines (5~28 mg/m3) (Wheless and Pierce, 2004). To reduce the potential risks and maintenance cost, the additional specific removal system for siloxane was necessary, and the suggest rez and was also agreed by most researchers (Alvarez-Fl o Egusquiza, 2015, Takuwa et al., 2009; Cabrera-Codony et al., 2014). Adsorption could be one possible mechanism for this scrapping system. Although little research referred to the coadsorption of siloxanes and other undesired components in biogas, it was common that the silicon compounds were easily trapped on or in the solid scrubbers. Based on experiments, Cabrera-Codony (Cabrera-Codony et al., 2014) found that CH4 and CO2 competed with D4 on the adsorption into activated carbon. It implied that the adsorption mechanism of siloxane could, to some extent, be similar to CH4 or CO2. It has also been
Table 3 e Concentrations of ambient siloxanes of this research and other references. Sampling sites
Concentrations of siloxane L2
D3
L3
D4
L4
D5
L5
D6
3
Air samples (mg/m ) Ambient air on the 0.0248 (0.0127e0.0358) surface of this landfill Ambient air in a e landfill of Guanzhou, China (Wang et al., 2001) Ambient urban air e in Guanzhou, China (Wang et al., 2001) Ambient industrial e air in Guanzhou, China (Wang et al., 2001) Water samples (mg/L) Leachate from this a landfill Municipal e wastewater in January (Xu et al., 2013) Solid samples (mg/g) Covered soil on the 0.418 (0.323e0.574) surface of this landfill Surface sediment e in river (Zhang et al., 2011) Indoor dusts (3e5 e electrical devices in room) (Lu et al., 2010) Personal care e products (Lu et al., 2011) a
0.246 (0.150e0.359) 0.117 (0.086e0.151) 0.311 (0.219e0.458)
a
a
0.273 (0.157e0.487)
a
0.0036
e
0.0114
e
e
e
e
0.0029
e
0.0009
e
e
e
e
0.0061
e
0.0135
e
e
e
e
10.4 (9.25e12.48)
51.3 (45.1e59.5)
34.6 (33.8e35.5)
a
a
58.6 (50.2e69.4)
a
0.48
ND
2.89
0.07
3.29
4.19 (2.45e5.54)
1.28 (1.02e1.62)
1.33 (0.99e1.45)
a
a
e
e
0.0072
e
0.0527 e
e
e
0.0104
0.0258 0.0202 0.0017
e
e
5.24
e
Unreliable data due to failed standard calibration curves..
29.7
e
1.56 (1.21e2.01)
e
2.20
a
0.0761
0.0175
18.3
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found siloxane was co-adsorbed when removing H2S with rubber particles (Siefers et al., 2010). Yu et al. (2013) found that adsorption capacity of D4 on activated carbon was positively related to BET surface area and micropore volume. But, Nam et al. (2013) reported that carbon-based adsorbents with well-developed micropore were more efficient for L2 than non-carbon-based adsorbents with macropore, which was reverse for D4 and D5. The later opinion was also supported by other researchers (Oshita et al., 2010). It could be deducted that the adsorption depended on molecular size and pore size. The pore size of Fe2O3 particles was much less than those carbon-based adsorbent, which might attribute to the significant removal of D4 and D5 in this research. Additionally, it was reported that non-carbon adsorbents might affect the conversion of siloxanes (Nam et al., 2013). It could explain why the higher L3 concentration was observed after desulfurizing. Another reason for decrease of the siloxanes could be the refrigerant drier, which aimed to remove the moisture from biogas. Freezing temperature and proper condensation system could decrease partial siloxanes in biogas and the efficiency depended on how low the temperature was (Soreanu et al., 2011). A previous laboratory study had shown that a refrigeration system could remove 20% siloxanes when removing 90% moisture (Piechota et al., 2012). Therefore, the performance of the refrigerant drier might contribute the loss of some siloxanes but not all of them because approximately as much as half siloxanes in the scrapper system were stopped and the operating temperature was not low enough to reach the efficiency (more than 50%). The concentrations of siloxanes in air (0.972 mg/m3) and soil (8.78 mg/g) on the landfill, and leaching water (155 mg/L) were also determined and recorded in Table 3, as well as other reports of siloxanes in various environments by other groups. Unlike landfill biogas, D4 did not dominate among the detected siloxanes in the environmental matrix samples, which were comparable with each other, and the profile was more like in artificial siloxane sources, such as personal care products, as stated previously. This implied that the siloxanes did not stayed in the matrix for a very long time or experience degradation. This was unlike the siloxanes in biogas, where D4 was much higher than others possibly due to the longer decomposition time (Gaj and Pakuluk, 2015). The concentrations of siloxanes (0.972 mg/m3) in the air over landfill were much larger than the reported values in ambient air (0.0038~0.0196 mg/m3 (Wang et al., 2001)), which could attribute to the possible evaporation from compacted waste rather than biogas leaking because of the different profile of siloxane species. Meanwhile, the concentrations of siloxanes from the leachate (155 mg/L) or covered soil (8.78 mg/g) were also much larger than those in reported municipal wastewater (8.93 mg/L (Xu et al., 2013))or dust/sediment (0.0756 mg/g (Lu et al., 2010)/0.136 mg/g (Zhang et al., 2011)). Due to the low aqueous solubility (Varaprath et al., 1996), the concentrations of siloxanes were very low in natural water. Higher concentration in the leachate also support the assumption that the siloxanes originated from the landfill, since the distance to pollution source was an increasing factor for D4-D6 in soil (Xu et al., 2017). All these facts suggested that landfill area was a potential important source of siloxane emission. Through the half-year sampling period, eight species of siloxanes were not always simultaneously detectable for each
59
sample, as shown in Table 3. To minimize unexpected deviation, L3 and D4, which were both successfully detected in all samples, were selected to represent linear and cyclic siloxane, respectively, in the following discussions. In the landfill, each sampled well was marked with a serial number and the smaller number implying an older garage filling time. Then, the profiles, shown as in Fig. 4, referred to the temporal (age of wells) variation of L3 and D4. The concentrations of L3 were lower than D4 in all the wells due to the reason discussed above. It was also found concentrations obtained from “older” well, like #13 and #14 wells, where garbage was filled in December 2015, were relatively lower than other recent wells, except for well #53 and #54, which might attribute to the low sampling frequency (only one sample from each well). Landfill age was reported to influence the concentration of siloxanes in biogas (Piechota et al., 2012); D4 and D5 from old landfills were found lower than recent ones, possibly resulting from more siloxane product used in recent life by residents or exhaustion of siloxanes in landfilled garbage over time (Badjagbo et al., 2010). According to our observation in this work, the latter explanation was preferred because the time span between the landfill well #13 and #54 is less than one year which is not long enough for society adapting new siloxane-contained products. As artificial compounds, polysiloxanes decreased through the landfill process without alternative sources. Less siloxane escaped out via biogas as time lapsing, due to the smaller biogas production and original pressure partition, both of which decreased the opportunity to effectively diffuse into biogas. Besides, as show in Fig. 4, the ratio D4/L3 was found higher from some “middle age” wells (#38 and #39) than earlier ones (#13 and #14) and later ones (#51-#53). It might attribute to the fact that largemolecule siloxane was broken into more stable one like D4 over a certain time, but the degradation was slow down and more siloxane escaped after a longer time. The ratio in well #54 was abnormal possibly due to the lack of replicated tests. Unlike the pervious report that concentrations of siloxanes in biogas from sewage sludge varied with the air temperature (Oshita et al., 2010), in this half-year study, the environmental temperature seemed not to influence the siloxanes as shown in Fig. 5. It perhaps attributed to the relatively stable temperature in the landfill.
Fig. 4 e Comparison of the concentrations between L3 and D4 in biogas from different sampling biogas wells.
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Fig. 5 e Comparison of the concentrations between L3 and D4 at different ambient temperature during sampling.
3.
Conclusions
A P&T-GC-MS method was developed to analyze siloxanes could decrease the sampling time of solvent method for biogas and enable the same analysis procedure for samples in different matrix, such as air, biogas, soil, and leachate from the same landfill, which potentially improve the reliability of the comparison between the samples. Smaller molecule of siloxanes had higher concentrations in biogas because of lower Gibb’s free energies, except for D4 and D5, which were stable due to the special molecular structure, resulting in high concentrations (63% of total siloxanes). More recent landfill wells produced more siloxanes in biogas and the concentrations in this investigation were different from reports in other sites in the world. Cyclic ones tended to be partially stopped by the current H2S scrubber and a refrigerant drier. It was possible to improve current scrubbing system for coexisting siloxanes to reduce the cost. Surrounding environmental matrix, such as air, leachate, and soil, was significantly influenced by the siloxanes from the landfill by escaping from waste rather than biogas due to the lower D4/L3 ratio in the matrix. The ratio could be used preliminary to estimate the “age” of landfill wells.
Acknowledgment This work was supported by the National Natural Science Foundation of China (No. 21407097) and Supported by Shandong Key Laboratory of Water Pollution Control and Resource Reuse (No. 2019KF14) and the Opening Project of Shanghai Key Laboratory of Atmospheric Particle Pollution and Prevention (LAP3) (No. FDLAP17001).
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Ajhar, M., Bannwarth, S., Stollenwerk, K.-H., Spalding, G., Yu¨ce, S., Wessling, M., et al., 2012. Siloxane removal using siliconeerubber membranes. Sep. Purif. Technol. 89, 234e244. rez, J., Egusquiza, E., 2015. Analysis of damage caused Alvarez-Fl o by siloxanes in stationary reciprocating internal combustion engines operating with landfill gas. Eng. Fail. Anal. 50, 29e38. Badjagbo, K., Heroux, M., Alaee, M., Moore, S., Sauve, S., 2010. Quantitative analysis of volatile methylsiloxanes in waste-toenergy landfill biogases using direct APCI-MS/MS. Environ. Sci. Technol. 44, 600e605. n, M.A., Sa nchez-Polo, M., Cabrera-Codony, A., Montes-Mora Martı´n, M.J., Gonzalez-Olmos, R., 2014. Biogas upgrading: optimal activated carbon properties for siloxane removal. Environ. Sci. Technol. 48, 7187e7195. Dewil, R., Appels, L., Baeyens, J., 2006. Energy use of biogas hampered by the presence of siloxanes. Energy Convers. Manage. 47, 1711e1722. Gaj, K., Pakuluk, A., 2015. Volatile methyl siloxanes as potential hazardous air pollutants. Pol. J. Environ. Stud. 24, 937e943. Kim, S.W., 1991. Silicone based polymer science. A comprehensive resource. J. Controlled Release 16, 366e369. Lamaa, L., Ferronato, C., Prakash, S., Fine, L., Jaber, F., Chovelon, J., 2014. Photocatalytic oxidation of octamethylcyclotetrasiloxane (D4): towards a better understanding of the impact of volatile methyl siloxanes on photocatalytic systems. Appl. Catal., B 156, 438e446. € ntela € , J., Rasi, S., Lehtinen, J., Rintala, J., 2012. Landfill gas La upgrading with pilot-scale water scrubber: performance assessment with absorption water recycling. Appl. Energy 92, 307e314. Li, Y., Zhang, W., Xu, J., 2014. Siloxanes removal from biogas by a lab-scale biotrickling filter inoculated with pseudomonas aeruginosa S240. J. Hazard. Mater. 275, 175e184. Liu, N., Shi, Y., Li, W., Xu, L., Cai, Y., 2014. Concentrations and distribution of synthetic musks and siloxanes in sewage sludge of wastewater treatment plants in China. Sci. Total Environ. 476, 65e72. Lu, Y., Yuan, T., Wang, W., Kannan, K., 2011. Concentrations and assessment of exposure to siloxanes and synthetic musks in personal care products from China. Environ. Pollut. 159, 3522e3528. Lu, Y., Yuan, T., Yun, S.H., Wang, W., Wu, Q., Kannan, K., 2010. Occurrence of cyclic and linear siloxanes in indoor dust from China, and implications for human exposures. Environ. Sci. Technol. 44, 6081e6087. Marine, S., Pedrouzo, M., Marce, R.M., Fonseca, I., Borrull, F., 2012. Comparison between sampling and analytical methods in characterization of pollutants in biogas. Talanta 100, 145e152. Nam, S., Namkoong, W., Kang, J.-H., Park, J.-K., Lee, N., 2013. Adsorption characteristics of siloxanes in landfill gas by the adsorption equilibrium test. Waste Manage 33, 2091e2098. Orhan, S., Berrin, T., 2012. Effect of persistent trace compounds in landfill gas on engine performance during energy recovery: a case study. Waste Manage 33, 74e80. Orthan, S., Berrin, T., 2013. Composition and source identification of deposits forming in landfill gas (LFG) engines and effect of activated carbon treatment on deposit composition. J. Environ. Manage. 128, 300e305. Oshita, K., Ishihara, Y., Takaoka, M., Takeda, N., Matsumoto, T., Morisawa, S., et al., 2010. Behaviour and adsorptive removal of siloxanes in sewage sludge biogas. Water Sci. Technol. 61, 2003e2012. Piechota, G., Hagmann, M., Buczkowski, R., 2012. Removal and determination of trimethylsilanol from the landfill gas. Bioresour. Technol. 103, 16e20. Ru¨cker, C., Ku¨mmerer, K., 2015. Environmental chemistry of organosiloxanes. Chem. Rev. 115, 466e524.
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Schweigkofler, M., Niessner, R., 2001. Removal of siloxanes in biogases. J. Hazard. Mater. 83, 183e196. Siefers, A., Wang, N., Sindt, A., Dunn, J., Mcelvogue, J., Evans, E., et al., 2010. A novel and cost-effective hydrogen sulfide removal technology using tire derived rubber particles. In: Proceedings of the Water Environment Federation 2010, pp. 4597e4622. Soreanu, G., Beland, M., Falletta, P., Edmonson, K., Svoboda, L., AlJamal, M., et al., 2011. Approaches concerning siloxane removal from biogasea review. Can. Biosyst. Eng. 53, 8.1e8.18. Sun, R.-D., Nakajima, A., Watanabe, T., Hashimoto, K., 2003. Decomposition of gas-phase octamethyltrisiloxane on TiO 2 thin film photocatalystsdcatalytic activity, deactivation, and regeneration. J. Photochem. Photobiol., A 154, 203e209. Surita, S.C., Tansel, B., 2015. Preliminary investigation to characterize deposits forming during combustion of biogas from anaerobic digesters and landfills. Renewable Energy 80, 674e681. Takuwa, Y., Matsumoto, T., Oshita, K., Takaoka, M., Morisawa, S., Takeda, N., 2009. Characterization of trace constituents in landfill gas and a comparison of sites in Asia. J. Mater. Cycles Waste Manage. 11, 305e311. Tansel, B., Surita, S.C., 2014. Differences in volatile methyl siloxane (VMS) profiles in biogas from landfills and anaerobic digesters and energetics of VMS transformations. Waste Manage 34, 2271e2277. Varaprath, S., Frye, C.L., Hamelink, J.L., 1996. Aqueous solubility of permethylsiloxanes (silicones). Environ. Toxicol. Chem. 15, 1263e1265. Wang, D., Alaee, M., Steer, H., Tait, T., Williams, Z.T., Brimble, S., et al., 2013a. Determination of cyclic volatile methylsiloxanes in water, sediment, soil, biota, and biosolid using largevolume injectionegas chromatographyemass spectrometry. Chemosphere 93, 741e748.
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Wang, J., Hu, Z., Chen, Y., Chen, Z., Xu, S., 2013b. Contamination characteristics and possible sources of PM10 and PM2.5 in different functional areas of Shanghai, China. Atmospheric Environment 68, 221e229. Wang, X., Lee, S., Sheng, G., Chan, L.Y., Fu, J.M., Li, X.-D., et al., 2001. Cyclic organosilicon compounds in ambient air in Guangzhou, Macau and Nanhai, Pearl River Delta. Appl. Geochem. 16, 1447e1454. Wheless, E., Pierce, J., 2004. Siloxanes in landfill and digester gas update. In: Proceedings of the 27th SWANA Landfill Gas Symposium, pp. 22e25. San Antonio, TX, March. Xu, L., Shi, Y., Cai, Y., 2013. Occurrence and fate of volatile siloxanes in a municipal wastewater treatment plant of Beijing, China. Water Res 47, 715e724. Xu, L., Xu, S., Zhang, Q., Xu, Q., Zhang, S., Cai, Y., 2018. Sources and fate of cyclic phenylmethylsiloxanes in one municipal wastewater treatment plant and biosolids-amended soil. Environ. Sci. Technol. 52, 9835e9844. Xu, L., Xu, S., Zhang, Q., Zhang, S., Tian, Y., Zhao, Z., et al., 2019. Chlorinated-methylsiloxanes in Shengli oilfield: their generation in oil-production wastewater treatment plant and presence in the surrounding soils. Environ. Sci. Technol. 53, 3558e3567. Xu, L., Xu, S., Zhi, L., He, X., Zhang, C., Cai, Y., 2017. Methylsiloxanes release from one landfill through yearly cycle and their removal mechanisms (Especially hydroxylation) in leachates. Environ. Sci. Technol. 12337e12346. Yu, M., Gong, H., Chen, Z., Zhang, M., 2013. Adsorption characteristics of activated carbon for siloxanes. J. Environ. Chem. Eng. 1, 1182e1187. Zhang, Z., Qi, H., Ren, N., Li, Y., Gao, D., Kannan, K., 2011. Survey of cyclic and linear siloxanes in sediment from the Songhua River and in sewage sludge from wastewater treatment plants, Northeastern China. Arch. Environ. Con. Tox. 60, 204e211.
Available online at www.sciencedirect.com
ScienceDirect www.elsevier.com/locate/jes
Investigation of volatile methyl siloxanes in biogas and the ambient environment in a landfill Ning Wang 1,2,3,*, Li Tan 3, Lianke Xie 4, Yu Wang 5,*, Timothy Ellis 6 1
School of Environmental Science and Engineering, Shandong Key Laboratory of Water Pollution Control and Resource Reuse, Shandong University, Jinan, 266237, China 2 Shanghai Key Laboratory of Atmospheric Particle Pollution and Prevention (LAP3), Shanghai, 200433, China 3 School of Municipal and Environmental Engineering, Shandong Jianzhu University, Jinan, 250100, China 4 State Grid Shandong Electric Power Company, Electric Power Science Research Institute, Jinan, 250100, China 5 Beijing Key Laboratory of Water Resources & Environment Engineering, China University of Geosciences (Beijing), Beijing, 100083, China 6 Department of Civil, Construction, and Environmental Engineering, Iowa State University, Iowa, USA
article info
abstract
Article history:
Landfill biogas is a potential alternative for fossil fuel, but the containing impurities, vol-
Received 12 November 2019
atile methyl siloxanes (simplified as siloxanes), often cause serious problems in gas tur-
Received in revised form
bines when applied to generate electricity. In this research, a collecting and analyzing
6 January 2020
method based on solvent adsorption and purge and trap-gas chromatography-mass
Accepted 6 January 2020
spectrometry was established to determine the siloxanes in biogas from a landfill in Jinan,
Available online 25 January 2020
China, and adjacent ambient samples, such as soil, air, and leachate of the landfill. The results showed that, octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane
Keywords:
(D5) accounted for 63% of total siloxanes; and without considering D4 and D5, the order of
Biogas
detected siloxanes in concentration was found relating to Gibbs free energies of molecules,
Siloxane
namely that higher abundant siloxane (except for D4 and D5) usually had lower Gibbs free
Landfill
energy. Additionally, the mass ratio between D4 and octamethyltrisiloxane (L3) in the
Purge and trap GC-MS
biogas varied with different garbage age in landfills, possibly revealing the breaking-down of larger siloxane molecules with time. The samples, which were collected from environmental samples adjacent to the landfill, such as soil, water, and air, presented much higher siloxane level than urban or rural area away from landfills. The current H2S scrubber of the landfill biogas could decrease the total siloxanes from 10.7 to 5.75 mg/m3 due to Fe2O3 and a refrigerant drier in a purification system and cyclic siloxanes were more easily removed than linear ones. © 2020 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.
* Corresponding authors. E-mail addresses: [email protected] (N. Wang), [email protected] (Y. Wang). https://doi.org/10.1016/j.jes.2020.01.005 1001-0742/© 2020 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.
j o u r n a l o f e n v i r o n m e n t a l s c i e n c e s 9 1 ( 2 0 2 0 ) 5 4 e6 1
Introduction Landfill is one of major means to deal with municipal solid wastes, while one common byproduct of this process is landfill gas, mostly consisting of methane and carbon dioxide (Nam et al., 2013). Currently, most of the landfill gas is usually used as fuel of reciprocating internal combustion engines or directly burned in a flare (Xu et al., 2019). Obviously, the latter treatment would risk of the leakage of unburned gas containing methane, which is a kind of powerful greenhouse gas; while the former should be a relatively economical and environment-friendly option. However, once burned, the trace impurity gas, siloxane, would form silica depositing on the combustion chamber inner surface and abnormally increase working temperature of the engine, therefore reducing efficiency. Siloxanes mostly originate from personal-care consumer products, antifoaming agents, and polymeric silicone products (as precursors), which are a group of linear or cyclic compounds with repeating silica-oxygen atom sequence surrounded by methyl groups (Xu et al., 2018). Long term exposure of siloxane causes engine failure by increasing the roughness of the cylinder and immobilizing the piston rez and Egusquiza, 2015). To rings in engines (Alvarez-Fl o prolong the life of the engines, it was necessary to prevent siloxane entering the biogas (Dewil et al., 2006). Researchers have tested and reported some methods to remove siloxanes from biogas, including photo-catalyzed decomposition with TiO2 thin films (Sun et al., 2003) or TiO2 coated particles (Lamaa et al., 2014); adsorptive removal with activated carbon (Oshita et al., 2010), silica gel (Oshita et al., 2010), and zeolite (Oshita et al., 2010); separation with siliconeerubber membranes (Ajhar et al., 2012); and biodegradation (Accettola et al., 2008; Li et al., 2014). Although the methods were efficient, most of them were based on siloxane removal under laboratory conditions; and little research focused on evaluating or improving current H2S or CO2 scrubbers for siloxane removal on economical purpose, which could be essential for practical large-scale usage. Besides the siloxane removal, researchers also investigated the distribution and profile of siloxanes in different landfill gas in different countries. Gas chromatography-mass spectrometry (GCeMS) was agreed for the quantification of siloxanes due to the low detection limit and high accuracy (Marine et al., 2012). However, there was no consensus on sampling methods or analysis methods. For sampling methods, Tedlar bags are easily to use but faced wall loss of target compounds (Marine et al., 2012); sorbent tubes have storage stability but expensive; traditional solvent method is robust for collecting of biogas but time consuming, normally more than three hours (Wheless and Pierce, 2004). In this research, a modified method, based on solvent method combining purge and trap (P&T)-GC-MS, was established, aiming at shrinking the collecting time and obtaining robust determination of siloxanes in different matrix, such as biogas, soil, and leachate. Compared to regular GC-MS, P&TGC-MS was more sensitive and dramatically decreased the sampling time of the solvent adsorption method. Solving the time-consuming problem and taking the advantage of solvent method, the method is a more economic option than sorbent
55
tube collecting, and more reliable than Tedlar bag method for siloxane analysis in biogas. Meanwhile, this P&T-GC-MS system is feasible for liquid and solid samples without changing the analysis instrument settings. Furthermore, since most related literatures forced on the pollution level and profiles of siloxane only in biogas from landfill, we performed a systematic investigation including siloxanes in surrounding environmental matrix, influence by biogas purifier, etc.
1.
Materials and methods
1.1.
Sampling
The sampling sites located in the Jinan No. 2 municipal solid waste landfill and disposal center, which dealt with 0.7 million tons of domestic garbage every year for Jinan, the capital city in Shandong Province, China and owned five 700 kw generators using biogas. The dumping in the landfill was a mixture of residential garbage, construction residue, and incineration ash. As shown in Fig. 1, 27 gas samples were collected from 10 biogas-observing wells and the main pipe conveying biogas to generator. Biogas samples were collected with two seriallyconnected impingers which were ice-rinsed in a cooler. Siloxanes in the biogas were absorbed and enriched in 10 mL methanol (AR, Hengxing Inc., China) in each impinger at a flowrate of 0.2 L/min, regulated by an active sampler (Model 224-PCXR8, SKC Inc., USA), for one hour. After absorption, the solution was accurately adjusted to 10 mL with methanol and partially transferred into vials at 4 C for further instrumental analysis. Three surrounding air sample was obtained via the same method. Surrounding soil and leachate samples (three for each matrix) were collected in clean metal box and widenecked bottle respectively and also stored at 4 C.
Fig. 1 e Biogas sampling sites (wells) in the landfill area.
56
j o u r n a l o f e n v i r o n m e n t a l s c i e n c e s 9 1 ( 2 0 2 0 ) 5 4 e6 1
1.2.
Instrument analysis
P&T-GC-MS applied in this research consisted of purge and trap device (P&T, Atomx Automated VOC Sample Preparation system, Teledyne Technologies Inc., USA) and gas chromatography-mass spectrometry (GC-MS, Trace™1300 þ ISQ, Thermo Fisher Scientific Inc., USA). The information for target siloxanes, such as quantization and characterized ions, retention time for GC-MS, etc., was listed in Table 1. The setting parameters were set as following:
Purge and Trap conditions Carrier gas Purge rate Purge duration preheating temperature for desorption Desorbing temperature Baking temperature Trapping temperature Sample inject volume GC-MS conditions Column IInjector temp Temperature program
Interface temperature Ion source temperature Scan mode Carrier gas
99.999% N2 40 mL/min 11 min 245 C 250 C for 2 min 330 C for 10 min ambient (<30 C) 10 mL sample solution in purge bottle filled with 40 mL water DB-5MS 40 C isothermal at 40 C for 2 min and programmed from 40 to 100 C at 20 C/min; isothermal at 100 C for 3 min and programmed from 100 to 200 C at 10 C/min; isothermal at 200 C for 5 min 250 C 250 C SIM 99.999% He
Forsoil samples, US Environmental Protection Agency method (EPA5030B and 5035) were referenced; 0.2 g soil were weighed and transferred into a 40-mL purge bottle with stir bar for purging and analysis.
1.3.
Calibration curve and QA&QC
The standard solution (100 mg/L, NEOCHEMA GmbH Co., Germany) containing 8 siloxanes was purchased and diluted into a series of solutions as external calibration curve,
including 0.2, 0.4, 0.6, 0.8, and 1.0 m/L in methanol (HPLC grade, Fisher, Inc., USA). Potential siloxane-containing personal products, such as cosmetics, were avoided to use by the analysts and silicon-containing cushion or seals materials were not allowed throughout the experiments. All glassware were cleaned with hexane and heated at 300 C overnight before employed. The filed blank and procedure blank test showed no detection of target chemicals, except for D4, D5, and D6, the amounts of which (0.03e0.09 mg/m3, 0.16e0.19 mg/m3, and 0.09e0.10 mg/m3,respectively as concentrations in biogas) were deducted from the final results for each instrument determination. The limit of detection (LOD) was 6.9e73.2 ng/L for liquid samples (including air, biogas, leachate in menthol), 7.7e50.5 ng/g for solid samples, which was triple times of S/N signal for 0.5 mg/L of standard solution. The recovery rates were 76.3%e113% for 0.5 mg/L of spike sample, 90.7%e122% for 1.0 mg/L, and 90.7%e116% for 2.0 mg/L. A repeatability test was carried out by determining five parallel 10 mL blank samples (level of siloxanes less than LOD), which was added a certain 0.5 mg/L of standard solution and the RSD for 8 siloxanes was 4.2%e9.5%. All the data referring to gas phase was adjusted to the values at standard conditions.
2.
Results and discussion
As show in Fig. 2, all eight specifies of target siloxane were detected in the biogas from landfill and the abundances of D4 and D5, accounting for approximately 63% of total siloxanes, were significantly larger than others. The results matched previous research, in which both siloxanes were present at the highest concentrations in biogas (Liu et al., 2014; Schweigkofler and Niessner, 2001; Wang et al., 2013a). All the siloxanes might mostly originate from evaporation or broken-up of polysiloxanes used as makeup ingredient or antifoaming additives in the anaerobic environment, and in which, the concentrations of linear siloxanes were normal higher or comparable to cyclic ones including D4 and D5 (Lu et al., 2011). It means that, in the commercial products, D4 and D5 are not significantly higher than other siloxanes, implying that the abundant D4 or D5 in the biogas could not all attribute to the evaporation from original sources. If considering the Gibb’s free energies of the siloxanes (Tansel and Surita, 2014), smaller molecules should be the more prevalent because of the lower energies barrier for molecule formation. Actually, the assumption was right for siloxanes only
Table 1 e Information of GC-MS analysis for siloxane standards. Component
1. 2. 3. 4. 5. 6. 7. 8.
Hexamethyldisiloxane Hexamethylcyclotrisiloxane Octamethyltrisiloxane Octamethylcyclotetrasiloxane Decamethyltetrasiloxane Decamethylcyclopentasiloxane Dodecamethylpentasiloxane Dodecamethylcyclohexasiloxane
Abbr.
Retention time (min)
R2
Qualitative mass to charge ratio (m/z)
Quantitative mass to charge ratio (m/z)
L2 D3 L3 D4 L4 D5 L5 D6
3.22 4.83 5.55 6.94 7.83 8.98 10.19 11.37
0.9903 0.9975 0.9986 0.9978 0.9931 0.9943 0.9915 0.9908
73, 147, 189, 149 96, 207, 208 73, 103, 221 281, 282 73, 207, 208 73, 267, 355 73, 147, 281 73, 341, 429
73 207 221 281 207 267 147 341
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Fig. 2 e Concentrations of siloxanes in biogas generated from the landfill.
if D4 and D5 were excluded. For example, the average concentration of L2 was larger than L3, then overwhelming L4 and L5 which were even below the LOD in some samples. The concentration of D3 was also found higher than D6 for the similar reason. The reason why D4 and D5 did not follow the rule could be that both siloxanes have more stable structure in molecule than others. Since the backbone of siloxane molecule is a chain of alternate O and Si atoms and the SieOeSi bond is relatively more flexible than OeSieO bond (Kim, 1991), once the O atoms locate inside a tetrahedral configuration, more energy is needed to bend the O atoms out. This could explain the high stability of D4 and D5, in which the O atoms faced insides the tetrahedral-like molecule and were protected from losing by the surrounding methyl groups (Tansel and Surita, 2014). Many previous studies support the structure of D4 and D5, both of which accounted for most siloxanes in landfill biogas. It is found that, excluding D4 and D5, other siloxanes decreased in concentration with the increasing molecular weight in different reports shown in Table 2. It is also found that the concentrations varied significantly from site to site, for example, D4 in Guangzhou, China was tenfold
€ skyla € , Finland. It could be attributed to more than that , in Jyva the variety of landfill age, waste profile, or even sampling method (Piechota et al., 2012; Ru¨cker and Ku¨mmerer, 2015; Wang et al., 2013b). Little systematic research was found in China and the current available data differed a lot for D3, D4, and total siloxane to our research, as shown in Table 2. The different garbage type may attribute to this since the landfill in this research contained lots of construction waste which could adsorb some siloxane or delaying the escaping, which needs further investigations. Nam et al. (Nam et al., 2013) has found that L3, L4, L5, and D6 formed when only L2, D4, and D5 passed an adsorption reactor loaded with activated carbon in lab test. Although the authors did not discuss the possibility of the catalysis of activated carbon, this may imply that besides the degrading of larger molecule siloxanes, small ones were able to polymerize over time (Soreanu et al., 2011); and the reaction might be reversible with the equilibrium decided by the Gibbs energy and molecular structure as mentioned above. In landfills, the biogas has to be purified to avoid CO2, moisture, and H2S before entering combustion chamber. This landfill employed Fe2O3 particles as H2S scrubber and dehydrated biogas with a refrigerant dryer. Although both of facilities were not designated to remove siloxane, it was noticed that the concentration decreased in the biogas in our research, as shown in Fig. 3. Based on the results of t-test (a ¼ 0.1), the concentrations of D4 (p ¼ 0.0598) and D5 (p ¼ 0.0674) were significantly decreased after traversing the scraping system, whereas other siloxanes were little influenced. The abnormal increase of L3 implies the possible breakdown of larger siloxanes passing the scrapper, which need more profound experiments and analysis on molecular level. After traversing the scrubber in this landfill, the ratio (0.08) between total linear siloxanes and cyclic ones was much lower than the value (0.24) before passing through the scrubber. It seemed that the cyclic ones were more easily trapped by the scrubbing process. The average remaining siloxane was total 5.75 mg/m3, larger than the suggested maximum values for microturbines (0.03 mg/m3), fuel cell
Table 2 e Concentrations of siloxanes in biogas of this research and other references. Concentrations of siloxanes (mg/m3) in landfill biogas
Sampling sites L2 This research € skyla € , Finland (La € ntela € et al., Jyva 2012) Istanbul, Turkey (Orhan and Berrin, 2012) Miami, USA (Tansel and Surita, 2014) Miami, USA (Surita and Tansel, 2015) Istanbul, Turkey (Orthan and Berrin, 2013) Guangzhou, China (Wang et al., 2001) China A (Takuwa et al., 2009) China B (Takuwa et al., 2009) Japan (Takuwa et al., 2009) Franki, Poland (Piechota et al., 2012)
D3
L3
D4
L4
D5
L5
D6
0.837 ± 0.151 0.874 ± 0.180 0.614 ± 0.089 4.159 ± 0.627 0.506 ± 0.104 2.896 ± 0.448 0.122 ± 0.041 0.659 ± 0.153 0.565 ± 0.154 0.129 ± 0.083 0.023 ± 0.007 1.083 ± 0.276 0.006 ± 0.004 1.135 ± 0.293 ND e <1.0
e
<0.5
4.9e6.8
<0.5
2.3e3.7
e
<1.0
0.680 ± 0.048 0.515 ± 0.007 0.097 ± 0.004 2.155 ± 1.135 ND
1.750 ± 0.750 ND
0.089 ± 0.008
1.35
0.39
0.12
5.78
0.04
2.98
ND
0.33
1.6 ± 0.1
<1.0
<0.5
5.0 ± 0.2
<0.5
2.9 ± 0.1
e
<1.0
e
3.6 ± 3.1
e
11.4 ± 5.5
e
e
e
Total Total Total Total
2.06e6.88 (L2, L3, L4, L5, D3, D4, D5, D6) 2.51e4.49 (L2, L3, L4, L5, D3, D4, D5, D6) 0.28e51.7 (L2, L3, L4, L5, D3, D4, D5, D6) 38.9 ± 0.1 (L2, D3, D4, D5, D6)
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Fig. 3 e Concentrations of siloxanes in biogas before and after the biogas purification system of the landfill.
engines (0.1 mg/m3) and gas turbines (0.7 mg/m3). However, it might be acceptable for internal combustion engines (5~28 mg/m3) (Wheless and Pierce, 2004). To reduce the potential risks and maintenance cost, the additional specific removal system for siloxane was necessary, and the suggest rez and was also agreed by most researchers (Alvarez-Fl o Egusquiza, 2015, Takuwa et al., 2009; Cabrera-Codony et al., 2014). Adsorption could be one possible mechanism for this scrapping system. Although little research referred to the coadsorption of siloxanes and other undesired components in biogas, it was common that the silicon compounds were easily trapped on or in the solid scrubbers. Based on experiments, Cabrera-Codony (Cabrera-Codony et al., 2014) found that CH4 and CO2 competed with D4 on the adsorption into activated carbon. It implied that the adsorption mechanism of siloxane could, to some extent, be similar to CH4 or CO2. It has also been
Table 3 e Concentrations of ambient siloxanes of this research and other references. Sampling sites
Concentrations of siloxane L2
D3
L3
D4
L4
D5
L5
D6
3
Air samples (mg/m ) Ambient air on the 0.0248 (0.0127e0.0358) surface of this landfill Ambient air in a e landfill of Guanzhou, China (Wang et al., 2001) Ambient urban air e in Guanzhou, China (Wang et al., 2001) Ambient industrial e air in Guanzhou, China (Wang et al., 2001) Water samples (mg/L) Leachate from this a landfill Municipal e wastewater in January (Xu et al., 2013) Solid samples (mg/g) Covered soil on the 0.418 (0.323e0.574) surface of this landfill Surface sediment e in river (Zhang et al., 2011) Indoor dusts (3e5 e electrical devices in room) (Lu et al., 2010) Personal care e products (Lu et al., 2011) a
0.246 (0.150e0.359) 0.117 (0.086e0.151) 0.311 (0.219e0.458)
a
a
0.273 (0.157e0.487)
a
0.0036
e
0.0114
e
e
e
e
0.0029
e
0.0009
e
e
e
e
0.0061
e
0.0135
e
e
e
e
10.4 (9.25e12.48)
51.3 (45.1e59.5)
34.6 (33.8e35.5)
a
a
58.6 (50.2e69.4)
a
0.48
ND
2.89
0.07
3.29
4.19 (2.45e5.54)
1.28 (1.02e1.62)
1.33 (0.99e1.45)
a
a
e
e
0.0072
e
0.0527 e
e
e
0.0104
0.0258 0.0202 0.0017
e
e
5.24
e
Unreliable data due to failed standard calibration curves..
29.7
e
1.56 (1.21e2.01)
e
2.20
a
0.0761
0.0175
18.3
j o u r n a l o f e n v i r o n m e n t a l s c i e n c e s 9 1 ( 2 0 2 0 ) 5 4 e6 1
found siloxane was co-adsorbed when removing H2S with rubber particles (Siefers et al., 2010). Yu et al. (2013) found that adsorption capacity of D4 on activated carbon was positively related to BET surface area and micropore volume. But, Nam et al. (2013) reported that carbon-based adsorbents with well-developed micropore were more efficient for L2 than non-carbon-based adsorbents with macropore, which was reverse for D4 and D5. The later opinion was also supported by other researchers (Oshita et al., 2010). It could be deducted that the adsorption depended on molecular size and pore size. The pore size of Fe2O3 particles was much less than those carbon-based adsorbent, which might attribute to the significant removal of D4 and D5 in this research. Additionally, it was reported that non-carbon adsorbents might affect the conversion of siloxanes (Nam et al., 2013). It could explain why the higher L3 concentration was observed after desulfurizing. Another reason for decrease of the siloxanes could be the refrigerant drier, which aimed to remove the moisture from biogas. Freezing temperature and proper condensation system could decrease partial siloxanes in biogas and the efficiency depended on how low the temperature was (Soreanu et al., 2011). A previous laboratory study had shown that a refrigeration system could remove 20% siloxanes when removing 90% moisture (Piechota et al., 2012). Therefore, the performance of the refrigerant drier might contribute the loss of some siloxanes but not all of them because approximately as much as half siloxanes in the scrapper system were stopped and the operating temperature was not low enough to reach the efficiency (more than 50%). The concentrations of siloxanes in air (0.972 mg/m3) and soil (8.78 mg/g) on the landfill, and leaching water (155 mg/L) were also determined and recorded in Table 3, as well as other reports of siloxanes in various environments by other groups. Unlike landfill biogas, D4 did not dominate among the detected siloxanes in the environmental matrix samples, which were comparable with each other, and the profile was more like in artificial siloxane sources, such as personal care products, as stated previously. This implied that the siloxanes did not stayed in the matrix for a very long time or experience degradation. This was unlike the siloxanes in biogas, where D4 was much higher than others possibly due to the longer decomposition time (Gaj and Pakuluk, 2015). The concentrations of siloxanes (0.972 mg/m3) in the air over landfill were much larger than the reported values in ambient air (0.0038~0.0196 mg/m3 (Wang et al., 2001)), which could attribute to the possible evaporation from compacted waste rather than biogas leaking because of the different profile of siloxane species. Meanwhile, the concentrations of siloxanes from the leachate (155 mg/L) or covered soil (8.78 mg/g) were also much larger than those in reported municipal wastewater (8.93 mg/L (Xu et al., 2013))or dust/sediment (0.0756 mg/g (Lu et al., 2010)/0.136 mg/g (Zhang et al., 2011)). Due to the low aqueous solubility (Varaprath et al., 1996), the concentrations of siloxanes were very low in natural water. Higher concentration in the leachate also support the assumption that the siloxanes originated from the landfill, since the distance to pollution source was an increasing factor for D4-D6 in soil (Xu et al., 2017). All these facts suggested that landfill area was a potential important source of siloxane emission. Through the half-year sampling period, eight species of siloxanes were not always simultaneously detectable for each
59
sample, as shown in Table 3. To minimize unexpected deviation, L3 and D4, which were both successfully detected in all samples, were selected to represent linear and cyclic siloxane, respectively, in the following discussions. In the landfill, each sampled well was marked with a serial number and the smaller number implying an older garage filling time. Then, the profiles, shown as in Fig. 4, referred to the temporal (age of wells) variation of L3 and D4. The concentrations of L3 were lower than D4 in all the wells due to the reason discussed above. It was also found concentrations obtained from “older” well, like #13 and #14 wells, where garbage was filled in December 2015, were relatively lower than other recent wells, except for well #53 and #54, which might attribute to the low sampling frequency (only one sample from each well). Landfill age was reported to influence the concentration of siloxanes in biogas (Piechota et al., 2012); D4 and D5 from old landfills were found lower than recent ones, possibly resulting from more siloxane product used in recent life by residents or exhaustion of siloxanes in landfilled garbage over time (Badjagbo et al., 2010). According to our observation in this work, the latter explanation was preferred because the time span between the landfill well #13 and #54 is less than one year which is not long enough for society adapting new siloxane-contained products. As artificial compounds, polysiloxanes decreased through the landfill process without alternative sources. Less siloxane escaped out via biogas as time lapsing, due to the smaller biogas production and original pressure partition, both of which decreased the opportunity to effectively diffuse into biogas. Besides, as show in Fig. 4, the ratio D4/L3 was found higher from some “middle age” wells (#38 and #39) than earlier ones (#13 and #14) and later ones (#51-#53). It might attribute to the fact that largemolecule siloxane was broken into more stable one like D4 over a certain time, but the degradation was slow down and more siloxane escaped after a longer time. The ratio in well #54 was abnormal possibly due to the lack of replicated tests. Unlike the pervious report that concentrations of siloxanes in biogas from sewage sludge varied with the air temperature (Oshita et al., 2010), in this half-year study, the environmental temperature seemed not to influence the siloxanes as shown in Fig. 5. It perhaps attributed to the relatively stable temperature in the landfill.
Fig. 4 e Comparison of the concentrations between L3 and D4 in biogas from different sampling biogas wells.
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j o u r n a l o f e n v i r o n m e n t a l s c i e n c e s 9 1 ( 2 0 2 0 ) 5 4 e6 1
Fig. 5 e Comparison of the concentrations between L3 and D4 at different ambient temperature during sampling.
3.
Conclusions
A P&T-GC-MS method was developed to analyze siloxanes could decrease the sampling time of solvent method for biogas and enable the same analysis procedure for samples in different matrix, such as air, biogas, soil, and leachate from the same landfill, which potentially improve the reliability of the comparison between the samples. Smaller molecule of siloxanes had higher concentrations in biogas because of lower Gibb’s free energies, except for D4 and D5, which were stable due to the special molecular structure, resulting in high concentrations (63% of total siloxanes). More recent landfill wells produced more siloxanes in biogas and the concentrations in this investigation were different from reports in other sites in the world. Cyclic ones tended to be partially stopped by the current H2S scrubber and a refrigerant drier. It was possible to improve current scrubbing system for coexisting siloxanes to reduce the cost. Surrounding environmental matrix, such as air, leachate, and soil, was significantly influenced by the siloxanes from the landfill by escaping from waste rather than biogas due to the lower D4/L3 ratio in the matrix. The ratio could be used preliminary to estimate the “age” of landfill wells.
Acknowledgment This work was supported by the National Natural Science Foundation of China (No. 21407097) and Supported by Shandong Key Laboratory of Water Pollution Control and Resource Reuse (No. 2019KF14) and the Opening Project of Shanghai Key Laboratory of Atmospheric Particle Pollution and Prevention (LAP3) (No. FDLAP17001).
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