Investigation of volatile methyl siloxanes in biogas and the ambient environment in a landfill
Graphical abstract
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 rings in engines (Álvarez-Flórez and Egusquiza, 2015). To 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 silicone–rubber 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 (GC–MS) 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&T-GC-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 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.
Section snippets
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
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
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
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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