Effect Of Toluene On CH₄ Oxidation Of Landfill Cover Soils And Its Mechanism

Landfill cover soil is the environmental interface between landfill gas (LFG) and the atmosphere, which plays an important role in reducing CH4 emission from landfills. In addition to CH4 and CO2, there are also many non-methane volatile organic compounds (NMVOCs) emit from landfills simultaneously. Although the concentration of NMVOCs in LFG is less than 1%, most of them are identified as toxic compounds, including benzene, toluene and trichloroethylene, and may have great effect on the growth and activity of microorganisms in landfill cover soils. Moreover, some methanotrophs can co-oxidize and transform halogenated and aromatic hydrocarbons. Thus, it is important to understand the response of CH4 oxdation activity to NMVOCs as well as its mechanism, which will be of considerable importance for cost-effective operation and optimization of landfill cover soils to mitigate CH4 and NMVOCs emissions from landfills. Toluene was chosen as the typical compound of benzenes in NMVOCs in this study. The function of microorganisms including CH4 oxidation and toluene degradation and the microbial community was investigated in landfill cover soils (LCS) and waste biocover soils (WBS) by using the molecular biotechniques such as real-time quantitative PCR (qPCR), cloning, terminal restriction fragment length polymorphism (T-RFLP) and stable isotope probing (SIP). The effect of toluene on the methanotrophic growth, CH4 oxidation activity, CH4-C flow and the functional gene expression was also studied. Meanwhile, simulated soil columns were used to study the transportation and emissions of CH4 and toluene in landfill covers. Based on the test of CH4 and toluene biodegradation in landfill cover microcosms, an analytical model was developed and employed to calculate CH4 and toluene emissions through landfill covers. The major results and conclusions are as follows:(1) The effect of toluene on the function and microbial community of cultures enriched with CH4 and/or toluene was investigated. Compared with the culture enriched only with CH4 or toluene, a higher environmental stress occurred in the culture enriched with CH4 and toluene. Higher contents of extracellular polysaccharose and extracellular protein were also detected in the culture enriched with CH4 and toluene, which inhibited the CH4 oxidation and toluene degradation. The co-existence of toluene and CH4 could lead to the decrease of the microbial diversity, due to its relatively higher selective stress to microbial community. Methylosinus was the dominant methanotrophs in the culture enriched with CH4 and toluene.(2) Methylosinus sporium was used as a model methanotroph to investigate the effect of toluene on the methanotrophic growth, CH4 oxidation activity and expression of the functional genes (pmoA and mmoX). The growth and CH4 oxidation activity of Methylosinus sporium was not obviously different at the toluene concentration of less than or equal to 300 mg m-3, while it was inhibited at the toluene concentration of more than 2000 mg m-3. When the toluene concentration was about 80000 mg m-3, the growth and CH4 oxidation of Methylosinus sporium was completely inhibited. The co-existence of toluene (～1000 mg m-3) and CH4 could also enhance the expression of sMMO in Methylosinus sporium in the short term (-40 h), which enhanced the CH4 oxidation activity of Methylosinus sporium to certain extent. The detection of toluene degradation intermediates by Methylosinus sporium and the molecular docking calculation showed that toluene could be mainly bound with sMMO by electrostatic interaction and hydrophobic effect and be oxidizd to o-cresol.(3) Compared with LCS, the effect of toluene on the CH4 oxidation activity and the methanotrophic community was investigated in WBS. In a short term, CH4 oxidation could be enhanced in the WBS samples at the toluene concentrations of less than or equal to 4000 mg m-3, while it would be inhibited in the LCS samples at the toluene concentration of 100 mg m-3. The completely inhibitory concentration of toluene on CH4 oxidation was-80000 mg m"3 in the WBS samples, which was about 10 times of that in the LCS samples. But the significant inhibition was observed in the WBS samples after exposed to the toluene concentration of >1000 mg m-3 for more than 50 days.After exposure to CH4 and toluene, methanotrophic abundance in both LCS and WBS was increased. Compared with the LCS samples, the methanotrophic abundance was 1-2 orders higher in the WBS samples. In the first 14 days, the methanotraphic abundance could be significantly enhanced in the WBS samples exposed to CH4 and the toluene concentration of 50-2000 mg m-3, but it significantly decrased in both WBS and LCS after exposure to CH4 and toluene for 61 days. Type I methanotrophs Methylocaldum, Methylobacter and Methylococcaceae and type II methanotrophs Methylocystis and/or Methylosinus were the dominant methanotrophs in the experimental soils, of which Methylocaldum dominated in the WBS samples and Methylocystis and/or Methylosinus dominated in the LCS samples. Compared with type II methanotroph Methylocystis and/or Methylosinus, type I methanotroph Methylocaldum might be more sensitive to the toxicity of toluene and/or its degradation intermiates.(4) The effect of toluene on the CH4-C flow and the functional microorganism in the landfill cover soils with different O2 concentration was studied in the landfill cover soil SIP-microcosms. The results showed that the co-existence of toluene and CH4 and low O2 concentration (2.5%) could enhance the conversion efficiency of CH4-C to organic matter-C and biomass-C, and thus led to the growth of microorganisms in the landfill cover soils.The dominant bacteria in the landfill cover soils beleng to Proteobacteria and Bacteroidetes. Compared with the high O2 concentration, toluene had greater effect on the microbial community of the landfill cover soils under the low O2 concetnration. The bacteria belonging to Proterbacteria, Gemmatimonadetes and Firmicutes were the major ones in CH4-C utilization in the experimental soils. Among them, type I methanotrophs including Methylobacter, Methylocaldum, Methylosarcina and type II methanotrophs Methylosinus were active in the landfill cover soils, of which type I methanotorphs were more abundant.(5) The transportation of CH4 and toluene in landfill covers and their removal mechanism was studied in simulated soil columns. The CH4 removal efficiency was about 45.4%-73.4% and 22.9%-29.6%, respectively, in the WBS and LCS columns at the LFG influent rate of 1.6 m3 m-2 d-1 and the toluene concentrations of 0-1336 mg m-3. The toluene removal efficiency was 98.7%-100% and 96.9%-98.1%, respectively, in WBS and LCS columns. The CH4 oxidation activity of both WBS and LCS was significantly inhibited by toluene, of which stronger inhibitive effect was observed in WBS. The toluene distribution was mainly affected by diffusion in the deeper depths of more than 21 cm, while it was influenced by both gas diffusion and biodegradation at the depths of 13-21 cm.Based on the diffusion, advection and first-order biodegrading reaction of CH4 and toluene in landfill covers, analytical model of CH4 and toluene emissions through landfill covers was constructed. The model fitted well with the measured values.