Characteristic of methane oxidation coupled to denitrification in cover soils of landfill

Abstract: Methane (CH4) is widely concerned because of its strong greenhouse effect. The CH4 generated from anaerobic biodegradation of waste in landfills is a significant source of atmospheric CH4. The CH4 generated transits the soil cover where it maybe be partly oxidized by CH4-oxidizing bacteria. Therefore, the landfill soil cover has a function on reducing CH4 emissions. On the other hand, N-pollution of leachate is the problem that must be controlled. According to documents, CH4 is a potentially inexpensive, widely available electron donor for biological denitrification of landfill leachate. Although no known methanotroph is able to denitrify, various consortia of microorganisms using methane as the sole carbon source which carry out denitrification both aerobically and anaerobically. Aerobic methane-oxidation coupled to denitrification (AME-D) is accomplished by aerobic methanotrophs oxidizing methane and releasing soluble organics that are used by coexisting denitrifiers as electron donors for denitrification. The work aimed to investigate the characteristics of AME-D in the cover soil of landfill. Thus, the batch assays were performed to investigate the effects of different factors such as CH4, O2 and NO3--N with different contents on AME-D. The concentration levels of CH4 were 0, 10, 20 and 30%, those of O2 were 0, 5,10 and 20%, and those of NO3--N were 0, 100, 200 and 300 mg/kg. In the assays, we first added 10 g of soil and NO3--N solution into 50 mL serum bottle with gas-tight rubber stoppers, then displaced the upper air in the bottle with argon gas and injected corresponding CH4 and O2 with syringe to make up the initial content, and finally incubated them at 30℃ for 7 d. The CH4, O2, CO2, N2 and N2O of gas samples in the third and seventh day were analyzed with a gas chromatograph (Fuli 9790, equipped with tandem connect of PorparkQ and 5? molecular sieve packed columns, and a thermal conductivity detector). Ar was used as carrier gas and its flow rate was 30 mL-1·min. Injector, oven, and detector temperatures were 50, 50, and 85℃, respectively. The results showed that the landfill cover soil was favorable for AME-D. The contents of CH4, O2 and NO3--N had significant influences on the CH4 oxidation (p<0.05, and followed the order of CH4>O2>NO3--N. Furthermore, the CH4 and O2 took significant interactions (p<0.05). The removal of CH4 increased as the increase of CH4 and O2, and was positively correlated with O2 concentration (n=144, r=0.786, p<0.01). The factor that had significant influences on the production of N2 was not NO3--N but CH4 and O2 (p<0.05). However, NO3--N and O2, CH4 and O2 took significant interactions (p<0.01) on the production of N2. The addition of NO3--N enhanced the product of N2 under low O2 content (<5%), while it took no effect under low O2 content (≥10%). The effects of C/O ratios on AME-D depended on the contents of O2 and CH4, the optimal range of C/O ratio was from 0.5 to 1.0. In the experiment, the AME-D coupled well as the contents of CH4, O2 and NO3--N were 20%, 20% and 100 mg/kg.