Environmental Factors Influencing Attenuation of Methane and Hydrochlorofluorocarbons in Landfill Cover Soils

The influence of different environmental factors on methane oxidation and degradation of hydrochlorofluorocarbons (HCFCs) was investigated in microcosms containing soil sampled at Skellingsted Landfill, Denmark. The soil showed a high capacity for methane oxidation resulting in a maximum oxidation rate of 104 μg CH₄ g⁻¹ h⁻¹ and a low affinity of methane with a half-saturation constant of 2.0% v/v. The hydrochlorofluorocarbons HCFC-21 (dichlorofluoromethane) and HCFC-22 (chlorodifluoromethane) were rapidly oxidized and the oxidation occurred in parallel with the oxidation of methane. The maximal HCFC oxidation rates were 0.95 and 0.68 μg g⁻¹ h⁻¹ for HCFC-21 and HCFC-22, respectively. Increasing concentrations of HCFCs resulted in decreased methane oxidation rates. However, compared with typical concentrations in landfill gas, relatively high HCFC concentrations were needed to obtain a significant inhibition of methane oxidation. In general, the environmental factors studied influenced the degradation of HCFCs in almost the same way as they influenced methane oxidation. Temperature had a strong influence on the methanotrophic activity giving high Q ₁₀ values of 3.4 to 4.1 over the temperature range of 2 to 25°C. Temperature optimum was around 30°C; however, oxidation occurred at temperatures as low as 2°C. A moisture content of 25% w/w yielded the maximum oxidation rate as it allowed good gas transport together with sufficient microbial activity. The optimum pH was around neutrality (pH = 6.5–7.5) showing that the methanotrophs were optimally adapted to the in situ pH, which was 6.9. Copper showed no inhibitory effect when added in relatively high concentrations (up to 60 mg kg⁻¹), most likely due to sorption of copper ions to soil particles. At higher copper concentrations the oxidation rates decreased. The oxidation rates for methane, HCFC-21, and HCFC-22 were unaltered in ammonium-amended soil up to 14 mg kg⁻¹ Higher ammonium concentrations inhibited the oxidation process. The most important parameters controlling oxidation in landfill cover soil were found to be temperature, soil moisture, and methane and oxygen supply.