Flux measurements of benzene and toluene from landfill cover soils

Carbon dioxide and CH4, C6H6 and C7H8 fluxes from the soil cover of Case Passerini landfill site (Florence, Italy) were measured using the accumulation and static closed chamber methods, respectively. Results show that the CH4/CO2, CH4/ C6H6 and CH4/C7H8 ratios of the flux values are relatively low when compared with those of the ‘pristine’ biogas produced by degradation processes acting on the solid waste material disposed in the landfill. This suggests that when biogas transits through the cover soil, CH4 is affected by degradation processes activated by oxidizing bacteria at higher extent than both CO2 and mono-aromatics. Among the investigated hydrocarbons, C6H6 has shown the highest stability in a wide range of redox conditions. Toluene behaviour only partially resembles that of C6H6, possibly because de-methylation processes require less energy than that necessary for the degradation of C6H6, the latter likely occurring via benzoate at anaerobic conditions and/or through various aerobic metabolic pathways at relatively shallow depth in the cover soil where free oxygen is present. According to these considerations, aromatics are likely to play an important role in the environmental impact of biogas released into the atmosphere from such anthropogenic emission sites, usually only ascribed to CO2 and CH4. In this regard, flux measurements using accumulation and static closed chamber methods coupled with gas chromatography and gas chromatography– mass spectrometry analysis may properly be used to obtain a dataset for the estimation of the amount of volatile organic compounds dispersed from landfills.