New isotopic (δ13CCO2-δ13CCH4) fractionation factor limits and chemical characterization of landfill gas

- We made 103 analyses of chemical and stable isotopic content of biogas.
- New fractionation factor limits bounding main methanogenesis pathways were defined.
- We have tried to recognize waste fermentation processes inside landfill body.
- Chemical and isotopic analyses do not give overlapping information.

103 gas samples collected at 7 different Italian landfills have analyzed to define their chemical (CO2, CH4, N2 and O2 + Ar,) and isotopic (δ13CCO2, δ13CCH4, δ2HCH4) composition. Chemical analysis was carried out by means of gas chromatography (Thermal Conductivity Detector - TCD and Flame Ionization Detector - FID), while isotopic parameters were analyzed by means of gas mass spectrometry.The N2/(O2 + Ar) and CH4/CO2 ratios have been utilized in order to obtain a primary classification of the samples. Following this approach, 7 groups, corresponding to different redox conditions and various degrees of interaction with air, have been individuated. This classification has been compared with isotopic indications.Isotopic contents (δ13CCO2 and δ13CCH4) have been processed to investigate fermentation processes by means of statistical data processing of fractionation factor (α), consisting in partitioning technique. This approach leads to a new definition of the fractionation factor (α) limits that define the domains of existence of major fermentation processes. These limits correspond to the maximum value expected for CO2 absorption and/or CH4 massive oxidation processes (α = 1.049), and to the minimum fractionation factor expected for CO2 biological reduction pathway (α = 1.067).Obtained results indicate that it is possible to assume which fermentation processes take place in LFG generation by combining information derived from the interpretation of chemical and isotopic data. Anyway, it is worth to note that the complexity of LFG generation processes, involving several matrices and a multitude of microbial associations, cannot be unwisely investigated just by chemical analysis or by a single isotopic parameter.