Utilizzo di tecniche isotopiche GC/C/IRMS per il monitoraggio del biogas in discarica: stato dell'arte e prospettive future

Methane is a trace gas in atmosphere, generally averaging at the 1.7 ppmv, but is commonly found in naturally occurring sediments, as the byproduct of thermal degredation of organic matter (natural gas) and as a byproduct of anthropogenic organic waste. The main sources of atmospheric methane in industrialized regions are: 1) natural gas (pipeline, stored hydrocarbon reservoirs, wells); 2) biogas (landifill gas); 3) incomplete combustion of biomass or fossil fuels. Resolving the source of methane in soil or at soil-atmosphere interface is not trivial, because the different CH4 sources often occur in concert. Landfill methane emissions are typically produced by organic waste degradation. They occur in association with CO2 emissions (carbon dioxide and methane are produced in roughly similar amounts upon waste degradation) and generally continue long after landfill capping, due to biochemical reactions at different depths in the landfill. These emissions are of great concern for landfill managing, and their characterization is crucial to define environmental policies and land use regulations. Stable isotope geochemistry can be used to characterize biogenic gas, and more in detail, landfill biogenic gas from biogenic gas from different sources. At now only few isotopic data on landfill CH4 have been reported in literature: the ?13C and ?D values varying from approximately -40 to -60 ?, and from -260 to -340 ?, respectively. The carbon and hydrogen isotopic composition of methane fingerprint the effects of isotopic fractionation upon methane released in the atmosphere, such as aerobic oxidation by methanotrophic bacteria. The ?13C and ?D values of CH4 collected from landfill in static chambers typically approach the upper range limits, and they become distinctively heavier due to methane oxidation. The difference between the isotopic composition of emitted gas and gas from the anaerobic zone can be used to estimate the methane oxidation in the soil. New analytical equipment now available allows for rapid sample processing and isotopic characterization of small gas samples. Gas-chromatographic procedures for gas separation coupled with combustion units and placed in line with mass spectrometers (GC-C-IRMS) allow for the so-called Compound Specific Isotope Ratio Mass Spectrometry (CS-IRMS). Monitoring the stable isotope composition of landfill gas is therefore a rapid and efficient method to control reactions in landfill covers and to estimate the contribution of anthropogenic methane to atmospheric pollutants.