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Effect of Temperature and Oxidation Rate on Carbon-isotope Fractionation during Methane Oxidation by Landfill Cover Materials

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The quantification of methane oxidation is one of the major uncertainties in estimating CH4 emissions from landfills. Stable isotope methods provide a useful field approach for the quantification of methane oxidation in landfill cover soils. The approach relies upon the difference between the isotopic composition of oxidized gas at the location of interest and anaerobic zone CH4 and knowledge of αox, a term that describes the isotopic fractionation of the methanotrophic bacteria in their discrimination against 13CH4. Natural variability in αox in different landfill soils and the effect of temperature and other environmental factors on this parameter are not well defined. Therefore, standard determinations of αox, batch incubations of landfill cover soils with CH4, were conducted to determine αox under a variety of conditions. When these results were combined with those of previous landfill incubation studies, the average αox at 25 °C was 1.022 ± 0.0015. αox decreased with increasing temperature (−0.00039 αox °C−1) over the temperature range of 3−35 °C. αox was found to be higher when determined after CH4-free storage and declined following CH4 pretreatment. αox declined nonlinearly with increasing methane oxidation rate, Vmax. Methane isotopic fractionation is defined as a function of temperature and rate during microbial oxidation; the technique is widely used to evaluate landfill cover efficiency.

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