Analytical modelling of stable isotope fractionation of volatile organic compounds in the unsaturated zone

Abstract

a b s t r a c tAnalytical models were developed that simulate stable isotope ratios of volatile organic compounds (VOCs) near a point source contamination in the unsaturated zone. The models describe diffusive transport of VOCs, biodegradation and source ageing. The mass transport is governed by Fick's law for diffusion. The equation for reactive transport of VOCs in the soil gas phase was solved for different source geometries and for different boundary conditions. Model results were compared to experimental data from a one-dimensional laboratory column and a radial-symmetric field experiment. The comparison yielded a satisfying agreement. The model results clearly illustrate the significant isotope fractionation by gas phase diffusion under transient state conditions. This leads to an initial depletion of heavy isotopes with increasing distance from the source. The isotope evolution of the source is governed by the combined effects of isotope fractionation due to vaporisation, diffusion and biodegradation. The net effect can lead to an enrichment or depletion of the heavy isotope in the remaining organic phase, depending on the compound and element considered. Finally, the isotope evolution of molecules migrating away from the source and undergoing degradation is governed by a combined degradation and diffusion isotope effect. This suggests that, in the unsaturated zone, the interpretation of biodegradation of VOC based on isotopic data must always be based on a model combining gas phase diffusion and degradation.

Chlorinated Ethene Degradation Rate Coefficients Simulated with Intact Sandstone Core Microcosms

“…The isotopes were modeled based on the governing equations described above. The rate coefficients were controlled by the kinetic isotope effect (KIE): , where k l and k h are rate coefficients for the light and heavy isotopes, respectively, and ε is the enrichment factor, indicating that the rate for the heavy isotope is slower by an average of ε.…”

“…In the model, TCE and cDCE molecules with different stable isotopic composition (i.e., 12 C-TCE and 13 C-TCE, 12 C-cDCE and 13 C-cDCE) were treated as separate species. 38 The isotopes were modeled based on the governing equations described above. The rate coefficients were controlled by the kinetic isotope effect (KIE): 38,39…”

Isotope Effects on the Vaporization of Organic Compounds from an Aqueous Solution–Insight from Experiment and Computations

“…In contrast, when an initial (liquid) state becomes enriched in lighter isotopologues, meaning that heavier moieties are more prone to evaporation, then we call such an effect inverse. Previous studies have substantiated the general rule that an observable kinetic isotope fractionation of a liquid phase-air transfer will reflect the bottleneck of the overall process. ,− If diffusion through the liquid phase is rate-limiting, the observable isotope effect that reflects a liquid phase diffusion will typically be very small. , If, however, diffusion through a stagnant air layer above the water surface is rate-limiting, the overall isotope effect may be non-negligible, and it will be a composite of the equilibrium air/solvent isotope effect and the kinetic isotope effect of diffusion through air. While isotope effects of diffusion in the gas phase can be estimated based on isotopologue masses (see, e.g., Bouchard 2011), the equilibrium isotope effect of solvent-air partitioning is less straightforward to predict.…”

“… 19 , 22 If, however, diffusion through a stagnant air layer above the water surface is rate-limiting, the overall isotope effect may be non-negligible, and it will be a composite of the equilibrium air/solvent isotope effect and the kinetic isotope effect of diffusion through air. While isotope effects of diffusion in the gas phase can be estimated based on isotopologue masses (see, e.g., Bouchard 2011 16 ), the equilibrium isotope effect of solvent-air partitioning is less straightforward to predict. Since such partitioning isotope effects arise from changes in internal motions of a compound that are caused by specific interactions with surrounding solvent molecules, one may expect a correlation with the nature of these noncovalent interactions.…”

Carbon Isotopic Fractionation via Diffusion in a Coarse Material

“…Several studies on the fate of contaminants (e.g., volatile organic compounds) in the vadose zone have addressed different effects such as diffusion, dilution, and biodegradation by modeling approaches (Bouchard et al, 2011(Bouchard et al, , 2008Hunkeler et al, 2004;Pasteris et al, 2002;Van De Steene & Hohener, 2009). Also, the stable carbon isotopic composition of carbon dioxide in soil gas (Cerling et al, 1991) has been determined.…”