Stable Isotope Fractionations of Chlorinated Ethenes Associated with Physical Processes
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Chlorinated ethenes are the widespread source of groundwater contamination, which mostly originate from industrial and dry-cleaning facilities. Since chlorinated ethenes are denser than water, they sink below the water table if spilled in large quantities and accumulate on top of low-permeability zones and bedrocks. In the subsurface, these contaminants undergo different processes such as advection, dispersion, diffusion, sorption and biodegradation. The knowledge of the processes affecting the movement of contaminant plume is beneficial to develop efficient remediation strategies. During the last two decades, compound-specific isotope analysis (CSIA) has become a powerful tool in identifying contamination sources and mechanism affecting the fate and transport of contaminants. There is a general acceptance that only transformation processes (i.e. degradation) contribute to stable isotope fractionation of organic compounds, while physical processes such as dissolution, volatilization, sorption, and diffusion have negligible effects on stable isotope fractionation. Most of the studies on the effect of physical processes on stable isotope fractionation of chlorinated ethenes have focused on stable carbon isotopes. In this study, controlled laboratory batch and column experiments were performed using different materials to evaluate the effect of sorption, desorption, diffusion, and back-diffusion on C, Cl, and H isotope fractionation of TCE and cis-DCE under static and dynamic conditions. The shift in H isotope fractionation during sorption batch experiments were significant and toward depletion of heavy isotopologues in the aqueous phase which was a counterintuitive phenomenon. The enrichment factors (εH) estimated for the sorption batch experiments were in the range of +32 ± 2.7 ‰ and +149 ± 31 ‰. Chlorine isotope data showed small enrichment in the aqueous phase. The enrichment factors (εCl) estimated for the sorption batch experiments ranged between -0.2 ± 0.06 ‰ and -0.8 ± 0.11 ‰ which were very small compared with the reported enrichment factors due to transformation processes. Carbon isotope results showed that sorption had a very small effect on isotope fractionation, which can be neglected when compared with isotope fractionations due to degradation process. The results from column experiments showed that sorption and desorption have a small effect on C and Cl isotope ratios of TCE even in the presence of a strong sorbent such as granular activated carbon (GAC). The isotope fractionations can be neglected compared with the ones during transformation processes. However, the shift in H isotope ratios was significant and showed a maximum isotope separation (∆2H) of -360 ‰ by the end of the experiment. As for the diffusion batch experiment, chlorine isotope separation of TCE and cis-DCE was observable and H isotope separation was significant. The H isotope separation ranged between -35 ‰ and -286 ‰. The Cl isotope separation ranged between -0.28 ‰ and -1.33 ‰. Results from the diffusion box experiment also showed significant H isotope separation of TCE and cis-DCE and observable chlorine isotope separation. The results from the current study showed that the effect of physical processes such as sorption, diffusion, and back-diffusion on H isotope fractionation of TCE and cis-DCE was significant. The reported value for H isotope fractionation of TCE due to biodegradation was small compared to H isotope fractionation values obtained in this study for physical processes. Therefore, compound-specific hydrogen isotope analysis is a promising tool to identify physical processes that affect the movement of chlorinated ethenes in the subsurface. Chlorine and carbon isotope fractionations due to physical processes were small compared with the isotope fractionations due to biodegradation.
Cite this work
Fatemeh Vakili (2017). Stable Isotope Fractionations of Chlorinated Ethenes Associated with Physical Processes. UWSpace. http://hdl.handle.net/10012/11268