Chlorine and Carbon Isotopes Fractionation during Volatilization and Diffusive Transport of Trichloroethene in the Unsaturated Zone

Abstract

To apply compound-specific isotope methods to the evaluation of the origin and fate of organic contaminants in the unsaturated subsurface, the effect of physicochemical processes on isotope ratios needs to be known. The main objective of this study is to quantify chlorine and carbon isotope fractionation during NAPL-vapor equilibration, air-water partitioning, and diffusion of trichloroethene (TCE) and combinations of these effects during vaporization in porous media. Isotope fractionation is larger during NAPL-vapor equilibration than air-water partitioning. During NAPL-vapor equilibration, carbon, and chlorine isotope ratios evolve in opposite directions although both elements are present in the same bond, with a normal isotope effect for chlorine (ε(Cl) = -0.39 ± 0.03‰) and an inverse effect for carbon (ε(C) = +0.75 ± 0.04‰). During diffusion-controlled vaporization in a sand column, no significant carbon isotope fractionation is observed (ε(C) = +0.10 ± 0.05‰), whereas fairly strong chlorine isotope fractionation occurs (ε(Cl) = -1.39 ± 0.06‰) considering the molecular weight of TCE. In case of carbon, the inverse isotope fractionation associated with NAPL-vapor equilibration and normal diffusion isotope fractionation cancel, whereas for chlorine both processes are accompanied by normal isotope fractionation and hence they cumulate. A source of contamination that aged might thus show a shift toward heavier chlorine isotope ratios.

Can Soil Gas VOCs be Related to Groundwater Plumes Based on Their Isotope Signature?

“…These results agree with the isotopic fractionation trends observed by Jeannottat and Hunkeler 24 for TCE, which showed the same opposite trends for carbon and chlorine isotope fractionation. The enrichment factors are higher for PCE than for TCE (ε Cl = −0.20 ± 0.04‰ vs −0.06 ± 0.05‰ and ε C = +0.46 ± 0.04‰ vs +0.38 ± 0.04‰, respectively).…”

“…The effect of air−water partitioning was investigated using a similar approach as the one previously developed by Jeannottat and Hunkeler. 24 The air−water partitioning experiments were carried out with a 50 mL gastight syringe partially filled with 30 mL of an aqueous solution of PCE at 5 mg/L with 20 mL of gas phase remaining. After an equilibration time of 20 min on a rotary shaker at 200 rpm in a horizontal position the gas phase was pushed out of the syringe and replaced by clean air.…”

“…Because isotopes and pairs of isotopologues differing in one heavy isotope fractionate the same way, isotope fractionation factors can thus be used to quantify isotopologue fractionation. 24 Only the mass 165.83 and 166.83 for carbon isotopes and 165.83 and 167.83 for chlorine isotopes were taken into account for the modeling. Equilibrium was assumed between concentrations in the gas and water phase and only aqueous-phase concentrations are reported.…”

Carbon isotopic fractionation during vaporization of low molecular weight hydrocarbons (C6–C12)

“…The theoretical fractionation factor between the diffusion coefficients is given by the following equation (Craig 1953;Cerling et al 1991;Bouchard et al 2008c;Jeannottat and Hunkeler 2012):…”

“…The evaporation-controlled process usually results in ''inverse isotope fractionation,'' characterizing of enriching 13 C in the vapor phase (Baertschi et al 1953;Balabane and Letolle 1985;Huang et al 1999;Poulson and Drever 1999;Wang and Huang 2001;Jeannottat and Hunkeler 2012;Xiao et al 2012), whereas diffusion-controlled vaporization, which depends on the system itself and intermolecular free energy due to the van der Waals attractive forces among molecules, results in the ''normal isotope fractionation,'' characterizing of enriching 13 C in the residual liquids (Shin and Lee 2010;Xiao et al 2012;Kuder et al 2009;Bouchard et al 2008a, b, c;Jeannottat and Hunkeler 2012;Hayes 1993;Wang and Huang 2001). Table 4 lists the carbon isotope enrichment factors of the LMWHs considered here and in previous studies, along with values calculated using Eq.…”