Stable Isotope (C, Cl, and H) Fractionation during Vaporization of Trichloroethylene

Abstract

Stable isotope fractionation during vaporization of trichloroethylene has been measured, with possible application as a technique to investigate subsurface behavior. The equilibrium value of Δ13Cvapor - liquid has been measured between 5 and 35 °C, and Δ13Cvapor - liquid, Δ37Clvapor - liquid, and ΔD vapor - liquid have been measured during progressive evaporation of liquid trichloroethylene at 22 ± 2 °C. Equilibrium values of Δ13Cvapor - liquid show a total range of 0.07−0.82‰, with a trend of decreasing Δ13Cvapor - liquid with increasing temperature, from approximately +0.7‰ at 5−15 °C to approximately +0.1‰ at 35 °C. Progressive evaporation experiments yield values of Δ13Cvapor - liquid = +0.35‰ and +0.24‰, Δ37Clvapor - liquid = −1.64‰, and ΔD vapor - liquid = +8.9‰. The positive values for carbon and hydrogen isotope fractionation, while unexpected, are consistent with available quantitative and qualititative data for trichloroethylene and other contaminant hydrocarbons, but a satisfactory explanation for these observations, particularly in combination with the negative value for chlorine, remains elusive. Vapor−liquid fractionation factors have application to the investigation of the behavior of trichloroethylene at contaminated sites, particularly sites undergoing remediation by techniques such as soil vapor extraction and soil bioventing.

Carbon isotope fractionation during aerobic biodegradation of n-alkanes and aromatic compounds in unsaturated sand

“…Thus, the non-reactive positions ''dilute" the isotope effect. Since no bond cleavage occurs during phase transfer processes, isotopic fractionation is expected to be absent or very small during sorption, dissolution, and volatilisation of compounds, as shown by several studies (Dempster et al, 1997;Harrington et al, 1999;Huang et al, 1999;Poulson and Drever, 1999;Slater et al, 1999;Hunkeler and Aravena, 2000). A small sorption isotope effect was recently reported for benzene and toluene in an organic matter rich matrix (Kopinke et al, 2005).…”

“…The trend to smaller AKIE for larger molecules could be due to rate limiting steps before the actual transformation step other than phase transfer processes such as sorption, dissolution, and volatilisation of compounds for which no or very small isotopic fractionation was shown (Dempster et al, 1997;Harrington et al, 1999;Huang et al, 1999;Poulson and Drever, 1999;Slater et al, 1999;Hunkeler and Aravena, 2000). Biodegradation of compounds is a multi-step procedure (O' Leary and Yapp, 1978) that includes transport of the compound into the cell (step 1), binding of the substrate to the enzyme (step 2), transformation at the reaction centre (step 3) and finally, dissociation of the product with the enzyme (step 4).…”

Hydrogen and Carbon Isotope Fractionation during Aerobic Biodegradation of Benzene

“…Usually the reaction rate is slightly faster for molecules with light isotopes ( 1 H, 12 C) as compared to molecules with heavy isotopes ( 2 H, 13 C), and as a result, the degrading compound becomes increasingly enriched in the heavy isotopes as the reaction progresses. In contrast, physical processes such as dissolution, volatilization, or sorption are usually not accompanied by significant isotope fractionation (9)(10)(11)(12). Large isotope effects with respect to carbon have been observed during reductive dechlorination (8,13,14) and oxidation (15,16) of chlorinated solvents while much smaller isotope effects occurred during biodegradation of BTEX (13,17,18).…”

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

“…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.…”

“…Previous experimental studies have investigated the evaporation of LMWHs mainly by simulating the evaporation of one or two pure components in each experiment and determining their composition and isotope fractionation at different stages during the evaporation (Huang et al 1999;Poulson and Drever 1999;Shin and Lee 2010). The effect of the matrix in which the vaporization of a hydrocarbon is studied has seldom been discussed in these experimental studies.…”