Effects of Volatilization on Carbon and Hydrogen Isotope Ratios of MTBE

Abstract

Contaminant attenuation studies utilizing CSIA (compound-specific isotope analysis) routinely assume that isotope effects (IEs) result only from degradation. Experimental results on MTBE behavior in diffusive volatilization and dynamic vapor extraction show measurable changes in the isotope ratios of the MTBE remaining in the aqueous or nonaqueous phase liquid (NAPL) matrix. A conceptual model for interpretation of those IEs is proposed, based on the physics of liquid-air partitioning. Normal or inverse IEs were observed for different volatilization scenarios. The range of carbon enrichment factors (epsilon) was from +0.7 per thousand (gasoline vapor extraction) to -1 per thousand (diffusive volatilization of MTBE from gasoline), the range of hydrogen epsilon was from +7 per thousand (gasoline vapor extraction) to -12 per thousand (air sparging of aqueous MTBE). The observed IEs are lower than those associated with MTBE degradation. However, under a realistic scenario for MTBE vapor removal, their magnitude is within the detection limits of CSIA. The potential for interference of those IEs is primarily in confusing the interpretation of samples with a small extent of fractionation and where only carbon CSIA data are available. The IEs resulting from volatilization and biodegradation, respectively, can be separated by combined carbon and hydrogen 2D-CSIA.

Linking Low-Level Stable Isotope Fractionation to Expression of the Cytochrome P450 Monooxygenase-Encoding ethB Gene for Elucidation of Methyl tert -Butyl Ether Biodegradation in Aerated Treatment Pond Systems

“…As expected, the predominant process taking place when purging a MTBE aqueous solution is the advective volatilization of the compound, which would produce a 13 C depletion in the water phase, leading to an inverse isotope fractionation rather than the passive diffusion which would produce an enrichment (ε C ϭ Ϫ1.0‰) (25). In our volatilization experiments, probably due to the absence of sediments and a broader range of concentrations, the inverse carbon isotope effect predicted for water-air equilibrium was detectable, in contrast to previous air sparging experiments through sand columns (ε C ϭ ϳ0‰ Ϯ 0.1‰) (25), whereas a small normal hydrogen isotope effect with a constant hydrogen fractionation for the whole studied range of concentrations was observed. The normal hydrogen isotope effect of MTBE in the liquid reflects the preferential binding of water to deuterium ( 2 H-MTBE) (25).…”

“…However, CSIA application at field scale must also account for some uncertainty related to potential fractionation caused by abiotic processes. Although MTBE adsorption on humic substances and the related isotopic effect can be considered negligible (23), other processes, such as diffusion or volatilization, can produce small but measurable normal or inverse isotopic effects (19,25). This fact becomes even more crucial when considering the diversity of fractionation already found among aerobic MTBE degraders, which possibly correlate with the employment of different MTBE-attacking enzymes (see the following explanation).…”

“…For that purpose, biofilms established in the pond system were sampled in October 2009 after almost 5 months of maximum aeration and 2 years of continuous operation. In addition, the potential isotopic effects of the volatilization processes taking place in such an open pond were tested in laboratory experiments in an attempt to better simulate field conditions (bubbling from aqueous solution) in comparison with those shown by previous studies performed in closed systems or sediment columns (19,25). In summary, a combination of CSIA and RNA-based molecular methods was used in this study to evaluate the removal of MTBE from a complex treatment system.…”

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.…”

“…For example, the enrichment of 13 C in the vapor fraction was reported for the evaporation of benzene, toluene, ethylbenzene, and xylene (collectively known as BTEX) (D 13 C vapor-liquid & ?0.2%) (Harrington et al 1999), trichloroethylene (D 13 C vapor-liquid = ?0.1% to ?0.7%) , chlorinated aliphatic hydrocarbons (D 13 C vapor-liquid = ?0.31% for trichloroethene and D 13 C vapor-liquid = ?0.65% for dichloromethane) (Huang et al 1999), and MTBE (tert-butyl methyl ether, D 13 C vapor-liquid = ?0.2-0.5% in different physical contexts) (Kuder et al 2009). The enrichment of 13 C in the vapor phase could be explained by higher vapor pressure of 13 C-substituted organic compounds relative to 12 C-substituted organic compounds (Baertschi et al 1953;Narten and Kuhn 1961;Jancso and Van Hook 1974).…”