Table of Contents:
- What are organic chlorinated solvents?
- How are chlorinated solvents analyzed using CSIA?
- How can CSIA be used to study chlorinated solvents?
- What methodologies are currently being developed at IT2?
- Additional information for using CSIA for chlorinated solvents.
1) What are organic chlorinated solvents?
Chlorinated solvents, including 1,2-dichloroethylene (DCE), trichloroethylene (TCE), perchloroethylene (PCE), and vinyl chloride (VC), are contaminants found in groundwater due to their use as degreasers in a variety of industries. Chlorinated solvents are dense non-aqueous phase liquids (DNAPLs) whose solubility is higher than what is acceptable in drinking water; PCE has a solubility of 150 mg/L and TCE has a solubility of 1.07 mg/L at 20°C in water (Shouakar-Stash et al., 2003). Due to their presence as DNAPLs, these contaminants are able to remain at the bottom of groundwater sources and be slowly released over time, resulting in large volumes of groundwater becoming contaminated.
2) How are they analyzed?
The importance and usefulness to perform CSIA on chlorinated organic compounds was recognized and attempted in the past with varying degrees of success. Early methods for determining chlorine isotopes for these compounds followed the methodology used to determine inorganic chlorine isotopes, where the sample is modified offline and injected into the GC as CH3Cl. Previous techniques all required time-consuming preparation steps and a relatively large amount of sample. There was also the potential for fractionation to occur due to the large number of preparation steps.
Shouakar-Stash et al. (2006) developed an online method for determining chlorine isotopes in these compounds. Their method allowed for less sample preparation, enhanced sensitivity, and improved precision. This method involves the measurement of mass fragment peaks in the mass spectra of the contaminating compounds. These fragments are produced by bombarding contaminant molecules with electrons, breaking them apart and producing charged fragments in a consistent fashion. By selecting certain fragments to be analyzed, one can determine δ37Cl values for these compounds.
- a) Trichloroethylene
Trichloroethylene is a chemical used primarily as a solvent today, but has historically been used as an anesthetic and a painkiller. It is toxic to humans and the Canadian government has set the limit of TCE in drinking water at 0.005 mg/L (http://healthycanadians.gc.ca/publications/healthy-living-vie-saine/water-trichloroethylene-eau/index-eng.php?page=3#expo).
TCE is analyzed for chlorine isotopes, using the method by Shouakar-Stash et al. (2006), by looking at the peaks at m/z 95 and 97 of the mass spectra. The fragment analyzed has lost one chlorine atom; the m/z 95 peak corresponds to the fragment (12C2 35Cl2 1H)+ and the m/z peak at 97 corresponds to (12C2 35Cl 37Cl 1H)+, as seen in the mass spectra below (Figure 1). The two other species that also contribute to the m/z 97 peak are negligible relative to the (12C2 35Cl 37Cl 1H)+ contribution.
Figure 1: Mass spectra of trichloroethylene, highlighting the peaks of interest (Shouakar-Stash et al., 2006)
By using the ratio of the m/z 95 and m/z 97 peaks, a value for δ37Cl can be found. This value is reported relative to δ37Cl of SMOC (standard mean ocean chloride).
- b) Perchloroethylene
Perchloroethylene is a chemical used in dry cleaning and as a solvent. It is toxic to humans and the Canadian government has set the limit of PCE in drinking water at 0.010 mg/L (http://healthycanadians.gc.ca/publications/healthy-living-vie-saine/water-tetrachloroethylene-eau/index-eng.php?page=3#s2.1).
Much like in the analysis of TCE, two peaks are analyzed for chlorine isotopes, m/z 94 and 96. The fragment has lost two chlorine atoms so the m/z 94 peak corresponds to the fragment (12C2 35Cl2)+ and the m/z 96 peak corresponds to the fragment (12C2 37Cl 35Cl)+, as seen in the mass spectra below (Figure 2).
Figure 2: Mass spectra of perchloroethylene, highlighting the peaks of interest (Shouakar-Stash et al., 2006)
By using the ratio of the m/z 94 and m/z 96 peaks, a value for δ37Cl can be found. This value is reported relative to δ37Cl of SMOC (standard mean ocean chloride).
3) How can CSIA be used to study chlorinated solvents?
Remediation techniques involving reductive dechlorination are commonly used to reduce the toxicity of chlorinated solvents, including TCE and PCE, through the replacement of chlorine atoms with hydrogen atoms. The level of detoxification is dependent on the organism’s ability to carry out complete dechlorination. The reaction mechanism controls the level of dechlorination, and when complete dechlorination is not achieved toxic daughter products, like cis-DCE, are formed (Cretnik et al., 2013).
Carbon and chlorine specific isotope analysis have been used in studies involving TCE to assess dual isotope fractionation effects during natural dechlorination for different microbial strains and chemical model reactions (cobalamin and cobaloxime; Cretnik et al., 2013). Different microbial strains were shown to possess similar dual isotope slopes during the biodegradation of TCE, which was also similar to the slope shown for abiotic degradation by cobalamin (Figure 3). These shared slopes, indicate that the transformation mechanism of TCE with cobalamin is similar to that of biodegradation. Cobalaxime showed a markedly different dual isotope slope, indicating a different reaction mechanism.
Figure 3: Dual isotope plots, δ37Cl and δ13C, of TCE during dechlorination by two microbial strains (Geobacter lovleyi and Desulf itobacterium hafniense) and chemical model reactions (cobalamin and cobaloxime; Cretnik et al., 2013).
A dual isotope approach, using δ37Cl and δ13C, was similarly used in a study involving the reductive dechlorination of PCE (Badin et al., 2014). Laboratory experiments were carried out using two bacterial strains, possessing different dehalogenase genes, carrying out reductive dechlorination on PCE. The resulting dual isotope slope values were different between strains and were compared to dechlorination slopes observed for two field sites (Figure 4). The data sets collected at the two field sites showed different isotopic values from each other, but both showed agreement with one of the two laboratory produced data sets, indicating different mechanisms for reductive dechlorination of PCE (Badin et al., 2014). Similar biotic and abiotic degradation pathway delineation has been carried out for other organic contaminants, including: 1,2 –dichloroethane (Palau et al., 2014), 1,1,1 – trichloroethane (Palau et al., 2014, Palau et al., 2016), cis-dichloroethene (Audi-Miro et al., 2015, Abe et al., 2009) and vinyl chloride (Abe et al., 2009).
These results are environmentally significant because they illustrate the potential to use CSIA to investigate and compare transformation mechanisms of different organisms and model agents in the reductive dechlorination of various chlorinated organic compounds. Reaction mechanism delineation is not only essential for increasing our understanding of the chemical and biological processes that these contaminants undergo, but also for looking at the effectiveness of remediation techniques used on contaminated sites.
Figure 4: Dual isotope plots of the reductive dechlorination of PCE from two field sites (denoted in red and blue). The dashed lines show the dual isotope slopes from bacterial species with different reductive dehalogenase genes in laboratory experiments (Badin et al., 2014).
4) What methodologies are currently being developed at IT2?
CSIA Methodologies are currently being developed for the following organic contaminants:
- a) Monochlorobenzene
- b) Dichlorobenzene
- c) 1,4-Dioxane
- d) Poly brominated diphenyl ethers (PBDEs)
5) Additional information on using CSIA for examining chlorinated solvents can be found in the following publications:
1) Abe, Y., Aravena, R., Zopfi, J., Shouakar-Stash, O., Cox, E. Roberts, J.D., Hunkeler, D. 2009. Carbon and chlorine isotope fractionation during aerobic oxidation and reductive dechlorination of vinyl chloride and cis-1,2-Dichloroethene. Environ. Sci. Technol. 43, 101-107.
2) Audi-Miro, C., Cretnik, S., Torrento, C., Rosell, M., Shouakar-Stash, O., Otero, N., Palau, J., Elsner, M., Soler, A., 2015. C, Cl and H compound-specific isotope analysis to assess natural versus Fe(0) barrier-induced degradation of chlorinated ethenes at a contaminated site. Journal of Hazardous Materials. 299, 747-754.
3) Badin, A., Buttet, G., Maillard, J., Holliger, C., Hunkeler, D., 2014. Multiple Dual C-Cl Isotope Patterns Associated with Reductive Dechlorination of Tetrachloroethene. Environmental Science & Technology. 48, 9179-9186.
4) Cretnik, S., Thoreson, K. A., Bernstein, A., Ebert, K., Buchner, D., Laskov, C., Haderlein, S., Shouakar-Stash, O., Kliegman, S., McNeill, K., Elsner, M., 2013. Reductive Dechlorination of TCE by Chemical Model Systems in Comparison to Dehalogenating Bacteria: Insights from Dual Element Isotope Analysis (C-13/C-12, Cl-37/Cl-35). Environmental Science & Technology. 47, 6855-6863.
5) Palau, J., Jamin, P., Badin, A., Vanhecke, N., Haerens, B., Brouyere, S., Hunkeler, D., 2016. Use of dual carbon-chlorine isotope analysis to assess the degradation pathways of 1,1,1-trichloroethane in groundwater. Water Research. 92, 235-243.
6) Palau, J., Cretnik, S., Shouakar-Stash, O., Höche, M., Elsner, M. and Hunkeler, D., 2014. C and Cl isotope fractionation of 1,2-dichloroethane displays unique δ13C/δ37Cl patterns for pathway identification and reveals surprising C-Cl bond involvement during microbial oxidation. Environmental Science & Technology. 48 (16), 9430–9437. DOI: 10.1021/es5031917.
7) Palau, J., Shouakar-Stash, O., Hunkeler, D., 2014. Carbon and Chlorine Isotope Analysis to Identify Abiotic Degradation Pathways of 1,1,1-Trichloroethane. Environmental Science & Technology. 48, 14400-14408.
8) Shouakar-Stash, O., Drimmie, R.J., Zhang, M., and Frape, S.K. 2006. Compound-specific chlorine isotopes ratio of TCE, PCE and DCE isomers by direct injection using CF-IRMS. Applied Geochemistry. 21, 766-781.