Do Mass Transformation Processes not Cause Isotope Fractionation Effect on Chlorinated Solvents?

Poster Presentation

Presented by T. Wang
Prepared by , Y. Chen, H. Hsu

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Compound specific isotope analysis (CSIA) has a great potential on environmental forensics, especially for sites contaminated by chlorinated solvents. Understanding the fractionation effects of natural transformation processes on those chlorinated compounds is beneficial to interpret CSIA data. The objective of this work is to investigate carbon, chlorine and hydrogen isotope compositions of six chlorinated compounds during the processes of evaporation, partitioning and dissolution by GC/MS and GC/IRMS. Evaporative results showed that carbon and chlorine stable isotope signature of all target compounds changed contrarily. TCE and EDC’s δ2H value declined as the evaporative percentage increased and the δ2H values of 1,1-DCE and DCM solvent residuals became enriched as the evaporation progressed with an enrichment factor of -13.15±3.99 and -12.41±9.98 ‰, respectively. Results of partitioning experiments showed that partitioning only caused significant isotope effect on PCE’s δ13C and TCE’s δ37Cl with an enrichment factor of -1.01±0.42 and -5.16±2.21, respectively, and there were no significant isotope effect observed on other target compounds’ carbon, chlorine or hydrogen stable isotope signatures. In the dissolution study, dissolution caused significant isotope effect on PCE, TCE, DCM’s δ13C and TCE’s δ2H. In addition, dissolution did not cause isotope effect on the chlorine stable isotope signatures of all target compounds because the variation of their δ37Cl values was not significant in 95% confidence interval and the difference of each signature was within the precision of analysis (1 ‰).Two-dimensional carbon and chlorine isotope analysis could help to discriminate between evaporation, partitioning and dissolution, even biodegradation and other reactions, owing to the isotope fractionation difference of each mechanism. Noted that the isotope effects caused by transportation processes were smaller than those caused by biological and chemical reactions. Therefore, transportation processes effects could be ignored when interpreting isotope signatures at most cases.