Isotopic fractionation of chlorinated ethenes during reductive dehalogenation by zero valent iron

Dayan, Hannah (1998) Isotopic fractionation of chlorinated ethenes during reductive dehalogenation by zero valent iron. Masters thesis, Memorial University of Newfoundland.

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Abstract

Many surface and groundwaters are contaminated by chlorinated ethenes as a result of their widespread use in different industries. One promising technique for the removal of these compounds is reductive dehalogenation using iron metal as the reducing agent. The method involves introducing an iron metal barrier into the flow of groundwater. Zero valent iron reacts with the chlorinated ethenes by either β-elimination or dehydrogenation, depending on the pH and Eh of the system. The purpose of this investigation was to determine the overall magnitude and direction of isotopic fractionation with respect to the stable isotopes of carbon (¹²C, ¹³C) during reductive dechlorination of selected chlorinated ethenes. By quantifying the fractionation of reductive dehalogenation as well as other reactions, the processes that may be dominant in the removal of the pollutants can be determined. Replicate experiments were done using low concentration solutions of trichloroethylene (TCE), perchloroethylene (PCE), 1,2-cis-dichloroethyelene (c-DCE) and 1,2-trans-dichloroethylene (t-DCE) and acid washed iron powder using compound specific isotopic analysis. The changes in the carbon isotope composition (δ¹³C) and the consumption of the ethenes were followed until they were no longer detectable by gas chromatography isotope ratio mass spectrometry (for 169, 345, 366 hours respectively). The presence of dissolved oxygen allowed for acidic conditions and the precipitation of ferric hydroxide. The consequence of this was the "natural" buffering of the system to pH between 5 and 6. The transition from β-elimination to dehydrogenation as the major reaction pathway also lies in this pH range. The pH also had an effect on the reaction rates. During the course of the reaction, enrichment in ¹³C was observed in the isotopic composition of each of the compounds. Kinetic or Rayleigh processes can be used to explain this effect. Fractionation factors were calculated using the Rayleigh model and found to be 1.0146, 1.0036, 1.0087, and 1.026 for c-DCE, t-DCE, TCE and PCE respectively. Future studies should include changing parameters such as temperature, as well as using buffer solutions; these studies will be useful in order to obtain a more complete picture of processes in natural waters.

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