Chlorinated Methane Hydrolysis Rates
EMSL Project ID
25679a
Abstract
The purpose of the work proposed here is to develop accurate physical chemical data for the hydrolysis rates of carbon tetrachloride (CT) and chloroform (CF) under conditions (temperature, pH, groundwater composition, sediment mineralogy) typical of the subsurface at Hanford. In addition to determining the homogeneous rates of hydrolysis, a significant effort will test the hypothesis that heterogeneous mechanisms can significantly increase hydrolysis rates at low temperatures. Heterogeneous effects on hydrolysis are more likely to be seen at low temperatures and consequently of high potential significance in determining contaminant behavior in the subsurface. These effects have not been observed previously due to the lack of hydrolysis experimentation at environmentally relevant temperatures. We will follow two experimental approaches. The first approach, which we have been using for the past several years, involves the preparation and long-term incubation of sealed glass ampules containing CT and CF in contact with various solutions (deionized water, Hanford groundwater) or suspensions (Hanford groundwater containing Hanford sediment or specific mineral phases). After incubation at temperatures of 20-70°C for periods of up to five years, the ampules are opened and the contents analyzed for chloride content by ion chromatography or for chlorinated methane content (including reductive degradation products such as dichloromethane and chloromethane) by gas chromatography-mass spectrometry. The rates of hydrolysis at each temperature are determined and the activation energies calculated. Heterogeneous effects, if seen, will be correlated with specific minerals present and used to develop a mechanistic understanding of the process.
The second approach involves a new application of isotope-ratio mass spectrometry (IRMS) and a different reactor system. We will use a zero-headspace reactor to incubate the sample. This reactor allows repetitive sampling while eliminating concerns about partitioning between gaseous and aqueous phases. We will incubate 13C-labeled CT or CF in this reactor, and samples from the reactor containing dissolved 13CO2 generated by hydrolysis will be diluted in a standard bicarbonate solution and then analyzed for δ13C after acidification to release CO2 and gas cleanup by standard cryo-trapping procedures. Our calculated instrumental sensitivity is about 22 fmol 13C, which should allow us to get data for groundwater temperatures in a matter of a few weeks to months, rather than years, and also will allow us to conduct experiments at the low concentrations typical of Hanford groundwater (i.e., 1 ppm or less).
This project represents the most comprehensive study of CT and CF hydrolysis rates ever attempted and should provide benchmark data for these reactions under environmentally relevant conditions. The results will have a significant and immediate impact on the remediation plans for the CT and CF plumes beneath the 200W Area at Hanford. We expect to publish several papers on the hydrolysis rates, and in particular, the search for heterogeneous effects on hydrolysis rates. Proof of such a phenomenon would represent a significant accomplishment.
Project Details
Project type
Large-Scale EMSL Research
Start Date
2008-08-12
End Date
2011-09-30
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members
Related Publications
Amonette JE, O Qafoku, TW Wietsma, PM Jeffers, CK Russell, and MJ Truex. 2009. Carbon Tetrachloride and Chloroform Attenuation Parameter Studies: Heterogeneous Hydrolytic Reactions -- Status Report . PNNL-18735, Pacific Northwest National Laboratory, Richland, WA. 44 pp.
Amonette JE, O Qafoku, TW Wietsma, PM Jeffers, CK Russell, and MJ Truex. 2010. Abiotic Degradation Rates for Carbon Tetrachloride and Chloroform: Progress in FY2009. PNNL-19142, Pacific Northwest National Laboratory, Richland, WA. 42 pp.