Chlorinated solvents, such as trichloroethene (TCE) and tetrachloroethene (PCE), are common contaminants at military and industrial facilities, and they represent one of the most difficult contamination scenarios facing site managers. Due to the combination of low aqueous solubility and mass transfer limitations, dense nonaqueous phase liquid (DNAPL)-contaminated aquifers serve as long-term sources of groundwater contamination that may persist for decades or even centuries. To achieve substantial DNAPL mass reduction within acceptable timeframes, several in situ remediation technologies have been developed, including chemical oxidation, thermal treatment, air sparging, co-solvent flushing, and surfactant flushing. Of these technologies, thermal treatment provides two distinct advantages in that no chemical agents are introduced into the subsurface and it has the potential to efficiently treat heterogeneous porous media.
Although elevated temperature has been shown to enhance rates of PCE and TCE reactivity, considerable uncertainty exists when attempting to estimate the fraction of contaminant mass that is degraded during thermal treatment. The goal of this research was to advance the understanding of abiotic and biotic reactions that promote in situ contaminant destruction during thermal treatment. Greater understanding of these reactions may lead to reductions in the duration and temperature required for thermal treatment, along with development of polishing methods to treat residual contamination. Specific objectives of the project included: (1) quantifying relationships between subsurface temperature, physical-chemical properties of chlorinated ethenes, and sorption-desorption parameters, (2) determining the rate and extent of chlorinated ethene destruction in contaminated field samples as a function of temperature, (3) assessing the effect of temperature on isolates, PCE-to-ethene consortia, and dechlorinating bacteria native to a contaminated field site, and (4) evaluating the destruction and recovery of chloroethenes during laboratory-scale thermal treatment of field-contaminated soil.
The project was structured around four tasks: (1) Contaminant Phase Distribution; (2) Chemical Reactivity and Byproduct Formation; (3) Microbial Reductive Dechlorination; and (4) Thermal Treatment Performance Evaluation. To investigate specific mechanisms and quantify causal relationships, batch and flow-through reactor experiments were performed in two- and three-phase (gas-liquid, solid-liquid, solid-liquid-gas) systems containing PCE or TCE over a temperature range of 25 to 800°C. In addition, laboratory-scale electrical resistive heating (ERH) studies were performed to assess contaminant mass recovery and chemical reactivity in DNAPL-contaminated field soils. In these studies, ERH was chosen as the thermal treatment method since it was used at each of the four sites from which field samples were obtained.
Results of this work demonstrate that although TCE and PCE may undergo abiotic transformation and degradation at temperatures typically encountered during thermal treatment (i.e., 25 to 120°C) the observed rates were slow, yielding disappearance half-lives that ranged from 40 to 7,000 days. In addition, microbial reductive dechlorination of TCE and PCE was negligible at temperatures above approximately 40°C. Thus, chlorinated solvent recovery during thermal treatment is likely to be dominated by enhanced mass transfer from the solid and liquid phases, while in situ transformation processes provide only minimal contributions to TCE and PCE treatment under most conditions. Although PCE and TCE mass transfer from field-contaminated soils to groundwater correlated to increasing temperature, substantial contaminant levels persisted in fine-grained soils even after heating at 95°C for up to 185 days. Thus, even after removal of the dissolved-phase and NAPL chlorinated ethenes, a substantial fraction of contaminant mass can remain associated with the solid phase, particularly in low-permeability soils with high clay and silt contents. This slowly desorbing fraction of contaminant mass may require prolonged heating combined with vapor and liquid extraction to achieve remediation goals and could result in the rebound of groundwater contaminant concentrations once thermal treatment ceases. One promising strategy to address such residual contamination is the coupling of thermal remediation efforts, either in series or in parallel, with compatible treatment technologies including bioremediation and oxidation/reduction processes.
This project provides important insights into the mechanisms governing chlorinated ethene fate under representative thermal remediation scenarios. A number of important conclusions were reached regarding the abiotic reactivity, microbial reductive dechlorination, phase distribution, and recovery of chlorinated ethenes at elevated temperatures, including: