Chlorinated volatile organic compounds (cVOCs), such as trichloroethylene (TCE) and perchloroethylene (PCE), represent one of the largest remediation challenges and costs at U.S. Department of Defense (DoD) sites. Anaerobic bioremediation via organic carbon source addition (with or without bioaugmentation with dechlorinating bacteria) is a commonly used approach to remediate cVOCs in situ. One significant issue with this approach is that reductive dechlorination processes are typically inhibited at pH values <~5.5. Aquifers with lower pH values are common, especially in the eastern United States. Raising the groundwater pH is often not feasible because of the large amount of buffer needed, the large size of many plumes, and the need for long-term treatment and repeated reinjections.
The primary goal of this project was to demonstrate a solar-powered technology to generate hydrogen (H2) in situ and reduce aquifer acidity to promote reductive dechlorination. During operation, Proton Reduction Technology (PRT) uses a low voltage potential applied across electrodes installed within an aquifer to impress a direct current (DC) in the subsurface. PRT was tested in a low pH cVOC-contaminated aquifer at Joint Base McGuire-Dix-Lakehurst, NJ (JB MDL). A successful demonstration was expected to result in sustainable aquifer pH control and contaminant degradation at significantly lower cost than conventional approaches that require the addition of buffers and organic electron donors. Successful application of this technology would allow the DoD to economically treat contaminated low pH aquifers and remote contaminant plumes where electrical power is not readily available or where long treatment times are expected.
Biological reductive dechlorination of cVOCs relies on bacteria that use H2 as an electron donor and the cVOC as an electron acceptor. The H2 may be supplied directly or by fermenting organic carbon electron donors. PRT generates H2 by electrolysis, with concurrent reduction of protons (hydrogen ions [H+]) on the surface of cathodes powered by an impressed current. In addition to producing H2, PRT consumes protons, thereby raising the pH of groundwater around and downgradient of the cathode. Thus, during this project, PRT technology was evaluated for its ability to foster dechlorination through in situ H2 generation while also raising the groundwater pH to favorable levels. In addition, PRT can also support biological remediation of several other common DoD contaminants, including RDX, hexavalent chromium (Cr[VI]), and perchlorate.
This field demonstration project used electrodes inserted into polyvinyl chloride (PVC) wells within the cVOC-contaminated low pH aquifer. The electrodes (three cathodes and two anodes) were operated to generate H2 to support biodegradation, and consume H+ to increase aquifer pH. The PRT system was operated for 507 days from start-up to shut-down. The demonstration was divided into four phases of operation, which included PRT-only operation, and PRT operation with varying groundwater recirculation configurations. The contaminated aquifer was inoculated with a bioaugmentation culture (SDC-9™) to ensure that the appropriate dechlorinating bacteria were present to support biodegradation. Electricity to operate the system was provided by solar panels and deep cycle 12 volt (V) batteries. During the demonstration, groundwater pH, contaminant concentrations, H2 production, distribution, and utilization, and electrode performance were monitored.
PRT resulted in partial reductive dechlorination of cVOCs in the low pH aquifer at JB MDL, but TCE dechlorination was not complete, at least not under the conditions of the demonstration. The lack of complete dechlorination, even after bioaugmentation, was likely due to the borderline pH and reducing conditions achieved in the aquifer. It is possible that dechlorination activity could have been improved if a higher pH (e.g., pH 6.5–7) or more reducing conditions (e.g., oxidation-reduction potential [ORP] < -100 millivolts [mV]) were consistently achieved.
Although PRT showed some potential for increasing pH and lowering ORP, the configuration of the system during this demonstration was not sufficient to achieve and maintain optimal geochemical conditions for extended periods. Because a circumneutral pH and highly-reduced environment could not be sustained, efficient dechlorination of TCE could not be achieved.
PRT was only partially successful in this test, but the results suggest it may be a useful component of an overall treatment system for remediating an acidic aquifer. However, additional treatments/amendments may be needed to better address and overcome the significant soil buffering capacity of many aquifers. For example, a large dose of buffer and a carbon substrate could be applied to a biobarrier at the start of treatment to overcome the initial acidity of the aquifer sediments and to produce a low ORP before applying current, and PRT could then be used as a long-term source of electron donor (H2) and hydroxide ion (OH-) to maintain aquifer pH.
Although this study showed that PRT can have significant limitations, it also has provided valuable guidance for the ongoing development of the technology. One recently-demonstrated strategy to overcome the limitations observed in this project is to use more closely-spaced electrodes, and to install the electrodes with metallurgical soil contact material (coke breeze) as backfill. This approach was tested successfully in the field under the U.S. Navy Environmental Sustainability Development to Integration (NESDI) program, during NESDI Project 501. This demonstration was conducted within a low pH cVOC-contaminated aquifer at Marine Corps Base Quantico, in Quantico, VA. During this one-year field demonstration, eight closely-spaced cathodes and two downgradient anodes were installed in a barrier configuration, and concentrations of cis-1,2-dichloroethene (cis-DCE) (the primary contaminant of concern) were reduced by 88–99% across the barrier.