Superfund Research Program
Electrolytic Strategies for Remediation of Chlorinated Solvents in Groundwater
Chlorinated solvents are among the most widespread contaminants of groundwater in this country. Several of these compounds, including vinyl chloride and trichloroethylene, are prominent pollutants at Superfund sites, where in many cases they have migrated into aquifers and seriously degraded the groundwater quality of surrounding communities. Most chlorinated solvents are harmful to humans - for example, some are known or probable human carcinogens, while others are associated with birth defects and liver damage - so, it is important to protect communities from exposure to these compounds. This is being accomplished in part by the extensive efforts on hazardous waste sites to remediate contaminated aquifers.
The task of cleaning up groundwater has proven to be one of the most difficult and expensive challenges facing remedial project managers and the environmental cleanup industry. Conventional methods of groundwater remediation use "pump-and-treat" strategies, which can be energy intensive and sometimes produce a contaminant disposal problem at ground surface. Experience has also shown that chemical residuals are very difficult to remove using this approach.
In the last few years, promising alternatives to pump-and-treat systems have been under development. One such approach is the use of permeable, reactive barriers that contain zero-valent iron (ZVI). These barriers are actually incorporated into the aquifer and can be thought of as subsurface reaction walls. This in situ form of treatment works by stripping the chlorine atoms from chlorinated hydrocarbons via reductive dehalogenation reactions and leaves behind innocuous products such as methane and ethane. However, this process has its own shortcomings. The long-term efficiency of ZVI barriers may be compromised by the oxidation of iron surfaces, which can lead to less favorable reaction kinetics and eventual contaminant breakthrough. New groundwater treatment methods are greatly needed.
Researchers at the University of Arizona are developing novel electrochemical methods for the in situ remediation of chlorinated contaminants. These methods offer the means to overcome the potential shortcomings of ZVI barriers. To begin, electrolytic reactors can be designed to take advantage of the "redox" properties of halogenated target compounds. Heavily chlorinated compounds such as perchloroethene and carbon tetrachloride can be completely dehalogenated at the cathode of an electrolytic reactor via processes that are analogous to reductions by ZVI. Easily oxidized compounds such as vinyl chloride can be oxidized at the anode. In addition, electrode materials can be selected to avoid corrosion, and process kinetics can be controlled to a degree that is absent in current ZVI systems.
Significant progress has been made in identifying materials - including copper, nickel, zinc and iron - that efficiently dehalogenate chlorinated solvents at the cathode terminal in electrolytic systems. Using these materials, the reaction products, kinetics and mechanisms have been characterized for the reductive dehalogenation of several common chlorinated groundwater contaminants.
Anodic oxidation of halogenated solvents has also been investigated. The oxidation of trichloroethene was examined using a conductive ceramic anode made of partially reduced titanium oxide. Trichloroethene was rapidly converted to carbon dioxide and carbon monoxide with the cogeneration of molecular oxygen. An investigation of the mechanistic details - for example, the dependence of trichloroethene transformation on hydroxyl radical formation - is underway.
Moreover, recent studies have shown that gas-phase chlorinated hydrocarbons, including perchloroethene, trichloroethene and carbon tetrachloride, can be electrolytically reduced. These findings open the way to reactor design for treatment of solvent-contaminated gases derived from soil-vapor extraction, bioventing, gas sparging and related processes. Ongoing studies are directed toward the selection of electrode materials and other design features that are relevant to the treatment of gas-phase contaminants.
Transfer of these electrolytic methods to the field for practical applications will depend on reactor design and scale up. Several bench- and pilot-scale electrolytic reactors have been constructed and evaluated under conditions that would likely be encountered in the field.
These recent findings show that the electrolytic decomposition of chlorinated solvents is promising for in situ treatment of contaminated groundwater. Both oxidative and reductive transformation strategies are available, and reaction kinetics, which are very fast, are to some extent under operational control via selection of electrode potential, material, and specific surface area, and manipulation of reactor hydraulics. Perhaps the most encouraging aspect of the research to date is the treatability of gas-phase contaminants in electrolytic reactors, which may be useful for the destruction of gases brought to the surface using solvent extraction techniques.
As a class, chlorinated solvents represent some of the most toxic, environmentally persistent compounds in groundwater. In some areas of the country these compounds have rendered the groundwater unsafe for human consumption, depriving some communities of a much-needed source of water. This well-recognized problem has been difficult to confront, so these studies represent exciting developments towards a potential new technology for groundwater remediation.
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To learn more about this research, please refer to the following sources:
- Chen G, Betterton EA, Arnold RG. 1999. Electrolytic oxidation of trichloroethylene using a ceramic anode. Journal of Applied Electrochemistry 29:961-970.
- Liu Z, Arnold RG, Betterton EA, Festa KD. 1999. Electrolytic reduction of CCl4-Effects of cathode material and potential on kinetics, selectivity, and product stoichiometry. Environ Eng Sci 16:1-13.
- Betterton EA, Arnold RG, Kuhler RJ, Santo GA. 1995. Reductive dehalogenation of bromoform in aqueous solution. Environ Health Perspect 103(S5):89-91. PMID:8565919
- Warren KD, Arnold RG, Bishop TL, Lindholm LC, Betterton EA. 1995. Kinetics and mechanism of reductive dehalogenation of carbon tetrachloride using zero-valence metals. J Hazard Mater 41(2-3):217-227. doi:10.1016/0304-3894(94)00117-Y
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