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IN-SITU TREATMENT OF GROUNDWATER CONTAMINATED WITH CHLORINATED SOLVENTS RECENT DEVELOPMENTS
An On-Line Version of a Column First Published in the:
National Environmental Journal May/June 1995 Vol. 5 No. 5
By: David B. Vance
The contamination of groundwater with chlorinated solvents is a well recognized problem that has proven resistant to resolution. Chlorinated solvents are denser than water which allows them to sink through groundwater aquifers, they have relatively high levels of solubility, and free phase chlorinated solvents react with clays causing shrinkage and cracking, permitting the solvents to pierce what were thought to be impenetrable aquitards. Lastly, chlorinated solvents are proven carcinogens with low (parts per billion) tolerance levels in drinking water.
Remediation has typically focused on pump and treat methods since chlorinated solvents are xenobiotic, meaning that they are man-made and do not serve as a ready source of "food" for bacteria. Irrespective, in-situ bioremediation of chlorinated solvents was intensely researched in the 1980's, with advances such as cometabolism. Cometabolism is the addition of a substrate that is naturally utilized by bacteria (such as methane) to generate enzymes that have the additional capacity to degrade chlorinated solvents. However, this approach was never totally satisfactory due to preferential affinity (typically hundreds of times greater) of generated degradation enzymes for the cometabolite substrate versus the targeted chlorinated solvents. Consequently the in-situ bioremediation of chlorinated solvents did not advance much beyond the stage of research, with few applications producing predictable results in the field.
In the last few years however, there have been a series of dramatic developments for the in-situ treatment of chlorinated solvents. The approach is based on the sequential reduction of chlorinated hydrocarbons to innocuous end products such as methane, ethane or ethene. In principal the process has been recognized in scientific circles since 1874. But, it is just beginning to be investigated for environmental application. The process includes microbiological systems, humic materials, biochemicals and most dramatically the use of zero valence state elemental metals.
The story should start with the microbiological methods. For years it was thought that the anaerobic degradation of chlorinated ethylene’s would terminate with vinyl chloride, a very toxic material. However, that assumption has been proven wrong. Anaerobic bacterial consortia have been found to dechlorinate perchloroethylene (PCE) completely to ethylene. Substrates that can act as an electron donor are required, which include formate, acetate, glucose or methanol. The key to the process is acetogenic catabolism of the substrate to produce hydrogen, which is in turn utilized for reductive dechlorination. This is a very different process than the production of cometabolic enzymes. Methanol has been found to be the most efficient substrate for the production of hydrogen under these conditions.
In these biological systems the rate limiting step to complete dechlorination to ethylene is the last stage conversion of vinyl chloride. The rate of that process has been found to be significantly enhanced by the presence of cyanocobalamin (vitamin B12) which contains cobalt at its molecular core.
This then leads to the most exciting development, the use of metals in absence of bacterial action to completely dechlorinate solvents.
The most common metal being utilized for this purpose is iron. But other metals including tin, zinc, palladium, and technetium have also shown to be effective. The process can be best described as anaerobic corrosion of the metal by the chlorinated hydrocarbon. During this process the hydrocarbon is adsorbed directly to the metal surface where the dehalogenation reactions occur. Most importantly, this process does not inactivate the metal surface, studies indicate that under anaerobic conditions metallic iron retains its reactivity over long periods of time. Increasing surface area (by reducing the size of iron filings) increases the effectiveness of the process.
The variations of the process are complex. Recent research on iron systems indicates three mechanisms at work in the reductive process.
- First the Feo acts as a reductant by supplying electrons directly from the metal surface to the adsorbed halogenated compound.
- Secondly, solubilized ferrous iron can act as a reductant, albeit at a rate at least an order of magnitude slower.
- Thirdly, metallic iron may act as a catalyst for the reaction of hydrogen with the halogenated hydrocarbon. The hydrogen is produced on the surface of the iron metal as the result of corrosion with water.
Theoretically without a catalyst the described reactions are not effective, it is thought that impurities in the iron or surface defects act as that catalyst. The reduction reactions have not been found to occur on iron oxides or oxyhydroxides.
The chemistry of these systems can be still further enhanced. Ferrous iron in the presence of humic material has been shown to dehalogenate carbon tetrachloride. As described above, ferrous iron alone can cause reduction, however the addition of humic material increases the rate of reaction by an order of magnitude (to near that achieved with metallic iron). As in the microbiological systems described earlier, the presence of vitamin B12 increases the rate of reaction in metal systems and improves the completeness of the reaction to ethene or ethane.
Iron metal is by far the most attractive for the implementation of this concept. Although, when utilizing iron, the presence of dissolved oxygen causes hydrodynamic problems as iron oxidation to oxyhydroxides causes plugging of the pore spaces and ultimately consumes the iron metal. However, the reductive dechlorination reactions still occur on the surface of unoxidized iron metal even under oxic conditions.
This technology has been field tested using iron filings mixed with sand in trenches, and it does work, although problematically with oxic groundwater. It may be advantageous to run such trench systems as a two stage treatment. The first would add a hydrogen generating carbon substrate to drive the system anoxic (and begin the dechlorination process), followed by a second stage reaction with iron metal under anaerobic conditions where the production of oxyhydroxides would not occur.
These developments have dramatic potential for future application. As the particulars for the hydrogen dehalogenation process in groundwater are resolved, it may be possible to exploit the technology fully in-situ by adaptation to the site specific saturated zone geologic matrix (in which iron species and humic materials are not rare), taking it beyond utilization in trenches or above ground treatment systems.
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