This invention relates to methods for enhancing the biotransformation of halogenated hydrocarbons and more particularly of chlorinated aliphatic hydrocarbons. This invention has direct application to both the in situ and ex situ bioremediation of contaminated soil, sediment and ground water.
Organic contaminants in soil, sediment and ground water may be biotransformed or broken down by naturally occurring microorganisms. Highly chlorinated aliphatic hydrocarbons may be biotransformed to lower chlorinated homologs by a process known as reductive dehalogenation. It has been found that such biotransformation of chlorinated aliphatic hydrocarbons proceeds best in an anaerobic environment as opposed to an aerobic environment, see Vogel et al., Environmental Science and Technology, Vol. 21, 722 (1987).
Of the highly chlorinated aliphatic contaminants, chlorinated ethylenes are among the most common. Compounds such as perchloroethylene and trichloroethylene have been used extensively as industrial degreasing solvents, dry cleaning fluids, fumigants and in a variety of other applications. In view of their widespread use these compounds have been found as contaminants in soil, sediment and ground water. Since perchloroethylene and trichloroethylene have relatively moderate water solubilities, such compounds are mobile in soils, sediments and ground water.
It is known that the addition of certain organic nutrients such as acetate and methanol stimulate the anaerobic biotransformation of chlorinated aliphatic compounds by various microorganisms, see Gibson et al., Applied and Environmental Microbiology, Vol. 58, 1392 (1992). Such stimulated biotransformation has been applied in bioremediation processes to reduce contamination of soil and ground water by converting highly chlorinated aliphatic hydrocarbons to their lower homologs, See U.S. Pat. No. 5,277,815. Research indicates that anaerobic processes reduce the degree of halogenation and produce compounds that are then more susceptible to further microbial attack by aerobic microorganisms.
However, currently known nutrients achieve only a limited degree of biotransformation. That is, the conversion of highly chlorinated aliphatic hydrocarbons such as perchloroethylene and trichloroethylene to their ultimate lower homolog, ethylene, is limited. Instead of complete or near complete conversion in a desirable period of time, currently known nutrients in conjunction with suitable microorganisms, are able to effect only partial conversion, thereby leaving intermediate homologs such as dichloroethylene and/or vinyl chloride in the contaminated soil or water undergoing treatment. Thus, there is a need for a method of enhancing the extent of biotransformation of highly chlorinated aliphatic hydrocarbons which results in a greater degree of conversion to lower homologs, most preferably ethylene, than currently known methods.
In addition to the extent of biotransformation which occurs, another factor important to the attractiveness of a bioremediation process is the rate at which biotransformation occurs. The rate of biotransformation using currently known nutrients is limited, and it is desirable to obtain higher rates of biotransformation. When planning or attempting to bioremediate large volumes of contaminated soil or water, the limited rate of biotransformation associated with currently known nutrients may render such bioremediation processes impracticable or commercially undesirable. Thus, there is a need for a method of stimulating biotransformation which has a higher rate of conversion than currently known processes.