In-situ Biodegradation of EDB and EDC in the Presence of Gasoline

摘要

Ethylene dibromide (EDB) and ethylene dichloride (EDC) are additives formerly used as lead scavengers in gasoline, among other applications. Regulatory levels of these compounds are exceeded at more than one-third of the gasoline-contaminated sites in the United States. In general, the concentration of EDB and EDC is small when compared with the concentration of petroleum hydrocarbons like benzene. To gain perspective on the fate of EDB and EDC in the subsurface, publications from peer-reviewed scientific journals and societies were compiled and critiqued. Articles were grouped according to four categories: 1) microorganism physiology and genetics, 2) enzyme-catalyzed mechanisms, 3) kinetics, and 4) applications. Before soil-microbes can use EDB or EDC as a carbon source, the halogens (bromide and chlorine) must be removed, a.k.a. dehalogenation. For the most part, researchers agree that dehalogenation is an enzyme reaction. In some microorganisms, the DNA components responsible for encoding and producing these enzymes consist of a plasmid/chromosome pair. This plasmid is independently reproducible and transmutable between microorganisms; therefore, consortia of dissimilar species can share the DNA necessary for enzyme production. Haloalkane dehalogenase (dhIA) has been identified as the enzyme that separates the first of the two halogens on the EDB and EDC molecules. Additionally, researchers have found that removal of the first halogen is the rate-limiting step in biodegradation of EDB and EDC. The catalytic mechanisms of dhIA has been observed at an atomic scale using crystallographic analysis of electron density maps (Nature, 1993). In this article, a six-step catabolic process between enzyme and substrate that results in the production of a free halogen ion and an alcohol byproduct has been unveiled. Common soil microbes like Psudonomas and Xanthobacter have been isolated and grown using only EDB or EDC as a carbon source. Degradation rate constants are reported for dhIA conversion of EDB and EDC. Large and pilot scale ground-water remediation that make use of trickle filters and recirculating reactors have been used to accelerate biodegradation of EDB and EDC. Based on this research and the results from a batch experiment using contaminated ground water from Albuquerque, New Mexico, it is proposed that biodegradation of low concentration EDC and EDB occurs as a pseudo first-order reaction that is related to the background concentration of the dhIA enzyme. Future research will include development of an in-situ method to determine degradation rate constants and an assay to quantify the background levels of dhIA.