ENHANCED CAH DECHLORINATION USING SLOW AND FAST RELEASING POLYLACTATE ESTERS

摘要

This paper compares the results of two separate field tests using slow and fast releasing polylactate esters to remediate chlorinated aliphatic hydrocarbon (CAH) contamination in groundwater by enhancing reductive dechlorination. The two polylactate esters produce soluble lactic acid at different rates upon hydration in groundwater. Microbes in the groundwater metabolize the lactic acid and in the process produce hydrogen molecules that serve as electron donors for reduction-oxidation reactions, including reductive dechlorination reactions. At both sites, both the fast and slow releasing polylactate esters have been shown to effectively change groundwater conditions to enhance reductive dechlorination of CAHs by first reducing competing electron acceptors. At Site 1 only the slow releasing compound was used and at Site 2 both compounds were used in conjunction. The time required to reduce the competing electron acceptor concentrations was greatly different at the 2 sites - approximately 1 month for Site 2 compared to 7 months for Site 1. Along with quickly providing conditions favorable for reductive dechlorination at Site 2, the high levels of hydrogen produced by the fast releasing compound also induced conditions favorable for methanogenesis. A dramatic increase in methane concentration was measured at Site 2 following the injections. At Site 1, low concentrations of hydrogen produced by the slow releasing compound only slightly increased methane levels. The higher concentrations of molecular hydrogen generated by the fast releasing compound is believed to be consumed more readily in methanogenesis and may limit the acceleration of the desired reductive dechlorinating microbial activity. However, the stimulation of the methanogenic microbes did not cause a detrimental effect on the dechlorinating microbes. The high methanogenic microbial activity observed at Site 2 did waste hydrogen in the production of methane, but reductive dechlorination of the CAHs continues.