Two different remediation methods, one biological and the other non-biological, but with potential compatibilities, were considered for treating groundwater containing high concentrations of 1,1,1-trichloroethane (TCA). One treatment strategy involves biological reductive dechlorination and bioaugmentation, and the second, abiotic treatment with nano-scale zero-valent iron (ZVI). The two approaches are potentially compatable due in part to the reductive nature of the processes as well to the release of molecular hydrogen by ZVI that could serves as an electron donor for down-gradient biological dehalogenation.
Biological reductive dechlorination was studied in microcosms constructed from site groundwater containing 14 mg/L TCA, and smaller amounts of 1,1-DCA, and 1,1-DCE. Four conditions were studied: (1) unamended, (2) amended with lactate, (3)amended with lactate and oil emulsion , and (4) lactate amended and bioaugmented with TCA-Degraders
The “bioaugmentation microcosm” was inoculated with a unique TCA and TCE degrading culture designated “NJa”which is able to convert TCA to chloroethane (CA), and TCE, DCE, and VC to ethene.
The data showed that the lactate-amended microcosm was able to convert TCA to 1,1-DCA, but was unable to convert 1,1-DCA to chloroethane. cis-DCE and 1,1-DCE were transformed completely to ethene, indicating that the native groundwater contained Dehalococcoides sp. (The groundwater was PCR-positive for Dehalococcoides ethenogenes). When the lactate amended microcosm was bioaugmented with NJa, TCA and 1,1-DCA were converted to chloroethane indicating that the native groundwater lacked the organism responsible for TCA degradation.
Using the groundwater with high TCA concentration (57 ppm), a second laboratory study was conducted on the reactions of three types of ZVI with groundwater containing TCA and “daughter” products. The ZVI samples differed in terms of surface area and the nature of the surface material. 100 ml portions of groundwater were transferred from 1 liter samples bottles under oxygen-free conditions and transferred to 160 cc serum bottles. ZVI (2.5 gm) was added to the groundwater as a powder. After an initial hand mix, test bottles were mixed on a slow rocking mixer for a period of 360 hr. For analysis of chlorinated compounds, and dissolved gasses, headspace gas samples were removed from serum bottles using a 100 ul gas tight syringe and injected directly into a gas chromatograph. The reaction of ZVI was also studied in the presence of groundwater amended with vegetable oil (a potential electron donor at this site). The results showed that ZVI reacted rapidly with TCA in the presence, and absence of vegetable oil, degrading 100 % of the TCA within 24 hr. In addition, 1,1- DCE and VC were converted to ethene. The T 1/2 for TCA was less than 4 hr for Type 2 ZVI iron. During this period, 18 % of the TCA was converted to DCA, 46 % to ethane, and 36% to non-volatile intermediates.