Enhanced reductive de-chlorination of a solvent contaminated aquifer through addition and apparent fermentation of...

In a field study, aqueous cyclodextrin (CD) was investigated for its ability to extract chlorinated volatile organic compounds (cVOC), such as trichloroethylene (TCE), 1,1,1-trichloroethane (TCA), and dichloroethene (DCE) through in-situ flushing of a sandy aquifer. After cessation of aquifer flushing, a plume of CD was left. Changes in CD, cVOC, and inorganic terminal electron acceptors (TEAs) (DO, nitrate, sulfate, iron) were monitored in four rounds of wellwater sampling (20, 210, 342, and 425days after cessation of active pumping). Post-CD flushing VOC levels rebounded (850% for TCE, 190% for TCA, and 53% for DCE) between the first two sampling rounds, apparently due to rate-limited desorption from aquifer media and dissolution from remaining NAPL. However, substantial reduction in the mass of TCE (6.3 to 0.11mol: 98%) and TCA (2.8 to 0.73mol: 74%) in groundwater was observed between 210 and 425days. DCE should primarily be produced from the degradation of TCE and is expected to subsequently degrade to chloroethene. Since DCE levels decreased only slightly (0.23 to 0.17mol: 26%), its degradation rate should be similar to that produced from the decaying TCE. Cyclodextrin was monitored starting from day 210. The mass of residual CD (as measured by Total Organic Carbon) decreased from 150mol (day 210) to 66 (day 425) (56% decrease). The naturally anaerobic zone within the aquifer where residual CD mass decreased coincided with a loss of other major potential TEAs: nitrate (97% loss), sulfate (31%) and iron (31%). In other studies, TCE and 1,1,1-TCA have been found to be more energetically favorable TEAs than sulfate and iron and their degradation via reductive dechlorination has been found to be enhanced by the fermentation of carbohydrates. Such processes can explain these observations, but more investigation is needed to evaluate whether residual levels of CD can facilitate the anaerobic degradation of chlorinated VOCs.