Remediating chlorinated solvents in groundwater to regulatory criteria is one of the Department of Defense’s (DoD) greatest environmental challenges. A variety of organic substrates have been used to stimulate reductive dechlorination of chlorinated compounds in groundwater. The use of vegetable oil has been developed as an economical, long-lasting alternative. However, it must be demonstrated in a laboratory setting that vegetable oil is a sufficient and adequate carbon source to drive anaerobic dechlorination. Field evidence and current knowledge of the anaerobic process indicates that this should be the case.
The overall objective of this project was to advance the VegOil Process to a point that DoD and others can evaluate its potential effectiveness and apply it routinely. To meet this objective, laboratory studies using vegetable oils were performed to (1) determine the anaerobic breakdown products of the oils, most importantly the dynamic profile of molecular hydrogen that results and the fractions of reducing equivalents that are channeled to dechlorination versus competing methanogenesis and (2) assess the nutritional sufficiency of the oils.
The use of vegetable oil as an organic substrate is intended to create the redox and electron donor conditions necessary to promote microbial reductive dechlorination of chlorinated solvents. Microcosm studies are useful to determine the effectiveness and nutritional sufficiency of vegetable oils to stimulate reductive dechlorination. An anaerobic mixed-culture was employed, consisting principally of a well-characterized dechlorinator (Dehalococcoides ethenogenes, Strain 195), various fatty-acid fermenters, acetotrophic methanogens, and hydrogenotrophic methanogens. Aliquots of the culture were transferred to serum bottles and amended with the following treatments: (a) tetrachloroethene [PCE] only; (b) PCE + oil; (c) PCE + oil + yeast extract + vitamin mixture; or (d) PCE + yeast extract + vitamin mixture. The time-course profiles of volatile fatty acids, hydrogen, chloroethenes, and methane were monitored. Successful degradation of the oils and reductive dechlorination was sustained, and the microcosms were re-spiked with PCE. Crude soybean oil, refined soybean oil, and palm kernel oil were tested and the microcosms were run for up to 140 days. Bottle types (a) and (d) served as controls without oils.
The laboratory microcosm studies were completed using crude soybean oil, refined soybean oil, and palm kernel oil. Limited microcosms using hydrogen release compound (a product of Regenesis, Inc.) were also conducted for comparison. Results of the microcosm studies have been reported in a Cornell University Thesis entitled Use of Vegetable Oil in Reductive Dechlorination of Tetrachloroethene by Sin Chit To, dated May 2001. A summary report entitled Summary of Laboratory Microcosm Studies and Engineering Implications of Using Vegetable Oils to Stimulate Reductive Dechlorination of Chlorinated Ethenes (Parsons, 2003) compares the results of the laboratory study to microcosm studies in the literature and to field observations. These studies document that vegetable oils are capable of stimulating complete reductive dechlorination of chlorinated ethenes over long periods of time in the field without prohibitive levels of methanogenesis. The biodegradation of vegetable oil is capable of sustaining reductive dechlorination without the addition of supplemental nutrients.
The primary benefit of using vegetable oil to stimulate reductive dechlorination is the relative cost of the substrate and the need for fewer applications. Vegetable oils are inexpensive, innocuous, food-grade carbon sources that cost from $0.20 to $0.50 per pound. Because of the low solubility of vegetable oil, the potential exists that a single low-cost injection could provide sufficient carbon to drive reductive dechlorination for many years. A secondary beneficial process is the partitioning of the dissolved chlorinated solvents into the vegetable oil, causing aqueous-phase concentrations to lower until steady-state conditions are reached. The chlorinated compounds are then slowly released back into groundwater within a reaction zone that is optimal for reductive dechlorination to occur.