Evidence suggests there is competition between reductive dehalogenators and methanogens in which the methanogens compete for the use of hydrogen in the conversion of carbon dioxide to methane. It is believed that a low concentration of hydrogen favors the reductive dehalogenators and starves out the methanogens. The objective, therefore, is to keep hydrogen concentrations low. The time release feature of HRC, which is based on the hydrolysis rate of lactic acid from the ester and the subsequent lag time to hydrogen conversion, facilitates this objective. HRC, therefore, becomes a passive form of enhanced natural attenuation in contrast to the more capital and management intensive alternatives now available. Laboratory and field results will be presented that will expand on the first uses of HRC by various members of the engineering and consulting firm community.
Hydrogen Release Compound (HRC) offers a passive, low-cost treatment option for in-situ anaerobic bioremediation of chlorinated aliphatic hydrocarbons. HRC is a proprietary, environmentally safe, food quality, polylactate ester specially formulated for the slow release of lactic acid upon hydration. Bioremediation with HRC is a multi-step process. Indigenous anaerobic microbes metabolize the lactic acid generated by HRC and produce hydrogen. The resulting hydrogen can be used by reductive dehalogenators which are capable of dechlorinating CAHs. Major target compounds in this group include PCE, TCE, TCA and their derivatives.
HRC is a high viscosity, flowable liquid that can be pressure injected using various direct push technologies. By providing a long-lasting, time-released hydrogen source, HRC can enhance anaerobic reductive dechlorination of CAHs. The following are some of the key advantages of HRC.
1) Low maintenance and low cost - unlike actively engineered systems, continuous mechanical operation and maintenance is eliminated, dramatically reducing overall operations and maintenance costs.
2) Constant and persistent hydrogen source - HRC is a semi-solid material that will remain where emplaced and generate highly diffusable hydrogen slowly over time. Since CAH plumes are difficult to locate, a continuous, highly diffusable hydrogen source increases the effectiveness of contact, containment, and remediation,
3) Enhance desorption of CAHs - The continuous hydrogen source provided by HRC can reduce dissolved phase CAH concentrations. This creates a larger concentration gradient which in turn facilitates desorption of CAHs from the soil matrix and
4) Favored reductive dechlorination over possible competing methanogenic activity - Results from several university studies suggest that there is competition for hydrogen between the reductive dechlorinators and methanogens While methanogen survival is favored under elevated hydrogen conditions, reductive dechlorinators are best supported in conditions of more moderate hydrogen concentration.
METHODS, RESULTS AND DISCUSSION
HRC Microcosm Studies with TCE. The use of HRC for remediation of TCE was studied in 200 ml test tube experiments in which the release of lactic acid from HRC was measured as a function of bacterial concentration and HRC concentration. In the experiments, 10 grams of sterilized sand was added to each test tube followed by a solution of TCE with a concentration up to 140 mg/L. Various quantities of bacteria, capable of metabolizing TCE, were then added. Finally, 0.5 or 1.5 grams of HRC was added to each test tube. Each day, 6 ml samples were taken and analyzed for TCE and lactic acid.
Results indicated that TCE was remediated under all conditions. Results from one representative experiment are presented in Figure 1, which shows that reduction in TCE follows an increase in lactic acid release. It is important to note that most of the initial drop in TCE (within the first hour) was due to adsorption of TCE on the sand. This TCE eventually desorbed from the soil as the dissolved phase TCE was remediated during the progression of the experiment.
Aquifer Simulation Vessel (ASV) Studies. The Aquifer Simulation Vessel (ASV) is used to establish the influence of important field-scale parameters on the efficacy of HRC. The ASV consists of a horizontal six inch diameter/six foot length pipe. The ASVs are designed to allow measurement at six inch intervals along the pipe. Each pipe is packed with actual contaminated soil from the field. HRC is placed in the system at the 'CAH-water' inlet side, such that the flowing water will pass through the HRC and then move through the pipe. The water can be added with various levels of CAHs and remediation rates measured. The distribution of lactic acid and its breakdown products can also be measured.
In the initial studies, the ability of HRC to facilitate the reductive dechlorination of TCE was measured. In the experiments, an ASV was filled with soil. Then, the TCE was added to the soil at the CAH-water inlet side at a concentration of approximately 6 mg/L. The ASV was allowed to acclimate over a period of 6 days during which time baseline TCE concentration profiles were developed. Finally, a 'slug' of HRC was added to the inlet side and the system was run at a flow rate of 0.5 ft/day for a period of 9 days. Results from one experiment, in which TCE levels were measured at days 1, 6, and 9 at each six inch interval along the ASV, are presented in Figure 2.
Field Study - Single Well Application. The effects of HRC-containing canisters in a single well were studied at a site in Florida. Eleven, four foot long canisters were placed in a five inch monitoring well containing moderate concentrations of TCE, cis-1,2-DCE, and VC. The contaminant plume, contained in a fine to medium grained sand aquifer, measured 120 feet in length by 60 feet in width. Due to a flat gradient, groundwater velocity is estimated to be less than 0.1 foot per day. Reductions in contaminant concentrations in the well are presented in Figure 3. Following three months of treatment, reductions in TCE, cis-1,2-DCE, and VC were 96%, 98%, and 99%, respectively. Absence of DO and highly negative redox levels confirmed the existance of a highly reduced environment.
Field Study - Recirculating Well System. As part of the USEPA SITE program, Harding Lawson Associates tested the efficacy of HRC in remediating CAHs in a recirculating well system. HRC canisters were inserted into injection wells; circulation was maintained between extraction and re-injection wells. Weekly monitoring over 189 days indicated changes in PCE, TCE, DCE, and VC concentrations as represented in Figure 4. Harding Lawson Associates concluded: 1) HRC can reduce redox conditions in wells containing HRC and along the downgradient flow path, 2) TCE biodegradation was demonstrated, 3) biodegradation products cis-DCE, VC, and ethene were detected.
Field Study - Full Scale Injection. The direct-push injection of HRC is currently being studied with Montgomery Watson at a site contaminated with high levels of PCE. 240 pounds of HRC were injected at 12 delivery points in a 60 square foot area. Approximately 8 months following the installation of HRC, PCE mass was reduced 126 grams, representing a reduction of 80%. PCE degradation rates in the HRC injected zone were 11.5 times faster than background rates at Day 70 and 4.9 times faster at Day 120.
Concurrent increases in PCE-degradation daughter products, TCE, DCE and VC, were also documented as was further sequential degradation through some of the daughter products themselves at different times. These total changes in mass over time for all the CAHs is presented in Figure 5. The continued effects of a single cost-effective application of HRC are demonstrated to be present for at least 253 days.
Although not shown, there were mass balances between parent and daughter products of between 27% and 46%; an important indicator that the HRC injections facilitated contaminant removal by biodegradation. There were several other indicators that the proper conditions for facilitating reductive dechlorination were established, including: highly negative redox levels, significant reductions in sulfate and nitrate, formation of acetic acid, increased dissolved hydrogen levels, and a substantial increase in total plate counts for anaerobic bacteria.