Soil and groundwater at many existing and former industrial areas and disposal sites is contaminated by halogenated organic compounds that were released into the environment. Halogenated organic compounds are heavier than water. When they are released into the subsurface, they tend to adsorb onto the soils and cause the appearance of DNAPL (dense-non-aqueous phase liquid) pool. Among those halogenated organic compounds, trichloroethylene (TCE), a human carcinogen, is one of the commonly observed contaminants in groundwater. Thus, TCE was used as the target compound in this study. One cost-effective approach for the remediation of the chlorinated-solvent and petroleum products contaminated aquifers is the installation of permeable reactive zones or barriers within aquifers. As contaminated groundwater moves through the emplaced reactive zones, the contaminants are removed, and uncontaminated groundwater emerges from the downgradient side of the reactive zones. The objective of this proposed study was to assess the potential of using a passive in situ carbon/hydrocarbon releasing barrier system to bioremediate TCE-contaminated groundwater. This passive active biobarrier system would have advantages over conventional systems including less maintenance, cost-effectiveness, no above-ground facilities, no groundwater pumping and reinjection, and groundwater remediation in situ. In this study, the unique biobarrier system has been developed. The biobarrier system included a carbon/hydrogen releasing barrier, which contained slow carbon/hydrogen-releasing materials. The slow carbon/hydrogen releasing material would cause the aerobic cometabolism and reductive dechlorination of TCE in aquifer. The carbon/hydrocarbon-releasing materials would release carbon when contacts with groundwater and release hydrogen after the anaerobic biodegradation of released carbon, thus cause the reductive dechlorination of TCE.
In this study, batch experiments were operated to test the feasibility of using vegetable oil as the slow-released substrate, molasses as fast-degrading substrate, and biosurfactant [simple green (SG) and lecithin] to produce emulsified oil. Several experimental conditions included the concentrations of contaminants and substrates, shacking speed, and percentage of each component. In the batch experiment, emulsified oil was prepared by blending with water, vegetable oil, lecithin, and SG. The stable emulsified oil was formed after 30 min of blending. Results show that formed emulsified oil globule had the smallest diameter with negative zeta potential. The negative zeta potential of the oil droplets would increase the inter-particle repulsion between the droplets and soil particles, which was beneficial for the transportation of emulsified oil within the soil pores. However, the emulsified oil and the vegetable oil would also adsorb onto the soil particles (with positive zeta potential), and cause the following clogging of the soil pores. The sorbed oil droplets would desorb after the equilibrium was reached. The sorption mechanism and soil heterogeneity would also cause the retardation of oil in the soils. Compared with vegetable oil, results show that the emulsified oil caused more uniform sorption onto soil particles and lower oil to water partition coefficient. Moreover, results show that TCE had higher affinity to emulsified oil. Thus, emulsified oil droplets would help to retard TCE molecules before the anaerobic biodegradation of TCE can occur. Thus, the removal of TCE in water phase could be due to the mechanisms of sorption and biodegradation.
Results from the microcosm study indicate that the addition of emulsified oil, cane molasses, SG, or lecithin would enhance the biodegradation rate of TCE under anaerobic conditions. However, addition of multivitamin would increase the bacterial population in the media but would not be able to enhance the TCE degradation rate. Results show that significant pH drop was observed due to the production of organic acids after the anaerobic biodegradation process of cane molasses. This also caused the inhibition of microbial growth in microcosms. Results reveal that higher TCE removal efficiency was observed in microcosms with lecithin addition followed by the addition of cane molasses, SG, emulsified oil, multivitamin, groundwater (without substrate addition). Moreover, reduced environmental conditions [with negative oxidation-reduction potential (ORP)] were observed after the addition of lecithin, cane molasses, SG, and emulsified oil. This would accelerate the reductive dechlorination of TCE in practical application. Results from the microcosm experiments also show that addition of cane molasses would significantly enhance the reductive dechlorination of TCE to the nontoxic end product (ethane). However, appearance of high nitrate concentration would inhibit the TCE degradation process due to the occurrence of denitrification. Compared with nitrate, high sulfate concentration would not have significant impact on the reductive dechlorination of TCE.
Results from the gene analysis show that phenol monooxygenase, toluene monooxygenase, and toluene dioxygenase were observed in the microcosms with lecithin, cane molasses, SG, and emulsified oil. This indicates that the addition of substrates would induce the potential of TCE-degrading enzyme. Addition of emulsified oil and emulsified oil in nitrate or sulfate-rich media would stimulate Dehalococcoides sp. to induce tceA, bvcA, and vcrA, enzymes for TCE reductive dechlorination. Results from the column study indicate that two major TCE removal mechanisms caused by the emulsified oil are as follows: (1) adsorption of TCE onto oil globules, and (2) release of hydrogen and acetate to enhance the reductive dechlorination process. The slowly released substrates from the emulsified oil would also consume dissolved oxygen (DO) and produce reduced environmental conditions to enhance the growth of TCE-degraders. Results from the column study also indicate that the remediation cost is approximately NT$9 for each mg of TCE removal. However, the cost would go up if operation and maintenance cost is considered. Results of this study will aid in designing an in situ biobarrier system containing slowly released carbon/hydrogen materials for remedial application. The proposed treatment scheme would be expected to provide a more cost-effective alternative to remediate TCE and other chlorinated-solvent contaminated aquifers. Knowledge obtained from this study will aid in designing a carbon/hydrogen releasing reactive barrier system for site remediation.
