BIOREMEDIATION OF TCE BY MULCH BIOBARRIERS: IMPACTS OF THE PRESENCE OF CO-CONTAMINANTS
Trichloroethene (TCE), a recognized human carcinogen, is a wide-spread groundwater contaminant and is commonly used as an industrial chlorinated solvent. It has been found in almost 34% of the nation’s drinking water supplies. Except for ethene, all other daughter products from reductive dechlorination of TCE are toxic to human health. Starting from the past few decades, people have been exploring effective and efficient removal technologies of TCE from water resources. Among them, biological PRBs (Permeable Reactive Barriers), also called biobarriers, are a promising and cost-effective bioremediation treatment that passively capture contaminants plume and then remove, transform, or destruct contaminants into nontoxic chemicals before they leave the target site. Despite the promise of applying biobarriers for bioremediation of chlorinated ethenes, no pilot- or field-scale research have demonstrated a complete dechlorination of TCE to benign ethene with a 100% conversion efficiency. Besides, several studies have demonstrated that the presence of nitrate and/or sulfate could preclude the reductive dechlorination of TCE under anaerobic conditions. Therefore, when remediating groundwater pollution with biobarriers, the presence of common nutrients including nitrate and sulfate and of other harmful industrial waste must be considered as complicating factors. In this research, column- and batch-scale tests were conducted to observe and measure the dechlorinating performance of KB-1TM mixed culture in simulated lab-scale biobarriers and in batch tests, with the presence of diverse co-contaminants: nitrate, sulfate and RDX. Considering the favorable market price and higher adsorption capacity for chlorinated ethenes, pine bark mulch was chosen to be filled into PRBs in this research to act as substrate and serve as the electron donor to favor the growth of microbes of interest. Butyric acid was injected into both columns and batch microcosms to enhance the reductive dechlorination by driving oxygen levels down and serving as extra electron donors. Gas chromatography was applied to compare methane production and quantify chlorinated ethenes inside columns and microcosms. PCR amplification and further DNA sequencing analysis were conducted to explore differences of microbial distributions along and between columns receiving varying electron acceptors. Column-scale experiments turned out that sulfate with a concentration of 0.25 mM had no obvious inhibitory impact on the dechlorination ability of mulch biobarrier when receiving 1 mg/L TCE. Batch-scale tests revealed that the presence of 1 mM nitrate would decline the dechlorination rate of chlorinated ethenes, while the presence of 1 mg/L RDX seemed to make no difference. Batch-scale tests also indicated that the presence of chlorinated ethenes would inhibit the denitrification process. Results from DNA sequencing showed that significantly higher amounts of dechlorinators existed in columns without the presence of nitrate and sulfate, indicating that these two alternative electron acceptors are driving the differences of microbial diversity between control and experimental columns.
Richardson, Ruth E.
Reid, Matthew Charles
Civil and Environmental Engineering
M.S., Civil and Environmental Engineering
Master of Science
dissertation or thesis