Graduate Thesis Or Dissertation
 

Abiotic and biological transformation of TBOS and TKEBS, and their role in the biological transformation of TCE and c-DCE

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  • At Site-300, Lawrence Livermore National Laboratory (LLNL), CA, trichloroethene (TCE) is present along with tetraalkoxysilanes such as tetrabutoxysilane (TBOS) and tetrakis(2-ethylbutoxy) silane (TKEBS), as subsurface contaminants. Intrinsic transformation of TCE to cis-dichloroethene (c-DCE) was observed in the groundwater at locations co-contaminated with TBOS or TKEBS. Attenuation of TBOS and TKEBS by abiotic hydrolysis and biological mineralization and the role played by TBOS and TKEBS in driving the TCE transformation were investigated. Under abiotic conditions, TBOS and TKEBS were found to slowly hydrolyze to 1-butanol and 2-ethylbutanol, respectively, and silicic acid. Hydrogen was produced as a result of the fermentation of the alcohols to the corresponding acids, and then subsequently to carbon dioxide. The hydrogen likely served as the electron donor for the microbially-mediated reductive dechlorination of TCE. The rates of hydrolysis of TBOS and TKEBS were determined and typical rates at pH 7, 30°C and 28 μM initial concentration, were 0.32 and 0.048 μ/day, respectively. The TBOS hydrolysis reaction was observed to be acid and base catalyzed and independent of temperature from 15 to 30°C. All hydrolysis experiments were conducted at concentrations above the solubility limit of TBOS and TKEBS and the rate of hydrolysis increased with concentration of TBOS or TKEBS. An aerobic microbial culture from the local wastewater treatment plant that could grow and mineralize the alkoxysilanes was enriched. The enriched culture rapidly hydrolyzed TBOS and TKEBS and grew on the hydrolysis products. The microorganisms grown on TBOS cometabolized TCE and c-DCE. TCE and c-DCE degradation was inhibited by acetylene indicating the stimulation of a monooxygenase enzyme. Acetylene did not inhibit the hydrolysis of TBOS.
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