Attenuation of silicon-based organic compounds (tetraalkoxysilanes) by abiotic hydrolysis and biological mineralization was investigated. At Lawrence Livermore National Laboratory site 300, tetraalkoxysilanes are present along with trichloroethene (TCE) as subsurface contaminants. Under abiotic conditions, the alkoxysilanes such as tetrabutoxysilane (TBOS) and tetrakis(2-ethylbutoxy)silane (TKEBS) hydrolyze to 1-butanol and 2-ethylbutanol, respectively, and silicic acid. The rates of hydrolysis of TBOS and TKEBS were determined to evaluate the significance of the hydrolysis reaction in the attenuation process, and typical rates at pH 7, 30 °C, and 28 µmol/L initial concentration were 0.32 and 0.048 µmol/L/day, respectively. The TBOS hydrolysis reaction was observed to be acidand 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 cis-1,2-dichloroethene (c-DCE). TCE and c-DCE degradation was inhibited by acetylene, indicating that a monooxygenase was involved in the cometabolism process. Acetylene did not inhibit the hydrolysis of TBOS or the utilization of 1-butanol, indicating that the above monooxygenase enzyme was not involved in the degradation of TBOS.

The biological reduction of trichlorofluoroethene (TCFE) was investigated in anaerobic groundwater microcosms. TCFE was reductively dehalogenated by microorganisms to produce three dichlorofluoroethene isomers, with cis-1,2-dichlorofluoroethene (c-DCFE) being the main isomer formed. Further sequential biological transformation of these compounds to mono-chlorofluoroethene isomers was incomplete and occurred at much slower rates. The rates of TCFE reduction were compared to the rates of reduction of two common chlorinated solvents, perchloroethene (PCE) and trichloroethene (TCE), when present at similar concentrations. Aqueous concentrations ranged from 7.0 to 14.0 mg/L for TCFE and from 7.5 to 15.0 mg/L for PCE and TCE. Similar rates of PCE and TCE transformation relative to TCFE were observed in single-compound tests (PCE, TCE, and TCFE in separate microcosms) and when the contaminants were present together as mixtures in the microcosms. The close similarities between the time course and kinetics of TCFE degradation and the degradation of both PCE and TCE, when present at comparable initial concentrations, suggest that TCFE could potentially be used as a benign reactive tracer to measure in-situ rates of PCE and TCE transformation in contaminated environments.

Biological reduction of trichloroethene (TCE), driven by the transformation products of tetraalkoxysilanes, was investigated in seasonal field monitorings and anaerobic groundwater microcosms. Under anaerobic conditions, tetraalkoxysilanes such as tetrabutoxysilane (TBOS) andtetrakis (2-ethylbutoxy) INTRODUCTIONTrichloroethene (TCE) is a common groundwater contaminant in aquifers throughout the United States (Westrick et al., 1984).TCE ranks in the top ten priority pollutants listed by the U.S. Environmental Protection Agency (EPA) (Federal Register, 1989). Over the past decade, microbial degradation of TCE has been extensively studied (Hopkins et al., 1993;Mars et al., 1996;Alvarez-Cohen & McCarty, 1991;Gibson & Sewell, 1992;Maymo-Gatell et al., 1997;Distefano et al., 1992;Fennell et al., 1997;Smatlak et al., 1996;Ballapragada et al., 1997;Sharma & McCarty, 1996;Semprini et al., 1995). Reductive dechlorination under anaerobic conditions and aerobic cometabolic processes are the predominant pathways for TCE transformation. In aerobic cometabolic processes, fortuitous oxidation of TCE is catalyzed by the enzymes induced and expressed for the initial oxidation of the growth substrates (Hopkins et al., 1993;Mars et al., 1996;Alvarez-Cohen & McCarty, 1991). In the reductive dechlorination process,TCE serves as an electron acceptor and chlorine atoms are replaced by hydrogen (Gibson & Sewell, 1992;Maymo-Gatell et al., 1997;Distefano et al., 1992;Fennell et al., 1997;Smatlak et al., 1996;Ballapragada et al., 1997 (Gibson & Sewell, 1992;Distefano et al., 1992;Fennell et al., 1997;Smatlak et al., 1996;Ballapragada et al., 1997) have been shown to support the dechlorination reaction. A range of dechlorinating cultures have been isolated and studied to determine the extent of dechlorination (Distefano et al., 1992;Holliger et al., 1993;Neumann et al., 1994;Smatlak et al., 1996;Maymo-Gatell et al., 1997;Fennell et al., 1997;Ballapragada et al., 1997;Holliger et al., 1998;Löffer et al., 2000). Incomplete dechlorination to cisdichlorethene (c-DCE) or vinyl chloride (VC) is often observed. However, some cultures are reported to dechlorinate completely to ethene (Distefano et al., 1992;Fennell et al., 1997;Smatlak et al., 1996;Ballapragada et al., 1997).The potential for enhancing anaerobic transformation processes for bioremediation is currently being tested and several recent studies have focussed on the role of H 2 (Fennell et al., 1997;Smatlak et al., 1996;Ballapragada et al., 1997) in the reductive dechlorination process. It has been shown that H 2 , generated from more complex organic substrates via fermentation, can serve as the ultimate electron donor in the dechlorination of TCE (Fennell et al., 1997;Smatlak et al., 1996;Ballapragada et al., 1997). In natural systems, H 2 -utilizing microorganisms include methanogens, acetogens, sulfidogens, and dechlorinators.The dechlorinators must compete with these other hydrogenotrophs for the evolved H 2 (Smatlak et al., 1996;Ballapragada et al., 1997). A significant advantage f...