Intrinsic remediation of trichloroethene driven by tetraalkoxysilanes as co-contaminants: Results of microcosm and field studies

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) and tetrakis(2-ethylbutoxy)silane (TKEBS) are abiotically hydrolyzed to their corresponding alcohols, 1-butanol and 2-ethylbutanol, respectively, and silicic acid. These alcohols are then fermented by subsurface microorganisms to their corresponding acids and hydrogen, which likely serves as the ultimate electron donor facilitating reductive dechlorination of TCE. At Site 300, Lawrence Livermore National Laboratory (LLNL), California, tetraalkoxysilanes are present along with TCE as subsurface contaminants. Intrinsic transformation of TCE to cis-dichloroethene (c-DCE) was observed at this site, and this was promoted by the transformation products of these tetraalkoxysilanes. Efficient transformation of high concentrations of TCE (150?300 mg/l aqueous concentration) was observed in anaerobic microcosms constructed with groundwater from this site. The lack of transformation of c-DCE to vinyl chloride (VC) and ethene in the microcosms is consistent with studies of the microbial community in the site groundwater reported by Lowe et al. (2002).