Abiotic and biological transformation of tetraalkoxysilanes and trichloroethene/cis-1,2-dichloroethene cometabolism driven by tetrabutoxysilane-degrading microorganisms

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 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 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.