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 abio tic hydrolysis and biological mineralization was investigated. At Lawrence Livermore National Laboratory site 300, tetraalkoxysilanes are present alon g with trichloroethene (TCE) as subsurface contaminants. Under abiotic cond itions, the alkoxysilanes such as tetrabutoxysilane (TBOS) and tetrakis(2-e thylbutoxy)silane (TKEBS) hydrolyze to l-butanol and 2-ethylbutanol, respec tively, and silicic acid. The rates of hydrolysis of TBOS and TKEBS were de termined to evaluate the significance of the hydrolysis reaction in the att enuation process, and typical rates at pH 7, 30 degrees C, and 28 mu mol/L initial concentration were 0.32 and 0.048 mu mol/L/day, respectively. The T BOS hydrolysis reaction was observed to be acid- and base-catalyzed and ind ependent of temperature from 15 to 30 degrees C. All hydrolysis experiments were conducted at concentrations above the solubility limit of TBOS and TK EBS, and the rate of hydrolysis increased with concentration of TBOS or TKE BS. 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 produ cts. The microorganisms grown on TBOS cometabolized TCE and cis-1,2-dichlor oethene (c-DCE). TCE and c-DCE degradation was inhibited by acetylene, indi cating that a monooxygenase was involved in the cometabolism process. Acety lene did not inhibit the hydrolysis of TBOS or the utilization of l-butanol , indicating that the above monooxygenase enzyme was not involved in the de gradation of TBOS.