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 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 (cDCE). 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. Introduction Tetraalkoxysilanes are a group of silicon-based compounds that contain four oxygen bridges from the central silicon atom to the corresponding organic (alkoxy) group. These compounds are widely used as heat-exchange fluids, sealants, and lubricants because they possess excellent thermal properties, which improve with length and branching of the organic substituent (1). The tetraalkoxysilanes of interest for this study are tetrabutoxysilane (TBOS) (CH3CH2CH2CH2O)4Si and tetrakis(2-ethylbutoxy)silane (TKEBS) (CH3CH2CH(CH2CH3)CH2O)4Si. At Lawrence Livermore National Laboratory site 300, these compounds along with trichloroethene (TCE) were used in heat-exchanger pipes at their materials testing facility. Subsurface contamination by these compounds resulted from leaking heat-exchanger pipes. TBOS and TKEBS are present as light nonaqueous phase liquids whereas TCE is present as a dense nonaqueous phase liquid in the subsurface and is also dissolved in both pure phase alkoxysilanes and groundwater. Available literature on TBOS and TKEBS mainly focuses on their thermal properties (1). Specific research work related to the transformation of these compounds under environmental conditions is limited. To our best knowledge, biological degradation of these compounds has not been investigated. However, numerous hydrolysis studies have been conducted on the lower homologues of the tetraalkoxysilanes such as tetramethoxysilane and tetraethoxysilane (2-5). These compounds hydrolyze abiotically to give the corresponding alcohols (2-5, 7) and silicic acid. Thus, the hydrolysis of TBOS and TKEBS was expected to yield 1-butanol and 2-ethylbutanol, respectively. The general equation for the complete hydrolysis of tetralkoxysilanes is as follows