Abiotic and Biological Transformation of Tetraalkoxysilanes and Trichloroethene/cis-1,2-Dichloroethene Cometabolism Driven by Tetrabutoxysilane-Degrading Microorganisms

Abstract

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.

Geochemistry and microbial diversity of a trichloroethene-contaminated Superfund site undergoing intrinsic in situ reductive dechlorination

“…Depletion of nitrate and sulfate in D3 relative to T5 also was consistent with alkoxysilane-induced anaerobiosis. Furthermore, the presence of 2-ethylbutanol and 2-ethylbutyrate^both representing transformation products of TKEBS previously observed in controlled laboratory experiments [15,17]^and the accumulation of acetate in D3 suggested that the microorganisms native to this subsurface habitat had adapted to the presence of TCE and were performing fermentative reactions including anaerobic transformation of the tetraalkoxysilane compound.…”

“…An additional bene¢t of these compounds^that could not be explored in this work^is their potential for triggering aerobic cometabolism of chloroethenes [17]. Tetraalkoxysilanes and their respective hydrolysis products have been shown to serve as the primary substrates of aerobic enrichment cultures capable of co-oxidizing TCE and cis-DCE [17].…”

Microbial Transformations at Aqueous-Solid Phase Interfaces: A Bioremediation Approach

“…4 Si], contain four oxygen bridges from the central silicon atom to the corresponding organic (alkoxy) groups. These compounds are widely used as heat-exchange fluids, sealants, and lubricants because of their excellent thermal properties [24,[421][422][423]. At Lawrence Livermore National Laboratory site 300, these compounds along with trichloroethylene (TCE) were used in heat-exchanger pipes at their materials testing facility [421][422][423].…”

“…Specific research work related to the transformation of these compounds under environmental conditions is limited, and biological degradation of these compounds has not been investigated [423]. However, numerous hydrolysis studies have been conducted on the lower homologues of the tetraalkoxysilanes such as tetramethoxysilane and tetraethoxysilane [229,423]. These compounds hydrolyze abiotically to give the corresponding alcohols and silicic acid [424].…”

“…Vancheeswaran et al [421][422][423] investigated the attenuation of silicon-based organic compounds (i.e., tetraalkoxysilanes) along with TCE as subsurface contaminants by abiotic hydrolysis and biological mineralization at Lawrence Livermore National Laboratory site 300. They reported, under abiotic conditions, the hydrolysis of the alkoxysilanes such as TBOS and TKEBS to 1-butanol and 2-ethylbutanol, respectively, and silicic acid.…”