Are Si-C bonds cleaved by microorganisms? A critical review on biodegradation of methylsiloxanes

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Methylsiloxanes, compounds that contain H 3C-Si-O subunits in their molecular structure, are emerging ubiquitous pollutants now detected in many environmental compartments. These compounds and generally Si-C bonds do not occur in living nature, but are industrially produced worldwide in millions of tons per annum and are widely used, resulting in their release to the environment. It is an open question whether or to what extent microorganisms are able to decompose these compounds. The presence of methylsiloxanes in many biogases adds to the economic relevance of this question. We here review and critically discuss, for the first time, the evidence obtained for and against degradation of methylsiloxanes by microorganisms, and in particular for microbial cleavage of Si-CH 3 bonds. As a result, no convincing demonstration of Si-C cleavage by native environmental microorganisms has been found.

Original languageEnglish
Article number137858
Publication statusPublished - 01.04.2023

Bibliographical note

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In the second paper of the series (Wang et al., 2014a), the same authors together with two colleagues described a BTF made of glass, filled with ceramic rings and inoculated with a purified strain of Phyllobacterium myrsinacearum that was isolated from the WWTP effluent of an industrial organosilicon manufacturer. During a D4 removal experiment (126 d), episodes of pH down to 2 and up to 12 occurred. Efficient D4 removal from an air stream (up to 60%), and products DMSD, dimerdiol, D5, oxalic acid, and Si(OH)4 were reported (GC trace of silylated product mixture given), but no MeSi(OH)3 or methanol. No explanation was given for oxalic acid formation, and no control experiment on possible formation of dissolved Si(OH)4 from glass and ceramics was reported. Si(OH)4 seemingly was identified via mass spectrum of a GC peak in the silylated THF extract of the crude reaction mixture that was ascribed to M4Q. That raises the possibility that this peak instead corresponded to the isomer L5, the silylation product of the trimerdiol, i.e. a hydrolysis product of D4. M4Q and L5 exhibit very similar mass spectra. This hypothesis is supported by the fact that oligomerdiols are extracted from an aqueous phase into THF, whereas inorganic silicic acid/silicate presumably is not.Santos-Clotas et al. (2019) studied removal of a mixture of D4, D5, hexane, toluene and limonene in an anoxic BTF made of glass and populated by a mixture of sludge bacteria and a strain of Pseudomonas aeruginosa adapted to D4. The BTF filling was initially lava rocks, later a layer of activated carbon was added “as a support (in order to enhance the mass transfer of biogas pollutants to the microbial community)”, as an adsorbent and reactor for catalyzed hydrolysis of the siloxanes. Finally the lava was removed leaving activated carbon as the only bed material. Thereby a beneficial effect of activated carbon on removal of these pollutants was seen. Products from the siloxanes, though, were those expected from hydrolysis (DMSD, dimerdiol, trimerdiol), no Me1Si or C1 product other than CO2 (from rapid biodegradation of toluene and limonene) was found. Silicic acid or silicate was found and quantified and was reported to well close the Si balance (no details given), but no attempts were reported to exclude possible silicic acid formation from glass and lava rocks. However, in a parallel abiotic BTF under the same conditions no removal of the target pollutants occurred.

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    Research areas

  • Chemistry - Decamethylcyclopentasiloxane, Dimethylsilanediol, Microbial cleavage, Organosiloxanes, Silicon–carbon bond, Silicon–oxygen bond