Decomposition of environmentally persistent cyclic methylsiloxanes in subcritical water

Abstract The cyclic methylsiloxanes octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5) were reacted in subcritical water at temperatures of 150–350 °C under argon, under O 2 , or in the presence of H 2 O 2 . When the reactions were performed in pure subcritical water (under argon) at 300 °C for 6 h, both D4 and D5 were almost completely hydrolyzed to water-soluble and hexane-non-extractable compounds such as dimethylsilanediol (DMSD). However, the high percentages of total organic carbon (TOC) remained in the aqueous phase [87% and 81% of the carbon atoms in the initial D5 (26.6 μmol) and D4 (30.6 μmol), respectively] and the low silicate yields (both 3% of the silicon atoms in the initial D5 and D4) indicated that little mineralization had occurred. Although the presence of O 2 had no significant effect on the reactivity of these substrates in subcritical water at temperatures up to 300 °C, the presence of H 2 O 2 enhanced the decomposition of these substrates. Raising the temperature to 350 °C greatly enhanced the mineralization. Specifically, when the reactions were performed in 2.0 M H 2 O 2 for 2 h, the percentages of TOC that remained in the aqueous phase decreased markedly (to 2% and 10% of the carbon atoms in the initial D5 and D4, respectively) and IR spectral measurements indicated that most of the substrates had been oxidized to silicate.