Fate of Cyclic Methylsiloxanes in Soils. 1. The Degradation Pathway

To determine the degradation pathway for cyclic volatile methylsiloxanes (cVMS) in soil, 40−200 mg kg-1 of 14C-labeled octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5), and dodecamethylcyclohexasiloxane (D6) were incubated in a soil sample at room temperature (∼22 °C) and ∼30% relative humidity. The organosilicon species extracted at different times were analyzed by gas chromatography/mass spectroscopy (GC/MS), gel permeation chromatography (GPC), and reverse-phase high-performance liquid chromatography (RP-HPLC). It was found that the degradation of cVMS was a multistep process, initiated by ring-opening hydrolysis of the cyclics to form linear oligomeric siloxane diols, followed by further hydrolysis of these oligomeric diols to the monomer dimethylsilanediol (DMSD). For D4, the ring-opening hydrolysis was faster than the subsequent degradation of tetramer and trimer diols {H[OSi(CH3)2]nOH, n = 3 and 4}, resulting in substantial accumulation of these oligomeric siloxane diols at early stages. In contrast, ring-opening was the rate-limiting step for D5 and D6, as indicated by the lack of oligomeric diol accumulation at any stage of the cVMS degradation. Nevertheless, all oligomeric siloxane diols were unstable in air-dried soil and ultimately hydrolyzed to DMSD within a few hours to a week, depending on the humidity. When the soil was re-wetted to water saturation, some of the intermediates continued to hydrolyze to DMSD, and some were converted back to cVMS, which then evaporated from wet soil.