Fate of cyclic methylsiloxanes in soils. 2. Rates of degradation and volatilization

Cyclic volatile methylsiloxane (cVMS) compounds are volatile, low-viscosity silicone fluids used as precursors in the synthesis of high molecular weig ht PDMS and as ingredients in certain personal care products. This study in vestigates cVMS degradation and evaporation rates in soils as influenced by molecular size, soil type, and moisture level. A temperate Michigan soil a nd a highly weathered Hawaiian soil were incubated with similar to 40 mu g of C-14-labeled cVMS/g of soil at similar to 22 degrees C; samples were kep t at 32%, 50%, 92%, and 100% relative humidity (RH) both in open and closed tubes. At each designated incubation time (from 0 to 21 days), the cVMS-co ntaining soils were extracted, and the extra cts were analyzed by liquid sc intillation counting (LSC) and reverse-phase high-performance liquid chroma tography (RP-HPLC). The results showed that cVMS degradation was more signi ficant than loss by volatilization in soil with low moisture levels. Degrad ation reactions followed pseudo-first-order kinetics. The half-life of cVMS fluids in air-dried soils ranged from 50 min to 5 days, depending on soil type and cVMS molecular sizes. At high humidity (particularly at 100% RM), the degradation slowed, while volatilization was accelerated and became a p redominant process in regulating the cVMS removal from sail. At any given m oisture level, the degradation rates of cVMS were much greater in highly we athered soils (e.g., Oxisols) than in temperate soils, and the differences were more profound for small cVMS (e.g., D-4) These findings demonstrate th at cVMS fluids are unlikely to persist in any soils within the wide range o f moisture conditions tested.