Degradation of silicone polymers in soil

Silicone polymers (polydimethylsiloxanes, or PDMS) are used in numerous personal care and household products, eventually enter wastewater treatment plants, and are later applied to the land as a component of sludge. The fate of silicones in soil is largely unknown, but this study shows that in a moist (0.2 MPa = 12% moisture) Londo sandy clay loam, 200 centi‐stoke (cs)¹⁴C‐labeled PDMS degraded slowly over six months to yield about 3% of applied ¹⁴C as low‐molecular‐weight, water‐soluble products. When the soil was allowed to dry in one week from 12 to 3% moisture, the degradation rate was much more rapid, and after several days at 3 % moisture about half of the applied ¹⁴C was water desorbable. HPLC‐GPC of tetrahydrofuran (THF) soil extracts showed that PDMS had been degraded to low‐molecular‐weight molecules of the general formula HO‐[Si(CH₃)₂O]_n‐H. The range of moistures in this experiment was measured in a field of Londo sandy clay loam during the summer of 1992, indicating that PDMS should be unstable in the soil environment. Further work on the identification and biological degradation of these small products is ongoing.