DEGRADATION OF SILICONE POLYMERS IN SOIL

Silicone polymers (polydimethylsiloxanes, or PDMS) are used in numerou s personal care and household products, eventually enter wastewater tr eatment plants, and are later applied to the land as a component of sl udge. 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 centistoke (CS) C-14-labeled PDMS degraded slowly over six months to yield about 3% of applied C-14 as low-molecular-weight, water-solu ble 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 sever al days at 3% moisture about half of the applied C-14 was water desorb able. HPLC-GPC of tetrahydrofuran (THF) soil extracts showed that PDMS had been degraded to low-molecular-weight molecules of the general fo rmula 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 199 2, indicating that PDMS should be unstable in the soil environment. Fu rther work on the identification and biological degradation of these s mall products is ongoing.