DEGRADATION OF SILICONE POLYMER IN A VARIETY OF SOILS

Previous studies on one soil showed that silicone polymer (polydimethy lsiloxane, or PDMS) degrades to dimethylsilanediol (DMSD). This study examines PDMS degradation on seven U.S. soils differing in pH, % organ ic matter, texture, mineralogy, and geographic origin. Moist soils wer e amended with 350-centistoke (cs) [C-14]PDMS at 100 mg kg(-1), and so ils were dried at 23 degrees C for 0, 2, 4, 7, 10, or 14 d. Foam plugs were inserted in tube necks to trap volatiles. Samples were extracted with water to monitor silanol formation, or with THF (tetrahydrofuran ) for analysis of molecular weight changes and identification of degra dates. In all soils, PDMS degraded extensively to low-molecular-weight , water-soluble products. Gas chromatography-mass spectrometry (GC-MS) identified the main product in all soils as DMSD. Other small silanol s and cyclic siloxanes were either not detected or were present in onl y trace amounts. No volatile C-14 was captured by the plugs, and quant itative recovery of C-14 showed no loss of unidentified volatiles. PDM S degradation was thus similar in a wide range of soils, and DMSD was the main degradate. A lower limit of 4,900 +/- 1,250 L kg(-1) for the kd of this PDMS suggests that the polymer should be immobile in soil.