Degradation of silicone polymer in a variety of soils

Previous studies on one soil showed that silicone polymer (polydimethylsiloxane, or PDMS) degrades to dimethyl-silanediol (DMSD). This study examines PDMS degradation on seven U.S. soils differing in pH, % organic matter, texture, mineralogy, and geographic origin. Moist soils were amended with 350-centistoke (cs) [¹⁴C]PDMS at 100 mg kg⁻¹, and soils were dried at 23°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 degradates. 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 silanols and cyclic siloxanes were either not detected or were present in only trace amounts. No volatile ¹⁴C was captured by the plugs, and quantitative recovery of ¹⁴C showed no loss of unidentified volatiles. PDMS 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 k_d of this PDMS suggests that the polymer should be immobile in soil.