Degradation of silicone polymer in a variety of soils

Abstract

Previous studies on one soil showed that silicone polymer (polydimethylsiloxane, or PDMS) degrades to dimethylsilanediol (DMSD). This study examines PDMS degradation on seven U.S. soils differing in pH, 070 organic matter, texture, mineralogy, and geographic origin. Moist soils were amended with 350-centistoke (cs) [I4C]PDMS at 100 mg kg-I, 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 I4C was captured by the plugs, and quantitative recovery of I4C 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 -I-1,250 L kg-' for the k, of this PDMS suggests that the polymer should be immobile in soil.

Temperature Dependence of the Organic Carbon/Water Partition Ratios (KOC) of Volatile Methylsiloxanes

“…This indicates that some of the low molecular weight breakdown products may be tightly bound to soil, consistent with the findings of Lehmann et al (1994Lehmann et al ( , 1995, Lehmann and Miller (1996), Xu (1998), andXu et al (1998). These show that as the soil dries binding of dimethylsilanediol to soil increases (i.e.…”

“…A follow-on study that used seven soils from the USA of differing pH, percentage organic matter, texture, mineralogy, and geographic origin demonstrates the general applicability of this degradation route (Lehmann et al, 1995). Moist soils (initial moisture between 8 and 31 per cent, depending on the soil) were amended with 14 C-PDMS (viscosity 350 cst and number average molecular weight 9440 g/mol) and maintained at 23°C for up to 14 days (during which the soils were allowed to dry naturally).…”

Volatilization and sorption of dimethylsilanediol in soil

“…Other soils had been collected in 1991 and 1992 and kept refrigerated since that time. They were used in a previous study on PDMS degradation [4] and were chosen to provide a wide range in soil properties (Table 1).…”

“…The hydrolysis of PDMS is believed to be abiotic, since the reaction is much faster as the soil dries than when it remains continuously moist [3]. In a variety of soils having different pH, organic matter contents, textures, and mineral types, DMSD was the sole reaction product [4]. The fact that dimer, trimer, and larger siloxanols (HO-[Si(CH 3 ) 2 O] n -H, n Ն 2) were not seen [4] is consistent with experiments showing that equilibrium in water favors the monomeric form over the various oligomers [5].…”

“…Dimethylsilanediol, or DMSD (HO-Si (CH 3 ) 2 -OH), forms in soil through hydrolysis [1,2] of silicone polymers, or polydimethylsiloxanes (PDMS), which have the structure (CH 3 ) 3 SiO-[Si(CH 3 ) 2 O] n -Si(CH 3 ) 3 , where n varies widely but averaged about 100 for our previous work [1,3,4]. The polymers are introduced into soil through the application of sewage sludge.…”