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.
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 kd of this PDMS suggests that the polymer should be immobile in soil.
Silicone materials are used in a wide variety of consumer/industrial products and process aids. Disposal to wastewater treatment is the primary source of entry for silicones (primarily poly(dimethylsiloxane) or PDMS) in aquatic environments. However, limited information is available on the concentration ranges, distribution, and fate of silicone materials in surface sediments. In this study, PDMS was measured in the surface sediments of four marine and four freshwater areas heavily impacted by municipal wastewater discharge to illustrate worst-case situations for the United States. Concentrations of PDMS were measured in 12−14 samples from each study area. Measured dry weight concentrations of PDMS ranged from below detection (0.2 μg·g-1) to 309 μg·g-1. Low level concentrations (≤0.6 μg·g-1) that were measured in 25% of the sediments may have been natural silicon materials rather than PDMS. Generally, concentrations of PDMS were greatest in sediments from depositional areas near effluent outfalls. Mean concentrations of PDMS (±SE) ranged from 0.6 ± 0.1 to 78 ± 20 μg·g-1 and were lowest in areas having advanced levels of wastewater treatment. Concentrations of PDMS in these “worst-case” sediments were less than the “no observable effects concentration” (NOEC) established in laboratory studies using sediment-dwelling organisms.
Dimethylsilanediol and other water‐soluble dimethysiloxanols undergo aqueous photolytic oxidative demethylation with tropospheric ultraviolet radiation in the presence of suitable chromophores, such as nitrogen oxides, to give silicic acid and carbon dioxide. The “biosuitability” of the resulting silicic acid was demonstrated by monitoring the growth of diatoms (Navicula pelliculosa) in the irradiated aqueous media. Thus, a plausible route for the further degradation of such water soluble hydrolysis products of poly(dimethyl‐siloxane) to naturally occurring materials was conclusively demonstrated. Furthermore, di‐ and monomethyl silanol species were shown not to be utilized by the diatom colony, even when present as completely water soluble components.
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