Silicone polymers (polydimethylsiloxanes, or PDMS) are used in numerous personal care and household products, eventually enter wastewater treatment plants, and are later applied to the land as a component of sludge. 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 centi‐stoke (cs)14C‐labeled PDMS degraded slowly over six months to yield about 3% of applied 14C as low‐molecular‐weight, water‐soluble 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 several days at 3 % moisture about half of the applied 14C was water desorbable. HPLC‐GPC of tetrahydrofuran (THF) soil extracts showed that PDMS had been degraded to low‐molecular‐weight molecules of the general formula 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 1992, indicating that PDMS should be unstable in the soil environment. Further work on the identification and biological degradation of these small products is ongoing.

Silicones (polydimethylsiloxanes) find use in a wide variety of industrial and consumer product applications because of their outstanding properties. Potential human exposure to silicones occurs in the work place during manufacturing and product formulation, as well as through the normal use of consumer products containing them.The entry of silicones into various environmental compartments raised health and safety concerns from potential exposure and mandated numerous environmental and toxicological studies. Such studies require qualitative and quantitative determination of silicone species at trace levels. However, the ubiquitous presence of silicones coupled with their unique chemistry renders their analysis at trace levels challenging.This paper provides a consolidated account of various aspects of silicones that must be borne in mind to obtain reliable data. The following topics are discussed: differences in the chemistry of silicones vs carbon; precautions in sample handling to avoid losses and inadvertent chemical transformation; potential sources for artifacts and interferences that could lead to systematic errors and data misinterpretation; sources for background and the need for matrix matched blank experiments; distinguishing silicones from silicates to avoid overestimation; potential for incorrect structural assignments; preventing inadvertent contamination; and questionable claims on the presence of silicones in biological matrices including that of silicone implants.

The aqueous solubility of several low mlecular weight linear, cyclic, and branched permethylsiloxanes was determined at room temperature. A method for preparing molecularly dispersed, colloid‐free saturated aqueous solutions is described. Solubilities, decreasing with increased molecular weights, ranged downward from one part per million to mere parts per trillion. The cyclic oligomers were slightly more soluble than their linear analogues. While the effect of branching was mixed, polar groups such as phenyl and hydroxyl moities sharply increased aqueous solubility. Semilog plots of the solubility/molecular weight yielded linear regressions, the extrapolation of which indicate the absence of any environmentally relevant water solubility (< 1 ppt) for the conventional higher molecular weight polydimethylsiloxanes of commerce.

were obtained using a high-pressure liquid chromatography (HPLC) system equipped with a radioisotope detector. The metabolite elution was carried out on a C 18 column using an acetonitrile/ water mobile phase. The structural assignments were based on GC-MS analysis of the tetrahydrofuran extract of urine containing the metabolites. Some of the metabolites in the extracts were first protected with trimethylsilyl groups prior to GC-MS analysis using bis(trimethylsiloxy)trifluoroacetamide or highly purified hexamethyldisiloxane. Hexamethyldisiloxane (MM, HMDS 1 ), the smallest member of the polydimethylsiloxane polymers, and decamethylcyclopentasiloxane (D 5 ), a cyclic siloxane are colorless volatile fluids. MM is quite volatile with a vapor pressure of 42.2 mm Hg at 25°C and a boiling point of 100°C (Flanningam, 1986). D 5 is relatively less volatile with a vapor pressure of 2 mm Hg at 50°C and a boiling point of 210°C. The aqueous solubilities of MM and D 5 are 930 and 17 ppb, respectively (Varaprath et al., 1996). The primary use of MM and D 5 is as intermediates in the manufacturing of high molecular weight siloxane polymers. MM and D 5 also find use as vehicles or ingredients in a wide range of consumer product formulations (Cameron et al., 1986) since they have several favorable properties such as low surface tension, adequate evaporation rate, lack of odor, high degree of compatibility with many consumer product ingredients, and low toxicity. Typical examples of applications include moisturizing creams, lotions, bath oils, colognes, shaving products, and perfumes. Besides these product applications, they are also used as cleaners, lubricants, and penetrating oils.The rigorously purified MM (Dow Corning OS-10, purity Ͼ99.9%) is one of the many ozone-safe volatile methylsiloxanes that is exempt from federal volatile organic compound regulations and hence is accepted as an alternative for other organic solvents. Another important industrial use of MM is as a chain-terminating agent in siloxane polymerizations. The use of MM and D 5 in various product formulations necessitated conducting chemical and environmental fate/effects tests of them.Potential human exposure to MM and D 5 can result at the work place during the manufacturing process, as well as through the normal use of consumer products that contain them. Only sparse toxicological information is available on these siloxanes since they are believed to be relatively inert and of low toxicity. However, octamethylcyclotetrasiloxane (D 4 ), a homolog of D 5 had been extensively studied. In rodents, inhalation exposure to D 4 results in dose-related hepatomegaly, transient hepatic hyperplasia, hypertrophy, and induction of hepatic cytochrome P450 enzymes in a fashion similar to phenobarbital (McKim et al., 1998(McKim et al., , 2001). Very limited toxicity data are available on HMDS and D 5 in biological systems. In a 13-week subchronic MM whole-body inhalation, renal histopathology consis-1 Abbreviations used are: MM, (HMDS) hexamethyldisiloxane; D 5 , decame...

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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.

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