Environ. Sci. Techno/. 1995, 29,864-868
Introduction
J O H N C . CARPENTER, JAMES A. CELLA,* AND STEVEN B. DORN General Electric Company, Corporate Research and Development, 1 River Road, Schenectady, New York 12301 ~~
The degradation of polydimethylsiloxanes (silicones) on a spiked standard soil matrix has been studied. Spiked soil was extracted to recover silicones, and the extract was examined by several analytical techniques, including gel permeation chromatography and high-pressure liquid chromatography with an inductively coupled plasma emission spectrometer as the detector. Silicones were found to degrade rapidly to low molecular weight a,o-siloxanediol oligomers. Within 4-6 weeks, more than half of the initial spiked silicone is converted to a water-soluble fraction consisting mostly of 1,l-dimethylsilanediol. Implications of this finding relative to the ultimate environmental fate of silicones is discussed.
Silicones are a class of polymers that possess unique and varied properties. These properties, coupled with their exceptional thermal and chemical stability, have led to a diversity of applications for these materials in the industrial and private sectors. Recently, silicones have found increasing application in consumer products and as such are being introduced into the environment at an increasing rate. An understanding of the environmental fate of these polymers is therefore important. In general, these silicones or polydimethylsiloxane (PDMS)derivatives are thought to be exceptionallyresistant to hydrolytic or oxidative breakdown under ordinary ambient conditions. A landmark paper by Buch et al. (1) however demonstrated that PDMS suffers extensive degradation when exposed to dry soils. The clay constituents of the soils were found to promote this rearrangement process, and degradation was found to be markedly inhibited by moisture in the soils. Lehmann (2) and coworkers have recently demonstrated that the level of 14C extracted into water from a [14C]PDMSspiked soil increased dramaticallywhen the soil moisture content dropped below 5%. Concomitant with this change, the molecular weight of 14Cin parallel THF extracts was found to decrease. The authors concluded that PDMS should be unstable at field soil moistures. We have studied the interaction of PDMS with an EPA standard soil matrix (SSM)using a combination of analytical techniques to further elucidate the nature of the soilinduced degradation products. Gel permeation chromatography (GPC) or high-pressure liquid chromatography (HPLC) interfaced to an inductively coupled plasma emission spectroscopy (ICP) detector has proven to be particularly instructive in this regard (3). In addition to providing sensitive,silicon-specificdetection of soil extracts, these techniques furnish useful structural information, such as the molecular weight and molecular weight distribution of high molecular weight species and identity of low molecular weight species. This paper details our findings regarding the degradation of PDMS on soils.
Experimental Section Equipment. Gel permeation chromatographylRIdetection was carried out using a Water’sAssociates Model 6000 LC pump connected to two Varian Associate’sTSK Gel GMHXL polystyrene, mixed bed gel columns and a Waters Associate’s Model 410 refractive index detector with chloroform as the eluant. GPC-ICPwas carried out using either aVarian 5000 LC or a Waters Associates Model 6000 pump connected to the same GPC columns noted above. The effluent from the GPC columns was introduced to a Jobin-Yvon38 Plus sequential inductively coupled plasma optical emission spectrometer, monitoring the 251.611-nmemission line of silicon. Xylene was used as the eluant. LC-ICP was carried out in the same fashion as described for GPC-ICP except that a YMC Y-Pack Polymer C-18 reverse phase HPLC column was connected to the ICP interface and the eluant used was acetonitrile-water either isocratically or as a * Corresponding author; FAX: 518-387-5592; e-mail address:
[email protected].
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 4, 1995
0013-936X/95/0929-0664$09.00/0
0 1995 American Chemical Society
gradient. Details of this technique including the operating parameters and equipment are described elsewhere ( 3 4 . G U M S was performed using a Jeol SX 102. LC/MS was carried out using a Waters 600 MS HPLC interfaced to a Jeol SX102 high-resolution magnetic sector mass spectrometer via the GE ultrasonic nebulizer-based particle beam HPLC-MSinterface. Gas chromatographywas carried out on a Perkin-Elmer Autosystem gas chromatogaph equipped with a flame ionization detector. Helium was used as the carrier gas at a flow rate of 30 mLlmin through a J&WScientific DB-5 Megabore column. For all quantitative analyses, appropriate reagent and equipment blank controls were carried out to determine background levels of detectable silicones from these extraneous sources. In general, these levels were insigmficant and, where chromatographic separation was involved, inconsequential. Combustion analysis was carried out on a Packard Instruments Model 307 thermal oxidizer. Scintillation counting was carried out on a LKB Model 1214Rackbeta scintillation counter. Reagents and Chemicals. PDMS standards were obtained from American Polymer StandardsCo., Mentor, OH. Polydimethylsiloxanegums and fluids were obtained from The General Electric Company, Silicones Products Business Division,Waterford, NY. 14C-LabeledPDMS (350cs, specific activity = 0.52 mCilg) was synthesized at the Dow Corning Corporationand made available by the Silicones Environmental Health and Safety Council PDMS task force. Hexamethyldisiloxane and hexamethyldisilazane were obtained from Aldrich and were used without further purification. Authentic samples of monomer,dimer, andtrimer diols were prepared accordingto literature procedures (4). Standard soil matrix (SSM)was obtained from the EPA and found to have a moisture content of about 2% by thermogravimetric analysis (TGA). It is a sieved blend comprised of 20% soil, 20% sand, 25% silt, 5% gravel, 22.5% kaolinite, and 7.5%montmorillonite. The soils, spiked and unspiked, were stored in glass jars with covers to maintain the moisture level at a constant value. Soil Amendment. SSM was amended with PDMS in three ways. Method A A weighed amount of soil was slurried with a hexane solution of PDMS followed by evaporation of the hexane in a stream of dry nitrogen. Method B: A weighed amount of soil was slurried with a solution of PDMS in hexane followed by filtration of the slurry. Method C: A weighed amount of soil was slurried with an aqueous PDMS emulsion followed by filtration and air drying until the soil reached its original weight. Regardless of the spiking method employed, the initially spiked soils were immediately extracted with either ethyl acetate or THF to verify the loading. In all cases 290% of the theoretical amount of PDMS was recovered in these initial extractions. When 14C-labeled PDMS was used, total mass balance was determined to be 295% in all cases. Extraction Procedure. For samples containing [I4C1PDMS, 0.1-g samples of soil were weighed into a 2.0-mL polypropylene microcentrifuge tube. To the tube was added 1.0 mL of the extraction solvent. The tube was shaken for 30 min on a wrist action shaker, and the sample was filtered through a 1.5-mLdisposable centrifugal microfilter. Samples (0.2 mL) of the extract were combusted, and the [14C]COzwas trapped and counted by liquid scintillation. The solids were also combusted to account for all silicones not extracted such that mass balance could be determined.
For samples spiked with unlabeled PDMS, ~ 1 . g0of soil or sediment was weighed into 4-dram glass vials. To the vial was added 10.0 mL of the extraction solvent. The vial was capped with a disposable Teflon cap and shaken for 30 min on a wrist action shaker. Aliquots (2.5 mL) of the extracts were filtered through 0.45 pm syringeless filters and evaporated to dryness under a stream of dry nitrogen to afford a residue that was analyzed by GPC-ICP. All extraction data presented is based on a single 30-minute extraction. Duplicate extractions afford roughly an additional 10%recovery of PDMS. Derivathtion Analysis. Analyses of the derivatized samples were carried out by GUMS. Soil samples were treated directly with hexamethyldisilazane (18 h, room temperature) followed by concentration and GUMS analysis.
Resub and Discussion In order to assess the nature and rate of PDMS degradation on SSM soil, it was necessary to determine the efficiency of extractionof siliconesfrom thismedium. Soilwas spiked with a nominal level of 450-600 ppm of either a blend of a gum (MW sz 400K)and a fluid (MWsz 13Q (E 40% gum/ 60% fluid) or a 350-cs fluid (SF96-3501, 14C-labeled or unlabeled. Initial spikings were carried out by slurrying the soil with a solution of the blend in hexane followed by removal of the solvent under vacuum with slight warming. Since it was suspected that warming might accelerate the soil-silicone chemistry, an alternative spiking procedure was employed that simplyinvolved stirring the soillhexane solution at ambienttemperaturefor 2 h followed by filtration and air drying of the soil. Finally, in order to affect soil amendment under conditions more closely modeling environmental situations, spiking was carried out by slurrying soil with an aqueous emulsion of the 350-cs PDMS fluid. Regardless of the spiking method employed, 295% of the PDMS adsorbed onto the soil within a few hours of exposure to the spiking solution. In general, no differences were noted in the behavior of PDMS on these soils as a function of the spiking method. A comparison of the extraction efficiency of a series of solventswas determinedusing a [l4C1-labeled (350-cs)fluid. For a freshly spiked soil, all solvents examined gave similar extraction efficiencies (Figure 1). GPC-ICP analysis of these extracts, however, revealed that significant change in the molecular weight of the PDMS had occurred even after brief exposure to the soil. For example, an ethyl acetate extract obtained just 2 h after soil amendment with the gum-fluid blend recovered approximately 90% of the PDMS. The molecular weight distribution of this extract,however, had changed from the bimodal distribution of the initial blend to a monomodal distribution,and the molecular weight was lower than either of the initial blend components (Figure 2). There was a steady decrease in the amount and M W of the recovered polymer as a function of the time elapsed since soil amendmentas indicated by an increase in the GPC retention time (Figure 3). After about 2 weeks at ambient temperature, less than 25% of the initial spike was recoverable by ethyl acetate extraction, and the average molecular weight was judged to be less than 10 000. Concomitant with this trend was an increase in the amount of water-extractable silicon detected by direct inlet ICP analysis (Figure4). This waterextractable fraction was not merely water soluble silicate, VOL. 29, NO. 4, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY I 8 6 5
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containing products, no cyclic siloxaneswere found in the extracts of the charcoal trap. Extraction of the same aged sample with wet THF recovered x200ppm of low M W oligomers. LC/MS analysis of this extract indicated that it contained a series of silanol-terminated oligomers, 1 ( n = 7 or less). It thus appears that in this soil sample, linear silanols, not cyclics are the principle products of degradation.
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Characterization of the PDMS breakdown products in aged soil was also obtained by direct silylation of the soil using hexamethyldisilazane. The major GUMS detectable component of this derivatization process was octamethyltrisiloxane (MDM),2 (see Figure 5). Smaller amounts of higher TMS-capped oligomers were also detected. Analysis of aqueous extracts of this soil by GUMS reveals that trimethylsilanol is also present in the aged soil.
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as these levels were separately determined (5)and found to be well below the levels detected by ICP. The degradation of silicones on soil can produce either cyclic siloxanes,linear silanols,or a mixture of each. When freshly amended SSM was allowed to “age”for 1 week in a vessel swept with nitrogen and the effluent was passed through a charcoal bed to trap any volatile silicon866 1 ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 4, 1995
(CH,) ,Si (OH) 3
Analysis of the aqueous extract from a 1 year-old sample of soil by HPLC-ICP is depicted in Figure 6. The major component of this extract is dimethylsilanediol, 3, the simplestmonomeric breakdown product of PDMS. Smaller amounts of dimer and trimer diols were also detected in this sample. These results indicate a fairly rapid breakdown of PDMS on SSM soil predominantly to low M W silanols. Conversion of the polymer to its simplest unit, dimethylsilanediol,3, is significant within the 6 month-1 yr time frame of this study. Low material balance obtained after long aging suggests that low molecular weight breakdown products may be tightly bound to the soil. The production of a low molecular weight, water-soluble fragment so readily from a hydrophobic polymeric species
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stantial portion of silicones entering the environmentwill be exposed to soils via septic systems or from the land application of sewage sludges. As these soils experience normalwetanddrycyclingduringnormalseasonalchanges it is expected, based on the observations reported here and VOL. 29. NO. 4.1995 I ENVIRONMENTAL SCIENCE 5 TECHNOLOGY m 887
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ronmentally relevant levels of nitrate (7) and this may provide a pathway for the ultimate mineralization of silicones.
Literature Cited
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elsewhere,that accumulation of these polymers is not likely in this medium. Rather, the fate of silicones in this case may be depend on the environmentalfate of dimethylsilane1,l-diol. It is reported that water-solublesilanols are rapidly degraded to silicate by sunlight in the presence of envi-
868 * ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 4, 1995
(1) Buch, R. R.; Ingebrigtson, D. N. Environ. Sci. Technol. 1979, 13, 676. (2) Lehrnann, R.; Varaprath, S.; Frye, C. L. Environ. Toxicol.Chem. 1994, 13, 1061. (3) (a) Dorn, S. B.; Skelly-Frame, E. M. Analyst 1994, 119, 1687. (b) Watanbe, N.; Nagase, H. Eisei Kagaku 1985, 31, 391. (c) Biggs, W. R.; Fetzer, I. C.; Brown, R. J. Anal. Chem. 1987, 59, 2798. (4) Cella, J. A.; Carpenter, J, C. 1.Organomet. Chem. 1994, 480, 23. (5) Standard Methods for theExamination of Water and Wastewater; Franson, M. A., Managing Ed.; American Public Health Association: Washington, DC, 1975; p 487. (6) Cella, J. A.; Carpenter, J. C. Abstracts, XXVII Organosilicon Symposium;March 1994; B-8. (7) (a) Buch, R. R.; Lane, T. H.; Annelin, R. B.; Frye, C. L. Environ. Toxicol. Chem. 1984,3,215.(b)Anderson, C.; Hochgeshwender, K.; Weidemann, R.; Wilmes, R. Chemosphere 1987,16,2567.See also ref 2.
Received for review April 1 1 , 1994. Revised manuscript received December 16, 1994. Accepted January 1 I , 1995.@ ES940222V @
Abstract published in AdvanceACSAbstracts, February 15, 1995.
