Environ. Sci. Technol. 1986, 20, 743-744
Evidence for a Sedimentary Siloxane Horizon Robert E. Pellenbarg" and David E. Tevault Chemistry Division, Naval Research Laboratory, Washington, D.C. 20375-5000
Selected samples from two Puget Sound sediment cores have been analyzed for poly(organo)siloxanes (silicones). One core was 60 years old at 30-cm depth (ages by lead-210 dating) and showed no evidence for silicones there. The second, 15 years old at depth, exhibited silicones at depth. Clearly shown is evidence for a siloxane horizon in these two cores, with the presence of the horizon directly related to the fact that silicones have been in widespread use only since World War 11. All samples were analyzed by solvent extraction and diffuse reflectance Fourier transform infrared spectrometry. Introduction Poly(organo)siloxanes (silicones) have been manufactured in yearly increasing quantities since World War 11. [1983 US.production estimated a t 1500 metric tons of silicone-based fluids, elastomers, and resins ( I ) ] . The unique physical, and surfactant, properties of silicones, coupled with their chemical inertness, have encouraged use of silicones in an ever lengthening list of consumer products. These products often obtain desirable characteristics from only trace additions of silicones, especially the most ubiquitous, polydimethyl silicones. However, product use and disposal, save incineration, in general does not degrade the totally anthropogenic silicone additives. Furthermore, as a class, silicones are not easily, if at all, biodegraded (2), so that silicones should be observable in appropriate environmental collection regions. It has been clearly demonstrated (3-6)that silicones are present in superficial sediments in a variety of aquatic systems. The surface-active silicones likely occupy the interface between liquid (water) and solid represented by the sediments and seston. Aquatic sediments, then, serve as a major environmental collection region for silicones. However, the question remains as to whether one would be able to observe a silicone horizon, or first appearance of silicones, in the sedimentary record. The potential extensive environmental dispersal of silicones began, in reality, around 1950-1955. Thus, it should be possible to detect this event horizon in the sedimentary record. Data are presented that demonstrate that this observation is, in fact, possible. Selected contemporary sediments, and deeper, older sediments in sedimentary columns, have been analyzed for characteristic silicone infrared absorption bands: only the uppermost, younger sediments exhibit silicones as assayed by Fourier transform IR spectroscopy. Experimental Section Sampling. Data concerning the samples analyzed in this study are listed in Table I. The sediment cores (Figure 1)were collected from Puget Sound, Washington, using standard metallic box coring equipment. Cores were divided soon after collection, dried, and stored dry pending further analyses. There is no evidence that the sedimentary record in the region studied has been disturbed once buried, so that chemical events recorded in the sediments, specifically the first appearance of silicones, should be well preserved (7).
Table I. Puget Sound Sediment Cores core
latitude
longitude
age at depth" in core, years
56 51A
47O29'N
122'23.8'W 122'27.5'W
60
47'40"
15
Ages determined by 210Pbdating (R. Carpenter, personal communication, May 1985). See ref 7 for details on dating technique. Samples at depth in core were 29-30 cm below sediment surface.
Sample Processing (A Qualitative Screening Procedure). Freeze-dried sediment samples (average weight 1g) were weighed out into separated new Pyrex test tubes. In all cases, new glassware was used to preclude any possibility of sample contamination by silicone residues from previous use of the equipment. All glassware had been precleaned with high-performance liquid chromatography (HPLC) grade glass distilled chloroform (Fisher) dedicated to this study, chosen as an appropriate solvent for the lipidlike silicones. The samples were combined with -5-mL portions of chloroform, sonicated for 5 min in a Model 450 ultrasonic cleaner (E/MC Corp.), and then centrifuged for 15 min at approximately 3500g in a Precision Scientific Model 10-R-9 centrifuge. Chloroform extracts were transferred to washed, new 10-mL Pyrex vials using new individual Pyrex Pasteur pipets. Sample volume was reduced to a few drops with gentle dry bottom heat and a stream of dry nitrogen. At no time did samples, glassware, or solvents come in contact with any organic polymeric materials that could contain traces of silicone. Blanks of two types were utilized in this study. Note that all sediment samples were extracted with solvent from the same new, dedicated bottle, so that any silicone containment would be added equally to all extracts. In no case, though, were characteristic silicone absorption bands seen in extracts from samples which, on the basis of their age, should have been accumulated prior to the widespread release of silicones. This finding implies a silicone-free reagent. However, as a confirmatory check on the solvent, 10 mL of the chloroform was evaporated to a few drops. This residue was scanned as described under Instrumental and yielded no evidence of infrared absorption bands characteristic of silicone. Instrumental. For analysis, the small volume liquid samples were transferred with new pipets onto approximately 1 cm3 of dry potassium bromide (Fisher),held a diffuse reflectance sample holder fitted to a Digilab Model FTS-15C Fourier transform infrared spectrophotometer. Samples were scanned (200 scans) at 8-cm-I resolution by using a liquid-nitrogen-cooledMCT detector to measure the low light levels inherent in diffuse reflectance experiments (8). Diffuse reflectance FTIR was used in this study for several specific reasons. Analysis is rapid, especially with regard to sample handling for introduction to the instrument. FTIR offers a sensitivity that detected characteristic silicone absorption bands in extracts from approximately 1-g samples. Additionally, for sample support, diffuse reflectance measurements utilize powdered potassium bromide, which is changed for each sample. This proce-
Not subject to U.S. Copyright. Published 1986 by the American Chemical Society
Environ. Sci. Technol., Vol. 20, No. 7, 1986
743
20
123"W
40'
20'
122"
48-n
48"N
40
40'
20
20
1200 I
20
I 123"W
I
40'
I 20
I
1000
E 0
WAVENUMBER
122"
Figure 1. Study area In Washington State.
Figure 2. FTIR spectra from selected Puget Sound sediment extracts.
dure precludes damage to expensive, delicate salt plates as required for transmission FTIR. Key analytical advantages utilized, then, were rapid sample turn around for both processing and instrumental measurement, and sensitivity which requires only small sample size.
pearance of silicone, is likely related to the fact that silicones have been widely used and released to the environment only since about the later 1940s to early 1950s. Future work will identify more precisely the vertical position of the horizon in a series of appropriate sediment column samples and attempt to quantify the increase of sedimentary siloxanes with time.
Results and Discussion Figure 2 presents the expanded, selected segments of the spectra from a 15-year-old (A) and 60-year-old (B) deep sediment, respectively. The important features of the spectra are indicated in the figure. Thus, for the younger core 56 sediment, there is clear evidence for highly characteristic SiOSi stretch at approximately 1000 cm-l. This band is characteristic of siloxanes (9) and is absent from the core 51A deep sample spectrum. The important point is that only the contemporary 15-year-old sediment gives an infrared signature for siloxanes. Future work will attempt to locate the actual silicone horizon in this and other sedimentary systems. It is important to consider the relationship of the core collection sites to potential silicone sources in the Puget Sound region. Both cores were collected within 10 km of Seattle's Westpoint Sewage Treatment Plant (see Figure 11, operational in 1966 (IO). This start-up date correlates well with the siloxane signal seen in the 15-year-old sediment from core 56. Westpoint releases some 475 metric tons of aliphatic hydrocarbons/year (IO), which, like silicones passing through the facility, end up in part in Puget Sound sediments. Indeed, these hydrocarbons account for strong absorption at 2950 cm-I, which is not shown in Figure 2.
Acknowledgments
Roy Carpenter, School of Oceanography, University of Washington, Seattle, WA, is thanked for the samples analyzed in this study and for the zloPbdating ages presented. Literature Cited (1) U S . International Trade Commission Synthetic Organic Chemicals, US.Production and Sales, 1983; USITC (2)
(3) (4) (5) (6) (7) (8) (9) (10)
Publication 1588; U.S. Government Printing Office: Washington, DC, 1983. Hobbs, E. J.; Keplinger, M. L.; Calandra, J. C. Enuiron. Res. 1975, 10, 397-406. Pellenbarg, R. E. Enuiron. Sci. Technol. 1979,13,565-569. Pellenbarg, R. E. Mar. Pollut. Bull. 1979, 10, 267-269. Pellenbarg, R. E. Mar. Pollut. Bull. 1982, 13, 427-429. Watanabe, N.; Nakamura, T.; Watanabe, E. Sci. Total Environ. 1984, 35, 91-97. Carpenter, R.; Peterson, M. L.; Bennett, J. T. Mar. Geol. 1985,64, 291-312. Fuller, M.; Griffiths, P. Anal. Chem. 1978, 50, 1906-1910. Smith, A. L. Analysis of Silicones;Wiley: New York, 1974; pp 1-408. Barrick, R. G. Enuiron. Sci. Technol. 1982, 16, 682-692.
Conclusions
This study shows the probable existence of a silicone horizon in a sediment column. The horizon, Le., first ap-
744
Environ. Sci. Technoi., Voi. 20, No. 7, 1986
Received for review July 22, 1985. Revised manuscript received January 21, 1986. Accepted March 10, 1986.
