Research Silicone in the Environment: A Worst-Case Assessment of Poly(dimethylsiloxane) (PDMS) in Sediments DAVID E. POWELL,* RONALD B. ANNELIN, AND ROBERT H. GALLAVAN Health & Environmental Sciences, Dow Corning Corporation, Midland, Michigan 48686
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 (e0.6 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 “worstcase” sediments were less than the “no observable effects concentration” (NOEC) established in laboratory studies using sediment-dwelling organisms.
Introduction Silicone is a generic term for a diverse class of polymeric compounds, which consist of an alternating silicon-oxygen (i.e., siloxane) backbone with organic substituents attached to the silicon to modify the polymer’s final properties (1, 2). Silicones are manufactured polymers and are not found in nature. They were first available commercially in the mid1940s and have become an important component in a wide variety of consumer and industrial products (3). The greatest potential for dispersal of silicones to aquatic environments is from consumer products and process aids discarded to wastewater (2), rather than from manufacture (1). However, limited information is available on concentration ranges, distribution, and fate of silicone materials in surface sediments. * Corresponding author phone: (517)496-8072; fax: (517)496-5595; e-mail:
[email protected]. 3706
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 33, NO. 21, 1999
In the United States, an estimated 17 690 metric tons of silicones were disposed to wastewater in 1993 (2). Of this amount, an estimated 77% (about 13 600 metric tons) was poly(dimethylsiloxane) (PDMS, also known as dimethicone), which is not soluble in water. The physical properties of PDMS (2, 4) suggest the material will be removed from the aqueous phase during sewage treatment by adsorption onto sludge, as demonstrated in the laboratory (5) and in field studies (6). Nonetheless, it is expected that some PDMS will be released with the wastewater stream adsorbed to suspended solids (i.e., sludge particles), which are discharged and thereby deposited to the bed sediments in receiving waters. Accidental spills of PDMS in surface waters, which could occur from such incidents as a pipeline failure or tank car accident, would also partition to suspended solids and be deposited to the sediments. Consequently, the highest concentrations of PDMS in aquatic environments will likely exist in sediments from areas that are most heavily impacted by wastewater treatment effluentssnotably effluents from facilities that discharge large quantities of suspended solids. Concentrations of PDMS would also be elevated in sediments from areas used for ultimate sludge disposal, such as the New York Bight (7). Reported concentrations of PDMS in sediments range from below detection (0.002-2.6 µg‚g-1 dry wt) to 126 µg‚g-1 dry wt (4). However, the experimental designs used to generate the existing data were not statistically robust nor developed to assess the distribution of PDMS in freshwater and marine sediments and the potential of toxic concentrations to exist in those sediments. In this paper, we report the concentrations of PDMS in sediments from four marine and four freshwater study areas in the United States (Table 1) that were selected to represent “worst-case” areas. We also discuss various factors that affect the distribution of PDMS in the study areas and compare observed concentrations to published toxicological data. The eight study areas are heavily impacted by municipal wastewater effluents and represent diverse geographic locations and basin geomorphometry.
Experimental Section Sample Collection and Analysis. With the exception of Hamilton Harbor (which is located in Ontario, Canada and not part of the discharge databases for the United States), all study areas were selected from a list of 30 potential locations provided by the U.S. Environmental Protection Agency (U.S. EPA) Office of Prevention, Pesticides, and Toxic Substances (OPPTS). The potential study locations were selected by the OPPTS, based upon the potential for large quantities of municipal wastes to be discharged into low flow streams or tidal areas with minimal flushing. Final selection of the study areas (Table 1) was based on recommendations by the U.S. EPA, the National Oceanic and Atmospheric Administration (NOAA), and individuals having experience with the study areas of interest. Additional considerations included geographic location, level of wastewater treatment, and results from other sediment monitoring projects (8-13). Two of the study areas, Little Calumet River and Hamilton Harbor, were both designated by the International Joint Commission as “areas of concern” within the Great Lakes (14). A fixed-grid sampling design (15, 16) was used to spatially allocate sampling effort within each study area (Table 1). Collection stations for river channels (e.g., Chattahoochee River) were spatially located along transects perpendicular 10.1021/es9903476 CCC: $18.00
1999 American Chemical Society Published on Web 09/24/1999
TABLE 1. Study Areas Where Samples Were Collected for Assessing Worst-Case Concentrations of Poly(dimethylsiloxane) (PDMS) in Surface Sedimentse 1994 effluent characteristicsa study area Mississippi River, navigation pool 2 (Minneapolis, MN) Chattahoochee River, West Point Lake (Atlanta, GA) Hamilton Harbor (Hamilton, ON) Little Calumet River (Chicago, IL) Puget Sound (Seattle, WA) Hillsborough Bay (Tampa, FL) San Francisco Bay (San Francisco, CA) Boston Harbor (Boston, MA)
flow (MLD)b
suspended solids (mg‚L-1)
(kg‚d-1)
sediment characteristics grid sizec (m)
water (%)
PDMS (µg‚g-1 dry wt)e
range
meand
range
meand
821
Freshwater Locations 10 8397 500 × 1000
31-65
49 ( 3
1.0-3.8
2.0 ( 0.2
293
18
5291
1000
28-73
47 ( 5
0.3-18.3
8.8 ( 2.3
345 143
21 18
7137 2575
750 × 750 1000
27-83 40-83
63 ( 1 55 ( 3
1.9-30.5 12.3-320
15.6 ( 1.4 78.0 ( 20.4
378 206 506 1548
63 2 1 95
23-69 26-85 28-61 19-60
36 ( 5 51 ( 7 54 ( 2 38 ( 4
0.1-3.5 0.2-2.8 0.7-2.6 1.1-34.2
0.6 ( 0.1 0.9 ( 0.2 1.8 ( 0.1 16.6 ( 3.3
Marine Locations 23 821 300 × 300 372 400 × 400 549 1000 × 1000 147 831 400 × 400
a Effluent characteristics are provided for the largest wastewater treatment facility in each study area. Reported data was obtained from NPDES permit records for 1994. b MLD is million liters per day. c Grid size is the linear distance between sample collection stations. The Chattahoochee and Little Calumet Rivers were sampled along transects perpendicular to the main channel of the river. Transects were located every river kilometer. Sample collection stations along a transect were separated by 200 m for the Chattahoochee river. Because the Little Calumet River was very narrow, only two sample collection stations were located along each transectsone on each side of the channel. d Mean values (( the standard error) are weighted for the proportion of the total surface area occupied by each depth or distance interval used to stratify each study area (15). e Study areas are classified as freshwater or marine and listed in order of increasing mean PDMS concentration within each classification. The analytical method detection limit for PDMS was 0.2 µg‚g-1 dry wt.
to the river channel. Sample collection stations for basins (e.g., Boston Harbor) were spatially located within known or suspected areas of sediment deposition. Surface sediments (uppermost 5 cm) were collected from 28 to 30 sample stations within each study area using a VanVeen or Ekmann dredge; sediments in direct contact with the dredge were not retained. After collection, samples were homogenized in the field, sealed in glass jars with Teflon-lined lids, placed on ice in the dark, and shipped (within 2-7 days of collection) to the laboratory where they were stored in the dark at 4 °C until analysis. Concentrations of PDMS were measured in 12-14 sediment samples from each study area. A stratified, random sampling design (15) was used to allocate analytical effort among the samples collected from the eight study areas. For this study, it was assumed a priori that concentrations of PDMS in sediments would be related to distance from the effluent outfall of the targeted wastewater treatment facility. Therefore, riverine study areas such as the Little Calumet River were subdivided (stratified) into smaller sections (strata) based on distance from the assumed primary source of PDMS entering the study area. Reservoirs were stratified by distance from the dam (Mississippi River, navigation pool 2) or distance from the headwaters (Chattahoochee River, West Point Lake). Study areas that were not reservoirs (Hamilton Harbor, Boston Harbor, Hillsborough Bay, Puget Sound, and San Francisco Bay) were stratified on the basis of water depth, because sediment accumulation in a basin is often related to water depth (17-19). Consequently, stratification based upon distance was thought inappropriate for study areas having a basin-type of geomorphometry. The number of samples analyzed from within each stratum was determined as a function of the surface area occupied by the depth interval or distance interval used to stratify each study area. Greater analytical effort was allocated to strata having greater water depth or closer proximity to the primary area of concern. Concentrations of PDMS were determined by extracting subsamples (5-10 g) of the wet sediments with tetrahydrofuran (THF) and measuring concentration of PDMS in the extract. Concentration and molecular weight of PDMS in the THF extracts were measured using size exclusion chromatography (also known as gel permeation chromatography or GPC) combined with inductively coupled plasma-atomic
emission spectroscopy (ICP-AES) for element-specific detection of silicon (20). Tetrahydrofuran was used as the mobile phase (0.1 mL‚min-1 flow rate) for separation of the PDMS polymer. The GPC column was calibrated for molecular weight using polystyrene standards. The GPC-ICP system was calibrated for PDMS analyses using external standards of a 350 cs PDMS fluid. All extracts were analyzed in duplicate, with calibration check standards run after every 3 samples to check for drift in instrument sensitivity. Quality Assurance. All equipment in contact with the sediment was decontaminated at each sample station before collecting any samples. To estimate the variability of PDMS concentration in the sediments, triplicate samples collected at individual stations and individual samples split in the field were analyzed as distinct samples. Field blanks, container blanks, and rinsing blanks were collected at each study area to verify that samples of sediment were not contaminated by collection, handling, shipment, or storage procedures. Accuracy of the PDMS determinations was estimated from procedural blanks, calibration standards, replicate samples, and spiked samples that were extracted and analyzed with each analytical batch. Measured amounts of PDMS in the field blanks (n ) 7), container blanks (n ) 8), rinsing blanks (n ) 8), and procedural blanks (n ) 13) were all less than or equal to the analytical method detection limit (21) of 0.8 µg. Mean field precision (relative standard deviation or RSD) estimated from analyses of eight sets of field triplicate samples was 17.6 ( 6.8% (95% CI). Mean field precision estimated from the analysis of eight sets of field split samples was 9.8 ( 10.4% (95% CI). Mean recovery of PDMS from analyses of 34 procedural spiked samples was 104 ( 4% (95% CI). Mean method precision (RSD) as estimated from analysis of 26 sets of replicate samples was 12.3 ( 3.7% (95% CI). Concentrations of PDMS in THF extracts of the sediments exceeded the calculated method detection limit (21) of 0.4 µg‚mL-1 in 119 of the 122 sediment samples analyzed. Data Analysis. Statistical analyses were performed using SAS (version 6.12). Mean concentrations of PDMS in the surface sediments of each study area were weighted for the proportion of the total surface area occupied by each depth or distance stratum (15). For statistical estimation, a concentration value equal to the method detection limit (0.2 VOL. 33, NO. 21, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
3707
FIGURE 1. Relationship between mean molecular weight and the mean concentration of PDMS in sediments from the eight study areas. Error bars are the standard errors for the estimated means. The mean concentration and molecular weight plotted for navigation pool 4 of the Mississippi River is natural, THF-extractable silicon measured in sediments deposited before calendar year 1940 (27). µg‚g-1 dry wt) was assigned to the three sediment samples and between study areas (Figure 1). Molecular weights for with PDMS concentrations less than the method detection the extracted materials could be placed into three molecular limit. A one-way analysis of variance (ANOVA) using pooled weight categories: low (Hillsborough Bay, Puget Sound, San standard errors was performed to test for differences in the Francisco Bay, and navigation pool 2 of the Mississippi River), weighted mean concentrations of PDMS among study areas. intermediate (Chattahoochee River, Hamilton Harbor, and If the global F-test indicated significant differences between Boston Harbor), and high (Little Calumet River). study areas, multiple t-tests (i.e., Fisher’s least significant Discussion difference test) were used to compare individual weighted means (22). Spearman (rank) correlation coefficients were Generally, the stratified random design used for this study used to examine the relationship between PDMS concentrayielded acceptable estimates of mean PDMS concentrations tion in the sediment and water depth from which the in sediments, as indicated by the standard errors of the sediment was collected. A type I error (R) of 0.05 was used estimates (Table 1). The rationale for stratification appears to judge the significance of all statistical tests. to have been justified for Boston Harbor, Hamilton Harbor, Little Calumet River, Puget Sound, and San Francisco Bay. Results In these study areas, standard errors based on weighted estimates of variance were 14-76% less than standard errors Water content of the sediments from the eight study areas based on simple estimates of variance. In contrast, stratificaranged from 19 to 85% (Table 1) and was typically less in tion had little impact on the precision of the estimated mean sediments collected from marine areas. Mean water content concentrations of PDMS in sediments from the Chattaof the sediments within each study area ranged from 36 to hoochee River (West Point Lake), Hillsborough Bay, or 54% in the marine areas and from 47 to 63% in the freshwater navigation pool 2 of the Mississippi River. In these study areas. areas, standard errors based on weighted estimates of Concentrations of PDMS in surface sediments varied variance were within 5% of standard errors based on simple widely within each study area (Table 1). With the exception estimates of variance. The fact that results incorporating the of sediments from the Little Calumet River, dry weight effects of stratification were not different from results concentrations of PDMS ranged from less than the analytical assuming simple random sampling indicates that the exmethod detection limit (0.2 µg‚g-1) to 34.2 µg‚g-1. Dry weight pected distribution of PDMS did not exist in these study concentrations of PDMS in the Little Calumet River ranged areas. Review of the final data demonstrated that concentrafrom 12.3 to 309 µg‚g-1. Except for sediments from the Little tions of PDMS in surface sediments of navigation pool 2 Calumet River, mean dry weight concentrations and standard were relatively uniform throughout the study area, while errors (SE) for PDMS in the surface sediments (Table 1) ranged distributions of PDMS in the Chattahoochee River (West Point from 0.6 ( 0.1 µg‚g-1 in Puget Sound to 16.6 ( 3.3 µg‚g-1 in Lake) and Hillsborough Bay were more complex than Boston Harbor. The mean dry weight concentration and SE originally suspected. of PDMS in sediments from the Little Calumet River was 78 Except for the Little Calumet River site, concentrations of ( 20 µg‚g-1. Based on multiple t-tests (ANOVA, F ) 12.96, PDMS in surface sediments were similar to concentrations degrees of freedom for the treatment ) 7, p > 0.01), mean reported for sediments collected from other locations in North concentrations of PDMS could be placed into three conAmerica, Europe, and Japan (4). Concentrations of PDMS in centration categories: low (Puget Sound, Hillsborough Bay, surface sediments from the Little Calumet River were similar San Francisco Bay, and navigation pool 2 of the Mississippi to those reported for New York Bight (7), an area that was River), intermediate (Chattahoochee River, Hamilton Harbor, used for off-shore sludge disposal. Of the 10 greatest and Boston Harbor), and high (Little Calumet River). Generconcentrations of PDMS in sediments from the eight study ally, concentrations of PDMS were lowest in sediments from areas (range 30.5-309 µg‚g-1 dry wt), eight were measured the marine study areas, with the notable exception of Boston Harbor. Concentrations of PDMS in the sediments were in sediments from the Little Calumet River. The greatest dry significantly correlated with water depth in only one of the weight concentrations of PDMS in sediments from that site study areas: Puget Sound (rs ) 0.77; p ) 0.005). were measured in samples collected near the confluence of the west branch of the Grand Calumet River (309 µg‚g-1) and Estimated molecular weights for organosilicon materials extracted from the sediments by THF were variable within near an effluent outfall of a sewage disposal plant (183 µg‚g-1). 3708
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 33, NO. 21, 1999
Essentially 100% of the flow in the west branch of the Grand Calumet River is municipal wastewater effluent (23). This stretch of the Calumet River system is essentially a constructed waterway used to divert municipal wastewater from the cities of Chicago, IL and Hammond, IN away from Lake Michigan. Organic carbon content of sediments in the west branch of the Grand Calumet River range from 7.4 to 22% dry wt (24, 25), demonstrating that the river system receives substantial amounts of sewage sludge. Similarly, organic carbon content of Little Calumet River sediments having the greatest concentrations of PDMS (i.e., 309 and 183 µg‚g-1 dry wt) were 12 and 6.8% dry wt, respectively. These results imply that concentrations of PDMS are considerably elevated in sediments collected from depositional areas receiving municipal sewage sludge. Molecular weight of the extracted PDMS polymer (as determined by GPC) decreased with decreasing concentration of PDMS in the sediment (Figure 1). The primary sources of PDMS in aquatic environments are consumer products and process aids discarded to municipal wastewater treatment (2). There are no natural sources of PDMS in the environment, as they occur only as manufactured polymers. For that reason, the observed decrease in molecular weight cannot logically be attributed to the use and disposal of different PDMS materials at the study areas. A more reasonable hypothesis for the observed relation between molecular weight and concentration is that small amounts of natural, THFextractable silicon may be present in the sediments. If so, the molecular weight profile of the natural material would become increasingly more predominant as the concentration of PDMS decreased. Other studies (26) have concluded that organic solvents (including THF) do not extract silicates and that chromatographic separation by GPC sufficiently isolates organosilicon polymers. However, results from one study (27) demonstrated that natural silicon materials were extracted from sediments deposited before calendar year 1940 (prior to the manufacture of silicones, including PDMS) by THF and could not be resolved from PDMS by size-exclusion chromatography (i.e., GPC). Moreover, the estimated molecular weight of the natural THF-extractable silicon material (2500 ( 170; 95% CI) was similar to the estimated mean molecular weight of PDMS in sediments from Puget Sound and Hillsborough Bay (Figure 1). These results suggest that reported low-level concentrations of “silicones” measured in sediments (4, 6) may not be anthropogenic in origin. For example, the estimated mean concentration of natural, THFextractable silicon in sediments of navigation pool 4 of the Mississippi River is 0.6 ( 0.1 µg‚g-1 dry wt (27). By comparison, the dry weight concentration of PDMS was e0.6 µg‚g-1 for 25% of the analyzed sediments collected from the worstcase study areas described here. Presumably, mean concentrations of PDMS in surface sediments of the study areas were a result of (1) type of wastewater treatment facilities discharging to the area; (2) volume of effluent; (3) dilution factor of receiving waters; and (4) retention time within the receiving waters. Review of 1994 NPDES permit data (Table 1) indicates that annual mean concentrations of total suspended solids in effluents from municipal wastewater treatment facilities were e2 mg‚L-1 (e550 kg‚d-1) in study areas having the lowest mean concentrations of PDMS in the surface sediments (Hillsborough Bay and San Francisco Bay). In contrast, the greatest mean concentrations of PDMS in the surface sediments were located in study areas where the annual mean concentrations of total suspended solids in municipal wastewater effluent were g18 mg‚L-1 or 2575 kg‚d-1 (Boston Harbor, Chattahoochee River, Hamilton Harbor, and Little Calumet River). Similarly, dry weight concentrations of PDMS in the sediments were strongly correlated (p < 0.001) with total organic carbon content (data not shown) in study areas having the
highest levels of PDMS, whereas the correlation did not exist (p g 0.1) in study areas having the lowest levels of PDMS. These results suggest that retention of solids during wastewater treatment is a principal component controlling the level of PDMS discharged and deposited to the sediments in the eight study areas. Laboratory and field studies (5, 6) have demonstrated that PDMS, like many sorbable contaminants, is removed from the aqueous phase during sewage treatment by adsorption onto sludge. Nonetheless, geomorphometry of the study area is also a significant factor determining concentrations of PDMS in the surface sediments. For example, the mean concentration of PDMS in Little Calumet River was 4.7 and 130 times greater than the mean concentrations in Boston Harbor and Puget Sound, respectively (Table 1). However, the annual mean concentration of total suspended solids in effluent discharged to Boston Harbor and Puget Sound were about 95 mg‚L-1 (147 831 kg‚d-1) and 63 mg‚L-1 (23 821 kg‚d-1), respectively, compared to only 18 mg‚L-1 (2575 kg‚d-1) for effluent discharged to the Little Calumet River (Table 1). Compared to Boston Harbor and Puget Sound, the Little Calumet River is a very small, low energy environment with minimal wave activity and currents. Hence, suspended solids in wastewater discharged to Little Calumet River are not susceptible to long-range transport and, presumably, are deposited relatively close to the sources. Clearly, suspended solids in wastewater discharged to Boston Harbor and Puget Sound are transported considerable distances and deposited over very large areas, due to surface and bottom currents and the effects of wave activity. As a result, concentrations of PDMS in sediments of Boston Harbor and Puget Sound are considerably less than would be expected if only the mass of sewage solids discharged to the areas were considered. Concentrations of PDMS in sediments collected from the eight study areas, including the Little Calumet River, were considerably less than concentrations reported to have no adverse biological effects (g350 to >2300 µg‚g-1 dry wt) in laboratory studies on sediment dwelling organisms (4). The concentration of PDMS was
