Correspondence/Rebuttal pubs.acs.org/est
Comment on “Unexpected Occurrence of Volatile Dimethylsiloxanes in Antarctic Soils, Vegetation, Phytoplankton, and Krill” n a recent paper, Sanchis et al.1 reported “unexpected” high concentrations of cyclic and linear volatile methylsiloxanes (VMS) in surface media (soils, vegetation, phytoplankton, and krill) from the Antarctic, which they attribute to long-range atmospheric transport and deposition by snow scavenging. Their results and interpretation of data are inconsistent with the current knowledge of the physicochemical properties,2,3 environmental behavior,4 and published monitoring data for these compounds.5−12 The only plausible explanation for the results reported by Sanchis et al.1 is contamination of their samples. The reasons for this conclusion are discussed below. The first concern is the analysis of the VMS. Extreme precautions are required when collecting samples from remote regions that are analyzed for these widely used volatile and hydrophobic substances. Unfortunately, the quality control (QC) procedures and results reported by Sanchis et al.1 are not sufficient to support their conclusion that samples were not contaminated during collection, storage, and processing procedures in the field and/or laboratory. The reported QC program is deficient in ensuring the integrity of samples analyzed for VMS, as follow: • Field blanks consisted of GFF and GFD filters that did not contain any organic matter and were inappropriate for the organic sample matrices that were collected. • Field blanks were simply exposed to ambient air conditions during sampling and thus were useful only for tracking contamination from airborne particulates but not for tracking partitioning of VMS between air and organic matter in the samples. • Laboratory matrix blanks were not appropriate for determining whether samples were compromised with VMS as a result of processing (e.g., lyophilization, grinding, and homogenization, etc.). • No field, transport, or storage positive controls were used in the study. • Laboratory matrix blanks consisting of tissue or soil dried at 60 °C for 48 h prior to extraction and analysis are not comparable to the sample matrices. • Recovery surrogates added prior to processing of the samples were m-xylene-d10 and naphthalene-d8, which are inappropriate for VMS analysis. 13C-VMS should have been used and added to samples in the field during collection. Because of the apparent lack of appropriate QC, there is no evidence to demonstrate that samples were not compromised during sampling and handling. Lines of evidence suggesting that collected/archived samples may have inadvertently been compromised include: • Samples were collected in 2005 and in 2009 but do not appear to have been analyzed for VMS until after adaptation of an analytical method published in 2013.13 Integrity of samples during storage was not characterized
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and cannot be evaluated because appropriate QC was not used. • Samples have been processed on the laboratory bench in open air; a clean room or similar technique for reducing background contamination14 was not used. Handling samples, especially grinding and homogenizing, would allow organic matter in the sample matrices to approach equilibrium concentrations with VMS in the ambient laboratory air, leading to gross contamination especially when sample temperature is low. • It appears that samples analyzed for VMS may have been archived samples that were collected and previously processed as part of earlier studies carried out to characterize POPs in the area.15,16 Extra handling of samples during POPs analysis would increase the chances of contamination by VMS. There are several other reasons why contamination of the samples is the only viable explanation for the observations reported by Sanchis et al.1 First, the reported concentrations of VMS are extraordinarily high for a “remote” region that has not received direct releases of VMSs. For example, reported concentrations of D4, D5, and D6 (all cyclic VMS, or cVMS) and all the linear VMS in “the Antarctic soil” were either greater than (e.g., D4) or in the similar range (e.g., D6 and D5) as concentrations in soils amended with VMS-containing biosolids.5,6 Lipid-equivalent concentrations (ng g−1 lipid weight) of the cVMS in krill calculated from the dry mass concentrations (ng g−1 dw) reported by Sanchis et al.1 assuming a generic lipid content of 0.25 g lipid g−1 dw are similar to cVMS concentrations in organisms at the base of food webs in areas directly impacted by wastewater effluents and emissions.7−12 Second, the “unexpected” high concentrations reported for Antarctic surface media1 could not be rationalized by deposition of airborne VMS via either air/surface partitioning or scavenging by snow. Volume-based concentrations of any given VMS in soil (CS) and air (CA) strive to reach equilibrium, and at equilibrium are related as CS/CA = ρS fOC K OC/KAW
(1)
where ρS, f OC, KOC, and KAW are the ratio of soil bulk density to the density of an equal volume of water, organic carbon content (OC), soil/water distribution constant and air/water partition coefficient, respectively. Using published KOC and KAW values2,3 and concentrations of OC in the soil samples from Antarctica (9 out of 11 are in the from 97% of the VMS from the snowmelt should be re-volatilized to air. The authors’ Figure 31 illustrates an inverse correlation between concentrations of VMS in “phytoplankton” and salinity of the surface seawater, and is cited as evidence that inputs of VMS occur with melting snow and ice. However, Figure 3 cannot be reproduced from the original data in the SI. The depicted apparent correlation is an artifact of a regression across mismatched data presented in Table S5-c and S2−C. Using the properly matched and uncensored concentrations of cVMS in Table S5-c and the corresponding salinity data in S2-c from the paper, no statistically significant correlations, implied by Figure 3, can be found. Similarly, a regression of the total concentration of 4 cVMS after removing the “outlier” (sample P11) as specified in the paper also produces no significant correlation (r2 of 0.0024), and thus there is no support for the authors’ speculations about scavenging of VMS by snow. The fourth issue relates to the bioaccumulation discussion in the paper. The authors report large BMF values (up to thousands) for cVMS in the Antarctic food web, and log BMF values of cVMS linearly related to their log KOW values (eq 5 and Figure S7). Here, BMF refers to the ratios of cVMS concentrations (ng g−1 dw) in krill and phytoplankton. However, Table 11 shows that concentrations in phytoplankton and krill differ greatly among replicates (e.g., by 90 fold for D5, 88 fold for D6 in phytoplankton). This produces very large errors in the BMF in addition to error due to the likely sample contamination. Furthermore, the krill and phytoplankton were sampled in two different years (see Table S2-c and S 2d) and at different locations separated by a large distance. Finally, using the data of the likely contaminated samples from S5-c and S5-d, we could not reproduce Figure S7 and eq 5. Based on the average concentrations of each cVMS in all samples in Table 1,1 ratios of cVMS concentrations in krill and phytoplankton (referred to as BMFs), are 8.6 for D3, 53 for D4, 11 for D5, and 8.9 for D6, which are much smaller than BMFs calculated from eq 5, which range between 96 for D3 (log KOW = 6.08) to 2,500 for D6 (log KOW = 8.87). It is not evident how eq 5 was derived because no statistically significant correlation (p ≫ 0.05) between log BMF and log KOW for cVMS is apparent, that is
More detailed examination of the Figure S7 reveals that eq 5 appears to be based on only four pairs of phytoplankton and krill samples (out of 11 pairs) and log KOW values from 6.6 to 9.3, which are significantly different from the cited published values. We conclude that the reported BMF values of VMS are likely flawed due to sample contamination, subject to large error due to large variability among replicates and unrepresentative sampling design and incorrectly described by eq 5.
Donald Mackay† Frank Gobas‡ Keith Solomon§ Matthew Macleod∥ Michael McLachlan∥ David E. Powell⊥ Shihe Xu*,⊥ †
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AUTHOR INFORMATION
Corresponding Author
*Phone: 989-496-5961; e-mail:
[email protected]. Notes
David Powell and Shihe Xu are employed by Dow Corninng Corporation, a manufacturer of methylsiloxanes. The other authors declare no competing financial interests.
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REFERENCES
(1) Sanchís, J.; Cabrerizo, A.; Galban, C.; Barcelo, D.; Farré, M.; Dachs, J. Unexpected occurrence of volatile dimethylsiloxanes in Antarctic soils, vegetation, phytoplankton and krill. Environ. Sci. Technol. 2015. DOI: 10.1021/es503697t. (2) Kozerski, G.; Xu, S.; Miller, J.; Durham, J. Determination of soil organic carbon-water sorption coefficients for volatile methylsiloxanes. Environ. Toxicol. Chem. 2014, 33, 1937−1945. (3) Xu, S.; Kozerski, G.; Mackay, D. Critical review and interpretation of environmental data for volatile methylsiloxanes: partition properties. Environ. Sci. Technol. 2014, 48, 11748−11759. (4) Xu, S.; Wania, F. Chemical fate, latitudinal distribution and long range transport of cyclic volatile methylsiloxanes in the global environment: A modeling assessment. Chemosphere 2013, 93, 835− 843. (5) Sanchez-Brunete, C.; Miguel, E.; Albero, B.; Tadeo, J. L. Determination of cyclic and linear siloxanes in soil samples by ultrasonic-assisted extraction and gas chromatography-mass spectrometry. J. Chromatogr. A 2010, 1217, 7024−7030. (6) Wang, D.-G.; Steer, H.; Tait, T.; Williams, Z.; Pacepavicius, G.; Young, T.; Ng, T.; Smyth, S. A.; Kinsman, L.; Alaee, M. Concentrations of cyclic volatile methylsiloxanes in biosolid amended soil, influent, effluent, receiving water, and sediment of wastewater treatment plants in Canada. Chemosphere 2013, 93, 766−773. (7) Borgå, K.; Fjeld, E.; Kierkegaard, A.; McLachlan, M. S. Food web accumulation of cyclic siloxanes in Lake Mjøsa, Norway. Environ. Sci. Technol. 2012, 46, 6347−6354.
log BMFcVMS = 0.089(± 0.20) log K OW + 0.63(± 1.53), n = 4, r 2 = 0.09, p = 0.71
Canadian Environmental Modeling Center, Trent University, Peterborough, Ontario K9J 7B8, Canada ‡ School of Resource and Environmental Management, Simon Fraser University, Vancouver, British Columbia V5A 1S6, Canada § Centre for Toxicology, School of Environmental Sciences, University of Guelph, Guelph, Ontario N1G 2W1, Canada ∥ Department of Environmental Science and Analytical Chemistry (ACES), Stockholm University, Stockholm 114 18, Sweden ⊥ Health and Environmental Sciences, Dow Corning Corporation, Midland, Michigan 48686, United States
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DOI: 10.1021/acs.est.5b01936 Environ. Sci. Technol. 2015, 49, 7507−7509
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(8) Borgå, K.; Fjeld, E.; Kierkegaard, A.; McLachlan, M. S. Consistency in trophic magnification factors of cyclic methyl siloxanes in pelagic freshwater food webs leading to brown trout. Environ. Sci. Technol. 2013, 47, 14394−14402. (9) Powell, D. E.; Woodburn, K. B.; Drottar, K.; Durham, J. Huff, D. W. 2009. Trophic dilution of cyclic volatile methylsiloxane (cVMS) materials in a temperate freshwater lake. Final Report, HES Study No. 10771−108, Dow Corning Corporation, Auburn, Michigan USA. Submitted to Centre Européen des Silicones, European Chemical Industry Council (CEFIC), Brussels, Belgium. http://www.epa.gov/ oppt/tsca8e/pubs/8ehq/2010/feb10/8ehq_0210_17834a.pdf (accessed 12 September 2013). (10) Powell, D. E.; Durham J.; Huff, D. W.; Böhmer, T.; Gerhards, R. Koerner, M. 2010. Bioaccumulation and trophic transfer of cyclic volatile methylsiloxanes (cVMS) materials in the aquatic marine food webs of the inner and outer Oslofjord, Norway. Final Report, HES Study No. 11060-108, Dow Corning Corporation, Auburn, Michigan USA. Submitted to Centre Européen des Silicones (CES), European Chemical Industry Council (CEFIC), Brussels, Belgium. http://www. epa.gov/oppt/tsca8e/pubs/8ehq/2010/feb10/8ehq_0210_17834a. pdf (accessed 12 September 2013). (11) McGoldrick, D. J.; Drouillard, K. G.; Keir, M. J.; Clark, M. G.; Backus, S. M. Concentrations and trophic magnification of cyclic siloxanes in aquatic biota from the Western Basin of Lake Erie, Canada. Environ. Pollut. 2014, 186, 141−148. (12) Jia, H.; Zhang, Z.; Wang, C.; Hong, W.-J.; Sun, Y.; Li, Y.-F. Trophic transfer of methyl siloxanes in the marine food web from coastal area of Northern China. Environ. Sci. Technol. 2015, 49, 2833− 2840. (13) Sanchís, J.; Martinez, E.; Ginebreda, A.; Farre, M.; Barceló, D. Occurrence of linear and cyclic volatile methylsiloxanes in wastewater, surface water and sediments from Catalonia. Sci. Total Environ. 2013, 443, 530−538. (14) Kierkegaard, A.; Adolfsson-Eric, M.; McLachlan, M. S. Determination of cyclic volatile methylsiloxanes in biota with a purge and trap method. Anal. Chem. 2010, 82, 9573−9578. (15) Cabrerizo, A.; Dachs, J.; Barceló, D.; Jones, K. C. Influence of organic matter content and human activities on the occurrence of organic pollutants in Antarctic soils, lichens, grass, and mosses. Environ. Sci. Technol. 2012, 46, 1396−1405. (16) Cabrerizo, A.; Dachs, J.; Barceló, D.; Jones, K. C. Climatic and biogeochemical controls on the remobilization and reservoirs of persistent organic pollutants in Antarctica. Environ. Sci. Technol. 2013, 47, 4299−4306. (17) Yucuis, R. A.; Stanier, C. O.; Hornbuckle, K. C. Cyclic siloxanes in air, including identification of high levels in Chicago and distinct diurnal variation. Chemosphere 2013, 92, 905−910. (18) Krogseth, I. S.; Kierkegaard, A.; McLachlan, M. S.; Breivik, K.; Hansen, K. M.; Schlabach, M. Occurrence and seasonality of cyclic volatile methylsiloxanes in Arctic air. Environ. Sci. Technol. 2013, 47, 502−509. (19) Roth, C. M.; Goss, K.-U.; Schwarzenbach, R. P. Sorption of diverse organic vapors to snow. Environ. Sci. Technol. 2004, 38, 4078− 4084. (20) Atapattu, S. N.; Poole, C. F. Determination of descriptors for semivolatile organosilicon compounds by gas chromatography and non-aqueous liquid-liquid partitioning. J. Chromatogr. 2009, 1216, 7882−7888.
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