EnVkOn. SCI. Techno/. 1988, 20, 201-204
NOTES Tributyltin in Seawater: Speciation and Octanol-Water Partition Coefficient Roy B. Laughlln, Jr,'t Harold E. Guard,' and Wllllam M. Coleman, 1115 University of California, Naval Biosciences Laboratory, Naval Supply Center, Oakland, California 94625
Analysis of chloroform extracts of tributyltin (TBT) dissolved in seawater shows that the equilibrium mixture of speciation products is composed of tributyltin chloride (TBTCl), tributyltin hydroxide (TBTOH), the aquo complex (TBTOH2+)(as a function of pH), and a tributyltin carbonato species. The equilibrium distribution is influenced by [Cl-1, dissolved COz, and pH and is easily displaced by variation within the environmental concentration range of these substances. Octanol-water partition coefficients (KO,)for tributyltin vary as a function of salinity. The lowest value, 5500, was measured in 2 5 7 ~and increased in higher or lower salinities to a maximum of 7000 in deionized water. The KO, values reported here are substantially higher than some previous reports in the literature but are in more reasonable agreement with published bioaccumulation measurements for tributyltin assuming partitioning processes are responsible.
Introduction Characterization of dissolved tributyltin species is important in assessing environmental effects of these organometallic compounds. However, because most quantitative methods such as hydridization irreversibly convert tributyltins to a common entity, TBTH, the identity of the original material is lost. There is little concensus in the literature regarding tributyltin speciation products in seawater. Monaghan et al. (2) first suggested that the most logical species is Bu,SnOH, then subsequently found that Bu,SnOH and Bu,SnC1 are present in seawater (3). Rzaev (4) contends that tributyltin occurs as the cation in water. No qualitative data exist to describe the speciation of tributyltin compounds in seawater. A knowledge of chemical speciation products can provide powerful insights into potential bioaccumulation pathways of a compound, particularly if the dissolved complexes are uncharged. The latter feature is essential for significant partitioning of the compounds into biota via dissolution into lipids. In simple partitioning processes of neutral compounds, solute-solvent interactions are the primary ones governing the equilibrium distribution of hydrophobic chemicals between aqueous and nonpolar compartments. Because simple partitioning is an intuitively obvious process, it has been applied in efforts to predict biological activity as a function of aqueous thermodynamic potential based upon the behavior of a chemical of interest in an octanol-water mixture (5). More recently, quantitative descriptions of the relationship between the KO,and bioaccumulation have been published (6-9).A Kowvalue may not accurately predict bioaccumulation in nonequilibrium situations where the organism's body burden is kinetically controlled by the rate of uptake, or when biotransforma+Present address: Harbor Branch Foundation, Division of Applied Biology, Route 1, Box 196, Fort Pierce, FL 33450. *Presentaddress: U.S. Office of Naval Research, Code 413 Organic Chemistry, Arlington, VA 22217. $Presentaddress: Dow Chemical USA, Texas Operations, B 3827 Freeport, TX 77541. 0013-936X/86/0920-0201$01.50/0
tion-depuration rates are high enough to influence chemical concentrations in water and/or tissue, or when specific binding occurs. The use of any model for prediction is based on analogy, but if its suitability is ambiguous, the burden of proof rests upon experimental determination. Tributyltin is a case in point. There is presently a contradiction in the literature regarding the relationship between reported KO,values and observed bioaccumulation. Thus it is difficult to assign these compounds in a specific model class, e.g., organic compounds or metals, for purposes of prediction of biophysical behavior. The purpose of this note is to report evidence for tributyltin speciation products occurring in seawater and their effect on KO, values. The values we report are substantially greater than those reported elsewhere but are sufficient to explain bioaccumulation as a partitioning process.
Materials and Methods Preparation of TBTO Solutions-Dispersions for NMR. Seawater solutions and dispersions of TBTO were prepared by either injecting acetone solutions of TBTO into seawater so that in one case TBTO solubility was exceeded, in a second it was not, or injecting neat TBTO (200 pL L-*)rapidly into seawater. Solutions were also prepared by the methods of Monaghan et al. (2). Chloroform extraction was carried out immediately, and the samples were concentrated by rotary evaporation. Identification of Tributyltin Speciation Products by NMR. NMR spectra of chloroform extracts of seawater were recorded in the pulse Fourier transform mode with proton noise decoupling using a Varian Model FT80A spectrometer with a broadband probe operating at 29.648 MHz (l19Sn) a t 35 "C. Tetramethyltin was used as the standard, and CDC13 served as the internal lock substance. NMR spectra of chloroform extracts of TBTO dissolved in seawater indicated the presence of several tributyltin compounds. Locations of spectral peaks were compared with and found to match chloroform solutions of tributyltin chloride (TBTCI), bis(tributy1tin) oxide (TBTO) (both purchased from Alfa Ventron and used without further purification), and a tributyltin carbonato compound, prepared by the reaction of carbon dioxide with TBTO in hexane (10). The precise identity of this carbonato complex remains enigmatic. Both the synthesized compound and material extracted from seawater are similar by NMR analysis, and their chemical shifts are consistent with a 4-coordinate butyltin carbonate (11). To confirm NMR identification, elemental analyses and electron impact mass spectra (EIMS), 70 eV, were performed by the Microanalysis Laboratory and Mass Spectroscopy Laboratory, Department of Chemistry, University of California, Berkeley, to characterize the composition of butyltins extracted from seawater. KO,Determinations. ['4C]Bis(tri-n-butyltin) oxide (TBTO) (3.67 mCi/mmol) was purchased from New England Nuclear. Sufficient neat material was dissolved in l-octanol (Fisher Chemicals) to yield a nominal 100 mg L-' solution. This solution was rinsed 5 times with filtered
0 1986 American Chemical Society
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seawater (0.45-pm membrane filters, Millipore). After each rinse, the aqueous phase was collected and centrifuged (3000 g) and decanted from any residual octanol. Radioactivity in 3.5 mL was determined by liquid scintillation counting. Then, 25 mL was extracted with methyl isobutyl ketone (MIBK), concentrated by evaporation under a stream of nitrogen, and spotted on a TLC plate (silica GF, Altech), which had first been treated with acetic acid. Plates were developed with a mixture of tetrahydrofuran/hexane/acetic acid (28:70:2). Eluted organic compounds were visualized before and after by oxidation with bromine. Then a 0.1% pyrocatechol violet 0.1% hexadecyltrimethylammonium bromide solution (Eastman Chemicals) was sprayed onto the plate subsequently to indicate the presence of tin by the appearance of a light blue product. (Original method is found in ref 12; enhancements for this application were suggested by David Evans, Duke University Marine Laboratory.) Authentic mono-, di-, tri-, and tetrabutyltin compounds were run under identical conditions for comparison with butyltin species from the MIBK extracts. While this identification does not provide unequivocal chemical characterization of butyltin compounds, it is, nevertheless, compelling evidence for identity given the simple system from which extracts were obtained. Serial dilutions of the rinsed octanol stock were subsequently prepared by the addition of sufficient octanol to make a series spanning a nominal range between 5 and 100 mg L-l. TBTO in octanol (5 mg L-I) yielded the lowest aqueous TBTO concentration that could reliably be determined (-850 pg L-l). Between 3 and 5 mL of the octanol from the dilution series was put into HC1-washed capped test tubes, and 5 mL of filtered seawater was added. The contents of each test tube were thoroughly mixed on a Vortex mixer until the octanol-water mixture became a transluscent mousselike emulsion. Mixing was repeated -6 h later, and the tubes were kept until the following morning in an incubator at 20 OC when the test tubes were centrifuged at 3000 g for 20 min. An aliquot of the octanol (0.2 mL) or water (3.5 mL) was added to Aquasol 2 (New England Nuclear) and counted to determine 14C content. All counts were corrected to disintegrations per minute (dpm) values using an external standard. This procedure was repeated using water phases of different salinities: 0 (deionized water), 2, 5, 15, 25, 32, and 45%. The last concentration was obtained by adding bulk sea salts (Instant Ocean) to seawater, followed by filtration as described above. The octanol phase minus that amount counted in the previous run was transferred to new acid-rinsed test tubes with 5 mL of seawater of the appropriate salinity. This slight reduction in octanol volume has no effect since the volume ratio of the two phases is not significant if the solute is not saturated in either one. Octanol-water partition coefficients were calculated as the slope of the regression line of the dpm mL-l seawater vs. dpm mL-l octanol.
Tetramethyltin
TBTO
+
Results and Discussion Identity of Speciation Products. NMR analyses show in normal seawater extracts that TBTCL, TBTO, and a carbonato species were present (Figure 1). Analysis by EIMS indicated 60% of the tributyltin was present as tributyltin chloride. That this mixture results from a true equilibrium is shown by the influence of pH on the NMR spectrum of solvent extracts (Figure 2). Below pH 7, predominant species are TBTOH2+and TBTC1, both of which extract as tributyltin chloride. A t a pH of -8, 202
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I50
IO0
0
50 PPm
Flgure 1. rrQSnNMR spectrum of a chloroform extract of a TBTO dispersion in 30%0 seawater (pH -8). The abscissa shows the displacement of the labeled peaks from a tetramethytin standard. Heights of the peaks are not proportlonal to concentration. Labels indicate species in HCCI,.
typical of seawater, three butyltin equilibrium products extract: TBTC1, TBTOH, and the carbonato species. Increasing the pH to 10 further shifts the equilibrium to favor the carbonato species and TBTOH. Similar analyses to specifically determine the effect of variations in [Cl-] were not performed during this phase of experimentation. Effects of pH on C1- exchange that have been shown by Aldridge (13) indicate the importance of this factor on physiologically relevant aspects of organotin speciation events and are in general agreement with the speciation scheme presented here. A qualitative summary of this equilibrium reaction in seawater at pH 8, incorporating the hypothesis discussed is ( B U ~ S ~+) ~HO 2 0 + 2Bu3SnOH
+
Bu3SnOH + H30+ + Bu3SnOH2+ H 2 0 Bu3SnOH2++ HOC02- ==Bu,SnOCO,Bu3SnOH2++ C1- ==Bu3SnC1
+ H30+
+ H20
The above equations assume a coordination number of 4 for Sn(1V). Coordination numbers of 5 or 6 are also possible. In principle, additional neutral or anionic ligands would have similar effects on Kowas outlined for the case of a coordination number of 4. KO,Determinations. The KO,for tributyltin varies as a function of salinity (Figure 3). The lowest KO,value, 5500, occurred in 25%. Values progressively increased as salinities were increased or decreased within the range tested. The highest value, 7000, occurred in deionized water. A t the other salinity extreme, 45%0,the KO,found was 6300. Variation of KO,with Salinity. The nonlinear relationship of KO, to salinity suggests that neither ionic strength nor pH changes associated with dilution of seawater exerts a predominant influence on the water solubility, which so dramatically alters the ratios of TBT in the two phases. In low salinity the increased stabilization of charged species with increasing [Cl-] is significant, resulting in a decreasing KO,, and in salinities above ~ 2 5 % 0 ,
Table I. Compilation of Published KO,Values for Tributyltin Derivatives
compd
conditions
bis(tri-n-butyltin) oxide (TBTO) distilled watera
KO,
method of analysis total tin: graphite furnace AAb using L’vov’s platform oxidation of organotins with acid followed by colorometric determination for total tin
tri-n-butyltin fluoride (TBTF)
distilled water
TBTO tri-n-butyltin chloride (TBTCl)
4 mL of octanol, 240 mL of water total tin in octanol phase aqueous 5 mL of octanol, 490 mL of water, concentration by difference: AAb pH 7.4, Tris buffer distilled water, pH 6.0, purified TBTO derivitization with n-pentylmagnesium bromide, gas chromatography with modified flame photometric detector [14C]TBT0 250/00 s [14C]TBT0 deionized water
TBTO TBTO TBTO
ref
200c 21
1400 22
2185 23 1300 24 1550 24 this work 7000 this work
5500
aAssumed. bAtomic absorbtion spectrometry. cBased on a pK,, estimated to be 2.3 from their Figure 5, p 487. 7000
TBTO
4000i
Carbonato species
1
TBTCl
I’
f
5000
-0003
0
10
20
30
40
50
Salinity %.
Figure Octanoi-water partition coefficients, KO,, of ti .Myitin as a function of salinity. Bars are fl standard deviation. Averages of 8-1 1 instantaneous slopes were used to calculate this standard deviation.
170
85
ppm
DOWNFIELD FROM ( C H 3 ) 4 S n Figure 2. ‘lsSn NMR spectrum of chloroform extracts of TBTO dispersions in seawater as a function of pH.
increasing [Cl-] shifts the equilibrium toward TBTCl with a concommitant increase in KO,. The difference between the highest and lowest measurement was 35 % . The significance of stabilization of tributyltin speciation products, such as the charged aquo complex and the carbonato species, by seawater ions as a function of salinity can be illustrated in a comparison with the behavior of dimethylmercury, a neutral organometallic compound. Wasik (14) reported that the KO,in seawater was 17% less than in freshwater. Additional evidence for the mechanism of salinity’s effect on partitioning behavior can be obtained from a consideration of stability constants of the putative speciation products. Approximate equilibrium constants for some speciation reactions occurring in seawater are known or can be inferred (15-18). The affinity of TBT+ for OHis -lo6 greater than for C1- (13). Undiluted seawater has a pH of -8 and [Cl-] -550 mmol, but salinity variation within the environmental range alters both buffering capacity and [Cl-] at a point where the equilibrium of tributyltin speciation products is very sensitive to displacement by these relatively small changes. Our results show the KO,to be the most stable between 15 and 32%0,with a major change occurring in salinities below 5%0. The initial drop in KO, presumably reflects a predominant effect of ionic strength in stabilization of the charged species. These data, showing that partitioning behavior is affected by local chemical composition of seawater, suggest that movements of tributyltin species and similar
charge-forming organometallics through the biosphere may be influenced strongly by covariant chemical factors in the milieu. Relationship of Reported KO,Values to Present Ones. The KO,values reported here are greater by at least a factor of 2 than those reported elsewhere, which vary from 200 to 2185 (Table I). We believe that the differences may be due to the presence of redistribution products, notably dibutyltin, in the mixture tested. Redistribution products are formed as follows: 2R3SnX R4Sn + R2SnX2 Redistribution may occur during synthesis, storage of neat material, or in solution. The law of mass action greatly favors redistribution under the first two conditions. The effect of dibutyltin, presumably a redistribution product, significantly reduces the measured KO, value attributed to tributyltin (Figure 4). KO,estimates of the first three washes were 600, 2500, and 3600. MIBK extracts of the aqueous phase developed on TLC plates indicated two tin-containing moieties. In the first wash, the major fraction matched dibutyltin, but its proportion decreased subsequently so that in third and subsequent washes, dibutyltin was not detected on TLC plates. No tetrabutyltin was detected in extracts of the aqueous phase. Tetrabutyltin is volatile and poorly water soluble, perhaps due to Henry’s law behavior, so its lack of detection is not surprising. These findings illustrate that unless an analysis method is specific for the organotin of interest, or starting material is of demonstrated purity, KO,values for R,SnX&,, may be significantly underestimated by redistribution to form the lower molecular weight homologues, Rn-lSnXP(n-l),which preferentially enriches the aqueous phase. In most of the previously reported determinations neither a tributyltin-specific method of analysis was used nor was the apparent purity of starting materials verified. KO,values find their primary use in the prediction of bioaccumulation potential of nonpolar organic chemicals. By use of MacKay’s equation (9),one would predict bioEnviron. Scl. Technol., Voi. 20, No. 2, 1986
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7006005001
I
2
3
4
5
WASH
Figure 4. Radioactivity (14C) of five aqueous washes of TBTO-octanol solution. Estimates of the KO, at each wash are shown for the first three washes. Dibutyltin was detected by thln-layer chromatography in the first two washes, but not subsequently.
concentration factors (BCF) of 400-500 for tributyltin. Measured BCF for marine bivalves and fish range from 1500 to 6000 for whole animals and somewhat higher for specific tissues (19,20). Thus, the values we report appear to be more consistent with the rather high bioconcentration factors observed.
Conclusions This note illustrates two points. First, when dissolved at low concentrations in seawater, TBTO forms chemical speciation products with all major seawater anions. The equilibrium distribution of these speciation products is mediated by anion concent,ration so that it can easily be displaced by salinity variation. Second, speciation events associated with salinity changes measurably influence Kow values. Thus, values in the literature may be significantly influenced by experimental methods that must be critically evaluated before acceptance of a KO,value for prediction purposes. Acknowledgments We thank A. Boyd, W. French, J. Ng, S. Lynn, and L. Fabiny for assistance.
Literature Cited Hodge, V. F.; Siedel, S. L.; Goldberg, E. D. Anal. Chem. 1979,51, 1256-1260. Monaghan, C. P.; Hoffman, J. F.; O’Brien, E. J.; Frenzel, L. M.; Good, M. L. “Proceedings of the 4th Annual Controlled Release of Pesticide Symposium”; Goulding, R. L., Ed.; 1977. Monaghan, C. P.; Kulkarni, V. I.; Ozcan, M.; Good, M. L. Office of Naval Research, Contract No. N00014-79-C-0487, Project No. NR356-709, Technical Report No. 2, July 22, 1980.
Rzaev, Z. M. 0. CHEMTECH 1979, 9, 58. Leo, A.; Hansch, C.; Elkins, D. Chem. Rev. 1971, 71, 525-550. Chiou, C. T.; Freed, V. H.; Schmeddling, D. W.; Kohnert, R. L. Environ. Sci. Technol. 1977, 11, 475-478. Veith, G. D.; DeFoe, D. L.; Bergstedt, B. V. J . Fish. Res. Board Can. 1979,36, 1040-1048. Neely, W. B.; Branson, D. R.; Blau, G. E. Environ. Sci. Technol. 1974,8,1113-1115. Mackay, D. Environ. Sci. Technol. 1982, 16, 274-278. Bloodworth, J. J.; Davies, A. G.; Basistha, S. C. J . Chem. SOC. C 1967, 309-1311. Blunden, S. J.; Hill, R.; Ruddick, J. N. R. J . Organornet. Chem. 1984,267, C5-C8. Omar, M.; Bowen, H. J. M. Analyst (London) 1982,107, 654-658. Aldridge, W. N. In “The Organometallic and Coordination Chemistry of Germanium, Tin and Lead”; Gielen, M., Harrison, P. G., Eds.; Nottingham, U.K., July 1977; pp 9-30. Wasik, S. P. In “Organometals and Organometalloids Occurrence and Fate in the Environment”; Brinckman, F. E., Bellama, J. M., Eds.; American Chemical Society: Washington, DC, 1978; pp 314-323. Jewett, K. L.; Brinckman, F. E.; Bellama, J. M. In “Organometals and Organometalloids Occurrence and Fate in the Environment”; Brinckman, F. E., Bellama, J. M., Eds.; American Chemical Society: Washington, DC, 1978; pp 158-187. Wieth, J. 0.;Tosteson, M. T. J . Gen. Physiol. 1979, 73, 765-788. S i l k , L. G. Chem. SOC.Spec. Pub. No. 25 Suppl. 1 1971. Tobias, R. S. In “Organometals and Organometalloids Occurrence and Fate in the Environment”; Brinckman, F. E., Bellama, J. M., Eds.; American Chemical Society: Washington, DC, 1978. Ward, G. S.; Cramm, G. C.; Parrish, P. R.; Trachman, H.; Slesinger, A. In “Aquatic Toxicology and Hazard Assessment: Fourth Conference, ASTM S T P 737”; Branson, D. R., Dichon, K. L., Eds.; Miami, FL, May 1981; pp 183-200. Waldock, M.; Thain, J. E. Mar. Pollut. Bull. 1983, 14, 41 1-415. Wong, P. T. S.; Chau, Y. K.; Kramar, 0.; Bengert, G. A. Can. J . Fish. Aquat. Sci. 1981, 39, 483-488. Slesinger, A. E.; Dressler, I. In “Report of the Organotin Workshop Report”; Good, M. L., Ed.; New Orleans, LA, Feb 1978. Wulf, R. G.; Byington, K. H. Arch. Biochem. Biophys. 1975, 167, 176-185. Maguire, R. J.; Carey, J. H.; Hale, E. J. J. Agric. Food Chem. 1983, 31, 1060-1065.
Received for review September 1 I , 1984. Revised manuscript received May 13,1985. Accepted October 16,1985. This research was supported by the U S . Office of Naval Research. Preliminary analyses of speciation products were presented to the 185th Annual Meeting of the American Chemical Society, Las Vegas, NV, March 28-April 2, 1981.
A Basic Program to Plot Stiff Diagrams Douglas Craft U S . Bureau of Reclamation, Denver, Colorado 80225
w Presented is a Basic language computer program that plots major ion water chemistry data on Stiff diagrams. The program is easy to use and allows the entry and editing of either milligrams per liter or milliequivalents per liter ion concentration data. The user may also select from three plot scaling options. The program was written for Hewlett-Packard Basic computers but may be modified to run on other systems. 204
Environ. Sci. Technol., Vol. 20, No. 2, 1986
An effective and straightforward method for evaluating multivariate water quality data involves the use of graphical plotting. Environmental scientists and hydrologists have long used Stiff diagrams (I) that plot major ion water data on four parallel axes with a cation and anion on each axis and the zero concentration for all axes located in the center of the plot. Using Stiff diagrams allows a quick visual evaluation of the major ion chemistry of a
Not subject to U.S. Copyright. Published 1986 by the American Chemical Society
