Environ. Sci. Technol. 2001, 35, 3151-3157
Sorption and Desorption Behavior of Organotin Compounds in Sediment-Pore Water Systems M I C H A E L B E R G , † C EÄ D R I C G . A R N O L D , ‡ STEPHAN R. MU ¨ LLER,† JU ¨ RG MU ¨ HLEMANN,† AND R E N EÄ P . S C H W A R Z E N B A C H * , † Swiss Federal Institute for Environmental Science and Technology (EAWAG) and Swiss Federal Institute of Technology (ETH), CH-8600 Du ¨ bendorf, Switzerland, and BMG Engineering Ltd., Ifangstrasse 11, CH-8952 Schlieren
Sediments contaminated with organotin compounds (OTs), in particular triorganotins (TOTs), are abundant in areas with high shipping activities. To assess the possible remobilization of these highly toxic compounds from such sediments, a profound understanding of their sorption/ desorption behavior is necessary. In this work the extent and reversibility of sorption of OTs to sediments has been investigated using contaminated freshwater harbor sediments and two certified OT containing marine sediments. Experiments conducted with perdeuterated OTs showed that sorption of OTs to sediments is a fast and reversible process involving primarily particulate organic matter (POM) constituents as sorbents. The organic carbonnormalized sediment-water distribution ratios (Doc, expressed in L/kgoc) determined in the laboratory were consistent with in-situ Docs obtained from OT concentrations measured in sediment and pore water samples from two dated sediment cores. For both butyl- and phenyltin compounds the log Doc values were in the range of 4.7-6.1, and the following sequence was observed: Doc (tri-OT) g Doc (diOT) g Doc (mono-OT). However, the differences were much less pronounced than would have been expected for hydrophobic partitioning of the corresponding compounds into POM. These results support our hypothesis from earlier work with dissolved humic acids that OT sorption to sediments occurs primarily by reversible formation of (innerspere) complexes between the tin atom and carboxylate and phenolate ligands present in POM. Because of the high Doc values (i.e. log Doc g 4) the diffusion of OTs from deeper sediments to the surface will be rather slow, and thus a major release from undisturbed sediments is not expected. However, because OTs readily desorb, any resuspension of contaminated sediments (e.g., by the tide, storms or dredging activities) will lead to enhanced OT concentrations in the overlaying water column. Furthermore, in contrast to polycyclic aromatic hydrocarbons (PAH) where large fractions may be tightly bound (in)to soot or other carbonaceous materials, OTs will be more readily bioavailable due to the fast and reversible sorption/ desorption behavior.
Introduction Because of their extensive use as biocides, particularly in antifouling boat paints, triorganotin compounds (TOTs), in 10.1021/es010010f CCC: $20.00 Published on Web 06/29/2001
2001 American Chemical Society
particular tributyltin (TBT) and triphenyltin (TPT), have been introduced in large quantities into the aquatic environment (1). Since TOTs are extremely toxic to aquatic organisms (24), their use has been increasingly restricted over the past decade. This has led to a significant decrease in TOT water concentrations in the freshwater as well as in the coastal marine environment (review in ref 5). However, because TOTs associate strongly with natural sorbents (6-8) and because they seem to be quite persistent under anoxic conditions (3, 9), they may accumulate in sediments (10). In fact, high levels of TOTs and of their degradation products (i.e. di- and monoorganotins) have been found in freshwater, estuarine and coastal marine sediments, with TOT concentrations up to 3 mg Sn/kg in various harbor sediments (3, 9, 11). Hence, as pointed out by several authors (12-15), such highly contaminated sediments may represent an important source for long-term pollution of the (aquatic) environment with organotin compounds (OTs). To assess the possible remobilization of OTs from sediments, a profound understanding of their sorption/desorption behavior is necessary. The present knowledge on the sorption and, particularly, on the desorption behavior of OTs in sediments is still rather limited. In some cases, very high apparent sediment water distribution ratios were obtained for TOTs from desorption experiments using contaminated sediments or by measuring actual sediment and pore water concentrations (16, 17). With respect to the possible input of OTs into the overlaying water column by diffusion, bioturbation, or resuspension, very little information is available (18, 19). There are some observations of water contamination with TOTs following dredging activities (20), whereas no substantial release of TBT from unshaken sediments could be observed (21). All these studies provide, however, very little insight to the kinetics and mechanism(s) of OT sorption in sediments. In earlier work it has been shown that the sorption to organic phases is significantly larger than sorption to mineral phases, including clay minerals, as well as Si-, Al-, and Fe(hydr)oxides (22-24). In particular, we have shown that TOTs associate strongly with humic acids primarily by formation of (innerspere) complexes with oxygen ligands (i.e. carboxyl and phenol groups). Hence, for sediments containing more than 1% of organic matter (i.e. 0.5% of organic carbon) the sorption to mineral phases should be negligible as compared to the sorption to organic matter (13, 23). In this paper we present the results of laboratory experiments and field measurements aimed (i) to determine the extent and reversibility of sorption of OTs, in particular of TBT, TPT, and of their major degradation products [i.e. dibutyltin (DBT), monobutyltin (MBT), diphenyltin (DPT), monophenyltin (MPT)] to lake sediments under typical natural conditions, (ii) to check whether the same sorption mechanism(s) as found for humic acids in earlier work govern the sorption of OTs to sediment organic matter, and (iii) to gain insight into the postdepositional fate of these chemicals in sediments. To this end, sorption-desorption batch experiments were conducted using contaminated freshwater harbor sediments and two certified OT containing marine sediments (PACS-1 and PACS-2). To distinguish between the OTs originally present in these sediments and the freshly sorbed OTs, sorption experiments were carried out using * Corresponding author phone: +41-1-823 51 09; fax: +41-1-823 54 71; e-mail:
[email protected]. † Swiss Federal Institute for Environmental Science and Technology (EAWAG) and Swiss Federal Institute of Technology (ETH). ‡ BMG Engineering Ltd. VOL. 35, NO. 15, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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perdeuterated standard compounds. The sediment-water distribution ratios determined in the laboratory are compared with in situ distribution ratios calculated from solid and pore water concentration depth profiles measured in two dated sediment cores from two different harbors.
Experimental Section Materials. The reference compounds tributyltin chloride TBTCl (>97%), dibutyltin dichloride DBTCl2 (97%), triphenyltin chloride TPTCl (>97%), tripropyltin chloride TPrTCl (>97%), tripentyltin chloride TPeTCl (>98%), and tetrabutyltin TeBT (g98%) were obtained from Fluka (Buchs, Switzerland). Diphenyltin dichloride DPTCl2 (>98%) was purchased from ABS (Basel, Switzerland). Butyltin trichloride MBTCl3 (95%) and phenyltin trichloride MPTCl3 (98%) were obtained from Aldrich (Steinheim, Germany). Perdeuterated organotin compounds (MBT-d9, DBT-d18, TBT-d27, MPTd5, DPT-d10, TPT-d15; as respective chlorides) were synthesized in our laboratories (23). Sodium tetraethylborate (>98%) was obtained from Strem Chemicals (Bischheim, France). Beware: Organotin compounds are toxic. Skin contact or inhalation of vapors must be avoided. Pure NaBEt4 is self-flammable, and water for preparing the solutions should be ready before weighing, to minimize the contact time of the NaBEt4 with air. All other chemicals and solvents were obtained in the best available quality from Merck (Darmstadt, Germany) or from Fluka. Reference sediments PACS-1 and PACS-2 were purchased from Promochem (Wesel, Germany). Organotin Analysis. The procedure for the analysis of OTs in water samples and in dried sediments is described in detail elsewhere (10). Briefly, the OT compounds in water samples were ethylated in the aqueous phase with NaBEt4 followed by liquid-liquid extraction with hexane. The dried sediment samples were extracted with accelerated solvent extraction (ASE) at 100 °C with a methanolic mixture of sodium acetate and acetic acid and further processed as described for the water samples. The OTs were quantified in the hexane extracts with GC/MS analysis. By using perdeuterated OTs as internal standards, excellent precision (relative standard deviations 100 µg Sn/g) Kdobs values were significantly smaller indicating nonlinear isotherms. Therefore, in the following, only Kdobs values determined at low concentrations (i.e. Cs ∼1 µg/kg) corresponding to the field situation (see below) will be discussed. Table 1 summarizes the Kdobs values determined for the various organotin species investigated. Also included are the particulate organic carbon normalized distribution ratios Doc
Doc ) Kdobs/foc
(1)
where foc is the fraction of particulate organic carbon (POC) present in the given sediment. For both butyl- and phenyltin compounds the following sequence was observed: Doc (triOT) g Doc (di-OT) g Doc (mono-OT). Note that the difference between the various species are much less pronounced than one would expect for hydrophobic partitioning of the corresponding compounds into particulate organic matter (POM). Unfortunately the pertinent physical-chemical properties (pKa, Kow, etc.) of the mono- and diorganotins are not accurately known, and a sound interpretation of their Doc values is therefore not possible. Consequently we will focus our following discussion on triorganotin compounds (TOTs). For the interpretation of the TOT data presented in Table 1, it is necessary to recall that in freshwater systems, TOTs are present primarily as positively charged TOT+ aquo ions and as neutral hydroxo complexes, TOTOH (26). Furthermore, it can be assumed that sorption to the POM is dominating the overall sorption of TOTs in organic rich soils and 3154
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FIGURE 2. Sediment-water distribution coefficients (Kdobs from first batch) as a function of TBT and TBT-d27 concentration for the sediments W2-18 and PACS-2. Solid line: TBT, broken line: TBTd27. Adsorption and desorption Kds are shown in the magnification for (A) desorption Kdobs of TBT, (B) adsorption Kdobs of TBT-d27, and (C) desorption Kdobs of TBT-d27.
FIGURE 3. Effect of hydrophobicity on the sediment-water distribution of TOTs (Cmax OTtot ) 25 ng OT/mg POC) and comparison with Aldrich humic acid (AHA)-water distribution (Cmax OTtot ) 60 000 ng OT/mg DOC) from ref 22. Plot of log DocTOT+ (Doc/rTOT+) determined in filtered lake water (buffered to pH 7.3) versus log Kow of the corresponding TOT-OH species. sediments (22, 24). Hence, at a given pH, the overall Doc can be expressed by
Doc ) RTOT+ ‚ Doc
TOT+
+ (1 - RTOT+)Doc
TOTOH
(2)
where RTOT+ ) 1/(1 + 10pH-pKa) is the fraction of TOT in acidic (cationic) form, and DocTOT+ and DocTOTOH are the natural organic matter-water distribution ratios of TOT+ and TOTOH respectively, at a specific pH. The pKa values of the TOTs investigated are given in Table 1. In earlier work on the association of TOTs with dissolved humic acid (HA) we found that even at high pH, the overall sorption was dominated by the association of the cationic form with the HA (22). We postulated that sorption of TOT+ to HA is governed primarily by the formation of complexes between the tin atoms and negatively charged HA ligands including carboxyl and phenolate groups. Consequently, Doc could be simply expressed by
Doc ) RTOT+ ‚ Doc
TOT+
(3)
where DocTOT+ is, however, a rather complex function of pH. The observed pH-dependence of DocTOT+ was described successfully with a semiempirical discrete log Kj spectrum model using four discrete complexation sites in HA exhibiting fixed pKa,j values of 4, 6, 8, and 10. Kj is the complexation constant of TOT+ with the ligand type j, i.e., a carboxyl (pKa,j ) 4 and 6) or phenolate (pKa,j ) 8 and 10) group. Kj values
TABLE 1. Sediment-Water Distribution Coefficientsa (log Kdobs and log Doc) of Alkyl- and Phenyltin Compounds Determined for the Harbor Sediment Layer W2-18 of Lake Zu1 rich and for the Marine Sediments PACS-1 and PACS-2 W2-18b (58 g/kgh)
PACS-1 (37 g/kgh)
compound
abbr
pKac
n
log Kdobs, L/kg
log Doc, L/kg
(RSD),d %
tributyltin dibutyltin monobutyltin tripropyltin tripentyltin triphenyltin diphenyltin monophenyltin
TBTe DBTe MBTe TPrTf TPeTf TPTg DPTg MPTg
6.3 n.a. n.a. ∼6.3 ∼6.3 5.2 n.a. n.a.
4 4 4 2 2 2 2 2
4.13 4.05 3.89 3.04 4.83 3.86 3.63 3.45
5.37 5.29 5.12 4.28 6.07 5.09 4.87 4.68
(5) (22) (13) (1) (1) (4) (31) (22)
PACS-2 (32 g/kgh)
n
log Kdobs, L/kg
log Doc, L/kg
(RSD),d %
2 2 2
4.03 3.94 3.68
5.46 5.37 5.11
(16) (23) (10)
n
log Kdobs, L/kg
log Doc, L/kg
(RSD),d %
4 4 4 2 2 4 4 4
3.61 3.38 3.15 2.22 4.10 3.21 2.97 2.53
5.11 4.88 4.65 3.73 5.60 4.73 4.47 4.05
(15) (17) (12) (12) (13) (22) (8) (34)
a Determined in laboratory experiments (72 h exposure, one batch) with 5.0 g of suspended sediment and 10 L of lake water buffered to pH 7.35 ( 0.1. b Sediment layer from core W2, 18-20 cm depth. c pKa values from ref 23; n.a. not available. d Because of the definition of Doc (eq 1), RSDs were identical for Kdobs and Doc. e Coefficients determined from desorption of sediment-sorbed butyltin compounds. f Coefficients determined from adsorption of TPrT and TPeT spiked to the water phase. g Coefficients determined from adsorption of perdeuterated phenyltin compounds spiked to the water phase. h POC.
FIGURE 4. Depth profiles of butyltin compounds, DOC, and POC, determined in sediment pore water (Cw) and in solid sediments (Cs) in the cores W2 and E2. The arrows (v) in the upper left corner of the charts indicate the OT concentrations measured in the harbor water. The year of deposition was determined from depth profile measurements of the radionuclides 137Cs and 134Cs. VOL. 35, NO. 15, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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two sediment cores from two different harbors in Lake Zurich, Switzerland. Also shown are the DOC and POC profiles. Some additional sediment and pore water characteristics of the two cores are given in Table 2. The database showing the OT concentrations analyzed in harbor waters and sediment cores of nine other Swiss harbors are available as Supporting Information. For the phenyltin compounds, similarly shaped profiles were obtained. However, compared to the butyltins, their concentrations were much smaller, and the relative abundance of the degradation products DPT and MPT were larger (see average values given in Table 3 and complete data set provided as Supporting Information). The profiles shown in Figure 4 nicely reflect the historical use record of TOT-based antifoulings in Switzerland. These compounds were introduced in the late 1960s, and their consumption reached a maximum in the mid 1980s and then decreased until they were banned in the early 1990s. Note that the differences in the two cores are due to the very different sedimentation rates in the two harbors. Since the original TBT and TPT contamination level of these sediments is not exactly known, the possible degradation of these compounds can be addressed only qualitatively. Nevertheless, the data show that both TBT and TPT remain in the sediments over long time periods. Furthermore, the fact that the fractions of the major degradation products do not increase significantly with increasing depth indicates that the TOTs are either degraded very slowly or that the di- and monoorganotins are readily degraded to inorganic tin species. The former assumption seems to be more likely, since it has been found in earlier studies that DBT and MBT are quite stable in sediments (27, 28). Table 3 summarizes the average in-situ Doc values determined for the various OTs in the two sediment cores. These values were determined from total sediment concentrations (Cs) and pore water concentrations (Cw,tot), that is, by neglecting the OTs associated with the DOC:
TABLE 2. Characteristic Parameters of the Sediment Cores from Harbor Wa1 denswila (W2) and Harbor Engea (E2) sediment core Wa1 denswil W2 average(SD)b Sediment core depth, cm 28 sedimentation rate,c cm/year 1.3 water content, % w:w 68 POC,d g C/kg 46 clay (63 µm), % 8 Pore Water 7.0 7.3
pH DOC,e mg/L
sediment core Enge E2 average(SD)b
(5) (0.7) (3) (5) (2)
28 0.5 65 34 14 84 2
(8) (0.6) (3) (3) (0.5)
(0.1) (3.5)
7.2 8.7
(0.2) (5.0)
a The pH of the harbor water was 8.1 at both locations. b Average values over the whole depth of the sediment cores, standard deviations in parentheses. c Determined from 137Cs measurements. d Particulate organic carbon in dried sediments. e Dissolved organic carbon in filtered pore water.
for complexation to phenolate ligands were found to be one to 2 orders of magnitude larger as compared to the Kjs obtained for the carboxyl groups. For more details see ref 22. As shown in Figure 3, when relating the DocTOT+ values determined at a given pH versus the octanol-water partition constant, Kow, of the corresponding TOTOH species, a very similar pattern is obtained for both sediment-water and for the HA-water systems. For the trialkyltin compounds, in all three cases, a linear correlation with a slope of about 0.5 is found between the logarithms of the respective parameters (lines in Figure 3). Furthermore, as compared to the trialkyltin cations, TPT+ exhibits an approximately 1 order of magnitude higher affinity to the sediment or HA, respectively. This finding can be rationalized by the fact that, as indicated by its lower pKa value (see Table 1), TPT+ forms stronger complexes with oxygen ligands than trialkyltin ions. A similar conclusion was also drawn in a recent study on the bioaccumulation of TBT and TPT by a larval midge (18, 19). In summary, together with our earlier findings on TOT sorption to HA (22), the results of these laboratory experiments suggest that, at ambient pH values, the sorption of TOTs (and probably also of the di- and monoorganotin compounds) to sediments is dominated by complex formations of the positively charged OT-species with oxygen ligands present in the POM. The fast kinetics and reversibility of sorption can be rationalized by imagining that in aqueous solution, the polar POM ligands are more easily accessible as compared to the more hydrophobic domains. Field Data. Figure 4 shows the sediment and pore water concentration profiles of TBT, DBT, and MBT determined in
Doc,in-situ )
Cs
(4)
Cw, tot ‚ foc
The association of the OTs with the DOC was neglected for two reasons. First, there was no observable trend in the Doc,in-situ values determined for various sediment depths, although the DOC concentrations ranged from 3 to 15 mg/L (Figure 4). Second, when using the average distribution ratio determined for TBT with two dissolved humic acids for pH values of 7.3 (i.e. DDOC ∼104.3 L/kgDOC, (22)), the fraction associated with the dissolved pore water organic matter is predicted to be less than 25% of the TBT mass present. Hence, the Doc values given in Table 3 should not be too different from the “true” Doc values. Inspection of Table 3 shows that the Doc values of the various OTs did not vary much within a given core (see
TABLE 3. Average Observed In Situ Distribution Coefficients (Log Kdobs and Log Doc) and OT Concentrations Determined in Pore Water and in Sediment Layersa of the Sediment Coresb from Harbor Wa1 denswilc (W2) and Harbor Engec (E2) sediment core W2b pore water, ng/L (SD) TBTf DBTf MBTf TPT DPT MPT
17 9.3 36 1.3 0.7 5.2
(14) (5.0) (21) (0.5) (0.4) (3.5)
sediment, ng/g (SD) 350 110 51 3.0 2.5 g
(280) (70) (34) (1.6) (1.7)
sediment core E2b log Kdobs, L/kg 4.34 4.11 3.26 3.58 3.64 g
Doc,d
log L/kg
5.69 5.47 4.61 4.94 5.00 g
(SD)e (0.1) (0.3) (0.4) (0.4) (0.3)
pore water, ng/L (SD) 9.8 3.8 6.6 0.9 0.8 g
(4.7) (2.5) (1.8) (0.1) (0.7)
sediment, ng/g (SD) 240 90 24 9.8 16 12
(170) (58) (11) (1.7) (7.7) (5.3)
log Kdobs, L/kg
log Doc,d L/kg
4.39 4.44 3.58 4.03 4.14 g
5.73 5.78 4.92 5.37 5.48 g
(SD)e (0.5) (0.4) (0.3) (0.1) (0.2)
a Average from 13 vertical layers of 1, 2, or 4 cm thickness. b Both cores were 28 cm deep and sliced into 13 layers. c The locations are shown in the Supporting Information. d Calculated from average POC values that are shown in Figure 4. e Identical standard deviations were obtained for Kdobs and Doc. f The corresponding depth profiles are shown in Figure 4. g Concentrations were below the detection limit in the majority of the sediment layers.
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standard deviations and corresponding concentration profiles in Figure 4). Furthermore, the average values compare favorably with the Docs determined in the laboratory experiments for the W2-18 sediment (Table 1). This indicates that (i) Doc (or Kdobs) did not depend on the water/sediment ratio, and (ii) sorption equilibrium was established in both cores (i.e., no aging phenomena were observable). The data also confirm the high Doc values found in the laboratory experiments for the much less hydrophobic TOT degradation products, in particular, the monoorganotins. Since very similar Doc values were determined in both the field and laboratory, the sorption mechanisms can be expected to be the same (i.e. readily reversible complexation of OTs by negatively charged POM-ligands). Environmental Significance. The results of this study show that sorption of tinorganic compounds to sediments is a fast and reversible process involving primarily particulate organic matter constituents as sorbents. Hence, in contrast to polycyclic aromatic hydrocarbons, where large fractions may be tightly bound to soot or other carbonacious material present in the sediment (29), OTs will, in general, readily desorb and, therefore, be more readily bioavailable. As a consequence, any resuspension of heavily OT-contaminated sediments (e.g. by the tide, storms, or dredging activities) will lead to enhanced OT concentrations in the water column. This is particularly important for the TOTs, which are toxic already at concentrations of a few nanograms per liter (3). In fact, as is illustrated by the arrows in Figure 4, more than 5 years after the ban of these compounds, OT concentrations similar to the ones in the pore water of the upper sediment layers were found in the water column of the harbors. On the other hand, when considering their rather high Doc values, the diffusion of OTs from deeper sediments to the surface will be rather slow, so that undisturbed sediments containing OTs in the lower layers should not be a major input source for such compounds in the overlaying water. However, OTs present in the sediment can be expected to be bioavailable to benthic organisms.
Acknowledgments We are indebted to Markus Hofer, Beat Mu¨ller, Andrea Ciani, and Jean-Marc Stoll for the assistance during sediment sampling. Michael Sturm, Alois Zwyssig, Alfred Lu ¨ ck, and Caroline Stengel are acknowledged for dating the sediments and for analyzing the sediment compositions. We thank Andrea Ciani, Beate Escher, Kai-Uwe Goss, Rene´ Hunziker, and Torsten Schmidt for critical comments on the manuscript.
Supporting Information Available A figure showing the locations of the 11 investigated harbors together with tables containing the full database of OT concentrations analyzed in the harbor waters and in 16 sediment cores. This material is available free of charge via the Internet at http://pubs.acs.org.
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Received for review January 5, 2001. Revised manuscript received May 7, 2001. Accepted May 7, 2001. ES010010F
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