New Model Calculations of pH-Depending Tributyltin Adsorption onto

Kandy 20 000, Sri Lanka. Interactions of the pollutant tributyltin (TBT) with mineral surfaces affects its distribution and transport in aqueous syste...
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Environ. Sci. Technol. 2005, 39, 844-849

New Model Calculations of pH-Depending Tributyltin Adsorption onto Montmorillonite Surface and Montmorillonite-Rich Sediment M A R I O N H O C H * ,† A N D R O H A N W E E R A S O O R I Y A †,‡ Institut fu ¨ r Geologie und Mineralogie, Universita¨t Erlangen-Nu ¨ rnberg, Schlossgarten 5, D-91054 Erlangen, Germany, and CML, Institute of Fundamental Studies, Kandy 20 000, Sri Lanka

Interactions of the pollutant tributyltin (TBT) with mineral surfaces affects its distribution and transport in aqueous systems. In the present work, model calculations are reported that quantify TBT adsorption onto pure-phase montmorillonite (sample SWy) under various pH and salinity conditions that are important from an environmental perspective. The pH level in the system is of substantial interest because it affects the speciation of TBT in solution as well as the surface properties of the solid phase, which are both important for adsorption reactions. The model is based on the generalized diffused layer model that includes >X- sites in order to account for the cation exchange effects of TBT attraction. The presence of >AlOH and >SiOH sites at the mineral surface was not considered separately during calculation. Instead, nonselective sites (>SOH) versus selective sites (>SsOH) were distinguished with respect to the sorptive sites on montmorillonite. The latter are characterized by a high affinity of TBT bonding. Both sorptive sites exhibit the same protolysis constants but different TBT binding constants [logKTBT/2 ) -1.18 for (>SOH), logKTBT/1 ) 3.98 for (>SsOH)]. LogKX/TBT for the cation exchange reaction was determined as between 3.05 and 4.14. The results indicate that the inclusion of selective sites during calculations is essential for quantifying pH-dependent TBT adsorption successfully. The parameters determined for the TBT adsorption onto purephase montmorillonite were subsequently used to calculate pH-dependent TBT adsorption onto a natural montmorillonite-rich sediment.

Introduction Highly toxic tributyltin (TBT) has entered various ecosystems during the past 40 years due to its widespread industrial application (1). To gain a better understanding of the global cycle of TBT, information on its distribution and fate is imperative. With regards to its distribution in aquatic systems, the strong affinity of TBT to particulate matter plays an important role. In previous studies sediments in an aquatic environment were identified as potential sinks for organotin compounds (2, 3). Other experiments also demonstrated that * Corresponding author phone: ++49/ (0)9131 8522660; fax: ++49/(0)9131 8529294; e-mail: [email protected]. † Universita ¨ t Erlangen-Nu ¨ rnberg. ‡ CML, Institute of Fundamental Studies. 844

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sediments could act as a source of renewed contamination (4, 5). Complicating matters, like the existence of various mineralogical (e.g. clay minerals, Fe- and Mn-hydroxides) and organic components in a sediment phase, influences the adsorption capacity for TBT. Previous studies showed that TBT is strongly adsorbed onto different clay minerals (6, 7). Furthermore, several parameters of the aqueous phase can affect the distribution and transport of this contaminant (e.g. pH, salinity, solid/solution-ratio). However, it was recognized that the pH level is one of the primary controlling factors of TBT partitioning, since it affects both the speciation of organotin compounds in aqueous solution as well as the surface properties of the sorbent material (for further discussion refer to refs 6 and 7). For this reason it is imperative to know details of the adsorption process and the interactions at the clay mineral-pollutant interface under various environmental conditions. The pH-dependent TBT adsorption onto kaolinite has previously been modeled (8), but the structure and the chemical composition of kaolinite and montmorillonite are different. Thus, their physical, chemical, and, especially, their adsorption properties are also dissimilar. The previous results have shown that TBT binding onto kaolinite can be calculated by generalized diffused layer model, considering the assumptions which are made for solid oxides (8). This is justifiable since kaolinite carries a small permanent charge. But the same assumption cannot be made for montmorillonite. In this case, the effect of the permanent charge has to be explicitly taken into account. Clays are not only one of the main sorbent components for pollutants in sediments and soils, but they also play an important role in technical applications, such as chemical barriers in landfills and toxic waste impoundments (9). For this reason a detailed knowledge about the adsorption ability of clays is of interest. The aim of the present study is to quantify TBT adsorption onto pure-phase montmorillonite under various experimental conditions that are important from an environmental perspective (pH, salinity). Because natural sediments are complex geological materials in their composition, a pure montmorillonite phase was used for adsorption experiments. Afterward, the resulting parameters determined for this system were applied to the calculation of the pH-dependent TBT adsorption of natural montmorillonite-rich sediment.

Experimental Section Sorbent Materials. A Na-montmorillonite sample (SWy) was chosen for the pure-phase clay during experimentation. It was obtained from Clay Mineral Repositors, Columbia, U.S.A. Montmorillonite is an expandable 2:1 layer silicate, with two silica tetrahedral sheets joined to a central octahedral sheet. The mineral has interlayer sites as well as ionizable hydroxyl sites on its external surfaces for ion adsorption. The mineral exhibits a pH-dependent variable charge in addition to a pH-nondependent permanent charge, which is caused by considerable substitution of Al by Fe and Mg in the octahedral sheets and substitution of Si by Al in the tetrahedral sheets. This creates a charge imbalance up to ∼ -0.66 per unit cell (10). The charge deficiency in the montmorillonite structure is satisfied by exchangeable cations (e.g. Na, Ca, Mg, Fe, and Li), which are adsorbed in the interstices of different unit layers, at the surface of the planes and around the edges of the mineral. In case of Na-montmorillonite SWy the predominantly exchangeable cation is sodium. These cations can be replaced by other ions, like heavy metal cations or 10.1021/es0400382 CCC: $30.25

 2005 American Chemical Society Published on Web 12/18/2004

TABLE 1. Physicochemical Parameters of the Sorbent Material parameter Montmorillonite (SWy) outer surface area [m2/g] total surface area [m2/g] total number of sites >SOH [µmol/L] cation exchange capacity (CEC) [mequiv/kg] surface charge density σ [µmol/m2] pHZPC TOC [%]

value

reference

31.82 662 36.8 764

(11) (12) (13) (11)

1.15 e2-3 0.02

(14) (15)

Natural Sediment (SED-M) bulk surface area [m2/g] 77.7 cation exchange capacity (CEC) 537 [mequiv/kg] total organic carbon (TOC) [%] 0.06 mineralogical composition clay minerals [%] 95 montmorillonite 80 kaolinite 10 illite 10 silica [%] 5

( 5) (5) ( 5) ( 5)

polar organic molecules. Montmorillonite has a high cation exchange capacity (CEC) and is an important sorbent for pollutants in sediments and soils. For quantifying the adsorption of TBT on natural sediments, a montmorillonite-rich sediment (sample SED-M) was used. To simplify matters, the experimental results of pH-depending TBT adsorption reported in ref 5 were used. The physicochemical properties of both sorbents which are relevant for the adsorption process are given in Table 1. In this table two types of surface area are given for pure phased montmorillonite, a total surface area and an outer surface area. Specific reasons, why the outer surface area was selected for modeling, are explained in the Results and Discussion section. Reagents. Unless otherwise mentioned, all chemicals used in this study were p.a. grade. The following organotin compounds were used: tetrabutyltin (TTBT, 98%) and tributyltin chloride (TBT, 97%) were obtained from Fluka (Darmstadt, Germany). Tripropyltin chloride (TPrT, 98%) was ordered from Merck Eurolab GmbH (Darmstadt, Germany). Stock solutions containing 2000 mg/L of TBT and TPrT as tin were prepared separately by dissolving respective organotin derivatives in methanol (adequate amounts of TTBT were dissolved in hexane) and stored in the dark at 4 °C to minimize dissociation and evaporation. Working solutions were prepared daily by dilution. Sodium tetraethylborate (NaBEt4) was purchased from Strem Chemicals (Bischheim, France), and working solution (1% w/v) was prepared daily. CHES (2-(cyclohexylamino) ethanesulfonic acid), TRICINE (N-[tris(hydroxylmethyl)methyl]glycine), MOPS (3-morpholinopropanesulfonic acid), and MES (2-morpholinoethansulfonic acid monohydrate) were obtained from Sigma-Aldrich and potassium hydrogen phthalate from Fluka. Hexane was purchased from Merck and Methanol (Chromasolv) from Riedel de Haen (Seelze, Germany). TBT Adsorption Experiments. The batch technique was used to determine the partitioning of TBT between the montmorillonite and the water phase. One gram of montmorillonite (SWy) was suspended into 50 mL of deionized water or artificial seawater (0.6 M) in a screw-capped Teflon flask. The composition of the artificial seawater is given in ref 13. The suspensions were equilibrated for 24 h after adjusting pH to desired values using the following buffers: CHES (pH ) 9), TRICINE (pH ) 8), MOPS (pH ) 7), MES (pH ) 6), sodium acetate/acetic acid buffer (pH ) 5), and potassium hydrogen phthalate (pH ) 4). The addition

of buffer solutions yielded an ionic strength, which was treated as “equivalent” to 0.06 M NaNO3. Subsequently, any changes of pH were corrected by dropwise addition of HNO3 or NaOH. The pH values were measured by Microprocessor pH Meter pH 196 with an accuracy of (0.05. Afterward, the suspensions were purged with nitrogen to remove atmospheric contaminants, like CO2, and spiked with aliquots of TBT working solution to yield a concentration of 400 ng(Sn)/mL (according to 1.377 µmol/L) in the water phase. The samples were shaken at 21 ( 1 °C for 24 h in the dark to ensure that the adsorption reaction reached a state of equilibrium (2, 4). After batching, the samples were centrifugated at 2500 rpm for a period of 50 min, and the TBT concentration in the supernatant was analyzed. NaBEt4 was used as a derivatization agent, and TTBT and TPrT were added as internal standards to correct the efficiency of the derivatization reaction and losses of evaporation. The organotin compounds were extracted with hexane and stored at -20 °C in the dark until analyzing. A more detailed description of sample preparation is given in ref 5. All concentrations of organotin in the present paper are given as tin. Experiments were carried out in triplicates, and the data presented herein are mean values. The amount of tributyltin adsorbed to the solid phase is calculated as the difference between the initial tributyltin concentration and the concentration determined in the equilibrium solution after 24 h batching. Sorptive losses of TBT to the flask walls were considered negligible (Xgroups bearing pH-nondependent charge which provides a solid with a fixed surface charge density (σfix) and (ii) pHdependent charge sites often designated as >SOH with the ability to protonate/deprotonate. Since there is no electrical charge inside the solid, the electrical potential is constant and fixed at the value of the surface potential. The total surface charge density is given as σtotal ) σfix + σvariable. Alternatively (model 2), the clay surface is treated as porous where interior pH-nondependent sites exist (i.e. >X-) that have a bulk charge density Pp and the internal electrolyte solution is chosen as a homogeneous phase containing fixed point charges. Since the solute ions can penetrate inside the solid, their distribution is not uniform and gives rise to a situation called double-double layer. The major difference between models 1 and 2 is the calculation of the internal potential resulting from the fixed charge. According to VOL. 39, NO. 3, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 2: Reactions at the Montmorillonite/Sediment-Water Interface and Water Reactions Needed for TBT Adsorption Modeling reaction

value

reference

Surface Reactions >XNa + H+ T >XH + Na+ logK ) 8.67 ( 0.07 >SOH + H+ T >SOH2+ logK ) 4.38 ( 0.45 >SOH T >SO- + H+ logK ) -5.26 ( 0.27 >SsOH + H+ T >SsOH2+ logK ) 4.38 ( 0.45 + >SsOH T >SsO + H logK ) -5.26 ( 0.27

(13) (13) (13) (13) (13)

Aqueous Phase Reaction TBT+ + H2O ) TBTOH + H+ logK ) -6.3 H+ + OH- ) H2O logK ) -14.0 Na+ + OH- ) NaOH logK ) -0.20

(20) (21) (21)

Kraepiel et al. (17), the proton or cation attraction caused by cation exchange process is calculated by

-FψP RT

[cation]adsorbed ) {cation}•e

where ψP is the internal potential inside the solid resulting from the fixed charge, R is the universal gas constant, F is the Faraday constant, and T is the absolute temperature. In model 1 the ψP is calculated from the internal charge density and the ionic strength according to the Gouy-Chapman theory (refer to ref 17 for the model derivation). Model 1 was used for calculations in the present study. The calculation of TBT adsorption onto a solid phase requires proton titration data. Data for SWy were obtained from ref 13 (Table 2). XRD data (Table 1) show that the sediment sample SED-M is strongly enriched in montmorillonite (80% of the clay fraction); hence we assumed that the TBT adsorption to this sediment is dominated by the montmorillonite phase (organic matter is less, 0.02%). Therefore, the protolysis constants determined for the pure mineral SWy were also applied to quantify TBT adsorption to sediment sample SED-M. The used generalized diffused layer model including ion exchange, required the optimization of following parameters: >SOH, >SsOH, KSOTBT, KSsOTBT, and KXTBT. All calculations were carried out by FITEQL and VMINTEQ computer codes (18, 19).

Results and Discussion The main reasons for using model 1 to quantify TBT+ adsorption onto montmorillonite are as follows: [a] TBT/ montmorillonite adsorption is strongly dependent on pH, hence it is believed that the TBT attraction on the clay surface is dominated by variable charge sites rather than permanent charge sites. [b] When compared to metal ions, the TBT+ ion has a significant larger size (374 Å3, (22)), so these molecules cannot easily be accumulated in the interlayer regions. [c] Although the variable charge sites, that exist mainly at the mineral edges, play an important role for TBT adsorption, the relative contribution of the ion exchange reaction at the surface planes cannot be neglected in the case of montmorillonite. This is in contrast to solids with low CEC, such as kaolinite. For kaolinite, the cation exchange reaction can be neglected for TBT adsorption modeling (8). The ion exchange effects of TBT attraction onto montmorillonite is accounted for by designating >X- sites, which abuts from the surface which is not penetrable. [d] Finally, a most simple chemical model is desired that can be used to interpret experimental data with the least number of parameters and is capable of predicting TBT adsorption over a range of experimental conditions as imposed by this study. This requirement is fulfilled with the election of the generalized 846

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diffused layer model with the inclusion of >X- sites to account for cation exchange effects. In analogous to Dzombak and Morel (23) and Barbier et al. (13), two different sorptive sites, namely >SOH (nonselective sites) and >SsOH (selective sites), were categorized to model TBT adsorption. These so-called “selective sites” are characterized by a very strong affinity for ion attraction. In modeling metal ion adsorption it is customary to invoke these high affinity sites (>SsOH), assuming a site density value typically around 100-fold lower than that of nonselective sites (>SOH), to obtain satisfactory fits (23). In the present study, the number of selective sites was calculated using the results of TBT adsorption experiments (refer to section TBT/ SWy-System). Generally, this calculation is based on the metal cation adsorption. A K+-ion attracted to the solid surface requires an area of approximately 0.05 nm2, whereas a TBT+ion has a significantly larger molecule size and requires an area of approximately 0.63 nm2 (6), which is 10-times higher in magnitude. The adsorbed TBT molecule can therefore not be treated as a point ion. It may, however, cover neighboring reactive sites that are no longer available for adsorption reaction. To get the best fits, it is suggested to reduce the number of selective sites (>SsOH) 1000-fold lower than nonselective sites (>SOH) for TBT adsorption instead of 100-fold lower, which is usually done for modeling metal ion adsorption. It is important to note that, no distinction was made between >AlOH and >SiOH sites. Instead, variable charge sites were categorized into two groups, namely >SOH and >SsOH, having the same protolysis constants. A somewhat similar approach was taken by Bradbury and Baeyens (24) when modeling Eu adsorption onto Na-montmorillonite. Their method applied differs from that presented herein with respect to the following points: [1] The proton binding data were collected from a strong affinity type site (>SsOH) and two weak affinity type sites (>Sw1OH and >Sw2OH). [2] Both the >Sw1OH and >Sw2OH have the same capacities but different protolysis constants. [3] The >SsOH and >Sw1OH are assumed to have the same protolysis constants. [4] The electrostatic term was not taken into account in both the mass action equations used to describe protolysis and the surface complexation reactions used to model the experimental data. In analogous to Dzombak and Morel (23) and Bradbury and Baeyens (24), it was assumed that >SsOH and >SOH have the same protolysis constants (refer to Table 2). However, the TBT binding constants for these sites are different and were calculated assuming the following reactions:

>SOH + TBT+ T >SOTBT + H+

(A)

>SsOH + TBT+ T >SsOTBT + H+

(B)

>XNa + TBT+ T >XTBT + Na+

(C)

The binding constants of both types of variable sites (>SOH and >SsOH) as well as the cation exchange reaction were calculated by means of FITQEL. Results are listed in Table 3. TBT/SWy-System. TBT adsorption density ΓTBT for montmorillonite was determined at pH 6. The results are presented as a Freundlich isotherm in Figure 1. As expected, ΓTBT increased with increasing TBT concentration having a range in value between ΓTBT ) 0.366 µmol/m2 × 10-3 (for an initial TBT concentration of 100 ng/mL (0.344 nmol/mL) in solution) and ΓTBT ) 3.47 µmol/m2 × 10-3 (for an initial TBT concentration of 1000 ng/mL (3.44 nmol/mL) in solution). Calculations of the TBT adsorption density corresponds to the outer surface area of montmorillonite (31.82 m2/g)

FIGURE 1. Variation of adsorption density ΓTBT of montmorillonite sample SWy as a function of TBT concentration in the equilibrium solution. The line illustrates the calculated Freundlich isotherm. Calculations are referred to a TBT molecular weight of 290.37 g/mol. Experiments were run with a particle concentration of 20 g/L.

TABLE 3: Optimized Values for Surface Sites and TBT Binding Constants reaction

value

Surface Sites in TBT/SWy-System >SOH [µmol/L] 63.3 >SsOH [µmol/L] 0.74 Surface Sites in TBT/SED-M-System >SOH [µmol/L] 12.4-63.3 >SsOH [µmol/L] 0.39-0.74 Surface Reactions >SsOH + TBT+ T >SsOTBT + H+ logKTBT/1 ) 3.98 >SOH + TBT+ T SOTBT + H+ logKTBT/2 ) -1.18 + + >XNa + TBT T XTBT + Na logKX/TBT ) 3.05-4.14

because previous diffractometer measurements of the TBTtreated sediment SED-M demonstrated that TBT is not accommodated into the interlayer space of a swelling montmorillonite (5). The density of reactive groups (>SOH groups) on the montmorillonite surface was determined by the experimental data of TBT adsorption. The calculation was conducted by means of the FITQEL program using the pK values reported by Barbier et al. (13) (Table 2) and data for CEC and specific surface area listed in Table 1. At a given particle concentration of 20 g/L montmorillonite in aqueous suspension, the number of reactive groups >SOH was calculated to be 63.3 µmol/L, which corresponds to a site density of 0.0599 sites/nm2. This value is lower than site densities of 0.4 to 1.6 sites /nm2 reported for montmorillonite having a specific surface area of 600-800 m2/g (14). But is has to be taken into account that these data are based on the adsorption of metal cations, while in our study the calculation of site density was based on the adsorption of the larger TBT molecules (see size discussion above). In case of sample SWy, 0.74 µmol/L of selective sites were calculated. Figure 2 illustrates the pH-dependent TBT adsorption onto montmorillonite sample SWy for two different ionic strengths (deionized H2O and artificial seawater, salinity ) 0.6 M). The maximum adsorption was found at pH 6. For deionized H2O, about 64% of the initial TBT concentration were adsorbed to the solid phase after 24 h of batching. TBT concentrations of 14.22 µg/g in the solid phase and 114.7 ng/mL in equilibrium solution was observed. The adsorption density ΓTBT on the montmorillonite surface was calculated to be 1.348 µmol/m2 × 10-3 (particle concentration 20 g/L). A reduction of adsorption was noted at higher and lower pH levels. Adsorption density at pH 4 and pH 9 decreased to 1.207 µmol/m2 × 10-3 and 1.069 µmol/m2 × 10-3, respectively. Afterward, adsorption experiments were run with artificial seawater to increase the ion strength. High concentrations

FIGURE 2. Variation of TBT adsorption density ΓTBT for montmorillonite sample SWy as a function of pH. Experiments were run with deionized H2O and artificial seawater (0.6 M) and each with 0.06 M buffer in addition. Symbols illustrate experimental results. Batch experiments were run with an initial TBT concentration of 400 ng/mL (1.377 µmol/L) and a particle concentration of 20 g/L. Error bars are standard deviations of a triplicate sample preparation. Lines represent the pH-depending TBT adsorption calculated by FITEQL. of divalent cations (Ca2+, Mg2+) affect ion exchange reactions and might cause problems in model calculations. The main component (∼75%) of the artificial seawater is NaCl with a concentration of 27 g/L, while the divalent cations are contained in a less concentration. Therefore, it should not be a problem for the model calculation. In all cases, the activity corrections were carried out using Davis model. In general, an increase of the ionic strength results in a lower TBT adsorption (about 58% of the initial TBT concentration at pH 6) (25). The reason is that metal ions included in the aqueous solution compete for the sorptive sites and the attraction of TBT+ ions decreases. Experiments with artificial seawater at pH 6 yielded TBT concentrations of 11.75 µg/g in the solid phase and 165.2 ng/mL in the solution. The TBT adsorption density was calculated to 1.272 µmol/m2 × 10-3. Lines in Figure 2 illustrate the modeled TBT adsorption behavior using parameters listed in Tables 2 and 3. For pH values e6.3 (pK value of TBT (20)), the dominant species in aqueous solution is TBT+. Data modeling was first carried out according to reaction (A), where it became immediately clear that the shape of the edge of TBT adsorption in Figure 2 and the plateau region could not be interpreted successfully by only considering this reaction. Hence, calculations were performed by the simultaneous consideration of both reactions (A) and (B). The application of these two reactions were sufficient to model the pH-dependent TBT adsorption onto kaolinite in a previous study (8). In the case of montmorillonite, modeling was somewhat successful, however except for the experimental data points between pH 4 and pH 5. For these pH values, ΓTBT were not affected by pH (in experiments with deionized H2O). This is interpreted as being due to cation exchange on the permanently charged planar sites according to reaction (C). The model performed best for deionized H2O but works also satisfactorily for a much higher ionic strength (0.6 M). The binding constant for reaction (A) is calculated as logKTBT/2 ) -1.182, which is in the order of magnitude of the constant logKTBT/2 ) -1.8 reported in ref 26. But in contrast to the present study, the selective sites were not included, and the calculated model was unable to be fitted to the experimental data of pH-dependent TBT adsorption onto montmorillonite. This points to the fact that the incorporation of selective sites is necessary to quantify TBT adsorption onto clay minerals successfully. Other values determined for a TBT/montmorillonite system were not available in the literature. In Figure 3, the distribution of surface site species in an aqueous TBT/SWy system as a function of pH is illustrated. Relative distribution of different surface species of montVOL. 39, NO. 3, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. a) Surface site species of the TBT/montmorillonite system as a function of pH calculated by means of FITEQL. b) Speciation of “selective sites” at the montmorillonite surface showing a high affinity for TBT attraction. c) Available surface sites for cation exchange reaction. The relative high values result from the high CEC of sample SWy (764 mequiv/kg). Particle concentration ) 20 g/L, TBT concentration ) 1.377 µmol/L. morillonite was calculated from the data given in Tables 2 and 3. Similar trends of species distribution were observed when sediment sample SED-M was included in the calculation (details are not presented here). As shown in Figure 3a, the general shape of the adsorption edge is mainly governed by the activity of (>SOTBT) species. However, the inclusion of both (SsOTBT) and (>XTBT) species is essential for a quantitative match of the ΓTBT values with the experimental data. It is also important to note that the distribution of free surface species is varied as (>XH) . (>SOH) > (>SsOH). As shown in Figure 3 the activity of (>SsOTBT) is higher than that of either (>XTBT) or (>SOTBT). The following distribution was calculated: in case of maximum adsorption (pH 6), about 40 µmol/L of negative charged reactive groups (>SO-) are available at the montmorillonite surface. Thereof, approximately 0.74 µmol/L of selective sites are occupied for TBT adsorption (SsOTBT) using an initial TBT concentration of 1.377 µmol/L in solution. TBT/SED-M System. After successfully modeling the TBT adsorption onto a pure-phase montmorillonite, parameters determined in this system were used to quantify TBT adsorption onto a natural sediment (sample SED-M) that contains 80% montmorillonite in the clay fraction (grain size e2 µm). Because the amount of other clay minerals and organic matter in this sample is low (Table 1), it was assumed that montmorillonite is the dominate component for adsorption processes. However, the surface area (77 m2/g) as well as CEC value (537 mequiv/kg) are quite different from those determined for the pure-phase montmorillonite SWy. The modeling of TBT/sediment-system required an adjustment of >SOH, >SsOH, and logKX/TBT. The range of usable data for these parameters is given in Table 3. To evaluate the success of the modeled data for describing the pH-dependent TBT adsorption, the results of experimental work running with montmorillonite-rich sediment (5) were taken for comparison. Figure 4 shows the TBT adsorption densities of experimental results as a function of pH. The maximum adsorption was found in the range between pH 6 and 7. At pH 6 an initial TBT concentration of 400 ng/mL in solution yielded concentrations of 13.67 µg/g in the sediment and 127.4 ng/mL in the equilibrium solution after 24 h of batching. The adsorption density at pH 6 was 0.606 µmol/m2 × 10-3. As illustrated in Figure 4, the calculated model data for pH-depending TBT adsorption fit well with the experimental data even though the clay fraction of this sample includes some amounts of kaolinite and illite 848

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FIGURE 4. Variation of TBT adsorption density ΓTBT for montmorillonite-rich sediment sample SED-M as a function of pH. Symbols illustrate the results of experimental work (5). Experiments were run with artificial seawater (0.6 M + 0.06 M buffer). Batch experiments were carried out with an initial TBT concentration of 400 ng/mL (1.377 µmol/L) and a particle concentration of 20 g/L. Error bars are standard deviations of a triplicate sample preparation. The line illustrates the TBT adsorption behavior calculated with FITEQL based on the model parameters determined for the TBT/ montmorillonite system. (Table 1). The line illustrating the modeled adsorption behavior lays completely within the error bars of the experimental data. The results of this work demonstrate a successful quantification of pH-dependent TBT adsorption onto montmorillonite and montmorillonite-rich sediment by means of model calculations based on the generalized diffused layer model that includes >X- sites to account for cation exchange reactions. Furthermore, the study emphasizes the necessity to integrate selective sites in model calculation for a best fit. The present work is seen as a contribution to the data set of TBT adsorption. Adsorption data are helpful to predict the distribution, transport, and bioavailability of pollutants in natural systems and to estimate the ecological risk. Especially the pH is of substantial interest because this parameter affects the speciation of organotin compounds in the water phase as well as the properties of the mineral surface and thus strongly affects the adsorption behavior. Results from modeling the adsorption for comparatively “simple systems” (e.g. TBT/SWy- and TBT/SED-M-system) are the basis modeling the adsorption behaviors of pollutants in association with complex materials with respect to mineralogical composition and the amount of organic matter.

Acknowledgments We want to thank J. Lu ¨ tzenkirchen, INE, Karlsruhe, for his valuable suggestions in data interpretation. We are grateful to J. P. Gustafsson, Department of Land and Water Resources Engineering, Stockholm for helpful advice in using VMINTEQ. R.W. wishes to thank DAAD for financial support at Erlangen abidance.

Literature Cited (1) Hoch, M. Organotin compounds in the environment - an overview. Appl. Geochem. 2001, 16, 719-743. (2) Langston, W. J.; Pope, N. D. Determinants of TBT adsorption and desorption in estuarine sediments. Mar. Pollut. Bull. 1995, 31, 32-43. (3) Harris, J. R. W.; Cleary, J. J.; Valkirs, A. O. Particle-water partitioning and the role of sediments as a sink and secondary source of TBT. In Organotin - Environmental Fate and Effects; Champ, M. A., Seligman, P. F., Eds.; Chapman & Hall: London, 1996. (4) Unger, M. A.; MacIntyre, W. G.; Huggett, R. J. Sorption behavior of tributyltin on estuarine and freshwater sediments. Environ. Sci. Technol. 1988, 7, 907-915. (5) Hoch, M.; Alonso-Azcarate, J.; Lischick, M. Adsorption behavior of toxic tributyltin to clay-rich sediments under various environmental conditions. Environ. Toxicol. Chem. 2002, 21, 1390-1397.

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(18) Herbelin, A.; Westall, J. C. Technical Report 96-01; Oregon State University, U.S.A., 1996. (19) Gustafsson, J. P. Visual MINTEQ (VMINTEQ) Version 2.14; KTH, Department of Land and Water Resources Engineering: Stockholm, Schweden, 2003. (20) Arnold, C. G.; Weidenhaupt, A.; David, M. M.; Mu ¨ ller, S. R.; Haderlein, S. B.; Schwarzenbach, R. P. Aqueous speciation and 1-octanol-water partitioning of tributyl- and triphenyltin: effect of pH and ion composition. Environ. Sci. Technol. 1997, 31, 2596-2602. (21) Lindsay, J. W. Chemical Equilibria of Soils; Wiley: New York, 1975. (22) Luedke, E.; Lucero, E.; Eng, G. Molecular volume as a predictor of organotin biotoxicity. Main Group Metal Chem. 1991, 14, 59-66. (23) Dzombak, D.; Morel, F. M. M. Surface Complexation Modeling: Iron Hydroxides; Wiley: U.S.A., 1990. (24) Bradbury, M. B.; Baeyens, B. Sorption of Eu on Na- and Camontmorillonites: experimental investigations and modeling with cation exchange and surface complexation. Geochim. Cosmochim. Acta 2002, 66, 2325-2334. (25) Hoch, M. Assessment of salinity variations in TBT adsorption onto kaolinite and montmorillonite at different pH levels. Water, Air, Soil Pollut. 2004, 152, 349-362. (26) Weidenhaupt, A. Trialkylzinnverbindungen: Speziierung im Oktanol/Wasser-System, Sorption an Mineraloberfla¨chen. Ph.D. Thesis No. 10940, ETH: Zu ¨ rich, 1995.

Received for review March 25, 2004. Accepted October 14, 2004. ES0400382

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