Environ. Sci. Technol. 1997, 31, 2603-2609
Sorption of Organotin Biocides to Mineral Surfaces A N D R EÄ W E I D E N H A U P T , † C EÄ D R I C A R N O L D , S T E P H A N R . M U ¨ LLER, STEFAN B. HADERLEIN,* 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
The sorption of triorganotin biocides (TOTs) from aqueous solution to mineral surfaces was investigated in batch sorption experiments using homoionic clay minerals (kaolinites, montmorillonites, illites), and aluminum, iron, and silicon (hydr)oxides. The TOTs studied include the two most widely used organotin pesticides, triphenyltin (TPT) and tributyltin (TBT), as well as shorter-chain trialkyltin homologues. In natural waters, these compounds are present predominantly as neutral TOT-OH species or as TOT+ cations (5.2 < pKa < 6.8). For all minerals investigated, sorption kinetics of TOTs were fast, and sorption was reversible. At clay minerals, sorption of TOTs was dominated by cation exchange of the TOT+ species. Adsorption of TOTs at homoionic clays increased with decreasing selectivity coefficients of the exchangeable cations (Na+ > K+ ≈ Rb+ . Cs+, Ba2+, Ca2+, Mg2+). On a surface area basis, TOT sorption to montmorillonite and illite was lower than to kaolinite, consistent with the surface charge densities of the clays and the absence of TOT+ intercalation. Since the dominating interaction of TOTs with all minerals was sorption of TOT+ cations to negatively charged surface sites, tXO-, sorption was strongly pH dependent, and sorption maxima occurred at the maximum overlap of TOT+ and tXOconcentrations. Thus, high TOT sorption to (hydr)oxide minerals occurred only if a significant fraction of negatively charged surface sites was present at pH values where TOT+ species predominate, i.e., to minerals exhibiting low pHZPC values such as silica. Consistent with recently published data from marine and estuarine systems, our results demonstrate that sorption of TOT+ cations to minerals may significantly contribute to the overall sorption of TOTs to natural solid matrices.
Introduction Tributyltin (TBT) and triphenyltin (TPT) are ubiquitous contaminants in freshwater and coastal seawaters, primarily due to their extensive use as anti-fouling agents in boat paints (1). In natural waters, these compounds are present predominantly as neutral TOT-OH or as cationic TOT+ species (2, 3), the pKa values of which range from 5.2 to 6.8 (see Table 1). Taking TBT as an example, Figure 1 compiles typical (total) concentrations of TOTs measured in aquatic systems. Note that even at low nanomolar aqueous concentrations, which are often exceeded in marine water and freshwater, TBT * Corresponding author phone: +41-1-823 55 24; fax: +41-1-823 54 71; e-mail:
[email protected]. † Present address: Chemical Engineering Department, ETH Zentrum CAB, CH-8092 Zu ¨ rich, Switzerland.
S0013-936X(97)00010-2 CCC: $14.00
1997 American Chemical Society
exhibits chronic and acute toxicity toward aquatic organisms such as algae, zooplankton, and mollusks (4). Considering the extremely high toxicity in combination with a wide-use pattern (e.g., agrochemicals, preservatives, stabilizers) and the relatively high persistence (1, 4), triorganotin compounds (TOTs) are of great concern, particularly in aquatic ecosystems. As is illustrated in Figure 1, the concentrations of TBT (and other TOTs) in aquatic sediments and organisms are generally orders of magnitude higher than the aqueous concentrations. Hence, sorption is a key process determining both the exposure (and, thus, the effects toward aquatic organisms) as well as the transport and fate of TOTs in aquatic environments. Although the application of TOTs as antifouling agents is now restricted in many countries, the question remains as to what extent (de)sorption of TOTs, which have accumulated in the sediments over decades, may control the aqueous concentrations of such compounds in the future. Hydrophobic partitioning into natural organic matter is often assumed to be the major sorption mechanism of TOTs (1, 4). However, when considering the existence of two major aqueous TOT species, TOT-OH and TOT+, sorption mechanisms other than hydrophobic partitioning may contribute to the overall sorption of such compounds. Thus, it is somewhat surprising that sorption processes of TOTs to naturally occurring solids have not yet been investigated systematically, although there is some experimental evidence indicating that sorption of TOTs to naturally occurring mineral phases might be significant. It has been shown that sorption of monobutyltin to various clay minerals proceeded by a cation exchange mechanism (5), however, at conditions that are not relevant to environmental systems (pH 2). Nevertheless, studies on the sorption of TBT to natural sediments containing a significant fraction of clay minerals but little organic carbon indicate that mineral surfaces might contribute to the overall sorption of TOTs to natural matrices (6-8). Furthermore, Sun et al. (9) recently postulated that ion exchange processes dominated the sorption of organotins to estuarine sediments. In this work, we report results on the adsorption of various TOT+ and TOT-OH species to a series of naturally occurring minerals. The TOTs studied included the two most widely used organotin pesticides, triphenyltin (TPT) and tributyltin (TBT), as well as other short-chain trialkyltin homologues. Batch adsorption experiments were conducted at various conditions with respect to the composition of the aqueous phase (e.g., pH, ionic strength, type of electrolytes) and the surface characteristics of the mineral sorbents used. The major goals of our study were (1) to evaluate whether TOTs may adsorb significantly to minerals of environmental relevance, (2) to identify the predominant sorption mechanisms of TOTs at such minerals, and (3) to determine the predominant environmental and compound specific factors that govern these surface interactions of TOT-OH and TOT+ species.
Experimental Section Chemicals. The TOTs investigated (see Table 1) were purchased as TOT+Cl- salts (purity g97%) and were used as received. Methanolic stock solutions of TOTs were prepared (0.1-1 M) and were kept refrigerated for further dilution. All other chemicals used were also of the highest purity available (>99.5%). Water was doubly distilled in quartz. Sorbents and Treatment of Surfaces. The sorbents used are characterized in Table 2. Prior to the sorption experiments, ∂-Al2O3, γ-Al(OH)3 (gibbsite), R-FeOOH (goethite), and amorphous SiO2 (Aerosil 90) were suspended for about 5 min
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FIGURE 1. Range of measured TBT concentrations in different compartments of aquatic environments (1, 4, 40). in 0.01 M aqueous HCl to remove acid soluble surface coatings. Subsequently the suspensions were centrifuged and resuspended repeatedly first in distilled water until a pH of 6.0 ( 0.5 was reached and then freeze-dried. Illite, kaolinite, and montmorillonite were treated similarly except for using 0.1 M solutions of the respective chloride salts in order to obtain homoionic clays. Prior to freeze-drying, excess electrolyte was removed at pH ) 6.0 ( 0.5 by repeated rinsing with distilled water. Sorption Experiments at Constant pH. Initial aqueous concentrations of the TOTs ranged from 1 to 150 µM, resulting in methanol cosolvent concentrations of less than 0.15 ‰ (v/v). Aqueous solutions of TOTs in a background electrolyte were spiked to known quantities of dry sorbent in 1.8-mL borosilicate glass vials (Omnilab AG, Mettmenstetten, Switzerland) that were sealed with aluminum foil liners and septum screw caps. The resulting suspensions were equilibrated for 12 ( 2 h on a rotary shaker (Reax 2, Heidolph, Kehlheim, Germany) in the dark at 20 °C and then centrifuged at 12000g for 1 min. The amount of mineral sorbents used was adjusted to result in a decrease of the initial aqueous TOT concentration of 20-80% due to adsorption. Consistent results of TOT adsorption were obtained with sorbent concentrations ranging from 5 to 100 g/L. The pH of the suspensions was measured with microglass electrodes (Glasbla¨serei Mo¨ller, Zurich, Switzerland) and was adjusted by dilute acid or base present in the TOT spiking solutions, buffered only by the amphoteric properties of the sorbents. Mass balances on the system were determined for selected TOTs by extracting the solid phase twice with a 0.1 M NaCl solution at pH ) 2.0. Since such desorption experiments showed very good recoveries (91-105%), sorbed concentrations of TOTs were routinely calculated from the difference between initial and equilibrium aqueous TOT concentrations. Every experiment was carried out in three replicates. Sorption Edge Experiments. The pH dependence of TOT sorption was investigated according to ref 10 in a dark jacketed-thermostated (20 °C) reaction flask attached to a titrator (Metrohm 614 and 665, Herisau, Switzerland) combined with a pH electrode (Ross Sure-Flow, Orion, Boston, MA). The 100-mL suspensions were kept at pH ) 4.0 (HClO4) for 2.5 ( 0.5 h before adding aliquots of TOT stock solutions. The suspensions were then titrated with 0.1 M NaOH in pH increments of 0.25-0.5. After equilibration (1.5 ( 0.5 h), three 1-mL aliquots were transferred into 1.8-mL borosilicate vials, centrifuged, and analyzed for TOTs. In both types of adsorption experiments, vials containing spike solutions of TOTs but no sorbents were processed similarly to the suspension samples and were used as external standards in
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the HPLC analyses. This was done to account for the minor TOT losses (
