Effects of Dissolved Carbonates and Carboxylates on the Sorption of

Mar 21, 2007 - Chemistry, University of Ioannina, Ioannina 45110, Greece. The sorption of a hydrophobic pesticide, thiram, on humic acid (HA) occurs v...
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Environ. Sci. Technol. 2007, 41, 2782-2788

Effects of Dissolved Carbonates and Carboxylates on the Sorption of Thiuram Disulfide Pesticides on Humic Acids and Model Surfaces PANAGIOTA STATHI,† MARIA LOULOUDI,‡ AND Y I A N N I S D E L I G I A N N A K I S * ,† Laboratory of Physical Chemistry, Department of Environmental and Natural Resources Management, University of Ioannina, Seferi 2, 30100, Agrinio, Greece, and Section of Inorganic and Analytical Chemistry, Department of Chemistry, University of Ioannina, Ioannina 45110, Greece

The sorption of a hydrophobic pesticide, thiram, on humic acid (HA) occurs via a specific pH-dependent binding of thiram at the deprotonated carboxylates of humic acid, forming a species thiram-{HACOO-} with K ) 0.69. Similarly, thiram was sorbed by two model polycarboxylate{SiO2COOH} materials via the formation of a surface species thiram-{SiO2COO-} with K ) 0.45 between thiram and the deprotonated carboxylates grafted on SiO2 particles. In all cases, allowance of presence of bicarbonate at natural concentration caused severe inhibition of thiram’s sorption. Oxalate and formate mimic the inhibitive effect of bicarbonate. Theoretical fit of the data showed that the inhibitive effect of HCO3- is due to the formation of the anionic species [thiram-HCO3]-1 (with K ) 0.90) which is water soluble and competes with the bound species thiram-{HACOO-}. The same phenomena were observed for the sorption of disulfiram. The specific interaction phenomena reported here bear relevance to the sorption properties of thiram and disulfiram on real soils and, therefore, may determine their environmental fate.

Introduction Humic substances are naturally occurring biogenic heterogeneous organic materials that complex strongly with heavy metals and organic compounds (1). The sorption of nonpolar organic compounds from aqueous solution is strongly influenced by the amount and properties of soil organic matter (1-5). This may be attributed to two primary mechanisms: phase partitioning and hydrophobic adsorption. Phase partitioning is the transfer of the solute from the aqueous phase to an organic phase (6). Hydrophobic adsorption is a surface reaction which occurs in competition with water on a region of the surface. The free energy of the sorbed complex is controlled by one- or two-dimensional contact with both the surface and the solvating fluid phase (6). Both of these mechanisms may be environmentally significant (2-6). The sorption of organic compounds to mineral-bound humic acids is, among other factors, influenced by the * Corresponding author e-mail [email protected]. † Department of Environmental and Natural Resources Management, University of Ioannina. ‡ Department of Chemistry, University of Ioannina. 2782

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chemical properties and conformation of the humic acids (3, 7-11). The bulk chemical properties of natural humic substances (elemental ratios and major functional group composition) influence the reactivity of humic substances toward organic and inorganic contaminants. The structural orientation of attached humic acids may be controlled by the distribution of hydroxyl groups on the oxide surface and their structure (8, 11). The sorption data of many nonpolar or low-polarity organics are believed to reflect phase partitioning (2, 7). This can be conceptualized by considering that the transferred solute is solvated by the soil organic matter that surrounds it in the three dimensions. Otherwise stated, humic material behaves as an organic solvent (2, 6). The physicochemical justification of a partitioning mechanism carries with it the assumption that nonpolar sorbates do not interact with specific sorption “sites” and that soil organic matter behaves as an homogeneous phase (6). Failure to meet either of these two conditions can be caused by specific interactions such as binding of solutes with soil organic matter. In certain cases, the occurrence of such specific interactions has been diagnosed or suggested indirectly by nonlinear sorption isotherms (2, 6, 7, 12-16). For example specific interactions of several nonpolar solutes with high-surface-area carbonaceous material were postulated by Chiou and Kile (12). Specific interactions were suggested by Spurlock and Biggar (13) to determine the sorption of several phenylureas onto soil organic matter. Xia and Ball (15) have analyzed an observed nonlinear isotherm based on a Polanyi-based modeling approach assuming that the used sorbent contained trace quantities of high surface area carbonaceous material, which concurs with the finding of Chiou and Kile (12). Pignatello et al. (16) suggested that that soil organic matter has an internal nanopore structure that provides specific sorption sites for organic compounds, regardless of their polarity (16). Within this model, SOM behaves as a dualmode (dissolution plus hole-filling) sorbent. Boyd et al. (17) considered that the observed nonlinear sorption of parathion on soil organic matter was due to specific interactions. Overall, in the existing literature data, specific interactions were diagnosed indirectly via nonlinear sorption isotherms for nonpolar compounds on soil organic matter. However, in all aforementioned cases these specific interactions were not identified at the molecular level. A proper assignment of the specific interactions would necessitate modeling of the reactions and fit to appropriate experimental data. Thiram (C6H12N2S4) [tetra-methyl-thiuram disulfide], and disulfiram (C10H20N2S4) [tetra-ethyl-thiuram disulfide] are hydrophobic nonpolar molecules widely used as pesticides or as rubber vulcanization accelerators (18). Both thiram and disulfiram are strongly sorbed on soils, making their elimination from the environment difficult (19). The mechanism of their sorption on oxides and montmorillonite clay was clarified in our recent work (20). We have shown that HCO3- at natural concentrations enhances dramatically the adsorption of the pesticides at pH below of point of zero charge of the oxide surfaces (20). Quantum mechanical density functional theory (DFT) calculations showed that this pervasive effect is due to the formation of a molecular complex [thiram-HCO3-] in aqueous solution which bears a net charge of -1; therefore, it behaves as an anion (20). These findings provided comprehensive insight into the physicochemical behavior of the thiuram disulfides in solution in conjunction with their sorption onto two of the main soil components, e.g., oxides and clay (18-20) explaining in part the reported strong sorption of these molecules 10.1021/es0630792 CCC: $37.00

 2007 American Chemical Society Published on Web 03/21/2007

in real soils (19). However the interaction of these molecules with the other important soil component, e.g., soil organic matter, still remains poorly understood. Given the acknowledged dominant role of soil organic matter in the sorption process of hydrophobic organics (2-4, 6, 8) this calls for a pertinent study of the sorption of thiuram disulfide pesticides on soil organic matter. Moreover, it is of interest to examine the effect of the association of carboxylates with thiram/ disulfiram in solution (20) on their sorption on soil organic matter. In this context, we present here a detailed physicochemical study of thiuram disulfide sorption on humic acid. A series of experiments were designed and conducted to gain insight into the relative importance of the structural units of humic acid. Particular emphasis was given on the elucidation of the role of the carboxylate groups of the humic macromolecule on the sorption phenomena. The potential role of mineralbound humic acid was also studied. Thus, in our experiments, the sorption was studied comparatively for the following sorbents: (a) natural humic acids in aqueous solution, (b) humic acid immobilized onto SiO2 particles. The third and fourth sorbents, were two new synthetic systems consisted of mono- or di-carboxylic moieties covalently immobilized onto SiO2 by grafting. The objectives of this work were (a) to investigate the sorption mechanism of thiuram disulfate pesticides on humic acid, (b) to elucidate the role of small carboxylates, particularly HCO3-, in solution on the sorption-partitioning mechanism, (c) to elucidate the role of immobile carboxyl groups on the humic macromolecule or on the SiO2 particle, and (d) to discuss the implications of the present results on a real soilperspective. Herein the term sorption is consistently used as a more general term, e.g., instead of adsorption or partition (6).

Materials and Methods All aqueous solutions were prepared in Milli-Q water. Leonardite humic acid standard was purchased from the International Humic Substances Society (IHSS lot 1S104H5) and used without further purification. Silica gel obtained from Fluka (specific surface area: 320 m2/g, particle size 0.063-0.2 mm), was activated by heating at 140 °C for 12 h. 3-aminopropyltriethoxysilane was obtained from Fluka (lot code 403608/1 53000). Iminodiacetic acid (IMDA) was obtained from Sigma-Aldrich (lot no. 18722AB-026). 3-Glycidyloxypropyl-trimethoxysilane was obtained from Fluka (lot code 453045/1). Thiram (C6H12N2S4) (tetra-methylthiuram disulfide), (cas no. 137-26-8) and disulfiram (tetraethyl-thiuram disulfide) (batch code 119H0936) obtained from Sigma-Aldrich, were used with no further purification. Sodium hydrogen carbonate, formic acid and oxalic acid were obtained from Fluka. N-(3-dimethylaminopropyl)-N′′-ethylcarbodiimide hydrochloride (EDC) was obtained from Alfa (lot code: FA031923). Thiram and disulfiram stock solutions of 120 mgL-1 were prepared in methanol as described previously (20) and stored at 4 °C for up to a week. Under these storage conditions in methanol the molecules are stable for more than 3 months, monitored by HPLC (20). Immobilization of Humic Acid onto SiO2. Immobilization of humic acid on aminopropyl silica has been obtained by the method of Koopal et al. (10), by formation of amide bonds between the amino groups of the aminopropyl silica and the carboxylic groups of HA activated by EDC (Figure 1A). To determine the stability of the SiO2-HA, 3 mg of material were suspended in 3 mL of water at different pH values, then the suspensions were stirred at room temperature for 2 h. The supernatants were separated from the precipitates by centrifugation. Then the absorbance at 254 nm was measured and the concentration of the humic acids released from the

FIGURE 1. (A) Schematic structure of HA immobilized on aminopropyl-silica by formation of amide bonds. (B) Schematic structure of SiO2-COOH and (C) SiO2-(COOH)2 materials. The structure of the thiram molecule shown was optimized with DFT. SiO2-HA was calculated. The SiO2-HA material was characterized by ATR-FTIR, TGA, and UV-vis, provided in part I of the Supporting Information. Preparation of the SiO2-COOH Material. Activated silica was modified by (CH2)3CN groups following the method of Clark et al. (21). Hydrolysis by H2SO4 results in modified silica which bears carboxylic acids as functional groups (Figure 2B). The loading achieved is ca. 0.95 mmol g-1 determined by thermogravimetric and elemental analysis. [drift-IR (KBr, cm-1, selected peaks) 3343:v(OH), 1719:v(CdO), 1375:δ(CH2), 1211:v(Si-O-Si asym), 1056: v(Si-O), 805:v(Si-O-Si sym)]. Preparation of SiO2-(COOH)2 Material. To a stirred solution of 60 mL toluene containing 1.0 mmol of IMDA, 1.0 mmol of (3-glycidyloxypropyl)-trimethoxysilane was added. The resulting mixture was allowed to react at 80 °C for 24 h. To this solution, 3.0 g of SiO2 and 5 mL of ethanol were added, and the slurred solution was maintained at 80 C for 24 h. The functionalized silica, SiO2-(COOH)2 (Figure 2C) was isolated by filtration, rinsed several times with methanol, pure water, and ethanol. The final product was dried under vacuum at 100 °C for 8 h and stored. For SiO2-(COOH)2 (%): C, 2.83; N, 0.33. The loading achieved is ca. 0.25 mmol g-1 determined by elemental and thermogravimetric analysis. [drift-IR (KBr, cm-1, selected peaks) 1735: stretching vibration of carboxylate ions, 1400:(CH2), 802:v(Si-O-Si)]. Surface ionization constants were estimated by potentiometric titration as described earlier (20, 22). Data are provided in the Supporting Information Cathodic Stripping Voltammetry (CSV) Measurements. Thiram and disulfiram were quantified by CSV as described previously (20) by using a Trace Master 5 polarograph by Radiometer Analytica. Typically, 0.25µM of thiram or disulfiram in 0.01M KNO3 resulted in a peak current Ip ) 420 nA, at Ep ) -544 mV at pH 7.0 (20, 23). Linear calibration curves were constructed for concentrations up to 10-6 M of each pesticide. Sorption Experiments on Dissolved Humic Acid. For the sorption studies on humic acids, batch samples were VOL. 41, NO. 8, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. (A) Sorption pH-edge for an initial concentration of 1.52 × 10-5M thiram on dissolved HA from Leonardite in CO2-free aqueous solution. (B) Theoretical fit of the sorption data (9). The open squares are the experimental sorption data shown in (A). (() Calculated concentration of free thiram, (b) Calculated concentration of the free deprotonated carboxylates of HA. The Schindler-type plot on top of (B) indicates two key-reactions determining the formation of the sorbed species thiram-{HACOO-}. The deprotonation of HA, symbolized by the vertical line, occurs at a range of pKa values listed in Table 1. prepared by spiking appropriate volumes of Milli-Q water with pesticide from the stock solution prepared in methanol. This resulted in a methanol concentration below 1%. In all samples 1.5-1.7 × 10-6 M thiram or disulfiram were used as initial concentration, for a ratio [HA:pesticide] 3:100 (w/w). The pH was adjusted to the desired value, using small volumes of HNO3 and NaOH. Screening experiments showed that under stirring at room temperature the sorption equilibrium was attained after 20 min. Then, the concentration of either thiram or disulfiram in solution was determined. Sorption on SiO2-HA, SiO2-COOH and SiO2-(COOH)2. Suspensions of SiO2-HA, SiO2-COOH, or SiO2-(COOH)2 were prepared by adding 3 mg of powder material in 20 mL Milli-Q water at the desired pH values, then the pH was readjusted and suitable volumes of pesticide solution were added from the stock solution in methanol. After stirring at room temperature for 20 min, the suspension was centrifuged and the concentration of thiram in the solution phase was determined. In all samples, to avoid CO2 contamination (samples without CO2), the suspension or solution was purged with 99.999% N2 gas for at least 30 min prior to addition of the pesticide (20). The CO2-free aliquots were stored in airtight Teflon tubes and used immediately. The experiments were performed in triplicate. Samples in the presence of CO2 were prepared in exactly the same way as for the samples without CO2 by allowing ambient CO2, e.g., a partial CO2 pressure PCO2 ) 10-3.5 atm, to be in contact with the solution under stirring during the sorption experiments. Theoretical Analysis. The theoretical analysis of the pHedge of sorption was performed by modeling the expected surface and solution reactions, taking into account the following experimental observations: (a) the binding of thiram involves is pH-dependent; (b) in the SiO2-[COOH] materials the surface carboxylates are the binding sites for thiram, while silica itself plays essentially no role; (c) thiram has an affinity for complexation with HCO3-, as we have demonstrated both experimentally as well as theoretically in our previous work (20). Thus, good fits to the data could be obtained by a minimum set of reactions which include (a) the standard protonation/deprotonation reactions for HA and SiO2, and (b) binding of thiram at the deprotonated carboxylates of HA or the {SiO2COO-}. Including interactions of thiram with additional sites such as the phenolic groups of HA with pKa 8-9 or the SiO- surface groups of silica did not improve the fit. The surface reactions used for the modeling are described in Table 1. FITEQL (24) was used to 2784

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determine the best fit of various surface reactions or combinations of reactions to the experimental sorption. The fitting procedure was the following. First, the pKa values for the protonation of the carboxylates in the model surfaces were calculated by fitting the potentiometric acid-base titration data. This analysis showed that the carboxylates in the {SiO2COOH} materials have distributed pKa values in the range 3.8-5.6 instead of a single value. In this respect they mimic the polyelectrolyte-type distributed pKa values typically observed in humic acids. The site densities for the carboxylates on SiO2 were estimated by the loading of the materials. The concentration of the carboxylates groups of Leonardite HA were taken from the value 6.25 mmol gr-1 HA reported by Westall et al. (27). The carboxylates of HA and of the {SiO2COOH} materials were introduced in FITEQL in a manner similar to that used by Westall et al. (27). Briefly, a weighted distribution of five discrete pKa values was assumed as in ref 27, listed in Table 1. Using more pKa values did not improve the fit. The five pKa values were found to reproduce satisfactorily the sorption pH-features, especially the smeared out profile. For simplicity, the assumed binding of thiram on the five types of COO- groups was modeled by using a single log K. Then, by using the site densities and the protonation pKa values fixed, the reaction constants for thiram were estimated from the fit to the sorption edge data by FITEQL. The relative experimental errors for the FITEQL input values of total pesticide concentration and log[H+] were 5%.

Results and Discussion Sorption of Thiram by Humic Acid in CO2-Free Solution. The sorption of thiram on dissolved Leonardite humic acid as a function of the pH is displayed in Figure 2A. In CO2-free solution at pH ∼ 5, about 30% of the dissolved thiram was sorbed, while at more alkaline pH the sorption approached 80%, Figure 2A. The theoretical analysis is shown in Figure 2B, and will be discussed in the following section. Sorption on Immobilized Carboxylates and Immobilized HA in CO2-Free Solution. The sorption of thiram on the material SiO2-COOH as a function of pH, in CO2-free solution, is presented in Figure 3A, open squares. The sorption of thiram rises rapidly at pH > 4.5 and attains a maximum of nearly 55% at pH > 6.0. The sorption of thiram on SiO2 was found to be low (Figure S5 in the Supporting Information). Thus the data in Figure 3A show that the immobilized carboxylates are responsible for the observed pH-dependent sorption of thiram. An analogous trend is observed for the

TABLE 1. Reactions and Stability Constants for the Surfaces Species for Sorption of Thiram on Humic Acid (HA), SiO2-HA, SiO2-COOH and SiO2-(COOH)2 reaction [OH-])[H+]-1 [HCO3-])[CO3-2][H+] [H2CO3])[CO3-2][H+]2 [thiram-HCO-3] ) [thiram][HCO3-] protonation of HA [{HACOOH}] ) [{HACOO-}][H+]

log K Solution Reactions

-14.0 10.35 16.35 -0.046 ( 0.002 4.1-6.1a

Surface Protonation Reactions protonation reactions of -COOH immobilized on SiO2 [{SiO2COOH}] ) [{SiO2COO-}][H+] 3.8-5.6b 3.4-6.1c [{SiO2(COOH)2}] ) [{SiO2(COO-)2}][H+] protonation reactions of silica [SiO-] ) [SiOH] [H+]-1 7.10 ( 0.2 [SiOH2+] ) [SiOH] [H+] -2.9 ( 0.0.2 Sorption Reactions sorption on {HA-COO-} groups [{HACOO-}-thiram] ) [[{HACOO-}][thiram] [SiO2{HACOO-}-thiram] ) [SiO2[{HACOO-}] [thiram] sorption on {SiO2COO} groups [thiram-{SiO2COO-}] ) [thiram][{SiO2COO-}] [thiram-{SiO2(COOH)2}] ) [thiram-{SiO2(COO-)2}]

reference

6 6 6 20 27

this work this work

26, this work 26, this work

-0.162 ( 0.002 -0.155 ( 0.002

this work this work

-0.347 ( 0.002 -0.345 ( 0.002

this work this work

a ,b,cWe have used five discrete pKa values according to the method of Westall et al. (27). (a) for the reaction [{HACOOH}] ) [{HACOO-}][H+], pKa ) 4.1, 4.5, 5.1, 5.8, 6.1, see ref 27. (b) for the reaction [{SiO2COOH}] ) [{SiO2COO-}][H+], pKa ) 3.8, 4.2, 4.6, 5.1, 5.6, estimated by the fit of the acid-base potentiometric data. (c) for the reaction [{SiO2(COOH)2}] ) [{SiO2(COO-)2}][H+], pKa ) 3.4, 3.9, 4.5, 5.3, 6.1, estimated by the fit of the acid-base potentiometric data. The carboxylate concentrations used were 6.25 mmol g-1 HA, 0.95 mmol g-1 on SiO-COOH.

FIGURE 3. (A) (0) Experimental and (9) theoretical sorption pH-edge for an initial concentration of 1.57 × 10-5M thiram on SiO2COOH material in CO2-free aqueous solution. (b) Calculated concentration of the free deprotonated SiO2-carboxylates. The deprotonation of the SiO2COO- is symbolized by the vertical line in the Schindler-type plot, occurs at the range of pKa values listed in Table 1. (B) (0) Experimental and (9) theoretical sorption pH-edge for an initial concentration of 1.68 × 10-6M thiram on SiO2HA material in CO2-free aqueous solution. (b) Calculated concentration of the free deprotonated carboxylates of the immobilized HA, SiO2HACOO-. The deprotonation of HA, symbolized by the vertical line in the Schindler-type plot plot, occurs at the range of pKa values listed in Table 1. sorption of thiram on immobilized HA, SiO2-HA, Figure 3B, open squares. In Figure 3B we observe that, at pH 5, more than half of thiram was sorbed while at more alkaline pH the sorption attains ∼70%. A similar strong pH-dependent sorption of thiram was observed for the dicarboxylate surface SiO2-[COO-]2 (see Figure S6 the in Supporting Information). Theoretical Analysis for Sorption of Thiram in CO 2-Free Conditions. The experimental data presented in Figures 2A, 3A, and 3B show a pH-dependent sorption of thiram sorption on humic acids and on immobilized carboxylates. The data for the sorption by the SiO2COOH materials show that thiram is specifically sorbed on the carboxylates. The pH dependence

of the sorption by the SiO2COOH implies that the deprotonated carboxylates should be involved in this sorption mechanism. In this context, theoretical fits to the data were obtained by FITEQL by assuming the reactions detailed in Table 1. The obtained fits to the data are shown as solid squares in Figures 2A, 3A, and 3B. At the top of each figure, pertinent reactions from Table 1 are presented in a Schindler type plot as an aid to the discussion. According to the derived speciation scheme, the key-reaction for the sorption is a specific binding of thiram on the deprotonated carboxylates. Thus, in Figure 3A, the theoretical analysis shows that the sorbed thiram by the {SiO2VOL. 41, NO. 8, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. (A) Sorption pH-edge for an initial concentration of 1.55 × 10-5M thiram on dissolved HA from Leonardite in the presence of ambient CO2. (B) (9) theoretical fit of the sorption data. The open squares are the experimental sorption data shown in (A). (() Calculated concentration of free thiram, (2) Calculated concentration of the species thiram-HCO3-. COOH} material can be accounted for by the formation of the species thiram-{SiO2COO-} resulting from binding of one molecule of thiram with one deprotonated carboxylate. The stability constant for the formation of thiram-{SiO2COO-} was estimated to be K ) 0.45, listed in Table 1. Furthermore, from the derived speciation scheme in Figure 3A, we see that the deprotonation of the surface carboxylates is responsible for the pH-dependent sorption profile that we observed experimentally. The sorption of thiram on the humic acids can also be analyzed by the same conceptual model. In Figures 2B and 3B we show that the key-reaction is the formation of the species thiram-{HACOO-} which can account for the observed pH-dependent sorption of thiram on humic acid. Accordingly, the pKa values of the carboxylates modulate the pH-edge for thiram’s sorption. The observed increase in the sorption at pH > 6, Figures 2 and 3 result from the complete deprotonation of the carboxylates and the formation of the species thiram-{HACOO-}. The distributed pKa values of the carboxylates of HA (27) are responsible for the smeared out pH-edge profile for the sorption of thiram on HA. Sorption Isotherms. In all the studied cases we have recorded nonlinear sorption isotherms (Figure S7 in the Supporting Information). The Langmuir constants estimated by the fitting of the Langmuir isotherms to the data are comparable to the stability constants for the species thiram{COO-} estimated from the sorption pH-edge data. Although the isotherm constants themselves cannot provide any insight about the molecular interactions involved, we may consider that the observed specific interactions, e.g., analyzed quantitatively by the fit to the pH-edge data, render support to the idea that the formation of the species thiram-{COO-} is the specific reaction which determines the nonlinear sorption isotherms. CO2 Inhibits Sorption of Thiram on HA. In the presence of CO2 under ambient pressure (PCO2 ) 10-3.5atm) the sorption of thiram is inhibited. In Figure 4A, the sorption of thiram on dissolved Leonardite HA in the presence of CO2 is significantly lower than the sorption under CO2-free conditions (Figure 2A). In Figure 5, the presence of CO2 during the sorption of thiram on SiO2HA, has a severe inhibitory effect, e.g., compare open squares in Figures 5 and 3B. This implies that the dissolved CO2, e.g., the carbonates, are implicated in the inhibition mechanism. This was further corroborated by another experiment where, by adding 10 mM NaHCO3 powder in CO2-free solutions containing thiram and HA at pH 6-8, the obtained sorption data were comparable with the open symbols in Figures 4 and 5 (not shown). Thus, for the theoretical analysis of the sorption inhibition caused by 2786

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FIGURE 5. (A) Sorption pH-edge for an initial concentration of 1.57 × 10-5M thiram on SiO2HA in the presence of ambient CO2. (B) (9) theoretical fit of the sorption data. (0) experimental sorption data. (() Calculated concentration of free thiram, (2) Calculated concentration of the species thiram-HCO3-. CO2, in addition to the solution and surface reactions used for the CO2-free systems, we have included (a) the solution reactions related to the pH-speciation of CO2 in solution and (b) one solution reaction describing formation of the species thiram-HCO3- (see Table 1 and top of Figures 4B and 5). In this way, good fit to the data could be obtained by using a stability constant, K ) 0.90, for the species thiram-HCO3(Figure 4B, solid squares). Recently, we have demonstrated that [thiram-HCO3]-1 is easily formed in solution (20) with a stability constant K ) 0.90 and bears a charge of -1, therefore, it behaves as an anion. Thus, in the presence of CO2 at pH > 6 where bicarbonate predominates, thiram is bound either by the carboxylates of HA, e.g., with K ) 0.69-0.70 or by bicarbonate in solution with a somehow larger stability constant of K ) 0.90, see Figures 4B and 5. Accordingly, at equilibrium the “solution” species [thiram-HCO3]-1 competes with the “bound” or “sorbed” species thiram-{HACOO-}. The theoretical speciation, Figures 4B and 5 shows that this competition can account for the observed decrease in the “sorbed” species thiram-{HACOO-} in the presence of CO2.

Inhibitory Effect of Other Carboxylates. In light of the present finding on the inhibitory role of HCO3-, we have examined the effect of other small carboxylate anions, e.g., formate and oxalate that have been shown to form stable complexes with thiram in solution (20). Sorption experiments for thiram or HA in the absence of CO2, show that oxalate or formate also inhibit thiram’s sorption on humic acid (see Figure S8 in the Supporting Information). Sorption of Disulfiram. We found the sorption of the other thiuram disulfide pesticide tested here, e.g., disulfiram, on HA and on the SiO2--carboxylates, parallels that for thiram. These experiments are described in detail in the Supporting Information. Overall the present data reveal two novel physicochemical phenomena. First, both thiram and disulfiram are sorbed via a specific binding onto the deprotonated carboxylate groups of humic acid or of the SiO2COO- material. As a result, the sorption isotherms are characteristically nonlinear, despite the hydrophobic nonpolar nature of the studied pesticides. Second, the specific binding of thiram and disulfiram on the carboxylates is strongly inhibited by bicarbonate at natural concentrations. Oxalate and formate mimic the inhibitive effect of bicarbonate. Physicochemical Mechanism Aspects. At the molecular level, all these phenomena can be understood by taking into account the proven affinity of thiram and disulfiram to form stable molecular complexes with carboxylates (20). Our quantum chemical density functional theory calculations (20) showed that, at the electronic level, this binding involves not only one but a multitude of specific weak interactions between the oxygens of the carboxylate moiety and the sulfur atoms of the thiuram disulfate molecule. Water molecules may play also a role, e.g., by forming H-bonds bridging thiram and the carboxylate moiety (20). In this context we suggest that in the case of the sorption of thiram by humic acid, the species thiram-{HACOO-} might be stabilized by analogous multiple weak bonds. Given that the exact structure and geometry of humic acids in solution is currently unknown, we cannot further detail the binding mechanism. It is of pertinence, however, to notice that the stability constant K ) 0.68 estimated for thiram-{HACOO-} is lower than the K value of 0.90 estimated for the solution species thiramHCO3-. This is the origin of the inhibitory effect of bicarbonate on thiram’s sorption by HA. The higher K value for thiramHCO3- might have the following mechanistic consequences. First, the adduct thiram-HCO3- is formed preferably in aqueous solution, i.e., in competition with the “bound” thiram-{HACOO-}. Second, the formed adduct thiramHCO3- is anionic (20) and is solvated by at least two water molecules (20); therefore, it is more water soluble than thiram itself (20). Thus, thiram-HCO3- would prefer the aqueous phase rather than the “organic phase” of humic acid (2, 6). This provides a simple comprehensive mechanistic frame for the observed inhibition of sorption. [SiO2-Carboxylates] as Model Surfaces. Finally, it is worth mentioning that, the novel materials [SiO2-COO-] and [SiO2-(COO-)2], possess several interesting properties. First, as we show here, is their ability to take up thiuram disulfide pesticides from aqueous solution. This is due to a specific interaction at the molecular level which renders them selective for the tested pesticides. The materials are solid powders, therefore, easy to separate from the aqueous phase by centrifugation. Second, the new materials can efficiently mimic certain aspects of the humic acid, that is the pKa distribution in the pH region 2-6. This renders them useful for further model studies of the carboxylates of humic acids. Environmental Implications: A Physicochemical Mechanism for the Sorption of Thiuram Disulfates in Real Soils. In our preceding paper (20), we revealed the pervasive role of the bicarbonate anion on the adsorption of thiuram

disulfides onto montmorillonite clay and Al2O3. Taken together, the previous paper (20) and the present data allow a comprehensive physicochemical picture of the interaction between thiuram disulfate pesticides and the main soil components, e.g., oxides, clay, and humic acid, to be anticipated. In waters rich in dissolved carboxylates, the formation of [HCO3-thiram]- in solution determines the fate of these pesticides. Depending on the composition of the soil particles and their charge, the anionic adduct [thiramHCO3]- will be either strongly adsorbed on positively charged oxides and clay (20), or repulsed from the negatively charged soil organic matter. This mechanism is expected to be dominant in surface waters rich in CO2. In a second scenario, in the absence of dissolved carboxylates, the thiuram disulfides would be sorbed exclusively on soil organic matter. Carboxylate-rich humic acids are expected to act as stronger sorbents than carboxylate-poor humic acids. Thus, in subsurface waters eventually devoid of CO2 and/or carbonates, the dissolved of immobilized humic substances will uptake the molecules of thiram or disulfiram. In any case, in a soil with mixed character, e.g., oxide, clay, and organic matter, the overall effect will be observed as a strong sorption of the pesticides on soil, with strong correlation with the content of soil in organic matter. This is in agreement with the previous observations for strong sorption of thiuram disulfides in real soils (18, 19).

Acknowledgments This work was supported by the University of Ioannina.

Supporting Information Available Figures S1, S2, and S3 show experimental characterization of the SiO2HA, SiO2COOH, SiO2[COOH]2 materials. Figure S4 shows surface charge determination. Figure S5 shows thiram’s sorption on SiO2, Figure S6 shows thiram’s sorption on SiO2[COOH]2, Figure S7 shows thiram’s sorption isotherms, Figure S8 shows inhibitory effect of oxalate, formate. Figure S9 shows sorption data for disulfiram. This material is available free of charge via the Internet at http:// pubs.acs.org.

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Received for review December 26, 2006. Revised manuscript received February 11, 2007. Accepted February 13, 2007. ES0630792