Environ. Sci. Technol. 2007, 41, 106-111
A Mechanistic Study of Methyl Parathion Hydrolysis by a Bifunctional Organoclay C H A I M R A V - A C H A , * ,† LUDMILA GROISMAN,† URI MINGELGRIN,‡ ZVI KIRSON,§ YOEL SASSON,| AND ZEV GERSTL‡ Research Laboratory of Water Quality, Ministry of Health, P.O.Box 8255, Tel-Aviv 61080, Israel, Institute of Soil, Water and Environmental Sciences, The Volcani Center, P.O.Box 6, Bet Dagan, 50250, Israel, Private consultant, P.O. Box 9050, Jerusalem, Israel, and Casali Institute of Applied Chemistry, The Hebrew University of Jerusalem, 91904 Israel
The mechanism for the hydrolysis of methyl parathion (MP) by a bifunctional quaternary-ammonium based longchained organclay (LCOC) containing an alkylamine (-CH2CH2-NH2) headgroup was elucidated. The pathway of the catalytic hydrolysis of methyl parathion by the LCOC was defined by following the effect of replacing H2O with D2O, by replacing the primary amino headgroup by a tertiary amino group, and by a detailed mathematical analysis of the proposed reaction scheme. A phosphorothioate isomer of MP was formed in the presence of the LCOC as an intermediate reaction product, initially increasing in concentration and then disappearing. The isotope effect was minimal and substituting a tertiary amine in the LCOC increased the rate of MP hydrolysis. A mechanism is proposed in which hydrolysis of MP can proceed via both a direct route (specific base hydrolysis) and through the formation of the isomer which then undergoes specific base hydrolysis more rapidly than the parent MP. The relative importance of each pathway is a function of pH with the direct hydrolysis of MP being predominant at higher pH values (pH > 10) and the isomer intermediate pathway predominating at lower pH values (pH ∼8-10).
Introduction Organoclays (OCs) may be used for pollution prevention and for remediation of the soil environment (1, 2). Altering the native clay’s properties to increase the retention of micropollutants and thus avoid their transport into groundwater is an important and innovative application of modified clays and OCs. However, a major drawback of this approach is that by sorbing a toxic material onto an organoclay one merely changes the environmental compartment in which the toxic material exists, and a second step is usually required to destroy or to detoxify it. A quaternary-ammonium based, long-chained organoclay (LCOC) that contained, in addition to the quaternary ammonium headgroup, an alkylamine (-CH2-CH2-NH2) as a second functional headgroup and that was capable of * Corresponding author phone: 972-3-6839763; fax: 972-36826996; e-mail:
[email protected]. † Ministry of Health. ‡ Institute of Soil, Water and Environmental Sciences. § Private consultant. | The Hebrew University of Jerusalem. 106
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 41, NO. 1, 2007
sorbing and catalyzing the hydrolysis of several organophosphate pesticides was described in a previous paper (3). The half-life of methyl parathion (MP) in an aqueous solution of the bifunctional organoclay was about 3-4 days, while the half-life of MP for hydrolysis in the absence of the OC at the same pH (9.0) was 40 days. The explanation given for the enhanced reaction rates was that the incorporation of the substrate into the organoclay matrix facilitates a proximity effect, namely the substrate and the catalytically active group of the OC (the amino group) are brought closer to each other by the adsorption process. In the present study experiments were conducted that, together with previously reported data (3), enabled the elucidation of the mechanism of catalysis by the bifunctional organoclay.
Materials and Methods Materials. The inorganic reagents were all analytical grade, and the organic reagents were of the highest grade of purity available (g98%). Water was purified by passing tap water through an ion-exchange resin and then through a Labconco “Water Pro PS” water purifier, equipped with a column of activated carbon and a 0.2 µm Millipore filter. Cation Synthesis. Synthesis of the N-decyl-N,N-dimethylN-(2-aminoethyl) ammonium cation (DDMAEA) has been described previously (3). A tertiary amine, long chain organic cation of the structure
was synthesized in the present study. The cation, N-decylN,N-dimethyl-N-(2-N,N-dimethylaminoethyl) ammonium (DDMAEDMA), was prepared by mixing 20 g of N,N,N′,N′tetramethylethylenediamine (Fluka, g99%) with 10 g of decylbromide (Aldrich) in 500 mL of absolute ethanol till the solution became turbid. The precipitate was filtered and redissolved in distilled water at pH 11 and extracted with diethylether to remove residues. The aqueous solution was freeze-dried and the product was re-crystallized from hot ethyl acetate. The results of elementary analysis were (%): C, 55.2; H, 10.8; N, 9.3; Br, 23.9 (Calculated C, 57; H, 11; N, 8.3; Br, 23.7). Organoclay Preparation. The synthesis of the TMAEA (N,N,N-trimethyl-N-(2-aminoethyl)ammonium)-andDDMAEA (N-decyl-N,N-dimethyl-N-(2-aminoethyl) ammonium)-clays were described previously in detail (3). The DDMAEDMAclay was prepared by mixing the free amino cation with sodium bentonite (Fisher Scientific) in a buffered solution at pH 11.0. After 48 h, the mixture was centrifuged, washed repeatedly with distilled water, and freeze-dried, after which it was kept in a vacuum desiccator. The results of elementary analysis (%) were: C 7.61; H 2.19; N 1.05, indicating 50-60% exchange. Methods. pKa Determination. The pKa values of the longchain quaternary ammonium cation, DDMAEDMA (tertiary amine) and of the short-chain quaternary ammonium cation, TMAEA (primary amine), were obtained by titrating a 0.1 M solution of the cations with 0.1 M HCl. In order to obtain a reasonably sharp endpoint, it was necessary for the concentration of the solution of the long-chain cation to be at least 0.1 M, namely, above its critical micelle concentration. Pillersdorf and Katzhendler (4), however, asserted that it is 10.1021/es060696h CCC: $37.00
2007 American Chemical Society Published on Web 11/18/2006
SCHEME 1 Schematic Description of the Hydrolysis of MP Catalyzed by the Bifunctional-OC (DDMAEA-) Clay.
FIGURE 1. Hydrolysis of MP catalyzed by the DDMAE bifunctional LCOC. Discrepancy between the rate of methyl parathion disappearance and the observed formation of p-nitrophenol at pH 10. possible to apply to micelles the same procedures used for the determination of the dissociation constants of acidic or basic monomers, but the apparent dissociation constant (Ka,app) may vary with the degree of dissociation (R) of the acid or base in accordance with the Lindenstrom-Lang equation:
pKa,app ) pH + log(1 - R)/R ≈ pKint + SR
(I)
where S is the slope of the line pKa,app vs R, and pKint is the intercept for R extrapolated to 0. Although one may expect the pKa value of micelles to depend on R, from the linear correlation between pKa,app and R (see Figure S2 in Supporting Information) presented in Figure 1 that this dependence is extremely weak and that the measured Ka,app, while not truly constant, is a very good approximation of the true pKa of the monomer. According to Figure S2, the pKa of the tertiary amino cation (DDMAEDMA) is in the range of 6.3-6.4. Since the pKa of the analogous primary amino cation, DDMAEA, was measured as 6.8 (5), it can be concluded that the tertiary amino cation, DDMAEDMA, is less basic than the primary amino cation, DDMAEA. A lower pKa value of the tertiary amine as compared to that of its analogue primary amine has been attributed to steric hindrance by the alkyl groups to the protonation of the tertiary amino group (4). The pKa of TMAEA was determined to be 7.0. Isotope Effect. The hydrolysis of MP in the presence of the bifunctional long-chain organoclay (DDMAEA-clay) was followed by mixing the organic cation with sodium bentonite in D2O. The hydrolysis of MP in the presence of the DDMAEA OC thus produced was carried out in D2O in parallel with the analogous experiments performed in H2O. The concentrated solution of methyl parathion added to the D2O was prepared in deuterated acetone, d6. The pD (equivalent to pH in aqueous solutions) of the D2O solution was raised to 10.3 (as measured by a pH meter) using metallic sodium. Based on the relationship pD ) pH* + 0.4, where pH* is the glass electrode reading (6), the pD value was actually 10.7. The pH of the aqueous control with the 0.05 M phosphate buffer was 10.2. This pH was chosen as control, bearing in mind the expected difference between dissociation constants in D2O and water by analogy to the difference between measured pKa values in these solvents (e.g (7)). Exactly the same conditions of temperature, component concentrations, and clay to solution ratio were employed in the deuterated system and the aqueous control.
Identification of Product and Intermediate. The product, p-nitrophenol (p-NP), was identified by comparing its GC retention time and MS spectra with that of a reference standard. The structure of the intermediate was deduced from its MS spectrum, obtained on a high-resolution magnetic sector instrument, Autospec (Micromass - Waters), attached to an Agilent GC 5890. Hydrolysis of MP by the Tertiary Amine Bifunctional DDMAEDMA-Clay. The hydrolysis of MP in the presence of the tertiary amine bifunctional organoclay (DDMAEDMAclay) was carried out in parallel with the hydrolysis in the presence of the primary amine organoclay (DDMAEA-clay) at pH 10.8 (where the amino headgroups of both cations are fully deprotonated), and the kinetics of hydrolysis were followed as described previously (3). The sorption kinetics of methyl parathion onto the bifunctional DDMAEA-OC was found to be very fast with the sorption practically completed in the first few hours (see the Supporting Information (SI)), so that sorption kinetics can be ruled out as a rate-limiting step. Calculations of the Degradation Rate Constants. The kinetic rate constants were calculated by integrating the kinetic equations (Scheme 1, eqs 1-3) and fitting the experimental data, using the proper initial conditions at t ) 0: namely, 1 for the concentration of the substrate (MP) and 0 for that of the isomer intermediate and the degradation product.
Results The rate of MP disappearance during its bifunctional LCOC (DDMAEA)-catalyzed hydrolysis (3) was not matched by the rate of appearance of the main metabolite, p-NP (at the pH values studied p-nitrophenol exists predominantly as the anionic phenolate species so p-NP denotes the sum of p-nitrophenol and p-nitrophenolate). Several studies have reported sorption of phenolate compounds (8-10). Sorption of the phenolate form occurs primarily for bulky molecules such as pentachlorophenol (8, 10) whereas for the smaller trichlorophenol, Dentel et al. (9) showed that sorption could be quantitatively attributed to the neutral species. Thus, for the even smaller p-NP molecule it is not surprising that p-NP sorption by the DDMAEA-OC was insignificant (See Supporting Information). The discrepancy between reactant disappearance and product appearance (Figure 1) indicates that the mechanism by which the bifuctional organoclay (DDMAEA-clay) hydrolyzes MP is not a straightforward, onestep reaction. Identification of the Intermediate Product. In addition to the discrepancy between MP disappearance and p-NP formation, an extra peak in the GC/MS chromatogram was observed only in samples containing the long-chained bifunctional organoclay (DDMAEA-clay). The peak first increased in area and then disappeared gradually. These findings lead to the conclusion that the degradation of MP on the long-chained bifunctional OC (DDMAEA-clay) proceeds through an intermediate. An isomer (O,S-dimethyl O-(4-nitrophenyl) thiophosphate) of the starting material, MP, with the structure (I) was deduced based on the mass spectra of this compound (see Supporting Information): High-resolution EIMS confirmed the empirical formula of C8H10NO5PS, the same as that of MP. The fragmentation of the isomer was, as expected, different (see Supporting Information). VOL. 41, NO. 1, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
107
FIGURE 2. Hydrolysis of methyl parathion catalyzed by the DDMAE bifunctional-LCOC at pH 10: calculated curves vs experimental data.
FIGURE 3. Calculated degradation of methyl parathion by the DDMAE bifunctional-LCOC at pH 9 and the experimental data of Groisman et al. (3).
d[p - NP] ) k2[I] + k3[MP] dt
(3)
The system of eqs 1-3 was solved using the initial conditions: [MP](t ) 0) ) Co; [I](t ) 0) ) 0; [p - NP](t ) 0) ) 0, to get explicit expressions for the concentrations of the species involved, as described below (for details see SI): The most significant differences between the mass spectrum of MP and its isomer are the following: (a) the isomer exhibits an ion at m/z 248.0020 (11.22%) which is absent from the mass spectrum of MP and is attributed to the cleavage of a methyl group. It is reasonable to expect that the S-Me bond cleaves more easily than O-Me due to the lower energy of the C-S bond compared to that of the C-O bond; (b) the base ion of MP at m/z 109.0124, attributed to C6H5S, is absent from the isomer. It can be explained by a five-membered ring rearrangement that enables the combination of the double-bonded S with the phenyl group. Such rearrangement was found, for instance, in the mass spectrum of methylphenyl thiocarbonate that also exhibits a base peak at m/z 109 attributed to C6H5S (11), but is hardly possible in the intermediate isomer (I). The concentrations of the starting material (MP), the intermediate (I), and p-NP as a function of time at pH 10 are presented in Figure 2 (data for other pH values can be found in the Supporting Information). MP degrades to p-NP and dimethylthiophosphate, one mole of MP producing one mole of each degradation product. Since we determined p-NP we included it in Scheme 1 and disregarded the second degradation product for the sake of clarity and brevity. The appearance of an intermediate product, and the fact that the curves in Figure 2 are typical of consecutive firstorder reactions, suggest reaction pathways as shown in Scheme 1 which conform to eqs 1-3. Kinetic Calculations. According to Scheme 1, MP can be converted to p-NP either directly or via the isomer, with both pathways being catalyzed by the bifunctional-OC (DDMAEAclay). The rate equations for Scheme 1 are
108
9
d[MP] ) -k1[MP] - k3[MP] dt
(1)
d[I] ) k1[MP] - k2[I] dt
(2)
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 41, NO. 1, 2007
[MP] ) c0e-(k1+k3)t [I] ) c0
(1b)
k1 [e-(k1+k3)t - e-k2t] k2 - k1 - k3
[
[p - NP] ) c0 1 -
(2b)
]
(k2 - k3)e-(k1+k3)t - k1e-k2t k2 - k1 - k3
(3b)
The values of the rate constants k1, k2, and k3 were obtained by computing the best fit between the calculated concentrations of the three components (1b-3b) and the experimental data by minimizing the sum of squares of the residuals. The optimization procedure employed the following constraints: k2 g k3 at all pH values, and all rate constants increase with pH. It is expected that k2 g k3 due to the higher polarity of the phosphoryl group of the isomer relative to the thiophosphoryl group of MP (e.g., (12)). The increase of the rate constants with pH is discussed below (eqs 4-8). The values of the rate constants thus obtained are presented in Table 1 and the calculated curves at pH 10 are given together with the experimental data in Figure 2. In order to express the rate constants in terms of pHindependent, universal constants, it was postulated that both reaction pathways (Scheme 1) can be catalyzed by a surfacebound amino group such as that of the DDMAEA bifunctional-OC and it is asserted that these reactions are analogous to the aminolysis of phenylesters, for which Bruice et al. (13) suggested the following equation:
κobs ) κ0 + κOH [OH] + κ1 [amine] + κ2 [amine]2 + κ3 [amine] [OH] (4) where κ0 represents the rate constant for the spontaneous hydrolysis of the organophosphate ester (the contribution of noncatalyzed hydrolysis in the aqueous phase); κOH represents the specific base catalyzed (by the hydroxyl ion) hydrolysis; κ1 represents the contribution of nucleophilic attack by the amino group (aminolysis); κ2 represents the contribution of self-assisted nucleophilic attack by amino
TABLE 1. Rate Constants (days-1) for the Hydrolysis of Methyl Parathion at Different pH Values (Standard Errors in Parentheses) pH
k1 MP f I
k2 I f p-NP
k3 MP f p-NP
9 10 11 12
0.10 0.24 (0.018) 0.31 (0.026) 1.34 (0.08)
0.21 0.48 (0.012) 0.60 (0.045) 2.04 (0.07)
0.05 0.12 (0.0) 0.31 (0.044) 1.53 (0.03)
groups (2 amino groups simultaneously); and κ3 represents the contribution of hydroxyl ions assisted catalysis through nucleophilic attack by amino groups. The overall hydrolysis of MP is a first-order reaction in MP concentration (3) so that the terms in eq 4 are all multiplied by the concentration of MP. Hence, each term in the expression for the observed rate constant for MP hydrolysis is one order higher than implied in eq 4. In a surface reaction, the likelihood of the involvement of 2 amine groups simultaneously in attacking a substrate molecule is negligible; hence, κ2 can be neglected. In the organoclay matrix it is expected that the second-order rate constant, κ1, is reduced to a first-order rate constant since practically the whole amount of the substrate is in the form of an intramolecular complex (i.e., an adsorbed species). Thus, adding (or subtracting up to a limit) amine groups to the surface will not affect the rate of the reaction. The thirdorder rate term, κ3, for the reaction involving a hydroxyl ion and catalyzed by the amine is, accordingly, reduced to a second-order rate constant. It appears that the rate law for organophosphate ester hydrolysis in the organoclay matrix should follow the equation of Rav-Acha et al. (5):
κobs ) κo + κcat
Ka Ka + H+
+ κOH[OH]
(5)
where κ0 is defined as in eq 4 above; κcat is the contribution to the rate constant of the reaction catalyzed by the amino group; Ka is the dissociation constant of the protonated amino group and the expression Ka/(Ka + H+) equals the fraction of the OC amino groups that is not protonated; κOH includes two contributions: (a) the specific base catalysis by hydroxyl and (b) a possible contribution of the hydroxyl ion to aminolysis by the amino groups of the bifunctional OC (for which the term Ka/(Ka + H+) can be assumed to be constant, because at OH- concentrations at which this contribution to the reaction is significant, [H+] becomes considerably smaller than Ka). In eq 5, the pH dependence of the rate constant is implicit in the inclusion of the [OH] term as well as in the dependence of the extent of dissociation of the amine on the pH. According to eq 5, each of the observed rate constants k1, k2, and k3 in eqs 1-3 can be expressed as pH-independent, universal rate constants as shown below:
k1 ) κisom + κisom o cat
Ka Ka + H+
k2 ) κisohyd + κisohyd o cat
k3 ) κdirhyd + κdirhyd o cat
+ κisom OH [OH ]
Ka +
Ka + H Ka Ka + H
+ κisohyd OH [OH ]
+ κdirhyd OH [OH ] +
(6)
(7)
TABLE 2. pH-Independent Rate Constants (Days-1) of the Various Reaction Steps (Scheme 1 and Eqs 6-8) isomerization (isom) k1 isomer hydrolysis (isohyd) k2 direct hydrolysis (dirhyd) k3
Ko
Kcat
KOH
0 0 0
0.24 0.55 0.12
108 150 142
organoclay’s amino group; and κisom OH is the rate constant of isomerization catalyzed by the hydroxyl ion. The terms κisohyd , κisohyd , and κisohyd and κdirhyd , κdirhyd , and κdirhyd have o cat OH o cat OH meanings analogous to the above-defined terms κisom , κisom o cat , and κisom OH for the hydrolysis of the isomer (k2 in Scheme 1), and for the direct hydrolysis of the organophosphate ester (k3 in Scheme 1), respectively. The universal, pH-independent rate constants in eqs 6-8 can be calculated by solving these three equations for three pH values (altogether 9 equations) using the κobs values (namely, the k1, k2, and k3 rate constants that were extracted for pH values 10, 11, and 12 using eqs 1b-3b). Thus, there are 3 equations for each set of unknowns κ0, κcat, and κOH. With k1 as κobs, one obtains the set of κisom terms, with k2 the set of κisohyd terms, and with k3 the set of κdirhyd terms. All κ0 constants were set a priori to zero as at low pH values (where the OC- amino group is fully protonated and the hydroxyl concentration is low) both isomerization and direct hydrolysis are negligibly small. This indicates that the pH-independent, spontaneous (i.e., noncatalyzed) hydrolysis is insignificant. Thus, only two unknowns are left in each of the three equations (eqs 6-8). We find them by solving two sets of three equations for all 6 unknowns (eqs 6-8) which allows us to check the robustness of the derived parameters against the experimental data at the third pH (e.g., Figure S6 in SI). Solving for all combinations of pH values (10 & 11; 11 & 12; 10 & 12) resulted in three values for κcat and κOH which were then averaged. These are the values reported in Table 2. Finally, these constants (Table 2) were used to predict the rate of MP hydrolysis catalyzed by the bifunctional LCOC (DDMAEA-clay) at pH 9. The calculated MP degradation curve and the experimental data reported by Groisman et al. (3) are presented in Figure 3. The excellent fit of the calculated curve to the experimental measurements attests to the robustness of the procedure for deriving the pH-independent constants. Isotope Effect. The isotope substitution effect on the kinetics of the hydrolysis was measured, i.e., the bifunctional OC (DDMAEA-clay) was prepared in D2O and the hydrolysis of MP in the presence of the deuterated OC was also measured in D2O. As compared with the hydrolysis of MP in water (k ) 0.392 d-1, at pH 10.2), the hydrolysis was somewhat slower in D2O (k ) 0.293 d-1). The ratio of the rates of hydrolysis defining the isotope effect is about 1.3. In interpreting the above results the relationship between the pH meter reading (10.3) and the actual pD should be considered. Catalysis by the Tertiary Amine OC. The overall rate constant for the hydrolysis of MP in the presence of the bifunctional OC containing the tertiary amine DDMAEDMA (k ) 0.618 d-1) at pH 10.8 was 1.7 times higher than in the presence of the bifunctional OC containing the primary amine DDMAEA (k ) 0.358 d-1).
Discussion (8)
where: κisom is the rate constant for spontaneous isomero ization; κisom cat represents the isomerization catalyzed by the
The importance of the catalytic effect of the bifunctional-OC (DDMAEA-clay) relative to that of specific base catalysis by the hydroxyl ion is most pronounced at the lower pH values employed, 9.0 and 10, as borne out by the relative magnitudes of κcat and κOH in all three sets of constants defined in eqs 6-8 and given in Table 2. At these pH values the contribution of VOL. 41, NO. 1, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
109
SCHEME 2 Conceptual Depiction of the Catalysis of MP Isomerization by the Bifunctional-OC (DDMAEA-) Clay.
the specific base catalysis by the hydroxyl ion (κOH[OH - ]) to the hydrolysis of the organophosphate esters is relatively small. A comparison of the rate constants (Table 1), indicates that at pH values of 9 and 10, approximately 65% of the reaction proceeds via the formation of the intermediate, and only 35% of the starting material hydrolyzes directly to the final products (k1/k3 ∼ 2). As the pH increases, the contribution of direct hydrolysis (k3), to the degradation of MP increases and eventually exceeds that of the isomerization pathway (Table 1). Thus, the direct hydrolysis pathway is more sensitive to the pH than the isomerization process. Overall, it is an interesting case in which the pH not only changes the rate of the catalytic reaction, but may also change its dominant pathway. The intermediate isomer hydrolyzes more rapidly than MP at all pH values (k2 > k3). Conversion of parathion and MP to the O, S isomer has been reported in the literature. For example, the O, S isomer of methyl parathion can be synthesized under relatively mild conditions (reflux of methanol at 67 °C) by a stepwise dealkylation-alkylation process (14). Fast isomerization of parathion (half-life around 15 min) was observed at high temperatures (110 °C) but also occurred at room temperature when the parathion was sorbed by montmorillonite or attapulgite (15, 16). Seger and Maciel (17) have shown that isomerization on Ca-montmorillonite can occur at room temperature over the course of several years. Recent work by Guo and Jans (18) has shown that in the presence of natural organic matter and hydrogen sulfide, methyl parathion undergoes a nucleophilic substitution at the methoxy-carbon bond resulting in the O, S isomer of desmethyl methyl parathion. Accordingly, the mechanism of catalytic isomerization of MP through an O-Me cleavage appears to be a logical explanation of the experimental observations. The isotope effect seems to indicate a nucleophilic attack of the DDMAEA OC’s amino group on the phosphoryl of the organophosphate ester, rather than a general base catalysis in which a water molecule participates in the transition state, because in the later case the isotope effect is usually >2 while the overall rate of the hydrolysis of MP was more rapid in water than in D2O by a factor of only 1.3. However, in the case of aminolysis of aryl esters, general base catalysis was reported not to show a significant isotope effect upon deuteration (19). Therefore, the observed isotope effect in the present case is not a sufficiently conclusive indication of a mechanism and one cannot exclude the existence of general base catalysis. The results of the experiments in which the catalytic efficiency of the tertiary amino-LCOC (DDMAEDMAclay) was examined were more helpful. These experiments were carried out to differentiate among 3 possible mechanistic pathways of the catalytic process as follows: (a) a nucleophilic 110
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 41, NO. 1, 2007
attack of the DDMAEA OC amino group on the reaction center of the organophosphate ester to form a transition state of a low activation energy, which in turn can be easily converted to the products; (b) a similar attack of the DDMAEA OC amino group via a water molecule (the amino group attracts the water proton and the hydroxyl is attracted to the reaction center of the ester, i.e., a general base catalysis); and (c) methylation of the amino group of the DDMAEA OC (due to cleavage of an OsME bond in the MP molecule) as a step in the production the isomer intermediate (Scheme 2). A nucleophilic attack by a tertiary amine is, as a rule, weaker than that by a primary amine because of steric hindrance. In light of the higher rate of degradation in the presence of the tertiary amine, mechanism (a) above can be excluded. Neither pathway (b) or (c) can be ruled out, but by analogy to related reactions (e.g., (18)), mechanism (c) seems more likely. Despite the lower basicity of the tertiary amine, it was more efficient (as part of the organoclay) in catalyzing the hydrolysis of MP. This unexpected result may be due to the fact that the basicity (or nucleophilicity) of free cations differs from that of the same cations when sorbed, as a result of the interaction between the sorbed cation and the clay surface. A molecular dynamics simulation of the layering behavior and structure of confined quaternary alkylammonium cations in organoclays was reported (20). The atomic density profile computations in the direction normal to the clay surface indicated a tendency of the methyl head groups to concentrate close to the clay surface. Thus, in the case of the tertiary amine DDMAEDMA-clay, such proximity of the headgroup methyls to the charged clay surface may result in a basicity or nucleophilicity considerably differing from that of the free cations. Using the pH-independent rate constants (Table 2), the degradation dynamics of MP in the presence of the bifunctional organoclay (DDMAEA-clay) can be predicted for a wide range of pH values. The results of such calculations for the hydrolysis of MP at pH 9 are presented in Figure 3 and the good fit between the experimental data and the calculated results demonstrates the utility of these pH-independent constants. Thiophosphoric acid esters, such as parathion, MP, and tetrachlorvinphos, are hazardous pollutants and their accumulation in the environment is a recognized ecological threat (e.g., (21)). Methods for their enhanced degradation are an urgent task of contemporary chemical technology and biotechnology. Due to the present, relatively high price of synthesizing the DDMAEA bifunctional OCs, it is not likely that they will be used in the foreseeable future for large scale applications. They can, however, already be used to treat point sources of pollution, such as effluents of production lines in the pesticide industry.
Possibly the main point of the present study is the demonstration that organoclays can also be made into catalysts for various chemical reactions useful in research as well as in the chemical industry by incorporating a functional headgroup that enhances the target reaction.
Acknowledgments We thank Prof. Zev Tashma (Department of Pharmaceutical Chemistry, The Hebrew University of Jerusalem, Israel) for his valuable insights during this study.
Supporting Information Available Mathematical derivation of eqs 1b-3b, MS data for isomer identification, pKa titration of the DDMAEDMA cation, MP sorption kinetics, p-NP sorption, calculated curves vs experimental data for the hydrolysis of MP by the DDMAE bifunctional-LCOC. This material is available free of charge via the Internet at http://pubs.acs.org.
Literature Cited (1) Gullick, R. W.; Weber, W. J. Evaluation of shale and organoclays as sorbent additives for low-permeability soil containment barriers. Environ. Sci. Technol. 2001, 35 (7), 1523-1530. (2) Prost, R.; Yaron, B. Use of modified clays for controlling soil environmental quality. Soil Sci. 2001, 166 (12), 880-895. (3) Groisman, L.; Rav-Acha, C.; Gerstl, Z.; Mingelgrin, U. Sorption and detoxification of toxic compounds by a bifunctional organclay. J. Environ. Qual. 2004, 33 (5), 1930-1936. (4) Pillersdorf, A.; Katzhendler, J. Dipolar Micelles .8. Hydrolysis of Substituted Phenyl Esters in A Hydroxamic Acid Surfactant. J. Org. Chem. 1979, 44 (4), 549-554. (5) Ravacha, C.; Ringel, I.; Sarel, S.; Katzhendler, J. The Catalytic Effect of Cationic Amino Micelles on the Hydrolysis of Substituted Phenyl-Esters. Tetrahedron 1988, 44 (18), 5879-5892. (6) Glasoe, P. K.; Long, F. A. Use of glass electrodes to measure acidities in deuterium oxide. J. Phys. Chem. 1960, 64, 188-189. (7) Bundi, A.; Wuthrich, K. H-1-Nmr Parameters of the Common Amino-Acid Residues Measured in Aqueous-Solutions of the Linear Tetrapeptides H-Gly-Gly-X-L-Ala-Oh. Biopolymers 1979, 18 (2), 285-297. (8) Christodoulatos, C.; Mohiuddin, M. Generalized models for prediction of pentachlorophenol adsorption by natural soils. Water Environ. Res. 1996, 68, 370-378.
(9) Dentel, S. K.; Bottero, J. Y.; Khatib, K.; Demougeot, H.; Duguet, J. P.; Anselme, C. Sorption of tannic acid, phenol, and 2,4,5trichlorophenol on organoclays. Water Res. 1995, 29, 12731280. (10) Stapleton, M. G.; Sparks, D. L.; Dentel, S. K. Sorption of pentachlorphenol to HDTMA-clay as a function of ionic strength and pH. Environ. Sci. Technol. 1994, 28, 2330-2335. (11) Eight Peak Index of Mass-Spectra, 4th ed.; Royal Society of Chemistry: Cambridge, 1991; Vol. 3. (12) Faust, S. D.; Gomaa, H. M. Chemical Hydrolysis of Some Organic Phosphorus and Carbamate Pesticides in Aquatic Environments. Environ. Lett. 1972, 3 (3), 171. (13) Bruice, T. C.; Donzel, A.; Huffman, R. W.; Butler, A. R. Aminolysis of Phenyl Acetates in Aqueous Solutions. 7. Observations on Influence of Salts Amine Structure and Base Strength. J. Am. Chem. Soc. 1967, 89 (9), 2106. (14) Thompson, C. M.; Frick, J. A.; Natke, B. C.; Hansen, L. K. Preparation, Analysis, and Anticholinesterase Properties of O,ODimethyl Phosphorothioate Isomerides. Chem. Res. Toxicol. 1989, 2 (6), 386-391. (15) Gerstl, Z.; Yaron, B. Stability of Parathion on Attapulgite As Affected by Structural and Hydration Changes. Clays Clay Miner. 1981, 29 (1), 53-59. (16) Mingelgrin, U.; Saltzman, S. Surface-Reactions of Parathion on Clays. Clays Clay Miner. 1979, 27 (1), 72-78. (17) Seger, M. R.; Maciel, G. E. NMR investigation of the behavior of an organothiophosphate pesticide, methyl parathion, sorbed on clays. Environ. Sci. Technol. 2006, 40 (2), 552-558. (18) Guo, X. F.; Jans, U. Kinetics and mechanism of the degradation of methyl parathion in aqueous hydrogen sulfide solution: Investigation of natural organic matter effects. Environ. Sci. Technol. 2006, 40 (3), 900-906. (19) Bender, M. C.; Bergeron, R. J.; Kamijama, M. Bioorganic Chemistry of Enzymatic Catalysis; John Wiley: New York, 1984. (20) Zeng, Q. H.; Yu, A. B.; Lu, G. Q.; Standish, R. K. Molecular dynamics simulation of organic-inorganic nanocomposites: Layering behavior and interlayer structure of organoclays. Chem. Mater. 2003, 15 (25), 4732-4738. (21) Kaloyanova, S.; Tarkowski, S. Toxicology of Pesticides, 1st ed.; WHO: Copenhagen, 1981.
Received for review March 23, 2006. Revised manuscript received September 26, 2006. Accepted September 26, 2006. ES060696H
VOL. 41, NO. 1, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
111
