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SHORT COMMUNICATIONS

Sorption and Detoxification of Toxic Compounds by a Bifunctional Organoclay

^a Research Laboratory of Water Quality, Ministry of Health, P.O. Box 8255, Tel-Aviv 61080, Israel
^b Institute of Soil, Water and Environmental Sciences, The Volcani Center, ARO, P.O. Box 6, Bet Dagan, 50250, Israel

^* Corresponding author (zgerstl{at}agri.gov.il).

Received for publication December 29, 2003.

	ABSTRACT

TOP ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

 
Organoclays are excellent sorbents for nonionic contaminants and therefore may have many environmental applications. A major limitation on the use of organoclays is that the contaminant merely changes its location from one environmental compartment to another while still remaining intact. In this study, a new type of organoclay, termed a bifunctional organoclay, has been prepared. It is able not only to sorb organophosphate pesticides, but also to catalyze their hydrolysis, and thereby detoxify them. The bifunctional organoclay prepared in this study is based on sodium montmorillonite, in which the inorganic counter ions are replaced by N-decyl-N,N-dimethyl-N-(2-aminoethyl) ammonium (DDMAEA). The detoxifying capacity of this organoclay for two organophosphate pesticides, methyl parathion [O,O-dimethyl O-(p-nitrophenyl) thionophosphate] and tetrachlorvinphos [2-chloro-1-(2,4,5-trichlorophenyl)ethenyl dimethyl phosphate], was demonstrated. It was shown that although the sorption of these pesticides on the bifunctional organoclay is very similar to that on N-decyl-N,N,N-trimethyl ammonium (DTMA) organoclay (the corresponding nonbifunctional organoclay), the hydrolysis of these pesticides is substantially enhanced only by the bifunctional organoclay. The half-life for the hydrolysis of the investigated pesticides in the presence of the bifunctional organoclay is about 12 times less than for their spontaneous hydrolysis, and the enhancement is even more pronounced relative to the hydrolysis of these pesticides in the presence of the DTMA organoclay (which actually inhibits their hydrolysis). Based on kinetic measurements, the pK_a of the ethylamino group of the bifunctional organoclay was estimated to be around 9.0. It is postulated that the catalytic effect of the bifunctional organoclay can be attributed to a nucleophilic attack of the unprotonated ethylamino group of the organoclay on the organophosphate ester.

Abbreviations: DTMA, N-decyl-N,N,N-trimethyl ammonium • DDMAEA, N-decyl-N,N-dimethyl-N-(2-aminoethyl) ammonium • TMAEA, N,N,N-trimethyl-N-(2-aminoethyl) ammonium

	INTRODUCTION

TOP ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

 
ORGANOCLAYS ARE CLAY MINERALS in which the inorganic counter cations have been replaced by organic cations, such as organic quaternary ammonium, pyridinium, or phosphonium ions, thus modifying the clay's surface properties from hydrophilic to hydrophobic (Boyd et al., 1988a, 1988b; Smith et al., 1990).

Quaternary ammonium organoclays may be divided into two groups depending on the structure of the organic cation and the resultant dominant mechanism of sorption on the organoclays (Boyd and Jaynes, 1994; Lo et al., 1998; Jaynes and Vance, 1999). The first group, called adsorptive organoclays, includes clays that contain short-chain quaternary ammonium ions, such as tetramethylammonium or trimethylbenzylammonium. Sorption of hydrophobic organic compounds by this type of organoclay from water displays Langmuir-type isotherms, which are commonly associated with two-dimensional surface interactions and therefore are susceptible to competitive sorption. The second group of organoclays, called organophilic organoclays, is composed of clays that contain long-chain quaternary ammonium ions, such as hexadecyltrimethyl ammonium or didodecyldimethyl ammonium. Sorption of hydrophobic organic solutes from water by this group of organoclays is characterized by linear isotherms over a wide range of solute concentrations and the sorption behavior resembles a partitioning process in which the organic long chains create a quasi-lipophilic three-dimensional phase, in which apolar organic compounds accumulate. Accordingly, sorption by organophilic organoclays is less affected by competitive sorption.

It has been shown that while the short-chain adsorptive organoclays sorb individual compounds that are only slightly hydrophobic (log K_ow up to 4.0, where K_ow is the octanol–water partitioning coefficient) more strongly than the organophilic organoclays, the long-chain organophilic organoclays are better sorbents from water for highly hydrophobic compounds (log K_ow > 4), and for competing sorbates from multicomponent solutions (Groisman et al., 2004).

Since organoclays have a strong capacity to sorb nonionic compounds from aqueous solutions (Boyd et al., 1988a; Sheriff et al., 1987; Smith and Jaffé, 1994), they have many environmental applications, such as liners for wastewater reservoirs, treatment of industrial wastewater, and site remediation. The major drawback of these treatment processes is that by sorbing a toxic material onto an organoclay, we merely change the environmental compartment in which the toxic material exists, and a second step is still required to destroy, immobilize, or detoxify it. To overcome this problem, a novel class of organoclays, termed bifunctional organoclays, has been prepared. These compounds are organoclays in which a second functional group was introduced on one of the hydrocarbon chains of the quaternary ammonium cation. The second functional group is chosen so as to catalyze the decomposition of target toxic organic compounds.

Bifunctional organoclays are not likely to be economically competitive for environmental applications in the foreseeable future because of their relatively high cost and specificity. They may be, however, immediately suitable for treating point sources of pollution such as production line effluents in pesticide manufacturing facilities.

Free alkylamines may attack carboxylic and organophosphate esters and catalyze their hydrolysis (Bruice and Benkovic, 1966; Rav-Acha et al., 1988). Accordingly, a bifunctional organoclay, in which the second functional group is ethylamine, should be capable of both sorbing and detoxifying organophosphate pesticides. As the investigated bifunctional organoclay was designed to catalyze the hydrolysis of organophosphate pesticides in an environmentally relevant pH range, it was decided to incorporate an ethylamine group into the organic cation, rather than a longer alkylamine group, since the intrinsic pK_a of the ethylamine moiety in the organoclay matrix was determined to be around 9.0, while the pK_a of larger alkylamines is expected to be higher.

In the present study, the bifunctional organoclay was prepared from a sodium montmorillonite in which the inorganic counter ion was replaced by the organic cation N-decyl-N,N-dimethyl-N-(2-aminoethyl) ammonium(DDMAEA) (Fig. 1).

View larger version (6K): [in this window] [in a new window]   Fig. 1. Structure of the organic cation N-decyl-N,N-dimethyl-N-(2-aminoethyl) ammonium (DDMAEA).

	Materials and Methods

TOP ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

 
Materials
The inorganic reagents were all of analytical grade, and the organic compounds were of the highest grade of purity available (98%). Water was purified by passing tap water through an ion exchange resin, and then through a Labconco (Kansas City, MO) Water Pro PS water purifier equipped with a column of activated carbon and a 0.2-µm Millipore (Billerica, MA) filter.

The Bifunctional Cation.
The compound N,N-dimethyl-N-(2-ethoxycarbonyl) ethylamine (Compound A) was prepared by adding 40 g (0.45 mol) of N,N-dimethyl-N-2-ethylamine (Aldrich) to 60 g (0.51 mol) of diethylcarbonate. The solution was kept at 70°C for 48 h and then distilled at 70°C and 0.5 Torr. Forty-four grams (0.27 mol) of Compound A were allowed to react with 82 g (0.37 mol) of decylbromide (Fluka, Buchs, Switzerland) at room temperature for 7 d. The product, N-decyl-N,N-dimethyl-N-(2-N-ethoxycarbonylethyl)-ammonium bromide (Compound B), was crystallized from acetone-ether. Hydrolysis of Compound B was performed in a 1:1 concentrated HBr and distilled water mix for 3 d under reflux. Water was evaporated and the remaining viscous oil was crystallized from ethanol-ether, yielding a white powder of the cation hydrogen dibromide (DDMAEA·HBr₂), which has a melting point in the range of 195 to 220°C. Elementary analysis (DNA Sequencer 377; PerkinElmer, Wellesley, MA) gave (%): C, 43.10; H, 8.77; N, 7.06; and Br, 41.15. Calculated values were (%): C, 43.07; H, 8.71; N, 7.10; and Br, 41.20.

The Long-Chain Bifunctional Organoclay.
A bifunctional organoclay was prepared by mixing 20 g of sodium bentonite (Fisher Scientific, Hampton, NH) with 6.2 g of DDMAEA·HBr₂ in 10.5 L of buffered solution (0.05 M phosphate) at pH 10.0 for 48 h using a mechanical shaker. These proportions were chosen to avoid micelle formation. The concentration of the cation was 1.5 mM, which is below its critical micellar concentration, while the total amount of the cation in solution was maintained at 16 mmol, equal to the cation exchange capacity of 20 g of the bentonite (80 cmol/kg clay). The pH was adjusted to 10.0 to obtain the ethylamino group of the cation in its free, rather than protonated, form. The pK_a of the DDMAEA cation in solution is 6.9 (Rav-Acha et al., 1988), but the pK_a of the sorbed DDMAEA cation in the bifunctional organoclay is about 9.0 (based on the kinetic measurements described in the Results and Discussion, below). After mixing at room temperature, the mixture was centrifuged and washed repeatedly with buffer solution (pH 10). The sample was dried by lyophilization (Freezone 4.5; Labconco) and then in a vacuum desiccator over P₂O₅. The elementary analysis was determined to be (%): C, 8.38; H, 2.15; N, 1.31; and Br, 0. This indicated about a 70% exchange relative to the cation exchange capacity (based on percent C).

The Short-Chain Bifunctional Organoclay (TMAEA Clay).
A 1.7-g sample of N,N,N-trimethyl-N-(2-aminoethyl) ammonium (TMAEA) chloride (Aldrich, St. Louis, MO) was mixed with 10 g sodium bentonite (Fisher Scientific) in 10.5 L of buffered solution (0.05 M phosphate) at pH 10.0 using a mechanical shaker. After 48 h mixing at room temperature, the mixture was centrifuged and washed repeatedly with buffer solution (pH 10). The sample was dried by lyophilization (Freezone 4.5; Labconco) and then in a vacuum desiccator over P₂O₅. The elementary analysis was determined as (%): C, 3.57; H, 1.97; N, 1.36; and Cl, 0. This indicated about 70% exchange.

A Conventional (Nonbifunctional) Long-Chain Organoclay.
A long-chain organoclay, containing the cation N-decyl-N,N,N-trimethyl ammonium (DTMA), was prepared by mixing sodium bentonite (Fisher Scientific) with 4.5 g of N-decyl-N,N,N-trimethyl-ammonium bromide (Fluka) in 15 L of distilled water. The procedure described above was designed to have a concentration of the cation monomer (1.07 mM) below its critical micellar concentration and a total amount of cation in the solution (16 mmol) equal to the cation exchange capacity of 20 g bentonite. After 48 h of mixing at room temperature, the mixture was centrifuged and washed repeatedly with distilled water until no bromide was detected by AgNO₃. The sample was dried by lyophilization (Freezone 4.5; Labconco) and then in a vacuum desiccator over P₂O₅. The results of elementary analysis (%) were: C, 9.64; H, 2.41; N, 0.82; and Br, 0.0. Elementary analysis for full exchange (C, 10.76; H, 2.07; N, 0.97; Br, 0) indicated about 90% exchange.

Methods
Sorption Isotherms.
Sorption of methyl parathion and tetrachlorvinphos (Fig. 2) on the bifunctional organoclay containing DDMAEA was compared with the sorption of the pesticides on the nonbifunctional organoclay (DTMA clay). Batch isotherms were performed in a series of 55-mL amber glass vials with Teflon-lined caps. The vials were filled with 1% (0.5 g/55 mL) suspensions of sorbent (either the bifunctional or the DTMA organoclay) in buffer solution (0.05 M phosphate) at pH 9.0 with no headspace. The vials were then spiked with various amounts of the pesticides (taken from a stock solution of the pesticide in acetone) and allowed to equilibrate for 24 h in an overhead shaker at 24°C. The aqueous phase was then separated by centrifugation at 4000 rpm (2500 x g) for 30 min, the concentration of residual pesticide in solution was determined by gas chromatography.

View larger version (12K): [in this window] [in a new window]   Fig. 2. The structure of the pesticides studied.

Degradation Studies.
The rate of methyl parathion hydrolysis on the bifunctional organoclay containing DDMAEA was measured in parallel with five controls: (i) methyl parathion on sodium bentonite (Fisher Scientific); (ii) methyl parathion on DTMA clay, a nonbifunctional organoclay; (iii) spontaneous hydrolysis of methyl parathion (without any solid or dissolved catalyst); (iv) methyl parathion hydrolysis in a solution of the DDMAEA cation monomer at a concentration of 5 mM, which is equivalent to the amount of DDMAEA contained on the bifunctional organoclay suspended in the same volume; and (v) hydrolysis of methyl parathion in a suspension of the short-chain bifunctional organoclay, TMAEA montmorillonite. Each kinetic experiment was performed in a series of amber glass vials of 55 mL volume with Teflon-lined caps, which were filled without headspace with 0.5 g sorbent (organoclay or control) and a buffer solution (0.05 M phosphate) of 30 mg/L methyl parathion at pH 9.0. The vials were kept in an overhead shaker at 24°C. At various times, the reaction in a subset of bottles was terminated by separating the solid from the liquid phase by filtration through a 0.7-µm sinter glass filter, after which the solution was acidified with HCl to pH 2 and concentrated by solid-phase extraction. The solid-phase extraction was performed by passing the acidified solution through a 47-mm C₁₈ disk, which was then eluted with 5 mL ethyl-acetate, 5 mL CH₂Cl₂, and 5 mL of a 1:1 mixture of the two solvents. The combined effluent was concentrated to 1 mL in a fine stream of nitrogen and analyzed by gas chromatography–mass spectrometry, using phenanthrene-d₁₀ as an internal standard for residual methyl parathion and p-nitrophenol produced by hydrolysis. Sorbed concentrations of methyl parathion and p-nitrophenol were similarly measured by extracting from the clay with a 1:1 CH₂Cl₂ and acetone mixture followed by gas chromatography–mass spectrometry analysis.

The kinetic runs involving all the liquid controls were performed under the same conditions with respect to pH, buffering, ionic strength, temperature, and initial concentration of methyl parathion as described above.

The degradation kinetics experiments with tetrachlorvinphos were performed in a similar manner, except that (i) the residual concentration of the pesticide was measured but not that of the degradation products and (ii) only three controls were used rather than five. The mineral clay and the short-chain bifunctional organoclay controls were excluded, as it was shown that these materials do not have any catalytic effect on organophosphate degradation (see Results and Discussion, below).

Analytical Procedures.
The concentrations of methyl parathion and tetrachlorvinphos were determined according to USEPA Method 525.2 on a ThermoQuest (Waltham, MA) Trace GC 2000 Polaris, equipped with a 30-m x 0.25-mm-i.d. fused silica capillary column coated with a 0.25-µm bonded film of polyphenylmethylsilicone (DB-5 MS; J&W Scientific, Folsom, CA). The column temperature was held at 130°C for 5 min and then raised to 180°C at a rate of 6°C/min and from 180 to 240°C at 7°C/min. The helium carrier gas flow was 33 cm/s. The injector temperature was 240°C.

	Results and Discussion

TOP ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

 
Sorption of Organophosphates on the Bifunctional Organoclay
Sorption isotherms of methyl parathion and tetrachlorvinphos on the bifunctional DDMAEA organoclay and the nonbifunctional DTMA organoclay at pH 9.0 are shown in Fig. 3. The sorption data were fit to a linear isotherm:

[1]

where Q is the solid-phase concentration of the sorbate per unit mass of the adsorbent (mg/g), C is the solute's equilibrium concentration (mg/L), and K_p is the adsorption coefficient (L/g).

View larger version (23K): [in this window] [in a new window]   Fig. 3. Sorption isotherms for methyl parathion and tetrachlorvinphos on the bifunctional (DDMAEA) and nonbifunctional (DTMA) organoclays.

When converted into K_oc values using the relationship (Schwarzenbach and Westall, 1981):

[2]

where f_oc is the organic carbon fraction of the sorbent, we find that the K_oc values for methyl parathion and tetrachlorvinphos on the bifunctional organoclay are 16.71 and 17.90 L/g, respectively, and on the DTMA organoclay they are 19.19 and 18.98 L/g, respectively. Hornsby et al. (1996) reported average K_oc values for the sorption of selected pesticides by soils, based on an extensive review of the relevant literature. For methyl parathion and tetrachlorvinphos the reported K_oc values are 5.1 and 0.9 L/g, respectively, considerably lower than those obtained in the present study. Similar differences in K_oc values between soils and organoclays have been reported previously. Celis et al. (2000) studied the sorption of tridimefon on a series of organoclays and found K_oc values ranging from 3.6 to 76 L/g, whereas the value reported for soils (Hornsby et al., 1996) is 0.3 L/g. The K_oc of prometon in soils is reported as 0.15 L/g (Hornsby et al., 1996), yet Socias-Viciana et al. (1998) obtained K_oc values for prometon on several organoclays in the range of 0.51 to 3.49 L/g. For fenuron, Hornsby et al. (1996) report a K_oc value of 0.042 L/g in soils, while the K_oc values reported by Aguer et al. (2000) for a series of organoclays range from 7.7 to 16.8 L/g.

While an in-depth analysis of these data is beyond the scope of the present study, it appears that in the organoclays, the sorbate molecules can more easily penetrate the organic volume than in soils, resulting in larger K_oc values.

The Catalytic Effect of the Bifunctional Organoclay on the Hydrolysis of Organophosphate Pesticides
The rate of methyl parathion hydrolysis in the presence of the long-chain bifunctional organoclay and the five controls is shown in Fig. 4. The catalytic effect of the bifunctional organoclay, relative to spontaneous hydrolysis, is readily apparent. The half-life of methyl parathion in the presence of the bifunctional organoclay is around 5 d, while the half-life for the spontaneous hydrolysis of methyl parathion reported in literature at the same pH (9.0) is 33 d (Tomlin, 1994) and a similar value (40 d) was calculated from the data of Fig. 4. The rate of methyl parathion hydrolysis on the long-chain bifunctional organoclay is also much higher when compared with its hydrolysis in the presence of the conventional (nonbifunctional) DTMA organoclay. The advantage of the long-chain bifunctional organoclay over the regular (nonbifunctional) organoclay for detoxifying contaminated waters is, therefore, obvious.

View larger version (26K): [in this window] [in a new window]   Fig. 4. The rate of methyl parathion hydrolysis in the presence of the long-chain bifunctional (DDMAEA) organoclay and in five controls (pH 9.0).

The fact that only the long-chain bifunctional organoclay exhibits a catalytic effect on the hydrolysis of the organophosphate ester indicates that for catalysis to take place, the relatively flexible long chain that can retain the substrate at a close proximity to the aminoethyl catalytic group must be present. The enhancing effect of sustaining a close distance between the substrate and the catalytic site is known in enzymatic chemistry as the proximity effect (Jencks, 1969, p. 30–35). The proximity effect has been demonstrated in several chemical matrixes such as micelles (Cang et al., 2001; Olmstead et al., 2001) and crown ethers (Robertus et al., 1999), and in this work in an organoclay matrix. Conversely, even though sorption on the short-chain bifunctional organoclay is greater than on the long-chain one, steric considerations dictate that the proximity effect does not occur on the former clay. The small catalytic effect of the cation monomer (Fig. 4) may be attributed to the formation of micelles into which the pesticide molecules partitioned just as they did into the organic core of the long-chained bifunctional clay.

A pH rate profile was constructed for the hydrolysis of methyl parathion in the presence of the long-chain bifunctional organoclay and the similar nonbifunctional DTMA organoclay to determine the pH at which the catalytic effect is maximum and to estimate the pK_a of the aminoethyl catalytic group in the organoclay matrix. In the following development we assume all kinetics exhibited first-order behavior. In light of the fact that the bifunctional and DTMA organoclay are identical except for the ethylamino group incorporated in the bifunctional organoclay, it is suggested that the catalytic effect can be expressed by the difference [k_bifunctional – k_DTMA (d^–1)] as shown in Eq. [3]:

[3]

where k_bifunctional and k_DTMA are the first-order rate constants for the hydrolysis of methyl parathion in the presence of the bifunctional organoclay and the DTMA organoclay, respectively. The k_cat term represents the rate of catalysis contributed by the alkylamino group of the bifunctional organoclay, and K_a is the dissociation constant of the protonated alkylamino group. The catalytic effect constants as defined in Eq. [3] are given in Table 1 and plotted against pH in Fig. 5. A small difference in the rate of spontaneous hydrolysis that may exist in the presence of the bifunctional or DTMA organoclays has been ignored in the derivation of Eq. [3].

View larger version (13K): [in this window] [in a new window]   Fig. 5. pH rate profile for the hydrolysis of methyl parathion [kbifunctional – kDTMA (d–1)].

One of the hydrolysis products of methyl parathion, p-nitrophenol, was measured in solution and in the sorbed state. The pK_a of p-nitrophenol is 7.15 (Serjeant and Dempsey, 1979) so that at the pH values where the bifunctional organoclay is active, p-nitrophenol should not be strongly sorbed and indeed was found entirely in the solution phase. The second hydrolysis product of methyl parathion, dimethylthiophosphate, was not determined but since at the pH values employed it exists predominantly as an anion, it is not expected to be significantly sorbed by the organoclay.

The same general trend as that observed for the enhanced hydrolysis of methyl parathion (Fig. 4) was found for tetrachlorvinphos (Fig. 6A). As the catalytic effect reached its plateau around pH 10, where the ethylamino group is fully unprotonated, the hydrolysis of methyl parathion and of tetrachlorvinphos was measured at pH 10 in the presence of the long-chain bifunctional organoclay. This rate of hydrolysis was compared with that in the presence of the DTMA clay, and to the rate of spontaneous hydrolysis and of hydrolysis in presence of the dissolved monomer, DDMAEA, at the same molarity of the monomer as in the suspension of the bifunctional organoclay. The results are shown in Fig. 6. As can be calculated from Fig. 6, the half-lives of tetrachlorvinphos and methyl-parathion in the presence of the bifunctional organoclay at pH 10.0 were about 10 h and 3 d, respectively, compared with a half-life of about 90 h and 36 d, respectively, for the spontaneous hydrolysis at that pH value.

View larger version (27K): [in this window] [in a new window]   Fig. 6. The hydrolysis of tetrachlorvinphos (A) and methyl parathion (B) in the presence of the long-chain bifunctional (DDMAEA) organoclay and controls (pH 10.0).

	Conclusions

TOP ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

The catalytic effect demonstrated in this study may have important, albeit limited, applications in the treatment of industrial and other types of wastewater (e.g., effluents of the pesticide manufacturing industry) as well as for the neutralization of organophosphate warfare chemicals. The reported work may open the gate for the preparation of other bifunctional organoclays that may be suitable as catalysts for many other specific purposes.

	REFERENCES

TOP ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

 

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