Oxidative Degradation of Organophosphorous Pesticides by N

May 22, 2009 - malathion, and reaction products were oxon compounds. The fabric containing imide and amide halamine structures were able to oxidize 90...
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KINETICS, CATALYSIS, AND REACTION ENGINEERING Oxidative Degradation of Organophosphorous Pesticides by N-Halamine Fabrics Xin Fei† and Gang Sun* DiVision of Textiles and Clothing, UniVersity of California, DaVis, California 95616

Cotton and cotton/polyester fabrics containing halamine structures were able to react with certain organophosphorus pesticides upon contact. The reaction occurred at thione group in methyl parathion and malathion, and reaction products were oxon compounds. The fabric containing imide and amide halamine structures were able to oxidize 90% of methyl parathion in less than 2 h of contact time under room temperature, while the amine halamine structure needed longer time to reach the same level of oxidation. The reaction was endothermic, and the oxidation rate was in first order to the concentrations of the pesticides. Introduction N-Halamine materials possess oxidative ability that can convert alcohols to ketones, sulfides to sulfoxides and sulfones, and cyanides to carbon dioxide and ammonium in water.1-4 Due to the oxidative ability, N-halamine fabrics could provide antibacterial functions and have been employed as biological protective materials in recent years.5 These fabrics also demonstrated oxidative power against toxic compounds such as carbamate pesticides, making these fabrics possible to be used as chemical and biological self-decontaminating clothing materials.6,7 However, these fabrics were unable to detoxify some carbamates containing aromatic rings.7 In the reaction with the carbamate pesticides, halamine structures oxidized sulfide groups in aldicarb and methomyl to their sulfoxide and sulfone analogues with reduced toxicity.7 Similar to some carbamate pesticides, many organophosphorus pesticides contain a sulfur element such as thione moiety (PdS).8 On the basis of the fact that N-halamine fabrics could effectively oxidize thiocarbamates,7 potential oxidative reactions between N-halamine fabrics and thione groups in organophosphorus pesticides were hypothesized. In fact, researchers have demonstrated that organophosphorothionate subgroups in the pesticides could be oxidized by aqueous chlorine solutions.9-14 The thiophosphate functional group (PdS) could be oxidized to the oxon form by the oxidant, and then the oxidized products may further be degraded quickly in environments since the hydrolysis rate of phosphate triester was about 160 times that of the corresponding phosphorothioates.15,16 Thus, the oxidation reaction between N-halamine fabrics and organophosphorus pesticides still could be an option for decontamination of organophosphorus pesticides. In this study, we intended to investigate potential reactions between halamine fabrics and organophosphate pesticides and understand the capability and limitation of such reactions, as well as potential reaction products and kinetics. Experimental Section Materials. Methyl parathion, malathion, malaoxon, and chlorpyrifos (98.5%, ChemServices) were selected as represen* To whom correspondence should be addressed. Tel.: 530-752-0840. Fax: 530-752-7584. E-mail: [email protected]. † Current address: Environmental Energy Technology Department, Lawrence Berkeley National Laboratory, Berkeley, CA 94720. E-mail: [email protected].

tative organophosphorus pesticides in this study. The structures and some properties of the three pesticides are listed in Table 1. N-Halamine fabrics were prepared by chemically finishing pure cotton (No. 400, TestFabrics, Inc. West Pittiston, PA) with 4% of 1,3-dimethylol-5,5-dimethylhydantoin (DMDMH) and cotton/polyester (35/65, No. 7436, Test Fabrics) with 4% of 1-monomethylol-2,2,5,5-tetramethyl-4-imidazolidin-4-one (MTMIO), respectively.20,21 The treated fabrics were chlorinated in a diluted bleach solution containing 0.01% (w/v) of Clorox bleach at 25 °C for 15 min and rinsed with water. The fabrics were air-dried and stored in a conditioning room (21 °C, 65% relative humidity). The DMDMH treated fabrics contain exclusively active imide and amide halamine structures, while MTMIO treated ones only contain amine halamine, which is the most stable and least reactive.5,21 Instrumentation. HPLC-UV was performed on a Waters 1500 series HPLC with a Waters W10681R 022 column (5 µm C18 film, 4.6 × 150 mm) and Waters 2487 Dual absorbance detector at the wavelength of 210 nm. HPLC grade acetonitrile was applied as an eluent at a flow rate of 1.00 mL/min. The responses of the resulting peak areas were integrated by software of the Breeze system. Reaction products were identified by using an Applied Biosystems Q Trap LC/MS/MS-ESI system and controlled by Analyst 1.4 Software. The positive ionization was performed by using reaction solutions doped with 0.1% formic acid and by a scan mode of Q1 MS or Enhanced MS with the following settings: ion spray voltage, 5500.0 V; curtain gas, 15.0; ion source gas flow rate 1 at 25.0 units; declustering potential, 50.0 V; entrance potential, 10.0 V. For Enhanced MS scan mode, collision energy was set at 20.0 V. Methods. Active chlorine content on fabric was measured in the following method. About 0.3 g of the treated fabric were cut into small pieces and soaked in 30 ( 0.001 mL of 0.001 N sodium thiosulfate solution containing 0.05 wt % nonionic wetting agent (Triton X-100), at room temperature under constant stirring for 30 min. The excess amount of sodium thiosulfate was titrated with a 0.001 N iodine solution. Unchlorinated DMDMH or MTMIO grafted fabrics, which were used as control samples in detoxification tests, were also titrated by using the same method. Available active chlorine of the bleached fabric (in parts per million) was then calculated from active chlorine (ppm) ) 35.45 × 1000[N(V2 - V1)/2]W

10.1021/ie801254u CCC: $40.75  2009 American Chemical Society Published on Web 05/22/2009

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Table 1. Physiochemical and Biological Characteristics of Selected Organophosphorus Pesticides

where V1 and V2 represent the volumes (mL) of iodine solution used in the titrations of the sodium thiosulfate solutions and the controls, respectively; N is the normality of the I2 solution; and W is the weight (g) of the halamine fabric. Organophosphorus pesticides were prepared as aqueous solutions at concentrations of 0.025, 0.125, 0.25, or 0.5 mM, respectively. A 5.000 ( 0.001 g amount of the DMDMH fabric or MTMIO fabric, containing 464.3 or 356.5 ppm of active chlorine, was immersed in the solutions at a controlled temperature, respectively. The reaction temperatures of 25 and 0 °C were controlled by a water bath (Precision Scientific, Chicago, IL) or an ice/water mixture, respectively. The contact time between the pesticide solution and N-halamine fabric was the time between the mixing and injection of the solutions into the HPLC. A volume of 20 µL of each sample was then injected into the HPLC or mass spectrometry. Results and Discussion Chlorinated DMDMH treated cotton fabric contains imide and amide halamine bonds, while chlorinated MTMIO treated cotton/polyester (35/65, No. 7436, Test Fabrics) only contains amine halamine bonds.20,21 The active chlorine contents on the

chlorinated DMDMH and MTMIO treated fabrics were all above 356 ppm, or 0.050 mM active chlorine/(5 g of the fabrics), measured by the iodometric titration method. Thus, the active chlorine/pesticide ratio was above 20:1 for 0.125 mM pesticide in 20 mL (0.0025 mMol), or higher than 100:1 for 0.025 mM pesticide in 20 mL aqueous solution, respectively. Thus, the active chlorine on the fabric was much more excessive than the pesticide in the reaction systems. As a result, the amount of active chlorine consumed in the reaction could be considered negligible under this condition, and the concentration of the active chlorine on the fabrics can be considered as unchanged. Reaction Products. Both unchlorinated DMDMH and MTMIO fabrics did not demonstrate any oxidative reactivity in contacting with methyl parathion solutions for more than 6 h, indicating that methyl parathion were not degraded by the unchlorinated fabrics. On the other hand, the halamine fabrics, both chlorinated DMDMH and MTMIO fabrics, were able to rapidly react with methyl parathion and malathion. However, both fabrics were unable to react with chlorpyrifos. HPLC-UV spectra of the mixture solutions of both malathion and methyl parathion with halamine fabrics showed new peaks of products of the reactions. The retention time of the only final

Figure 1. Mass spectrum of pure malathion (where, for example, 4.1e6 represents 4.1 × 106).

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Figure 2. Mass spectrum of reaction products of malathion and chlorinated DMDMH pure cotton fabric (where, for example, 7.2e4 represents 7.2 × 104). The contact time was more than 7 h.

Figure 3. Mass spectrum of reaction products of methyl parathion and chlorinated DMDMH pure cotton fabric (where, for example, 9.0e6 represents 9.0 × 106). The contact time was more than 7 h.

product in malathion solution reacted with halamine fabrics was 8.0 min, similar to oxidized malathion, malaoxon, indicating that the possible product of malathion with N-halamine fabric was an oxidation product. Mass spectra of pure malathion and malathion treated by chlorinated DMDMH fabric (after 7 h of reaction) are presented in Figures 1 and 2. The special m/z of 331.2, 348.2, and 353.2 representing malathion-H, malathionH2O, and malathion-Na (Figure 1) were hardly found in the mass spectra of the halamine fabric treated malathion solution (Figure 2), showing that malathion was almost totally consumed

by the halamine fabric. But, the obvious peaks at the m/z of 314.7 and 337.1, which coincide with molecular ions of malaoxon-H (m/z 315.3) and malaoxon-Na (m/z 337.3), appeared in the reaction mixture, confirming the oxidation reaction between malathion and the N-halamine fabrics. Similar to malathion, methyl parathion was also oxidized by the halamine fabrics, since a strong peak on m/z 248.3 (Figure 3, which is corresponding to methyl paroxon-H) was found after the reaction of methyl parathion and chlorinated DMDMH pure cotton fabric.

Ind. Eng. Chem. Res., Vol. 48, No. 12, 2009 Scheme 1. Proposed Mechanism of the Oxidation of Methyl Parathion by N-Halamines

Proposed Reaction Mechanism. The active chlorine on the N-halamine fabrics could react with sulfur atoms in malathion and methyl parathion, shown in Scheme 1. After isomerization, PdSdO will become PdO bonds and the sulfur will be released from the parent compounds.22,23 If the oxidants are more

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reactive, such as aqueous chlorine, the sulfur could be further oxidized to sulfate.9 Compared to malathion and methyl parathion, chlorpyrifos exhibited totally different behavior, very resistant to oxidative halamine. The ethoxyl groups connected to phosphorus atom are bulkier than the methoxyl groups and could block access of the halamine structure to the sulfur in the PdS group. Such steric hindrance by ethoxyl groups inhibits the oxidation reaction of chlorpyrifos by the N-halamine structures on the fabrics. However, chlorpyrifos was reported to be oxidized by aqueous chlorine solutions.10 This difference is due the oxidative power of free chlorine (Cl+), which could easily react with sulfur atom in PdS bonds, while the halamine mostly contains combined chlorine (N-Cl) instead of free chlorine. Detoxification Kinetics. Methyl parathion was employed in reaction kinetic study with the fabrics containing different halamine structures. At room temperature (25 °C) methyl parathion was rapidly decomposed by the chlorinated DMDMH fabric, which contains very active imide and amide halamine

Figure 4. Residual methyl parathion after reaction with chlorinated DMDMH No. 400 fabric.

Figure 5. Residual methyl parathion after reaction with chlorinated MTMIO No. 7436 fabric at room temperature.

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Thus, the reaction rate of methyl parathion is expressed as -d[MeP]/dt ) kr[MeP]a[halamine]b

(3)

Here a and b are the orders of the two reactants. Since the consumed halamine is negligible due to the excessive amount of active chlorine on the fabrics, [halamine] could be regarded as a constant in eq 3. Furthermore, if a ) 1 (first-order reaction in terms of methyl parathion concentration), ln[MeP] - ln[MeP]0 ) -kobst

Figure 6. Logarithm concentration of methyl parathion versus contact time on chlorinated DMDMH No. 400 fabrics under different pesticide concentrations (5 g of the fabric; 20 mL of methyl parathion solution) and different temperatures (0 and 25 °C).

Here kobs ) kr[halamine]b. The plots of ln[MeP] under different methyl parathion concentrations and different reaction temperatures versus reaction time are shown in Figures 6 and 7, respectively. All of the plots show linear correlations between ln[MeP] and contact time, proving the reactions are first order to the concentration of methyl parathion. Table 2 lists the kinetic parameters for the reactions between methyl parathion and chlorinated DMDMH and MTMIO fabrics. The activation energy of the reaction can be calculated by using the van’t Hoff-Arrhenius equation.24 ln(k1 /k2) ) Ea(T2 - T1)/RT1T2

Figure 7. Logarithm concentration of methyl parathion versus contact time on chlorinated MTMIO No. 7436 fabrics under different pesticide concentrations (5 g of the fabric; 20 mL of methyl parathion solution at 25 °C). Table 2. Observed Kinetic Parameters of Methyl Parathion with Halamine Fabrics temp (°C)

halamine

initial concn (mM)

kobs (min-1)

R2

25 25 0 0 25 25

DMDMH DMDMH DMDMH DMDMH MTMIO MTMIO

0.125 0.025 0.125 0.025 0.125 0.025

0.02 0.0573 0.0086 0.0184 0.0077 0.0063

0.9619 0.9942 0.9932 0.9915 0.9606 0. 7155

bonds (Figure 4).21 At a concentration of 0.025 mM, almost all methyl parathion was eliminated by the halamine fabric within a contact time of 1 h. When the concentration was increased to 0.125 mM, more than 95% of methyl parathion was eliminated by the fabric within 2 h, indicating powerful detoxification ability of the imide and amide halamine structures. Under a lower temperature (0 °C), the detoxification speed was reduced but almost all methyl parathion could still be decomposed by the fabrics in a longer contact time (Figure 4). The oxidative ability of amine halamine structure (MTMIO treated fabric) is not as strong as that of amide or imide ones. But amine halamine still could react with methyl parathion in a slower speed and needs a much longer time to degrade methyl parathion (Figure 5). Even after 6 h of contact, still 40 or 20% residual methyl parathion pesticide remained for concentrations of 0.125 and 0.025 mM, respectively. In fact, after 90 min of reaction the amount of methyl parathion was almost unchanged for the concentration of 0.125 mM. The reaction between methyl parathion and N-halamine functional groups can be written as a(MeP) + b(halamine) f c(products)

(2)

(4)

(5)

Here k1 and k2 are the apparent rate constants for the reaction under temperatures T1 and T2; Ea is the activation energy for the reaction; R is the gas constant (1.987 cal/(K mol)); and T is the absolute temperature (kelvin). On the basis of eq 5, the activation energies for oxidation of methyl parathion by the chlorinated DMDMH fabrics were 5.47 and 7.36 kcal/mol for initial pesticide concentrations of 0.125 and 0.025 mM in a temperature range of 0-25 °C, respectively. Actually, the activation energies should be the same even though the initial concentrations are different. Thus, the activation energy of methyl parathion by imide halamine fabrics should be around the range of 5.47-7.36 kcal/mol, which is quite low compared with hydrolysis reaction of methyl parathion (9.9 kcal/ mol).25 Thus, the oxidation reaction should be a dominating reaction during the contact of halamine fabrics with methyl parathion. Malathion was easily oxidized by the halamine fabrics, with HPLC-UV results indicating that the malathion peak completely disappeared within a contact time of 6 h. However, due to the peaks of malathion overlapping with the products of the reaction, resulting in difficulties in calculating the degradation rates of malathion by N-halamines. Conclusions This research revealed that halamine fabrics could be employed in pesticide protective clothing materials against certain organophosphorus pesticides. Malathion and methyl parathion were easily oxidized to oxon forms by cotton and cotton/polyester fabrics containing halamine structures. The imide and amide halamines (chlorinated DMDMH fabrics) were much more reactive than the amine halamine (chlorinated MTMIO fabric). The reaction between methyl parathion and halamine fabrics were pseudo-first-order to the concentrations of the pesticide. The activation energy of oxidizing methyl parathion is lower than that of the hydrolysis of this pesticide. Acknowledgment We are grateful for the financial support from the National Science Foundation (Grant DMI 9733981) and the National Textile Center (Grant C02-CD06).

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ReceiVed for reView August 17, 2008 ReVised manuscript receiVed January 7, 2009 Accepted May 2, 2009 IE801254U