Two-Stage Decontamination of Organophosphorus Compounds on

Dec 26, 2012 - malathion, paraoxon, parathion, and other organophosphorus pesticides were ... Organophosphorus (OP) compounds are a large group of...
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Two-Stage Decontamination of Organophosphorus Compounds on Sensitive Equipment Materials Vladimir Blinov,* Konstantin Volchek, Wenxing Kuang, Carl E. Brown, and Akshay Bhalerao Environment Canada, 335 River Road, Ottawa, Ontario, K1A 0H3, Canada S Supporting Information *

ABSTRACT: Decontamination of sensitive equipment materials contaminated by organophosphorus (OP) compounds by a two-stage process was evaluated. The process involved extraction of OP from contaminated materials with methanol (first stage), followed by decomposition of OP by heterogeneous catalytic methanolysis (second stage). In a series of tests diazinon, malathion, paraoxon, parathion, and other organophosphorus pesticides were spiked onto sensitive equipment materials and extracted with methanol after a presented contact time. After extraction, the lowest quantity of residue was found for diazinon, while parathion was the most persistent. High-impact polystyrene (HI-PS) retained 10-fold or more mass of OP than other tested materials. The surface of the HI-PS was deteriorated after contacting with OP due to a plasticizing effect. A longer contact time with HI-PS and lower polarity factor of OP resulted in higher residual amounts of OP after extraction. In the second stage of the process, diazinon and paraoxon were decomposed by methanolysis in presence of palladium- and ytterbium-based solidsupported catalysts, respectively. Complete destruction of diazinon was achieved within 30 s, while 60 min was required to destroy paraoxon.

1. INTRODUCTION

A new method based on metal-ion-catalyzed methanolysis was developed for the destruction of OP CWAs in solutions. It reportedly provides a rapid and complete destruction and does not generate toxic byproducts.10−18 Several methanolysis catalytic systems have been developed for the decomposition of various types of OPs that contain ether phosphorus−oxygen (PO) or phosphorus−sulfur (PS) double bonds.10−18 In particular, a solid-supported catalytic system containing complexes of lanthanide ions (i.e., La3+, Sm3+, Eu3+, Yb3+) attached to silica or polystyrene was designed for the destruction of PO type OPs in buffered methanol solutions.16 Another solid-supported catalytic system containing palladium ions (Pd2+) attached to polystyrene was selective to PS type pesticides, such as diazinon.18 This paper describes a study whose objective was to evaluate the applicability of catalytic methanolysis to the decontamination of OPs from solid surfaces. Several OP pesticides, including parathion, paraoxon, malathion, and diazinon, were chosen for the study. Their selection was based on two factors. First, as indicated, the pesticides are listed as potential chemical terror agents. In case of an attack, the affected areas and infrastructure must be decontaminated to reduce the human health hazard. Second, the pesticides, such as malathion and parathion, are reported to be surrogates for the OP CWAs.19,20 Since the possession and handling of CWAs are very strictly regulated and is possible in only a handful of designated chemical laboratories worldwide, the surrogates are an easier choice to be used to study the properties and behaviors of respective CWAs. The focus of the present work was on the

Organophosphorus (OP) compounds are a large group of substances with toxicities ranging from mild to very high. Examples of OPs include pesticides and chemical warfare agents (CWAs). OP CWAs are volatile and extremely toxic. They have been banned under the Chemical Weapons Convention; however, significant stockpiles remain in several countries, and there have been cases of OP CWA use in terrorist attacks.1 In contrast, OP pesticides have a lower volatility and are much less toxic than OP CWAs. This enables the broad application of pesticides in agriculture, mosquito control, and many other areas. Despite their lower human health hazard, there have been numerous reports of OP pesticide poisoning, including fatal incidents, mostly as result of mishandling.2−4 OP pesticides present a threat because of their relative stability, persistence, and accumulation in the environment. Easy access to commercial OP pesticides makes them a likely weapon of choice in the hands of terrorists.5−7 Once OPs are dispersed in the environment, following either a technological accident or a deliberate act, they can enter the human body via inhalation, ingestion, or dermal contact. When low-volatile OPs end up on solid surfaces, such as equipment or building materials, they can pose a prolonged threat. The surfaces must therefore be decontaminated to eliminate or minimize the threat and to ensure a safe environment. Even though the malicious use of OP pesticides may not result in serious injuries or deaths, it will very likely lead to a panic and a major disruption of normal activities in the affected areas. There are many decontamination technologies available for OP CWA destruction,8,9 however, most of them are corrosive to surfaces and may cause damage to sensitive equipment. In addition, they may generate toxic byproducts of OP degradation. © 2012 American Chemical Society

Received: Revised: Accepted: Published: 1405

July 27, 2012 December 20, 2012 December 26, 2012 December 26, 2012 dx.doi.org/10.1021/ie302012y | Ind. Eng. Chem. Res. 2013, 52, 1405−1413

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decontamination of sensitive equipment materials via a twostage process. The first stage included immersing the contaminated sensitive equipment materials into a methanol bath for a given period of time to extract OPs from the surfaces. The second stage, i.e., catalytic destruction of the OPs extracted from equipment, utilized the solid-supported catalyst. OP pesticides were involved as surrogates of real CWAs. The specific objectives of this study were to evaluate (a) the effectiveness of methanol extraction of OPs from sensitive equipment materials and (b) the efficiency of solid-supported catalysts for the decomposition of OPs in extracts.

Scheme 1. Flow Diagram of the Two-Stage Decontamination Tests Procedure

2. EXPERIMENTAL SECTION 2.1. Materials. Components of a desktop computer were used to fabricate samples (coupons) representing different types of materials used in sensitive equipment manufacturing. The coupons were cut from a keyboard (high-impact polystyrene), computer case (painted steel), and electronic circuit board (fiberglass/epoxy resin composite). Other coupons were made from CD/DVD compact discs (polycarbonate plastics) and from a polarizing film specimen. The circuit board (CB) coupons represented composite materials that are used in a variety of electronic devices. The polycarbonate plastics are used in sensors, digital cameras, optical scanners, etc. Polarizing film (PF) is used in LCD monitor manufacturing. Polarizing film specimen (PFSC-NA) was purchased from Alight Company (Hollywood Park, TX, USA). Key plastic and PF coupons had a surface size of 1 cm × 1 cm. The samples of the other materials had a surface area of 2 cm × 3 cm. Methanol (99.8%, anhydrous) and hexane (99.5% of total hexanes, distilled in glass) were purchased from Caledon Laboratories Ltd. (Georgetown, ON, Canada). Both the Pd- and Yb-based solid-supported catalysts16,18 were synthesized and provided by Queen’s University (Kingston, ON, Canada). Neat pesticides, including diazinon (O,O-diethyl O-2isopropyl-6-methylpyrimidin-4-yl phosphorothioate, 99.3%), etrimfos (O-6-ethoxy-2-ethylpyrimidin-4-yl O,O-dimethyl phosphorothioate, 64.2%), fenthion (O,O-dimethyl O-4-methylthiom-tolyl phosphorothioate, 98.6%), parathion (O,O-diethyl O(4-nitrophenyl) phosphorothioate, 99.3%), phorate (O,O− diethyl S-(ethylthiomethyl) phosphorodithioate, 99.5%), and thionazin (O,O-diethyl O-pyrazin-2-yl phosphorothioate, 99.4%) were purchased from Sigma−Aldrich Canada, Ltd. (Oakville, ON, Canada). Paraoxon (O,O-diethyl O-p-nitrophenyl phosphate, 98%) was obtained from Toronto Research Chemical, Inc. (Toronto, ON, Canada). Commercial-grade malathion (O,O-dimethyl-S-(1.2-dicarbethoxy) ethyl phosphorodithioate, 95%) was supplied by Agrium, Inc. (Calgary, Alberta, Canada). Parathion, paraoxon, malathion, and diazinon were the predominant pesticides used in the study. Other pesticides (phorate, etrimfos, fenthion, and thionazin) were only used in a few confirmatory tests undertaken to confirm the study conclusions. Paraoxon and diazinon were used in the catalytic decontamination tests. The physical data for pesticides21 used in this work are listed in the Supporting Information. 2.2. Methods and Procedures. The procedures utilized in the study are described in Scheme 1. 2.2.1. OP Spiking Procedure. Five different types of coupons were precleaned in hexane for 10 min, allowed to air-dry, and spiked with 3.0−3.5 mg of neat OP. A Sartorius MC-1, RC210D analytical balance was used to measure the sample

mass. The contaminated surface area was typically in the range of 0.6−0.9 cm2 and was determined by taking photographs of samples using a digital camera (Model D ELPH SD780IS, Canon Canada, Inc., Mississauga, Canada). 2.2.2. Exposure to OP and Extraction with Methanol. After the spiked coupons were allowed to stand for 4−76 h in covered glass vials (or jars) at room temperature (21 °C) and under a relative humidity of 30%−45%, the OP was extracted with 25 mL of methanol for 10 min. As found in preliminary tests, 10 min was determined to be sufficient for complete and consistent extraction of OP from glass coupons. Glass was considered as a control material, being nonporous and inert, with respect to both OPs and solvents. After extraction, the coupons were visually inspected. Photographs of the cross sections of selected HI-PS coupons were taken after the tests, using a ZEISS AXIOSKOP microscope (Model EL-EINSATZ 451485, Carl Zeiss, Jena, Germany). 2.2.3. Extraction of Residual OP. After drying for 45 min under ambient conditions, the coupons were further extracted with 10 mL of hexane for 24 h on a rotary (100 rpm) shaking table to determine the amount of residual OP remaining on the materials. 2.2.4. Catalytic Decomposition of OP. Solutions of diazinon and paraoxon generated in the first stage (section 2.2.2) of the process were treated with Pd- and Yb-based solid-supported catalysts, respectively. Pd-based catalyst (0.2 g) and Yb-based catalyst (0.04 g) were added to 2.0-mL methanol solutions of diazinon and paraoxon, respectively. After vortexing for a period of time from 30 s to 60 min, 1.0 mL of deionized water was added, to quench the reaction. One and a half milliliter (1.5 mL) samples were taken and transferred into 15-mL conicalbottom test tubes for solvent exchange. Methanol was then evaporated under nitrogen flow (120 mL/min) at 40 °C for 1 h and OPs were subsequently extracted with 10 mL of hexane under vortexing. 2.2.5. Analysis of OP. A gas chromatograph equipped with a pulsed-flame photometric detector (GC/PFPD) (Varian CP3800, Varian Canada, Inc., Mississauga, Canada) was used for the analysis of OP compounds. The GC/PFPD system was 1406

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Table 1. OP Residues on Sensitive Equipment Materials and Extraction Recoveries after 10 min of Contact with Methanola Paraoxonb

Parathion sensitive equipment materials computer key (HI-PS) painted steel (case) CD circuit board

2

residue (mg/m ) 3150 8.0 150 90

± ± ± ±

275 0.7 58 14

recovery (%) 93.10 99.98 99.62 99.75

± ± ± ±

residue (mg/m2)

0.77 0.00 0.14 0.03

920 120 5 5

± ± ± ±

68 16 1 1

Malathion

recovery (%) 98.10 99.63 99.99 99.99

± ± ± ±

0.25 0.05 0.00 0.00

residue (mg/m2) 457 6.0 6.0 40

± ± ± ±

39 0.8 0.6 9

recovery (%) 98.99 99.99 99.98 99.89

± ± ± ±

0.16 0.00 0.01 0.02

a OP/material contact time = 24 h; triplicate samples; initial OP loading = 40−55 g/m2. bData for paraoxon/HI-PS are based on testing of 18 samples.

Figure 1. Coupons before spiking (left), after spiking with parathion (middle), and after treatment with methanol (right). Parathion-material contact for 24 h; extraction with methanol for 10 min. Materials: (a) HI-PS, (b) painted steel, (c) CD plastic, (d) circuit board, and (e) polarizing film. Area of coupon surfaces: 1 cm × 1 cm (HI-PS and polarizing film); 2 cm × 3 cm (painted steel, CD, and circuit board).

with an inner diameter of 0.25 mm and 0.25-μm film. The temperature program was 50 °C, held for 1 min, and ramped at a rate of 25 °C/min to 300 °C, then held for 1 min. The MSD was operated in scan mode (40−400 amu). Helium was used as the carrier gas in the GC/PFPD and GC/MSD systems. 2.2.6. Quality Assurance and Quality Control. Calibration of the GC device was performed with 0.050−1.0 mg/L OP standard solutions and verified for each batch of samples. Triplicate samples were processed in all extraction tests. Duplicate samples were processed in each catalytic decontamination test. Control samples were processed simultaneously with every series of samples for solvent exchange to monitor the recovery of OPs. Control and blank samples were also processed during decontamination tests. Separate studies were

equipped with a 30-m VF-5MS column with an inner diameter of 0.25 mm and 0.25-μm film thickness. The temperature program was 50 °C, held for 0.5 min, and ramped at a rate of 30 °C/min to 290 °C. The instrument was calibrated using OP laboratory standards. A linear response was obtained over a range of concentrations from 0.015 mg/L to 1.0 mg/L. The detection limits were as follows: 0.015 mg/L for diazinon; 0.020 mg/L for etrimfos and fenthion; 0.018 mg/L for malathion; 0.012 for phorate; 0.045 mg/L for paraoxon; 0.060 mg/L for parathion; and 0.040 mg/L for thionazin. A gas chromatograph mass spectrometric detector (MSD) system (Model 6890 GC/5973 MSD, Agilent, Englewood, CA, U.S.A.) was used to confirm the identity of the OP compounds. The GC/MSD system was equipped with a 30-m HP-5MS column 1407

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cracks and crevices. The residues of parathion on HI-PS were the highest among all the materials tested. The changes on the HI-PS coupon surfaces were also the most pronounced. Similar observation was found for other OPs. A more-intense white color corresponded to higher residues of OP on HI-PS plastic in the following order: parathion > paraoxon > malathion > diazinon. Figure 3 shows photographs of the cross-section of the HIPS plastic coupons exposed to different OPs for 24 h, followed

undertaken to evaluate reproducibility of the results. Eighteen (18) tests were conducted involving one type of the materials and procedures that were identical in the processing of all other materials. The standard deviation (SD) and relative standard deviation (RSD) of these results did not exceed 7% for the extraction procedure and were less than 12% when solvent exchange was involved.

3. RESULTS AND DISCUSSION 3.1. Removal of OP by Methanol Extraction. No detectable residues of OP were found after extraction from polarizing film coupons. Diazinon residue of 56 ± 19 mg/m2 was found on HI-PS, but it was not detectable on other materials. As seen from Table 1, the residues of OP on other materials were in the range of 5−3150 mg/m2 (99.99% to 93.10% recovery). Parathion was the most persistent on the materials studied, excluding painted steel. The most significant residues were detected on the computer key plastic material (HI-PS), which appeared to have the highest affinity for OPs, among the tested sensitive equipment materials. Total recoveries of OP from the high-impact polystyrene (HI-PS) coupons including extraction and solvent exchange, were determined to be in the range from 88.3% to 101.7% after contacting for 76 and 24 h, respectively. The recovery values over 100% may be caused by uncertainties/errors in analytical procedures. The values from 70% to 110% are considered as acceptable when analyzing pesticides at concentrations of 0.01−1 mg/L.22 3.2. Visual Observations. Visual inspection revealed that the contact with all tested OPs caused significant changes on HI-PS, painted metal, and CD coupons but there were no visible changes on circuit board and polarizing film coupons. For example, testing with parathion resulted in notable changes, as shown on photographs (see Figures 1a, 1b, and 1c). In contrast, Figures 1d, and 1e show the circuit board and polarizing film coupons that were not affected by parathion. The most noticeable changes were found on HI-PS plastic (Figure 1a). Therefore, the balance of the study was focused on this material. The changes on HI-PS coupons appeared as white surface layers in places where the material was contacted with parathion and became visible after the methanol extraction stage (see section 2.2.2). In comparison, the HI-PS coupon remained black when it was treated with methanol but not exposed to parathion (see Figure 2). Therefore, it was concluded that alterations to the surface were caused not by solvent but by the parathion, which reacted with the plastic, resulting in a partial plasticization of its surface. Subsequent treatment with solvent leached the plasticized material and left

Figure 3. Cross sections of HI-PS coupons after the tests with (a) diazinon, (b) malathion, (c) paraoxon, and (d) parathion. Contact with OP for 24 h; extraction with methanol for 10 min.

by extraction of the OP with methanol. One can observe surface layers with different structures and thicknesses. The thickness of the layers was in the range of 0.04 mm (diazinon) to 0.11 mm (paraoxon). More-porous layers were observed in samples affected by paraoxon than other OPs. Holes with diameters in the range of 0.03−0.07 mm were found in the layers formed on the coupons affected by paraoxon (see Figure 3c). The pictures of the top surfaces of the layers (Figure 4) demonstrate a difference in the structure of layers formed after interaction with parathion (Figure 4a) and paraoxon (Figure 4b). 3.3. A Physical Model of OP-Plastic Interaction, Surface Layer Formation, and OP Extraction. A general

Figure 4. Top view of the layers on HI-PS coupons after the tests with (a) parathion and (b) paraoxon. Contact with OP for 24 h; extraction with methanol for 10 min.

Figure 2. Pictures of unspiked HI-PS coupons (size: 1 cm × 1 cm) taken (a) before and (b) after treatment with methanol for 10 min. 1408

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phenomenological picture of interaction of OP with plastic is illustrated in Figure 5 for parathion on HI-PS.

Figure 6. Residual concentrations of OPs in HI-PS plastic versus the contact time between OP and plastic. Initial loadings: 40−55 g/m2. Extraction with methanol for 10 min. Figure 5. Changes to HI-PS material surface during contact with parathion, followed by methanol extraction.

increasing the contact time from 4 h to 72 h caused an almost 40-fold increase in the residual concentration of parathion. Since the penetration of OP into the plastic appears to be a time-dependent parameter, the residual quantities of OP increased with time. The experimental results shown in Figure 6 are welldescribed by the equation

After spiking, a pool of parathion was formed on the surface of the HI-PS coupon (see Figures 1a and 5a). Possible fates of parathion on the surface of plastic, over a contact time of 24 h, can include evaporation, natural degradation, and penetration into the plastic. As noted in the control tests, evaporation and natural degradation only made a minor contribution ( paraoxon > malathion > diazinon. The residual concentrations of parathion were found to be ∼2 orders of magnitude higher than those of diazinon. This can be explained by the differences in molecular structures of the OP that affect their ability to penetrate into plastic. The molecular structure affects the plasticizing strength of compounds,26 which can be estimated by empirical parameters, such as the Hildebrant solubility parameter, the Flory−Huggins interaction parameter, and polarity parameter.27,28 The latest one was chosen to describe the experimental results obtained in this work. It is simple and does not require knowledge of specific properties of the plasticizer (the heat of evaporation, mole fraction of plasticizer in a mixture with plastic, glass-transition temperature), which may not be available. The polarity parameter was reportedly25−28 used to assess and compare the strength of the plasticizers−plastic interaction, within a homologous series of plasticizers. Polar and polarizable groups in the plasticizer facilitate an interaction with polymer molecules, because of a high cohesion at many points along the polymer chain.26 In addition, a higher polarity of the plastic molecule increases the migration of plasticizers.26 It is believed that a varying impact of different OP on HI-PS observed in the study was caused by differences in the OP structures. The polarity parameter (POL) was calculated for each OP by dividing the number of nonpolar carbon atoms in the plasticizers by the number of polar groups present and multiplying this number by the molar mass of the plasticizer:27,28 1409

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POL =

Article

M × Ap P0 × 1000

the molecular structures of thionazin, etrimfos, diazinon, fenthion, paraoxon, and parathion may be considered as analogues (excepting nitrogen in rings instead of carbon). The molecule structure of phorate is somewhat similar to that of malathion, in that neither of them have rings in their structure. The polarity parameters of fenthion (1.11), thionazin (0.99), etrimfos (1.75), and phorate (1.82) were found to be between the values for diazinon (2.43) and parathion (0.58), paraoxon (0.55), and malathion (0.88). Additional data may improve the reliability of the correlation. As seen from Figure 7, residual OP quantities decreased as the OP polarity parameters increased. The effect of polarity parameter on the residual surface concentration of OP can be described by the following equation:

(3.2)

where M is the molar mass of a plasticizer; Ap is the number of carbon atoms in the plasticizer excluding aromatic and carboxylic group carbon atoms; and P0 is the number of polar groups present in a plasticizer molecule. For instance, for parathion with a molar mass of 291.26, four carbon atoms (Ap), and two polar groups (P0), the polarity parameter is 0.58. The calculated empirical coefficients k and n, correlation coefficients (eq 3.1), and numerical values of polarity parameters of OP (eq 3.2) are shown in Table 2. Table 2. Coefficients k and n, Correlation Coefficients (eq 3.1), and Polarity Parameters (eq 3.2) for the Studied OP

log R s = 4.28 − 0.63 × POL

Coefficients

a

OP compound

k

parathion paraoxon malathion diazinon

42.94 20.27 9.11 5.09

n

correlation coefficient, Ra

polarity parameter, POLb

1.26 1.23 1.29 0.94

0.99 0.97 0.98 0.88

0.58 0.55 0.88 2.43

(3.3)

where Rs is the residual surface concentration of OP on the material (mg/m2) and POL is the polarity parameter. The correlation coefficient (R = 0.85) calculated for the experimental data set (22 pairs) indicates with 95% confidence that there is a correlation between the variables,29 which is described by eq 3.3. The square of the correlation coefficient (R2 = 0.72) shows that 72% of the dependent variables (log Rs) can be attributed to the independent variables (POL).29 Deviations of experimental data from the linear fit (Figure 7) may be caused by differences in structure of these compounds (see the Supporting Information) and their actual polarities. For instance, residual concentrations were always lower for paraoxon than those of parathion (Figure 7), despite their polarity parameters being very similar. This may be caused by a more significant difference in the actual polarities, in comparison with the calculated polarity parameters of paraoxon and parathion. Thus, the residual concentrations of OP on HI-PS plastic correlate with the polarity parameters of OP compounds. The polarity parameters can be used to predict the ability of OP to penetrate in HI-PS plastic and to be retained after extraction with methanol. 3.5. Decontamination of Paraoxon and Diazinon in the Methanol Extracts with Solid-Supported Catalysts. Paraoxon and diazinon were decomposed in the solution using Yb-based and Pd-based solid-supported catalysts designed for the destruction of PO and PS type compounds, respectively.16,18 As found, paraoxon and diazinon were not degraded in the control samples without catalyst additives. The efficiency of decontamination of paraoxon in solution versus the solution/catalyst contact time is shown in Figure 8. Both the standard solutions of paraoxon and methanol extracts (section 2.2.2) were used in the experiments. Figure 8 shows that more than 60 min were required to destroy more than 95.0% of paraoxon in methanol solutions. One can see from Figure 8 that the experimental data corresponding to the tests with extracts are in good agreement with data obtained in tests with standard solutions. This means that there was no significant effect of impurities that could be present in the extract due to leaching from plastic. To evaluate the catalytic reaction kinetics, i.e., order of the reaction and the rate constant, experimental data were plotted versus time in accordance with the following integrated rate equations:30

See Figure 6. bSee eq 3.2.

The effect of polarity parameters on residual concentrations of OP after 72 h contact with HI-PS plastic is shown in Figure 7. In addition to the results for parathion, paraoxon, malathion,

Figure 7. Logarithms of residual concentrations of OP in HI-PS plastic after 72 h of contact between OP and plastic versus polarity parameters. Initial surface concentrations: 44−58 g/m2. Two or three repeats for each OP are shown.

and diazinon, data for thionazin, fenthion, etrimfos, and phorate are plotted in Figure 7. These data were obtained in tests undertaken exclusively to determine the residues of these pesticides after 72 h of contact with HI-PS plastic. The need for additional data was caused by the significant difference in polarity parameters of diazinon (2.43) and other tested OP (0.55, 0.58, and 0.88 for paraoxon, parathion, and malathion, respectively). This made finding a reliable correlation between polarity parameters and the residues of OP compounds problematic. Therefore, thionazin, fenthion, etrimfos, and phorate were selected for additional tests taking into consideration their molecular structures and calculated polarity parameters. One can see (in the Supporting Information) that

Zero-order: Ct = C0 − kt 1410

(3.4)

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eqs 3.4−3.7). The data fit the integrated first-order rate (eq 3.5) well with the square correlation coefficient of R2 = 0.97 (see Figure 9). However, the experimental values showed nonlinear trends and did not fit the other rate eqs 3.4, 3.6, and 3.7. Thus, the methanolysis reaction followed first-order kinetics. The first-order rate constant k was found to be 8 × 10−4 s−1. The first-order rate constant for catalytic methanolysis of paraoxon determined in our study seems to be somewhat less than the kobs value reported by Andrea et al.16 for the similar reaction and catalyst. This discrepancy can be explained by the difference in experimental conditions. The specific shape of 15mL conical-bottom test tubes and agitation by vortexing used in our tests kept the catalyst on the bottom of test tube. In contrast, the catalyst was suspended in the bulk solution by manual shaking of experimental cell in the reported tests.16 As it was reported,16 the reaction rate sharply dropped when the shaking was stopped and catalyst was settled down. A factor-of100 decrease in the reaction rate was observed in another study of catalytic methanolysis when solid-supported Pd-based catalyst was settled to the bottom of the experimental cell.18 The fact that the rate of reaction is affected by agitation is evidence that the catalytic methanolysis of paraoxon is most likely controlled by a transport stage. The quantity of paraoxon consumed per gram of solid supported catalyst versus the square root of contact time (t1/2) is shown in Figure 10. A fit of the experimental data (R2 = 0.97) correlated well with the intraparticle diffusion kinetic model developed by Weber and Morris31 and expressed by eq 3.8:

Figure 8. Efficiency of decontamination of paraoxon in methanol solution versus extract/catalyst contact time. (Legend: (△) data for extract of paraoxon from HI-PS plastic and (◆) data for standard solution of paraoxon.) Catalyst/paraoxon mass ratio = 3030/1.

First-order: ln Ct = ln C0 − kt

(3.5)

Second-order: 1 1 = + kt Ct C0

(3.6)

Third-order: 1 1 = 2 + kt 2 Ct C0

Q t = K idt 1/2 + C (3.7)

(3.8)

where Qt is the amount of reactant taken up per gram of solid supported catalyst at time t, (M g−1), Kid is the rate constant of intraparticle transport (M g−1 s−1/2), and C is an intercept (M g−1). According to Weber and Morris,31 if the rate-limiting step is intraparticle diffusion, a plot of solute adsorbed against the square root of the contact time (t1/2) should yield a straight line

where t is time (s), C0 is the initial concentration of reactant (M); Ct is the reactant concentration at time t (M); and k is the reaction rate constant (in units corresponding to reaction order). The experimental data were plotted versus time in coordinates corresponding to the above kinetics models (see

Figure 9. Logarithms of residual concentrations of paraoxon versus extract/catalyst contact time. Initial paraoxon concentration: C0 = 2.45 × 10−5 M. Catalyst/paraoxon mass ratio = 3030/1. 1411

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the contact is ∼5−6 h. After a longer contact, the residual concentrations are increased by more than an order of magnitude. Decontamination of the methanol extract with solidsupported catalysts, the second step of the two-stage method, was determined to be effective. Diazinon was completely destroyed within 30 s of contact with a Pd-based solidsupported catalyst. A Yb-based solid catalyst, designed for the destruction of paraoxon, was somewhat less active. At least 60 min were required to decompose ∼95% of paraoxon in the solution. The lower destruction is explained by a slow transport of OP to the reaction sites.

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ASSOCIATED CONTENT

S Supporting Information *

Figure 10. Amount of paraoxon decomposed by catalyst versus the square root of contact time (t1/2). The trend line corresponds to intraparticle diffusion model (eq 3.8). Initial concentration, C0 = 2.45 × 10−5 M; solid-supported catalyst = 40 mg.

Organophosphorus compounds used in the study, molecular structures, and physical properties. This material is available free of charge via the Internet at http://pubs.acs.org.

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passing through the origin. As seen from Figure 10, the experimental data obtained in our study meet this hypothesis. The intraparticle diffusion model was used by Senthilkumaar32 for study of adsorption of pesticides on carbon from water solution. We applied the model in our study assuming that the catalytic process with a solid-supported catalyst in general consists of consecutive stages, including transport of paraoxon through pores of the catalyst and following catalytic chemical reaction. Thus, the catalytic methanolysis process involving solid-supported catalyst is controlled by diffusion of paraoxon through the pores of silica-supported catalyst. The intraparticle rate constant determined from the slope of the plot (Figure 10) was found to be 1.0 × 10−5 M g−1 s−1/2. Decontamination of diazinon in methanol extracts using the Pd-based solid-supported catalyst was quick and effective. Diazinon was not detectable in any of the 18 samples initially containing 158−250 mg/L of the pesticide after 0.5 min of treatment with the catalyst.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support for this research by the Chemical Biological, Radiological-Nuclear, and Explosives (CBRNE) Research and Technology Initiative (CRTI), Project Charter CRTI-06-0170RD. The authors are grateful to Drs. R. Stan Brown, Alexei A. Neverov, and Mark F. Mohamed (all from Queen’s University) for synthesizing the catalysts and providing helpful recommendations on their application. We are grateful to Sana Louie, a co-op student, for her significant contribution to the experiments.

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REFERENCES

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4. CONCLUSIONS Two-stage decontamination can be used for sensitive equipment contaminated with organophosphorus (OP) compounds. OPs can be effectively removed from sensitive equipment by methanol extraction. The highest extraction efficiency was achieved for diazinon on all materials, except polarizing film. Parathion was the most persistent compound on all tested materials, excluding polarizing film and painted steel. The highest residual amounts of OP were found on high-impact polystyrene (HI-PS) plastic. Correlation between polarity factors of OP and efficiency of extraction from HI-PS was observed. The lowest extraction efficiency was found for parathion, which has a low polarity factor (0.58). A much higher polarity factor of diazinon (2.43) caused the lowest attraction of this OP toward plastic and the highest extraction efficiency. The polarity parameter is thus recommended for predicting of the persistence of OP on polystyrene-based polymer materials. The residual concentrations of OP and the extraction efficiencies depend on the contact time between OP and HIPS plastic. Increasing the duration of parathion-plastic contact from 4 h to 72 h caused a reduction in the average extraction efficiency from 99.5% to 76.0%. Avoiding a long contact between OP and plastic material is recommended for effective removal of OP by methanol extraction. The critical duration of 1412

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