Decontamination of Paraoxon and Parathion on Sensitive Equipment

Aug 18, 2014 - Sensitive equipment materials were decontaminated from organophosphorus compounds paraoxon or parathion by immersion into a catalytic ...
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Decontamination of Paraoxon and Parathion on Sensitive Equipment Materials by Catalytic Methanolysis Vladimir Blinov,* Konstantin Volchek, Wenxing Kuang, Akshay Bhalerao, and Carl E. Brown Environment Canada, 335 River Road, Ottawa, Ontario K1A 0H3, Canada ABSTRACT: Sensitive equipment materials were decontaminated from organophosphorus compounds paraoxon or parathion by immersion into a catalytic reactive solution and by spraying with the same solution. Immersion of contaminated material samples into methanol-based catalytic solutions resulted in an effective decontamination. Greater than 99% decontamination was observed for paraoxon on high-impact polystyrene (HI-PS) over 15 min of reaction time. Under the same process conditions, the decontamination from parathion did not exceed 95%. The catalytic decompositions of paraoxon and parathion followed firstorder reaction kinetics with rate constants of 5.4 × 10−3 and 1.3 × 10−3 s−1, respectively. These values were by an order of a magnitude lower than the respective rate constants reported for homogeneous reactions in a methanol solution. Decontamination by spraying the catalyst on the contaminated surface revealed that multiple applications would be required to overcome a rapid evaporation of methanol from the surface and an associated loss of catalytic activity. a catalytic system containing La3+ ions, which were introduced into methanol solution as triflate or perchlorate salts, accelerated methanolysis of paraoxon by a factor of up to 109. This acceleration was relative to a control reaction with sodium methoxide at near-neutral pH and ambient temperature. The process resulted in a complete decomposition of phosphonate ethers, which were used as simulants of OP CWA, and did not generate toxic byproducts.12−14 Another catalytic system, which comprised an ortho-palladated complex{Pd(dmba)(py)(OTf)} in buffered methanol, was designed to promote the methanolysis of phosphorothioate ethers, such as fenitrothion, diazinon, coumaphos, and dichlorphenthion.15 Kinetic studies revealed that this catalytic system rapidly and effectively destroyed fenitrothion and other PS pesticides in methanol solution with reaction rate constants in the range 1.65 × 10−2 to 6.86 × 10−2 s−1 at optimal pH conditions.15 Furthermore, a heterogeneous catalyst, which contained immobilized orthopalladated complexes attached to solid supports such as amorphous silica gel or macroporous polysterene, effectively decomposed PS pesticides in methanol solutions.16,17 The above-mentioned nonaqueous catalytic systems are far less corrosive than common decontamination formulations based on strong oxidizers or high-alkaline solutions. This feature makes them attractive for use in the decontamination of sensitive equipment. Examples of this equipment are detection and analytical instruments, communication and optical devices, electronics, and personal protection equipment. Although the kinetics and the mechanism of the catalytic degradation of phosphonates and phosphorothioates in methanol solutions have been thoroughly investigated,12−16,18−20 the capability and effectiveness of the newly developed catalytic systems to decontaminate surfaces have never been tested. The

1. INTRODUCTION Chemical nerve agents, such as sarin, soman, and VX, are extremely toxic organophosphorus (OP) compounds that were developed and produced in the past as chemical weapon agents (CWAs). While they have been banned under the Chemical Weapons Convention, significant stockpiles remain in several countries suggesting there is a threat, at least hypothetical, of these agents being stolen and used by terrorist organizations.1−3 In fact, sarin and soman were used during the Iraq− Iran war in 1980, in Iraqi Kurdistan in 1988, in the Tokyo subway chemical attack in 1995, and most recently, during the civil war in Syria in 2013.1−4 OP pesticides are much less toxic than CWAs yet may cause poisoning and, in extreme cases, death. One of the risk factors associated with OP pesticides is their availability. For example, paraoxon, whose toxicity is comparable with that of sarin, is available from several commercial suppliers.5 A deliberate release of OP CWAs, their precursors, or OP pesticides for purposes of terrorism or sabotage in buildings and other urban environments would cause contamination of the affected areas. In the case of semivolatile and low-volatile OP compounds, which may penetrate into building materials and/or reversibly interact with them,6−8 such contamination can last a long time and cause a prolonged health threat. A decontamination of building interiors, including construction materials and valuable sensitive equipment, will therefore be required to reduce the associated risks. A number of technologies are available to decontaminate solid surfaces from OP CWAs.9,10 Most of these are based on chemical decomposition of OP materials via hydrolysis, oxidation, and reaction with α-nucleophiles. The main disadvantages of these chemical decontamination methods are (a) material corrosion due to the use of strong oxidizers and (b) possible incomplete decomposition of CWAs accompanied by the generation of highly toxic byproducts. An alternative method for the destruction of neutral OP (phosphonate) ethers (OP CWAs) was recently developed that is essentially free of the above drawbacks.11 It was reported that Published 2014 by the American Chemical Society

Received: Revised: Accepted: Published: 13856

June 16, 2014 August 11, 2014 August 18, 2014 August 18, 2014 dx.doi.org/10.1021/ie502427y | Ind. Eng. Chem. Res. 2014, 53, 13856−13861

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rotary (100 rpm) shaking table (Gyrotory shaker model G2, New Brunswick Scientific Co., Inc., Edison, NJ, USA) to assess the quantities of residual OP remaining on the materials. The samples were inspected visually. The runoff liquids after the decontamination procedure were also analyzed to determine possible residues of OP, as described in our previous work.17 2.2.4. Decontamination of Paraoxon on HI-PS by Spraying the La-Based Catalytic System. The La-based liquid catalytic system was applied by spraying on HI-PS coupons contaminated with paraoxon. The catalytic formulation (0.2 mL) was sprayed onto the HI-PS coupons (1.2 × 4.5 cm2) placed in Petri dishes. The liquid was injected by 1 mL syringe in a homemade nozzle with 0.5 mm inner diameter outlet and sprayed from a distance of about 5 cm in a nitrogen flow. A volume of 0.2 mL was enough to wet the coupon and form a thin layer of liquid catalytic system. The extraction procedure (Section 2.2.3) was then implemented after 15 min contact with the catalyst. The extraction of paraoxon from the coupon was performed without quenching of the reaction, as the sample surface was visibly dry. The catalytic system was reapplied up to three times in multiplied application tests. Once the previous volume of 0.2 mL catalyst was sprayed on the coupon, the same amount of catalyst was reapplied after an interval of 5 min. The contact time was reduced in multiplied application tests (in comparison with initially selected 15 min) because the surface was found to be visibly dry even after 5 min contact due to a rapid evaporation of methanol. 2.2.5. Analysis of OP. A gas chromatograph equipped with a pulsed flame photometric detector (GC/PFPD) was used for the analysis of paraoxon and parathion in the extracts.17 Analytical results were used to calculate total residual amounts of OP on tested material surfaces. Efficiencies of decontamination were then calculated as a percentage of the decomposed amounts of OP over initially loaded amounts. 2.2.6. Quality Assurance and Quality Control. As in our previous study, quality assurance/quality control measures included calibration of the GC, using triplicate samples, processing blank and control samples, and reproducibility control experiments for different test procedures.17 Standard deviation and relative standard deviation did not exceed 7% for the extraction and decontamination procedure and were less than 12% when solvent exchange was involved.

authors of this paper have previously investigated a two-stage decontamination method involving the above systems.17 The method included an extraction of OP from surfaces with methanol followed by their decomposition in the extract solution using a solid-supported catalyst. In comparison, the present work focused on a direct decontamination of surfaces using liquid catalytic formulations. The objectives of this study were to (1) evaluate the effectiveness of liquid catalytic systems in the decomposition of paraoxon and parathion on sensitive equipment materials, (2) study kinetics of the catalytic decomposition of paraoxon and parathion on surfaces, and (3) evaluate the application of liquid catalytic formulation on surface by spraying. OP compounds paraoxon and parathion, which respectively represented phosphonate and phosphorothioate ethers, were selected in this study. Paraoxon and parathion were used in decontamination studies in methanol solutions in the presence of various selective metal-ion-based catalytic systems.12−16,18−20 In addition to being toxic themselves, paraoxon and parathion are also considered surrogates for chemical warfare agents (e.g., GA, G-B, G-D, and VX),21,22 so that a certain finding of this work may be applicable to decontamination of CWAs.

2. EXPERIMENTAL SECTION 2.1. Materials. Samples (coupons) of materials were fabricated from computer parts, CD/DVD compact discs, and commercial polarizing film. The same materials were used in our previous work.17 Coupons of high-impact polystyrene (HIPS) and polarizing film (PF) had a surface size of 1 × 1 cm2. Samples of painted steel (PS), circuit board (CB), and CD/ DVD discs had a surface area of 2 × 3 cm2. HI-PS samples with a surface area of 1.2 × 4.5 cm2 were used in spray tests. Both the La-based and Pd-based liquid catalytic formulations12−16,18−20 were prepared using procedures and components provided by the developers of the methanol catalytic process at Queen’s University, Kingston, ON, Canada. Further information on test materials can be found in our previous publication.17 2.2. Methods and Procedures. A generic decontamination test procedure included (1) spiking OP compounds onto coupons, (2) exposure of contaminated coupons to catalytic solutions, (3) extraction of residual OP from the coupons, (4) analysis of the extracts by a gas chromatograph (GC), and (5) calculation of OP residues and decontamination efficiency. Test procedures and analytical methods utilized in this work were identical or similar to those used in our recent study of twostage decontamination.17 2.2.1. Spiking Procedure. The coupons were precleaned in hexane for 10 min, allowed to air-dry, and spiked with 3.0−3.2 mg of OP. The contaminated surface area was typically in the range 0.9−1.5 cm2 and was determined by taking photographs of samples and measuring the affected areas.17 The spiked coupons were allowed to sit in covered glass vials or jars at room temperature (21 °C) and relative humidity of 30−45% for 24 h. 2.2.2. Exposure of Coupons to Liquid Catalyst. Two milliliters of liquid catalytic formulations were added to the vials, and the coupons were kept immersed in the catalytic system over time periods ranging from 1 to 20 min. The catalytic reaction was quenched by adding 1 mL of deionized water. Coupons were then removed, rinsed by methanol, and dried under ambient conditions for 45 min. 2.2.3. Extraction of Residual OP. Following drying, the coupons were extracted with 10 mL of hexane for 24 h on a

3. RESULTS AND DISCUSSION 3.1. Overall Effectiveness of Decontamination of Paraoxon and Parathion by Immersion. The overall efficiency of paraoxon decontamination from various sensitive equipment materials by immersion was compared taking into account residual amounts shown in Table 1. Table 1. Paraoxon Residues on Sensitive Equipment Materials after Contact with the La-Based Liquid Catalyst for 10 mina material high-impact polystyrene painted steel circuit board compact disc polarizing film

paraoxon residue (μg) 80.0 4.3 1.5 0.5 0.3

± ± ± ± ±

10.0 1.0 0.8 0.1 0.1

a

Paraoxon/materials contact time 24 h; average of triplicate samples; initial paraoxon loading: 3.0−3.2 mg on each sample.

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Table 2. Residual Masses and Decontamination Efficiencies of Paraoxon and Parathion on HI-PSa paraoxon contact time (min)

residual mass (mg)

1 2 5 6 10 15 20 a

0.951 0.657 0.584 0.395 0.080 0.010 N/D

± ± ± ± ± ±

0.086 0.078 0.107 0.107 0.010 0.002

parathion

decontamination efficiency (%) 74.2 ± 80.4 ± 82.3 ± 88.0 ± 97.6 ± 99.7 ± N/D

1.7 3.0 3.1 3.1 0.3 0.1

residual mass (mg) 0.849 ± 0.600 ± 0.426 ± N/Db 0.267 ± 0.221 ± 0.158 ±

decontamination efficiency (%)

0.118 0.108 0.045 0.012 0.021 0.008

73.4 ± 79.5 ± 86.5 ± N/D 91.3 ± 92.6 ± 94.7 ±

4.0 4.0 1.3 0.5 0.7 0.2

OP/HI-PS contact time 24 h; triplicate samples; initial loadings: 3.0−3.8 mg per sample. bN/D: not determined.

corresponding to the above kinetics models (eqs 1−4). It was found that the data best fit with the integrated first-order rate model (eq 2). Square correlation coefficients were 0.942 for paraoxon and 0.938 for parathion (Figures 1 and 2). The fits

It is clearly seen that HI-PS coupons retained much more residual paraoxon than other tested materials. These results correlate well with findings of our study of the two-step catalytic decontamination.17 A high residual concentration of paraoxon was explained by its penetration into plastic (HI-PS). This resulted in HI-PS plasticizing23−26 and an alteration of HI-PS structure. No residues of paraoxon were found in a runoff liquid. Total recoveries of OP from HI-PS coupons after extraction and solvent exchange were in the range from 88.3% to 101.7%. Recovery values of 70−110% are considered acceptable in analyzing pesticides at concentrations in a range from 0.01 to 1 mg/L.27 Table 2 shows the residual masses of paraoxon and parathion on HI-PS and the corresponding decontamination efficiencies for various contact times with catalytic solutions (La-based for paraoxon and Pd-based for parathion). It is evident that longer contact times resulted in smaller residual masses and higher decontamination efficiencies. 3.2. Kinetics of Decontamination of Paraoxon and Parathion on Plastic. To evaluate the catalytic reaction kinetics, including the reaction order and the rate constant, experimental data were plotted versus time in accordance with integrated rate equations:28 Zero-order:

Ct = C0 − k × t

Figure 1. Concentration of paraoxon Ct vs contact time. Natural logarithms of concentrations correspond to the following: (○) residues after catalytic decontamination; (■) initial loaded amount of paraoxon on HI-PS in the tests; (▲) residual amounts of paraoxon after wiping; and (×) paraoxon residues in control extraction tests. See explanation in Section 3.2.1.

(1)

First-order: ln Ct = ln C0 − k × t

(2)

Second-order: 1 1 = +k×t Ct C0

(3)

Third-order: 1 1 = 2 +k×t 2 Ct C0

(4)

where t is time (s), C0 is the initial concentration of the reactant (M), Ct is the reactant concentration at time t, (M), and k is the reaction rate constant (in units corresponding to reaction order). To apply formal kinetic models, the “hypothetical” residual concentrations of OP were considered in assumption that residues of paraoxon and parathion in HI-PS plastic were a result of a homogeneous catalytic reaction. These concentrations were calculated based on the volume of the catalytic system used (2 mL), the mass of OP spiked onto the surfaces, and the experimentally found residues on HI-PS surfaces. Concentrations were plotted versus time in coordinates

Figure 2. Residual concentrations of parathion vs contact time. Average initial mass of parathion is 3.06 mg; (C0 = 5.27 × 10−3 M; ln C0 = −5.246). Logarithms of concentrations correspond to the following: (○) residues after catalytic decontamination; (■) initial loaded amount of parathion; and (▲) parathion residues in control extraction tests.

with the other rate eqs 1, 3, and 4 were significantly worse. It was therefore concluded that the catalytic destruction of paraoxon and parathion followed first-order kinetics. 13858

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3.2.1. Paraoxon. Logarithms of residual concentrations of paraoxon are plotted in Figure 1 (○ marks) versus duration of contact with the La-based catalytic system. The observed firstorder rate constant (k o 1 ) of paraoxon decomposition determined from the graph slope (Figure 1) was found to be 5.4 × 10−3 s−1. This is only about 4.5% of the rate constant for the homogeneous reaction of paraoxon in methanol solution under similar experimental conditions (1.2 × 10−1 s−1) reported in the literature.11,19 This means that the reaction of catalytic decomposition of paraoxon on the surface of plastic is much slower than the reaction in the homogeneous system (methanol solution). This is likely caused by a slower transport of paraoxon through the surface layer compared to the transport in the bulk liquid. The formation of the surface layer is a result of HI-PS plasticization due to interaction with penetrated paraoxon.17 The value of ln C0 obtained by extrapolation using eq 2 to 0 s was determined to be −5.727. In comparison, the natural logarithm of concentration corresponding to the initially loaded amount (ln C0(i)) was equal to −5.093 (indicated as ■ in Figure 1). One can theorize that the ln C0 value, calculated from eq 2, was related to the mass of paraoxon associated with plastic but not to the initial mass of paraoxon spiked onto the coupon. According to the model proposed in the two-step catalytic decontamination study,17 only a part of the originally spiked paraoxon penetrates into plastic, alternates its structure, and forms bonds with polymer molecules. The other part of paraoxon does not react with the plastic. This unbound liquid paraoxon on the coupon surface is rapidly decomposed by a catalytic methanolysis reaction with a rate constant likely similar to the first-order rate constant of 1.2 × 10−1 s−1, as reported for paraoxon decomposition in a homogeneous system.11,19 It can therefore be theorized that the initially loaded amount of paraoxon, which corresponds to ln C0(i), dropped quickly to the amount associated with plastic, which corresponds to ln C0. The catalytic decomposition of paraoxon on the surface occurred at a much slower rate (ko1 = 5.4 × 10−3 s−1) that was caused by a decreased activity of paraoxon bonded to plastic as well as by a slower transport within the surface layer. In order to verify the above hypothesis, the mass of paraoxon that remained on the surface and did not associate with the plastic was estimated in a separate experiment. Triplicate HI-PS samples were exposed to paraoxon for 24 h, and then, bulk paraoxon was carefully wiped from the surface of each coupon by a small cotton swab wet with methanol. The swabs were then placed into vials filled with 10 mL of methanol for extraction. The masses of paraoxon that were removed from the surface and those remaining on the plastic were determined using the procedures described in Section 2. Test results based on triplicate samples can be summarized as follows: initial mass of paraoxon spiked onto plastic is 3.05 ± 0.04 mg; mass of paraoxon removed by wiping is 1.32 ± 0.17 mg (43.2 ± 5.5%); and mass of paraoxon retained on plastic is 1.83 ± 0.13 mg (60.0 ± 5.2%). On an average, approximately 60% of the initial mass of paraoxon loaded on the plastic surface became bonded to the plastic after 24 h contact while 40% of paraoxon remained on the surface as a bulk liquid. One can see from Figure 1 (▲) that the value of ln C0(r) corresponding to the mass of paraoxon remaining on the surface was equal to −5.705. It is close to the value of ln C0 = −5.727 obtained by extrapolation of experimental data to a zero contact time using eq 2.

Residues of paraoxon on HI-PS after extraction with methanol were determined in control tests under similar conditions. Data plotted in Figure 1 and depicted as × marks revealed that the extraction of paraoxon from the materials was faster than the catalytic decomposition. For example, following a 5 min extraction with methanol, the residues of paraoxon on plastic were only about 5% of the residues after catalytic decomposition under the same test conditions. Thus, the rate of catalytic decomposition of paraoxon on the surface of HI-PS plastic (ko1 = 5.4 × 10−3 s−1) was approximately 1/22 of the rate of the reaction occurred in a bulk methanol solution (k1 = 1.2 × 10−1 s−1).11,19 As already suggested in this paper, this was most likely caused by the interaction of paraoxon with the plastic resulting in a decreased paraoxon activity and a slower, compared to the liquid phase, transport of reactants through the layer of altered plastic. Furthermore, a greater than 97% decomposition of paraoxon on HI-PS plastic can be achieved by immersing the plastic in the La-based catalytic formulation for at least 10 min. We believe that the advantage of immersion in the catalytic solution, as opposed to immersion in just solvent (methanol) is twofold: (a) paraoxon bonded to the surface is decomposed even though at a slower rate; and (b) the extracted paraoxon is rapidly decomposed in the liquid phase so there is no need for an additional treatment of the extract.17 3.2.2. Parathion. Figure 2 shows the results of decomposition of parathion using the Pd-based catalytic formulation. It is found that the reaction of parathion decomposition on HIPS fits first-order reaction kinetics. The rate constant determined from the slope of the ln Ct versus time plot was found to be 1.3 × 10−3 s−1. This value is more than 1/30 lower than the first-order rate constant of 4.2 × 10−2 s−1 reported by Lu et al.15 for decomposition of fenitrothion, an analogue of parathion, in a methanol solution under similar test conditions. Data in Figure 2 also suggest that, as similar to paraoxon, the logarithm of initial concentration of parathion did not fit perfectly to the first-order rate equation. It dropped sharply in the first minute of contact with the catalytic system and then fits to eq 2. It is believed that the same phenomena, that is, bonding to plastic and a slower decomposition of the bonded parathion on surface (compared to that in the bulk solution), are responsible for this kinetic behavior. Table 2 shows that the decomposition of parathion on HI-PS plastic with 94% efficiency can be achieved by catalytic reaction with Pd-based catalytic formulation over a 20 min contact time. The rate of decomposition is about 4 times lower than the rate of decomposition of paraoxon obtained in this study using the La-based catalyst (Figure 1). Extraction of parathion with methanol (in control tests) was faster than the catalytic destruction, but the difference in rates of extraction versus decomposition for parathion was not as great as that for paraoxon (Section 3.2.1). It is believed that parathion, whose molecular structure contains sulfur, can form stronger (than paraoxon) bonds with the plastic material. This reduces the rates of both extraction and decomposition, but the extraction rate is affected to a greater extent. 3.3. Decontamination of Paraoxon on HI-PS by Spraying the La-Based Catalytic System. The residual amounts of paraoxon and calculated decontamination efficiencies obtained after a single and repeated spraying of the catalyst are shown in Table 3. One can see that the average residual masses of paraoxon decreased from 1.371 to 0.195 mg as the number of spraying 13859

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Table 3. Residual Masses of Paraoxon, Decontamination Efficiencies, and “Equivalent Time” in Spraying Decontamination Testsa no. of applications

no. of replicates

1 2 3 1b

5 5 5 3

residue of paraoxon (mg) 1.371 0.807 0.195 0.020

± ± ± ±

0.131 0.087 0.044 0.017

decontamination efficiency (%)

equivalent timec (min)

± ± ± ±

0.8 2.4 6.8 13.8

54.6 73.3 93.5 99.3

4.5 4.9 1.5 0.1

a

Paraoxon/HI-PS contact time: 24 h; plastic/catalyst contact time: 15 min. bCoupons were exposed to the catalyst in a closed dish. cTime required to produce the same residual mass when the sample was immersed in the liquid catalytic system (calculated using data shown in Section 3.2.1).

Figure 3. Relative mass of the catalytic system on HI-PS coupons vs time. Exposed surface area of coupons is 1.2 × 4.5 cm2. Initial mass of catalyst on the surfaces in mg: 73.1 in open container test (■); 72.6 in closed container test (▲).

applications increased from 1 to 3. The corresponding efficiencies of decontamination increased from 54.6% to 93.5%. One of the tests was conducted with a modified procedure when the coupons were kept for 15 min in a closed Petri dish to minimize the evaporation of methanol. In this case, the sample surface remained wet for 15 min after spraying the catalyst. The coupons were kept for an additional 5 min to evaporate methanol, followed by extraction. As seen from Table 3, the residual mass of paraoxon of 0.02 mg was found on the coupon when it was kept in a closed container after a single spray application. Table 3 also shows the calculated “equivalent time”, which would be required to obtain the corresponding residual masses of paraoxon, if decontamination was performed by immersing of samples into the bulk catalytic system (as described in Sections 2.2.2 and 2.2.3). Experimental data presented in Section 3.2.1 was used for the calculations. The equivalent times were in the range 0.8−6.8 min for single, double, and triple spraying application tests. The discrepancy between the calculated equivalent times and the actual contact time of 15 min may be explained by a rapid evaporation of methanol (the main component of the catalytic system) from the open surfaces. The catalytic action requires the presence of solvent; therefore, the loss of solvent would hinder the catalytic activity. Control tests were conducted to estimate the mass of the catalytic system remaining on the surface of HI-PS samples over time, when methanol evaporation occurred. For this purpose, a portion of the catalyst solution was applied to the surface of the HI-PS sample with a syringe and evenly spread by a pipet tip; then, the net weight of applied catalyst was monitored for a period of time from 5 to 15 min. The relative mass of the catalytic solution on the coupon was then calculated by dividing the remaining mass over the initially loaded amount. The relative weight (in percent) of the catalyst solution on the surface of coupons is plotted versus time in Figure 3. One can see that the relative masses of the catalyst solutions were rapidly decreasing with time due to methanol evaporation. In the open container test, less than 50% and 10% of the catalyst solution remained on the surface 3 and 5 min after spraying, respectively. It should be noted that changes in the solution composition due to methanol evaporation may also affect the reaction rate. It was reported that the rate constant of the La-catalyzed methanolysis reaction of paraoxon in the liquid phase is strongly dependent on the pH. For example, changing the pH from 8.9 to either 7.0 or to 10.0 caused a decrease in the rate constant by an order of the magnitude.29 It is thus believed that quick methanol evaporation from a surface in the spray tests caused reduction of reactive catalytic components in the methanolysis catalytic system. Therefore, the reaction rate is

decreased in comparison with the rate of the reaction in immersing tests, in which the composition of the catalytic system is close optimal one, which is recommended by the inventors.29 The mass of the catalyst solution remaining on the surface of the coupons in closed containers decreased with time at a much slower rate, compared to the tests with an open container (Figure 3) due to less evaporation of methanol. Nearly 50% of the initial mass of the catalyst solution still remained on the surface 13 min after the catalyst was applied. When the coupons were kept in closed Petri dishes after the catalyst was sprayed, only 0.02 mg of paraoxon remained after decontamination. It was calculated that 13.8 min would be required to achieve the same level of decontamination if the sample was immersed into the bulk catalyst. The actual duration of spraying test was 15 min that is close to the calculated time. Therefore, methanol evaporation plays a major negative role in decontamination by spraying. Any measures that prevent or minimize evaporation would improve decontamination effectiveness.

4. CONCLUSIONS Toxic OP compounds, such as paraoxon and parathion, can be effectively decomposed on sensitive equipment materials by catalytic methanolysis. Greater than 99% decontamination was observed for HI-PS plastic that had been previously in contact with paraoxon for 24 h. This decontamination was achieved within 15 min using a lanthanum-based catalytic system. For parathion, a decontamination efficiency of almost 95% was observed within 20 min using a palladium-based liquid catalyst. Catalytic decomposition of both paraoxon and parathion on solid HI-PS plastic surfaces followed first-order reaction kinetics. The respective rate constants were found to be 5.4 × 10−3 and 1.3 × 10−3 s−1. The rate of decontamination is affected by interaction of OP with solid plastic material, mainly due to a slower mass transfer within the plastic-bonded OP layer. In view of this, the above rate constants observed on the coupon surface were only 1/22 and 1/32 of the respective rate constants for paraoxon and parathion in the liquid phase. HI-PS could also be decontaminated from paraoxon by spraying the catalytic solution onto the surface; however, a repeated application was required due to a rapid evaporation of methanol from the surface and a loss of catalytic activity.

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Corresponding Author

*E-mail: [email protected]. 13860

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Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support for this study was provided 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. Stanley Brown, Alexei A. Neverov, and Mark F. Mohamed of Queen’s University for synthesizing the catalysts and providing recommendations on their application.

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dx.doi.org/10.1021/ie502427y | Ind. Eng. Chem. Res. 2014, 53, 13856−13861