Transformation of Organophosphorus Pesticides in the Presence of

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Environ. Sci. Technol. 2009, 43, 2335–2340

Transformation of Organophosphorus Pesticides in the Presence of Aqueous Chlorine: Kinetics, Pathways, and Structure-Activity Relationships STEPHEN E. DUIRK,* LISA M. DESETTO,† AND GARY M. DAVIS† U.S. Environmental Protection Agency, Office of Research and Development, National Exposure Research Laboratory, 960 College Station Rd., Athens, Georgia 30605

Received October 9, 2008. Revised manuscript received December 22, 2008. Accepted January 27, 2009.

The fate of organophosphorus (OP) pesticides in the presence of aqueous chlorine was investigated under simulated drinking water treatment conditions. Intrinsic rate coefficients were found for the reaction of hypochlorous acid (kHOCl,OP) and hypochlorite ion (kOCl,OP) for several OP pesticides. The reaction of hypochlorous acid (HOCl) with each OP pesticide was relatively rapid near neutral pH, kHOCl,OP ) 0.86 - 3.56 × 106 M-1h-1. HOCl reacts at the thiophosphate (P ) S) moiety of the OP pesticide resulting in the formation of the corresponding oxon (P ) O), which is more toxic than the parent pesticide. Hypochlorite ion (OCl-) was found not to oxidize OP pesticides but act like a nucleophile accelerating hydrolysis, kOCl,OP ) 37.3-15 910 M-1h-1. Both the kHOCl,OP and the kOCl,OP were found to correlate well with molecular descriptors within each subgroup of the OP pesticide class. A model was developed to predict the transformation of OP pesticides in the presence of aqueous chlorine. With hydrolysis rate coefficients, the transformation of OP pesticides under drinking water treatment conditions was found to be adequately predicted. The structure-activity relationships and model developed here could be used by risk assessors to determine exposure to OP pesticides and their transformation products in potable water.

Introduction The Food Quality Protection Act of 1996 (FQPA) requires that all pesticide chemical residuals in or on food be completely reassessed for all existing tolerances. Drinking water is considered a potential pathway for dietary exposure, but there is reliable monitoring data for only the source water (1). When assessing potential pesticide exposure due to drinking potable water, all potential transformation pathways need to be addressed. Under drinking water treatment conditions, hydrolysis and chemical oxidation are the most relevant transformation pathways for the class of organophosphorus (OP) pesticides (2). The OP pesticides were chosen for their widespread use and measured concentrations in drinking water supplies (3-5). However, there is * Corresponding author phone: (706) 355-8206; fax: (706) 3558202; e-mail: [email protected]. † Student Services Authority. 10.1021/es802868y

Not subject to U.S. Copyright. Publ. 2009 Am. Chem. Soc.

Published on Web 03/09/2009

little quantitative rate coefficient information for either the oxidation or hydrolytic pathway. Chlorination is the most commonly used chemical disinfection process for community water systems (6), and it is known to react with numerous pesticides. For example, four s-triazines were found to degrade in the presence of aqueous chlorine (7, 8). Atrazine was also found to be significantly degraded by ozone (9); however, subsequent chlorination of the ozonated effluent had very little effect on the concentration of residual atrazine or its ozone degradation products (10). Also, some carbamate pesticides have been shown to react with chlorine while other members of this pesticide class were found to be stable in chlorinated water. For example, carbaryl and propoxur do not react with chlorine; but aldicarb, methomyl, and thiobencarb do exhibit significant reactivity (11-13). These findings demonstrate that chlorine reactivity with different members in a specific class of pesticides can vary significantly due to chemical structure variations. Therefore, it is prudent to study the fate and transformation pathways of entire chemical classes, using class members that exhibit systematic structural variations and employing carefully selected experimentation and numerical modeling. When chlorine reacts with the phosphorothioate and phosphorodithioate subgroups of the organophosphorus (OP) pesticide class, the thiophosphate functionality (P ) S) can be oxidized to its corresponding oxon (P ) O) (14-16). The resulting oxons are typically more potent than the parent as an inhibitor of acetlycholinesterase, an enzyme necessary for regulating nerve impulse transmission (16). Duirk and Collette (2), elucidated the fate of chlorpyrifos (CP) and its transformation products over the pH range of 6-11. They were able to model the loss of CP and chlorpyrifos oxon (CPO) to the stable end-product of 3,5,6-trichloro-2-pyridinol (TCP) over this pH range in buffered deionized water systems, as well as in the presence of naturally occurring aqueous constituents such as bromide and natural organic matter (NOM) (17). The purpose of this study was to further elucidate the kinetics and transformation pathways of OP pesticides, as a class, in the presence of aqueous chlorine. The additional OP pesticides chosen for this study contained the thiophosphate moiety from both the phosphorothioate subgroup (chlorethoxyfos (CE), chlorpyrifos (CP), diazinon (DZ), parathion (PA), and tebupirimfos (TE)) and the phosphorodithioate subgroup (malathion (MA), methidathion (ME), and phosmet (PM)) (Figure 1). Neutral and alkaline hydrolysis rate coefficients were determined for CE, ME, PM, and TE, since these hydrolysis rates coefficients could not be obtained from literature. Since all these OP pesticides can form oxons in the presence of chlorine, reaction rate coefficients for both hypochlorous acid (HOCl) and hypochlorite (OCl-) were found for both oxidants.

Experimental Procedures Experimental procedures can be found in Supporting Information (SI).

Results and Discussion Hydrolysis of Select OP Pesticides. Discussion of OP pesticide hydrolysis can be found in the SI. Neutral (kN,OP) and alkaline (kB,OP) hydrolysis rate coefficients can be found in Table 1. OP Pesticide Reaction Order with Aqueous Chlorine. In the presence of aqueous chlorine, the oxidation of CP was VOL. 43, NO. 7, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 2335

FIGURE 1. Structures of eight OP pesticides from both the phosphorothioate and phosphorodithioate subgroups. found to be first order resulting in an overall second order reaction (2). Therefore, observed loss of each OP pesticide in the presence of aqueous chlorine was assumed to be first order with respect to the OP pesticide. If ln([OP]/[OP]o) versus time (t) plots are linear, then this assumption would be valid when there is a molar excess of chlorine. The observed firstorder rate coefficients (kobs) for all the OP pesticides were determined from these plots via the slope of the regression line as shown in the following expression. ln

[OP] ) -kobst [OP]o

(1)

Under pseudo first-order chlorination conditions, all the OP pesticides exhibited a first order dependency with respect to the OP pesticide in the presence of excess aqueous chlorine at pH 6.5 (SI Figures S5-11). The reaction order of the aqueous chlorine reacting with each OP pesticide was determined by plotting the log of kobs versus the log of the initial chlorine concentration at pH 6.5 (SI Figure S12). Since the slope of the regression line is approximately 1 for all eight pesticides, this indicates that the loss of OP pesticides can be described as a second-order reaction. The apparent loss of each OP pesticide in the presence of aqueous chlorine at a specific pH could then be described by the following rate expression where kapp is the apparent second-order rate coefficient at a specific pH (eq 2). d[OP] ) -kapp[HOCl]T[OP] dt

(2)

The observed first-order rate coefficients at each pH were then assumed to linearly increase with increasing chlorine concentration (equation 3). The kapp for each OP pesticide was determined by plotting kobs versus the initial total aqueous chlorine concentration. kobs ) kapp[HOCl]T

(3)

Figure 2 shows that kobs increased linearly with increasing chlorine concentration at pH of 6.5. Since approximately 91% of the active chlorine is in the HOCl form, the apparent 2336

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rate coefficient will be very close to the intrinsic rate coefficient of hypochlorous acid reacting with each OP pesticide. The range of kapp,OP was found to be 0.59 × 106 3.08 × 106 M-1h-1 for CE to DZ respectively. As pH increased from 6.5 to 9, kobs decreased for all the OP pesticides (Figure 3). When the pH of the aqueous system shifts from neutral to alkaline pH, the chlorine species also shifts from HOCl to OCl-, pKa of 7.5 (18). The decrease in the kobs would be expected if hypochlorous acid is significantly more reactive than OCl-, which has been previously observed with OP pesticides and other anthropogenic chemicals (2, 19). The transformation of OP pesticides at alkaline pH in the presence of chlorine can become relatively complicated. CP and its corresponding oxon, CPO, were found to have several transformation pathways occurring simultaneously (i.e., oxidation, chlorine-assisted hydrolysis, and alkaline hydrolysis) at alkaline pH (2, 17). Since only CP and DZ have been thoroughly studied (2, 14, 20), the other OP pesticides were investigated at alkaline pH in the presence of chlorine. At pH 9, approximately 97% of the active chlorine will be in the hypochlorite form resulting in a slower observed transformation rate (Figure 3). All the OP pesticides degraded rapidly in the presence of aqueous chlorine (SI Figure S13). Just like at pH 6.5, the observed rate coefficients increased linearly with increasing chlorine concentration. The kapp,OP at pH 9 ranged from 0.78 × 104 to 8.8 9 × 104 M-1h-1 for CE to PM respectively, which is 2 orders of magnitude slower than pH 6.5. PM degraded faster than all the OP pesticides, including DZ, due to PM being more susceptible to alkaline hydrolysis (Table 1). Chlorine-Assisted Hydrolysis of OP Oxons. Chlorineassisted hydrolysis was first observed by Edwards et al. (21), investigating the factors that determine the reactivity of nucleophiles, which are basicity, polarizability, and the presence of unshared pairs of electrons on the adjacent atom to the nucleophilic atom (i.e., the alpha effect). The three lone-pairs of electrons on both the chlorine and oxygen atom enhance hypochlorite’s nucleophilicity toward specific moieties such as a tetrahedral phosphorus (i.e., phosphoesters) and carbonyls (21, 22). At pH 9, diazoxon (DZO), malaoxon (MAO), and paraoxon (PAO) loss was observed in the presence of aqueous chlorine (Figure 4). With subgroup differences aside, the loss of each oxon increased linearly with increasing chlorine concentration. DZO and PAO were significantly more stable than MAO at pH 9 because phosphorodithioates are more susceptible to alkaline hydrolysis (23). The rate coefficients for chlorine-assisted hydrolysis (kOCl,OPO) for the four oxons were found to only be slightly larger than their corresponding parents (Table 1). This was first reported with CP and CPO in the presence of chlorine and bromine (2, 17). This is due to the structure of the nucleophile, OCl-, and not minor structural differences between phosphorothioate and phosphate analogs (22). Since tetrahedral phosphorus moieties are susceptible to SN2 attack by OCl- (21), the difference in the partial positive charge at the tetrahedral phosphorus atom between (P ) S) and (P ) O) does not significantly influence the rate of nucleophilic attack. OP Pesticide Degradation Pathway Model. Degradation pathway models have been shown to be effective tools for determining rate coefficients and predicting transformation product speciation in complex systems. Previously, a degradation pathway model for CP was developed in order to determine the intrinsic rate coefficients for both HOCl and OCl- (2). With comprehensive mass balances over the pH range of 6.5-9, the reaction of HOCl with CP resulted in CPO formation. OCl- was found to accelerate the hydrolysis of CPO. Using the OP-chlorine degradation pathway model and mass balances, the different chlorine species resulted in different transformation products. HOCl oxidized CP to CPO

TABLE 1. Chlorination and Hydrolysis Rate Coefficients for Eight OP Pesticides and Four Oxon Transformation Productsa kN,OP(h-1)

OP pesticide CE CP CPO DZ DZO MA MAO ME PA PAO PM TE

4.68((0.69) × 10 d 3.72 × 10-4 e 2.13 × 10-3 f 1.56 × 10-4 f 9.99 × 10-4 b,g 7.92 × 10-5

kB,OP(M-1h-1) -4

1.42((1.01) × 10-3 2.66 × 10-4 f 2.00 × 10-4 c 5.55((0.11) × 10-4 1.10((0.0.01) × 10-3 b f

kHOCl,OP(M-1h-1)

1.25((0.30) × 10 37.1 230.2 f 18.9 f 165.6 g 1.98 × 103

3

3.56((0.65) × 106 1.72((0.36) × 106

2.22((0.04) × 102 f 4.3 f 46.1 2.73((0.08) × 105 6.0((0.1)

a

0.86((0.18) × 10 e 1.72 × 106

kOCl,OP or kOCl,OPO(M-1h-1) 6

1.89((0.12) × 106 2.20((0.53) × 106 2.84((0.80) × 106 1.76((0.43) × 106

15,900 ( 2100 e 990 e 1340 627 ( 30 914.1 ( 54.2 382 ( 26 565 ( 99 252 ( 47 37 ( 10 48 ( 10 1000 ( 100 71 ( 13

95% confidence intervals shown in parentheses. b MA hydrolysis rate coefficient at pH 4. coefficient at pH 2. d From ref 29. e From ref 2. f From ref 23. g From ref 30.

FIGURE 2. Observed first-order rate of loss for eight OP pesticides versus chlorine concentration at pH 6.5. [OP]o ) 0.5 µM, [PO4]T ) 10 mM, Temperature ) 25 ( 1 °C, and [HOCl]T ) 0-100 µM. Error bars represent 95% confidence intervals. CP data from Duirk and Collette (2).

c

PM hydrolysis rate

FIGURE 4. Second-order rate coefficients of three OP pesticide oxons at pH 9. [OP]o ) 0.5 µM, [CO3]T ) 10 mM, temperature ) 25 ( 1 °C, and [HOCl]T ) 0-200 µM. Error bars represent 95% confidence intervals for both data points and regression lines. following equations, kOCl,OPO is the intrinsic rate coefficient for hypochlorite assisting in the hydrolysis of the OPO (oxon transformation product), and kh,OP and kh,OPO are the hydrolysis rate coefficients for both OP and OPO, and OPH represents the hydrolysis product. d[HOCl]T ) -5kHOCl,OP[HOCl][OP] - kOCl,OP[OCl-][OP] dt kOCl,OPO[OCl-][OPO] (4) d[OP] ) -kHOCl,OP[HOCl][OP] - kh,OP[OP] dt kOCl,OP[OCl-][OP] (5) d[OPO] ) kHOCl,OP[HOCl][OP] - kh,OPO[OPO] dt kOCl,OPO[OCl-][OPO] (6)

FIGURE 3. The pH dependency of kobs for the loss of the OP pesticides in the presence of aqueous chlorine. [OP]o ) 0.5 µM, [HOCl]T ) 25 µM, [buffer]T ) 10 mM, and temperature ) 25 °C. Error bars represent 95% confidence intervals. CP data from Duirk and Collette (2). and OCl- acted like a nucleophile accelerating CP hydrolysis resulting in 3,5,6-trichloropyridinol (TCP) (2). Knowing the pH dependency on the observed rate of OP loss is due to chlorine speciation, pooled data from pH 6.5-9 will be used to parametrize the intrinsic rate coefficients for HOCl (kHOCl,OP) and OCl- (kOCl,OP) reacting with each OP pesticide using the following system of ordinary differential equations. In the

d[OPH] ) kh,OP[OP] + kOCl,OP[OCl-][OP] + kh,OPO[OPO] + dt kOCl,OPO[OCl-][OPO] (7) The rate equations were written based on the stoichiometric equations in Table 2. The stoichiometric coefficient 5 in equation 1 of Table 2 assumes that for the loss of one mole of each OP pesticide there is a subsequent loss of 5 moles of chlorine. One mole of chlorine oxidizes the OP pesticide to the corresponding oxon (P ) O) and 4 more moles react with sulfide, released from the parent OP pesticide (16), resulting in the formation of sulfate (SO42-), which has been VOL. 43, NO. 7, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 2. Stoichiometric Equations Used in the OP-Chlorine Pesticide Transformation Pathway Model reaction stoichiometry kHHOC ,OP

5HOCl + OP 98 OPO + 5H+ + 5Cl- + SO42l

1

kh,OP

2

OP 98 OPH

3

OPO 98 OPH

4

OP + OCl- 98 OPH

5

OP + OCl- 98 OPH

6

kh,OPO

kOC ,OP l

kOC ,OP l

HOCl a H+ + OCl-

verified when investigating chlorination of DZ (14). Although the mechanism and reactivity of aqueous chlorine with reduced sulfur species is not fully known, the reaction of HOCl and OCl- with sulfite is very rapid, kHOCl,S ) 2.74 × 1012 M-1h-1 and kOCl,S ) 8.28 × 107 M-1h-1, respectively (24). This agrees with our experimental results (SI Table S1), which an average of 4.8 moles of chlorine were consumed for each mole of OP pesticide. Therefore, the stoichiometric coefficient 5 was used in the model to account for the loss of chlorine and allow for more accurate determination of intrinsic rate coefficients through mass balances and nonlinear regression analysis of pooled data sets for each OP pesticide. The model was then used to determine kHOCl,OP and kOCl,OP for the OP pesticides not previously investigated. Table 1 shows the intrinsic rate coefficients for both chlorine species, the intrinsic rate coefficient for chlorine-assisted hydrolysis for four oxons, and the neutral and alkaline hydrolysis rate coefficients, which were discussed in the SI. The kHOCl,OP were found to be very similar to the second-order apparent rate coefficients at pH 6.5 and were generally 2-4 orders of magnitude greater than kOCl,OP with the exception of CE, which appears to be very susceptible to nucleophilic attack. The kOCl,OP for the phosphorothioate subgroup was found to be an order of magnitude faster than kB,OP. However, OCl- does not appear to increase the rate of hydrolysis as significantly for the phosphorodithioate subgroup. This could be due to the fact that this subgroup is generally found to be extremely susceptible to nucleophilic attack from hydroxide ion. To verify that HOCl is the only chlorine species that results in oxon formation, the transformation of PA to PAO was modeled at pH 9.0 (Figure 5). Since alkaline hydrolysis and reaction with OCl- was found to relatively slow compared to oxidation, PAO should be very stable at pH 9. Although only 3% of the active chlorine was in the HOCl form, the slow transformation of PA to PAO was adequately predicted. This indicates that HOCl is the only chlorine species that oxidizes OP pesticides to the corresponding oxons. Intrinsic rate coefficients for DZ and CP have been previously reported (14, 20). Zhang and Pehkonen (14), found intrinsic rate coefficients for DZ with aqueous chlorine: kHOCl,DZ ) 4.7 × 105 M-1h-1 and kOCl,DZ ) 972 M-1h-1. Although the hypochlorite intrinsic rate coefficients were similar, HOCl rate coefficients were found to be an order of magnitude different. The difference can be explain because kHOCl,DZ was approximated from data gathered over the pH range of 9.5-11, and it was thought that both HOCl and OCl- resulted in oxon formation. Since parallel reaction pathways can occur at any pH, the most accurate method to parametrize reaction rate coefficients of OP pesticides in the presence of chlorine would be to use a comprehensive transformation pathway model and data sets relevant to drinking water treatment conditions. Acero et al. (20), reported intrinsic rate coefficients 2338

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rate/equilibrium coefficient (25 °C)

kHOCl,OP ) Table 1

reference

this work

kh,OP ) kN,OP + kB,OP[OH-] kN,OP ) Table 1 kB,OP ) Table 1 kh,OPO ) kN,OPO + kB,OPO[OH-] kN,OPO ) Table 1 kOCl,OP ) Table 1

this work

kOCl,OPO ) Table 1

this work

pKa ) 7.5 (HOCl/OCl-)

(18)

for both CP and DZ: kHOCl,CP ) 5.2 × 105 M-1h-1 and kHOCl,DZ ) 9.5 × 105 M-1h-1, which are also an order of magnitude less than the what has been presented in Table 1. The difference is due to the incorporation of an acid catalyzed reaction (i.e., H2OCl+), which has been used explain the oxidation of anthropogenic chemicals below neutral pH (25). Raman and UV spectroscopy have been employed to verify the existence of the H2OCl+ species in chlorine solutions that contained equal molar concentrations of chlorine and chloride and a solution where the active chlorine to chloride molar ratio was 11.6:1 (26). Aqueous diatomic chlorine, Cl2(aq), and HOCl were the only chlorine species identified below neutral pH. When trying to model the reaction of trimethoprim below neutral pH (19), it was concluded that Cl2(aq) is more likely responsible for the increase in the observed loss of trimethoprim below pH 6. Therefore, the intrinsic rate coefficients for CP and DZ calculated by Acero et al. (20), would have been underestimated due to incorporating the high reactive low pH chlorine species, H2OCl+. Frontier molecular orbital theory has been used to correlate oxidation rate coefficients with the easily calculated energy of highest occupied molecular orbital (EHOMO) (27). EHOMO was initially thought to be able to correlate all eight OP pesticides; however, subgroup differences were quickly unveiled (Figure 6). The phosphorodithioate subgroup was found to have a slightly higher potential than the phosphorothioate subgroup. This is most likely due to the sulfur linkage and the methyl esters at the tetrahedral phosphorus atom allowing the phosphorodithioates examined to be more

FIGURE 5. PA transformation to PAO in the presence of chlorine at pH 9.0. [PA]o ) 0.47 µM, [HOCl]T ) 10-50 µM, [CO3]T ) 10 mM, and temperature ) 25 ( 1 °C. Error bars represent 95% confidence intervals. Lines represent model results.

FIGURE 6. Relationship between kHOCl,OP with EHOMO as a function of OP pesticide subgroup. Error bars about the regression line represent 95% confidence intervals. CP data from Duirk and Collette (2).

FIGURE 8. Experimental and model results for DZ loss in the presence of chlorine at pH 7.0. [DZ]o ) 0.6 µM, [HOCl]T ) 20 µM, [PO4]T ) 10 mM, and temperature ) 25 ( 1 °C. Error bars represent 95% confidence intervals. Lines represent model results. the presence of aqueous chlorine can be predicted under drinking water conditions. Diazinon (DZ) and parathion (PA) were chosen because their corresponding oxons and hydrolysis products are commercially available. At a pH and chlorine concentration typical of drinking water treatment, DZ was rapidly transformed to diazoxon (DZO) (Figure 8). DZO has been found to be stable in the presence of chlorine for over 48 h at neutral pH (15). At pH 8, PA was also rapidly transformed to paraoxon (PAO) (SI Figure S14). PAO was also found to be relatively stable at alkaline pH in the presence of chlorine (Figures 4 and 5). Since exposure assessments account for concentrations of anthropogenic chemicals and transformation products in the plant effluent, regulators could potentially use this model to access potential exposure to OP pesticides and their more toxic oxon products.

FIGURE 7. Relationship between kOCl,OP with alkaline hydrolysis rate coefficient (kB,OP) as a function of OP pesticide subgroup. Error bars about the regression line represent 95% confidence intervals. CP data from Duirk and Collette (2). easily oxidized by chlorine than the phosphorothioate subgroup, which primarily had ethyl and phenyl esters. EHOMO was found to be a good molecular descriptor describing the oxidation of OP pesticides within each subgroup. Other molecular descriptors were needed in order to correlate the reactivity of hypochlorite with each OP pesticide. Cross correlations use rate coefficients as molecular descriptors with a well understood reaction mechanism (i.e., SN2 alkaline hydrolysis) and infer a mechanistically similar reaction with a different reactant (i.e., chlorine-assisted hydrolysis) (28). Therefore, correlating kOCl,OP with alkaline hydrolysis rate coefficients for all eight OP pesticides would then confirm that OCl- acts as a nucleophile accelerating OP pesticide hydrolysis. Subgroup differences were evident as the phosphorothioates generally hydrolyze slower at alkaline pH than the phosphorodithioates (Figure 7); however, chlorine appeared to have a greater effect accelerating phosphorothioate hydrolysis as indicated by the slope of the regression line being an order of magnitude greater than the slope for the phosphorodithioates. Even with the differences between the two OP pesticide subgroups, this cross correlation appears to validate that OCl- acts like a nucleophile attacking the tetrahedral phosphorus atom accelerating the hydrolysis of OP pesticides. With the available rate coefficients determined here and in cited literature, OP pesticide transformation pathways in

Acknowledgments We thank Jimmy Avants, Dane Primerano, and Lidia Samarkina for their technical assistance, Paul Winget for calculating EHOMO energies, and Wayne Garrison and Jackson Ellington for their consultation and expertise. Disclaimer: Although this work was reviewed by the U. S. Environmental Protection Agency and approved for publication, it may not necessarily reflect official Agency policy.

Supporting Information Available Hydrolysis discussion and figures, as well as additional figures discussed in this article. This material is available free of charge via the Internet at http://pubs.acs.org.

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