Chemical Hydrolysis and Oxidation of Parathion and Paraoxon in

10 Chemical Hydrolysis and Oxidation of Parathion and Paraoxon in Aquatic

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Environments HASSAN M . G O M A A and S A M U E L D . F A U S T Research and Development Division, Keramchemie (Canada) Ltd., Don Mills, Ontario, Canada, and Department of Environmental Sciences, Rutgers, The State University, New Brunswick, N. J. 08903

The kinetics of oxidation of parathion by Cl , ClO , and KMnO was evaluated. Several products were detected: paraoxon, p-nitrophenol, 2,4-dinitrophenol, 2-hydroxy-5nitrobenzoic acid, and 2,2'-dihydroxy-5,5'-dinitrodiphenyl. Paraoxon and 2,4-dinitrophenol are significant because of their toxic properties. KMnO was the oxidant for parathion. Preadjustment of the pH to alkaline values was essential to increase the rate of the reaction and to prevent accumulation of paraoxon. p-Nitrophenol is the major product of parathion—KMnO reaction at pH 9.0. Oxidation of parathion by KMnO under neutral or acidic conditions produces paraoxon and not p-nitrophenol. Large dosages of Cl or ClO are required for the oxidation of parathion at a pH value of 7.4 with accumulation of paraoxon. The efficiencies of Cl and ClO was increased by raising the pH from 7.4 to 9.0 and was decreased by lowering the pH from 7.4 to 3.1. 2

2

4

4

4

4

2

2

2

2

/ ^ r g a n i c pesticides have been found in drinking, recreational, irrigational, fish, and shellfish waters and in the attendant sediments and bottom muds since 1945. The early evidence for this overall distribution was mainly obtained from physiological responses of aquatic organisms. From 1961 to date, however, direct analyses by gas-liquid chromatography and other techniques indicate that most natural waters and their equilibrium solid phases contain trace amounts of organic pesticides ( I , 2, 3, 4, 5, 6,7,8,9). Since pesticides enter water resources regardless of 189 Faust; Fate of Organic Pesticides in the Aquatic Environment Advances in Chemistry; American Chemical Society: Washington, DC, 1972.


190

FATE OF ORGANIC PESTICIDES

precautions in their use, public health and water treatment officials must have the knowledge and the plant facilities to remove them. Concentrations of these chemicals in public water supplies should conform to the appropriate quality criteria (10). Technology for removing organic pesticides from water has not paced the problems that have resulted from the increased quantities used, new products and formulations placed on the markets, and new sources for their entry into the environment. Some organic pesticides are resistant or unaffected by conventional water and wastewater treatment practices (11). Also, many organic pesticides resist biological degradation i n aquatic environments, hence they may persist for some time (7, 12, 13, 14). Methods to remove trace organic compounds of all types from aqueous solution (15, 16, 17) have been studied extensively. One was to deal with pesticide pollution of water supplies by using an oxidant for chemical degradation. Among the many chemical oxidants available, certain ones have shown that they reduce the concentration of organic contaminants—e.g., ozone, chlorine dioxide, chlorine, the peroxides, and potassium permanganate (18, 19, 20, 21). Under certain environmental conditions, different organic compounds w i l l react differently to any particular chemical oxidant. No general statement is made concerning the oxidation of organic materials by a given oxidant. The interaction of environmental conditions, types and concentrations of organic compounds, and reaction time drastically affects the efficiency of chemical oxidation processes. Kinetic studies may be used to evaluate the factors affecting the efficiency of removing trace organic pesticides from water by chemical oxidants. The rates of oxidative reactions and the optimum conditions under which the reactions occur are determined in the laboratory before applying them to pilot plant or field studies. Kinetic data also compares the efficiencies of different oxidants with the economy of different treatment processes. Kinetic studies are also required to define more clearly the mechanisms inherent in the chemical reactions and the end products inherent in utilizing a specific chemical oxidant under different environmental conditions with various types of organic pesticides. The rates of accumulating and degrading any toxic intermediate oxidation product(s), formed during oxidation of the parent compound, are evaluated to determine the optimum conditions of removal. Organophosphorus Insecticides vs. Chlorinated Hydrocarbons Two general classes of insecticides, chlorinated hydrocarbons and organic phosphorus compounds, are used. The former have been recog-

Faust; Fate of Organic Pesticides in the Aquatic Environment Advances in Chemistry; American Chemical Society: Washington, DC, 1972.


10.

G O M A A

A N D

FAUST

Parathion and Paraoxon

191

nized as having high residual activity and have often been blamed for fish mortality and other deteriorations of wildlife activity (3, 22, 23, 24). Recent concern in the United States is leading to decreased use of persistent chlorinated hydrocarbons—e.g., D D T . The organic phosphorus insecticides are usually believed to have a low residual life in aqueous environments and are not particularly regarded as water contaminants. However, hydrolysis data (25, 26) indicate the stability and persistence of parathion and Diazinon. These compounds under natural water conditions are characterized by residual lives that vary between 3-6 months. During the last decade parathion has been the most used organophosphorus insecticide. It has been proved to be valuable in crop protection (27). However, using this compound so much has also resulted in numerous accidental intoxications, and many have been lethal (28). In aquatic environments parathion hydrolyzes to yield p-nitrophenol or oxidizes to yield paraoxon (25, 26). Baker (29) has shown that substituted phenols affect the odor quality of drinking water. p-Nitrophenol may be chlorinated at a water treatment plant to produce an odorous product. The U . S. Public Health Service has adopted 1 ^g/liter as a limit for phenolic compounds in water (10). Paraoxon is more toxic to insects and mammals than the parent compound parathion (27). The lethal dose ( L D ) for male white rats is 14 m g / k g for parathion while that determined for paraoxon is only 3 m g / k g (30). Bioassay studies with fathead minnows indicated a Median Tolerance Limit ( T L ) (96 hours) for parathion of 1.4 mg/liter and 0.3 mg/liter for paraoxon. 5 0

m

To evaluate the overall oxidation reaction of parathion, the kinetics of chemical hydrolysis of parathion and paraoxon first had to be investigated. Then the kinetics of chemical oxidation of p-nitrophenol, paraoxon, and parathion were studied with K M n 0 . This oxidant was chosen partially because of its accepted use to reduce tastes and odors caused by organic compounds (31, 32, 33, 34); potassium permanganate also is easily applied, and its reduction products are filtered from the finished water. Chlorine and chlorine dioxide were tested for their efficiencies in oxidizing parathion and paraoxon. 4

Chemical

Hydrolysis

of Parathion

and

Paraoxon

Experimental Procedure. Hydrolysis reactions were conducted in a thermostatically controlled water bath ( ± 0 . 2 ° C ) in diffused light. Into a one liter volumetric flask, an aliquot of the insecticide stock solution was pipetted. The solution was reduced to near dryness with a gentle stream of N gas. Water (900 m l ) , distilled twice, was added to the flask and placed on a shaker for at least one hour to dissolve the insecticide completely. Into another flask, exactly 100 m l of the orthophosphate 2



192


buffer solution ( 0.2M ) was added. Both flasks were placed i n the water bath to reach temperature equilibrium, after which the buffer solution was mixed with the insecticide solution. A stop watch was started at the initial pouring. A t various time intervals, aliquots of the reaction solution were withdrawn to determine residual insecticide concentrations. These aliquots were immediately poured into a 250-ml separator to which was previously added enough acid ( 5N H3PO4 ) to adjust the p H to 1.0-1.5 before solvent extraction. This separator also contained a desired volume of ethyl acetate for extraction of the residual insecticide (1:1 solution-solvent). After 5 minutes of shaking, the lower aqueous layer was removed. The solvent layer was passed through a two-inch column containing approximately 5 grams of granular anhydrous sodium sulfate into a 50-ml volumetric flask. The separator and the column were washed three times with 2 m l of solvent. These washings were added to the eluate. The final volume of the extract was adjusted to 50 m l . A Microtek MT-200 gas chromatograph was used with a 10-mc N i electroncapture detector at 250°C. Operating and chromatographic conditions are as follows: 1. L i q u i d phase = 1.5% silicone ( G E , X E - 6 0 ) plus 1.5% silicone ( D C , QF-1) 2. Solid support = gas-chrom Q, 80-100 mesh 3. Inlet temperature — 200°C 4. Column aging = 4 days at 240°C under normal flow rate. 5. Column temperature = 185 °C 6. Column dimension = 4-feet glass, y4-inch O.D., 40-mm I.D. 7. Detector temperature = 200 °C 8. F l o w rate of carrier gas ( N ) = 65 m l per minute 9. Recorder speed = 0.3 inch per minute Figure 1 shows the isothermal separation of parathion and paraoxon. The insecticide concentration was calculated in each sample in the hydrolysis series from standard curve of peak area vs. concentration. Each experiment was repeated three times. Results. The rates of hydrolysis of parathion and paraoxon were dependent upon p H : 6 3

2

S(O)

IIÔC H 2

\

/

5

[

3 0] +

H

ÔC.Hs

Parathion or Paraoxon S(O) O N-(' a

x

)-OH

+

[I ^ O C H H O — P ^ ÔC^He 2

5

p-Nitrophenol


(1)

10.

G O M A A

A N D FAUST


193

50

PARAOXON

(0.008*g)

40 PARATHION

(0.005*g)

A

Ο

RESPONSE


30

10

; \J \

0

0

1

1

5

10

I

Λ

1

1

15

20

v

.

T I M E - Minutes

Figure 1.

Isothermal separation of parathion and paraoxon

B y the law of mass action, the velocity depends on the concentration of the two reactants: ^ —

=

^Wns^Cat

\

Δ

)

where, C i . is the residual concentration of insecticide (mole/liter) at time, t. Ccat. is the concentration of catalyst—i.e., either H 0 or O H " in solution (mole/liter). Since the amount of catalyst used i n the reac tion is negligible, its concentration is constant. The velocity equation then takes the form: n s

+

3

T7— =-ô&Îns

\°)

dt so that the rate is proportional to the residual concentration of the insecti cide. Thus, the process should be first-order kinetics which was experi mentally verified. Integrating Equation 3 yields: logC .= In

h g C °

l

m

- - ^ t

(4)

where, C ° . is the initial concentration of insecticide (mole/liter). K is the observed rate constant ( t' ) and t is the time ( hours ). Table I shows the data obtained upon hydrolyzing parathion at five p H values. The calculated first-order rate constants (Equation 4) at Ius

ob

1


194


Table I.


Sampling Time (hours)

Effect of p H on the Rate

pHS.l

(V

0 2 4 6 8 12 24 48 96 144 192 240 480

K

11.50

ρΗδ.Ο o b

χ

c

io-*'

—

11.41 11.31 11.18 11.14 11.07 10.66

K

t

χ ίο-*

o b

11.50 —

1.94 1.90 1.86 1.92 1.84 1.80

11.39 11.30 11.20 11.09 11.00 10.55

1.96 1.72 1.64 1.68 1.60 1.59

° Average value of three experimental runs at the indicated p H . Ct is the parathion concentration at time t (mg/liter). h

each sampling time at the indicated p H values are nearly constant. Since first-order kinetics are observed, relative rates are represented by the half life—i.e., the time in which one-half of the original concentration has been hydrolyzed. Table II shows the hydrolysis rate constants and half-lives of parathion and paraoxon at five p H values and at 20°C. The hydrolysis of parathion proceeds slightly faster under acidic or neutral conditions of p H . The reverse is observed under alkaline condi tions where the hydrolysis of paraoxon is approximately 7.5 times faster than parathion at p H 9.0 and 5.5 times faster at p H 10.4. The reaction order was confirmed by repeating the reaction at p H values of 3.1 and 10.4 using three insecticide concentrations. Equation 4

Table II.

Rate Constants of Hydrolysis of Parathion and Paraoxon at 20°C (25) Parathion*

pH

K

3.1 5.0 7.4 9.0 10.4

1.65 1.88 2.66 1.32 2.08

o b

(hrr )

b

K

ti/2 (hrs.)

1

Χ Χ Χ χ Χ

Paraoxon

10" 10" 10" 10" 10"

4 4

4 3

2

4182 3670 2594 523 33.2

o b

1.46 1.66 2.00 9.87 1.15

(hrr )

U/2 (hrs.)

1

Χ Χ Χ Χ Χ

10" 10" 10" 10" 10"

4 4 4 3

1

4726 4156 3450 69.9 6.0

° Initial parathion concentration was 3.95 Χ 10" Af. Initial paraoxon concentration was 4.81 X 10" Af. 5

b

5


10.

G O M A A

A N D

FAUST

195


Constant of Hydrolysis of Parathion"

C

K

t


11.50

11.22 11.06 10.92 10.79 10.15

c

χ w~

4

o b

— — — — — — — 2.68

— — — — — — 11.36

pH 104

pH 9.0

VH 74

2.62 2.72 2.70 2.66 2.60

K

t

o b

X 10~

C

3

11.50

— — — — — 11.14 10.81 10.07 9.54 9.01

K

o b

χ

W'

— — — — — 1.34

11.50 11.04 10.63 10.07 9.75 9.00 6.90

2.06 1.98 2.21 2.07 2.04 2.13

— —

— — — — — —

— — — — — —

1.30 1.39 1.30 1.27

— —

t

3b is the observed first order rate constant (hour

x

—

).

shows that the hydrolysis kinetics still obey the first-order expression by constant K values (Table III). Another objective of this study determined how temperature affects the rate constants, which resulted i n calculating the activation energy of the hydrolysis reactions. Reactions were conducted in a thermostati cally controlled water bath to ± 0 . 2 ° C in diffused light. The rate con stant, K , was evaluated at 10°, 20°, 40°, and 60°C from the slope of a plot of log Cins as a function of time (Equation 4) for each experimental run. Table I V shows that the velocity of the reactions vary with temperature. The Arrhenius equation was used to calculate the observed activa tion energy, E , from a plot of the logarithm of the rate constant against the reciprocal of the absolute temperature: ob

oh

ob

Table III.

Verification of Order of Hydrolysis Reaction" K >

(hour ) 1

ob

Insecticide Parathion Paraoxon

Insecticide Cone, M 7.90 3.95 1.97 9.62 4.81 2.41

Χ χ Χ Χ Χ χ

10" 10" 10" 10" 10" 10"

5

5 5 5 5

5

pH 3.1 1.71 1.65 1.61 1.40 1.46 1.52

Χ Χ Χ X Χ χ

pH 104

10" 10" 10" 10' 10" 10"

4 4

4

4 4 4

"Temperature = 20° ± 0.2°C. Average values of three experimental runs at the indicated p H . b


.021 .021 .019 .126 .115 .121

196


log™ K

ob

= logio Aob — 2 303 R T

^

The observed activation energy is calculated from the slope 2.303RT (Table V ) .

—E / ob

It seems that the reaction rate approximately doubled for each 10degree temperature increase. The parent compound and its oxon are characterized by nearly equal activation energies at p H 3.1. However, under alkaline condition the E value calculated for paraoxon is approxi mately 2.5 kcal mole" smaller than that calculated for the parent com pound, parathion.


ob

1

Table IV.

Hydrolysis Rate Constants of Parathion" and Paraoxon at Different Temperatures 6

Temp., °C Parathion 10 20 40 60 Paraoxon 10 20 40 60

pH9.0

pHS.l " K , hours'

1

o b

5.20 1.65 8.58 3.83

X Χ Χ Χ

10" ΙΟ" 10" 10"

5.01 1.46 8.05 4.02

Χ Χ Χ Χ

10" 10" 10" 10"

t

5 4

4 3

5 4

4 3

J / 2

, hours

K , hours o b

1

13327 4182 808 181

5.41 1.32 5.47 2.65

Χ X Χ Χ

10" 10' 10" 10"

13832 4726 861 172

3.83 9.87 3.81 1.19

Χ Χ Χ Χ

10" 10" 10" 10"

t / / , hours

4 3 3 2

3

3 2 1

2

1281 523 127 26.2 181 69.9 18.2 5.8

Initial parathion concentration was 3.95 X 10" M. Initial paraoxon concentration was 4.81 X 10' M. Average values of three experimental runs at the indicated p H . Ionic strength was 0.02AÎ. α

5

5

b

c

Ketelaar (35) reported an activation energy of 16.6 kcal mole" for parathion hydrolysis i n 5 0 % alcohol-water mixture IN in sodium hydroxide. Ketelaar and Gersmann (36) studied the hydrolysis of parathion and paraoxon in 5 0 % acetone-water mixture. The activation energies were reported for parathion's hydrolysis as 22.7 kcal mole" and for paraoxon as 20.5 kcal mole" . The calculated activation energies for parathion and paraoxon in buffered water (Table V ) are much lower than those reported by the two investigators for the mixed solvent systems. 1

1

1

Potassium Permanganate Oxidative Kinetics Experimental Procedure. Reactions were conducted in a thermostatically controlled chamber at 20° =b 0.2°C. in diffused light. To a one-liter volumetric flask was added the desired concentration of oxidant


10.

G O M A A

A N D FAUST

197


and buffer solution. The desired quantity of compound under investigation was added to another one-liter flask. After both flasks reached temperature equilibrium, the contents of each flask were simultaneously poured into the reaction vessel. A blank solution of oxidant was used under the same conditions. A stop watch was started at the initial pouring. After various time intervals aliquots of the reaction and blank solutions were withdrawn to determine the residual oxidant and pesticide concentrations. Each experiment was repeated three times. Concentrations of parathion and paraoxon were determined using a Microtek MT-200 gas chromatograph equipped with a 10-mc N i electron-capture detector at 250°C. The operating and chromatographic conditions used for determining p-nitrophenol and its oxidative products, using a flame ionization detector, are described above. Two exceptions are inlet temperature of 225°C and detector parameters of 225°C, air at 0.6 C F M and hydrogen gas at 30 m l / m i n . Figure 2 shows the isothermal separations of p-nitrophenol and two of its intermediate oxidative products, 2,4-dinitrophenol and 2-hydroxy5-nitrobenzoic acid. A standard curve of peak area vs. concentration was constructed for p-nitrophenol and 2,4-dinitrophenol. After determining the residual concentrations of the compound under investigation and its major intermediate oxidative products in the extract with gas-liquid chromatography techniques, the volume of each sample was reduced to 2 m l by flash evaporation. These concentrates were subjected to other chromatographic and spectrophotometric analyses to identify further any intermediate and final products of oxidation. Results. R E D O X R E A C T I O N M O D E L S O F P A R A T H I O N , P A R A O X O N , A N D P - N I T R O P H E N O L O X I D A T I O N S B Y KMn0 . Before proceeding to the kinetic studies, computations and experiments were conducted to determine the fate of the reactants under acidic and alkaline conditions. Assumptions were made concerning the oxidation states of the various elements: ( a ) M n 0 ~ is reduced to M n 0 and not to M n , ( b ) carbon is oxidized to C 0 , ( c ) the aromatic nitro group is oxidized to N 0 " , and ( d ) equilibrium is obtained between the reactants and the products. Steward (37) reported that many organic compounds are degraded to C 0 by potassium permanganate oxidation although i n basic solution oxalate is isolated frequently as a major reaction product. This is because oxalate suffers further rapid oxidation only in acid solution. Assuming that organic phosphorus and sulfur groups become P 0 ~ and S 0 ~ , the following hypothetical redox reactions represent the degradation of


6 3

4

4

2 +

2

2

3

2

4

Table V.

4

Activation Energies of the Hydrolysis Reactions

Compound

pH

Parathion

3.1 9.0 3.1 9.0

Paraoxon

3

E

o b j

kcal. mole

16.4 14.5 16.4 12.0


2

198


2,4-DINITROPHENOL

Chemical Hydrolysis and Oxidation of Parathion and Paraoxon in

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