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Environmental fate & pathways

Hydrolysis

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Administrative data

Endpoint:
hydrolysis
Type of information:
experimental study
Adequacy of study:
key study
Study period:
12 February 1988 - 30 May 1988
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
comparable to guideline study with acceptable restrictions

Data source

Reference
Reference Type:
study report
Title:
Unnamed
Year:
1988
Report Date:
1988

Materials and methods

Test guideline
Qualifier:
according to
Guideline:
EPA Guideline Subdivision N 161-1 (Hydrolysis)
Version / remarks:
The study was conducted according to the EPA Pesticide Assessment Guidelines, Subdivision N, Environmental Fate: Chemistry Series §161-1
Deviations:
no
Principles of method if other than guideline:
The hydrolytic degradation of fenitrothion was determined at pHs 5, 7 and 9 using [phenyl-14C]fenitrothion at a concentration of 1 mg/l. The buffer solutions and test vessels were sterilised by autoclaving at 120°C and 1.03kPa for an hour and by heating at 160°C in an oven for 2 hours, respectively. All of the procedures up to extraction were conducted aseptically in a clean bench. Volatile radioactivity was not trapped. Duplicate samples at each pH were incubated at 25C°±1 for 30 days in the dark.
GLP compliance:
yes
Remarks:
Test material: [phenyl-14C]fenitrothion (Batch No.: C-88-008, Radiochemical purity: 99.9%, Specific radioactivity: 8.07MBq/mg (60.5 mCi/mmol)

Test material

Reference
Name:
Unnamed
Type:
Constituent
Test material form:
liquid
Radiolabelling:
yes
Remarks:
14C

Study design

Analytical monitoring:
yes
Details on sampling:
The test solutions were sampled at the specified intervals (0, 3, 5, 7, 14, 21 and 30 days after incubation). The 0.5 ml aliquot was radioassayed in
duplicate to determine the concentration of 14c and then 20 ml of the buffer solution was sampled to estimate the hydrolysis rate of fenitrothion and to identify the degradation prod ucts, The pH 's of test solutions were measured at the beginning of the study and on each samppling day prior to extraction.
Buffers:
The pure water was used to prepare buffer solutions at pH's of 5, 7 and 9. The buffer solutions were prepared by dissolving the components in the pure water. HEPES buffer proposed as pH 7 buffer in the protocol was not used because of buffer catalysis at 0.01 M . The pH's of the solutions were checked with a pH meter model F-7AD C Horiba Co. , Ltd. l calibrated with standard buffer solutions at 25°c. The buffer solutions were sterilized by autoclaving at 120°c and 1.5 psi for 1 hr with a Model SD-JO N autoclave (Tomy Seiko) before use. The buffer solutions were sterilized by autoclaving at 120°c and 1.5 psi for 1 hr.
Estimation method (if used):
The half-life of fenitrothion was estimated by a
least-square approximation method
Details on test conditions:
pH's of 5, 7 and 9 and incubated at 25 ± 1°c under
dark conditions .
Duration of test
Duration:
30 d
pH:
5
Temp.:
25 °C
Initial conc. measured:
1 mg/L
Number of replicates:
2
Positive controls:
no

Results and discussion

Test performance:
The pH of the buffer solutions was maintained within  0.04 pH units during the course of the study. Sterilized conditions were also maintained during the study.
Transformation products:
yes
Identity of transformation productsopen allclose all
No.:
#1
Reference
Reference substance name:
Unnamed
IUPAC name:
3-methyl-4-nitrophenol
Identifier:
CAS number
Identity:
2581-34-2
Molecular formula:
C7H7NO3
Molecular weight:
153.137
SMILES notation:
CC1=C(C=CC(=C1)O)[N+](=O)[O-]
InChl:
PIIZYNQECPTVEO-UHFFFAOYSA-N
No.:
#2
Reference
Reference substance name:
Unnamed
IUPAC name:
hydroxy-methoxy-(3-methyl-4-nitrophenoxy)-sulfanylidene-$l^{5}-phosphane
Identifier:
CAS number
Identity:
4321-64-6
CAS number:
4321-64-6
Molecular formula:
C8H10NO5PS
Molecular weight:
263.204
SMILES notation:
CC1=C(C=CC(=C1)OP(=S)(O)OC)[N+](=O)[O-]
InChl:
LNGYYGXBMGRUKZ-UHFFFAOYSA-N
Details on hydrolysis and appearance of transformation product(s):
Recoveries were 97.1-103.8% with unextractable/aqueous radioactivity being 0.7% of applied radio activity.
Fenitrothion mainly hydrolyzed to demethylated fenitrothion (DM-SNT) at pH' s of 5 and 7, and to 3-methyl-4-nitrophenol (NMC) at pH 9.
The sole major product at pHs of 5 and 7 was DM-FNT, which occurred at maximums of 6.6 and 10.3% of the applied 14C respectively after 30 days. At pH9, the sole major metabolite was NMC at a maximum of 14.4-15.1 % of the applied 14C after 30 days. Other products and unidentified radioactivity did not exceed 10 % of the applied 14C in any buffer solution tested throughout the experiments

Total recovery of test substance (in %)open allclose all
% Recovery:
88.9
pH:
5
Temp.:
25 °C
Duration:
30 d
% Recovery:
88.9
pH:
7
Temp.:
25 °C
Duration:
30 d
% Recovery:
81.4
pH:
9
Temp.:
25 °C
Duration:
30 d
Dissipation DT50 of parent compoundopen allclose all
pH:
9
Temp.:
25 °C
Hydrolysis rate constant:
> 6.88 - < 6.96 d-1
DT50:
> 100 - < 101 d
Type:
(pseudo-)first order (= half-life)
pH:
7
Temp.:
25 °C
Hydrolysis rate constant:
> 3.73 - < 3.85 d-1
DT50:
180 - 186 d
Type:
(pseudo-)first order (= half-life)
pH:
5
Temp.:
25 °C
Hydrolysis rate constant:
> 3.46 - 3.63 d-1
DT50:
> 191 - 200 d
Type:
(pseudo-)first order (= half-life)
Details on results:
Distribution of 14C.
During the course of experiments, more than 97.1% of the applied 14c was extracted from any buffer solution, and the unextractable radiocarbon was less than 0.7% of the applied 14C . The total 14C recoveries were 97.1-103.8% of
the applied 14C. These findings indicate that the chemical or physical adsorption of radioactive compounds onto the reaction
vessels was negligible and little volatile 14C was produced.
Hydrolysis rate.
The half -lives were estimated. Fenitrothion gradually hydrolyzed in water with the half -lives of 100-200 days in the pH range of 5-9 (ubject to considerable
inaccuracy.).
The hydrolysis rate of fenitrothion decreased in the order of pH 9 >> pH 7 > pH 5. The hydrolysis at pH 9 was about 2 times as rapid as those at pH's of 5 and 7.
Fenitrothion showed the greatest hydrolytic stability at pH 5 in the pH range of 5-9. Thus, fenitrothion was indicated to be relatively unstable under basic conditions, as compared with neutral and acidic conditions .

Degradation products were analysed by TLC .
Two major products were formed via hydrolytic reactions of fenitrothion. NMC and DM-SMT
Fenitrothion was also identified in the same way.Major product
at pH's of 5 and 7 was DM-SMT, which amounted to 6.6-10.3% of "'
the applied 14C after 30 days. At pH 9, the formation of NMC was predominant, amounting to 14.4-15.1% of the applied 14C
after 30 days. Any of other unidentified products was of minor importance, not exceeding 10% of the applied 14c in any buffer
solution tested throughout the experiments.

Any other information on results incl. tables

Hydrolysis rates and corresponding half-lives of fenitrothion in aqueous buffer solutions at 25°C

 

pH

Exp. No.

Rate constant (day-1)

Regression coefficient (r2)

Half-life (days)

5

1

2

3.46´10-3

3.63´10-3

0.986

0.934

200

191

7

1

2

3.73´10-3

3.85´10-3

0.979

0.960

186

180

9

1

2

6.88´10-3

6.96´10-3

0.975

0.980

101

100

Applicant's summary and conclusion

Validity criteria fulfilled:
not applicable
Conclusions:
Fenitrothion gradually hydrolyzed in water with half-lives of 100-200 days at 25 °C in the pH range of 5-9. Fenitrothion is mainly hydrolyzed to DM-FNT via cleavage of the P-O-methyl linkage at pHs 5 and 7 (9.8-10.3% at pH 5 after 30 days), and to NMC via cleavage of the P-O-aryl linkage at pH 9 (14.4-15.1% after 30 days).
Executive summary:

Hydrolysis study of organo-phosphorous insecticide fenitrothion [Sumithion (R) –O,O –dimethyl O-(3-methyl-4-nitro- phenyl > phosphorothioate1 CAS No. 122-14-5] was conducted according to the EPA Pesticide Assessment Guidelines, Subdivision N, Environmental Fate: Chemistry Series §161-111 , with the use of 14C preparation uniformly labeled in the phenyl ring.

14C-Fenitrothion was dissolved at 1.0 ppm in the buffer solutions at pH's of 5, 7 and 9 and incubated at 25 ± 1°C under dark conditions for 30 days. Experiments were conducted in duplicate and the test solutions were sampled at 0, 3, 5, 7, 14, 21 and 30 days after incubation . Degradation products as well as fenitrothion were tentatively identified by two-dimensional thin-layer co-chromatography with authentic standards and their chemical identities were confirmed by high-performance liquid chromatography.

The buffer solutions used were free from the microbial contamination during the course of experiments, as evidenced by a dilution plate method. Fenitrothion was relatively unstable at pH 9 with the half -lives of 100-101 days, compared with those of 180-186 days and 191-200 days at pH's of 7 and 5, respectively. Cleavage of P-Q-methyl linkage to form the demethylated fenitrothion (DM-SMT) was predominant at pHs of 5 and 7, while at pH 9 cleavage of P-Q-aryl linkage to form 3-methyl-4 -nitrophenol (NMC) was the major hydrolytic pathway