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Environmental fate & pathways

Hydrolysis

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Reference
Endpoint:
hydrolysis
Type of information:
experimental study
Adequacy of study:
key study
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
other: GLP Guideline study
Qualifier:
according to guideline
Guideline:
OECD Guideline 111 (Hydrolysis as a Function of pH)
GLP compliance:
yes (incl. QA statement)
Analytical monitoring:
yes
Details on sampling:
Time course concentrations were determined from duplicate sampling at any observation time. Samples were taken at the beginning of the test and after 6, 24, 48, 75, 96 and 120 hours.
Buffers:
- pH: 4
- Type and final molarity of buffer: 0.05M citrate buffer
- pH: 7
- Type and final molarity of buffer: 0.05M phosphate buffer
- pH: 9
- Type and final molarity of buffer: 0.05M borate buffer
Details on test conditions:
TEST SYSTEM
- Type, material and volume of test flasks, other equipment used: Amber glass vials with PTFE seals were used as test vessels
The test was carried using a thermostatically controlled enclosure at ¿ 0.5 ºC the chosen temperature, which was kept within ¿ 0.1 ºC. Photolytic interference was avoided all precautions were taken to exclude dissolved oxygen.
Duration:
120 h
pH:
4
Temp.:
50 °C
Initial conc. measured:
14.4 mg/L
Duration:
120 h
pH:
7
Temp.:
50 °C
Initial conc. measured:
14.4 mg/L
Duration:
120 h
pH:
9
Temp.:
50 °C
Initial conc. measured:
14.4 mg/L
Number of replicates:
Each sample was prepared in duplicate
Preliminary study:
A preliminary test was performed with sufficient number of samples in order to be able to estimate whether the hydrolysis was lower than 10 % after 120 hours or not
Transformation products:
no
% Recovery:
95.6
pH:
4
Temp.:
50 °C
Duration:
120 h
% Recovery:
99.3
pH:
7
Temp.:
50 °C
Duration:
120 h
% Recovery:
99.3
pH:
9
Temp.:
50 °C
Duration:
120 h
pH:
4
Temp.:
25 °C
DT50:
> 1 yr
Type:
(pseudo-)first order (= half-life)
pH:
4
Temp.:
50 °C
DT50:
ca. 91 d
Type:
(pseudo-)first order (= half-life)
pH:
7
Temp.:
25 °C
DT50:
> 1 yr
Type:
(pseudo-)first order (= half-life)
pH:
7
Temp.:
50 °C
DT50:
ca. 477 d
Type:
(pseudo-)first order (= half-life)
pH:
9
Temp.:
25 °C
DT50:
> 1 yr
Type:
(pseudo-)first order (= half-life)
pH:
9
Temp.:
50 °C
DT50:
ca. 425 d
Type:
(pseudo-)first order (= half-life)

Almost no degradation of Diuron at the tested pH was observed to occur

Table 1: Time course of Diuron tech. concentrations at 50 ºC and different pH values

 

pH 4

pH 7

pH 9

Time

[h]

pH

[mg/L]

[%] of initial

pH

[mg/L]

[%] of initial

pH

[mg/L]

[%] of initial

0

4.01

13.7

100

7.01

14.4

100

9.01

14.0

100

2

4.01

13.7

100

-

-

-

9.01

14.0

100

4

4.02

13.7

100

-

-

-

9.01

14.0

100

6

4.02

13.7

100

7.01

14.4

100

9.01

14.0

100

24

4.01

13.7

100

7.00

14.4

100

9.01

14.0

100

48

4.01

13.6

99.3

7.02

14.4

100

9.00

14.0

100

75

4.01

13.6

99.3

7.01

14.4

100

9.02

13.9

99.3

96

-

-

-

7.02

14.3

99.3

-

-

-

120

4.02

13.1

95.6

7.02

14.3

99.3

9.01

13.9

99.3

 

 

 

Table 2:   Degradation rate and half-life of Diuron tech. at 25 ºC / 50 ºC and different pH values *

Temperature

[ ºC]

pH

Kobs

t1/2

25

4

< 0.693 y-1

> 1 year

50

4

0.00762 d-1

2181 hours

25

7

< 0.693 y-1

> 1 year

50

7

0.00145 d-1

11436 hours

25

9

< 0.693 y-1

> 1 year

50

9

0.00163 d-1

10206 hours

*             Degradation rate calculated according to Arrhenius equation

 

Executive summary:

A GLP guideline study (OECD 111) is available for this test substance. The hydrolytic stability of Diuron under abiotic conditions is not dependent on pH. The degradation of Diuron being less 10 % at Ph 4, 7 and 9 and the corresponding half life at ambient temperature (calculated using Arrhenuis equation) estimated to be longer than 1 yr.

Description of key information

Diuron is not susceptible to abiotic hydrolysis, is hydrolytically stable at pH 5, 7 and 9 for 120 hours at 50 °C, in the dark. Therefore, chemical hydrolysis can be excluded as a dissipation mechanism of Diuron in environmental systems.

Key value for chemical safety assessment

Additional information

A GLP guideline study (OECD 111) is available for this test substance. The hydrolytic stability of Diuron under abiotic conditions is not dependent on pH. The degradation of Diuron being less than 10 % at pH 4, 7 and 9 and the corresponding half life, at ambient temperature (calculated using Arrhenuis equation), estimated to be longer than 1 yr (Wilfinger, 2007)