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Phototransformation in water

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Endpoint:
phototransformation in water
Type of information:
experimental study
Adequacy of study:
other information
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
other: GLP Guideline study
Study type:
direct photolysis
Qualifier:
according to guideline
Guideline:
EPA Guideline Subdivision N 161-2 (Photodegradation Studies in Water)
GLP compliance:
yes
Analytical method:
high-performance liquid chromatography
other: TLC
Details on sampling:
Irradiated samples & dark controls – 2 replicates sampled at 0, 3, 7, 11 and 15 days.
Samples for collection of volatiles were analysed after 15 days
Light source:
Xenon lamp
Light spectrum: wavelength in nm:
< 290
Details on light source:
Xenon arc simulated sunlight source fitted with UV filter to prevent light of wavelength < 290 nm to reach the solution
Details on test conditions:
TEST SYSTEM
The experiment was conducted in Teflon vessels of 3 cm diameter and 9 cm height. Irradiated vessels were placed in sockets in a water-cooled steel block positioned in the Suntest unit. The irradiated solutions were magnetically stirred continuously. Additional vessel (control and exposed) were modified for the collection of volatile degradation products.
The irradiation was continuous over time except for a period of 40 hours between 3-day and 7-day (cut in the electric supply). The time sampling was extended to allow for this dark period. Control vessels were maintained in darkness in an oscillating frame within a temperature controlled water bath. Temperatures were recorded at 1 hour intervals.
The vessels fitted for the collect of volatiles were incorporated into separate gas lines so that sterile, humidified, CO2-free air was drawn through the test vessels. Two traps containing ethanediol to trap organic volatiles and two traps containing 1 M NaOH to trap 14CO2 were used.

TEST SOLUTION
[Phenyl(U)-14C]Diuron was used without radio-dilution. It was dissolved in methanol and made up to a volume in a 5 mL volumetric flask. Duplicated aliquots (50 µL) of this stock solution were transferred to 50 mL flasks which were made up to volume with methanol and triplicate aliquots radio assayed by LSC. The stock solution contained 30.81 mg Diuron. An aliquot of 1.66 mL was transferred to a 5mL volumetric flask and made up to volume with methanol. This solution was passed through a 0.2 µm filter prior to use.
An aliquot of 4 mL of the filtered solution was added to 800 mL of buffered solution. The final concentration of Diuron in the buffered solution was 10 mg/L (methanol 0.5 % v/v). Aliquots of 30 mL were transferred to the test vessels and placed in the sun-test unit ( or dark control)

REPLICATION
10 dark controls, 10 irradiated and 4 for the collection of volatiles (2 control and 2 irradiated)

OTHER:
Temperature: Irradiated and control solution were maintained at 25 ºC.
pH: The study was conducted in aqueous solutions buffered at pH 7
Duration:
15 d
Temp.:
25 °C
Initial conc. measured:
10 mg/L
Reference substance:
yes
Remarks:
Diuron; N’ – (3-4-dichlorophenyl)-N-methylurea (DCPMU); N-(3,4-dichlorophenyl) urea (DCPU); 3,4-dichloroaniline (DCA); 3,3’4,4’-tetrachloroazobenzene (TCAB); 3,3’4,4’-tetrachloroazoxybenzene (TCAOB)
Dark controls:
yes
% Degr.:
87
Sampling time:
15 d
Test condition:
Direct photolysis
DT50:
9 d
Test condition:
actual irradiation
DT50:
43 d
Test condition:
equivalent under natural sunlight (latitude 30 -50 ºN), assuming 12 h of daylight
Details on results:
Thirteen minor non-volatile degradates were formed. None of which amounted for more than 9%, and in most cases no more than 5%
Results with reference substance:
Neither TCAB nor TCAOB were found at 0.002 ppm. DCA was not detected at 0.006 ppm.

TLC analysis of irradiated test solutions showed that Diuron was extensively degraded. After 7 days of continuous irradiation (equivalent to 30 days of natural light) 60% of the radioactivity could be identified either as Diuron or CO2. By the end of the study only 33% of the initial Diuron remained. The rest had been degraded to CO2or more polar fragments. None of the polar fragments comprised more than 8.5% of the initial radioactivity at any time, and there was no evidence from the later analysis to suggest that this compound would accumulate further.

The total quantity of radioactivity decreased to about 87% of the %AR after 15 days. This decline was balanced by trapped volatile radioactivity which amounted to 16.4%

Table 2:           Concentrations of the different components in irradiated test solutions resolved by TLC in solvent system B

 

Irradiation time

[days]

Components

(expressed as mg Diuron equivalent/L)

Polars

B1

B3

B4, B5

Diuron

B6, B7

Zone a *

0

0.06

9.98

0.08

2.95

1.14

0.20

0.15

0.13

7.86

0.19

0.04

7.00

2.54

0.39

0.18

0.19

5.23

0.24

0.06

10.70

3.43

0.41

0.22

0.20

3.97

0.20

0.05

14.67

3.57

0.51

0.22

0.17

3.35

0.19

0.01

Irradiation time

[days]

Components

(expressed as % applied radioactivity)

Polars

B1

B3

B4, B5

Diuron

B6, B7

Zone a *

0

0.6

98.8

0.8

2.95

11.2

2.0

1.5

1.2

77.8

1.9

0.3

7.00

25.1

3.9

1.8

1.9

51.7

2.3

0.6

10.70

33.9

4.1

2.2

1.9

39.3

2.0

0.5

14.67

35.3

5.1

2.2

1.7

33.2

1.9

0.1

*             Zone a: Area of chromatograms between component B7 and solvent front


 

Table 3:           Material balance of the radioactivity in solutions

 

Sampling time

Actual Irradiation time

Equivalent natural time

[% ]AR

[days]

[days]

[days]

Test solution

Volatiles

Total

0

0

0

100.3

Not detected

100.3

0

0

0

100.2

Not detected

100.2

3

2.95

14.2

97.1

Not detected

97.1

3

2.95

14.2

97.1

Not detected

97.1

7

7.00

33.6

92.4

Not detected

92.4

7

7.00

33.6

92.0

Not detected

92.0

11

10.70

51.4

89.8

Not detected

89.8

11

10.70

51.4

89.6

Not detected

89.6

15

14.67

70.4

86.5

Not detected

86.5

15

14.67

70.4

86.8

Not detected

86.8

15

14.67

70.4

78.8

16.4

95.2

15

14.67

70.4

79.4

16.4

95.8

Executive summary:

In irradiated solution Diuron degraded with a half-life of 9 days under the study conditions, equivalent to a calculated half-life of about 43 days under natural (latitude 30 -50 ºN) sunlight assuming 12 hours of daylight. Trapped volatile radioactivity, mainly14CO2, amounted to 16% AR by the end of the study. Thirteen minor photoproducts were formed, none of which accounted for more than 9% applied radioactivity at any time, and most of which were considerably more polar than Diuron. Diuron was not degraded in dark control solutions.

Endpoint:
phototransformation in water
Type of information:
experimental study
Adequacy of study:
other information
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
other: GLP Guideline study
Study type:
direct photolysis
Qualifier:
according to guideline
Guideline:
other: Photo-transformation of Chemicals in Water. Part A: Direct Photo-transformation. UBA, Berlin
Deviations:
no
GLP compliance:
yes
Analytical method:
high-performance liquid chromatography
Details on sampling:
Concentrations of Diuron were measured at 0, 0.5, 0.66, 1.0, 1.5, 2.0, 3.0, 4.0, 5.0, 5.08, 6.0, 6.88 and 7.0 hours
Light source:
other: Polychromatic Light (according to ECETOC Method)
Light spectrum: wavelength in nm:
ca. 195 - ca. 405
Details on light source:
Hg-lamp TQ 150 (Original Hanau Co) fitted with Duran 50 filter to prevent light of wavelength < 290 nm to reach the solution
Details on test conditions:
TEST MEDIUM
An amount of 55.5 mg test substance was weighted into a 250 mL flask and made up to volume with 250 mL highly pure water. The solution was stirred, for 24 hours, filtered and analysed by HPLC.
For the UV-absorption solutions with different concentrations were prepared. For the quantum yield calculation, the spectrum from a solution of 11.17 mg Diuron/L was used.
For the photo-degradation experiments, the stock solution was diluted in water to concentrations of 6.61 and 6.58 mg/L.

REPLICATION
- No. of replicates: 2

Duration:
7 h
Temp.:
25 °C
Initial conc. measured:
6.61 mg/L
Duration:
7 h
Temp.:
25 °C
Initial conc. measured:
6.58 mg/L
Computational methods:
The environmental half-life was estimated using the models of the GC-SOLAR program and the Frank/Klöpffer program.
Parameter:
max lambda
Value:
248 nm
Parameter:
max epsilon
Value:
18 030
Parameter:
epsilon 295 nm
Value:
730
Quantum yield (for direct photolysis):
0.024
DT50:
2.253 - 2.51 h
Test condition:
in laboratory
DT50:
>= 2.5 - <= 48 d
Test condition:
calculated environmental half life
Predicted environmental photolytic half-life:
2.5 to 48 days
Transformation products:
not measured

UV-absorption properties:
The UV-absorption spectrum of Diuron in highly pure water
(11.17 mg/l) shows a maximum at 211 nm (epsilon = 28151;
band width: up to 219 nm) and a maximum at 248 nm (epsilon =
18030; band width: from 232 to 259 nm). The absorption of
Diuron with epsilon = 730 l/mole x cm at 295 nm to epsilon =
20 l/mole x cm at 343 nm ends in the environmentally
relevant range of wavelenghts.

Direct interactions of Diuron in aqueous solution with the sunlight in the troposphere are
therefore well possible.

Quantum yield:
The quantum yield of direct photodegradation of Diuron was determined according to the ECETOC-method in polychromatic light. From the UV adsorption data and the kinetic results of two
photodegradation experiments in a merry-go-round apparatus
the quantum yield was calculated to be 0.0243.
Assessment of half-life:
The results of modelling (GC-Solar program and Frank &
Kloepffer program) indicate that direct photodegradation in
water contributes to the overall elimination of Diuron in
the environment and that the half-lives for the typical
application periods range from 3 days to one month.

 

Table 2:    Concentrations of Diuron in irradiated test solutions resolved by HPLC

 

Irradiation time

[hours]

Diuron

[mg/L]

Experiment 1­

Experiment 2

0

6.61

6.58

0.5

-

6.06

0.66

5.96

-

1.0

5.60

5.70

1.5

5.18

5.22

2.0

4.63

4.75

3.0

3.78

3.71

4.0

2.88

2.58

5.0

-

1.81

5.08

2.36

-

6.0

1.40

1.31

6.88

0.88

-

7.0

-

0.70


   

Table 3:           Environmental half-lifeof Diuron for different latitudes according to the software GC-SOLAR

 

 

Half-life

[days]

Latitude

30th

40th

50th

60th

Spring

2.5

2.9

3.5

4.4

Summer

2.2

2.3

2.5

2.8

Fall

3.7

5.1

8.3

17.0

Winter

5.4

9.2

20.4

69.2

 

 

Table A4:           Environmental half-life of Diuron for different latitudes according to the Frank and Klöpffer program

 

 

Half-life *

[days]

 

Photolysis constant

Minimum

Mean

Maximum

[s-1]

 

 

 

April

0.156 x 10-5

2.8

5.1

21

July

0.215 x 10-5

2.5

3.7

12

October

0.547 x 10-6

7.7

15

67

January

0.167 x 10-6

23

48

220

*             Conditions: cloudiness in central Europe

 

Executive summary:

In irradiated solution Diuron degraded with a half-life of 2.25 – 2.51 h under the study conditions, equivalent to a range of 2.2 to 48 days of environmental half life according to the modelling calculations. From the UV absorption data and the kinetics results the quantum yield (the number of photons destroyed/the number of photons absorbed) was calculated to be 0.0243.

The UV absorption spectra of Diuron indicate that interactions of Diuron in aqueous solution with sunlight are possible and the model assessments show that a contribution to the elimination of Diuron in the environment from direct photodegradation is to be expected

Description of key information

Photodegradation processes are capable of contributing significantly to the elimination of Diuron from the aquatic environment.

Key value for chemical safety assessment

Additional information

In irradiated solution Diuron degraded with a half-life of 9 days under the study conditions, equivalent to a calculated half-life of about 43 days under natural (latitude 30 -50 ºN) sunlight assuming 12 hours of daylight. Trapped volatile radioactivity, mainly14CO2, amounted to 16% AR by the end of the study. Thirteen minor photoproducts were formed, none of which accounted for more than 9% applied radioactivity at any time, and most of which were considerably more polar than Diuron. Diuron was not degraded in dark control solutions (Hawkins et al., 1989). The quantum yield (the number of photons destroyed/the number of photons absorbed) of direct photodegradation of Diuron in pure water was calculated to be 0.0243 (Hellpointner, 1991).