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Environmental fate & pathways

Phototransformation in water

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Reference
Endpoint:
phototransformation in water
Type of information:
experimental study
Adequacy of study:
key study
Study period:
02/2013
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
study well documented, meets generally accepted scientific principles, acceptable for assessment
Study type:
direct photolysis
Principles of method if other than guideline:
Investigation of the photolytic stability in aqueous solution.
GLP compliance:
no
Radiolabelling:
no
Analytical method:
high-performance liquid chromatography
Details on sampling:
Samples were taken at 0, 27, 48, 72 and 144 hours. The samples were analysed in a series and are therefore stored deep frozen at approximately -70°C and protected from light until the experiment was terminated.
Light source:
other: visible light
Light spectrum: wavelength in nm:
400 - 700
Details on light source:
Continuous uniform illumination was provided in the spectral range of 400 to 700 nm. A photon flow density ranging from 110 to 140 μE x m-2 x s-1 , or an equivalent range of 7300 to 9300 lux, was measured.
Details on test conditions:
After preparation, the test solutions were deposited in a climate chamber in which a temperature in the range of 21°C to 24°C (+/- 2°C) was maintained. Temperature was measured and recorded daily.
Continuous uniform illumination was provided in the spectral range of 400 to 700 nm. A photon flow density ranging from 110 to 140 μE x m-2 x s-1 , or an equivalent range of 7300 to 9300 lux, was measured. The light intensity was checked before the start of the test. The controls in the dark were protected from light by wrapping the flaks with aluminum foil.
Duration:
144 h
Temp.:
21 °C
Initial conc. measured:
6.2 mg/L
Dark controls:
yes
% Degr.:
50
Sampling time:
3.2 d
Test condition:
OECD nutrient medium at a concentration of 10 mg/l
% Degr.:
50
Sampling time:
7.7 d
Test condition:
OECD nutrient medium at a concentration of 32 mg/l
% Degr.:
50
Sampling time:
9.2 d
Test condition:
Deionized water at a concentration of 10 mg/l
% Degr.:
50
Sampling time:
9.6 d
Test condition:
Deionized water at a concentration of 32 mg/l
Transformation products:
not measured

Results of photolysis pH 8-9 at 21 °C at a concentration of 10 mg/L

      Test 1 (Deionised water)     Test 2 (OECD nutrient medium)
Sampling time  Under illumination: Amount of the substance [mg/L]  In the dark: Amount of the substance [mg/L]  Under illumination: Amount of the substance [mg/L]  In the dark: Amount of the substance [mg/L]
  0 h  4.710  4.506  6.178  5.626
  24 h  4.041  4.613  3.588  5.316
  48 h  3.300  4.836  3.526  5.759
  72 h  4.986*  4.110  2.743  6.267
 144 h  2.876  5.911  1.511  6.070

* This value is regarded as outliner and is not taken into account for calculation of the half-life time.

t (1/2) = 9.2 days in deionised water

t (1/2) = 3.2 days in OECD nutrient medium

Photolysis rate constant: k = 8,749 x 10-7 / s (deionised water)

Photolysis rate constant: k = 2.496 x 10-6 / s (OECD nutrient medium)

Results of photolysis pH 8-9 at 21 °C at a concentration of 32 mg/L

      Test 1 (Deionised water)     Test 2 (OECD nutrient medium)
Sampling time  Under illumination: Amount of the substance [mg/L]  In the dark: Amount of the substance [mg/L]  Under illumination: Amount of the substance [mg/L]  In the dark: Amount of the substance [mg/L]
  0 h 24.00 22.40  24.31  24.25
  24 h  21.79 21.60  19.60  24.23
  48 h  22.54 22.56  19.16  21.21
  72 h  19.53 25.68  18.59 24.67
 144 h  15.51 23.44  13.37  24.27

t (1/2) = 9.6 days in deionised water

t (1/2) = 7.7 days in OECD nutrient medium

Photolysis rate constant: k = 8.377 x 10-7 / s (deionised water)

Photolysis rate constant: k = 1.046 x 10-6 /s (OECD nutrient medium)

The longer half-life time obtained in the tests at the concentration of 32 mg/l in the presence of light may be due to hindered radiation of the molecules of the substance, as the solution was dark coloured.

Validity criteria fulfilled:
yes
Conclusions:
In deionised water the half-life times were 9.2 days at 10 mg/L and 9.6 days at 32 mg/L, and in OECD nutrient medium a half-life time of 3.2 days at 10 mg/L and 7.7 days at 32 mg/L was obtained. The longer half-life time at the concentration of 32 mg/l in the presence of light may be due to hindered radiation of the molecules of the substance, as it is dark coloured.
Executive summary:

The stability of Nigrosin WLF in aqueous test solutions was investigated.

The tests were performed in order to examine the stability under conditions simulating those in the environment (pH 7, presence of light and air. Under these conditions, chemicals may be exposed to abiotic degradation by several chemical and physical processes, e.g. hydrolysis, oxidation and photolysis. Test 1 was performed under these conditions in pure water at a temperature of 21°C at two concentrations. Under the same conditions, but in the dark, test solutions at the same concentrations were performed to serve as controls. Additionally, naturally occurring traces of inorganic compounds may influence the chemical stability of the test substance in the environment. Test 2 was performed at two concentrations in an algae nutrient medium containing traces of salts. Conditions for radiation, ventilation and nutrient medium corresponding those conditions used in ecotox testing in accordance with Commission Regulation (EC) No 761/2009 amending Regulation No 440/2008, Method C.3 ‘Freshwater Alga and Cyanobacteria, Growth inhibition test’ (2009) which is equivalent to OECD Guideline for Testing of Chemicals No. 201 (2006) ‘Alga, Growth Inhibition Test’. Nigrosin WLF is a dark coloured test substance, which reduce the light emission for the algae cells because of shading effects. Enough light is essential for the growth of the algae, so for a reduction of shading effects, the test was carried out with strong photon flow density (110-140 µE x m-2 x s-1; measured in the range 400 to 700 nm) and with a reduced test volume of 25 ml in 300 mL Erlenmeyer flasks. Under the same conditions, but in the dark, test solutions at the same concentrations were analysed to serve as controls. The samples were analysed using a HPLC method with UV detection. The degradation of the investigated test substance can be described by first order kinetics. Half-life times and degradation rates of Nigrosin WLF were calculated from the degradation curves.

The results of the test series in different media clearly indicate that Nigrosin WLF is photolytically unstable in the presence of light. By comparison, the results of the test series conducted in the dark show stability of the test substance under exposure conditions. When performing the tests in pure water (test 1), the half-life times were 9.2 and 9.6 days. In test 2, a nutrient-enriched medium containing salts was taken to examine the stability. In comparison to test 1, catalytic processes might reduce the stability of the substance to a half-life time of 3.2 and 7.7 days. The longer half-life time obtained in the tests at the concentration of 32 mg/l in the presence of light may be due to hindered radiation of the molecules of the substance, as Nigrosin WLF is a dark coloured substance. In conclusion, it is clearly demonstrated, that Nigrosin is rapidly degradable by photolysis. As no additional peaks are shown in the chromatograms transformation products were not detected via HPLC. Due to the complex nature of photochemical processes with a large number of possible transformation pathways, and sub-sequent reactions of produced intermediate species, confirms the difficulty on identification of transformation products.

Description of key information

In deionised water the half-life times were 9.2 days at 10 mg/L and 9.6 days at 32 mg/L, and in OECD nutrient medium a half-life time of 3.2 days at 10 mg/L and 7.7 days at 32 mg/L was obtained. The longer half-life time obtained in the tests at the concentration of 32 mg/l in the presence of light may be due to hindered radiation of the molecules of the substance, as the solution was dark coloured.

Key value for chemical safety assessment

Additional information

The stability of the substance in aqueous test solutions was investigated.

The tests were performed in order to examine the stability under conditions simulating those in the environment (pH 7, presence of light and air). Under these conditions, chemicals may be exposed to abiotic degradation by several chemical and physical processes, e.g. hydrolysis, oxidation and photolysis. Test 1 was performed under these conditions in pure water at a temperature of 21°C at two concentrations. Under the same conditions, but in the dark, test solutions at the same concentrations were performed to serve as controls. Additionally, naturally occurring traces of inorganic compounds may influence the chemical stability of the test substance in the environment. Test 2 was performed at two concentrations in an algae nutrient medium containing traces of salts. Conditions for radiation, ventilation and nutrient medium corresponding those conditions used in ecotox testing in accordance with Commission Regulation (EC) No 761/2009 amending Regulation No 440/2008, Method C.3 ‘Freshwater Alga and Cyanobacteria, Growth inhibition test’ (2009) which is equivalent to OECD Guideline for Testing of Chemicals No. 201 (2006) ‘Alga, Growth Inhibition Test’. The substance is a dark coloured test substance, which reduce the light emission for the algae cells because of shading effects. Enough light is essential for the growth of the algae, so for a reduction of shading effects, the test was carried out with strong photon flow density (110-140 µE x m-2 x s-1; measured in the range 400 to 700 nm) and with a reduced test volume of 25 ml in 300 mL Erlenmeyer flasks. Under the same conditions, but in the dark, test solutions at the same concentrations were analysed to serve as controls. The samples were analysed using a HPLC method with UV detection. The degradation of the investigated test substance can be described by first order kinetics. Half-life times and degradation rates of the substance were calculated from the degradation curves.

The results of the test series in different media clearly indicate that the substance is photolytically unstable in the presence of light. By comparison, the results of the test series conducted in the dark show stability of the test substance under exposure conditions. When performing the tests in pure water (test 1), the half-life times were 9.2 and 9.6 days. In test 2, a nutrient-enriched medium containing salts was taken to examine the stability. In comparison to test 1, catalytic processes might reduce the stability of the substance to a half-life time of 3.2 and 7.7 days. The longer half-life time obtained in the tests at the concentration of 32 mg/l in the presence of light may be due to hindered radiation of the molecules of the substance, as the substance is a dark coloured substance. In conclusion, it is clearly demonstrated, that the substance is rapidly degradable by photolysis. As no additional peaks are shown in the chromatograms transformation products were not detected via HPLC. Due to the complex nature of photochemical processes with a large number of possible transformation pathways, and sub-sequent reactions of produced intermediate species, confirms the difficulty on identification of transformation products.