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Environmental fate & pathways

Phototransformation in water

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Administrative data

Endpoint:
phototransformation in water
Type of information:
migrated information: read-across from supporting substance (structural analogue or surrogate)
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: public available literature (good documentation)
Cross-reference
Reason / purpose for cross-reference:
reference to other study

Data source

Reference
Reference Type:
publication
Title:
Photolysis of aqueous chlorine at sunlight and ultraviolet wavelengths - I. Degradation rates
Author:
Nowell, L. N.; Hoigne, J.
Year:
1992
Bibliographic source:
Wat. Res. vol. 26, No. 5, pp. 593-598, 1992

Materials and methods

Study type:
direct photolysis
Test guideline
Qualifier:
no guideline followed
Principles of method if other than guideline:
Photolysis study in water. For details see materials and methods section in IUCLID5 dossier.
GLP compliance:
not specified

Test material

Constituent 1
Chemical structure
Reference substance name:
Sodium hypochlorite
EC Number:
231-668-3
EC Name:
Sodium hypochlorite
Cas Number:
7681-52-9
Molecular formula:
ClO.Na
IUPAC Name:
sodium hypochlorite
Details on test material:
Chlorine was applied as hypochlorite obtained from Siegfried AG.
Specific details on test material used for the study:
Details on properties of test surrogate or analogue material (migrated information):not indicated.

Study design

Radiolabelling:
no
Analytical method:
other: Uvikon 810 double-beam spectrophotometer
Details on sampling:
Raw filtered lake water and groundwater were either used as they were (pH ~ 8) or used after pH adjustment with H2SO4 or HCl or NaOH. For experiments with model aqueous solutions, bidistilled or Nanopure water was buffered with 0.01 or 0.05 M phosphate, and either 1-octanol or acetate was added as a free radical scavenger.
Buffers:
lake water with pH adjusments only
Light source:
other: Mercury lamp, sunlight and UV light
Light spectrum: wavelength in nm:
280 - 2 800
Details on light source:
IrradiationSample solutions were irradiated in 1.5 cm i.d. quartz tubes in a Merry-Go-Round Reactor (MGRR) equipped with a Hanau TQ 718 high pressure mercury lamp operated at 500 and 700 W and a solidex borosilicate glass filter shielding the light <300 nm (Haag and Hoigné, 1986). The temperature was maintained at 20°C. Irradiations in sunlight were performed in the same quartz tubes held at a 30° angle from the horizon. A conversion factor of 1.5 was used to estimate the light intensity for flat water irradiation and for monitoring based on a horizontal pyranometer (Haag and Hoigné, 1986). This pyranometer (LI-COR Model LI-200 SB) measures both direct and diffuse irradiation (lambda = 400-1100 nm) reaching a flat surface, and is calibrated to give total sun- and skylight from 280 to 2800 nm. This reading gives about 1.05 kW/m2 for noon June clear sky sunlight. Ultraviolet (u.v.) irradiation was performed in quartz tubes positioned parallel to, and at a distance of 8 cm from a conventional low pressure mercury lamp (110 V, 400 mA) which emits 95% of its total energy (95% = 14.5 W) at 255 nm and yields 0.14 W of u.v. light 1/cm of lamp height. (Full u.v. absorption in sample tubes gave 3.7 μEinstein/l*s when based on ferrioxalate actinometry.)
Details on test conditions:
no further details given
Reference substance:
not specified
Dark controls:
not specified

Results and discussion

Preliminary study:
no preliminary study
Test performance:
no data
Dissipation half-life of parent compoundopen allclose all
DT50:
12 min
Test condition:
at pH 8 when exposed as a horizontal water layer to solar irradiation of 1.05 kW m-2
DT50:
37 min
Test condition:
at pH 7 when exposed as a horizontal water layer to solar irradiation of 1.05 kW m-2
DT50:
1 h
Test condition:
at pH 6 when exposed as a horizontal water layer to solar irradiation of 1.05 kW m-2
Predicted environmental photolytic half-life:
Based on these kinetic measurements, aqueous chlorine formed in cloudwater by (slow) ozonation of chloride is expected to have a photolytic lifetime of about a couple of hours during day-time in summer (pH 4.5-5).
Transformation products:
not specified
Details on results:
The photolysis of aqueous chlorine can be approximated by a kinetic law that is first-order in chlorine residual. Its rate at the surface does not vary in the presence of different OH (or Cl) radical scavengers or in different types of fresh waters when measured at constant pH.The photolytic half-life of aqueous chlorine at the surface of a flat water body is about 12 min at pH 8, 37 min at pH 7, and 1 h at pH 6 when exposed as a horizontal water layer to solar irradiation of 1.05 kW m-2. This dose of irradiation corresponds to clear sky, June noon sunlight at 47° latitude. These half-lives will vary with latitude, time of day, season, and weather conditions, all of which affect incident light intensity. In sunlight, absorbance in the wavelength region of lambda = 320-340 nm controls for the most part the rate of chlorine photolysis. The lifetime of chlorine will increase with depth in a water column, due to shielding in this wavelength range by chromophoric water constituents associated with dissolved organic material. In natural water with high DOC content (e.g. eutrophic Greifensee lake water, 4.0mg l-1 DOC and α330nm= 1.8 m-1), significant photolysis will occur only within the top 0.5 m. In contrast, in water of very low DOC content, such as that originating from some aquifers and which are typical for very transparent drinking water, cooling waters and swimming pools (e.g. Seewerben groundwater, 0.4 mg l-1 DOC and α 330mim = 0.23 m-1), chlorine photolysis within 3 m depth of a well-mixed column still will occur at a mean rate of 50% of that measured at the surface. A reference value can be expressed as follows: for a water of α 330mim = 1 m-1, the apparent rate-constant for chlorine photolysis in a mixed column of 2 m (pH = 8) is 1.2% min-1 during June noon sunlight exposure. This value would decrease with the depth of the water column and with increasing color content of the water at lambda = 330 nm.Where chlorine is applied in open systems for water disinfection, photodegradation will reduce the lifetime of chlorine, especially for waters with low DOC and with pH >= 7. To maintain a certain chlorine residual, one approach is to increase the total dose of chlorine applied. However, for ecological as well as economic reasons, a better approach would be to minimize the amount of chlorine photodegraded, e.g. by shielding from sunlight, by avoiding use of shallow water reservoirs, or by operating at a lower pH region.Shock treatment with chlorine should preferably be performed at night in all cases where the pH is above 7 or when sun-shielding is not possible.On UV irradiation (255 nm), HOCI and OCl- will photolyze at approximately the same rate, but chlorine depletion by u.v. requires a much longer irradiation time than that which is generally applied for u.v. disinfection.Based on these kinetic measurements, aqueous chlorine formed in cloudwater by (slow) ozonation of chloride is expected to have a photolytic lifetime of about a couple of hours during day-time in summer (pH 4.5-5).
Results with reference substance:
no data

Applicant's summary and conclusion

Validity criteria fulfilled:
not specified
Conclusions:
Based on these kinetic measurements, aqueous chlorine is expected to have a photolytic lifetime of about a couple of hours during day-time in summer (pH 4.5-5).
Executive summary:

The photolysis half-life of aqueous chlorine in, exposed to summer noon sunlit with clear sky (47°N) at a pH 8 is 12 min when measured at the surface. The half-life

increases with decreasing pH due to the decreasing ratio of OCl-/HOCl to 60 min at pH 5. The pseudo-first-order rate constant for the photolysis of HOCl becomes 2 x 10-4 s-1 and that of OCl- 1.2 x 10-3 s-1 The variation of the rate of photolysis with depth was calculated for water columns exhibiting different light absorption coefficients by taking into account that, for both HOCl and OCl-, the most effective wavelength for photolysis in sunlight is approx. 330 nm. These results show that in water treatment, chlorine photolysis should be minimized whenever possible by operating at low pH, sun shielding or night-time addition of chlorine or avoiding storage in shallow reservoirs. The rate of chlorine photolysis controls the formation of OH radical which acts as a secondary highly reactive photooxidant.

On u.v. (255 nm) irradiation both HOCl and OCl- photolyze at comparable rates and slowly enough that chlorine depletion will not occur during the time of irradiation typical in UV disinfection.

Photolysis can also contribute to the depletion of chlorine in atmospheric waters whenever chlorine is formed by (slow) ozonation of chloride.

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