diethyl 1-(2,4-dichlorophenyl)-5-methyl-4,5-dihydro-1H-pyrazole-3,5-dicarboxylate

Phototransformation in water

Link to relevant study record(s)

Reference

Endpoint:

phototransformation in water

Type of information:

experimental study

Adequacy of study:

key study

Study period:

15 Jun 1992 - 15 Jun 1994

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

guideline study

Study type:

direct photolysis

Qualifier:

according to guideline

Guideline:

EPA Guideline Subdivision N 161-2 (Photodegradation Studies in Water)

GLP compliance:

yes

Findings:

The total recovery of samples for all series was in a range of 99.6 to 106.6% of applied radiolabel with the exception for one sample of dark controls (85.7% for final sampling interval).

By the end of the incubation, volatile components were detected between 0.5% (dark controls) and 13.4% (mean value of the two irradiation series).¹⁴C-carbon dioxide was formed at 0.1% (dark controls) to 6.3% (mean value for irradiated samples) of applied radioactivity.

Besides the parent compound, one predominant photodegradation product with reporting title “Peak 9”was observed at peak values of approx. 40% by the end of the experiment. Co-chromatography of samples showed that P9 was identical with a major unknown observed in soil photolysis with reporting title “M8”.

Efforts on the structural elucidation of “Peak 9” resulted in three structural proposals of which an 2,3-oxaziridine structure in the heterocycle was proposed as the preferred structure explaining best all results from spectroscopical work .

At least nine additional components were observed, none of them amounting to more than 8.4% in maximum in the course of the experiment.

The UV spectrum of the parent compound AE F107892 shows very limited adsorption at wave lengths of more than 290 nm. This is supported by findings of low mean values for the quantum yield with 2.53 x 10^-5and 2.45 x 10^-5for the two Suntest photoreactors used.

 

The experimental half-lives for photolysis were corrected by the relative photon flux of the photoreactors given as hours sunshine at 52° Northern latitude per instrument hours. In addition, the half-lives were normalised to a daily sunshine duration of 12 hours. The calculated half-lives for photolysis (DT_{50 calc}) of mefenpyr-diethyl are summarized in the Table

 

Corrected half-lives of photolytic degradation of AE F107892 for the Northern Hemisphere

Sterile aqueous acetate buffer, pH 5	DT50 calc. [hours]^1	DT50 calc. [days]^1
Photoreactor SUNTEST II	193.5	16.5
Photoreactor SUNTEST III	200	17

¹:  rounded values

 

Conclusion:

The contribution of aqueous photolysis to the overall degradation of AE F107892 is regarded as moderate to low due to the low quantum yields (2.5 x 10^-5) of the compound. The dissipation of the parent compound (DT-50 = 16.5 days) is accompanied by the formation of a large number of minor products, each observed at peak values of less than 10% of the applied radiolabel besides the formation of¹⁴C-carbon dioxide and other non-polar volatiles.

A major product was observed at maximum values of 40%. The investigations for structural elucidation resulted in structural proposals very similar to the parent compound, see 2,3-oxaziridine derivative, of which the formation can be explained by an addition of an oxygen atom to the parent compound.

This suggests that reactive oxygen species may be formed in the irradiated water. The oxygen species may be the major reactants with the dissolved molecules of AE F107892 rather than a direct interaction of the compound with the photons of the light. Under environmental conditions, the formation of photolytically induced metabolites is less pronounced due to the very low half-life of the parent compound as a precursor under biotic conditions.

 

Photodegradation products are therefore not regarded as a significant degradation pathway for the elimination of mefenpyr-diethyl from the aquatic environment.

Description of key information

Photodegradation processes are not regarded to contribute significantly to the elimination of the substance from the aquatic environment.

Key value for chemical safety assessment

Half-life in water:

17 d

Additional information

The photogedradation of the substance in water was determined in a study following EPA Guideline Subdivision N 161-2 (1994). The determined environmental half-life (52° Northern latitude) was 17 days for sterile aqueous photolysis. The photolysis is accompanied by the formation of a number of minor products one major product as well as the formation of 14C-carbon dioxide and other non-polar volatiles. Considering the rapid biotic hydrolysis of the parent substance in surface water and the low quantum yield determined for photolytic conversions, the formation of photolytic induced transformation products is much less pronounced for the outdoor aquatic environment. The direct photolysis is therefore not of significance for the elimination of the substance from the aquatic environment.