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Environmental fate & pathways

Phototransformation in water

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Description of key information

Half-life (t1/2) values for carbon-14 labeled test substance in distilled water and pH 5 and pH 9 solutions under simulated sunlight:

Aqueous Media

Half-life (t1/2)

(days)

Based on 24 h light/day

Based on 12 h light/day

A.     Carboxl Carbon-14 Label

Distilled Water

1.9

3.7

pH 5 Buffer

2.7

5.3

pH 9 Buffer

1.3

2.5

B.     Carbonyl Carbon-14 Label

Distilled Water

 2.3

 4.5

Key value for chemical safety assessment

Additional information

Key study (American Cyanamid Company, PD-M Volume 20-20, 1983)

The photodegradation in water of the test substance was investigated in a laboratory study equivalent to OPPTS 835.2240. The photolysis of the carboxyl carbon-14C and carbonyl carbon-14C labeled test substance was studied in distilled water, potassium acid phthalate buffer (pH 5.0), and boric acid buffer (pH 9.0). The test substance was dissolved in each media at a concentration of 25 ppm and exposed continuously at 25 °C in an environmental chamber to simulated sunlight (borosilicate-filtered Xenon arc lamp) for a period of 10 days.

The test substance photodegrades rapidly in various aqueous media under simulated sunlight and follows first order kinetics. The half-life of the carboxyl carbon-14 labeled test substance in distilled water, pH 5 and pH 9 buffer solutions under the tested conditions were calculated as 1.9 days (3.7 days on 12 hour exposure), 2.7 days (5.3 days on 12 hour exposure), and 1.3 days (2.5 days on 12 hour exposure), respectively. The half-life of the carbonyl carbon-14 labeled test substance in distilled water was calculated as 2.3 days (4.5 days on 12 hour exposure). The photodegradation rate of the test substance was slightly higher at higher pH.

The amount of nonvolatile carbon-14 radioactivity from carboxyl carbon-14 and carbonyl carbon-14 labeled test substance remaining in distilled water at the end of 10-day continual exposure were 92.5% and 23.4% of the total applied radioactivity, respectively. The nature of the lost carbon-14 radioactivity from distilled water was determined to be carbon-14 carbon dioxide in separate trapping experiments.

The carboxyl carbon-14 labeled test substance found in distilled water at the end of 10-day continual exposure was 2.7% of the total carbon-14 radioactivity. Carboxyl carbon-14 labeled test substance in distilled water photodegraded into Furo[3,4 -b]pyridin-5(7H)-one-7-hydroxy-, 2 (29.7%), 2,3 -Pyridinedicarboxylic acid (22.7%), Quinolinimide (trace) and Furo[3,4 -b]pyridine-5(7H)-one (trace). There were about 25 other unknowns comprising 37.7% with none exceeding 10 % of the total carbon-14 radioactivity.

The carbonyl carbon-14 labeled test substance found in distilled water at the end of 10-day continual exposure was 4.4% of total carbon-14 radioactivity. The only major degradation product derived from carbonyl carbon-14 labeled test substance in distilled water was carbon dioxide. There were about 24 other unknowns comprising 19.0% with none exceeding 10% of the total carbon-14 radioactivity. The test substance was stable in the dark during 10-day incubation period.

In another study the residues of the test substance and the two degradation products, 7-hydroxyfuro[3,4-b]pyridine-5(7H)-one and 2,3-pyridinecarboxylic acid, in aquatic field dissipation for Arsenal 2AS herbicide applied to freshwater ponds in Florida, were determined during summer months (American Cyanamid Company, RES 99-059, 1999). Arsenal 2AS herbicide was applied to the ponds at a rate of 1.6 Ib ae/A.
The residues of the test substance, in pond water from treated pond 11 averaged 25 ppb at 1 hour after treatment and declined to <1.00 ppb at the 42 days after treatment (DAT). The residues of the test substance, in pond water from treated pond 21 averaged 92.9 ppb at 1 hour after treatment and declined to <1.00 ppb at the 42 DAT. The test substance in pond water for pond 21, resulted in a half-life of 3.9 days in this study. The residues of the test substance, in pond sediment from treated pond 11 averaged 4.3 ppb at 1 DAT and declined to <1.00 ppb at the 14 DAT. The residues of the test substance, in pond sediment from treated pond 21 averaged 11.3 ppb at 6 hour after treatment and declined to <1.00 ppb at the 90 DAT. Residues of 7-hydroxyfuro[3,4-b]pyridine-5(7H)-one and 2,3-pyridinecarboxylic acid were also identified in all ponds (< 2.00 ppb after 14 DAT).
Because this study was performed during the summer months, the rapid dissipation observed in this study may not be representative of dissipation rates at other aquatic environments.