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Environmental fate & pathways

Phototransformation in water

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Description of key information

Photodegradation in water (DTPMP-H, CAS 15827-60-8): 4 - 91% transformation (phosphonate to orthophosphate) after 17 days under a range of conditions.

Key value for chemical safety assessment

Additional information

Two studies are available focussing on stability in water due to photodegradation mechanisms. Whilst this is not a conventional pathway for study it brings useful evidence for environmental fate in the real environment.

Photodegradation of DTPMP-H (15827-60-8) in water was examined (Saeger, Monsanto (undated, believed to be 1979), reliability 2). 14% transformation (phosphonate to orthophosphate) was measured after 17 days at pH 7 (4% at pH 4 and 10). Levels of degradation in the presence of ferric nitrate were higher, with 36% transformation by day 3 in the presence of ferric nitrate at pH 7 (by day 17: 70% at pH 7, 91% at pH 4 and 47% at pH 10). The effect of other metals (chromic, zinc and cupric ions) was insignificant.

In a separate test, half-lives less than 1 hour were measured in water at pH 3, pH 5-6 and at pH 10, irradiated by a middle pressure mercury lamp emitting between 190 and 600 nm. Half-lives were found to be shorter in the presence of iron ions at environmentally relevant concentrations.

Degradation in the presence of ferric (Fe III) ions reflects the ability of that ion to absorb light, and because it can be strongly complexed by DTPMP, that energy can be transferred to the complexing anion, resulting in degradation. It is possible that ferrous (Fe II) ions would be formed in this process, and, due to the presence of oxygen, ferric ions would be regenerated.

The degradation product identified in this study is orthophosphate. No specific reaction pathway is proposed by the study authors.

Discussion of trends in the DTPMP category

Photodegradation in the presence of common metal ions has been observed. Based on evidence from a number of studies members of this group are considered to be partially degradable over short time periods, and with evidence of mineralisation, particularly in the light, over longer periods.