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Environmental fate & pathways

Phototransformation in water

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Description of key information

Photodegradation in water (DTPMP-H): 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 degradation pathway for study it brings useful evidence for environmental fate in the real environment.

Photodegradation of DTPMP-H 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 with DTPMP (1-3Na) (Lesueur, 2005), 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.

In summary, photodegradation in the presence of common metal ions has been observed. Based on evidence from a number of studies, DTPMP-H and its salts are considered to be partially degradable over short time periods, and with evidence of mineralisation, particularly in the light, over longer periods.

The acid and salts in the DTPMP category are freely soluble in water and, therefore, the DTPMP anion is fully dissociated from its cations when in solution. Under any given conditions, the degree of ionisation of the DTPMP species is determined by the pH of the solution. At a specific pH, the degree of ionisation is the same regardless of whether the starting material was DTPMP-H, DTPMP (1-3Na), DTPMP (5-7Na), DTPMP-xK, DTPMP (xNH4) or another salt of DTPMP.


Therefore, when a salt of DTPMP is introduced into test media or the environment, the following is present (separately):

1. DTPMP is present as DTPMP-H or one of its ionised forms. The degree of ionisation depends upon the pH of the media and not whether DTPMP-H, DTPMP (1-3Na), DTPMP (5-7Na), DTPMP-xK, DTPMP (xNH4), or another salt was used for testing.

2. Disassociated ammonium, potassium or sodium cations. The amount of ammonium, potassium or sodium present depends on which salt was added.

3. Divalent and trivalent cations have much higher stability constants for binding with DTPMP than the sodium, potassium or ammonium ions so would preferentially replace them. These ions include calcium (Ca2+), magnesium (Mg2+) and iron (Fe3+). Therefore, the presence of these in the environment or in biological fluids or from dietary sources would result in the formation of DTPMP-dication (e.g. DTPMP-Ca, DTPMP-Mg) and DTPMP-trication (e.g. DTPMP-Fe) complexes in solution, irrespective of the starting substance/test material.