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Environmental fate & pathways

Phototransformation in water

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Description of key information

As there is no data available for tetrasodium hydrogen 2-phosphonatobutane-1,2,4-tricarboxylate, results of the parent acid, 2-phosphonobutane-1,2,4-tricarboxylic acid are taken into account for this endpoint.
The results of modelling based on the irradiation of 2-phosphonobutane-1,2,4-tricarboxylic acid in buffer pH 9 and in presence of FeCl3 indicated that the mean photolysis half-life should range from 2-3 days in summer to 15-65 days in winter. The results of modelling based on the irradiation of 2-phosphonobutane-1,2,4-tricarboxylic acid in pure water and stored in brown glass prior to irradiation indicated that the mean photolysis half-lives should range from 0.2-0.3 days in summer to 1-10 days in winter.

Key value for chemical safety assessment

Additional information

Based on the absence of data for tetrasodium hydrogen 2-phosphonatobutane-1,2,4-tricarboxylate ("PBTCNa4"), the read-across approach is proposed with 2-phosphonobutane-1,2,4-tricarboxylic acid ("PBTC").

In aqueous media, PBTCNa4 and PBTC dissociate into the corresponding anion (2-phosphonatobutane-tricarboxylate ion) and the sodium ion and hydrogen ion (proton), respectively. Fate, behavior and the ecotoxicological properties of PBTC and its tetrasodium salt are thought to be an effect of the phosphonato-carboxylate ion rather than of the sodium ion or the hydrogen ion (proton), which are normal constituents in environmental systems and have no relevant ecotoxic properties in low concentrations.

Therefore a read-across between PBTCNa4 and PBTC is justified.

In a comprehensive study, the irradiation of [14C]PBTC with simulated sunlight (artificial light source) in a merry-go-round apparatus showed different results. Generally, the formation of a reactive photosystem, probably by complexation of ions [e.g. Fe (III)], was found to be necessary for an absorption of environmentally relevant light (wave length > 290 nm) as well as for a transformation of PBTC to a slightly less polar main photoproduct. Depending on the marginal conditions in the irradiated solutions [pH and ion concentration, e.g. Fe (III)] an equilibrium between PBTC and photoproduct at quite different ratios was to be observed. The most effective transformation at a low level of remaining PBTC in the irradiated solutions was determined for pH 9 in the presence of Fe(III), but still more transformation was observed in pure water stored together with PBTC in a brown glass vessel prior to irradiation. Higher amounts of ions being present in the test solution, e. g. Fe (III), probably decreased the transformation rate or enhanced the back-reaction to PBTC. This influence was already described by Kleinstück [Degradation of phosphonic acids in aqueous solution by light (KSK 4410). BAYER AG, Leverkusen, AC, unpublished report no. AC-6-1252-/m (1990).]. The butane-1,2,4-tricarboxylic acid (BTC) was found to be a final product of PBTC photolysis, but the amounts at the respective sampling periods could not be measured, because a poor chromatographic separation from the main photoproduct. Therefore, no further information about the stability of the main photoproduct could be given. The corresponding irradiation in natural water (Hönniger Weiher) resulted in a PBTC half-life of about 30 minutes and a PBTC steady state concentration of less than 10% of initial after 420 minutes. This proved that photolysis can contribute to the overall degradation of PBTC in natural waters. The estimates of "environmental photolysis half-lives" based on two different arithmetic models (GC-SOLAR and Frank & Klöpffer) by means of the resulting quantum yields and the light absorption data in the environmentally relevant range of wavelengths were well comparable when considering identical marginal conditions. The results of modelling based on the irradiation of PBTC in buffer pH 9 in presence of FeCl3 indicated that the mean photolysis half-life should range from 2-3 days in summer to 15-65 days in winter. The results of modelling based on the irradiation of PBTC in pure water, stored in brown glass prior to irradiation indicated that the mean photolysis half-life should range from 0.2-0.3 days in summer to 1-10 days in winter. The before-mentioned assessments did not consider any indirect photodegradation mechanisms which could increase the degradation in natural waters.

For a computer-based assessment of the so-called environmental photolysis half-life in water the quantum yield of direct photoreaction of [3,4-14C]PBTC, should be determined according to the ECETOC method in polychromatic light.

The irradiation of [14C]PBTC with simulated sunlight (filtered Xenon light) in a merry-go-round apparatus showed different results. Generally, the formation of a reactive photosystem, probably by complexation of ions [e.g. Fe(III)], was found to be necessary for an absorption of environmentally relevant light (yield > 290 nm) as well as for a transformation of PBTC to a slightly less polar main photoproduct.

Depending on the marginal conditions of irradiation [pH and ion concentration, e.g. Fe(III)] an equilibrium between PBTC and main photoproduct at quite different ratios was observed. An effective transformation at a low level of remaining PBTC in the irradiated solutions was determined for pH 9 in the presence of Fe(III). The quantum yield was calculated to be 0.0022. More effective was the transformation in pure water being stored together with PBTC in a brown glass vessel prior to irradiation. In that case, only traces of e.g. Fe or Mn ions can have been dissolved, but the quantum yield was calculated to be 1.84. Higher amounts of ions being present in the test solution, e.g. Fe(III), probably decreased the transformation rate or enhanced a back-reaction to PBTC.

The butane-1,2,4-tricarboxylic acid (BTC) was found to be a final product of photolysis of PBTC. The quantities of BTC at the respective sampling periods could not be determined, because of a bad chromatographic separation from the main photoproduct. Therefore, no further information about stability of the main photoproduct could be given.

The corresponding irradiation of [14C]PBTC in a natural water (Hönniger Weiher, NRW) resulted in a PBTC half-life of about 30 minutes and a PBTC steady state concentration of less than 10% of the initial concentration of 1 mg/L. This proved that photolysis can contribute to the overall elimination of PBTC in natural waters. The measured fast photo-transformation was unexpected, because not any absorption of light above 230 nm was measurable. Therefore, not any calculation of a quantum yield according to the above-mentioned method was possible. Nevertheless, the acting quantum yield must have been extremely high (yield >> 1), because that fast photo-transformation was measured.

The estimates of "environmental photolysis half-lives" based on two different arithmetic models (GC-SOLAR and Frank & Klöpffer) by means of the resulting quantum yields and the light absorption data in the environmentally relevant range of wavelengths were well comparable when considering identical marginal conditions. The results of modelling based on the irradiation of PBTC in buffer pH 9 and in presence of FeCl3 indicated that the mean photolysis half-life should range from 2-3 days in summer to 15-65 days in winter. The results of modelling based on the irradiation of PBTC in pure water and stored in brown glass prior to irradiation indicated that the mean photolysis half-lives should range from 0.2-0.3 days in summer to 1-10 days in winter.