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Environmental fate & pathways

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No indication for a hydrolysis after 5 days at 50 °C can be found in the surrogate substance tetrasodium hydrogen 2-phosphonatobutane-1,2,4-tricarboxylate (Holzaht-Grimme, 2021).

Based on the chemical structure of 2-phosphonobutane-1,2,4-tricarboxylic acid, hydrolysis is not expected under temperatures and pH values occurring in the environment (Allmendinger, 2008). Based on practical industrial experiences for decades, phosphonic acids are known to be stable towards hydrolysis even at extreme pH values and temperatures. Phosphonic acids are appropriate chemicals to agents in cooling-water treatment and for production of alkine industrial cleaners also because of this property (Grohmann, 1988).


Phototransformation in water

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, the active substance 2-phos-phonobutane-1,2,4-tricarboxylic acid in Bayhibit AM, should be determined according to the ECETOC method in polychromatic light and based on a test guideline ("Phototransformation of Chemicals in Water, Part A" (Umweltbundesamt, Berlin, Germany, December 1992) and other information (Wilmes, R. (1988): BAYER AG, unpublished PF-Report No. 2974). 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 ( lambda > 290 nm) as well as for a transformation of 2-phosphonobutane-1,2,4-tricarboxylic acid 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 2-phosphonobutane-1,2,4-tricarboxylic acid and main photoproduct at quite different ratios was observed. An effective transformation at a low level of remaining 2-phosphonobutane-1,2,4-tricarboxylic acid 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 2-phosphonobutane-1,2,4-tricarboxylic acid.

The butane-1,2,4-tricarboxylic acid (BTC) was found to be a final product of photolysis of 2-phosphonobutane-1,2,4-tricarboxylic acid.

The quantification of BTC at the respective sampling periods was technically not feasible. Therefore, no further information about stability of the main photoproduct could be given. The corresponding irradiation of [14C] 2-phosphonobutane-1,2,4-tricarboxylic acid in a natural water resulted in a 2-phosphonobutane-1,2,4-tricarboxylic acid half-life of about 30 minutes and a 2-phosphonobutane-1,2,4-tricarboxylic acid 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 2-phosphonobutane-1,2,4-tricarboxylic acid 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 ( >> 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 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 (Hellpointner, 1993).