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Environmental fate & pathways

Biodegradation in soil

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Description of key information

As there is no data available for tetrasodium hydrogen 2-phosphonatobutane-1,2,4-tricarboxylate ("PBTCNa4") results of the parent acid, 2-phosphonobutane-1,2,4-tricarboxylic acid ("PBTC") are taken into account for this endpoint.
Degradability of radioactive labelled [14C]PBTC was investigated in three soils following the OECD TG 304A. Main degradation product was 14CO2, on average between 15.5 and 27.4 % after 133 d. DT50 between 102 and 142 days were calculated. The recoveries (material balances) for the different test vessels ranged from 101.7 to 105.6 %. Nevertheless, it was shown that PBTC is moderately degradable and thoroughly metabolised to CO2 in soil.

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Additional information

As there is no data available for tetrasodium hydrogen 2-phosphonatobutane-1,2,4-tricarboxylate ("PBTCNa4") a read-across approach with the corresponding parent acid 2-phosphonobutane-1,2,4-tricarboxylic acid ("PBTC") is proposed. 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.

The degradability of [3,4-14C]PBTC was investigated in three agricultural soils following the OECD TG 304A. The test soils maintained under aerobic conditions were German standard soils 1.) BBA 2.1 (sand), 2.) BBA 2.2 (loamy sand), 3.) silt loam from Bayer farm Laacherhof. Start concentration was 0.92 µg PBTC/100 g DW of soil (0.92 ppm). Temperature and soil moisture during total testing period of 133 days were 20°C and about 50% of the respective maximum water holding capacity. The recoveries (material balances) for the different test vessels ranged from 101.7% to 105.6%.

The [14C]PBTC was thoroughly metabolised to 14CO2, the main degradation product, accounting for 21.3% , 27.4%, 15.5% of the applied radioactivity in the soils 1.), 2.) and 3.) after 133 days, respectively. During the incubation period a constant increase of 14CO2 was measured. However, the formation rates of 14CO2 decreased with increasing time parallel to the decrease of the active biomass of soils.

After 133 days low portions of PBTC were recovered by two extractions using aqueous CaCl2 solution (2.1%, 1.4% and 0.2% for soils BBA 2.1, BBA 2.2 and Laacherhof, respectively) indicating a correlation to the textural class of soil. The main portion of radioactivity (pre-dominantly as PBTC) was extracted by extensive HCl extraction. The portion of not-extracted (bound) residue resulting from the [14C]PBTC treatment amounted to 16.8%, 31.8% and 42.1% for the soils BBA 2.1, BBA 2.2 and Laacherhof, respectively. Correlation to the textural class of soil (lowest bound residues in the sand, highest in the silt loam) was observed. The predominant portion of radioactivity as well as of PBTC remaining in soil after 133 days of incubation was not easy to extract indicating a low mobility or leaching potential of PBTC in soils.

The time for disappearance of 50% of PBTC (DT50 value) calculated (1st order) from the results of HPLC (on realistic worst case assumption for peak evaluation) was 142 days, 102 days and 107 days for the soils BBA 2.1, BBA 2.2 and Laacherhof, respectively.

Due to known limitations of laboratory test systems (not all the processes relevant for degradation under outdoor conditions are reflected) the degradation rates reported here do not necessarily reflect the real situation in a natural environment. Nevertheless, it was shown that the PBTC is moderately degradable and is thoroughly metabolised to CO2 in soil.