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Concerning biodegradability of tetrasodium hydrogen 2-phosphonatobutane-1,2,4-tricarboxylate ("PBTCNa4"), also results of the parent acid, 2-phosphonobutane-1,2,4-tricarboxylic acid ("PBTC"), are taken into account and a read-across approach is proposed with 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.

Biodegradation in water: screening tests

Following the EU Method C.4-B (Determination of the "Ready" Biodegradability - Modified OECD Screening Test) a degradation rate of 4 % within 28 days was determined for PBTCNa4. Therefore, the test substance is considered to be "Not Readily Biodegradable". Inherent biodegradability of PBTCNa4 was assessed according to the OECD TG 203 B, showing 17% degradation after 28 days. Based on this result the substance is considered to be not inherently biodegradable. Anaerobic biodegradation was determined with a method equivalent or similar to EPA OTS 796.3140 (Anaerobic Biodegradability of Organic Chemicals). As no degradation was observed within 56 days PBTCNa4 is considered as not biodegradable under anaerobic conditions.

Biodegradation in water and sediment: simulation tests

Opposed to OECD guideline 308, not a water-sediment system but the inocula gained from river water and river sediment were separately tested for their ability to degrade PBTC. The test material PBTC was used as recrystallised disodium salt. PBTC as sole source of carbon, both with PBTC and orthophosphate as sources of phosphorus, was found to be not biodegradable by enrichment cultures from river water and river sediment. Biodegradability was found either by certain strains gained from these cultures or if an alternative source of carbon is available. In the latter case degradation was observed, even if in the presence if inorganic phosphate. Both (alternative carbon source, inorganic phosphate) are present in many environmental surface water. Thus, PBTC is biodegradable under environmental conditions equivalent / similar to the test conditions. The biodegradation was shown to be more effective and faster if a certain strain or strain combination gained from this both ecosystems was used. Slow degradation under anaerobic conditions for cultures from rivers sediment and river water is not clearly stated by the publication but can be strongly be assumed based on the presented information. Abiotic degradation was not observed.

The study has shown that biodegradation of PBTC in river water and river sediment under environmental conditions primarily depends on the presence of an alternative carbon source and could be optimized certain strains that can easily be enriched and isolated from these both compartments.

Biodegradation in soil

The degradability of [3,4-14C]PBTC was investigated in three agricultural soils following the OECD guideline 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.