2-phosphonobutane-1,2,4-tricarboxylic acid

Endpoint summary

Additional information

(1) Stability

Hydrolysis

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 (PBTCNa4, Holzaht-Grimme, 2021) and no half-life times and hydrolysis rates were determined. No further tests at other temperatures or pH values are required.

Based on the chemical structure of 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTC), 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 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in Water, Part A" (Umweltbundesamt, Berlin, Germany, December 1992) and other information (Wilmes, R. (1988): BAYER AG, unpublished PF-Report No. 2974).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 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 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]PBTC in a natural water 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 ( >> 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 (Hellpointner, 1993).

 

(2) Biodegradation

Biodegradation in water

PBTC was examined for biodegradability according to the "Modified OECD-Screening-Test" (Guideline 301 E) and according to the "Modified SCAS-Test as described in OECD Guideline 302 A. In both tests, no (inherent) biodegradation was observed under test conditions (Horstmann, Grohmann, 1988). Also no biodegradation was observed in the "Closed Bottle Test" according to OECD Guideline 301 D (Kästner, Gode, 1983). In another test PBTCNa4 was examined for inherent biodegradability according to the " Zahn-Wellens test". Again, no biodegradation was observed (BUA, 1994).

Thus, PBTC is not ready biodegradable in water.

Biodegradation in water and sediment

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. PCBT 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 (Rasche et al., 1994).

Biodegradation in soil

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

It was shown that the PBTC is moderately degradable and is thoroughly metabolised to CO2 in soil (Hellpointner, 1996). However, with a worst-case DT50 of 142 d the substance must be classified as persistant (P).

 

(3) Bioaccumulation

The logarithmic octanol-water partition coefficient (log Kow) for PBTC was predicted using the QSAR calculation of the Estimation Program Interface (EPI) Suite v 4.11. The log Kow was estimated to be -1.66.

A substance is assessed as potentially bioaccumulative, if its log Pow is higher than 3.

Therefore, PBTC is considered as not potentially bioaccumulative.

 

(4) Transport and distribution

According to REGULATION (EC) No 1907/2006 (ANNEX VIII) a study or screening test on adsorption / desorption has not to be conducted if based on the physicochemical properties the substance can be expected to have a low potential for adsorption (e.g. the substance has a low octanol water partition coefficient).

The logarithmic octanol-water partition coefficient (log Kow) for PBTC was predicted using the QSAR calculation of the Estimation Program Interface (EPI) Suite v 4.11. The log Kow was estimated to be -1.66.

Therefore, PBTC is expected to have a low potential for adsorption.