A mixture of: triphenylthiophosphate and tertiary butylated phenyl derivatives

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Partitioning of substances into the different environmental compartments depends mainly on their physico-chemical properties. Since the water solubility of mixture of triphenyl-thiophosphate and tertiary butylated phenyl derivatives is very low (<= 0.1 mg/L) and the log Pow values high (6.5 – 14.1), the components of the UVCB substance are expected to mainly distribute to sediment and soil. The test substance is not expected to evaporate into the atmosphere due to the low vapor pressure of < 0.0001 Pa at 20 °C. Thus, long-range transport through the atmospheric compartment is not expected. Additionally, an estimated rate of hydrolysis is low as indicated by a respective study with the whole UVCB substance (Ciba 1998). This is supported by a hydrolysis study (OECD 111) with the main component O,O,O-triphenyl thiophosphate (CAS 597-82-0, structure A). After up to 30 days of incubation, the half-life was determined to be 24.2 days (pH 9), 102.4 days (pH 7) and 115.8 days (pH 4) at 25°C. The 30d-DT50 of the expected transformation product (triphenyl phosphate) was determined to be 1.7 days (pH 9), 10.9 days (pH 7) and 77.9 days (pH 4) at 25°C. Increasing phenol concentrations show that both O,O,O-triphenyl thiophosphate and triphenyl phosphate are further hydrolyzed under separation of phenol. However, the abiotic transformation of O,O,O-triphenyl thiophosphate in water to triphenyl phosphate is considered to be low as the hydrolysis half-lives of the triphenyl phosphate are much lower than the half-lives of triphenyl thiophosphate. Furthermore, data on triphenyl phosphate show a difference in the ecotoxicity profile and in the environmental behaviour. Triphenyl phosphate is readily biodegradable and shows acute and chronic effects in toxicity studies to aquatic organisms leading to a classification as Aquatic Acute 1 and Aquatic Chronic 2 according to Regulation (EC) No 1272/2008 (CLP/GHS) (ECHA dossier CAS 115-86-6, January 25, 2016). As O,O,O triphenyl thiophosphate does not fulfill the criteria for ready biodegradability and shows no toxic effects to aquatic organisms up to the solubility limit it can be assumed that the transformation to triphenyl phosphate takes place to a minor extent in aquatic media.   

During use as lubricant additive Triphenyl thiophosphate (structure A) degrades to triphenyl phosphate which forms a multilayered solid film on the metal or metal oxide which will be further degraded. In a tribotesting study at 150 °C with poly-α olefin (PAO) it could be shown that the reaction products are pyrophosphate, organo-phosphate and sulfate species. This degradation is metal catalyzed. Oxygenated compounds produced by the oxidation of the base oil adsorbed onto the iron surface and reacted with it to form carbonates and carboxylates (Mangolini et al. 2011, 2012). The same mechanism is assumed for the butylated constituents.

Based on biodegradation screening and simulation tests, the substance is considered to be persistent in the environment. The substance to register was found to be poorly biodegradable in a Closed bottle test according to OECD guideline 301D (Huntington 1996). In addition, several biodegradation data are available for the main component Triphenyl thiophosphate (structure A). 17.8 and 19.3% mineralization (CO2 evolution) was observed in 28 days in a modified Sturm test (OECD 301B). Further investigations during experimental phase using radiolabelled test item showed that only 51.5% and 60.8% of the radioactivity consisted of the test item, whereas the remaining radioactivity consisted of its degradation products. Thus, the test item is not completely biodegraded over a 28-day period under the test conditions but up to 19% is mineralized and additionally up to 48.5% of the test item is transformed into its transformation and degradation products. Phenol can be identified from the chromatogram of the degradation products. This is supported by the results of the hydrolysis study, which showed that different transformation products of the test substances are generated in parallel once hydrolysis has started. The results indicate that the test item cannot be classified readily biodegradable but well primarily degradable under conditions of the test. An inherent biodegradability test with Structure A according to OECD 302C confirmed this result. The test material showed up to 59.5 and 66.8 % biodegradation after 28 days. These results indicate that the test item can be degraded if sufficient degrading microorganisms are available; however, this is not necessarily be the case in surface water. A study investigating the biodegradation of O,O,O-triphenyl phosphorothioate under environmental conditions according to GLP and OECD guideline 309 using radiolabelled test material showed only very low mineralization of 3.1 – 3.7%. No organic volatiles were detected (≤ 0.1 % AR). The amount of parent after 61 days was in the range of 80.2 % to 90.5 % AR. The DT50 value is determined to be > 61 days.  Reason for the low degradation are assumed to be low microorganism concentrations and adsorption of the components to DOC which might lower bioavailability of the substances to microorganisms. High adsorption to the solid phase is expected based on measured data and QSAR calculations (log Koc > 5). The adsorption potential increases with increasing butylation.

In a weight of evidence approach using experimental data on the whole substance and QSAR on the different constituents the bioaccumulation potential of the constituents was assessed. The main component O,O,O-triphenyl phosphorothioate (structure A) was identified as bioaccumulative (BCF > 2000); however, the study was performed with test concnetrations above the solubility in water. The other butylated constituents have a lower bioaccumulation potential in the experimental study supported by QSAR. Supported by the Diammax Average it can be concluded that constituents B to I are not bioaccumulative (BCF < 2000). A new bioaccumulation test with the main component at relevant concentrations would be needed to conclude on the bioaccumulation potential of the substance.

 

Mangolini F, Rossi A, Spencer ND. 2012. Tribochemistry of triphenyl phosphorothioate (TPPT) by in situ attenuated total reflection (ATR/FT-IR) tribometry. J Phys Chem 116: 5614 - 5627

Mangolini F, Rossi A, Spencer ND. 2011. Influence of metallic and oxidized iron/steel on the reactivity of triphenyl phosphorothioate in oil solution. Tribol Int 44: 670 - 683