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Assessment of Aquatic Toxicity of Triphenyl Phosphite Based on Assessment of Hydrolysis Products

In attempting to design appropriate studies to investigate the acute ecotoxicity of TPP in algae, daphnids and fish, it was discovered that the testing would be difficult to conduct in compliance with OECD Guidance Document #23 entitled “Guidance Document on Aquatic Toxicity Testing of Difficult Substances and Mixtures” (OECD 2000) because of the inherent physical/chemical properties of the test substance (i.e., extremely poor water solubility and rapid hydrolysis at the calculated limit of solubility). After consultation with experts, it was concluded that, since direct measurements of the ecotoxicity of the parent test substance would likely be very difficult the testing program should focus on quantifying the toxicity of the combination of hydrolysis by-products of "aged" TPP.


Measured and calculated aquatic toxicity values (EC50) for the individual primary hydrolysis by-products of TPP (phenol and phosphorous acid) were identified. With these data, estimates of the acute ecotoxicity values (solution toxicity values) based on the sum of the ratios of the maximum theoretical concentration of the individual hydrolysis by-products (i.e., “aged” TPP) in solution (at the limit of solubility) and their representative ecotoxicity values (EC50) were found to be 2.2 x 10-3(algae), 2.7 x 10-2(daphnids) and 3.1 x 10-2(fish). Since a derived solution toxicity value of 1.0 is considered to be equivalent to anEC50for the “aged” solutions, the further below a value is from 1 the lower the inferred ecotoxicity hazard. On this basis, these findings indicate that the mixture of hydrolysis by-products arising from a standard solution of TPP would not be toxic to aquatic organisms.


It is possible to quantitatively predict the maximum theoretical concentration of hydrolysis by-products and resulting toxicity of the solution of these by-product based on the known toxicity of the individual by-products, assuming additive toxicity.


On this basis, a detailed literature search was conducted to assess the available database of information pertaining to the acute toxicity of phenol and phosphorous acid to fish (rainbow trout), daphnids and algae. Where sufficient data were unavailable in the scientific literature, ECOSAR (US EPA 2001) was used to model the potential ecotoxicity of the hydrolysis by-products of TPP. A summary of the acute ecotoxicity data for these byproducts to these organisms is provided in the report.


By this method, a solution toxicity value of 1.0 is considered equivalent to an acute EC50value (i.e.,itwould predict 50% acute mortality /growth inhibition in the test species, based on the theoretical concentrations). 




Based on the following stoichiometry:


one mole TPP (mw= 310.29 g/mole) yields three moles phenol (mw = 94.11 g/mole) and one mole phosphorous acid (mw= 82.00 g/mole);


At its aqueous solubility limit of 0.3 mg/L, TPP would hydrolyse to 2.9 x 10-6moles/L phenol and 9.7 x 10-7moles/L phosphorous acid. These molar concentrations equate to mass concentrations of 0.27 mg phenol/L and 0.079 mg phosphorous acid/L respectively.


The modeling results of the solution toxicity values for the mixture of hydrolysis by-products of TPP at its aqueous solubility limit in fish, daphnids and algae are summarized below.

 Aquatic Toxicity Values  Phenol EC50 (mg/L) Calculated Phenol Concentration (mg/L)  Phosphorus Acid EC50 (mg/L) Calculated Phosphorus Acid Concentration(mg/L)    Calculated Solution Toxicity Values
 Fish 8.9  0.27  383  0.079  0.031
 Daphnia  10.1  0.27  387  0.079  0.027
 Algae  144.2  0.27  230  0.079  0.0022




The predicted solution toxicity values attributable to the mixture of TPP hydrolysis by-products were calculated to be2.2 x 10-3(algae), 2.7 x 10-2(daphnids) and 3.1 x 10-2(fish). These values are well below 1.0 indicating that the mixture of hydrolysis by-products arising from a saturated solution of TPP would essentially be not toxic to aquatic organisms. While a classification of R50 -53 was established for TPP, there is no clear basis for this classification and due to the rapid hydrolysis of TPP it is simply not possible to test its aquatic toxicity reliability. The hydrolysis products for TPP, phenol and phosphorous acid, are not classified as dangerous to the environment, further supporting the conclusion that TPP will not be toxic to aquatic organism.