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Assessment of Aquatic Toxicity Based on Assessment of Hydrolysis Products

 

Based on experiences with attempting to design and conduct appropriate studies to investigate the ecotoxicity of alkyl and aryl phosphites, it was determined that conducting aquatic toxicity studies on diphenyl isodecyl phosphite (DPDP) in algae, daphnia and fish would not be possible. This conclusion is consistent 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 hydrolysis). Based on the expert approach that was developed for alkyl and aryl phosphites, it was concluded that, since direct measurements of the ecotoxicity of the parent test substance (DPDP) would not feasible, the evaluation should focus on quantifying the toxicity of the combination of hydrolysis by-products of DPDP – namely phenol, isodecanol and phosphorus acid.

 

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. To achieve this, measured and calculated aquatic toxicity values for the individual primary hydrolysis by-products of DPDP (phenol, isodecanol and phosphorous acid) were identified. Using the estimated water solubility of DPDP of 2.3 x 10-4mg/L maximum theoretical concentrations of the individual hydrolysis by-products in water were calculated. These maximum estimated concentrations were then compared to the aquatic toxicity values for the hydrolysis products and ratios for each were summed to develop a maximum theoretical solution toxicity. A derived solution toxicity value of 1.0 would be considered to be equivalent to a toxicity value for an “aged” (hydrolyzed) solutions. The further this value is below 1, the lower the anticipated ecotoxicity hazard.

 

Results

 

one mole DPDP (mw= 374.46 g/mole) yields two moles phenol (mw = 94.11 g/mole), one mole of isodecanol (avg. mw = 158.29 g/mole) and one mole phosphorous acid (mw= 82.00 g/mole);

 

 

At its aqueous solubility limit of 2.3 x 10-4mg/L = 2.3 x 10-7g/L = 5.6 x 10-10 moles/L (2.3 x 10-7g/L ÷ 374.46 g/mole), DPDP would hydrolyze to 1.12 x10-9 moles/L phenol, 5.6 x 10-10 moles/L isodecanol and 5.6 x 10-10 moles/L phosphorous acid. These molar concentrations equate to mass concentrations of:

Phenol: 1.1 x10-7 g/L or1.1 x10-4mg/L

Isodecanol: 8.86 x 10-8g/L or8.86 x 10-5mg/L

Phosphorous Acid 4.6 x 10-8g/L or4.6 x 10-5mg/L

 

These concentrations were then compared to the aquatic toxicity values for phenol, isodecanol and phosphorus acid. 

 

Conclusions

 

Phenol

 

The phenol EC50 values for daphnia is 8.9 mg/L, fish is 10.1 mg/L, and algae is 144.2 mg/L. These are all approximately 4-5 orders of magnitude higher than the maximum estimated concentration of phenol in water from the hydrolysis of DPDP. It should also be noted that the aquatic PNEC for phenol (used in the DPDP CSR) is 7.7 µg/L and this value is 70 times higher than the maximum achievable concentration of phenol from DPDP.

 

Isodecanol

 

The lowest aquatic toxicity value for isodecanol was a NOEC of 0.4 mg/L for 16-day exposure to daphnia. This value is approximately 4-5 orders of magnitude higher than the highest estimated theoretical concentration for isodecanol from DPDP.

 

Phosphorus Acid

 

The EC50 values for phosphorus acid are 383 mg/L for acute fish, 387 mg/L for acute daphnia, and 230 mg/L for algae. These values are all approximately 8 orders of magnitude higher than the highest estimated theoretical concentration for phosphorus acid from DPDP. 

 

Overall

 

Based on these results, it does not appear to be possible to achieve a sufficient concentrations of the hydrolysis products of DPDP to achieve aquatic toxicity.

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