Thermal cracking oil from blends of rubber, fuel oils and paraffin waxes, steam-stripped

Toxicity to aquatic algae and cyanobacteria

Description of key information

It is concluded that the hydrocarbon fraction of the Thermal cracking oil from blends of rubber, fuel oils and paraffin waxes, steam-stripped drives the acute aquatic toxicity of the oil as the nitrile fractions are predicted to have no acute effect at saturation, based on the high log Kow (log Kow of hexadecanitrile was predicted to be 6.73, whilst the log Kow of octadecanitrile was predicted to be 7.71). The chronic toxicity to aquatic plants of the oil as a whole substance was predicted to be 0.035 mg/l (Chronic Value), based on the expected additive effect of the two nitrile components of the oil (at a concentration of 4 % w/w of the oil). The 72 hour EC50 of the hydrocarbon fraction of the UVCB to aquatic algae was estimated to be 0.48 mg/l. 

Key value for chemical safety assessment

EC50 for freshwater algae:

0.48 mg/L

EC10 or NOEC for freshwater algae:

0.12 mg/L

Additional information

The acute and chronic toxicity to aquatic algae of the hydrocarbon fraction of the UVCB substance Thermal cracking oil from blends of rubber, fuel oils and paraffin waxes, steam-stripped, was calculated using the PETROTOX model, made available by CONCAWE, which uses the Hydrocarbon Block Model to predict the eco-toxicological endpoints. The default systems parameters of PETROTOX are designed to mimic the design of typical experimental systems. Therefore the simulations were performed assuming a 10% headspace volume to account for volatilization of some components. Additionally the particulate organic carbon loading was set at 2 mg/L for algae and SSTP organisms, whereas it was left at 0 mg/L for the fish and daphnia models. It is assumed that the presence of organic carbon would result in the absorption of some of the test substance and hence lower the bioavailability. This approach was consistent with the initial validation of the model. The output of the PETROTOX model gives “Toxic Unit” (TU) values at different loadings that are converted to an LD50. This acute toxicity value is then converted to the chronic NOEL using standard acute to chronic ratios. The 72 hour EC50 of the hydrocarbon fraction of the UVCB to aquatic algae was estimated to be 0.48 mg/l. The 72 hour NOEC of the hydrocarbon fraction of the UVCB to aquatic algae was estimated to be 0.12 mg/l.

The potential aquatic toxicity of the non-hydrocarbon components of the UVCB was determined via a literature search for applicable data on the substances. The only non-hydrocarbon components in the substance that have significantly greater toxicity than the hydrocarbon elements are the two nitriles, hexadecanitrile and eicosanitrile. These components make up a combined fraction of 1.7 % w/w of the substance. If the recommended assessment factors applied to the nitriles the toxicity of the overall substance will be mainly due to the nitriles. Therefore the PNEC of the substance will be based on the total nitrile content of the substance.

The acute and chronic toxicity to aquatic plants of the nitrile fraction of the substance was calculated using the ECOSAR model (v 1.11), which was developed by the US EPA and uses a database of measured data to calculate the baseline ecotoxicity of organic compounds. The model calculates the baseline toxicity of a substance by assuming that the substance behaves as a narcotic and the toxicity is correlated to the partition co-efficient. It then highlights functional groups that have been shown to display excess toxicity. Neither nitrile contains functional groups that display excess toxicity and are therefore viewed as “neutral organics”. If a substance has a very high partition co-efficient, it is assumed that the substance is unlikely to be soluble enough for aquatic organisms to be exposed to the substance. The chronic toxicity to aquatic plants of the oil as a whole substance was predicted to be 0.035 mg/l (Chronic Value), based on the expected additive effect of the two nitrile components of the oil (at a concentration of 4 % w/w of the oil). It is concluded that the hydrocarbon fraction of the Thermal cracking oil from blends of rubber, fuel oils and paraffin waxes, steam-stripped drives the acute aquatic toxicity of the oil as the nitrile fractions are predicted to have no acute effect at saturation, based on the high log Kow (log Kow of hexadecanitrile was predicted to be 6.73, whilst the log Kow of octadecanitrile was predicted to be 7.71).