Ecotoxicity assessment of phthalic acid di-alkyl ester mixtures toward aquatic organisms based on results from a series of subchronic toxicity tests

Phthalic acid di-alkyl esters (PAEs) constitute a class of industrial compounds with a wide range of carbon chain lengths and physicochemical properties that are predominantly used as plasticizers. Although multiple PAEs co-occur in water environments, which poses a combined exposure risk to aquatic organisms, no studies have quantitatively demonstrated the mixture ecotoxicity of multicomponent PAEs. This study performed a series of subchronic toxicity tests on three aquatic organisms (green alga Raphidocelis subcapitata, daphnid Ceriodaphnia dubia, and fish Danio rerio), using a total of 12 PAEs with the number of carbons in the alkyl chains ranging from C1 to C11 to determine their toxicity profiles and the combined effects. Shorter-chain PAEs (C1-C6) generally exhibited increasing toxicity associated with a logarithm of the octanol-water partition coefficient (log KOW), whereas for fish, C5 and C6 PAEs deviated from the log KOW-toxicity relationship. Longer-chain PAEs (C8-C11) were nontoxic below their solubility limits. Mixture toxicity tests with six shorter-chain PAEs at the equivalent toxic unit (calculated by exposure concentration/50% inhibition concentration) demonstrated good agreement between the observed concentration-response curves and predicted curves using the concentration addition (CA) model across all test organisms. These results provide a practical dataset to assess the mixture toxicity of multiple PAEs as well as insights into the applicability of the CA model to chemical groups based on structural similarities and toxicity profiles.