Mixture predicted no-effect concentrations of surfactants and antibiotics: Modeling based on experimental testing

Environmental transport pathways of antibiotics resemble those of surfactants, and the two often co-exist in aqueous environments, represent a significant ecological risk. Although the individual toxicities of these chemicals are well-documented, their combined effects and the corresponding mixture predicted no-effect concentrations (PNECs) have not been adequately investigated. This study examined the combined toxicity of fluoroquinolone antibiotics (levofloxacin (LEV) and enrofloxacin (ENR)) and nonionic surfactant (octylphenol ethoxylate (TritonX-100)) through acute toxicity tests on Daphnia magna, a widely used model organism for freshwater toxicity assessment due to its high sensitivity to pollutants and ecological relevance. The concentration addition and independent action models were used to assess the combined toxicity interactions, while the interspecies correlation estimation (ICE) and species sensitivity distribution (SSD) models were employed to estimate the mixture PNECs of surfactants and antibiotics at different mixing ratios. The results indicated that synergistic effects were observed in both sets of binary mixtures. The differences of PNECs caused by varying mixing ratios within the same mixture system were as high as 24.7 %, with values ranging from 260.73 to 325.23 μg/L for the Triton X-100-LEV and from 247.85 to 284.57 μg/L for the Triton X-100-ENR. The study highlights the importance of mixture effects in ecological risk assessments, as ignoring these factors may lead to inaccurate predictions of environmental risks.