Sour doesn´t always make you smile

Abstract

The report investigates how pH influences the toxicity and bioaccumulation of ionizable chemicals in aquatic environments. Ionizable substances switch between neutral and ionic forms, depending on the pH, which directly affects their uptake and toxicity. Using zebrafish embryos (Danio rerio) as a model, the study showed that both neutral and ionic species contribute to toxicity in a similar manner, driven primarily by differences in their pH-dependent distribution (logD). The toxicity of 24 ionizable substances, including pharmaceuticals (e.g., diclofenac, ibuprofen, fluoxetine) and other chemicals, was tested across 4 different pH levels. A strong correlation was observed between chemical uptake (logD) and toxicity, suggesting that toxicity is driven by uptake differences (toxicokinetics) rather than internal processes (toxicodynamics). Both neutral and ionic chemical species contribute to toxicity in a quantitatively similar manner, with the ionic species simply contributing less. The resulting ΔlogD-based model reliably predicts toxicity across varying pH levels, offering a powerful tool for assessing chemical risks under worst-case scenarios. This approach has already been applied in deriving Environmental Quality Standards for ibuprofen and provides significant potential for improving chemical regulation and environmental risk assessment.