Highlights
- Chronic diclofenac (DCF) exposure was tested in Daphnia magna at pH 7.0 and 8.7.
- An unexpected pH increase in M4 medium was observed during the experiment.
- The NOEC (no observed effect concentration) of DCF was 5 mg l-1 at pH 8.7.
- Initial pH 7.0 prolonged the time to release of the first hatchlings.
- At 1.3 mg l-1 DCF, fewer neonates were observed at pH 7.0 than at pH 8.7.
Abstract
Pharmaceuticals, as ionizable compounds, are a challenging group of pollutants to analyze because the pH of the environment can alter its ecotoxicological features. However, changes in the toxicity of pharmaceuticals toward aquatic organisms were observed even when there was no change in the ionization of the molecules with a pH shift. Therefore, we conducted a study that aimed to check how pH influences the chronic toxicity of diclofenac (DCF, pKa ≈ 4.0) toward Daphnia magna at two pH levels, 7.0 and 8.7, where DCF is obtained as relatively polar anion. The performance of the experiment with OECD 211 was found to be challenging because of the medium pH shift during the exposure test. Acidification increased the toxicity of DCF, reducing the number of neonates (at a concentration of 1.3 mg l-1) and showing a tendency towards delayed hatching and reduced number of hatchings. In addition, the acidification itself also changes D. magna reproduction, affecting the number of hatchings and the first day of hatching. We conclude that the pH, as a factor of toxicity modulation for ionizable compounds, needs to be evaluated and presented in scientific protocols.
Bethke K. & Caban M., 2025. Effect of acidification on the chronic toxicity of diclofenac to Daphnia magna. Aquatic Toxicology 287: 107497. doi: 10.1016/j.aquatox.2025.107497. Article (subscription required).
