Oxidation of methylparaben (MeP) and p‑hydroxybenzoic acid (p-HBA) by manganese dioxide (MnO2) and effects of iodide: Efficiency, products, and toxicity.

It is reported that methylparaben (MeP, a widely used phenolic preservative) and its major metabolite p‑hydroxybenzoic acid (p-HBA) display estrogenic activity and are frequently detected in various environmental settings. Naturally occurring manganese dioxide (MnO2) plays an important role in attenuation of contaminants released into the environment, and the presence of iodide (I-) may affect these processes. In this work, it was found that both MeP and p-HBA displayed considerable reactivity towards MnO2 with their half-lives increased with decreasing MnO2 concentrations or increasing pH. The presence of I- obviously accelerated the transformation efficiency of MeP and p-HBA by MnO2 with stronger enhancement at higher I- concentrations or lower pH. Dimeric products (e.g., dimeric MeP or p-HBA) were generated from MeP/p-HBA treated by MnO2, and iodinated aromatic products (e.g., mono-/di-iodinated MeP/p-HBA) were additionally identified in the presence of I-. Higher concentrations of these iodinated aromatic products were generally formed at higher I- or lower MnO2 concentrations or lower pH. Ecotoxicity analysis showed that dimeric and iodinated aromatic products were more eco-toxic than parent MeP/p-HBA. This work shows that MnO2 may greatly affect the fate of MeP and p-HBA released into the environment, and the presence of I- can significantly affect these processes.