Operational parameters affecting MB/Red-light photosensitized degradation of pharmaceuticals

The methylene blue photosensitization under red light irradiation (MB/Red-light) is a promising and powerful tool for removal of pharmaceuticals from wastewater. To further develop this new technology, the present work aimed at studying the effect of operational parameters on the performance of MB/Red-light pharmaceuticals removal processes. Three pharmaceuticals, i.e. diclofenac (DFN), propranolol (PRP), and sulfamethoxazole (SFZ), were used as model compounds, and degradation rate constants and total compound removal were examined. The three operational parameters studied were initial MB concentration (0–5.0 mg/L), initial pharmaceutical concentration (0.1–2.0 mg/L), aeration rate (0–5.0 L/min), and for DFN also the pH. The results show that degradation of pharmaceuticals was promoted with increasing initial MB concentration at values of [MB] below 0.5 mg/L, and leveled off to constant values at [MB] values higher than 2.0 mg/L. Initial pharmaceutical concentration and aeration rate were found to have no significant impact. Moreover, rapid degradation of pharmaceuticals can take place even at low initial dissolved oxygen concentrations (2.0 mg/L, i.e. situations without aeration). In order to better understand the effect of pH on the MB/Red-light pharmaceutical degradation processes, DFN was chosen for more detailed investigation, with identification of the degradation products formed under neutral and alkaline conditions identified by LC–MS/MS. The pH was found to play an important role on the transformation pathways and formation of degradation products.