Unraveling the time evolution and post mortem changes of nanometric MnOOH during in situ chemical oxidation of ciprofloxacin by activated PMS

Water contamination by synthetic organic compounds and effective methods to remove them are hot topics that deserve special attention due to their impact on human’s life. In this work, nanometric MnOOH compound was synthesized (using a greener approach), characterized, and used to remove ciprofloxacin (CIP) antibiotic by in situ chemical oxidation using peroxymonosulfate (PMS) as oxidizer. The concentration of MnOOH (0.5, 1, and 2 g L^–1), PMS (1, 2, and 4 g L^–1), pH (3, 7, and 10), and morphological, structural, chemical, and electrochemical changes during/after use of MnOOH were investigated. The CIP molecule was completely oxidized and mineralized (> 60%) after 6 h under acidic conditions, without significant contributions of dissolved Mn(II) species. Low amount of intermediate compounds derived from CIP were detected by liquid chromatography coupled to mass spectrometry (LC-MS/MS). CIP degradation was carried out by produced oxidants (such as HO^• detected by electron spin resonance and LC-MS/MS) from PMS activation or, to a lesser extent, directly on the surface of MnOOH by direct electron transfer. The former was attested by high-resolution transmission electron micrographs due to formation of an amorphous shell structure (MnO₂) over MnOOH crystallites (using X-ray photoelectron spectroscopy measurements) with the consequent increase of the charge transfer resistance that hindered further electron transfer to the PMS oxidant.