Manganese Dioxide - Based Photocatalytic Oxidation Processes for the Removal of Ciprofloxacin from Water: Materials, Mechanisms and Perspectives

Ciprofloxacin (CIP), a broad-spectrum fluoroquinolone, is one of the most commonly detected antibiotics in surface waters and wastewater, contributing to ecological risks and the selection of antibiotic-resistant bacteria and genes. Various methods, including adsorption, photocatalysis, persulfate-based oxidation, and electrochemical oxidation, have been employed for its removal. Manganese dioxide (MnO2) is an earth-abundant, low-toxicity, polymorphic oxide characterized by a narrow band gap (typically 1–2 eV), a rich Mn(IV)/Mn(III)/Mn(II) redox chemistry, and strong surface reactivity towards phenolic, anilinic, and amine-containing micropollutants. This review synthesizes evidence on (i) the occurrence, risk, and resistance associated with CIP, (ii) adsorptive and photocatalytic benchmarks for CIP, (iii) the structure–property relationships of α-, β-, γ-, δ-, and ε-MnO2, (iv) the dark and light-driven transformation of pharmaceuticals on MnO2, and (v) Mn-based catalysts that directly degrade CIP, including Mn/Co oxide photocatalysts and MnO2-activated peroxymonosulfate (PMS) systems. In the literature, δ-MnO2 and ε-MnO2 are identified as the most reactive polymorphs. The oxidation process, including piperazine-ring cleavage, hydroxylation, and F/OH substitution in CIP transformation, is driven by reactive oxygen species (h+, OH, O2 -, 1O2) and Mn(III) intermediates. However, there is a significant gap in research regarding direct, light-driven CIP degradation by pristine MnO2 polymorphs. We propose research priorities focusing on the mechanism, toxicity, real-water testing, and scale-up.