Effective reduction of metronidazole over the cryptomelane-type manganese oxide octahedral molecular sieve (K-OMS-2) catalyst: facile synthesis, experimental design and modeling, statistical analysis, and identification of by-products.

High concentrations of antibiotic compounds within pharmaceutical wastewater have hazardous impacts toward environment and human health. Therefore, there is an immediate requirement of efficient treatment method for removal of antibiotics from aquatic environment. In the present study, the cryptomelane catalyst-type manganese oxide octahedral molecular sieve (K-OMS-2) was synthesized in the presence of benzyl alcohol as a reducing agent and cetyltrimethylammonium bromide as a structure-directing agent and then utilized to reduce the metronidazole. The central composite design method was the experimental design adopted. The FESEM analysis revealed that the K-OMS-2 surface contained many uniformly cylindrical aggregates less than about 40 nm in diameter and about 80-100 nm in length. Besides, a high specific surface area of 129 m/g and average pore size of 45.47 nm were recorded. According to the TGA/DTA analysis, the prepared catalyst revealed high thermal stability. The maximum metronidazole degradation (95.36%) was evident at conditions of pH = 3, catalyst mass = 0.97 g/L, contact time = 200 min, and metronidazole concentration = 20 mg/L. Metronidazole did not form a complex with nitrate, fluoride, sulfate, or hardness. These ions exerted a negligible effect on metronidazole reduction using the K-OMS-2 catalyst, except for hardness, which reduced the removal efficiency of metronidazole by 17%. The FTIR and LC-MS revealed a complex mechanism involved in the metronidazole degradation by the K-OMS-2 involving the formation of an amino group, a hydroxyelated compound via N-denitration, and hydrogenation process on the K-OMS-2 catalyst surface.