Efficiency And Mechanism Of Ciprofloxacin Degradation By Peroxymonosulfate/Cobalt Sulfide Hollow Nanospheres Activation System

Antibiotics are frequently occurred in the various natural water systems owing to human activities.As one class of emerging pollutants,they are hard to remove effectively in the traditional wastewater treatment plants which are designed to remove conventional contaminants.In addition,antibiotic resistance is an urgent threat to water resource sustainability and global health.Thus,highly efficient and low-cost technologies for antibiotics removal from wastewater have drawn significant attention.Sulfate radical based advanced oxidation processes hold great promise for removal of emerging pollutants among the purification technologies.The heterogeneous cobalt-based catalysts are of great concern among PMS activators due to high efficiency,facile operation,low cost and good resusablity.Unfortunately,some disadvantages do exist,including nanoparticle aggregation,cobalt leaching,complex preparation.Thus,it is of significance to develop new Co-based nanostructure catalysts to activate peroxymonosulfate（PMS）and degrade typical antibotics.In this study,ciprofloxacin（CIP）is selected as the model pollutant.Co S₂,Co₃S₄,and Co₉S₈ hollow nanospheres（Co S_x HNSs）were synthesized at200°C for 9 h via a one-step facile hydrothermal method using cobalt-ethylenediamine complex as a precursor and carbon disulfide（CS₂）as a soft template and sulfur source.Co S_x HNSs with different phases and compositions were realized by simply changing the molar ratio of cobalt（II）acetate to CS₂.SEM and TEM images indicate that the Co₉S₈,Co₃S₄,and Co S₂ nanospheres consist of randomly assembled nanosheets.As the molar ratio of cobalt（II）acetate to CS₂ increases,the resulting product’s phase changes from Co S₂to Co₃S₄ and eventually Co₉S₈,with the increasing nanosphere diameter,tighter nanostructure and decreased surface roughness.The Brunauer–Emmett–Teller（BET）specific surface area is measured to be 22.30,51.79,and 29.88 m² g^-1 and the average pore diameter is 22.8,22.9,28.4 nm for Co₉S₈,Co₃S₄,and Co S₂ HNSs,respectively.XPS Co 2p spectra indicate Co（II）/Co follows the order Co S₂HNSs>Co₃S₄HNSs>Co₉S₈HNSs.The ciprofloxacin（CIP）adsorption capacity follows the order Co₃S₄HNSs>Co₉S₈HNSs>Co S₂HNSs,while the CIP degradation efficiency follows the order of Co S₂HNSs>Co₃S₄HNSs>Co₉S₈HNSs.The Co S₂ HNSs/PMS system is proved to be the best system to degrade CIP.The possible reasons for different catalytic activities of Co S_xHNSs are related to their chemical composition,elemental valence,and crystal structure.The order of those factors is chemical composition<elemental valence<crystal structure.The effect of operating parameters on performance of Co S₂ HNSs/PMS system was explored,including catalyst dosage,CIP concentration,PMS dosage,p H,temperature,anions and natural organic matter（NOM）.Results showed that reaction rate constant varied inversely with the initial CIP concentration,while it increased with the catalyst dosage increasing.It initially increased and decreased with PMS concentration increasing afterwards.Co S₂ HNSs exhibited highly catalytic activity in a wide p H range of 3–10.Complete removal of 10 mg/L CIP was achieved by Co S₂ HNSs in 20 min at initial p H of 8.0 with 62.6%CIP mineralization.At the reaction temperature of 35℃,CIP was removed totally in 3 min.The catalytic performance dropped in the presence of chloride,phosphate,nitrate ions and humic acid due to radical scavenging effects.Above 97%CIP removal was achieved even in the sixth run,with the cobalt leaching content of 0.07 mg/L.This demonstrates the catalytic stability and reusability for CIP degradation.Unique properties of Co S₂ make it a promising strategy for popularization and application.To gain insight into the reaction mechanism,radical quenching tests and electron paramagnetic resonance（EPR）were performed to identify the primary reactive oxygen species involved in the Co S₂ HNSs/PMS system.Both·OH and SO₄^·-were generated and the latter played a key role in CIP degradation.The degradation pathway of CIP was proposed based on HPLC–MS/MS analysis of fourteen CIP intermediates,and two new intermediates,namely C₁₅H₁₈O₄N₃F（m/z 323）and C₂₉H₃₁O₄N₆F（m/z 546）,were identified for the first time.There exists the redox cycle between Co（II）and Co（III）,which further enhances the persistence of Co S₂ HNSs/PMS system.This work demonstrates phase and coordination dependent catalytic properties and will provide insight into the future development of high-performance nanocatalyst for pollutant removal.A systematic study on the influencing factors,universality,reusability and stability of the Co S₂ HNSs/PMS was carried out.The mechanism on the CIP degradation in the water was analyzed and discussed.It is of significance to provide the thereotical basis and technical support for efficient and economic treatment of refractory organic pollutants in industrial and pharmaceutical wastewater.