Preparation Of Multi-interface MnO₂-based Heterojunctions For Catalytic Degradation Of Ciprofloxacin Under Visible Light

Photocatalytic oxidation is regarded as a potential energy-saving,efficient and safe water purification technology.The activity of photocatalyst plays a crucial role during contaminants degradation.The development of green catalysts with stable performance is the key to the application of photocatalytic technology in water treatment.MnO₂ is one of the commonly used photocatalyst components with advantages of strong response to visible light,large specific surface area,low isoelectric point and low price.However,at present,the MnO₂ based composite photocatalysts exist problems such as low visible light utilization rate,weak redox ability and low photocurrent density of doped components.Moreover,the influence rule of water quality factors on the corresponding photocatalytic performance and the photocatalytic mechanism of the catalyst is still unclear.In view of the above problems,this study firstly prepared MnO₂/Fe₃O₄ and MnO₂/Mn₃O₄ composite photocatalysts using Fe₃O₄ and Mn₃O₄ as doping components with potentially excellent performance,which were further used as photocatalysts to remove ciprofloxacin in water under visible light.The presence of MnO₂ component could increase the specific surface area and reduce the isoelectric point of the composite catalyst,promoting the removal efficiency of ciprofloxacin with positive charge.The removal efficiencies of ciprofloxacin by MnO₂/Fe₃O₄（molar ratio of MnO₂ and Fe₃O₄ was 3:1） and MnO₂/Mn₃O₄（molar ratio of MnO₂ and Mn₃O₄ was 5:2） reached 74.2% and 95.2% respectively under visible light irradiation for 60 min.Active species capture confirmed that superoxide radicals,holes and hydroxyl radicals played roles in MnO₂/Mn₃O₄ photocatalytic removal of ciprofloxacin,among which superoxide radicals played the prominent role.The enhanced absorption of visible light and higher redox potential in Z-type heterojunction are the main reasons for the enhanced photocatalytic activity of MnO₂/Mn₃O₄.MnO₂/Mn₃O₄/Mn₂O₃ heterojunction photocatalyst was developed to further improve the density and separation performance of photo-induced carriers.The mass fraction of MnO₂,Mn₂O₃ and Mn₃O₄ components in the composite catalyst with superior photocatalytic performance was determined to be 19.15%,52.47% and 23.50% respectively by the thermogravimetric analysis.The photocatalytic activity of MnO₂/Mn₃O₄/Mn₂O₃ was significantly enhanced.The removal efficiency of ciprofloxacin by MnO₂/Mn₃O₄/Mn₂O₃ was as high as 95.6% with dosage of 0.2 g/L under visible light for 40 min,which was equivalent to the catalytic efficiency by MnO₂/Mn₃O₄ composite catalyst under visible light for 60 min.In addition,MnO₂/Mn₃O₄/Mn₂O₃ was found to possess excellent reusability and stability.In order to investigate the influence rules of water quality factors on the photocatalytic degradation of ciprofloxacin by MnO₂/Mn₃O₄/Mn₂O₃ and to evaluate the potential application,the effects of solution pH,co-existing ions and natural organic matter on ciprofloxacin removal were studied.The MnO₂/Mn₃O₄/Mn₂O₃ activity was outstanding under neutral solution condition,which was 20.6% and 6.6%higher than those under pH 5.5 and 8.5,respectively.Coexisting cations mainly reduced the removal rates of ciprofloxacin through charge shielding effect.The inhibitory effect of coexisting Ca²⁺ and Mg²⁺on the removal of ciprofloxacin in solution was greater than that of Na⁺,and the negative effect of coexisting Ca²⁺was higher than that of Mg²⁺.Coexisting Fe³⁺ at low concentrations（5.0μM） facilitated the removal of ciprofloxacin by trapping electrons and promoting the formation of holes and oxygen-containing free radicals.The removal rate of ciprofloxacin was less affected by coexisting Mn²⁺,which was only reduced by 0.5-3.8% when 5.0-15.0μM Mn²⁺ was added.Coexisting anions mainly reduced the removal rate of ciprofloxacin by charge shielding effect and trapping holes and hydroxyl radicals.The inhibitory effect of anions with the same concentration presented the following order:SiO3^2->HCO₃^->HPO₄^2->SO₄^2->Cl^->NO₃^-.Humic acid hindered the adsorption of ciprofloxacin on the catalyst surface and affected the photon transfer,thus reducing its removal efficiency.The removal efficiency of ciprofloxacin decreased by 1.8-8.7% with coexisted humic acid of 2.0-6.0 mg/L.Active species capture and electron paramagnetic resonance（EPR） measurement showed that superoxide radicals and holes dominated during ciprofloxacin degradation.Fluorescence spectra,photocurrent and electrochemical impedance spectra further proved that the density and separation efficiency of photo-induced carriers in MnO₂/Mn₃O₄/Mn₂O₃ composite were significantly improved.In addition,the intermediates and pathways of ciprofloxacin degradation by MnO₂/Mn₃O₄/Mn₂O₃ photocatalysis were analyzed.Nitrogen and fluorine removed from the molecular structure of ciprofloxacin accounted for 71.9%and 55.8%respectively after irradiation for 40 min,suggesting the destruction of quinoline ring.The intermediates with m/z of 364,362,334,316,306,291 and 263 were mainly detected.It was reasonable to speculate that the ciprofloxacin degradation began with the destruction of piperazine by active species.Then the product of m/z=334 partially transformed into another product of m/z=316 by defluorination.The other product of m/z=334 removed ketone group and transformed into the product of m/z=306,which further transformed into the product of m/z=263 by losing N atom via oxidation.Finally,the piperazine ring were completely destroyed.In conclusion,this study developed a MnO₂/Mn₃O₄/Mn₂O₃ heterojunction with multifold electron transport paths,among which visible-light responsive MnO₂ was the core component.The density and separability of photo-induced carrier in MnO₂/Mn₃O₄/Mn₂O₃ were significantly enhanced,thus improving the photocatalytic activity.The effects of water quality factors on ciprofloxacin removal by MnO₂/Mn₃O₄/Mn₂O₃ photocatalysis were systemically investigated and the possible degradation pathways of ciprofloxacin were proposed.This research is of significance to promote the development and application of visible-light-driven MnO₂ based photocatalyst.