| Due to the extensively utilization of fluoroquinolones(FQs)and insufficient metabolism by human or animal bodies,a large number of antibiotics are excreted into the water environment,which seriously threatens the drinking water and human health due to the induced drug-resistant bacteria and resistant genes.The heterogeneous Fenton process is one of advanced oxidation processes based on the generation of hydroxyl radicals(·OH)through reaction between Fe-based catalysts and oxidant(H2O2).Hydroxyl radical is non-selective,which can attack and mineralize most organics.Heterogeneous Fenton catalysis has the advantages of catalyst recovery,low energy consumption,and simple operation process,and it has been widely investigated.Among various catalysts,the Fe-based catalysts have received the most attention owing to their large natural reserves,low toxicity,and convenient preparation.However,traditional Fe-based catalysts,own some disadvantages such as the poor stability and single active site of Fe species,which limit the heterogeneous Fenton catalytic activity.To overcome the defect of limited active site,the dual-center heterogeneous Fenton system has become a new development direction of Fe-based catalysts.Recently,MnFe2O4 has shown great application potential in photo-Fenton catalysis,persulfate activation,ozone catalysis,and heterogeneous Fenton catalysis.They dual active sites of Mn and Fe promote the catalytic rate,and its spinel structure ensures the stability in catalytic process.It can also be separated and recovered by external magnetic field.However,MnFe2O4 is easy to agglomerate,and the rate of reduction of Fe3+/Mn3+ to Fe2+/Mn2+ is very slow,further limit the overall reaction rate of heterogeneous Fenton.Hence,it is urgent to the develop effective modification methods to reduce the agglomeration of MnFe2O4 particles and overcome the rate limitation of Fenton reaction.In order to further enhance the catalytic activity of MnFe2O4,three MnFe2O4 modified catalysts,including MnFe2O4/CeO2,MnFe2O4/S-ZnO,and MnFe2O4/Cu-Al2O3,were constructed by combining with metal oxide,modifying surface Lewis acid sites,coupling the adsorption and multi-metal catalysis.Their morphology structure,composition,and surface properties were systematically investigated.Fluoroquinolones antibiotics were selected as typical pollutants to evaluate the optimal doping rate of composite.The effects of experimental parameters,such as temperature,H2O2 concentration,catalyst concentration,initial pH of solution,inorganic anions and HA,were studied.Under the optimal condition,the Fenton catalytic performance of different catalysts were evaluated.Combined with free radical quenching experiments,the H2O2 consuming rate and the ·OH concentration were measured,and the XPS result,the enhanced catalytic mechanism of the modified composites were proposed.The recyclability and stability of three modified MnFe2O4 catalysts were evaluated.Finally,the best modification strategy was obtained by comparing the Fenton catalytic performance of the three modified MnFe2O4 catalysts.The main conclusions of this study are as follows:(1)MnFe2O4/CeO2 composites were synthesized using metal oxides as carriers through simple coprecipitation and high-temperature calcination.The loading of CeO2 increased the surface area and pore volume of MnFe2O4/CeO2,successfully reducing the agglomeration of MnFe2O4.Under the optimized conditions(10 mg/L OFX,0.1 g/L of catalyst,29 mM of H2O2 at pH 4.5 and 25℃),MnFe2O4/CeO2 with the Ce/Mn molar ratio of 0.3 showed the highest catalytic performance,where the degradation rate of OFX reached 92.0%in 120 minutes,and its catalytic activity(fisrt-order reaction constant,k=0.025 min-1)was 2.2 times that of pure MnFe2O4(k=0.011 min-1).The results of radical quenching experiments showed that·OHsurface was the main ROS,and the effects of·O2-and 1O2 were weak.Based on the results of XPS characterization,H2O2 consumption rate,and ·OH concentration measurements,the Ce accelerate the redox cycle of Mn(Ⅲ)/Mn(Ⅱ)and Fe(Ⅱ)/Fe(Ⅲ),and promote the rate of free radical production.The rapid redox cycle of Ce(Ⅳ)/Ce(Ⅲ)leads to the generation of a large number of oxygen vacancies on the surface of CeO2,which were conducive to the efficient decomposition of H2O2 and the formation of.OHsurface.The two factors jointly enhance the Fenton catalytic activity of CMFO-0.3.Based on the LC-MS test results and DFT calculations,three pathways of OFX degradation were proposed.The toxicity assessment results showed that the "acute toxicity" and "developmental toxicity" of most intermediates were significantly reduced,while the "bioaccumulation factor" and "mutagenicity" of some products were increased.It is still necessary for sufficient mineralization to ensure safe discharge.Studies have also showed that CMFO-0.3 is active at a wide pH range of 4.5～9.0,has good stability and can remove different types of antibiotics.(2)MnFe2O4/S-ZnO composites were synthesized by coprecipitation and calcination methods,and S-doped ZnO was used as carrier to regulate the surface Lewis acidity of MnFe2O4.The optimal loading amount of S-ZnO was 150 mg(150S-ZnO@MFO).Under the optimized conditions(10 mg/LNOF,0.3 g/L of catalyst,30 mM of H2O2 at pH 6.5),the 150S-ZnO@MFO showed the highest catalytic performance,where the degradation rate of NOF reached 98.3%in 60 minutes,and its catalytic activity was 3.6 times that of pure MnFe2O4.The contribution of reactive species on NOF degradation followed the order of OH>·O2->1O2.FT-IR result demonstrated the abundant-OH groups on the surface of 150S-ZnO@MFO strengthen its surface Lewis acidity,which made the degradation rate of NOF by 150S-ZnO@MFO remain above 90%under neutral(pH=6.5)or even alkaline conditions(pH=9-10),XPS result indicated that the doping of S accelerated the transfer rate of electrons from the-OH groups to Fe3+/Mn3+,thereby accelerating the reduction of Fe3+/Mn3+to Fe2+/Mn2+,ultimately improving the utilization efficiency of H2O2 and promoting the formation of ·OH.(3)MnFe2O4/Cu-Al2O3 composites were synthesized by coprecipitation and calcination methods,and porous Cu-doped Al2O3 obtained by a gaseous template method was used as carriers.The optimal loading amount of Cu-Al2O3 was 150 mg(150Cu-Al2O3@MFO).Under the optimized conditions(10 mg/L CIP,0.25 g/L of catalyst,30 mM of H2O2 at pH 5.4 and 25℃),150Cu-Al2O3@MFO showed the highest removal performance on CIP as the removal rate of CIP by adsorption and degradation was 90,8%within 30 minutes,and its catalytic activity was 7.8 times that of MnFe2O4.The free radical quenching experiment showed that the contribution of ·OHsurface was higher than that of ·OHfree,followed by·O2-.The adsorption rate of CIP on 150Cu-Al2O3@MFO was up to 46.4%.XPS results showed that there were electronic cycles between the active sites of Cu,Mn,and Fe.Adsorption and muti-metals synergistic catalysis jointly enhanced the removal effect of 150Cu-Al2O3@MFO on CIP.(4)The catalytic activity of three modified MnFe2O4 composites were evaluated with CIP as target pollutant.The results showed that 150Cu-Al2O3@MFO had the highest heterogeneous Fenton catalytic activity.Under the identical experimental conditions,its catalytic activity was 5.1 times and 6.7 times that of CMFO-0.3 and 150Cu-Al2O3@MFO,respectively.Therefore,the best modification strategy for MnFe2O4 is the synergistic effect of adsorption and mutimetals catalysis.This study aims to provide modification methods for enhancing the heterogeneous Fenton catalytic activity of MnFe2O4,and provide the theoretical and experimental references for promoting its practical application in wastewater treatment. |
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