| Antibiotics,which is biologically toxic and difficult to be degraded naturally,may enter into aquatic environment in the process of production or use and then pose a great threat to the water safety.Cephalosporins,which are harmful to aquatic organisms and human health,are the most widely used antibiotics and the typical pollutants in antibiotic wastewater.In this paper,cefalexin(CLX)is selected as the representative of cephalosporin antibiotic pollutants and is subjected to the catalytic ozonation with catalyst composited by manganese dioxide as main catalyst and graphene as supporting catalyst so as to evaluate the catalytic reaction and develop an efficient and stable catalytic ozonation technology for antibiotic wastewater treatment.Hydrothermal temperature and hydrothermal time have effect on the crystallinity and morphology of the synthesized manganese dioxide,thus affecting,the catalytic performance of manganese dioxide.When hydrothermal temperature is 140 ? and hydrothermal time is 12 h,the synthesis of manganese dioxide catalytic performance is best.When the dissolved ozone concentration in water is 0.12 mg/L,and the dosage of manganese dioxide is 25 mg/L,the CLX removal rate is 39.1%after 5 min reaction,which is 3.4 times higher than the removal rate of ozone oxidation.Graphene can improve the electron transfer efficiency of the catalyst.Composite manganese dioxide-graphene catalyst(MnO2-GO)is prepared and the amount of graphene added is optimized.The appropriate amount of graphene is 1%wt.After 5 min reaction,the removal rate of CLX with MnO2-GO is 50.8%,which is 1.3 times higher than that of MnO2.However,the catalytic stability of MnO2-GO is poor.In order to improve the stability and performance of the MnO2-GO,we adopt two methods of hetero-atom(nitrogen)modification and amino modification to further optimize the catalyst design.Nitrogen atom modification can improve the catalytic performance of MnO2-NGO.After 5 min reaction,the removal rate of CLX with Mn02-NGO is 57.9%,However,nitrogen atom modification can't improve the catalytic stability of MnO2-NGO.The reason may be that the nitrogen atoms on the surface of the graphene and metal oxide metal atoms can't form stable chemical bonds,which is not possible to strengthen the adhesion of graphene and manganese dioxide.Amino modification can improve the catalytic performance of MnO2-NH2-GO.After 5 min reaction,the removal rate of CLX of MnO2-NH2-GO is 55.5%,and the catalytic stability of it increases significantly.Even if the repeated use of the five times,the catalytic degradation effect of CLX of MnO2-NH2-GO is slightly changed.In the MnO2-NH2-GO,it is found that the MnO2 is tightly attached to the graphene surface.After several reactions,MnO2 and graphene are still closely connected.Free radical quenching experiments show that superoxide free radicals played a leading role in the catalytic ozonation of CLX by MnO2-NH2-GO.Phosphate can compete with ozone molecules to adsorb on the acidic site of catalyst surface.With the addition of phosphate,the catalytic degradation efficiency of CLX decreases by 38%,which proves that the active site of MnO2-NH2-GO is its acidic site on the surface.After catalytic ozonation,the valence state of manganese on the surface of MnO2-NH2-GO changes.The Mn3+/Mn4+decreases,and Mn3+loses electrons,which are transferred to the ozone molecules adsorbed on the catalyst surface through manganese dioxide and graphene,and react with ozone to generate superoxide radicals and degrade CLX.The study shows that for catalytic ozonation CLX in water,the efficiency of MnO2-GO catalyst is better than MnO2.Nitrogen-atom modified catalyst MnO2-NGO can improve the catalytic performance,'but it does not help improve the stability of the catalyst,while the amino-modified catalyst MnO2-NH2-GO can significantly improve the catalytic perfomance and stability at the same time.MnO2-NH2-GO catalytic ozonation can efficiently degrade CLX in water,which is expected to be widely used in the treatment of antibiotic wastewater. |
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