Efficiency And Mechanism Of Levofloxacin Degradation By Se@Co₃O₄/Peroxymonosulfate Activation System In Water

With the rapid development of medical,agriculture,breeding and other industries,antibiotics have been widely used in various fields,but their abuse has caused severe environmental problems.Quinolones have gradually become one of the main contaminants in major rivers,lakes and even oceans of China.Levofloxacin（LEV）,as a representative of the third generation of quinolone drugs,its dosage has leapt to the forefront of the world.Most of the traditional wastewater treatment plants rely on biochemical methods,but they are not suitable for these emerging pollutants which are difficult to biodegrade.Among a series of efficient technologies,sulfate radical based advanced oxidation process holds great promise because of its excellent performance and less by-products.Cobalt-based catalysts are regarded as one of the most effective materials among lots of ways to activate persulfate,but the simple cobalt oxides,such as Co₃O₄,possess weaker catalytic activity for long reaction time and instability.A variety of ameliorated strategies（e.g.transition metal-doping）have been reported as yet,however,no studies have shown that metalloid-doping can play the same role,and this can also avoid the toxicity caused by metal ion leaching.Selenium（Se）displays more metallic properties compared to other metalloids.And selenides exhibit promising catalytic performance because of their diverse oxidation states and nice electrical conductivity for charge transfer.Thus,a new catalyst named Se@Co₃O₄ was synthesized by calcining and used to activate peroxymonosulfate（PMS）for the degradation of LEV in water.The main works and conclusions are as follows:（I）A new catalyst named Se@Co₃O₄ was synthesized.Cobalt nitrate hexahydrate and urea were used to prepare basic cobalt carbonate as precursor,then doped with selenium powder,and calcined in air atmosphere to obtain the best performance catalyst（the mass ratio of precursor to selenium powder was 1:2 and the calcination temperature was 400℃）.The characterization results showed that Co₃O₄ and Se@Co₃O₄ were successfully synthesized.The micromorphologies of two catalysts changed from an urchin-like sphere to a coral-like sphere,but the crystal structure did not change obviously.Both of them were mesoporous structure,and BET specific surface area were 34.745 and25.344 m²/g,respectively.The XPS results show that the molar ratio of Co²⁺/Co³⁺in the Se-doped catalyst was apparently increased（from 0.81 to 1.62）,and different results showed that the proportion of oxygen vacancy in total oxygen（hydroxyl oxygen,oxygen vacancy and lattice oxygen）was also increased.（II）The excellent performance of Se@Co₃O₄/PMS system was investigated.The degradation efficiency of LEV（10 mg/L,50 m L）by Se@Co₃O₄/PMS system could reach95.6%within 25 min under the optimal conditions（40 mg/L Se@Co₃O₄,0.2 g/L PMS,initial p H 6.6,25℃）,and the apparent rate constant was 0.3905 min^-1.The activation effect of Se@Co₃O₄ was much better than that of the catalysts prepared before doping or other studies.It was found that the degradation efficiency varied positively with the dosage of catalysts,the concentration of PMS and the reaction temperature,as for another factor,it initially increased and then decreased with p H increasing afterwards.The unilateral increase of catalyst or PMS concentration could not significantly improve the catalytic performance when reached a certain degree.And the effect of p H on the system was related to the surface charges of catalysts and pollutants.Due to the radical scavenging effects,Cl^-,NO₃^-,HPO₄^2-and humic acid（HA）would interfere with the system.The mineralization of LEV increased gradually with the increase of PMS concentration.In addition,the Se@Co₃O₄/PMS system showed outstanding degradation performance for other organic pollutants and could be reused for 5 times,which fully verified the universality and reusability of this system,and these could make it a promising strategy for popularization and application.（III）The reasons for the improvement of catalytic performance and the mechanism of LEV degradation were revealed.The XPS results showed that the significant improvement could be attributed to the increase of active Co²⁺sites and oxygen vacancy density which was contributed to the directional electron transfer.In addition,the introduction of Se（IV）helped to the catalysis of PMS,and was conducive to the reduction of Co³⁺.DFT calculation results showed that Se@Co₃O₄ possesses better adsorption and electron transfer ability.It could stretch the O-O bond length of PMS and was prone to cleavage to produce reactive oxygen species（ROS）.Radical quenching and EPR tests indicated that the primary ROS in this system were sulfate radicals(SO₄^·-),also,hydroxyl radicals（^·OH）,superoxide radicals(O₂^·-)and singlet oxygen（¹O₂）manifested a key role.Sixteen possible intermediates were identified by HPLC-MS and three pathways of LEV degradation were inferred,among which the devastation of quinolone structure and piperazine group,defluorination,benzene ring and morpholine ring opening were important steps.A comprehensive study on the performance,influencing factors,universality and reusability of Se@Co₃O₄/PMS were carried out.The reasons for the improvement of catalystic performance caused by metalloid-doping and the degradation mechanism of LEV were scrutinized,which could provide theoretical basis and technical support for green and highly effective solution of antibiotic pollution in aquatic environment.