| In recent years,the excessive usage and disorderly discharge of tetracycline antibiotics have caused a continuous adverse impact on ecological balance and human health.How to deal with such "emerging pollutants" conveniently and effectively has become one of the research hotspots in the environmental field.Traditional Fenton oxidation technology is an important member of advanced oxidation technology and has been widely applied in real water treatment enginering.The reaction of divalent iron ions(Fe2+)and hydrogen peroxide(H2O2)can generate highly reactive hydroxyl radicals(·OH)for reducing the biological toxicity of wastewater and improving the biodegradability of refractory organic matter.On this basis,a series of Fenton-like technologies have emerged via assisting or replacing Fe2+.Heterogeneous Fenton-like using solid-phase catalysts performs well,which can not only solve the problem of nonrecyclable homogeneous ions and easy formation of secondary pollutants,but also weaken the limitations of reaction conditions especially pH and enhance the degradation efficiency.In addition,benefiting from the surface properties of the catalyst,the active oxidative species generated by the heterogeneous Fenton-like system are not limited to ·OH,but also generate new radicals and non-radicals such as superoxide radicals(·O2-),singlet oxygen(1O2)and active H2O2 compounds(H2O2*).Among them,non-radicals exhibit strong anti-interference ability to environmental substrates and selective degradation of electron-rich pollutants due to their great electrophilicity.However,the existing heterogeneous Fenton-like catalysts generally have problems such as high cost,low catalytic efficiency,and poor recycling rate.At the same time,there is currently a lack of understanding of the generation,enhancement and regulation of new reactive oxidative species,the degradation reaction mechanism is not clear enough as well,and the follow-up research on pollutants such as degradation intermediates,degradation pathways and product toxicity has not been followed up in time.Based on the above,this thesis aims to construct a cobalt-based heterogeneous Fenton-like system for the degradation of three model pollutants including doxycycline(DC),oxytetracycline(OTC),and chlortetracycline(CTC)in view of the current situation of difficult degradation and high residues of tetracycline antibiotics.Cobaltiron oxide(CoFeO),cobalt carbide(Co/C)and copper-cobalt selenide(CuCoSe)catalysts with different surface properties were designed and prepared by a facile synthesis and modification method for enhanced activation of H2O2 to produce different types of active oxidative species.Through the combination of characterization and experiments,the degradation behavior,interference factors,material stability and reaction mechanism in the reaction process were deeply studied,and the degradation pathway and product toxicity were reasonably predicted by means of quantum chemical calculations.The main research contents and conclusions are as follows:(1)A spinel-structured bimetallic CoFeO was designed and prepared.Compared with single metal oxides such as cobalt oxide(CoO),iron oxide(Fe2O3)and mixed phase CoO/Fe2O3,CoFeO can activate H2O2 in a short time to generate a large amount of ·OH for fast removal of DC.The CoFeO catalyst is magnetic and easy to separate from water,and the material is stable and strong ion leaching is low.The CoFeO+H2O2 system weakened the interference of pH,and the degradation efficiency of DC reached 92%under neutral conditions,and which also exceeded 70%after 5 cycles.Through mechanism analysis,it is found that the Co2+/Co3+ species in CoFeO are the main active sites for the reaction,and the abundant structural hydroxyl groups(-OH)on the surface of the material and the synergistic effect between Co and Fe metals both promote the decomposition of H2O2 to OH.And ·OH is the only oxidative active species in the CoFeO+H2O2 system.The 10 main intermediates produced during the degradation of DC were detected by liquid chromatography-mass spectrometry(LC-MS),and the possible degradation pathways of DC molecules under radical attack were proposed by combining with the calculation of the condensed Fukui function.(2)Amorphous Co/C composites(Co/C-1,Co/C-2,Co/C-3 and Co/C-4)were synthesized via one-step solvothermal method by changing the dosage of cobalt salts and using low-cost glucose as the carbon source.Their degradation performances of OTC in the Fenton-like oxidation system were systematically investigated.The surface structures and properties of the four materials are obviously different.But it is found that there is no lattice oxygen,and a large number of oxygen-containing functional groups(C-O and OH)are retained.Co exists in the material in the form of Co-O-C and Co-C-H coordination.Among them,Co/C-3 has the highest ratio of Co-O-C/H,and its catalytic performance is also the best.Under neutral conditions,0.20 g/L Co/C-3 activated 10 mM H2O2 can completely degrade 10 ppm OTC in 30 minutes,and after 5 cycles,not only the structure is intact,but the removal rate of OTC still exceeds 80%.In addition,inorganic anions,natural organic matters and actual water samples did not affect its degradation performance,and Co/C-3+H2O2 showed good anti-interference ability and application prospects.It is suggestted that 1O2 is the dominant active species in the reaction system,Co-O-C and Co-C-H promote the rapid transfer of electrons on the surface of the material,and accelerate the transform of H2O2 to ·O2-that is converted to 1O2 again through the valence cycle between Co2+-O-C/H and Co3+-O-C/H.Owing to the special amorphous structure,·OH is not generated in the system.The possible degradation pathways of OTC under electrophilic attack were put forward based on the degradation intermediates which were identified by LC-MS and the calculation of the condensed Fukui function.The toxicity assessment software(T·E·S·T)was used to simulate the toxicity of OTC and its degradation intermediates,the overall toxicity of that is weakened with the treatment of Co/C-3+H2O2 system.(3)A heterogeneous CuCoSe material was prepared by solvothermal selenization and air calcination,and its catalytic activity is significantly higher than that of copper cobalt oxide(CuCoO)and copper cobalt glyceride precursor(CuCo-gly).At neutral pH,0.10 g/L CuCoSe can activate 5 mM H2O2 within 10 minutes to achieve fast and efficient removal of 20 ppm CTC(100%)with good reusability and stability.In addition,the CuCoSe+H2O2 system showed good anti-interference ability against background substrates(common inorganic anions and natural organic matters),and also achieved excellent CTC degradation performance in practical water application.Mechanistic analysis revealed that 1O2 was the dominant oxidative active species in this system,and the active H2O2 compounds(H2O2*)formed by H2O2 on the surface of CuCoSe also played an important role in the direct oxidation of CTC.Cu+ is the main active site to promote H2O2 to generate 1O2,Co promotes the turnover efficiency of Cu+/Cu2+,and Co/Cu bimetals synergistically accelerate the activation and decomposition of H2O2.The introduction of Se changes the local electron density of the material and also creates Se/O vacancies to further enhance the electron-mediated process to generate nonradicals.Based on the intermediates identified by LC-MS and the calculation results of the condensed Fukui function,a possible pathway for CTC undergoing non-radical action was raised.Besides,the toxicological assessment shows that the environmental risk of the degradation products is reduced. |
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