Behavior And Mechanism Of Typical Antibiotics Removal In Aqueous Solution By MOFs-modified TiO₂-NTs In Photo（Electro） Catalysis

Antibiotics are widely used to treat or prevent human and animal diseases caused by bacterial and fungal infections.Due to their relatively low biodegradability,most antibiotics end up in the aquatic environment,posing long-term threats and potential risks to human health.Therefore,effective control of antibiotic pollution in water environment has become one of the hot issues concerned by environmental researchers.In recent years,the development of nanotechnology has promoted the exploration of nanomaterial-based adsorbents and photocatalysts for antibiotic removal.However,the effective recovery of catalyst and the recombination of photogenerated carriers are still urgent problems to be solved.Electrochemically assisted photoelectrocatalysis is another effective strategy for the mineralization removal of antibiotics besides photocatalysis.Based on synthetic tunability and"semiconductor-like properties",metal-organic frameworks（MOFs）can enrich this important research field by constructing photoelectrodes,and their derived metal oxides can form he terojunctions with semiconductors to effectively promote photogenerated carriers’transmission.In addition,the photoelectrodes modified by MOFs and their derivatives can not only solve the problems of difficult powder recycling and easy recombination of carriers,but also reasonably adjust the photoelectrocatalytic activity at the molecular level,increase the contact area between the electrolyte and the electrode,thereby promoting the diffusion of electrolyte and reaction substrate in the electrode.Therefore,considering that semiconductor TiO₂,Zn O and Fe₂O₃ have rich optical research basis,in this paper,TiO₂ nanotubes（TiO₂-NTs）are used as conductive substrates,and uses in-situ growth,electrochemical deposition,electrochemical reduction,microwave and hydrothermal self-assembly to modify MOFs with Zn and Fe as metal centers on conductive substrate.A variety of typical antibiotics in water such as sulfamethazine（SMZ）,norfloxacin（NOR）,tetracycline（TC）was selected as target pollutants to evaluate the performance of the photoelectrode,and the the removal behavior and mechanism of the photoelectrode modified by MOFs and their derivatives on typical antibiotics in water was systematically studied.The specific research work and achievements of this paper were summarized into the following four aspects:In the first part,TiO₂-NTs was used as the conductive substrate,and the light absorption ability of TiO₂-NTs was improved by external co-doping of N and F.Meanwhile,ZIF-8 was selected as a photosensitizer,and ZIF-8/NF-TiO₂-NTs photoelectrodes were prepared by growing ZIF-8 on the NF-TiO₂-NTs in the manner of simultaneous in-situ growth of metal centers and organic ligands.The removal behavior and mechanism of SMZ and other antibiotics by the unmodified TiO₂-NTs and modified ZIF-8/NF-TiO₂-NTs were comparatively studied.The reusability and water stability of the target photoelectrode were evaluated.The results showed that after exogenous co-doping of N and F and modification of ZIF-8,the light absorption capacity of the target photoelectrode is significantly improved,and the light absorption edge extend ed to 554 nm from 374 nm of TiO₂-NTs.In addition,the N-Ti-O bonds formed by the in-situ growth of ZIF-8 on the surface of NF-TiO₂ contributed to the transfer of photogenerated electrons generated by ZIF-8 to NF-TiO₂,and further to the external circuit,effectively avoiding the recombination of photogenerated electron-hole pairs（e^--h⁺）,thereby generating enough active radicals for photo-electrocatalysis.Based on the above advantages,the first-order reaction kinetic constant and degradation efficiency of ZIF-8/NF-TiO₂ for SMZ were increased by 21.7 times and 11.6 times higher than those of TiO₂-NTS,respectively,and the synergistic factor between photoelectricity was up to 3.5.After 8 cycles,the phase structure of ZIF-8/NF-TiO₂-NTs did not change.The research in this part firstly demonstrated the promoting effect of doping on enhancing the p hotosensitivity of TiO₂-NTs,and provided a design idea for the construction of semiconductor-MOFs composite photoelectrodes.（Chapter 2）In the second part,considering that the doping method of TiO ₂ in part 1 might cause the introduction of exogenous imp urities,the light absorption capacity of TiO₂ was improved by self-doping of Ti³⁺.Specifically,Ti⁴⁺was partially reduced to Ti³⁺by constructing a reducing atmosphere.Meanwhile,using MIL-100（Fe）as photosensitizer,which was derived into Ar-Fe₂O₃ in an inert atmosphere（Ar gas）and combined with Ti³⁺-TiO₂to form an Ar-Fe₂O₃/Ti³⁺-TiO₂ heterojunction.The internal structure,thermal stability,element valence,optical and photoelectrochemical properties of the heterojunction were characterized in d etail.The existence of oxygen vacancies was characterized by electron paramagnetic resonance（EPR）,which confirmed the partial reduction of Ti⁴⁺to Ti³⁺.The removal behavior of NOR and other antibiotics by Ar-Fe₂O₃/Ti³⁺-TiO₂heterojunction was evaluated,and the possible catalytic mechanism was proposed based on trapping experiment,valence band spectrum,band gap energy and Mott-Schottky curve analysis.The results showed that Ti³⁺self-doping could form an intermediate band gap below the conduction band（CB）to improve the light absorption capacity,and the resulting oxygen vacancies were beneficial to provide more photogenerated e^-for generating active radicals.After forming a heterojunction with Ar-Fe₂O₃,the photogenerated e^-in the CB of Ar-Fe₂O₃ could migrate to the CB of Ti³⁺-TiO₂,and the photogenerated h⁺in the valence band（VB）of Ti³⁺-TiO₂ could also migrate to the VB of Ar-Fe₂O₃,facilitating the separation of e^--h⁺pairs.Finally,the Ti³⁺-TiO₂/Ar-Fe₂O₃/visible light/PDS system could achieve a degradation efficiency of 97.8%for NOR within 5 min,and the first-order kinetic constants were 21 and 12 times higher than those of the pure PDS system and the photocatalytic system.The research in this part confirmed the promoting effect of Ti³⁺self-doping on the enhancement of light absor ption capacity for TiO₂,and revealed an efficient heterojunction carrier transport path between Ti³⁺-TiO₂ and Ar-Fe₂O₃.（Chapter 3）The third part refered to the research idea of modifying MOFs on TiO ₂-NTs in part 1,and according to the research conclusi on of part 2 that the self-doping of Ti³⁺could effectively improve the light absorption capacity of TiO ₂ and could form efficient heterojunction with Ar-Fe₂O₃.Firstly,the self-doping of Ti³⁺on TiO₂-NTs was realized by electrochemical reduction,and the self-assembly of MIL-100（Fe）was modified on Ti³⁺-TiO₂-NTs by means of electrochemical pulse deposition of metal center firstly and then providing organic ligands.After the high temperature reduction process,the study of part 2 was extended to Ar-Fe₂O₃/Ti³⁺-TiO₂-NTS photoelectrode.The removal behavior and mechanism of TC and other antibiotics by the target photoelectrode were studied,and the photostability and water stability were also evaluated.The results showed that the band gap of Ti³⁺-TiO₂-NTs was shortened from 3.08 e V of TiO₂-NTs to 2.80 e V after electrochemical reduction.Benefit from the narrow bandgap brought about by Ti³⁺self-doping,the high dispersion of Fe metal centers,the enhanced compactness and high mass transfer efficiency caused by MOFs self-assembly,the improved adsorption of MOFs derivatives,and the high carrier separation rate formed by recombination,the modified Ar-Fe₂O₃/Ti³⁺-TiO₂-NTs photoelectrode could achieve 100%degradation of TC within 90 min.Compared with photocatalysis（24.87%）and electrocatalysis（57.76%）,photo-electrocatalysis exhibited a significant synergistic effect with a synergy factor of 4.20.The research in this part confirmed the effectiveness of electrochemical reductive self-doping of Ti³⁺for the enhancement of light absorption capacity,and provided an effective electrochemical deposition method for improving the compactness and st ability of MOFs self-assemblyon on conductive substrates.（Chapter 4）The fourth part drawed on the research ideas of the part 1,synthesized the research conclusions of part 2 and the experience of part 3 in successfully using electrochemical reduction and pulse deposition to orderly modify MOFs on TiO₂-NTs.MOF-5 with Zn as the metal center was selected as the photosensitizer.After optimizing the electrochemical deposition and changing the traditional hydrothermal self-assembly to the microwave hydrothermal self-assembly,the study in part 1 was extended to Ar-Zn O/Ti³⁺-TiO₂-NTs photoelectrodes.The removal behavior and mechanism of TC and other antibiotics by the target photoelectrode were studied,and the photostability,water stability and p H stability were comprehensively investigated.Besides applied voltage and p H,the effects of environmental factors such as pulse deposition time,microwave hydrothermal time,and photon flux on the performance of the photoelectrode were evaluated.The results showed that without electrochemical modification and subsequent MOF-5modification,only 11.18%of TC degradation could be achieved by the photoelectrode.After modification,the photoelectric catalytic activity of Ar-Zn O/Ti³⁺-TiO₂-NTs was significantly improved,the removal rate of TC reached98.63%within 60 min,and the first-order kinetic constant was increased from0.0018 min^-1 of TiO₂-NTs to 0.0677 min^-1.In addition,the mineralization efficiency of Ar-Zn O/Ti³⁺-TiO₂-NTs for TC reached 74.27%within 60 min,which was better than the performance of the Ar-Fe₂O₃/Ti³⁺-TiO₂-NTs in part 3.The photo-electrocatalytic efficiency of SMZ was 90%within 60 min,which was also better than that of ZIF-8/NF-TiO₂-NTs in part 1.The research in this part confirmed the validity and versatility of the study in previous parts,and provided more exploration ideas for MOFs and their derivatives to participate in photoelectrocatalysis in the form of photoelectrodes.（Chapter 5）...