【作者】 刘鑫；

【导师】 邱建丁；

【作者基本信息】 南昌大学 ， 化学， 2023， 博士

【摘要】 随着能源需求的增加,铀作为战略资源被广泛开发。由于铀的固有毒性和放射性,采矿过程中产生的大量含铀废水会对生态系统和公共安全产生不利影响。铀通常以四价（U（IV））和六价（U（VI））的形式存在,与U（IV）相比,U（VI）具有较高的流动性、溶解度和毒性。因此探索有效去除U（VI）的方法成为研究的关键问题。吸附法是最简便的铀固定化方法,可以通过活性基团的配位作用捕获铀,其中复合材料可解决单一材料选择性差和亲和力不足的缺点。此外光催化还原U（VI）为不溶性U（IV）是最有前途的铀去除方法,而且复合材料可解决单独半导体催化剂光吸收范围窄、电子-空穴易复合和活性位点密度不足的缺点。因此开发新型U（VI）固定化材料和方法对解决水体中的铀污染问题具有重要意义。本论文主要通过有机分子或共价有机框架（COFs）对单一材料进行改性或复合,从而改变电子流动方向和调制带隙宽度以提高吸附和光催化反应效率,制备了多种结构稳定、比表面积大、孔隙分布规则和活性基团多的复合材料,用于复杂水体环境中U（VI）的吸附、光催化还原和等离子体光催化还原,实现了对真实水环境中U（VI）的有效去除,主要研究内容如下:1.碳纳米管/共价有机框架复合材料用于去除U（VI）。针对单一碳纳米管（CNTs）吸附性能不足的问题,本文在CNTs表面原位生长共价有机框架COF-OH,合成了结晶度高、结构稳定、有特异性U（VI）结合位点的氧化还原型复合材料CNT/COF-OH,将其用于去除稀土尾矿废水中的铀。COF-OH的引入增加了U（VI）的活性吸附位点,CNTs良好的电子传递能力和π共轭性能增加了吸附官能团的电子密度,保证了还原反应的进行。CNT/COF-OH通过静电吸引、化学络合和化学还原三种作用协同,实现了对U（VI）的固定化,其U（VI）去除能力比CNT、COF-OH和混合CNT+COF-OH分别提高了390.7%、54.6%和84.5%。同时CNT/COF-OH的选择性分离系数是COF-OH的1.6倍。采用CNT/COF-OH对稀土尾矿废水进行了铀去除实验,U（VI）去除率可达96.7%。因此,共价有机框架改性碳纳米管的协同效应对解决废水中有害离子引起的安全问题具有重要的理论和实际意义。2.Bi₄Ti₃O₁₂-三嗪-醛基苯骨架的D-π-A结构增强光催化还原铀（VI）。针对Bi₄Ti₃O₁₂缺乏吸附位点、带隙宽的问题,在Bi₄Ti₃O₁₂表面后接枝吸电子有机小分子以重构和优化D-π-A结构。本文先合成了Bi₄Ti₃O₁₂（B1）颗粒,再将B1与6-氯-1,3,5-三嗪-二胺交联得到B2,最后利用B2和4-甲酰苯甲醛的席夫碱反应合成具有D-π-A阵列结构的B3以探究D-π-A阵列结构对光催化去除U（VI）的影响。B3中Bi₄Ti₃O₁₂-（供体）-三嗪单元（π-电子桥）-醛基苯（受体）交联,可以有效延伸D-π-A结构,并通过苯环末端的π-π相互作用形成[-A-π-D-π-A-（π-π）-A-π-A-]_n阵列,导致在B3内部形成了多个极化电场,缩短了电子单次传递距离。而且,电子推拉效应增加了载流子传递的驱动力,致使带隙进一步减小。因此,通过能带匹配效应,U（VI）更易在B3的吸附位点处得到电子被还原为U（IV）,充分提高了光催化效率。在模拟太阳光下,B3对U（VI）的去除容量可达684.9 mg g^-1,是B1的2.5倍和B2的1.8倍。D-π-A阵列结构为调制带隙宽度和提高光催化还原U（VI）的性能提供了设计思路。3.Sn S₂-共价有机框架Z型范德华异质结增强光催化还原铀（VI）。针对单组分催化剂光能利用效率低以及全无机异质结催化剂活性位点不足、光生电子和空穴易复合的问题,本文合成了具有三嗪结构的共价有机框架与Sn S₂紧密堆积的有机-无机复合材料Sn S₂COF,用于稀土尾矿废水中U（VI）的光催化还原。Sn S₂COF是一种Z型范德华异质结光催化剂,且范德华相互作用形成了高速电子传输通道。两组分之间费米能级的差异导致复合材料的电子在界面处自发扩散,形成内建电场。在光激发下,处于内建电场中的电子流向发生逆转,进一步促进了光生电子-空穴的分离,保持了导带的高还原性,避免了Sn S₂的光腐蚀。Sn S₂COF具有较高的还原去除U（VI）的能力,达到1123.3 mg g^-1,远远超过了Sn S₂和COF对U（VI）的还原能力。在稀土尾矿废水中,Sn S₂COF对U（VI）的去除率高达98.5%,且具有良好的选择性和循环性。有机-无机异质结复合材料的设计理念为提高光催化还原U（VI）的性能提供了新策略。4.双肖特基势阱和氧空位对增强等离子体光催化还原U（VI）的协同作用。针对单独光催化作用中电子供给不足的问题,等离子体光催化是提高电子密度的有效策略。本文在Bi/Bi₂O_3-x表面原位生长共价有机框架制备了等离子体光催化剂Bi/Bi₂O_3-x@COFs,用于稀土尾矿废水中U（VI）的吸附和光催化还原。Bi/Bi₂O_3-x中的氧空位以及Bi与Bi₂O_3-x界面形成的肖特基势阱增加了自由电子的数量,从而产生了局域表面等离子体共振（LSPR）,增强了Bi/Bi₂O_3-x@COFs的光吸收性能。此外,氧空位提高了Bi/Bi₂O_3-x的费米能级,使Bi₂O_3-x与COFs界面之间的肖特基势垒成为另一个势阱,电子转移方向发生逆转,从而增加了COFs层的电子密度。外层COFs为N型半导体且具有合适的能带结构和特定的结合U（VI）基团作为活性反应位点。由于光生电子和热电子的协同作用,Bi/Bi₂O_3-x@COFs对U（VI）的去除量高达1411.5 mg g^-1。通过循环实验探究了复合材料对稀土尾矿废水中U（VI）的去除性能,再生后的Bi/Bi₂O_3-x@COFs对U（VI）的去除率仍高于93.9%。因此,引入LSPR和肖特基势阱的设计方案为增加电子密度和提高光催化性能提供了有效途径。5.双功能MOF525@BDMTp光催化还原U（VI）和降解毒死蜱（CP）的协同作用。污水中的放射性离子U（VI）和有机磷污染物会对生态环境和人体健康造成极大危害,采用一对二的手段在光催化还原U（VI）的同时降解毒死蜱（CP）可有效解决此问题。本文通过共价键桥接原位合成了以金属有机框架MOF525为核、共价有机框架BDMTp为壳的双功能光催化剂MOF525@BDMTp。在黑暗条件下,因费米能级的差异导致电子由BDMTp自发流向MOF525,致使BDMTp具有正电性并于界面形成内置电场。CP表面带负电更易与复合材料的正电部分发生相互作用,提高了吸附亲和力。光照条件下,因内置电场的作用电子由MOF525转移至BDMTp,导致BDMTp电子密度增大,活化了孔壁上U（VI）的结合位点。界面形成的II-型异质结,提高了电子-空穴分离效率,保障了BDMTp上电子还原U（VI）和MOF525上空穴氧化CP的同时进行。而且U（VI）和CP的存在提高了MOF525@BDMTp对对应物种的光催化性能。MOF525@BDMTp对U（VI）的去除容量达625.0 mg g^-1,对CP的去除率达99.8%。因此,功能集成复合材料的设计是提高复杂环境中光催化性能的有效手段。更多还原

【Abstract】 With the increase of energy demand,uranium has been widely developed as a strategic resource.Due to the inherent toxicity and radioactivity of uranium,a large amount of uranium-containing wastewater produced in the mining process will adversely affect the ecosystem and public safety.Uranium is usually available in quadrivalent（U（IV））and hexavalent（U（VI））forms,and U（VI）has higher fluidity,solubility,and toxicity compared to U（IV）.Therefore,exploring effective methods to remove U（VI）has become a key issue in the research.The adsorption method is the simplest method for uranium immobilization,which can capture uranium through the coordination of active groups.The composites can solve the shortcomings of poor selectivity and insufficient affinity of a single material.In addition,photocatalytic reduction of U（VI）to insoluble U（IV）is the most promising method for uranium removal,and composites can solve the shortcomings of narrow optical absorption range,easy electron-hole recombination and insufficient density of active sites of single semiconductor catalysts.Therefore,it is of great significance to develop new U（VI）immobilized materials and methods to solve the problem of uranium pollution in water.In this paper,organic molecules or covalent organic frameworks（COFs）were used to modify or compound single materials,so as to change the direction of electron flow and modulate band gap width to promote adsorption and photocatalytic reaction efficiency.A variety of composites with stable structure,large specific surface area,regular pore distribution and many active groups were prepared.It was used for U（VI）adsorption,photocatalytic reduction and plasmonic photocatalytic reduction in complex water environment,realizing effective removal of U（VI）in real water environment.The main research contents are as follows:1.Carbon nanotubes/covalent organic frameworks composites were used to remove U（VI）.In view of the insufficient adsorption performance of single carbon nanotubes（CNTs）,covalent organic framework COF-OH was grown in-situ on the surface of CNTs,then oxidized reduction composite CNT/COF-OH with high crystallinities,stable structure,and specific U（VI）binding site was synthesized,which was used to remove uranium from rare earth tailings wastewater.The introduction of COF-OH increased the active adsorption site of U（VI）,and the good electron transport capacity andπ-conjugation property of CNTs increased the electron density of adsorption functional groups,ensuring the reduction reaction.The U（VI）immobilization of CNT/COF-OH was realized through three synergies of electrostatic attraction,chemical complexation,and chemical reduction.The U（VI）removal capacity of CNT/COF-OH was 390.7%,54.6%,and 84.5%higher than those of CNT,COF-OH,and mixed CNT+COF-OH,respectively.Meanwhile,the selective separation coefficient of CNT/COF-OH was 1.6 times that of COF-OH.The uranium removal experiment of rare earth tailings wastewater was carried out by using CNT/COF-OH,and the removal rate of U（VI）was up to 96.7%.Therefore,the synergistic effect of carbon nanotubes modified by covalent organic frameworks has important theoretical and practical significance to solve the safety problems caused by harmful ions in wastewater.2.Photocatalytic reduction of U（VI）was enhanced by D-π-A structure of Bi₄Ti₃O₁₂-triazine-aldehyde benzene skeleton.The D-π-A structure was reconstructed and optimized by grafting electron-absorbing organic molecules onto the surface of Bi₄Ti₃O₁₂ due to its lack of adsorption sites and large band gap width.In this paper,Bi₄Ti₃O₁₂（B1）particles were synthesized first,and then B1 was crosslinked with6-chloro-1,3,5-triazine-diamine to obtain B2.Finally,B3 with D-π-A array structure was synthesized by Schiff base reaction of B2 and 4-formylbenzaldehyde to investigate the effect of D-π-A array structure on photocatalytic removal of U（VI）.B3,Bi₄Ti₃O₁₂（donor）-triazine unit（π-electron bridge）-aldehyde benzene（acceptor）crosslinking,can effectively extend the D-π-A structure,and by the end of the benzene ring ofπ-πinteraction to form[-A-π-D-π-A-（π-π）-A-π-A-]_n array.As a result,multiple polarized electric fields were formed inside B3,shortening the single transmission distance of electrons.Moreover,the electron push and pull effects increased the driving force of carrier transfer,which further reduced the band gap.Therefore,through the band matching effect,U（VI）was more likely to obtain electrons at the adsorption site of B3 and be reduced to U（IV）,which fully improved the photocatalytic efficiency.Under simulated sunlight,the U（VI）removal capacity by B3 reached 684.9 mg g^-1,which was 2.5 times greater than B1 and 1.8 times greater than B2.The structure of D-π-A array provides a design idea for modulating band gap width and improving the performance of photocatalytic U（VI）reduction.3.Sn S₂-covalent organic framework Z-scheme van der Waals heterojunction enhanced photocatalytic reduction of U（VI）.In order to solve the problems of low light-energy utilization efficiency of single-component catalysts,and insufficient active sites and easy recombination of photoelectrons-holes of all-inorganic heterojunction catalysts,a covalent organic framework with triazine structure and Sn S₂ closely packed composite Sn S₂COF were synthesized for the photocatalytic reduction of U（VI）in rare earth tailings wastewater.Sn S₂COF was a Z-scheme van der Waals heterojunction photocatalyst,and van der Waals interaction formed high speed electron transport channel.Due to the difference of Fermi energy level between the two components,the electrons of the composite spontaneously diffused at the interface,forming a built-in electric field.Under photoexcitation,the flow direction of electrons in the built electric field would be reversed,which further promoted the separation of photogenerated electron-hole,maintained the high reducibility of the conduction band,and avoided the photocorrosion of Sn S₂.Sn S₂COF had a high U（VI）reduction and removal capacity of 1123.3 mg g^-1,which far exceeded the U（VI）reduction capacity of Sn S₂ and COF.In rare earth tailings wastewater,the removal rate of U（VI）by Sn S₂COF was up to 98.5%,and Sn S₂COF had good selectivity and recycling.The design concept of organic-inorganic heterojunction composites provides a new strategy for improving the performance of photocatalytic U（VI）reduction.4.The double Schottky well and oxygen vacancy had a synergistic effect on plasmonic photocatalytic reduction of U（VI）.Plasmonic photocatalysis is an effective strategy to increase electron density due to the shortage of electron supply in single photocatalysis.Plasmonic photocatalyst Bi/Bi₂O_3-x@COFs was prepared by in-situ growing covalent organic framework on Bi/Bi₂O_3-x surface for U（VI）adsorption and photocatalytic reduction in rare earth tailings wastewater.The oxygen vacancy in Bi/Bi₂O_3-x and the Schottky potential well formed at the Bi/Bi₂O_3-x interface increased the number of free electrons,resulting in localized surface plasmon resonance（LSPR）,which enhanced the light absorption performance of Bi/Bi₂O_3-x@COFs.In addition,the oxygen vacancy increased the Fermi level of Bi/Bi₂O_3-x,making the Schottky barrier between Bi₂O_3-x and COFs interface became another potential well,and the electron transfer direction was reversed,thus increasing the electron density of the COFs layer.The outer COFs was N-type semiconductor with appropriate band structure and specific binding U（VI）groups,which could be used as active reaction sites.Due to the synergistic action of photogenerated charge and thermoelectric charge,the removal rate of U（VI）by Bi/Bi₂O_3-x@COFs was as high as 1411.5 mg g^-1.The U（VI）removal performance from rare earth tailings wastewater by composite was investigated through cyclic experiments,and removal rate of U（VI）by regenerated Bi/Bi₂O_3-x@COFs was still higher than 93.9%.Therefore,the design scheme of LSPR and Schottky potential well provides an effective way to increase electron density and improve photocatalytic performance.5.Dual-function MOF525@BDMTp had a synergistic effect of photocatalytic U（VI）reduction and chlorpyrifos（CP）degradation.Because radioactive ion U（VI）and organophosphorus pollutants contained in sewage will cause great harm to the ecological environment and human health,the one-to-two method of photocatalytic reduction of U（VI）and degradation of chlorpyrifos（CP）can effectively solve this problem.In this paper,a bifunctional photocatalyst MOF525@BDMTp with MOF525 as core and BDMTp as shell was synthesized by covalent bond bridging in-situ.In the dark condition,electrons flowed spontaneously from BDMTp to MOF525 due to the difference of Fermi energy level,resulting in the positive charge of BDMTp and the formation of internal electric field at the interface.The negatively charged surface of CP was more likely to interact with the positively charged part of the composite material,which improved the adsorption affinity.Under light conditions,electrons transferred from MOF525 to BDMTp due to the action of the internal electric field,which resulted in the increase of the electron density of BDMTp and activated the U（VI）binding site on the pore wall.The type-II heterojunction formed at the interface improved the efficiency of electron-hole separation and ensured the simultaneous electron reduction of U（VI）on BDMTp and the oxidation of CP on MOF525.Moreover,the presence of U（VI）and CP improved the photocatalytic performance of MOF525@BDMTp to the corresponding species.MOF525@BDMTp had a removal capacity of 625.0 mg g^-1 for U（VI）and a removal rate of 99.8%for CP.Therefore,the design of functional integrated composites is an effective means to improve the photocatalytic performance in complex environments.更多还原