g-C3N4/Gr/TiO2 Z型光催化体系仿生硝化-反硝化去除水中氨氮的性能及机理研究

Development of photocatalysis technology is of great significance for resolving the problems of biological ammonia removal processes, such as long reaction period, carbon source addition and sludge pollution, and so on. However, how to control the generation of N2 product in photocatalytic ammonia removal process is a major scientific issue, as the majority of ammonia was converted into toxic NO3- and NO2- in the photocatalytic reactions. In this project, a g-C3N4 (PS I)/Gr/TiO2 (PS II) Z-scheme photocatalytic system with spatially separated oxidation sites and reduction sites is proposed to simulate biological nitrification-denitrification process to convert ammonia into N2. The oxidation potential of PS II (2.9 V) and the reduction potential of PS I (-1.39 V) are higher than the needed reactive potentials of ammonia oxidation and NO3- reduction, respectively, indicating the reactions are theoretically feasible. By constructing the TiO2/Gr/g-C3N4/Gr/TiO2 structure with oxygen-less PS I and oxygen-rich PS II, the nitrification of ammonia on PS II will be promoted by photogenerated holes/oxygen, while effects of these active species on denitrification of NO3- on PS I will be avoided. The functions and reaction mechanisms of each step reaction as well as the overall reaction in the ammonia degradation process are to be studied, and the photocatalytic performance will be evaluated. This project is expected to provide a simple method for harmlessly removing ammonia, and provide experimental reference and theoretical basis for other pollutants degradation by Z-scheme system via oxidation-reduction two-step processes.

发展光催化氨氮去除技术对解决生物法周期长、需外加碳源和污泥污染等问题具有重要意义。然而光催化氧化氨氮的产物多为NO3-和NO2-，如何控制反应过程中氨氮选择性生成N2是目前面临的主要科学问题。本项目设计了氧化还原位点空间分离的g-C3N4(PSI)/Gr/TiO2(PSII) Z体系来模拟生物硝化-反硝化完成氨氮到N2的转化。所采用的PSII氧化电位(2.9V)和PSI还原电位(-1.39V)分别高于氨氮氧化和NO3-还原所需的反应电位，证明理论上可行；TiO2在外(富氧)、g-C3N4在内(缺氧)的结构构建可有效促进空穴/氧在PSII上对氨氮进行氧化同时避免二者对PSI上NO3-还原过程的影响。项目将对氨氮转化过程中各阶段/总反应的协作及机理进行研究并进行效果调控。拟开展工作有望为水中氨氮的无害化去除提供一种简单可行的方法，并为Z体系氧化-还原两步降解其他污染物提供实验参考和理论依据。

氨氮的过量排放易造成水体的富营养化及饮用水安全问题。光催化去除氨氮技术具有环境友好、节能、运行周期短的优势，然而，其氧化氨氮的产物绝大多数为NO3-和NO2-。针对以上问题，本项目制备了氧化还原位点空间分离的g-C3N4(PSI)/rGO/TiO2(PSII) Z体系来模拟生物硝化-反硝化完成氨氮的去除。实验通过调控反应过程中氨氮的初始浓度、pH、材料的比例等不同影响因素，优化了复合催化剂对氨氮的降解效果。结果表明，在pH为9.5、氨氮初始浓度为50 mg/L且GO: g-C3N4为1:10时，g-C3N4/rGO/TiO2光催化剂对氨氮的去除率最高，为96.80%。其光催化动力学速率为0.705 h−1，分别是g-C3N4/TiO2、g-C3N4和TiO2催化反应动力学速率的1.7倍、1.9倍和1.5倍。目标污染物氨氮和中间产物NO3-的交互实验表明二者的去除存在相互促进作用，进而证实了在g-C3N4/rGO/TiO2 Z体系中存在着光催化氨氮氧化和NO3-还原的同步去除机制。活性物种的捕获以及自由基测试实验结果证明了g-C3N4/rGO/TiO2 Z体系去除氨氮的反应机理为：氨氮在PS II的价带上发生光催化硝化反应产生NO3-，而中间产物NO3-在PS I的导带上发生光催化反硝化反应产生气态产物。然而，项目研究发现，上述过程中反硝化阶段的效率仍然比较低，为了进一步提高g-C3N4/rGO/TiO2反硝化的效果，构建了还原位点加强的g-C3N4(Ag)/Gr/TiO2 Z体系光催化材料，该材料在4 h内对NO3-的还原率达到50.46%，活性物种测试实验证明了Ag纳米颗粒促进光生电子与O2结合形成·O2-是该材料效果提高的主要原因。最后，项目将g-C3N4/rGO/TiO2用于实际焦化废水中的氨氮去除，结果表明，其对三种不同焦化废水的氨氮去除率均保持在85%以上，且材料连续使用6个循环后仍能够保持≥90%的氨氮去除效率（焦化B厂），证明了上述Z体系材料具有一定的实际应用性。以上工作为水中氨氮的无害化去除提供了一种简单可行的方法，并为Z体系氧化-还原两步降解其他污染物提供了实验参考。