地下水中硝酸盐向氮气定向转化的多级电解工艺及机理研究

The electrochemical reduction of nitrate in groundwater to nitrogen confronts several problems, especially slow mass transfer kinetic and low selectivity. In this project, a flow-through multistage electrolysis process including three steps of enrichment, reduction and oxidation is designed to solve the two major problems. The flow-through mode of the process will improve the mass transfer kinetic. Besides, this process uses stepwise transformation strategy to directionally reduce nitrate to nitrogen, which includes nitrate enrichment, nitrate reduction, nitrite reduction, and ammonia oxidation. In details, 1) a Sn/Pd-based porous composite cathode loaded on carbon fibre felt will be prepared with the functions of enrichment and reduction. The preparation of the Sn/Pd-based porous composite cathode will be optimized according to key factors that control the cathode performance in water permeability, nitrate adsorption and removal rate of nitrate. 2) Based on the optimized cathode, a flow-through multistage electrolysis process will be developed to include three steps of enrichment, reduction and oxidation. Operating parameters will be defined and optimized to achieve efficient enrichment and directional transformation of nitrate. 3) Mechanisms of enrichment and directional transformation of nitrate in the process will be illustrated. This study will benefit for contamination remediation of nitrate in groundwater.

针对电化学还原法去除地下水中硝酸盐存在的传质差、还原副产物多、氮气生成的选择性低的问题，本项目拟构建“富集-还原-氧化”过滤式多级电解工艺。利用过滤方式提高传质效率；采用分段转化，通过“硝酸盐富集-硝酸盐还原-亚硝酸盐还原-氨氧化”多工艺的协同运作，实现硝酸盐向氮气的定向转化。具体的：1）以碳纤维毡为基底，制备集富集和还原功能于一体的Sn/Pd基多孔复合阴极；以透水能力、硝酸盐的吸附、去除效果为指标，明确控制电极特性的关键因子，优化电极制备方法。2）以Sn/Pd基多孔复合阴极为基础，构建“富集-还原-氧化”过滤式多级电解工艺，研究工艺参数对硝酸盐富集与定向转化过程的影响及其方式。以硝酸盐向氮气高效定向转化为目标，对工艺参数进行优化。3）探究硝酸盐在所开发工艺中的富集、转化行为和场所，解析工艺定向去除硝酸盐的机理。项目的研究成果为电化学技术修复硝酸盐污染的地下水提供理论指导和技术支持。

电化学还原法因其简便、高效的特点成为水体硝酸盐污染控制领域的重要技术之一，不过仍存在传质差、氮气生成的选择性低等弊端。为解决上述问题，本项目首先研究了几类典型催化剂以及电极构型对硝酸盐还原反应的影响，发现Cu催化剂具有优异的硝酸盐还原性能而三维多孔电极结构可以有效提高硝酸盐的传质。此外，粒径、缺陷以及氧化态等催化剂特性是调控硝酸盐还原反应性能的关键因子。随后，制备了几种高效还原硝酸盐的多孔阴极并构建了多级电解工艺，发现氮气生成的选择性得到了明显提升。另外，水体中常见共存离子以及地下水、自来水等水体对硝酸盐还原性能存在影响，但硝酸盐去除性能仍然较高。最后，探究了硝酸盐去除机理，发现直接电子转移是硝酸盐转化为氨的主要机理，而氨向氮气转化是活性氯的作用。上述研究结果为电化学技术修复硝酸盐污染的水体提供理论指导和技术支持。