The increase in nitrogen, especially nitrate, in the water environment poses a great threat to human health and makes the need to develop ways to efficiently remove nitrate from water. Here, we established a novel time-delayed denitrification system for the first time using dark photocatalyst, the cyanamide-functionalized heptazine-based polymer (NCN-CN_x). Interestingly, the dark photocatalysis system can store electrons when being irradiated, and release them as needed when being coupled with a conductive co-catalyst in dark environment. The changes in concentration of nitrate, nitrite and ammonia were measured to state the improved denitrification performance, and the enzyme activities and electrochemical behaviors were investigated to explore the mechanism of the denitrification system. The results showed that NCN-CN_x had a positive effect on the microbial nitrate removal process because of the release of stored photogenerated electrons. Electrons released by a co-catalyst (graphene) on demand could promote microbial denitrification with a 77% nitrate removal efficiency, which was almost 4.8 times and 3.1 times higher than that of the photocatalytic group and the microbial group. Correlation analysis indicated that the activities of nitrate reductase, nitrite reductase, and catalase as well as electron transport system were the most important relative factors affected by NCN-CN_x during the denitrification process. Furthermore, bacterial community analysis revealed that NCN-CN_x increased the relative abundance of denitrifiers, including Sphingomonadaceae, Xanthomonadaceae and Cyclobacteriaceae, in the treated communities compared with the original community in river sludge. These findings provided some new insights into the mechanism by which dark photocatalyst can enhance the denitrification process and the establishment of day/night photocatalytic systems for sustainable removal of nitrate in water.

水环境中氮（尤其是硝酸盐）的增加对人类健康构成了巨大威胁，因此需要开发有效地从水中去除硝酸盐的方法。在这里，我们首次使用深色光催化剂，氰胺官能化的庚嗪类聚合物（NCN-CN _x）。有趣的是，黑暗的光催化系统可以在被辐射时存储电子，并在黑暗的环境中与导电助催化剂结合时根据需要释放电子。通过测量硝酸盐，亚硝酸盐和氨的浓度变化来说明反硝化性能的提高，并研究了酶的活性和电化学行为以探讨反硝化系统的机理。结果表明，NCN-CN _x由于释放了储存的光生电子，因此对微生物硝酸盐去除过程具有积极影响。助催化剂（石墨烯）按需释放的电子可以促进微生物的反硝化作用，硝酸盐的去除效率为77％，比光催化基团和微生物基团的效率高出近4.8倍和3.1倍。相关分析表明，硝酸盐还原酶，亚硝酸盐还原酶和过氧化氢酶的活性以及电子传输系统是脱氮过程中受NCN-CN _x影响的最重要的相对因素。此外，细菌群落分析显示，NCN-CN _x增加了反硝化剂（包括鞘氨醇单胞菌科，在处理过的社区中，黄原花科和环细菌科与河泥中的原始社区相比。这些发现为暗光催化剂可以增强反硝化过程的机理以及建立昼夜光催化系统以可持续去除水中的硝酸盐提供了一些新的见解。