Time-delayed photocatalysis enhanced microbial nitrate reduction via solar energy storage in carbon nitrides

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-CNx). 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-CNx 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-CNx during the denitrification process. Furthermore, bacterial community analysis revealed that NCN-CNx 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.