*4.6. Kinetics*

The kinetics of nitrate removal complied with the two-compartment first-order kinetic equations. In the rapid removal stage, that is, within 2 h of reaction, the concentration of NO<sup>3</sup> −-N decreased almost in a straight line, indicating that the reaction was strong and rapid at this time. In the slow removal stage, that is, within 2 h to 24 h after the reaction, the concentration of nitrate nitrogen decreased extraordinarily little, indicating that the reaction in the system was extremely slow and that the reaction was almost complete. At the beginning of the removal reaction, ZB12-500 reacted rapidly with NO<sup>3</sup> −-N in solution when it just entered the system. This is because the initial concentration of nitrate is relatively high and the mass transfer driving force is large, so NO<sup>3</sup> −-N is easily sent to the active site on the surface of the composite materials. This is manifested by the film diffusion of nitrate ions from the bulk liquid phase to the external surface of ZB12. As the reaction produced a large amount of iron oxide, the active site of nZVI was blocked so that nZVI could not be exposed to the system to react with the remaining NO<sup>3</sup> −-N. Moreover, as the concentration of nitrate in the solution decreased, the mass transfer driving force also decreased, resulting in a slow reaction in the later stage.
