*4.4. Effect of Initial Nitrate Concentration*

The initial nitrate concentration is negatively correlated with its removal efficiency. The study of Sparis et al. also showed that the higher the initial nitrate nitrogen concentration, the lower the reduction rate constant [15]. In fact, when nitrate concentration in solution is low, the active sites on the ZB12-500 composite are relatively abundant, which makes the utilization rate of ZVI very high. However, for higher concentrations of nitrate in solution, nitrate ions compete for limited active sites on ZB12-500 composites, which makes it easier for nitrate anions to squeeze onto the ZB12-500 surface and then rapidly oxidize nZVI particles on the ZB12-500 surface, eventually seriously blocking the porous structure of ZB12-500. As a result, the material loses more active sites more quickly, which inevitably hinders nitrate reduction. In the experiment, N<sup>2</sup> conversion ratio was positively correlated with the initial nitrate concentration, which might be the result of the influence of solution ion concentration on the surface charge density of the material. Mikami et al.'s study also showed that increased nitrate concentration was conducive to the transformation of reduced products into N<sup>2</sup> rather than NH<sup>4</sup> + -N [76]. As the density of N-species on the surface of the composite became high with the increased in NO<sup>3</sup> −-N concentration, the selectivity to N<sup>2</sup> increased.
