*4.2. Effect of the Dosage of ZB12-500*

When the dosage is low, the removal efficiency increased rapidly with increasing dosage because the increased dosage provided more Fe<sup>0</sup> active sites, which is the reason for the positive correlation. At the same time, there were sufficient reaction raw materials for ZB12-500 in the whole reaction process, and the active site of ZB12-500 could be fully utilized in the reduction process. Therefore, when the dosage was less than 5 g/L, ZB12-500 experienced the same reaction conditions and environment, resulting in the same nitrogen product selectivity. When the dosage was greater than 5 g/L, nitrate dilution significantly inhibited the reactivity of the residual nitrate, leading to incomplete reduction of more nitrate, i.e., an increased proportion of intermediates accumulated, resulting in a slight change in nitrate removal efficiency and a slight reduction in N<sup>2</sup> selectivity. Some previous studies have found similar results [68,69]. Accordingly, a dosage of 5 g/L is optimum for this study.
