*4.3. Effect of pH*

From the changing trend of NO<sup>3</sup> −-N removal efficiency with the initial pH, it can be seen that the reaction between the ZB12-500 material and NO<sup>3</sup> −-N is an acidophilic reaction. Hao et al. also reached this conclusion in their research results [70]. The reasons are as follows: Under acidic conditions, H<sup>+</sup> in the solution is increased, the adsorption of NO<sup>3</sup> −-N on the biochar surface is enhanced, and protons also participate in the reduction process of nitrate nitrogen [71]. Under alkaline conditions, the increase of OH− leads to more metal hydroxide precipitation (Fe(OH)<sup>2</sup> and Fe(OH)3) and metal carbonate (FeCO3). This led to increased corrosion of the nZVI. These iron oxides limited the diffusion of nitrate ions and coated the zero-valent iron, reducing the active sites on the surface of ZB12-500 composite and reducing the removal efficiency of NO<sup>3</sup> −-N [72]. Many studies have found that pH can affect the reduction of nitrate by ZVI particles [19,73,74]. However, in this section, the composite material can achieve good selectivity on N<sup>2</sup> (21.92–27.13%) within the initial pH range of 5–10. This may be related to the rich functional groups, huge specific surface area, and abundant active sites of biochar carriers with pyrolysis temperature of 500 ◦C [75]. The reason N<sup>2</sup> conversion ratio of ZB12-500 was the lowest under neutral conditions was that the content of active sites or functional groups on biochar surface decreases under neutral conditions.
