*4.5. Effect of Co-Existing Ions*

In this study, Na<sup>+</sup> , Mg2+, Ca2+ and Cl<sup>−</sup> had insignificant effects on the removal efficiency of NO<sup>3</sup> −-N, while SO<sup>4</sup> <sup>2</sup><sup>−</sup> and HCO<sup>3</sup> − significantly reduced the removal efficiency

of NO<sup>3</sup> −-N. General cations and low concentrations of Cl− have a negligible effect on the removal of anionic NO<sup>3</sup> −. However, the outer-spherically sorbing anions, especially SO<sup>4</sup> <sup>2</sup>−, have a significant interference effect on nitrate adsorption, and adsorption competition will occur on the limited adsorption sites on the surface of ZB12-500 [77,78]. HCO<sup>3</sup> − can be ionized to form CO<sup>3</sup> <sup>2</sup><sup>−</sup> and H<sup>+</sup> or hydrolyzed to form OH<sup>−</sup> and H2CO3. However, the degree of hydrolysis of HCO<sup>3</sup> − is greater than the degree of ionization, so more OH− is generated by hydrolysis, which can form Fe(OH)2, Fe(OH)<sup>3</sup> and FeCO<sup>3</sup> with iron ions in the system. Thus, the corrosion of nZVI was aggravated, and the formation of iron oxides will cover the outer surface of nZVI particles, preventing it from continuing to react with NO<sup>3</sup> <sup>−</sup>-N, and ultimately reducing the reduction efficiency of NO<sup>3</sup> −-N. The reason HCO<sup>3</sup> <sup>−</sup> obviously improved the product selectivity of N<sup>2</sup> may be related to the atomic structure of HCO<sup>3</sup> <sup>−</sup>. The atomic arrangement of HCO<sup>3</sup> − is planar, and the carbon in the center is bonded with three oxygen atoms (one C=O, one C–OH, and one C–O–). All of these functional groups are involved in the reduction process of NO<sup>3</sup> −-N. If the ZB12-500 composite is applied to actual groundwater, the influence of HCO<sup>3</sup> <sup>−</sup> and SO<sup>4</sup> <sup>2</sup><sup>−</sup> should be considered.
