## 3 Results and Discussion
### Characterization of Zr-EPRC
#### 3.3.3 Single Impact Factor Adsorption Experiments  
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significantly when the pH exceeds 7. The findings from the pH single-factor adsorption experiment appear to contradict the optimal pH obtained from the substrate's modification experiment. The apparent discrepancy arises from the distinction between the optimal pH during substrate modification and the pH during the subsequent adsorption stage. The modification experiment identifies the optimal pH for substrate modification, which may differ from the ambient pH observed during the adsorption experiment. This observation can be attributed to two factors:  
Phosphate Speciation: Under acidic conditions, phosphorus mainly exists in the form of H2 PO3\({}^{-}\), while under alkaline conditions, it primarily exists as HPO3\({}^{2-}\) or PO3\({}^{3-}\). H2 PO3\({}^{3-}\) has a lower free energy of adsorption and is more prone to adsorption reactions compared to HPO3\({}^{2-}\) and PO3\({}^{3-}\).  
Surface Reactivity: The adsorption of zirconium metal for phosphorus removal occurs through the exchange of ligands between the hydroxyl groups on the surface of zirconium oxide and phosphate in water. The reactivity of the hydroxyl groups on the surface of hydrated zirconium oxide depends on the solution's pH. Acidic conditions favor the protonation of the hydrated zirconium oxide surface and the attraction of phosphate through electrostatic forces. However, under higher pH conditions, the deprotonation of hydrated zirconium oxide reduces the reactivity. Additionally, the increased OH\({}^{-}\) ions in the solution can compete with phosphate for active sites, further weakening the adsorption capacity of the adsorbent.  
In practical water treatment processes, it is advisable to avoid treating highly alkaline wastewater directly. Instead, the alkalinity of the wastewater can be adjusted via pH modification prior to treatment. This ensures the effective removal of phosphorus from the wastewater.  
* Co-existing anions  
Figure 20 demonstrates that Zr-EPRC particles exhibit a strong resistance to interference from co-existing anions, maintaining effective adsorption capacity for phosphorus removal even in the presence of competing ions. When analyzing the impact of each interfering ion on the adsorption capacity, the following observations can be made:  
Figure 19: Effect of pH on phosphorus removal effect of Zr-EPRC particles.  
NO3\({}^{-}\), SO4\({}^{2-}\), and HCO3\({}^{-}\) have some inhibitory effects on the adsorption and phosphorus removal capabilities of Zr-EPRC particles. Among these, the inhibitory effect of HCO3\({}^{-}\) is relatively pronounced. This is because the introduction of HCO3\({}^{-}\) into the solution raises its pH, resulting in an increase in the OH\({}^{-}\) concentration. This decrease in the surface protonation of hydrated zirconia hampers the effective adsorption of phosphorus. Adjusting the solution pH to neutral prior to the adsorption experiments resulted in only a 3.3% decrease in the adsorption capacity compared to the blank group, confirming that HCO3\({}^{-}\) primarily affects Zr-EPRC particles through pH alteration.  
Low concentrations of Cl\({}^{-}\) appear to enhance the adsorption of phosphorus by Zr-EPRC, while high concentrations of Cl\({}^{-}\) inhibit this process. The change in the adsorption capacity could be attributed to the increased ion concentration on the material's surface upon adding chloride ions, reducing the \(\zeta\)-potential on the material's surface and promoting adsorption. However, as the concentration of chloride ions increases, their competitive strength with phosphoric acid as an anion also increases, leading to the inhibition of adsorption.  
In summary, the inhibitory effect of anions on Zr-EPRC particles is limited, and these particles display excellent ion selectivity and resistance to interference during practical applications.