# Research on a Reductive Deep Chlorine Removal Process for Breaking Through the Solid Film Barrier
## 3 Results and Discussion
### Copper Ion Addition  
To investigate the effect of copper ion addition on the dechlorination effect of ascorbic acid. We conducted the experiments under the following conditions: an initial pH of 3.6,  
Figure 4: Effect of different n(H\({}_{2}\)A)/n(Cl\({}^{-}\)) ratios on dechlorination effect.  
an \(\rm n(H_{2}A)/n(Cl^{-})\) ratio of 0.75, a reaction temperature of 20 \({}^{\circ}\)C, and a duration of 20 min. The molar ratios of \(\rm n(Cu^{2+})\) to \(\rm n(Cl^{-})\) were 0.75:1, 0.9:1, 1.1; 1.25:1, 1.5:1, and 1.75:1.  
Figure 5 illustrates that the addition of copper ions significantly impacts the efficiency of chloride ion removal from the solution. As the ratio of \(\rm n(Cu^{2+})/n(Cl^{-})\) increases, the removal rate of chloride ions gradually rises, and the concentration of chloride ions in the post-treatment solution decreases. When the ratio of \(\rm n(Cu^{2+})/n(Cl^{-})\) reaches 1.25, the removal rate of chloride ions reaches its maximum value of 90.8%, and the concentration of chloride ions decreases from 1 g/L to 0.0917 g/L. The reduction of copper ions by ascorbic acid in the chloride removal process is an oxidation-reduction reaction, where ascorbic acid acts as a reducing agent and is oxidized. The oxidation rate is positively correlated with the concentration of copper ions. As the ratio of \(\rm n(Cu^{2+})/n(Cl^{-})\) increases, the concentration of copper ions rises, and the chloride ions are removed more rapidly. The increase in copper ion concentration enhances the likelihood of \(\rm Cu^{2+}\) interacting with ascorbic acid anions, which promotes the formation of \(\rm Cu^{+}\) and effectively removes chloride ions from the solution.  
As the ratio of \(\rm n(Cu^{2+})/n(Cl^{-})\) increases to 1.5, the removal rate of chloride ions decreases to 87.2%. This phenomenon occurs because the increase in copper ion concentration leads to extensive hydrolysis of copper ions in the solution, forming \(\rm Cu(OH)^{+}\) and \(\rm Cu(OH)_{2}\) and releasing H\({}^{+}\). On one hand, the hydrolysis products cannot participate in the reduction coordination reaction, lowering the concentration of \(\rm Cu^{+}\). On the other hand, the local decrease in pH weakens the reducing power of ascorbic acid, hindering the reduction reaction. Furthermore, the high concentration of copper ions increases the viscosity of the solution, negatively affecting the mass transfer process between reactants and reducing the reaction rate, which, in turn, impacts the chloride ion removal efficiency. When the copper ion concentration is further increased, no significant change is observed in the chloride ion removal rate compared to the \(\rm n(Cu^{2+})/n(Cl^{-})\) ratio of 1.5. This is because the amount of ascorbic acid is fixed, and as the reaction progresses, the ascorbic acid is gradually consumed and can no longer participate in redox reactions with the excess copper ions. Therefore, the further increase in copper ion concentration has no significant impact on the chloride ion removal efficiency. Based on these experimental results, an \(\rm n(Cu^{2+})/n(Cl^{-})\) ratio of 1.25 was selected for subsequent experiments to achieve the optimal chloride ion removal effect.  
Figure 5: Effect of different \(\rm n(Cu^{2+})/n(Cl^{-})\) ratios on dechlorination effect.