# Research on a Reductive Deep Chlorine Removal Process for Breaking Through the Solid Film Barrier
## 3 Results and Discussion
### Initial Reaction pH  
The principle behind ascorbic acid dechlorination mainly involves the reduction of Cu\({}^{2+}\) in the solution by ascorbic acid, thereby achieving chloride ion removal. To better investigate the effect of pH on the dechlorination efficiency of ascorbic acid, we conducted a detailed analysis of the hydrolysis equilibrium of Cu\({}^{2+}\) in the solution. As shown in Figure 6, we used HSC 6.0 software to calculate the thermodynamic parameters of the Cu\({}^{2+}\) hydrolysis reaction and performed equilibrium calculations based on the law of mass conservation. The results indicate that as pH increases, the hydrolysis reaction of Cu\({}^{2+}\) gradually intensifies, eventually forming Cu(OH)\({}_{2}\). Based on the equilibrium concentration distribution of copper ions at different pH values shown in Figure 6, it can be observed that when the pH exceeds 2, Cu\({}^{2+}\) begins to hydrolyze, and when the pH exceeds 4, Cu\({}^{2+}\) is fully hydrolyzed to Cu(OH)\({}_{2}\). To prevent copper ion hydrolysis from reducing dechlorination efficiency, the pH of the dechlorination reaction should theoretically be controlled below 4 to ensure that Cu\({}^{2+}\) primarily exists in its ionic form and can effectively undergo a redox reaction with ascorbic acid. Based on the above analysis, we conducted experiments under the following conditions: ratio of n(Cu\({}^{2+}\))/n(Cl\({}^{-}\))/n(H\({}_{2}\)A) = 1.25:1:0.75, with a reaction temperature of 20 \({}^{\circ}\)C and a reaction time of 20 min. The experiments investigated the chloride ion removal efficiency at initial pH values of 1.8, 2.4, 3.0, 3.6, and 4.2. As shown in Figure 7, the initial reaction pH has a significant impact on the removal efficiency of chloride ions from the solution. The removal rate of chloride ions initially increases and then decreases as the initial pH value rises. When the initial pH increases from 1.8 to 3.6, the concentration of chloride ions in the post-reaction solution significantly decreases from 0.4661 g/L to 0.0917 g/L, resulting in a chloride ion removal rate of 90.8%. In conjunction with the experimental observations in Figure 8, it is clearly seen that as the pH increases, the turbidity of the solution gradually intensifies. At an initial pH of 3.6, a noticeable white precipitate forms in the solution. As the initial reaction pH was raised from 1.8 to 3.6, the removal of chloride ions increased with the rising pH. The reason is that in the redox reaction, the electrode potential of the redox reaction determines the feasibility and rate of the redox reaction. Based on the mechanism of chloride ion removal through the reduction of divalent copper ions by ascorbic acid, when copper ions are reduced to cuprous ions, the reaction process  
Figure 6: Distribution of hydrolytic equilibrium concentration of Cu\({}^{2+}\) at different pH levels. depended on the driving force of the redox reaction in the aqueous solution, i.e., the potential difference between C6H8O6 and Cu2+. The redox reaction between ascorbic acid and Cu2+ is shown in Equation (10), and the electrode reaction formula for ascorbic acid is shown in Equation (11):  
\[2\text{Cu}^{2+}+\text{ C}_{6}\text{H}_{8}\text{O}_{6}=\text{ 2 Cu}^{+}+\text{C}_{6}\text{H}_{6}\text{O}_{6}+ \text{2H}^{+} \tag{10}\]  
\[\text{C}_{6}\text{H}_{8}\text{O}_{6}+2\text{e}^{-}=\text{ C}_{6}\text{H}_{6}\text{O}_{6}+\text{2H}^{+} \tag{11}\]  
where the standard electrode potential of C6H6O6/C6H8O6, \(\varphi^{0}\) = 0.08 V. According to the Nernst equation, the electrode potential of C6H6O6/C6H8O6 can be obtained from the relationship between the electrode potential of C6H6O6/C6H8O6 and pH using Equation (12):  
\[\varphi=\text{ }\varphi^{0}+2.303\text{ }\times\frac{\text{RT}}{\text{nF}} \text{lg}\frac{\text{c}(\text{C}_{6}\text{H}_{6}\text{O}_{6})\cdot\text{c} \left[\text{H}^{+}\right]^{2}}{\text{c}(\text{C}_{6}\text{H}_{8}\text{O}_{6})} \tag{12}\]  
where c(C6H8O6) and c(C6H6O6) are the concentrations of C6H8O6 and C6H6O6, and \(n\) = 2.