Research on a Reductive Deep Chlorine Removal Process for Breaking Through the Solid Film Barrier

Abstract

Chloride ions in zinc refining accelerate equipment corrosion and anode and cathode losses, increase lead content, and reduce zinc quality. Therefore, the removal of chloride ions has become a research priority. The existing copper slag dechlorination process has problems such as the solid film barrier leading to impeded mass transfer, product wrapping triggering active site coverage, and incomplete reactions due to insufficient reaction-driving force, leading to low utilization of copper slag, poor dechlorination efficiency, and long reaction times. To address these issues, a new method of deep dechlorination based on the reduction of Cu²⁺ by liquid-phase mass transfer is proposed in this paper. The process utilizes ascorbic acid as a reducing agent, establishes a homogeneous aqueous phase reaction system, breaks through the solid membrane barrier, and avoids the encapsulation of the product layer, achieving efficient dechlorination. The enol structure of ascorbic acid promotes rapid dechlorination through proton-coupled electron transfer (PCET). Thermodynamic calculations show that compared to the current copper slag dechlorination process, this method increases the reaction-driving force by 18.6%, reduces the Gibbs free energy (ΔG^θ) by 59.3%, and increases the equilibrium constant by 6.7 × 10⁹ times, making the reaction more complete and achieving a higher degree of purification. The experimental results show that under optimized conditions, the chloride ion concentration in the solution decreases from 1 g/L to 0.0917 g/L within 20 min, with a removal rate of 90.8%. The main precipitate is CuCl. This process provides a more efficient solution to the chloride ion contamination problem in the hydrometallurgical zinc refining process.

1. Introduction

As one of the important non-ferrous metals, zinc has a wide range of industrial applications and significant economic value [1,2]. Hydrometallurgical zinc refining is the method predominantly used in the zinc smelting process, accounting for 80% of the world’s zinc production [3]. However, with the acceleration of industrialization and the overexploitation of mineral resources, the quality of ores has gradually declined, leading to a significant increase in the impurity content in zinc ores, especially the content of chlorine [4,5]. In the hydrometallurgical zinc refining process, the continuous accumulation of chloride ions can corrode equipment and electrodes [6,7]. In an acidic environment, the high concentration of chloride ions easily reacts with the PbO₂ protective layer on the anode, leading to anode degradation [8]. Additionally, the generated Pb²⁺ can react with chloride ions to form PbCl₂, which not only affects the stability of the electrodes but also reduces the purity of the final zinc ingots, thereby impacting product quality [9,10]. Therefore, effectively controlling the chloride ion concentration in the hydrometallurgical zinc refining system has become a key step in ensuring process stability and product quality.

At present, there have been extensive studies on chlorinated wastewater treatment at home and abroad, and the main methods include ion exchange [11,12], adsorption [13,14], electrochemical dechlorination [15,16,17], chemical precipitation, etc. Although ion exchange does not introduce new impurities, it generally shows poor dechlorination performance and requires a long processing time. Adsorption methods can efficiently remove chloride ions but often suffer from high adsorbent loss rates. Electrochemical dechlorination offers excellent removal efficiency. In recent studies, Zhang et al. [18] conducted an electrochemical chlorine evolution reaction (CER) using Ti/RuO₂ electrodes, which enhanced the chloride removal efficiency and reduced long-term chlorine release. Noman Khalid Khanzada et al. [19] developed conductive RO membranes (CNT-RO) embedded with carbon nanotubes, which were effective in reducing membrane degradation through the electrochemical reduction of chlorine, significantly improving chlorine resistance and desalination efficiency. However, the relatively high energy consumption of this method limits its feasibility for long-term industrial application. The chemical precipitation method is often used as the main method for removing chloride ions because of its easy operation and fast effect, and common precipitation methods include silver chloride precipitation [7], bismuth oxychloride precipitation [20], and cuprous chloride precipitation [21]. However, the price of silver and bismuth is usually high, which makes the dechlorination cost of the chemical precipitation method high and limits its application. Currently, companies typically use copper slag produced during the hydrometallurgical zinc refining process for dechlorination [22]. However, the copper slag dechlorination reaction is a typical solid–liquid reaction. In practical applications, when copper slag is left for a long time, a solid film forms on its surface, which hinders the mass transfer process. Meanwhile, the product CuCl tends to accumulate on the surface of the solid reactant, covering the active sites and further hindering the reaction. In addition, the copper slag dechlorination reaction makes a special redox reaction, and is characterized by a low reaction-driving force and incomplete reactions. These factors lead to low dechlorination efficiency, long dechlorination times, and high consumption of copper slag in this process [23]. Therefore, it is of great significance to explore a more efficient and rapid dechlorination method.

In response to the issues existing in current dechlorination processes, a new method of deep dechlorination using ascorbic acid to reduce Cu²⁺ is proposed in this study. Ascorbic acid (H₂A) is a natural green reductant [24,25], commonly used in processes such as metal ion leaching, reduction precipitation, and the removal of toxic and harmful substances [26,27,28]. In this process, ascorbic acid forms a homogeneous aqueous phase reaction system that overcomes the limitations of solid membrane mass transfer. Ascorbic acid effectively diffuses and interacts with Cu²⁺ in the solution, facilitating electron transfer reactions. This homogeneous liquid-phase reaction enhances the contact efficiency between the reactants and the reducing agent while also avoiding the issue of solid reaction products being wrapped by deposits, which can block active sites. This process solves the problems of high consumption and low reaction efficiency in the current copper slag dechlorination process. Figure 1. compares the mass transfer processes of the two methods, highlighting the advantages of the new approach in terms of mass transfer efficiency. Meanwhile, dissolved oxygen in the solution can cause the oxidation and re-dissolution of cuprous chloride (CuCl). In traditional copper slag dechlorination methods, a large amount of copper slag is typically required to ensure the effective removal of chloride ions from the solution. In contrast, this study uses ascorbic acid as a reducing agent, which effectively removes dissolved oxygen during the reaction, preventing the re-dissolution of CuCl and improving chloride ion removal efficiency. In addition, the unique enol structure of ascorbic acid provides a high electron density, making it an excellent electron donor. The enediol group (-C(OH)=C(OH)-) is present in its molecular structure. It promotes spontaneous charge transfer through the proton-coupled electron transfer (PCET) mechanism [29,30], significantly improving the electron transfer efficiency. This results in the rapid reduction of Cu²⁺ to Cu⁺, effectively reducing the dechlorination reaction time. In order to investigate the behavior of the process in the removal of chloride ions, experimental studies on key factors such as ascorbic acid dosage, copper ion dosage, initial pH, reaction temperature, and reaction time were carried out in this paper.

2. Materials and Methods

2.1. Experimental Reagents

The reagents used in this experiment were sodium chloride (NaCl), copper (II) sulfate pentahydrate (CuSO₄∙5H₂O), and sodium hydroxide (NaOH) produced by Tianjin Zhiyuan Chemical Reagent Co. (Tianjin, China); ascorbic acid (C₆H₈O₆) produced by Shanghai Aladdin Biochemical Technology Co. (Shanghai, China); and sulfuric acid (H₂SO₄) produced by Chongqing Chuandong Chemical Group Co. (Chongqing, China) All of the above reagents were analytically pure.

2.2. Experimental Procedures and Methods

Ascorbic acid and copper (II) sulfate pentahydrate with different molar ratios were added to the chlorine-containing solution to be treated, and the pH of the reaction solution was adjusted. The reaction was carried out in a collector-type thermostatic magnetic stirrer, with a constant temperature and the same stirring speed maintained for a certain time. After the reaction was completed, solid–liquid separation was performed using a circulating vacuum pump, resulting in a dechlorinated solution and a chlorine-containing precipitate. The mass concentration of chloride ions in the dechlorinated liquid was measured by ion chromatography. After washing, the precipitate was dried in an electric heating blast dryer. A schematic diagram of the experimental apparatus and the process flow are shown in Figure 2. The removal efficiencies of Cl⁻ in the solution are shown by the following formula:

$$\mathsf{\delta }={\displaystyle \frac{{\mathrm{C}}_0-{\mathrm{C}}_{\mathrm{t}}}{{\mathrm{C}}_0}}\times 100\%$$

In the above formula, δ denotes the chloride ion removal efficiency; C₀ refers to the initial concentration of chloride ions in the solution; and C₁ indicates the concentration of chloride ions left in the solution after the reaction is completed (g/L).

2.3. Analytical Detection Methods

The chloride ion concentration in the solution was determined using ion chromatography. The crystalline phases of the precipitate were identified through X-ray diffraction (XRD) analysis using a Miniflex 600 diffractometer (Rigaku, Tokyo, Japan), with diffraction patterns interpreted via Jade software (Jade 6.5.0.0). The precipitate’s morphology and elemental composition were examined using scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS) on a Sigma 300 microscope (ZEISS, Oberkochen, Germany). The elemental content was further quantified using X-ray fluorescence spectroscopy (XRF) with an SX Primus III+ spectrometer (Rigaku, Tokyo, Japan). The surface chemical properties and elemental composition of the solid powder were analyzed using X-ray photoelectron spectroscopy (XPS) with a K-Alpha spectrometer (Thermo Scientific, Waltham, MA, USA).

2.4. Calculations Related to Thermodynamic Parameters

In redox reactions, the driving force of the reaction depends on the potential difference between the reactants [31]. The standard Gibbs free energies for oxidation and reduction reactions determine the spontaneity of the reaction. The more negative the standard Gibbs free energy, the greater the spontaneity of the reaction. Similarly, the larger the standard equilibrium constant, the more complete the reaction tends to be.

2.4.1. Ascorbic Acid Dechlorination

Ascorbic acid dechlorination uses ascorbic acid as a reducing agent, which undergoes a redox reaction with copper ions in the solution to generate cuprous ions for dechlorination. The reaction equation is shown in Equation (1).

$$2{\mathrm{Cu}}^{2+}+{\mathrm{C}}_6{\mathrm{H}}_8{\mathrm{O}}_6+2{\mathrm{Cl}}^{-}\to 2\mathrm{CuCl}+{\mathrm{C}}_6{\mathrm{H}}_6{\mathrm{O}}_6+{2\mathrm{H}}^{+}$$

(1)

The reaction of ascorbic acid with Cu²⁺ involves the transfer of electrons from the reducing agent to Cu²⁺. In the redox process, two half-reactions take place: one in which Cu²⁺ gains electrons to reduce to Cu⁺, which then combines with Cl⁻ to form CuCl, and another in which the reducing agent (ascorbic acid) loses electrons and is oxidized to dehydroascorbic acid [32]. The electric potential ΔE, the standard Gibbs free energy ΔG^θ, and the standard equilibrium constant K were calculated for this reaction from the data in

Table 1. Standard electrode potentials of ascorbic acid dechlorination electrochemical pairs at 25 °C and under standard conditions.

Redox Electric Pair	Reduction Half-Reaction Equation	Eθ (V)
Cu2+/CuCl	Cu2+ + e− + Cl−→CuCl	0.57
C6H8O6/C6H6O6	C6H8O6 − 2e−→C6H6O6 + 2H+	0.08

, using Equations (2), (3) and (5).

$$∆{\mathrm{E}}_1={\mathrm{E}}_{{\mathrm{Cu}}^{2+}/\mathrm{CuCl}}-{\mathrm{E}}_{{\mathrm{C}}_6{\mathrm{H}}_8{\mathrm{O}}_6/{\mathrm{C}}_6{\mathrm{H}}_6{\mathrm{O}}_6}=0.57-0.08=0.49\mathrm{V}$$

(2)

$${∆{\mathrm{G}}_1}^{\mathsf{\theta }}=-\mathrm{zF}{\mathrm{E}}^{\mathsf{\theta }}=-2\times 96,484.5\times 0.49=-94,554.8\mathrm{J}/\mathrm{mol}=-94.6\mathrm{KJ}/\mathrm{mol}$$

(3)

The calculations were performed according to isothermal Equation (4):

$${∆\mathrm{G}}^{\mathsf{\theta }}=-\mathrm{RTlnK}$$

(4)

$$\lg {\mathrm{K}}_1={\displaystyle \frac{\mathrm{zF}{\mathrm{E}}^{\mathsf{\theta }}}{2.303\mathrm{RT}}}={\displaystyle \frac{94,554.8}{2.303\times 8.314\times 298}}=16.572$$

(5)

$${\mathrm{K}}_1=3.733\times {10}^{16}$$

2.4.2. Copper Slag Dechlorination

The core mechanism of the copper slag dechlorination method involves the reduction of Cu²⁺ in the solution by elemental copper in the copper slag, resulting in the formation of Cu⁺ and the removal of chloride ions. The Cl⁻ in the chlorinated solution ultimately precipitates as cuprouschloride (CuCl) and remains in the slag. The reaction mechanism for copper slag dechlorination is shown in Equation (6).

$${\mathrm{Cu}}^{2+}+\mathrm{Cu}+{2\mathrm{Cl}}^{-}=2\mathrm{CuCl}\downarrow$$

(6)

Copper slag dechlorination contains two half-cell reactions: one in which Cu²⁺ gains electrons to be reduced to Cu⁺ and combined with Cl⁻ to form CuCl, and the other in which Cu⁰ loses electrons to be oxidized to Cu⁺ and combined with Cl⁻ to form CuCl. The electric potential ΔE, the standard Gibbs free energy ΔG^θ, and the standard equilibrium constant K of the reaction were calculated from the data in

Table 2. Standard electrode potentials of copper slag dechlorination electrochemical pairs at 25 °C and under standard conditions.

Redox Electric Pair	Reduction Half-Reaction Equation	Eθ (V)
Cu2+/CuCl	Cu2+ + e− + Cl−→CuCl	0.57
CuCl/Cu	Cu0 − e− + Cl−→CuCl	0.171

using Equations (7)–(9).

$${∆\mathrm{E}}_2={\mathrm{E}}_{{\mathrm{Cu}}^{2+}/\mathrm{CuCl}}-{\mathrm{E}}_{\mathrm{CuCl}/\mathrm{Cu}}=0.57-0.171=0.399\mathrm{V}$$

(7)

$${{∆\mathrm{G}}_2}^{\mathsf{\theta }}=-{\mathrm{z}}_2\mathrm{F}{\mathrm{E}}^{\mathsf{\theta }}=-1\times 96,484.5\times 0.399=-38,497.3\mathrm{J}/\mathrm{mol}=-38.5\mathrm{kJ}/\mathrm{mol}$$

(8)

$$\lg {\mathrm{K}}_2={\displaystyle \frac{{\mathrm{z}}_2\mathrm{F}{{\mathrm{E}}_2}^{\mathsf{\theta }}}{2.303\mathrm{RT}}}={\displaystyle \frac{38,497.3}{2.303\times 8.314\times 298}}=6.747$$

(9)

$${\mathrm{K}}_2=5.58\times {10}^6$$

2.4.3. Comparison of Thermodynamic Parameters

The calculation results of the ascorbic acid dechlorination method and copper slag dechlorination method were compared and analyzed, as shown in

Table 3. Thermodynamic parameter comparison of ascorbic acid dechlorination and copper slag dechlorination methods.

Dechlorination Process	Thermodynamic Parameter
	Reaction Potential Difference (ΔE)	Standard Gibbs Free Energy (ΔGθm)	Standard Equilibrium Constant (K)
Ascorbic acid dechlorination	0.49 V	−94.6 kJ/mol	3.733 × 1016
Copper slag dechlorination	0.399 V	−38.5 kJ/mol	5.58 × 106
Process comparison	ΔE1 > ΔE2	$∆{{\mathrm{G}}^{\mathsf{\theta }}}_{\mathrm{m}}({\mathrm{C}}_6{\mathrm{H}}_8{\mathrm{O}}_6/{\mathrm{Cu}}^{2+})<∆{{\mathrm{G}}^{\mathsf{\theta }}}_{\mathrm{m}}(\mathrm{Cu}/{\mathrm{Cu}}^{2+})<0$	K1 $\gg$ K2

. According to the thermodynamic calculation results of the two methods, corresponding thermodynamic diagrams were drawn, as shown in Figure 3.

The thermodynamic calculations show that the potential difference (ΔE₁) for ascorbic acid dechlorination is 0.091 V higher than that of copper slag dechlorination (ΔE₂), and its reaction-driving force is increased by 18.6%. The standard Gibbs free energy (ΔG₁^θ) for ascorbic acid dechlorination is 56.1 kJ/mol lower than that of copper slag dechlorination (ΔG₂^θ), suggesting that ascorbic acid dechlorination had stronger reaction spontaneity. The standard equilibrium constant (K₁) for ascorbic acid dechlorination is significantly greater than that of copper slag dechlorination (K₂), with K₁ being 6.7 × 10⁹ times larger than K₂. This indicates that the reaction tendency for ascorbic acid dechlorination is much stronger than that for copper slag dechlorination under standard conditions, and the reaction is more complete.

3. Results and Discussion

3.1. Ascorbic Acid Addition

Figure 4 illustrates how varying amounts of ascorbic acid impact its dechlorination effect. We conducted the experiment under the following conditions: an initial pH of 3.6, a molar ratio of Cu²⁺ to Cl⁻ of 1.5, a reaction temperature of 20 °C, and a reaction time of 20 min. We carried out a one-way experiment with an n(H₂A)/n(Cl⁻) ratio of 0.4, 0.5, 0.75, 1, 1.25.

As shown in Figure 4, different amounts of ascorbic acid significantly affect the removal of chloride ions from the solution. As the n(H₂A)/n(Cl⁻) ratio increases, the chloride ion removal rate initially rises and then gradually decreases. As the n(H₂A)/n(Cl⁻) ratio rises from 0 to 0.5, the rate of chloride ion removal markedly improves, resulting in a decrease in chloride ion concentration in the solution from 1 g/L to 0.1837 g/L, which corresponds to a removal rate of 81.4%. As the n(H₂A)/n(Cl⁻) ratio rises from 0.5 to 0.75, the concentration of chloride ions decreases to 0.1363 g/L, resulting in a peak removal rate of 86.2%. However, as the n(H₂A)/ (Cl⁻) ratio is further increased to 1 and 1.25, the chloride ion removal rates drop to 84.6% and 83.6%, respectively, which are lower than the rate observed at an n(H₂A)/n(Cl⁻) ratio of 0.75.

These experimental results indicate that increasing the amount of ascorbic acid significantly enhances the removal rate of chloride ions. This is because ascorbic acid dissociates into ascorbate anions and hydrogen ions in the solution. As the n(H₂A)/n(Cl⁻) ratio increases, the concentration of ascorbate anions in the solution rises, which increases the frequency of interactions between reactants, thereby promoting the reduction in more copper ions and significantly improving chloride ion removal efficiency. However, when the amount of ascorbic acid is too high, the chloride ion removal rate decreases. This is because, under high concentrations of ascorbic acid, hydrogen ions gradually accumulate during the reaction, and the increasing hydrogen ion concentration inhibits the production of ascorbate anions. This, in turn, affects the coordination between copper ions and ascorbate anions, hindering chloride ion removal. Therefore, to optimize the experimental results, an ascorbic acid addition ratio of n(H₂A)/n(Cl⁻) = 0.75 was chosen for subsequent studies.

3.2. 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, an n(H₂A)/n(Cl⁻) ratio of 0.75, a reaction temperature of 20 °C, and a duration of 20 min. The molar ratios of n(Cu²⁺) to 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 n(Cu²⁺)/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 n(Cu²⁺)/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 n(Cu²⁺)/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 Cu²⁺ interacting with ascorbic acid anions, which promotes the formation of Cu⁺ and effectively removes chloride ions from the solution.

As the ratio of n(Cu²⁺)/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 Cu(OH)⁺ and Cu(OH)₂ and releasing H⁺. On one hand, the hydrolysis products cannot participate in the reduction coordination reaction, lowering the concentration of 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 n(Cu²⁺)/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 n(Cu²⁺)/n(Cl⁻) ratio of 1.25 was selected for subsequent experiments to achieve the optimal chloride ion removal effect.

3.3. Initial Reaction pH

The principle behind ascorbic acid dechlorination mainly involves the reduction of Cu²⁺ 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 Cu²⁺ in the solution. As shown in Figure 6, we used HSC 6.0 software to calculate the thermodynamic parameters of the Cu²⁺ hydrolysis reaction and performed equilibrium calculations based on the law of mass conservation. The results indicate that as pH increases, the hydrolysis reaction of Cu²⁺ 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, Cu²⁺ begins to hydrolyze, and when the pH exceeds 4, Cu²⁺ 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 Cu²⁺ 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(Cu²⁺)/n(Cl⁻)/n(H₂A) = 1.25:1:0.75, with a reaction temperature of 20 °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 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):

$${2\mathrm{Cu}}^{2+}+{\mathrm{C}}_6{\mathrm{H}}_8{\mathrm{O}}_6={2\mathrm{Cu}}^{+}+{\mathrm{C}}_6{\mathrm{H}}_6{\mathrm{O}}_6+{2\mathrm{H}}^{+}$$

(10)

$${\mathrm{C}}_6{\mathrm{H}}_8{\mathrm{O}}_6{+2\mathrm{e}}^{-}={\mathrm{C}}_6{\mathrm{H}}_6{\mathrm{O}}_6+{2\mathrm{H}}^{+}$$

(11)

where the standard electrode potential of C₆H₆O₆/C₆H₈O₆, φ^θ = 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):

$$\phi ={\phi }^{\mathsf{\theta }}+2.303\times {\displaystyle \frac{\mathrm{RT}}{\mathrm{nF}}}\lg {\displaystyle \frac{{\mathrm{c}\left({\mathrm{C}}_6{\mathrm{H}}_6{\mathrm{O}}_6\right)·\mathrm{c}\left[{\mathrm{H}}^{+}\right]}^2}{\mathrm{c}\left({\mathrm{C}}_6{\mathrm{H}}_8{\mathrm{O}}_6\right)}}$$

(12)

where c(C₆H₈O₆) and c(C₆H₆O₆) are the concentrations of C₆H₈O₆ and C₆H₆O₆, and n = 2. The Equation (13) is obtained through the following calculation.

$$\phi ={\phi }^{\mathsf{\theta }}+0.0591\lg \left({\mathrm{H}}^{+}\right)$$

(13)

According to the calculations based on the Nernst equation, when the initial reaction pH value is low, the concentration of hydrogen ions is high, which leads to the high electrode potential of ascorbic acid, thus weakening the reducing ability of ascorbic acid, which is unfavorable for the reduction reaction to take place. When the pH value gradually increases, the electrode potential of ascorbic acid decreases, which enhances the reducing ability of ascorbic acid and, thus, is more conducive to the removal of chloride ions. Additionally, as shown in Figure 9. the pH of the solution significantly affects the species and concentration of ascorbic acid. As the pH increases, ascorbic acid dissociates into ascorbate ions and hydrogen ions, which increases the concentration of ascorbate ions in the solution. This promotes greater binding of divalent copper ions to their ligands, enhancing the reduction efficiency and ultimately improving the chloride ion removal rate.

The removal of chloride ions from the solution gradually decreases as the reaction pH was further increases. When the initial reaction pH increases to 4.2, the removal rate of chloride ions is only 79.8%. According to the experimental observations in Figure 8, the color of the precipitate changes from white to yellow. This suggests that when the pH reaches 4.2, the following side reactions may occur, as shown in Equations (14) and (15). Additionally, the resulting cuprous chloride product reacts with OH⁻, leading to the re-dissolution of chloride ions, as shown in Equation (18), which decreases the chloride ion removal efficiency.

$${\mathrm{Cu}}^{2+}+{\mathrm{C}}_6{\mathrm{H}}_8{\mathrm{O}}_6+{3\mathrm{OH}}^{-}\to \mathrm{CuOH}\downarrow +{2\mathrm{H}}_2\mathrm{O}+{\mathrm{C}}_6{\mathrm{H}}_6{\mathrm{O}}_6$$

(14)

$$4\mathrm{CuOH}+{\mathrm{O}}_2+{2\mathrm{H}}_2\mathrm{O}={4\mathrm{Cu}\left(\mathrm{OH}\right)}_2$$

(15)

$${\mathrm{Cu}\left(\mathrm{OH}\right)}_2+{\mathrm{C}}_6{\mathrm{H}}_8{\mathrm{O}}_6=\mathrm{Cu}+{\mathrm{C}}_6{\mathrm{H}}_6{\mathrm{O}}_6+{2\mathrm{H}}_2\mathrm{O}$$

(16)

$$2{\mathrm{Cu}\left(\mathrm{OH}\right)}_2+{\mathrm{C}}_6{\mathrm{H}}_8{\mathrm{O}}_6={\mathrm{Cu}}_2\mathrm{O}+{\mathrm{C}}_6{\mathrm{H}}_6{\mathrm{O}}_6+{3\mathrm{H}}_2\mathrm{O}$$

(17)

$$2\mathrm{CuCl}+{2\mathrm{OH}}^{-}=\mathrm{CuOH}\downarrow +{2\mathrm{Cl}}^{-}$$

(18)

Based on the equilibrium concentration distribution of copper ions at different pH levels shown in Figure 6. It can be observed that as the pH increases, the concentration of OH⁻ ions rises, enhancing the hydrolysis of copper ions. This leads to the gradual conversion of divalent copper ions into copper hydroxide. This conversion competes with the target reaction (i.e., the reduction of copper ions), causing more copper ions to form insoluble copper hydroxide, which hinders the formation of cuprous ions (Cu⁺). Therefore, when the initial reaction pH exceeds 3.6, according to Equations (16) and (17), insoluble impurities such as cuprous hydroxide, copper metal, and cuprous oxide may form in the solution, resulting in a decrease in chloride ion removal efficiency. To optimize the experimental results, an initial reaction pH of 3.6 was selected for subsequent experiments.

3.4. Reaction Temperature

To explore the influence of different temperatures on the dechlorination effect of ascorbic acid, we conducted experiments under the following conditions: n(Cu²⁺)/n(Cl⁻)/n(H₂A) = 1.25:1:0.75, with an initial reaction pH value of 3.6 and a reaction time of 20 min. In addition, the experiment was conducted at different reaction temperatures, including 20 °C, 30 °C, 40 °C, 50 °C, 60 °C, and 70 °C.

As shown in Figure 10, with an increase in reaction temperature, the removal rate of chloride ions in the solution decreases. When the temperature rises from 20 °C to 70 °C, the chloride ion removal rate decreases from 90.8% to 51.7%. This indicates that the best chloride ion removal performance is achieved at 20 °C. The reasons for the decline in chloride ion removal rate with increasing temperature are as follows: (1) Decreased stability of ascorbic acid at higher temperatures: Ascorbic acid is relatively heat-resistant in its crystalline form, but it is highly sensitive to high temperatures in aqueous solutions. In the presence of O₂ and divalent copper ions, it is easily oxidized and converted into dehydroascorbic acid, which further degrades into biologically inactive diketogulonic acid. As the temperature increases, the degradation rate of ascorbic acid accelerates, leading to a decrease in the ascorbic acid available for reduction, thus reducing the chloride ion removal rate. (2) The increase in temperature affects the precipitation process of cuprous chloride. According to Figure 11, it can be seen that the solubility product constant of CuCl increases gradually with an increase in temperature, which leads to an increase in its solubility, which is unfavorable for the precipitation of cuprous chloride and makes the re-solubilization of cuprous chloride occur.

3.5. Reaction Time

To study the effect of reaction time on the dechlorination efficiency of ascorbic acid, we conducted experiments with reaction times ranging from 0.5 to 40 min under the following conditions: an initial pH of 3.6, a molar ratio of n(Cu²⁺)/n(Cl⁻)/n(H₂A) = 1.25:1:0.75, and a reaction temperature of 20 °C.

From Figure 12, it can be observed that as the reaction time increases, the concentration of remaining chloride ions in the solution initially decreases and then increases. At a reaction time of 3 min, the removal rate of chloride ions is 70.0%, and the concentration of chloride ions decreases from 1 g/L to 0.3002 g/L, with a rapid decrease in chloride ion content. At the 10 min reaction time, the removal rate reaches 78.8%, and the chloride ion concentration drops from 1 g/L to 0.2116 g/L. When the reaction time is extended to 20 min, the removal rate of chloride ions reaches its highest value of 90.8%, and the remaining chloride ion concentration is only 0.0917 g/L. This is mainly due to the unique enol structure of ascorbic acid, which has a high electron density. The enediol group (-C(OH)=C(OH)-) of ascorbic acid drives spontaneous charge transfer through the proton-coupled electron transfer (PCET) mechanism, significantly enhancing the electron transfer efficiency, which, in turn, rapidly reduces Cu²⁺ to Cu⁺, accelerating the chlorination removal process.

When the reaction time exceeds 20 min, the chloride ion removal rate starts to decline. This is because after the reaction is complete, as the reaction progresses, the concentration of ascorbic acid gradually decreases, weakening the reducing environment of the entire system. The dissolved oxygen in the solution promotes the oxidation of the generated cuprous chloride (as shown in reaction Equation (19)), and as some cuprous chloride is oxidized and re-dissolved, chloride ions are released back into the solution, leading to an increase in the remaining chloride ion concentration and ultimately reducing the chloride ion removal efficiency.

$$2\mathrm{CuCl}+{\displaystyle \frac{1}{2}}{\mathrm{O}}_2+{2\mathrm{H}}^{+}\to {2\mathrm{Cu}}^{2+}+{2\mathrm{Cl}}^{-}+{\mathrm{H}}_2\mathrm{O}$$

(19)

3.6. Ascorbic Acid Dechlorination Reaction Mechanism

3.6.1. Analysis of Products of Dechlorination

The effect of different reaction factors on the reductive dechlorination reaction of ascorbic acid was investigated, and the optimized experimental conditions were determined to be as follows: an initial pH of 3.6, a molar ratio of n(Cu²⁺)/n(Cl⁻)/n(H₂A) = 1.25:1:0.75, a reaction temperature of 20 °C (room temperature), and a reaction time of 20 min. Under these optimized conditions, 90.8% removal of chloride ions was achieved, and the chloride concentration was reduced from an initial value of 1 g/L to 0.0917 g/L. Under this condition, the slag samples of dechlorination products were characterized by XRD, XPS, XRF, and SEM-EDS.

In order to investigate the substance type and composition of the precipitate product in the dechlorination of ascorbic acid, the precipitate product was analyzed for its physical phase using X-ray diffraction (XRD). The XRD technique is an effective analytical method that can provide information about the crystal structure in the precipitate product and help to identify its main physical phase. The XRD pattern of the precipitate is shown in Figure 13. By comparing this pattern with the standard cuprous chloride pattern (JCPDS NO.06-0344), it can be found that the characteristic diffraction peaks in the XRD pattern match well with the diffraction peaks of the standard cuprous chloride pattern, which proves that the precipitation product produced during the experiment is mainly cuprous chloride (CuCl). An additional diffraction peak was generated at the 2θ position at 36.7°, and this peak corresponds to basic copper chloride. This indicates that the effects of exposure to air and water during precipitate washing resulted in the conversion of a small amount of cuprous chloride to basic copper chloride through oxidation.

To further explore the reaction mechanism of ascorbic acid dechlorination, we analyzed the precipitation products formed during the reaction using X-ray photoelectron spectroscopy (XPS), as illustrated in Figure 14. The XPS spectra were corrected for surface charge by taking the carbon C1s peak (284.8 eV) as a reference. From the 2p3/2 XPS spectrum of Cu, it can be observed that the binding energy peak at 932.4 eV corresponds to monovalent copper Cu(I) (from CuCl). In addition, satellite peaks of Cu²⁺ were detected at 938.4 eV and 945.2 eV, and the presence of the satellite peaks indicates the presence of copper in an oxidized state (Cu²⁺) in the sample. This suggests that the generated precipitation product came in contact with air during washing as well as drying, resulting in oxidation of the generated CuCl to basic copper chloride.

To investigate the surface morphology and elemental distribution of the precipitate produced in the ascorbic acid dechlorination process, scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) were employed for analysis, as shown in Figure 15. From the image, it can be observed that the precipitate exhibits an irregular morphology. Further analysis using the EDS spectrum reveals that the main constituent elements of the precipitate are Cu and Cl. According to the EDS elemental mapping, the atomic percentage of Cu is 43.51% and the mass percentage is 60.88%, while the atomic percentage of Cl is 44.90% and the mass percentage is 35.04%. Oxygen (O) is also present with an atomic percentage of 11.59% and a mass percentage of 4.08%. The distribution of Cu and Cl elements is nearly identical, indicating that these elements are uniformly distributed within the precipitate. Based on this distribution pattern, it can be inferred that the precipitate is primarily CuCl. This conclusion is further supported by the elemental composition ratios of the dechlorination product in

Table 4. Percentage of elemental content of ascorbic acid dechlorination products.

Element	Cu	Cl	O
Concentration (%)	67.82	32.09	0.09

, where Cu and Cl are the dominant elements. Their ratio is consistent with the theoretical composition of cuprouschloride (CuCl). This finding not only corroborates the results from the EDS analysis but also provides strong quantitative evidence for CuCl as the main product. This conclusion is consistent with the findings from the X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analysis.

3.6.2. Dechlorination Process Through Reduction of Cu²⁺ with Ascorbic Acid

Based on the study of the experimental procedure and the analysis of the precipitation products, the reaction process for the reduction of Cu²⁺ by ascorbic acid to remove chlorine is shown in Figure 16. Ascorbic acid (C₆H₈O₆) is a reducing agent with an enol structure in which three carbon atoms and one oxygen atom form double bonds, all of which are sp²-hybridized. The π, π-conjugated structure that forms between the carbon atoms in the enol structure increases the electron cloud density, making the carbon–carbon double bond highly reactive. The p-electron pair on the hydroxyl oxygen of the enol structure slightly biases the charge toward the carbon end, increasing the polarity of the hydroxyl–oxygen–hydrogen bond and making it easier for hydrogen to dissociate. The redox reaction of ascorbic acid follows a proton-coupled electron transfer (PCET) mechanism, in which proton (H⁺) dissociation and electron (e⁻) transfer occur simultaneously. Specifically, the enediol structure of ascorbic acid facilitates the dissociation of a proton to form an ascorbate monoanion. The negatively charged oxygen atom in this species can attract positively charged particles such as Cu²⁺, enabling the transfer of an electron from the oxygen to the Cu²⁺ ion, thereby reducing it to Cu⁺. Throughout the process, proton transfer and electron transfer occur in a concerted manner, reflecting the core characteristics of the PCET mechanism, thereby contributing to the effective reduction of Cu²⁺ and removal of chloride ions by ascorbic acid. The hydroxyl group of another enolic structure of ascorbic acid can also ionize hydrogen ions and participate in the reduction of Cu²⁺. The reduced Cu⁺ reacts with chloride ions (Cl⁻)-in solution to form a cuprous chloride precipitate (CuCl). During this process, ascorbic acid itself is oxidized to dehydroascorbic acid (C₆H₆O₆).

When dissolved oxygen exists in the solution, the CuCl produced by the reaction is very easily oxidized. However, this process uses ascorbic acid as a reducing agent, which effectively removes dissolved oxygen during the dechlorination reaction, preventing the oxidation and re-dissolution of cuprous chloride.

4. Conclusions

In response to the issues with the current copper slag dechlorination process, this study proposes a new deep dechlorination process based on liquid-phase reduction. Through a comparative analysis of thermodynamic parameters and experimental validation, the following conclusions are drawn.

(1)

The ascorbic acid dechlorination process showed significant advantages over copper slag dechlorination. Its potential difference ΔE was 0.091 V higher, the reaction-driving force was enhanced by 18.6%, the standard Gibbs free energy ΔG^θ was 59.3% lower, and the equilibrium constant K was 6.7 × 10⁹ times higher. This indicates that the dechlorination of ascorbic acid has stronger spontaneity and a more complete reaction, and a higher purification depth can be achieved.

(2)

Under optimized conditions (initial pH 3.6, n(Cu²⁺)/ n(Cl⁻)/n(H₂A) = 1.25:1:0.75, 20 °C, 20 min), the chloride concentration was reduced from 1 g/L to 0.0917 g/L with 90.8% removal. The ascorbic acid dechlorination process achieved rapid dechlorination through its unique enol structure, and the reaction process was a homogeneous aqueous phase reaction, with simple and easy-to-control diffusion steps, which avoided the limitations posed by the solid film barrier and possessed higher flexibility and reaction efficiency than copper slag dechlorination.

The experimental study on the dechlorination of ascorbic acid showed that ascorbic acid as a reducing agent has a good removal effect on chloride ions. Compared to the traditional copper slag dechlorination method, the ascorbic acid method demonstrates higher reaction efficiency. Additionally, for the removal of the same molar amount of chloride ions, the reagent consumption of the copper slag method is significantly higher than that of ascorbic acid. Therefore, from a reagent cost perspective, the ascorbic acid dechlorination method offers a certain economic advantage. In future industrial applications, attention should be paid to the impact of temperature on dechlorination efficiency. At the same time, given the organic nature of ascorbic acid, the subsequent organic substances produced need to be properly treated according to the specific production process of the enterprise, such as through adsorption, oxidation, or other technologies, to reduce the accumulation of organic substances.