Removal of Cl(−I) from acidic industrial wastewater through oxidation: a review on methods and mechanisms

Abstract

An abundance of acidic wastewater containing high concentrations of Cl(−I) is produced annually in industries. The traditional precipitation, ion exchange and physical separation methods for Cl(−I) removal possess the major limitation of low efficiency, making it necessary to develop new approaches with high efficiency. Recently, the technology of advanced oxidation process (AOPs) and ligand-to-metal charge transfer process (LMCT) has attracted considerable attention for the oxidation removal of Cl(−I), where the various intermediate active species with extremely high oxidability exhibit the ability to oxidize Cl(−I). In this article, the efficiency, mechanisms, advantages, limitations, developmental trends and potential improvements of the electrolysis, UV-induced and strong-oxidant-dominated oxidation approaches for Cl(−I) removal are reviewed in detail. The oxidation approaches exhibit the common advantage of high efficiency. However, the potential secondary pollution of Cl₂ gas and the high requirement for wastewater quality limit the actual application. Before the electrolysis and UV-induced oxidation methods can be applied, the energy consumption must be further reduced. Additionally, the slow release and stability of the Na₂S₂O₈, NaBiO₃ and PbO₂ agents should be further studied in the strong-oxidant-dominated oxidation approaches. This study provides the effective removal of Cl(−I) from acidic industrial wastewater with the theoretical foundation and technical support.

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Wenyue Dou

Wenyue Dou (corresponding author) works at Xuzhou University. Her main research interests involve environmental water chemistry of chloride pollutants, the control of water pollution, and resource recovery based on adsorption, oxidation and reduction, coagulation and separation processes of chloride pollutants in water environments.

Linghao Kong

Linghao Kong works at the Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. His main research interests involve the development of technology for heavy metal detection, the control of heavy metal pollution and the reuse of industrial wastewater.

Water impact

A large quantity of acidic wastewater containing high concentrations of Cl(−I) is produced in industries annually. Cl(−I), the most stable form of the element chlorine in aqueous conditions, is hard to remove by common physicochemical methods. Recently, technology based on AOPs and LMCT processes has attracted considerable attention for the oxidation removal of Cl(−I) from acidic industrial wastewater. In this article, oxidation approaches for Cl(−I) were divided into electrolytic, UV-induced and strong-oxidant-dominated methods. The efficiency, mechanisms, advantages and limitations of the traditional electrolysis methods and the newly-developed Cu(0)/Cu(II)/UV, Bi₂O₃/UV, Na₂S₂O₈, NaBiO₃ and PbO₂ methods were reviewed in detail. Finally, the developmental trends and potential improvements for these oxidation-removal approaches of different kinds were discussed separately, providing the theoretical foundation and technical support for the effective removal of Cl(−I) and the purification of industrial wastewater.

1. Introduction

Cl(−I), the most stable form of the element chlorine, widely exists in surface waters. Except for the natural sources of seawater intrusion and mineral corrosion, the artificial source of industrial wastewater is also a major source for Cl(−I) in nature.^1–3 In industries, numerous materials and additive agents containing Cl(−I) are frequently employed, leading to the generation of acidic wastewater with high concentrations of Cl(−I) and large quantities.^4–8 For example, the use of a kind of major additive agent, hydrochloric acid (HCl), results in a high content of Cl(−I) in acidic wastewater produced from the rare earth extraction process.⁹ The content of the Cl(−I) impurity in the mineral materials has been greatly enhanced in recent years with the continuous exploitation of mineral resources. Thus, the amount of Cl(−I) released during the hydrometallurgical process has also increased. In the strongly acidic wastewater produced from nonferrous smelting industries, the concentration of Cl(−I) can even reach over ten thousand milligrams per liter.

Without the appropriate treatment on the above-mentioned acidic industrial wastewater containing high concentrations of Cl(−I), a sharp increase in the Cl(−I) content of surface waters will be induced. Furthermore, the environment and organisms will be damaged to varying degrees. On the one hand, the discharge of Cl(−I)-containing wastewater may influence the normal growth of organisms. For instance, the accumulation of Cl(−I) in soils can lead to salinization problems, which will then inhibit plant growth and induce crop death due to poisoning.¹⁰ Additionally, drinking water with high Cl(−I) concentrations may affect the normal metabolism of animals and lead to dramatic poisoning symptoms.^11,12 On the other hand, for the living facilities and industrial settings, Cl(−I) is also extremely unfavorable because of its corrosion function. For example, Cl(−I) in the concrete can lead to the corrosion of steel, the expansion of concrete and the decrease in the compressive capacity, ultimately inducing damages to building structures.^13–16 Meanwhile, Cl(−I) possesses the ability of an inter-crystalline, crevice and pitting corrosion on metals through destroying the passive films composed of metallic oxide. As a result, the abnormal operations of equipment and security risks may occur.^17–19 Therefore, to avoid the environmental and biological risks caused by the discharge of Cl(−I), the effective removal of Cl(−I) from various acidic industrial wastewater is of particular significance.

Cl(−I) cannot be utilized by microorganisms effectively. Worse still, a high concentration of Cl(−I) will enhance the osmotic pressure, destroy the cell membrane, and inhibit the normal growth of microorganisms. Given this, the removal of Cl(−I) through biological methods seems to be nearly impossible, and physicochemical methods are commonly employed for the removal of Cl(−I). At present, the widely-studied methods for Cl(−I) removal under aqueous conditions can be divided into precipitation, ion exchange, physical separation and electrolysis. Cl(−I) has the strong ability to combine with some specific metal ions or organic compounds to generate precipitates, making precipitation become the most widely used Cl(−I)-removal approach. The major precipitants employed for the removal of Cl(−I) include copper slag, bismuth oxide (Bi₂O₃), silver nitrate (AgNO₃), calcium–aluminum salts and thiourea.^17,20–28 Meanwhile, the negative ion charge makes it possible for Cl(−I) to be removed through ion exchange approach using ion exchange resins or layered double hydroxides (LDHs).^29–35 In addition, because of the extremely high stability of Cl(−I), removing Cl(−I) without any changes to its ionic form through physical separation methods, including electrosorption, electrodialysis, evaporation concentration and solvent extraction,^36–46 has also been widely studied.

As we mentioned above, Cl(−I) possesses extremely high stability because of its fully saturated outer electrons, which indicates that the valence changes of Cl(−I) are rather difficult. At the same time, the electrode potential (φ₀) of Cl(−I)/Cl₂ is up to approximately 1.36 eV,⁴⁷ further demonstrating that through general oxidation methods, Cl(−I) is hard to transform into Cl₂ gas. Naturally, the electrolysis approach, where the redox reactions can easily occur under the function of a current, was studied for the oxidation removal of Cl(−I) in the form of Cl₂ gas in the past few decades.^48–51

Although various Cl(−I)-removal methods have been studied and proposed, the common disadvantage of low efficiency in these approaches makes it necessary to develop new Cl(−I)-removal methods with higher efficiency. In addition, the limitations of different Cl(−I)-removal technologies, for instance, the high dosage of precipitants in the precipitation method, the high regeneration frequency of ion exchangers in the ion exchange method, and the demand of the low Cl(−I) concentration in the physical separation method, also show the importance of further research studies on Cl(−I) removal from wastewater.

It can be determined from the φ₀ of Cl(−I)/Cl₂ that, as long as φ₀ of the dosed oxidant and its corresponding reduction product exceeds 1.36 eV, it is probable that Cl(−I) will be oxidized to produce Cl₂ gas by this kind of oxidant. Nowadays, along with the rapid development of water treatment technologies, various active species with extremely high oxidability (for instance, hydroxyl (˙OH) radical, sulfate (˙SO₄⁻) radical, photo-induced holes (h⁺), Pb(IV) and Bi(V)) have begun to attract much attention for the oxidation-removal process of Cl(−I) with high concentration. Accordingly, the technology of advanced oxidation process (AOPs) and ligand-to-metal charge transfer process (LMCT) was systematically studied for the removal of Cl(−I) from acidic industrial wastewater, and was proved to be effective.

In this article, the oxidation approaches for Cl(−I) removal are divided into electrolysis, ultraviolet (UV)-induced and strong-oxidant-dominated methods, as shown in Fig. 1. The research studies on the oxidation-removal approaches of Cl(−I) from acidic industrial wastewaters, including the traditional electrolysis methods, the newly developed Cu(0)/Cu(II)/UV, Bi₂O₃/UV, sodium persulfate (Na₂S₂O₈), sodium bismuthate (NaBiO₃) and lead dioxide (PbO₂) methods, were reviewed in detail. The corresponding technology, mechanisms, Cl(−I)-removal efficiency, advantages and limitations were all systematically analyzed. Finally, the developmental trends and potential improvements for the oxidation-removal approaches of Cl(−I) in acidic industrial wastewater were discussed separately, providing the effective removal of Cl(−I) under aqueous conditions and the purification of industrial wastewater with a theoretical foundation and technical support.

Fig. 1 Oxidation approaches for Cl(−I) removal from acidic wastewater.

2. Oxidation-removal methods of Cl(−I)

During the treatment of acidic Cl(−I)-containing wastewater using the oxidation-removal approaches, Cl(−I) can be oxidized to generate Cl₂ gas under the function of an electric current or strong oxidants. Additionally, the chlorine (˙Cl) radical, a kind of intermediate species produced with the help of various active species, including h⁺, ˙OH radical, ˙SO₄⁻ radical, Pb(IV) and Bi(V), exhibits the ability to directly combine with some coexisting solids to produce precipitates, resulting in the effective removal of Cl(−I) from acidic industrial wastewaters through two paths of oxidation and precipitation.

2.1. Electrolysis oxidation method

As shown in Fig. 2, during the treatment of wastewater containing Cl(−I) conducted in an electrolytic cell, the directional movement of Cl(−I) to the anode under the driving of potential difference (PD) firstly occurs between the cathode and anode. Then, Cl(−I) will be oxidized to produce Cl₂ gas on the anode according to eqn (1). Simultaneously, many H⁺ ions in the solutions will move to the cathode and be reduced, leading to the formation of H₂ gas as shown in eqn (2). For the wastewater containing a high concentration of metal ions (M^x+), M^x+ will replace H⁺ to be reduced by electrons (e⁻), and the generated zero-valent metal will deposit gradually on the cathode according to eqn (3).⁴⁹

2Cl(−I) − 2e⁻ → Cl₂↑ (on the anode) (1)

2H⁺ + 2e⁻ → H₂↑ (on the cathode) (2)

M^x+ + xe⁻ → M↓ (on the cathode) (3)

Fig. 2 Mechanisms for Cl(−I) removal through the electrolysis oxidation process.

During the treatment using the electrolysis method, the removal efficiency of Cl(−I) depends on the electrolytic conditions of the wastewater composition, electrolytic cell structure, electrode material, current density and power duration, among which the electrode material is an important factor for the Cl(−I)-removal process. The study on the removal of Cl(−I) from zinc sulfate aqueous solutions showed that, after 180 min of dichlorination conducted under the applied voltage of 0.6 V and the ultrasonic agitation power of 50 W, 54.5% of Cl(−I) with the initial concentration of 300 mg L⁻¹ can be removed by the copper electrode.⁵⁰ In addition, a kind of ruthenium–titanium–activated carbon (Ru–Ti–AC) combined anode electrode was confirmed to exhibit excellent Cl(−I)-removal performance during the electrolysis process. For the desulfurization wastewater containing 12 000 mg L⁻¹ of Cl(−I), 80% of Cl(−I) can be removed under the current density of 400 mA after 60 min.⁵¹ Recently, a kind of porous Ni–Ce-doped lead dioxide (Ni–Ce-doped PbO₂) electrode was applied on the electrolysis removal process of Cl(−I), and was verified to have excellent performance. Under a current intensity of 50 mA, a Cl(−I)-removal efficiency of approximately 87.4% can be achieved after 90 min of treatment on the simulated wastewater with an initial Cl(−I) concentration of 4000 mg L⁻¹.⁴⁹

Owing to the high Cl(−I)-removal efficiency, strong stability, wide suitable acidity range and the avoidance of introducing secondary impurities, the electrolysis method can be well applied to the treatment of acidic wastewater with high Cl(−I) concentrations. However, the long-time duration of power consumption directly leads to the high cost, thus limiting the application scope to the treatment of wastewater at a small volume. Additionally, coexisting ions with higher reducibility than Cl(−I) in the wastewater, including Fe(II), I(−I) and Br(−I), can be oxidized on the anode preferentially. Thus, the electrolysis approach is not suitable for acidic wastewater containing multiple impurities.

2.2. UV-induced oxidation methods

In the traditional Cl(−I)-removal precipitation methods using copper slag and Bi₂O₃, the involved solids of cuprous chloride (CuCl), Bi₂O₃ and bismuth oxychloride (BiOCl) are all excellent photocatalysts that can be excited under UV light to produce active species.^52–59 These active species with strong oxidability make it possible for Cl(−I) to be oxidized and removed using copper slag and Bi₂O₃ under UV light. In view of this, the mechanisms and efficiency of Cl(−I) removal from acidic industrial wastewater under UV light began to attract much attention.

2.2.1 Cu(0)/Cu(II)/UV oxidation & precipitation method. In the traditional copper slag precipitation method for Cl(−I) removal, comproportionation first happens between Cu(II) and Cu(0) contained in the copper slag with the presence of Cl(−I). Then, the generated Cu(I) will precipitate with Cl(−I) to produce CuCl solid according to eqn (4), ultimately leading to the removal of Cl(−I) through the precipitation process.^20–22 The copper slag precipitation method has been widely used in zinc electrolysis and metallurgical industries for Cl(−I) removal. Despite this, the limited efficiency, high dosage of copper slag and the high residual concentration of Cu(II) have always resulted in the rather high cost, agent waste and the risks of secondary pollution, indicating that further improvement on this approach is necessary.

Cu(0) + Cu(II) + 2Cl(−I) → 2CuCl (Solid) (4)

In recent research, an improved approach described as Cu(0)/Cu(II)/UV oxidation & precipitation method was proposed for Cl(−I) removal. Because of the involvement of UV light, the mechanisms change completely for Cl(−I) removal, where the ˙Cl radical is a kind of critical intermediate substance. According to the proposed mechanisms shown in Fig. 3, the ˙Cl radical can form in two pathways. One pathway is that the LMCT occurs in the complex compound of Cu(II) and Cl(−I) ([Cu(II)Cl_x]^2−x) under UV light to produce the ˙Cl radical (eqn (5)). The other is that the ˙Cl radical forms from the oxidization reaction of Cl(−I) by h⁺ and ˙OH radical, the products of CuCl photocatalysis process under UV light (eqn (6)–(9)). Next, the generated ˙Cl radical can directly combine with Cu(0) to produce CuCl as shown in eqn (10), leading to the removal of Cl(−I) in the form of CuCl precipitate. Meanwhile, the ˙Cl radical can also rapidly combine to produce Cl₂ gas (eqn (11)), which is another form of the removed Cl(−I).⁶⁰ Overall, in this improved approach, owing to the formation of the ˙Cl radical in the LMCT of [Cu(II)Cl_x]^2−x and the photocatalysis process of CuCl, Cl(−I) can be removed in the form of CuCl precipitate and Cl₂ gas through oxidation and precipitation processes.

h⁺ + OH⁻ → ˙OH (7)

Cl(−I) + ˙OH → OH⁻ + ˙Cl (8)

h⁺ + Cl(−I) → ˙Cl (9)

Cu(0) + ˙Cl → CuCl (Solid) (10)

˙Cl + ˙Cl → Cl₂ (Gas) (11)

Fig. 3 Mechanisms for Cl(−I) removal through the Cu(0)/Cu(II)/UV oxidation & precipitation process.

As reported in the relevant study, after 40 min of treatment under UV light provided by a Hg lamp (28 W), 87.9% of Cl(−I) was effectively removed from the actual acidic zinc smelting wastewater with an initial Cl(−I) concentration of 428.8 mg L⁻¹.⁶⁰ Compared with the traditional method, the mole ratio of Cu(II)/Cl(−I) was reduced from 8 : 1 to 2 : 1, indicating that the dosage of copper slag and the residual Cu(II) can be greatly reduced. Despite the high efficiency and low copper slag dosage, the high energy consumption and complex operation make this approach far from practical. For instance, the layout and maintenance of UV lamps, the handling of the generated Cl₂ gas and the further removal of residual Cu(II) will bring some difficulties to practical operation.

2.2.2 Bi₂O₃/UV oxidation & precipitation method. In the traditional Bi₂O₃ precipitation method for Cl(−I) removal, Bi₂O₃ is firstly dissolved to release Bi(III) in the presence of H⁺ (eqn (12)). Then, Bi(III) continues to react with Cl(−I) and H₂O to produce BiOCl (eqn (13)), finally resulting in the removal of Cl(−I) in the form of BiOCl precipitate.^23–25 Bi₂O₃ exhibits excellent Cl(−I)-removal performance. However, this approach needs a rather strong acidity of wastewater owing to the dissolution process of Bi₂O₃, the most critical reaction determining the Cl(−I)-removal efficiency. Moreover, the high dosage of Bi₂O₃ also limits the further application of this method during actual wastewater treatment. Therefore, an improvement was proposed to solve the abovementioned problems.

Bi₂O₃ + 6H⁺ → 2Bi(III) + 3H₂O (12)

Bi(III) + H₂O + Cl(−I) → 2H⁺ + BiOCl (Solid) (13)

Owing to the excellent photocatalysis performance of the Bi₂O₃ agent and BiOCl product, new reactions occur under UV light and help the removal of Cl(−I). During the treatment of wastewater through the recently proposed Bi₂O₃/UV oxidation & precipitation method as shown in Fig. 4, the dissolved Bi(III) is firstly produced from Bi₂O₃ under the action of both sulphuric acid (H₂SO₄) and UV light. Next, Bi(III) combines with Cl(−I) and hydrolyzes to BiOCl on the surface of Bi₂O₃, leading to the formation of the Bi₂O₃/BiOCl heterojunction, which transforms to Bi₂O₃(e⁻)/BiOCl(h⁺) under the excitation by UV light. Then, h⁺ in the VB of BiOCl oxidizes OH⁻ to ˙OH radical. Simultaneously, the ˙Cl radical is produced from Cl(−I) under the oxidation function of both ˙OH radical and h⁺ according to eqn (7)–(9). Finally, BiOCl will form directly from Bi₂O₃, e⁻ and ˙Cl radical as shown in eqn (14).⁶¹ After the wastewater treatment, Bi₂O₃ is regenerated from BiOCl using sodium hydroxide (NaOH) solution as shown in eqn (15). As a result, Cl(−I) is ultimately transferred from the wastewater to the sodium chloride/sodium hydroxide (NaCl/NaOH) concentrate. In general, because of the photocatalysis process of the Bi₂O₃/BiOCl heterojunction under UV light, the ˙Cl radical can be produced effectively and play an important role in removing Cl(−I) in the form of Cl(−I)-containing concentrate.

Bi₂O₃ + 6H⁺ → 2Bi(III) + 3H₂O (14)

2BiOCl + 2NaOH → Bi₂O₃ + 2NaCl + H₂O (15)

Fig. 4 Mechanisms for Cl(−I) removal through the Bi₂O₃/UV oxidation & precipitation process.

For the actual acidic nonferrous smelting wastewater containing 10 400 mg L⁻¹ of Cl(−I), a high efficiency of 98.1% was reached after 80 min of treatment under UV light with the energy consumption being calculated to be 11.2 kW h m⁻³. After further treatment on BiOCl using NaOH solution, approximately 97.4% of Bi₂O₃ was regenerated and reused in the next batch of Cl(−I)-removal experiments.⁶¹ Because of the addition of UV light, the mole ratio of dosed Bi₂O₃ to Cl(−I) was lowered from 1.5 : 1 to 0.5 : 1. Meanwhile, the required initial H₂SO₄ concentration of the wastewater was also lowered from 70 to 40 g L⁻¹, signifying that the traditional Bi₂O₃ precipitation method has been significantly improved. Additionally, the regeneration of Bi₂O₃ greatly lowered the cost of the agent. However, the high energy consumption and complex operation brought by the employment of UV lamps limited further research on this approach.

2.3. Strong-oxidant-dominated oxidation methods

To avoid the complex operation and high energy consumption brought from the employment of UV light, it becomes meaningful to search for a kind of new strong oxidant exhibiting the ability to directly oxidize Cl(−I) or to induce the generation of an active species for Cl(−I) removal without the addition of UV light.

2.3.1 Na₂S₂O₈ oxidation method. The φ₀ of SO₄²⁻/S₂O₈²⁻ is 2.01 eV,^62,63 which is much higher than that of Cl(−I)/Cl₂. In addition, Na₂S₂O₈ can be activated by heating, acid or metal ions to generate ˙SO₄⁻ radical with a redox potential of 2.5–3.1 eV,^64–67 indicating that the oxidation of Cl(−I) by Na₂S₂O₈ can be realized theoretically. In the study on the Cl(−I) removal by Na₂S₂O₈, the involved mechanisms (Fig. 5) were described as follows. Firstly, S₂O₈²⁻ decomposes to produce the ˙SO₄⁻ radical under the activation of heating according to eqn (16). Then, the sulfate (˙HSO₄) radical will form in the combination reaction between the ˙SO₄⁻ radical and H⁺ and oxidize OH⁻ to form the ˙OH radical (eqn (17) and 18). Under the function of the ˙SO₄⁻ and ˙OH radicals, the ˙Cl and chlorine (˙Cl₂⁻) radicals will thus form from Cl(−I) as shown in eqn (8), (19) and (20).⁶⁸ Finally, owing to the self-combination process, Cl₂ gas can be produced from the ˙Cl and ˙Cl₂⁻ radicals according to eqn (11) and (21), resulting in the removal of Cl(−I).

˙SO₄⁻ + H⁺ → ˙HSO₄ (17)

˙HSO₄ + H₂O → ˙OH + HSO₄⁻ + H⁺ (18)

Cl(−I) + ˙SO₄⁻ → ˙Cl + SO₄²⁻ (19)

˙Cl + Cl(−I) → ˙Cl₂⁻ (20)

˙Cl₂⁻ + ˙Cl₂⁻ → 2Cl⁻ + Cl₂ (Gas) (21)

Fig. 5 Mechanisms for Cl(−I) removal through Na₂S₂O₈ oxidation process.

In the treatment of actual strongly acidic wastewater containing various metal ions and 1000 mg L⁻¹ of Cl(−I), where the mole ratio of dosed Na₂S₂O₈ to Cl(−I) was 0.6 : 1, a high efficiency of 96–98% can be achieved at 25 min under 70 °C with the residual Cl(−I) concentration below 158 mg L⁻¹. Additionally, the removal efficiency of Cl(−I) for the actual wastewater was 8–10% higher than that for the simulated wastewater. This probably resulted from the activation function of metal ions on Na₂S₂O₈ to produce the ˙SO₄⁻ radical as shown in eqn (22). This novel Cl(−I)-removal approach using Na₂S₂O₈ shows the advantages of high efficiency and the avoidance to introducing foreign ions. However, Na₂S₂O₈ can easily decompose with the presence of H₂O as shown in eqn (23), indicating that the dry, airtight and dark storage conditions for Na₂S₂O₈ are required. Meanwhile, once Na₂S₂O₈ is added to the wastewater, the violent reaction between Na₂S₂O₈ and H₂O makes the decomposition of Na₂S₂O₈, leading to the waste of the agent.

S₂O₈²⁻ + M^x+ → M^x+1 + ˙SO₄⁻ + SO₄²⁻ (22)

2Na₂S₂O₈ + 2H₂O → 2Na₂SO₄ + 2H₂SO₄ + O₂ (Gas) (23)

2.3.2 NaBiO₃ oxidation & precipitation method. The compounds of Bi(V) possess extremely high oxidability under acidic conditions. Furthermore, the φ₀ of Bi(III)/bismuthate (Bi(III)/BiO₃⁻) is 1.59 eV,⁶⁹ which is higher than that of Cl(−I)/Cl₂, signifying the possibility for Cl(−I) to be oxidized by NaBiO₃ under acidic conditions. According to the proposed mechanisms for Cl(−I) removal from strongly acidic wastewater using NaBiO₃ shown in Fig. 6, Cl(−I) can be removed in the form of Cl₂ gas and BiOCl precipitate.⁷⁰ After being dosed, NaBiO₃ will be dissolved gradually to produce BiO₃⁻, which further oxidizes Cl(−I) and OH⁻ to generate ˙Cl and ˙OH radicals, respectively (eqn (24) and (25)). Simultaneously, the ˙Cl radical can also form from the oxidation of Cl(−I) by the ˙OH radical (eqn (8)). Thus, the abundant ˙Cl radical will self-combine to produce Cl₂ gas (eqn (11)). In addition, dissolved Bi(III), the reduction product of BiO₃⁻, will react with Cl(−I) and H₂O to produce the BiOCl solid as shown in eqn (13). As a result, Cl(−I) will be efficiently removed through two paths of oxidation by BiO₃⁻ and precipitation by Bi(III).

BiO₃⁻ + 2Cl(−I) + 6H⁺ → Bi(III) + 2˙Cl + 3H₂O (24)

BiO₃⁻ + 4H⁺ → Bi(III) + 2˙OH + H₂O (25)

Fig. 6 Mechanisms for Cl(−I) removal through the NaBiO₃ oxidation & precipitation process.

Through the treatment on actual strongly acidic smelting wastewater (10 L) containing 2128 mg L⁻¹ of Cl(−I) using NaBiO₃, approximately 98.5% of Cl(−I) was effectively removed at a NaBiO₃ to Cl(−I) mole ratio of 1 : 3 at 180 min under 30 °C. Furthermore, the generated Cl₂ gas was continuously absorbed by NaOH solution to avoid the secondary pollution and produce sodium hypochlorite (NaClO) for the regeneration of NaBiO₃ (eqn (26)). After the regeneration process of NaBiO₃ from BiOCl using the absorption solution for Cl₂ gas, 94.3% of the dosed NaBiO₃ agent was regenerated and reused, and Cl(−I) was finally transferred from acidic wastewater into the NaCl/NaOH concentrate.⁷⁰ This approach has the major advantages of high efficiency and low cost for the regenerative agent. Owing to the regenerative ability, NaBiO₃ shows greater potential for the realization of practical application on Cl(−I) removal in a short time than Na₂S₂O₈. In view of the decomposability under high temperature and the reactivity with H₂O of NaBiO₃, the strict cryogenic, dry and dark storage conditions of the agent are also necessary to this approach.

BiOCl + 2NaOH + NaClO → 2NaCl + NaBiO₃ + H₂O (26)

2.3.3 PbO₂ oxidation method. Pb(IV) with strong oxidability possesses a Pb(IV)/Pb(II) φ₀ value of 1.69 eV under acidic conditions,⁷¹ which is higher than the 1.36 eV φ₀ value of Cl(−I)/Cl₂. In the newly proposed approach for Cl(−I) removal using a PbO₂ agent,⁷² the ˙Cl radical can form through two pathways of surface oxidation and ˙OH radical oxidation according to the following mechanisms (Fig. 7). In the former surface oxidation process, the ≡Pb(IV)–Cl(−I) complex firstly forms because of the absorption of Cl(−I) on the surface of PbO₂ as shown in eqn (27). The two-step LMCT then happens in the ≡Pb(IV)–Cl(−I) complex, where two e⁻ transfer from Cl(−I) to Pb(IV) along the covalent bond successively, leading to the generation of the ˙Cl radical and ≡Pb(II) (eqn (28) and (29)). Meanwhile, in the ˙OH radical oxidation process, PbO₂ first oxidizes OH⁻ to produce the ˙OH radical as shown in eqn (30), which can further oxidize Cl(−I) and form the ˙Cl radical (eqn (8)). Ultimately, many of these ˙Cl radicals will self-combine, leading to the removal of Cl(−I) in the form of Cl₂ gas. Pb(II), the reduction product of Pb(IV), will react with the coexisting SO₄²⁻ and be precipitated in the form of lead sulfate (PbSO₄) solid.

≡Pb(IV)–OH⁻ + Cl(−I) + H⁺ → ≡Pb(IV)–Cl(−I) + ˙Cl (27)

≡Pb(IV)–Cl(−I) → ≡Pb(III) + ˙Cl (28)

≡Pb(III)–Cl(−I) → ≡Pb(II) + ˙Cl (29)

PbO₂ + 2H⁺ → Pb(II) + 2˙OH (30)

Fig. 7 Mechanisms for Cl(−I) removal through the PbO₂ oxidation process.

During the treatment of the simulated wastewater containing 1000 mg L⁻¹ of Cl(−I) and 200 g L⁻¹ of H₂SO₄, a high Cl(−I)-removal efficiency of 93.4% was obtained at the PbO₂/Cl(−I) mole ratio of 2 : 1 under 50 °C at 30 min. Additionally, the PbO₂ agent was regenerated from PbSO₄ using the NaOH absorption solution of Cl₂ gas according to eqn (31). After six-times regeneration and reuse, PbO₂ still showed excellent Cl(−I)-removal ability with the efficiency above 90%.⁷² PbO₂ shows the major advantages of high efficiency and regenerative ability. Additionally, the much lower price of PbO₂ compared to that of NaBiO₃ brings PbO₂ greater advantages in the treatment cost. However, similar to NaBiO₃, PbO₂ easily decomposes under high temperature, which indicates that the strict dry and dark storage conditions of the agent are essential in this approach. Meanwhile, the precipitation of Pb(II) with SO₄²⁻ will lead to the waste of H₂SO₄, a kind of valuable resource in the strongly acidic wastewater.

PbSO₄ + 2NaOH + NaClO → Na₂SO₄ + PbO₂ + NaCl + H₂O (31)

3. Conclusions and perspectives

In this article, the three major oxidation means for Cl(−I) removal from acidic industrial wastewater, including electricity, UV light and strong oxidant, are reviewed in detail. Table 1 summarizes the advantages, limitations and potential improvements of these oxidation-removal approaches for Cl(−I) in the acidic industrial wastewater as follows.

Table 1 Oxidation-removal approaches for Cl(−I) from acidic industrial wastewater

Methods	Mechanisms	Advantages	Disadvantages	Potential improvements
Electrolysis	Oxidation by electric current	Strong stability	High cost	Reducing of energy consumption
		Wide suitable acidity range
		Avoidance of secondary impurities
UV-induced oxidation methods
Cu(0)/Cu(II) + UV	Oxidation & precipitation by LMCT & AOPs	Low dosage of copper slag	High energy consumption
			Complex operation
Bi2O3 + UV	Oxidation & precipitation by AOPs	Low dosage of Bi2O3	High energy consumption
		Regenerative ability of Bi2O3	Complex operation
Strong-oxidant-dominated oxidation methods
Na2S2O8 + heating	Oxidation by AOPs	Avoidance to foreign ions	High cost for Na2S2O8	Stabilization and slow releasing of reagents
		Low energy consumption	Strict storage conditions of Na2S2O8
NaBiO3 + heating	Oxidation & precipitation by AOPs	Low energy consumption	Strict storage conditions of NaBiO3
		Regenerative ability of NaBiO3	High price of NaBiO3
PbO2 + heating	Oxidation by LMCT & AOPs	Low energy consumption	Strict storage conditions of PbO2
		Regenerative ability of PbO2	Waste of valuable H2SO4 resource
		Low price of PbO2

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1. All of the oxidation methods for Cl(−I) removal exhibit the advantage of high efficiency. Given this, these oxidation means are likely to be employed for the treatment of wastewater containing extremely high concentrations of Cl(−I). Despite this, some Cl₂ gas will be produced and may induce secondary pollution without appropriate treatment countermeasures. Meanwhile, to avoid the waste of agents and the generation of byproducts, the oxidation technology is unsuitable for the wastewater containing multiple reducing substances;

2. The high energy consumption of current and UV light leads to the high cost of electrolysis and UV-induced oxidation methods. More research studies to lower the cost (for instance, determining a suitable UV lamp type and layout, optimized electrolysis cell configuration and modified electrode material) are required before the actual application;

3. The slow-release technology of the Na₂S₂O₈, NaBiO₃ and PbO₂ agents must be further developed to avoid the agent waste and the safety risks. For instance, the porous supporting skeleton and hard shell can be considered to be applied on the modification of the Cl(−I)-removal agents for the sustained release effect.

In the field of industrial wastewater treatment, the removal of Cl(−I) has always been a technical challenge. Although many methods have been developed for the removal of Cl(−I) in recent decades, the traditional precipitation approach using copper slag is still the most widely applied one because of the low cost and easy operation. In this case, the newly developed oxidation methods show the major advantage of high efficiency. Before the application of these oxidation methods, a series of practical problems along with the removal of Cl(−I), including the high energy consumption, the strict storage conditions of reagents and the absorption of Cl₂ gas, need to be taken into consideration.

Conflicts of interest

The authors declare no competing financial interests.

Acknowledgements

This work was supported by the Scientific Research Foundation for the PhD (Xuzhou University of Technology, No. 02900284). We are grateful to the editors and anonymous reviewers for their valuable comments and suggestions for our paper.

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