Removal of Chloride Ions from Strongly Acidic Wastewater Using Cu(0)/Cu(II): Efficiency Enhancement by UV Irradiation and the Mechanism for Chloride Ions Removal

A review of chloride ions removal from high chloride industrial wastewater: Sources, hazards, and mechanisms

To reduce metal pipe corrosion, improve product quality, and meet zero liquid discharge (ZLD) criteria, managing chloride ion concentrations in industrial wastewaters from metallurgical and chemical sectors has become increasingly important. This review provides detailed information on the sources, concentration levels, and deleterious effects of chloride ions in representative industrial wastewaters, and also summarizes and discusses various chloride ion removal techniques, including precipitation, ion exchange, physical separation, and advanced oxidation (AOPs). Among these, AOPs are particularly promising due to their ability to couple with other technologies and the diversity of their auxiliary technologies. The development of dechlorination electrode materials by electro-adsorption (CDI) can be inspired by the electrode materials used in chloride ion battery (CIB). This review also provides insights into exploring the effective combination of multiple chloride removal mechanisms, as well as the development of environmentally friendly composite materials. This review provides a theoretical basis and development direction for the effective treatment and secondary utilization of chlorine-containing industrial wastewater in the future.

Study on the Removal of Chloride Ions in an Acidic Solution of Zinc Smelting by Green Method

In the process of zinc smelting, when the chloride ion concentration exceeds 100 mg/L, it continuously corrodes the electrode plate and affects the stability of the electrodeposition process. Therefore, the chloride concentration must be reduced below 100 mg/L. Compared with other methods used to control the reactions of Cu(II), the use of the copper slag produced in zinc smelting without other additives does not cause reverse dissolution; to reduce the cost, turn the waste into treasure, and protect the environment, research was carried out on chloride removal by the copper slag via a synergistic valence control process. In this study, the influencing factors, such as the amount of copper slag, the reaction time, and reaction temperature, were systematically investigated. The results showed that the optimum dechlorination conditions were as follows: the copper: copper(II): chloride molar ratio was 6:5:1, the reaction time was 60 min, and the reaction temperature was 20 °C. The chloride ion concentration was decreased from 1.6 g/L to 0.05 g/L, the dechlorination efficiency was 96.875%, and the residual chloride ion concentration was less than 100 mg/L, which provides a basis for industrial use.

Chloride removal from flue gas desulfurization wastewater through Friedel's salt precipitation method: A review

As a high efficiency method for chloride removal, Friedel's salt precipitation (FSP) method has attracted much attention in zero liquid discharge (ZLD) of flue gas desulfurization (FGD) wastewater. This review provides comprehensive knowledge of FSP method for chloride removal through analysis of the evolution, reaction mechanisms and influential factors, and describes the recent research progress. FSP method is a cost-efficient technology to remove chloride from saline wastewater by adding lime and aluminate. Chloride ions react with the precipitants by adsorption or/and ion exchange to form Friedel's salt, which is affected by the reaction conditions including reaction time, temperature, interferential ions, etc. The effluent of this process can be reused as the makeup water of desulfurization tower, and the dechloridation precipitates can be reclaimed as adsorption materials and sludge conditioners. That can not only offset a fraction of the treatment cost, but also avoid secondary pollution, so ZLD of FGD wastewater can be achieved. This paper summarizes the deficiencies and potential improvement measures of FSP method. We believe this technology is a promising way to achieve ZLD of FGD wastewater and other wastewater containing chloride, and expect FSP method would become more mature and be widely applied in hypersaline wastewater treatment in the foreseeable future.

Aquatic photochemistry of Cu(II) in the presence of As(III): Mechanistic insights from Cu(III) production and As(III) oxidation under neutral pH conditions

Surface complexation between arsenite (As(III)) and colloidal metal hydroxides plays an important role not only in the immobilization and oxidation of As(III) but also in the cycle of the metal and the fate of their ligands. However, the photochemical processes between Cu(II) and As(III) are not sufficiently understood. In this work, the photooxidation of As(III) in the presence of Cu(II) under neutral pH conditions was investigated in water containing 200 μM Cu(II) and 5 μM As(III) under simulated solar irradiation consisting of UVB light. The results confirmed the complexation between As(III) and Cu(II) hydroxides, and the photooxidation of As(III) is attributed to the ligand-to-metal charge transfer (LMCT) process and Cu(III) oxidation. The light-induced LMCT process results in simultaneous As(III) oxidation and Cu(II) reduction, then produced Cu(I) undergoes autooxidation with O₂ to produce O₂^•⁻ and H₂O₂, and further the Cu(I)-Fenton reaction produces Cu(III) that can oxidize As(III) efficiently (k_{Cu(III)+As(III)} = 1.02 × 10⁹ M^-1 s^-1). The contributions from each pathway (ρ_{rCu(II)-As(III)+hv} = 0.62, ρ_{rCu(III)+As(III)} = 0.38) were obtained using kinetic analysis and simulation. Sunlight experiments showed that the pH range of As(III) oxidation could be extended to weak acidic conditions in downstream water from acid mine drainage (AMD). This work helps to understand the environmental chemistry of Cu(II) and As(III) regarding their interaction and photo-induced redox reactions.

Analysis of the chloride ion removal mechanism from simulated wastewater by discarded vitamin tablets

Vitamin (VM) tablets are often discarded or incinerated as medical waste, and untreated highly chlorinated wastewater is discharged, polluting the environment. In this study, Cu²⁺ was reduced by vitamin C (VC, a component of VM), and the precipitate formed by the reaction of its product with Cl^- in water was used to remove Cl^- from simulated wastewater. This allows for the resourceful use of waste VM, while also achieving the goal of dechlorinating wastewater. Meanwhile, the effect of various parameters on dechlorination was studied, and the dechlorination mechanism was analyzed. According to the results, the removal rate of Cl^- increased first and then decreased with pH, removal time and reaction temperature. Using VC in VM to dechlorinate simulated wastewater, the removal rate of Cl^- was 94.31% under optimum conditions: pH 2.5, temperature 30 °C and reaction time 10 minutes. According to the dechlorination process, it can be inferred that Cu²⁺ is reduced to Cu⁺ by VC, and Cu⁺ and Cl^- coprecipitate to remove Cl^-. Therefore, it is feasible to use discarded VM to treat high concentration chlorine-containing wastewater.

Heterostructured α-Bi2O3/BiOCl Nanosheet for Photocatalytic Applications

Photocatalytic degradation of organic pollutants in wastewater is recognized as a promising technology. However, photocatalyst Bi₂O₃ responds to visible light and suffers from low quantum yield. In this study, the α-Bi₂O₃ was synthetized and used for removing Cl^- in acidic solutions to transform BiOCl. A heterostructured α-Bi₂O₃/BiOCl nanosheet can be fabricated by coupling Bi₂O₃ (narrow band gap) with layered BiOCl (rapid photoelectron transmission). During the degradation of Rhodamine B (RhB), the Bi₂O₃/BiOCl composite material presented excellent photocatalytic activity. Under visible light irradiation for 60 min, the Bi₂O₃/BiOCl photocatalyst delivered a superior removal rate of 99.9%, which was much higher than pristine Bi₂O₃ (36.0%) and BiOCl (74.4%). Radical quenching experiments and electron spin resonance spectra further confirmed the dominant effect of electron holes h⁺ and superoxide radical anions ·O₂^- for the photodegradation process. This work develops a green strategy to synthesize a high-performance photocatalyst for organic dye degradation.

Construction of bismuth-based porous carbon models by 3D printing technology for light-enhanced removal of chloride ions in wastewater

The bismuth oxide (Bi₂O₃) based chloride (Cl^-) removal method is one of the chemical precipitation methods possessing good selectivity and high removal efficiency of Cl^- ions, but Bi₂O₃ often appears in the powder form, which is difficult to be recovered for regeneration. In this work, the combination of 3D printing technology and the Bi₂O₃ method was explored to construct the resin model including the Bi-precursors. In the optimum carbonization process at 400 °C for 30 min, the Bi³⁺ ions of the Bi-precursor were reduced into the metallic Bi⁰ nanoparticles, whose surfaces were covered by the thin Bi₂O₃ layers to form the heterostructured Bi⁰/Bi₂O₃ core/shell nanoparticles with an average size of 43 nm. These Bi⁰/Bi₂O₃ nanoparticles were tightly adhered to the internal and external surfaces of the hierarchical porous carbon model (Bi-PCM), which greatly facilitated their regeneration and ensured the stable Cl^- removal performance. After five cycles of Cl^- removal, the chloride removal efficiency over the multiple Bi-PCMs in the dark and pH 1 conditions maintained at about 26%, which then largely increased to 63.6% with UV light irradiation. The light-enhanced mechanism was related to the improved release rate of Bi³⁺ ions caused by photocorrosion and the Cl^• radicals produced from the holes and the ^•OH and O₂^•- radicals, which quickly reacted with Bi₂O₃ to form BiOCl. The construction of Bi-PCMs by using 3D printing technology provides a very promising strategy for the removal of Cl^- ions from wastewater.

Use of catalytic wet air oxidation (CWAO) for pretreatment of high-salinity high-organic wastewater

Catalytic wet air oxidation (CWAO) coupled desalination technology provides a possibility for the effective and economic degradation of high salinity and high organic wastewater. Chloride widely occurs in natural and wastewaters, and its high content jeopardizes the efficacy of Advanced oxidation process (AOPs). Thus, a novel chlorine ion resistant catalyst B-site Ru doped LaFe_1-xRu_xO_3-δ in CWAO treatment of chlorine ion wastewater was examined. Especially, LaFe_0.85Ru_0.15O_3-δ was 45.5% better than that of the 6%RuO₂@TiO₂ (commercial carrier) on total organic carbon (TOC) removal. Also, doped catalysts LaFe_1-xRu_xO_3-δ showed better activity than supported catalysts RuO₂@LaFeO₃ and RuO₂@TiO₂ with the same Ru content. Moreover, LaFe_0.85Ru_0.15O_3-δ has novel chlorine ion resistance no matter the concentration of Cl^- and no Ru dissolves after the reaction. X-ray diffraction (XRD) refinement, X-ray photoelectron spectroscopy (XPS), transmission electron microscope (TEM), and X-ray absorption fine structure (XAFS) measurements verified the structure of LaFe_0.85Ru_0.15O_3-δ. Kinetic data and density functional theory (DFT) proved that Fe is the site of acetic acid oxidation and adsorption of chloride ions. The existence of Fe in LaFe_0.85Ru_0.15O_3-δ could adsorb chlorine ion (catalytic activity inhibitor), which can protect the Ru site and other active oxygen species to exert catalytic activity. This work is essential for the development of chloride-resistant catalyst in CWAO.

Clean and effective removal of Cl(-I) from strongly acidic wastewater by PbO2

Recycling strongly acidic wastewater as diluted H₂SO₄ after contaminants contained being removed was previously proposed, however, Cl(-I), a kind of contaminant contained in strongly acidic wastewater, is difficult to remove, which severely degrades the quality of recycled H₂SO₄. In this study, the removal of Cl(-I) using PbO₂ was investigated and the involved mechanisms were explored. The removal efficiency of Cl(-I) reached 93.38% at 50℃ when PbO₂/Cl(-I) mole ratio reached 2:1. The identification of reaction products shows that Cl(-I) was oxidized to Cl₂, and PbO₂ was reduced to PbSO₄. Cl₂ was absorbed by NaOH to form NaClO, which was used for the regeneration of PbO₂ from the generated PbSO₄. Cl(-I) was removed through two pathways, i.e., surface oxidation and •OH radical oxidation. •OH generated by the reaction of PbO₂ and OH^- plays an important role in Cl(-I) removal. The regenerated PbO₂ had excellent performance to remove Cl(-I) after six-time regeneration. This study provided an in-depth understanding on the effective removal of Cl(-I) by the oxidation method.

Peroxymonosulfate enhanced photoelectrocatalytic oxidation of organic contaminants and simultaneously cathodic recycling of silver

Degradation of organic contaminants with simultaneous recycling of Ag⁺ from silver-containing organic wastewater such as photographic effluents is desired. Although photoelectrocatalysis (PEC) technology is a good candidate for this type of wastewater, its reaction kinetics still needs to be improved. Herein, peroxymonosulfate (PMS) was employed to enhance the PEC kinetics for oxidation of phenol (PhOH) at the anode and reduction of Ag⁺ at the cathode. The degradation efficiency of phenol (PhOH, 0.1 mmol/L) was increased from 42.8% to 96.9% by adding 5 mmol/L PMS at a potential of 0.25 V. Meanwhile, the Ag (by wt%) deposited on the cathode was 28.1% (Ag₂O) in PEC process, while that of Ag (by wt%) was 69.7% (Ag⁰) by adding PMS. According to the electrochemistry analysis, PMS, as photoelectrons acceptor, enhances the separation efficiency of charges and the direct h⁺ oxidation of PhOH at the photoanode. Meantime, the increasing cathode potential avoided H₂ evolution and strongly alkaline at the surface of cathode, thus enabling the deposition of Ag⁺ in the form of metallic silver with the help of PMS. In addition, PMS combined with PEC process was effective in treating photographic effluents.

A review on the removal of Cl(-I) with high concentration from industrial wastewater: Approaches and mechanisms

Large quantities of wastewaters containing high concentrations of Cl(-I) can be generated in several industries when chloride-containing materials and additive agents are employed. Because Cl(-I) is unavailable to microorganisms, physicochemical methods are generally used for the removal of Cl(-I); however, as the most stable form of chlorine under aqueous conditions, Cl(-I) in wastewaters is difficult to remove to achieve low residual concentrations through common physicochemical methods. This paper provides new insights into traditional precipitation, oxidation, ion exchange and physical separation methods, as well as newly developed approaches, for Cl(-I) removal from various industrial wastewaters through analysis of the mechanisms, applicable conditions, optimum parameters, and method advantages and disadvantages. Moreover, the developmental trends and potential improvements to these approaches are also presented. Currently, precipitation is the most common and efficient Cl(-I) removal method, for which ultraviolet (UV) light is regarded as an effective means of improvement. Additionally, advanced oxidation processes (AOPs), where Cl(-I) can be oxidized to generate Cl radicals, Cl₂^- radicals, Cl₂ gas, etc., show great promise for Cl(-I) removal. This review provides a theoretical foundation for the effective treatment and for the secondary utilization of industrial wastewaters containing Cl(-I).

The noteworthy chloride ions in reclaimed water: Harmful effects, concentration levels and control strategies

Chloride ions (Cl^-), which are omnipresent in reclaimed water, can cause various problems in water reuse systems, especially during water transmission and at end use sites. Although reverse osmosis (RO) is considered as an effective technology to reduce chloride, its high investment and complex maintenance requirements hinder its application in many water reclamation plants (WRPs). Recently, several technologies bringing new options to better deal with chloride have gained increased attention. This review provides detailed information on the harmful effects, concentration levels, and sources of chloride in reclaimed water and summarizes and discusses various chloride removal technologies, including non-selective methods (e.g., membrane filtration, adsorption and ion exchange, oxidation, and electrochemical methods) and selective methods (e.g. precipitation and specially designed electrochemical methods). Among these, Friedel's salt precipitation and capacitive deionization showed attractive development potential. This review also proposes a holistic framework for chloride control from aspects of "Fit-for-Purpose" planning, technical system development, and whole process optimization, which could facilitate the planning and operation of long-term sustainable water reuse practices.

Removal of chloride from water and wastewater: Removal mechanisms and recent trends

Increased chloride concentration can cause salinization, which has become a serious and widespread environmental problem nowadays. This review aims at providing comprehensive and state-of-the-art knowledge and insights of technologies for chloride removal. Mechanisms for chloride removal mainly include chemical precipitation, adsorption, oxidation and membrane separation. In chemical precipitation, chloride removal by forming CuCl, AgCl, BiOCl and Friedel's salt. Adsorbents used in chloride removal mainly include ion exchangers, bimetal oxides and carbon-based electrodes. Oxidation for chloride removal contains ozone-based, electrochemical and sulfate radical-based oxidation. Membrane separation for chloride removal consists of diffusion dialysis, nanofiltration, reverse osmosis and electrodialysis. In this review, we specifically proposed the factors that affect chloride removal process and the corresponding strategies for improving removal efficiency. In the last section, the remaining challenges of method explorations and material developments were stated to provide guidelines for future development of chloride removal technologies.

Multi-array wax paper-based platform for the pre-concentration and determination of silver ions in drinking water

In this work, a wax-patterned chromatographic paper has been utilized as a holistic platform to 1) synthesize Prussian Blue Nanoparticles (sensing species), 2) load the reagents for the assay, 3) concentrate the sample through multistep, and 4) visualize the determination of silver ions. Waters are continuously affected by changes in the composition, thus the utilization of reagent-free analytical tools is of huge interest for smart drinking water monitoring. Herein, we report the characterization and application of a multi-array paper-based platform for the colorimetric determination of silver ions based on the conversion from Prussian Blue to its silver-based analogue, namely Ag₄[Fe(CN)₆]. In particular, the platform highlights the increase of sensitivity due to paper pre-concentration of sample, that can be easily adapted to the analytical necessities. Within the proposed experimental setup, Ag⁺ is visualized down to a detection limit of 0.9 μM, with high repeatability and satisfactory recoveries in the range comprised between 90 and 113%.

Sulfate radical-based removal of chloride ion from strongly acidic wastewater: Kinetics and mechanism

Specific to strongly acidic wastewater, the traditional lime neutralization produces massive hazardous waste and present serious environmental risks. Thus, the recycling of purified wastewater after the contained contaminants being removed has been proposed. However, among these contaminants, chloride ion (Cl(-I)) is rather difficult to remove. This study proposes a new method to remove Cl(-I) using thermal activated persulfate (PS). Under optimized conditions, above 96% of initial Cl(-I) was removed from the actual wastewater, and the residual Cl(-I) was below 158 mg/L, which satisfies the requirement of Cl(-I) concentration for wastewater recycling. Furthermore, the mechanism was investigated. In the strongly acidic wastewater, the high concentration of H⁺ prompted the thermal activation process of PS through two pathways. (1) H⁺ prompted the transformation of S₂O₈^2- into HSO₄^- and SO₄, and then into HSO₅^- that was finally transformed into ·OH and ·SO₄^- at above 70 ℃. (2) H⁺ prompted the production of ·OH through the transformation of ·SO₄^- into ·HSO₄ and the cleavage of ·HSO₄. The key step for Cl(-I) removal was identified as the formation of ·Cl or ·Cl₂^- from the oxidation of Cl(-I) by ·SO₄^- and ·OH, and their contribution ratios were estimated to be 67.4% and 32.6%, respectively.

A novel precipitant for the selective removal of fluoride ion from strongly acidic wastewater: Synthesis, efficiency, and mechanism

The strongly acidic wastewater containing fluoride [F(-I)] is generally neutralized using lime, producing massive hazardous solid waste, which may present serious environmental risks. In this study, a novel precipitant, N,N'-Bis(3-(triethoxysilyl)propyl)thiourea, was developed for the selective removal of F(-I) from strongly acidic wastewater. The precipitant was synthesized using (3-aminopropyl)triethoxysilane and thiourea at a molar ratio of 2:1 under 160 ℃. More than 90% of initial F(-I) was removed by the prepared precipitant from strong acidic wastewater produced by nonferrous metal smelting industry, and the residual F(-I) concentration decreased to below 100 mg/L. The F(-I) removal performance is almost free from the interference of coexisting ions. Only 6 kg/m³ of fluoride slag, which can be recycled as a concrete waterproofing agent, was produced. The F(-I) removal mechanism including substitution, polycondensation, ion exchange and complexation was clarified: ‒OH on Si atoms in the hydrolysis product of BTPT was substituted by F(-I), and a fluoro-substituted product formed; the polycondensation of BTPT and fluoro-substituted product produced polymer precipitates; the specific adsorption of F(-I) on the polymer precipitates occurred through ion exchange with ‒OH and complexation with -NH₂⁺-.

Removal of Cl(-I) from strongly acidic wastewater containing Cu(II) by complexation-precipitation using thiourea: Efficiency enhancement by ascorbic acid

Strongly acidic wastewater produced by copper smelting industries contains high concentrations of Cl(-I), Cu(II) and H₂SO₄. The common method for the treatment of this type of wastewater is neutralization, which produces large amounts of solid waste. To avoid the production of solid waste, it was proposed to selectively remove contaminants and then recycle the wastewater as diluted sulfuric acid. This study proposed a new complexation-precipitation method to effectively remove Cl(-I) using thiourea (TU) under the promotion of ascorbic acid (AC). The Cl(-I) removal efficiency was optimized, important effecting factors were investigated and the mechanisms of the AC-improved removal of Cl(-I) were studied. The results showed that, Cl(-I) removal efficiency reached 87.4 % under a TU/AC/Cl(-I) mole ratio of 1:3:1 and the residual Cl(-I) concentration was lowered from 1000 mg/L to 126.4 mg/L. The mechanism investigation showed that, AC first reduces Cu(II) to Cu(I), then, the produced Cu(I) is quickly complexed by TU to form the [Cu(I)_x(TU)_y]^x+; finally, [Cu(I)_x(TU)_y]^x+ precipitates with Cl(-I) in the form of [Cu(I)_x(TU)_y]Cl_x. This study provided a theoretical foundation of complexation-precipitation of Cl(-I) under strongly conditions and developed an effective method for removal of Cl(-I) from strongly acid waster.

Study of interferences and procedures for their removal in the spectrophotometric determination of ammonium and selected anions in coloured wastewater samples

Ammonium and selected anions were determined in wastewater samples with highly complex matrices by spectrophotometry using the reagent-kit method. For this purpose, the interferents of coloured compounds and S^2-, SO₃^2-, CO₃^2- and Cl^-, which are often present in wastewater samples, were systematically investigated in the spectrophotometric determination of ammonium, nitrate, chloride, sulphate, fluoride and phosphate. After this, innovative procedures for their removal were proposed. For sample decolourization, a DEAE column was used to determine ammonium, while a Florisil column was used for the colour removal and anions' determination. S^2- and CO₃^2- were eliminated from the samples by adding HCl or HNO₃, which transformed them into gases H₂S and CO₂. The stepwise addition of CaCl₂ to the sample, adjusted to pH 8, initiated the formation of CaSO₃, which was removed by filtration. Cl^- was removed by the addition of Ag₂O, which formed a AgCl precipitate that was removed from the solution by filtration. The accuracy of the determination was tested with spike-recovery tests, which showed recoveries for the analytes in the spiked samples ranging from 95 to 105%. The repeatability of the measurements of nitrate, chloride, sulphate and phosphate in the wastewater samples was better than ±1%, while that for the ammonium and fluoride samples was ±2 and ±5%, respectively. The data from the present investigation revealed that the developed procedures for the decolourization and stepwise removal of interferents enabled accurate spectrophotometric determination of ammonium, nitrate, chloride, sulphate, fluoride and phosphate by using cuvette tests in complex wastewater and environmental water samples.

UV-Improved Removal of Chloride Ions from Strongly Acidic Wastewater Using Bi2O3: Efficiency Enhancement and Mechanisms

Strongly acidic wastewater generated from nonferrous metal smelting industries can be recycled as sulfuric acid after the contaminants have been removed, and among which, Cl^- is rather difficult to remove. Although previous studies showed that Cl^- can be removed from acidic Zn electrolyte by Bi₂O₃, this method still suffers from low efficiency when being employed for strongly acidic wastewater recycling. Otherwise, very high Bi₂O₃ dosage and H₂SO₄ concentration are required, leading to the need for improvement. In this study, UV irradiation was employed to improve the removal, and it was found that Cl^- removal efficiency was substantially enhanced from 63.9 to 98.3%, the optimum Bi₂O₃/Cl^- mole ratio was lowered from 1.5:1 to 0.5:1, and to achieve the maximum removal efficiency, the required H₂SO₄ concentration was lowered from 70 to 40 g/L. The mechanisms were also elaborated. First, Bi₂O₃ dissolves under the function of UV and H⁺, and the produced Bi³⁺ combines with H₂O and Cl^- to form BiOCl. Then, Bi₂O₃/BiOCl transforms into BiOCl(h⁺)/Bi₂O₃(e^-) under UV irradiation, and the generated h⁺ oxidizes Cl^- to Cl^•. Finally, Cl^• reacts with Bi₂O₃/e^- to produce BiOCl. This study offered a theoretical foundation for the improvement of Cl^- removal from strongly acidic wastewater.