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

Ascorbic acid-induced digenite (Cu9S5) formation: A strategy to enhance sulfidation efficiency for copper recovery from acidic wastewater

Sulfide precipitation is an effective method for copper recovery from acidic wastewater. However, excessive use of sulfide reagents leads to secondary pollution, which poses a significant challenge. This study demonstrates that leveraging the reducing properties of ascorbic acid (AA) and adding Na2S to acidic copper-containing solutions (pH 2-4) at a Cu:S molar ratio of 1.6:0.8 enhances Cu(II) removal efficiency from 50 % to 100 %, doubling sulfide reagent utilization. Solution chemistry, electron microscopy, and X-ray spectroscopy analyses indicate that AA primarily promotes copper sulfide precipitation with higher copper content. Addition of AA at pH 2 at a Cu:S:AA molar ratio of 1.6:0.8:1 lowers the oxidation-reduction potential (ORP) from 0.5 V to 0.1 V, forming digenite (Cu9S5). Compared to covellite (CuS) formed without AA, digenite exhibits a higher Cu oxidation state and a lower S oxidation state. Furthermore, coagulation kinetics studies show that solution pH, Na2SO4, and FeCl2 concentrations influence copper sulfide aggregation. At pH 2, Cu9S5 coagulates at least 4.2 times faster than CuS within 20 min. Cost analysis shows that the cost per ton of copper recovered from wastewater using this method is about one-third of the cost of conventional methods. More importantly, this study minimizes residual sulfide, offering a novel strategy for dose control in copper sulfide recovery.

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.

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.

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.

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).

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.

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.

ZIF-8 engineered bismuth nanosheet arrays for boosted electrochemical reduction of nitrate

Removal of nitrate in wastewater is of great importance to environmental protection and humanity. However, the competitive reaction of hydrogen evolution (HER), which could occupy most active sites of the electrocatalyst, is one of the big challenges for nitrate removal. In this study, a novel zeolitic imidazolate framework-8 film engineered bismuth nanosheet electrocatalyst (ZIF-8/Bi-CC) was designed and synthesized for the electrochemical reduction of nitrate. The water contact angle and electrochemical tests demonstrated that the construction of the hydrophobic ZIF-8 film effectively weakened the competition of HER. And the nitrate removal efficiency and ammonium selectivity increased by 25.9% and 34.2% respectively after bismuth nanosheets were embedded into the ZIF-8 film. Besides, the bismuth concentration detection results indicated that the ZIF-8 film as the protective shell could effectively prevent the leaching of bismuth into the solution. More importantly, the final nitrate removal rate of ZIF-8/Bi-CC was close to 90% after 5 h when treating actual garbage fly ash wastewater, the NITRR efficiency stability and the obtained product were confirmed by five electrochemical cycles. The metal-organic framework film engineered electrocatalyst is a promising strategy for designing a new catalyst for the removal of nitrate in industrial wastewater.

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.