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

Emerging Contaminants: An Important But Ignored Risk Factor for Psoriasis

Industrialization and modernization have changed the environment. A group of emerging contaminants (ECs) has been defined recently. Psoriasis, whose incidence has increased in recent years, is a relapsing immune-mediated disease carrying a heavy disease burden. The erythematous scaly plaque is a typical symptom and occurs on several parts of the body. In addition, psoriasis has many comorbidities, such as psoriatic arthritis, diabetes, and depression, damaging the quality of life of patients. IL-17, IL-12, IL-23, and TNF-alpha are important related cytokines. ECs can influence psoriasis through the immune system and inflammatory responses. Specific mechanisms include increasing pro-inflammatory cytokines such as TNF-α and IL-17, and activating immune cells such as macrophages. And for psoriasis patients, it is suggested to reduce the exposure of most ECs. However, the complex mechanisms involved have not been discussed together and concluded. In this review, we summarize the relationship between ECs and psoriasis, focusing on the immune system, especially the immune cells and cytokines. These results can help guide clinical treatment and long-term management of psoriasis.

Unraveling the Structural Evolution of Cobalt Sulfides in Electrocatalytic NO3RR: the Inescapable Influence of Cl. Inorg Chem 2025;64:2787-2794. [PMID: 39915902 DOI: 10.1021/acs.inorgchem.4c04780] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Electrochemical nitrate reduction (NO3RR) to ammonia is an attractive approach for mitigating NO3- pollution and producing valuable NH3. Cobalt-sulfur compounds are widely considered to be potential electrocatalysts for NO3RR. However, there is still a lack of research on the probable structural evolution, long-term stability, and reactive sites of cobalt-based sulfides during catalysis. Herein, we have employed three cobalt sulfides (CoSx, where x = 8/9, 2, 1.097) with different sulfur contents as catalysts for electrocatalytic NO3RR under alkaline conditions. At -0.8 V vs RHE, all these CoSx show promising performances that Faradaic efficiencies of >80% and a high yield of >1780 mmol h-1 gcat-1 for NH3 production are achieved. Through a combination of X-ray diffraction (XRD), transmission electron microscopy (TEM), and other characterizations, it is revealed that all these cobalt sulfides are easily converted into cobalt hydroxide during the NO3RR. This phenomenon is seemingly contradictory to the thermodynamic prediction that, according to the Pourbaix diagram, these CoSx compounds should be stable even under the catalytic condition. We suggest that this is due to the presence of Cl- ions in the electrolyte that promote the transformation of CoSx toward Co(OH)2. Chloride ions are commonly found in both industrial settings and natural water bodies and are challenging to remove. The evolved Co(OH)2 species is proposed to be responsible for catalyzing NO3RR, especially during a long-term catalytic process. This study highlights the inevitable structural evolution of CoSx catalysts under current alkaline electrocatalytic NO3RR conditions, offering theoretical guidance for the judicious selection and design of future catalysts.

Removal of high concentration chlorides and organic pollutants from incineration fly ash and sewage sludge hydrothermal liquid by Friedel's salt preparation

Hydrothermal liquid, produced in the process of coordinated hydrothermal harmless disposal of incineration fly ash (IFA) and sewage sludge (SS), contains a large number of organic pollutants and chloride. There is no report on the treatment of wastewater produced by hydrothermal treatment. To address the aforementioned issue, this study employs Friedel's salt precipitation for the dechlorination of the hydrothermal liquid. Experimental results show that the application of a reagent ratio of n(Al):n(Ca):n(Cl) = 4:8:1 effectively removed 76.77% of Cl- and 99.76% of SO42- from the hydrothermal liquid at an optimized temperature of 25 °C. Additionally, not only a 36.85% reduction of total organic carbon (TOC) was achieved through flocculation with sodium aluminate (NaAlO2), but also merely 1% of Al3+ from the original dosage remained in the filtrate. Moreover, combined with the analysis of the phase structure of the precipitate, Friedel's salt precipitation primarily removes Cl- via interlayer ion exchange and charge balance adsorption, providing insights into the operational mechanisms and the influence of interference factors. This work provides technical support for the application of synergistic hydrothermal treatment technology of IFA and SS, also has reference value for the treatment of other high chlorine-containing organic wastewater.

Mangrove fungi in action: Novel bioremediation strategy for high-chloride wastewater

Bioremediation of extremely high-chloride wastewater poses significant challenges due to the adverse effects of elevated salt concentrations on most microorganisms, where chloride levels can be as high as 7% (w/v). Mangrove wetlands derived fungus, Aspergillus aculeatus, emerged as a promising candidate, capable of removing approximately 40% of chloride ions in environments with concentration of 15% (w/v), representative of industrial wastewater conditions. Transcriptomics and biochemical assays conducted under increasing salt conditions revealed that elevated chloride concentrations induce the expression and activity of S-adenosyl methionine-dependent methyltransferase, which facilitates the conversion of chloride into chloromethane. This is the first report characterizing the biological mechanism behind high salt tolerance and chloride removal capacity of Aspergillus aculeatus. This salt remediation mechanism may work as a starter for developing future bioremediation strategies to treat high-chloride wastewater using fungi, offering an eco-friendly alternative to traditional physical or chemical methods.

Cutting edge technology for wastewater treatment using smart nanomaterials: recent trends and futuristic advancements

Water is a vital component of our existence. Many human activities, such as improper waste disposal from households, industries, hospitals, and synthetic processes, are major contributors to the contamination of water streams. It is the responsibility of every individual to safeguard water resources and reduce pollution. Among the various available wastewater treatment (WWT) methods, smart nanomaterials stand out for their effectiveness in pollutant removal through absorption and adsorption. This paper examines the application of valuable smart nanomaterials in treating wastewater. Various nanomaterials, including cellulose nanocrystals (CNC), cellulose nanofibrils (CNF), nanoadsorbents, nanometals, nanofilters, nanocatalysts, carbon nanotubes (CNTs), nanosilver, nanotitanium dioxide, magnetic nanoparticles, nanozero-valent metallic nanoparticles, nanocomposites, nanofibers, and quantum dots, are identified as promising candidates for WWT. These smart nanomaterials efficiently eliminate toxic substances, microplastics, nanoplastics, and polythene particulates from wastewater. Additionally, the paper discusses comparative studies on the purification efficiency of nanoscience technology versus conventional methods.

Interlayer Structure Manipulation of FeOCl/MXene with Soft/Hard Interface Design for Safe Water Production Using Dechlorination Battery Deionization

Suffering from the susceptibility to decomposition, the potential electrochemical application of FeOCl has greatly been hindered. The rational design of the soft-hard material interface can effectively address the challenge of stress concentration and thus decomposition that may occur in the electrodes during charging and discharging. Herein, interlayer structure manipulation of FeOCl/MXene using soft-hard interface design method were conducted for electrochemical dechlorination. FeOCl was encapsulated in Ti3C2Tx MXene nanosheets by electrostatic self-assembly layer by layer to form a soft-hard mechanical hierarchical structure, in which Ti3C2Tx was used as flexible buffer layers to relieve the huge volume change of FeOCl during Cl- intercalation/deintercalation and constructed a conductive network for fast charge transfer. The CDI dechlorination system of FeOCl/Ti3C2Tx delivered outstanding Cl- adsorption capacity (158.47 ± 6.98 mg g-1), rate (6.07 ± 0.35 mg g-1 min-1), and stability (over 94.49 % in 30 cycles), and achieved considerable energy recovery (21.14 ± 0.25 %). The superior dechlorination performance was proved to originate from the Fe2+/Fe3+ topochemical transformation and the deformation constraint effect of Ti3C2Tx on FeOCl. Our interfacial design strategy enables a hard-to-soft integration capacity, which can serve as a universal technology for solving the traditional problem of electrode volume expansion.

Reactive P and S co-doped porous hollow nanotube arrays for high performance chloride ion storage

Developing stable, high-performance chloride-ion storage electrodes is essential for energy storage and water purification application. Herein, a P, S co-doped porous hollow nanotube array, with a free ion diffusion pathway and highly active adsorption sites, on carbon felt electrodes (CoNiPS@CF) is reported. Due to the porous hollow nanotube structure and synergistic effect of P, S co-doped, the CoNiPS@CF based capacitive deionization (CDI) system exhibits high desalination capacity (76.1 mgCl- g-1), fast desalination rate (6.33 mgCl- g-1 min-1) and good cycling stability (capacity retention rate of > 90%), which compares favorably to the state-of-the-art electrodes. The porous hollow nanotube structure enables fast ion diffusion kinetics due to the swift ion transport inside the electrode and the presence of a large number of reactive sites. The introduction of S element also reduces the passivation layer on the surface of CoNiP and lowers the adsorption energy for Cl- capture, thereby improving the electrode conductivity and surface electrochemical activity, and further accelerating the adsorption kinetics. Our results offer a powerful strategy to improve the reactivity and stability of transition metal phosphides for chloride capture, and to improve the efficiency of electrochemical dechlorination technologies.

Single-crystal vs polycrystalline boron-doped diamond anodes: Comparing degradation efficiencies of carbamazepine in electrochemical water treatment

The ongoing challenge of water pollution by contaminants of emerging concern calls for more effective wastewater treatment to prevent harmful side effects to the environment and human health. To this end, this study explored for the first time the implementation of single-crystal boron-doped diamond (BDD) anodes in electrochemical wastewater treatment, which stand out from the conventional polycrystalline BDD morphologies widely reported in the literature. The single-crystal BDD presented a pure diamond (sp3) content, whereas the three other investigated polycrystalline BDD electrodes displayed various properties in terms of boron doping, sp3/sp2 content, microstructure, and roughness. The effects of other process conditions, such as applied current density and anolyte concentration, were simultaneously investigated using carbamazepine (CBZ) as a representative target pollutant. The Taguchi method was applied to elucidate the optimal operating conditions that maximised either (i) the CBZ degradation rate constant (enhanced through hydroxyl radicals (•OH)) or (ii) the proportion of sulfate radicals (SO4•-) with respect to •OH. The results showed that the single-crystal BDD significantly promoted •OH formation but also that the interactions between boron doping, current density and anolyte concentration determined the underlying degradation mechanisms. Therefore, this study demonstrated that characterising the BDD material and understanding its interactions with other process operating conditions prior to degradation experiments is a crucial step to attain the optimisation of any wastewater treatment application.

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.

Influence of Inorganic Anions on the Chemical Stability of Molybdenum Disulfide Nanosheets in the Aqueous Environment

Chemical stability is closely associated with the transformations and bioavailabilities of engineered nanomaterials and is a key factor that governs broader and long-term application. With the growing utilization of molybdenum disulfide (MoS2) nanosheets in water treatment and purification processes, it is crucial to evaluate the stability of MoS2 nanosheets in aquatic environments. Nonetheless, the effects of anionic species on MoS2 remain largely unexplored. Herein, the stability of chemically exfoliated MoS2 nanosheets (ceMoS2) was assessed in the presence of inorganic anions. The results showed that the chemical stability of ceMoS2 was regulated by the nucleophilicities and the resultant charging effects of the anions in aquatic systems. The anions promote the dissolution of ceMoS2 by triggering a shift in the chemical potential of the ceMoS2 surface as a function of the anion nucleophilicity (i.e., charging effect). Fast charging with HCO3- and HPO42-/H2PO4- was validated by a phase transition from 1T to 2H and the emergence of MoV, and it promoted oxidative dissolution of the ceMoS2. Additionally, under sunlight, ceMoS2 dissolution was accelerated by NO3-. These findings provide insight into the ion-induced fate of ceMoS2 and the durability and risks of MoS2 nanosheets in environmental applications.

Process optimization and mechanism for removal of high-concentration chlorine from municipal solid waste incineration fly ash washing wastewater by Friedel's salt

Waterwashing is an important pretreatment method for the reuse of municipal solid waste incineration (MSWI) fly ash. However, the presence of high levels of chlorides and small amounts of sulfides in the waterwashing solution makes it difficult to treat and reuse. Therefore, in this study, a calcium sulfate- Friedel's salt precipitation method was used to dechlorinate and desulfurize in the MSWI fly ash washing solution. This paper mainly focused on the chloride removal, and the effects of factors such as reagent ratios, temperature, and reaction time on chloride removal rate and the two-stage dechlorination and desulfurization process were optimized. The experimental results indicated that when the ratio in the first stage was n(Ca):n(Al):n(Cl) = 3:1.5:1, and in the second stage, n(Ca):n(Al):n(Cl) = 8:4:1, a chloride removal rate of up to 90.5% and a sulfide removal rate of over 99.88% could be obtained. The deposited particles were analyzed by using FE-SEM, XRD, and FTIR to investigate their size, morphology, phase composition, and functional groups. The study revealed that excessively high or low reagent ratios could reduce the interlayer spacing of Friedel's salt. Additionally, high temperatures led to the decomposition of Friedel's salt, and the dechlorination efficiency was affected.

Simultaneous detection and removal of mercury (II) using multifunctional fluorescent materials

Environmental problems caused by mercury ions are increasing due to growing industrialization, poor enforcement, and inefficient pollutant treatment. Therefore, detecting and removing mercury from the ecological chain is of utmost significance. Currently, a wide range of small molecules and nanomaterials have made remarkable progress in the detection, detoxification, adsorption, and removal of mercury. In this review, we summarized the recent advances in the design and construction of multifunctional materials, detailed their sensing and removing mechanisms, and discussed with emphasis the advantages and disadvantages of different types of sensors. Finally, we elucidated the problems and challenges of current multifunctional materials and further pointed out the direction for the future development of related materials. This review is expected to provide a guideline for researchers to establish a robust strategy for the detection and removal of mercury ionsin the environment.

Advancements and sustainable strategies for the treatment and management of wastewaters from metallurgical industries: an overview

Metallurgy is pivotal for societal progress, yet it yields wastewater laden with hazardous compounds. Adhering to stringent environmental mandates, the scientific and industrial sectors are actively researching resilient treatment and disposal solutions for metallurgical effluents. The primary origins of organic pollutants within the metallurgical sector include processes such as coke quenching, steel rolling, solvent extraction, and electroplating. This article provides a detailed analysis of strategies for treating steel industry waste in wastewater treatment. Recent advancements in membrane technologies, adsorption, and various other processes for removing hazardous pollutants from steel industrial wastewater are comprehensively reviewed. The literature review reveals that advanced oxidation processes (AOPs) demonstrate superior effectiveness in eliminating persistent contaminants. However, the major challenges to their industrial-scale implementation are their cost and scalability. Additionally, it was discovered that employing a series of biological reactors instead of single-step biological processes enhances command over microbial communities and operating variables, thus boosting the efficacy of the treatment mechanism (e.g., achieving a chemical oxygen demand (COD) elimination rate of over 90%). This review seeks to conduct an in-depth examination of the current state of treating metallurgical wastewater, with a particular emphasis on strategies for pollutant removal. These pollutants exhibit distinct features influenced by the technologies and workflows unique to their respective processes, including factors such as their composition, physicochemical properties, and concentrations. Therefore, it is of utmost importance for customized treatment and disposal approaches, which are the central focus of this review. In this context, we will explore these methods, highlighting their advantages and characteristics.

Dechlorination of wastewater from shell-based glucosamine processing by mangrove wetland-derived fungi

Wastewater from processing crustacean shell features ultrahigh chloride content. Bioremediation of the wastewater is challenging due to the high chloride ion content, making it inhospitable for most microorganisms to survive and growth. In this study, mangrove wetland-derived fungi were first tested for their salt tolerance, and the highly tolerant isolates were cultured in shrimp processing wastewater and the chloride concentration was monitored. Notably, the filamentous fungal species Aspergillus piperis could remove over 70% of the chloride in the wastewater within 3 days, with the fastest biomass increase (2.01 times heavier) and chloride removal occurring between day one and two. The chloride ions were sequestered into the fungal cells. The genome of this fungal species contained Cl- conversion enzymes, which may have contributed to the ion removal. The fungal strain was found to be of low virulence in larval models and could serve as a starting point for further considerations in bioremediation of shell processing wastewater, promoting the development of green technology in the shell processing industry.

Generation and engineering applications of sulfate radicals in environmental remediation

Sulfate radical (SO4•-)-based advanced oxidation processes (AOPs) have become promising alternatives in environmental remediation due to the higher redox potential (2.6-3.1 V) and longer half-life period (30-40 μs) of sulfate radicals compared with many other radicals such as hydroxyl radicals (•OH). The generation and mechanisms of SO4•- and the applications of SO4•--AOPs have been examined extensively, while those using sulfite as activation precursor and their comparisons among various activation precursors have rarely reviewed comprehensively. In this article, the latest progresses in SO4•--AOPs were comprehensively reviewed and commented on. First of all, the generation of SO4•- was summarized via the two activation methods using various oxidant precursors, and the generation mechanisms were also presented, which provides a reference for guiding researchers to better select two precursors. Secondly, the reaction mechanisms of SO4•- were reviewed for organic pollutant degradation, and the reactivity was systematically compared between SO4•- and •OH. Thirdly, methods for SO4•- detection were reviewed which include quantitative and qualitative ones, over which current controversies were discussed. Fourthly, the applications of SO4•--AOPs in various environmental remediation were summarized, and the advantages, challenges, and prospects were also commented. At last, future research needs for SO4•--AOPs were also proposed consequently. This review could lead to better understanding and applications of SO4•--AOPs in environmental remediations.

Ecotoxicological response surface analysis of salt and pH in textile effluent on Bacillus subtilis and Lactuca sativa

Textile effluents, although their composition can vary considerably, typically contain high levels of dissolved salts and exhibit wide variations in pH. Ecotoxicological studies regarding the effects of these parameters, however, have been limited owing to the need for sensitive and easy-to-handle bioindicators that require low amounts of sampling, are cost-effective, time-efficient, and ethically endorsed. This kind of study, additionally, demands robust multi-factorial statistical designs that can accurately characterize the individual and combined relationship between variables. In this research, Response Surface Methodology (RSM) was used to calculate the individual and interaction effects of NaCl concentration and pH value of a Simulated Textile Effluent (STE) on the development rate (DR) of the bioindicators: Bacillus subtilis bacteria and Lactuca sativa lettuce. The results demonstrated that the bioindicators were sensitive to both NaCl and pH factors, where the relative sensitivity relationship was B. subtilis > L. sativa. The quadratic equations generated in the experiments indicated that increased concentrations of 50-250 mg L-1 of NaCl caused a perturbance of 1.40%-34.40% on the DR of B. subtilis and 0.50%-12.30% on L. sativa. The pH factor at values of 3-11 caused an alteration of 27.00%-64.78% on the DR of the B. subtilis and 51.37%-37.37% on the L. sativa. These findings suggest that the selected bioindicators could serve as effective tools to assess the ecotoxicological effects of textile effluents on different ecological systems, and the RSM was an excellent tool to consider the ecotoxicological effects of the parameters and to describe the behavior of the results. In conclusion, the NaCl and pH factors may be responsible for disrupting different ecosystems, causing imbalances in their biodiversity and biomass. Before discharge or reuse, it is suggested to remove salts and neutralize pH from textile effluents and, mostly, develop novel, eco-friendlier textile processing techniques.

Preparation of stable and long-lived source samples for the stand-alone beam program at the Facility for Rare Isotope Beams

At the Facility for Rare Isotope Beams (FRIB), an oven-ion source combination was used to create rare isotope beams in support of the stand-alone user beam program of the ReAccelerator (ReA) facility. This ion source, called Batch-Mode Ion Source (BMIS), was loaded with enriched stable nuclides (30Si, 50Cr, and 58Fe) and long-lived radionuclides (26Al, 32Si). The introduced samples, herein designated as source samples, were thermally volatilized in the BMIS oven, and then ionization was used to generate the required beams. Owing to the different chemical behavior of the used samples, it was important to tailor the sample loading process for each desired beam species. An important parameter here is the volatility of the introduced species, which influences the adequate release of the isotope of interest. Additionally, any co-present, volatile components will affect the ion yields of the desired isotope, while isobaric contaminants will decrease the beam purity. To manufacture isotope source samples that meet these characteristics, various chemical methodologies were developed. All prepared samples were successfully used in BMIS to deliver beams for various user beam experiments. The here-established sample preparation techniques will greatly aid future efforts in developing offline rare-isotope beams.

Dual step-scheme heterojunction with full-visible-light-harvesting towards synergistic persulfate activation for enhanced photodegradation

The occurrence of micropollutants in aquatic media raises great concern because of their biological toxicity and persistence. Herein, visible-light-driven photocatalyst titanium dioxide/graphitic carbon nitride/triiron tetraoxide (TiO2-x/g-C3N4/Fe3O4, TCNF) with oxygen vacancies (Ov) was prepared via a facile hydrothermal-calcination method. The complementary visible-light co-absorption among semiconductors enhances light-harvesting efficiency. The built-in electric field formed during Fermi level alignment drives photoinduced electron transfer to improve charge separation across the interfaces. The increased light-harvesting and favorable energy band bending significantly enhance the photocatalytic performance. Therefore, TCNF-5-500/persulfate system could effectively photodegrade bis-phenol A within 20 min under visible-light irradiation. Moreover, the superior durability, non-selective oxidation, adaptability, and eco-friendliness of the system were confirmed by different reaction conditions and biotoxicity assessment. Furthermore, the photodegradation reaction mechanism was presented according to the major reactive oxygen species produced in the system. Thus, this study constructed a dual step-scheme heterojunction by tuning visible-light absorption and energy band structure to increase the charge transfer efficiency and photogenerated carrier lifetime, which has great potential for environmental remediation using visible photocatalysis.

Advancement of biochar-aided with iron chloride for contaminants removal from wastewater and biogas production: A review

The use of fossil fuels, emission of greenhouse gases (GHG) into the atmosphere, and waste pose a problem to the environment and public health that urgently needs to be dealt with. Among numerous chemical activating agents that can be added to anaerobic digestion (AD) to enhance nutrient removal and increase the quality and quantity of biomethane, iron chloride (FeCl₃) is the one that has the lowest cost and is the most environmentally friendly. This state-of-the-art review aims to revise the influence of FeCl₃ on the Brunauer-Emmett-Teller (BET) surface area of biochar and its ability to increase methane (CH₄) yield and remove contaminants from biogas and wastewater. The novelty of the study is that FeCl₃, an activating agent, can increase the BET surface area of biochar, and its efficacy increases when combined with zinc chloride or phosphoric acid. Regarding the removal of contaminants from wastewater and biogas, FeCl₃ has proven to be an effective coagulant, reducing the chemical oxygen demand (COD) of wastewater and hydrogen sulfide in biogas. The performance of FeCl₃ depends on the dosage, pH, and feedstock used. Therefore, FeCl₃ can increase the BET surface area of biochar and CH₄ yield and remove contaminants from wastewater and biogas. More research is needed to investigate the ability of FeCl₃ to remove water vapor and carbon dioxide during biogas production while accounting for a set of other parameters, including FeCl₃ size.

Comprehensive understanding of electrochemical treatment systems combined with biological processes for wastewater remediation

The presence of toxic pollutants in wastewater discharge can affect the environment negatively due to presence of the organic and inorganic contaminants. The application of the electrochemical process in wastewater treatment is promising, specifically in treating these harmful pollutants from the aquatic environment. This review focused on recent applications of the electrochemical process for the remediation of such harmful pollutants from aquatic environments. Furthermore, the process conditions that affect the electrochemical process performance are evaluated, and the appropriate treatment processes are suggested according to the presence of organic and inorganic contaminants. Electrocoagulation, electrooxidation, and electro-Fenton applications in wastewater have shown effective performance with high removal rates. The disadvantages of these processes are the formation of toxic intermediate metabolites, high energy consumption, and sludge generation. To overcome such disadvantages combined ecotechnologies can be applied in large-scale wastewater pollutants removal. The combination of electrochemical and biological treatment has gained importance, increased removal performance remarkably, and decreased operational costs. The critical discussion with depth information in this review could be beneficial for wastewater treatment plant operators throughout the world.

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.

Evaluation of Forward Osmosis and Low-Pressure Reverse Osmosis with a Tubular Membrane for the Concentration of Municipal Wastewater and the Production of Biogas

Currently, freshwater scarcity is one of the main issues that the world population has to face. To address this issue, new wastewater treatment technologies have been developed such as membrane processes. Among them, due to the energy disadvantages of pressure-driven membrane processes, Forward Osmosis (FO) and Low-Pressure Reverse Osmosis (LPRO) have been introduced as promising alternatives. In this study, the behavior of a 2.3 m² tubular membrane TFO-D90 when working with municipal wastewater has been studied. Its performances have been evaluated and compared in two operating modes such as FO and LPRO. Parameters such as fouling, flow rates, water flux, draw solution concentration, organic matter concentration, as well as its recovery have been studied. In addition, the biogas production capacity has been evaluated with the concentrated municipal wastewater obtained from each process. The results of this study indicate that the membrane can work in both processes (FO and LPRO) but, from the energy and productivity point of view, FO is considered more appropriate mainly due to its lower fouling level. This research may offer a new point of view on low-energy and energy recovery wastewater treatment and the applicability of FO and LPRO for wastewater concentration.

Effects of nitrate and Fe/As molar ratio on direct iron(III)-arsenite precipitation in high-sulfate-chloride wastewaters

The addition of an arsenite-chloride solution into an arsenite-sulfate solution is extremely beneficial for the removal of As(III) via Fe(III) salt precipitation at pH 2.3. However, the applicability of this method to complicated high-As(III) metallurgical wastewaters still requires further verification. This work investigated the effects of nitrate and Fe/As molar ratio on As(III) immobilization using Fe(III) in three acid radical media including sulfate, chloride, and nitrate at pH 2.3. Our results indicated that 72.1‒93.5% of As(III) was precipitated, which was 5‒10% less than those obtained in the nitrate-free systems. The Fe/As molar ratio of 4 was the optimal condition with an average of 93% As(III) removal based on a broad sulfate/chloride molar ratio range (1:1‒16). However, a maximum of 96% As(III) removal was observed under the Fe/As molar ratio of 1.5 and the sulfate/chloride condition of 1:16. The negative correlation between complexation and precipitation was attributed to the enhanced initial complexation by the synergistic effect of the mononitratoiron complex and FeH₂AsO₃²⁺. The variation of Fe/As molar ratios resulted in the diverse solid species, thus further affecting the As(III) removal efficiency. Despite producing tooeleite as a major As(III) host phase, ferrihydrite and poorly crystalline ferric arsenite hydroxysulfate formed simultaneously at the Fe/As molar ratio of 4 participated in As(III) immobilization compared with the solid products at Fe/As molar ratios ≤ 2.

Coupled microwave hydrothermal dechlorination and geopolymer preparation for the solidification/stabilization of heavy metals and chlorine in municipal solid waste incineration fly ash

To improve the degradation efficiency of persistent organic pollutants (POPs) in municipal solid waste incineration fly ash (MSWIFA), as well as to overcome the difficulties of subsequent hydrothermal liquid and hydrothermal slag treatment, a two-step treatment strategy of microwave hydrothermal degradation coupled with geopolymer immobilization was proposed. Results showed that the optimal process parameters for microwave hydrothermal dechlorination were a temperature of 220 °C, a time of 1 h, and NaOH addition of 10 wt%. Microwaves accelerated the OH^- mediated hydrolysis reactions and promoted the breaking of CCl bonds, leading to dechlorination. The compressive strength of the 20 % MSWIFA-based geopolymers reached 75.79 MPa, and the immobilization rate of the heavy metals (HMs) and Cl^- surpassed 90 %. Alkaline environment provided by microwave hydrothermal promoted the formation of Ca(OH)₂, which subsequently formed Friedel's salt (3CaO•Al₂O₃•CaCl₂•10H₂O) with Cl^- in the geopolymer. The charge density difference and density of states (DOS) of Friedel's salt were analyzed by first-principles calculations, confirming that the existence of strong interactions between Ca-s, Al-p, O-p, and Cl-p states was the chemical mechanism of Cl^- immobilization. The Friedel's salt and HMs were encapsulated by geopolymers with dense silica-alumina tetrahedral frameworks, achieving the solidification/stabilization (S/S) of HMs and Cl^-. This work provided a new approach for the environmentally sound and resourceful treatment of MSWIFA.

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.

Water reuse from wastewater treatment: The transition towards circular economy in the water sector

Water is crucial for economic development since it interacts with the agricultural, production, and energy sectors. However, the increasing demand and climate change put pressure on water sources. This paper argued the necessity of using reclaimed water for irrigation within the scope of a circular economy. The barriers (i.e., technological and economic, institutional/regulatory, and social) to water reuse practices were revealed. Lessons on how to overcome the barriers were learned from good practices. The roadmaps adopted in the European Union for the transition towards the circular economy were reviewed. It has been observed that these roadmaps are generally on the circularity of solid wastes. However, water is too important for the economy to be ignored in the transition towards circular economy. Research needs and perspective for a comprehensive roadmap to widen water-smart solutions such as water reuse were drawn.

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.