Process optimization to enhance utilization efficiency of precipitants for chloride removal from flue gas desulfurization wastewater via Friedel's salt precipitation

Purification of waste liquids from alkaline washing pretreatment of municipal solid waste incineration fly ash using Friedel's salt

Washing is a widely used pretreatment method for the resource utilization of municipal solid waste incineration fly ash (MSWI FA). However, the high concentrations of heavy metals and chlorides in alkaline washing pretreatment waste liquids can adversely affect the environment and human health, necessitating proper disposal. Therefore, in this study, the heavy metal removal and ultrasonic-assisted two-stage dechlorination process from MSWI FA alkaline washing waste liquids using Friedel's salt were developed. When the end-point pH was 9 and Friedel's salt dosage was 4 g/L, the removal rate of key heavy metals from MSWI FA alkaline washing waste liquids exceeded 95 %. A series of characterization showed that Friedel's salt can effectively remove heavy metals from waste liquids through isomorphic substitution, ion exchange, surface adsorption and precipitation formation. Experimental results demonstrated that when the molar ratio of Ca: Al: Cl was adjusted to 2.5:1.2:1 in the first stage and 7:3:1 in the second stage, combined with an ultrasonic power of 600 W, the ultrasound-assisted two-stage dechlorination method achieved a chloride removal efficiency exceeding 93.8 % from the alkaline washing solution of MSWI FA. And, 272.6 g of Friedel's salt was obtained while treating 1 L of waste liquids. This study revealed that ultrasonic-enhanced dechlorination occurs through mechanisms that promote the dissolution and dispersion of CaO while inhibiting the formation of Ca3Al2(OH)12. In addition, the combination of alkaline washing pretreatment and waste liquids reuse, an integrated idea was provided, which was helpful for the comprehensive utilization of MSWI FA.

Enhancing the Early Hydration of Supersulfate Cement: The Effect of Sodium Aluminate

Supersulfate cement (SSC) has received significant attention in the construction industry due to its extensive utilization of solid wastes and low carbon emissions. However, the low carbonation resistance and early strength of SSC greatly restricted its application, which was attributed to early insufficient alkalinity and slow hydration. Facilitating early hydration alkalinity is critical to promote early hydration and improve early performance for SSC. Thus, sodium aluminate (SA), an admixture with concentrations ranging from 0% to 4%, was adopted to enhance early alkalinity and investigate its impact on the initial hydration process. The results indicated that incorporating SA into SSC enhances its early performance by balancing high alkalinity and AFt stability. The addition of 2% SA accelerates hydration procession, reducing initial and final setting times by 76% and 42%, respectively, while increasing viscosity by 50% for improved structural stability. At 2% SA, 1-day and 7-day compressive strengths rose from 3.7 MPa to 8.4 MPa and from 15.1 MPa to 18.5 MPa, respectively, representing gains of 127% and 22.5%, which were facilitated by accelerated GGBFS dissolution and needle-like AFt formation, which densifies the crystal-gel network microstructure.

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.

Investigation on the rotary atomization evaporation of high-salinity desulfurization wastewater: Performance and products insights

Spray drying of concentrated wastewater epitomizes a harmonious convergence of technological progress, economic viability, and practicality within the realm of zero liquid discharge. Nevertheless, elevated salinity may influence the atomization and evaporation processes, along with the storage and transportation of evaporation products. This study systematically examines the influence of salinity on rotary atomization evaporation performance and evaporation products of wastewater through a series of experimental investigations. The results indicate that, under identical conditions, the overall droplet size of high-salinity wastewater is approximately 20-50% larger compared to conventional wastewater. Salinity significantly influences the atomization particle size (D32), followed by rotation speed, and then influent flow rate. The high-salinity wastewater droplets manifest a multi-bubble growth pattern with earlier shell expansion, where the reduction of free water dominates the overall process dynamics. Despite the diminished evaporation rate, the total evaporation duration shortens with elevating salinity, reducing flue gas consumption by about 10%. With elevated crystalline salt content, high-salinity wastewater evaporation products exhibit pronounced hygroscopicity, manifesting as a viscous powder with suboptimal flowability (FF 3.57) at a 2 wt% moisture content. This study bridges the gap in rotary spray drying technology for high-salinity wastewater treatment, contributing to sustainable water conservation and energy-efficient management.

Application of waste iron in wet flue gas desulfurization (WFGD) wastewater treatment

The wet flue gas desulfurization (WFGD) procedure results in wastewater containing a complex mixture of pollutants, including heavy metals and organic compounds, which are hardly degradable and pose significant environmental challenges. Addressing this issue, the proposed approach, incorporating waste iron shavings as a heterocatalyst within a modified Fenton process, represents a sustainable and effective solution for contaminants degrading in WFGD wastewater. Furthermore, this study aligns with the Best Available Techniques (BAT) regulations by meeting the requirement for compound oxidation-replacing the chlorine utilization with the generation of highly reactive radicals-and coagulation, which completes the treatment process. This method introduces an innovative use of waste-derived iron shavings in a BAT-compliant technology, providing a sustainable and cost-effective alternative to conventional treatments. The study focuses on process kinetics and optimization parameters, achieving approximately 48% total organic carbon (TOC) removal in 90 min at an optimal pH 3, using 1998 mgL-1 H2O2 under UV light. Analysis of variance revealed that the process efficiency depended more significantly on pH than time duration or the H2O2 dose. Catalyst's characterization, including the use of microscopic techniques, including electron microscopy, laser diffraction, X-ray fluorescence, Raman spectroscopy, and UV spectroscopy, indicates its stability and great reusability with consistent TOC decrease across three process cycles. This research demonstrates a cost-effective, environmentally friendly approach to wastewater treatment, advancing sustainable methodologies through the repurposing of waste materials and underscoring the catalyst's reuse potential.

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.

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.

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.

Removal of high concentration of chloride ions by electrocoagulation using aluminium electrode

Wastewater containing a high concentration of chloride ions (Cl^- ions) generated in industrial production will corrode equipment and pipelines and cause environmental problems. At present, systematic research on Cl^- removal by electrocoagulation is scarce. To study the Cl^- removal mechanism, process parameters (current density and plate spacing), and the influence of coexisting ions on the removal of Cl^- in electrocoagulation, we use aluminum (Al) as the sacrificial anode, combined with physical characterization and density functional theory (DFT) to study Cl^- removal by electrocoagulation. The result showed that the use of electrocoagulation technology to remove Cl^- can reduce the concentration of Cl^- in an aqueous solution below 250 ppm, meeting the Cl^- emission standard. The mechanism of Cl^- removal is mainly co-precipitation and electrostatic adsorption by forming chlorine-containing metal hydroxyl complexes. The current density and plate spacing affect the Cl^- removal effect and operation cost. As a coexisting cation, magnesium ion (Mg²⁺) promotes the removal of Cl^-, while calcium ion (Ca²⁺) inhibits it. Fluoride ion (F^-), sulfate (SO₄^2-), and nitrate (NO₃^-) as coexisting anions affect the removal of Cl^- ions through competitive reaction. This work provides a theoretical basis for the industrialization of Cl^- removal by electrocoagulation.

Mapping the research on desulfurization wastewater: Insights from a bibliometric review (1991-2021)

Desulfurization wastewater in coal-fired power plants (CFPPs) is a great environmental challenge. This study aimed at the current status and future research trends of desulfurization wastewater by bibliometric analysis. The desulfurization wastewater featured with high sulfate (8000 mg/L), chlorite (8505 mg/L), magnesium (2882 mg/L) and calcium (969 mg/L) but low sodium (801.82 mg/L), and the concentrations of the main contaminants were critically summarized. There was an increasing trend in the annual publications of desulfurization wastewater in the period from 1991 to 2021, with an average growth rate of 15%. Water Science and Technology, Desalination and Water Treatment, Energy & Fuels, Chemosphere, and Journal of Hazardous Materials are the top 5 journals in this field. China was the most productive country (58.3% of global output) and the core country in the international cooperation network. Wordcloud analysis and keyword topic trend demonstrated that removal/treatment of pollutants dominated the global research in the field of desulfurization wastewater. The primary technologies for desulfurization wastewater treatment were systematically evaluated. The physicochemical treatment technologies occupied half of the total treatment methods, while membrane-based integrated processes showed potential applications for beneficial reuse. The challenges and outlook on desulfurization wastewater treatment for achieving zero liquid discharge are summarized.

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.

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.