Industrial Waste Salts: Characteristics, Impurity-Oriented Treatment Pathways, and Resource Utilization Strategies

Abstract

The large-scale generation of industrial waste salts (IWSs) across sectors such as coal chemical, pesticide, pharmaceutical, and dye manufacturing has raised increasing environmental and regulatory concerns. These IWSs often exhibit complex physicochemical profiles—featuring high concentrations of inorganic salts, persistent organic pollutants, and trace heavy metals—that pose significant challenges for both safe disposal and resource recovery. This review provides a comprehensive and pollutant-oriented overview of industrial waste salts, focusing on their sector-specific characteristics, dominant contaminant types, and tailored treatment strategies. Removal pathways for organic matter (e.g., thermal decomposition, advanced oxidation) and inorganic impurities (e.g., precipitation, ion exchange) are systematically analyzed, followed by technical pathways for salt separation based on crystallization and membrane processes. Resource utilization routes for major salt components, particularly NaCl and Na₂SO₄, are critically assessed in terms of technical feasibility, impurity tolerance, and end-use compatibility. The emergence of reclaimed salt quality standards and sector-specific impurity thresholds reflects a paradigm shift from purity-based to performance-based reuse evaluation. Finally, the review highlights future priorities including adaptive impurity control, downstream-specific salt grading, and enforceable regulatory frameworks to ensure the safe, scalable, and circular deployment of reclaimed salts in industrial systems. This study supports the coordinated advancement of control technologies and reuse standards, enabling the transformation of waste salts from environmental liabilities to secondary resources.

1. Introduction

The rapid shift of global chemical manufacturing toward developing economies has led to a geographic concentration of industrial residues, particularly hypersaline waste salts. China, producing over 40% of the world’s chemical output (European Chemical Industry Council), generates an estimated 20 million tons of industrial waste salts (IWSs) annually [1]. These salts typically originate from the pharmaceutical, agrochemical, coal chemical, and fine chemical sectors through processes such as neutralization, salting-out, and crystallization.

Unlike conventional saline wastes, IWSs are characterized by a complex matrix of organic pollutants, heavy metals (e.g., Cr(VI), Pb²⁺), and toxic anions (e.g., CN⁻, F⁻). Their high variability across industries—ranging from organic-rich pharmaceutical salts to multi-component coal chemical residues—renders generic treatment approaches ineffective [2]. This compositional heterogeneity presents both technical and regulatory challenges.

Current regulatory frameworks further complicate waste salt management. In China, revised national standards (GB 18599; GB 18598) [3,4] introduce a salinity-based classification system: non-hazardous wastes containing >5% water-soluble salts are banned from conventional landfills, while hazardous wastes exceeding 10% salinity must be disposed of in engineered concrete landfills, often at costs exceeding USD 500/ton. Despite their environmental intent, these regulations have imposed substantial economic and logistical burdens. The scarcity of licensed concrete landfills and the steady increase in waste salt production have led to severe disposal bottlenecks. Simultaneously, national policy directives promote reduced landfill dependence, creating a regulatory paradox: landfill restrictions are tightening while sustainable alternatives remain underdeveloped.

Available detoxification strategies face trade-offs. Thermal treatments (e.g., incineration, pyrolysis) effectively decompose organics but pose risks of dioxin generation, heavy metal volatilization, and equipment corrosion in halogenated matrices [5]. Membrane-based separations offer high-purity salt recovery but remain capital intensive. Meanwhile, precipitation and oxidation methods often generate secondary residues. These challenges are compounded by the fact that many existing treatment processes were primarily developed for pollutant removal and regulatory compliance, rather than enabling downstream resource recovery or reuse [6], limiting opportunities for circularity.

While numerous reviews have addressed IWS purification or resource recovery, they typically examine these aspects in isolation—either focusing on individual treatment techniques or exploring valorization potential without linking it to upstream salt characteristics [5]. Moreover, the regulatory context is often omitted, despite its decisive role in defining classification, treatment feasibility, and reuse thresholds.

Given that China accounts for over 40% of global chemical output and faces acute waste salt management challenges due to its inland geography and water scarcity, this review primarily focuses on the Chinese context while providing a pollutant-oriented framework that can be adapted to other regions with similar constraints. This review aims to bridge these disconnected domains by offering a cross-sectoral, system-oriented framework for IWS management. Specifically, it (i) classifies IWSs based on industrial origin and compositional profiles, (ii) critically evaluates treatment and detoxification strategies, (iii) examines how regulatory frameworks constrain or enable technical pathways, and (iv) proposes an integrated roadmap linking safe disposal, effective treatment, and resource recovery. By connecting compositional diversity with process design and policy dynamics, this review provides a holistic foundation for more sustainable, circular waste salt management.

2. IWS Generation and Characteristics

Geographical and regulatory disparities significantly influence IWS management strategies. In coastal regions of Europe or Southeast Asia, the primary focus remains on advanced purification for compliant marine discharge. Conversely, in landlocked industrial hubs—such as central China, inland India, and parts of North America—stringent Zero Liquid Discharge (ZLD) mandates necessitate the transition from “treatment for disposal” to “purification for reuse” [7]. The adoption of ZLD technologies has surged in high-polluting sectors. These systems recover water from high-salinity streams via membrane or thermal concentration, but concurrently generate vast quantities of solid waste salts. As ZLD deployment expands alongside rising chemical production, waste salt output has increased sharply.

Current estimates suggest that the fine chemical and coal chemical sectors alone produce over 10 million tons of waste salts annually [8]. Other major contributors include pharmaceuticals, pesticides, dyes, resins, and incineration residues. The composition of these IWSs is highly variable, reflecting the diversity of feedstocks and reaction pathways across industries. In general, IWSs contain a mixture of sodium chloride or sulfate as primary components, combined with organic pollutants, heavy metals, and other inorganics [9].

Importantly, the formation mechanisms differ by sector. In coal chemical processes, evaporation of high-salinity wastewater yields mixed salts rich in cyanide, phenols, and ammonia. In pharmaceutical and pesticide manufacturing, salting-out steps and neutralization reactions produce complex mixtures of chlorides, phosphates, and recalcitrant organics such as nitrogen heterocycles or halogenated aromatics [10]. These sector-specific profiles result in distinct technical challenges for downstream treatment, especially in organic removal, impurity separation, and salt recovery. Understanding the origin-specific characteristics of waste salts is thus essential for designing effective treatment pathways and enabling targeted resource utilization.

2.1. Pesticide Industry

Pesticide IWSs are primarily composed of chloride salts, with NaCl and KCl being the dominant components, alongside inorganic impurities such as CaCl₂, MgCl₂, and MgSO₄ [11]. These salts often contain organic compounds that are both difficult to biodegrade and highly toxic.

IWS composition is inherently shaped by the upstream chemical process design. Taking glyphosate as an example, two major production routes—namely the IDA and glycine methods [12] (Figure 1a,b). The first involves targeted neutralization of acidic or alkaline streams, followed by evaporation and crystallization, resulting in relatively uniform salts with predictable composition [13]. The second route combines multiple wastewater streams before crystallization, producing more heterogeneous salts with complex impurities. These generation pathways fundamentally shape the composition, variability, and downstream treatability of waste salts. The pesticide industry generates between 0.2 to 2.0 tons of waste salt per ton of product, depending on the pesticide type, production process, and reaction efficiency. Glyphosate waste salts account for 46.6% of the total pesticide IWSs, followed by paraquat (6.8%) and imazethapyr (5.6%), collectively making up more than half of the IWS output (Figure 1c).

The organic pollutant content of pesticide waste salt is generally higher than that of other industrial waste streams, often exceeding 8500 mg/kg [11]. However, significant variations exist among pesticides due to differences in synthesis pathways and raw materials [14]. For example, glyphosate, chlorpyrifos, and fluorinated pyrethroids exhibit distinct elemental compositions [15]. Based on the elemental analysis of five representative pesticides—glyphosate, chlorpyrifos, prochloraz, nicosulfuron, and fluorinated insecticides—pesticide waste salt can be classified into three categories (Figure 1d,e):

• High-chloride salts (e.g., glyphosate), characterized by high Cl⁻ content (e.g., 54.3%) and low organic residues (C+N+O ≤ 10%);
• High-organic-content salts (e.g., prochloraz), with significant organic loads (C+N+O = 36.35–66.16%) dominated by complex compounds such as imidazoles or sulfonylureas;
• High-fluoride salts (e.g., fluorinated insecticides), featuring elevated fluoride levels (e.g., 39.52% F⁻) and minimal organic matter (C+N+O ≤ 2.77%).

Generally, pesticide waste salt with high organic content are typically treated using incineration, pyrolysis, or advanced oxidation to remove the majority of organic pollutants. However, these treatment methods do not negate the necessity of classification-based strategies, as the physicochemical properties of each category dictate specific process optimizations. This classification guides targeted treatment strategies. High-chloride salts, typical of glyphosate production, are treated via fractional crystallization to recover sodium chloride (>90% purity), combined with wet oxidation to degrade residual organics such as phosphorus intermediates. High-organic-content salts, like those from chlorpyrifos and prochloraz, undergo solvent extraction (e.g., ethyl acetate) to isolate lipophilic compounds, followed by advanced oxidation processes [16] (e.g., Fenton or electrochemical oxidation), achieving over 90% degradation of refractory organics. High-fluoride salts, represented by pyrethroid wastes, are managed by fluoride precipitation (CaCl₂ to CaF₂) and thermal decomposition (>600 °C), enabling the recovery of reusable fluoride compounds (e.g., NaF) [17].

Despite these advances, challenges remain. Minor fractions (1–4%) may contain trace metals or emerging pollutants such as perfluorinated compounds, requiring further study. Moreover, high costs of advanced treatments like supercritical water oxidation (SCWO) and membrane separations limit widespread application, especially in resource-limited settings [18]. Future efforts should explore cost-effective alternatives, including biochar-based adsorbents, and employ machine learning to optimize treatment efficiency and resource recovery.

A classification-based approach combined with innovative technologies is essential for sustainable pesticide waste salt management. This strategy not only mitigates environmental risks but also transforms hazardous by-products into valuable resources, aligning with circular economy principles and global sustainability goals.

2.2. Pharmaceutical Industry

Pharmaceutical waste salt, generated primarily from API and intermediate synthesis via halogenation, neutralization, condensation, and crystallization, consist mainly of inorganic salts (NaCl, Na₂SO₄, NaBr, KF, CH₃COONa) with complex organic residues. Similar to pesticide waste salts, waste salts are characterized by high organic content, complex compositions, and environmental persistence, yet they show greater compositional diversity due to the multitude of synthetic routes across different drug categories [19].

Owing to their typically high organic content (>10%) and chemical complexity, thermal treatments are widely applied, with method selection guided by calorific value and organic nature [20]. Incineration is suited for wastes with high calorific value (>10,500 kJ·kg⁻¹), enabling complete oxidation at 600–1100 °C under oxygen-rich conditions, though it demands stringent flue gas controls to mitigate dioxins and heavy metals [21]. Pyrolysis, conducted at 400–600 °C under low-oxygen atmospheres, is preferred for wastes with moderate calorific values or structurally complex organics, facilitating selective by-product recovery while reducing secondary pollution [22].

Representative case studies demonstrate tailored approaches: sulfate-rich waste salt containing amine or urea derivatives have been effectively treated via low-temperature carbonization and filtration [23], phosphorus-rich salts purified through solvent elution and activated carbon achieving >98% recovery, and halogenated organics from vitamin B₆ synthesis better managed by combustion than pyrolysis [24].

In summary, the pharmaceutical industry’s waste salts demand targeted, composition-driven treatment strategies, particularly for organic pollutant removal, reflecting their higher variability relative to pesticide wastes.

2.3. Coal Chemical Industry

The coal chemical industry is a major source of industrial waste salts in China, especially in arid regions like Inner Mongolia and Ningxia, where water scarcity and ecological sensitivity necessitate stringent environmental controls [25]. To meet the regulatory and resource-efficiency goals, many coal chemical enterprises have adopted “near-zero discharge” strategies [26], typically involving pretreatment, deep concentration, and evaporation-crystallization processes. These not only facilitate extensive water reuse but also generate large volumes of mixed solid salts as waste [19].

Waste salts from coal chemical processes are generally complex mixtures primarily composed of NaCl and Na₂SO₄ [27]. Recent studies employing source-tracing techniques have identified 47 distinct species in coal chemical salt-bearing wastewater, spanning both inorganic and organic compounds. As shown in Figure 2a, brine, crystallization mother liquor, and other salt-laden streams are the dominant sources of inorganic ions, while organic pollutants are chiefly introduced via process wastewater. Among the inorganic ions, Na⁺ and SO₄²⁻ exhibit the highest detection frequencies—75% and 79%, respectively—with concentrations reaching up to 135,560 mg/L and 79,452 mg/L. This points to a sulfate-rich impurity environment, especially in high-salinity streams derived from wet oxidation, sulfur recovery, and other sulfur-intensive units. Calcium (Ca²⁺) and magnesium (Mg²⁺) ions are also prevalent (>60% detection frequency), potentially complicating crystallization and contributing to scaling during evaporation. On the organic side, phenol is the most prominent contaminant, detected in 11% of samples at concentrations up to 5500 mg/L, reflecting residual aromatics from gasification and synthesis reactions.

In contrast to pharmaceutical and pesticide waste salts, those from coal chemical processes are typically inorganic-dominant with low organic content (TOC ≤10,000 mg/L) [28], thanks to effective upstream biodegradation and oxidation. Consequently, thermal treatment is less emphasized, and research attention has shifted toward the separation, purification, and classification of inorganic salts. A composition-based classification framework is thus more suitable than one based on organic load, and can be broadly categorized into:

• Sulfate-dominant mixed salts, where Na₂SO₄ is the major phase;
• Chloride-dominant mixed salts, primarily composed of NaCl;
• High-impurity mixed salts, containing heavy metals or toxic residues requiring special handling.

In essence, the core challenge lies in inorganic impurity removal and phase-specific salt separation [29]. Advanced techniques such as nanofiltration have been employed to selectively separate monovalent and divalent ions, enhancing salt purity while mitigating fouling and energy consumption [30]. Moreover, process optimization strategies have shown promising results—for instance, controlling the Cl⁻/SO₄²⁻ ratio in feedwater streams has been demonstrated to significantly improve selective crystallization efficiency, enabling the more effective separation of sodium sulfate and sodium chloride fractions [31].

Despite these advances, several challenges persist. Many waste salts contain fine particulates, scaling precursors, or exhibit strong hygroscopicity, all of which complicate handling and reuse. Furthermore, standardized classification criteria for regulatory purposes remain underdeveloped. Future efforts should focus on the pre-separation of critical ions, the development of scalable impurity removal pathways, and the use of process simulation tools to optimize salt recovery and valorization.

Figure 2. (a) Water quality analysis of saltwater with different sources from the coal chemical industry (authors’ own work, data source: Ref. [32]). (b) The main components of waste liquid after washing incineration fly ash (authors’ own work, data source: Ref. [33]). (c) Process flow diagram for the washing of incineration fly ash. (d) Summary of IWS sources, commonalities, distinctive features, and a generalized pretreatment and purification route.

Figure 2. (a) Water quality analysis of saltwater with different sources from the coal chemical industry (authors’ own work, data source: Ref. [32]). (b) The main components of waste liquid after washing incineration fly ash (authors’ own work, data source: Ref. [33]). (c) Process flow diagram for the washing of incineration fly ash. (d) Summary of IWS sources, commonalities, distinctive features, and a generalized pretreatment and purification route.

2.4. Municipal Solid Waste (MSW) Incineration—Fly Ash-Derived Salts

MSW incineration fly ash is a major secondary source of industrial waste salts in China, contributing over 50% of landfilled hazardous waste in 2023 [34]. In response, the Ministry of Ecology and Environment issued a 2025 directive promoting reduced landfilling and enhanced pretreatment for resource recovery. While fly ash itself is classified as hazardous waste, the focus of this study is on the salt-rich by-products generated during its treatment—particularly those produced through water washing, which accounts for a substantial portion of the fly ash mass.

Water washing—especially for cement kiln co-processing—has become the standard pretreatment method [35]. Chinese fly ash is particularly rich in chlorine, with total Cl sometimes exceeding 20% (Figure 2b). The salt fraction, comprising 20–30% of dry mass, mainly consists of chlorides of Na, K, and Ca, and contains various heavy metals such as Zn, Pb, Cu, Cr, and Ni [33]. These chlorides are poorly immobilized by conventional solidification methods and can compromise the mechanical stability of treated products.

To address these issues, multi-stage washing—typically a three-stage countercurrent process—is employed to reduce the chloride content to below 1%, resulting in a high-salinity brine known as fly ash washing effluent (Figure 2c) [36]. Compared to conventional industrial waste salts, fly ash-derived salts are characterized by their urban origin, high halide levels, and potential heavy metal contamination. Organic micropollutants such as dioxins (PCDD/Fs) are generally not detected in the brine due to their low solubility and strong affinity for solid matrices [37]. Thus, the primary environmental concerns in salt recovery processes are associated with heavy metal ions and alkaline impurities.

The management of fly ash washing brine presents both disposal and resource recovery challenges. If not properly treated, it may lead to secondary pollution. Current treatment strategies typically involve the chemical precipitation of heavy metals followed by evaporative crystallization. Studies have demonstrated that the combined use of inorganic (e.g., Na₂S) and commercial organic precipitants (e.g., MT103) can achieve over 99% removal efficiency for heavy metals [38]. The subsequent crystallization yields sodium and potassium chloride products with purities exceeding 95%.

In this context, the selective recovery of Na⁺ and K⁺ salts from washing brine represents the final step in the resource valorization of fly ash. It not only facilitates the high-value utilization of residual salts but also significantly mitigates the environmental risks associated with fly ash disposal.

2.5. Other Industries

In the petrochemical sector, IWSs mainly arise from pollutant neutralization and wastewater treatment, typically containing NaCl, Na₂SO₄, and Na₂CO₃, along with persistent organics (e.g., PAHs, benzene derivatives) and heavy metals. High salinity, toxic co-contaminants, and strong matrix heterogeneity complicate treatment. For instance, SO_x and NO_x scrubbing produces saline effluents with recalcitrant organics, overloading conventional systems. Targeted technologies such as wet oxidation, membrane separation, and selective crystallization have shown promise, underscoring the need for integrated management from source control to resource recovery.

In the dyeing industry, IWSs often derive from spent mother liquors and washing brines rich in dye intermediates and residual organics. Effluents typically exhibit salinity levels of 30–90 g·L⁻¹ and contain structurally diverse compounds—azo, anthraquinone, nitroaromatics—that resist removal by conventional crystallization or oxidation [39]. Sun et al. demonstrated that a combined process—incorporating filtration, freeze crystallization, electrodialysis, and evaporation—can yield high-purity NaCl and Na₂SO₄ from dyeing wastewater [40].

In the epoxy resin industry, IWSs emerge from epoxidation and neutralization stages, often accompanied by organic residues and unreacted raw materials [41]. These high-salinity streams contain corrosive by-products such as bisphenol A, glycerol, and aromatic solvents. Recovery processes involving neutralization, washing, filtration, and centrifugation have enabled the reuse of NaCl and NaOH within internal production loops, supporting a closed-loop resource model with TOC levels below 10⁻⁴ in recovered salts.

2.6. Summary of IWS Sources and Characteristics

In summary, while IWSs share similar core generation mechanisms—primarily arising from the neutralization of acidic or alkaline streams and subsequent treatment of high-salinity wastewater—their physicochemical characteristics diverge significantly across sectors. Although all IWSs contain mixtures of organic contaminants, inorganic ions, and metal species, the relative abundance and complexity of these constituents vary considerably depending on the industrial source. For instance, IWSs from pesticide and pharmaceutical industries are often dominated by persistent organic pollutants, whereas those from coal chemical processes and fly ash residues exhibit elevated concentrations of inorganic and metallic impurities, including complex mixed salts. These disparities directly shape the technical challenges and treatment strategies required (Tables S1 and S2).

To enable effective waste salt valorization, a structured and stepwise approach is indispensable—precisely as illustrated in Figure 2d. The process begins with identifying the industrial origin to inform the expected pollutant profile. This is followed by the targeted removal of organic matter, selective elimination of inorganic and metal ions, and finally, determination of whether the salt is composed of single or multiple species. Through such sequential refinement, IWSs can be progressively transformed into purified forms suitable for reuse—thus achieving the ultimate objective of resource-oriented waste salt management.

This also underscores the necessity of a graded, source-specific strategy to ensure precise control throughout the entire life cycle of waste salt—from pollutant characterization to resource recovery.

3. Technologies for IWS Treatment and Resource Utilization: A Pollutant-Oriented Perspective

IWS treatment technologies at home and abroad primarily aim to reduce environmental risks through pollutant removal or solidification, followed by either harmless disposal (e.g., incineration, landfill) or resource utilization. Among these, resource utilization involves secondary treatment to reprocess salt as industrial feedstock or additives, contributing to a circular economy. Traditional reviews typically categorize treatment methods by technology types—thermal treatment, oxidation, washing, precipitation, and membrane separation (Figure 3a). However, this approach may overlook the critical influence of pollutant characteristics on treatment selection. In practice, the selection of technical routes is largely governed by the type and concentration of pollutants present, as different contaminants impose distinct removal mechanisms, operational constraints, and cost implications [42]—particularly organics, heavy metals, and soluble/inert inorganic salts. To better reflect this reality, this section reclassifies mainstream strategies based on pollutant characteristics, forming a more application-oriented and mechanism-driven framework. Based on Section 2, most IWSs consist of major salts (often mixed), organic pollutants, and inorganic impurities (including metals). Therefore, treatment is discussed in three categories: (1) removal of organic pollutants, (2) removal of inorganic impurities, and (3) salt separation and purification.

3.1. Removal of Organic Pollutants

Effective removal of organic pollutants is essential for the safe disposal and resource recovery of IWSs. These pollutants—often quantified by TOC (Total Organic Carbon)—include halogenated organics, aromatic hydrocarbons, and residual solvents. Their complex structures and strong interactions with the salt matrix (via adsorption, encapsulation, or bonding) make them difficult to eliminate.

This section highlights three representative treatment strategies widely adopted in practice: (1) thermal decomposition, (2) advanced oxidation processes (AOPs), and (3) biological treatment, typically as a pretreatment. Other methods—such as solvent extraction or adsorption—are also used under specific conditions but are not discussed here. Among these, thermal and oxidative approaches remain the most robust and versatile.

3.1.1. Thermal Treatment

Thermal treatment is a cornerstone technology for degrading organic pollutants in IWSs, particularly those with high TOC and refractory characteristics. Most organic contaminants decompose or volatilize at 200–500 °C [44]. Heating beyond these thresholds induces thermal cracking into smaller gas-phase molecules. Depending on the salt matrix and organic load, three main approaches are employed: incineration, pyrolysis, and high-temperature melting [5].

Incineration is suitable for IWSs with a high organic content (e.g., TOC > 100,000 mg/kg or >10 wt%) and calorific values exceeding 10,500 kJ/kg [22]. Pretreatment steps such as filtration and concentration are typically needed to prevent feed pipe blockage and improve combustion efficiency. Fluidized-bed incineration has achieved near-complete TOC removal at 750–780 °C [45]. However, challenges remain, including low fuel value, equipment corrosion, slagging, and the risk of dioxin formation from halogenated organics.

Pyrolysis refers to the thermal decomposition of organic pollutants in the absence or limitation of oxygen, typically at temperatures below the melting point of the IWS. This process transforms organics into volatile gases (e.g., CO, CH₄) or carbonaceous residues, making it especially effective for low-calorific, high-TOC waste salts such as those generated in the pesticide and pharmaceutical industries. Key operational parameters—including the salt composition, carrier gas atmosphere, heating rate, and residence time—substantially influence the efficiency of pyrolysis [46]. For instance, Li et al. investigated the thermal treatment of pesticide-derived waste salts and found that the mass-loss patterns under pyrolysis and combustion were similar [43] (Figure 3b), with oxygen presence accelerating the degradation process. Their findings also confirmed that such waste salts exhibit low calorific value and complex composition, making pyrolysis a suitable disposal route. Similarly, Zhao et al. examined the influence of pyrolysis atmosphere and duration, revealing that nitrogen-based atmospheres yielded slightly slower TOC removal than air, but above 500 °C, both heating time and gas atmosphere had a minimal effect on organic degradation. Under oxygen-free conditions at ≥500 °C and treatment times under 20 min, organic removal efficiencies reached 96% or higher [47].

According to the National Advanced Pollution Control Technology Catalog (Solid Waste and Soil Pollution Control Sector, 2023) [48], pyrolysis is particularly recommended for waste salts with TOC levels exceeding 5000 mg/kg, where low-temperature pyrolysis can achieve organic removal rates above 99%. These results underscore pyrolysis as a reliable and high-efficiency option for TOC-rich waste salts, provided that operational parameters are carefully optimized to prevent secondary pollution, such as dioxin formation [49].

High-temperature melting treats IWSs at 800–1200 °C, exceeding their melting points to achieve full liquefaction and the complete thermal destruction of organics. This prevents sintering and adhesion to furnace linings seen in low-temperature processes while enhancing product purity. The method is suitable for complex, low-calorific salts with high organic loads. Dong et al. applied this technique at 850–900 °C, achieving effective organic removal [50]; Flue gas was further treated in a secondary combustion chamber. Given its high energy demand, integration of waste heat recovery is essential for improving energy efficiency and cost-effectiveness.

Melting behavior varies with salt composition, requiring case-specific process tuning [51]. Compared with pyrolysis, high-temperature melting ensures more thorough organic degradation and tolerates broader feedstock variability. However, its drawbacks include elevated energy consumption, large flue gas volumes, and significant salt particle entrainment, which can hinder downstream resource recovery. As summarized in

Table 1. Comparative analysis of thermal treatment technologies for industrial waste salts (IWSs).

	Incineration	Pyrolysis	High-Temperature Melting
Applicable conditions a	TOC > 100,000 mg/kg (or >10 wt%)	TOC > 5000 mg/kg	Complex waste salts with high organic loading
Key parameters & TOC removal rate b	▪ 873 K (~600 °C), 30 min → TOC removal ~93% [52]; ▪ 750–780 °C → near-complete TOC removal (fluidized bed) [45]	▪ 500 °C, 20 min → TOC removal ~96% [47] ▪ 550 °C, 0.5 h → TOC removal ~99% [41]	Temperature 600–650 °C → TOC removal rate 98.5% [53]
Energy consumption c	High energy demand, requiring auxiliary fuel supply	Moderate energy input	Very high (energy-intensive operation)
Environmental risks	Risk of dioxin/furan formation under high chlorine conditions; flue gas treatment required	Low dioxin formation (anaerobic conditions); risk of toxic gas release (e.g., HCN) under incomplete pyrolysis	Potential formation of flue gas-borne pollutants (e.g., dioxins); risk of secondary pollution; strict gas treatment required
Advantages	Significant volume reduction; effective organic destruction; applicable to complex waste compositions; reduced secondary contamination	Mature and industrially proven; energy recovery from organics; high thermal efficiency; moderate energy demand; resistance to salt caking; broad applicability	High organic removal efficiency; high-purity recrystallized salts; low sensitivity to feed composition and morphology
Limitations	High capital and maintenance costs; corrosion risk; complex flue gas treatment; high auxiliary fuel demand	Temperature constraints (below salt melting point); variable process stability; limited removal of refractory organics (e.g., polymerized/coked species); potential incomplete detoxification	High energy consumption; lower thermal efficiency; risk of secondary pollution from flue gas; high equipment requirements; corrosion and slagging/agglomeration issues; limited large-scale application

Notes: ^a Applicable conditions represent general guidelines; actual applicability depends on impurity composition, salt matrix, and reuse requirements; ^b Key parameters and TOC removal efficiencies are derived from representative studies; performance varies with operating conditions (e.g., temperature, residence time, atmosphere), and no universal values apply; ^c Energy consumption is expressed qualitatively (e.g., High/Medium/Very high) due to significant variability across feedstocks, scales, and energy sources; reported quantitative values are case-specific and not directly generalizable.

, the three thermal treatment technologies exhibit distinct trade-offs in terms of TOC removal efficiency, energy demand, environmental risk, and engineering feasibility. Incineration is suitable for high-TOC waste salts but involves high energy consumption and emission control requirements. Pyrolysis offers a more balanced performance, while high-temperature melting provides superior stabilization and product purity at the expense of significantly higher energy input.

3.1.2. Biological Treatment

Compared to the pesticide and pharmaceutical industries—where thermal treatment is more suitable due to high TOC levels—low-TOC waste salts from sectors such as coal chemical production are more amenable to advanced oxidation, biological pretreatment, or solvent extraction, considering both technical feasibility and economic viability.

Biological treatment serves as an effective pretreatment strategy for biodegradable organic contaminants. It relies on microbial metabolism, including assimilation and mineralization, to reduce organic loads [54]. Although high salinity poses a challenge, recent advances in halotolerant strain selection and process optimization (e.g., sludge retention time control) have expanded its applicability [55]. In engineering practice, the feasibility of biological treatment is evaluated based on factors such as the BOD₅ (5-day Biochemical Oxygen Demand)/COD (Chemical Oxygen Demand) (ratio with >0.3 indicating good biodegradability, reference HJ 576-2010) [56], salinity (<10% for aerobic systems), toxicity, and cost. Aerobic processes (e.g., BAF (Biological Aerated Filter), SBR (Sequencing Batch Reactor)) are suitable for low-salinity waste streams, while anaerobic systems (e.g., UASB (Up-flow Anaerobic Sludge Blanket), AF (Anaerobic Filter)) can handle higher-strength wastewaters, offering the added benefit of biogas recovery. In many cases, combined anaerobic–aerobic configurations are adopted to enhance COD, nitrogen, and phosphorus removal [57]. As a low-cost and environmentally friendly option, biological treatment plays an important role in front-end conditioning prior to thermal or oxidative processes.

According to the typical characteristics summarized in Section 2, many IWS streams are characterized by high salinity (often exceeding 10%) and low biodegradability (BOD₅/COD < 0.3). As a result, only a limited fraction of IWSs is inherently suitable for direct biological treatment. Nevertheless, biological processes can still play an important role as a supplementary pretreatment for low-salinity and readily biodegradable fractions, thereby justifying their inclusion as a mainstream auxiliary strategy.

3.1.3. Advanced Oxidation Processes (AOPs)

Advanced oxidation processes (AOPs) represent one of the most widely adopted and versatile approaches for degrading persistent organic pollutants, particularly in moderate- to high-TOC waste salt systems. These processes employ oxidants (e.g., H₂O₂, O₃), catalysts (e.g., Fe²⁺, TiO₂, Ni–Cu alloy), and external energy sources (e.g., UV irradiation, electric current) to generate reactive oxygen species (ROS), such as hydroxyl radicals (·OH) and superoxide anions (O₂·⁻). These ROS initiate rapid and non-selective oxidation, ultimately mineralizing organics into CO₂, water, and inorganic salts (Figure 3c). Among these species, ·OH exhibits a redox potential of 2.8 V—significantly higher than ozone (2.07 V)—making it especially effective in breaking down stable and recalcitrant organics under advanced oxidation conditions [58]. Major AOP techniques applied in waste salt treatment include Fenton oxidation, ozonation, and electrochemical oxidation, wet air oxidation (WAO), and supercritical water oxidation (SCWO).

However, AOP performance is severely compromised in high-chloride industrial waste brines (IWSs) and concentrated brines. Chloride ions act as strong hydroxyl radical scavengers, drastically reducing oxidation efficiency and lowering the TOC removal rates [59]. Additionally, chloride-mediated reactions promote the formation of toxic chlorinated by-products (CBPs), representing a critical practical limitation for Fenton and ozonation systems. Notably, electrochemical oxidation is less inhibited because chloride can be converted in situ to reactive chlorine species (RCS) that contribute to organic degradation [2].

Fenton oxidation relies on the Fe²⁺-catalyzed decomposition of hydrogen peroxide (H₂O₂) under acidic conditions to generate hydroxyl radicals (·OH), which rapidly degrade organic pollutants [60]. This method is particularly effective for low-salinity and highly toxic waste salts, though it inevitably generates iron-containing sludge. For example, Ateş et al. reported that the treatment of dyeing wastewater using Fenton reagents (2.7 g/L H₂O₂, 0.5 g/L FeSO₄, pH 3, 65 °C, 30 min) achieved 93% COD and 69% color removal, highlighting its applicability to high-toxicity, low-salt effluents [61]. To overcome the limitations of conventional Fenton systems—particularly sludge generation and operational complexity—recent advancements have focused on [62]: (1) enhancing Fe³⁺/Fe²⁺ redox cycling to improve electron transfer efficiency; (2) optimizing reaction conditions (e.g., pH, H₂O₂/Fe²⁺ ratio, temperature) to maximize ·OH yield; and (3) substituting soluble iron salts with heterogeneous catalysts (e.g., Fe₃O₄@SiO₂, biochar-supported iron oxides) to reduce secondary pollution and improve catalyst recyclability [63].

These innovations collectively improve the feasibility and sustainability of Fenton oxidation for large-scale industrial applications.

Ozonation removes organic pollutants through two principal mechanisms: (1) direct electrophilic attack on electron-rich functional groups (e.g., C=C bonds, aromatic rings) and (2) indirect oxidation via reactive species such as hydroxyl radicals (·OH) and singlet oxygen (¹O₂), generated during ozone decomposition [64]. This dual-mode action makes ozonation particularly effective for degrading refractory organics with unsaturated structures. Efficiency can be further enhanced by integrating hydrogen peroxide to form the peroxone process, which promotes ·OH generation. For example, Wang and Zhang reported that treating coking wastewater at pH 8.15 for 40 min achieved a COD removal efficiency of 48.15% [65]. In another study, a combined ozonation–air flotation-enhanced electrocoagulation process achieved 89% COD removal within 15 min for coal chemical wastewater [66].

In engineering applications, ozonation performance depends on parameters such as catalyst type and dosage, ozone concentration, reaction time, and reactor configuration. Optimizing these factors is essential for maximizing degradation efficiency while minimizing energy consumption and operating costs [67].

Electrochemical oxidation degrades pollutants via two pathways: (1) direct oxidation, where organic molecules lose electrons at the anode surface, and (2) indirect oxidation, involving the in situ generation of reactive species (e.g., ·OH) through electrochemical reactions [68]. Electroactive anodes not only catalyze oxidation but also enhance adsorption and electron transfer. Meanwhile, cathodic reduction can facilitate the dechlorination of organohalogens, reducing their toxicity. Key operational parameters—such as current density, electrolyte concentration, and pH—critically influence process efficiency and energy consumption [69]. Unlike chemical oxidation, electrochemical systems typically produce no secondary pollution, making them environmentally advantageous. Under optimized conditions (pH 6.06, 18.2 V, 23.5 min), Chavalparit and Ongwandee [70] achieved 55.4% COD and 96.6% SS (Suspended Solids) removal, highlighting the technology’s efficacy. Integration with membrane technologies further enhances removal performance by enabling the simultaneous elimination of organic matter, heavy metals, and colloidal particles [71], offering a synergistic approach to complex waste salt treatment.

In addition to the mainstream AOPs discussed above, technologies such as supercritical water oxidation (SCWO) and wet air oxidation (WAO) have also demonstrated effectiveness for specific IWS streams. SCWO achieves near-complete organic destruction (>99.9%) at high temperatures and pressures, making it suitable for high-TOC, refractory waste salts, though its application is limited by high capital costs and corrosion challenges. WAO operates under milder conditions and is well-suited for moderate-TOC, high-salinity waste streams, with industrial deployments reported in the petrochemical and coal chemical sectors.

3.1.4. Organic Matter Removal Strategy

In summary, the choice of organic pollutant removal strategy in waste salt treatment is governed primarily by the TOC level, biodegradability, and salinity of the matrix (

Table 2. Comparative analysis of advanced oxidation processes (AOPs).

Technology	Reaction Conditions & Removal Efficiency	Energy Consumption	Advantages	Limitations
Fenton/Fenton-like	FeSO4 = 5.0 × 103 mg/L, H2O2, 2 h; TOC removal = 28.70% [72]	Electricity = 96.12 kWh/kg TOC	Well-established, simple operation; Moderate energy input for basic configurations	Iron sludge production and disposal burden; Potential formation of toxic chlorinated by-products
	FeSO4·7H2O = 0.15 mmol/L, CoPc-modified electrode; TOC removal = 89.9% [73]	Energy = 1013 kWh/kg organic matter
	pH = 3, 4 bar, H2O2 = 100 mg/L, H2O2: FeSO4= 1:4; TOC removal = 76% [74]	—
	H2O2 694.7 mg/L, Fe2 + = 67.3 mg/L, pH = 3, COD removal 79.6%, TOC removal 73.2% [75]	Electric energy per order EE/O (oxidant) = 6.97 kWh/m3
Ozonation	O3 = 5.00 g/h, 8 h; TOC removal = 6.20% [72]	Electricity = 611.05 kWh/kg TOC	No chemical sludge generation	Low TOC mineralization efficiency in non-optimized configurations
	O3 = 1.95 g/h, pH = 3–5, 5 bar; TOC removal = 96% [76]	—
Electrooxidation (EO)	Ru-Ir/Ti-EO (83 mA/cm2, 8 h); COD = 92.57%, TOC = 78.76% [77]	—	Excellent compatibility with high-Cl− matrices	Electrode fouling and degradation over time
	Electricity = 100.80 Wh, 2 h; TOC removal = 11.30% [72]	Electricity = 82.03 kWh/kg TOC
Wet air oxidation (WAO)	O2 = 160.00 g/h, 2 h; TOC removal = 49.30% [72]	Electricity = 364.55 kWh/kg TOC	Minimal sludge production; Compatible with neutral pH operation	High pressure and temperature requirements; material corrosion
	pH = 8.5, 150 °C, 3 MPa, 24 h; COD removal 62%, TOC removal 37% [78]	—
Supercritical Water Oxidation (SCWO)	23 MPa, 600–700 °C; TOC removal 81%, COD removal 74% [79]	—	Near-complete mineralization of refractory organics	Extreme operating conditions; Prohibitive capital and operational costs

Note: Energy consumption and operational costs are highly dependent on the specific wastewater matrix, organic load, and treatment scale. To ensure comparability, data from the same literature source or pilot-scale studies were prioritized where possible. A dash (—) indicates that the specific metric was not reported in the cited reference or is not applicable under the described conditions.

). Solvent extraction offers a selective pre-treatment option for recovering hydrophobic organic contaminants from high-salinity matrices, particularly in pesticide and pharmaceutical waste salts, before subsequent destruction or resource recovery. Thermal decomposition—including incineration, pyrolysis, and high-temperature melting—is favored for TOC-rich waste salts, particularly those containing refractory compounds or hazardous organics. Biological treatment, although constrained by salinity, offers an energy-efficient and environmentally benign option when paired with halotolerant strains and optimized operation. Advanced oxidation processes (AOPs) serve as a flexible and powerful toolkit, capable of degrading a wide range of persistent organics across salinity levels, especially in moderate- to high-TOC systems. However, issues such as sludge generation (Fenton), ozone utilization efficiency, and electrode cost (electrochemical oxidation) remain as practical bottlenecks. Future advancements are expected to focus on hybrid processes, catalyst recovery, and improved energy integration, enabling more robust and cost-effective front-end conditioning for downstream resource utilization.

3.2. Removal of Inorganic Impurities

Inorganic impurities in IWSs refer to non-target components other than the major salts (e.g., NaCl, Na₂SO₄), such as toxic anions (F⁻, NO₃⁻), hardness ions (Ca²⁺, Mg²⁺), and heavy metals (e.g., Fe³⁺, Pb²⁺, Cd²⁺). Their removal is essential for improving salt purity and enabling downstream resource recovery. Common techniques include precipitation, adsorption, membrane separation, and electrochemical methods.

3.2.1. Precipitation Method

The precipitation method is a widely applied technique that removes specific inorganic contaminants through chemical reactions forming insoluble precipitates. By introducing appropriate reagents, target ions react to form compounds with low solubility products (K_sp), which can then be separated via sedimentation, filtration, flotation, or centrifugation [80]. This method is particularly effective for the removal of heavy metals, hardness ions, and fluoride. Its performance is closely linked to wastewater composition and operating conditions such as pH and reagent type. For instance, hydroxide precipitation—commonly using lime or caustic soda—relies on precise pH control to achieve optimal removal efficiency [81]. As shown in Figure 3d, Fe³⁺ can be almost completely precipitated at pH > 9. However, metals like Ag⁺ and Pb²⁺ are poorly removed by hydroxide precipitation alone and are more effectively treated via sulfide precipitation. Ye et al. achieved a Pb removal rate of 98.5% from bioleaching solutions of lead-zinc tailings using sodium sulfide as the precipitant [82]. Similarly, Yao et al. reported 99.65% arsenic removal from acidic wastewater via sulfide precipitation, followed by the hydrothermal stabilization of the precipitate as amorphous As₂S₃ [83].

Selective precipitation based on ion-specific chemistry is also increasingly adopted. In treating high-fluoride wastewater from the fluorine chemical industry, Chen et al. applied a pre-precipitation strategy using calcium chloride and flocculants [84]. This process effectively removed fluoride prior to evaporative crystallization, enabling the selective recovery of NaCl and Na₂SO₄ and significantly reducing the complexity and cost of subsequent treatment steps. Precipitation also plays a role in resource recovery from IWSs. Zhao developed a hydrolysis–neutralization–recrystallization route for chloride-rich residues from TiO₂ production [85]. After alkaline treatment and precipitation, the clarified solution was reused for salt recovery. In another case, Zou et al. treated phosphate-rich pharmaceutical intermediate salts via coprecipitation, recovering hydroxyapatite as a high-value product while enabling NaCl reuse through subsequent crystallization [86]. This dual recovery approach exemplifies a circular strategy that integrates waste minimization with material valorization.

Overall, precipitation remains a robust and cost-effective method for removing diverse inorganic impurities from IWSs. While its applicability depends on specific waste characteristics and may generate secondary sludge, its adaptability and potential for by-product recovery make it a valuable component of sustainable waste salt treatment systems.

3.2.2. Ion Exchange Method

Ion exchange is a highly effective technique for the selective removal of trace inorganic impurities from waste salts, particularly for low-concentration or targeted contaminants such as heavy metals, nitrate, and fluoride. This method is based on reversible ion exchange reactions between target ions in the liquid phase and functional groups on a solid-phase resin, typically composed of synthetic organic polymers.

Cation exchange resins, containing sulfonic acid groups (–SO₃H), exchange hydrogen or sodium ions for positively charged contaminants (e.g., Ni²⁺, Cu²⁺, Pb²⁺), while anion exchange resins, bearing quaternary ammonium groups (–NR₃⁺), exchange chloride or hydroxide ions for anionic species such as NO₃⁻ or F⁻. Through this mechanism, ion exchange achieves the deep purification of salt solutions with high selectivity.

The technique has been widely demonstrated in the removal of heavy metals from industrial wastewater. For example, acidic cation exchange resins have been employed to remove Ni²⁺ [87]; Purolite C100 has shown effectiveness in extracting Pb²⁺; INDION 225H resin exhibited strong selectivity for Cu²⁺; and strong cation exchange resins (AMBERJET 1200 Na) were used for the simultaneous removal of Ni²⁺ and Pb²⁺ [88]. More advanced applications involve chelating resins and selective anion exchangers tailored for specific ions in complex multi-ion systems. Such specificity is especially valuable for the removal of toxic trace metals or for meeting stringent impurity thresholds in salt reuse scenarios [89]. However, the economic viability of ion exchange is closely linked to resin cost, regeneration efficiency, the treatment and disposal of regenerant effluents, and the value of the recovered salt. Due to its high operational cost and limited capacity, ion exchange is typically employed as a polishing step following bulk impurity removal methods such as precipitation. It is also frequently coupled with membrane separation or crystallization processes to maximize recovery efficiency and product quality in integrated waste salt treatment systems.

3.2.3. Other Methods

In addition to precipitation and ion exchange, techniques such as adsorption and electrochemical methods also play important roles in the removal of inorganic impurities from IWSs. These approaches are often applied under specific conditions or integrated with primary treatment processes to enhance overall performance. Adsorption typically utilizes porous materials—such as activated carbon, activated alumina, biochar, or metal oxides—for the targeted removal of heavy metals. Electrochemical techniques, including electrodeposition, enable selective metal recovery by tuning current density, pH, and electrode potential [90].

Moreover, electrodialysis (ED) and electrodialysis reversal (EDR) play dual roles in this framework: they are utilized for the precise removal of multivalent inorganic impurities and also facilitate the concentration and separation of monovalent/divalent salts, enhancing the overall purity of the recovered crystalline products.

The choice of the optimal strategy for impurity removal fundamentally depends on the physicochemical properties, concentration, and speciation of the target pollutants, the composition and ionic strength of the salt matrix, and the desired purity level of the recovered salts. Each method presents distinct advantages and limitations. In practice, coupled or hybrid systems are increasingly adopted to achieve specific removal goals. For example, Guan et al. employed an electrocatalytic oxidation-electrodeposition process to recover nickel from ammonia-nickel complex wastewater [91], while Zhang demonstrated the recovery of copper from electroplating effluent via ion exchange coupled with electrodeposition [92]. Such integrated approaches not only enhance removal efficiency but also offer greater flexibility in adapting to the diverse characteristics of IWS streams.

3.3. Salt Separation and Recovery

Following the removal of organic contaminants and inorganic impurities, IWSs typically exist as concentrated brines dominated by mixed ions (e.g., Cl⁻/SO₄²⁻, Na⁺/K⁺). Salt separation is a critical step to transform these complex solutions into purified, marketable salt products such as NaCl, Na₂SO₄, or KCl. This process relies on the precise control of crystallization pathways or selective ion transport to overcome thermodynamic constraints and ionic interference [93]. This section focuses on two representative strategies: crystallization-based and membrane-assisted salt separation.

Crystallization is a widely applied method that utilizes differences in solubility and phase equilibrium to sequentially recover target salts. Major operational modes include evaporative crystallization and cooling crystallization. For instance, in the NaCl–KCl–H₂O ternary system, Li et al. employed a mechanical vapor recompression (MVR) process to separate salts through evaporative crystallization [94]. By controlling the outlet concentration of a falling-film evaporator to ~22% and reducing the brine temperature to 40 °C, high-purity KCl crystals were obtained. In another case, Jiao et al. combined MVR and TVR (thermal vapor recompression) with freeze crystallization to treat reverse osmosis concentrate from industrial effluents, producing Na₂SO₄ and NaCl crystals with purities of 99.09% and 98.52%, respectively [95]. For freeze crystallization in the NaCl–Na₂SO₄–H₂O system, the selective crystallization of Na₂SO₄·10H₂O occurs at 0 °C, where Na₂SO₄ solubility is 5.2 g/100 g H₂O while NaCl solubility remains 35.7 g/100 g H₂O [96]. Effective separation can also be achieved at ≤−2 °C, with sulfate reduced from 74.3 g/L to 6.9 g/L without NaCl coprecipitation [97]. The process is efficient when NaCl < 80 wt%; above this threshold, evaporation becomes more favorable [96]. High NaCl concentrations depress the freezing point and hinder Na₂SO₄ crystallization [97].

Crystallization-based separation typically leverages both graded energy utilization and phase transition control. Evaporative crystallization is suitable for systems with high boiling-point elevation and allows for energy recovery, while cooling crystallization is more energy-efficient for brines with low solubility differences. However, organic contaminants tend to concentrate in the NaCl fraction, resulting in relatively lower product purity—a key limitation of this route.

To address this, membrane-assisted salt separation has emerged as a complementary strategy. Nanofiltration membranes operate based on size exclusion and electrostatic repulsion, allowing for the selective separation of monovalent and divalent ions. Nanofiltration membranes can effectively retain organics and divalent salts (e.g., SO₄²⁻) on one side, allowing for the recovery of high-purity Na₂SO₄ through freeze-dissolution-evaporation crystallization [98]. Monovalent salts (e.g., NaCl) pass into the permeate, from which high-grade NaCl can be recovered by evaporation. However, in hypersaline NaCl–Na₂SO₄ mixed systems, residual organic and inorganic scaling significantly accelerate membrane degradation and shorten service life. Even when operated properly, conventional nanofiltration and reverse osmosis cannot avoid salt co-crystallization, resulting in a product purity of only 50–80%, which fails to meet the industrial requirements [99]. Although hydrophobic PVDF and PP membranes in membrane-assisted crystallization can promote heterogeneous nucleation and reduce the induction time of NaCl crystals, membrane wetting and surface crystal deposition remain persistent challenges [100].

Thermal and membrane-based salt separation each present distinct advantages and limitations. Thermal processes are simple, robust, and cost-effective but may produce salts with lower purity. Membrane-assisted separation offers higher-purity NaCl and is particularly suited for NaCl-rich brines, though it requires higher capital and operational expenditures and may suffer from long-term fouling. In practical applications, hybrid systems—such as nanofiltration integrated with evaporative and cooling crystallization—are increasingly adopted to improve separation efficiency and product quality. Such integrated approaches are also recommended in technical standards for high-salinity wastewater treatment and represent the mainstream direction of waste salt valorization.

3.4. Pollutant-Oriented Treatment Framework

Overall, the selection of appropriate treatment technologies for IWSs is strongly governed by the type, concentration, and refractory nature of the target pollutants, as well as the desired purity of the final recovered salt (Table S3). To clarify this logic, a pollutant-oriented treatment framework is summarized in Figure 3e, which systematically maps organic and inorganic contaminants to suitable removal technologies and links fractionation strategies to targeted salt recovery objectives.

In the context of sustainability and environmental impact, limited but valuable life cycle assessment (LCA) studies have been reported for IWS treatment. For organic pollutant removal routes, pyrolysis exhibits a lower global warming potential (GWP) of 1144 kg CO₂ eq per ton of waste salt, whereas solvent elution generates a substantially higher carbon footprint of 4702 kg CO₂ eq, mainly due to organic solvent production and distillation energy consumption [101]. Meanwhile, among the advanced oxidation processes (AOPs), electrooxidation (EO) demonstrates the lowest environmental impact in terms of carbon emissions, ecotoxicity, and fossil resource scarcity, while ozone-based processes show relatively higher burdens [72]. Although salt recovery from IWSs can partially offset the environmental impacts of virgin salt production, the net environmental benefit is closely related to the treatment pathway, energy structure, and process efficiency. Residual trace contaminants in reclaimed salts may also bring potential long-term environmental risks in downstream applications, which have rarely been quantified in existing LCA investigations. Therefore, combined LCA-techno-economic analysis (TEA) for different treatment technologies represents a prominent research gap toward sustainable industrial waste salt valorization.

4. Resource Utilization of Treated IWS

Building on the preceding sections on impurity removal and salt separation, this section explores how the purified salt fractions can be reintegrated into industrial systems through targeted resource utilization strategies. Following the effective removal of organic and inorganic impurities and the successful separation of major salt constituents, IWS can be transformed into valuable secondary resources. The feasibility of their reuse depends not only on the purity and chemical composition of the reclaimed salts, but also on their alignment with the technical requirements and quality standards of downstream applications. This section focuses on the two predominant salt components in Chinese IWSs—sodium chloride (NaCl) and sodium sulfate (Na₂SO₄)—and provides a critical overview of their main utilization pathways, associated technical challenges, and current application potentials.

4.1. Utilization Pathways of Sodium Chloride (NaCl)

NaCl is a principal component of IWSs in China and represents a key target for resource recovery. When adequately purified, reclaimed NaCl can be reused across multiple non-food-grade applications, including deicing agents, cement additives, and dyeing auxiliaries. For instance, it can react with acetic acid to produce calcium magnesium acetate (CMA), or be co-crystallized with KCl for inorganic deicers. It has also been explored as a performance enhancer in concrete, facilitating the formation of calcium silicate hydrate (C-S-H) gel [102]. However, these low-value applications offer limited capacity for large-scale consumption and are often constrained by impurity tolerances and low product acceptance.

In contrast, ion-exchange membrane caustic soda production offers a high-value and high-volume pathway for NaCl reuse. In China, this sector accounts for over 50% of raw salt consumption, with annual caustic soda output reaching ~43 million tons in 2024 (Figure 4a) [103]. This presents a major opportunity for industrial-scale salt reuse. Policy incentives further reinforce this direction: according to the Industrial Structure Adjustment Guidance Catalogue (2024 edition) [104], new membrane electrolysis facilities are required to use brine containing at least 40% industrial waste salt to avoid classification as restricted or obsolete. Furthermore, diaphragm cell facilities are only permitted if they incorporate comprehensive waste salt reuse.

Nevertheless, using reclaimed waste salt instead of virgin NaCl introduces unique technical barriers that are rarely encountered with pure raw salt. These include variable TOC levels, unstable feed supply, seasonal fluctuations, and inconsistent ionic composition, all of which threaten stable operation. In particular, long-term exposure to complex impurities—including heavy metals, residual organics, and fluctuating ratios of Ca²⁺/Mg²⁺/SO₄²⁻—can accelerate cation-exchange membrane (CEM) fouling, increase cell voltage, reduce current efficiency, and shorten membrane service life.

Ion membrane electrolysis splits purified brine into NaOH, Cl₂, and H₂ through the selective transport of Na⁺ and Cl⁻ ions (Figure 4b). The process is highly sensitive to brine purity, with current specifications requiring total organic carbon (TOC) below 10 mg/L and minimal concentrations of divalent cations and heavy metals [105]. Impurities such as Mg²⁺ and Ca²⁺ can form insoluble carbonates (e.g., MgCO₃, CaCO₃) that block ion transport, while organics may adsorb onto ion-exchange membranes, causing fouling and performance degradation [106]. For instance, molecular weight and functional group chemistry have been shown to significantly affect membrane resistance and electrochemical efficiency [107]. Recent studies demonstrate the feasibility of using reclaimed NaCl in membrane electrolysis when stringent pretreatment is applied. Yin et al. implemented a two-stage refining process to treat vanadium–titanium waste salt, achieving stable electrolysis over 20 months and treating over 6000 tons of salt [108]. The process was later scaled to accommodate 40,000 tons annually from upstream sources including MDI and PPS by-product salts [109]. Other case studies have reported consistent electrolysis performance using wet oxidation–pretreated epoxy resin salt (TOC < 10 mg/L) and thermally treated SPI salt (TOC < 3.5 mg/L), with current efficiency exceeding 93.5% and negligible cell voltage fluctuation over two years of operation [110].

To fully enable NaCl resource utilization via membrane electrolysis, further advances are needed in deep impurity removal technologies, membrane fouling resistance, and the establishment of tiered quality grading systems for reclaimed salts. Developing application-specific reuse standards will also be essential to facilitate broader market acceptance and ensure the safe and effective deployment of recycled salts in high-value processes.

4.2. Utilization Pathways of Sodium Sulfate (Na₂SO₄)

Following impurity removal and salt fractionation, reclaimed sodium sulfate typically exists in the form of anhydrous Na₂SO₄ or decahydrate (Na₂SO₄·10H₂O). Its utilization potential depends on purity, residual contaminants, and compliance with application-specific standards. Compared to NaCl, sodium sulfate exhibits a distinct application profile, being widely used as a functional additive in industries such as textiles, glass, cement, and chemical processing. In the textile sector, Na₂SO₄ acts as a leveling and exhausting agent in reactive dyeing processes by increasing the ionic strength of dye baths, promoting dye-fiber binding [111]. In glass manufacturing, it serves as a refining agent and flux, promoting bubble removal via SO₂/O₂ gas release, stabilizing Fe oxidation states, and lowering melting temperatures. However, impurities in reclaimed sulfate (e.g., organics or insoluble particles) may hinder glass clarity and strength, limiting direct reuse unless stringent purification is ensured. In addition, Na₂SO₄ also finds niche applications in cement as a grinding aid, enhancing early compressive strength by accelerating hydration kinetics [112]. In advanced material research, it has been employed in synthesizing α-hemihydrate gypsum gel materials using agricultural by-product sulfates, demonstrating potential in sustainable construction materials [113].

Despite these pathways, the overall market for sodium sulfate remains limited in scale, with major consumption focused on Glauber’s salt production and sulfur-based chemical intermediates. To expand its utility, value-added chemical conversion routes have gained attention. One promising route involves the reaction of Na₂SO₄ with ammonium bicarbonate to produce sodium carbonate (Na₂CO₃) and ammonium sulfate ((NH₄)₂SO₄). Na₂CO₃, a key feedstock in glassmaking and food additives, precipitates due to low solubility and can be recovered by the thermal decomposition of intermediate bicarbonates. Simultaneously, (NH₄)₂SO₄—an important nitrogen fertilizer—can be crystallized from the mother liquor via evaporation (Figure 4d). Experimental studies have demonstrated this approach’s feasibility. Liu et al. reported that under optimized conditions (Na₂SO₄ concentration ~330 g/L, molar ratio 1:1, reaction time 1 h), the process achieved ~90% conversion to Na₂CO₃ and ~85% yield of (NH₄)₂SO₄, offering a viable dual-product valorization path [114]. Another emerging pathway involves bipolar membrane electrodialysis (BMED), enabling the electrochemical conversion of Na₂SO₄ solutions into H₂SO₄ and NaOH. Joo et al. achieved a conversion rate of 98.1% from 20 wt% Na₂SO₄ waste brine, generating sulfuric acid (96.1% purity) and caustic soda (99.6%) at current efficiencies exceeding 80% [115]. Recent advances in BMED-5 (five-compartment bipolar membrane electrodialysis metathesis) enable the simultaneous production of HCl, NaOH, and (NH₄)₂SO₄ within an integrated system, significantly improving resource reclamation efficiency from sulfate-laden industrial effluents (Figure 4e) [116].

Overall, while Na₂SO₄ appears less economically attractive than NaCl for large-scale reuse, these chemical transformation routes and niche industrial applications represent promising directions for its high-value utilization. The success of these pathways hinges on achieving consistent product quality, removing inhibitory impurities, and developing application-specific reuse standards to expand market acceptance.

Furthermore, as these salts originate from waste sources, it is crucial to prevent their direct entry into food-related applications to minimize public health risks. This calls for stricter regulatory oversight and the establishment of clear contaminant thresholds to ensure safe and responsible reuse.

5. Standards and Technical Requirements for Reuse of Reclaimed IWS

The practical reuse of reclaimed waste salts not only requires technical feasibility but also strict compliance with environmental regulations and rigorous product quality standards. In China, recent years have seen a clear regulatory shift: from basic hazardous waste control toward promoting safe, large-scale reuse of reclaimed IWSs.

At the national level, IWSs have been identified as priority targets in policies such as the Reform Plan for Hazardous Waste Management Capacity (2021) [117] and the Major Hazardous Waste Infrastructure Plan (2023–2025) [118]. These frameworks emphasize centralized treatment, risk control, and the development of reuse pathways, particularly for low-value but high-volume wastes such as salts. Technical guidance documents—including the Environmental Management Guidelines for Hazardous Waste salts and the Recycling Pollution Control Technical Standards—further define requirements for storage, handling, and reuse, aiming to minimize secondary contamination, ensure traceability throughout the recycling chain, and establish a controllable pathway for safe and circular reuse.

More critically, safe reuse hinges on adherence to product quality standards. While traditional industrial salt standards (e.g., GB/T 5462-2015 for NaCl; GB/T 6009-2014 for Na₂SO₄) [119,120] were developed for mined or evaporated salts, they do not adequately address the trace-level organic and inorganic contaminants typically present in reclaimed salts, which can compromise downstream processing or product integrity. To address this gap, China has developed a series of application-specific standards for reclaimed salts (e.g., T/ZGZS 0302-2023, T/ZGZS 0303-2023) [121,122]. These standards incorporate dual constraints: high-purity requirements for target components (NaCl ≥ 97.5%, Na₂SO₄ ≥ 97%) and stringent limits on impurity indicators such as Ca²⁺, Mg²⁺, TOC, and heavy metals, tailored to downstream use in industries like chlor-alkali production or textile dyeing.

As illustrated in

Table 3. Comparison of key quality indicators in industrial salt, reclaimed salt, and specialized application.

Standards Name	Graded Specifications	Basic Indicators	Extended Indicators
General Industrial Salt Standard
Industrial salt GB/T 5462-2015 [119]	Industrial Dried Salt (Grade 2)	NaCl ≥ 97.50; Moisture Content ≤ 0.80; Water Insolubles ≤ 0.20; Ca2++Mg2+ ≤ 0.60; SO42− ≤ 0.90	—
Anhydrous sodium sulfate for industrial use GB/T 6009-2014 [120]	Type II Qualified Product	Na2SO4 ≥ 97.0; Moisture Content ≤ 1.0; Water Insolubles ≤ 0.20; Ca2++Mg2+ ≤ 0.40; Cl− ≤ 0.90; Fe ≤ 0.004	—
Cross-Industry Reclaimed Salt Standards
Reclaimed industrial salt-Sodium chloride T/ZGZS 0302-2023 [121]	Industrial Dried Salt	NaCl ≥ 97.50; Moisture Content ≤ 0.80; Water Insolubles ≤ 0.20; Ca2++Mg2+ ≤ 0.60; SO42− ≤ 0.90	pH 6–9; TOC ≤ 8.0 mg/L; NH3-N ≤ 1.0 mg/L; Total Phosphorus ≤ 0.2 mg/L; F− ≤ 1.0 mg/L; CN− ≤ 0.2 mg/L; Volatile Phenols ≤ 0.005 mg/L; Cu ≤ 1.0 mg/L; Zn ≤ 1.0 mg/L; Se ≤ 0.01 mg/L; As ≤ 0.05 mg/L; etc.
Reclaimed industrial salt-Sodium sulfate T/ZGZS 0303-2023 [122]	Industrial Sodium Sulfate	Na2SO4 ≥ 98; Moisture Content ≤ 0.5; Water Insolubles ≤ 0.10; Ca2++Mg2+ ≤ 0.30; Cl− ≤ 0.70; Fe ≤ 0.010; Whiteness (R457) ≥ 82	pH 6–9; TOC ≤ 8.0 mg/L; NH3-N ≤ 1.0 mg/L; Total Phosphorus ≤ 0.2 mg/L; F− ≤ 1.0 mg/L; CN− ≤ 0.2 mg/L; Volatile Phenols ≤ 0.005 mg/L; Cu ≤ 1.0 mg/L; Zn ≤ 1.0 mg/L; Se ≤ 0.01 mg/L; As ≤ 0.05 mg/L; etc.
Industry-specific Reclaimed Salt Standard
Glyphosate by-product industrial salt Part 1: Sodium chloride HG/T 5531.1-2019 [125]	—	NaCl ≥ 94.0; Moisture Content ≤ 5.50; Water Insolubles ≤ 0.30;	Glyphosate (C3H8NO5P) ≤ 0.05 w/%; Dimglyphosate (C5H10NO7P) ≤ 0.1 w/%; Total Phosphorus ≤ 0.15 w/%; TOC ≤ 0.03 w/%
Dicamba by-product industrial salt of sodium chloride T/CAPDA 058-2023 [126]	—	NaCl ≥ 98.50; Moisture Content ≤ 0.50; Water Insolubles ≤ 0.10; Ca2+ ≤ 0.15; Mg2+ ≤ 0.10; SO42− ≤ 0.50	Dicamba ≤ 0.05%; 2,5-Dichlorophenol ≤ 0.01%; 3,6-Dichlorosalicylic Acid ≤ 0.01%; TOC ≤ 0.01%
Coal chemical industry—By-product industrial sodium chloride T/CCT 002-2019 [127]	Industrial Dried Salt (Grade 2)	NaCl ≥ 97.5; Moisture Content ≤ 0.8; Water Insolubles ≤ 0.20; Ca2++Mg2+ ≤ 0.60; SO42− ≤ 0.90; Whiteness (R457) ≥ 67	TOC ≤ 40 mg/kg
Coal chemical industry—By-product industrial sodium sulfate T/CCT 001-2019 [128]	Type A Qualified Product	Na2SO4 ≥ 97.0; Moisture Content ≤ 1.0; Water Insolubles ≤ 0.20; Ca2++Mg2+ ≤ 0.40; Cl− ≤ 0.90; Fe ≤ 0.04	TOC ≤ 50 mg/kg
Industrial sodium chloride produced by epoxy resin T/CPCIF 0068-2020 [123]	Type II	NaCl ≥ 93.3; Moisture Content ≤ 4; Water Insolubles ≤ 0.20; Whiteness (R457) ≥ 60	TOC ≤ 800 mg/kg; pH 7.0–10.0; Epichlorohydrin ≤ 30 mg/kg; Toluene ≤ 1.0 mg/kg
Sector-Specific Application Standards
Salt for ion-exchange membrane caustic soda QB/T 5270-2018 [124]	Refined Dry Salt	NaCl ≥ 98.5; Moisture Content ≤ 0.3; Water Insolubles ≤ 0.10; Ca2+ ≤ 0.15; Mg2+ ≤ 0.10; SO42− ≤ 0.30;	I ≤ 2.0 mg/kg; Ba ≤ 15 mg/kg; Fe ≤ 2 mg/kg; NH4+ ≤ 4 mg/kg; Potassium Ferrocyanide ≤ 2 mg/kg
Salt for printing and dyeing QB/T 4890-2015 [129]	—	(NaCl + Na2SO4) ≥ 98.0; Moisture Content ≤ 0.8; Water Insolubles ≤ 0.20; Ca2++Mg2+ ≤ 0.30;	Fe ≤ 50mg/kg; I ≤ 5.0 mg/kg; Potassium Ferrocyanide ≤ 10 mg/kg
Salt for water treatment QB/T 5685-2022 [130]	Environmental Grade (Class 1)	NaCl ≥ 98.5; Moisture Content ≤ 1.0; Water Insolubles ≤ 0.10; Ca2++Mg2+ ≤ 0.06; SO42− ≤ 0.50;	I ≤ 5.0 mg/kg; Potassium Ferrocyanide ≤ 10 mg/kg

Notes: GB/T, HG/T, QB/T, and association standards are recommended standards. HJ-series standards are implemented as mandatory requirements only when cited by national laws or mandatory national standards. No dedicated mandatory national standard (GB) for industrial waste salt is currently available.

, industry-specific waste salt standards establish specific limit values for characteristic pollutants based on the compositional characteristics of the salts and their intended industrial applications. For instance, the standard for Industrial Sodium Chloride Produced from Epoxy Resin (T/CPCIF 0068-2020) [123] mandates strict limits on residual epichlorohydrin (≤30 mg/kg) and toluene (≤1.0 mg/kg)—key impurities originating from the epoxy resin production process. Similarly, Salt for Ion-Exchange Membrane Caustic Soda (QB/T 5270-2018) [124] imposes a tight constraint on ammonium ions (NH₄⁺ ≤ 4 mg/kg), primarily due to the safety risk of ammonia nitrogen reacting with chlorine during electrolysis to form explosive trichloramine (NCl₃). These regulatory requirements directly reflect the need to mitigate process hazards and ensure product quality in specialized industrial scenarios.

In this context, achieving the practical reuse of reclaimed salts—especially in high-value or sensitive industrial applications—demands not only restoring compositional purity but also tailoring impurity control strategies to meet stringent, application-specific quality standards (Tables S4 and S5). This reinforces the need for both robust refining technologies and clear regulatory frameworks to support the safe, scalable, and compliant reuse of waste-derived salts. Furthermore, establishing enforceable reuse boundaries—especially to prevent entry into food or pharmaceutical domains—is essential from a risk governance perspective.

6. Conclusions and Future Perspectives

Industrial waste salts constitute a growing challenge in hazardous waste management, especially in large-scale chemical economies such as China. Their heterogeneity—reflected in diverse organic pollutants, inorganic impurities, and salt matrices—calls for pollutant-oriented, sector-specific strategies. This review highlights that effective treatment hinges on salt composition: organic-laden salts necessitate thermal or oxidative degradation; inorganic-rich salts benefit from targeted precipitation, ion exchange, or membrane separation; and mixed-salt systems require integrated crystallization or fractionation to enable reuse.

The valorization of reclaimed salts, particularly NaCl and Na₂SO₄, is technically viable and policy-supported but limited by impurity thresholds and application-specific standards. High-value pathways—such as membrane caustic soda production and chemical transformation into soda ash or ammonium sulfate—offer scalability, contingent on stringent impurity control and grading protocols. The emergence of sector-specific quality benchmarks signals a shift toward performance-based regulation, yet broader market acceptance and cross-sector harmonization remain underdeveloped.

Sustainable IWS management must be built upon four pillars: (i) upstream classification based on source and composition; (ii) modular detoxification guided by pollutant profiles; (iii) precision recovery technologies with adaptive impurity control; and (iv) enforceable regulatory frameworks balancing safety, circularity, and economic viability. Crucially, reclaimed salts must be restricted from entering food or pharmaceutical chains to safeguard public health. Bridging technical innovation with policy reform will be key to positioning IWSs as strategic secondary resources in a circular economy.

Research Methodology

This review systematically summarizes and analyzes the research progress on industrial waste salt (IWS) treatment and resource utilization. The literature collection and screening process followed a structured review approach to ensure the comprehensiveness, reliability, and timeliness of the cited studies. Relevant publications were retrieved from major international and Chinese databases, including Web of Science, Scopus, ScienceDirect, Google Scholar, and SpringerLink for English-language literature, as well as China National Knowledge Infrastructure (CNKI), Wan fang Database, and VIP Database for Chinese literature. The search primarily focused on studies published in the last 10–15 years, while earlier seminal works were included where necessary to provide essential background. The final reference selection reflects a balance between foundational studies, recent technological developments, and policy-related research, with particular emphasis on literature providing quantitative performance data, process comparisons, or sector-specific analyses.

The literature selection was based on the following criteria. First, priority was given to peer-reviewed journal articles, authoritative review papers, and technical reports issued by governmental agencies and industry associations. Second, studies with clear experimental design, complete technical parameters, and measurable performance indicators (e.g., impurity removal efficiency, salt purity, and energy consumption) were preferentially included. Third, for similar research topics, recent publications (within the past 5–7 years) were prioritized to reflect the current state of technological development, while highly cited classical studies with fundamental significance were retained to ensure continuity of the research framework. Finally, studies with insufficient relevance, inconsistent research scope, or incomplete technical information were excluded to maintain the overall quality and consistency of the review.