A Review on the Preparation of Catalysts Using Red Mud Resources

Abstract

The production of alumina produces red mud (RM), a highly alkaline solid waste. The majority of it is disposed of in landfills, which seriously pollutes the environment. It needs to be recycled and handled with care to protect the environment. RM is a promising raw material for wastewater and waste gas treatment owing to its high alkalinity and abundant metal compounds. It can efficiently remove diverse pollutants while facilitating large-scale utilization of RM resources. Reviews of the use of RM resources to create catalysts for environmental governance are, nevertheless, scarce. Therefore, this paper analyzes and summarizes the pertinent research on RM-based catalysts to remove pollutants from the environment based on journal literature related to RM resource utilization from 2015 to 2025. This study reviews the application of RM-based catalysts for degrading pollutants in wastewater and exhaust gases via advanced oxidation processes (AOPs)—including photocatalysis, Fenton-like catalysis, ozonation catalysis, and persulfate catalysis—as well as catalytic oxidation, chemical looping combustion (CLC), and selective catalytic reduction (SCR). The paper emphasizes the analysis of modification strategies and catalytic mechanisms of RM-based catalysts in environmental remediation and examines the environmental risks and corresponding mitigation measures related to their preparation from RM resources. Finally, it outlines that future research should prioritize green, low-energy modification processes; catalytic systems for the synergistic removal of multiple pollutants; and efficient, recyclable separation and recovery technologies. These directions aim to promote the sustainable application of RM in large-scale environmental remediation and to achieve the integrated advancement of resource utilization and ecological protection.

1. Introduction

The solid waste known as RM is produced during the Bayer process of making alumina. There are many valuable metal elements in it. It has seriously contaminated the environment because of its high alkalinity, pH of 10–13, fine particles, and trace levels of radioactive and heavy metals [1]. According to reports, 1–5 tons of RM are produced for every ton of alumina produced [2]. Currently, there are an estimated 4 billion tons of accumulated inventory, and the annual global production of RM can reach 175.5 million tons [3]. RM is classified as a highly alkaline industrial solid waste, and its applications face numerous restrictions. Therefore, a comprehensive analysis of its chemical composition and mineralogical characteristics is essential. RM exhibits high plasticity, characterized by large particle size, high density, and considerable porosity [4,5]. As a complex solid waste, RM’s chemical and mineralogical compositions vary with the bauxite source and the alumina production process. Table 1 presents a comparison of RM samples from different mining regions. Chemical analysis indicates that its primary components are Fe₂O₃, Al₂O₃, SiO₂, TiO₂, Na₂O, and CaO [6]. Although the overall chemical composition remains stable, the proportions vary among RM samples. Given its favorable properties, RM should be regarded not merely as waste but as a valuable material rich in metallic constituents [7]. However, RM is currently managed mainly through temporary storage, resulting in land resource wastage and soil pollution [1]. During storage, severe environmental problems may occur: strongly alkaline leachate mixing with rainwater can contaminate groundwater and surface water to varying degrees; in addition, wind can disperse fine RM particles, worsening air pollution [8]. Furthermore, RM contains heavy metals such as V, Cd, and Cr, which may threaten soil, aquatic systems, and plants [9]. Consequently, researchers are actively exploring its potential applications to mitigate RM’s environmental impact.

Currently, RM is mainly applied in producing building materials, extracting precious metals, and environmental protection [10,11]. Among these, applications in the building materials and chemical industries are most prevalent, particularly in producing concrete [12], bricks [13], and ceramics [14]. Incorporating RM into concrete can enhance hydration and reduce setting time [15]. RM can also be used to produce geopolymer bricks with excellent mechanical strength and durability under harsh conditions [13]. As a construction material, RM can substantially reduce the need for extracting virgin raw materials. However, RM contains heavy metals and is strongly alkaline, and its products may leach toxic substances, posing environmental risks [16]. Additionally, RM is rich in metals such as aluminum, iron, titanium, and scandium, which can be recovered as high-value resources through various extraction processes [17,18]. However, challenges remain, including limited extraction technologies, high energy and cost demands, and environmental hazards from slag handling. More importantly, none of these methods can achieve large-scale resource utilization of RM [10]. Given its high metal content, porosity, large specific surface area, and pore-like structure, numerous studies have confirmed that RM—owing to its high alkalinity and abundant metal compounds—is a promising raw material for wastewater and exhaust gas treatment, capable of removing various pollutants while enabling large-scale resource utilization. In water treatment, RM is modified into composite catalysts to remove and adsorb organic compounds and heavy metals from wastewater [19,20]. In exhaust gas treatment, RM’s reactive, alkaline, and inert components enable high catalytic activity for degrading methane, volatile organic compounds, CO, and nitrogen oxides [21]. RM-based catalysts can effectively remove air pollutants and significantly reduce greenhouse gas emissions [22].

Previous reviews on RM-based catalysts have primarily addressed either the comprehensive utilization of RM resources (e.g., building materials, metal recovery) or individual catalytic applications, such as thermal catalytic oxidation of VOCs, adsorption, or photocatalysis. Environmental risk discussions are often brief or fragmented, with no dedicated sections for systematic assessment or targeted mitigation strategies. This review is based on the Web of Science database, using keywords such as “red mud,” “catalyst,” “wastewater,” and “exhaust gas,” and prioritizes English-language, peer-reviewed articles published between 2015 and 2025. It systematically summarizes recent advances in RM-based catalyst applications for various environmental remediation technologies, forming the core of this review. While earlier reviews typically addressed only 1–3 major catalytic applications, we propose a more comprehensive framework for RM-based catalyst use in environmental remediation, summarizing progress across seven major catalytic systems: (1) AOPs—photocatalysis, Fenton-like reactions, ozone catalysis, and persulfate activation (Section 3.1, Section 3.2, Section 3.3 and Section 3.4). (2) Thermal catalytic oxidation—catalytic combustion of volatile organic compounds (VOCs) (Section 3.5). (3) Chemical looping combustion (CLC)—clean combustion of fuels, such as methane, with concurrent CO₂ capture (Section 3.6). (4) Selective catalytic reduction (SCR)—removal of nitrogen oxides (NO_x) (Section 3.7). In addition, a dedicated chapter (Section 4) addresses “Environmental Risks and Mitigation Measures for Red Mud Catalyst Preparation.” This chapter provides an in-depth analysis of RM’s environmental hazards, including high alkalinity, potential leaching of heavy metals or radioactive elements, and dust emissions, as well as the challenges these pose during catalyst preparation and application. It systematically proposes and discusses targeted “pretreatment-followed-by-utilization” strategies along with specific mitigation measures. This paper provides a detailed discussion on the design, modification, and environmental remediation applications of RM-based catalysts, along with associated environmental risks, aiming to offer theoretical insights and technical support for RM resource utilization and pollution control.

Table 1. Composition of RM in different regions (wt%).

RM Source	Main Chemical Composition (wt%)						Ref.
	Fe2O3	Al2O3	CaO	SiO2	TiO2	Na2O
Stade, Lower Saxony (Germany)	35.3	15.7	6.7	14.0	11.4	-	[23]
Xi’an, China	40.91	19.55	0.59	17.99	5.35	6.24	[24]
Fine waste particles (<0.1 mm) of RM	42.6	22.5	-	14	7	6.6	[25]
LuBei Chemical Industry Co	37.16	25.13	0.09	33.13	3.55	0.82	[26]
National Aluminum Company (NALCO), India	29.47	27.74	-	24.53	18.2	-	[27]
Tan Rai Aluminum Factory, Lam Dong province, Vietnam	64.2	12.6	-	3.8	9.4	4.6	[28]
Shandong, China	52.59	25.47	1.24	7.57	5.91	5.38	[29]
Barcarena	31.45	35.47	1.81	12.68	5.84	-	[30]

Table 1. Composition of RM in different regions (wt%).

RM Source	Main Chemical Composition (wt%)						Ref.
	Fe2O3	Al2O3	CaO	SiO2	TiO2	Na2O
Stade, Lower Saxony (Germany)	35.3	15.7	6.7	14.0	11.4	-	[23]
Xi’an, China	40.91	19.55	0.59	17.99	5.35	6.24	[24]
Fine waste particles (<0.1 mm) of RM	42.6	22.5	-	14	7	6.6	[25]
LuBei Chemical Industry Co	37.16	25.13	0.09	33.13	3.55	0.82	[26]
National Aluminum Company (NALCO), India	29.47	27.74	-	24.53	18.2	-	[27]
Tan Rai Aluminum Factory, Lam Dong province, Vietnam	64.2	12.6	-	3.8	9.4	4.6	[28]
Shandong, China	52.59	25.47	1.24	7.57	5.91	5.38	[29]
Barcarena	31.45	35.47	1.81	12.68	5.84	-	[30]

2. Modification Methods for RM

In catalysis, RM can function as a carrier, co-catalyst, additive, or be directly employed as a bulk catalyst by utilizing the specific metal and metal oxide components within its structure as active sites. Accordingly, selectively tailoring its composition and increasing its specific surface area are critical. Furthermore, before RM is used as a bulk catalyst or support, its properties should be modified via pretreatment. These modification techniques primarily aim to increase surface area and remove impurities, thereby transforming the parent material into a more effective catalyst. The following sections outline several of these modification methods.

(1) Acid treatment

Acid treatment is the most widely used method for modifying RM. In this process, acids such as hydrochloric, nitric, and sulfuric acid are typically employed to activate RM [31]. Key parameters, including acid concentration and solid-liquid ratio, play a decisive role in determining the properties of the final product. Recent studies consistently indicate that acid concentration is the dominant factor affecting RM performance, surpassing the influence of other parameters. Acid concentration produces markedly different effects on treated RM. For example, Wang et al. [32] applied acid leaching and precipitation to RM, producing materials suitable as NH₃-SCR catalysts. Systematic studies revealed that at acid concentrations below 3.1 mol⋅L⁻¹, the acidic environment markedly increases the leaching rates of Fe, Ti, Al, Ca, and Na, while promoting interactions between Fe and Ti species in RM-based catalysts. At 3.1 mol⋅L⁻¹, more Fe and Ti are leached, and Ti can be incorporated into the Fe₂O₃ phase, reducing its grain size. The resulting catalyst exhibits superior low-temperature activity and enhanced SO₂ resistance. Notably, the 3.1 mol⋅L⁻¹ catalyst showed the strongest reducing ability, the highest surface chemisorbed oxygen content, and moderate surface acidity. Acid pretreatment markedly alters the structural characteristics of RM. Acids modify RM mainly by leaching Ca, Si, and Al, thereby generating surface pores. This process increases the specific surface area and average pore size of the modified RM. However, a larger surface area does not necessarily improve adsorption capacity, as excessive acidification may remove active components that promote pollutant adsorption. Fundamentally, adsorption capacity is determined by the number of effective adsorption sites. Although acid modification can improve RM performance and broaden its applications, it also entails environmental impacts, high energy and cost demands, and low reusability [33]. When using acid-washing waste liquid as an acid source, the effects of its complex components on RM must be evaluated.

(2) Heat treatment

Heat treatment is a fundamental approach for preparing RM for catalytic applications, typically involving high-temperature calcination and drying. Its effectiveness primarily depends on two key parameters: temperature and duration. The optimal calcination temperature and duration should be determined according to the specific target pollutant. For instance, Yu et al. [34] prepared a series of RM catalysts at varying calcination temperatures using a simple method for the catalytic treatment of biodiesel wastewater. Studies indicate that calcination increases the specific surface area of RM catalysts, thereby providing more active sites. When the calcination temperature is ≤400 °C, the catalyst develops a well-defined porous structure conducive to adsorption and diffusion. Most Fe₂O₃ exists in an amorphous form, while crystalline α-Fe₂O₃ shows greater dispersion compared with catalysts calcined at other temperatures. The catalyst surface also contains abundant Fe³⁺ and metal-oxygen species, which lower the reduction temperature and enhance reducibility. When the calcination temperature exceeds 400 °C, sintering occurs, increasing the α-Fe₂O₃ grain size. This change rapidly decreases the specific surface area while increasing mechanical strength, and also reduces hydroxyl group dispersion. Conventional heat treatment is typically conducted in static air to remove impurities that could contaminate active metal sites, as well as physically and chemically bound water. This process also increases the material’s specific surface area. Heat treatment is often combined with acid modification. High-temperature calcination enhances RM stability by evaporating and decomposing organic matter and moisture. Studies have shown that thermal treatment can alter the RM structure, for example, converting Fe₂O₃ into Fe₃O₄ and Fe⁰, thereby improving adsorption performance and facilitating recovery [35]. Although high-temperature calcination improves RM properties, it also presents drawbacks including high energy consumption, environmental pollution, resource waste, and substantial equipment demands. Therefore, selecting an RM treatment method requires comprehensive consideration of multiple factors to achieve a more sustainable and environmentally friendly solution.

(3) Other modification methods

Besides acid and heat treatments, RM can also be modified by ball milling. During ball milling, kinetic energy from moving balls breaks chemical bonds between molecules, leading to particle size reduction. However, prolonged ball milling may cause particle agglomeration [36]. Key factors affecting particle size include ball-to-sample ratio, grinding chamber volume, grinding speed, and mass ratio [37]. Other modification methods include ultrasonic treatment [38] and hydrogenation [39]. Novel approaches involve adding various materials to RM [40] or utilizing RM to synthesize new materials, such as zeolites [41]. Beyond single activation methods, combined activation processes have been explored in recent studies. These combined processes integrate multiple activation steps, such as acidification and calcination, to improve the physicochemical properties of activated sludge [42]. Compared with single activation, combined methods capitalize on the advantages of each step, thereby enhancing RM’s adsorption performance.

3. Catalytic Applications of RM in Environmental Remediation

3.1. Photocatalysis

An eco-friendly technique called photocatalytic organic matter degradation uses light energy to power catalysts, which in turn cause redox reactions and transform organic pollutants into innocuous compounds (like CO₂ and H₂O) [43]. While the abundant Fe₂O₃, TiO₂, ZnO, and other components in RM provide redox capabilities, its fundamental mechanism is based on the photoelectric effect of semiconductor materials like graphite carbon nitride (g-C₃N₄) and TiO₂ [44]. In order to improve the photocatalytic activity of organic pollutant degradation, semiconductor materials can be further introduced through composite modification, which can increase the range of light absorption, effectively reduce agglomeration, and provide more active sites [45]. As an illustration, Chegeni et al. [46] calcined RM-modified g-C₃N₄ to create a composite material (RM-CN). The findings demonstrated that the increase in surface area, which successfully inhibited electron-hole recombination and eliminated rhodamine B (RhB) entirely in nearly two hours, was the cause of the high activity of RM-CN under visible light. In light of this, some research has effectively removed several organic materials at once using RM-CN. Compared to pure g-C₃N₄, RM addition improves RM-CN’s adsorption capacity, broadens its visible-light photoresponse, and grants dual functions of adsorption and photocatalysis under visible light [47]. Additionally, Wang et al. [48] synthesized a novel Z-type heterojunction RM-CN composite via a one-step calcination method. Comprehensive characterization confirmed the formation of a Z-type heterojunction between g-C₃N₄ and α-Fe₂O₃. This significantly enhanced spatial separation of photogenerated carriers, enabling efficient formaldehyde degradation. After 2 h of illumination, degradation efficiency reached 63.04%, and the composite showed excellent recyclability. This study is the first to apply Z-type heterojunctions composed of RM and non-metallic semiconductors for degrading air pollutants like formaldehyde, establishing a foundation for future RM-based photocatalysts. However, RM/g-C₃N₄ composites generally suffer from low RM content, costly raw materials, and limited photocatalytic efficiency in degrading organic pollutants such as antibiotics. TiO₂ is a promising material for RM-based composites due to its large-scale RM consumption, visible-light responsiveness, and cost-effectiveness. Meng et al. [49] prepared a highly stable TiO₂/RM composite via hydrolysis-calcination to study its effect on photocatalytic degradation of the antibiotic chloramphenicol B. At a TiO₂/RM mass ratio of 0.5, the composite achieved up to 80.3% removal efficiency within 2 h. Its high performance results from a narrow bandgap (2.03 eV), matching RM’s and lower than TiO₂’s, which significantly enhances visible-light absorption. Electrochemical analysis showed lower impedance and higher photocurrent density, confirming enhanced separation efficiency of photogenerated charges.

Based on experimental and theoretical studies of representative composite photocatalytic systems, including RM/g-C₃N₄ and RM/TiO₂, a schematic diagram of the charge transfer mechanism in RM/semiconductor composite photocatalysts was constructed (Figure 1). This diagram integrates the reported band structures, charge-carrier migration pathways, and radical generation mechanisms [44,45,47,49]. More specifically, when active components like α-Fe₂O₃ in RM and semiconductor materials are composited with RM, they form a heterojunction that causes the Fermi levels of the two to be rebalanced. When exposed to visible light, the heterojunction structure causes holes to collect in the RM valence band (VB) and electrons to move from the RM conduction band (CB) to the semiconductor conduction band. Adsorbed organic pollutants can be oxidized by holes on the α-Fe₂O₃ valence band, while Fe³⁺ ions in the solution can be reduced by all of the electrons accumulated on the semiconductor CB. The RM/semiconductor composite materials’ photocatalytic activity is greatly increased by this synergistic effect. Furthermore, it is possible to prepare magnetic components like Fe₂O₃ in RM as magnetic catalysts that have both catalytic activity and magnetic responsiveness. These catalysts can be rapidly separated and recovered when exposed to a magnetic field, greatly increasing catalytic efficiency and lowering separation costs [50]. Liu et al. [51] optimized reaction conditions to incorporate carbon quantum dots (CQDs), derived from waste rice noodles (WRN), into Fe³⁺-containing RM leachate via hydrothermal reaction. This process not only facilitated the transformation of Fe³⁺ ions into the magnetic γ-Fe₂O₃ phase, but also allowed CQDs to attach to the γ-Fe₂O₃ surface, forming heterojunctions. Coupling CQDs with γ-Fe₂O₃ enhances electron-hole separation and suppresses recombination, thereby producing abundant ·OH and O₂⁻ radicals that efficiently photodegrade dyes such as methylene blue. Additionally, the composite preserves the magnetic properties of γ-Fe₂O₃, achieving over 98% recovery and 96% photocatalytic efficiency after 10 cycles. This composite meets industry demands for cost-effective and recyclable photocatalysts, highlighting its significant commercial potential. Moreover, RM, rich in Al₂O₃ and SiO₂, can be transformed into photocatalyst carriers, including zeolite molecular sieves [52] and ceramic substrates with tunable porosity [44]. Loading photocatalysts onto these carriers enables efficient removal of organic pollutants, including phenols and dyes. For instance, Hellna et al. [52] synthesized RM-based ZSM-5 using the hydrothermal method and subsequently prepared ZnO/ZSM-5 composites via wet impregnation. ZnO did not disrupt the ZSM-5 framework, causing only physical interactions, but led to a slight reduction in surface area. ZnO significantly enhanced MB decolorization, with the optimal effect achieved at a 34% w/w loading.

3.2. Fenton-like Oxidation

The high redox potential of the highly active ·OH generated by H₂O₂ makes the Fenton process very promising for the breakdown and mineralization of organic pollutants. For instance, Chen et al.’s [53] novel acid-modified RM Fenton catalyst (RM4) has a high phenol removal efficiency of up to 96.8% under ideal circumstances and is abundant in pore structures and functional groups. Due to its abundance of components like Fe₂O₃ and FeOOH, RM has been used extensively in Fenton-like catalytic removal of organic pollutants in recent years. One of the simplest to make of them is biochar based on iron. In a Fenton-like process, Chen et al. [54] efficiently degraded phenol using a Fe-based catalyst (FRM/2%A) treated with activated carbon and H₂O₂. The ferrous polymetallic oxides, their mesoscopic particles, and the microcellular structures that formed on the catalyst surface were responsible for the study’s findings, which demonstrated that under ideal circumstances, the degradation rate could reach 99.3% in just 5 min. Furthermore, RM-doped metals used as electrodes in the electro-Fenton (EF) system have demonstrated special benefits in the management of heavy metal complex pollution. As an illustration, Wang et al. [55] constructed a three-dimensional electro-Fenton (3D-EF) system using nickel-doped RM (Ni-RM) as a particle electrode to decompose Ni-EDTA. According to the findings, the Ni-RM-based 3D-EF system’s decomposition efficiency was substantially higher than the RM electrode’s 56.3%, reaching 97.8% in 120 min. Research has demonstrated that by catalytically activating H₂O₂ produced at the cathode, the Ni-RM electrode efficiently encourages the formation of reactive oxygen species (such as ¹O₂ and ·OH). Some researchers have also investigated the removal effect of EF on organic pollutants in wastewater using RM doped with metal oxides as electrodes. In a 3D-EF system, Lu et al. [56] employed CuO-doped RM (CuO/URM) as particle electrodes to break down ciprofloxacin (CIP). Under ideal circumstances, the CIP degradation efficiency increased to 66.80% after 80 min of treatment. Cl⁻ significantly accelerated the breakdown of CIP, whereas NO₃⁻, HCO₃⁻, and H₂PO₄⁻ significantly slowed it down, according to the study.

Conventional Fenton catalysts require an acidic environment (pH < 3) to function and typically employ iron salts or iron oxides as active ingredients. They also generate a significant amount of iron sludge. Fe²⁺ and light work together as a synergistic catalytic effect in RM to effectively decrease Fe²⁺ consumption and iron sludge production, significantly increase the generation of ·OH, and improve the oxidative degradation efficiency of organic pollutants. Of these, RM can be combined with semiconductor materials to increase the photo-Fenton catalytic effect. For instance, An et al. [57] employed a composite material of acid-modified graphene oxide (rGO) (RM-H/rGO) as a photo-Fenton catalyst to eliminate RhB dye. By reducing the impurity content in red mud through acid leaching and calcination treatment, α-Fe₂O₃ became active for the breakdown of H₂O₂. The composite material with 1% rGO had the highest activity, according to experiments, removing up to 99.8% of the material in 20 min at pH = 3. In addition to preventing charge carrier recombination, rGO modification can quicken the Fe³⁺/Fe²⁺ cycle in the heterogeneous Fenton process. Fe-based biochar’s high specific surface area facilitates pollutant adsorption, and its oxygen-containing functional groups and Fe constituents can activate H₂O₂ to generate reactive oxygen species (ROS). Li et al. [58] created a zero-valent iron-based biochar composite (RMBC) by co-pyrolyzing RM and Maotai distiller’s grains (DGs) in a single step. After five consecutive TC degradations, the material remained stable and demonstrated excellent degradation performance for TC, RhB, MB, and acid orange 7 (AO7) under visible light when the pyrolysis temperature reached 900 °C. Fe⁰, a crucial element in the photo-Fenton process, is much more active than Fe₃O₄ and Fe₂O₃. XRD analysis revealed that DGs totally reduced the iron oxides in RM to Fe⁰. The primary ROS responsible for TC degradation were O₂⁻, ·OH, h⁺, and ¹O₂. Furthermore, S-type heterojunction materials have been extensively researched in the field of organic pollutant degradation because they are thought to offer notable benefits in facilitating the separation of photogenerated carriers and demonstrating exceptional redox ability. He et al. [59] created a novel RM-based heterojunction catalyst (Fe/Co-Al-LDH/RM) using a straightforward mechanochemical synthesis method (MCS) that is made of Fe₂O₃ and cobalt–aluminum-layered double hydroxide (Co-Al-LDH). The degradation of gatifloxacin (GAT) in a visible light-driven photo-Fenton system is significantly impacted by this material, and under ideal circumstances, the removal rate reaches 94% after 120 min of reaction. Co(IV) and Fe(IV) are the primary active species in weakly acidic environments, whereas h⁺ and ·OH are dominant in strongly acidic environments, according to the study.

Table 2. Application and operating conditions of RM-based catalysts in Fenton-like catalysis.

Catalyst	Fenton Type	Pollutant and Concentration	Reaction Conditions	Removal Rate	Ref.
RM4	Traditional Fenton	Phenol; 100 mg/L	0.02 M H2O2; 0.05 g RM4; 120 min	96.8%	[53]
FRM/2%A	Traditional Fenton	Phenol; 100 mg/L	1 g/L FRM/2%A; 5 mM H2O2; initial pH = 3–6	99.3%	[54]
Ni-RM	Electro-Fenton	Ni-EDTA; 0.1 mM	pH range: 3.3–11; current density: 10–30 mA cm−2	97.8%	[55]
CuO/URM	Electro-Fenton	CIP; 0.1 g/L	4 g/3 LCuO/URM; pH = 7; Applied voltage: 10 V; aeration intensity: 5 L/min	80.66%	[56]
RM-H/rGO	Photo-Fenton	RhB; 10–50mg/L	pH = 3.0; 10 mM H2O2; 1g/L RM-H/rGO; 20 min	99.8%	[57]
RMBC	Photo-Fenton	TC, RhB, MB, AO7; -	Pyrolysis temperature of 900 °C; under visible light conditions	91.6%, 94.5%, 77.2%, 94.2%	[58]
Fe/Co-Al-LDH/RM	Photo-Fenton	GAT; 20 mg/L	120 min; 0.03 g/L Fe/Co-Al-LDH/RM; 90 mmol/L H2O2; pH = 6.5	94.0%	[59]

summarizes the application of RM-based catalysts in Fenton-like catalysis along with their operating parameters.

3.3. Ozonation

Strong oxidizing agents like ozone (O₃) break down organic pollutants by destroying their molecular structures. The breakdown of ozone can be catalyzed by metal oxides like Fe₂O₃ and MnO₂ in RM to produce ·OH with potent oxidizing capabilities, which can efficiently break down organic pollutants. As an illustration, Huang et al. [45] demonstrated the high efficiency and stability of RM by using it directly as a catalyst to catalyze the ozone oxidation degradation of ciprofloxacin (CIP) in wastewater. The system enhanced the removal rates of CIP and TOC by 44.8% and 21.6%, respectively, in comparison to the single ozone system. The hydroxyl groups and oxygen vacancies on the RM surface function as active sites to break down ozone and produce reactive oxygen species, while the Fe^2+/Fe³⁺ redox pair encourages the formation of oxygen vacancies. In addition to efficiently mineralizing CIP, the RM/O₃ system drastically lowers the toxicity of the intermediates and offers a fresh approach to reducing antibiotic pollution. RM’s ozone catalytic activity against organic pollutants can be further increased by appropriately loading metals onto its surface. As an illustration, Sun et al. [60] created a Co-Ce@RM ozone catalyst for tetracycline (TCN) degradation using RM as the carrier. Following five hours of calcination at 400 °C, the removal rate of TCN from Co-Ce@RM with a 1:3 Co and Ce doping ratio could reach 87–91 percent. To describe the catalyst, the researchers employed a range of analytical methods. The optimal pollutant and chemical oxygen demand removal rates in the Co-Ce@RM catalytic ozone reaction were 94.17% and 75.27%, respectively, when the ozone aeration rate was set at 0.4 L/min, the catalyst loading was set at 9 percent, and the pH of the solution was set at 9. Experiments using free radical quenching also demonstrated that the primary active groups responsible for TCN degradation were O₂⁻ and ¹O₂. The ozone catalytic activity can also be increased by loading metals on RM as metal oxides. As an illustration, Wang et al. prepared and optimized an Mn/Ce@RM catalyst for effective catalytic ozonation degradation of coal chemical biochemical tailwater by doping and calcining RM [61]. The pore structure of Ce@RM was greatly enhanced when Mn and Ce were loaded onto the catalyst as MnO₂ and CeO₂. The optimized Mn/Ce@RM catalyst’s COD removal rate under ideal preparation conditions was 84.96%, and after 25 reuses, it was still able to maintain a removal rate of 72.72%. According to the degradation mechanism study, adsorption and direct ozone oxidation are supporting processes in the breakdown of organic matter, whereas ·OH is the primary player. In addition to increasing ozonation catalysis’s catalytic efficiency, activating RM with biochar can efficiently mineralize organic matter in organic wastewater and lessen its toxicity. Zhang and associates [62] by merely pyrolyzing RM with peanut shells, created a stable and effective peanut shell-RM catalyst that can remove 98% of the levofloxacin (LEV) in 50 min. This catalyst exhibits significant promise in the area of environmental governance and offers the benefits of high efficiency, stability, and multifunctionality.

The catalytic efficiency of ozonation catalysis can also be increased by hydrogenation modification of the primary active ingredients in RM, such as Fe₂O₃ and TiO₂. For instance, Yan et al. [39] applied simple hydrogenation and thermal modification to RM to produce a catalyst (H-RM) for catalytic ozonation, achieving efficient degradation of LEV. According to the experimental findings, H-RM’s catalytic activity in LEV degradation was noticeably superior to RM’s, and the degradation rate could surpass 90% in just 50 min. Simple hydrogenation modification promotes the formation of Fe₃O₄, significantly enhancing the catalytic activity of RM [39]. Owing to its high alkalinity and catalytic efficiency, the dealkalization step can be omitted. Therefore, H-RM serves as an effective ozone catalyst for degrading organic wastewater. According to safety tests, the H-RM catalyst’s total Cr(VI) concentration was lower, and the aqueous solution’s leaching concentration of water-soluble Cr(VI) was likewise low. Mechanism experiments demonstrated that the oxidation effect was enhanced by a significant increase in the concentrations of dissolved O₃ and ·OH. One important factor in the breakdown of LEV is ·OH. Building on previous research, the regulatory patterns of active sites under different modification strategies were summarized and, together with kinetic studies and free-radical quenching experiments, elucidated the formation pathways of key oxide species, thereby establishing a mechanistic framework, as illustrated in Figure 2 [39,45,60,61,62]. More specifically, metal oxides in RM (e.g., Fe₂O₃, MnO₂, and CeO₂), along with multivalent metals introduced via modification, facilitate ozone adsorption and decomposition through redox cycles and oxygen vacancy effects. This process generates highly reactive oxygen species, including ·OH, ·O₂, and O₂⁻, with ·OH playing the dominant role in pollutant degradation. Surface hydroxyl groups and oxygen vacancies synergistically enhance ozone activation, leading to efficient mineralization of organic pollutants. Various modification methods, including metal loading, biochar activation, hydrogenation, and ultrasonic enhancement, can increase specific surface area, the number of active sites, electron mobility, and gas-liquid mass transfer efficiency. These improvements significantly enhance catalytic activity and adaptability for practical wastewater treatment. H-RM also maintains high activity, is recyclable, and has good catalytic stability. Tang et al. [38] synthesized H-RM ozone catalysts via ultrasonically enhanced hydrogenation of RM and integrated them with a ceramic membrane treatment system. Compared with unmodified RM, the hydrogenated RM displays agglomerated surface particles. Ultrasonication enhanced the dispersion of H-RM on the ceramic membrane and promoted vortex formation, facilitating gas–liquid mass transfer and improving the catalytic efficiency of the powder for ozone oxidation. Under optimal conditions, a 2 L RhB solution (40 mg/L) achieved a removal rate of up to 90%. However, the system showed lower efficiency in treating actual dye wastewater compared to RhB solution, likely due to the presence of complex and recalcitrant compounds. Therefore, future studies should focus on precise control of RM surface structures and active sites, anti-poisoning strategies, and composite modifications to improve adaptability to real wastewater.

3.4. Persulfate Activation

Because of its high oxidation potential, SO₄⁻ based advanced oxidation technology has garnered a lot of attention lately for the treatment of organic matter loads, including antibiotics. SO₄⁻ has a longer half-life, a wider pH range, and a higher redox potential than ·OH [63]. Research has demonstrated that the activity and redox stability of RM-based catalysts can be enhanced by the synergistic interaction of RM with other metal oxides. Chen et al. [64] synthesized and employed a composite material made of RM doped with MnO₂ to activate peroxymonosulfate (PMS) for M-cresol degradation. With a starting pH of 3–8, 2 g/L catalyst, 10 mM PMS, and 50 mg/L M-cresol, the 0.1 M/RM@G/PMS system was able to eliminate 71.4 percent of COD and fully degrade M-cresol in 90 min. According to studies, the catalyst surface’s abundant mesoporous structure and rich Mn and Fe active components can efficiently encourage electron transfer and further speed up the redox cycles of Mn(III)-Mn(II) and Fe(III)-Fe(II), which in turn activate PMS. By using coprecipitation technology, cobalt species are uniformly loaded onto the inside or outside of RM to create a composite material with a synergistic catalytic effect that can be applied widely in advanced oxidation and other fields. Wu et al. [65] used the coprecipitation method to create a cobalt-loaded (Co-RM) RM-based catalyst for effective persulfate (PS) activation. The Co-RM/PDS system’s maximum OFL removal efficiency was 80.06% when 15 mg/L ofloxacin (OFL), 0.4 g/L Co-RM, 4 g/L PDS, pH = 3.0, and 40 °C were present. Experiments using free radical scavenging verified that SO₄⁻ was the primary active ingredient in the reaction system. Another efficient way to evenly load cobalt active components onto the surface of RM carriers is to prepare cobalt-loaded RM-based catalysts using the impregnation method. As an illustration, Yue et al. [66] optimized the preparation conditions and created a cobalt-loaded RM catalyst using the impregnation method. A tetracycline removal efficiency of 89.5% was attained with pH = 7, 0.3 g/L Co-RM, and 3 g/L PDS. According to the study, SO₄²⁻ encouraged degradation, whereas anions like Cl⁻ and CO₃²⁻ inhibited it. Experiments using free radical scavenging demonstrated that SO₄⁻ and ·OH jointly took part in the oxidation process.

Biochar’s catalytic activity can be greatly increased by combining biomass with iron oxide-rich RM, which has promising application potential in the field of persulfate advanced oxidation technology (PS-AOPs). As an illustration, for the degradation of RhB, Deng et al. [67] prepared RM treated with activated persulfate (PDS) and stripped RM-biochar composite (RMBC) catalysts. The findings demonstrated that RMBC had stronger catalytic oxidation activity than RM because it had a larger specific surface area (10 times), smaller pore size, and a higher proportion of catalytically active metals (Fe, Al, and Ti) than RM. Second, the RhB degradation rate of RM was 76.70% under ideal dosage conditions, and it decreased to 41% after three cycles. In contrast, the degradation rates of RMBC were 89.98% and 67%, respectively. Similarly, Yang et al. [68] prepared a modified RM biochar catalyst (MRBC) using RM and corn stover (CS) by acid pretreatment and co-pyrolysis. With a removal rate of 88.59% in 30 min and an exceptionally low iron leaching rate of 0.049 mg/L, the catalyst demonstrated high stability and efficiency in the degradation of levofloxacin (LFX) under PDS activation. The primary mechanism of degradation was the free radical pathway (SO₄⁻ and ·OH). An inventive composite material with significant catalytic activity is RM-modified Fe⁰-based biochar. The activation efficiency of persulfate is greatly increased by the combination of the reducing capacity of Fe⁰ and the adsorption-conductive network of biochar, demonstrating special benefits in the area of pollution control. For example, Ma et al. [69] used RM and industrial syrup (IS) as raw materials to prepare a RM-modified Fe0-based biochar by co-pyrolysis for PDS activation to generate SO₄⁻ and O₂⁻. In the experiment, sulfadiazine (SDZ) was removed by 99.7% within 20 min. In the Fe_3/2 spectrum of XPS, the ratio of Fe(II) and Fe(III) increased after the reaction, reflecting that Fe⁰ participated well in the activation reaction of PDS as an active site [70]. The RM/IS mass ratio of 1:1 exhibits superparamagnetism and can be recovered by magnetic separation. In addition, RM can also be used to prepare magnetic biochar catalysts, which can be quickly recovered by applying an external magnetic field, solving the engineering bottleneck of difficult separation of powdered catalysts. For example, Wang et al. [71] used magnetic biochar (RSDBC) prepared from red mud and sewage sludge to activate PMS and achieved 82.5% degradation of sulfamethoxazole (SMX) within 50 min. The biomass-RM composite catalyst achieved the dual goals of solid waste resource utilization and efficient catalysis through the synergistic effect of biochar and iron oxide. The catalytic degradation and operating parameters of the RM-based persulfate catalyst are shown in

Table 3. Catalytic degradation and operating parameters of RM-based persulfate catalysts.

Catalyst	Reaction Time	Pollutant and Concentration	Reaction Conditions	Removal Rate	Ref.
MnO-RM	90 min	M-Cresol; N-50 mg/L	2 g/LMnO-RM; 10 mM PMS; Initial pH = 3~8	100%	[64]
Co-RM	-	OFL;15 mg/L	0.4 g/L Co-RM; 4 g/L PDS; pH = 3.0, 40 °C	80.06%	[65]
Co-RM	-	TC; -	pH = 7; 0.3 g/L Co-RM; 3 g/L PDS	89.5%	[66]
RMBC	-	RhB; 20 mg/L	pH = 4.6	89.98%	[67]
MRBC	30 min	LFX; 10 mg/L	8 mM PDS; 1.6 g/L MRBC	88.59%	[68]
RM/IS	20 min	SDZ; -	-	99.7%	[69]
RSDBC	50 min	SMX; 20 mg/L	1.0 g/L RSDBC; pH = 2.65~10.86	82.5%	[71]

.

3.5. Catalytic Oxidation

Catalytic oxidation lowers the activation energy for organic waste gas oxidation by using a catalyst, thereby increasing the reaction rate and allowing oxidation to occur at lower temperatures. This technology is currently widely applied for VOC removal. Catalytic oxidation allows VOCs to be converted into CO₂ and H₂O at relatively low temperatures (typically 200~500 °C). This process effectively removes VOCs while reducing energy consumption and byproduct formation, providing high safety and efficiency. According to studies, pure RM has low catalytic activity for removing volatile organic compounds (VOCs); however, calcination and acid modification can greatly increase the catalytic activity of pure RM for VOCs [72]. Liang et al. [73] prepared acid-modified RM (MRM) that exhibited significantly higher catalytic performance for toluene oxidation than untreated RM, achieving up to 99.2% efficiency at 300 °C. Acid-assisted UV irradiation (MRM-UV) of RM induced the formation of ferrous oxalate precipitates, which reduced the catalytic temperature to 238 °C. The study further revealed that acid treatment increased both the specific surface area and the amount of reducible, mobile lattice oxygen in RM, thereby enhancing toluene oxidation activity. RM prepared via the acid dissolution–alkali precipitation method showed higher activity than that produced by calcination [72]. Ryu et al. [74] synthesized acid-modified RM-supported manganese catalysts via the impregnation method and evaluated their performance. The RM-supported manganese catalyst achieved 100% toluene conversion at an optimal temperature, indicating that manganese addition significantly enhances catalytic activity. Acid modification decreases the alkaline content in RM, increases its specific surface area, and improves the catalyst’s efficiency in toluene adsorption. Pande et al. [42] studied the catalytic oxidation performance of MRM for toluene by calcination, hydrochloric acid, and mixed acid (oxalic acid and L-ascorbic acid) modification methods. At 360 °C, the samples modified with oxalic acid and ascorbic acid achieved complete conversion of toluene, while the calcined RM and HCl-treated MRM reached complete conversion at temperatures above 450 °C. The performance difference was attributed to the higher specific surface area of the RM treated with mixed acid and the enhanced reducibility of Fe³⁺ in the catalyst, which would improve the catalytic oxidation activity of toluene. Fang et al. [75] also studied the effect of the type of acid used in the modification process (HNO₃, H₂SO₄, HCl) on the toluene oxidation performance of MRM catalysts. The toluene oxidation performance of these RM was in the following order: HNO₃-treated > HCl-treated >> H₂SO₄-treated > untreated RM. The superior performance of the HNO₃-treated RM was attributed to its higher specific surface area, higher Fe³⁺ content, enhanced surface oxygen content and a large number of surface acid sites [75,76].

When RM is used as a carrier to load other metals (Cu, Mn, Pd, Co, Pt, etc.) [76,77], these metals interact with Fe₂O₃ in RM, promote the migration of lattice oxygen in the catalyst [21], and the loaded metals increase the Lewis acid sites of RM-based catalysts. Nguyen et al. [28] created a support material (RR) by treating rice husk ash and RM with acid. In the oxidation reaction of p-xylene, the 3MnRR1 catalyst, which was made by doping 3.0 weight % Mn into RR, demonstrated good performance, achieving a 100% CO conversion at around 280 °C and a 93% conversion at 400 °C. Being a cyclic hydrocarbon, p-xylene is more difficult to oxidize than CO, which is likely why its conversion temperature is higher. The conversion of p-xylene slightly drops when CO and p-xylene are present together. According to this phenomenon, the RM-supported manganese catalyst is anticipated to be a viable candidate material for the simultaneous removal of CO and p-xylene. Additionally, Meng et al. [76] synthesized a series of catalysts by incorporating various transition metals (Mn, Cu, Ce, Co, and Cr) onto an active metal reduction surface (ARM). Under identical experimental conditions, all catalysts effectively improved catalytic activity for toluene oxidation. Notably, the Mn-doped acid-pretreated RM (Mn/ARM) catalyst exhibited superior low-temperature toluene oxidation activity compared to conventional Mn/TiO₂ and Mn/Al₂O₃ catalysts. Comprehensive characterization indicates that Fe-Mn interactions are markedly enhanced via the Mn⁴⁺⇌Mn³⁺ electron transfer mechanism. Surface electronic structure reconstruction not only activates chemically adsorbed oxygen on Fe₂O₃ but also promotes the migration of MnO_x lattice oxygen. Based on the aforementioned studies, during the catalytic oxidation of VOCs, gas molecules are adsorbed to the acid sites on the catalyst surface and react with the surrounding active oxygen to produce H₂O and CO₂. Finally, gaseous oxygen molecules are activated at oxygen vacancies to replenish the consumed active oxygen and prepare for subsequent catalytic cycles [78]. The redox reaction of Fe³⁺/Fe²⁺ in RM accelerates the migration of active oxygen during the entire catalytic cycle, thereby enhancing the oxidation catalytic ability of RM-based catalysts. Therefore, the higher the Fe₂O₃ content in RM, the stronger its catalytic activity against VOCs. However, the catalytic activity of RM-based catalysts does not increase proportionally with manganese loading. Previous studies have shown that, within an optimal range, increasing metal loading enhances the number of active sites on the catalyst surface, thereby facilitating toluene adsorption and conversion. Excessive metal loading, however, causes metal species to aggregate on the catalyst surface, thereby reducing the accessibility and utilization of active sites [79]. Figure 3. Drawing on studies of the catalytic activity of acid-modified RM and supported transition metals (e.g., Mn, Cu, and Fe-Mn composites) for VOC degradation, the adsorption—activation—oxidation cycle of VOC molecules on the catalytic surface is summarized. This schematic integrates experimental findings on active components (surface acidity, oxygen vacancies, and the Fe³⁺/Fe²⁺ redox cycle) with thermal catalytic kinetics and active oxygen migration mechanisms [72,73,74,75,76,79].

3.6. Catalytic Chemical Looping Combustion

Chemical Looping Combustion (CLC) is an emerging clean and efficient combustion technology. Unlike conventional combustion, which burns fossil fuels in the air, CLC employs metal oxides as oxygen carriers to provide oxygen for combustion while simultaneously capturing and separating CO₂ from flue gas [80]. RM is regarded as an ideal oxygen carrier for CLC systems because of its low cost. Currently, RM application in CLC technology is mainly focused on methane removal. This section discusses the catalytic degradation of methane using RM-based catalysts in chemical looping combustion. Deng et al. [81] used two types of RM to prepare composite RM-based catalysts with different ratios and evaluated the activity and stability of these RM-based catalysts by catalyzing CH₄. Among them, V-RM is rich in Fe₂O₃, while W-RM is rich in inert and alkaline components. The results show that a 7:3 mixture of V-RM and W-RM exhibits a CH₄ catalytic activity of up to 90%, an average CH₄ conversion of 81%, and an average CO₂ selectivity of 89%. In addition, even after ten cycles, the CO₂ selectivity remains around 90%. This enhanced activity can be attributed to the synergistic effect between the active species (Fe₂O₃) and the inert support (Al₂O₃), which not only improves the activity but also enhances the stability of the composite RM-based catalysts. Studies have shown that composite RM-based catalysts achieve higher methane combustion conversion rates, lower coking tendencies, and better cycle stability than either of the two single RM-based catalysts [80]. RM improves the activity and redox stability of composite catalysts through synergistic interactions with other metal oxides. For example, Liu et al. [82] synthesized composite oxygen carriers with varying ratios of pyrite slag and RM via a simple mechanical mixing method. The 5R5P sample (50 wt% RM and 50 wt% pyrite slag) exhibited the highest performance in the reduction half-cycle, with methane conversion rates 396% and 61.79% higher than those of pure RM and pure pyrite slag, respectively. Over 20 consecutive cycles, the 5R5P sample maintained stable activity, consistently achieving methane conversion rates of approximately 44%. Synergistic interactions between inert RM (Al₂O₃, SiO₂, Na₂O) and active pyrite slag components (Fe₂O₃) significantly enhance the methane combustion activity and cycle stability of the composite oxygen carrier [81,82]. For example, Lin et al. [83] prepared a series of RM-based catalysts loaded with NiO and MnO_x by wet impregnation method, and compared the activity of pure RM-based catalysts with those loaded with NiO and MnO_x in CH₄ catalytic conversion. Studies have shown that the CH₄ conversion rate and CO₂ selectivity of RM-based catalysts loaded with NiO and MnO_x are higher than those of pure RM-based carriers. Among these, the pure RM, 10Mn-RM, and 15Ni-RM samples after 20 oxidation cycles showed the highest activity. The RM-based catalyst loaded with NiO had an average CH₄ conversion rate of 62% and a CO₂ selectivity of 77%, while the RM-based catalyst loaded with 10 wt% manganese showed the best activity among the manganese-containing samples.

Cu–Fe bimetallic oxides have been identified as promising oxygen carriers for CLC systems owing to their high reactivity and strong resistance to sintering [84]. Therefore, CuO can also be introduced to interact with the inherent iron oxide of RM, and the reaction activity and redox stability of the oxygen carrier are significantly improved. Deng et al. [85] modified the RM oxygen carrier with CuO and used it as a cost-effective oxygen carrier for CH₄ chemical looping combustion. The sample containing 20 wt% CuO showed the highest CH₄ conversion, CO₂ selectivity and oxidation efficiency in multiple redox tests, which was greatly improved compared with the original RM. Comprehensive characterization showed that free CuO and CuFe₂O₄ were detected in the CuO-modified RM after calcination at 900 °C. The copper components in these two oxides can be preferentially reduced to metallic copper when reacting with methane. The reduced copper species may serve as active sites for CH₄ activation and oxygen release, thereby promoting the reaction of iron oxides in RM with methane. Gu et al. [86] prepared a composite catalyst mixed with copper ore and RM in proportion, and tested the performance of the catalyst by CH₄ CLC. The results showed that the composite catalyst with a copper ore to RM ratio of 7:3 had the highest catalytic activity for CH₄, with a CH₄ conversion rate and CO₂ selectivity of 86% and 90%, respectively. The redox cycle stability test showed that the composite catalyst was more stable than pure copper ore in the redox cycle. The addition of RM improved the anti-sintering ability and the actual oxygen concentration in the copper ore, and significantly improved the oxidation activation of CH₄. Cheng et al. [87] integrated chemical looping with steam methane reforming (SMR) to synthesize a series of CeO₂-Ni co-doped RM oxygen carriers via the impregnation method. conversion rate reached 67.6%. Under steady-state operation over 50 cycles, the catalysts achieved an average methane conversion rate of 67.6%. This performance is attributed to the synergistic interaction between cerium oxide and Ni dopants, which react with RM to form CeO₂-based Ce-Ni-O and Ce-Fe-O solid solutions, as well as the perovskite-type CeFeO₃ phase. These phases substantially promote oxygen vacancy formation and facilitate lattice oxygen migration. The catalytic performance of RM-based catalysts for methane CLC, along with their operating parameters, is summarized in

Table 4. Catalytic degradation of CH₄ by RM-based catalysts and operating parameters.

Catalysts	Calcination Environment	Dosage	Reaction Conditions	CH4 Concentration and Flow Rate	CH4 Conversion Rate	Catalytic Stability	Ref.
Two RMs were mixed at a mass ratio of 7:3.	900 °C; 2 h	5 g	T = 900 °C	5%; 100 mL/min	Average 81%; Maximum 90%	Ten cycles; 90% CO2 selectivity.	[81]
RM containing 15 wt% NiO and 10wt % MgO.	900 °C; 2 h	5 g	T = 900 °C	5%; 200 mL/min	65%	Twenty cycles; CH4 conversion rate:75–40%; 60% CO2 selectivity.	[83]
RM with 20 wt% copper oxide.	900 °C; 2 h	2 g	T = 800 °C	5%; 200 mL/min	90%	Twenty cycles; 100% CO2 selectivity; over 60% conversion rate.	[85]
Copper ore and RM were mixed at a mass ratio of 7:3.	900 °C; 6 h	2 g	T = 900 °C	5%; 200 mL/min	86%	Twenty cycles; CH4 conversion rate exceeds 75%, CO2 selectivity exceeds 90%.	[86]
RM and pyrite were mixed at a mass ratio of 1:1.	900 °C; 2 h	2 g	T = 900 °C	5%; 300 mL/min	45.82	Twenty cycles; conversion rate: approximately 44%.	[82]

.

3.7. Catalytic Selective Catalytic Reduction

Selective Catalytic Reduction (SCR) is a process in which a reductant selectively reacts with NO_x in flue gas over a catalyst, producing non-toxic, pollution-free N₂ and H₂O [88]. Currently, SCR technology provides highly effective NO_x control in boiler flue gas and is technically mature, making it the most widely used and effective flue gas denitrification method worldwide. Under optimal configuration and temperature conditions, NO_x removal efficiencies of 80~90% can be achieved [88]. RM is rich in Fe₂O₃, which offers abundant acidic sites and moderate redox properties. These properties make RM a promising candidate for NH₃-SCR reactions targeting nitrogen oxide reduction [89]. However, alkali metal oxides (e.g., Na₂O, CaO) in RM readily sinter at high temperatures, reducing surface acidity and catalytic activity, and thereby severely inhibiting the NH₃-SCR reaction for NO_x removal [90]. Therefore, physical and chemical modifications are necessary to enhance its catalytic performance [91,92]. Wang et al. [93] prepared a series of RM-based catalysts by acid washing and calcination using sesbania powder (SP), guar gum (GG), and sodium carboxymethyl cellulose (CS) as binders. After the addition of GG and CS, the number of Fe³⁺, surface adsorbed oxygen and Brønsted acid sites decreased, resulting in a decrease in NH₃-SCR activity. In contrast, the RM-15sp-550 catalyst calcined at 550 °C and added with 15% sesbania powder as a binder showed the highest catalytic activity, with a NO_x conversion rate of more than 90% in the temperature range of 325~450 °C. Lin et al. [94] prepared an RM-based catalyst by mixing sulfuric acid with RM and using a hydrothermal method, and then neutralized the pH with a 40% ammonium carbonate solution. The RM catalyst showed high catalytic activity, with a NO_x conversion rate of more than 90% at 300~450 °C. If the treated RM slurry is made into an RM-coated catalytic filter, dust in the flue gas can be removed at the same time. In addition, when H₂O and SO₂ are present, SO₂ is oxidized and forms SO₄²⁻ on the catalyst surface, increasing the acidic sites, thereby promoting ammonia adsorption and driving the NH₃-SCR reaction [95]. Another application for RM is as a carrier for NH₃-SCR catalysts. The redox cycle of various transition metal element valence states enhances RM’s ability to store and release oxygen as well as its redox performance, which raises catalyst activity and expands the active temperature range. Qi et al. [96] prepared CuO/RM catalyst by the wet impregnation method and found that when the CuO loading was 7%, CuO/PRM showed the best NH₃-SCR performance and achieved 98.4% NO conversion at 325 °C. CuO/PRM has the highest CuO dispersion, which promotes the generation of more Cu+ and surface oxygen species. In addition, the highly dispersed CuO is conducive to the activation of NH₃ by Lewis acid sites, while weakening the adsorption of NO_x, thereby enhancing the NH₃-SCR reaction through the Eley-Rideal mechanism. In addition, RM can be prepared into ZSM-5 zeolite, which is an ideal carrier for NH₃-SCR catalyst.

Additionally, RM can be converted into zeolites (e.g., BEA [97], ZSM-5 [98]) to serve as efficient carriers for NH₃-SCR catalysts. Yang et al. [97] synthesized a copper-based BEA zeolite NH₃-SCR catalyst (Cu-BA0.5-H) using acid-washed and alkali-melted RM as the aluminum source. At an NH₃/NO ratio of 1, Cu-BA0.5-H achieved ~99% NO_x removal efficiency in the low-temperature range of 200~300 °C. Studies have shown that potassium peroxide reduction in SCR follows the Eley-Rideal (E-R) mechanism, with the critical intermediate varying according to reaction temperature. Acid washing and alkali fusion are inferred to convert insoluble silicon- and aluminum-containing components into soluble forms, thereby influencing the catalyst’s denitrification performance. When RM-derived zeolites are used, metal loading is critical; excessive loading reduces specific surface area and suppresses catalytic activity [67]. In summary, acid leaching removes alkaline components from RM, whereas calcination opens its pore structure and transforms the needle-like magnetite phase into uniform α-Fe₂O₃. However, these studies present several limitations: (1) high-temperature calcination (≥500 °C) and long processing times (≥5 h); (2) complex metal loading procedures—powdered RM becomes sticky upon contact with water during impregnation or hydrothermal treatment, increasing separation and washing workload; (3) limited enhancement in low-temperature (100~200 °C) denitrification performance—although metal addition improves efficiency at 200~300 °C, achieving optimal performance at lower temperatures often requires multiple metals [99]. Furthermore, research on catalyst poisoning resistance and long-term stability remains limited. Song et al. [100] developed a denitrification catalyst (R5B5-450) from RM and biochar via acidification, grinding, and calcination, effectively addressing the aforementioned issues. R5B5-450 achieved >90% NO conversion efficiency over 225~400 °C, demonstrating strong potential for application in real coal-fired flue gas treatment. Its superior NO_x removal performance is attributed to: (1) dispersed iron oxide particles with small crystal grains and redox pairs that drive NO/NH₃ activation; (2) oxygen vacancies and surface-adsorbed oxygen species that promote NO oxidation to NO₂, facilitating the Fast-SCR pathway; (3) high specific surface area and moderately strong acid sites that enhance mass transfer and ammonia adsorption, synergizing with Fe species to enable efficient E-R and L-H reactions. This study offers a reference for the effective utilization and reduction of RM, while also helping to lower denitrification catalyst production costs. The applications and performance of RM-based SCR catalysts are summarized in

Table 5. Preparation and performance of RM-based SCR denitrification catalyst.

Catalyst	Preparation Condition			Catalytic Property and Condition			Ref.
	Solution for Leaching	Calcination Condition	Dopants	NOx Conversion	Temperature Range	GHSV (h−1)
RM-based	HCl	550 °C	sesbania powder (binder)	>90%	325~450 °C	30,000	[93]
RM-based	H2SO4	500 °C, 5 h	-	>90%	300~450 °C	60,000	[94]
RM-based	HNO3	-	-	>90%	275~475 °C	-	[92]
Ce0.3/RM	HNO3	500 °C	Ce0.3	>80%	275~400 °C	30,000	[91]
Ce/RM	HCl	500 °C, 5 h	Ce	100%	200~400 °C	30,000	[99]
CuO/RM	HNO3	500 °C, 6 h	7% CuO	>90%	300~375 °C	12,000	[96]
Cu-BA0.5-H	NH4Cl	600 °C, 3 h	Cu	99%	200~300 °C	-	[97]
Co-Mn/ ZSM-5	-	550 °C, 3 h	5%wt Mo and 10%wt Mn	98.8%	150 °C	40,000	[98]
BC/RM	HNO3	300 °C, 2h	BC	>90%	225~400 °C	23,000	[100]

.

3.8. Long-Term Performance Comparison of RM Modification Strategies

Based on the above research, various modification strategies play a crucial role in enhancing the physicochemical properties and catalytic performance of catalysts derived from RM resources. Acid washing effectively removes alkaline components and impurities, markedly increasing specific surface area and surface acidity. Heat treatment improves pore structure and regulates the phase state of iron oxides, thereby enhancing redox capacity. Metal loading and semiconductor composites broaden the light response range, facilitate electron transport, and improve the synergistic degradation of multiple pollutants. Biochar composites and magnetic modification enhance adsorption activity and enable facile recovery. These modification approaches have shown notable success in photocatalysis, Fenton-like oxidation, ozonation, persulfate activation, VOCs catalytic oxidation, chemical looping combustion, and NH₃-SCR. The most effective and durable strategy involves metal doping or codoping (e.g., Cu, Ce, Ni, Mn) combined with composite oxygen carriers and metal–oxide interactions, which maintain cyclic stability for years and suppress sintering during CLC and high-temperature cycling. For example, RM modified with 20 wt% CuO and CeO₂-Ni codoping retains high catalytic activity over multiple cycles. Biochar- or carbon-based composites and magnetic modifications for aqueous advanced oxidation processes (AOPs) exhibit excellent activity retention and facile recovery after 10~25 cycles, while markedly reducing metal leaching risk. For instance, magnetic γ-Fe₂O₃ composites achieve a recovery rate exceeding 98% after 10 cycles, while maintaining a photocatalytic degradation efficiency of 96%. Similarly, the Mn/Ce@RM ozone system retains approximately 72.7% of its activity after 25 cycles. Acid treatment markedly enhances initial activity, specific surface area, and surface acidity; however, without subsequent stabilization or sealing (e.g., calcination, carrier coating, biochar compositing), it increases metal leaching and reduces long-term reusability. Thermal treatment stabilizes the active phase and mitigates leaching; however, excessive temperatures can cause particle sintering and reduce specific surface area, with long-term effectiveness depending on temperature and calcination atmosphere control. Using RM as a carrier and integrating it with more stable supports (e.g., zeolites, ceramic membranes) significantly improves structural integrity and cycling stability under industrial conditions, making it particularly suitable for high-temperature applications such as SCR and VOCs catalytic oxidation. This section consolidates previously fragmented descriptions of methods—such as acid washing, heat treatment, ball milling, biochar incorporation, metal loading, and magnetic modification—into a coherent sequence: pretreatment (dealkalization/impurity removal) → activation/surface enhancement (acid, heat, ball milling, ultrasound) → functionalization/loading (metal, semiconductor, biochar) → encapsulation/stabilization (coating, carrier, curing) → recycling/disposal. This sequence highlights the interdependence of processing steps, facilitating clearer understanding of process selection.

4. Environmental Risks and Mitigation Measures for RM Catalyst Preparation

RM, an alkaline-rich industrial by-product, has recently attracted considerable attention in environmental catalysis owing to its low-cost carrier potential and valuable metal oxides, such as Fe₂O₃ and γ-Al₂O₃ [101]. However, its high alkalinity (pH = 10~13), elevated Na content, fine particle size, and potentially mobile trace elements (e.g., Cr, V, As, and occasionally low-level radioactive nuclides) present notable environmental risks when RM is repurposed as a catalyst or carrier [102]. First, RM typically exhibits a high pH (>10), releasing large amounts of OH⁻ in aquatic or humid environments. This alters the chemical conditions of surrounding soils and water bodies, disrupts ecosystems, and increases the mobility of certain metals. High alkalinity can also compromise catalyst stability under aqueous or humid conditions, leading to leaching of active components or phase transformations [103]. Second, RM contains various heavy metals and toxic elements (e.g., As, Cr, V, Pb) that may leach into wastewater or groundwater under acidic, alkaline, or saline conditions, leading to secondary pollution, poisoning of active sites, or contamination of products. Low-level radioactive elements have also been detected in RM from certain regions, necessitating risk assessment. Such leachates may compromise the long-term safety of catalyst recycling and disposal [104]. Additionally, fine-grained RM can be readily dispersed by wind or water. During transport and processing, RM dust can enter ecosystems, increasing exposure risks and potentially blocking or degrading catalytic bed layers or membrane components [103]. Therefore, direct use without proper treatment may alkalize surrounding soils and waters, promote harmful metal migration, and cause dust exposure during transport. Furthermore, alkaline ions and metal oxides in RM may exchange, coat, or form inactive phases with introduced active metals (e.g., precious or transition metals), reducing surface area, blocking active sites, and thereby decreasing catalytic efficiency or causing active component loss under wet or cyclic conditions [105,106]. Notably, even if RM-based catalysts perform well, they may still become hazardous waste upon disposal or replacement if not effectively solidified or detoxified, necessitating long-term storage or recycling strategies to prevent risk transfer to downstream processes [107].

Due to the strong alkalinity, high heavy metal content, and potential leaching and environmental risks of RM, a “pretreatment followed by utilization” strategy is essential before applying RM in wastewater or exhaust gas purification. (1) Pretreat RM to neutralize alkalinity and remove toxic components, thereby mitigating leaching risks. Weak acids or specific salt solutions can dissolve soluble Na⁺/alkaline species and leachable heavy metals, after which valuable components in the residue can be recovered [33]. It should be noted that acid washing generates saline and acidic effluents, which require appropriate downstream treatment [106]. (2) Employ solidification or stabilization techniques to reduce RM leaching and facilitate safe disposal. High-temperature treatment or incorporation of silicate- or lime-based binders can immobilize hazardous components within a less leachable matrix, enabling the preparation of stable catalytic carriers; however, energy use and carbon footprint must be carefully balanced [106,107]. Additionally, alkaline activation or silica–alumina polymer systems can encapsulate RM, yielding mechanically robust carriers with reduced leaching [107]. (3) Activate or modify RM to minimize exposure to harmful constituents and improve catalytic performance. Controlled calcination temperature and atmosphere can remove volatile or soluble components, produce a more stable carrier surface, and promote the incorporation of active metals into stable phases [105]. (4) Design “safe catalytic systems” and closed-loop processes. Use RM as a composite carrier rather than a single active phase by combining it with ceramic, carbonaceous supports, or stable metal oxides, thereby reducing RM exposure while exploiting its structural framework [105]. Additionally, harmful or valuable elements can be first recovered to reduce residue toxicity and improve economic feasibility, after which the remaining material can be processed into catalysts [103,108]. In recent years, numerous studies have focused on resource recovery and extraction processes. (5) Implement on-site management and monitoring measures, including standardized leaching and long-term durability tests, during catalyst preparation and application. Systems with high release potential require secondary treatment or solidification before disposal [104]. Life cycle assessment (LCA) and environmental risk assessment (ERA) are recommended to prevent transferring pollution from the production stage to the disposal stage [107].

5. Conclusions and Outlook

This review systematically summarizes the progress in utilizing RM for environmental catalyst preparation, emphasizing modification strategies applied in environmental remediation. RM, containing abundant active components such as Fe₂O₃ and Al₂O₃, has demonstrated significant catalytic performance in applications including photocatalysis, Fenton-like oxidation, ozonation, persulfate activation, VOC oxidation, chemical looping combustion, and NH₃-SCR following modifications via acid treatment, thermal treatment, and metal loading. Research on RM-based catalysts has progressed from basic solid waste utilization to systematic design approaches aimed at multi-pollutant synergistic treatment, environmental safety, and practical engineering applications. Despite recent improvements in activity, selectivity, and stability via various modification strategies, key challenges and research opportunities remain for achieving scalable, durable, and low-risk applications: (1) high-performance RM-based catalysts currently rely heavily on high-temperature calcination, strong acid treatments, or multi-step chemical synthesis, creating substantial energy and environmental bottlenecks. Future research should prioritize green preparation methods, including low-temperature solid-phase reactions, mechanochemical synthesis, microwave-assisted synthesis, and ionic liquid media, to lower carbon footprints and minimize secondary pollution. (2) Real wastewater and exhaust streams often contain complex mixtures of pollutants, which are challenging to treat efficiently via a single reaction pathway. There is an urgent need for composite catalytic systems capable of concurrently activating multiple oxidants (e.g., ozone, persulfates, H₂O₂) or integrating photothermal/electrochemical synergistic functions to achieve efficient simultaneous degradation of organic compounds, nitrogen oxides, VOCs, and other pollutants. (3) Future work should establish unified durability evaluation standards and conduct long-term stability tests spanning over 50 or even hundreds of cycles. In situ or quasi-in situ characterization techniques (e.g., in situ XRD, XPS, DRIFTS) should be employed to investigate deactivation mechanisms and quantify the contributions of factors such as sintering, metal leaching, and surface poisoning under various operational conditions. (4) Environmental safety and LCA should quantitatively evaluate environmental risks during catalyst preparation, operation, and disposal (particularly alkaline and heavy metal leaching), and integrate LCA with ERA throughout catalyst design and process optimization to ensure sustainable technology deployment. (5) Magnetic separation, ceramic membrane immobilization, 3D printing, and related technologies should be integrated to develop modular catalyst configurations that are recoverable, mechanically robust, and adaptable to diverse reactor conditions, facilitating the translation of RM-based catalysts from laboratory research to industrial implementation. In summary, future research should prioritize green preparation, functional integration, long-term operational stability, and lifecycle safety, aiming to develop RM-based catalysts capable of efficient, stable, and low-risk performance across diverse environmental media, thereby promoting the synergistic advancement of solid waste utilization and environmental protection.