Sustainable Remediation: Advances in Red Mud-Based Synergistic Fabrication Techniques and Mechanistic Insights for Enhanced Heavy Metal(Loid)s Sorption in Wastewater

Abstract

Rapid growth in the alumina industry generates vast amounts of highly alkaline red mud (RM), posing significant environmental risks. However, RM shows great promise as a resource for environmental remediation, particularly through its conversion into effective adsorbents. This research reviews recent advancements in developing RM-based adsorbents for sustainable wastewater treatment, especially targeting heavy metal(loid)s (HMs). We examine key modification mechanisms to enhance RM’s properties, summarize synthesis methods for various RM- based adsorbents, and evaluate their performance in removing HMs from water, guiding the design of subsequent new materials. Crucially, this review highlights studies on adsorbent reusability, HM leaching, and economic feasibility to address economic and safety concerns. Finally, we discuss adsorption mechanisms and prospects for these materials.

1. Introduction

In trace amounts, heavy metal(loid)s (HMs) are poisonous and non-biodegradable, harming the ecosystem [1,2]. HM poisoning causes cancer, neurological disabilities, organ failure, growth delays, and gastrointestinal disorders [3,4]. Battery and mining effluents increase metal(loid) concentrations in the environment [5,6]. As(III) [7], Cd(II) [8], Cr(VI) [9], Hg(II) [10], Pb(II) [11,12], Cu(II) [13], Ni(II) [14], Mn(II) [13], Sb(III, V) [15,16,17,18,19,20,21] and other hazardous ions are found in wastewater. Species, concentration, pH, wastewater type, and HM ion solubility affect HMs’ toxicity [22].

Many water treatment methods have been developed to purify HMs in wastewater, including ion exchange [23], chemical precipitation [24], adsorption [25,26], membrane separation [27], etc. These technologies have advantages and disadvantages in practice. Adsorption is optimal for removal capacity, energy consumption, operating conditions, and secondary pollutants. Adsorption is simple to handle, regenerable, and non-toxic [28,29], making it a promising method for HMs’ removal from water. However, conventional adsorbents (e.g., activated carbon, zeolites, molecular sieves, and resins) are cost prohibitive. Thus, developing low-cost, efficient adsorbents has garnered interest. Red mud (RM), a porous structural material, is frequently used to remove heavy metal cations (Pb, Cu, Zn), inorganic anions (phosphate, nitrate, fluoride), and organic contaminants (dyes and antibiotics) [30].

RM is an aluminum smelting byproduct. It may be Bayer RM, sintered RM, or mixed RM, depending on the manufacturing technique. RM’s chemical composition is globally similar, but its specific content varies. Bayer RM has high quantities of alumina, iron oxide, and alkali because alumina is reacted with caustic soda to make aluminum salts [31], whereas sintered RM has high CaO and little iron and alkali [32]. Bauxite composition, alumina production, RM storage technique, and storage period impact RM form and composition. RM typically comprises Fe, Al, Na, and Ti oxides [33].

The application scope of red mud encompasses construction materials [34,35,36,37], ceramics [38], environmental remediation [39], acid mine drainage (AMD) treatment [13], catalysis [38,40], neutralizers [41], and soil rehabilitation [42,43,44,45]. However, RM’s high alkalinity and massive stockpiles pose severe environmental risks to soil and groundwater [46,47]. It has been utilized as an adsorbent for HM ions to improve red mud treatment, which is efficient in removing Pb [11], Cu [48,49], Zn [50,51], Ni [52], Cd [52,53], and Cr [54]. O-H, C-O-C, Si-C, and Si-O surface chemical groups play a crucial role [55].

Red mud’s tiny particle size, huge specific surface area, and rich pore structure make it ideal for HMs’ removal. In practice, strong alkalinity, tendency for particle agglomeration, weak selective adsorption capacity when multiple HMs coexist, and other issues must be addressed to meet water quality standards [56,57,58]. Many modifications have been devised to improve HMs’ removal, resulting in a variety of tailored red mud-based adsorbents.

Previous research has reviewed the application of red mud and red mud-based adsorbents in the removal of HMs, inorganic anions, and organic compounds; however, there is a lack of guiding principles for modified synthesis processes and specialized analyses of the removal mechanisms for individual target pollutants. This study aims to summarize the application of red mud adsorbents in heavy metal removal to further advance sustainable remediation research. Additionally, it emphasizes the need to consider economic feasibility and production processes when designing new adsorbents, with the goal of providing guidance for establishing a comprehensive standard framework.

The primary aim of this review is to present the latest advancements in the application of red mud-based adsorbents in the field of water and wastewater treatment. Firstly, this review provides the modification processes of red mud, along with the synthesis techniques, advantages, and disadvantages of several red mud-based adsorbents. Secondly, the adsorption mechanisms of HMs by red mud-based adsorbents are elucidated, with emphasis on their reusability, potential metal leaching, and economic feasibility. Thirdly, this review examines the possible use of red mud-based adsorbents in water treatment and future research paths based on the literature to give theoretical justification and practical evidence for enhancing their performance and sustainable treatment. Subsequently, this review covers the mechanism, limits, and future developments of HM ions’ adsorption.

2. Modification of Red Mud

Red mud requires processing to diminish its toxicity and enhance its physicochemical qualities before its use as a wastewater adsorbent, owing to its elevated alkalinity, potential leaching toxicity, and inadequate adsorption capacity. Heat treatment [59], neutralization [60], organic modification [61], and metal compound modification [62] are common practices. These modification approaches are systematically compared in

Table 1. Comparison of different modification methods.

Method	Advantages	Disadvantages	Practical Applicability
Heat treatment	Significant improvement in activity	High energy consumption, risk of secondary pollution	Acidic wastewater treatment agent
Neutralization	Significant reduction in alkalinity, low cost, and easy to operate	Salt accumulation limits performance improvement	Building material pretreatment, soil improvement
Organic modification	Good ecological compatibility, carbon sequestration, and emission reduction	Low intensity, long cycle	Ecological restoration
Metal compound modification	Excellent adsorption performance, functional design	High cost, complex process	Remediation of water contaminated with heavy metals or arsenic

.

2.1. Heat Treatment

At high temperatures (300–1000 °C), heat treatment optimizes red mud’s adsorption ability by reorganizing its physical structure and changing its chemical phase [59]. At the physical level, thermal processes contribute to the grain size adjustment and pore morphology evolution of hematite: dewatering (removal of structural water, bound water) reduces the adsorption resistance before 500 °C, while carbonate decomposition releases CO₂ to form a rough surface, and the specific surface area and porosity are significantly increased; however, after the temperature exceeds 600 °C, the enhanced crystallinity of hematite leads to the sintering shrinkage of the material, and a sharp decrease in the specific surface area above 700 °C triggers a decline in adsorption efficiency [63,64,65]. At the chemical level, the phase transition sequence dominates component activation: dehydration of alumina trihydrate to produce Al₂O₃ at 300–550 °C, decomposition of calcite to CaO with CO₂ at 600–800 °C, and formation of new phases, such as tricalcium aluminate, at 800–900 °C, where 700 °C may be the critical threshold temperature [66]. Although heat treatment at 700 °C reduces specific surface area, it has been shown that 700 °C annealed red mud (ARM700) exhibits optimal adsorption properties compared to other annealing temperature conditions due to the simultaneous activation of the multicomponent synergistic effects of Fe₂O₃, Al₂O₃, SiO₂, and sodium–aluminum–silicon carbonate (Equations (1) and (2)), which is attributed to the high-temperature-stimulated directional generation of reactive phases, such as CaO, Al₂O₃, and Fe₃O₄, resulting in a highly reactive crystalline composite structure [67,68]. The temperature of heat treatment is a crucial control parameter that profoundly influences the efficacy of red mud-based adsorbents: moderate heat treatment can enhance the physicochemical properties of the materials, whereas elevated temperatures tend to compromise the pore structure and diminish adsorption capacity, while high energy consumption restricts scalability [59,69]. Therefore, future research should prioritize low-temperature synergistic modification approaches (e.g., coupled chemical activation) to simultaneously optimize structural stability, adsorption performance, and cost-effectiveness.

$${\mathrm{N}\mathrm{a}}_8{\mathrm{A}\mathrm{l}}_6{\left({\mathrm{S}\mathrm{i}\mathrm{O}}_4\right)}_6{\left(\mathrm{O}\mathrm{H}\right)}_2·2{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{s}\right)}+{\mathrm{C}\mathrm{O}}_{2\left(\mathrm{a}\mathrm{q}\right)}\to {\mathrm{N}\mathrm{a}}_5{\mathrm{A}\mathrm{l}}_3{\left({\mathrm{S}\mathrm{i}\mathrm{O}}_4\right)}_3{\mathrm{C}\mathrm{O}}_{3\left(\mathrm{s}\right)}+3{\mathrm{N}\mathrm{a}\mathrm{A}\mathrm{l}\mathrm{S}\mathrm{i}\mathrm{O}}_{4\left(\mathrm{s}\right)}+3{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{l}\right)}$$

(1)

$$2{\mathrm{A}\mathrm{l}\left(\mathrm{O}\mathrm{H}\right)}_{3\left(\mathrm{s}\right)}\to {\mathrm{A}\mathrm{l}}_2{\mathrm{O}}_{3\left(\mathrm{s}\right)}+3{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{l}\right)}$$

(2)

2.2. Neutralization

RM neutralization treatment is a high-quality solution for reducing environmental damage. Neutralization works by adding gypsum or phosphogypsum, acid activation, CO₂, or seawater to decrease the material’s pH and change its micromorphology to enhance its specific surface area [70,71,72,73,74]. Amorphous material in the RM portion may be increased to promote HMs’ binding. pH neutralization makes red mud useful for cleanup [60].

(1)

Acid activation. Through acid solution and RM reaction, acid treatment modulates surface properties and component activities to improve pollutant adsorption, dissolve/reconstruct mineral phases, and functionalize surfaces and surface functionalization [75]. Existing studies have mostly used inorganic acids like HCl, HNO₃, and H₂SO₄ (0.05–1.0 M concentration range) to modify samples. Two typical processes are acid solution reflux followed by liquid ammonia precipitation and drying [76] and direct acid leaching followed by washing and drying [77,78,79]. Both methods effectively remove alkali metal and impurity phases, expose fresh active surfaces, and optimize pore structure [80]. Acid treatment increased the specific surface area of red mud (e.g., from 13.15 to 23.80 m²/g [75], or from 33.5 to 67.10 m²/g [81]), dissolved mineral phases like hematite and gibbsite [82], and increased Fe₂O₃ content by 3–4% [75,83,84]. Additionally, surface hydroxyl and metal–oxygen group density increased, forming high-affinity metal binding [75,85]. Acid treatment increases electrostatic adsorption capacity by modulating surface charge characteristics (e.g., pH decreased from 11.00 to 2.12, isoelectric PZC decreased), but the acid concentration and reaction time must be tightly controlled to avoid the over-dissolution of Fe/Al oxides [85]. Moreover, the buffering capacity of red mud is intrinsically linked to its alkaline constituents (e.g., calcium hydroxide, carbonates, etc.). However, some studies have shown that specific surface area is not necessarily correlated with cation adsorption efficiency, emphasizing the importance of surface chemical properties (e.g., functional group type and charge distribution) [70]. In conclusion, acid treatment optimizes adsorption performance through physical–chemical synergistic modification, but the source and mineral composition of red mud affect its effect, which must be combined with the surface reaction mechanism for targeted process design. Acid neutralization of red mud has the potential for low cost and “waste for waste,” but is limited by location dependence, the risk of dilute salt contamination, and transportation costs. Its adsorption efficiency is regulated by chemical composition, specific surface area, etc., but the correlation between the latter and cation adsorption is still controversial.

(2)

Neutralization with seawater. Seawater neutralization reduces pH to 8.5–8.8 by replacing Na⁺ in red mud with Ca²⁺ and Mg²⁺ while maintaining residual neutralization potential and also converts soluble wastes to solids (such as hydrotalcite, calcite, and brucite; Equation (3)) and increases the surface area [86,87,88,89]. Rai et al. [90] demonstrated that pH is mainly controlled by seawater dosage and that the technique is very efficient and cost-effective but is limited by geographical location.

$$6{\mathrm{M}\mathrm{g}}_{\left(\mathrm{a}\mathrm{q}\right)}^{3+}+8{\mathrm{O}\mathrm{H}}_{\left(\mathrm{a}\mathrm{q}\right)}^{-}+2{\mathrm{A}\mathrm{l}\left(\mathrm{O}\mathrm{H}\right)}_{4\left(\mathrm{a}\mathrm{q}\right)}^{-}+{\mathrm{C}\mathrm{O}}_{3\left(\mathrm{a}\mathrm{q}\right)}^{2-}+4{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{l}\right)}\to {\mathrm{M}\mathrm{g}}_5{\mathrm{A}\mathrm{l}}_2{\left(\mathrm{O}\mathrm{H}\right)}_{16}·{\mathrm{M}\mathrm{g}\mathrm{C}\mathrm{O}}_3·4{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{l}\right)}$$

(3)

(3)

Neutralization with gypsum. Gypsum neutralization of red mud induces the precipitation of Ca(OH)₂, tricalcium aluminate, and CaCO₃, and the addition of 5–8% gypsum releases Ca²⁺ and reduces pH to 8.6 [80,91] via the mechanism that Ca²⁺ reacts with carbonate to form calcite to improve buffering capacity (Equation (4)) [92,93,94]. At the same time, carbonate precipitation provides adsorption sites for the immobilization of toxic elements [95], and excess Ca²⁺ may react directly with aluminate to form phases such as hydrocalumite Ca₂Al(OH)₇·2H₂O (Equation (5)) [96]. Although this method is limited by the efficiency of carbonate precipitation in the presence of Ca²⁺/Mg²⁺ deficiency [97], it has been popularized in the field of construction materials and soil remediation due to its ease of operation.

$${\mathrm{C}\mathrm{a}\mathrm{S}\mathrm{O}}_4·2{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{s}\right)}+{\mathrm{C}\mathrm{O}}_{2\left(\mathrm{a}\mathrm{q}\right)}+2{\mathrm{O}\mathrm{H}}_{\mathrm{a}\mathrm{q}}^{-}\to {\mathrm{C}\mathrm{a}\mathrm{C}\mathrm{O}}_{3\left(\mathrm{s}\right)}+{\mathrm{S}\mathrm{O}}_{4\left(\mathrm{a}\mathrm{q}\right)}^{2-}+3{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{l}\right)}$$

(4)

$$2{\mathrm{C}\mathrm{a}\mathrm{S}\mathrm{O}}_4·{2\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{s}\right)}+{\mathrm{A}\mathrm{l}\left(\mathrm{O}\mathrm{H}\right)}_{4\left(\mathrm{a}\mathrm{q}\right)}^{-}+3{\mathrm{O}\mathrm{H}}_{\left(\mathrm{a}\mathrm{q}\right)}^{-}\to {\mathrm{C}\mathrm{a}}_2{\mathrm{A}\mathrm{l}\left(\mathrm{O}\mathrm{H}\right)}_7·2{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{s}\right)}+2{\mathrm{S}\mathrm{O}}_{4\left(\mathrm{a}\mathrm{q}\right)}^{2-}+2{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{l}\right)}$$

(5)

(4)

Neutralization with carbon dioxide. CO₂-neutralized red mud lowers pH via liquid-phase carbonation, where CO₂ dissolution produces carbonic acid that interacts with alkaline components. However, a pH rebound occurs due to the ongoing dissolution of solid-phase alkalinity, such as tricalcium aluminate (TCA), which releases OH^- ions [80,98,99,100]. Equations (6) and (7) represent interactions between CO₂ and hydroxide ions in the caustic solution, whereas Equations (8)–(10) depict the dissolution of TCA in CO₂. TCA interacts with CO₂ to produce calcite and aluminum hydroxide; however, the kinetics are sluggish, and the reaction is reversible. Additionally, residual alkaline minerals, such as kaolinite, present in the solid phase after incomplete conversion, elevate the pH back to the alkaline range by dissolution [98,101]. Yadav et al. [102] determined that a particle size of 30 μm for red mud maximized carbonation efficiency owing to a substantial proportion of the active phase, but larger particles hindered full neutralization due to inadequate solid–liquid interaction. This method sequesters CO₂ and diminishes the necessity for subsequent acid leaching [99]; however, the solid-phase alkalinity buffering effect renders the neutralization effect transient, necessitating the integration of subsequent stabilization treatments (e.g., calcium carbonate generation) to mitigate pH fluctuations [101]. Therefore, it is critical to elucidate the mechanisms of solid-phase alkalinity conversion and develop sustainable neutralization technologies for red mud, which could effectively remediate its persistent high alkalinity [103].

$${\mathrm{CO}}_2+{\mathrm{OH}}_{\left(\mathrm{aq}\right)}^{-}\leftrightarrow {\mathrm{HCO}}_{3\left(\mathrm{aq}\right)}^{-}$$

(6)

$${\mathrm{HCO}}_{3\left(\mathrm{aq}\right)}^{-}\leftrightarrow {\mathrm{H}}_{\left(\mathrm{aq}\right)}^{+}+{\mathrm{CO}}_{3\left(\mathrm{aq}\right)}^{2-}$$

(7)

$${\mathrm{Al}\left(\mathrm{OH}\right)}_{4\left(\mathrm{aq}\right)}^{-}\leftrightarrow {\mathrm{Al}\left(\mathrm{OH}\right)}_{3\left(\mathrm{s}\right)}+{\mathrm{OH}}_{\left(\mathrm{aq}\right)}^{-}$$

(8)

$${\mathrm{NaAl}\left(\mathrm{OH}\right)}_{4\left(\mathrm{aq}\right)}+{\mathrm{CO}}_{2\left(\mathrm{aq}\right)}\leftrightarrow {\mathrm{NaAl}\left(\mathrm{OH}\right)}_2{\mathrm{CO}}_{3\left(\mathrm{s}\right)}+{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{l}\right)}$$

(9)

$${\mathrm{Ca}}_3{\mathrm{Al}}_2{\left(\mathrm{OH}\right)}_{12\left(\mathrm{s}\right)}+3{\mathrm{CO}}_{2\left(\mathrm{aq}\right)}\leftrightarrow {\mathrm{NaAl}\left(\mathrm{OH}\right)}_{3\left(\mathrm{s}\right)}+{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{l}\right)}$$

(10)

2.3. Organic and Metal Compound Modification

Organic modification may increase functional groups (hydroxyl groups, amino groups, etc.) and specific surface area to improve metal ion adsorption in red mud (RM), which tends to agglomerate [104,105,106]: The modification (RM/CS) increased the specific surface area from 68.6 to 105.7 m²/g and the maximum adsorption capacities for nickel (II) and lead (II) reached 31.66 mg/g and 208.48 mg/g, respectively [107,108]. A new method of in situ graft polymerization was used in a reverse suspension system to synthesize red mud/polyacrylic acid (RM/PAA) composite materials. The maximum adsorption capacity reached 96.15 mg/g, which is much higher than that of RM (21.70) [109]. Red mud activated with cetyltrimethylammonium bromide (CTAB) can greatly improve the removal ratio of Cr(VI) up to four-times higher than that of original red mud [61]. Organic matter agglomeration reduces the specific surface area, which must be adjusted. In addition, metal compound modification has improved RM adsorption capability significantly. MnO₂ can complex and remove HMs ions like Cd(II), Cu(II), Zn(II), and Pb(II) due to its excellent specific surface area, high surface activity, and chemical stability [62], but its poor dispersibility and tendency to agglomerate limit its use [110]. Loading it onto red mud carriers improves its dispersibility and adsorption efficiency [111]. MnO₂ alteration increased RM’s specific surface area, pore volume, and surface functional groups [11]. Fe₂O₃ has also been utilized to modify red mud because of its superior adsorption ability and selective removal of numerous HM ions [112]. In research findings, Fe₂O₃-coated red mud adsorbent removed 91.74% of As(III) [113]. Some investigations employed FeCl₃-modified RM to create MRM with variable arsenate adsorption at various pH levels [114]. La-RM, made by loading rare earth element lanthanum (La) over red mud, adsorbs Cr(VI) at 17.35 mg/g [115]. The L-PR composites made by combining phosphogypsum and red mud and modified via La impregnation optimized the structure and composition of the raw materials and significantly increased the specific surface area and pore volume with a uniform pore size distribution. The La-associated oxygen-containing functional groups may be involved in As(V) adsorption, which improve performance [7,116]. In summary, while organic functionalization and metal composite modification markedly improve red mud’s adsorption capacity, their practical application is constrained by material agglomeration, cost-effectiveness, and pollutant selectivity, necessitating the future development of aggregation-resistant composite architectures and integrated optimization frameworks to simultaneously enhance performance, economic viability, and environmental sustainability.

3. Preparation Method of Red Mud-Based Adsorbent

Red mud-based adsorbent synthesis techniques vary depending on the method, from a single hydrothermal approach, alkali fusion, pyrolysis, etc. to a diversity of processes.

Table 2. Advantages and disadvantages of various preparation techniques.

Method	Advantages	Disadvantages	References
Alkali fusion	Effortless operation and rapid reaction time.	Equipment degradation, energy consumption, and alkali use rise.	[117]
Hydrothermal	Superior crystal quality and minimal reaction temperature.	Elevated equipment prerequisites and inadequate safety features.	[118]
Pyrolysis	Elevated resource recovery rate and ecological advantages.	High energy use, complex products, and complicated technology.	[119]
Cured Foam Method	Pollution reduction and productivity boost.	Process complexity, energy, equipment, and material restrictions.	[120]
Carbothermal reduction	High output, simple process control, repeatability.	High reaction temperature, equipment needs, and manufacturing costs.	[121]
CVD method	Very good coverage, scalability, and performance.	Lack of precision, contamination, and material restrictions.	[122]

lists the advantages and disadvantages of several preparation procedures, and Figure 1 shows their process parameters.

3.1. Alkali Fusion

High-temperature melting with alkali fusion may react with alumina and other metal oxides in red mud to create corrosion-resistant aluminate crystals, Figure 1a. Coal fly ash (CFA) is a powdery mineral residue formed as a byproduct of coal combustion. In the past several years, RM and CFA have often been used as adsorbents to remove HMs. Al₂O₃ and SiO₂ are the primary active ingredients in both CFA and RM; different proportions of these two ingredients combined can produce the diverse radioactivity of Si and Al [123]. Zhao et al. [123] adjusted the ratio of CFA to RM and the ratio of silica–alumina synthesis in conjunction with NaOH calcination at 300 °C to create composite C1R4 with a high specific surface area, adsorption capacity, and magnetic characteristics to improve adsorption efficiency and economics. Xie et al. [117] demonstrated that optimal magnetic 4A zeolite synthesis—achieving maximum SiO₂ and Al₂O₃ solubility and high adsorption efficiency—was obtained at a red mud/NaOH mass ratio of 1:2, an alkali fusion temperature of 800 °C, and a reaction duration of 90 min. The alkali fusion method’s essential process parameters (temperature, percentage of alkali, melting duration, etc.) require optimization to achieve the synthesis reaction and maintain the balance between melting conditions and material activity for the optimum benign adsorbent.

3.2. Hydrothermal

Hydrothermal technique: Creating a high-temperature and high-pressure environment promotes the transformation of mineral phases in red mud, Figure 1b, especially the synthesis of zeolite-like (tetrahedral unit framework composed of SiO₄ and AlO₄) adsorbent materials, which have a strong affinity for HM ions due to their excellent ion-exchange capacity and large specific surface area. Researchers found that acid-leaching pretreatment and hydrothermal modification dissolved and recrystallized red mud alumina and silica into a zeolite structure, improving adsorption performance [14,118]. Low-temperature hydrothermal zeolites were synthesized by Belviso et al. [124] to remove HM ions. Other substances can be loaded onto red mud using the hydrothermal method, such as Bai et al.’s manganese dioxide-modified red mud [11], which showed enhanced adsorption effects. Mg-Fe-Al LDHs were prepared by Chai et al. [125], expanding the utilization of red mud-based adsorbents. Optimal preparation conditions were the pH of the reaction precursor (10.0), 4:1 molar ratio (Mg²⁺: (Fe³⁺ + Al³⁺)), 100 °C crystallization temperature, and 12 h crystallization duration. Wang et al. [126] prepared integrated microelectrolytic functional materials (IMCs) by combining hydrothermal and co-pyrolysis processes, and red mud’s high iron content (>30 wt%) can be used to make IMCs [32], suggesting that hydrothermal methods can be enhanced by other methods. They prepared red mud/black liquor (RM/BL) for Cr(VI) removal beforehand [9]. RM/BL had average pore volumes of 0.05, 0.10, and 0.15 cm³/g at 350, 650, and 850 °C, demonstrating that higher water bath temperatures promote porous structures. In addition, Yan et al. [127] produced magnetic zeolites from red mud and coal gangue (CG) using the alkali calcination–hydrothermal technique. Alkali calcination has been used as a pretreatment to improve feedstock aluminum and silicon conversion, and iron in RM can be reduced by carbon in CG under a non-oxidizing atmosphere to form the magnetic phase. Zeolite A diffraction peaks rose at 90 °C hydrothermal temperature. Zeolite A and magnetite diffraction peaks developed simultaneously at 3 h. These diffraction peaks rose steadily with hydrothermal time and peaked at 9 h. Zeolite A and magnetite diffraction peaks increased with increasing hydrothermal time [128]. Liu et al. [129] produced pure single-crystal four molecular sieves from red mud utilizing alkali–melt–hydrothermal crystallization. Hydrothermal methods can produce uniform and defect-free crystals and nanostructures, but the pore structure and morphology of red mud-based hydrothermal products are sensitive to many factors, so the parameters must be finely tuned for high-performance design. Additionally, the synthesis cycle, reaction condition constraints, and solvent selection must be addressed. Thus, future studies should investigate ways for technological integration to increase preparation efficiency and material attributes.

3.3. Pyrolysis

Pyrolysis converts biowaste to biochar through anaerobic high temperature and the co-pyrolysis modification of transition metals (Fe, Al, etc.) in red mud to enhance adsorption performance, Figure 1c. Biochar removes dyes/HMs and recovers energy (pyrolysis of oil, syngas) [119,130,131]. Surface modification using transition metal ion (Fe, Al, and Co) precipitation or co-pyrolysis with metal oxides improves biochar adsorption [119]. As a reaction medium, CO₂ accelerates the cleavage of volatile organic compounds (VOCs) through the thermal degradation of the feedstock, generating high volumes of synthesis gases (particularly CO) [132]. This process provides ideas for the application of Fe in combination with biochar. As mentioned above, adding transition metal oxides from red mud to CO₂ changes pyrolysis and liquid/gas product characteristics, which makes the combined use of iron and biochar more feasible [133]. Furthermore, CO₂ intervention controls Fe phase formation [134], since CO₂ pyrolysis only creates magnetite, whereas N₂ pyrolysis produces both magnetite and Fe⁰ (reason: hematite reduces to magnetite [132]). Other favored redox processes, including CO₂, may potentially lead to the creation of various Fe compounds. For instance, CO₂ with FeO creates CO and Fe₃O₄ [135]. Pyrolysis temperature impacts the material structure, with 700 °C promoting Fe⁰ formation and pore development (e.g., MMBC specific surface area increases with temperature [136,137]; SRMBC pore structure changes from microporous to dense at 600–900 °C [138]). Pyrolysis can produce high-purity, evenly dispersed materials by choosing precursors and managing reaction conditions. Pyrolysis processes are unsuitable for heat-sensitive materials because they may destabilize the material structure by causing excessive crystal growth or disintegration [139,140]. Novel precursors and reaction mechanisms to lower reaction temperature and increase pyrolysis’ application are still needed.

3.4. Cured Foam Method

The cured foaming approach enhances the pore structure and mechanical characteristics of the material, as seen in Figure 1d, by meticulously controlling the foaming parameters, namely the dose of the foaming agent. Geopolymers possess inherent mesoporosity and are capable of adsorbing HMs [141,142]. Porous adsorbent materials were synthesized through integrating red mud and geopolymer. For instance, Yan et al. [120] employed red mud/slag as the precursor, utilizing H₂O₂ foaming to create a microcracked structure that augmented the adsorption sites. Gonçalves et al. [143] achieved a controllable enhancement in pore quantity and size by varying the sodium dodecyl sulfate blowing agent content (1.2–1.7 wt%), although an excessive concentration (2.2 wt%) resulted in phase separation and diminishing porosity [144]. Carvalheiras et al. [145] produced mechanically stable foams with a peak lead adsorption capacity of 30.96 mg/g by optimizing the metakaolin/red mud ratio relative to the blowing agent. In summary, some recent research has concentrated on the impact of blowing agent dose on the material’s adsorption characteristics; however, the challenging management of the material’s internal void structure presents many issues, rendering the foaming technique acceptable only for systems with high mechanical strength. Further optimization of the process is necessary for general use.

3.5. Carbothermal Reduction

The carbothermal reduction process converts Fe₂O₃ in red mud to zero-valent iron (ZVI) using a high-temperature carbon source, such as biomass, while offering the benefits of low cost and environmental sustainability, as seen in Figure 1e. Du et al. [121] synthesized a granular red mud adsorbent (ZVI@GRM) under anoxic conditions at 900 °C, employing corn stover as a pore-forming agent and carbon source, along with a biochar framework to mitigate iron agglomeration and enhance specific surface area. Concurrently, Li et al. [146] implemented carbothermal regeneration at 800 °C to convert Fe₂O₃ into nanozero-valent iron (nZVI), utilizing Al₂O₃/SiO₂ as a dispersant to stabilize nZVI, thereby enabling the regenerated material to effectively remove and immobilize Cr(VI) through the formation of FeCr₂O₄. This method lowers production costs by substituting graphite with biomass; however, the carbon/red mud ratio and temperature must be meticulously controlled to prevent excessive reduction or product inactivation, and the process conditions require optimization to improve stability and application potential.

3.6. CVD Method

In carbon nanotube (CNT) synthesis, chemical vapor deposition (CVD) demonstrates simplicity and cost-effectiveness compared to arc discharge and laser ablation, enabling operation at low temperatures and ambient pressure while achieving high purity, yield, and precise control over growth parameters and structures (Figure 1f). Utilizing CVD for CNT synthesis from red mud, thermally stable oxides (e.g., Al₂O₃, SiO₂, TiO₂) in red mud disperse Fe phases during CVD reactions and promote amphiphilic characteristics in products [147]. Mesgari Abbasi et al. [122] synthesized RM/CNT composites via methane (CH₄) decomposition catalyzed by red mud at 650 °C and 750 °C. The enhanced specific surface area was attributed to irregular CNTs, with optimal growth observed at 750 °C, accompanied by sequential Fe₂O₃→Fe₃O₄→Fe₃C phase transitions and carbon formation [148]. Researchers further explored CNT synthesis on alkali-activated materials (AAMs) derived from red mud, where AAMs (formed by alkali activation of alumina/silica precursors) enhance geopolymers’ mechanical properties [149,150,151]. XRD analysis revealed hematite-to-magnetite/metallic iron transformations, iron carbide phases, and graphite carbon, confirming metallic iron in red mud as catalytic sites for CNT growth [122,151]. While CVD demonstrates potential for synthesizing CNTs from red mud, its large-scale application for producing red mud-derived adsorbents necessitates overcoming key challenges, particularly in process simplification and the development of continuous deposition technologies.

3.7. Comparison and Prospects

Beyond conventional approaches, red mud-based adsorbent preparation encompasses several specialized methods: (1) enhanced precipitation techniques for Fe nanoparticle loading [152]; (2) synthesis of porous geopolymers through alkali-activated red mud-coal gangue systems, utilizing negatively charged aluminosilicate networks for metal ion adsorption [153]; (3) reverse suspension polymerization incorporating carboxyl groups to boost adsorption capacity [109]; and (4) sol–gel-derived red mud/MCM-41 composites exhibiting exceptional Cr(VI) adsorption and reduction capabilities, attributed to their high specific surface area (465 m²/g) [154]. Each synthesis method has its advantages and disadvantages, and the appropriate process should be selected based on the characteristics of the target material and practical application requirements. The solidification foaming method is suitable for low-cost large-scale applications; hydrothermal or CVD methods are suitable for the synthesis of high-precision functional materials; hydrothermal, alkali fusion, or carbon thermal reduction methods are more suitable for preparing materials with high crystallinity and complex structures. In the future, leaching risks can be reduced through functional ligand modification, waste co-utilization, and encapsulation methods such as phosphate solidification or silane coupling agents. Combined with carbon thermal reduction, metal recovery, and machine learning-guided optimization, an “adsorption regeneration” closed-loop, and green sustainable development can be achieved.

4. Key Parameters Impacting HMs’ Removal

pH, coexisting ions, reaction temperature, starting concentration, adsorbent dose, and contact duration impact red mud-based adsorbent adsorption equilibrium. In addition, HMs leaching from the adsorbent, the adsorption–desorption cycle time, and the adsorbent type dose might affect red mud-based adsorbent performance on target pollutants. These and other factors affect the selectivity, adsorption capacity, and adsorption rate of red mud-based adsorbents for specific HMs as well as their stability and competitive adsorption behaviors in complex environments. Understanding these influences is crucial for optimizing the practical application effects of these adsorbents, especially in the process of treating complex wastewater. Researchers and practitioners may customize red mud-based adsorbents to increase their efficiency and efficacy in real-world situations, improving HM pollution mitigation in industrial and environmental settings, as shown in Table S1 in the Supplementary Materials. Utilizing these findings may hasten the development of breakthrough treatment solutions, guaranteeing safer water supplies for HM-polluted areas.

4.1. pH

The pH is a critical factor influencing the efficacy of red mud-based adsorbents [155]. The pH of the solution markedly influences the adsorption efficacy of red mud-based adsorbents by altering their surface charge and the shape of HM ions; the transition between protonated and deprotonated states impacts electrostatic interactions [156]. Various red mud-based materials exhibit distinct pHpzc values, defined as the pH at which the zeta potential of the adsorbent equals zero [157], as shown in Table S2 in the Supplementary Materials. Under acidic conditions (pH < pHpzc), protonation results in an augmented surface positive charge, leading to electrostatic repulsion with cations (e.g., Pb²⁺, Cd²⁺) and competition for adsorption sites with H⁺ [123,158]. Consequently, the adsorption of heavy metal cations in water diminishes, obstructing their binding to the material [7]. Conversely, under alkaline conditions (pH > pHpzc), deprotonation increases the negative surface charge, facilitating the electrostatic attraction of cations; however, OH⁻ can induce the precipitation of metal hydroxides [155,159]. Additionally, anionic contaminants (e.g., CrO₄^2- from Cr(VI)) are not adsorbed in water at pH > pHPZC due to blocked electrostatic repulsive adsorption, necessitating chemical reduction (e.g., Cr(VI) → Cr(III)) to improve the removal rate [160]. The variations in pHZC among various materials dictate the suitable pH range, as seen in Figure 2e. Zhao et al. [123] discovered that Pb/Cu/Cd adsorption reached saturation at pH 6–7, whereas optimum Cr(VI) adsorption mostly occurred under acidic circumstances to avoid electrostatic repulsion. Moreover, variations in the affinity of ionic species for functional groups (e.g., the extent of COOH ionization escalates with pH [109]) and the potential for material component dissolution (e.g., under low-pH conditions, the adsorption capacity of arsenic (V) is relatively low, which may be related to the dissolution of magnetite/Fe⁰ under highly acidic conditions, resulting in the elimination of adsorption sites for arsenic (V) in the adsorption material [134]) further limit adsorption efficacy. Therefore, it is imperative to optimize pH conditions according to the properties of the target pollutants to attain effective adsorption.

4.2. Coexisting Ion

Through the complex formation of coexisting ions and competitive adsorption, ionic strength has a significant impact on the performance of red mud-based adsorbents. The authors comment on the results from 15 references. For example, Cu²⁺ and Mn²⁺ compete for adsorption sites based on differences in hydration radii (0.419 nm and 0.438 nm) and hydration energies (Cu²⁺: 2121 kJ/mol and Mn²⁺: 1862 kJ/mol) [163]; the actual adsorption may be related to the electronegativity and hydrated ionic radius of Cu²⁺ and Mn²⁺. In addition to having a smaller hydrated radius and a charge center that is closer to the material’s surface, Cu²⁺ has a greater electronegativity (1.90) than Mn²⁺ (1.55). This gives the surface of the adsorbent based on red mud more affinity for Cu²⁺ than for Mn²⁺. The efficacy of heavy metal adsorption is further reduced by high quantities of K⁺, Na⁺, Ca²⁺, and Mg²⁺ in the water column, which compete with heavy metal cations [164]. Among these, highly charged/small-radius ions (e.g., Ca²⁺) demonstrate stronger competitiveness [11,123,165], while anions modify adsorption mechanisms through complexation—As(V)-Ni(II) complexes synergistically enhance adsorption [166], and specific anions like HCO₃⁻ improve Sb(III) uptake via surface coordination [167]. Through competition or inhibition of reduction, SO₄²⁻/PO₄³⁻, etc., lower the removal of Cr(VI)/Sb(III) [152,168,169,170,171]. However, the adsorption capacity or removal efficiency of the adsorbent reduces when the system comprises two or more cations compared to a single system [129]. As illustrated in Figure 2f, the order of adsorption inhibition is associated with the ionic charge/hydration characteristics under multi-ion systems (e.g., Pb-Cu-Cd coexistence) [123]. To reduce the interference of the coexisting ions in the practical application, this must be combined with pHpzc modulation and the optimization of the adsorbent selectivity.

4.3. Temperature

The adsorption of HMs by the red mud-based adsorbent was mostly negative ΔG°, positive ΔH°, and positive ΔS°, as shown in Table S3 in the Supplementary Materials. This suggests that the adsorption process was spontaneous. The capacity of the adsorbent is strongly influenced by the adsorption temperature. Adsorption is improved by moderate heating because it raises the collision between the adsorbent and the pollutant and improves molecular thermal activity [172]. Within a certain range, the adsorption capacity rises as the temperature rises [121]. The authors comment on the results from five references. Fe_xO_y-BC(RM) was prepared by Qi et al. [161] for the adsorption of Cd(II), as shown in Figure 2b. At the same concentration, the adsorbent’s adsorption capacity rose as the temperature rose. Cui et al. [8] showed that under ΔG° < 0, ΔH° > 0, and ΔS° > 0 circumstances, the adsorption of Cd(II) by RM@HC was enhanced by the temperature rise. Yao et al. [173] demonstrated that elevated temperatures promote both the dispersion and mass transfer of Cd(II) in aqueous solutions, while simultaneously enhancing its adsorption and immobilization onto the adsorbent surface. Furthermore, Awual et al. [174] proposed that adsorbate ions diffuse from the outer layer to the adsorbent active sites more quickly at higher temperatures due to increased kinetic energy. A higher temperature may result in more surface active sites, which would improve the adsorbent’s adsorption capacity [175]. Despite enhanced adsorption at elevated temperatures, practical applications should prioritize energy efficiency by minimizing temperature variations.

4.4. Initial Concentration and Adsorbent Dosage

Some researchers posit that the initial concentration and adsorbent dosage significantly influence the adsorption and removal of HMs by red mud-based adsorbents. When the red mud-based adsorbent is not saturated, HM adsorption rises with the starting concentration [138]. Concentration gradients cause pollutants to interact with adsorbent active sites [176]. Higher initial concentrations of HMs enhance mass transfer but saturate adsorption sites [11], reducing efficiency. Excessive initial concentrations of HM coverage inhibit further adsorption on red mud-based materials. Rising concentrations also shift adsorption from monolayer to multilayer, affecting isotherm model fitting. An alternative approach for HMs’ removal involves optimizing the dosage of red mud-based adsorbents. Cui et al. [8] demonstrated that RM@HC achieved maximum Cd(II) adsorption capacity of 113.55 mg/g at an optimal dose of 1.5 g/L (Figure 2c). Due to strong reactive sites in solution, adsorption saturated at 2 g/L with a reduction efficiency of 99.82% (143.68 mg/g) [177]. Due to the adsorbent cost and high metal ion removal, high dosages are required.

4.5. Contact Time

Concerning the effect of contact time, the authors comment on the results from one reference. Red mud-based adsorbents’ adsorption capacity and active site count determine their equilibrium adsorption time. Adsorption begins with HM ions binding quickly to the adsorbent active sites. Adsorption decreases over time due to site saturation. Gonçalves et al. [162] created 3D-printed lattices for Ni, Cu, Cd, Zn, and As adsorption and found equilibrium after 24 h (Figure 2d). High-concentration gradients and abundant active sites promote rapid initial adsorption, which slows as binding sites become saturated. Other modifiers may raise the specific surface area and negative charge density of the red mud-based adsorbent to boost its affinity for HM ions.

4.6. Types of Adsorbents

Concerning the effect of types of adsorbents, the authors comment on the results from one reference. Different adsorbents have different structural characteristics and functional groups that impact adsorption. Adsorbents with oxygen-containing groups (e.g., -COOH, -OH) have more adsorption active sites, improving adsorption. Adsorbents with smaller particle sizes and larger specific surface areas have more interaction with contaminants, improving adsorption. As illustrated in Figure 2a, Shang et al. [7] observed that PG, RM, PR, and L-PR had varied clearance rates in individual and binary Cd(II)/As(V). Functional groups (La-O) formed complexes with Cd(II)/As(V) in monoadsorption and ternary complexes in coadsorption after lanthanum (La) alteration. Specific surface area, pore volume, and pore size rose increase adsorption.

4.7. Reusability and Regeneration

Recovery and reuse are crucial to cost-effective and appealing adsorbents. Recycling adsorption performance is an important parameter since it affects treatment cost. Red mud-based adsorbents typically exhibit declining efficiency after multiple regeneration cycles due to permanent site occupation and loss of unstable functional groups (e.g., carboxyl and hydroxyl) during adsorption–desorption processes [138,178].

The commonly used desorbents primarily include NaCl, NaOH, HCl, HNO₃, EDTA, etc. The binding mechanism between different desorbents and HMs affects the regeneration of red mud-based adsorbents. Firstly, NaCl desorbs cations (e.g., Cu²⁺, Cd²⁺, and Pb²⁺) through ion exchange, but the active site loses its activity after multiple cycles, reducing the adsorption capacity (e.g., MZ material maintains 52.89–82.87 mg/g after five cycles [127], porous sphere three cycles, removal efficiency at 54.60% [120]). Secondly, NaOH can desorb As(V) [134], but strong alkaline conditions (pH > 12) deteriorate the adsorbent [179]. Thirdly, strong acids (HCl, HNO₃) dissolve metal precipitates but damage adsorbent pores and cause contamination. HNO₃-desorbed NaSi-MGP lost 60% Pb²⁺/Cu²⁺ capacity after five cycles [153], while solid–liquid separation incurs recovery losses [152]. H ions have a considerable affinity for the active sites on the adsorbent surface [180]; therefore, large amounts will compete with metal ions for active sites. Fourth, EDTA gently desorbs HMs (e.g., Pb²⁺, Hg²⁺) under its chelating effect [153], and regeneration efficiencies outperform acids/bases (e.g., RM-nZVI maintains high regeneration for Hg after EDTA desorption [10]). Therefore, optimizing the adsorbent to balance desorption efficiency, material stability, and environmental risks and exploring the multi-cycle regeneration strategy to mitigate functional group loss and structural collapse are needed in the future.

4.8. Leaching of HMs and Economic Viability

Existing studies show that red mud-based adsorbents’ leaching concentrations are usually below the national safety standard (GB5085.3-2007) [181], and geopolymers can further inhibit leaching by encapsulating HMs through dense calcium carbonate hydrates [182]. However, most studies lacked systematic leaching tests, and only a few studies verified their results. Most studies lacked comprehensive leaching testing, and few proved their safety (

Table 3. Leaching of HMs in red mud-based adsorbents (mg/L⁻¹).

Test Methods	Absorbent	Cd	Pb	Cr	As	Zn	Cu	Ni	References
GB5085.3-2007	RM	0.0081	0.28	0.291	2.267	0.43	0.132	ND	[11]
	RM	ND	0.576	1.327	NO	NO	0.0104	NO	[153]
	Mn-RM	0.016	0.21	0.54	ND	0.07	0.126	ND	[11]
	Red mud	ND	ND	ND	ND	0.37	0.06	0.06	[143]
	NaSi-MGP	ND	0.327	ND	NO	NO	0.0104	NO	[153]
	Norm	1	5	15	5	100	100	5	[181]

Note: ND: Not detected; NO: The item was not tested.

). Gonçalves et al. [143] found that Na, Ca, and Si were significantly released in the first 24 h at pH 3, for 28 days, and slowly accumulated, while K, Al, and Fe were released in lower amounts and posed little risk of leaching trace HMs like Mn, Zn, and Ni. Nevertheless, the environmental risks of long-term release behaviors in complex systems require rigorous assessment. Ensuring ecological safety demands standardized leaching protocols and advanced adsorbent encapsulation technologies.

Economic feasibility is a primary consideration in adsorbent design for large-scale applications. Red mud, as a solid waste material with readily available characteristics, has led to the development of a series of red mud-based adsorbents. However, to date, summaries and analyses of red mud-based adsorbents have primarily focused on their removal efficiency for target pollutants, with limited detailed analysis from an economic feasibility perspective. Given the diversity in raw material selection and production processes for red mud-based adsorbents, calculating costs presents challenges. Therefore, this section summarizes the wholesale prices and advantages/disadvantages of red mud compared to several low-cost commercial adsorbents through

Table 4. The unit chemical cost, and the advantages and disadvantages of using low-cost adsorbents.

Materials	Material Price (CNY kg−1) c	Advantages	Disadvantages
Soil a	0.43–6.34	Richness, accessibility	Limited affinity, requiring special handling and disposal
Rock a	0.035–8.33
Clay a & b	0.12–11.10	Excellent porosity, large surface area	High doses require special handling and disposal.
Iron oxide a & b	0.034–300.00	Inherent magnetism	Dependent on environmental conditions
Apatite a & b	8.00–380.00	Availability and efficiency	Eutrophication, ion (NH4+) leaching
Struvite a & b	4.00–2400.00
Zeolite a & b	0.15–5.00	Honeycomb porous structure with a large specific surface area	Recycling is challenging and inefficient, using solvent heat/hydrothermal treatment, which increases material costs.
Cellulose a & b	0.99–94.1	Unique structure and function provide specific active adsorption sites	The processing procedure is complex, the scope of application is limited, and selectivity and affinity are limited.
Alginate a & b	10.00–120.00	Rich in functional groups (-OH and -COOH)	Has a certain solubility and needs to be modified.
Chitin/chitosan a & b	1.00–900.00	Rich in amino and hydroxyl groups	Poor mechanical stability, easily soluble in acidic solutions
Biochar a & b	0.80–130.00	Large specific surface area, rich in oxygen-containing functional groups	The properties may fluctuate depending on process factors (temperature) and contain toxic substances (HMs, etc.).
Agricultural and local wastes b	0.001–11.00	Low cost, easy to obtain	May release toxic elements, have limited mechanical strength, and have adsorption capacity

Note: ^a, wholesale price from https://www.1688.com/ on 8 September 2024. ^b, wholesale price from https://b2b.baidu.com/ on 8 September 2024. ^c, CNY: Chinese Renminbi.

and

Table 5. Overview of current wholesale prices for red mud in several public markets.

Iron Content (Fe2O3)	Price (CNY/ton)	Price (CNY/kg)	Source Company	Time
≥64%	80.00	0.080	China Aluminum (Zhengzhou) Aluminum Industry	2025.05
60~64%	Base price + floating rate *	0.070~0.075 *	China Aluminum (Zhengzhou) Aluminum Industry	2025.03
56~60%	Base price + floating rate *	0.065~0.070 *	China Aluminum (Zhengzhou) Aluminum Industry	2025.03
50~56%	1.20	0.0012	China Aluminum (Zhengzhou) Aluminum Industry	2024.12
Not specified (industrial waste residue)	46.46	0.046	Tianjin Jinyu Zhenxing Environmental Protection	2025.02

Note: * For every 4% increase in iron content, the price rises by approximately 5 RMB/ton (i.e., 0.005 RMB/kg) based on the benchmark price (not disclosed). The quoted price is for as-received material. The actual dry basis price must be calculated based on the moisture content.

. The aim is to provide insights for designing new materials by combining red mud with these low-cost adsorbents. As shown in Table 5, the wholesale price of red mud (0.0012–0.08) increases with the increase in iron oxide content, but it is still lower than the prices of several adsorbents in Table 4. Red mud, rich in aluminum oxide, silicon dioxide, and iron oxide, can be used as raw material for synthetic zeolite and zero-valent iron. In addition, appropriate synthesis processes can be selected based on material characteristics. This content was discussed in previous sections.

5. Mechanism of HMs’ Removal

HMs are highly toxic environmental pollutants that pose significant health risks, including acute poisoning and chronic diseases, even at minimal concentrations. HMs are always present in different forms, such as cations and anions, when they enter water bodies. Research on red mud is becoming more and more comprehensive, ranging from single materials to composites, and many studies have shown that various red mud-based adsorbents are effective in removing HMs from wastewater (Table S1). So, the differences in the removal mechanisms of HMs will be specifically analyzed in terms of adsorption models, adsorption kinetics, and adsorption mechanisms.

5.1. Sorption Isotherm Model

Adsorption equilibrium occurs when adsorption and desorption rates balance. Researchers utilize adsorption isotherms and kinetic models to describe adsorption [183,184]. Temperature governs the distribution ratio of adsorbed versus free-phase contaminants in aqueous adsorption systems [185]. HM adsorption on red mud-based adsorbents is studied using the Langmuir and Freundlich isotherms. Supplementary Table S4 shows that the HM adsorption study using terracotta-based materials uses the Langmuir isotherm, indicating chemical monolayer adsorption with consistent surface adsorption [123], homogeneous monolayer adsorption on the adsorbent surface with a few adsorption sites. Adsorption capacity is maximized when all adsorption sites are monolayers [19]. The Freundlich isotherm is an empirical equation for multilayer adsorption without assumptions [185]. Freundlich isotherm analysis reveals three adsorption regimes: ① physisorption (n > 1) dominates in MZ-Cu²⁺ (n = 6.47), Fe-C-CO₂-As(V) (n = 2.88), and SRMBC-750 systems [138]; ② chemisorption occurs when n ≤ 1, as demonstrated by L-PR (1/n = 0.141) [7]; ③ adsorption becomes unfavorable when n < 0.5 [186]. Notably, Cu²⁺/Cd²⁺/Pb²⁺ adsorption shifts from multilayer to monolayer with increasing ionic radius, while Fe-C-CO₂ shows metal-specific n values (Ni(II): 6.05) [134]. Ni(II) absorption by RM/CS supports the Sips theory. As temperatures rise, α_s remains non-zero, indicating adsorption via monolayer and multilayer processes. The weaker van der Waals interactions of Ni(II) ions promote a transition from multilayer to monolayer adsorption [107].

5.2. Adsorption Kinetic Model

To understand adsorption kinetics, it is necessary to study changes in adsorption quantities or rates. The adsorption of HMs onto red mud-based adsorbents follows both pseudo-first-order and pseudo-second-order kinetic models, with the latter being predominant (Table S4). This prevalence indicates that chemisorption is the dominant adsorption mechanism. The pseudo-first-order model implies that the adsorption process is mainly driven by the adsorbent concentration, while the pseudo-second-order model represents the rate of chemical reaction between the adsorbent and the adsorbate [187]. To better understand the adsorption mechanism of red mud-based adsorbents, several studies have used the intraparticle diffusion model, the Elovich kinetic model, and the Boyd equation, which suggest that film diffusion also controls the adsorption rate [120,153,188]. The intraparticle diffusion model describes the kinetic process as three stages: transient boundary diffusion, inner-surface diffusion, and equilibrium adsorption [153]. In these kinetic models, the contact time certainly affects adsorption. Since various HMs reach equilibrium at different times, the contact time should be considered, especially in binary or polymeric heavy metal systems.

5.3. Adsorption Mechanism

Heavy metal(loid) adsorption by red mud-based adsorbents has several mechanisms (Figure 3). ① Ion exchange: K, Na, Ca²⁺, and Mg²⁺ are exchanged with heavy metal cations thanks to numerous adsorption sites [14] (Figure 4d). ② Complexation: Surface oxygen-containing functional groups complex with Pb²⁺ [137], surface complexation of amorphous iron oxides (nFe₃O₄ and its byproducts FeO(OH) and Fe(OH)₃) with Sb³⁺ [152], and hydrogen bonding between hydroxyls and antimony molecules remove Sb(III) (Figure 4a). Metal ions may form inner-sphere ternary complexes with sulfate and mineral surface hydroxyl groups to increase adsorption [189,190,191]. ③ Chemical precipitation: Precipitation of metal ions as hydroxides or carbonates occurs. The oxidation of Fe⁰ produces OH− by reacting with water, and the generated OH⁻ reacts with Pb ions in the solution to form hydroxide precipitates (Pb(OH)₂). Subsequently, Pb(OH)₂ reacts with Pb²⁺ and CO₃²⁻ to form Pb(CO₃)·Pb(OH)₂ [137], and C-H hydrolysis releases Ca²⁺ and OH^- into the water [13] (Figure 4c). ④ Adsorbent pore size distribution and surface area determine physical adsorption [10,192,193]. Jiang et al. [138] used adsorption isotherms and kinetic experiments. First, zeta potential analysis was used to determine the material’s surface charge distribution under different pH conditions. The Freundlich constant n values were all greater than 1, indicating that physical adsorption was dominant. The negatively charged surface of SRMBC-750 is ideal for Cu²⁺ adsorption at pH > 3. Electrostatic adsorption has a potential influence throughout [194]. FTIR and XRD spectra showed that oxygen-containing functional groups complexed to modify the C-O peaks from the O-H peaks [195]; these changes reflect the stretching vibrations of -OH and C-O, indicating that the functional groups on SRMBC-750 effectively adsorbed Cu²⁺. The presence of C-C/C=C in C1_s spectra shows that the ‘cation-π’ may also be engaged in Cu²⁺ adsorption. ⑤ Electrostatic attraction: solution pH and adsorbent pH_PZC alter electrostatic interactions [194]. FTIR study showed that Cd(II)/As(V) adsorption on L-PR includes alkoxy C=H groups, indicating electrostatic attraction [7]. ⑥ Reduction converts hazardous high-valence metal ions to low-valence states. Fe⁰ reduces Pb(II) and Cr(VI) in redox processes (Figure 4b), according to XRD, SEM, and XPS [9,10,121].

6. Conclusions and Outlook

Due to its porous structure, elevated specific surface area, and abundant Fe/Al/Si oxides, the red mud-based adsorbent effectively eliminates HMs from wastewater through ion exchange, chemical precipitation, and complexation, with the adsorption process primarily aligning with pseudo-second-order kinetics and the Langmuir model. It can be combined with industrial/biological wastes (e.g., gangue, phosphogypsum, biochar) and technologies such as hydrothermal and co-precipitation to make composites with high adsorption capacity and regenerability, such as zeolites, magnetic materials, geopolymers, and so on. Practical applications still face problems, such as high preparation costs, poor scale-up practicality, and environmental hazards (e.g., leaching of modifiers). Future goals include the following: pore/group optimization for enhanced adsorption, low-energy synthesis, resource diversification, improved risk assessment, and scalable wastewater treatment validation.