Efficient Inorganic Stabilization Materials for Chromium and Arsenic Pollution in Water and Soil

Abstract

Chromium and arsenic, as prevalent heavy metal contaminants in water environments, pose significant threats to ecological systems and public health, necessitating urgent remediation measures. Conventional remediation techniques face challenges including high costs, prolonged remediation cycles, limited durability, and secondary contamination risks. While stabilization materials have emerged as promising solutions, the complex stabilization mechanisms for chromium and arsenic remain diverse and have not yet been fully elucidated. With reference to previous research, this paper systematically reviews inorganic stabilization materials for chromium and arsenic contamination remediation, with particular emphasis on elucidating their stabilization mechanisms and influencing factors. This review extensively evaluates various material types to inform practical applications, while highlighting investigations into novel composite materials, which advance technological innovation in water environmental remediation. It offers novel perspectives for addressing chromium and arsenic pollution challenges, potentially driving the development of more sustainable remediation strategies.

1. Introduction

Chromium and arsenic are widely present in nature as toxic heavy metals. Chromium is usually found in two forms: Cr(III), an essential trace element in mammalian organisms, and Cr(VI), classified as a Group I human carcinogen [1]. Figure 1 shows the diseases caused by chromium and arsenic on the human body. The presence of chromium and arsenic in drinking water has been proven to pose multiple hazards to human health. The relationship between arsenic exposure and cardiovascular disease has been confirmed by multiple studies. For example, Smith et al. [2] found that arsenic exposure is related to an increased incidence rate of carotid atherosclerosis. In addition, chronic arsenic poisoning may lead to skin lesions, such as pigmentation and keratosis, as well as neurological problems, such as tingling sensations in the hands and feet. Hexavalent chromium (Cr(VI)) has been classified as a carcinogen, and its toxic mechanisms include DNA damage, genomic instability, and the generation of reactive oxygen species (ROS). Chromium exposure may lead to diseases of the skin, liver, kidneys, reproductive system, and nervous system. These studies indicate that exposure to chromium and arsenic has significant toxic effects on multiple organ systems, emphasizing the importance of reducing exposure to these heavy metals [3]. Arsenic is distributed in both organic and inorganic forms in nature, mainly as As(III) and As(V) [4]. The sources of chromium and arsenic are primarily identified as industrial emissions, mining, agricultural fertilization, and wood anti-corrosion treatment (Figure 2). Chromium and arsenic could penetrate soil and groundwater through precipitation, soil erosion, and fertilization. Exceeding natural background levels, they trigger soil acidification/salinization, plant growth is inhibited to alter microbial community structures, and the balance of the ecosystem is ultimately disrupted. For example, oxidative imbalance and mutations could be caused by Cr(VI), significantly suppressing plant biomass production and reproductive development [5]. Furthermore, aquatic chromium and arsenic undergo biomagnification through food chains, demonstrating dose-dependent correlations with human hepatorenal dysfunction and carcinogenesis [6]. At present, chromium and arsenic contamination is still recognized as a serious problem worldwide; over 200 million people are exposed to arsenic-contaminated groundwater globally [7,8,9,10]. Specifically, the World Health Organization (WHO) estimates that at least 150 million people worldwide drink water with arsenic levels exceeding the WHO safe limit of 10 μg/L in over 70 countries [11]. In terms of chromium pollution, these polluted sites are primarily concentrated in South Asia, where about 75% are associated with the leather industry; a survey by Karunanidhi et al. [12] found that 66% of the groundwater samples collected from 23 sampling points near the Para River in India had chromium levels that severely exceeded the safe range set by the World Health Organization, highlighting the urgency for remediation innovation.

Conventional remediation technologies such as electrochemical treatment, chemical precipitation, coagulation-flocculation, membrane filtration, ion exchange, bioremediation, and adsorption have been extensively applied [13]. Electrochemical remediation is used to drive heavy metal ions to precipitate in weakly acidic or neutralized electrolytes as hydroxides, thus allowing pollutants to be removed [14]. In contrast, adsorption via biochar or zeolites capitalizes on high specific surface areas and active sites [13]. Microbial remediation employs enzymes for reduction of Cr(VI) to less toxic species [15].

Despite the fact that these traditional methods have their own advantages in practical applications, their long-term effectiveness remains a significant issue to be addressed in chromium and arsenic pollution treatment. For example, electrochemical treatment may face the problem of increased long-term operating costs due to electrode degradation [16]. Biochar or zeolite adsorbents may experience decreased adsorption efficiency after multiple uses, making long-term stable operation challenging [17]. Although microbial remediation is green and environmentally friendly, its long remediation cycle may not meet the demand for rapid pollution control [18]. These issues highlight the need for further exploration of more sustainable and long-term stable remediation technologies in the treatment of chromium and arsenic pollution. In this context, the emergence of chromium and arsenic stabilizers has provided a new solution to solve this problem. Stabilization is more widely used in the remediation of chromium and arsenic contaminated soil than other remediation technologies because it is fast, effective, convenient, and cheap, especially in large-scale applications [19]. This strategy introduces stabilizers into contaminated media to initiate physical encapsulation, chemical precipitation, surface complexation, and ion exchange [20], effectively reducing the bioavailability of chromium and arsenic [21]. In recent years, with the improvement of solidification technology, chromium and arsenic stabilizers have been widely used in soil remediation, sewage treatment, and other aspects. For example, the mixture of poly ferric sulfate (PFS) and calcium hydroxide (Ca(OH)₂) was used to stabilize arsenic in abandoned arsenic plants [22]. In addition, extensive exploration was carried out by researchers in the preparation and performance optimization of stabilizers to improve their fixation ability and stability to pollutants. The co-pyrolysis of red mud and rice straw achieved the removal of Cr(VI) [23]. Chromium and arsenic stabilizers were found to not only be applicable in actual projects of heavy metal pollution remediation but also have good economy and practicality, and they are expected to become an important technical means of chromium and arsenic pollution treatment. Through continuous optimization and innovation, chromium and arsenic stabilizers are anticipated to play a more important role in future environmental governance, to promote the sustainable use of water and soil resources and ensure ecological security and human health.

In the previous era of emerging technologies, the stabilization mechanism of various repair materials for chromium and arsenic was explored in depth, and their performance was optimized, making it a research hotspot. Many new materials, such as nano metal oxides and MXenes, were demonstrated to have excellent stabilization potential due to their unique structures and properties. Nanometal oxides, because of their large specific surface area, were able to efficiently remove chromium and arsenic through various methods, such as electrostatic adsorption and redox reactions. Meanwhile, MXenes, as a novel two-dimensional material, provide for ion diffusion and adsorption due to their adjustable interlayer spacing and abundant surface functional groups. Clay mineral materials are found to be stable and able to function under mild conditions, and some materials are gradually emerging in practical applications due to their wide range of sources and low cost. However, research on these new materials is still in the primary stage, and many challenges in terms of long-term stability, large-scale preparation processes, and environmental compatibility in practical applications are present, which urgently need further research. Zhang et al. [24] showed that the morphology and stability of arsenic changed with the growth of Fe(III)-As(V) co-precipitate over time, which indicated that long-term stability needs to be further studied. Some materials are costly to prepare and may cause secondary contamination [25].

Globally, chromium and arsenic pollution not only affects soil and water quality but may also pose potential hazards to human health through food chain transmission. Therefore, developing efficient, economical, and sustainable pollution remediation technologies is of significant global importance. This article reviews the latest progress of inorganic stabilization materials in the field of chromium and arsenic pollutant stabilization. The structure–performance correlation of typical inorganic stabilizers such as phosphates, layered double hydroxides (LDHs), zero-valent iron, and their modified materials was summarized. The stabilization mechanisms of chromium and arsenic based on surface complexation, co-precipitation, ion exchange, and redox reactions were analyzed. And the influence of environmental pH, stabilization time, and coexisting ions on the stabilization effect was discussed. This review will provide theoretical support and a decision-making basis for the development of high-performance and low environmental disturbance stabilization materials and technologies.

2. Inorganic Stabilization Materials

During the past decade, the chemical stabilization and remediation of chromium and arsenic in contaminated soil have been extensively investigated. For example, the leaching behavior of chromium in contaminated soil stabilized using Portland cement and slag was studied by Lindh et al. [26]. A variety of remediation techniques for heavy metal contaminants in soils were reviewed by Azhar et al. [27]. In addition, the effectiveness of sulfate-reducing bacteria in the removal of chromium from acidic mine wastewater was investigated by Dong et al. [28]. The stabilization materials of chromium and arsenic are shown in Figure 3. A range of stabilization materials were continuously developed and researched. These materials are predominantly designed to alter the existing forms of contaminants in soil through physicochemical interactions with Cr/As, resulting in reduced mobility, bioavailability, and toxicity of the target elements. Within the domain of chromium and arsenic pollution mitigation, inorganic stabilizing agents are recognized as pivotal components; they produce effects in chromium and arsenic contamination remediation through multifaceted mechanisms. These materials not only demonstrate excellent technical performance but also offer significant economic advantages. For example, Sekula et al. [29] utilized 150 kg of low-cost iron-based materials to treat 360 L of mine wastewater per hour. Additionally, zinc peroxide material can simultaneously remove chromium and arsenic, thereby further reducing processing costs [30]. Although graphene has a high production cost, its efficient adsorption performance and renewability make it cost-effective in treating pollutants [31].

The principal categories of inorganic stabilization materials, including clay minerals, lime-based substances, phosphate compounds, and nanoscale metal oxides, are systematically examined in subsequent sections. A comprehensive analysis was conducted to evaluate the stabilization mechanisms and treatment efficiencies of these materials for chromium and arsenic, along with their operational advantages and technical constraints in field applications. This critical assessment was undertaken to establish a robust theoretical framework and practical guidelines for chromium/arsenic contamination management, with the dual objectives of promoting fundamental research in environmental remediation technologies and addressing this pressing global ecological challenge.

2.1. Phosphate Compounds

Calcium phosphate, aluminum phosphate, and iron phosphate are all recognized as common phosphate compounds, which are stabilized by generating metal phosphate precipitation of Cr and As. Soluble phosphates such as KH₂PO₄ or H₃PO₄ are used as sources of phosphate [36]. CaCl₂·2H₂O, FeCl₃·6H₂O, and (NH₄)₂HPO₄ were adopted for synthesizing iron-doped hydroxyapatite (Fe-HAP) by Yang et al. [37]. XRD characterization showed that the main components of Fe-HAP were HAP, Fe₂O₃ and FeOOH, and the characteristic peak of Fe₃(AsO₄)₂·6H₂O appeared after adsorption of As(V) (Figure 4a). The residual state after repair increased from 7% to 18.1% (Figure 4b). The results show that Fe-HAP is a monolayer adsorption and precipitation adsorption process for As(V) (Figure 4c). Fe⁰ powder, hydroxyapatite (HAP), and citric acid were ball milled by Jiang et al. [38] to obtain iron and phosphate slow-release materials of CA-Fe⁰/HAP. The slow-release generated iron was combined with As, while the rest was formed into iron oxide (hydroxide), which could also combine with As. This raised the residual state of As from 58.01% to 93.82% (Figure 4d). Magnesium ascorbate phosphate (MAP) and phytase were used by Han et al. [39] to reduce and stabilize Cr(VI). Phosphate stabilizers were applied to soil contaminated with Cr(VI) to remove 99.9% of 100 mg/kg Cr(VI) after 15 days (Figure 4e). Phosphate stabilizers for soil remediation can also meet the needs of plant growth, achieving ecological and stabilization synergistic treatment.

2.2. Nano-Metal Oxides

Nanometal oxides are referred to as metal oxide materials with particle sizes ranging from 1 to 100 nm. They include TiO₂, ZnO, Fe₂O₃, CeO₂, etc. The specific surface area of nanometal oxides is usually considered to be very large. It is noted that the large specific surface area not only provides more reaction sites but also enhances its adsorption capacity. Yan et al. [40] mechanically activated limonite by a ball mill. After mechanical activation, the specific surface area of limonite increases. Compared to untreated brown iron ore, the adsorption rates of As(V) and As(III) have increased by 12.23 and 2.99 times, respectively (Figure 5a). Due to the formation of a relatively stable inner-ball complex, the addition of 10% ball milled limonite increased the residual state F5 of As in the soil by 16.25% (Figure 5b).

Nano metal oxide dual functional materials have been developed, which combine photocatalytic performance and adsorption function and have received high attention. The Fe₃O₄ magnetic core prepared by Yu et al. [41] was encapsulated by a dual functional p-n heterojunction constructed by FeOOH modified carbon-doped TiO₂ (FeOOH@C/TiO₂@Fe₃O₄), which is spherical in shape (Figure 5c). Under visible light irradiation, the carbon doping of TiO₂ was found to reduce the bandwidth, promote the separation of photo-generated electrons and holes, and generate active substances (such as ·OH and ·O₂⁻) that could oxidize As(III) to As(V). The adsorption process conforms to chemical adsorption. Constructing a material model, the adsorption energy was calculated to be only −1.9732 eV (Figure 5d). Nickel-modified zinc oxide (ZnO-Ni) was synthesized by Ravbar et al. [42]. Within 150 min, 22% of Cr(VI) was removed under illumination via zinc oxide. Under visible light irradiation, it was found that nickel oxygen clusters promoted the transfer of interfacial charges from the valence band of ZnO to nickel. The photo-generated electrons could be used to directly reduce Cr(VI) to trivalent chromium (Cr(III)) (Figure 5e). Ma et al. [43] also found that photo-generated electrons play a major role in the reduction of Cr(VI).

2.3. Iron/Aluminum Based Materials

Iron/aluminum-based stabilization materials utilize their high specific surface area and surface functional groups to reduce the toxicity of heavy metals in soil and water. The most typical iron-based material is zero-valent iron (ZVI). Cr(VI) removal was enhanced by Wu et al. [44] through the use of carboxymethyl cellulose (CMC) and zero-valent iron nanoparticle complex (CMC-nFe⁰). Adding CMC can reduce 53% of Cr(VI) on the surface of nFe⁰ (Figure 6a). However, it was found that using nFe⁰ alone resulted in an agglomeration phenomenon. Iron/aluminum-based materials also include ferrous sulfate (FeSO₄), ferrous chloride (FeCl₂), aluminum oxide (Al₂O₃), etc., which have unique dispersibility. Alidokht et al. [45] used FeSO₄·7H₂O, Fe₂(SO₄)₃·6H₂O, and NaOH as raw materials to prepare hydroxy sulfate green rust (GR_SO4) as a stabilizer to stabilize Cr. It was found that due to the presence of manganese oxides in the soil, Cr(III) can be oxidized again. Hui et al. [46] mixed soil dissolved organic matter (DOM) with three iron-based NPs (nZVI, α-Fe₂O₃, and Fe₃O₄) to explore the effect of their interactions on Cr adsorption. Fe₃O₄ adsorbs more strongly with soil DOM. In contrast, nZVI with DOM aging can remove 79.87% of Cr(VI). When aluminum-based materials are brought into contact with water, hydroxides of aluminum tend to be formed. The hydroxide of aluminum is combined with Cr to form insoluble chromium hydroxide precipitates and with arsenic to form insoluble arsenic compounds such as Al₂(AsO₄)₃. Aluminum sludge, a byproduct from water treatment plants containing amorphous Al(OH)₃ as its primary component, was utilized for arsenic adsorption; the adsorption capacity is about 19 mg/g [47]. Yang et al. [48] ball-milled magnetite to transform irregular particles into spherical shapes (Figure 6b). The successful adsorption of As reduced the exchangeable state of As in the soil by 30.25% (Figure 6c). Hydroxyl/carboxyl groups formed on ball-milled magnetite effectively removed As(III)/As(V) via hydrogen bonding and complexation, while synergistic oxidation with reactive oxygen species converted As(III) to more removable As(V) (Figure 6d).

2.4. Polymetallic Sulfide

Metal sulfides have the characteristics of high surface reactivity, strong reducibility, good selectivity, wide sources, and easy synthesis. Currently, their application in the treatment of chromium/arsenic pollution has received widespread attention. Common metal sulfides include pyrite (FeS₂), molybdenum disulfide (MoS₂), etc. Fe(II) and S²⁻ were alternately injected by Xie et al. [49] to prepare Fe sulfide coatings. Firstly, As(III) reacted directly with Fe sulfide coatings to form As sulfide precipitate, effectively removing As. Secondly, the surface composite reaction of Fe sulfide coatings also promoted the immobilization of As, forming As–S bonds and As–O–H bonds, which enhanced the removal ability of As and achieved a removal rate of As(III) at 109.7 mg/g. Potassium sulfide (K₂S) was added by Zhu et al. [50] to promote a combination of As³⁺ with S²⁻ to form an insoluble arsenic sulfide (As₂S₃) precipitate. Finally, As was removed through a co-precipitation mechanism, with the reduced As content below 0.1 ppm. However, these metal sulfide stabilization materials also have some drawbacks, such as difficulty in solid-liquid separation and the need to improve their adsorption capacity for chromium and arsenic pollutants. Therefore, it is necessary to further study the modification of these metal sulfides to solve the problems faced in the promotion of practical application. Compounding of β-cyclodextrin (β-CD) with Fe₃S₄, CD-Fe₃S₄ was prepared by Kong et al. [51]. Compared with Fe₃S₄, CD-Fe₃S₄ has an enhanced removal rate of Cr(VI), reaching over 99%. Applied to actual wastewater containing Cr(VI) 5.61 mg/L, 99.91% of chromium can be removed.

2.5. Graphene

Graphene was identified as a two-dimensional material composed of single-layer carbon atoms arranged in the form of sp² hybridization [52]. Its two-dimensional structure was found to endow the material with a series of unique physicochemical properties, including a large specific surface area and stability. Graphene with a specific surface area of 1300–1400 m²/g was synthesized by Chesnokov et al. [53], which greatly improved its adsorption capacity. Graphene was considered relatively stable in most chemical environments to resist acid and alkali [54]. Das et al. [55] investigated the arsenic removal ability of graphene oxide iron nanomaterials (GFEN), achieving a 95% removal rate for As(V) and a 99% removal rate for As(III) within the pH range of 3–9. The removal capacities of As(III) and As(V) were found to be 306 mg/g and 431 mg/g, respectively. Polyethyleneamine (PEI) was decorated onto graphene oxide (GO) by Geng et al. [56]. The adsorption performance of GO and PEI was improved after cross-linking; GO and PEI are connected together through covalent bonds, hydrogen bonds, and electrostatic interactions. As the molecular weight of PEI increases, its adsorption capacity for Cr(VI) gradually increases. The best adsorption performance is achieved using a molecular weight of 70K, with an adsorption capacity of 436.2 mg/g (Figure 7a). XPS shows partial reduction of Cr(VI) adsorbed onto the material (Figure 7b).

2.6. MXenes

Mxenes were identified as a new class of two-dimensional materials with a layered structure [57], which was usually composed of multiple connected layers. Its basic chemical composition could be expressed as M_n+1X_nT_x (M = transition metal element, x = C and/or n, and T = surface termination, e.g., –O, –OH, –F) [58]. The interlayer spacing of Mxenes was adjustable [59], which provided a feasible channel for the diffusion of ions and small molecules between their layers. The Mxenes were mainly prepared from their parent materials by selective etching [60]. Ammonium fluoride (NH₄F) was used by Abbasi et al. [61] as an etchant to produce Mxenes. A clear layered structure was generated (Figure 8a). The adsorption capacity of Cr(VI) reaches 59.8 mg/g. Ti₃AlC₂ was used by Liu et al. [62] as a precursor to prepare Ti₃C₂, which was then complexed with FES, polyimide (PEI), and polydopamine (PDA) to generate Ti₃C₂@FeS-PDA/PEI composites, removing 107.2 mg/g of Cr(VI). Titanium aluminum carbide (Ti₃AlC₂) and multi-walled carbon nanotubes (MWCNTs) were dispersed by Li et al. [63] into FeSO₄·7H₂O solution to produce nZVI/MXene@CNTs. The maximum adsorption capacity for As(III) is 443.32 mg/g. The entire process involves adsorption, catalytic oxidation, and co-precipitation (Figure 8c). Zang et al. [64] doped MXenes with Fe and Zn. Used for actual wastewater treatment, all meet the discharge standards, and remove over 80% of As(III) at 0.1–1 mg/L (Figure 8d). Jamluddin et al. [65] obtained layered MXenes. For Cr adsorption, ultraviolet photocatalytic synergy was carried out, contributing to the greatly improved removal efficiency of Cr. Compared with pure adsorption, it was increased by 3.1–28.9% (Figure 8b).

3. Mechanism of Chromium and Arsenic Stabilization

The physical and chemical properties of polluted sites are adjusted and changed through stabilization materials of chromium and arsenic, and the chemical form of chromium or arsenic elements is changed through adsorption, oxidation–reduction, chemical precipitation, and ion exchange, thereby reducing the mobility and bioavailability of chromium and arsenic to achieve the goal of stabilization. For example, studies have shown that iron oxide, due to its rich surface functional groups and high specific surface area, can effectively stabilize chromium and arsenic in soil through adsorption [66,67]. Sulfides (such as Na₂S) can reduce hexavalent chromium (Cr(VI)) to trivalent chromium (Cr(III)) via redox reactions, while fixing arsenic in the form of insoluble sulfides [68,69]. Phosphates can form chromium phosphate precipitates with chromium and arsenate precipitates with arsenic, thereby reducing their solubility and mobility in water [70]. Zeolite can effectively remove chromium and arsenic from water through the ion exchange mechanism [71,72]. Figure 9 shows the mechanism types of the stabilization process, which is complex and involves multiple mechanisms working together [73,74,75,76]. These studies indicate that inorganic stabilization materials hold significant potential for application in the treatment of chromium and arsenic pollution, thereby providing a scientific basis for the remediation of contaminated sites.

3.1. Adsorption Mechanism

The adsorption performance of stabilization materials for chromium and arsenic depends on their surface functional groups and pore structure, mainly including physical adsorption and chemical adsorption. Physical adsorption refers to the adsorption of chromium or arsenic on material surfaces caused by non-specific van der Waals forces. Chemical adsorption is caused by chemical reactions between pollutants and materials, forming covalent or ionic bonds [77]. Zhang et al. [78] prepared Schwertmannite by oxidizing Fe²⁺ solution with different H₂O₂ supply rates. A low supply rate of H₂O₂ results in higher crystallinity, larger specific surface area (SSA), and larger particle size of Schwertmannite. The adsorption capacity of Cr(VI) reaches 1.89 mmol/g. This study highlights the importance of controlling the oxidation rate to optimize the material’s adsorption properties. LDH-Pb was prepared by Khandelwal et al. [79] via in situ doping of Pb²⁺ onto LDHs. After doping with Pb²⁺, the particles perpendicular to it are formed, and the surface of LDH-Pb is fluffier and porous. Due to the large amount of protonated hydroxyl groups generated, it is electropositive over a wide pH range. The addition of Pb²⁺ reduces the influence of other negative ions and organic matter and improves the adsorption performance of LDH, with an adsorption capacity of 188.7 mg/g. This research underscores the role of surface modification in improving adsorption efficiency. Lignin and Fe(NO₃)₃ were added in the tetrahydrofuran/water solution by Zhang et al. [80] to prepare LC-FLG@Fe⁰ particles with graphene shells. At a pH of 3, the maximum adsorption was reached for As(III). The adsorption kinetics indicate that it is a chemical adsorption, and the maximum adsorption capacity is 107.2 mg/g. This study demonstrates the potential of using organic–inorganic hybrid materials to enhance adsorption capacity. Montmorillonite modified with amorphous ferritic iron ore (amFe@Mont) was synthesized by Jiang et al. [81]. Its specific surface area was measured to be 176.82 m²/g, and it was indicated by the H4 hysteresis loop that it had a microporous and mesoporous structure, with a majority of micropores. This material’s high surface area and porous structure contribute to its effective adsorption properties. A modified lime mortar binder, called C5 lime concentrate, was used by González-Sánchez et al. [82] for mine tailings treatment. Abundant needle-shaped C-S-H structures were shown in the SEM image. This enabled As elements to form fixed and non-leachable compounds under different conditions. This study highlights the potential of using modified binders for the effective immobilization of heavy metals in tailings.

3.2. Oxidation-Reduction Reaction Mechanism

Relying solely on adsorption is difficult to eliminate the toxicity of chromium and arsenic, so researchers have found that Cr and As were also converted into non-toxic forms through redox reactions by stabilization materials. Wang et al. [83] modified the maceration of green tea with attapulgite (ATP, Mg₅Si₈O₂₀(OH)₂(OH₂)·4H₂O) to obtain GATP. FT-IR spectroscopy showed the presence of polyphenols and other tea components on GATP (Figure 10a). Small molecular weight polyphenols (SMWP) undergo redox reactions with Cr(VI); C-O is oxidized to C=O, and 62.98% of Cr(VI) is reduced to Cr(III) (Figure 10b,c). Applying materials to actual contaminated sites, Li et al. [84] used a mixture of clay minerals (CMs) and pyrogenic carbons (PCs) to remediate Cr-contaminated soil. There are a large number of Fe(II) species in CM, which directly transfer electrons to Cr(VI) to form dissolved and solid Cr(III). PC can be used as an electron medium to facilitate this process, and PC surface functional groups can also participate in the reduction of Cr(VI). The resulting Cr(III) after reduction is fixed on the CM surface (Figure 10d). The Fe(III) in Fe₃O₄ NPs can be used to oxidize the more toxic form of arsenic, As(III), into the less toxic form, As(V) [85].

3.3. Precipitation Mechanism

Stabilization materials reduce the mobility and bioavailability of chromium and arsenic by adjusting soil pH or generating anions such as sulfate, carbonate, hydroxide, and phosphate ions, which precipitate with chromium or arsenic ions. For high-concentration chromium and arsenic pollution, the generation of low-solubility precipitates through chemical reactions is an important way to achieve long-term stabilization. Wang et al. [86] treated As(V) with the synergistic effect of lime, limestone, and clay minerals. The addition of lime causes clay minerals to produce calcium silicate hydrate (CSH) and calcium aluminum hydrate (CAH) as heavy metal stabilizers. The produced Ca₃(AsO₄)₂·4H₂O precipitation was revealed by XRD (Figure 10e), to prove the chemical fixation effect of the calcium-based system on As. The leaching rate of soil As stabilized with this material is only 7% (Figure 10f). Zhou et al. [87] impregnated sepiolite (SEP) in Fe(NO₃)₃ to form ferric nitrate-modified sepiolite (NIMS), and SEP was impregnated in KMnO₄ and FeSO₄ to generate iron–manganese modified sepiolite (FMS). In NIMS, FeOOH adsorbs and precipitates As(III) and As(V), with a strong adsorption capacity for As. NIMS and FMS reduced As in brown rice by 30% and 25%, respectively. Shen et al. [88] prepared MnFe₂O₄@Fh-EDA by combining manganese ferrite nanoparticles (MnFe₂O₄) with ethylenediamine-functionalized ferrite (Fh-EDA) for Cr(VI) adsorption. Some Cr(VI) undergo redox reactions with Mn(II) to generate Cr(III)–Mn(III) precipitates. MnFe₂O₄@Fh-EDA has an adsorption capacity of 51.36 mg/g, reducing Cr(VI) concentration from 20.0 mg/L to below 0.5 mg/L. Fe₃S₄ was successfully composited with MXenes_x [89]. The highly toxic Cr(VI) was reduced to less toxic Cr(III) through redox reactions via Fe(II) and S(–II). Final Cr(III) being precipitated as Cr(III)–Fe(III) hydroxide.

3.4. Ion Exchange Mechanism

Ion exchange is the process of removing chromium or arsenic ions from contaminated sites by exchanging them with ions of stabilization materials. Ion exchange is a reversible and stochastic chemical reaction in which pollutant ions exchange with charged ions attached to the surface of a stabilizing material to maintain electrical neutrality. Wang et al. [90] extracted biogenic Mn oxide (BMO) by microbial incubation. The main component of BMO is MnO₂, which has poor crystallinity. The hydroxyl group on the surface of BMO undergoes ion exchange with As to form the Mn–As complex. The bioavailable As decreased from 4.56 to 2.04 mg/kg. Ana et al. [91] used a synthetic zeolite with a cation exchange capacity of 188.72 meq/100 g to remove 56% of Cr(VI). The surface of graphene oxide, which contained abundant oxygen-containing functional groups (such as hydroxyl and epoxy groups), was found to electrostatically attract Cr(VI) under acidic conditions and to participate in the redox reaction of reducing Cr(VI) to Cr(III) in the lower valence state, as reported by Gao et al. [92]. However, as the alkalinity increases, these oxygen-containing groups become negatively charged due to ion exchange, resulting in the Cr removal rate decreasing from 99.8% to 10.4%.

3.5. Ligand Exchange and Surface Complexation Mechanism

There are usually some oxygen-containing functional groups on the surface of chromium and arsenic stabilized materials, such as hydroxyl (–OH), carboxyl (–COOH), etc. These functional groups can act as ligands to interact with chromium or arsenic ions. For example, pollutant ions replace hydrogen atoms in hydroxyl groups to form coordination bonds with oxygen atoms, thereby being fixed on the surface of materials. Luo et al. [93] used Mn salt to connect cellulose nanocrystal (CNC) and polyethyleneimine (PEI) to prepare CNC–Mn–PeEI. Mn was introduced as a “bridge junction” to form O–Mn–O bonds with cellulose nanocrystals (CNC) and polyethyleneimine (PEI), to enhance the stability of the material and the adsorption capacity of As(III) by successfully connecting CNC and PEI. In the adsorption process, As(III) was attracted to the CNC–Mn–PEI surface through electrostatic attraction and interacted with oxygen-containing groups on the material surface to form stable Mn–O–As bonds, thus achieving removal. The adsorption capacity for As(III) is 78.026 mg/g.

Some atoms or groups on the surface of materials can form surface coordination sites through chemical reactions or physical adsorption, and form surface complexes with chromium or arsenic ions through coordination bonds. During this process, heavy metal ions provide empty orbitals, and the coordinating atoms on the surface coordination sites provide lone pair electrons, forming a stable coordination structure. Chitosan magnetic graphene oxide (CMGO) nanocomposites were synthesized by Sherlala et al. [94] to form surface complexes through the interaction of surface functional groups (such as –NH₂ and –OH) with As(III). CaSO₄·H₂O is encapsulated around ferric arsenate to further stabilize the arsenic. Zhou et al. [95] reported that when steel slag is added as a stabilizer, As forms complexes with Fe and Ca elements in the steel slag.

4. Factors Affecting the Stabilization Process of Chromium and Arsenic Stabilized Materials

The stabilization process of chromium and arsenic stabilized materials is influenced by various factors, such as environmental pH, interfering ions, and stabilization time.

4.1. The Influence of Environmental pH on the Stabilization Process of Chromium and Arsenic

The pH of soil or groundwater is an important factor affecting the stabilization process for treating chromium- and arsenic-contaminated soil. The acidity or alkalinity of the contaminated site determines the valence state of chromium or arsenic. Chromium mainly exists in the form of Cr³⁺ under acidic conditions, and in the form of CrO₄²⁻ and Cr₂O₇²⁻ under alkaline conditions. Arsenic mainly exists in the form of H₃AsO₃ under acidic conditions, and in the form of H₂AsO₄⁻, HAsO₄²⁻, and AsO₄³⁻ under alkaline conditions. Figure 11 shows the valence states of As and Cr at different pH values [96]. Therefore, the environment in which stabilization materials are used also affects the efficiency of stabilization. K₂FeO₄-FeCl₃ was prepared by Kong et al. [97], and the adsorption mechanism of p-arsenic acid (p-ASA) was analyzed. As the pH increases, the removal rate of p-ASA gradually decreases, from 98.58% at a pH of 5 to 41.83% at a pH of 9 (Figure 12a). Kim et al. [98] combined polyethylene oxide (PEI) with graphene oxide (GO) to prepare polyethylenimine-coated graphene oxide composite material (GO@PEI). At a pH of 3, the pseudo-first-order removal rate constant of Cr(VI) is 3.7 × 10⁻³ min⁻¹. As the alkalinity increases, the removal rate constant decreases to 3.0 × 10⁻⁷ min⁻¹. Aluminum-containing waste (AWR) was used by Yang et al. [99] to form aluminum arsenate (AlAsO₄) and aluminum hydrogen arsenate (AlHAsO₄⁺) to effectively remove arsenic. The surface of AWR has small irregular flocs, which provided good adsorption sites and promoted the fixation of arsenic. By adjusting the pH value, especially when the pH was greater than 7, the aluminum and calcium ions in AWR were precipitated in the form of hydroxide to improve the stability of solid arsenic.

4.2. The Influence of Stabilization Time on the Stabilization Process of Chromium and Arsenic

In the initial stage of adding stabilization materials, due to sufficient active sites on the surface of the material, chemical reactions (such as ion exchange, surface complexation, and chemical precipitation) occur rapidly between the material and chromium or arsenic ions. Next, the active sites on the surface of the material are occupied, and the reaction rate gradually slows down. Finally, the adsorption and chemical reactions of pollutants reach dynamic equilibrium. Wei et al. [100] combined montmorillonite with glucose to generate carbon-coated montmorillonite nanocomposite (CMt) under acidic conditions. Adsorption equilibrium can be quickly reached within 48 h. However, the stabilization products formed by short-term remediation are prone to re-release with environmental changes. Hou et al. [101] used SIF as a stabilizer for As. The process of SIF stabilizing As is divided into three stages. The first stage is from 0 d to 2 d, where F4 (crystalline hydrous oxides-bound As) is converted to F3 (amorphous hydrous oxides bound As). As a result of the reductive dissolution of the Fe–As co-precipitate, As is released. In the second stage from 2 d to 90 d, the transition from F3 to F5 (residual As) mainly occurs. Through diffusion within the particles, a stable complex or mineral is finally formed. The third stage occurs between 90 d and 180 d, featuring mainly transition from F3 to F4, where the crystallization process of iron oxides and hydroxides is performed (Figure 12d). Shan et al. [102] selected Ca(OH)₂ as the core material and zeolite as the coating to prepare core–coating wrapping balls. Then, FeSO₄·7H₂O was added to obtain a novel granular half-wrapping structured amendment (HWA). When the ratio of FeSO₄·7H₂O:zeolite:Ca(OH)₂ is 6.5:1:1, the stabilization rate of As is 95.5%. After 365 days, the stability rate decreases to 92.1%. The stabilization rate is calculated as the percentage decrease in the leaching concentration of heavy metals after stabilization compared to the leaching concentration before stabilization. Specifically, it is determined using the following formula:

$$\mathrm{Stabilization} \mathrm{Rate}={\displaystyle \frac{C_0-C_t}{C_0}}\times 100$$

(1)

Among them, $C_t$ represents the heavy metal leaching concentration after $\mathrm{t}$ days of the stabilization process, while $C_0$ represents the heavy metal leaching concentration before the stabilization treatment. However, current research primarily focuses on short-term stabilization effects, with relatively limited research on long-term stabilization. For instance, studies have found that iron-modified graphene oxide can maintain a Cr(VI) removal rate of over 70% even after five repeated uses [103]. Additionally, the ability of modified zero-valent iron to capture pollutants decreases by only 4.8% after six cycles. These findings indicate that these materials hold significant potential for long-term stabilization [104]. Therefore, further exploration of the application of such materials in long-term stabilization could contribute to the development of more efficient and sustainable pollution control technologies.

4.3. The Influence of Coexisting Ions on the Stability Process of Chromium and Arsenic

In polluted sites, there may be other ions present in water or soil. To some extent, they will also affect the effectiveness of the stabilization process. Some ions, due to their similar properties to chromium or arsenic ions, will first react with the stabilizing material, occupying the active sites on the surface of the material and causing competitive adsorption, resulting in a decrease in the stabilization efficiency of chromium or arsenic. Some ions react with functional groups on the surface of the material to form new groups that promote the stability of chromium or arsenic. Therefore, before applying stabilization materials, it is necessary to investigate the influence of coexisting ions on their stabilization effect. The {201}TiO₂-ZrO₂ composite materials were prepared by Yu et al. [105] and were used for As removal in drinking water. Under acidic conditions, SO₄²⁻ and PO₄³⁻ inhibited the adsorption of 20–30% As(III). Under alkaline conditions (pH greater than 9), SO₃²⁻ and CO₃²⁻ also have inhibitory effects. For As(V), Mg²⁺ and Ca²⁺ increase removal efficiency by 20% at pH value greater than 10 (Figure 12e). Xi et al. [106] prepared a mixture of CMC-FeS@EPS (carboxymethyl cellulose-stabilized ferrous sulfide@extracellular polymeric substance) from CMC (carboxymethyl cellulose), EPS (extracellular polymeric substance), and FeSO₄ 7H₂O to remove Cr(VI). The addition of Ca²⁺, Mg²⁺, and Na⁺ changed the surface charge characteristics of the material or bound with functional groups, which resulted in a decrease in the removal rate of Cr(VI) from 89.67% to 65.33%, 66.46%, and 74.42%. A precipitate was formed by HCO₃⁻ with FeS, which reduced the removal rate to 56.67%. Electrostatic repulsion with chromium ions was enhanced by SO₄²⁻ and NO₃⁻, and the removal rates were decreased to 72.46% and 78.83%, respectively (Figure 12b). Wei et al. [107] prepared MnS-encapsulating biochar-dispersed zero-valent iron (MnS-nZVI@BC). Adding different interfering ions, except for Na⁺, other cations (K⁺, Cu²⁺, Ni²⁺, Ba²⁺, Ca²⁺, Cd²⁺, Cr³⁺, and Zn²⁺) promoted the stability of Cr(VI) due to increased solution conductivity or enhanced surface positive charge. The addition of Cl⁻ and NO₃⁻ had no effect on the stabilization process, while SO₄²⁻ promoted removal due to the enhanced electrostatic attraction between Cr(VI) and the material (Figure 12c).

Figure 12. (a) The removal efficiency of (p-ASA) by K₂FeO₄-FeCl₃ at different pH values [97]; (b) coexisting ion pairs CMC-FeS@EPS remove the influence of Cr(VI) [106]; (c) Cl⁻, NO³⁻, SO4²⁻, and metal cations MnS-nZVI@BC remove the influence of Cr(VI) [107]; (d) the changes in the occurrence form of As over time after correcting the soil with SIF [101]; (e) competitive adsorption of NO₃⁻, SO₄²⁻, SiO₃²⁻, CO₃²⁻, PO₄³⁻, Mg²⁺, Ca²⁺, and Fe³⁺ on {201}TiO₂-ZrO₂ using As(III/V) [105].

Figure 12. (a) The removal efficiency of (p-ASA) by K₂FeO₄-FeCl₃ at different pH values [97]; (b) coexisting ion pairs CMC-FeS@EPS remove the influence of Cr(VI) [106]; (c) Cl⁻, NO³⁻, SO4²⁻, and metal cations MnS-nZVI@BC remove the influence of Cr(VI) [107]; (d) the changes in the occurrence form of As over time after correcting the soil with SIF [101]; (e) competitive adsorption of NO₃⁻, SO₄²⁻, SiO₃²⁻, CO₃²⁻, PO₄³⁻, Mg²⁺, Ca²⁺, and Fe³⁺ on {201}TiO₂-ZrO₂ using As(III/V) [105].

5. Conclusions

Chromium and arsenic heavy metal pollution is one of the most severe environmental problems facing the world today. Therefore, the preparation of materials with efficient fixation and long-term stability to remove chromium and arsenic elements from natural environments has become a research hotspot. The use of heavy metal stabilization materials to remove chromium and arsenic to improve soil and groundwater is a widespread, efficient, and cost-effective approach. However, the effectiveness and applicability of stabilization materials need to be comprehensively evaluated based on the forms of chromium and arsenic, environmental conditions, and material characteristics. Research has shown that using chromium and arsenic stabilization materials to remediate contaminated sites is effective. Chromium and arsenic stabilization materials remove pollutants through mechanisms such as adsorption, ion exchange, redox reactions, surface complexation, and electron transfer [108]. Most stabilization processes are favorable under acidic conditions and are influenced by different coexisting ions in environment [109]. During different stable cycles, the stabilization products are altered with the environmental changes [110]. Despite the promising prospects of chromium and arsenic stabilization materials, there are still some issues that need to be considered, The following summarizes eight future development prospects for chromium and arsenic stabilization Materials (Figure 13):

• In practical applications, remediation should be carried out based on the form of pollutants and the characteristics of the site. The forms of pollutants, such as the valence and occurrence forms of chromium and arsenic, directly affect toxicity and mobility, determining the stabilization pathway (reduction, adsorption, or physical fixation). The characteristics of the site (pH, organic matter, permeability, interfering ions, and soil type) and climatic conditions (temperature, humidity, and precipitation) determine the suitability and reaction efficiency of the material [117], avoiding stability failure or secondary release caused by unsuitable environmental conditions.
• The selection and targeting of stabilization materials directly determine the stabilization efficiency of chromium and arsenic. Different materials have different mechanisms of action on chromium (such as zero-valent iron reducing Cr(VI)) and iron-based materials adsorbing As(V)), and precise matching of pollutant characteristics is required. In addition, material costs and engineering feasibility also need to be comprehensively balanced.
• When using stabilization materials, the feed material, reaction time, pH range, and cost range should be considered. If these parameters are mismatched, it may result in substandard processing or resource waste. Optimizing parameters can simultaneously improve efficiency and engineering sustainability.
• Risk control and long-term management should be considered before applying chromium and arsenic stabilization materials. It is necessary to prevent and control the risk of secondary release after stabilization and reduce environmental exposure through monitoring frequency and emergency plans (such as supplementing iron-based materials). At the same time, long-term stability depends on the anti-aging ability of materials (such as sulfide failure under oxidation conditions) and environmental fluctuations (such as pH sudden changes).
• Future research should pay more attention to evaluating chromium and arsenic stabilization materials in practical complex environments. Most current research focuses on static experiments, whereas real environments should simulate dynamic processes such as wet–dry alternation, freeze–thaw cycles, and thermal aging. It is recommended that wet–dry cycle experiments be utilized to simulate the stabilization capacity of chromium and arsenic pollutants and structural changes of materials under periodic wetting–drying effects.
• Currently, research on chromium and arsenic stabilization materials requires the establishment of unified performance evaluation standards and engineering application guidelines. In the future, a standardized framework systematically covering material screening, stabilization efficiency verification, and environmental risk control should be developed. Firstly, it is necessary to clarify material application parameters for different contaminated sites (such as acidic mine drainage, alkaline tailings ponds, etc.) and establish standardized stabilization experimental methods. Secondly, an evaluation standard should be developed to define reasonable thresholds for long-term leaching concentration, occurrence forms, and ecological toxicity of chromium and arsenic.
• The breakthrough of chromium and arsenic stabilization technologies requires the deep integration of environmental engineering, materials science, and geology. Future research requires the construction of a multidisciplinary cross-disciplinary framework: the discipline of environmental engineering should focus on analyzing the migration and transformation laws of pollutants, providing environmental data for material design; materials science needs to be combined with geological mineralization principles to develop mineralized materials and enhance their ability to stabilize chromium and arsenic; geology needs to intervene in the study of hydrogeochemical characteristics of the site and predict the geological compatibility of stabilization materials through models.
• The green transformation of chromium and arsenic stabilization materials has become an important trend. Future research will focus on developing environmentally friendly and low-energy inorganic stabilizers, with priority given to using industrial solid waste such as red mud and fly ash or natural minerals (such as attapulgite and sepiolite) as matrix materials, and improving their ability to stabilize heavy metals of chromium and arsenic through modification, to achieve the goal of “treating waste with waste”.

This article systematically introduces the remediation techniques and materials for chromium and arsenic pollution, with a focus on analyzing the stability mechanism, treatment efficiency, and application potential of inorganic stabilization materials. Research should prioritize the development of multifunctional composite materials, evaluate their long-term stability under dynamic environmental conditions, and optimize material properties through in-depth research on environmental compatibility, large-scale production processes, and cost reduction. These efforts will advance chromium/arsenic remediation technology, promote sustainable use of water and soil resources, and maintain ecological security and human health.