Fabrication of FeAl-LDH onto sponge for efficient chromium(VI) immobilization and reduction in soil remediation

Liyun Hu; Yang Xiao; Xuan Xia; Qinghan Ye; Mengjia Shi; Hui Zhang; Wenying Yuan; Jing Cheng; Xinhong Qiu

doi:10.4491/eer.2025.330

Environ Eng Res > Volume 31(2); 2026 > Article

Hu, Xiao, Xia, Ye, Shi, Zhang, Yuan, Cheng, and Qiu: Fabrication of FeAl-LDH onto sponge for efficient chromium(VI) immobilization and reduction in soil remediation

Research

Environmental Engineering Research 2026; 31(2): 250330.

Published online: August 28, 2025

DOI: https://doi.org/10.4491/eer.2025.330

Fabrication of FeAl-LDH onto sponge for efficient chromium(VI) immobilization and reduction in soil remediation

Liyun Hu¹, Yang Xiao¹, Xuan Xia¹, Qinghan Ye¹, Mengjia Shi¹, Hui Zhang¹, Wenying Yuan¹, Jing Cheng^1,^†

, Xinhong Qiu^1,^2,^3,^†

¹School of Chemistry and Environmental Engineering, Wuhan Institute of Technology, Wuhan, 430074, China

²Wuhan Institute of Technology Jingmen Research Institute of New Chemical Materials Industry Technology, Wuhan 430070, China

³Hubei Three Gorges Laboratory, Yichang 443008, China

^†Corresponding author: E-mail: Jeannie1204@126.com (J. C.), qxinhong@gmail.com (X. Q.), Tel: +86 27 8719 4560 (J. C.), +86 27 8719 5680 (X. Q.), Fax: +86 27 8719 4560 (J. C.), +86 27 8719 5680 (X. Q.), ORCID: 0009-0001-0551-9675 (J. C.), 0000-0002-9523-8374 (X. Q.)

Received June 10, 2025 Revised August 25, 2025 Accepted August 27, 2025

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Efficient remediation of Cr(VI)-contaminated soils is facilitated by materials possessing both high adsorption potential and reductive functionality. In this work, a novel composite, FeAl-LDH@PS, was synthesized by hydrothermally anchoring Fe-Al layered double hydroxides (LDHs) onto a porous, elastic polyurethane sponge (PS), serving as a recoverable support to enhance LDH dispersion, structural stability, and reusability. A Cr(VI) concentration of 1000 mg/kg was effectively treated by FeAl-LDH@PS, resulting in 74.97% immobilization within 60 minutes and a substantial decline in leachable Cr(VI) from 43.01 mg/L to 4.45 mg/L under varied environmental scenarios. Mechanism analysis indicated that Fe²⁺ and Al³⁺ were released from the FeAl-LDH@PS. The released Fe²⁺ reduced Cr(VI) to Cr(III), while Al³⁺ underwent hydrolysis to form AlOOH. Subsequently, the newly generated Fe₂O₃ and AlOOH co-precipitated or surface-adsorbed Cr species. The composite maintained high stability and remediation efficiency across a broad pH range and under repeated dry–wet and freeze–thaw cycles. Phytotoxicity assays further confirmed its low environmental risk and suitability for safe application in soil systems. Microbial community structure and predicted metabolic functions revealed that FeAl-LDH@PS remediation not only shifted the bacterial composition toward that of clean soil but also restored core metabolic pathways associated with carbon and nitrogen cycling.

Keywords: Cr(VI) Immobilization, FeAl-LDH, Polyurethane sponge, Reductive remediation

Graphical Abstract

Keywords: Cr(VI) Immobilization, FeAl-LDH, Polyurethane sponge, Reductive remediation

1. Introduction

Chromium (Cr) is a pervasive soil contaminant originating from natural processes and anthropogenic activities (e.g., metallurgy, mining, electroplating) [1, 2]. It primarily exists as Cr (VI) and Cr(III), exhibiting stark differences in toxicity and mobility [3]. Cr(III) forms insoluble precipitates with soil colloids, whereas Cr(VI) is highly soluble, oxidizing, and readily transported in soils, posing significant teratogenic and mutagenic risks via plant uptake and food chain transfer [4, 5]. Due to these hazards, remediation of Cr(VI)-polluted soils has become a pressing concern.

Layered double hydroxides (LDHs), commonly referred to as anionic clays or hydrotalcite-like compounds, are 2D nanostructured materials composed of positively charged brucite-type layers and charge-compensating interlayer anions [6]. The tunable composition of LDHs allows for diverse physicochemical properties, and their ability to undergo anion exchange endows them with enhanced adsorption capacity and functional adaptability [7]. Our group has also employed LDHs, leveraging their unique structural memory effect, to passivate multiple heavy metals in soil. Experimental results, including heavy metal toxicity leaching tests, plant uptake analysis, and assessments of soil microbial community dynamics, confirmed that LDH amendments significantly reduced the translocation of Cr(VI) and Cd into plant tissues and promoted the restoration of soil microbial diversity. These outcomes indicate a substantial decline in the bioavailability and ecological toxicity of heavy metals post-treatment. Furthermore, long-term field studies conducted by Wang et al. [8] and Li et al. [9] have validated the practical efficacy of LDHs in reducing heavy metal accumulation in edible crops under real agricultural conditions.

Cr(VI) is far more toxic and mobile than Cr(III), and its reduction to Cr(III) is crucial for mitigating ecological and health risks. Layered double hydroxides (LDHs) possess variable-valence metal cations, enabling multifunctional Cr(VI) immobilization and reduction. Specifically, Fe²⁺ within LDHs can thermodynamically transfer electrons to Cr(VI) (E°Fe³⁺/Fe²⁺ = 0.77 V; E°Cr₂ O₇²⁻/Cr³⁺= 1.33 V), facilitating its conversion to the less toxic Cr(III). Previous work confirmed the dual stabilization and reduction ability of Fe–Al LDHs in soil remediation. However, conventional powdered LDHs suffer from agglomeration [10], limiting their practical application in large-scale or long-term remediation.

To address the poor recoverability and agglomeration issues of powdered LDHs, various solid supports (e.g., activated carbon fibers, biochar, cellulose) have been explored to enhance LDH dispersion and stability. For instance, Yuan et al. [11] anchored FeAl-LDH onto activated carbon fibers, achieving improved Cr(VI) adsorption capacity. However, these rigid supports often exhibit limited flexibility and adaptability in heterogeneous soil environments. Polyurethane sponges (PS) present a promising alternative due to their highly interconnected porous framework, large specific surface area, and inherent elasticity. Such structural features facilitate uniform LDH anchoring, increase the accessibility of active sites, and enable intimate contact with soil particles while preserving mechanical integrity under environmental stresses. Moreover, sponges can be easily retrieved from treated soils, reducing the risk of secondary contamination. Previous studies have demonstrated that hybridizing sponges with nanomaterials markedly improves environmental remediation performance [12]. Li et al. fabricated a lightweight magnetic polyurethane sponge by loading magnetic hydroxyapatite onto its framework, achieving high Pb²⁺ adsorption while maintaining a low density (0.46 g/cm³) for easy flotation and magnetic separation [13]. Similarly, Jiang et al. used a polyurethane sponge as a sacrificial scaffold to hydrothermally synthesize an anatase TiO₂–reduced graphene oxide composite. The sponge’s uniform pore channels provided abundant nucleation sites, prevented nanoparticle agglomeration, and ensured uniform dispersion on RGO sheets, thereby enhancing contaminant adsorption and electron transport [14]. These examples highlight the unique advantages of sponge matrices, including high porosity, structural flexibility, and excellent particle dispersion capability, which make them ideal scaffolds for functional composite fabrication. Despite these advances, the application of FeAl-LDH loaded Polyurethane sponge (FeAl-LDH@PS) composites for simultaneous Cr(VI) immobilization and reduction in soils has not been systematically investigated.

In this work, a novel FeAl-LDH@PS composite was fabricated by hydrothermally anchoring FeAl-LDH onto a Polyurethane sponge substrate. The potential of FeAl-LDH@PS for immobilizing Cr(VI) in contaminated soils was systematically evaluated by examining key operational factors, including composite loading, soil pH, initial Cr(VI) concentration, and dosage. Comprehensive characterization analyses were performed to elucidate the mechanisms governing Cr(VI) reduction and stabilization. To assess environmental robustness, remediation performance was further tested under simulated dry–wet and freeze–thaw cycles. Moreover, phytotoxicity assays were conducted to evaluate ecological safety, providing an integrated assessment of FeAl-LDH@PS’s feasibility for practical soil remediation applications.

2. Materials and Methods

2.1. Materials

Polyurethane sponge (PS, density: 20 kg/m³; porosity: 92%; average pore size: ~0.95 mm) was purchased from Yongsheng Sponge Factory (Nanjing, China). Ferrous chloride tetrahydrate (FeCl₂· 4H₂O, ≥99%), aluminum chloride hexahydrate (AlCl₃·6H₂O, ≥99%), potassium dichromate (K₂Cr₂O₇, ≥99.8%), sodium hydroxide (NaOH, ≥96%), and hydrochloric acid (HCl, 36–38%) were obtained from Sinopharm Chemical Reagent Co., Ltd. (China). All chemicals were of analytical reagent grade and used as received without further purification.

2.2. Preparation of FeAl-LDH@PS Composites

The low-density PS was cut into small pieces of 1 cm × 1 cm and immersed in a 150 mL mixed solution containing 4.8 g AlCl₃·6H₂O and 8 g FeCl₂·4H₂O, sealed and stirred. After PS completely absorbed the salt solution, the pH value was adjusted to 9 with 4 mol/L NaOH and 1 mol/L HCl solution, and then the solution was quickly transferred to the reactor and reacted at 120 °C for 24 h to make iron and aluminum type of layered double hydroxides (FeAl-LDH) grow on PS. After the reaction, the obtained composite material (FeAl-LDH@PS) was washed once with a mixed solution with an alcohol-water ratio of 3 : 7, and then washed twice with an ethanol solution. Finally, the product was placed in a vacuum drying oven at 50 °C for 3 h. According to the different mass of PS, FeAl-LDH@PS_x with different loadings could be obtained. When the mass of PS was 0.35 g, 0.7 g and 1.4 g, FeAl-LDH@PS_x was denoted as FeAl-LDH@PS_0.35, FeAl-LDH @PS_0.7 and FeAl-LDH@PS_1.4.

2.3. Preparation of Test Soils

Potassium dichromate (0.56 g, 1.13 g, 2.26 g, and 2.83 g) was individually weighed into beakers, dissolved in a small volume of deionized water, and then diluted to 1 L using volumetric flasks to prepare Cr(VI) stock solutions with concentrations of 200, 400, 800, and 1000 mg/L, respectively. The test soil was collected from Suqian City, Jiangsu Province, China. After the removal of surface debris, the soil was air-dried and sieved through a 100-mesh screen. To prepare Cr(VI)-contaminated soils, the corresponding Cr(VI) solutions were added to the soil at a fixed soil-to-solution ratio of 1 g:1 mL. The mixtures were stirred thoroughly and then dried to yield chromium-contaminated soils with Cr(VI) concentrations of 200, 400, 800, and 1000 mg/kg. For pH-specific treatments, 1000 mg/L Cr(VI) solution was added to the soil at the same solid–liquid ratio (1 g:1 mL). The pH of the mixture was adjusted using appropriate volumes of NaOH or HCl to obtain Cr(VI)-contaminated soils with initial pH values of 3, 4, 7, and 10.

2.4. Passivation of Cr(VI)-Contaminated Soils Using FeAl-LDH@PS

The passivation performance of FeAl-LDH@PS toward Cr(VI)-contaminated soil was evaluated under varying experimental conditions, including different composite loadings (FeAl-LDH@ PS_0.35, FeAl-LDH@PS_0.7, FeAl-LDH@PS_1.4), initial Cr(VI) concentrations in soil (200, 400, 800, and 1000 mg/kg), amendment dosages (0.025, 0.05, 0.075, and 0.1 g), and soil pH values (3, 4, 7, and 10). The composites were thoroughly mixed with Cr(VI)-spiked soils in the presence of deionized water and subjected to uniform agitation using a rotary mixer. After the designated reaction period, the mixtures were centrifuged at 4000 rpm to facilitate phase separation. The supernatants were collected by syringe filtration using a 0.45 μm membrane and subsequently analyzed for residual Cr(VI) concentration. The solid residues were dried in an oven and used for toxicity leaching tests to assess Cr(VI) release behavior post-treatment. To ensure experimental reliability and reproducibility, all tests were performed in parallel.

2.5. Toxic Leaching Tests

According to the U.S. EPA Method 1311 (SW-846), the TCLP procedure was conducted using Extractant 2. Both untreated and FeAl-LDH@PS-treated Cr(VI)-contaminated soil samples were mixed with Extractant 2 at a liquid-to-solid ratio of 20:1 (v/w). The mixtures were placed on a rotary shaker and agitated at room temperature for 18 hours at a speed of 30 ± 2 rpm. After leaching, the suspensions were centrifuged, and the supernatants were filtered through 0.45 μm syringe filters. The Cr(VI) concentration in the filtrates was then determined to evaluate the leaching behavior.

2.6. Dry-wet and Freeze-thaw Cycle Experiments

To evaluate the environmental stability of Cr(VI) immobilization, both untreated and FeAl-LDH@PS-treated chromium-contaminated soils were subjected to simulated dry–wet and freeze–thaw cycles. For the dry–wet treatment, soil samples were first air-dried in centrifuge tubes, followed by the addition of ultrapure water to adjust the moisture content to 30%, 50%, or 70% of the soil’s water-holding capacity. The moistened soils were equilibrated at room temperature for 3 hours and then oven-dried at 50 °C for another 3 hours. For the freeze–thaw treatment, soil samples were adjusted to 70% water-holding capacity and stored in a freezer at −10 °C, −15 °C, or −25 °C for 3 hours. Subsequently, the samples were thawed at room temperature for 3 hours. This freezing–thawing cycle was repeated four times. After treatment, all soil samples were oven-dried and subjected to the TCLP test to assess Cr(VI) leachability under environmental stress conditions.

2.7. Assessment of Phytotoxicity in Remediated Soils

To evaluate the phytotoxicity and chromium uptake after remediation, a pot experiment was conducted using four identical culture dishes (equal diameter). Each dish was filled with 5 g of one of the following soil types: clean soil, Cr(VI)-contaminated soil, PS-treated soil, and FeAl-LDH@PS-treated soil. Thirty wheat seeds and 25 mL of deionized water were added to each dish. The cultivation was maintained at 25 °C under natural light, with timely replenishment of water to maintain appropriate moisture content. The growth status of wheat seedlings was observed and documented photographically. After 15 days of cultivation, the root and shoot lengths were measured, followed by drying at 60 °C to a constant weight to determine dry biomass. Subsequently, the dried plant tissues were subjected to ashing in a muffle furnace at 550 °C for 6 h. The resulting ash was digested in a mixture containing 2 mL HNO₃, 1 mL of 30% H₂O₂, and 1 mL of deionized water. The mixture was allowed to stand for 2 h, sealed in a digestion vessel, and heated at 100 °C for 4 h. After cooling, the solution was filtered, and the total chromium content in the digestate was analyzed using flame atomic absorption spectrophotometry (FAAS).

2.8. Characterizations

The morphological characteristics of the composites before and after the reaction were examined using scanning electron microscopy (SEM, JSM-5510LV, JEOL, Japan) coupled with energy-dispersive X-ray spectroscopy (EDS). The crystalline structures were identified via X-ray diffraction (XRD, D8 Advance, Bruker, Germany), while functional groups were characterized using Fourier-transform infrared spectroscopy (FTIR, Nicolet-6700, Thermo Electron, USA). Elemental composition and chemical states were further investigated by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Fisher Scientific, USA), providing insights into elemental distribution and corresponding binding energies. The Illumina MiSeq/NovaSeq platform was employed to perform paired-end sequencing of community-derived DNA fragments. Subsequent analysis of microbial community composition and diversity was conducted using the QIIME2 bioinformatics pipeline.

3. Results and Discussion

3.1. Characterization of FeAl-LDH@PS

The morphology and composition of pristine PS and FeAl-LDH@PS are shown in Fig. 1. SEM images indicate that PS features a highly porous structure with pore sizes of several hundred micrometers (Fig. 1a), and a smooth surface under higher magnification (Fig. 1b). After FeAl-LDH loading, the porous skeleton is retained (Fig. 1c), while abundant flake-like LDH particles are distributed across the sponge matrix (Fig. 1d). EDS confirms the presence of Fe, Al, O, Cl, and C in the composite (Fig. 1e), with C content at 38.74 wt%, consistent with the carbon-rich sponge framework. XRD analysis further verifies the synthesis of FeAl-LDH@PS. Characteristic peaks at 11.61° and 23.46° match the (003) and (006) planes of FeAl-LDH, indicating high crystallinity [15, 16], while PS shows a broad amorphous peak at 20.49°. The coexistence of both features in the composite confirms LDH deposition on PS (Fig. 1f). In the FTIR spectrum (Fig. 1g), PS shows characteristic bands at 3200–3600 cm⁻¹ (O–H), 2867 cm⁻¹ (C–H), 1085 cm⁻¹ (C–O–C), and 1532 cm⁻¹ (N–H) [17, 18]. FeAl-LDH displays bands at 3300–3600 cm⁻¹ and 1639 cm⁻¹, assigned to interlayer O–H stretching and H–O–H bending [19]. The composite spectrum combines features from both components, confirming successful FeAl-LDH integration onto the PS framework. Gravimetric analysis was conducted to quantify the FeAl-LDH loading on PS with varying masses (0.35 g, 0.70 g, and 1.40 g) by comparing pre- and post-hydrothermal synthesis weight measurements. The results revealed a progressive reduction in LDH loading: FeAl-LDH@PS_0.35 exhibited the highest loading at 0.51 g/g, followed by 0.34 g/g for FeAl- LDH@PS_0.7, and 0.26 g/g for FeAl-LDH@PS_1.4.

3.2. Immobilization Cr(VI) by FeAl-LDH@PS in Soil

3.2.1. Immobilization Cr(VI) by FeAl-LDH@PS with different loading

As shown in Fig. 2a, pristine PS shows negligible Cr(VI) adsorption, confirming that FeAl-LDH is the main active component in the composite. Therefore, evaluating the effect of LDH loading is essential for optimizing the material’s performance [20]. As the PS mass increases from 0.35 g to 1.40 g, the Cr(VI) immobilization efficiency in 1000 mg/kg soil drops significantly from 99.94% to 55.56%, due to a lower FeAl-LDH proportion. To balance removal efficiency and material usage, FeAl-LDH@PS_0.7 was selected for further experiments. As shown in Fig. 2b, increasing composite dosage enhances Cr(VI) removal, reaching 99.74% at 0.075 g. This trend is attributed to more active sites and improved contact between the material and pollutants. However, further increasing the dosage provides only marginal benefits. Therefore, a dosage of 0.050 g was adopted in subsequent tests, considering both treatment effectiveness and material economy.

3.2.2. The soil remediation with different pollution levels by FeAl-LDH@PS

To evaluate remediation under different contamination levels, initial Cr(VI) concentrations in soil were varied. As shown in Fig. 3a, FeAl-LDH@PS achieved rapid immobilization under low contamination (200 mg/kg), removing 97.48% of Cr(VI) within 10 min. At higher levels (e.g., 800 mg/kg), the system required more time to reach equilibrium, attaining 94.84% removal after 30 min.

Comparative analysis with LDH-based adsorbents reported in the literature (Table S1) shows that FeAl-LDH@PS achieved the highest and most stable Cr(VI) removal efficiency (99.08–74.97%) within the 200–1000 mg/kg concentration range at a dosage of 0.05 g/g soil. While certain calcined or delaminated LDHs, such as calcined MgAl-LDH at 500 °C or delaminated CaAl-LDH, displayed near-complete removal under low contamination, their performance declined sharply as Cr(VI) levels increased. In contrast, FeAl-LDH@PS exhibited a comparatively smaller reduction, attributable to the PS support maintaining active site accessibility and enhancing mass transfer even under high contaminant stress. Moreover, Fe incorporation into the LDH framework not only increased anion-exchange capacity but also introduced reductive sites capable of converting Cr(VI) to the less mobile and less toxic Cr(III), thereby ensuring sustained immobilization across a broad contamination gradient.

Kinetic modeling using pseudo-first-order and pseudo-second-order equations (Eqs. (1) and (2)) revealed that the pseudo-second-order model best described the process (Fig. 3b, 3c and Table S2), as indicated by higher R² values. This suggests a two-step removal mechanism involving initial surface adsorption followed by intraparticle diffusion [21]. The kinetic constants (K₁ and K₂) further confirmed that FeAl-LDH@PS retained a high adsorption capacity even at 1000 mg/kg Cr(VI), underscoring its applicability in highly contaminated soils.

(1)

ln (Q_{e} - Q_{t}) = ln Q_{e} - K_{1} t

(2)

\frac{t}{Q_{t}} = \frac{1}{K_{2} Q_{e}^{2}} + \frac{t}{Q_{e}}

3.2.3. Influence of soil pH on Cr(VI) immobilization

As shown in Fig. S1, FeAl-LDH@PS maintained high Cr(VI) immobilization efficiency across a broad pH range, with a maximum of 82.49% at neutral pH (7.0). When the pH decreased to 4.0, efficiency declined slightly to 75.58%, primarily due to the dominance of HCrO₄⁻ under acidic conditions [22, 23], which interacts less strongly with positively charged LDH layers than CrO₄²⁻ [24]. Acidic environments also promote LDH dissolution, but the PS matrix provides structural protection. Interestingly, at pH 3.0, a minor increase in efficiency was observed, likely due to enhanced Fe²⁺ release from FeAl-LDH, promoting Cr(VI) reduction to Cr(III). In alkaline conditions (pH 10.0), efficiency dropped to 80.25%, which may result from OH⁻ competing with CrO₄²⁻ for adsorption sites[24]and the formation of Fe/Al hydroxide precipitates that block active sites [25].

3.2.4. Fe and Cr(VI) leaching after immobilization

Iron speciation in soil is strongly influenced by redox dynamics and pH, with excess Fe potentially causing phytotoxic effects [26]. To assess the risk of Fe leaching from FeAl-LDH@PS during Cr(VI) immobilization, total Fe concentrations in the leachate were monitored (Fig. S2a). Although a gradual increase was observed, reaching 2.27 mg/L at 60 min, the concentrations remained low, suggesting minimal environmental concern. To evaluate the long-term stability of Cr(VI) in treated soils, the TCLP method (EPA SW-846 1311) was employed, which assesses heavy metal mobility in solid matrices. As shown in Fig. S2b, Cr(VI) leaching was significantly suppressed by FeAl-LDH@PS in both the immediate filtrate and the TCLP extract. In the untreated control, Cr(VI) levels reached 43.01 mg/L, whereas the treated soil dropped sharply to 4.45 mg/L within 5 min and remained stable. These results confirm that FeAl-LDH@PS enables rapid and stable immobilization of Cr(VI), reducing its environmental mobility and risk.

3.2.5. Effect of dry-wet and freeze-thaw cycles on stability

Environmental variations, particularly dry–wet and freeze–thaw cycles, significantly affect soil physicochemical characteristics [27] and the stability of heavy metal remediation processes. In Cr(VI)-contaminated soils without treatment, increased moisture led to fluctuating Cr(VI) leaching levels (44.83, 38.44, and 38.68 mg/L; Fig. S3a), while freeze–thaw treatments also resulted in high Cr(VI) concentrations (42.66–42.47 mg/L; Fig. S3b). In contrast, FeAl-LDH@PS-treated soils exhibited consistently low Cr(VI) leaching (<5 mg/L) under all conditions, demonstrating the composite’s strong immobilization efficiency and structural resilience. This stabilization is attributed to the robust interaction between Cr(VI) and the LDH component, as well as the PS matrix’s ability to buffer environmental stress. The minimal variation in leaching across changing water content and freezing temperatures indicates that FeAl-LDH@PS maintains reliable performance under environmental fluctuations, supporting its practical application in dynamic field scenarios.

3.3. Plant growth and Microorganism Analysis

To assess the phytotoxicity and ecological safety of FeAl-LDH@PS, four soil treatments were designed: clean soil (S1), Cr(VI)-contaminated soil (S2), FeAl-LDH@PS-treated contaminated soil (S3) and PS-treated contaminated soil (S4) . Wheat seeds were cultivated in each group and their growth was recorded (Fig. S4a). Normal germination occurred in S1 and S3, indicating that seed viability was not a limiting factor. In contrast, no wheat survived in S2 and S4, suggesting that PS alone could not alleviate Cr(VI) toxicity. In FeAl-LDH@PS-treated soil (S3), wheat developed robust roots and shoots, with survival rate reaching 93%. Morphological analysis showed increased root length (172.57 mm) and comparable shoot growth (161.82 mm) relative to clean soil (S1: 135.91 mm roots, 163.95 mm shoots, Fig. S4b). Dry biomass data further supported these findings, with S3 plants yielding higher root and shoot dry weights (257.5 mg and 505.8 mg, respectively) than those in S1 (194.4 mg and 369.7 mg, Table S3). Total chromium concentrations (Table S4) revealed greater Cr accumulation in S3 than S1 due to residual soil Cr, but the majority remained confined to roots, with minimal translocation to shoots. These results confirm that FeAl-LDH@PS can mitigates Cr(VI) phytotoxicity, thereby enhancing plant tolerance while reducing potential food chain risks.

As shown in Fig. 4a, Cr(VI) contamination markedly reduced microbial diversity, with Proteobacteria and Myxococcota dominating under stress conditions. Meanwhile, beneficial phyla related to nutrient cycling, such as Actinobacteriota, Firmicutes, and Acidobacteriota, were significantly suppressed. Clean soils exhibited high microbial diversity, with abundant Actinobacteriota and Acidobacteriota, indicating stable nutrient cycling and ecosystem functionality. After FeAl-LDH@PS treatment, microbial communities partially recovered. The abundance of stress-tolerant Myxococcota decreased, while beneficial phyla increased, suggesting enhanced ecological conditions. This recovery is attributed to Cr(VI) immobilization by FeAl-LDH@PS, which reduced pollutant toxicity and bioavailability. Additionally, Fe²⁺ and Al³⁺ release helped buffer soil pH and redox conditions, promoting microbial stabilization.

The alpha diversity analysis shows clear variation among the three soil types (Table S5). Contaminated soil had the lowest microbial richness and diversity, with a Chao1 index of 171.09 and a Shannon index of 3.22, indicating a highly disturbed and simplified community. Clean soil exhibited much higher richness and diversity, with a Chao1 index of 1697.30 and a Shannon index of 5.72, reflecting a healthy and stable microbial ecosystem. After remediation, the soil showed notable improvement, with the Chao1 index increasing to 1677.41 and the Shannon index to 5.40, suggesting that the remediation strategy effectively restored microbial community complexity and functional potential. From the Fig. 4b, in clean soils, microbial communities exhibited robust metabolic profiles with active energy and biosynthetic pathways, including the TCA cycle, fatty acid metabolism, carbon fixation, and amino acid biosynthesis. Stable expression of cell maintenance pathways (e.g., protein export, aminoacyl-tRNA biosynthesis) and low activity of antibiotic biosynthesis reflected cooperative and balanced community dynamics. In contrast, Cr(VI)-contaminated soils showed suppressed core metabolic functions and enhanced expression of stress-related pathways, such as DNA repair, cell cycle control, and antibiotic biosynthesis, suggesting microbial adaptation to environmental stress. Following FeAl-LDH@PS treatment, key metabolic pathways (e.g., fatty acid metabolism, TCA cycle, amino acid synthesis) gradually recovered toward clean-soil levels. The reduced activity in antibiotic-related genes further indicated alleviated microbial competition and improved community stability. This recovery is attributed to the effective Cr(VI) immobilization and environmental buffering provided by FeAl-LDH@PS, which mitigated toxicity and supported microbial resilience. Overall, structural and functional analyses confirmed that FeAl-LDH@PS not only stabilized pollutants but also facilitated microbial ecological restoration in contaminated soils.

3.4. Immobilization Mechanism of Cr(VI) by FeAl-LDH@PS

The SEM image (Fig. 5a and b) shows that after Cr(VI) stabilization, the overall framework of FeAl-LDH@PS remains largely intact, with only localized network fractures observed. EDS analysis further confirms a reduction in iron content, indicating a certain degree of Fe leaching during the reaction [28]. Meanwhile, Cr was detected on the surface of the composite, with an atomic concentration of approximately 0.14% (Fig. 5c), demonstrating successful Cr(VI) immobilization. Elemental mapping (Fig. 6a) reveals the spatial distribution of Fe, C, O, Al, and Cr in the material [29]. A strong overlap between Fe and C (Fig. 6b) further verifies the uniform anchoring of FeAl-LDH on the PS matrix. More importantly, Cr and Fe exhibit significant colocalization (Fig. 6c), with nearly identical spatial distributions (Fig. 6d–h), indicating that Cr(VI) is primarily immobilized through interaction with FeAl-LDH.

In the XRD pattern after reaction (Fig. S5a), the typical reflections of FeAl-LDH disappeared, indicating that its layered structure was no longer preserved. Instead, new diffraction features appeared, which can be attributed to secondary phases formed during the passivation process. These include α-Fe₂O₃, confirming that Fe(II) species in the LDH layers were oxidized to stable iron oxides, and AlOOH, indicating partial hydrolysis of the aluminum component. The emergence of these phases demonstrates that FeAl-LDH@PS undergoes structural transformation during the reaction. These transformation products are not only evidence of LDH decomposition but also play an active role in subsequent remediation.

The FTIR spectrum of Cr(VI) passivated by FeAl-LDH@PS is shown in Fig. S5b. At 3290 cm⁻¹, there is an absorption peak for hydrogen bonding between interlayer water molecules and hydroxyl groups [30, 31],while the peak at 1690 cm⁻¹ corresponds to the hydroxyl groups in the water molecules between the layers of the layered hydroxides [32]. The absorption peak of Cr appears at 873 cm⁻¹ [33].

XPS results (Fig. 7) strongly support the Cr(VI) immobilization mechanism. After treating Cr-contaminated soil, Cr-specific peaks appeared in the XPS survey spectrum (Fig. 7a), confirming successful adsorption. The Cr 2p₃/₂ spectrum shows peaks at 576.92 eV (Cr(III)) and 578.40 eV (Cr(VI)) [34, 35] (Fig. 7b), indicating partial reduction of Cr(VI), attributed to Fe(II) in FeAl-LDH acting as an electron donor. The Al 2p binding energy remained stable (Fig. 7c), while Fe 2p₃/₂ exhibited shifts from Fe(II) (710.30 eV) to Fe(III) (711.50 eV) [36] (Fig. 7d). Quantitative analysis showed Fe(II) content decreased from 43.56% to 13.87%, confirming a redox process. Thus, Cr(VI) stabilization involves both adsorption and Fe(II)-mediated reduction. A schematic mechanism is illustrated in Fig. 8.

In slightly acidic soil conditions (pH ≈ 6.5), hexavalent chromium mainly exists as Cr₂O₇²⁻ and HCr₂O₇⁻ species [37]. At the onset of the reaction, Fe²⁺ released from the FeAl-LDH participates in the reduction of Cr(VI) to Cr(III) [38]. Correspond to Eqs. (3) and (5) below.

(3)

C r_{2} O_{7}^{2 -} + 6 F e^{2 +} + 14 H^{+} \to 2 C r^{3 +} + 6 F e^{3 +} + 7 H_{2} O

(4)

H C r_{2} O_{7}^{-} + 6 F e^{2 +} + 14 H^{+} \to 2 C r^{3 +} + 6 F e^{3 +} + 7 H_{2} O

(5)

F e_{2} A l {(O H)}_{6} C l \to 2 F e^{2 +} + A l^{3 +} + C l^{-} + 6 O H^{-}

As the immobilization process continues, Fe²⁺ released from the FeAl-LDH layers is gradually oxidized to Fe³⁺. Part of the Fe³⁺hydrolyzes to Fe(OH)₃, which subsequently dehydrates to form Fe₂O₃. Meanwhile, the release of Fe destabilizes the LDH lattice, causing Al³⁺ to hydrolyze and transform into AlOOH [39]. These secondary phases, Fe₂O₃ and AlOOH, provide active sites for co-precipitation and surface adsorption of Cr species, thereby reinforcing its stabilization. Eqs. (6) and (7) illustrate the overall process.

(6)

F e^{3 +} + 3 O H^{-} \to F e {(O H)}_{3} \to F e_{2} O_{3} + 3 H_{2} O

(7)

A l^{3 +} + 2 H_{2} O \to A l O O H + 3 H^{+}

4. Conclusions

In this work, FeAl-LDH@PS composites were hydrothermally synthesized and applied as functional materials for the immobilization of Cr(VI) in contaminated soils. The integration of PS significantly enhanced remediation efficiency by increasing surface area, preventing particle agglomeration, and facilitating material recovery—overcoming key limitations of conventional LDHs. A systematic evaluation of operational parameters, including composite dosage, initial Cr(VI) concentration, material loading, and soil pH, revealed optimal conditions for stabilization. Toxicity leaching tests confirmed a substantial decrease in Cr(VI) mobility after treatment. The superior performance of FeAl-LDH@PS is primarily attributed to the sponge matrix, which offers high porosity, elasticity, water retention, and structural stability, enabling uniform LDH dispersion and improved contact with soil contaminants. Long-term remediation potential was validated under simulated dry–wet and freeze–thaw cycles, while phytotoxicity tests demonstrated good plant compatibility and ecological safety. Microbial community analyses further indicated that FeAl-LDH @PS not only suppressed stress-tolerant taxa but also facilitated the recovery of nutrient-cycling microorganisms and key metabolic functions, supporting ecosystem restoration. After reaction, characterizations provided mechanistic insights, indicating that Cr(VI) immobilization proceeds through Fe²⁺-mediated reduction to Cr(III) following the release of Fe from the LDH layers, accompanied by the hydrolysis of Al into AlOOH. The resulting Fe₂O₃ and AlOOH phases, together with residual LDH surfaces, act as active sites for co-precipitation and surface adsorption of chromium. These synergistic processes collectively ensure the effective and stabilization of Cr(VI) in soil environments.

Supplementary Information

eer-2025-330-supplementary.pdf

Notes

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51504170), Innovation and Entrepreneurship Training Program for College Students of Hubei Province (S202410490013), the Open Founding of Wuhan Institute of Technology Jingmen Research Institute of New Chemical Materials Industry Technology (JM2023001), and the Natural Science Foundation of Hubei Province (2024AFD190). We are also grateful to Shiyanjia Lab (https://www.shiyanjia.com) for conducting the XPS analysis.

Conflict of Interest Statement

The authors declare that they have no conflict of interest.

Author Contributions

L. Hu (Undergraduate) conceived and designed the experiments, conducted all investigation and wrote the original draft. Y. Xiao (Undergraduate) conceived and designed the experiments, implemented methods and analyzed the data. X. Xia (Undergraduate) performed the experiments, implemented methods and analyzed the data. Q. Ye (Undergraduate) performed the experiments, implemented methods and analyzed the data. M. Shi (Undergraduate) performed investigation, validation and data curation. H. Zhang (Associate Professor) supervised the study and edited the manuscript. Y. Ying (Postgraduate) performed investigation, validation and data curation. J. Cheng (Associate Professor) conceptualized the study, provided resources, supervised the work and reviewed the manuscript. X. Qiu (Associate Professor) conceptualized the study, acquired funding, provided resources, supervised the work and reviewed the manuscript.

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Fig. 1

(a-b) SEM image of PS; (c-d) SEM image of FeAl-LDH@PS; (e) EDS data of FeAl-LDH@PS; (f) XRD patterns of PS, FeAl-LDH and FeAl-LDH@PS; (g) FTIR spectra of PS, FeAl-LDH and FeAl-LDH@PS.

Fig. 2

Immobilization performance of FeAl-LDH@PS with different loading (a) and dosage (b). ([Cr(VI)=1000 mg/kg], soil pH=6.53)

Fig. 3

(a) The immobilization of different concentration Cr(VI) in soil by FeAl-LDH@PS; (b) Pseudo first-order kinetic model; (c) Pseudo second-order kinetic model. (the dosage of FeAl-LDH@PS is 0.05 g, pH=6.53)

Fig. 4

(a) Microbial community composition and (b) predicted metabolic functions in contaminated, clean, and remediated soils

Fig. 5

SEM images and EDS analysis of FeAl-LDH@PS after Cr(VI)-contaminated soil remediation. (a) (b) SEM images; (c) EDS.

Fig. 6

(a) EDS mapping of all elements, (b) and (c) 2d-histograms (scatterplots) of Fe-C and Fe-Cr, 2d-histograms EDS mapping of (d) O, (e)C, (f)Al, (g)Fe, and (h)Cr, respectively.

Fig. 7

XPS spectrum of FeAl-LDH@PS before and after Cr(VI)-contaminated soil remediation. (a) XPS survey; (b) Cr 2p; (c) Al 2p; (d) Fe 2p.

Fig. 8

Reaction mechanism of FeAl-LDH@PS remediation of Cr(VI) in soil