Effect of Modified Illite on Cd Immobilization and Fertility Enhancement of Acidic Soils

Abstract

Cadmium pollution in the soil induces significant hazards to agricultural growth and public health. Therefore, new routes are needed to develop low-cost soil amendments that can effectively stabilize cadmium and improve soil fertility. This study introduces modified illite (MIT) with the enhanced ability to stabilize heavy metal Cd through the mixing of illite with calcium carbonate, silicate and sulfate in specific proportions by using the well-known calcination-hydrothermal process. The characterization findings revealed that the modified MIT is predominantly composed of gehlenite and akermanite, with greatly improved specific surface area, pore structure and cation exchange capacity. The main purpose of the present study is to unravel a plausible mechanism on how MIT can stabilize Cd(II) in the soil and to investigate the effect of MIT on the fertility of the contaminated soil. Experiments on soil remediation revealed that MIT has a more profound stabilizing impact on Cd(II) compared to natural illite, resulting in a 22.84% reduction in acid-soluble Cd of the soil when MIT was dosed at 0.5%. The potential mechanism of Cd(II) immobilization by MIT involves the ionic exchange of Cd²⁺ with the exchangeable Ca²⁺ and Mg²⁺. In addition, the hydrolysis products of gehlenite and akermanite are prone to form silicate precipitates with free Cd, leading to soil pH increment. The pot experiments showed that MIT significantly reduces the cadmium content of soil and improves the growth of water spinach organs. Meanwhile, enhancement of the indicators in rhizosphere soil further proved that MIT effectively increases the nutrient content, improves the soil structure and promotes microorganism growth.

1. Introduction

Human exploitation of world mineral resources, modernized industrialization and developed agricultural activities have increasingly induced serious leakage of waste gases and sewage, misemployment of chemical fertilizers and discharge of acidic wastewaters in recent decades, leading to the accumulation of cadmium pollution as a significant factor in decreasing the quality of agricultural soils in the Pearl River Delta region [1,2].

Cadmium poisoning severely harms vegetable growth and biomass by inhibiting photosynthesis and metabolism of living organisms [3]. In South China’s red soil regions, which have deficient minerals, Cd may severely diminish nutrients, such as Ca, Mg and K, in crops and lead to crop malnutrition [4,5,6]. Moreover, the lifestyle of the microorganisms in soil are much vulnerable than plants and are strongly affected by the presence of Cd [7]. Cadmium reduces the activity of soil enzymes, restricts the availability of organic and inorganic nutrients and disrupts soil ecology [8,9]. Notably, plants can easily absorb and accumulate cadmium before the entrance of this toxic element into the human body through the food chain. Excess cadmium is highly dangerous to human health and leads to various cancers and a series of diseases in the kidneys, lungs and brain [10]. Therefore, pollution of soils with cadmium is an inevitable and urgent environmental issue.

There are primarily three physical, chemical and biological techniques for heavy metal remediation from soils, including soil replacement and vitrification, immobilization and soil washing, microbial and phytoremediation techniques [11]. In situ immobilization is a well-known and indispensable strategy for treating soils polluted with heavy metals. This technology offers superior immobilization efficacy, preferred economy and excellent environmental benefits, among other methods [12]. The in situ immobilization technique is preferentially much better than other methods, such as precipitation, ion exchange, adsorption, redox and organic complexation, which effectively reduces the active content, mobility and bioavailability of heavy metals in soil [13].

Organic, inorganic and clay minerals are frequently used as amendments for the in situ immobilization of soil with remediation techniques [14]. Organic wastes, such as animal manure, crop straw and food residues, are commonly organic amendments in agriculture that reduce the concentration of heavy metals through adsorption and/or complexation with heavy metals [15,16]. Besides, natural organic wastes can improve soil properties by enhancing the nutrients and humus of the soil together with increasing the pH and CEC [17,18]. However, these organic wastes comprise various pollutants and pathogens that cannot be disregarded and will exacerbate soil contamination if not managed appropriately [19,20,21]. Inorganic amendments primarily reduce Cd mobility through different mechanisms, such as adsorption, complexation and precipitation [22]. Lime is one of the most commonly used natural materials that significantly improves soil pH and promotes cadmium precipitation through the interaction of cadmium with carbonates. Moreover, calcium and cadmium ions in the soil can reduce the uptake of cadmium by plants through reducing the mobility of cadmium [14]. In addition, phosphate compounds facilitate the immobilization of Cd through adsorption and precipitation reactions [23]. Meanwhile, oxides of Fe, Al and Mn can decrease Cd migration through co-precipitation and adsorption processes [24,25]. Moreover, other inorganic wastes, such as fly ash and red mud, have been considered to effectively remediate cadmium-contaminated soils [26]. However, the feasibility and cost effectiveness of the inorganic additives are inevitably challenging because of their secondary pollution, nutrient imbalance and soil salinization effects if used in large quantities over a long time.

Because of the environmental friendliness, low cost and the plentiful resources of natural minerals, they are increasingly being exploited as soil amendments to remediate heavy metals. Natural minerals reduce the bioavailability of heavy metals through various physicochemical interactions [27]. Although, single natural minerals have weak activity and require a heavy dose, composite or modified mineral amendments are increasingly desirable [28,29,30].

Illite is a layered 2:1 silicate mineral that is intercalated with a substantial amount of alkaline earth K⁺ and is widely used in ceramics [31]. There are a number of studies on the adsorption of heavy metals by illite [32], while recent studies have shown the outstanding capability of illite in the adsorption of radioactive elements [33,34,35]. Additionally, illite is a potassium-rich aluminosilicate that is advantageous for manufacturing potash fertilizers [36]. Due to the low specific surface area of illite, and the considerable hydrophilicity and stable interlayer potassium ions, this compound is inappropriate for heavy metal remediation. Encouragingly, several recent studies have shown that illite would be significantly improved by specific modifications. For instance, the hydrophobic interaction of illite would be enhanced by covering its surface with CTAB surfactant, thus improving the adsorption capacity of this mineral for anionic pharmaceuticals [37]. Direct chlorination treatment can significantly improve the thermal stability, flame retardancy and mechanical properties of illite [38]. NaOH-modified illite has a high removal efficiency for iron and manganese ions with medium concentrations, and is capable of increasing groundwater standards to meet the requirements for normal drinking water [39]. The adsorption capacity of illite is significantly improved for chromium removal in water through its combination with organic modifiers and mechanochemical treatments [40]. However, little research has been published on the rehabilitation of heavy metal-contaminated soils using modified illites. Exceptionally, studying the immobilization of cadmium with modified illite is indispensable for the remediation of contaminated farmland.

This report investigated the incorporation of natural illite with calcium carbonate, calcium sulfate, wollastonite and dolomite in specific proportions. The synthetic ameliorant was prepared using the well-known calcination-hydrothermal method. The calcination process destroys the illite structure and converts the raw material into intermediate products, mainly composed of metasilicate and metallic aluminates. This process also exchanges the interlayer of potassium ions with cations, such as calcium and magnesium, increasing the illite layer spacing. Subsequently, the intermediate products undergo crystalline reconstructions through a hydrothermal process at a high temperature and pressure, resulting in the modified illite. Due to the high proportion of exchangeable alkaline earth cations, such as Ca, Mg and K, in illite and its superior structural characteristics, it is a potential remediating agent to amend soils by stabilizing heavy metals and increasing soil quality.

Consequently, it is essential to study the remediation capability of modified illite (MIT) on contaminated soils and investigate its impact on the soil quality and fertility. In this paper, a series of MIT minerals are synthesized using the abovementioned process, and the structural composition of illite is studied before and after modification. Subsequently, an exhaustive experiment was conducted on a sample of soil and crops enriched with different doses of MIT to investigate the remediation capacity of the material with the primary objectives of: (1) probing the efficacy and mechanism of MIT in fixing Cd, as an important heavy metal, in soil; (2) assessing the regulating effect of MIT on the transformation, migration and accumulation of Cd in soil and crops; and (3) exploring the ability of MIT to improve the soil properties and microbial growth.

2. Materials and Methods

2.1. Experimental Reagents and Soil Samples

Nitric acid (HNO₃), sodium hydroxide (NaOH), perchloric acid (HClO₄), 30% hydrogen peroxide (H₂O₂), calcium chloride (CaCl₂), calcium carbonate (CaCO₃) and calcium sulfate (CaSO₄) were purchased from Guangzhou Chemical Reagent Company. Diethylenetriaminepentaacetic acid (C₁₄H₂₃N₃O₁₀), triethylamine (C₆H₁₅N), hexamethylammonium cobalt trichloride ([Co(NH₃)₆]Cl₃) and cadmium nitrate (Cd(NO₃)₂) were purchased from the Aladdin Corporation. Acetic acid (CH₃COOH), hydroxylamine hydrochloride (NH₃OHCl) and ammonium acetate (CH₃COONH₄) were purchased from Tianjin Damao Chemical Reagent Company. Hydrofluoric acid (HF) was obtained from Shanghai Macklin Biochemical Co., Ltd. The wollastonite, dolomite and illite all came from Antu County, Jilin Province. All the chemicals were of analytical grade.

The original soil used for the experiments was attained from Xin’er Village, Zhongshan City, Guangdong Province, China (113° 28′ 26″ E, 28° 42′ 57″ N). The soil was mixed thoroughly and spread evenly in a well-ventilated greenhouse, air dried for two weeks, ground and passed through a 2 mm sieve. After adding a sufficient amount of deionized water and complete mixing, 0.18 g of Cd(NO₃)₂ powder was added to 90 kg of soil and thoroughly mixed. Maintaining a certain amount of water content, the samples were placed in a greenhouse for two months to perform natural weathering and grounding, then passed through a 2 mm sieve. As shown in Table S1, a portion of the contaminated soil was extracted using the five-point approach, ground and sieved through a 0.074 mm mesh, before characterization of its physical and chemical properties.

2.2. Preparation of Amendment

In the first step, the illite was modified using the calcination-hydrothermal method. Unmodified illite, CaCO₃, CaSO₄, wollastonite and dolomite were uniformly mixed (5:8:1:2.5:4, w/w) with 35% w/w deionized water and ground for 30 min. Next, the mixture was dried at a low temperature (60 °C) and calcined at 1000 °C for 1 h in a muffle furnace. After calcination, the activated intermediate was cooled and ground to the particle size ≤0.074 mm. Thereafter, the filtered intermediate was aged at 25 °C for 48 h by adding deionized water and thoroughly mixed. In the 4th step, deionized water is mixed with the aged product in a 10:1 ratio and transferred to a PTFE-lined hydrothermal reactor set at 150 °C for 4 h. The final product was washed with deionized water, dried at 60 °C and then, ground to a particle size ≤0.074 mm to obtain the final MIT. The prepared MIT was used for subsequent experiments, as well as for characterization experiments.

2.3. Adsorption Experiments

The potential of the amended soil was investigated for the adsorption of cadmium leachates from a natural environment. The original soil was incubated with 0.1, 0.3, 0.5 MIT and 0.5% illite, respectively. In this paper, the experimental group was set up according to the dosage ratio, which was defined by the mass ratio (w/w%) of the amendment to the soil. The amended soils of the five experimental groups were packaged in 50 mL polyethylene bottles weighing 0.12 g, each containing 40 mL of 1–20 mg/L cadmium nitrate solution. The pH was adjusted to 5 using 0.1 M NaOH and HNO₃. The groups were then put in an orbital shaker (180 rpm) for 24 h and, subsequently, centrifuged at 4000 rpm, which was repeated three times for each experiment. Atomic absorption spectrophotometry (AAS) was used to determine the cadmium content in the solutions obtained after centrifugation and filtration. After filtration, the obtained amendments were utilized for subsequent characterization and analysis experiments. The adsorption behavior of the amended soils was investigated using the Langmuir and Freundlich isotherm models. The Langmuir model assumes uniform monolayer adsorption of contaminants by the adsorbent. Whereas, the Freundlich model was developed to describe the adsorption of contaminants by multiple layers at limited sites of uneven energy, which can be expressed as [41]:

$${\mathrm{Q}}_{\mathrm{e}}={\mathrm{K}}_{\mathrm{F}}\times {{\mathrm{C}}_{\mathrm{e}}}^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$\mathrm{n}$}\right.}(\mathrm{F}\mathrm{r}\mathrm{e}\mathrm{u}\mathrm{n}\mathrm{d}\mathrm{l}\mathrm{i}\mathrm{c}\mathrm{h})$$

(1)

$${\mathrm{Q}}_{\mathrm{e}}=\frac{{\mathrm{Q}}_{\mathrm{m}}\times {\mathrm{C}}_{\mathrm{e}}}{{\mathrm{K}}_{\mathrm{L}}+{\mathrm{C}}_{\mathrm{e}}}(\mathrm{L}\mathrm{a}\mathrm{n}\mathrm{g}\mathrm{m}\mathrm{u}\mathrm{i}\mathrm{r})$$

(2)

where in Q_e (mg kg⁻¹) is the quantity of the adsorbed Cd²⁺, C_e (mg L⁻¹) is the concentration of the equilibrium solution, K_F is the Freundlich equilibrium constant, K_L (mg⁻¹) is the Langmuir constant associated with the bonding energy interaction and n is the heterogeneity indicator.

2.4. Characterization

The surface area and pore volume of the MIT were analyzed using the Brunauer–Emmett–Teller (BET) method, using a fully automated specific surface area porosity analyzer (Micromeritics ASAP 2460, Norcross, GA, USA). Subsequently, an X-ray diffractometer (Rigaku Ultima IV, Tokyo, Japan) was applied to analyze the physical phases of the MIT by X-ray diffraction (XRD, Rigaku Ultima IV, Tokyo, Japan), before and after Cd adsorption. Fourier transform infrared spectroscopy (FT-IR) was performed using a Thermo Fisher Nicolet iN10 (Waltham, MA, USA) to unravel the changes in the influential functional groups before and after adsorption of the Cd by the MIT and the modified soil. The components and chemical composition of the MIT and amended soil before and after adsorption of the heavy metal were investigated by X-ray photoelectron spectrometry (XPS) using a Thermo Scientific K-Alpha instrument. Scanning electron microscopy (SEM) was used to observe the morphology of the MIT and amended soil before and after Cd adsorption (Tescan MIRA LMS, Brno, Czech Republic).

2.5. Soil Remediation Experiments

Soil remediation experiments were performed using the in situ immobilization technique with four replicate samples. Each clay pot (16 cm diameter × 15 cm height) was filled with 1 kg of the contaminated soil. Then, 0.1, 0.3 and 0.5% MIT and 0.5% illite were added sequentially, and then finally, thoroughly mixed with the contaminated soil. After that, a certain amount of deionized water was added every five days and the water content of each pot was maintained at 70%. Then, the samples were collected on days 3, 7, 14 and 40. According to Tests S1 and S2, the collected soils were analyzed for pH, CEC, EC and total Cd. The European Community Bureau of Reference (BCR) was used to measure the cadmium partitioning between the different fractions. Table S2 details the exact testing methodology.

2.6. Pot Experiment

The pot experiment was conducted in a greenhouse at South China University of Technology, using a completely randomized design with five replicate samples. Each pot (18 cm diameter × 20 cm height) contained 3 kg of the contaminated soil. Before sowing, CK, 0.1, 0.3 and 0.5% MIT were mixed well with the four groups of soils. Then, the soil in each pot was maintained at 70% water content for two weeks. Urea was added to the experimental group, as the nitrogen fertilizer, at a rate of 0.03 g/kg of soil. Six seeds of healthy water spinach were planted in each container at a depth of approximately 1.5 cm. After two weeks of seedling growth, three plants were diluted out of the pots and retained for plant analysis. Water spinach was harvested after two months and the growth of each group was measured. Then, the spinach was dried at 65 °C for three days, crushed and analyzed for heavy metal concentration. Finally, the soil within a 5 mm radius around the roots of the plant was carefully collected and used to measure various indicators of rhizosphere soil.

Each organ of the collected plant samples was cut off with scissors, measured with a measuring tape and then, the concentration of the Cd was determined with a graphite furnace digestion instrument (Test S3). The DTPA extractable Cd concentration; the alkaline-N, Olsen-P, and NH₄OAc-K; the available concentration of calcium and magnesium; the urease, alkaline phosphatase and catalase activities; the soil organic matter (SOM); the microbial biomass carbon (MBC) and microbial biomass nitrogen (MBN); and the bacterial and fungal population of rhizosphere soil, were all determined as shown in Test S4.

2.7. Statistical Analysis

All data were statistically analyzed using the Microsoft Excel 2019 software. The XRD spectra were inspected using the Jade 6.0 software. The XPS spectra were probed with the Advantage 5.52 and XPSpeak software 4.1. The Omnic 8.0 software was used to analyze the FT-IR spectra. The explored data were plotted using the Origin 9.5.1.195 software. The significance level was set at a p-value < 0.05, n = 5.

3. Results and Discussion

3.1. Characteristics of Illite before and after Modification

The physical and chemical properties of the illite were changed through modification (

Table 1. Physical and chemical parameters of illite before and after modification.

Parameter	Illite	MIT
pH	6.8	11.2
CEC	23.11	98.37
BET surface area (m2/g)	10.32	37.49
Adsorption average pore diameter (nm)	6.51	10.81
Single point adsorption total pore volume (cm3/g)	0.017	0.101

). The pH of the illite grew by 4.38 units after modification, because of the presence of calcium carbonate and a lot of alkaline earth elements in the reaction medium. The CEC value of the MIT was 4.28 times greater than that of the illite and the ion exchange capacity of the former was significantly enhanced in the soil. As shown in Figure 1, the N₂ adsorption–desorption isotherm of the MIT conformed to a type IV adsorption curve with a significant H3-type hysteresis loop [42]. According to the pore size distribution analysis, the pore diameter of the MIT was mainly 2.3–20.7 nm, indicating that the MIT had a mesoporous structure and was greatly beneficial to the adsorption and transportation of particles in the soil [43]. The N₂ adsorption–desorption isotherms and pore size distribution analyses confirmed that the MIT has a higher adsorption capacity compared to illite (Figure S1). In addition, BET analysis revealed that the specific surface area of the MIT was 3.63 times greater than that of illite and its adsorption average pore diameter was increased from 6.51 to 10.81 nm. Moreover, the single point adsorption total pore volume of the MIT increased from 0.017 to 0.101 cm³/g. Therefore, the modification process significantly enhanced the pore structure of the illite, resulting in a greatly enhanced adsorption capacity that may be attributed to the hydrothermal process and ion exchange of the interlayer cations [44,45].

The XRD patterns of the illite before and after modification are shown in Figure 2a. Before modification, the diffraction peaks of the illite were observed at around 8.78°, 19.78°, 30.08° and 34.86° [46]. Notably, the most sharp diffraction peaks were due to quartz. After modification, strong diffraction peaks were detected at the 2θ of 16.08°, 21.04°, 23.97°, 29.00°, 31.23°, 36.45°, 44.46° and 52.04°, which are characteristic peaks of akermanite (Ca₂MgSi₂O₇). Meanwhile, the characteristic peaks of gehlenite (Ca₂Al₂SiO₇) appeared at 23.99°, 31.41°, 37.51°, 52.09° and 68.18°. The substances in the illite–carbonate–sulfate–silicate system were progressively activated throughout the calcination process [47]. When the temperature reached 1000 °C, CaO and MgO were generated due to the thermal decomposition of the calcium carbonate and magnesium carbonate, which further underwent a series of reactions with SiO₂ in the molten state to achieve akermanite and gehlenite [48,49]. Due to the ionic exchange of Ca²⁺, Mg²⁺ and K⁺, the electrical valence of the reaction became unbalanced, enhancing the instability of the reactants and promoting the synthesis of new clay minerals. Meanwhile, the silicon and aluminum in the illite structure were leached out at the high temperature, leading to an alteration in the Si-O tetrahedra and Al-O octahedra. Furthermore, the subsequent hydrothermal process was extremely beneficial to the reconstruction of the crystalline minerals, which enhanced the crystallinity and stability of the products.

The microscopic morphology and structural characteristics of the illite before and after modification were observed by SEM (Figure 2c,d). As shown in the Figures, the surface of the illite was smooth and composed of many irregular scale-like and feather-like structures piled in a relatively dense structure before modification. Whereas, the microstructure of the MIT consisted of irregular rod-like and block-like agglomerates after modification, and the surface became depressed and uneven. Moreover, the layer spacing of the MIT was expanded due to the replacement of Ca²⁺ and Mg²⁺ that results in the expansion of the original pore structure; therefore, the surface involves plenty of minute pores. The above findings demonstrated that the modification process substantially improved the specific surface area and pore volume of the MIT.

Figure 2b displays the FT-IR spectra of the illite before and after modification. This Figure shows a strong stretching vibration corresponding to the hydroxyl groups near 3550 cm⁻¹. The HOH bending vibration at 1630 cm⁻¹ indicated a certain amount of interstratified water molecules in the illite and MIT [50]. The OH group of the hydrated cation represented a bending vibration at 1480 cm⁻¹, indicating that K⁺ should be detached from the MIT structure by ion exchange with the hydrated cation in solution during the modification process [51,52]. The spectral bands at 1040, 1020, 977 and 938 cm⁻¹ are attributed to the stretching vibrations of the Si-O or Si-O-Si, because most aluminosilicate minerals contain these spectral bands. The change in portion of the spectral bands indicated that the modification process induced conversion of the original silica-aluminate mineral into many new substances. The spectral band at 854 cm⁻¹ is attributed to the Al-OH deformation vibration, while another band near 537 cm⁻¹ belongs to the Si-O-Al [53]. There were significant changes when comparing the FT-IR spectra before and after the modification, resulting from the replacement of the Si in the illite structure by more reactive Al³⁺ cations during the modification process. In addition, the spectral bands near 470 cm⁻¹ belong to the bending vibrations of the Si-O and Si-O-Si [54].

The full XPS spectral scans before and after modification of the illite (Figure 3a) showed that the Mg1s, O1s, Ca2p, C1s, Si2p and Al2p peaks were assigned to the binding energies of 1303, 531, 346, 284, 102 and 73 eV, respectively. Comparing the elemental spectra before and after modification showed a slight change in the binding energy of Si and Al, while the intensity of the characteristic bands was significantly increased due to the presumable synthesis of a new active mineral. During modification, a portion of Si and Al in the reaction system was converted into water-soluble salts and, therefore, they had been removed from the crystalline structure (Figure S2). Since Ca and Mg are not contained in the original structure of the illite, the significant increase in intensity of their characteristic peaks proved that these two elements had been successfully replaced with K in the MIT structure. The XRD analysis also demonstrated that akermanite and gehlenite are the principal constituents of the MIT.

3.2. Mechanism of Cadmium Stabilization

3.2.1. Adsorption Isotherms of Amended Soil

After calculation, the Langmuir adsorption model was used to describe the adsorption isotherm for Cd in solution on amended soils with different treatments (Figure 4). The parameters of the isothermal adsorption equation (R² > 0.93) are shown in

Table 2. Isotherm parameters of cadmium adsorption from aqueous solutions of amended soils containing different amounts and types of passivators.

Treatments		Langmuir			Freundlich
	Qe (mg g−1)	KL (L mg−1)	R2	1/n	KF (L mg−1)	R2
CK	3.66	0.347	0.94	0.532	1.30	0.97
0.1% MIT	3.87	0.283	0.93	0.588	1.27	0.98
0.3% MIT	4.05	0.348	0.95	0.564	1.42	0.98
0.5% MIT	4.30	0.412	0.95	0.545	1.62	0.98
0.5% Illite	3.84	0.252	0.94	0.603	1.21	0.96

. All soil samples exhibited nonlinear sorption isotherms and the nonlinearity of the isotherms increased after the addition of the MIT. The experimental results showed that the adsorption of Cd²⁺ by the amended soils increased significantly with the addition of the MIT. The maximum adsorption capacities were 3.79 mg g⁻¹, 3.85 mg g⁻¹, 4.05 mg g⁻¹ and 3.64 mg g⁻¹ for the amended soils with the additions of 0.1%, 0.3%, 0.5% MIT and 0.5% illite, respectively. The MIT improved the adsorption capacity of the soil for cadmium in solution.

As mentioned previously, the interlayer or pores of the MIT contain a large amount of exchangeable Ca²⁺ and Mg²⁺, and the modification increased the illite’s specific surface area and improved the illite’s pore structure, which enhanced its ion exchange capacity. Moreover, the Si-OH and Al-OH formed on the molecular fracture surfaces of the silica-aluminate minerals provide the MIT with numerous active adsorption sites for complexation; electrostatic interactions occur between the negatively charged sites and groups on the surface and in the pores of the MIT; the SiO₃^2ࢤ generated by hydrolysis of the major components in the MIT reacts with the Cd²⁺ by precipitation. All these potential mechanisms suggest that the calcination-hydrothermal method is a promising way to increase the adsorption capacity of illite for heavy metal Cd. However, the stabilization mechanism of the MIT on Cd needs to be further confirmed, so it is necessary to characterize the MIT materials after adsorption.

3.2.2. Characterization of MIT after Adsorption

The characteristic peaks of akermanite and gehlenite were slightly changed after Cd(II) stabilization, due to hydrolysis during the adsorption process; however, new characteristic peaks of quartz emerged. Meanwhile, the generated SiO₃^2ࢤ ions from the hydrolysis of akermanite and gehlenite in the MIT reacted with free Cd²⁺ to form cadmium metasilicate precipitate, which reduced the mobility of the cadmium in the soil [55].

The shape of the MIT peaks in the FT-IR spectrum after Cd stabilization were nearly identical to that before stabilization, suggesting that the structure of the MIT had not deteriorated. The intensity of the HOH bending vibration at 1630 cm⁻¹ was increased after Cd stabilization, indicating that more interlayer water had been adsorbed by the MIT. The characteristic spectral band at 1480 cm⁻¹ was changed under the acidic conditions, due to the ion exchange of the hydrated cations in the MIT with Cd²⁺. After adsorption of Cd²⁺ in the Cd(NO₃)₂ solution, a strong vibration at 1385 cm⁻¹ was found, which might be attributed to the asymmetric stretching of NO₃⁻ [56]. Most likely, the alteration of the band around 1000 cm⁻¹ after adsorption of the Cd²⁺ heavy metal can be attributed to the coordination with many amorphous silicates on the MIT surface that generates stable silicate precipitates involving Si-O-Cd bonds. In addition, the decreasing intensity of the Al-OH bending vibration was most likely related to the changes in pH during adsorption [57].

The rod-like aggregates vanished after adsorption and stabilization of Cd²⁺. Numerous flaky and flocculent agglomerates were irregularly attached to the surface and pore channels of the MIT after adsorption (Figure 2e). The agglomerates on the surface of the MIT consisted of metasilicate and silicoaluminate precipitates. Similarly, the same outcomes can be drawn for the amended soil before and after Cd loading by comparing the morphological properties (Figure S3). These studies clearly confirmed that the MIT had successfully immobilized Cd²⁺.

The two characteristic peaks of Cd3d appeared around 405 and 412 eV, as shown in the XPS spectrum (Figure 3b). This study also demonstrated the successful adsorption of cadmium ions on the surface of the MIT. After the stabilization of Cd(II), the binding energies and characteristic peaks of Si2p, Al2p and O1s changed, which may indicate the involvement of Si, Al and O in the immobilization of Cd(II) through precipitation or complexation reactions. The modification of the illite enhanced the specific surface area of the framework, because of the numerous activated mineral components with high CEC values in the MIT that resulted in the substantial adsorption and enhanced ion exchange capacity for Cd. Furthermore, the intensity of the characteristic peaks of Ca2p and Mg1s decreased, which clearly indicated the replacement of the exchangeable Ca²⁺ and Mg²⁺ in the MIT with Cd²⁺ and the stabilization of the target Cd heavy metal (Figure 3c,d).

3.2.3. Analysis of Soil Remediation

Soil pH directly affects the migration and bioavailability of heavy metals and essentially indicates the capacity of soils to remediate heavy metals. As shown in Figure 5a, the addition of MIT to the soil resulted in the considerable enhancement of the pH and acidification capacity of the soil. The addition of 0.1, 0.3 and 0.5% MIT increased the pH value by 0.97, 1.22 and 1.38 units, respectively, after 40 days, whereas 0.5% illite increased the pH by 0.66 units. CEC denotes the total amount of cations that can be adsorbed and exchanged by soil colloids at a certain pH. This parameter is principal to assessing the buffering capacity of soils. EC is another indispensable parameter related to the physicochemical properties of soil that indicates the number of salts in the sample. All of the performed remediation experiments in this research increased the soil CEC. This parameter was enhanced by 43.30, 70.86, 80.86 and 25.21% when dosed at 0.1, 0.3 and 0.5% MIT together with 0.5% illite, respectively, as compared to the CK group (Figure 5b). Compared to the CK group, the EC was enhanced by 7.69, 24.48 and 63.64% in the groups with 0.1, 0.3 and 0.5% MIT dosage, respectively, whereas it decreased by 11.19% in the group with 0.5% illite. Eventually, the CEC and EC values in the soil showed the same trend as observed for the pH. Considering the composition and crystalline structure of the MIT, this compound was more effective than natural illite in growing the pH, CEC and EC at the same concentration and under the same experimental conditions.

The distribution of the different chemical forms of cadmium in the soil is directly related to its biological toxicity, as described in Figure 6a. The proportion of acid-soluble cadmium decreased by enhancing the restoration time. While the amount of reducible, oxidizable and residual cadmium increased with the restoration time. The extent of the acid-soluble Cd was significantly reduced as the MIT dosage was enhanced from 0.1 to 0.5%. After 40 days, the percentage of acid-soluble Cd decreased by 11.58, 17.66 and 22% in the soils with 0.1, 0.3 and 0.5% MIT, respectively, while this amount decreased by 4.37% in the soil with 0.5% illite. The percentage of reducible Cd was eventually increased by 6.27, 7.78 and 8.06% in the presence of 0.1, 0.3 and 0.5% MIT, respectively, while the amount of reducible Cd was 3.30% for 0.5% illite. Similarly, the quantity of oxidizable cadmium was increased by 0.77, 2.33 and 2.96% in the presence of 0.1, 0.3 and 0.5% MIT, respectively, this amount was increased by 0.61% for 0.5% illite. Finally, the residual Cd was increased by 4.60, 7.61 and 11.82% for 0.1, 0.3 and 0.5% MIT, respectively, while 0.5% illite showed a 2.19% increase. Acid-soluble Cd is considered as the most bioavailable component of cadmium. When the MIT and illite were mixed at the same dose level, the MIT most prominently reduced the acid-soluble Cd. However, a slight increase was observed in the percentage of reducible and oxidizable Cd. The majority of the removed acid-soluble Cd was transformed into the residual Cd.

DTPA-extractable Cd is considered to have a significant corporation with the bioavailable Cd in soil, and it is extensively used to assess the toxicity of Cd in soil for plants. DTPA leaching was performed on the rhizosphere soils collected after crop harvesting. When the MIT dosage was increased from 0.1% to 0.3 and 0.5%, the DTPA-extractable Cd in the soil declined by 21.94, 26.29 and 32.56%, respectively, when compared to the CK group (Figure 6b). The concentration of DTPA-extractable Cd in the soil was significantly reduced by the MIT dosage. In summary, the application of the MIT effectively reduced the bioavailability of cadmium, and the impact of the MIT became more pronounced as the dosage increased.

3.2.4. Stabilization Mechanism

Numerous studies have shown that pH greatly affects the bioavailability of Cd, such that the effectiveness of Cd in the soil is higher when the pH is lower, thus intensifying its damage to crops [58,59]. Natural illite contains many alkaline earth metals, such as potassium. Akermanite and gehlenite both contain a large amount of active Ca and Mg, which are generated during the modification process. The Ca²⁺ and Mg²⁺ cations can be quickly replaced by H⁺ and Al³⁺, decreasing the soil acid content and effectively improving the acidity of the soil. The hydrolysis reaction of akermanite and gehlenite could be related to the elevation of soil pH (Equations (3) and (4)). Thus, further hydrolysis of akermanite and gehlenite induces more and more available Ca²⁺ and Mg²⁺ ions in the soil that could be easily exchanged by other cations and promote the hydrolysis reaction. As a result, the OH⁻ content would neutralize the acidity of the soil. It has been reported that pH increment leads to the enhancement of the soil colloids and agglomerates, resulting in more negative charges and adsorption sites on the surface of the soil particles that are prone to effectively reducing the biological productivity of Cd [60]. This report revealed that the stabilization of Cd(II) by amendment was more pronounced in the higher pHs, however, the bioavailability of Cd was significantly reduced, consistent with the reported findings [61].

$${\mathrm{C}\mathrm{a}}_2\mathrm{M}\mathrm{g}{\mathrm{S}\mathrm{i}}_2{\mathrm{O}}_7+{\mathrm{H}}_2\mathrm{O}\to 2{\mathrm{C}\mathrm{a}}^{2+}+{\mathrm{M}\mathrm{g}}^{2+}+{{2\mathrm{S}\mathrm{i}\mathrm{O}}_3}^{2-}+2{\mathrm{O}\mathrm{H}}^{-}$$

(3)

$${\mathrm{C}\mathrm{a}}_2{\mathrm{A}\mathrm{l}}_2{\mathrm{S}\mathrm{i}\mathrm{O}}_7+{\mathrm{H}}_2\mathrm{O}\to 2{\mathrm{C}\mathrm{a}}^{2+}+2{\mathrm{A}\mathrm{l}{\mathrm{O}}_2}^{-}+{\mathrm{S}\mathrm{i}\mathrm{O}}_2+{2\mathrm{O}\mathrm{H}}^{-}$$

(4)

$${\mathrm{C}\mathrm{a}}^{2+}{\mathrm{M}\mathrm{g}}^{2+}\left(\mathrm{M}\mathrm{I}\mathrm{T}\right)+{\mathrm{C}\mathrm{d}}^{2+}\to {\mathrm{C}\mathrm{d}}^{2+}\left(\mathrm{M}\mathrm{I}\mathrm{T}\right)+{\mathrm{C}\mathrm{a}}^{2+}+{\mathrm{M}\mathrm{g}}^{2+}$$

(5)

$${\mathrm{C}\mathrm{d}}^{2+}+2{\mathrm{O}\mathrm{H}}^{-}\to {\mathrm{C}\mathrm{d}\left(\mathrm{O}\mathrm{H}\right)}_2\downarrow$$

(6)

$${\mathrm{C}\mathrm{d}}^{2+}+2{\mathrm{A}\mathrm{l}{\mathrm{O}}_2}^{-}+4{\mathrm{H}}_2\mathrm{O}\to {\mathrm{C}\mathrm{d}\left(\mathrm{O}\mathrm{H}\right)}_2\downarrow +{2\mathrm{A}\mathrm{l}\left(\mathrm{O}\mathrm{H}\right)}_3\downarrow$$

(7)

$${\mathrm{C}\mathrm{d}}^{2+}+{{\mathrm{S}\mathrm{i}\mathrm{O}}_3}^{2-}\to \mathrm{C}\mathrm{d}{\mathrm{S}\mathrm{i}\mathrm{O}}_3\downarrow$$

(8)

The characterization data showed that Ca²⁺ and Mg²⁺ can easily replace the stable K⁺ between the illite layers during the modification process, and these cations are weakly correlated with the structure of the MIT and can be simply detached from it. Consequently, the ion exchange capacity of the MIT was further enhanced via modification of the specific surface area and pore structure [62], which is crucial for the immobilization of Cd(II). As stated previously, owing to the continual hydrolysis of the silica-aluminate minerals, large quantities of Ca²⁺ and Mg²⁺ will be released into the soil solution that can be exchanged by free Cd²⁺ (Equation (5)). Simultaneously, the concentration of OH⁻, AlO₂⁻and SiO₃²⁻ was increased continuously and these anions can immobilize Cd through the precipitation process (Equations (6)–(8)), which further promotes the hydrolysis of akermanite and gehlenite.

The performed experimental results showed that the addition of MIT increased the soil EC, possibly due to the presence of soluble silicate and meta-aluminate in the MIT. Since MIT has a superior specific surface area and pore structure, free Cd can be easily converted into precipitate in the soil and, therefore, can simply be captured by the MIT and quench its mobility. In addition, the silanol (Si-OH) and aluminol (Al-OH) groups could be generated at the fracture surface of the layered silica-aluminate and may form many active adsorption sites to immobilize Cd through complexation. Finally, the adsorption caused by the mutual attractions between Cd²⁺ and many negatively charged adsorption sites and groups on the surface of the MIT are also believed to be responsible for the capturing of Cd²⁺ by the MIT.

3.3. Enhancement of Soil Fertility

3.3.1. Effect on Plant Growth

As shown in

Table 3. The growth index of each organ of water spinach at different MIT dosing levels.

Dosage	Fresh Biomass (g plant−1)			Size (cm plant−1)
	Root	Shoot	Leaves	Root	Shoot		Leaves
					Length	Diameter	Length	Width
CK	0.24 c	3.03 c	1.29 c	5.53 d	59.13 b	0.36 d	6.58 d	1.70 c
0.1%	0.58 bc	4.81 b	2.20 b	9.45 c	63.73 b	0.46 c	8.16 c	2.21 b
0.3%	0.87 b	6.37 a	3.30 a	11.76 b	74.32 a	0.51 b	9.03 b	2.54 ab
0.5%	1.91 a	6.59 a	3.54 a	17.59 a	74.37 a	0.56 a	10.15 a	2.58 a

Different lowercase letters indicate significant differences between each treatment (p < 0.05, n = 3).

, the MIT was beneficial to increasing the fresh weight of water spinach. For the roots, the fresh weight of the CK group was 3.03 g per plant. When the MIT dose was increased from 0.1 to 0.3 and 0.5%, the fresh weight of the roots increased to 4.81, 6.37 and 6.59 g, respectively. Similarly, the fresh weight of the stems was 1.59–2.18 times higher than that of the CK group; while the fresh weight of the leaves was 1.50–2.75 times higher than that of CK group. However, there was no statistically significant difference between the two experienced groups under the MIT dosing of 0.3 and 0.5% for both stem and leaf fractions. In addition, the results revealed that with the most vigorous development attained at 0.5% MIT, the growing size of the water spinach was significantly increased in all the experienced groups compared to the CK group. The length of roots was 1.71, 2.12 and 3.18 times longer than that of the CK group when the dosage was 0.1, 0.3 and 0.5%, respectively. The substantial promotion of root growth by the MIT is probably the critical reason for the significant differences in the growth of the water spinach. With the increase in MIT dosage from 0.1% to 0.3 and 0.5%, the length of stem was increased by 7.78, 25.69 and 25.77%, respectively. The leaves increased by 24.01, 37.23 and 54.26% in length, while they decreased by 30, 49.41 and 51.76% in width, both for the MIT dosing of 0.1, 0.3 and 0.5%, respectively. Overall, the MIT showed a significant increment in the growth of all parts of the spinach, especially the roots. While for the stems and leaves, the enhancement effects were similar at 0.3 and 0.5% doses (Figure S4).

Figure 6c shows the changes in Cd(II) concentration in each organ of water spinach with increasing MIT dosage. In general, the accumulation of Cd(II) in all the organs was reduced. When the amount of MIT was increased from 0.1% to 0.3 and 0.5%, the concentration of Cd(II) in the roots was reduced by 8.32, 28.05 and 52.19%, respectively. While for the stems and roots, the maximum reduction in Cd(II) concentration was 22.65 and 41.22%, respectively. Interestingly, 0.5% MIT resulted in the most significant reduction in Cd(II) concentration in each organ, due to the changes in the distribution and reduction in the bioavailability of the different Cd(II) forms in the soil that prevent the transportation of this toxic ion to all the organs of the plant.

3.3.2. Enrichment of Soil Nutrients

Alkaline-N, Olsen-P and NH₄OAc-K are essential nutrients in the soil that directly impact the growth of crops and are critical indicators when evaluating soil fertility. As shown in Figure 7a, the alkali-N concentration was decreased by 13.39, 15.18 and 25.44% as the MIT dosage was enhanced from 0.1% to 0.3% and 0.5%, respectively, because the MIT itself does not contain N, which is critical for the growth of the roots and is responsible for the nutrient uptake by plants [63,64]. The results of the plant growth showed that more nitrogen can be absorbed by the plant when the roots grow better and leads to the diminishment of the nitrogen concentration. In contrast, Olsen-P was increased by 31.85, 53.06 and 60.10% by enhancing the MIT dosage from 0.1% to 0.3 and 0.5%, respectively. Similarly, NH₄OAc-K was increased by 14.10, 35.90 and 42.31% with the same mentioned amount of MIT, respectively. The addition of MIT increases the concentration of nutrient P in the soil, probably because MIT promotes P cycling in the soil [65]. However, we know that excess uptake of P by plants is not desirable. Furthermore, the enhancing of the concentration of nutrient K was due to the modification process, which made it easier for large amounts of K to be released into the soil from the illite itself.

Calcium and magnesium ions are significant plant nutrients and they are present in the soil in an exchangeable state. When MIT was added to soil, a lot of calcium and magnesium will be undoubtedly brought to the soil, which was confirmed by the performed experiments in this study (Figure 7b). With the addition of 0.1, 0.3 and 0.5% MIT, the concentration of exchangeable calcium was increased by 26.67, 55.56 and 77.78%, respectively, and the amount of the exchangeable magnesium was enhanced to 18.18, 54.55 and 136.36%, respectively, compared to the CK group. The increase in exchangeable calcium and magnesium concentration was attributed to the release of the corresponding active ions from the MIT into the soil.

The results of the pot experiments showed that the growth of water spinach became more vigorous with the increase in MIT dosage. One of the reasons for this finding was that MIT significantly reduces the bioavailability of Cd(II) in soil. The experimental results showed that the MIT significantly reduced the Cd(II) concentration in all the organs of water spinach, particularly the roots. Since the mineral amendment MIT was abundant in practical mineral elements, K, Ca, Mg, Si and S, it neutralizes acidic soils and supplements the lack of nutrients in the soil. Notably, potassium and calcium are essential for plants and the former ion promotes photosynthesis, stabilizes the osmotic pressure of the cells and conserves water in plants. Moreover, potassium plays a crucial role in plant growth [66]. In addition, calcium is an integral part of plant cell walls and cell membranes, contributing to the structure of plant cells and improving their resistance to diseases [67]. On the other hand, magnesium is also an indispensable component of chlorophyll, which influences various physiological functions and promotes the conversion and metabolism of sugars in plants [68]. Additionally, sulfur is another essential nutrient that affects the synthesis of proteins, chlorophyll and various hormones in plants [69]. According to previous studies, silicon not only increases the content of the available nutrients in the soil, but also promotes the uptake of the various nutrients by plants and even strengthens the structure of cell walls and enhances the defense response of plants [70].

3.3.3. Improvement of Soil Quality

Soil organic matter (SOM) is an essential component of healthy soils [71]. The experimental results showed that the addition of MIT increases the SOM. However, the increase in SOM was negligible when enhancing the MIT dose (Figure 7c). According to previous studies, the carbon stock in the soil gradually depleted. However, clay minerals can be regarded as a component closely related to SOM, having a considerable role in preserving SOM [72]. The increase in the SOM was attributed to the application of the mineral amendment MIT, but it was not related to the addition of new SOM and presumably associated with the effective lowering of SOM loss [73].

Soil microbial community structure and enzyme activity are critical indicators for evaluating soil health (Figure 7d). The activity of urease, alkaline phosphatase and catalase was significantly elevated compared to the CK group. At the MIT dosage of 0.1, 0.3 and 0.5%, urease activity was increased by 40.44, 71.56 and 94.76%, respectively, whereas the activity of alkaline phosphatase was enhanced by 24.40, 30.15 and 68.90%, respectively, and the catalase activity was enhanced by 15.22, 32.61 and 47.83%, respectively, according to the mentioned MIT dosage. In conclusion, the MIT significantly enhanced the activity of urease, alkaline phosphatase and catalase in the soil.

Soil microbial biomass carbon (MBC) and microbial biomass nitrogen (MBN) are the most active components of organic carbon and organic nitrogen in soil. They are essential sources of available carbon and nitrogen in soil, which reflect the turnover and capacity of soil carbon and nitrogen pools [74]. As shown in Figure 7e, the MBC and MBN in the inter-root soil were significantly increased after the addition of MIT. At the MIT dosage of 0.1, 0.3 and 0.5%, the value of MBC was 1.80, 2.69 and 6.95 times higher, respectively, than that of the CK group, while the amount of MBN was 2.50, 3.14 and 6.36 times higher, respectively, than that of the CK group. These results indicate that when MIT is added to the soil, the soil will be more favorable for microbial growth and the microbial life activities will be increasingly active. Microorganisms in the soil can regulate nutrient cycling, stimulate phytohormones and improve plant immunity, thus promoting crop growth [75,76]. Soil microbial biomass is the driving force of soil nutrient transformation, cycling and preserving of the available plant nutrients in the soil [77]. The experiments proved that both the bacterial and fungal populations in rhizosphere soil were elevated with MIT dosage (Figure 7f). The bacterial population was enhanced by 45.66, 67.63 and 93.06% with the MIT dosage of 0.1, 0.3 and 0.5%, respectively, while the fungi was increased by 2.01, 4.11 and 7.83 times, respectively, compared to the CK group.

In this paper, the illite was modified using the calcination-hydrothermal method and provided a 2:1 type of clay mineral structure. The performed modifications enabled the MIT to involve many loose pore channels and a large specific surface area, in which the water molecules existed in a relatively free state. Therefore, the MIT enhanced the permeability and water-holding capacity of the soil. At the same time, the MIT facilitated the stability of the organic content and promoted the formation of soil aggregates. Thus, the MIT further improved the clay structure and buffer capacity of the soil. The MIT increased the SOM, which was a source of nutrients and energy needed for microbial growth [78] and thus, enhanced the life activity of the microorganisms in the soil. Plants interact with the microorganisms (bacteria and fungi) in soil in a mutually beneficial symbiotic relationship [79]. Microorganisms play an essential role in plant growth by increasing the effectiveness of the nutrients and producing the hormones needed for plant growth, as well as protecting plants from pathogens and diseases [80]. In turn, plants can provide a habitat for microorganisms and provide nutrients, thus promoting their growth and reproduction [81]. Moreover, enzymes are considered to be significant indicators of biochemical processes and the metabolic status of microorganisms in soil [82]. Many studies have studied the effects of amendments and pollutants on soil quality [83]. Urease, phosphatase and catalase activity are the most commonly used indicators to evaluate heavy metal-contaminated soils [84]. In the present study, the addition of MIT to the Cd-contaminated soil significantly increased the population of microorganisms and the activity of all the abovementioned enzymes. Therefore, MIT could be very beneficial in mitigating the harmful effects of Cd on microbial and enzyme activities. Consequently, MIT improved the soil structure and the ameliorates physicochemical properties and, more importantly, soil fertility. The MIT promoted the ecological remediation of the Cd-contaminated soils by reinforcing well-functioning soil–plant–microbial tripartite interactions.

4. Conclusions

In this study, MIT was successfully synthesized by the calcination-hydrothermal method. The modification of MIT increased the specific surface area and optimized the pore structure of this mineral clay. MIT has many exchangeable active cations and has excellent stabilization potential for the heavy metal Cd. The soil remediation experiments showed that the CEC, EC and pH of the amended soil were significantly increased. Acid-soluble Cd can be converted to residual Cd, thus reducing the mobility of Cd(II). Ion exchange is the main reason for the immobilization of the Cd with MIT. Moreover, surface precipitation as well as surface adsorption are also crucial stabilization mechanisms. Finally, pot experiments were used to study the growth indicators of water spinach. The MIT significantly promoted water spinach growth due to a reduction in Cd(II) bioavailability. The presence of MIT significantly improved the physicochemical properties of the soil and increased its effective nutrient content. The changes in the SOM, MBC and MBN were studied on the microbial population and the activity of the three enzymes in the rhizosphere soil and it was evident that the growth environment of the plant roots was significantly improved. The capabilities of mineral MIT were evidenced by its excellent Cd(II) stabilization ability and the improvements made to the soil quality. Due to the advantages of low-cost and environmental friendliness of mineral materials, MIT is a promising choice for remediation of acidic Cd-contaminated soils and provides a theoretical basis for the application of multifunctional mineral amendments. However, due to the limited scale of the experiments, subsequent large-scale field experiments on heavy metal-contaminated lands are needed to investigate the stability of MIT for the long-term immobilization of different heavy metals and detailed processing parameters are necessary in this field.