Preparation of Zeolite A from Ion-Adsorbing Rare Earth Tailings for Selective Adsorption of Pb²⁺: An Innovative Approach to Waste Valorization

Abstract

Ion-adsorbing rare earth tailings (IRETs) contain a large amount of clay minerals, which are a potential source of silicon and aluminum for the preparation of zeolite materials. The complexity of the tailings’ composition and the impurity composition are the main difficulties in the controllable preparation of zeolite. Herein, IRETs were treated by classification activation technology for the preparation of IRET-ZEO, which was used for the removal of heavy metal Pb²⁺ in water. A new method of resource utilization of ion-type rare earth tailings is realized by “treating waste with waste”. The results showed that the IRETs were classified and then thermally activated, and the optimal activation parameter was calcination at 850 °C for 1 h. The optimal NaOH concentration used in the crystallization process was 5 mol/L, with a crystallization time of 3 h and a crystallization temperature of 85 °C, and the crystallization product was zeolite A. The removal rate of the Pb²⁺ solution with an initial concentration of 100 mg/L was as high as 96.7% in an acidic solution with a pH value from 2 to 5.5. In particular, when the solution pH was higher than 4.2, the adsorption rate of Pb²⁺ was close to 100%. The IRET-ZEO showed a fast adsorption rate (5 min to reach adsorption equilibrium), a large adsorption capacity (378.35 mg/g), excellent acid resistance, and selectivity and regenerability for Pb²⁺. This work provides a new strategy for the green resource utilization of IRETs and the treatment of lead-containing wastewater.

1. Introduction

Heavy metal ion contamination is currently one of the most significant forms of water pollution, posing a notable threat to both ecological safety and human health [1]. Large volumes of wastewater produced during mineral extraction and processing contain heavy metal ions such as Cu²⁺, Pb²⁺, Cd²⁺, and Cr²⁺. Once these heavy metal ions enter the aquatic environment, toxic heavy metal ions enter the food chain and accumulate in the human body, which are difficult to eliminate naturally [2]. Currently, the primary methods for treating heavy metal ions include chemical precipitation, biological methods, ion exchange, membrane separation, and adsorption [3,4,5]. However, chemical precipitation requires substantial amounts of chemicals and may lead to secondary pollution [6]; biological treatment is difficult to control [7]; the ion-exchange method has lower regeneration efficiency of the ion-exchange agents [8]; and membrane separation is costly and associated technologies are still being developed [9]. As environmental protection standards become increasingly stringent, the development of novel, green, and efficient technologies for treating heavy-metal-ion-contaminated wastewater is imperative. Among these methods, adsorption is distinguished by low cost, simple operation, the absence of secondary pollution, and high efficiency [10,11,12,13]. Consequently, the development of high-performance adsorbent materials has emerged as a major research focus.

Zeolite materials are known for their large specific surface area, unique pore structure, and ease of modification, which makes them widely used in adsorption, separation, and catalysis [14]. Compared to natural zeolites, synthetic zeolite provides advantages such as higher purity, uniform particle size, controllable morphology, and superior performance. At present, the methods for synthetic zeolite can be divided into three categories based on the type of raw materials used: the first method uses pure chemical reagents as the source of silica and alumina for zeolite material synthesis; the second method employs waste containing silicon and aluminum as the raw material, which undergoes pre-activation treatment prior to use in zeolite synthesis; and the third method combines waste with a small amount of chemical reagents for material synthesis. The zeolite prepared by the first method has consistent physicochemical properties and the greatest level of control is achieved during this synthesis process. However, it requires a large consumption of chemical reagents, resulting in high production costs and the potential generation of toxic by-products during synthesis. In contrast, the latter two methods effectively utilize the silicon and aluminum components in the waste, significantly reducing production costs and fulfilling the objective of turning waste into valuable resources [15].

The use of solid waste as a raw material for zeolite synthesis is an important approach to the resource reutilization and high-value utilization of waste [16,17,18,19]. Common types of solid waste used are electrolytic manganese residue [20], crushed stone [21], waste glass [22], fly ash [23], solar panels [24], and lithium leach residue [25]. Existing research mainly focuses on utilizing the silicon and aluminum components in solid waste. This process involves activating and leaching the inert silicon and aluminum components in the waste before using them for zeolite material synthesis [26]. Generally, the synthesis of zeolite from tailings requires a complex multi-step process, including “activation-silicon and aluminum dissolution-polymerization and zeolitization” [27]. However, this process is characterized by issues such as reaction complexity, stringent reaction conditions (high temperature and pressure), high consumption of acids and bases, and significant energy consumption [28].

Ion-adsorbing rare earth ores (also known as weathered crust elution-deposited rare earth ores) are primarily distributed across southern China. Due to their unique mineral occurrence characteristics, in situ leaching is widely adopted for extraction [29]. However, this method entails considerable risks of geological disasters, such as landslides, and poses substantial challenges to ecological safety. The tailings from ion-adsorbing rare earth mines mainly consist of minerals such as quartz, clay, mica, and feldspar [30,31], which are abundant in silicon and aluminum components. These tailings serve as potential sources of silicon and aluminum for the preparation of zeolite and other mineral materials. Yang et al. [32] successfully synthesized 33 types of functionalized red pigments from IRETs subjected to mechanical grinding and high-temperature calcination by doping with iron and lanthanum elements. Zhou et al. [30] treated ion-adsorbing rare earth mine tailings to study the adsorption of La³⁺ and Y³⁺. Kinetic studies showed that the adsorption process of La³⁺ and Y³⁺ onto the treated tailings follows a pseudo-second-order kinetic model. In the field of zeolite synthesis from tailings, Cheng et al. [33] activated the silicon and aluminum components in the tailings via an alkali fusion method, followed by hydrothermal synthesis with additional aluminum species to produce zeolite A. This material exhibited excellent adsorption performance towards typical pollutants from rare earth mining operations, such as Pb²⁺, Cd²⁺, Cu²⁺, NH₄⁺, PO₄³⁻, and F⁻. Current studies involve stringent reaction conditions for the synthesis of zeolite A, leading to high energy consumption and poor control over product quality. These problems primarily stem from the significant variability in the reactivity of different silicon and aluminum minerals, posing challenges for the efficient utilization of the tailings through a single activation and crystallization process. Based on the difference in particle size distribution of the components in the IRETs, different active minerals are fractionally activated by grading, which can be used for the synthesis of different types of zeolite materials, such as A-type zeolite and high-silica zeolite. The use of IRETs for zeolite preparation based on classification activation technology has not been reported in the previous literature.

In this work, silicon–aluminum minerals from IRETs were employed as raw materials, and a classification activation method was utilized to separate the tailings into highly active clay-rich components and inert silica-rich (quartz) components. This work focused on the highly active clay-rich components, using thermal activation to stimulate the reactivity of the clay minerals and facilitate the utilization of the high-activity silicon and aluminum components in the zeolitization process. Based on experimental conditions, a straightforward, cost-effective method was developed for the preparation of IRET-based zeolite (IRET-ZEO), which exhibited excellent adsorption performance. The adsorption performance of IRET-ZEO for Pb²⁺ was thoroughly investigated. The effects of solution pH, IRET-ZEO dosage, adsorption time, and initial concentration of Pb²⁺ on adsorption behavior were examined, and the optimal adsorption conditions were determined. Additionally, adsorption kinetics, competitive adsorption, and cyclic regeneration performance were also analyzed. Therefore, this study offers a new strategy and significant practical potential for the green resource utilization of IRETs and their application in the treatment of heavy-metal-containing wastewater.

2. Results and Discussion

2.1. Particle Size Distribution and Composition of IRETs

The main chemical composition of the IRETs is provided in

Table 1. Main chemical composition of IRETs.

SiO2	Al2O3	MgO	CaO	K2O	Na2O	Fe2O3	SO3	Rb2O	Y2O3	LOI
61.60	24.86	0.32	0.09	4.41	0.39	1.95	0.21	0.10	0.05	6.02

LOI: lost on ignition.

. The content of SiO₂ was highest in the IRETs, followed by Al₂O₃, with the combined content of the silicon oxides and aluminum oxides accounting for 86.46%. The concentrations of other oxides were relatively low. The XRD pattern of the IRETs is shown in Figure 1, indicating that the dominant mineral present in IRETs is quartz, along with small amounts of kaolinite and illite. Therefore, these rare earth tailings serve as an adequate source of Si and Al for the synthesis of zeolite materials. The schematic diagram of resource utilization for IRETs was shown in Figure 2. The particle size distribution of the IRETs is shown in

Table 2. Particle size distribution of IRETs.

Particle Size/mm	Mass/g	Distribution Ratio/%
>0.8	114.00	57.00
0.074~0.8	48.12	24.09
0.044~0.074	4.88	2.44
<0.044	32.94	16.47
Total	200.00	100.00

. The particle size distribution of the tailings is uneven, with a predominance of coarse particles. Particles larger than 0.8 mm accounted for 57.00% of the total, while the smallest particle size components, 0.044 mm to 0.074 mm, accounted for only 2.44%. The XRD analysis of each particle size component of the IRETs (Figure 3) revealed that the main mineral phases are quartz, kaolinite, and illite [32]. The clay minerals, kaolinite and illite, are primarily concentrated in particles smaller than 0.074 mm, whereas quartz is mainly distributed in the coarser particles. Therefore, screening and classification can effectively separate the silica-rich quartz components from the clay minerals, facilitating the subsequent classification and activation process.

2.2. Effect of Crystallization Conditions on the Phase of IRET-ZEO

The effects of thermal activation temperature, crystallization temperature, alkali concentration, and crystallization time on the phase of the IRET-ZEO are shown in Figure 4. As can be seen from Figure 4A, the thermal activation temperature of the IRETs had little effect on the physical phase composition of the zeolite products, the characteristic diffraction peaks of the kaolinite in the activated products at each temperature disappeared and transformed into an amorphous structure, and its reactivity was significantly improved. The characteristic diffraction peak of illite gradually decreased with the increase in calcination temperature. In order to obtain better crystallization activity of the IRETs, 850 °C was chosen as the optimal calcination temperature.

When the crystallization temperature was lower than 65 °C, no obvious zeolite phase appeared in the crystallization product. This might be due to the sufficient depolymerization of the silica and aluminum components. When the crystallization temperature reached 85 °C, obvious A-type zeolite characteristic diffraction peaks appeared in the product, which indicated that the silica–aluminum component in the IRETs crystallized to form the A-type zeolite phase at this crystallization temperature. Therefore, 85 °C was chosen as the optimal crystallization temperature (Figure 4B).

In order to study the influence of the alkali concentration of the crystallization reaction on the phase composition of zeolite in the crystallization products, the phase compositions of the crystallization products of activated tailings in 1 mol/L, 3 mol/L and 5 mol/L NaOH were investigated. As can be seen from Figure 4C, the characteristic diffraction peaks of zeolite in the crystallization products were gradually enhanced with the increase in alkali concentration, indicating that the crystallinity of zeolite in the crystallization products increased significantly with the increase in alkali concentration, and therefore 5 mol/L was selected as the optimal alkali concentration.

The effect of crystallization time on the physical phase of the crystallization products is shown in Figure 4D; with the increase in crystallization time, the phase of the crystallization products was changed insignificantly. Moreover, considering the time cost of synthesis, 3 h was chosen as the optimal crystallization time. The SEM image (Figure 5) of the crystallization product showed that its micro-morphology was dominated by cubic A-type zeolite, which was consistent with the XRD results. And there were obvious defects on the surface of the zeolite, which provided abundant action sites for the adsorption of heavy metal ions, which was conducive to the improvement of the adsorption performance of the material.

2.3. Effect of Factors on Adsorptive Properties of Synthesized Zeolite

The solution pH is one of the important factors for the adsorption performance of the materials [34]. The removal rate and adsorption capacity of Pb²⁺ by the IRET-ZEO under different pH conditions is shown in Figure 6A. In the acidic solution with pH 2–5.5, the removal rate of the IRET-ZEO for Pb²⁺ with an initial concentration of 100 mg/L was higher than 96.72%. Especially when the pH was higher than 4.0, the concentration of Pb²⁺ was reduced by the IRET-ZEO from the initial 100 mg/L to less than 0.0020 mg/L, and the adsorption capacity of Pb²⁺ was 53.91 mg/g. The removal rate and adsorption capacity of Pb²⁺ under different pH conditions are shown in Figure 6A. This indicates that the IRET-ZEO showed excellent acid resistance under a wide range of pH conditions, and the IRET-ZEO exhibited a greater potential for application in the removal of Pb²⁺ from drinking water and Pb²⁺-containing wastewater with a low concentration of Pb²⁺.

Figure 6B shows the removal rate and adsorption capacity of Pb²⁺ under different dosages of the IRET-ZEO. When the dosage of the IRET-ZEO was 0.2 g/L, the removal rate of the Pb²⁺ solution with an initial concentration of 100 mg/L was 83.11%, and with the increase in the IRET-ZEO dosage, the removal rate of Pb²⁺ also increased. When the dosage of the IRET-ZEO was more than 0.4 g/L, the removal rate of Pb²⁺ was close to 100%. This was due to the increase in the capacity of the IRET-ZEO, which increases the adsorption sites and the surface area of the IRET-ZEO to achieve a better adsorption effect. The adsorption of Pb²⁺ by the IRET-ZEO decreased with the increase in dosage, and an adsorption capacity of 448.05 mg/g was achieved at a dosage of 0.2 g/L, which confirmed that the IRET-ZEO exhibited an excellent adsorption capacity of Pb²⁺.

The effect of different adsorption times on the removal rate and adsorption capacity of Pb²⁺ is shown in Figure 6C. After 1 min of adsorption, the removal rate of Pb²⁺ by the IRET-ZEO was 98.2% in the solution with an initial concentration of 100 mg/L, and the adsorption capacity was 52.94 mg/g. With the increase in adsorption time, the adsorption capacity of Pb²⁺ by the IRET-ZEO reached equilibrium quickly, in 5 min, which indicated that the IRET-ZEO displayed a faster adsorption rate for Pb²⁺.

In order to explore the adsorption capacity of the IRET-ZEO, the effects of different initial concentrations of Pb²⁺ on the removal rate and adsorption capacity are shown in Figure 6D. When the initial concentration of Pb²⁺ was less than 500 mg/L, the removal rate of the IRET-ZEO of Pb²⁺ was higher than 99% in all cases, and the removal rate remained at 75.67% when the initial concentration of Pb²⁺ was 1000 mg/L. The adsorption capacity of the IRET-ZEO of Pb²⁺ at this time reached 378.35 mg/g, which might be owing to the abundant surface sites and pore structure properties of the IRET-ZEO. A comparison of the adsorption performance of IRET-ZEO for Pb²⁺ with other adsorbents in previous literature is summarized in

Table 3. Comparison between the results of this work and others found in the literature.

Adsorbents	pH	Dosage (g/L)	Adsorption Equilibrium Time (min)	Maximum Adsorption Capacity (mg/g)	References
Chitosan–zeolite	4.5	5	1440	275	[35]
LFS + FA	5	1	180	292.8	[36]
MZA	4~5	-	120	330.72	[37]
Zeolite NaY	6	1	2880	527.12	[38]
Metakaolin-based geopolymer	4.0	-		147.06	[39]
HCl-treated Egyptian kaolin	5.5	-	90	34.5	[40]
Na-T13	8	0.2	5	170.12	[41]
MoS2@kaolin	2~5.5	1.6	40	280.39	[42]
IRET-ZEO	2~5.5	0.4	5	378.35	This work

. IRET-ZEO exhibited significant advantages in adsorption capacity, adsorption rate, and synthetic cost compared with other adsorbents in the previous literature. This indicated that IRET-ZEO exhibited excellent adsorption capacity when treating solutions containing high initial concentration of Pb²⁺, and showed an excellent potential in practical Pb²⁺-containing wastewater treatment.

2.4. Adsorption Kinetics

The adsorption kinetics of the IRET-ZEO to Pb²⁺ were investigated using the pseudo-first-order kinetic model, pseudo-second-order kinetic model, Elovich kinetic model, and intraparticle diffusion model. The results are shown in Figure 7, and the corresponding kinetic parameters are listed in

Table 4. Kinetic parameters of Pb²⁺ adsorption by IRET-ZEO.

Models	Kinetic Parameters
Pseudo-first-order kinetic model	k1 (min−1) 0.0267	qe (mg·g−1) 0.18	R2 0.2393
Pseudo-second-order kinetic model	k2 (g·mg−1·min−1) 1.1532	qe (mg·g−1) 53.76	R2 0.9999
Intraparticle diffusion model	ki (mg·g−1·min−1/2) 0.6634	C (mg·g−1) 52.28	R2 1.0000
	0.0116	53.79	0.5734
Elovich equation	α (mg·g−1·min−1) -	β (mg·g−1) 0.18	R2 0.6067

. The linear fit of the pseudo-second-order kinetic model (R² = 0.9999) was markedly superior to that of the other models, indicating that the adsorption of Pb²⁺ by the IRET-ZEO follows the pseudo-second-order kinetic model, suggesting that the adsorption process is controlled by chemical interactions. The rate constant k₂ obtained from the pseudo-second-order model was 1.3486 g·mg⁻¹·min⁻¹, demonstrating a remarkably rapid adsorption rate. The equilibrium adsorption capacity q_e was 53.76 mg/g, closely matching the experimental value of 53.88 mg/g, further confirming the accuracy of the adsorption kinetic model. In addition, the linear fit of the intraparticle diffusion model showed that the adsorption process was not influenced by a single diffusion factor but consisted of a rapid adsorption process and an adsorption process that tended toward saturation. The efficiency and cost in the practical application of the adsorbent was determined by the faster adsorption kinetics [43], which was one of the important parameters for the practical application of adsorption materials.

2.5. Selective and Regeneration Adsorption

To investigate the selective adsorption rate of the IRET-ZEO in complex metal ion solutions, its adsorptive behavior was studied in a mixed solution containing five coexisting metal ions. The results are shown in Figure 8. In the mixed ion solution, the IRET-ZEO achieved a removal rate of 74.63% for Pb²⁺. This was followed by the removal of Zn²⁺ and Cr³⁺, with removal rates of 30.33% and 15.3%, respectively. The adsorption performance of the IRET-ZEO for Cu²⁺ and Cd²⁺ in the mixed solution was less effective. The IRET-ZEO maintained strong adsorption performance for Pb²⁺ regarding the complexity of the metal ion mixture, demonstrating excellent selective adsorption capabilities and excellent adsorption potential for complex solutions.

The regeneration adsorption performance of the IRET-ZEO was also studied (Figure 9). The results showed that with increasing regeneration cycles, the Pb²⁺ removal rate gradually decreased, which could be attributed to the partial destruction of adsorption sites in the acidic environment. However, after eight adsorption–desorption regeneration cycles, the Pb²⁺ removal rate remained at approximately 83%, indicating that the IRET-ZEO exhibits excellent acid resistance and regeneration performance. The competitive adsorption and regeneration adsorption results confirm that the IRET-ZEO has substantial potential for practical applications [44].

3. Materials and Methods

3.1. Materials

NaOH, H₂SO₄, Ethylenediaminetetraacetic acid disodium salt (Na₂EDTA), Pb(NO₃)₂, Zn(NO₃)₂·6H₂O, Cr(NO₃)₃·9H₂O, Cd(NO₃)₂·4H₂O, and CuSO₄ were all purchased from Sinopharm Group and are analytical grade. The IRETs used in the experiments were sourced from a rare earth mine in Changting, Fujian, China, following in situ leaching.

3.2. Activation of Tailings and Zeolite Crystallization

In this study, clay minerals from IRETs served as the silicon and aluminum sources for zeolite synthesis. First, the tailings were screened using a 200-mesh Tyler standard sieve (0.074 mm), and the undersized portion was subjected to thermal activation to stimulate the reactivity of the silicon and aluminum components. Specifically, a specific quantity of rare earth tailings was placed in a muffle furnace and heated at a rate of 10 °C/min. The samples were held at different calcination temperatures (550 °C, 650 °C, 750 °C, and 850 °C) for 1 h. After thermal activation, the samples were cooled to room temperature at a rate of 5 °C/min, yielding thermally activated IRETs. A specific quantity of the activated tailings was then placed into a conical flask, and a NaOH solution with varying concentrations (1 mol/L, 3 mol/L, and 5 mol/L) was added, ensuring a liquid-to-solid ratio of 10:1. The beaker was placed on a thermostatic heating magnetic stirrer, and the crystallization process was conducted at different temperatures (45 °C, 65 °C, and 85 °C) for predetermined times (3 h, 8 h, and 10 h). After crystallization, a vacuum pump was used to separate the solid and liquid phases by filtration. The crystallized product was rinsed repeatedly with deionized water until the pH was neutral. Finally, the product was dried in an oven.

3.3. Adsorption Experiments

A Pb²⁺ solution with an initial concentration ranging from 100 to 1000 mg/L was prepared for adsorption studies. A 50 mL Pb²⁺ solution was placed in a triangle flask, and 0.01 mol/L NaOH and H₂SO₄ solutions were used to adjust the adsorption pH to specific values. A specified mass (0.01 to 0.1 g) of IRET-ZEO was added. The flask was then placed in a thermostatic water bath shaker (150 rpm) where adsorption occurred for a designated time (10 to 90 min) at a controlled temperature of 25 °C. Upon completion of the adsorption process, the supernatant solution was collected and the Pb²⁺ concentration was measured. Three parallel samples were averaged for each group of experiments. The removal efficiency (%) and adsorption capacity (mg/g) of the Pb²⁺ were calculated using Equations (1) and (2), respectively.

$$\mathrm{\eta }={\displaystyle \frac{{\mathrm{C}}_0-{\mathrm{C}}_{\mathrm{t}}}{{\mathrm{C}}_0}}\times 100\%$$

(1)

$$\mathrm{q}={\displaystyle \frac{({\mathrm{C}}_0-{\mathrm{C}}_{\mathrm{e}})\times \mathrm{V}}{\mathrm{m}}}$$

(2)

• η: Removal efficiency (%);
• C₀: Initial concentration of Pb²⁺ (mg/L);
• C_t: Concentration of Pb²⁺ present in solution after adsorption time t (mg/L);
• C_e: Concentration of Pb²⁺ remaining in solution at adsorption equilibrium (mg/L);
• q: Adsorption capacity (mg/g);
• V: Volume of solution used in the adsorption experiment (L);
• m: Mass of adsorbent material used (g).

3.4. Adsorption Kinetics

The adsorption kinetics of the Pb²⁺ from the solution mixed with the IRET-ZEO were fitted to the pseudo-first-order kinetic equation (Equation (3)), pseudo-second-order kinetic equation (Equation (4)), Elovich kinetic equation (Equation (5)), and the intraparticle diffusion model (Equation (6)). The adsorption kinetic model of Pb²⁺ by the IRET-ZEO was evaluated through the linear correlation coefficient (R²) for each model. The expressions of the kinetic models are as follows [45,46,47,48]:

$$\ln ({\mathrm{q}}_{\mathrm{e}}-{\mathrm{q}}_{\mathrm{t}})=\ln {\mathrm{q}}_{\mathrm{e}}-{\mathrm{k}}_1\mathrm{t}$$

(3)

$${\displaystyle \frac{\mathrm{t}}{{\mathrm{q}}_{\mathrm{t}}}}={\displaystyle \frac{1}{{\mathrm{k}}_2{\mathrm{q}}_{\mathrm{e}}^2}}+{\displaystyle \frac{1}{{\mathrm{q}}_{\mathrm{e}}}}\mathrm{t}$$

(4)

$${\mathrm{q}}_{\mathrm{t}}=\mathrm{\beta }\ln (\mathrm{\alpha }\times \mathrm{\beta })+\mathrm{\beta }\ln \mathrm{t}$$

(5)

$${\mathrm{q}}_{\mathrm{t}}={\mathrm{k}}_{\mathrm{i}}{\mathrm{t}}^{0.5}+\mathrm{C}$$

(6)

• t: Adsorption time (min);
• k₁: Adsorption rate constant obtained from the pseudo-first-order kinetic model (min⁻¹);
• k₂: Adsorption rate constant obtained from the pseudo-second-order kinetic model (g·mg⁻¹·min⁻¹);
• q_e: Adsorption capacity at equilibrium (mg·g⁻¹);
• q_t: Adsorption capacity at time t (mg·g⁻¹);
• α: Elovich coefficient (mg·g⁻¹·min⁻¹);
• β: Elovich coefficient (g·mg⁻¹);
• k_i: Adsorption rate constant obtained from the intraparticle diffusion model (mg·g⁻¹·min^−1/2);
• C: Constant (mg·g⁻¹).

3.5. Selective Adsorption Experiments

A mixed solution of metal ions was prepared by dissolving Pb(NO₃)₂, Zn(NO₃)₂·6H₂O, Cr(NO₃)₃·9H₂O, Cd(NO₃)₂·4H₂O, and CuSO₄ in deionized water with the initial concentration of each metal ion set to 100 mg/L. An amount of 0.1 g IRET-ZEO was added to 50 mL of the mixed ion solution and placed in a thermostatic water bath shaker for 10 min at 25 °C. After adsorption, the supernatant was collected and the concentrations of the metal ions were measured. The removal efficiency of each metal ion in the mixed solution was calculated using Equation (1).

3.6. Regeneration Adsorption Experiment

The regeneration experiments were carried out under optimal adsorption conditions (pH was 4.0, adsorbent dosage was 0.4 g/L, contact time was 5 min, and initial concentration of Pb²⁺ was 100 mg/L). The desorption agent of the lead ions adsorbed on the zeolite was a 0.5 mol/L Na₂EDTA solution, the desorption time was 60 min, and the liquid–solid ratio was kept consistent with the adsorption experiments. The desorbed zeolite was washed to neutral with deionized water and then dried to constant weight at 100 °C in an oven. The regenerated zeolite was used for the next adsorption–desorption cycle experiment.

3.7. Characterization Methods

3.7.1. X-Ray Diffraction Analysis (XRD)

An X-Ray diffraction (XRD) analysis was conducted using an Ultima IV X-Ray diffractometer for phase identification of the samples at scanning ranges from 5° to 80°. The Cu target was used as the target material, and the voltage and current were set to 25 mA and 40 kV, respectively.

3.7.2. Scanning Electron Microscope (SEM)

The microstructure of the samples was examined using an SU8020 scanning electron microscope (SEM) (Thermo Fisher Scientific Inc., Waltham, MA, USA). The IRET-ZEO was first dried in a vacuum drying oven, and the sample was then mounted on conductive tape and coated with gold prior to SEM observation. The morphology and microstructure of the IRET-ZEO were observed using SEM.

3.7.3. Inductively Coupled Plasma Emission Spectrometer (ICP-OES)

The concentration of Pb²⁺ and other metal ions in the solution before and after adsorption was measured using an Optima 8000 ICP-OES spectrometer (PerkinElmer, Inc., Waltham, MA, USA). The operating conditions were set as follows: cooling gas flow rate at 17.0 L/min, solution uptake rate at 2 mL/min, and observation height at 17 mm.

3.7.4. X-Ray Fluorescence Spectrometer (XRF)

The main chemical composition of the IRETs was analyzed by PANalytical Axios. Ag was used as the target material, and a test voltage of 50 kV and a current of 100 μA were selected.

4. Conclusions

In this study, IRETs were used as raw materials, and a classification activation method combined with thermal activation was applied to activate the tailings. A mild alkali dissolution process was then employed to synthesize zeolite materials. The effects of thermal activation temperature, crystallization temperature, alkali concentration, and crystallization time on the phase composition of the zeolite were investigated. The synthesized IRET-ZEO was then tested for its adsorption performance of Pb²⁺ in aqueous solutions. The main conclusions are as follows:

(1)

The primary mineral phases of the IRETs are kaolinite, quartz, and illite. The tailings contain 61.60% SiO₂ and 24.86% Al₂O₃, with lower concentrations of other impurities. IRETs are a potential source of silica–aluminum for zeolite preparation. The clay minerals used in this study were primarily concentrated in particles smaller than 0.074 mm, and the clay-rich components could be easily separated by sieving. However, further work is required to explore the resource utilization of the coarse, silica-rich components.

(2)

Based on a phase analysis of the crystallization products, the optimal conditions for synthesizing the IRET-ZEO from the tailings through alkali dissolution were identified. The optimal activation parameter was calcination at 850 °C for 1 h. The optimal crystallization parameters were a NaOH concentration of 5 mol/L, a crystallization time of 3 h, and a crystallization temperature of 85 °C. The synthesized product was cubic A-type zeolite.

(3)

In acidic solutions with a pH range of 2 to 5.5 and an initial Pb²⁺ concentration of 100 mg/L, the removal rate of Pb²⁺ by the IRET-ZEO remained consistently above 96.7%. Notably, when the solution pH exceeded 4.2, the Pb²⁺ removal rate reached 100%. This demonstrated that the IRET-ZEO exhibited a fast adsorption rate (reaching equilibrium within 5 min), a high adsorption capacity (378.35 mg/g), and excellent acid resistance, selectivity, and regeneration performance. The adsorption kinetics results indicated that the adsorption of Pb²⁺ by the IRET-ZEO follows a pseudo-second-order kinetic model, with the process being primarily governed by chemical interactions. This work provides a new strategy for the resource and valorization utilization of IRETs with “treating waste with waste” applications.