Removal of Cd²⁺ and Pb²⁺ from an Aqueous Solution Using Modified Coal Gangue: Characterization, Performance, and Mechanisms

Abstract

The impact of various modification methods on enhancing the adsorption performance of coal gangue (CG) for hazardous heavy metals has not been thoroughly investigated. In this study, three CG samples were first modified by calcination, followed by acid washing, alkali washing, and hydrothermal treatment, to obtain modified CG samples. The adsorption performance was assessed based on the adsorption capacities for Cd²⁺ and Pb²⁺ (i.e., q_e,Cd and q_e,Pb), and the kinetics of the adsorption processes were analyzed using kinetic equations. XRD, SEM-EDX, FTIR, and N₂ adsorption–desorption isotherms were used to elucidate the adsorption mechanisms. Results indicated that q_e,Cd and q_e,Pb of raw CG samples were approximately 10 and 25 mg/g, respectively, with only slight changes observed after calcination, acid washing, and alkali washing. In contrast, hydrothermal treatment yielded NaP and NaA zeolites, which significantly enhanced q_e,Cd and q_e,Pb to values of 48.5–72.7 and 214.9–247.5 mg/g, respectively. The hydrothermally treated CG samples primarily adsorbed Cd²⁺ and Pb²⁺ through ion exchange with Na⁺ within the zeolite structure, facilitating the entry of these ions into the zeolite’s pore channels. The adsorption processes were effectively described by the pseudo-second-order kinetic model. By optimizing the conditions of hydrothermal modification, the adsorption performance of CG samples is anticipated to further improve due to the creation of additional adsorption sites.

1. Introduction

Heavy metal pollution from industrial wastewater has become a global concern. Due to their tendency to bioaccumulate in living organisms, heavy metals in aquatic environments pose significant threats to both ecological systems and public health [1,2]. These metals also adversely affect soil and plants, leading to issues, such as stunted growth, altered enzyme activities, and disrupted photosynthesis. Cadmium (Cd) and lead (Pb), as prominent examples of heavy metals, have become widely distributed in the environment due to their extensive use in industrial processes and commercial applications [3,4]. For example, excessive Pb levels in the human body can damage the nervous system and kidneys and affect the cardiovascular, skeletal, reproductive, and immune systems. Current treatment technologies for heavy-metal-containing wastewater include ion exchange, membrane separation, electrolysis, chemical precipitation, and adsorption. Among these, adsorption is widely promoted due to its low cost, simplicity, and lack of secondary pollution [5,6]. The core of adsorption technology lies in developing adsorbents with high adsorption capacity and low cost. Researchers have investigated a variety of adsorbents, mainly including zeolite materials [7], carbon-based materials [8], and clay minerals [9].

Coal gangue (CG) is a solid waste generated during coal mining, accounting for 10% to 20% of the raw coal yield. The primary mineral constituents of CG include clay minerals, such as kaolinite, montmorillonite, and illite, along with quartz, feldspar, mica, and pyrite. Modifying CG can enhance its porosity and ion exchange capacity, facilitating the development of modified CG adsorbents [10]. High-temperature calcination is a widely used method for CG modification [11,12]. For instance, Qiu et al. [11] incorporated sodium tetraborate (Na₂B₄O₇·10H₂O) into the calcination process, resulting in a more than seven-fold increase in the Mn²⁺ adsorption capacity of the modified CG compared to raw CG. They also introduced a hydrothermal modification technique that produces zeolite from CG, which significantly boosts its adsorption capacity for Cd²⁺ to 183.7 mg/g [13]. Additionally, acid washing and alkali washing following calcination have been shown to improve the adsorption performance of CG samples [14,15,16]. Researchers have also developed various methods to create adsorption sites on CG samples [17,18]. For example, Shang et al. [17] devised a technique to introduce mercapto groups (–SH) onto CG, enhancing its adsorption capacity for Pb²⁺, Cd²⁺, and Hg²⁺ to 332.8, 110.4, and 179.2 mg/g, respectively.

Coal gangue is rich in SiO₂ and Al₂O₃, with concentrations ranging from 60% to 90%, making it a natural raw material for the production of zeolites. Zeolites exhibit excellent adsorption properties. However, traditional production methods typically use chemical raw materials, such as Al(OH)₃, NaOH, and Na₂SiO₃·H₂O, which are relatively costly. Using coal gangue as a synthesis source can significantly reduce production costs. Alkaline-fusion-assisted hydrothermal crystallization is the most common method used to synthesize zeolite [7]. NaX zeolite, NaY zeolite, and NaA zeolite were prepared, and their adsorption behaviors for heavy metal ions were explored [5,6,19]. For instance, Ge et al. [5] synthesized NaX zeolite from coal gangue, and the maximum Pb²⁺ adsorption capacity reached was 457 mg/g. Jin et al. [6] demonstrated a low-cost method of preparing zeolite NaA from coal gangue, which involves using a low alkali concentration (1.8 mol/L) and a longer crystallization time (10 h). In order to improve adsorption performance, Bu et al. [20] developed a sequential alkaline and ultrasonic post-treatment method that facilitates the creation of hierarchical mesopores, thereby enhancing the adsorption performance of mesoporous zeolites.

Due to economic considerations, the modification of CG presents a more feasible approach for producing absorbents compared to the synthesis of zeolite, which requires extensive amounts of sodium hydroxide and prolonged high-temperature calcination. Researchers have explored various modification techniques, including calcination, acid washing, alkali washing, hydrothermal treatment, and intercalation. However, there is limited research comparing the effectiveness of these methods in enhancing the adsorption performance of CG for heavy metal ions. This study addressed this gap by modifying three CG samples with different SiO₂/Al₂O₃ molar ratios through calcination, followed by acid washing, alkali washing, and hydrothermal treatment, to produce modified CG samples. The adsorption performance was evaluated based on the capacities to adsorb Cd²⁺ and Pb²⁺, and the kinetics of the adsorption processes were analyzed using appropriate kinetic models. Adsorption mechanisms were elucidated using X-ray diffraction (XRD), scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX), Fourier-transform infrared spectroscopy (FTIR), and nitrogen adsorption–desorption isotherms. This study aimed to provide a comprehensive understanding of how different modification methods affect the compositional properties and adsorption capabilities of CG samples, offering guidance for future efforts to develop high-capacity, cost-effective heavy metal adsorbents.

2. Experimental Section

2.1. Materials

In this study, CG samples were collected from three different locations, the Fumin mine in Inner Mongolia, the Yangshita mine in Inner Mongolia, and the Sihou mine in Shanxi Province, which were denoted as FM, YST, and SH, respectively. The chemical compositions of these samples were analyzed using X-ray fluorescence spectroscopy (XRF-1800, Shimadzu, Kyoto, Japan), with the results summarized in

Table 1. Main chemical composition of the CG samples.

Sample	SiO2	Al2O3	Fe2O3	K2O	Na2O	CaO	MgO	SO3	P2O5	Others	SiO2/Al2O3
FM	66.59	20.64	4.10	2.13	1.25	1.97	1.99	0.12	0.25	0.96	5.49
YST	64.64	21.43	3.56	3.59	0.87	2.51	0.92	0.30	0.18	2.00	5.13
SH	56.34	34.71	1.05	4.15	1.19	0.24	0.55	0.22	0.05	1.50	2.86

. The molar ratios of SiO₂ to Al₂O₃ were 5.49 for FM, 5.13 for YST, and 2.86 for SH, indicating that SH has a higher aluminum content compared to FM and YST.

The proximate analysis of CG samples was conducted following the Chinese national standard GB/T 212-2008 [21], and the results are shown in

Table 2. Proximate analysis of the CG samples.

Sample	Mad	Ad	Vd	FCd
FM	7.06	94.05	5.78	0.17
YST	2.39	86.18	8.42	5.40
SH	0.97	87.91	7.61	4.47

. The lowest ash content in the YST sample indicated that this sample contained the highest amount of organic matter among all the samples. The raw CG samples were initially crushed, screened to particles smaller than 180 μm, and then dried at 105 °C for 24 h. The dried CG powder was subsequently used for modification. Additional chemicals used in this study, including HCl, NaOH, hexadecyl trimethyl ammonium bromide (HTAB), CdCl₂·2.5H₂O, and Pb(NO₃)₂, were purchased from Sinopharm Chemical Reagent Co., Ltd. Solutions of Cd²⁺ and Pb²⁺ at a concentration of 1000 mg/L were prepared by dissolving precise amounts of CdCl₂·2.5H₂O and Pb(NO₃)₂ in distilled water.

2.2. Preparation of Modified Coal Gangue

The raw CG samples underwent various modification treatments, including high-temperature calcination, acid washing, alkali washing, and hydrothermal treatment, as illustrated in Figure 1. The procedures for each treatment are detailed next:

(1) Calcination modification: FM, YST, and SH were placed in a crucible and calcined at 600 °C for 5 h in a muffle furnace (DC-B08/12, Dotrust, Beijing, China). The resulting samples were designated as FM-C, YST-C, and SH-C, respectively.

(2) Acid washing treatment: In total, 10 g of the calcined CG was stirred with 100 mL of 1 M HCl solution at ambient temperature for 24 h. Afterward, the solution was removed by filtration under reduced pressure, and the insoluble residue was washed with distilled water until the pH of the filtrate approached 7.0. The filtered residue was then dried at 105 °C for 12 h, yielding the acid-modified samples FM-C-AC, YST-C-AC, and SH-C-AC.

(3) Alkali washing treatment: This treatment followed a procedure similar to that of acid washing, except that the calcined CG was stirred with 100 mL of 1 M NaOH solution at ambient temperature for 24 h. The resulting alkali-modified samples were designated as FM-C-AL, YST-C-AL, and SH-C-AL.

(4) Hydrothermal treatment: Hydrothermal modification was conducted, as described by Qiu et al. [13]. Briefly, 10 g of calcined CG, 100 mL of 2 M NaOH solution, and 1 g of hexadecyl trimethyl ammonium bromide (HTAB) were added to a 200 mL three-necked round-bottom flask equipped with a reflux condenser. The mixture was continuously stirred at 100 °C for 24 h. The resulting colloidal suspension was then filtered under vacuum and washed with distilled water, and the residue was dried at 105 °C for 12 h, resulting in the hydrothermally modified samples FM-C-HY, YST-C-HY, and SH-C-HY.

2.3. Adsorption of Cd²⁺ and Pb²⁺ in Aqueous Solution

2.3.1. Adsorption Capacity Test

For the adsorption capacity experiment, 50 mL each of 100 mg/L Cd²⁺ and Pb²⁺ solutions was mixed in a conical flask to simulate wastewater. These solutions were prepared by diluting a 1000 mg/L stock solution with distilled water, adjusting the final pH to approximately 7.0. To investigate adsorption, 0.1 g of the modified coal gangue was introduced into the wastewater. The mixture was incubated at room temperature for 1800 min on a horizontal shaker (HY-2, Supo, Shaoxing, China). After the adsorption period, the samples were filtered through a 0.45 μm membrane filter. The concentrations of Cd²⁺ and Pb²⁺ in the filtrate were determined using a UV–visible spectrophotometer (UV-1100, Mapada, Shanghai, China) [22]. In the tests involving hydrothermally modified CG samples, it was observed that both Cd²⁺ and Pb²⁺ ions were fully adsorbed by a 0.1 g sample. Consequently, the dosages were adjusted to 0.04 g for Cd²⁺ and 0.01 g for Pb²⁺. The adsorption capacity was calculated using Equation (1) [17]:

$$q_e={\displaystyle \frac{\left(c_0-c_e\right)\times V}{m}}$$

(1)

where q_e represents the adsorption capacity (mg/g) and c₀ and c_e denote the initial and equilibrium concentrations of the heavy metal ions, respectively (mg/L). The volume of the wastewater is represented by V (L), and m is the mass of the solid adsorbent added to the flask (g). Each adsorption test was performed in triplicate, and the average values were used for the final data analysis. Specifically, q_e,Cd and q_e,Pb refer to the adsorption capacities for Cd²⁺ and Pb²⁺, respectively.

2.3.2. Adsorption Kinetic Test

For adsorption kinetic tests, 1.6 g of hydrothermally modified CG samples was combined with 2000 mL of Cd²⁺ solution, while 0.4 g of the same samples was mixed with 2000 mL of Pb²⁺ solution. Both solutions had an initial concentration of 100 mg/L. The pH of the solutions was approximately 7.0 and was not adjusted further. The mixtures were incubated at room temperature on a horizontal shaker (HY-2, Supo, Shaoxing, China). At specified time intervals (0, 2, 5, 10, 20, 30, 60, 90, 120, 180, 360, 540, 1440, 1560, 1680, and 1800 min), 1 mL of the samples was withdrawn from the conical flasks. The total volume of the samples taken did not exceed 2% of the initial solution volume. The extracted samples were filtered through 0.45 μm membrane filters, and the ion concentrations were measured using a UV–visible spectrophotometer (UV-1100, Mapada, Shanghai, China) [22]. The amount of ions adsorbed at time t (q_t, mg/g) was calculated using Equation (2) [13]:

$$q_t={\displaystyle \frac{\left(c_0-c_t\right)\times V}{m}}$$

(2)

where q_t represents the adsorption amount (mg/g) and c_t denotes the concentration of heavy metal ions at time t (mg/L). Each adsorption test was performed in triplicate, with the average values used for final data analysis. Specifically, q_t,Cd and q_t,Pb refer to the adsorption amounts for Cd²⁺ and Pb²⁺, respectively. Following the adsorption tests, the CG samples were filtered, washed, and dried using the previously described procedures. The samples were then labeled as follows: FM-C-HY (Cd), YST-C-HY (Cd), and SH-C-HY (Cd) for the hydrothermally modified CG samples after Cd²⁺ adsorption and FM-C-HY (Pb), YST-C-HY (Pb), and SH-C-HY (Pb) for the hydrothermally modified CG samples after Pb²⁺ adsorption.

2.4. Modified CG Characterization

The crystal phase of the modified CG samples was analyzed using XRD with a Bruker D8 diffractometer, which used Cu Kα radiation (λ = 0.15418 nm) and operated at 40 kV and 100 mA. XRD scans were conducted over a 2θ range of 5–60° at a scanning rate of 10°/min. The surface morphology of the modified CG samples was examined with a scanning electron microscope (Gemini SEM 300, Zeiss, Oberkochen, Germany).

To explore the adsorption mechanism, EDX, FTIR and N₂ adsorption–desorption isotherms were performed on hydrothermally modified CG samples both before and after adsorption. The distribution of heavy metals in the modified CG samples was analyzed using an energy-dispersive spectrometer (XFlash Detector 5010, Bruker, Ettlingen, Germany). FTIR spectra were recorded in the mid-infrared range from 400 to 4000 cm⁻¹ with a Fourier transform infrared spectrometer (Nicolet 6700, ThermoFisher, Waltham, MA, USA). N₂ adsorption–desorption measurements were carried out using an automatic surface and porosity analyzer (ASAP 2460, Micromeritics, Atlanta, GA, USA).

3. Results and Discussion

3.1. Properties of Modified CG Samples

Figure 2 displays the XRD patterns for both raw and modified CG samples. The primary mineral phases identified in the raw CG samples included quartz, kaolinite, and muscovite. Additionally, nontronite and illite were observed in the FM and SH samples, respectively. The diffraction peaks corresponding to kaolinite and nontronite were notably diminished following calcination at 600 °C. This reduction is attributed to the decomposition of hydroxyl groups within these clay minerals, which leads to the release of structural water and the subsequent collapse of the interlayer structure [15]. It has been proposed that kaolinite transforms into disordered metakaolinite upon calcination at this temperature [11]. In comparison to the calcined CG samples, both acid and alkali washing procedures enhanced the peak intensities of the mineral crystal phases. Acid treatment effectively dissolves metal oxides and carbonate minerals, such as Fe₂O₃, CaO, and CaCO₃, while alkali treatment removes some amorphous silicates by forming soluble sodium silicate. The resulting increase in the purity and concentration of quartz and kaolinite thus improved the overall crystallinity of the coal gangue components [14].

Figure 2 also illustrates that the hydrothermal modification of the calcined CG samples resulted in the formation of different zeolites. Specifically, NaP zeolite was synthesized in the FM-C-HY and YST-C-HY samples, whereas NaA zeolite was produced in the SH-C-HY sample. Qiu et al. [13] also observed the formation of zeolite materials, such as Na₆Al₆Si₁₀O₃₂ and Na₁₂Al₁₂Si₁₂O₄₈, following the hydrothermal modification of combusted coal gangue. It is hypothesized that calcination and hydrothermal treatment partially disrupted the structures of the CG samples, generating AlO₄ and SiO₄ tetrahedral units. Under the structural guidance of HTAB, these active units condensed into relatively ordered aluminosilicate gels, which subsequently crystallized into zeolite structures [7]. The SiO₂/Al₂O₃ ratio in the aluminosilicate gel plays a crucial role in determining the type of zeolite synthesized. Higher SiO₂/Al₂O₃ ratios generally lead to the formation of NaP, NaX, and NaY zeolites, whereas lower ratios favor the production of NaA zeolite [23,24]. In this study, NaP zeolite was obtained from the FM and YST samples, which had high SiO₂/Al₂O₃ ratios of 5.49 and 5.13, respectively, while NaA zeolite was synthesized from the SH sample, which had a lower SiO₂/Al₂O₃ ratio of 2.86. These zeolite crystals were thought to improve the adsorption performance of modified CG samples.

Figure 3, Figure 4 and Figure 5 present SEM images of both raw and modified CG samples. As depicted in Figure 3a, Figure 4a and Figure 5a, the raw CG samples exhibited irregular shapes with a dense texture and were characterized by fine particles and layers adhering to their surfaces. Generally, raw CG samples exhibit poor adsorption performance due to their low pore volume and specific surface area [25]. Despite undergoing calcination, acid washing, and alkali washing, no significant changes in the morphological characteristics of the CG samples were observed. During calcination at 600 °C, the combustion of inherent organic matter and the decomposition of minerals, such as kaolinite, are expected to contribute to changes in the CG morphology. For instance, Jablonska et al. [10] reported that thermal modification at 600 °C increased the mesopore and macropore volumes of a Polish CG sample, while decreasing the total specific surface area. Similarly, Qiu et al. [11] found that the addition of Na₂B₄O₇·10H₂O during CG calcination weakened and fractured CG bonds, resulting in an increase in the BET surface area from 9.29 to 20.05 m²/g. Furthermore, Gao et al. [14] observed that treatment with HCl and KOH solutions reduced the particle size and generated irregular layered structures, enhancing the surface area. The effectiveness of calcination, acid washing, and alkali treatment in developing the pore structure of CG is likely influenced by various factors, including the organic matter content, mineral composition, concentration of the acid/alkali solution, and processing temperature. In this study, however, no significant changes in the surface morphology of CG samples were detected from SEM images.

Figure 3e, Figure 4e and Figure 5e display the SEM images of FM-C-HY, YST-C-HY, and SH-C-HY, respectively. Compared to the raw and other modified CG samples, these hydrothermally modified CG samples exhibited aggregates of fine particles with a loose and porous texture. Additionally, cubic particles, highlighted by red circles in Figure 4e and Figure 5e, were observed. These aggregates and cubic particles were likely zeolite precursors and crystals, respectively. To achieve high-crystallinity zeolites from CG, alkali fusion is commonly used to activate the CG, promoting the conversion of CG into soluble aluminosilicate gel [7]. It is presumed that the 2 M NaOH solution used in this study effectively dissolved only the amorphous silicate and aluminate minerals present in the CG samples. Due to the resulting solution’s low Al–Si supersaturation, the growth of zeolite nuclei during the crystallization phase was inhibited. Consequently, fewer zeolite crystals were formed, and the majority of the zeolite phase exhibited low crystallinity.

3.2. Adsorption of Cd²⁺ and Pb²⁺ by Modified CG Samples

3.2.1. Adsorption Capacity

Figure 6 illustrates the adsorption capacities of raw and modified CG samples for Cd²⁺ and Pb²⁺ in aqueous solutions. The raw CG samples exhibited relatively poor adsorption performance, with q_e,Cd and q_e,Pb values of approximately 10 and 25 mg/g, respectively. In contrast, the FM sample demonstrated the highest adsorption capacity for both Cd²⁺ and Pb²⁺, likely due to its higher clay mineral content. Clay minerals, such as kaolinite and nontronite, can adsorb metal ions through ion exchange [9]. After thermal modification at 600 °C, YST-C showed a greater ion adsorption capacity compared to FM-C and SH-C, though the differences were minimal. This enhancement is likely attributed to the significant amount of intrinsic organic matter in YST, which resulted in a more developed porous structure in YST-C following the combustion of the organic matter. Acid and alkali washing resulted in only minor changes in q_e,Cd and q_e,Pb, making it difficult to draw a definitive conclusion due to the complexity of the adsorption process. Gao et al. [14] reported that KOH washing increased the adsorption capacity of CG for U(VI) from 44 to 140 mg/g, while HCl washing had negligible effects. They noted that lower-binding-energy Al has a higher affinity for U(VI), and the increased Al/Si ratio after alkali washing contributes to better adsorption performance. Based on these findings, calcination, acid washing, and alkali washing appear to be ineffective methods for improving adsorption performance.

Figure 6 also shows significant enhancement in the adsorption performance of CG samples following hydrothermal treatment. The equilibrium adsorption capacities for Cd²⁺ (q_e,Cd) of FM-C-HY, YST-C-HY, and SH-C-HY were 48.5, 72.7, and 65.6 mg/g, respectively. The q_e,Pb values of FM-C-HY, YST-C-HY, and SH-C-HY were 214.9, 247.5, and 242.3 mg/g, respectively. This improvement in adsorption capacity can be attributed to the formation of zeolite, which offers a high surface area, a stable crystal structure, and a substantial cavity volume. The zeolite facilitates the accommodation of Cd²⁺ and Pb²⁺ ions through physical adsorption and ion exchange [6,26]. Notably, the adsorption capacity for Pb²⁺ was higher than that for Cd²⁺ in the hydrothermally modified samples. Shang et al. [17] observed similar results, finding that the maximum adsorption capacities of mercapto-modified CG for Cd²⁺ and Pb²⁺ were 110.4 and 332.8 mg/g, respectively. Wingenfelder et al. [27] noted that the hydrated forms of Cd²⁺ and Pb²⁺ slightly exceed the width of zeolite pore channels, indicating that only dehydrated cations can penetrate these channels. The lower hydration energy of Pb²⁺ accounts for its preferential sorption over Cd²⁺. The series YST-C-HY > SH-C-HY > FM-C-HY showed a decrease in both q_e,Cd and q_e,Pb despite YST-C-HY and FM-C-HY containing the same type of zeolite (NaP). SEM analysis suggested that the lower adsorption capacity of FM-C-HY may be due to the lower crystallinity of the NaP zeolite it contains. Well-crystallized zeolites possess a more developed pore channel system, which enhances their ability to retain more Cd²⁺ and Pb²⁺.

3.2.2. Adsorption Kinetics

Given that hydrothermal treatment significantly enhanced the adsorption performance for Cd²⁺ and Pb²⁺, our primary focus was on investigating the adsorption kinetics of the hydrothermally modified CG samples. Figure 7 illustrates the adsorption profiles of heavy metal ions over varying time periods. It is evident that both Cd²⁺ and Pb²⁺ ions exhibited a significant increase in adsorption within the initial 180 min, followed by a gradual approach to equilibrium by 540 min. This initial rapid uptake can be attributed to the high availability of vacant adsorption sites on the modified CG samples and the steep concentration gradient of metal ions between the solution and the adsorbent [26]. As these sites become occupied, the rate of adsorption slows down [13]. Additionally, a slight decrease in q_t,Cd and q_t,Pb toward the end of the adsorption period suggests a possible desorption of metal ions.

Adsorption kinetic models are valuable tools for elucidating the mechanism of adsorption and assessing the effectiveness of adsorbents in pollutant removal. Among these models, the pseudo-first-order and pseudo-second-order models are commonly used across various adsorption systems. The linear representation of the pseudo-first-order kinetics is given by Equation (3) [13,17]:

$$\ln \left(q_e-q_t\right)=\ln q_e-k_{1}t$$

(3)

where k₁ represents the rate constant for the pseudo-first-order model (1/min). The values of q_e and k₁ are obtained from the slope and intercept of the linear plot of ln(q_e − q_t) versus t, respectively. The linear form of the pseudo-second-order model is expressed by Equation (4) [13,17]:

$${\displaystyle \frac{t}{q_t}}={\displaystyle \frac{1}{k_2{q}_e^2}}+{\displaystyle \frac{t}{q_e}}$$

(4)

where k₂ (g/(mg·min)) denotes the rate constant for the pseudo-second-order model. The values of q_e and k₂ are derived from the slope and intercept of the plot of t/q_t versus t, respectively. Figure 8 presents the kinetic fitting for the adsorption of Cd²⁺ and Pb²⁺ onto the modified CG samples.

As presented in

Table 3. Kinetics parameters for Cd²⁺ and Pb²⁺ adsorption by modified CG samples.

Adsorption	Experimental qe (mg/g)	Pseudo-First-Order Model			Pseudo-Second-Order Model
		k1	R2	Calculated qe (mg/g)	k2 (×10−4)	R2	Calculated qe (mg/g)
FM-C-HY (Cd2+)	48.6	0.0024	0.879	30.9	3.48	0.987	48.8
YST-C-HY (Cd2+)	72.8	0.0066	0.942	40.7	6.03	0.999	73.9
SH-C-HY (Cd2+)	65.6	0.0033	0.877	46.3	2.27	0.988	67.0
FM-C-HY (Pb2+)	215.0	0.0024	0.931	108.4	1.12	0.997	216.9
YST-C-HY (Pb2+)	247.5	0.0030	0.831	95.5	1.61	0.999	250.0
SH-C-HY (Pb2+)	242.4	0.0016	0.836	122.8	0.92	0.995	239.2

, the high correlation coefficients and the close agreement between the calculated q_e values and the experimental q_e values suggest that the pseudo-second-order model is highly suitable for describing the adsorption kinetics of Cd²⁺ and Pb²⁺ on the modified CG samples. These findings align with those reported by other researchers [11,13,28]. The pseudo-second-order model typically accounts for both the diffusion process and the surface reaction process. This implies that the rate-limiting step in the adsorption process is surface adsorption, which is influenced by the availability of adsorption sites. Some researchers have also highlighted that chemisorption is the predominant mechanism during the adsorption process [1].

3.3. Adsorption Mechanism

To gain a deeper understanding of the adsorption mechanisms of modified CG samples, FM-C-HY, YST-C-HY, and SH-C-HY were characterized both before and after the adsorption of Cd²⁺ and Pb²⁺. The characterization techniques used included XRD, SEM-EDX, FTIR, and nitrogen adsorption–desorption isotherms.

3.3.1. XRD

Figure 9 illustrates that the XRD patterns of the modified CG samples before and after the adsorption of Cd²⁺ and Pb²⁺ were largely similar, suggesting that the overall zeolite structure is not significantly altered by the adsorption processes. However, minor variations in the XRD patterns were observed post adsorption. Specifically, Figure 9a,b demonstrates that the peak at 2θ = 21.5° (1 1 2) shifted to a higher diffraction angle with Cd²⁺ adsorption, while it shifted to a lower diffraction angle with Pb²⁺ adsorption. Similar shifts in diffraction peaks were reported by Pu et al. [29], who observed a shift of the peak at 2θ = 4.67° (NaP zeolite) to a higher diffraction angle (2θ = 4.83°). They attributed this shift to the replacement of Na⁺ (r = 1.02 Å) in the NaP zeolite lattice by the smaller Cu²⁺ (r = 0.73 Å), which resulted in a reduction in unit cell dimensions and an increase in the diffraction angle. Considering that the ionic radii of Cd²⁺ and Pb²⁺ are 0.95 Å and 1.19 Å, respectively, it is hypothesized that Cd²⁺ adsorption leads to a decrease in unit cell dimensions, while Pb²⁺ adsorption results in an increase.

Figure 9 also demonstrates that the peaks corresponding to the (1 0 1) lattice plane of NaP zeolite, as well as the (2 2 0) and (2 2 2) lattice planes of NaA zeolite, nearly vanished following the adsorption of Cd²⁺ and Pb²⁺. Similar observations were reported by Steinike et al. [30], who noted that bivalent heavy metal cations, such as Zn²⁺, Cd²⁺, and Pb²⁺, could occupy the Na⁺ positions within the α-cages and β-cages of the NaA zeolite. The disappearance of these diffraction peaks indicates the disruption of the corresponding ordered stacks of unit cells. This effect may be attributed to the low crystallinity of the derived NaP and NaA zeolites, which were partially decomposed during the adsorption process in aqueous solutions. It is suggested that cation exchange plays a significant role in the removal of Cd²⁺ and Pb²⁺ by hydrothermally modified CG samples.

3.3.2. SEM-EDX

Figure 10 presents the SEM images and EDX spectra of modified CG samples after the adsorption of Cd²⁺ and Pb²⁺. These images confirmed the presence of Cd²⁺ and Pb²⁺ on the surface of the modified CG samples, indicating that adsorption has occurred. Additionally, the content of Na significantly decreased or was nearly undetectable following adsorption. This reduction is attributed to the ion exchange process involving NaP and NaA zeolites in the modified CG samples, where Na⁺ ions are replaced by Cd²⁺ and Pb²⁺ ions. Similar findings have been reported by other researchers [31,32]. For instance, Lv et al. [31] observed that after the adsorption of Cd²⁺ and Pb²⁺ by NaA zeolite, the distribution of Na decreases relative to other elements.

3.3.3. N₂ Adsorption–Desorption

Table 4. Pore structure parameters of modified CG samples before and after adsorption.

Sample	SBET (m2·g–1) a	Smicro (m2·g–1) b	Sext (m2·g–1) c	Vtotal (cm3·g–1) d	Vmicro (cm3·g–1) b	Dpore (nm) e
FM-C-HY	23.27	0	23.27	0.076	0	3.399
FM-C-HY (Cd)	13.84	0	13.84	0.067	0	3.403
FM-C-HY (Pb)	20.57	0	20.57	0.077	0	3.400
YST-C-HY	20.22	0	20.22	0.081	0	3.402
YST-C-HY (Cd)	12.11	0	12.11	0.072	0	3.405
YST-C-HY (Pb)	9.11	0	9.11	0.054	0	3.403
SH-C-HY	13.01	0	13.01	0.073	0	3.404
SH-C-HY (Cd)	20.31	2.04	18.27	0.088	0.001	3.402
SH-C-HY (Pb)	14.41	0	14.41	0.058	0	3.402

^a Determined by the multipoint BET method. ^b Measured by the t-plot method. ^c Calculated by the difference. ^d Calculated from the absorbed volume of N₂ at a relative pressure P/P₀ of 0.99. ^e Determined by the BJH method using the adsorption branches of N₂ isotherms.

summarizes the pore structure parameters of the modified CG samples before and after adsorption. For both FM-C-HY and YST-C-HY, the adsorption of Cd²⁺ or Pb²⁺ resulted in a decrease in the total surface area (S_BET) and the total pore volume (V_total), indicating that the heavy metal ions occupied some pore channels. Notably, FM-C-HY has a relatively larger surface area, but it does not exhibit better adsorption performance, indicating that the adsorption process is not solely physical. Since ion exchange was identified as an important adsorption mechanism, exchangeable Na⁺ within the pore channels of zeolites were thought to be active adsorption sites. FM-C-HY should contain more non-active surface area formed by the stacking of inherent CG minerals, such as quartz and muscovite. By contrast, SH-C-HY showed an increase in both S_BET and the total pore volume, V_total, after the adsorption process. The possible reason is that Cd²⁺ or Pb²⁺ adsorption leads to an enlargement of the pore system of NaA zeolite.

3.3.4. FTIR

Figure 11 displays the FTIR spectra of modified CG samples before and after the adsorption of Cd²⁺ and Pb²⁺ ions. The peak at approximately 3400 cm⁻¹ corresponded to the stretching vibration of O–H bonds, while the peak at around 1640 cm⁻¹ was attributed to the bending vibration of O–H groups. The band near 960 cm⁻¹ was assigned to the asymmetric stretching of Si–O–Si or Si–O–Al bridges [6]. Peaks observed at about 2920 and 2850 cm⁻¹ were attributed to the asymmetrical and symmetrical stretching of alkyl C−H, indicating the introduction of hexadecyltrimethylammonium in the CG samples post-modification [33]. Importantly, the characteristic peaks of the CG samples after adsorption remained largely unchanged, although the peak near 960 cm⁻¹ shifted to a higher wavenumber. The presence of exchangeable cations can influence the IR spectra of zeolite frameworks due to factors such as mass, charge, ion size, and the cation’s environment [34]. This shift is primarily due to the ion exchange between Na⁺ and Cd²⁺ or Na⁺ and Pb²⁺, causing a slight alteration in the zeolite structure. Some researchers have also noted shifts in the O–H peaks around 3400 and 1600 cm⁻¹, indicating the involvement of hydroxyl groups in the removal of heavy metal ions by NaA and NaP zeolites [29,31,35].

3.4. Limitations and Implications of This Study

From the perspective of adsorption capacities for Cd²⁺ and Pb²⁺, hydrothermal treatment emerged as the preferred method for modifying CG. In this study, the hydrothermal modification of CG was conducted under fixed experimental conditions. Further research is needed to explore how variables such as the NaOH concentration, HTAB dosage, and processing time, as well as their interactions, affect adsorption performance. Additionally, ion exchange was identified as the principal mechanism for Cd²⁺ and Pb²⁺ adsorption by hydrothermally modified CG samples. Given that cation exchange sites are predominantly associated with aluminum–oxygen tetrahedra, enhanced adsorption performance could potentially be achieved by introducing an additional aluminum source to increase the number of adsorption sites. This approach will be a central focus of our future research. These findings are significant for the development of high-performance, cost-effective adsorbents derived from CG through hydrothermal treatment.

4. Conclusions

This study investigated three CG samples with different SiO₂/Al₂O₃ molar ratios, subjected to four distinct modification methods, to assess and compare their compositional properties and adsorption capabilities. Additionally, the adsorption kinetics and mechanisms of the hydrothermally modified CG samples were examined. The principal findings are as follows:

(1)

The initial adsorption capacities of the unmodified CG samples for Cd²⁺ and Pb²⁺ were approximately 10 and 25 mg/g, respectively. These values remain largely unchanged following calcination, acid washing, and alkali washing. In contrast, hydrothermal treatment produces NaP and NaA zeolites, leading to substantial increases in the adsorption capacities for Cd²⁺ and Pb²⁺, reaching values of 48.5–72.7 and 214.9–247.5 mg/g, respectively.

(2)

Hydrothermally modified CG samples primarily remove Cd²⁺ and Pb²⁺ through ion exchange with Na⁺ within the zeolite framework, facilitating the ingress of these ions into the zeolite’s pore channels. The adsorption data are well described by the pseudo-second-order kinetic model, indicating that chemisorption is the dominant mechanism.

(3)

The study highlights the effective removal of Cd²⁺ and Pb²⁺ by hydrothermally modified CG samples, establishing hydrothermal treatment as an efficient and cost-effective modification strategy. Future research should aim at optimizing the hydrothermal process and exploring methods to increase the number of adsorption sites on the CG samples.