Adsorption Cd(Ⅱ) metal ions from aqueous solution using an economical and feasible zeolite from coal gangue: Adsorption performance, mechanism and economic assessment

Yuzhi Zhou; Pian Hu; Linli Long; Jingyu Sun; Boyu Wang; Jingwei Niu; Min Chen; Xiaoyang Chen

doi:10.4491/eer.2024.693

Environ Eng Res > Volume 31(1); 2026 > Article

Zhou, Hu, Long, Sun, Wang, Niu, Chen, and Chen: Adsorption Cd(II) metal ions from aqueous solution using an economical and feasible zeolite from coal gangue: Adsorption performance, mechanism and economic assessment

Research

Environmental Engineering Research 2026; 31(1): 240693.

Published online: May 11, 2025

DOI: https://doi.org/10.4491/eer.2024.693

Adsorption Cd(II) metal ions from aqueous solution using an economical and feasible zeolite from coal gangue: Adsorption performance, mechanism and economic assessment

Yuzhi Zhou^1,^2,³, Pian Hu¹, Linli Long¹, Jingyu Sun¹, Boyu Wang¹, Jingwei Niu¹, Min Chen⁴, Xiaoyang Chen^1,^2,^3,^†

¹School of Earth and Environment, Anhui University of Science and Technology, Huainan 232001, PR China

²Anhui Province Engineering Laboratory of Water and Soil Resources Comprehensive Utilization and Ecological Protection in High Groundwater Mining Area, Huainan 232001, PR China

³State Key Laboratory for Safe Mining of Deep Coal Resources and Environment Protection, Huainan 232001, PR China

⁴Taizhou Institute of Zhejiang University, Zhejiang University, Taizhou 318000, PR China

^†Corresponding author: E-mail: chenxy@aust.edu.cn, Tel: +86-554-6668045 Fax: +86-554-6668045, ORCID: 0000-0001-9166-7895

Received December 11, 2024 Revised April 18, 2025 Accepted May 10, 2025

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

Abstract

In this work, a novel coal gangue-derived zeolite (ZCG) was successfully fabricated through alkali fusion and hydrothermal processes, and was used as an adsorbent for the elimination of Cd(II) from aqueous solution. Batch adsorption studies were performed at varying temperature (298.15 – 318.15 K), dosage (0.1 – 1.0 g/L), pH (2.0 – 7.0), time (15 – 1440 min) and Cd(II) initial concentration (20 – 300 mg/L), respectively. The transformation of CG into ZCG resulted in an enhancement of surface area and ion-exchange sites, thus significantly enhancing adsorption performance. The Cd(II) adsorption by ZCG followed the Langmuir model and pseudo second-order kinetic models, which indicated Cd(II) adsorption of ZCG was monolayer chemisorption. The maximum adsorption capacity of the synthesized ZCG towards Cd(II) was 66.49 mg/g, which was higher than that of CG (48.01 mg/g). SEM and XPS results demonstrated that the predominant mechanism of Cd(II) removal by ZCG was ion exchange, and Cd²⁺ ions were mainly anchored on the Si-OH sites on ZCG. In a word, we envision that the ZCG will provide new pathway for developing cost-effective and high-efficient adsorbent materials for the purification of Cd(II) and other pollutants, while minimizing the environmental footprints of current utilization of coal gangue.

Keywords: Adsorption mechanism, Cd(II) removal, Coal gangue, Langmuir isotherm, Thermodynamics, Zeolite

Graphical Abstract

Keywords: Adsorption mechanism, Cd(II) removal, Coal gangue, Langmuir isotherm, Thermodynamics, Zeolite

1. Introduction

Environmental pollution is one of the most pressing issues of the 21st century, especially heavy metal pollution. Among metal ions pollutants, Cadmium (Cd) are classified as one of the most dangerous heavy metal ions because of their undegradability, toxicity and carcinogenicity [1, 2]. To date, the most widely used methods in the field of removing heavy metals from contaminated water are chemical precipitation, ion exchange, membrane filtration, ion-exchange, as well as adsorption [3–5]. Among the above-mentioned methods, adsorption method is considered as an attractive method for encapsulating heavy metal due to cost effective, easy-design and efficiency [6]. Currently, zeolite as a common material has been widely developed for cleaning up heavy metals due to its advantages such as simple preparation, non-toxicity, good thermal stability, and environmental friendliness. However, the high synthetic cost of zeolite from standard chemical reagents and mineral resources greatly limits its extensive application [7, 8]. Thus, some researchers have been concerned with seeking affordable alternative raw materials for synthesizing zeolite, such as perlite [9], fly ash [10], and oil shale ash [11], and so on.

Coal gangue (CG) is a kind of the largest industrial solid waste in China, which is a byproduct produced during coal mining and processing process. Nowadays, it is expected that the annual production of CG in China has reached 0.3 billion tons and accumulated amount of CG up to 4.5 billion tons [12]. A great quantity of CG stockpiling not only cause land occupation, but led to severe environmental problems [13]. Randomly stacked CG will produce toxic chemicals, thus contaminating water and soil, and causing serious damage to human health [14]. Presently, the utilization of coal gangue is still low value-added applications, such as concrete production, making bricks and backfill mining [15]. Given that CG mainly consists of SiO₂ and Al₂O₃, Coal gangue (CG) is extremely suitable as a raw material for synthesizing zeolite. Once implemented, the preparation cost of zeolite will be reduced while its environmental risks will also be reduced [16]. At present, the studies on zeolite synthesis from CG and its application as adsorbent are insufficient. Jin et al. [17] converted coal gangue into coal-analcime composite, and found that the maximum Pb²⁺ exchange capacity was as high as 268 mg/g. Qian et al. [18] successfully prepared the Na-A zeolite by coal gangue with the maximum CEC value of 358 mg/g. Lu et al. [19] found that the adsorption capacity of coal gangue-derived zeolite was 45.05 mg/g and 44.53 mg/g for Cu²⁺ and Co²⁺, respectively, and suitable as an economical adsorbent for pollutant removal. However, the composition of coal gangue varied in different regions, which affected the adsorption performance of CG-derived zeolite. Therefore, more research still is needed for the comprehensive and systematic information of the adsorption technique of Cd(II) on CG-derived zeolite for accelerating the industrialization of CG-derived zeolite.

In addition, the conventional hydrothermal route had drawbacks that were not conducive to large-scale zeolite synthesis [20]. For this reason, this work firstly activated coal gangue via moderate NaOH solid, and followed by the hydrothermal processes to fabricate economical and high efficient zeolite materials, and applied to Cd(II) removal. The purposes of this work are to: (1) understand the crystal structure, morphologies and chemical composition of the synthesized zeolite through different characterization techniques including TGA, SEM, FT-IR, BET, XRD and XPS; (2) investigate the influence of contact time, solution pH, initial concentration and coexisting ions on adsorption performance of ZCG; (3) reveal the immobilization mechanism of Cd(II) by ZCG.

2. Materials and Methods

2.1. Materials

Coal gangue (CG) was taken from Huainan City in Anhui province. Specific chemical composition characterized by XRF were shown in Table S1. As shown in supplementary material Table S1, it could be seen that CG was mainly composed of SiO₂ and Al₂O₃, accounting for more than 90%, and the molar ratio of SiO₂ to Al₂O₃ was calculated to be around 2.22. Reagent NaOH and CdCl₂ were supplied from Sinopharm Chemical Reagent Co., Ltd. All chemicals used in experiments were of analytic grade and without further purification.

2.2. Preparation of Coal Gangue-derived Zeolite

Coal gangue-derived zeolite was produced by alkaline fusion-hydrothermal method referencing to relevant literature and making minor changes [21, 22], including CG pretreatment, alkaline fusion, aging, crystallization, water washing, drying, etc (Fig. S1). (1) CG pretreatment: Briefly, the CG was grinded down by a ball grinding mill, and sieved through a 100-mesh sieve; (2) alkaline fusion: the CG power (20 g) and NaOH (24 g) were mixed and calcined at 500°C for 3h; (3) aging and crystallization: A given amount of CG was dispersed into 80 mL of 3 mol/L NaOH solution, and continuous stirring for 12 h. Then, the slurry was transferred to a stainless-steel reaction tank and heated at 105°C for 12 h. (4) washing and drying: After the reaction, the product was filtered, rinsed and dried at 60°C for 24 h.

2.3. Characterization Techniques

The morphologies of the CG and ZCG were displayed by SEM images (Hitachi Regulus8100). The chemical composition of the CG was obtained by X-ray fluorescence spectrometer. The crystal phase and mineral composition of the adsorbents were analyzed by XRD patterns. The functional groups of ZCG were determined using Fourier-transform infrared spectrometer. The textural surface properties of the CG and ZCG were determined by analyzing the N₂ adsorption-desorption isotherm. The thermal stability analysis of ZCG was performed by using TA Discovery TGA 550, USA. Zeta potential of ZCG were determined by Malvern Zetasizer Nano ZS90ζ potential analyzer.

2.4. Adsorption Experiment

To determine removal efficiency and adsorption capacity of zeolite, Cd(II) adsorption were studied by equilibrating a quantitative dose of zeolite powders with Cd(II) solution. The pH values of solutions were adjusted with dilute solution of HCl or NaOH. The effect of the solution pH was examined by adjusting Cd(II) solution pH ranging from 2.0 to 7.0 at C₀=50 mg/L. Except for pH effect experiment, all the adsorption experiments were conducted at a fixed pH 7.0. The adsorption kinetics was determined using different time intervals (from 15 min to 1440 min). The adsorption isotherm at pH 7.0 was studies using the initial concentration ranging 20 to 300 mg/L for ZCG. To investigate the selectivity of ZCG for Cd(II) ions, the effect of some competing ions (NH₄⁺, Na⁺, K⁺, Ca²⁺, Mg²⁺), whose concentration was between 0~400 mg/L, was performed at pH 7.0. The mixtures of solution were placed on the agitator and stirred at 25°C. After adsorption equilibrium, the mixture was filtered by 0.22 μm membrane filter. The concentration of Cd(II) was determined by atomic adsorption spectrophotometer. All the adsorption experiments were executed three times to validate the results. The adsorption capacity (q_e, mg/g) and the removal percentage (R_e, %) were calculated by Eqs. (1)~(2):

(1)

R_{e} (%) = \frac{C_{0} - C_{e}}{C_{0}} \times 100

(2)

q_{e} = \frac{V (C_{0} - C_{e})}{W}

where C₀ is the Cd(II) initial concentration (mg/L), C_e is the Cd(II) equilibrium concentration (mg/L). q_e is the amount of Cd(II) adsorbed on ZCG (mg/g), V (L) is the Cd(II) solution volume, W (g) stands for the mass of the ZCG.

3. Results and Discussion

3.1. Characterization of Samples

SEM micrographs for CG and ZCG samples was displayed in supplementary material Fig. S2. The morphology of CG had sheet structure (Fig. S2a). In Fig. S2b, we could clearly identify that the synthesized product contained columnar crystals and cubic well-crystallized zeolite particles among the amorphous phase. The reason for this phenomenon may be due to the thermodynamical instability of the smooth cubic particles, which may be transformed into the more stable phase for a longer reaction time base on the Oswald’s rule of successive transformation [18]. As shown in Fig. S2c, columnar crystals appeared with the length of about 1um. Moreover, the smooth cubic particles presented in Fig. S2d was relatively uniform with the average size of 1.7 um±0.2, which indicated the presence of well-developed zeolites particles.

Considering the essential influence of the difference sources of Si and Al on zeolite synthesis [23, 24], it is very necessary to analyze the mineral composition of CG. The XRD pattern of raw CG was displayed in Fig. 1a. The XRD results shown that CG mainly consisted of kaolinite, quartz and pyrite. After the CG underwent hydrothermal synthesis, the diffraction peaks of kaolinite and pyrite disappeared, whereas the characteristic peaks of a type of zeolite (Chemical formula: Na₂(AlSiO₄)₆·4H₂O, PDF No. 42-0216) were observed in Fig. 1b. This results showed the successful preparation of zeolite.

Fig. 1c exhibited the FT-IR spectra of the as-synthesized zeolite (ZCG). Two peaks centered at 3483 cm⁻¹ and 1647 cm⁻¹ were attributed to the stretching vibration of hydroxyl group (H-O bond) [25]. The peaks located in 1471, 1410 and 977 cm⁻¹ were observed, corresponding to the characteristic peaks of ZCG, which belonged to the interior asymmetric and symmetric stretching vibration of T-O-T bonds (T=Si/Al) [26]. The peaks at 569 cm⁻¹ was assigned to the exterior vibration of double four-membered ring [27]. The peaks at 719 cm⁻¹ and 662 cm⁻¹ were attributed to the symmetric stretching vibration of Si-OH and Al-O bonds [28].

From Fig. 1d, it could be seen that the TG curve of ZCG included three weight loss stages. With the first stage before 200°C, the ZCG showed rapid weight loss rate with about 8.86%wt, associating to the evaporation of water. Within the range of 200~500°C, the weight loss rate was 3.92%, which was attributed to the removal of water from the ZCG samples. Further heating to around 650°C led to only a relative lower degradation (around 1.22%), which indicated that this type of zeolite was very stable in high temperature. After 800°C, the total mass loss of the ZCG was 14%. The zeta potential of the ZCG particle suspension at different pH values was displayed in Fig. S3. According to Fig. S3, it was observed that the pH_PZC of ZCG was determined to be 7.08.

To further obtain more information about the textural characteristic of the CG and ZCG samples, N₂ adsorption-desorption isotherms and corresponding pore size distribution curves, were illustrated in Fig. 1e and f. N₂ adsorption-desorption isotherms curves of the CG and ZCG samples belonged to IV isotherm with the H₃ hysteresis loop, which indicated the presence of typical meso-porous structure. From Fig. 1e, it could be seen that the pores of the CG and ZCG were mainly concentrated between 2~20 nm with a peak appearing at 4 nm for the CG and two peaks appearing at 4 nm and 10 nm for the ZCG, respectively. The specific surface area, pore volume and pore size of the CG and ZCG were shown in Table S2. The surface area, pore volume and pore size of ZCG were determined to be 25.15 m²/g, 0.0616 cm³/g and 19.15 nm, respectively, which were higher than those of CG. After conversion to zeolite, the surface area, pore volume and pore size ascended and ion exchange site were formed, indicating that ZCG had much stranger combining capacity and more potential binding sites, which was beneficial for adsorption [29].

3.2. Adsorption Performance of the CG and ZCG for Cd(II) Removal

3.2.1. Effect of adsorbent dosage

The changes of Cd(II) adsorption capacity and removal percentage with adsorbent dosage were displayed in Fig. S4a and b. Obviously, when the ZCG addition increased from 0.1 g/L to 1 g/L, the adsorption capacity decreased rapidly, while the removal percentage firstly increased sharply, then gradually approach a constant value. This was because with increasing the ZCG dosage, a greater number of adsorption sites were provided to adsorption Cd(II), whereas the amount of adsorption sites became in excess compared to present metal ions, thus causing incomplete adsorption [30].

3.2.2. Effect of solution pH

Fig. S5 was plotted with Cd(II) adsorption capacity and removal percentage against solution pH. As illustrated in Fig. S5, with the increase of pH value from 2.0~7.0, Cd(II) adsorption capacity and removal rate increases rapidly, and at pH 7.0, Cd(II) adsorption capacity and removal percentage reach 38.64 mg/g and 15.45% for the CG, 63.20 mg/g and 25.28% for the ZCG, respectively. This was because when solution pH was low, there was intense competition between H⁺ and Cd²⁺ for adsorption sites, so that Cd(II) adsorption capacity and removal percentage were low. However, with pH increasing, H⁺ concentration gradually decreased, and competitive adsorption became weaken, which was beneficial for adsorption [31].

3.2.3. Effect of adsorption time and adsorption kinetics

The time-dependent adsorption behaviors of Cd(II) by the CG and ZCG were studied and the results were displayed in Fig. 2a. The accumulation of Cd(II) onto the CG and ZCG increased initially and then reached equilibrium state, and at 1440 min, the adsorption capacity of the CG and ZCG were determined to be 38.60 mg/g and 67.30 mg/g, respectively. Apparently, the adsorption performance of the ZCG for Cd(II) was significantly better than that of the CG. The fast adsorption initially was attributed to the presence of substantial numbers of available adsorption sites for Cd(II) uptake on the adsorbent and large concentration gradient between solid and liquid phases [10]. As time progressed, the adsorption process became slow, which was caused by the electrostatic repulsion between adsorbed and unabsorbed Cd(II) became stronger and the adsorption sites on the CG and ZCG surface was gradually occupied, which limited the migration of Cd(II) from solution to the surface of the CG and ZCG, causing the removal percentage gradually slowed down with the prolongation of adsorption time [32].

In order to study the control mechanisms of the adsorption process, pseudo first-order, pseudo second-order and intra-particle diffusion kinetic models were used to fitting with adsorption data. The kinetic models of pseudo first-order and pseudo second-order were as follows [33, 34]:

(3)

ln (q_{e} - q_{t}) = ln q_{e} - \frac{k_{f}}{2.303} t

(4)

\frac{t}{q_{t}} = \frac{t}{k_{s} q_{e}^{2}} + \frac{t}{q_{e}} t

where q_e is the adsorption capacity at equilibrium (mg/g), q_t is the adsorption capacity at time t (mg/g), k_f is the pseudo-first-order rate constant (g/mg·min), k_s is the pseudo second-order rate constant. The fitting curves were displayed in Fig. 2b and c, and the kinetic fitting parameters were listed in Table 1. As shown in Fig. 2b, c and Table 1, it was clear that the adsorption data were in conformity with the pseudo second-order kinetic model with the higher correlation coefficient (R²=0.998 for the CG and R²=0.999 for the ZCG). In addition, the adsorption capacity (38.84 mg/g for the CG and 68.12 mg/g for the ZCG, respectively) calculated by pseudo second-order kinetic model were closer to experimental values. Therefore, Cd(II) adsorption by CG and ZCG was governed by chemisorption process [35].

To identify the mechanism of rate-limiting step for Cd(II) adsorption process, we simultaneously used intra-particle diffusion model for fitting the kinetic data, intra-particle diffusion kinetic model was expressed by Eq. (5) [36]:

(5)

q_{t} = k_{id} t^{1 / 2} + C

where k_id is the intra-particle diffusion rate constant (mg/g·min^0.5), and C usually reflects the boundary layer thickness. As shown in Fig. 2d, the plot of q_t vs t^1/2 exhibited multi-linear relationship, indicating the occurrence of two steps during the adsorption process. The first stage corresponded to instantaneous adsorption stage. At this stage, Cd(II) rapidly diffused to the adsorbent surface under electrostatic action. The adsorption rate in the second stage was relatively slow, corresponding to adsorption equilibrium stage, and Cd(II) could only bind to the adsorption sites inside the adsorbent material through slow diffusion process within the particles, mainly through complexation or chemical precipitation. Furthermore, the fitting plot did not pass through the origin, which indicated that Cd(II) adsorption by the CG and ZCG was not only controlled by intra-particle diffusion.

3.2.4. Effect of Cd(II) concentration and adsorption isotherms

The effect of the initial concentration on Cd(II) adsorption by the CG and ZCG was investigated, and was shown in Fig. S6. From Fig. S6, it was found that the adsorption capacity increased, whereas Cd(II) removal descends as the initial concentration increased. For interpretation of the adsorption data, Langmuir, Freundlich, Tempkin and Dubinin-Radushkevich isotherm models were used for fitting the isotherm data. Langmuir, Freundlich, Tempkin and Dubinin-Radushkevich isotherm models were expressed by Eqs. (6)~(9)[37–40]:

(6)

\frac{C_{e}}{q_{e}} = \frac{1}{q_{max} K_{L}} + \frac{C_{e}}{q_{max}}

(7)

ln q_{e} = ln K_{F} + \frac{1}{n} ln C_{e}

(8)

q_{e} = \frac{RT}{b_{T}} ln α + \frac{RT}{b_{T}} ln C_{e}

(9)

ln q_{e} = ln q_{m} - β ɛ^{2}

where C_e represent the Cd(II) equilibrium concentration (mg/L), q_e is the adsorbed Cd(II) amount, K_L is the Langmuir constant (L/mg), K_F and n are the temperature-dependent adsorption indexes, R refer to the ideal gas constant (8.314J/(mol·K)), α is the equilibrium binding constant (mg/L), T is the Kelvin temperature (K), b_T is the constant related to adsorption heat, ɛ is the Polanyi potential, β is a constant related to the mean free energy change (mol²/kJ²). The fitting curves were displayed in Fig. S7 and the fitting parameters were comprised in Table 2. From Fig. S6, Langmuir model was in well correspondence with the adsorption data, and the maximum equilibrium adsorption capacity of Cd(II) by the CG and ZCG were 48.01 mg/g and 66.49 mg/g, respectively. In addition, higher coefficient R² obtained by Langmuir model than other isotherm models also indicated that Cd(II) adsorption by the CG and ZCG was mainly controlled by the monolayer adsorption process [41]. According to Table 2, 1/n values of CG and ZCG were 0.16 and 0.18, respectively, indicating Cd(II) adsorption by ZCG was prone to occur.

We summarized the adsorption capacity of Cd(II) by ZCG, natural zeolite and previously reported low-cost adsorbents, as listed in Table 3. Obviously, the adsorption capacity of the ZCG was much higher than that of previously reported low-cost adsorbents, which indicated that the zeolite derived from the CG exhibited excellent adsorption performance to uptake Cd(II) compared to other previously reported low-cost adsorbents.

3.2.5. Effect of temperature and adsorption thermodynamics

Fig. S8 showed the Cd(II) adsorption capacity onto the CG and ZCG with the adsorption temperature increasing. From Fig. S8, it was observed that Cd(II) adsorption capacity by the CG and ZCG enhanced as temperature increasing, which shown that high temperature was beneficial for Cd(II) adsorption. To predict the nature of Cd(II) adsorption on the CG and ZCG and related regularities of the adsorption process, the thermodynamic parameters, such as the changes of Gibbs free energy (ΔG°), enthalpy (ΔH°) and entropy (ΔS°), were calculated according to Eqs. (10) and (11) [54, 55]:

(10)

lnK = - \frac{Δ H^{0}}{RT} + \frac{Δ S^{0}}{R}

(11)

Δ G^{0} = - RTlnK

where ΔH° is the enthalpy change (kJ/mol), ΔG° is the Cibbs free energy (kJ/mol), ΔS° is the entropy change (J/mol K). Specifically, the ΔH° value was positive (9.74 kJ/mol and 32.46 kJ/mol for the CG and ZCG), indicating that the adsorption was endothermic, which could be illuminated by the following reasons: (1) Higher temperature promoted the diffusion and migration of Cd(II) through the external boundary layer and internal pores of the adsorbents [56]. (2) Cd(II) coordinated water needed be dehydrated for the complexation with surface sites of the CG and ZCG prior to adsorption, and this process was endothermic [57]. (3) The increasing temperature was propitious to the availability of more active sites [58]. The positive value of ΔS° indicated the enhanced disorderliness at the solid-water interfaces after adsorption [59]. The ΔG° value was negative and shifted to a high negative value with temperature increased from 25.15°C to 45.15°C, and high temperature was helpful to improve the adsorption performance of ZCG [60, 61].

3.2.6. Effect of coexisting ions

Because of the simultaneously existence of various components in actual wastewater, it was essential to investigate the interference of competitive ions containing NH₄⁺, K⁺, Mg²⁺, Ca²⁺ and Na⁺ as compared by without any interference ions (Fig. 3). From Fig. 3, it could see that the order of the adsorption efficiency of ZCG was K⁺>Na⁺>NH₄⁺>Mg²⁺>Ca²⁺. Compared with monovalent cations, divalent cation showed greater inhibition effect on the adsorption efficiency of Cd(II), which could be explained by the following reasons: Ca(II) (0.100 nm) and Mg(II) (0.072 nm) has smaller ionic radius than Na(I) (0.102 nm) and K⁺(I) (0.138), which would result in a stronger ions-active groups interactions [62]. More importantly, the presence of 400 mg/L Ca²⁺ had a greater impact on the Cd(II) adsorption, which resulted in more than 83% reduction in the removal efficiency. This phenomenon might be attributed to form out-sphere surface complexes with water molecules ‘Ca²⁺-(H₂O)n’, which could cover the adsorbents surface, reducing available adsorption sites for Cd(II) and producing repulsive force between ZCG and Cd(II) [63, 64]. Therefore, it was necessary to remove superfluous Ca²⁺ to ensure the efficiency of the ZCG.

3.3. Adsorption Mechanism

To explore the interaction mechanism between the ZCG and the target ions, ZCG and ZCG-Cd(II) were analyzed by SEM-EDS and XPS. The elemental analysis in Fig. S9 displayed that the ZCG after the adsorption of Cd(II) (named as ZCG-Cd(II)) mainly consisted of O, Na, Al, Si and Cd, indicating that Cd ions was captured from aqueous solution. Through SEM-EDS analysis, we also found that Cd(II) was successfully adsorbed by ZCG (Fig. S10).

For further explore the Cd(II) removal mechanism by the ZCG, XPS spectra were introduced to measure ZCG before and after adsorption. According to Fig. 4a, the characteristic peaks of Na1s, O1s, Si2p and Al2p obtained for the ZCG were located at 1071.8 eV, 531.4 eV, 102.1 eV and 74.2 eV, respectively. After Cd(II) adsorption, it was found that the conspicuous Cd3d (405.8 eV) absorbance peak appeared, and the high-resolution Cd3d spectrum presented two peaks around the binding energies of 412.4 eV and 405.7 eV, respectively, corresponding to Cd3d_3/2 and Cd3d_5/2 (Fig. 4b). However, Na1s peak of the ZCG-Cd(II) was weakened greatly compared with the ZCG, indicating the adsorption process of Cd(II) by the ZCG involved ion-exchange interaction between Cd(II) and Na⁺ ions, which could express as follows [65, 66]:

({Na}^{+} - Zeolit e) + {Cd}^{2 +} \to ({Cd}^{2 +} - Zeolite) + {Na}^{+}

Fig. 4c and d displayed the high-resolution XPS spectra of O1s. From Fig. 4c and d, O1s absorbance peak was deconvoluted into three characteristic peaks with binding energies of approximately 530.8 eV, 531.7 eV and 532.6 eV, corresponding to the oxygen in Al-O-, Si-O-Si and Si-OH, respectively. After Cd(II) adsorption, the binding energy of 532.6 eV of the ZCG migrated to a higher binding energy of 532.9 eV, and binding energy of Si-O-Si shifted from 531.7 to 531.8 eV. This phenomenon may be due to the replacement of the heavier atom could cause a shift in O1s peak to higher binding energy [67]. the deconvolution of Si2p spectrum was presented in Fig. 4e and f. the peaks of the ZCG centered at 101.5 and 102.8 eV, relating to two SiO_x(x=0~4) signal peaks [12]. After Cd(II) adsorption, the peak at 101.5 eV shifted to a lower binding energy 101.3 eV, suggesting the interaction between silicon and Cd²⁺. Thus, the above results confirmed the ion-exchange mechanism for the Cd(II) adsorption, and that Cd²⁺ was adsorbed on the Si-OH sites on the ZCG [68]. The interactions between Cd(II) and ZCG were illustrated in Fig. 5.

3.4. Economic Assessment

To assess the practicality of employing ZCG adsorbent in the treatment of wastewater containing Cd(II), a cost analysis was conducted, and analysis results was presented in supplementary material Section A. The production cost of coal gangue-derived zeolite was mainly composed of chemical reagents and electricity charges. According to estimation results, the total cost of coal gangue-derived zeolite was ¥0.12/g (0.0166 USD), which was much lower than that of commercial zeolite, making it competitive. Consequently, we have reason to believe that synthetic zeolites have enormous application and market prospects, which was suitable for treating wastewater containing Cd(II).

4. Conclusions

A low-cost zeolite (ZCG) was successfully prepared from coal gangue (CG) via alkaline fusion-hydrothermal method. Cd(II) adsorption performance by ZCG was comprehensively investigated. The synthesized ZCG exhibited the maximum Cd(II) adsorption capacity of 66.49 mg/g at pH=7.0. The whole adsorption reaction was in accordance with the peudo second-order kinetic and Langmuir model, indicating that the adsorption process was controlled by chemisorption and the monolayer adsorption. The thermodynamic studies revealed that the adsorption process was an endothermic and spontaneous process. This work not only provided a potential avenue to purify heavy metal ions from effluent, but also solve the environmental problems of coal gangue accumulation. Coal gangue derived zeolites has preliminarily shown certain potential in pollutant removal. Therefore, in the future, further research on coal gangue derived zeolites is necessary and worthwhile to carry out the large-scale application in the field of environmental remediation.

Supplementary Information

eer-2024-693-supplementary.pdf

Notes

Acknowledgements

The authors are grateful for the financial support provided by Scientific Research Foundation for High-level Talents of Anhui University of Science and Technology (No. 2022yjrc13), the Opening Foundation of Anhui Province Engineering Laboratory of Water and Soil Resources Comprehensive Utilization and Ecological Protection in High Groundwater Mining Area (No.:2022-WSREPMA-05), Natural Science Research Project of Anhui Educational Committee (No. 2023AH051177), the Opening Foundation of Anhui Province Engineering Laboratory for Mine Ecological Remediation (No. KS-2022-002). Related characterization analysis has been supported by Shiyanjia Lab (www.shiyanjia.com). The authors express their sincere thanks and gratitude to the anonymous reviewers due to their positive comments and constructive suggestions.

Author contributions

Y.Z.Z. (Lecturer) conceptualized, wrote, and revised the manuscript. P.H. (Master's student) provided suggestions for the manuscript. L.L.L. (Master's student) conducted all experiments. J.Y.S. (Master's student) analyzed all data. B.Y.W. (Master's student) conducted some experiments. J.W.N. (PhD Student) provided suggestions for the manuscript. M.C. (Associate Professor) wrote the manuscript. X.Y.C. (Professor) guided this paper.

Conflict-of-Interest Statement

The authors declare no competing interests.

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Fig. 1

XRD analysis of (a) coal gangue and (b) the as-synthesized zeolite; (c) FT-IR spectra and (d) TGA curves of the ZCG; (e) N₂ adsorption-desorption isotherm and (f) pore size distribution of the CG and ZCG.

Fig. 2

(a) Adsorbed quantity of Cd(II) by the CG and ZCG samples as a function of time t; The linearized fitting of (b) pseudo-first order, (c) pseudo-second order and (d) intra-particle diffusion kinetic models (C₀=50 mg/L, T=30°C, solution pH=7, V=50 mL, adsorbent dosage=0.2 g/L).

Fig. 3

Effect of coexisting ions with different concentration on Cd(II) adsorption by CG and ZCG (Cd(II) ions concentration =50 mg/L, T=30°C, solution pH=7, V=50 mL, adsorbent dosage=0.2 g/L).

Fig. 4

(a) XPS spectra of the ZCG and ZCG-Cd(II), XPS spectra region of (b) Cd3d, O1s for (c) ZCG and (d) ZCG-Cd(II), and Si2p for (e) ZCG and (f) ZCG-Cd(II).

Fig. 5

Diagrammatic sketch of encapsulating Cd(II) from aqueous solution by an economical and feasible zeolite from coal gangue.

Table 1

Kinetic fitting parameters for Cd(II) adsorption on the CG and ZCG

Kinetic models	Parameter	CG	ZCG
Pseudo first-order	q_e (mg/g)	33.83	57.73
	k_f(min⁻¹)	0.0322	0.0156
	R²	0.7394	0.4350

Pseudo second-order	q_e (mg/g)	38.84	68.12
	k_s(g/(mg·min))	0.0006	0.0004
	R²	0.9984	0.9988
Intra-particle diffusion	k_id1(mg/(g·min^1/2))	0.9357	1.3542
	C₁(mg/g)	12.4286	33.0286
	R₁²	0.8795	0.9929
	k_id2(mg/(g·min^1/2)	0.2546	0.2356
	C₂(mg/g)	28.6636	57.7096
	R₂²	0.9412	0.8916

Table 2

Adsorption isotherms fitting parameters of the CG and ZCG

Isothermal models	Parameter	CG	ZCG
Langmuir	q_max(mg/g)	48.01	66.49
	K_L(L/mg)	0.0889	0.0744
	R_L	0.1837	0.2120
	R²	0.9995	0.9974
Freundlich	K_F(mg⁽¹⁻ⁿ⁾Lⁿ/g)	19.5565	24.1632
	n	6.1200	5.6500
	R²	0.8674	0.9509
Tempkin	α	1.5557	1.8466
	b_T(kJ/mol)	413.4605	284.8840
	R²	0.9082	0.9606
Dubinin-Radushkevich	b(mol²/J²)	1.53E-05	1.54E-05
	E(kJ/mol)	0.1806	0.1800
	q_max (mmol/g)	43.1270	56.5853
	R²	0.7863	0.7296

Table 3

Comparison of Cd(II) adsorption capacities of various adsorbents

Adsorbent	q_max (mg/g)	Isothermal models	Refs.
Modified sewage sludge	14.7	Langmuir isotherms model	[42]
Nalco Plant Sand	58.13	Langmuir isotherms model	[43]
Coal gangue	38.61	Langmuir isotherms model	[44]
Bamboo charcoal	12.08	Langmuir isotherms model	[45]
Eucalyptus Camaldulensis	6.173	Langmuir isotherms model	[46]
Palm oil boiler mill fly ash	15.823	Langmuir isotherms model	[47]
Iranian natural zeolite	3.4	Freundlich isotherms model	[48]
Ukraine clinoptilolite	4.22	Freundlich isotherms model	[49]
Zeolite-based geopolymer	26.25	Langmuir isotherms model	[50]
HM-CFB-FA	183.7	Langmuir isotherms model	[51]
Zeolite NaX	38.61	Langmuir isotherms model	[52]
FM-C-HY	48.5	Langmuir isotherms model	[53]
ZCG	66.49	Langmuir isotherms model	In this study