Adsorption of Cd²⁺ ions onto zeolites synthesized from a mixture of coal fly ash and oil shale ash in aqueous media

Abstract

Contaminated water, especially water containing high concentrations of toxic metal ions, is frequently discharged into the environment. Thus, developing advanced adsorbents with high adsorption capacities is essential for efficient pollutant removal. In this study, X- and A-type zeolites were synthesized using industrial waste materials, specifically oil shale ash and coal fly ash. Their physicochemical properties were characterized using XRD, SEM, and nitrogen adsorption–desorption analyses. The adsorption behaviors of Cd²⁺ ions onto the synthesized zeolites from aqueous solutions were thoroughly investigated, including kinetic and isotherm analyses. The results showed that the adsorption process adhered to pseudo-second-order kinetics and followed the Langmuir isotherm model for all zeolites. Maximum adsorption capacities were reached within 60 min, with a dosage of 1 g L⁻¹ and at pH 7. Among the zeolites, OSA75 demonstrated the highest Cd²⁺ adsorption capacity, reaching 236.41 mg g⁻¹, indicating that the incorporation of a small amount of coal fly ash significantly enhanced the adsorption performance. The dominant mechanism governing Cd²⁺ adsorption was identified as cation exchange.

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1. Introduction

Contaminated water, particularly water containing high concentrations of toxic metal ions, is often discharged into the environment. The persistence, accumulation, and toxicity of heavy metals in wastewater necessitate the development of effective removal strategies. Among these pollutants, cadmium is particularly detrimental to aquatic ecosystems and human health, as it is classified as a carcinogen.¹ In recent years, various treatment technologies, such as chemical precipitation, flocculation, and electrochemical processes, have been proposed to mitigate the presence of heavy metals in wastewater.^2,3 However, many of these approaches are hindered by limitations such as high operational costs and suboptimal efficiency. In contrast, adsorption has gained prominence as a preferred approach for removing toxic metals, owing to its straightforward operation, cost-efficiency, and remarkable effectiveness.⁴

Various adsorbents, such as clay materials,^5–7 activated carbon,² and geopolymers^8,9 have been tested for Cd²⁺ uptake from wastewater.^10,11 Despite these efforts, the adsorption capacities of these materials remain insufficient for treating Cd²⁺-contaminated wastewater. Therefore, there is an urgent need to develop advanced adsorbents with high adsorption capacity. Zeolites, in particular, have garnered significant interest as promising adsorbents, attributed to their extensive surface area and highly porous structure.^12–16

Zeolites can be derived from a range of sources, including both natural and synthetic materials, as well as pure chemical reagents. However, natural zeolites often exhibit low adsorption capacity and are limited by their reserves.^13,17 On the other hand, synthetic zeolites are costly to produce due to the use of chemical reagents.¹⁸ To tackle these challenges, numerous researchers have explored the synthesis of zeolites from cost-effective solid waste materials, including red mud, construction waste, coal fly ash (CFA), and oil shale ash (OSA), among others.^19,20 These waste materials share mineralogical composition and structural similarities with the precursors of natural zeolites, making them viable sources of alumina and silica for zeolite synthesis. Various techniques, such as the hydrothermal method, alkaline fusion method, alkaline fusion-hydrothermal method, and microwave-assisted method, have been employed to convert these materials into zeolites. This approach not only provides a sustainable waste utilization strategy but also offers an efficient solution for the removal of heavy metals from wastewater.²¹ This approach not only reduces the need for solid waste disposal but also helps mitigate heavy metal pollution. Several studies have investigated the potential of zeolites synthesized from CFA and OSA for heavy metal removal.^{11,13,22–26} However, research on synthesizing zeolites from solid wastes with varying silicon and aluminum contents remains limited.

The high silicon content in OSA and the elevated aluminum levels in CFA offer a promising opportunity for their combined use in zeolite synthesis. This study aims to synthesize zeolites from these materials and evaluate their effectiveness in removing Cd²⁺ from aqueous solutions. The effects of pH, contact time, and initial Cd²⁺ concentration on adsorption were systematically examined. Kinetic models were employed to determine adsorption parameters, while isotherm models were applied to characterize equilibrium behavior. Additionally, the mechanisms underlying Cd²⁺ adsorption onto the synthesized zeolites were investigated. This research provides a promising approach to mitigating environmental challenges related to both solid waste disposal and heavy metal-contaminated wastewater, aligning with the “waste-for-waste” treatment concept.

2. Experimental

2.1. Materials

This study utilized CFA and OSA as the primary raw materials, sourced from an oil-shale-fired power plant in Shandong and a coal-fired station in Shanxi, China. The chemical compositions of OSA and CFA were determined using XRF, and the results are presented in Table S1.† The SiO₂/Al₂O₃ weight ratio for OSA was found to be 3.59, in contrast to 1.17 for CFA, indicating a higher silicon content in OSA and a higher aluminum content in CFA. X-ray diffraction (XRD) analysis (Fig. 1) revealed that OSA primarily contained quartz, while CFA exhibited a glassy amorphous phase, characterized by a broad peak between 15° and 30°. Scanning electron microscopy (SEM) images (Fig. 2) showed that OSA consisted mainly of dense phases with occasional glass beads, whereas CFA particles were granular with smooth surfaces.

Fig. 1 XRD patterns of OSA and CFA.

Fig. 2 SEM images of CFA and OSA.

Analytical-grade sodium hydroxide (NaOH) and cadmium chloride (CdCl₂·2.5H₂O) were procured from Beijing Chemical Works. Deionized water was employed consistently as the solvent. A 1000 mg L⁻¹ Cd²⁺ stock solution was prepared by accurately dissolving CdCl₂·2.5H₂O in deionized water, and working solutions were subsequently prepared by appropriate dilution. The pH levels of the working solutions were adjusted using HNO₃ and NaOH solutions at concentrations of 0.01, 0.1, and 1 mol L⁻¹.²⁷

2.2. Methods

2.2.1. Synthesis of zeolites. Zeolite was synthesized from OSA using the alkaline fusion hydrothermal method described in our previous study.²⁸ In the present work, this same procedure was employed for zeolite synthesis. Specifically, 2 g of OSA was mixed with 2.4 g of sodium hydroxide and subjected to fusion at 600 °C for 1 h. After cooling, the fused mixture was ground and added to 20 mL of deionized water. The resulting slurry was aged at room temperature with mechanical stirring for 3 h, then transferred to a Teflon-lined autoclave for crystallization at 80 °C for 12 h under static conditions. Following crystallization, the mixture was filtered, and the solid product was washed with deionized water until the pH neared 7. Finally, the product was dried at 105 °C for 2 h. The resulting zeolite, designated as OSA100, was used as the adsorbent.

Other zeolites were synthesized following the same procedure. In addition to the reference zeolite (OSA100), blended zeolites were also produced, in which OSA was partially replaced by 25%, 50%, 75%, and 100% CFA, resulting in the zeolites labeled as OSA75, OSA50, OSA25, and CFA100, respectively.

2.2.2. Adsorption experiments. Adsorption experiments were performed by introducing 0.05 g of zeolite into 50 mL of Cd²⁺ aqueous solution in uniform beakers at 30 °C. The influence of pH on adsorption capacity was assessed across a pH range of 3 to 9. Kinetic studies were conducted at intervals of 5, 10, 20, 40, 60, 80, 100, 120, and 150 min. Adsorption isotherms were determined using various initial concentrations of Cd²⁺ aqueous solutions (20, 40, 80, 100, 120, 150, 180, 200, 250, 300, 400, and 500 mg L⁻¹). The solutions were agitated at 150 rpm using a shaker, and following each experiment, the mixtures were filtered through 45 μm membranes. The concentration of residual Cd²⁺ in the filtrates was analyzed using an Inductively Coupled Plasma Emission Spectrometer (ICP). Calculations of adsorption capacity (q_e) and uptake efficiency (E%) are given in ref. 29.

2.2.3. Characterization and analysis. The chemical composition of the raw materials was determined using XRF with an Empyrean instrument. Mineralogical analysis was conducted through XRD employing a Rigaku instrument, utilizing Ni-filtered Cu Kα radiation (λ = 0.15418 nm). XRD patterns were recorded over a 2θ range of 2.5° to 70° at a scan rate of 4°·min⁻¹, and the data were processed using JADE software with the ICSD minerals database. Particle morphology was examined using SEM (ZEISS EVO18). The surface area and other surface properties of the synthesized zeolites were determined with a Micromeritics ASAP 2020 analyzer, after degassing at 300 °C for 12 h. The residual Cd²⁺ concentration in the solutions was quantified using inductively coupled plasma spectroscopy (PerkinElmer 8300).²⁷

3. Results and discussion

3.1. Characteristics of adsorbents

Fig. 3 presents the XRD patterns of the synthesized zeolites (OSA100, OSA75, OSA50, OSA25, and CFA100). The XRD pattern of OSA100 corresponds to X zeolite, which aligns closely with PDF#12-0246 (Na₂Al₂Si₂.₄O₈.₈·6.7H₂O). Similar diffraction patterns were observed for OSA75 and OSA50 (Fig. 3). In contrast, the XRD profiles of CFA100 and OSA25 closely match those of Na-A zeolite (PDF#38-0241, Na₂Al₂Si₁.₈₅O₇.₇·5.1H₂O). Furthermore, the synthetic zeolites exhibited well-defined crystallinity, as evidenced by the sharp and intense diffraction peaks. The Si/Al ratio of precursor materials plays a crucial role in determining the specific types of zeolites formed during hydrothermal synthesis. As the Si/Al ratio increases, the aluminum content in the synthesis system decreases, promoting the formation of X-type zeolite. Conversely, a lower Si/Al ratio creates conditions more favorable for the formation of A-type zeolite.^30,31 Consequently, when synthesizing composite zeolites by blending oil shale ash and coal fly ash, an increase in the proportion of coal fly ash reduces the overall Si/Al ratio of the raw materials. This gradual decrease shifts the synthesis conditions from favoring X-type zeolite formation to those more conducive to A-type zeolite formation. In this study, the higher aluminum content in CFA compared to OSA led to the formation of Na–A zeolite when the CFA content exceeded 50%, rather than Na–X zeolite.

Fig. 3 XRD patterns of OSA100 (a), OSA75 (b), OSA50 (c), OSA25 (d) and CFA100 (e).

Fig. 4 presents the N₂ adsorption–desorption isotherms of the synthesized zeolites, displaying type IV profiles accompanied by H3 hysteresis loops, indicative of mesoporous structures.³² The summarized specific surface areas and pore properties of the zeolites are presented in Table 1. The BET surface area (S_BET) values, as listed, decrease in the following order: OSA75 > OSA100 > OSA50 > OSA25 > CFA100. Additionally, both micropores (<2 nm) and mesopores (2–50 nm) were identified in the zeolite samples. However, significant differences were observed in the micropore-to-mesopore volume ratio and the average pore diameter among the various zeolites. The ratio of mesopore volume was higher in X-type zeolite (OSA100, OSA75 and OSA50), whereas the ratio of micropore volume was higher in A-type zeolite (OSA25 and CFA100). Furthermore, the pore size of X zeolite (OSA100, OSA75 and OSA50) were larger than that of A zeolite (OSA25 and CFA100). The variation in the specific surface area of oil shale ash/coal fly ash composite zeolites is primarily attributed to differences in pore type and pore volume. Compared to A-type zeolite, X-type zeolite has a higher density of micropores, leading to a larger pore volume and, consequently, a greater specific surface area. The differences in pore size within these composite zeolites are influenced not only by the type of pores but also by the structural characteristics of the zeolite framework. A-type zeolite belongs to the cubic crystal system, where SOD cages are arranged in a simple cubic configuration. This structure forms a three-dimensional 8-membered ring pore system with a pore diameter of 0.41 nm. In contrast, X-type zeolite falls under the hexagonal crystal system, characterized by a larger pore volume and a three-dimensional 12-membered ring pore system with a pore diameter of 0.72 nm. As a result, X-type zeolite generally exhibits a larger specific surface area, higher pore volume, and greater average pore size, whereas A-type zeolite possesses a lower specific surface area, smaller pore volume, and reduced average pore size.

Fig. 4 N₂ adsorption–desorption isotherms of OSA100, OSA75, OSA50, OSA25 and CFA100.

Table 1 Surface characteristics of OSA100, OSA75, OSA50, OSA25 and CFA100

Samples	SBET (m^2 g^−1)	Micropore volume (cm^3 g^−1)	Mesopore volume (cm^3 g^−1)	Pore volume (cm^3 g^−1)	Average pore diameter (nm)
OSA100	252.22	0.109	0.110	0.229	2.914
OSA75	288.55	0.086	0.132	0.221	3.068
OSA50	215.89	0.038	0.104	0.145	3.975
OSA25	199.04	0.073	0.033	0.111	2.330
CFA100	170.83	0.055	0.031	0.104	2.444

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3.2. Adsorption tests

3.2.1. Effect of solution pH. Fig. 5 illustrates the effect of solution pH on Cd²⁺ removal by the synthesized zeolites, with uptake efficiency plotted against pH. The results highlight the critical role of pH in Cd²⁺ adsorption. Uptake efficiency increased with rising pH, stabilizing as pH ranged from 3 to 9. Additionally, the Cd²⁺ uptake efficiency of the zeolites (OSA50, OSA25, and CFA100) increased rapidly and approached that of OSA75 when the pH exceeded 7. At lower pH values, adsorption capacity was reduced, likely due to competitive adsorption by H⁺ ions. As pH increased, the efficiency of Cd²⁺ removal improved, which can be attributed to two main factors: first, the competitive adsorption of H⁺ ions progressively diminished and ultimately ceased, and second, the precipitation of Cd²⁺ occurred at pH values above 7.³³ Consequently, a pH of 7 was selected for subsequent experiments, as it optimized adsorption capacity while avoiding precipitation.

Fig. 5 Influence of solution pH on Cd²⁺ adsorption by zeolites (m = 1 g L⁻¹, t = 120 min, C₀ = 100 mg L⁻¹, T = 30 °C).

3.2.2. Adsorption kinetics. Fig. 6 demonstrates the effect of contact time on Cd²⁺ adsorption by the synthesized zeolites. The adsorption capacity increased steadily within the 5-150 min timeframe. The initial adsorption rate was rapid, gradually slowing as the system neared equilibrium, where the Cd²⁺ adsorbed by the zeolites balanced with the residual Cd²⁺ concentration in the solution. As shown in Fig. 6, no significant variation in Cd²⁺ removal was observed for any of the zeolites after 60 min.

Fig. 6 Impact of contact time on Cd²⁺ adsorption onto zeolites (m = 1 g L⁻¹, pH 7, C₀ = 100 mg L⁻¹, T = 30 °C).

To investigate the adsorption kinetics of Cd²⁺ on synthetic zeolites, three models were evaluated: pseudo-first-order, pseudo-second-order, and intraparticle diffusion models.^10,34,35 The relevant parameters and correlation coefficients are provided in Table S2.† Among these, the pseudo-second-order model demonstrated the best agreement with the experimental data, as illustrated in Fig. 7. For all synthetic zeolites, the correlation coefficients (R²) for the pseudo-second-order model were greater than 0.999, substantially outperforming those of the other models. Furthermore, the experimentally measured adsorption capacities (q_e,exp) closely matched the theoretical values (q_e,cal) predicted by this model. These findings confirm that the adsorption process of Cd²⁺ onto synthetic zeolites is most accurately described by the pseudo-second-order model, indicating a mechanism predominantly governed by chemisorption.³⁶

Fig. 7 Pseudo-second-order kinetic model for Cd²⁺ adsorption onto zeolites.

3.2.3. Adsorption isotherm. Fig. 8 illustrates the influence of the initial Cd²⁺ concentration on its adsorption behavior by synthetic zeolites. With an increase in the initial concentration from 20 to 500 mg L⁻¹, the equilibrium adsorption capacities exhibited a marked enhancement. Among the zeolites, OSA75 exhibited the highest adsorption capacity, attributable to its larger BET surface area and higher availability of exchangeable cations. These factors significantly enhanced its Cd²⁺ uptake efficiency.³⁷

Fig. 8 Influence of initial Cd²⁺ concentration on adsorption behavior of zeolites (m = 1 g L⁻¹, pH 7, t = 60 min, T = 30 °C).

Adsorption isotherms are crucial for understanding adsorption mechanisms, as they reveal the interactions between adsorbate molecules and the active sites of the adsorbent surface. In this study, the adsorption behavior of Cd²⁺ on synthetic zeolites was assessed using the Langmuir, Freundlich, and Dubinin–Radushkevich (D–R) models.^38–40 Fig. 9 presents the Langmuir adsorption isotherm curves for Cd²⁺ on synthetic zeolites along with their linear fits. The corresponding isotherm parameters and correlation coefficients are summarized in Table S3.† The Langmuir model showed the highest correlation coefficients (R²) for all zeolites, followed by the Freundlich and D–R models, indicating that the Cd²⁺ adsorption is most accurately described by the Langmuir isotherm. This suggests that adsorption occurs in a monolayer on adsorbents with a uniform surface energy distribution. According to the Langmuir model, the maximum Cd²⁺ adsorption capacities ranked as follows: OSA75 > OSA50 > OSA100 > OSA25 > CFA100, aligning closely with experimental data in Table S3.† The highest monolayer adsorption capacity was found for OSA75 at 236.41 mg g⁻¹, followed by 211.86 mg g⁻¹ for OSA50. These results underscore the superior adsorption performance of OSA75, which cannot be replicated by the individual raw materials. This enhanced performance can be attributed to the incorporation of a small amount of CFA, which improved the crystallinity and pore structure of the synthesized zeolite, thereby increasing its surface area and the number of exchangeable cations available for adsorption.

Fig. 9 Langmuir isotherm model describing Cd²⁺ adsorption onto zeolites.

To evaluate the adsorption efficiency of zeolites for Cd²⁺ removal from aqueous systems, their maximum adsorption capacities were compared with those of other reported materials, as summarized in Table 2. It is evident from the data that the zeolites synthesized in this study exhibit significantly higher adsorption capacities than other adsorbents. This finding highlights the superior performance of zeolites derived from a mixture of coal fly ash and oil shale ash, making them a promising candidate for the treatment of cadmium-contaminated wastewater.

Table 2 Adsorption results of Cd²⁺ onto different adsorbents

Adsorption materials	Adsorption capacity (mg g^−1)
Iranian natural zeolite	4.01 (ref. 12)
Activated carbon	111.78 (ref. 2)
Modification of nano-clays	94.6 (ref. 5)
Magnetic nano-zeolite	159.39 (ref. 17)
OSA100	187.18 (this study)
OSA75	227.24 (this study)
OSA50	205.87 (this study)
OSA25	179.54 (this study)
CFA100	156.21 (this study)

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3.3. Adsorption mechanisms

The results indicate that Cd²⁺ uptake was primarily governed by ion-exchange processes on the synthetic zeolites, as supported by both the adsorption kinetics and isotherms. Despite the identical adsorption mechanism, significant variations in the adsorption capacities of the synthetic zeolites were observed. This suggests that the maximum Cd²⁺ adsorption capacity was not solely determined by specific surface area and exchangeable cations, but also influenced by additional factors, such as the structural characteristics of the zeolites. Metal ions in aqueous solutions typically exist in a hydrated form, with Cd²⁺ possessing an ionic radius of 0.97 Å and a hydrated ionic radius of 4.29 Å. When comparing the crystal radii of X-type zeolite (7.4 Å) and A-type zeolite (4.5 Å) channels, it becomes evident that both hydrated and unhydrated Cd²⁺ ions can easily traverse the channels of X-type zeolite. In contrast, the hydrated Cd²⁺ ions are similar in size to the A-type zeolite channels, which may hinder their exchange unless some of the hydration water is removed. Consequently, OSA75 exhibited the highest adsorption capacity, whereas CFA100 displayed the lowest.

4. Conclusions

The X- and A-type zeolites were synthesized by recycling industrial solid wastes, specifically OSA and CFA, through the alkaline fusion hydrothermal method. These zeolites exhibited significant adsorption affinity for Cd²⁺. Maximum adsorption capacities for all zeolites were achieved within 60 min at a concentration of 1 g L⁻¹ and pH 7. The adsorption kinetics of Cd²⁺ were most accurately described by the pseudo-second-order model. Additionally, adsorption isotherm analysis revealed that the Langmuir isotherm model provided the best fit, indicating monolayer adsorption behavior. Among the zeolites investigated, OSA75 exhibited the highest Cd²⁺ adsorption capacity, reaching 236.41 mg g⁻¹, with adsorption capacities following the order: OSA75 > OSA50 > OSA100 > OSA25 > CFA100. These results suggest that incorporating a small proportion of CFA during zeolite synthesis enhanced adsorption performance. The Cd²⁺ uptake by the synthetic zeolites was primarily attributed to ion-exchange processes, suggesting a chemical adsorption mechanism. Additionally, the structural and surface properties of the zeolites significantly contributed to their enhanced adsorption capacities. These findings suggest that synthesizing zeolites from a combination of solid wastes offers a promising approach for wastewater treatment applications.

Data availability

The data supporting this article have been included as part of the ESI.†

Author contributions

Shuxia Bai: writing – original draft, visualization, validation, methodology, investigation, formal analysis, data curation, conceptualization. Zhibing Chang: writing – review & editing, supervision, validation, methodology. Zhengchun Ren: visualization, investigation, data curation. Yongqin Zhao: visualization, validation, methodology, investigation, formal analysis, data curation, conceptualization, supervision. Laixue Pang: writing – review & editing, validation, methodology, funding acquisition.

Conflicts of interest

The authors have no conflicts of interest to declare.

Acknowledgements

The authors are grateful to the financial support of the Natural Science Foundation of Shandong Province (ZR2020ME231).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5ra00752f

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