Adsorption of Pb(II) and brilliant green dye onto geopolymer/zeolite hybrid composites

Geopolymers, aluminosilicate materials formed by alkali activation, have drawn interest because of their unique mechanical, chemical, and thermal characteristics. They are interesting for adsorption applications due to their similar chemical structure to zeolite. This study investigates the synthesis and characterization of hybrid geopolymer/zeolite composites to remove lead ions (Pb(II)) and brilliant green (BG) dye from aqueous solutions. Sodium hydroxide and sodium silicate were used to activate fly ash and blast furnace slag blends. This was followed by hydrothermal treatment to encourage the conversion of amorphous geopolymeric gel to crystalline zeolites. Several variables were systematically changed, such as foaming agents, alkali molarity, and bead size to compare adsorption performance. The formation of zeolite phases was confirmed by structural and morphological investigations, such as XRD, FT-IR, SEM, and BET, which also shed light on the porous character of the composite. The geopolymer/zeolite composites demonstrated notable removal efficiency for Pb(II) (up to 123 mg/g) and BG dye (up to 115 mg/g) in adsorption studies. Importantly, this work reveals that average pore diameter plays a more critical role than surface area in determining adsorption capacity of bulk-type adsorbents, contrasting conventional assumptions in the field. The work provides possibilities for creating long-lasting, efficient adsorbents for the treatment of water by highlighting the roles that pore size and surface area play in the adsorption mechanism. Given the structural similarity between heavy metals and certain radionuclides, these findings have broader implications for developing geopolymer-based materials for radioactive waste treatment applications.

1 Introduction

Geopolymers, often referred as low-Ca alkali-activated binders, are inorganic polymers that are produced by alkali-activation of aluminosilicate materials, like metakaolin, blast furnace slag, and fly ash (Davidovits, 2008). The reaction starts with the dissolution of monomers upon exposure to alkali solution, followed by oligomerization, polymerization, and gelation (Van Deventer et al., 2007). The result is a dense, partially semi-crystalline gel that has excellent mechanical strength, chemical resistance, and thermal stability (Davidovits, 2013). Hence, geopolymers are considered alternative binders to traditional Portland cement in concrete (Ahmad et al., 2023; Ślosarczyk et al., 2023).

The chemical structure of geopolymers consists of randomly oriented Si and Al tetrahedra, having charge-balancing cations supplied by the alkali-activator (Na⁺/K⁺) (Elhadi et al., 2025). They are often considered amorphous analogous of zeolites owing to their similar chemical structure (De Silva and Sagoe-Crenstil, 2008). Moreover, nano-structured zeolites (e.g., zeolite Na-P1 and sodalite) are often detected in geopolymers, particularly in the samples activated with low molarity sodium hydroxide (NaOH) solution and/or cured at higher temperatures/pressures (Park et al., 2018; Wang Z. et al., 2021; Kim and Khalid, 2022). Studies have reported the possible conversion of geopolymeric gel to crystalline zeolite upon exposure to high temperature and pressure hydrothermal conditions (Ma et al., 2016; Lee et al., 2017; Khalid et al., 2018; Proust et al., 2024). Since zeolites are popular adsorbents in the industry because of their high cation exchange capacity and surface area, combining the structural integrity of geopolymers with the adsorptive capacity of zeolites positions these hybrid materials as promising candidates for specialized environmental remediation tasks, including the treatment of radioactive wastewater where both mechanical stability and selective ion-exchange are required (Luukkonen et al., 2019; Ettahiri et al., 2023).

Consequently, several studies reported the adsorption potential of geopolymers or geopolymer/zeolite hybrid composites which are covered in recently published review articles (Rożek et al., 2019; Asim et al., 2022; Ren et al., 2022; Xu et al., 2022; Lin et al., 2024). For instance, Duan et al. (2016) reported a Cu(II) uptake capacity of 113.41 mg/g of fly ash and iron ore tailing-based porous geopolymers. A Pb(II) uptake capacity of 118.6 and 143.3 mg/g was reported for geopolymer and geopolymer/zeolite hybrid particle adsorbents (<74 µm) by Liu et al. (2016), respectively. Lan et al. (2019) reported that the metakaolin-based particle geopolymers (<0.5 mm) showed about 70.3 mg/g capacity for Cd(II). Recently, Yan et al. (2023) reported uptake values of 24.15 and 33.3 mg/g for Cu(II) and methylene blue dye onto a geopolymer particle adsorbent (150–450 µm), respectively. Moreover, geopolymer spheres showed an adsorption capacity of 24.6 and 79.7 mg/g for methylene blue (Yan et al., 2018; Novais et al., 2019). He et al. (2021) derived different granular zeolite species (0.15–0.32 mm) from geopolymers and obtained Pb(II) adsorption capacities of 160.70, 239.5, and 252.70 mg/g for Li-ABW, K-F, and Phillipsite zeolites, respectively.

Despite a promising potential of being used a self-supported adsorbent, studies on geopolymer/zeolite are limited. The structural and functional characteristics of geopolymer/zeolite composites are largely determined by experimental factors such as raw materials composition, molarity of alkali activator, and thermal treatment settings. For example, changing the concentration of NaOH during synthesis can have a major impact on how the zeolite phases grow and how porous the composites are (Rożek et al., 2019; Ren et al., 2022). Furthermore, adding macro-porosity to the composites using foaming agents may increase the accessibility of adsorbate species and boost the total adsorption capacity (Sithole et al., 2024). There remains a significant knowledge gap regarding the relative importance of different pore characteristics (size versus volume versus surface area) in determining adsorption performance of bulk-type adsorbents, which this study addresses systematically. The use of geopolymer/zeolite composite for removal of industrial dyes is also rarely reported. Hence, the potential of geopolymer/zeolite hybrid composites for the adsorption of Pb(II) ions and BG dye was explored in this study.

Class F fly ash and ground granulated blast furnace slag were used as the main raw materials for the composites’ synthesis. A combination of sodium hydroxide (NaOH) and sodium silicate (SS) solution was used to activate the materials. To maximize the adsorption performance, the effects of several experimental parameters were methodically examined, including the molarity of NaOH, the size of the beads, the foaming agents, and the hydrothermal treatment. The structural and morphological characteristics of the composites were examined using characterization methods such as X-ray diffraction (XRD), nitrogen physisorption (BET/BJH), scanning electron microscopy (SEM), and Fourier-transform infrared spectroscopy (FT-IR). Adsorption experiments were performed to assess the composites’ effectiveness in eliminating Pb(II) and BG dye from the aqueous solutions.

The purpose of this work is to shed light on the synthesis, characterization, and adsorption capabilities of hybrid composites made of geopolymer and zeolite to aid in the development of novel materials for water treatment applications. By establishing clear relationships between synthesis parameters, pore characteristics, and adsorption performance, this study provides fundamental insights that can guide the rational design of geopolymer-based adsorbents for diverse applications, from industrial wastewater treatment to radioactive contaminant removal. It is anticipated that the results would provide insightful direction for creating long-lasting and efficient adsorbents to combat dye and heavy metal contamination.

2 Experimental

2.1 Materials and experimental parameters

Class F fly ash (80 mass%) and ground granulated blast furnace slag (20 mass%) were used as binders for synthesis of geopolymer-zeolite beads. Their chemical compositions are given in Table 1. A mixture of NaOH and SS solutions was used for activation of raw materials. The SiO₂, Na₂O and H₂O contents in SS were about 29, 10, and 61 mass%, respectively. An activator/binder mass ratio of 1 and SS/NaOH solutions mass ratio of 0.33 were used for all the samples following author’s previous studies (Khalid et al., 2019; Wang Z. et al., 2021).

Table 1

Table 1. Chemical composition of fly ash and slag (wt%).

The experimental parameters include molarity of NaOH solution (6 or 8 M), beads diameter (5 or 7 mm), use of hydrogen peroxide (H₂O₂) foaming agent, and hydrothermal treatment for in situ conversion of geopolymeric gel to zeolites to get geopolymer-zeolite composite beads. The experimental plan along with designation of samples is summarized in Table 2. The molarity of NaOH solution for control sample (MS) was 6 M, while 8 M NaOH solution was used for sample MS-N8. To study the size effect on adsorption efficiency, 7 and 5 mm diameter beads were prepared. Moreover, some of the samples were foamed (MS-F4) with 10% concentrated H₂O₂ solution to study the effects of porosity enhancement which can potentially increase the diffusion of adsorbate solutions within the beads, possibly resulting in higher adsorption rate/capacity. Half of the beads from each sample type were hydrothermally treated for phase transformation from amorphous alimunisilicate gel to crystalline zeolitic phases (represented by letter “H” at the end of sample’s designation).

Table 2

Table 2. Details of experimental parameters.

2.2 Preparation and curing of geopolymer-zeolite beads

Customized molds were fabricated for casting bead samples. The schematic of the molds and prepared samples are shown in Figure 1. After addition of alkali activator, the mixtures were stirred for 5 min to get uniform slurry which was then injected into the molds using a syringe. H₂O₂ was added after initial stirring and slurry was stirred for another 3 min for the foamed samples. Molds were then put at 50 °C for 24 h initial curing. Following, beads were demolded and half of the beads from each sample type were shifted to 500 mL autoclave reactor for hydrothermal treatment at 150 °C for 48 h. The other half of the beads from each sample type were immersed in water for extended curing. The water was changed on daily basis so that the unreacted alkali can leach out as it can increase the pH of adsorbate solution during adsorption experiments (Lee et al., 2017). After hydrothermal treatment, those beads were also cured under same conditions. Once the pH of wash water stabilized around 8, samples were dried in vacuum oven for 24 h followed by characterization and adsorption experiments.

Figure 1

Figure 1. Schematic of the molds used for casting geopolymer-zeolite beads (a) and prepared bead samples (b).

2.3 Characterization of geopolymer-zeolite beads

Samples were characterized by means of XRD, FT-IR, nitrogen gas (N₂) physisorption (BET/BJH) and SEM analyses. For XRD and FT-IR, samples were grinded to pass a 64 μm sieve. The powder was immersed in IPA solution for 5 min to arrest the reaction, followed by vacuum filtration and drying at 50 °C overnight to get dried powder. The XRD data were recorded on EMPYREAN machine using Cu-Kα radiation at 40 kV and 30 mA and at a scanning rate of 0.2°/min from 5° to 65° in the 2 $\mathrm{\theta }$ mode. Considering the amorphous nature of geopolymers, rutile (TiO₂) was added in the powdered XRD samples (20 mass%) as an internal standard to precisely identify the crystalline phases. International Center for Diffraction Data (ICDD) PDF database was used for phase identification.

The FT-IR spectra were recorded on a device manufactured by Jasco, Japan (FT-IR 4100). Three spheres of each sample type were used for N₂ physisorption analysis, conducted using a Micromeritics TriStar II 3020 device. The surface area, pore size distribution (PSD), and pore volume were evaluated at −195.85 °C using nitrogen adsorption–desorption isotherms. The Brunauer–Emmett–Teller (BET) method was applied to calculate the specific surface area, whereas the cumulative pore volume, average pore diameter, and PSD were obtained through Barrett–Joyner–Halenda (BJH) adsorption analysis. For SEM images, spheres were cracked, and internal surfaces were analyzed using an ultra-high-resolution FE-SEM device (SU8230) manufactured by Hitachi High-Technologies Corp., Japan. Samples were coated with platinum to increase the conductance of the surfaces.

2.4 Adsorption tests

Adsorption experiments were conducted for Pb(II) and BG dye. About 0.4 g of adsorbent (two 7 mm or five 5 mm diameter spheres) were used for 500 mL adsorbate solution of 100 mg/L concentration for each batch. The exact weight of adsorbent for each batch was noted and used for calculation of adsorption capacity ($q_{\mathrm{e}}=\left(C_o-C_e\right)V/m$) (Lee et al., 2017). Samples were extracted after 120 h of adsorbent-adsorbate interaction in a shaking incubator.

Lead nitrate (Pb(NO₃)₂) compound was used to prepare 100 mg/L solution of Pb(II). For BG, a 1000 mg/L stock solution was prepared which was then diluted to 100 mg/L. The residual concentrations of Pb(II) were measured by ICP-MS, while spectrophotometer was used for BG measurement at λ = 625 nm (Rehman et al., 2013). The 10, 25, and 50 mg/L reference solutions of BG were also prepared to draw calibration curve to measure the residual concentrations of BG after adsorption experiments using spectrophotometer.

3 Results and discussion

3.1 Characterization of geopolymer-zeolite beads

3.1.1 X-ray diffraction (XRD)

XRD patterns of control, foamed, and hydrothermally treated samples are shown in Figure 2. All the untreated samples (MS, MS-F4, MS-N8) showed the common phases of geopolymers, i.e., quartz (PDF #01-086-1629, #01-089-8936), mullite (PDF #01-074-4145, #01-079-1450), and a typical diffused hump representing aluminum substituted C-A-S-H gel (Kim et al., 2017; Park et al., 2018), in addition to the peaks of the internal standard. It is well known that the quartz and mullite are the inherited phases of fly ash which mostly stay inert under normal curing conditions, therefore, these phases are often detected in XRD of geopolymers (Ma et al., 2016).

Figure 2

Figure 2. XRD patterns of samples. The annotations indicate: Q, Quartz; M, Mullite; P, Zeolite Na-P1; C, CSH type gel; R, Rutile.

The hydrothermal treatment of samples resulted in formation of zeolite Na-P1, confirmed through relevant XRD peaks (PDF #01–074-1787, #01-071-0962). Additionally, quartz peaks were significantly reduced in these samples showing its involvement in the reaction. This suggests that the reactivity of geopolymers can be enhanced under high temperature and pressure conditions. This enhanced reactivity under hydrothermal conditions is particularly significant as it demonstrates the potential for accelerated zeolite formation, a property that could be exploited in radioactive waste treatment where rapid immobilization of contaminants is often critical. The dissolution and recrystallization of quartz under these conditions also suggests that even relatively inert phases can be mobilized to contribute to the formation of more reactive zeolitic structures.

Lastly, it should be noted that no significant differences were observed in the XRD spectra of un-foamed, foamed, and different NaOH molarity samples. All the samples showed same crystalline phases. This could be attributed to the fact that the alkali activation of fly ash under hydrothermal conditions mainly results in the formation of zeolite Na-P1, irrespective of the minor changes in the mixture composition as has been reported in the literature (Aldahri et al., 2016; Zheng et al., 2019). Moreover, H₂O₂ foaming agents used in this study only interacted with the liquid matrix, decomposing to H₂O and O₂ (Ducman and Korat, 2016), hence only the physical changes were expected as observed in SEM images shown in the next section.

3.1.2 Scanning electron microscopy (SEM)

SEM images were taken to study the morphology of synthesized zeolite Na-P1 crystals as well as the formation of pores in foamed samples. SEM images of samples before and after hydrothermal treatment are shown in Figure 3. It can be clearly seen that the control samples mainly had amorphous gel phases. In addition, different-sized pores were easily seen in foamed samples due to the foaming action. Alternatively, zeolite Na-P1 crystals were seen in all the hydrothermally-treated samples. Similar morphology of zeolite Na-P1 crystals were also reported by Cardoso et al. (2015b), Cardoso et al. (2015a).

Figure 3

Figure 3. SEM micrographs of geopolymer-zeolite spheres.

Another observation made from SEM is that the Na-P1 crystals in MS-N8-H samples were not fully developed yet. This might be attributed to the relatively high molarity of NaOH solution used in these samples. High molarity alkali solutions improve geopolymer strength by forming denser gel matrices (Mustafa Al Bakri et al., 2012; Palanisamy and Kumar, 2018), which inhibit the diffusion and reorganization required for complete zeolite crystallization. Hence, prolonged hydrothermal treatment might be required to fully develop Na-P1 crystals in these samples. This reveals an important trade-off: higher NaOH molarity provides more Na⁺ for ion exchange but simultaneously retards zeolite formation. Similar observations were made in previous studies (Lee et al., 2016; Khalid et al., 2018). Understanding this balance is crucial when optimizing materials for specific applications.

3.1.3 Nitrogen physisorption (BET/BJH)

The pores characteristics (i.e., BET surface area, BJH pore volume and average pore diameter, and pore size distribution (PSD) of samples are summarized in Table 3; Figure 4. The control samples (not treated under hydrothermal conditions) showed lower surface area and pore volume compared to the hydrothermally-treated samples, e.g., the surface area and pore volume of sample MS-H were 101.1 m²/g and 0.16 cm³/g compared to only 25.5 m²/g and 0.08 cm³/g, respectively, for respective control sample MS. This confirms that the formation of zeolite crystals results in substantial increase of surface area which is normally desired for adsorbents (Abazari et al., 2019). Moreover, it can be noted that the samples with foaming agent (MS-F4) and high molarity NaOH solution (MS-N8) had less surface area and pore volume compared to the control samples MS. This can be because of foaming agent mainly give rise to the formation of macropores which are out of the detection range of nitrogen sorption method (>100 nm). The lower surface area and pore volume of high molarity samples can be related to the discussion in the preceding section i.e., high molarity of NaOH is known to form denser matrix (Mustafa Al Bakri et al., 2012; Palanisamy and Kumar, 2018) that potentially resulted in decrease of surface area and pore volume.

Table 3

Table 3. Results of N₂ physisorption (BET/BJH).

Figure 4

Figure 4. Pore size distribution (PSD) of geopolymer-zeolite beads as measured by the BJH method.

It was further noted from Figure 4 that the PSD curves of hydrothermally-treated samples shifted a little to the left side, i.e., towards the smaller pore sizes, thus the average pore size of these samples was less than the control samples (Table 3). It can be stated that the formation of zeolitic crystals under hydrothermal conditions caused the pore refinement, resulting in smaller connected pores; consequently, giving rise to the surface area and pore volume contrary to the foaming action. This pore refinement phenomenon has significant implications: while it increases surface area that is typically considered beneficial for adsorption, it simultaneously restricts the diffusion pathways for adsorbate species, particularly in bulk materials. This creates a nuanced relationship between pore structure and adsorption performance that is explored in detail in Section 3.2. The pore refinement represents restructuring at the mesopore scale (2–50 nm), where zeolite formation creates new channels while blocking original gel pores. This explains how materials can simultaneously exhibit higher surface area but reduced accessibility.

The adsorption-desorption isotherms of samples are shown in Figure 5. The isotherms were of type IV with hysteresis loops somewhat similar to H4 type (Sing et al., 1982). This confirms the existence of mesoporous networks, characteristic of materials with both microporous zeolite crystals and mesoporous geopolymer gel. This hierarchical pore structure is particularly advantageous for applications requiring both high selectivity (from micropores) and efficient mass transport (from mesopores), such as in radioactive waste streams containing multiple contaminant species of different sizes.

Figure 5

Figure 5. Adsorption-desorption isotherm of geopolymer-zeolite beads.

3.1.4 Fourier-transform infrared spectroscopy (FT-IR)

Infrared spectra of samples are shown in Figure 6. The bands with peaks around 668, 876, 980–995, 1475, and 1635 cm⁻¹ were observed. The small peak at 668 cm⁻¹ is due to the symmetrical stretching of Si-O-T (T: Si or Al) bonds (Bernal et al., 2010). The peak around 877 cm⁻¹ is representing asymmetric stretching of AlO₄⁻ groups (Bernal et al., 2010). The broad band around 1000 cm^-1 is the characteristics band of geopolymers and zeolites due to asymmetric stretching of Si-O-T in SiO₄ tetrahedra (Lecomte et al., 2006; Visa and Popa, 2015). The band around 1475 cm^{− 1} and shoulder around 870 cm⁻¹ represent anti-symmetric stretching and out-of-plane bending vibrations of O-C-O bonds in the carbonate group (CO₃^2-) (Lecomte et al., 2006; Siyal et al., 2016). The band at 1635 cm⁻¹ represents the bending vibration of O-H bonds in molecular water bound in the hydrated products (Siyal et al., 2016).

Figure 6

Figure 6. FT-IR spectra of geopolymer-zeolite beads.

3.2 Adsorption efficiency

3.2.1 Adsorption capacity

The adsorption capacities of geopolymer-zeolite beads for Pb(II) and BG dye are summarized in Table 4. The maximum adsorption capacities of about 130 and 115 mg/g were noted for Pb(II) and BG, respectively. These values represent a significant advancement over most bulk-type geopolymeric adsorbents reported in the literature, demonstrating that careful control of synthesis parameters can yield materials with performance approaching that of pulverized adsorbents while maintaining the practical advantages of bulk forms.

Table 4

Table 4. Adsorption capacities of geopolymer-zeolite beads.

Among the samples studied, it was found that the hydrothermally-treated samples exhibited relatively less adsorption capacities for both the contaminates, compared to their respective control samples. Both the geopolymers and zeolites have shown high adsorption potential in the pulverized form, owing to the direct interaction with adsorbate ions (He et al., 2021; Wang C. et al., 2021). In the case of bulk-type samples, low permeability hinders the transportation of adsorbate species within the bulk matrix. The conversion of some of the amorphous gel in geopolymers to crystalline zeolites results in increased surface area and pore volume, which should help in the diffusion of adsorbate species within the matrix of these bulk-type samples. Thus, the adsorption capacity was expected to increase. However, the current results were found to be the opposite. This counter-intuitive finding represents a key novelty of this study and challenges the conventional wisdom that higher surface area automatically translates to better adsorption performance in bulk adsorbents. This highlights that some other parameters also play a critical role in the adsorption efficiency of bulk-type samples.

In our previous study (Khalid et al., 2018), the average pore diameter was found to play a key role in the adsorption capacity of these bulk-type adsorbents, regardless of the presence of zeolites and/or higher surface area. Therefore, adsorption capacities were plotted against pore diameter, pore volume, and surface area in Figure 7. The comparison gave conclusive results in the case of Pb(II) with relatively high R-square values. The adsorption capacity was directly related to the average pore diameter, while it decreased with the pore volume and BET surface area. The same trend was followed by BG, despite a lower R-square value. This significant finding establishes a clear hierarchy in the importance of pore characteristics for bulk-type adsorbents: pore diameter > pore volume > surface area. This hierarchy can be explained by mass transport limitations, i.e., larger pores facilitate easier diffusion of adsorbate species deep into the material, allowing more of the internal surface area to be accessed during the experimental timeframe. The dominance of pore diameter can be further explained by tortuosity effects: smaller pores increase diffusion resistance, preventing adsorbate species from reaching interior sites within the 120 h experimental timeframe. This is particularly significant for larger molecules like BG dye, which experience steric hindrance in narrow pores. This insight has profound implications for rational material design, suggesting that for bulk applications, strategies to maintain or increase pore diameter should be prioritized over those that simply maximize surface area.

Figure 7

Figure 7. Adsorption capacity against average pore diameter, pore volume, and BET surface area.

It was observed that the 5 mm bead samples either showed comparable (MS5-F4-H) or better (MS5-H) adsorption efficiency compared to their 7 mm counterpart samples. The smaller beads tend to offer more exposed areas, potentially resulting in enhanced interaction with the adsorbate solution. This can potentially result in better adsorption efficiency. The improved performance of smaller beads also reflects shorter diffusion pathways, reducing the time required for adsorbate species to reach interior sites. This effect becomes particularly important when designing column reactors or packed beds for continuous treatment systems.

Moreover, the use of foaming agent generally resulted in marginal increase of adsorption capacity compared to their unformed counterparts (e.g., MS and MS-F4) in the case of Pb(II) while opposite results were observed for BG. The differential response of Pb²⁺ and BG to foaming likely reflects the different nature of these adsorbates, i.e., Pb²⁺ as a small cation benefits from any increase inaccessibility, while BG as a larger organic molecule may require specific pore sizes for optimal adsorption. This suggests that pore engineering strategies need to be tailored to the target contaminant’s molecular characteristics.

Lastly, the MS-N8 sample showed the highest adsorption capacity. It can be inferred that the presence of more Na⁺ species in this sample potentially resulted in more cation exchange, thus a higher adsorption capacity was seen. This finding is particularly relevant for radioactive waste applications, as many radionuclides (such as cesium (Cs⁺) and strontium (Sr²⁺)) are cationic species that would be captured through similar ion-exchange mechanisms. The ability to enhance cation exchange capacity by simply adjusting NaOH molarity provides a straightforward approach to tailoring materials for specific radionuclide capture.

It should be noted that adsorption is a complex process. In geopolymeric or geopolymer-zeolite composites, adsorption can occur through physical capture, ion-exchange with charge balancing cations (mostly sodium), and/or chemical bonding with non-bridging Si–${\mathrm{O}}^{-}$ or Al–${\mathrm{O}}^{-}$ (El-Eswed et al., 2017; Luukkonen et al., 2019). FT-IR spectra of Pb(II) and BG-adsorbed samples were taken to understand the adsorption mechanism and are plotted along with the spectra of control samples in Figure 8. A change in transmittance was observed in the Si-O-T band around 1000 cm^-1 after adsorption of Pb(II) and BG. This confirms the involvement of SiO₄ tetrahedra in the adsorption process by ion-exchange and/or chemical bonding. Another major change was observed in AlO₄⁻ group band around 876 cm⁻¹. The Si–${\mathrm{O}}^{-}$ or Al–${\mathrm{O}}^{-}$ in geopolymers/zeolites are known to interact with the adsorbate ions (El-Eswed et al., 2017; Luukkonen et al., 2019). Similar behavior was confirmed through the FT-IR spectra of Pb(II) and BG-adsorbed samples. This was also confirmed by the higher adsorption capacity of MS-N8 compared to MS, owing to higher content of exchangeable Na + cations in this sample. The spectroscopic evidence of chemical interaction between the adsorbates and the aluminosilicate framework suggests that these materials offer not just physical capture but also strong chemical binding, which is essential for long-term immobilization. This dual mechanism, combining ion exchange with chemical bonding, provides multiple barriers against contaminant release, enhancing the safety and reliability of waste forms.

Figure 8

Figure 8. Comparison of FT-IR spectra of control and pollutant-adsorbed sample: (a) MS, (b) MS-H, (c) MS-F4, (d) MS-F4-H, (e) MS-N8, and (f) MS-N8-H.

3.2.2 Comparison with literature

A comparison of the adsorption capacities observed in this work is made with the values reported in the literature in Table 5. Only the studies reporting bulk-type geopolymeric adsorbents (i.e., solid blocks, not pulverized/grinded) are covered. It is evident that the adsorption capacity of studied adsorbents was much higher than the values reported in the literature. The superior performance achieved in this study validates the effectiveness of the synthesis approach and parameter optimization undertaken. This advancement is attributed to the combination of optimized NaOH molarity, controlled bead size, and strategic balance between zeolite formation and pore structure preservation. The potential reason for such behavior could be their mesoporous characteristic and high molarity of NaOH solution used during the synthesis.

Table 5

Table 5. Comparison of adsorption capacitates of bulk-type geopolymer/zeolite adsorbents.

3.3 Implications for radioactive waste treatment

While this study focused on Pb(II) and BG dye, the findings have significant implications for radioactive waste treatment. The structural similarities between heavy metals and radionuclides suggest that this adsorption mechanisms would effectively capture radioactive cations like Cs⁺ and Sr²⁺.

The hybrid geopolymer/zeolite composites offer specific advantages for this application. The ion-exchange capacity demonstrated here directly relates to radionuclide capture, and the author’s previous work on cesium removal using similar mesoporous geopolymer-zeolite composites (Lee et al., 2017) validates this approach. The ability to enhance cation exchange through NaOH molarity adjustment provides a straightforward optimization pathway. Additionally, the bulk bead format enables direct deployment in column reactors, avoiding the difficulty handling of powdered adsorbents.

The finding that pore diameter dominates over surface area is particularly relevant for radioactive applications, as radionuclides exist as hydrated species requiring specific pore sizes for optimal access. Future work should focus on radiation stability testing, competitive adsorption studies with mixed waste streams, and pilot-scale demonstrations to advance these materials toward practical deployment in radioactive waste treatment facilities.

4 Conclusion

The geopolymer/zeolite composite spheres were prepared in this work. Hydrothermal treatment, foaming agent, molarity of NaOH solution in the alkali activator were the parameters studied. Microstructural characterization of the synthesized samples was done through XRD, SEM, N₂ physisorption, and FT-IR. Finally, the adsorption potential of the samples was studied for Pb(II) and BG dye. The following conclusions can be drawn from the results.

1. Amorphous geopolymeric gel underwent a partial transformation into zeolitic Na-P1 crystals after hydrothermal treatment. XRD and SEM confirmed this phase transition. Nitrogen physisorption analysis (BET/BJH) showed that the development of zeolite crystals greatly increased the surface area and pore volume of the beads.

2. Despite the higher surface area and pore volume, hydrothermally-treated samples showed a reduced ability to adsorb Pb(II) and BG. This counter-intuitive finding reveals that for bulk-type adsorbents, average pore diameter is a more critical parameter than surface area for determining adsorption capacity. This was explained by the smaller pore diameters of these samples which probably made it more difficult for adsorbate species to diffuse into bulk material.

3. Due to their higher surface area-to-volume ratio, which improved the interaction between the adsorbent and adsorbate species, smaller beads (5 mm) performed better in terms of adsorption than bigger beads (7 mm).

4. Although the pore structure was enhanced by the addition of a foaming agent, the adsorption capacity was not consistently increased. This suggests that the distribution of pore sizes, rather than the overall porosity, is a crucial factor in adsorption effectiveness, and that pore engineering strategies must be tailored to the specific molecular characteristics of target contaminants.

5. The best adsorption capabilities for Pb(II) and BG were found in beads synthesized with a greater NaOH molarity (8 M). This is probably because the beads included more Na⁺ ions, which promoted ion exchange during adsorption. This finding has direct implications for designing materials for radioactive cesium and strontium capture, where high cation exchange capacity is essential.

6. The present study’s adsorption capacities (122.8 mg/g for Pb²⁺ and 114.9 mg/g for BG) substantially exceeded those documented in earlier research on bulk-type geopolymeric adsorbents, representing significant advancement in the field. The utilization of high-molarity NaOH solutions and the mesoporous structure, confirmed by the type IV isotherms having H4 type hysteresis loops, of the produced beads were responsible for the enhanced performance.

7. FT-IR analysis revealed that adsorption occurs through multiple mechanisms including ion exchange and chemical bonding with Si–O^- and Al–O^- groups, providing robust contaminant retention. This dual mechanism is particularly advantageous for long-term immobilization applications such as radioactive waste treatment.

This study demonstrates the potential of hybrid geopolymer/zeolite composites as efficient adsorbents for dyes and heavy metal ions in water treatment applications. The key finding reveals that pore diameter, rather than surface area, is the dominant parameter controlling adsorption in bulk-type materials, establishing that optimizing pore structure is essential for improving adsorption efficiency. These findings provide a foundation for developing effective adsorbent materials for environmental cleanup, with potential applications in radioactive waste treatment where mechanical integrity, chemical stability, and high adsorption capacity are required. However, the regeneration capacity and reusability of these materials were not evaluated in this study. Future work should focus on regeneration performance through multiple adsorption-desorption cycles, optimizing the synthesis procedure, scaling up for industrial applications, and conducting radiation stability testing and competitive adsorption studies with mixed ionic solutions to advance these materials toward practical deployment.

Data availability statement

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

Author contributions

HK: Funding acquisition, Validation, Methodology, Investigation, Conceptualization, Writing – review and editing, Formal Analysis, Data curation, Writing – original draft.

Funding

The author(s) declared that financial support was received for this work and/or its publication. The author would like to acknowledge the support provided by the Interdisciplinary Research Center for Construction and Building Materials (IRC-CBM) at King Fahd University of Petroleum and Minerals (KFUPM), Saudi Arabia, for funding this work through project No. INCB2417.

Conflict of interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Generative AI statement

The author(s) declared that generative AI was used in the creation of this manuscript. Generative AI was used only to proofread the manuscript and improve the writing.

Any alternative text (alt text) provided alongside figures in this article has been generated by Frontiers with the support of artificial intelligence and reasonable efforts have been made to ensure accuracy, including review by the authors wherever possible. If you identify any issues, please contact us.

Publisher’s note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

References

Abazari, R., Mahjoub, A. R., Shariati, J., and Noruzi, S. (2019). Photocatalytic wastewater purification under visible light irradiation using bismuth molybdate hollow microspheres with high surface area. J. Clean. Prod. 221, 582–586. doi:10.1016/j.jclepro.2019.03.008

CrossRef Full Text | Google Scholar

Ahmad, S., Khalid, H. R., Shaqraa, A. A., Bahraq, A. A., Maslehuddin, M., and Al-Dulaijan, S. U. (2023). Properties of natural pozzolan-based geopolymer concrete: effects of natural pozzolan content, type of alkaline activator, and silicate/alkali ratio. Eur. J. Environ. Civ. Eng. 27, 356–373. doi:10.1080/19648189.2022.2043941

CrossRef Full Text | Google Scholar

Aldahri, T., Behin, J., Kazemian, H., and Rohani, S. (2016). Synthesis of zeolite Na-P from coal fly ash by thermo-sonochemical treatment. Fuel 182, 494–501. doi:10.1016/j.fuel.2016.06.019

CrossRef Full Text | Google Scholar

Asim, N., Badiei, M., Shakeri, M., Emdadi, Z., Samsudin, N. A., Soltani, S., et al. (2022). Development of green photocatalytic geopolymers for dye removal. Mater Chem. Phys. 283, 126020. doi:10.1016/j.matchemphys.2022.126020

CrossRef Full Text | Google Scholar

Bernal, S. A., Mejía, R., Gutierrez, D., Provis, J. L., and Rose, V. (2010). Effect of silicate modulus and metakaolin incorporation on the carbonation of alkali silicate-activated slags. Cem. Concr. Res. 40, 898–907. doi:10.1016/j.cemconres.2010.02.003

CrossRef Full Text | Google Scholar

Bhuyan, M. A. H., and Luukkonen, T. (2024). Adsorption of methylene blue by composite foams containing alkali-activated blast furnace slag and lignin. Int. J. Environ. Sci. Technol. 21, 3789–3802. doi:10.1007/s13762-023-05245-5

CrossRef Full Text | Google Scholar

Brahmi, A., Ziani, S., AitAli, S., Benkhaoula, B. N., Yu, Y., Ahouari, H., et al. (2024). Porous metakaolin geopolymer as a reactive binder for hydroxyapatite adsorbent granules in dye removal. Hybrid. Adv. 5, 100134. doi:10.1016/j.hybadv.2023.100134

CrossRef Full Text | Google Scholar

Bumanis, G., Novais, R. M., Carvalheiras, J., Bajare, D., and Labrincha, J. A. (2019). Metals removal from aqueous solutions by tailored porous waste-based granulated alkali-activated materials. Appl. Clay Sci. 179, 105147. doi:10.1016/j.clay.2019.105147

CrossRef Full Text | Google Scholar

Cardoso, A. M., Horn, M. B., Ferret, L. S., Azevedo, C. M. N., and Al Pires, M. (2015a). Integrated synthesis of zeolites 4A and Na–P1 using coal fly ash for application in the formulation of detergents and swine wastewater treatment. J. Hazard Mater 287, 69–77. doi:10.1016/j.jhazmat.2015.01.042

PubMed Abstract | CrossRef Full Text | Google Scholar

Cardoso, A. M., Paprocki, A., Ferret, L. S., Azevedo, C. M. N., and Pires, M. (2015b). Synthesis of zeolite Na-P1 under mild conditions using Brazilian coal fly ash and its application in wastewater treatment. Fuel 139, 59–67. doi:10.1016/j.fuel.2014.08.016

CrossRef Full Text | Google Scholar

Davidovits, J. (2008). Geopolymer chemistry and applications.

Google Scholar

Davidovits, J. (2013). Geopolymer cement, a review. Saint-Quentin, France: Geopolymer Institute, 1–11.

Google Scholar

De Silva, P., and Sagoe-Crenstil, K. (2008). Medium-term phase stability of Na2O-Al2O3-SiO2-H2O geopolymer systems. Cem. Concr. Res. 38, 870–876. doi:10.1016/j.cemconres.2007.10.003

CrossRef Full Text | Google Scholar

Duan, P., Yan, C., Zhou, W., and Ren, D. (2016). Development of fly ash and iron ore tailing based porous geopolymer for removal of Cu(II) from wastewater. Ceram. Int. 42, 13507–13518. doi:10.1016/j.ceramint.2016.05.143

CrossRef Full Text | Google Scholar

Ducman, V., and Korat, L. (2016). Characterization of geopolymer fly-ash based foams obtained with the addition of Al powder or H2O2 as foaming agents. Mater Charact. 113, 207–213. doi:10.1016/j.matchar.2016.01.019

CrossRef Full Text | Google Scholar

El-Eswed, B. I., Aldagag, O. M., and Khalili, F. I. (2017). Efficiency and mechanism of stabilization/solidification of Pb(II), Cd(II), Cu(II), Th(IV) and U(VI) in metakaolin based geopolymers. Appl. Clay Sci. 140, 148–156. doi:10.1016/j.clay.2017.02.003

CrossRef Full Text | Google Scholar

Elhadi, K. M., Raza, A., Ghazouani, N., and Khan, D. (2025). Physical properties and material characterisation of fly ash-based geopolymer composites having nano silica and micro glass. Eur. J. Environ. Civ. Eng. 0, 1–22. doi:10.1080/19648189.2025.2474577

CrossRef Full Text | Google Scholar

Ettahiri, Y., Bouna, L., Hanna, J. V., Benlhachemi, A., Pilsworth, H. L., Bouddouch, A., et al. (2023). Pyrophyllite clay-derived porous geopolymers for removal of methylene blue from aqueous solutions. Mater Chem. Phys. 296, 127281. doi:10.1016/j.matchemphys.2022.127281

CrossRef Full Text | Google Scholar

Gonçalves, N. P. F., Olhero, S. M., Labrincha, J. A., and Novais, R. M. (2023). 3D-printed red mud/metakaolin-based geopolymers as water pollutant sorbents of methylene blue. J. Clean. Prod. 383, 135315. doi:10.1016/j.jclepro.2022.135315

CrossRef Full Text | Google Scholar

He, P., Zhang, Y., Zhang, X., and Chen, H. (2021). Diverse zeolites derived from a circulating fluidized bed fly ash based geopolymer for the adsorption of lead ions from wastewater. J. Clean. Prod. 312, 127769. doi:10.1016/j.jclepro.2021.127769

CrossRef Full Text | Google Scholar

Hertel, T., Novais, R. M., Murillo Alarcón, R., Labrincha, J. A., and Pontikes, Y. (2019). Use of modified bauxite residue-based porous inorganic polymer monoliths as adsorbents of methylene blue. J. Clean. Prod. 227, 877–889. doi:10.1016/j.jclepro.2019.04.084

CrossRef Full Text | Google Scholar

Khalid, H. R., Lee, N. K., Park, S. M., Abbas, N., and Lee, H. K. (2018). Synthesis of geopolymer-supported zeolites via robust one-step method and their adsorption potential. J. Hazard Mater 353, 522–533. doi:10.1016/j.jhazmat.2018.04.049

PubMed Abstract | CrossRef Full Text | Google Scholar

Khalid, H. R., Lee, N. K., Choudhry, I., Wang, Z., and Lee, H. K. (2019). Evolution of zeolite crystals in geopolymer-supported zeolites: effects of composition of starting materials. Mater Lett. 239, 33–36. doi:10.1016/j.matlet.2018.12.044

CrossRef Full Text | Google Scholar

Kim, H.-J., and Khalid, H. R. (2022). The effects of calcium aluminate cement substitution on physicochemical properties of geopolymer–zeolite composites. Arab. J. Sci. Eng. 47, 5073–5078. doi:10.1007/s13369-021-06394-w

CrossRef Full Text | Google Scholar

Kim, G. M., Khalid, H. R., Park, S. M., and Lee, H. K. (2017). Flow property of alkali-activated slag with modified precursor. ACI Mater J. 114, 867–876. doi:10.14359/51700794

CrossRef Full Text | Google Scholar

Lan, T., Li, P., Rehman, F. U., Li, X., Yang, W., and Guo, S. (2019). Efficient adsorption of Cd2+ from aqueous solution using metakaolin geopolymers. Environ. Sci. Pollut. Res. 26, 33555–33567. doi:10.1007/s11356-019-06362-w

PubMed Abstract | CrossRef Full Text | Google Scholar

Lecomte, I., Henrist, C., Liégeois, M., Maseri, F., Rulmont, A., and Cloots, R. (2006). (Micro)-structural comparison between geopolymers, alkali-activated slag cement and Portland cement. J. Eur. Ceram. Soc. 26, 3789–3797. doi:10.1016/j.jeurceramsoc.2005.12.021

CrossRef Full Text | Google Scholar

Lee, N. K., Khalid, H. R., and Lee, H. K. (2016). Synthesis of mesoporous geopolymers containing zeolite phases by a hydrothermal treatment. Microporous Mesoporous Mater. 229, 22–30. doi:10.1016/j.micromeso.2016.04.016

CrossRef Full Text | Google Scholar

Lee, N. K., Khalid, H. R., and Lee, H. K. (2017). Adsorption characteristics of cesium onto mesoporous geopolymers containing nano-crystalline zeolites. Microporous Mesoporous Mater. 242, 238–244. doi:10.1016/j.micromeso.2017.01.030

CrossRef Full Text | Google Scholar

Lin, H., Zhang, J., Wang, R., Zhang, W., and Ye, J. (2024). Adsorption properties and mechanisms of geopolymers and their composites in different water environments: a comprehensive review. J. Water Process Eng. 62, 105393. doi:10.1016/j.jwpe.2024.105393

CrossRef Full Text | Google Scholar

Liu, Y., Yan, C., Zhang, Z., Wang, H., Zhou, S., and Zhou, W. (2016). A comparative study on fly ash, geopolymer and faujasite block for Pb removal from aqueous solution. Fuel 185, 181–189. doi:10.1016/j.fuel.2016.07.116

CrossRef Full Text | Google Scholar

Luukkonen, T., Heponiemi, A., Runtti, H., Pesonen, J., Yliniemi, J., and Lassi, U. (2019). Application of alkali-activated materials for water and wastewater treatment: a review. Rev. Environ. Sci. Biotechnol. 18, 271–297. doi:10.1007/s11157-019-09494-0

CrossRef Full Text | Google Scholar

Ma, X., Zhang, Z., and Wang, A. (2016). The transition of fly ash-based geopolymer gels into ordered structures and the effect on the compressive strength. Constr. Build. Mater 104, 25–33. doi:10.1016/j.conbuildmat.2015.12.049

CrossRef Full Text | Google Scholar

Mustafa Al Bakri, A. M., Kamarudin, H., Bnhussain, M., Rafiza, A. R., and Zarina, Y. (2012). Effect of Na2SiO3/NaOH ratios and NaOH molarities on compressive strength of fly-ash-based geopolymer. ACI Mater J. 109, 503–508. doi:10.14359/51684080

CrossRef Full Text | Google Scholar

Novais, R. M., Buruberri, L. H., Seabra, M. P., and Labrincha, J. A. (2016). Novel porous fly-ash containing geopolymer monoliths for lead adsorption from wastewaters. J. Hazard Mater 318, 631–640. doi:10.1016/j.jhazmat.2016.07.059

PubMed Abstract | CrossRef Full Text | Google Scholar

Novais, R. M., Ascensão, G., Tobaldi, D. M., Seabra, M. P., and Labrincha, J. A. (2018). Biomass fly ash geopolymer monoliths for effective methylene blue removal from wastewaters. J. Clean. Prod. 171, 783–794. doi:10.1016/j.jclepro.2017.10.078

CrossRef Full Text | Google Scholar

Novais, R. M., Carvalheiras, J., Tobaldi, D. M., Seabra, M. P., Pullar, R. C., and Labrincha, J. A. (2019). Synthesis of porous biomass fly ash-based geopolymer spheres for efficient removal of methylene blue from wastewaters. J. Clean. Prod. 207, 350–362. doi:10.1016/j.jclepro.2018.09.265

CrossRef Full Text | Google Scholar

Novais, R. M., Carvalheiras, J., Seabra, M. P., Pullar, R. C., and Labrincha, J. A. (2020). Highly efficient lead extraction from aqueous solutions using inorganic polymer foams derived from biomass fly ash and metakaolin. J. Environ. Manage 272, 111049. doi:10.1016/j.jenvman.2020.111049

PubMed Abstract | CrossRef Full Text | Google Scholar

Palanisamy, P., and Kumar, P. S. (2018). Effect of molarity in geo polymer earth brick reinforced with fibrous coir wastes using sandy soil and quarry dust as fine aggregate. (Case study). Case Stud. Constr. Mater. 8, 347–358. doi:10.1016/j.cscm.2018.01.009

CrossRef Full Text | Google Scholar

Park, S. M., Khalid, H. R., Seo, J. H., Yoon, H. N., Son, H. M., Kim, S. H., et al. (2018). Pressure-induced geopolymerization in alkali-activated fly ash. Sustainability 10, 3538. doi:10.3390/su10103538

CrossRef Full Text | Google Scholar

Proust, V., Leybros, A., Gossard, A., David, T., Mao, Z., Li, Y., et al. (2024). Influence of porous aluminosilicate grain size materials in experimental and modelling Cs+ adsorption kinetics and wastewater column process. J. Water Process Eng. 66, 106066. doi:10.1016/j.jwpe.2024.106066

CrossRef Full Text | Google Scholar

Rehman, M. S. U., Munir, M., Ashfaq, M., Rashid, N., Nazar, M. F., Danish, M., et al. (2013). Adsorption of brilliant green dye from aqueous solution onto red clay. Chem. Eng. J. 228, 54–62. doi:10.1016/j.cej.2013.04.094

CrossRef Full Text | Google Scholar

Ren, Z., Wang, L., Li, Y., Zha, J., Tian, G., Wang, F., et al. (2022). Synthesis of zeolites by in-situ conversion of geopolymers and their performance of heavy metal ion removal in wastewater:a review. J. Clean. Prod. 349, 131441. doi:10.1016/j.jclepro.2022.131441

CrossRef Full Text | Google Scholar

Rożek, P., Król, M., and Mozgawa, W. (2019). Geopolymer-zeolite composites: a review. J. Clean. Prod. 230, 557–579. doi:10.1016/j.jclepro.2019.05.152

CrossRef Full Text | Google Scholar

Sing, K. S. W., Everett, D. H., Haul, R. A. W., Moscou, L., Pierotti, R. A., Rouquerol, J., et al. (1982). Reporting physisorption data for gas/solid systems — with special reference to the determination of surface area and porosity. Pure Appl. Chem. 57, 603–619. doi:10.1351/pac198254112201

CrossRef Full Text | Google Scholar

Sithole, T., Orero, B., Ntuli, F., and Okonta, F. (2024). Modification of geo-filters (basic oxygen furnace slag - NaOH/Na2SiO3/FA) for the removal of Fe3+ and SO42− ions from acidic mine effluent in fixed bed column: artificial intelligence modeling. J. Water Process Eng. 61, 105305. doi:10.1016/j.jwpe.2024.105305

CrossRef Full Text | Google Scholar

Siyal, A. A., Azizli, K. A., Man, Z., Ismail, L., and Khan, M. I. (2016). Geopolymerization kinetics of fly ash based geopolymers using JMAK model. Ceram. Int. 42, 15575–15584. doi:10.1016/J.CERAMINT.2016.07.006

CrossRef Full Text | Google Scholar

Ślosarczyk, A., Fořt, J., Klapiszewska, I., Thomas, M., Klapiszewski, Ł., and Černý, R. (2023). A literature review of the latest trends and perspectives regarding alkali-activated materials in terms of sustainable development. J. Mater. Res. Technol. 25, 5394–5425. doi:10.1016/j.jmrt.2023.07.038

CrossRef Full Text | Google Scholar

Tang, Q., Ge, Y. Y., Wang, K. T., He, Y., and Cui, X. M. (2015). Preparation and characterization of porous metakaolin-based inorganic polymer spheres as an adsorbent. Mater Des. 88, 1244–1249. doi:10.1016/j.matdes.2015.09.126

CrossRef Full Text | Google Scholar

Van Deventer, J. S. J., Provis, J. L., Duxson, P., and Lukey, G. C. (2007). Reaction mechanisms in the geopolymeric conversion of inorganic waste to useful products. J. Hazard Mater 139, 506–513. doi:10.1016/j.jhazmat.2006.02.044

PubMed Abstract | CrossRef Full Text | Google Scholar

Visa, M., and Popa, N. (2015). Adsorption of heavy metals cations onto zeolite material from aqueous solution. J. Membr. Sci. Technol. 5, 133. doi:10.4172/2155-9589.1000133

CrossRef Full Text | Google Scholar

Wang, C., Yang, Z., Song, W., Zhong, Y., Sun, M., Gan, T., et al. (2021a). Quantifying gel properties of industrial waste-based geopolymers and their application in Pb2+ and Cu2+ removal. J. Clean. Prod. 315, 128203. doi:10.1016/j.jclepro.2021.128203

CrossRef Full Text | Google Scholar

Wang, Z., Khalid, H. R., Park, S. M., Bae, S. J., and Lee, H. K. (2021b). MgO-induced phase variation in alkali-activated binders synthesized under hydrothermal conditions. Mater. Structures/Materiaux Constr. 54, 111. doi:10.1617/s11527-021-01689-8

CrossRef Full Text | Google Scholar

Xu, J., Li, M., Zhao, D., Zhong, G., Sun, Y., Hu, X., et al. (2022). Research and application progress of geopolymers in adsorption: a review. Nanomaterials 12, 3002. doi:10.3390/nano12173002

PubMed Abstract | CrossRef Full Text | Google Scholar

Yan, S., He, P., Jia, D., Wang, Q., Liu, J., Yang, J., et al. (2018). Synthesis of novel low-cost porous gangue microsphere/geopolymer composites and their adsorption properties for dyes. Int. J. Appl. Ceram. Technol. 15, 1602–1614. doi:10.1111/ijac.13045

CrossRef Full Text | Google Scholar

Yan, C., Guo, L., Ren, D., and Duan, P. (2019). Novel composites based on geopolymer for removal of Pb(II). Mater Lett. 239, 192–195. doi:10.1016/j.matlet.2018.12.105

CrossRef Full Text | Google Scholar

Yan, S., Feng, X., Huang, K., Ren, X., Zhao, Y., and Xing, P. (2023). Synthesis and adsorption performance of fly ash/steel slag-based geopolymer for removal of Cu (II) and methylene blue. Int. J. Appl. Ceram. Technol. 20, 1591–1605. doi:10.1111/ijac.14341

CrossRef Full Text | Google Scholar

Zheng, Z., Ma, X., Zhang, Z., and Li, Y. (2019). In-situ transition of amorphous gels to Na-P1 zeolite in geopolymer: mechanical and adsorption properties. Constr. Build. Mater 202, 851–860. doi:10.1016/j.conbuildmat.2019.01.067

CrossRef Full Text | Google Scholar