Adsorption/Desorption Performances of Simulated Radioactive Nuclide Cs⁺ on the Zeolite-Rich Geopolymer from the Hydrothermal Synthesis of Fly Ash

Abstract

The operation of nuclear power plants generates a large amount of low- and intermediate-level radioactive waste liquid. Zeolite-rich geopolymers, which are synthesized under hydrothermal conditions from industrial waste fly ash, can effectively immobilize radioactive nuclides. In this study, the synthesis law of zeolite-rich geopolymers and the adsorption/desorption performances of radioactive nuclide Cs⁺ were researched using XRD, SEM and ICP. The results show that the increase in curing temperatures and NaOH concentrations leads to the transformation of Y-type zeolite to chabazite and cancrinite at low NaNO₃ concentrations. However, at high NaNO₃ concentrations, NaOH above 2 M has no obvious effect on the phase transformation of the main zeolite of chabazite and cancrinite. In the adsorption and desorption experiment of Cs⁺ on the chabazite/garronite-rich geopolymer, it was found that the adsorption of Cs⁺ in the low initial concentration range is more suitable for the Freundlich equation, while the Langmuir equation fits in the adsorption process at the high initial concentration range. Moreover, the desorption kinetics of Cs⁺ are in good agreement with the pseudo-second-order rate equation. Thus, the adsorption of Cs⁺ on chabazite/garronite-rich geopolymers is controlled by both physical and chemical reactions, while desorption is a chemical process.

1. Introduction

With the rapid growth of global energy demands, nuclear energy is considered a clean and reliable energy source in the world [1]. Subsequently, a large amount of radioactive waste liquid will be generated. ¹³⁷Cs is one of the most important fission products of low- and intermediate-level radioactive waste liquid (LILW) due to its short half-life (t_1/2 = 30.2 a) and high yield. Compared with ⁹⁰Sr, ¹³⁷Cs is an alkali metal element, and its high solubility in water gives it strong fluidity [2]. Geopolymers are often used to solidify radionuclide ions, which are usually prepared from fly ash activated by alkaline, sometimes called alkali-activated materials [3]. The radionuclide ions are solidified in the geopolymer in the form of physical adsorption or chemical bonding [4]. Geopolymers are superior to Portland cement in mechanical strength, acid/salt resistance, and thermal stability; they are conducive to the sustainable development of the construction industry and also have a great application prospect in the solidification of hazardous wastes [5]. Furthermore, a large amount of fly ash will be produced every year in the world [6], and geopolymers prepared from fly ash do not need to emit CO₂, which alleviates some of the greenhouse effect [7,8]. Thus, the utilization of fly ash is also of great significance for the disposal of LILW [9].

At present, there are many technologies to produce zeolite, such as precipitation, hydrothermal, microwave, and sol–gel synthesis [10,11]. On the basis of traditional alkali melting–hydrothermal synthesis of zeolites, the steps of the calcination activation pretreatment of fly ash, water-soluble desiliconization, alkali-soluble aluminosilicate, filtration, and other steps to synthesize various types of high-purity zeolite products have emerged. Many researchers [12,13,14] have investigated the effects of preparation methods on the synthesis of fly-ash-based geopolymers and discussed the effects of preparation conditions such as the alkalinity, liquid/solid ratio, and crystallization time on the synthesis products. He et al. [5] systematically studied the effect of hydrothermal treatment parameters on the phase composition and microstructure of hydrothermal products of the amorphous sodium-based geopolymer (NaGP), and they concluded that the best hydrothermal method for preparing NaGP was treated with a 1M NaOH solution at 160 °C for 6 h. The products of fly-ash-based geopolymers are various, with different raw materials and curing conditions. Wang et al. [15] and Wdowin et al. [16] found that the products of geopolymers include amorphous geopolymer gel and zeolite N (C)-A-S-H, NaA-X zeolite, NaP zeolite, NaP1 zeolite, cesium garnet, A zeolite, and so on.

Lei et al. [17] investigated the adsorption characteristics of zeolite microspheres in metakaolin/slag-based geopolymers for Cs⁺ and Sr²⁺, and the results showed that the adsorption kinetic process of Cs⁺ and Sr²⁺ on zeolite microspheres was in good agreement with the pseudo-second-order kinetic equation. Lee et al. [18] prepared fly ash/slag-based mesoporous geopolymers using the hydrothermal method, and the results showed that when the initial concentration of Cs⁺ was 100 mg/L, the maximum adsorption capacity of mesoporous geopolymers was 15.24 mg/g. Hui et al. [19] prepared 4A zeolite from fly ash and investigated the adsorption kinetics of different metal ions on fly-ash-based 4A zeolite using pseudo-first-order and pseudo-second-order kinetic equations; it was found that the pseudo-first-order model fits in Ni²⁺ well, and the pseudo-second-order model fits in Co²⁺ and Cr³⁺ well.

Large-volume geopolymer solidified matrices have been used in the treatment of LILW with high levels of alkali (NaOH) and salts (NaNO₃), while the high center temperature of the matrices has prompted the generation of many crystal products. However, the transformation law of the fly-ash-based geopolymer products formed under different curing temperatures and sodium salt concentrations is under researched, and the adsorption/desorption mechanism of the simulated nuclide Cs⁺ is not clear. This study aims to clarify the relationship between the nuclides and a single phase of fly-ash-based geopolymers. The influence of the curing temperature, NaOH concentrations, and NaNO₃ concentrations on the transformation of fly-ash-based geopolymer products in the system was explored. Moreover, the isothermal adsorption characteristics and desorption kinetics of Cs⁺ by the main zeolites were studied. The Langmuir and Freundlich isotherm adsorption equations were used for the fitting analysis, while the pseudo-first-order and pseudo-second-order kinetic models were used to fit the desorption test data.

2. Materials and Methods

2.1. Materials

Fly ash, mainly composed of SiO₂ and Al₂O₃, was used as the raw material for the synthesis of zeolite materials [20]. The oxide composition of fly ash (wt.%) was analyzed by an X-ray fluorescence spectrometer, and the results are listed in

Table 1. Oxide composition of fly ash.

Oxide Composition	SiO2	Al2O3	CaO	Fe2O3	K2O	MgO	Other
Content (%)	59.79	21.99	5.55	5.13	3.21	1.35	2.98

. The D₅₀ of the fly ash was 18.75 µm, determined by a laser particle analyzer (Mastersizer2000, Malvern, UK). The fly ash used in this study was obtained from the Jiangyou Power Station, China. The analytically pure reagents, such as CsCl, NaOH, NaNO_3, etc._, are all from Kelong Chemical Co., Ltd., Chengdu, China. The Cs⁺ solution used in the experiment refers to the CsCl solution, and its concentration refers to the concentration (mg/L) of the cesium element in the solution.

2.2. Sample Preparation Methods

A fly-ash-based geopolymer was prepared using the hydrothermal synthesis method. Fly ash was fixed at 5 g, and the water/fly ash ratio was 8. The mass concentrations of NaNO₃ solution (100~500 g/L), NaOH molar concentrations (0.66~8 M), and curing temperatures (60~150 °C) were varied to prepare the fly-ash-based geopolymer. The curing time was constant at 24 h. The specific design ratios are shown in

Table 2. Design ratios with different NaOH concentrations and different NaNO₃ concentrations.

60 °C Water Bath for 8 h; Curing at 90 °C for 24 h
Sample Number	NaNO3 (g/L)	NaNO3 (g)	W/F	NaOH (g)	Fly Ash (g)
100N-0.66H	100	4.15	8	1.06	5
100N-2H	100	4.15	8	3.23	5
100N-4H	100	4.15	8	6.45	5
100N-6H	100	4.15	8	9.68	5
100N-8H	100	4.15	8	12.90	5
300N-0.66H	300	13.55	8	1.06	5
300N-2H	300	13.55	8	3.23	5
300N-4H	300	13.55	8	6.45	5
300N-6H	300	13.55	8	9.68	5
300N-8H	300	13.55	8	12.90	5
500N-0.66H	500	24.90	8	1.06	5
500N-2H	500	24.90	8	3.23	5
500N-4H	500	24.90	8	6.45	5
500N-6H	500	24.90	8	9.68	5
500N-8H	500	24.90	8	12.90	5

Notes: N means NaNO₃ solution concentration, and H means NaOH molar concentration.

and

Table 3. The ratios of sample 100N-2H at different curing temperatures.

Stirring in Water Bath at 60 °C for 8 h, Curing at Different Temperatures for 24 h
Sample Number	NaNO3 (g/L)	NaNO3/g	W/F	NaOH/g	Fly Ash/g
100N-2H-60 °C	100	4.15	8	3.23	5
100N-2H-90 °C	100	4.15	8	3.23	5
100N-2H-120 °C	100	4.15	8	3.23	5
100N-2H-150 °C	100	4.15	8	3.23	5

.

2.3. Isothermal Adsorption Experiment

The initial concentration of the adsorbent is a key parameter that determines the final adsorption behavior of the adsorbent, and the concentration gradient between the adsorbent surface and the adsorbate solution controls the mass transfer rate [18]. Thus, a 50 mL CsCl solution with mass concentrations of 50, 100, 150, 200, 300, 400, 500, 600, and 800 mg/L (Cs⁺) was added with 0.2 g products after washing and drying in several centrifuge tubes and then shaken in a constant temperature shaker (300 r/min) for 6 h. The supernatant was filtered with a 0.45 um needle filter, and the mass concentration of the remaining Cs⁺ was detected to calculate the adsorption rate (Equation (1)) and adsorption capacity (Equation (2)) [21]. The adsorbed product was dried for further tests.

$$R=\frac{C_0-C_e}{C_0}\times 100$$

(1)

$$Q_e=\frac{\left(C_0-C_e\right)}{m}\times V$$

(2)

where R is the adsorption rate, %; C₀ is the initial mass concentration of Cs⁺, mg/L; C_e is the residual mass concentration of Cs⁺, mg/L; Q_e is the equilibrium adsorption capacity, mg/g; m is the amount of the products added, g; V is the volume of Cs⁺ solution, L.

2.4. Desorption Kinetics Test

A total of 0.2 g of the test products was added in multiple centrifuge tubes, then 50 mL of the Cs⁺ solution with a mass concentration of 150 mg/L was added. The tubes were placed in a constant temperature oscillation box (300 r/min) to conduct an oscillation adsorption for 6 h, and finally, they were moved to a centrifuge. The remaining solid samples in the centrifuge tube were washed with deionized water to remove non-specifically adsorbed metal ions on the products, and then they were placed in a new centrifuge tube. A total of 50 mL of a hydrochloric acid solution (pH = 1) was added in the centrifugal tube as the eluate, and the tubes were put into a constant temperature oscillation box (300 r/min). At different times (5, 10, 15, 30, 45, and 60 min), the centrifuge tube was taken out, and the supernatant was filtered with a 0.45 μm needle filter. The remaining Cs⁺ concentration was detected to calculate the desorption capacity Q_j (Equation (3)) and desorption rate R_j (Equation (4)), and the results were used to fit the subsequent desorption kinetic curve.

$$Q_j=C_{je}{V}_j/m$$

(3)

$$R_j=\frac{Q_j}{Q_e}\times 100$$

(4)

where R_j is the desorption rate, %; Q_e is the equilibrium adsorption capacity in 150 mg/L Cs⁺ solution, mg/g; m is the additional amount of the products, g; Q_j is the desorption capacity of Cs⁺ from the products, mg/g; C_je is the desorption equilibrium mass concentration of Cs⁺, mg/L; V_j is the volume of desorption solution, L.

2.5. Characterization

The composition of oxides in fly ash was determined by X-ray Fluorescence (XRF) (Axios, PANalytical, Almelo, The Netherlands). The products of fly-ash-based geopolymers were detected by X-ray Diffractometry (XRD) (Smartlab, Rigaku, Tokyo, Japan) from 5º to 80º of 2θ with a rate of 20°/min using Cu K radiation. The morphology of fly-ash-based geopolymers was analyzed by Scanning Electron Microscope (SEM) (TM4000, Hitachi, Tokyo, Japan), and the surfaces of the fly-ash-based geopolymers were sprayed with a gold conductive layer and pasted onto conductive double-sided tape. The Fourier Transform Infrared Spectrometer (FTIR) (IS5, Thermo Fisher Scientific, Waltham, MA, USA) was used to detect the evolution of the H–O, Si–O–T (T = Si and Al), N–O, and C–O bond, and the fly-ash-based geopolymers were soaked in anhydrous ethanol and then dried. Subsequently, the dried fly-ash-based zeolite was mixed with KBr crystal and ground into a powder, which was then compressed in a mold (12–14 MPa) for 15 s to form an FTIR sample. The concentration of Cs⁺ was detected by an Atomic Absorption Spectrometer (AAS) (AA700, PerkinElmer, Waltham, MA, USA).

3. Results and Discussion

3.1. Products Analysis

3.1.1. XRD

The XRD diffraction patterns of the products of the fly-ash-based geopolymers synthesized based on Table 2 and Table 3 are shown in Figure 1 and Figure 2. It can be seen from Figure 1a that in the system with the NaNO₃ concentration of 100 g/L and NaOH concentration of 2 M, the diffraction peaks of the Y-type zeolite (PDF # 89-1629) and chabazite (PDF # 44-0248) appear, while the diffraction peak intensity of chabazite is rather low. At the same time, the diffraction peaks of faujasite (PDF # 28-1036) are observed. With the increase in NaOH concentrations (4~8 M), the diffraction peaks of faujasite increase, and the diffraction peaks of cancrinite (PDF # 46-1332) also occur, while the characteristic peaks of cancrinite become stronger and stronger. It can be seen from Figure 1b that in the system with a NaNO₃ concentration of 300 g/L and a low NaOH concentration (0.66 M), a small number of diffraction peaks of chabazite emerge, but the intensity of the diffraction peaks is low. As the NaOH concentrations varied from 2 M to 8 M, the diffraction peaks of SiO₂ (PDF # 85-0796) show a decreasing trend. The crystal phase types in the fly-ash-based geopolymer products increase obviously, and the diffraction peaks of cancrinite and chabazite rise. Indeed, the characteristic peak of NaNO₃ (PDF # 72-0025) with a high diffraction peak intensity also appear, which indicates that there is too much unreacted NaNO₃ in the system. It can be seen from Figure 1c that in the system with the NaNO₃ concentration of 500 g/L, the diffraction peak of SiO₂ has disappeared, and the main phase is NaNO₃. Meanwhile, the diffraction peaks of cancrinite, chabazite, and faujasite zeolites still exist in the system, indicating that high NaNO₃ concentrations have no inhibition effect on the growth of zeolites. However, chabazite is more suitable for growth in high NaNO₃ (300~500 g/L) environments compared with cancrinite. When NaOH concentrations are in the range of 6~8 M, an increase in NaNO₃ will promote the transformation of cancrinite into chabazite.

In the following experiment, the concentration of the NaOH solution in the system was controlled at 2 M as no obvious zeolite formed at 0.66 M, and the NaNO₃ concentration was set as 100 g/L, which is near to the normal concentration of NaNO₃ in the LILW. The phase transformation of alkali-activated fly-ash-based geopolymer products at different curing temperatures from 60 °C to 150 °C was studied. High curing temperatures are another key parameter affecting hydrothermal conversion that promote the grains to crystallize and grow during hydrothermal treatment [5]. From the analysis of Figure 2, it can be seen that there is an obvious crystallization peak of silica near 2θ = 26.5° in the sample after the curing treatment of low temperatures (60 °C), while no characteristic diffraction peaks of other crystal phases are found, which indicates that no crystal phase emerges under this condition. When the curing temperature rises to 90 ℃, the diffraction peaks of silica decrease obviously, and the characteristic diffraction peak of Y-type zeolites occurs. However, the crystallinity is low, and there are still unreacted mullite and silica. When the temperature reaches 120 °C, the characteristic diffraction peaks of Y-type zeolites in the product are weakened, and the characteristic diffraction peaks of chabazite and garronite (PDF # 39-1374) appear. However, the crystallinity of chabazite is not high, and the silica diffraction peaks weaken. This indicates that a curing temperature of 120 °C is beneficial to the crystal growth and transformation into chabazite and garronite, but the reaction under hydrothermal conditions is still incomplete. The weakening of the characteristic diffraction peak of silica was caused by the dissolution of silica by a high concentration of the NaOH solution [2]. By further increasing the curing temperature to 150 °C, the characteristic diffraction peaks of cancrinite emerge, and the crystallinity is high. Moreover, the characteristic diffraction peaks of silica disappear. Thus, it can be concluded that the increase in temperature is beneficial to the crystallization and phase transformation of zeolites. Also, Palomo et al. [22] reported the promoting effect of high temperatures on the formation of the zeolite phase. Zheng et al. [23] studied the effect of curing temperatures on the evolution of the crystalline phase of geopolymers, and the results showed that a relatively high curing temperature (>60 °C) was a necessary condition for the phase transition of fly-ash-based geopolymers.

3.1.2. SEM

XRD diffraction pattern analysis has roughly determined the zeolite generated at the NaOH concentration of 2 M and NaNO₃ concentration of 100 g/L, as Y-type zeolite, chabazite/garronite, and cancrinite have been obtained at the curing temperatures of 90 °C, 120 °C, and 150 °C, respectively. Figure 3, Figure 4 and Figure 5 are the SEM images of the fly-ash-based geopolymer products prepared by the above three conditions.

In Figure 3, it can be seen that the fly-ash-based geopolymer has a polyhedron-shaped product under the NaOH concentration of 2 M and 100 g/L of NaNO₃ at the curing temperature of 90 °C, which is the characteristic morphology of Y-type zeolites [24]. The particle size and shape of Y-type zeolites are relatively uniform. Indeed, it can also be observed that the surface presents a wrinkled spherical particle, which is the characteristic morphology of the chabazite. Moreover, the spherical particles are larger than the polyhedron-shaped particles. At the same time, dissolved and broken spherical particles can be observed. Combined with the results of XRD, it can be concluded that Y-type zeolites and chabazite were formed in the alkali-activated fly-ash-based geopolymer products under this condition. From Figure 4, it can be seen that the fly-ash-based geopolymer has well-crystallized particles under a NaOH concentration of 2 M and a NaNO₃ concentration of 100 g/L at a curing temperature of 120 °C. Most of the particles are regularly spherical, and the surface has obvious folds. The particle size is small, but the size is uniform, and the typical chabazite crystal shows an aggregation growth morphology [25]. There is also a small part of the particles with a cross-shaped columnar structure, which is larger than chabazite. The well-crystallized phase indicates that new crystals have been generated, and it is also proven that the fly ash in the raw material is fully involved in the reaction. Combined with the XRD results, it was determined that chabazite and garronite were formed under this condition. From Figure 5, it can be seen that the fly-ash-based geopolymer has thorny spherical particles under the NaOH concentration of 2 M and NaNO₃ concentration of 100 g/L at the curing temperature of 150 °C, which are the characteristic morphology of cancrinite. The particle size is uniform, the crystallization is good, and the small irregular particles are grown around the spherical particles. Thus, it can be concluded that the alkali-activated fly-ash-based geopolymer generates cancrinite under this condition. Above all, the fly-ash-based geopolymer product of chabazite and garronite formed at 120 °C (sample T120: chabazite/garronite-rich geopolymers) was selected for the subsequent isothermal adsorption test and desorption kinetic test to explore the adsorption and desorption characteristics of Cs⁺, as the main crystal phases of chabazite and garronite obtained excellent directional adsorption ability to Cs⁺ [26,27].

3.1.3. FTIR

Figure 6 shows the FTIR results of T120 products before and after the adsorption of Cs⁺. The sharp absorption peak at 462.76 cm⁻¹ belongs to the internal bending vibration of the silicon aluminum oxide tetrahedron, while its symmetrical tensile vibration is at 572.99 cm⁻¹ [24]. The absorption peak near 1635.74 cm⁻¹ is generated by the stretching and bending vibrations of O–H and H–O–H. As the hydration of fly-ash-based geopolymer was terminated by soaking in absolute ethanol and the test product was dried sufficiently to eliminate the influence of free water on the absorption peak, the wider peaks in these two places are related to the hydration gel generated during the reaction process, such as N–A–S–H gel. The absorption peak at 1423.67 cm⁻¹ is caused by the asymmetric stretching vibration of O–C–O [28,29]. An amount of 1384.70 cm⁻¹ is the stretching vibration of N–O, and the source of nitrogen in the experiment is only NaNO₃. It can be concluded that there is still unreacted NaNO₃ in the fly-ash-based geopolymer under the condition of alkali excitation [25], which is consistent with the phenomenon that a strong NaNO₃ diffraction peak is found in the XRD pattern of the fly-ash-based geopolymer products generated at high concentrations of NaNO₃. The small peak at 685.61 cm⁻¹ is the bending vibration of T–O–T (T = Si, Al). Corresponding to the Si-O bond at 995.97 cm⁻¹, it can be inferred that the T120 products contain a small amount of cancrinite [30]. The infrared spectra of chabazite/garronite-rich geopolymers before and after adsorption show no obvious change in the peak. No evidence has shown the chemical bond fracture and new bond formation in the adsorption process of nuclide Cs⁺ on the products; therefore, it can be deduced that the adsorption of Cs on chabazite/garronite-rich geopolymer is physical adsorption.

3.2. Adsorption Performances

3.2.1. Adsorption Capacity and Adsorption Rate

Figure 7 outlines the adsorption rate curves of T120 products with different initial concentrations of the Cs⁺ solution, and the adsorption time is 6 h. From Figure 7, it can be seen that as the concentration of Cs⁺ in the system varies from 50 mg/L to 800 mg/L, the adsorption capacity of chabazite/garronite-rich geopolymers to Cs⁺ increases from 11.73 mg/g to 82.95 mg/g. The increasing initial mass concentration of Cs⁺ elevates the ion concentration in the unit volume solution and extends the contact time between the adsorbate and the adsorbent to a certain extent. From Figure 7, it can also be seen that the adsorption rate of Cs⁺ by chabazite/garronite-rich geopolymers decreases with the increase in the initial concentration of Cs⁺, and the adsorption rate of Cs⁺ by chabazite/garronite-rich geopolymers decreases from 93.80% to 41.48%. When the concentration of Cs⁺ is 50 mg/L, the adsorption removal rate of Cs⁺ by chabazite/garronite-rich geopolymers is the highest at 93.80%.

3.2.2. Adsorption Isotherm Model

The adsorption data were analyzed, and the fitting parameters were obtained to infer the adsorption reaction mechanism. For the adsorption of heavy metals in a solution, the Langmuir isothermal adsorption model [31] and the Freundlich isothermal adsorption model [32] were used to fit the experimental data. The Langmuir equation is a model based on monolayer adsorption (chemical adsorption), while the Freundlich equation is suitable for describing the adsorption process of uneven surfaces (physical adsorption). Figure 8 demonstrates the Langmuir and Freundlich isothermal adsorption equation fitting curve of Cs⁺ on chabazite/garronite-rich geopolymers. The fitting parameters of Langmuir and Freundlich’s isothermal adsorption equations of Cs⁺ on the chabazite/garronite-rich geopolymer are shown in

Table 4. Isothermal adsorption equation and fitting parameters of Cs⁺ on T120 products (25 °C).

	Langmuir			Freundlich
	Qm (mg/g)	KL	R2	n	KF (mg/g)	R2
T120	87.91	1.75 × 10−2	0.95	3.36	12.64	0.96

.

According to the fitting parameters of the isothermal adsorption curve, the isothermal adsorption curve of Cs⁺ is fitted well with the Freundlich equation and Langmuir equation, both of which are suitable for describing the isothermal adsorption behavior of Cs⁺ on the chabazite/garronite-rich geopolymer. It indicates that the adsorption of Cs⁺ on the chabazite/garronite-rich geopolymer is dominated by monolayer adsorption and the adsorption of uneven surfaces at the same time. Moreover, the isothermal adsorption fitting curve shows that the isothermal adsorption data of Cs⁺ in the low initial concentration range (C_o at 50~300 mg/L) are more suitable for the Freundlich equation, while the Langmuir equation fits the adsorption data at the high initial concentration range (C_o at 400~800 mg/L). When the residue concentration of Cs⁺ is in the range of 50~800 mg/L, the maximum theoretical equilibrium adsorption capacity of chabazite/garronite-rich geopolymers to Cs⁺ is 87.91 mg/g. The calculated separation coefficients (K_L) using the Langmuir model fall within the range of 0–1, and the anisotropy coefficients (n) obtained from the Freundlich model are all greater than 1. These coefficients imply that the adsorption of Cs⁺ from aqueous solutions by fly-ash-based zeolites is the favorable equilibrium adsorption region [33].

3.3. Desorption Performances

In the desorption test of Cs⁺, a hydrochloric acid solution with a pH = 1 was used as the eluent, and the mass percentage of concentrated hydrochloric acid was 37%.

Table 5. Desorption rate and amount of Cs⁺ from T120 products at different desorption times.

Desorption Time (min)	5	10	15	30	45	60
Desorption rate (%)	83.38	90.63	80.52	87.73	88.25	101.74
Desorption capacity (mg/g)	26.28	28.56	25.38	27.65	27.81	32.06

highlights the desorption capacity and desorption rate of Cs⁺ on the T120 products at different desorption times under the condition of 25 °C. The initial concentration of Cs⁺ is selected as 150 mg/L (to ensure sufficient adsorption and low concentrations, as the Cs⁺ in real LILW is rather low). It can be seen from the adsorption isotherm curves in Figure 7 that the equilibrium adsorption capacity of chabazite/garronite-rich geopolymers at 150 mg/L Cs⁺ solution is 31.51 mg/g. However, Table 5 shows that when the desorption time is 60 min, the desorption capacity of Cs⁺ (32.06 mg/g) in the solution after desorption is greater than the equilibrium adsorption capacity of 31.51 mg/g. This result may be caused by a deviation during the dilution process of the test samples. Therefore, the pseudo-first-order and pseudo-second-order kinetic fittings were performed using the desorption data of the (5~45 min) time period.

Table 6. Desorption kinetic equation and fitting parameters of Cs⁺ from T120 products.

Kinetic Equation	Desorption Kinetic Equation	Desorption Rate	Theoretical Equilibrium Desorption Capacity/mg·g−1	R2
Pseudo-first-order	lg(Qe − Qt) = 0.68 − 2.28 × 10−3 t	5.25 × 10−3 min−1	4.74	0.09
Pseudo-second-order	t/Qt =1.71 × 10−2 + 3.576 × 10−2 t	7.48 × 10−2 mg/(g·min−1)	27.96	0.99

outlines the fitting parameters of the pseudo-first-order and pseudo-second-order kinetic models [34,35]. Figure 9 and Figure 10 outline the pseudo-first-order and pseudo-second-order kinetic fitting curves at the time period of 5~45 min, respectively.

From the fitting parameters of Table 6, it can be seen that the desorption data of Cs⁺ on the chabazite/garronite-rich geopolymers after saturated adsorption at 150 mg/L have a good correlation with the pseudo-second-order kinetic equation during the desorption period of 5~45 min. The correlation is 0.99, and the pseudo-second-order desorption rate is 7.48 × 10⁻² mg/(g·min⁻¹). The theoretical equilibrium desorption capacity during the 5–45 min period is 27.96 mg/g, which is close to the desorption capacity of 27.81 mg/g at 45 min, and the relative error is 0.55%. Combined with the adsorption model mentioned above, it can be inferred that the adsorption and desorption processes of Cs⁺ on chabazite/garronite-rich geopolymers are mainly chemical processes [36,37].

4. Conclusions

The products of fly-ash-based geopolymers and the adsorption performance of the products to the simulated radionuclide Cs⁺ were controlled by NaNO₃ concentrations, NaOH concentrations, and curing temperatures. Specific findings from this work include the following:

(1)

In the low NaNO₃ system (100 g/L), an increase in the NaOH concentration from 0.66 M to 2 M promoted the formation of Y-type zeolites and chabazite, while an increase in the NaOH concentration from 2 M to 8 M led to the transformation of zeolites into cancrinite. In the high NaNO₃ (300~500 g/L) system, the increase in NaOH concentrations above 2 M had no obvious effect on the product transformation, and the products were mainly cancrinite and chabazite.

(2)

In the low NaNO₃ system (100 g/L) with NaOH concentrations of 2 M, Y-type zeolite was formed at 90 °C. With the increase in curing temperatures (90~150 °C), the Y-type zeolite was firstly transformed into garronite and chabazite, and then cancrinite at last. It can be concluded that NaNO₃ concentrations, NaOH concentrations, and curing temperatures all promote the crystallization of cancrinite and chabazite.

(3)

At the NaNO₃ concentration of 100 g/L and 120 °C, the adsorption capacity of Cs⁺ decreased with the increase in the initial concentration of Cs⁺, and the adsorption rate of Cs⁺ increased with the increase in the initial concentration of Cs⁺ initial. The adsorption of Cs⁺ in the low initial concentration range was more suitable for the Freundlich equation, while the Langmuir equation fit the adsorption process at the high initial concentration range. The adsorption of Cs⁺ on the chabazite/garronite-rich geopolymer was dominated by physical and chemical adsorption at the same time.

(4)

In the range of 5~45 min, the desorption kinetic process of Cs⁺ on the chabazite/garronite-rich geopolymer was in good agreement with the pseudo-second-order equation. Indeed, the desorption of Cs⁺ on the chabazite/garronite-rich geopolymer was a chemical desorption.