Natural zeolite as a replacement for resin in the cation exchange process of cesium on post-irradiated nuclear fuel

Zeolites are hydrated aluminosilicate crystals containing alkali or alkaline earth cations in a three-dimensional framework. The basic framework of the zeolite structure consists of tetrahedral units of AlO₂ and SiO₂ interconnected with O atoms; therefore, zeolite has the empirical formula Mⁿ⁺[(AlO₂)_x(SiO₂)_y]·zH₂O [2]. The M notation was the alkali or alkaline earth metal cations, where the particular numbers were x, y, and z as n was the change of the metal ions. From the above formula, zeolite can be seen the existence of the three components, such as frame [(AlO₂)_x(SiO₂)_y], alkali metal Mⁿ⁺ (¹⁴⁴Ba, ¹³⁷Cs, and ⁹⁰Sr), and H₂O [2], which is part of zeolite. Zeolite structures consist of four O atoms that are interconnected with all Al atoms to form a tetrahedron. Si⁴⁺ atoms are replaced by Al³⁺ so that the zeolite is negatively charged and will be neutralized by alkali or alkaline earth cations, such as NH4+{\rm{NH}}_4^+ , Na⁺, K⁺, Mg²⁺, and Ca²⁺, in the cavity of the zeolite structure, thus producing a stable compound [3]. Therefore, zeolite is likely to be a good substitute for the resin in the ion exchange process. Zeolite structures are porous and contain water molecules that are easily separted from the rest of the structure by the heating process. This phenomenon means that zeolite can be used as a specific adsorbent in molecular sieves, ion exchangers, and as a catalyst. The unique characteristics of zeolite cause it to be used widely in various industries [4, 5].

The cation exchange capacity (CEC) of zeolites is the maximum number of Cs ion in milli-equivalent (mEq) that can be absorbed by 1 g of zeolite under equilibrium conditions, as given in Eq. (1) [6] (1)CEC=Cs0-CsiMass Zeo{\rm{CEC}} = {{{\rm{C}}{{\rm{s}}_0} - {\rm{C}}{{\rm{s}}_i}} \over {{\rm{Mass}}\,{\rm{Zeo}}}} where CEC is the cation exchange capacity of the zeolite (mEq/g); Cs₀ is the mole equivalent weight of Cs before the cation exchange process; Cs₁ is the mole equivalent weight of Cs, after the cation exchange process; and Mass Zeo is the weight of zeolite used for the cation exchange process (g).

The CEC of natural zeolite varies from 1.5 mEq/g to 6 mEq/g and is dependent upon the amount of Al⁺ and Si⁺ atoms in the structure of the zeolite. Zeolites have a higher CEC than clay rocks such as kaolin (0.03–015 mEq/g), bentonite (0.80–1.50 mEq/g), and vermiculite (1–1.50 mEq/g) [7, 8].

Most zeolite has a chemical composition consisting of SiO₂, Al₂O₃, Fe₂O₃, K₂O, TiO₂, MgO, CaO, and Na₂O. Zeolites have a variety of structures, with specific characteristics and depending on how they were formed [9]. Zeolite is a very common mineral in Indonesia, particularly in Bayah, Lampung, and Tasikmalaya; it is also present in Java and Sumatra, where it is generally composed of clinoptilolite and mordenite in varying proportions, depending on its source.

The results of the composition analysis of zeolites from Bayah, Lampung, and Tasikmalaya using X-ray diffractometer (XRD)-Panalytical product with cobalt sources showed that 74% of the three types of zeolites were clinoptilolite with mordenite, while others were quartz, clay and gips, as shown in Fig. 1 [10, 11].

Fig. 1

Various types of zeolite have different absorption capacities for different molecules. The selectivity of each type of zeolite depends on its structure. Therefore, zeolite can be used as filters for ions or molecules and can also be used as ion exchange materials in chemical analysis as a substitute for synthetic resin as a catalyst in chemical processes. These materials are important because ion exchanges occur in a solution containing anions, cations, and water molecules, and one or more of these ions must be absorbed by the solid-phase microporous matrix. Water molecules contained in the matrix ion will cause ionic equilibrium and neutralize the solution. Ions in the solution can also move freely within the microporous matrix; this phenomenon caused by zeolite composed of alkali or alkaline earth cations with the empirical formula Mⁿ⁺[(AlO₂)_x(SiO₂)_y]·zH₂O. Mⁿ⁺ is a cation source that can move freely and can be exchanged, in part or completely, with other cations [12, 13, 14].

Zeolites are generally composed of mordenite and clinoptilolite in varying proportions. Mordenite acts as an adsorbent or ion exchange and is highly selective toward Cs ions. Cation exchange occurs selectively in the order Cs > Sr = Ba > U, while the selective cation exchange of clinoptilolite ions occurs in the order Cs > Sr > Ba > U [13]. Previous studies tested the selectivity of zeolite cation exchanges with fission products in nuclear fuel of post-irradiation, especially for the Cs, Sr, and U isotopes. The results of the test showed that zeolites were very selective for the separation of Cs > Sr > U isotopes [13, 14]. Properties of zeolite, such as the size of its cations and hydrated cations, surface area, ion radius, and chemical composition, affect the process of cation exchange between zeolite with the isotope of fission products. When the surface area and ion radius differ, the cations cannot be exchanged completely. A phenomenon that occurs in the process of cation exchange was used to separate ¹³⁷Cs from the other fission products in the post-irradiation nuclear fuel [15].

In addition to the properties of the zeolite that affect the ion exchange and absorption processes, other important properties of zeolite are its CEC and the diffusion of ions [16]. The processes of ion exchange and absorption by zeolite follow the kinetics of ion diffusion mechanisms. This phenomenon occurs because different types of natural zeolites consist of various different minerals. Ion diffusion processes on the structure of zeolite can control the ion exchange and absorption processes. The CEC values can be used to determine the effectiveness of ion exchange and absorption processes. The ion exchange capacity of zeolite particles is usually expressed as diffusion coefficients (D_i). The D_i can be determined using Eq. (2); the value of the diffusion coefficient (D_i) is then used to determine the kinetics parameters, such as activation energy (E_a), using the Arrhenius formula shown in Eq. (3) [17]. (2)Ft/F~=6/rDi⋅t/π{F_t}/{F_\sim} = 6/r\sqrt {{D_i}} \cdot t/\pi where F_t is fraction values of ions exchanged at time t; F_~ is the fraction of ions exchanged in 24 h; r is the ion radius of zeolite particles; D_i is the diffusion coefficient (m²/s); t is the contact time (s); π – 22/7. (3)Di=D0exp (-Ea/RT){D_i} = {D_0}\exp \,(- {E_a}/{\rm{RT}}) where D_i is the diffusion coefficient at temperature T_i (m²/s); D₀ is the diffusion coefficient at temperature T₀ (m²/s); E_a is the activation energy (J/mole); R is Boltzmann constant (1.36 × 10⁻²³ J/K); T is the temperature (K).

The magnitude of the E_a is obtained from the value of the slope, as obtained from the equation of linearity between the D_i and temperature (T). The amount of E_a is the energy needed for Cs ions to be absorbed onto zeolites. If the value of E_a is low, Cs ions diffuse easily into the zeolite framework because the ion exchange process in the zeolite is not blocked by the effects of the ion sieve [17, 18].

This study aimed to determine the properties of natural zeolites from the Bayah, Lampung, and Tasikmalaya regions (particle diameter −270 + 400 mesh) and determine their effects on the kinetics of the diffusion process and CEC of ion Cs. These properties will determine whether the character of natural zeolite can be used as a substitute for synthetic resins in cation exchange during the separation of ¹³⁷Cs isotope from uranium in nuclear fuel of U₃Si₂/Al post-irradiation.

The zeolites from Bayah, Lampung, and Tasikmalaya were activated with saturated NH₄Cl and used to obtain monocationic zeolite (NH₄-Z), which is readily usable as a cation exchange material. After the activation process, all cations in zeolite can be replaced by ammonium cations homogeneously or as a monocation. The NH4+{\rm{NH}}_4^+ -zeolite powder is used as a cation exchange material to absorb ¹³⁷Cs isotopes in standard CRM. An NH4+{\rm{NH}}_4^+ -zeolite was obtained from the results of the activation of zeolite with NH₄Cl, as shown in the following reaction (4)NH4Cl+‐zeolite (Mn+)→NH4+‐zeolite+MCln{\rm{N}}{{\rm{H}}_4}{\rm{C}}{{\rm{l}}^+} - {\rm{zeolite}}\,({{\rm{M}}^{n +}}) \to {\rm{NH}}_4^+ - {\rm{zeolite}} + {\rm{MC}}{{\rm{l}}_n} where M is the alkali and alkaline earth metals (cations in zeolites) and n is the valence electron of the metal. The cation exchange process between NH4+{\rm{NH}}_4^+ -zeolite and ¹³⁷Cs isotope occurs as the following reaction. (5)NH4Cl+‐zeolite+ 137Cs→ 137Cs‐zeolite+NH4+{\rm{N}}{{\rm{H}}_4}{\rm{C}}{{\rm{l}}^+} - {\rm{zeolite}} + {\,^{137}}{\rm{Cs}} \to {\,^{137}}{\rm{Cs}} - {\rm{zeolite}} + {\rm{NH}}_4^+

The hydrated ionic radius of NH4+{\rm{NH}}_4^+ = 331 pm, ¹³⁷Cs = 329 pm, U⁴⁺ = 80 pm, U⁶⁺ = 97 pm, and Sr²⁺ = 412 pm so that the Cs⁺ ion is easier to exchange with NH4+{\rm{NH}}_4^+ when compared with Sr²⁺ ion or U in the zeolite framework.

The results of the XRF analysis of the chemical composition of zeolites from Bayah, Lampung, and Tasikmalaya showed that they contained Si, Al, Ca, Fe, Mg, Na, K, and Ti, as shown in Table 1. Table 1 shows 68.07% silica and 16.52% alumina are present in zeolite from Lampung, which is greater than the quantity of these elements found in zeolite from Bayah and Tasikmalaya.

In addition, the P and Mn elements were obtained as impurities in zeolites from Lampung and Bayah, while the element P was not obtained as impurities in zeolites from Tasikmalaya. The zeolite used as a cation exchange material must contain Si and Al in the ratio (Si/Al) >1, because Si⁴⁺ atoms are replaced by Al³⁺ and the unstable charge of Si⁺ is stabilized by ¹³⁷Cs isotope [16]. Each of the three zeolites can be used as a cation exchange material because they each have a (Si/Al) ratio >1, the most potential zeolite from Lampung.

The results of the surface area analyses, pore size, and adsorption, which underwent heat treatment up to 200°C, are shown in Table 2. Table 2 shows that zeolite from Lampung had a larger surface area (10.048 m²) than those from Tasikmalaya (8.332 m²) and Bayah (6.353 m²). For zeolites from Lampung, Tasikmalaya, and Bayah, the three types of zeolites each have the same pore size of 16.065, 16.801, and 16.235 Å, respectively.

Apart from the surface area and pore size analyses, adsorption analysis of each type of zeolite against N₂ at room temperature under isothermal conditions can be used to draw a correlation between partial pressure (P/Po, mmHg) and volume (ml/g), as shown in Fig. 2.

Fig. 2

Figure 2 and Table 2 show that zeolite from Lampung had the highest adsorption capacity, i.e., 24.500 ml/g, followed by zeolites from Tasikmalaya and Bayah at 13.800 ml/g and 13.250 ml/g, respectively, at a partial pressure (P/Po) of 1 mmHg [8]. Analyses of the surface area, specific pore size, and absorption of the three types of zeolite showed that the zeolite from Lampung was the most suitable as an absorbent of fission isotopes in nuclear fuel.

Table 3 shows the results of the optimization of the time taken for the cation exchange process from ¹³⁷Cs isotope to NH4+{\rm{NH}}_4^+ -zeolite (that’s from Bayah, Lampung, and Tasikmalaya) that was stirred for 1, 2, 3, 4, 5, and 24 h [19].

Table 3 shows that the optimization of the Cs ion exchange process by NH₄-zeolite occurred at a stirring time of 1 h. The process of Cs ion exchange by zeolite from Lampung is greater when compared to zeolites from Bayah and Tasikmalaya. The decrease in the milli-equivalent value of Cs ions which can be exchanged with NH₄-zeolite occurs very significantly at stirring for up to 1 h, both for zeolites from Bayah, Lampung, and Tasikmalaya. At the contact times greater than 1 h, i.e., (2, 3, 4, 5, and 24 h), there was a decrease in the value of milli-equivalent of Cs ions exchanged by NH4+{\rm{NH}}_4^+ -zeolites from each of the three regions.

When the stirring time is more than 1 h, there is a decrease in the exchange of Cs ions by zeolites. Increased stirring time up to 24 h causes the temperature of the solution to increase. This phenomenon becomes a barrier or disturbance to the strength of the Cs ion bonding, especially for the absorption process, so that the Cs ions are easily separated from the zeolite structure. The cation exchange process occurs at 84.54%, while the absorption process is only 15.46%, as explained in the next section. The decrease in absorption of Cs ions by zeolites from Bayah and the process is very comparable to the decrease obtained for zeolites from Lampung and Tasikmaya.

The effective CEC of Cs with NH4+{\rm{NH}}_4^+ -zeolite was determined by stirring for 1 h. The CEC of zeolite from Lampung was 1.448 mEq/g, which was larger than that of the zeolites from Bayah and Tasikmalaya, which had CEC values of 1.427 and 1.404 mEq/g with standard deviation (SD) and relative standard deviation (RSD), respectively as shown in Table 4. The CEC values of zeolites from Bayah, Lampung, and Tasikmalaya did not have significant differences when compared with the CEC values obtained by other researchers [5, 20].

The diffusion coefficient (D_i) of zeolites from Bayah, Lampung, and Tasikmalaya was determined in a manner similar to how CEC was measured. The ion exchange process was conducted using contact durations of 1, 2, 3, 4, 5, and 24 h at temperatures of 30°C and 50°C. The weight fractions of the Cs ions (F_t/F_~) variables with contact durations during the ion exchange process and the rate of Cs sorption into the zeolite are listed in Tables 5 and 6.

This suggested that the diffusion process probably occurred fastest within 1 h. D_i values were obtained from the slope of F_t/F versus √t shown in Fig. 3 for a temperature of 30°C and Fig. 4 for a temperature of 50°C.

Fig. 3

Figures 3 and 4 show that the zeolite from Lampung had a higher sorption rate than those from Bayah and Tasikmalaya, but the difference was not very large. The D_i values for the three types of natural zeolite at temperatures of 30°C and 50°C are listed in Table 7.

Fig. 4

The D_i values of these three zeolites were almost identical. This was probably because the compositions of these natural zeolites were also almost the same, as indicated by the XRF analysis of their chemical compositions (Table 1). The E_a value was determined by calculating the relationship between Ln D_i and 1/T using Eq. (2). This yielded E_a values of 36.61, 36.61, and 31.09 kJ/mole for the zeolites from Bayah, Lampung, and Tasikmalaya, respectively. These E_a values were greater than that of the standard zeolite (mordenite), which has an E_a value of 9.06 kJ/mole. This is because the natural zeolites have complex compositions that made it possible for them to contain more than one type of crystal structure. Therefore, it could be concluded that the Cs cation exchange in the natural zeolite was more difficult than that in the synthesized zeolite.

The results of the characterization showed that zeolite from Lampung had better character compared to zeolites from Bayah and Tasikmalaya, so zeolite from Lampung was used as cation exchanged for the separation of ¹³⁷Cs in nuclear fuel.

The stability of the ¹³⁷Cs-zeolite bond was tested heat treatment in 25, 200, 500, and 600°C. The results showed that there was no significant release of ¹³⁷Cs isotopes from the structure of the zeolite. Heat treatment up to 600°C allowed the ¹³⁷Cs isotope to become separated from the structure of the zeolite, after which a leaching test was performed on the ¹³⁷Cs-zeolite solids in the water to determine the effect of the heat treatment on the absorption of ¹³⁷Cs by the zeolites. Upon heating to 600°C, ¹³⁷Cs isotopes did not leach into the water. When the leaching did occur, the cation exchange and adsorption processes were at 84.54% and approximately 15.46%, respectively, as shown in Table 8.

However, Table 8 shows that the separation of ¹³⁷Cs isotope from zeolite did not enable the leaching process to occur in order not to disturb the ¹³⁷Cs isotope because the ¹³⁷Cs isotope bonding by the zeolite reached 84.54%. When the heating at 200°C was followed by the leaching process, a 99.73% fraction yield of ¹³⁷Cs isotope in the zeolite was obtained. This indicated that the heating process allowed the ¹³⁷Cs isotope to be bound to the inner structure of zeolite, but also that the ability of the zeolite to bind ¹³⁷Cs did not increase as the temperature increased.

Nuclear fuel of post-irradiation contains isotopes of fission products as gamma radiation transmitters such as ¹³⁴Cs, ¹³⁷Cs, ⁹⁰Sr, ¹⁴⁰Ba, and ¹⁴⁴Ce. The amount of isotopes in nuclear fuel varies depending on the fission yield and half-life, as shown in Table 9.

Competition between isotopes can occur when the ¹³⁷Cs isotope is separated from other isotopes using zeolite from Lampung. Therefore, it is necessary to test the selectivity of NH₄-zeolite cation with Cs, Sr, Ba, and Ce [21]. The selectivity test results of the NH₄-zeolite cation with Cs, Sr, Ba, and Ce are shown in Fig. 5.

Fig. 5

Figure 5 shows that ¹³⁷Cs isotope is more selective toward NH₄-zeolite compared to ⁹⁰Sr, ¹⁴⁴Ba, and ¹⁴⁴Ce isotopes. This is because the size of the NH₄ ion radius is 148 pm, while the size of the Cs⁺ and Sr²⁺ ions is 167 pm and 112 pm, but the hydrated radii of NH₄, Cs⁺, and Sr²⁺ are 331, 329 and 412 pm, so Cs⁺ will be easier to exchange with NH₄ compared to Sr²⁺ in the zeolite framework. This is supported by the results of other researchers regarding the calculation of the selectivity coefficient of zeolites from Lampung for the Cs⁺, Sr²⁺, Ba, and Ce ions as shown in Table 10 [21].

The results of the separation of ¹³⁷Cs isotope in nuclear fuel of U₃Si₂/Al post-irradiation by a cation exchange method using zeolites from Lampung are shown in Fig. 6 and Table 11, while the separation of ¹³⁷Cs using Dowex resin (synthetic resin) is shown in Table 12.

Fig. 6

Figure 6 shows the isotopes spectrum of ¹³⁴Cs and ¹³⁷Cs bound in the solid phase at 604.7 keV and 661.7 keV, respectively. The isotope content of ¹³⁴Cs obtained by measuring cesium using a gamma spectrometer is very small while the content of ¹³⁷Cs is very large. This is due to a half-life of ¹³⁴Cs of only around 2.1 years and ¹³⁷Cs of around 30.17 years.

From Tables 11 and 12 obtained recovery separations for ¹³⁷Cs isotope in U₃Si₂/Al fuel post-irradiation using zeolite from Lampung and resin Dowex was about the same around 98% to 99%. This shows that zeolite Lampung can replace resin Dowex as a cation exchange material for the separation of ¹³⁷Cs isotope.