稀有金属(英文版) 2015,34(10),752-756

收稿日期：5 April 2015

基金：financially supported by the National Natural Science Foundation of China(No.51272114);the Young and Middle-aged Scientist Reward Foundation of Shandong Province,China(No.2008BS09004);the Scientific Research Foundational Projects of Qingdao,China(No.12-1-4-3-(8)-jch);the Scientific Research Starting Foundation of Qingdao University of Science and Technology,China(No.0022322);

Synthesis and ion exchange properties for Cs⁺ of ammonium-type mordenite

Dian-Quan Dong Jian-Guo Zheng Xian-Lian Han

College of Chemical Engineering,School of Qingdao University of Science and Technology

Abstract：

Cesium content in the seawater is 3.0 9 10-4 mg·L-1, and ammonium-type mordenite was used to exchange cesium from cesium source. K-type mordenite was obtained by hydrothermal synthesis method. After ammonium-modified, ion exchange properties such as saturated ion exchange capacities and selectivity coefficients for alkali metal ions were determined. At the same time, ion exchange isothermal curves of ammonium-type mordenite for Cs+ and NH4+ were determined at 25°C.Pitzer electrolyte solution theory was used to work out activity coefficients of the system, and the Kielland diagram of the ion exchange system was obtained. The calculated equilibrium constant(Ka) and ion free enthalpy(DG0) of mordenite for Cs+ and NH4+ are 1.455 and -930.3J·mol-1, respectively, indicating that ion exchange process of ammonium-type mordenite for cesium is a spontaneous process.

Keyword：

Ion exchange; Thermodynamics; Mordenite; Cesium;

Author： Dian-Quan Dong,e-mail:dongdianquan@sohu.com;

Received： 5 April 2015

1 Introduction

Cesium resource is widespread in nature, and its abundance in the crust is 3.0 mg L^-1; the average content of seawater is about 3.0 9 10^-4mg L^-1. Cesium atoms are the ideal cosmic ion rocket engine fuel as its outermost electron is very unstable and can easily be stimulated to emit out to become positively charged cesium ions. Calculation shows that with this kind of cesium ion space rocket propellant, thrust is one hundred times higher than that of liquid or solid fuel. This cesium ion rocket can roam in the universe space for 1 year or more.

In Tibet, some salt lake brine like zhabuy, etc., contained rich cesium resource [1–3], but the concentration is low, so ion exchange absorption is a promising method to separate cesium from liquid phase. Phosphomolybdate adsorption was used to separate Cs?, and its ion exchange property was assayed [4–7]. Na A zeolite was synthesized, and the performance of the exchange of cesium was studied by Yi et al. [8]. Gao et al. used clinoptilolite to extract potassium, rubidium and cesium [9]. And the nature and location of silver in Ag-exchanged mordenite catalysts were studied [10]. In this study, K-type mordenite was synthesized by hydrothermal synthesis method [11, 12], and after ammonium retrofit, cesium ion exchange property was determined.

2 Experimental

2.1 Synthesis of K-type mordenite and ammonium retrofit

Na-type mordenite synthesis: Na Al O₂, Na OH, silica sol and water were used as raw material to synthesize the product which was determined by X-ray diffraction (XRD, D/max-c A-type X-ray diffractometer) to confirm the structure and scanning electron microscopy (SEM, S-4800) to confirm the crystal size of the material, and then the results identified were compared with the literature’s.

K-type mordenite synthesis: shaking and leaching the obtained Na-type mordenite in 0.1 mol L^-1hydrochloric acid for 24 h at room temperature, following centrifuging and separating, then using hydrochloric acid to leach 10 times, finally obtaining the H?-type mordenite.

KAl O₂, KOH, silica sol and water were mixed withmole ratio of and 1 wt% H⁺type mordenite was added as seed crystal. After stirring evenly, the mixture was added into a reaction kettle, and following maturing 24 h, crystallization reaction at 180–190 °C for 5 days, centrifuge-washing to p H 8–9, drying the product at 100 °C, finally K-type mordenite was obtained. XRD pattern of K-type mordenite was then compared with that in Ref. [13].

Ammonium retrofit of mordenite: weighing certain quality of K-type mordenite, oscillating and leaching using 2.0 mol L^-1ammonium chloride solution (hydrochloric acid solution volume/zeolite quality = 5 ml:1 g) for 24 h at room temperature, centrifugal separating, removing liquid phase, replacing the ammonium chloride solution to leach and wash repeatedly ten times and drying at 100 °C [10, 11]. The potassium content of mordenite was analyzed before and after transformation.

2.2 Saturated exchange capacity

0.1 g NH₄-type mordenite was added to 50 ml 0.1 mol L^-1Li Cl solutions, likewise added to 50 ml 0.1 mol L^-1Na Cl, KCl, Rb Cl and Cs Cl solutions, respectively. After oscillation leaching at 25 °C and saturated exchange for 10 days, the supernatant was obtained to measure its cations concentration and conduct blank experiment. According to the reduction of alkali ion in the supernatant, the saturated alkali ion exchange capacity of ammonium-type mordenite was calculated.

2.3 Distribution coefficient(Kd)

0.1 g NH₄-type mordenite was mixed with 0.2 ml Li Cl, 0.2 ml Na Cl, 0.2 ml KCl, 0.2 ml Rb Cl, 0.2 ml Cs Cl (all concentrations of the chloride solutions are 0.05 mol L^-1), and then ammonium chloride solution and distilled water up to 10 ml were added to get ammonium ion solution of 1 x10^-3mol L^-1. By the same method, ammonium ion solutions of 1 x 10^-2, 1 x 10^-1, 1 mol L^-1were obtained. After intermittent oscillation for 10 days at room temperature [14], the ion concentration in the supernatant was measured, and the distribution coefficient was calculated according to the change in the metal ion concentration before and after the balance.

2.4 NH4+and Cs+ion exchange isotherm

0.1 g ammonium-type mordenite was immerged into 0.1 mol L^-1total binary mixture with different mole ratios of Cs⁺to NH₄⁺(9/1, 8/2, 7/3, 6/4, 5/5, 4/6, 3/7, 2/8, 1/9), respectively. The ammonium-type mordenite was oscillated at temperature of 25 °C until equilibrium, and four samples were configurated under the same condition (same liquid-phase composition and same quality of mordenite); a sample was taken to check, respectively, after 3, 5, 7 and 10 days oscillated equilibrium at 25 °C; after centrifugal separation, the concentration of Cs⁺in supernatant was measured. Equilibrium data were used to calculate the mole fractions of Cs⁺in liquid and mordenite phases [7, 11].

3 Results and discussion

3.1 Synthesis of K-type mordenite and ammonium retrofit

Figures 1 and 2 show XRD patterns and SEM image of synthetic zeolite. Compared with the results in Ref. [12], the synthesized product is mordenite, and the crystal size is about 5–10 lm. Through analyzing the ammonium mordenite potassium content before and after retrofit, the potassium contents of mordenite are 1.82 mmol g^-1zeolite and 0.43 mmol g^-1mordenite, respectively; 78 % potassium transforms into ammonium which can be thought to be ammonium-type mordenite.

3.2 Saturated exchange capacity

The relation between saturated exchange capacity of ammonium mordenite to alkali ion and ionic radius at 25 °C is shown in Fig. 3. As can be seen from Fig. 3, mordenite saturation capacity exchange to cesium is higher than that to other alkali metal ions. Saturation exchange capacity increases with the increase in alkali metal ion radius (bare ion radius); namely, the mordenite saturation capacity exchange decreases with the increase in alkali metal hydrated ionic radius (hydration ionic radius of alkalimetal ions ascending order: Cs?\ Rb?\ K?\ Na? \ Li?) [15]. The results show that mordenite is exchanged with hydrated ion and the obtained mordenite has a high saturation exchange capacity to Cs?up to 166.1 mg g^-1.

Fig.1 XRD patterns of synthetic zeolite

Fig.2 SEM image of synthetic zeolite

Fig.3 Relation between saturated ion exchange capacity and ion radius of NH4?mordenite for alkali metal ions (25 °C)

3.3 Distribution coefficient(Kd)

The distribution coefficient (K_d) of mordenite to alkaline metal ion and separation factor (a) with ammonium concentration of 1 x 10^-3mol L^-1are shown in Table 1. The separation factor calculated from the data is 1229.2. The mordenite separation factor to cesium and sodium is very high, so cesium exchange agent has an application prospect [16].

In the measured range of ammonium concentration, the selective sequence of ammonium-type mordenite to alkali metal ion is Cs⁺> Rb⁺> K⁺> Na⁺> Li⁺.

It shows that the mordenite has good ion exchange selectivity for cesium ion. This conclusion is consistent with the conclusion of Sherry [17] in the study of naturalmordenite. Natural zeolite contains impurities which will block the channel of mordenite and affect the ion exchange performance. The synthetic mordenite has better performance in ion exchange.

Table 1 Distribution and selectivity coefficients of mordenite for alkali metal ions (c(NH₄⁺) = 0.001 mol L^-1)  下载原图

Table 1 Distribution and selectivity coefficients of mordenite for alkali metal ions (c(NH₄⁺) = 0.001 mol L^-1)

In addition, effective hydrated ionic radii of Cs⁺and NH₄⁺are similar [12], and pore and cavity of ammonium mordenite in size are more suitable for the entrance of Cs⁺; therefore, synthetic mordenite has a higher saturation exchange capacity and ion exchange selectivity for Cs⁺.

3.4 Ion exchange isotherm diagram of mordenite

Ion exchange isothermal diagram of mordenite on NH₄⁺– Cs⁺at 25 °C is shown in Fig. 4. As can be seen from Fig. 4, 25 °C isothermal adsorption line is located above the diagonal, showing that ammonium-type mordenite has good selectivity for Cs⁺.

Mass action quotient can be calculated from the experimental data in Fig. 4 by the following equation:

where is equilibrium coefficient of Cs⁺to NH4⁺represented by mass ratio (mass action quotient),is the mole fraction of Cs⁺in mordenite phase, is the mole fraction of Cs⁺in liquid phase, is the mole 4 fraction of NH₄⁺in mordenite phase, and is the mole fraction of NH₄⁺in liquid phase.

Fig.4 Ion exchange isothermal diagram of mordenite on NH4+–Cs+at 25 °C

Fig.5 Kielland diagram of NH4+–Cs+ion exchange system at 25 °C

For NH₄⁺–Cs⁺ion exchange system, the activity coefficient of electrolyte solution and the system can be calculated by Pitzer electrolyte solution theory model [18, 19]. Further NH₄⁺–Cs⁺ion exchange equilibrium constant can be calculated from the following equation:

where is equilibrium coefficient of Cs⁺to NH₄⁺represented by concentration (Kielland quotient), is the activity coefficient of Cs Cl in the equilibrium liquid phase, and is the activity coefficient of NH₄Cl in the equilibrium liquid phase. Kielland diagram was obtained by plotting ln [7]. Ion exchange isothermal Kielland diagram of mordenite on NH₄⁺–Cs⁺at 25 °C is shown in Fig. 5.

By the conclusion of Gains and Thomas [20], ion exchange atomicity was one to one:

where K_a^A_Bis equilibrium coefficient of A to B represented by activity coefficient, K_cB^Ais equilibrium coefficient of A to B represented by concentration, and X_Ais the mole fraction of A in mordenite phase.

The NH₄⁺–Cs⁺exchange of thermodynamic equilibrium constant ( ) can be calculated as 1.455. And the standard enthalpy of exchange reaction ( ) was calculated as follows:

where is exchange free energy, K_ais thermody namic equilibrium constant, R is universal gas constant, 8.314 J K^-1mol^-1, and T is the temperature, K.

The standard enthalpy of exchange reaction is negative, showing that the direction of ion exchange reaction reduces system enthalpy in ideal state [21]; namely, the mordenite selectivity for Cs⁺is larger than that for the original exchangeable cation NH₄⁺.

4 Conclusion

The ammonium mordenite saturated exchange capacity to Cs⁺is 166.1 mg g^-1. The separation coefficient () of synthetic ammonium-type mordenite is 1229.2, and the mordenite selectivity for Cs⁺is higher than that for other alkali metal ions. The calculated mordenite exchange equilibrium constant () and exchange free energy () are 1.455 and -930.3 J mol-1, respectively. The is negative, showing that the process of ammo nium mordenite-exchanged cesium is a spontaneous process.