(666g) Preparation of H8Nb22O59•8H2O and Ion-Exchange Properties for Na+ and K+

The ion-exchanger H₈Nb₂₂O₅₉·8H₂Owith almost the same structure of Rb₈Nb₂₂O₅₉ was synthesized. The uptake amount for Na⁺ and K⁺ is very close to theoretical value of 2.55 mmol·g^-1 calculated from the composition of H₈Nb₂₂O₅₉·8H₂O. The affinity of H₈Nb₂₂O₅₉·8H₂O for Na⁺ and K⁺ is similar, while the results of distribution coefficient (K_d) showed the K⁺ selectivity is higher than that of Na⁺at pH about 2.5.

Keywords: Ion-exchanger; Rb₈Nb₂₂O₅₉; distribution coefficient; protonated compound

1. Introduction

High-purity sodium chloride (99.99%) could be extensively applied on food industry, medicine, the medical field, etc. Very similar properties for Na⁺ and K⁺ results in difficulty for separation K⁺ from sodium chloride solution. It is impossible to obtain a high-purity sodium chloride though conventional methods, such as precipitation, ion exchange, and extraction ^[1-3].

In order to improve the selectivity of inorganic ion-exchanger, more attention is focused on the ion-sieve compound with high selectivity for K⁺ in recent decades. Ion-sieve compound is a metal oxide porous crystals (MOPCS)^[4]. The ion-sieve compound show high selectivity for specific ion depending on the template ion. Because of the differences of occupying sites, Na₂Ti₂O₃SiO₄·2H₂O exchanger with a tunnel structure exhibited a high ion exchange capacity for small alkali metal ions and extremely high affinity for large alkali metal ions Cs⁺ and Rb⁺. It could be regarded as a promising treatment material for nuclear waste ^[5]. MnO₂·0.5H₂O showed a markedly high ion-exchange capacity (5.3 mmol/g) for lithium ions due to its ion-sieve property and is the most promising adsorbent for lithium in seawater ^[6]. Rb₈Nb₂₂O₅₉ was synthesized since 1960s, however, more attention was paid for the structure and physic-chemical properties of Rb₈Nb₂₂O₅₉^[7,8].

The present paper describes the synthesis of Rb₈Nb₂₂O₅₉ and its ion-exchange properties for Na⁺ and K⁺. The structural characteristics are studied by X-ray diffraction (XRD), Thermogravimetric differential scanning calorimetry (TG-DSC), and Scanning electron microscope (SEM), respectively.

2. Experimental

2.1 Preparation and protonated Rb₈Nb₂₂O₅₉

Compound containing Rb₂CO₃ (99.9%, m.p.723 °C) and Nb₂O₅ (99.99%, m.p. 1520 °C) with Rb₂CO₃/ Nb₂O₅ molar ratio of 4/11 were completely mixed, followed by ground, and then calcined at different temperatures from 700 °C to 1300 °C for 8 h in air before use.

About 0.5 g Rb₈Nb₂₂O₅₉ was first treated with 50 mL of 10 M HNO₃ solution for 72 h and filtered, treated Rb₈Nb₂₂O₅₉was then washed by deionized water, and finally dried at 70 °C for 12 h.

2.2 physic-chemical analysis

XRD patterns of the different Rb₈Nb₂₂O₅₉samples was carried out on a XD-3 X-ray powder diffractometer (Purkinje, China) equipped with Cu Kα radiation (λ=0.15406 nm) and scanning rate of 4 °/min.

TG-DSC analysis was performed using a TGA/DSC1/1100 instrument.

The morphology of various samples was obtained using a SU-1510 (Hitachi, Japan) scanning electron microscope (SEM).

The amount of Na⁺ and K⁺in the different solutions was measured by atom absorption spectrometry (AAS) using WFX-120 Elemental analyzer (China).

2.3. Ion-exchange capacity

The mixed solution was prepared using 0.1 M NaCl and 0.1 M NaOH (pH>12.40) or 0.1 M KCl and 0.1 M KOH (pH>12.40), respectively. The uptake of Na⁺ and K⁺ from above mixed solution (5 mL) was carried out by intermittent shaking 50 mg of the H₈Nb₂₂O₅₉·8H₂O sample for 7 days at room temperature. The concentration of Na⁺ and K⁺ in the supernatant was determined by titration with 0.1 M HCl. The ion-exchange properties were evaluated from the difference of Na⁺ and K⁺concentration in the initial solution and in the supernatant solution, ion-exchange capacity was calculated by the following equation:

Q=(5×c(OH^-) - 0.1×V_HCl)/m                         (1)

2.4. pH titration

A 50 mg H₈Nb₂₂O₅₉·8H₂O sample was immersed in a mixed solution (5 mL) of 0.1 M NaCl and 0.1 M NaOH (or 0.1 M KCl and 0.1 M KOH) with different pH values. After intermittent shaking for 7 days at room temperature, the pH of the supernatant solution was measured with a pH meter (HANNA, Italy).

2.5. Distribution coefficient (K_d)

The K_d values were calculated according to the uptakes for Na⁺ or K⁺according to the following formula:

K_d (mL / g) =mental ion uptake (mmol/g) / mental ion concentration after adsoption (mmol/mL)                                            (2)

3. Results and Discussion

3.1 The characteristic of Rb₈Nb₂₂O₅₉ and H₈Nb₂₂O₅₉·8H₂O

The characteristic peaks of Rb₂CO₃ and Nb₂O₅ completely disappeared when Rb₈Nb₂₂O₅₉ samples are calcined at different temperatures. Simultaneously, diffraction peaks of Rb₈Nb₂₂O₅₉ which have the characteristic peaks at 2θ values of 12.33°, 14.23°, 15.90°, 26.51°, 26.75°, 27.42°, 28.58°, 29.35°, 30.16°, 31.14°, and 34.47° are distinctively observed. The XRD results show that the characteristic peaks of Rb₈Nb₂₂O₅₉ calcined at 700 °C are weak, and the all intensities of the characteristic peaks of Rb₈Nb₂₂O₅₉ increase with the increase of the calcination temperature. The durations have little effect on the crystal structure when Rb₈Nb₂₂O₅₉ calcined at 1200 °C.

3.2 Effect of calcination temperature

Ion-exchange capacities of H₈Nb₂₂O₅₉·8H₂O at different calcination temperatures for Na⁺ and K⁺ were determined by equation (1) from mixed solutions of 0.1 M NaCl and 0.1 M NaOH (pH>12.40) or 0.1 M KCl and 0.1 M KOH (pH>12.40), respectively. The capacity of Na⁺ and K⁺ of H₈Nb₂₂O₅₉·8H₂O samples increases with the increase of the calcination temperatures. The results suggest that affinity of H₈Nb₂₂O₅₉·8H₂O for Na⁺ and K⁺ with ion-exchange sites also enhances with the increase of the calcination temperatures. The uptake amount of H₈Nb₂₂O₅₉·8H₂O for K⁺ is slightly higher than that of Na⁺, independent of calcination temperatures. The uptake amount of TH ₁₂₀₀ for Na⁺ and K⁺ is 2.34 mmol·g^-1 and 2.50 mmol·g^-1, respectively, which is very close to theoretical value of 2.55 mmol·g^-1 calculated from the composition of H₈Nb₂₂O₅₉·8H₂O.

To avoid the steric interactions between the inserted metal ions, the mixed solution of NaOH and KCl (Na⁺/K⁺=1, mole ratio) was prepared with low concentration of Na⁺ and K⁺ (<12mmol/L) for K_d measurement. K_d reflects the intrinsic affinity between ion-exchange sites and guest ions, because of extremely low concentration of Na⁺ and K⁺. K_d is suitable for evaluating the ion-sieve property of an inorganic ion exchanger. The distribution coefficients (K_d) of H₈Nb₂₂O₅₉·8H₂O samples for Na⁺ and K⁺ at different calcination temperatures were measured according to the equation (2) in above mixed solution. The results show that K_d values of K⁺ is far higher than that of Na⁺ at pH about 2.5, and the selectivity of H₈Nb₂₂O₅₉·8H₂O for K⁺ is higher than that of Na⁺independent of calcination temperatures.

The separation factor  show the higher selectivity of H₈Nb₂₂O₅₉·8H₂O for K⁺, and increases with the increase of the calcination temperature. The crystallinity of H₈Nb₂₂O₅₉·8H₂O increases with the increase of the calcination temperature. The above results indicate that the crystallinity of H₈Nb₂₂O₅₉·8H₂O might be relative to the selectivity of H₈Nb₂₂O₅₉·8H₂O, the high crystallinity of H₈Nb₂₂O₅₉·8H₂O is beneficial to the K⁺adsorption.

3.3 Effect of pH values for adsorptive capacity of H₈Nb₂₂O₅₉·8H₂O

The H₈Nb₂₂O₅₉·8H₂O sample has similar exchange capacities for Na⁺ and K⁺ over the pH range studied, indicating the similar affinity. The adsorptive capacity for Na⁺ and K⁺, evaluated from the amount of OH^- added, increases with pH values, which indicating the solutions with high pH values is helpful for the protonic dissociation from H₈Nb₂₂O₅₉·8H₂O. And this is a general feature of inorganic ion exchanger.

4. Conclusion

The synthesis of Rb₈Nb₂₂O₅₉ and its ion-exchange properties for Na⁺ and K⁺ was studied. The structural characteristics are studied by XRD, TG-DSC, and SEM, respectively. The all intensities of the characteristic peaks of Rb₈Nb₂₂O₅₉ increase with the increase of the calcination temperature. The durations have little effect on the crystal structure when Rb₈Nb₂₂O₅₉ calcined at 1200 °C. The protonated sample H₈Nb₂₂O₅₉·8H₂O shows markedly high selectivity for the adsorption of K⁺ at pH about 2.5. Ion-sieve exchanger Rb₈Nb₂₂O₅₉ is the promising adsorbent for K⁺for purification from sodium chloride solution.

Acknowledgement

We thank Tanggu Municipal Science and Technology Commission of Tianjin Binhai New Area (China) for financial support (special funds).

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