265129 Preparation of H8Nb22O598H2O and Ion-Exchange Properties for Na+ and K+

Thursday, November 1, 2012: 2:18 PM
405 (Convention Center )
Xianglong Zhang1, Yanlong Zhai1, Chang Wang2 and Qinglan Hao1, (1)College of Material Science & Chemical Engineering, Tianjin University of Science & Technology, Tianjin, China, (2)College of Marine Science & Engineering, Tianjin University of Science & Technology, Tianjin, China

Abstract

The ion-exchanger H8Nb22O59·8H2O with 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 (Kd) 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×VHCl)/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 (Kd)

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

Kd (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 Kd measurement. Kd reflects the intrinsic affinity between ion-exchange sites and guest ions, because of extremely low concentration of Na+ and K+. Kd is suitable for evaluating the ion-sieve property of an inorganic ion exchanger. The distribution coefficients (Kd) 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 Kd 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).

 References

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