Crystallographic Studies of Sr2+ and K+ Ion-exchanged Zeolite Y (FAU, Si/Al = 1.56) from Binary Solution with Different Mole Ratio of Sr2+ and K+

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Crystallographic Studies of Sr2+ and K+ Ion-exchanged Zeolite Y (FAU, Si/Al = 1.56) from Binary Solution with Different Mole Ratio of Sr2+ and K+

Abstract

Zeolite is one of the most significant synthetic zeolites which is a commercial product, and it is an additive in most household cleaning agents. It is mainly used in softening of water at homes. This compound has a very high ion exchangeability as a result of its low silver to ammonium ratio. In the previous work, studies were done mostly on the structure of the different cation exchange of the zeolite. As a result, therefore, the investigation of the thermal steadiness as well as the crystal structure of hydrated zeolite becomes an area of concern, especially to the study. The powder and the single crystal of both the Sr2+ and K+ exchange zeolite are prepared using static exchanging methods. The crystalline structure of the zeolite is investigated using the x-ray diffraction methods at ambient temperature. To study the Sr2+-ion selectivity of zeolite Y (Si/Al = 1.56) in binary solution with different Sr2+ and K+ concentration during exchange, two single-crystals of fully dehydrated, Sr2+- and K+-exchanged zeolites Y were prepared by the flow method using a mixed ion-exchange solution whose Sr(NO3 )2:KNO3 mole ratios were 1:1 (crystal 1) and 1:100 (crystal 2), respectively, with a total concentration of 0.05 M, followed by vacuum dehydration at 723 K. Their crystals were determined by single-crystal synchrotron X-ray diffraction techniques in the cubic space group Fd3m, respectively, and were refined to the final error indices R1/wR2 = 0.0429/0.1437 and 0.0356/0.1239 for crystals 1 and 2, respectively. In the structure of |Sr28K19|[Si117Al75O384]-FAU (crystal 1), 28 Sr2+ ions per unit cell occupy four different acupoints; 15, 2, 2, and 9 are at sites I, I, II, and II, respectively, whereas, the K+ ions occupy only one site: 19 are at site II. In the structure of |Sr17K41|[Si117Al75O384]-FAU (crystal 2), 17 Sr2+ ions per unit cell occupy three equipoints; 10.5, 2, and 4.5 are at sites I, I, and II, respectively. The residual 41 K+ ions per unit cell are found at four different sites; 8, 24, 3, and 6 are at sites I, II, IIIa, and IIIb, respectively. The degrees of ion exchange are 74.7 and 45.3% for crystals 1 and 2, respectively. This result shows that the degree of Sr2+ exchange decreased sharply by decreasing the initial Sr2+ concentration and increasing the initial K+ concentration in the given ion-exchange solution.

Keywords: Strontium, Zeolite Y, Ion exchange, Competing for cation

Introduction

Zeolites are a larger class of the microporous crystalline aluminosilicate materials which is made up of a three-dimensional structure of silicon IV oxide and aluminum IV oxide tetrahedral. In general, the predominant of the research that it has been done on the zeolites focused majorly on the synthetic zeolites. Usually, the natural zeolites have differences in the chemical composition since it is formed from the different elements altogether. However the synthetic zeolites are used mainly due to their purity, the pore size uniformity as well due to their degree of the crystalline structure.

Over many decades, the zeolite played a significant role in many different factories as well in different industries. Zeolite has many applications as well. The fields of application are the three specific field's courtesy of their exclusive characters. The properties which make the compound more unique are its thermal stability, the acidic nature the hydrophobicity of the surfaces and finally the ion exchange capacity. When it is used as adsorbents, the zeolites are very excellent in the removal of minor polar solvents and the heavy metals which includes the copper ions, lead, barium, and the silver ions.

On the other hand, the zeolites can be used as a catalyzer. When it is used as such, it reacts with the petroleum products, thus enabling them to undergo reactions such as cracking, isomerization and hydrogenation as well as dehydrogenation among other essential reactions which takes place in the industry. In a study that was conducted recently, the report indicates that most of the zeolites are used as detergents. Additionally, the report did mention that China is the largest consumer of the natural zeolites in the world followed by other nations such as the United States of America.

The radioactive liquid wastes containing 90Sr have been considered as hazardous environmental pollutants, and their treatment has received particular attention all over the world for decades [1, 2]. Because the chemical properties of 90Sr are very similar to Ca, when radioactive Sr enters the human body, 90Sr can easily replace Ca and cause genetic changes, leukemia, anemia, and other diseases [3-5]. Therefore, separation and removal of Sr with high fission yield and the long half-life is significant.

Over the past years, various methods such as chemical precipitation, thermal treatment, membrane, ion exchange, solvent extraction, and adsorption have been developed and applied to remove radiotoxic ions from aqueous solution [2, 3, 5, 6]. Among these methods, ion exchange technique by zeolites has extensively been used as the most effective method due to its simplicity, specific cation selectivity, high efficiency, low cost, chemical stability, and thermal resistance [2,7-9].

A large number of natural or synthetic zeolites such as mordenite, chabazite, clinoptilolite, zeolites A, X, and Y have been widely used for removal of radionuclides [5, 9-17]. Generally, the removal of the radioactive cations by the zeolites is also influenced by the Si/Al ratios and the framework structure of zeolites and by the chemical species and concentration of cations in aqueous solution [5,18-20].

In the environment, various species of cations exist including Na+, K+, Ca2+, and Mg2+ as a mixture and their total amount is much more significant than the targeted radioactive Sr2+[5, 21-23]. These ions have the potential to af...