Ion-exchange properties of zeolite/glass hybrid materials

Nobuyuki TAIRA³, Kohei YOSHIDA and Takuya FUKUSHIMA

National Institute of Technology, Gunma College, 580 Toriba-machi, Maebashi 371–8530, Japan

Hybrid materials were prepared from ground glass powder and various zeolites such as A-type, mordenite, X-type, and Y-typezeolites, and their ion removal effect was investigated. The hybrid materials of A-type, Y-type, and mordenite zeolites showedsimilar Sr2+ removal rates from aqueous solutions. The removal rate of Sr2+ ions increased as the amount of zeolite in the hybridmaterials increased. Compared with other hybrid materials, the hybrid materials of X-type zeolite showed higher Sr2+ removalrates, especially for zeolite content greater than 25%. As the amount of X-type zeolite in the hybrid materials increased, the Sr2+

removal rate increased greatly, with a 100% removal rate when the content of X-type zeolite exceeded 62.5%.©2017 The Ceramic Society of Japan. All rights reserved.

Key-words : Hybrid materials, Zeolite, Ground glass, Ion exchange

[Received February 13, 2017; Accepted March 10, 2017]

Zeolites are microporous crystalline materials with uniform andmolecular-sized pores and have high cation-exchange capacitysimilar to other clay minerals. Since they have particularly highselective exchange characteristics, natural zeolites have been stud-ied extensively for treating water containing radioisotopes.1)­4)

Natural zeolites such as mordenite, erionite, chabazite, andclinoptilolite have shown an extremely high radioisotope removalrate in radioactive wastewater,5) although they have disadvan-tages such as small exchange capacity and collapse of particles.Radioactive substances can be removed with high efficiency usingartificial zeolites instead of natural zeolites.6) However, artificialzeolites have poor sinterability and have a disadvantage that theirhandling, for example, in recovery from water, is difficult becausethey are mainly obtained as a powder. As an alternative to sinter-ing, it is possible to solidify zeolites using various binders, but thebinders may clog the characteristic pores of zeolites, interferingwith the removal of radioactive substances. Kamitani et al. havereported that a pellet-like sample of hybrid materials of soda-limeglass and A-type zeolite can remove cesium and strontium ions.7)

Although hybrid materials from zeolites and soda-lime glass areinteresting and attractive as an ion-exchange material, there is noreport on hybrid materials using other types of zeolites. The Si/Alratio of A-type zeolite is equal to 1, which is the minimum valuefor zeolites. X-type and Y-type zeolites have the same crystallo-graphic structure of faujasite with a difference in the Si/Al ratiogiving rise to distinct chemical properties. The Si/Al ratio is 1­1.5for X-type, 1.5­3 for Y-type, and 5.0 for mordenite zeolites.8)

Thus, the use of other zeolites in hybrid materials is expected tolead to further progress in the ion-exchange field. In this study,therefore, hybrid materials of soda-lime glass with mordenite,X-type, and Y-type zeolites in addition to A-type zeolite wereprepared and their ion removal effect was investigated.Samples of hybrid materials were prepared from zeolites and

soda-lime glass following the work reported by Kamitani et al.7)

As starting zeolite sources, A-type zeolite (Wako Pure Chemical,99.9%), X-type zeolite (Wako Pure Chemical, 99.9%), Y-typezeolite (Wako Pure Chemical, 99.9%), and mordenite zeolite(Wako Pure Chemical, 99.9%) were used. Soda-lime glass pow-

der was obtained from dry ball milling of commercial windowglass. The chemical composition of soda-lime glass powder usedin this study was analyzed by fluorescent X-ray analysis (RigakuPrimus II). The powders of zeolite/glass were mixed in variousratios of 0/100, 12.5/87.5, 25/75, 37.5/62.5, 50/50, 62.5/37.5,75/25, 87.5/12.5, and 100/0 (wt/wt%) in an agate mortar.The mixtures (0.5 g) were pressed into pellets, slowly heated(4 °C/min) to 700°C, and kept at 700°C for 2 h. The productswere characterized by X-ray powder diffraction (XRD; CuK¡radiation, Rigaku RINT 2100). Scanning electron microscopy(SEM) was performed on the samples with a JEOL JSM-5600LVelectron transmission microscope operated at 5 kV. The thermalbehavior of the samples was measured with thermogravimetry-differential thermal analysis (TG­DTA) equipment (Rigaku TG­DTA 8121). The ion-exchange performance of the hybrid zeolite/glass samples was evaluated by a 6 h experiment using 20mL of10­100 ppm Sr2+ standard solution. The Sr2+ standard solutionswere prepared by dissolving strontium nitrate Sr(NO3)2 (KantoChemical, 99.5%) in deionized water. The pH values for thestandard solutions are 5.9­6.0. The quantity of Sr2+ remained inthe recovered solution was measured by inductively coupledplasma atomic emission spectroscopy (ICP-AES, Seiko Instru-ments Inc. SPS7800).The results of fluorescent X-ray analysis of the soda-lime glass

used for preparing the hybrid materials are summarized inTable 1. The chemical composition of the soda-lime glass usedas the starting material is close to that of the common soda-limeglass, with silicon and sodium as the main components. Figure 1shows the results of TG­DTA for the glass powder sample andvarious zeolite samples. For the glass powder sample, an endo-thermic peak was observed around 700°C, suggesting that theglass powder sample begins to soften at around 700°C. Sincean endothermic peak was observed near 800°C for A-type andX-type zeolites and around 900°C for Y-type zeolite, it is

Table 1. Chemical composition of soda-lime glass powder used in thisstudy

element Si Na Ca Mg Al others

composition (%) 45 31 7.2 5.8 1.9 9.1³ Corresponding author: N. Taira; E-mail: taira@chem.gunma-ct.ac.jp

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expected that the crystal structure will be broken near thesetemperatures. For mordenite zeolite, since a clear peak was notobserved, the crystal phase was identified by XRD measurement.The XRD patterns for mordenite zeolite are shown in Fig. 2. Asthe XRD pattern of mordenite zeolite heated at 1000°C for 2 hdid not show a clear diffraction peak, indicating amorphous crys-tal structure. On the other hand, the mordenite zeolite heated at900°C for 2 h showed almost the same diffraction pattern asbefore heating, which suggests that heating at or below 900°Cmaintains the crystal structure. Therefore, the hybrid materialswere prepared by heating at 700°C, which is a temperature atwhich glass softening starts to occur but the crystal structure ofvarious zeolites is not destroyed.The prepared hybrid materials were crushed into a powder and

subjected to XRD measurements to confirm the presence of thecrystal phase. Since all hybrid materials showed the same ten-dency, the XRD patterns of the X-type zeolite/glass samples areshown in Fig. 3 as an example. The diffraction patterns derivedfrom the zeolite did not change with change in the zeolite/glassratio. As the glass content was increased, the background fromglass increased, but the XRD patterns showed the presence ofzeolite crystals in the hybrid materials. Since the hybrid materialscontain zeolite crystals, they can be expected to retain the ionexchange capacity.Figure 4 shows SEM images of the X-type and Y-type zeolite

samples. All the hybrid materials showed similar features in SEMimages. The diameter of the particles for the X-type zeolite is 2­5¯m and that of the particles for the Y-type zeolite is less than0.5¯m. For zeolite content less than 25%, a small number of

zeolites were seen on the surface of the hybrid samples [Figs. 4(a)and 4(d)]. With increasing zeolite content, the number of zeoliteson the surface of hybrid samples increased [Figs. 4(b) and 4(e)].Finally, for zeolite content of at least 50%, almost all of the samplesurface was covered with zeolite, as shown in Figs. 4(c) and 4(f ).The hybrid materials were immersed in Sr2+ standard solutions

at a concentration of 10 ppm for 6 h to conduct an adsorption test.After the hybrid materials were recovered, quantification of Sr2+

remaining in the solution was carried out by ICP-AES. Figure 5shows the results of removal of Sr2+ with various hybrid mate-rials. The hybrid materials of A-type, Y-type, and mordenitezeolite showed similar Sr2+ removal rates. Samples containing nozeolite (i.e., zeolite content is 0%) did not remove Sr2+ ions, andthe removal rate of ions increased with increase in the amount of

Fig. 1. Results of TG­DTA measurement for various zeolites andground glass. (a) ground glass, (b) A-type zeolite, (c) X-type zeolite,(d) Y-type zeolite, and (e) mordenite zeolite.

Fig. 2. XRD patterns for mordenite zeolite. (a) no heat treatment,(b) heat at 900°C for 2 h, and (c) heat at 1000°C for 2 h.

Fig. 3. XRD patterns for X-type zeolite/glass hybrid materials.(a) ground glass, (b) 25% X-type zeolite/75% glass, (c) 50% X-typezeolite/50% glass, (d) 75% X-type zeolite/25% glass, and (e) X-typezeolite.

Fig. 4. SEM images of X-type and Y-type zeolite/glass hybridmaterials. (a) 25% X-type zeolite/75% glass, (b) 37.5% X-type zeolite/62.5% glass, (c) 50% X-type zeolite/50% glass, (d) 25% Y-type zeolite/75% glass, (e) 37.5% Y-type zeolite/62.5% glass, and (f ) 50% Y-typezeolite/50% glass.

Fig. 5. Results of Sr2+ removal for various zeolite/glass hybridmaterials after 6 h immersion. (a) A-type zeolite/glass, (b) mordenitezeolite/glass, (c) X-type zeolite/glass, and (d) Y-type zeolite/glass.

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zeolite in the hybrid materials. This evolution of removal rate issimilar to that reported by Kamitani et al.7) using A-type zeolitematerials, with our study showing higher removal rates. Thehybrid materials with X-type zeolite showed different removalrates compared with the other hybrid materials, especially forzeolite content greater than 20%. As the content of X-type zeoliteincreased beyond 25%, the Sr2+ removal rate increased greatlyand a 100% removal rate was observed for X-type zeolite contentexceeding 62.5. The Sr2+ removal experiment was carried out onthe hybrid material containing X-type zeolite by changing theconcentration of the Sr2+ standard solution. As seen in Fig. 6,even when the concentration of the Sr2+ standard solution wasincreased from 10 ppm to 20, 50, and 100 ppm, it was possible toremove almost all Sr2+ ions with a hybrid material containingmore than ³50% zeolite content.It is considered that the X-type zeolite hybrid materials show

higher Sr2+ removal rate than other samples because of the char-acteristics of X-type zeolite. For the samples of A-type, Y-type,and mordenite zeolites, after the immersion experiment, thesamples with 100% zeolite sometimes collapsed, while the hybridmaterials containing glass powders did not collapse. The sampleswith 100% zeolite broke because the zeolite is difficult to sinterand contains no binder glass. On the other hand, hybrid materialscontaining X-type zeolite and 12.5% glass collapsed easily. It isthought that this is caused by the large number of water mole-cules in X-type zeolite. The porosity of the hybrid materialsincreases by heating water as the water enters the zeolite phase,and the air in the composite breaks the material when it tries toescape to the outside during immersion.9) Therefore, we preparedhybrid materials with the zeolite sufficiently heated. There was nosignificant change in the Sr2+ removal property of these hybrid

materials. The X- and Y-type zeolites have large pores with three-dimensional network structure. The pores are called supercagesand they connected to one another through 0.74 nm windows inthe tetrahedral arrangement. The supercages have much largeradsorption sites than other zeolites. Although Y-type zeolite hasthe same skeletal structure as X-type zeolite, the removal ratesof Y-type/glass hybrid materials were inferior to X-type/glasshybrid materials. This may be because the hydrophilicity of Y-type zeolite is low and therefore, it shows poor adsorption fromthe aqueous phase compared with the more hydrophilic X-type zeolite.10) Although A-type zeolite is hydrophilic, the Sr2+

removal rates of A-type zeolite/glass samples are equivalentto those of mordenite zeolite/glass hybrid samples, which arehydrophobic. This is probably because A-type zeolite has smallerpores than mordenite zeolite does. The morphology of the usedzeolites can be effective for the ion-exchange properties ofzeolite/glass hybrid materials. Both the X-type zeolite and the Y-type zeolite are almost spherical, and the particle diameter of theY-type zeolite is smaller than that of the X-type zeolite as shownin Fig. 4. From the viewpoint of the specific surface area, it isexpected that the size of Y-type zeolite is smaller, so that thespecific surface area increases and the ion-exchange capacity willincrease. However, the ion-exchange capacity of Y-type zeolite/glass hybrid samples is inferior to that of the X-type zeolite/glasssamples. Although the reason is not clear, the hybrid materialswith X-type zeolite and ground glass powder showed better ion-exchange properties. Further investigation of this system isneeded to clarify the nature of ion-exchange properties.

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Fig. 6. Results of Sr2+ removal for X-type zeolite/glass hybridmaterials after 6 h immersion. (a) 10 ppm, (b) 20 ppm, (c) 50 ppm, and(d) 100 ppm Sr2+ standard solution.

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