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Journal of Radioanalytical and Nuclear Chemistry, Vol. 258, No. 3 (2003) 551–556 ... toward cerium and thorium from nuclear wastewaters. Column experiments ...

Journal of Radioanalytical and Nuclear Chemistry, Vol. 258, No. 3 (2003) 551–556

Iranian natural clinoptilolite and its synthetic zeolite P for removal of cerium and thorium from nuclear wastewaters H. Kazemian,1 H. Modarres,2 H. Ghasemi Mobtaker1 1 Jaber

Ibn Hayan Research Labs, Atomic Energy Organization of Iran (AEOI), North Amirabad St., Tehran, Iran 2 Department of Chemistry, Amir Kabir University (Tehran Poly-Technique), Tehran, Iran (Received April 24, 2003)

The ion-exchange behaviors of an Iranian natural clinoptilolite and its modified forms as well as a relevant synthetic zeolite P were investigated toward cerium and thorium from nuclear wastewaters. Column experiments were performed on different exchangers in various conditions and the effect of parameters such as particle size, pH, temperature, and time were considered. The distribution coefficient, cation exchange capacity and some thermodynamic parameters were calculated. Ion-exchange isotherms and break-through curves were plotted. As a result, the selectivity of synthetic zeolite P from Iranian natural clinoptilolite toward cerium and thorium was compared with that of natural and cationic forms of clinoptilolite.

Introduction Zeolites are microporous high-internal-surface crystalline and hydrated aluminosilicates of alkali and alkaline earth cations with an infinite open and rigid three-dimensional structure. The three-dimensional framework consists of AlO45– and SiO44– tetrahedral units linked through shared oxygens. Synthetic zeolites are usually produced under varying conditions and techniques. Hydrothermal technique is a well-known method. Natural and synthetic zeolites have been widely studied as exchangers for the treatment of liquid radioactive wastes. So far, most studies in this field have been focused on the removal of mono- and di-valent cations of fission products (especially 137Cs and 90Sr) from nuclear wastes using both natural and synthetic zeolites.1,2 Moreover, zeolites can be considered as alternative materials for the removal of multivalent and heavier radioisotopes like actinides and lanthanides. Lanthanides such as cerium (Ce) are commonly found in the waste streams of nuclear power plants and research centers.3 On the other hand, thorium-232 appears as an impurity in the production of nuclear fuel. The conventional processes for the removal of this radionuclide are solvent extraction and organic ion exchanger resins.4 This paper is an early work of a great project under progress entitled “Nuclear Waste Treatment Using Inorganic Ion-Exchangers” in the Jaber Ibn Hayan Research Labs (JIHRL) of the Atomic Energy Organization of Iran (AEOI). The results as bench scale tests were promising.

Experimental Materials The natural zeolite used in this study was a clinoptilolite from Meyaneh region of Iran. The zeolite was ground and wet sieved to a particle size of 224– 500 µm. The powder was then washed with distilled water in order to remove probable soluble salts and dried at 323 K for about 24 hours. The powder was stored in a desiccator over a saturated NaCl solution, in order to maintain a constant vapor pressure during the whole period of experiments. Various cationic forms of clinoptilolite were prepared by treating the zeolite powder with a normal solution of KNO3, NH4NO3 and NaNO3 at 353 K for 24 hours. The ratio of the solution volume to the mass of the zeolite was adjusted at 1:25 in all experiments. The product was then washed, dried, and stored in the desiccator as before. The clinoptilolite H-form was prepared by heating clinoptilolite NH4-form at 653 K for 3 hours. The natural and modified samples were characterized using XRD (Philips PM 9920105, Cu K , 40 kV), XRF (Philips PW 1480) and STA (Rheometric Scientific thermal analyzer). The clinoptilolite and its cationic forms were used in preliminary cerium and thorium experiments. The experiments indicated that clinoptilolite and its cationic forms do not have significant selectivity toward both cerium and thorium. For both cations, the distribution coefficient (Kd, ml/g) at the concentration of 0.01N was in the range of 10–30 ml/g.

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H. KAZEMIAN et al.: IRANIAN NATURAL CLINOPTILOLITE AND ITS SYNTHETIC ZEOLITE P

In order to improve the ion exchange capability of the natural sample, a zeolite P was synthesized from natural clinoptilolite and characterized using the above considered methods. The synthetic zeolite P was hydrothermally synthesized by treatment of powdered natural clinoptilolite with sodium hydroxide solution.5 In this regard, a powder sample of clinoptilolite was sieved to a particle size of 224–500 µm and placed in a Teflon vessel in contact with a solution of 3N NaOH. The ratio of solution volume to the mass of zeolite powder (V/m) was held at 10 ml/g. The vessel was sealed in a stainless steel container. After heating up to 373 K for 2 days, it was shaken. Using a paper filter, the solid phase was separated, washed, and dried at 333 K for about 24 hours. The synthesized sample was stored in a desiccator over a saturated sodium chloride solution. The formation of zeolite P was ascertained by thermal analysis (STA) and X-ray analysis (XRD/XRF) in comparison with references. Ion-exchange experiments According to the preliminary tests, the synthesized zeolite P was selected for further ion-exchange investigations. Sets of batch experiments were carried out on a sample portion of 0.2 g of synthesized zeolite to elucidate the maximum equilibration period (kinetic tests) for the uptake of cerium from 25 ml cerium nitrate (0.01N) and thorium nitrate (0.02N). All experiments were carried out at 298 K, with a shaking rate of 250 rpm. Filter-separating of solid phase from liquid was followed by centrifuging (5000 rpm for 30 minutes) in order to separate colloidal particles. A portion of supernatant was taken for cerium measurement using ICP-AES technique (Liberty 150 AX Turbo) and thorium using a colorimetric method. The colorimetric method was carried out with Thoron as a specific reagent for Th determination using UVVisible spectrophotometry (CARY 3) with 1.0 cm quartz cuvette at 454 nm and a pH range of 0.4–1.2. For calculating the Th concentration, the sorption of solution was compared with a working curve that was a plot of absorbance versus standard concentration of thorium.6 Kd (ml.g–1) values were calculated by: Kd = {(Ci–Cf)/Cf}.V/W where Ci and Cf are the initial and the final concentration of considered here cations in the solution, respectively, and V is the volume of the solution (ml) in contact with W grams of the zeolite powder. The effects of various parameters such as pH, temperature and cation concentration on Kd were investigated.

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The ion-exchange isotherms were obtained by analyzing iso-normal solutions in contact with zeolite powder before and after equilibrium and plotted in terms of the equivalent fraction of cation in the zeolite phase (Az) against the equivalent fraction in the solution phase (As). Experimental isotherms were obtained at total normality of 0.1N and 0.05N for Th and Ce, respectively, to keep the ionic strength of the solution constant. The total volume of the solutions was 20 ml and the isotherms were plotted in different temperatures. The thermodynamic parameters of the ion exchange reaction ( G, Ka) were calculated with a Microsoft Excel compatible software.7 For the column experiments, a glass tube of 20 cm in length and 0.8 cm in diameter was used and 1 gram of powdered zeolite was packed in the column (about 5 cm in height). The flow rate of the solution was adjusted at 20 ml/h. The 5 ml fractions of effluent were collected and each portion was analyzed for cerium (using ICPAES technique) and thorium (using UV-Visible spectrometry).

Results and discussion The XRD and XRF results of the natural clinoptilolite sample are shown in Tables 1 and 2, respectively. From the compatibility of the results with the data reported in Reference 8, it can be concluded that the investigated zeolite is clinoptilolite. Preliminary experiments were carried out on the natural clinoptilolite and its cationic forms9 to determine the distribution coefficients (Tables 3 and 4). All experiments were carried out in concentrations of 0.05 and 0.005N for thorium and 0.01N for cerium. For enhancing the absorption property of natural clinoptilolite, zeolite P was synthesized, characterized and tested.

Table 1. Chemical composition of the natural clinoptilolite sample and Reference 8 (XRF data) Fragment SiO2 Al2O3 Na2O MgO K2O TiO2 MnO CaO P2O5 Fe2O3

Reference, wt. % 65.17 13.38 1.62 0.53 2.82 – – 3.22 – 1.06

Sample, wt. % 65.87 12.10 1.73 1.23 3.43 0.302 0.06 2.62 0.069 2.49

H. KAZEMIAN et al.: IRANIAN NATURAL CLINOPTILOLITE AND ITS SYNTHETIC ZEOLITE P

hydrogen ions are readily absorbed by the solid and excess positive charge is created especially at the surface of the crystal.11 Therefore, electrostatic interactions (repulsion) take place between the cationic species present in the solution and the zeolite leading to a lower metal uptake. In solutions with pH greater than 4.5 and up to 5.5, the uptake is increased rapidly (Fig. 3). This observation may be explained by a hydrolysis process of cerium ions in the zeolite interlayer spaces, i.e.:12

Table 2. X-ray diffraction data of the investigated zeolite P and the Reference 8 Reference

Sample

d, Å

I/I

8.96 7.94 6.81 5.26 5.12 5.08 4.65 3.98 3.564 3.429

100 12 6 10 16 14 32 65 9 21

d, Å

I/I

8.93 7.83 5.09 4.67 3.96 3.53 3.17 2.97 2.84

100 16 12 16 94 17 68 44 27

[Ce(OH2)x]3+

Table 3. Distribution coefficient (Kd) of cerium (0.01N solution) on the natural and cationic forms of clinoptilolite Natural

Na-form

K-form

H-form

NH4-form

25

25

26

18

27

Preliminary kinetic experiments were performed to determine the required time for establishing the equilibrium with the considered concentration at 298 K and the solid to liquid ratio. The results indicated that an uptake plateau was reached after 48 hours for cerium (Fig. 1) and after 24 hours for thorium (Fig. 2). The equilibrium times showed a relatively slow rate of ion exchange reaction of these cations on zeolite P. Ion-exchange studies of cerium on zeolite P indicated that the uptake of cerium on this zeolite depends on the pH of the solution at equilibrium. The uptake of cerium increased with increasing pH up to 3.5. The absolute uptake of cerium from the solution with an initial concentration higher than 450 ppm in pH

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