The removal of radioactive cesium from aqueous solutions using clinoptilolite modified by various modifiers (ferrocyanides Ni, Cu, Fe and H3BO3) was carried out. The influence of chemical composition of solution, concentration of components in solution and ratio of solution volume and amount of adsorbent on the sorption process was studied. We showed high selectivity towards 137Cs (Kd = 4.38×103 – 3.1×104 ml/g). The mechanism of increasing the selectivity with respect to 137Cs when using H3BO3 as a structure-forming additive was considered

Masherova, N. R.

Panasyugin, A. S.

Tsyganov, A. R.

Belarusian State Technological University

Panasyugin A.S.1, Tsyganov A.R.2, Masherova N.P.2 EFFECT OF STRUCTURE-FORMING ADDITIVES  ON ION-EXCHANGE PROPERTIES  OF FERROCYANIDE-ALUMINOSILICATE SORBENTS 1Belarusian National Technical University, Belarus 2Belarusian State Technological University, Belarus niilogaz@tut.by  The removal of radioactive cesium from aqueous solutions using clinoptilolite modified by various modifiers (ferrocyanides Ni, Cu, Fe and H3BO3) was carried out. The influence of chemical composition of solution, concentration of components in solution and ratio of solution volume and amount of adsorbent on the sorption process was studied. We showed high selectivity towards 137Cs (Kd = 4.38×103 – 3.1×104 ml/g). The mechanism of increasing the selectivity with respect to 137Cs when using H3BO3 as a structure-forming additive was considered. The use of radioactive materials for medical, scientific and industrial purposes produces large amounts of low- and intermediate-level radioactive waste. Cesium-137 is one of the most dangerous radionuclides because of its long half-life and high compound solubility. Natural aluminosilicate materials, particularly clinoptilolite, are widely used to remove cesium from radioactive solutions [1]. Clinoptilolite is a natural zeolite. It has a low cost and is readily available. The spent solutions have  a complex composition. This imposes stringent requirements for the selectivity of adsorbents. Natural ion-exchange materials are characterized by sufficiently high exchange capacity, but low selectivity. It is well known that ferrocyanides possess high selectivity towards cesium [2-8]. The aim of the present study was to increase the selectivity of clinoptilolite. For this purpose, various additives (nickel, iron, and copper ferrocyanides and boric acid as a structure-forming agent) were applied to the surface of clino-ptilolite. The ability of the synthesized composite ion-exchangers 20to absorb cesium from active solutions was investigated depending on the chemical composition of the solutions, the concentration of the components in the solution, and the ratio between the solution volume and the sample amount. All reagents used were brands pure for analysis. Clinoptilolite from the Sokirnitsa deposit (Ukraine) was chosen as a matrix. The chemical composition of clinoptilolite is as follows:  SiO2 – 68.64%; Al2O3 – 11.50%; Fe2O3 – 1.57%; MgO – 0.89%; CaO – 2.38%; Na2O – 0.79%; K2O – 2.98%; H2O – 4.12%; SiO2/Al2O3 – 10.13%. The clinoptilolite samples were crushed, ground, and sieved to a particle size in the range of 250-1000 μm. The samples were then converted to sodium form by shaking in 1 M sodium chloride solution for 24 hours, then filtered, washed with distilled water, and air dried. Clinoptilolite modified with nickel, iron, and copper ferro-cyanides was synthesized as follows. First, Na+-clinoptilolite was converted to the Met+-form (Met+ = Ni2+, Fe3+, Cu2+) by adding solutions of salts of the corresponding metals. After 2 hours of exposure, the mixture was filtered off and washed with distilled water. Then 0.1 M solution of K4[Fe(CN)6] was added to clino-ptilolite treated with the corresponding cation. Then samples were filtered, washed and dried for two days at room temperature.  Ferrocyanide-containing samples were synthesized in  the presence of boric acid using structure-forming additives.  The clinoptilolite, which was in Fe-, Cu-, or Ni-form, was treated with a 10% boric acid solution before the stage of treatment with potassium ferricyanide. Then the obtained samples were washed with water to remove BO33+ ions. Completeness of washing was checked by infrared spectroscopy by the absence of characteristic lines corresponding to borate ions (1200 cm-). The details of preparation of composite ion-exchangers based on clinoptilolite were described in previous works [5,6]. Adsorption of 137Cs long-lived isotopes was performed under static conditions on model solutions containing calcium, 21magnesium, sodium and chlorine ions as salt background at their weight ratio of Na:Ca:Mg:Cl=1:5:5:25. The total content  of background salt ions was 360 mg/l, which corresponds to the chemical composition of drinking water in Minsk (Belarus). In experiments with radioactive solutions, 10 ml of a solution containing 137Cs was added to batches of 0.1 g of test samples. The suspensions were shaken for 24 hours at 25 oC. After separating the solid sample from the liquid phase, the activity  of the solution was measured. The initial activity of 137Cs in  the solutions was 4.44-102 Bq l-1. Activity of solutions before and after establishment of equilibrium was measured by means  of multichannel radiometer RUS-91 equipped with spectrometric scintillation detectors. Activity of radionuclide 137Cs was determined by measuring intensity of gamma radiation. Cesium removal experiments were performed under dynamic conditions. 137Cs. The experiments were carried out in a glass column with a diameter of 10 mm at a filtration rate  of 8-10 column volumes per hour. The height and volume of the adsorbent layer were 60 - 65 mm and 5.5 - 6.0 cm3, respectively. According to the experimental values of the activity  of the solution before and after passing through the sorbent,  the purification factor (Koch.) and the degree of sorption (S, %) were determined. The data presented in Fig. 1 and Table 1 show different effects of the modification on the ion-exchange properties of clinoptilolite. Thus, the exchange capacity (E.C.) for cesium ions of ferrocyanide samples synthesized in the absence of H3BO3 did not change significantly. We observed only a slight increase in its value for the sample containing iron ferrocyanide, which, at first sight, contradicts the literature data [7] indicating an increase in the exchange capacity of individual ferrocyanides in the series FCNi > FCu > FCFe. However, it should be taken into account that the E.C. of the individual ferrocyanides essentially depends on the method, the conditions of their production, and,  as a consequence, on the crystal structure being formed.  22The deposition of ferrocyanides on the external and internal surfaces of the aluminosilicate is accompanied, as in the case of adsorption of alkali metals on individual ferrocyanides [2], by the distortion of their crystal structure, especially under the conditions of the limited intracrystalline space of the zeolite. This circumstance determines to a greater extent the changes in the E.C. values for cesium of ferrocyanide-alumosilicate samples that we have established.  Fig.1. Kinetic curves of alkali metals exchange, where  a - clinoptilolite modified with Cu ferricyanide, b - clinoptilolite modified with Cu ferricyanide and H3BO3,  F = Qτ/Q∞ - absorption degree of incoming cation Table 1. Ion-exchange characteristics with respect to cesium  of ferricyanide-alumosilicate sorbents depending on the nature  of ferricyanide and structure-forming additive Sample + structure-forming additive S, % Кoch Kd , mg/g Exchange capacity, by Cs+, mg-eq/g Initial clinoptilolite 97.9 46.9 4.59×103 1.51 Ferrocyanide Fe 98.1 54.1 5.0×103 1.6 Ferrocyanide Fe + H3BO3 98.8 84.4 8.3×103 1.49 Ferrocyanide Cu 99.2 142 1.4×104 1.51 Ferrocyanide Cu + H3BO3 99.7 309 3.1×104 1.46 Ferrocyanide Ni 98.8 88.0 8.7×103 1.55 Ferrocyanide Ni + H3BO3 99.4 162 1.6×104 1.33 0 50 100 150 200 2500,00,20,40,60,81,0(a)F H+-Li+ H+-Na+ H+-Cs+t, мин.23The modification of clinoptilolite with ferricyanides in the presence of boric acid causes an increase in the selectivity of the samples, which is associated with the increasing manifestation of their ionocyte properties due to the features of the forming struc-ture. In spite of the fact that the presence of BO33+ ions in the modified samples was not detected by X-ray diffraction analysis and IR spectroscopy (which testifies to the absence of their interaction with exchangeable zeolite cations and ferrocyanide anions), they nevertheless play the role of a structure-forming additive, which forms, after their removal in the synthesis process, intercrystalline cavities similar in size to hydrated cesium ions (rCs+ = 3.1 Ǻ и rBO33+ = 2.96 Ǻ) preventing for steric reasons the adsorption of mineral background ions. The influence of the steric factor on the process of alkali metal ions exchange can be confirmed by the kinetic curves of exchange on clinoptilolite modified with copper ferricyanide (Fig. 1), which was chosen as the object of study due to its highest selectivity to Cs as compared to the original zeolite.  To quantify the exchange rate and determine its limiting stage, kinetic parameters of exchange were calculated. Comparison  of the rate constants of the external diffusion process (R) with the exchange constant of the intradiffusion process (B) shows that  at a given solution concentration the exchange rate of H -Li and H -Na is mainly determined by the intradiffusion process  (Table 2), because R>>B. It should be noted that the values of the rates of the external and intra-diffusion processes, as well as the effective diffusion coefficient D' increase with decreasing counte-rion size. Consequently, the mobility of alkaline cations in the modified samples decreases, as for the original clinoptilolite (7), in the series Li > Na > Cs , which is the reverse of the selectivity series. From the kinetic curves of H -Li , H -Na, and H -Cs ion exchange on clinoptilolite modified with copper ferricyanide (Fig. 1) it is seen that up to 80%, 65%, and 55%, respectively, of the exchange capacity is realized by the external diffusion mechanism. 24Table 2. Dependence of kinetic parameters of exchange  of monovalent cations on clinoptilolite on the nature of structure-forming additive (t=0C; fraction 1.5-2.5 mm; СМeОН = 0.1 n) Structure-forming additive Ex-change ions External diffusion region Intradiffusion region dQ/dτ,  mg-eq/g с R, с- D́, cm2/с В, с- Ferro-cyanide Cu  H+- Li+ 0.003 8.04×10-3 4.08×10-8 1.0×10-5 H+-Na+ 0.0028 6.11×10-4 5.14×10-8 1.27×10-5 H+-Cs+ 0.041 3.43×10-4 6.15×10-8 1.52×10-5 Ferro-cyanide  Cu + H3BO3 H+- Li+ 0.0017 5.16×10-3 1.54×10-8 3.79×10-6 H+-Na+ 0.0018 3.50×10-4 2.43×10-8 6.00×10-6 H+-Cs+ 0.0097 8.00×10-5 5.04×10-8 1.24×10-5 For the samples modified by copper ferricyanide in the pre-sence of boric acid (Fig.1, Table 2), a decrease in almost all kinetic parameters of ion exchange is observed. From the data of Fig.1 it is clear that in the case of H -Cs exchange by the fast external diffusion mechanism, only 30% of the exchange capacity is realized. The remaining 60-70% of the exchange capacity can be realized by the slow intradiffusion process. Thus, the introduction of boric acid changes the ratio between the rates of external and intradiffusion processes, causing the course of H -Cs exchange by the mixed mechanism. From the data in Table 2 it can be concluded that the introduc-tion of structure-forming additive in clinoptilolite significantly affects the rate constant (B) of the intradiffusion process. Thus, when hydrogen ions are exchanged for lithium ions, the B value decreases from 1.0×10 s to 3.79×10 s, i.e. by 2.7 times. For the exchange of H-Na, it is observed to decrease 2.1 times. In case of exchange for cesium ions, the value of the rate constant of the int-radiffusion process decreases only by 20%. This character of the change in the B constant indicates a slowing down of the ex-change rate of Na and Li ions on ferrocyanide samples modified in the presence of boric acid, and, as a consequence, an increase 25in their selectivity in relation to cesium. The data in Table 3 confirm the above, since the samples modified with ferricyanides in the presence of H3BO3 possess the distribution (Kd) and purification (Koch) ratios 1.8-6.7 times higher compa-red to the initial clinoptilolite and 1.5-2.2 times higher compared to the sorbents synthesized without the structure-forming additive. Table 3. Influence of the structure-forming additive (H3BO3)  on the values of distribution coefficients of 137Cs Sample Kd, ml/g Na-clynoptylolite 4.38×103 Clynoptylolite + Cu ferrocyanide 1.4×104 Clynoptylolite + Cu ferrocyanide + H3BO3 3.1×104 Thus, comparative tests on removal of cesium-137 by means of initial clinoptilolite and modified clinoptilolite modification by copper ferricyanide and boric acid as a structure-forming agent have been performed. It was found that modification of the clinoptilolite surface with a structure-forming additive leads to  a 1.5-2.2-fold increase in selectivity. As a result of dynamic experiments, it was found that a column filled with clinoptilolite, a ferrocyanide Cu exchanger, allows the deactivation of 137Cs from approximately 900 column volumes of solution with  an initial activity of 4.44-102 Bq l-1. References 1. Jacobi D.L., Streat M. Preparation and properties of hexacyano-ferrates for the removal of caesium from radioactive waste streams: a review. // C.L.Casarci, L.Pietrelli, ed. New separation chemistry techniques for radioactive waste and other specific applications. Elsevier Applied Science, 1991. Р. 231-243 2. Vesely V., Pekarek V. Synthetic inorganic ion exchangers. I. Salts of heteropolyacids, insoluble ferrocyanides, synthetic aluminosili-cates and miscellaneous exchangers // Talanta, 1972. V. I9. P 219-262  3. Mekhail F.M., Benyamin K. Sorption of Cesium on Zinc Hexacyanoferrate (II), on Zinc Hexacyanoferrate (III) and Hexacyanocobaltate (III) // Radiochimica Acta. 1991. V.55 P. 95-107  264. Szeglowski Z., Constantinescu O., Hussonnois M. Sorption Behaviour of I to V Group Elements on a Nickel Hexacyanoferrate (II) Composite Ion Exchanger // Radiochimica Acta. 1994. V.64 P. 127-139  5. Panasugin A.S., Komarov V.S., Ratko A.I., Trofimenko N.E., Masherova N.P. Ion exchange properties of clynoptylolite modified by metal ferrocyanides // Vesti ANB. Ser. khim. Nauk. 1993. No.2 P.30-34 6. Panasugin A.S., Trofimenko N.E., Komarov V.S., Ratko A.I., Masherova N.P. Influence of structure-forming additives on the ion exchange properties of ferrocyanide-aluminosilicate adsorbents // Neorgan. mater. 1994. V. 30. P. 1083-1086  7. Panasugin A.S., Trofimenko N.E., Masherova N.P., Ratko A.I., Golikova N.I. Sorption behaviour of cesium and strontium from minera-lised aqueous solutions on natural aluminosilicates modified by heavy metal ferrocyanides //Zhurn. prikl. khim. 1993. V.66. P.2119-2122  8. Sipos-Galiba I., Lieser K.H. Comparison of the properties of inorganic ion exchangers with respect to the separation of the fission products cesium and strontium // Radiochem. and Radioanal. Lett. 1980. V.42 P.329-343  Панасюгин А.С.1, Цыганов А.Р.2, Машерова Н.П.2 ВЛИЯНИЕ СТРУКТУРООБРАЗУЮЩИХ ДОБАВОК  НА ИОНООБМЕННЫЕ СВОЙСТВА ФЕРРОЦИАНИД-АЛЮМОСИЛИКАТНЫХ СОРБЕНТОВ 1Белорусский национальный технический университет,  2Белорусский государственный технологический университет Проведено удаление радиоактивного цезия из водных растворов  с использованием клиноптилолита, модифицированного различ-ными модификаторами (ферроцианидами Ni, Cu, Fe и H3BO3). Изучено влияние химического состава раствора, концентрации компонентов в растворе и соотношения объема раствора и коли-чества адсорбента на процесс сорбции. Показана высокая селек-тивность по отношению к 137Cs (Kd = 4,38×103 - 3,1×104 мл/г). Рассмотрен механизм увеличения селективности по отношению  к 137Cs при использовании H3BO3 как структурообразующей добавки. 27

Effect of structure-forming additives on ion-exchange properrties of ferrocyanide-aluminosilicate sorbents

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Effect of structure-forming additives on ion-exchange properrties of ferrocyanide-aluminosilicate sorbents

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Belarusian State Technological University