the sorption of some radiocations on microporous titanosilicate ets-10

Journal of Radioanalytical and Nuclear Chemistry, Vol. 258, No. 2 (2003) 243–248

0236–5731/2003/USD 20.00 Akadémiai Kiadó, Budapest© 2003 Akadémiai Kiadó, Budapest Kluwer Academic Publishers, Dordrecht

The sorption of some radiocations on microporous titanosilicate ETS-10C. C. Pavel,* K. Popa, N. Bilba, A. Cecal, D. Cozma, A. Pui

Faculty of Chemistry, “Al. I. Cuza” University, Iasi, Romania(Received December 11, 2002)

The microporous titanosilicate ETS-10 synthesized from gel with following molar composition: 1.0 Na2O:1.49 SiO2 : 0.2 TiO2 : 0.6 KF : 1.28 HCl : 39.5 H2O was subjected to sorption of radioactive cations 115Cd2+, 204Hg2+, 60Co2+ and 137Cs+ (M) fromaqueous solution, in the absence of ionic competition. The uptake of these cations on the ETS-10 was compared by means of the distributioncoefficient (Kd) versus contact time and sorption capacity (R) at equilibrium. The FT-IR spectra of M-ETS-10 sorption products exhibit amodification of the absorption band, principally at 381 cm–1.

Introduction

After the Chernobyl accident a great amount ofradioactive substances, mainly the artificialradionuclides 137Cs and 90Sr, were emitted in theEuropean’s water and atmosphere. The total surface inEurope, affected by 137Cs was estimated at twopercents.1 The presence of the radioactive cations suchas 115Cd2+, 204Hg2+ and 60Co2+ in the environment is apotential health hazard.

The general goal of the treatment of medium and lowlevel radioactive liquid wastes is to reduce their activityto a level suitable for storage and disposal. The use ofthe cation exchange proprieties of zeolite has receivedmuch attention over the past decades in water andindustrial waste treatment.

Zeolites constitute a major class of natural andsynthetic aluminosilicate, crystalline, microporousmaterials,2,3 exhibiting, due to their structure, uniquephysical and chemical properties. Because of theseproperties, the zeolites have found a wide spectrum ofenvironmental and industrial applications (e.g.,molecular sieves, ion exchangers, adsorbents, catalystsand detergent builders). Of special importance for theenvironmental applications is their ability to retainheavy metal species4 and radionuclides from aqueousmedia. The removal and purification of radioactivecesium and strontium from the environmental andaqueous systems by clay minerals and zeolites startedalmost simultaneously with the development of thenuclear power industry.5,6

Microporous crystalline titanosilicates with corner-sharing TiO62– octahedral and SiO4 tetrahedral asbuilding units constitute a novel zeotype family. ETS-10is undoubtedly the most interesting one of thesematerials, possessing a three-dimensional 12-ring poresystem, and presenting considerable potential for beingused as catalyst, desiccant and ion-exchanger.7–9 Thepresence of tetravalent titanium in octahedralcoordination generates a “–2” formal negative charge on

* E-mail: [email protected]

framework, which is balanced by exchangeable cations.A typical unit cell composition of the as-synthezisedmaterial is Na1.5K0.5TiSi5O13.

Due to octahedral titanium [TiO6]2– and relativelyhigh amounts of titanium in their structure(TiO2/SiO2 = 0.2), ETS-10 has high ion exchangecapacity (~4.5 meq/g ETS-10 anhydrous). The ionexchange capacity, chemical stability, resistance toradiation and the pore size (~8 Å is in the size ofhydrated ions) recommended this material as suitable forthe retention of toxic and radioactive metal ions fromwastewaters.10–13

The purpose of this research was to investigate thesorption behavior of ETS-10 against some radionuclidesas well as to compare the corresponding distributioncoefficient values, which can be used as a measure oftheir removal/retention ability.

Experimental

Materials

Institute of Atomic Physics Magurele, Bucharest,supplied radioactive cations 115Cd2+, 204Hg2+, 60Co2+(as chlorides) and 137Cs+ (as nitrate); as non-radioactivecarriers CdCl2, HgCl2 and CsNO3 (Fluka), CoCl2(Chemapol Prague) were used. Other reagents used inthis work were of analytical reagent grade and wereobtained from chemical commercial suppliers.

InstrumentsThe X-ray diffraction (XRD) patterns of the ETS-10

samples were recorded on a Philips PW 1830 X-raypowder diffractometer, using Cu Kα radiation; the scanrate was 0.02° per second, in a 2θ range of 5–45°. Themorphology and the dimensions of the crystals wereevaluated by scanning electron microscopy (SEM) on aMicrospec WDX-2A microscope at 25 kV.

C. C. PAVEL et al.: THE SORPTION OF SOME RADIOCATIONS ON MICROPOROUS TITANOSILICATE ETS-10

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The elemental chemical analysis for the determination ofion exchange capacity was performed using a ShimadzuAA-660 atomic absorption spectrometer. The FT-IRspectra were taken on a Jasco 660-Plus Fouriertransform infrared spectrometer in the transmissionmode using KBr-diluted samples against a KBr standard.The measured wave number range was 350–1400 cm–1with a resolution of 4 cm–1 and a scanning speed of2 mm/s. The pH of the filtrates was determined using aRadelkis Model OP−208/1 pH-meter with a calomelelectrode. Centrifuging was carried out using a MSEOberwil 2000-rpm centrifuge. The reagent systems werethermostated at 20 or 40 °C using a Messgeräte – WerkLauda water bath thermostat, and at 4 °C using acommercial refrigerator. The radioactivitymeasurements were carried out using a scintillationdetector connected to a multichannel analyzer CanberraOmega 1.

Synthesis of titanosilicate ETS-10The hydrothermal synthesis of ETS-10 titanosilicate

was carried out according to the previous procedure,14starting from a gel with following molar composition:1.0 Na2O : 1.49 SiO2 : 0.2 TiO2 : 0.6 KF : 1.28 HCl : 39.5H2O. The gel was prepared at ambient temperature bymixing an acid solution (A) containing TiCl4 (Merck),KF (Panreac) and HCl (37 wt.%, Riedel-De-Haen) witha basic solution (B) containing sodium silicate (27 wt.%SiO2, 8 wt.% Na2O, Merck) and NaOH (Carlo Erba)under constant stirring at room temperature. Thecrystallization was carried out at 190 °C for 3 days. Theproduct recovered by filtration was washed with distilledwater and dried at 105 °C overnight.

ProcedureThe sorption of ETS-10 titanosilicate for the Cd2+,

Hg2+, Co2+ and Cs+ cations was measured in theabsence of competing ions by isotope dilution analysis.The characteristics of used radioactive solutions aregiven in Table 1.

Two ml from every initial radioactive solution115CdCl2, 204HgCl2, 60CoCl2 and 137CsNO3 wasintroduced in a 500-ml calibrated flask and bidistilledwater was added to the mark. The concentrations ofsolutions were fixed to 5.10–3M, using the abovementioned non-radioactive carriers.

Three parallel solution series were prepared inBerzelius flasks, containing 100 ml 115CdCl2, 204HgCl2,60CoCl2 and 200 ml 137CsNO3. These volumes werechosen for heaving the same total number of mili-equivalent gram of cation (namely one) in each sample.

Every solution series was thermostated to 4, 20 and40 °C. Afterwards, 0.1 g ETS-10 titanosilicate was

added. This moment was considered as the starting timefor the sorption experiment. In order to maximize thesolid/liquid interaction, the reaction Berzelius flaskswere shaken discontinuously. After 12, 24, 48, 72 and148 hours, 1 ml from each solution were taken out.These liquid aliquots were centrifuged (for theelimination of the rest of ETS-10 from the suspension),the supernates were introduced in the porcelain cruciblesand evaporated to dryness. In order to determine thesorption capacity, standard samples were prepared underthe same conditions, using 1 ml from each startingsolution.

The activity of the dried samples was measured bymeans of a multichannel analyzer. The areas of the1.28 MeV (for 115Cd2+), 0.28 MeV (for 204Hg2+),1.17 MeV (for 60Co2+) and 0.66 MeV (for 117Cs+) peakswere taken into account. All the measurements ofstandards and samples were carried out of the end of theexperiment in order to eliminate the error coming fromthe radionuclides decay.

The uptake of these cations were expressed in termsof the distribution coefficients (Kd, ml/g) and sorptioncapacities (R, meq/g). The distribution coefficient (Kd) isdefined as the ratio between the activity of sorbedspecies (directly proportional with concentration of thecations) per gram of titanosilicate and the activity of thesame species per ml in the liquid phase, at equilibrium:

MV

AAAK

ttid ⋅−= ml/g (1)

where Ai and At are the initial solution radioactivity andat time t, respectively, V represents the volume (ml) ofthe solution and M is the initial weight of the dryETS-10.

The sorption capacity (R), defined as the differencebetween the initial (Ci) and at time t (Ct) concentrationof solutions, in mili-equivalent ion retained per gram(meq/g) of dry ETS-10 was used, too.

The FT-IR spectra were recorded in order todetermine the vibrational energy that is the mostdisturbed by the presence of the sorbed cation.

Results and discussion

Characterization of ETS-10 titanosilicate

The X-ray powder diffraction pattern of the as-synthesized ETS-10 showed to be identical to thosegiven in the literature7 (Fig. 1a). The stability of ETS-10structure to γ-irradiation was tested using a 60Co sourceof 3.104 Ci with a dose rate of 8.3 kGy/h. The structureof ETS-10 did not change after 72 hours of γ-irradiation(Fig. 1b).


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Fig. 1. XRD pattern of ETS-10 before (a) and after (b) γ-irradiation

Fig. 2. SEM images of ETS-10 crystals

Figure 2 shows the morphology and size of the pureETS-10 crystals. Their quasi-cubic form and theirtendency to agglomerate can be observed (Fig. 2a). Thesmall particle size (∼1 µm, Fig. 2a) has a positiveinfluence on their adsorption property.11

The ion exchange capacity was determined by AAS,after the dissolution of the sample in HF and distilledwater. A value of 3.85 meq/g ETS-10 hydrated wasobtained (3.14 meq/g Na+ and 0.71 meq/g K+) or4.47 meq/g based on the anhydrous mass of the ETS-10titanosilicate.

Fig. 3. Dependence of Kd of 115Cd2+ (a), 204Hg2+ (b), 60Co2+ (c) and137Cs+ (d) on ETS-10 vs. contact time, at 4, 20 and 40 °C

Study of the sorption processThe sorption of radioactive cations on ETS-10

titanosilicate was studied as a function of contact time atthree different temperatures: 4, 20 and 40 °C. Takinginto account the ionic charge, the batch factor (V/M) was


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1000 ml/g and 2000 ml/g for the divalent cations and themonovalent one (137Cs+), respectively. The results ofthe experiments are presented in Fig. 3.

For all sorption systems and the studied cations, arapid increase of Kd in the first minutes of contact can beobserved. The higher is the exchange temperature, thehigher is the value of Kd, further, the equilibrium valueis established more quickly. The maximum distributioncoefficient and sorption capacity at equilibrium, stated inml/g (Kd) and meq/g (R), respectively, are summarizedin Table 2.

The distribution coefficient is increasing in the seriesof:

137Cs+>115Cd2+ ≅ 204Hg2+>60Co2+ at 40 °C and137Cs+>115Cd2+>60Co2+>204Hg2+ at 20 and 4 °C.After 168-hour contact time, the sorption equilibrium

is attained for all the studied cases, except for 60Co2+and 137Cs+ at 4 °C.

Increasing the exchange temperature from 20 to 40 °Cdoes not generate an important augmentation of 115Cd2+,60Co2+ and 137Cs+ sorption on ETS-10. For 204Hg2+ aregularity between the established temperature and themaximum distribution coefficient can be observed: at4 °C, Kd, max = 45% of the corresponding value at 40 °C,while at 20 °C, the value of Kd, max represents already∼75% of the last value.

The maximum exchange levels for the used cations,obtained at 20 °C, on ETS-10 correspond approximatelyto the theoretical exchange capacity of dehydrated ETS-10 titanosilicate (4.47 meq/g) (Fig. 4). However, at20 °C the maximum sorption capacity (R) values vary inthe order of:

137Cs+ > 115Cd2+ > 60Co2+ > 204Hg2+

as shown in Table 2.

Fig. 4. Variation of sorption capacity (R, meq/g) for 115cd2+ (■),204Hg2+ (●), 60Co2+ (▲) and 137Cs+ (▼) on ETS-10 titanosilicate vs.contact time at 20 °C

Thermodynamic consideration of sorptionTo calculate the thermodynamic constants ∆H°, ∆S°

and ∆G°, the sorption experiments were studied at 277,293 and 313 K. The values of ∆H° and ∆S° werecalculated from the slopes and intercepts of the linearvariation of lnKd vs. 1/T using the relation:15

RTH

RSKd

oo ∆−∆=ln (2)

where Kd is the distribution coefficient (ml/g), T is theabsolute temperature (K) and R is the gas constant(kJ/mol.K).

The standard free energy values were calculatedfrom:

ooo STHG ∆⋅−∆=∆ (3)The values of ∆H°, ∆S° and ∆G° are given in Table

3. As it can be seen, ∆H° is positive for the sorption of115Cd2+, 204Hg2+, 60Co2+ and 137Cs+ cations on ETS–10titanosilicate and the process is endothermic.

Table 1. Characteristics of the radioactive solution usedCharacteristics 115Cd2+ 204Hg2+ 60Co2+ 137Cs+

Compensation anion Cl– Cl– Cl– NO3–Molar concentration, cM 5.10–3 5.10–3 5.10–3 5.10–3pH 6.1 6.0 5.8 5.1Ionic radius, Å 0.97 1.10 0.74 1.69


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Table 2. The maximum distribution coefficient (Kd) and sorption capacity (R) atequilibrium after 168-hour contact time of the solution with the ETS-10

Radioactive ion Temperature, °C Kd, ml/g R, meq/g4 °C 631 3.87

20 °C 791 4.43115Cd2+

40 °C 811 4.484 °C 341 2.54

20 °C 627 3.85204Hg2+

40 °C 808 4.454 °C 436 3.03

20 °C 733 4.2360Co2+

40 °C 760 4.314 °C 1258 3.83

20 °C 1636 4.50137Cs+

40 °C 1670 4.55

Table 3. Thermodynamic parameters for the sorption of 115Cd2+, 204Hg2+, 60Co2+ and 137Cs+ on ETS-10∆G°, kJ/molRadiocation ∆H°,

kJ/mol∆S°,

kJ/mol.K 277 K 293 K 313 K115Cd2+ 5.26 0.072 –14.68 –15.83 –17.27204Hg2+ 14.57 0.102 –13.68 –15.31 –17.3560Co2+ 11.23 0.092 –14.25 –15.72 –17.56137Cs+ 5.57 0.079 –16.31 –17.57 –19.17

Fig. 5. The FT-IR spectra of the M-ETS-10 samples(M=Co2+, Hg2+, Cd2+ and Cs+)

The free energy values for all the systems are negative,and the increase of the absolute value of ∆G° withincreasing temperatures shows that higher temperaturesfavour the ionic exchange.

The cation influence on ETS-10 structureThe ETS-10 titanosilicate structure was reconfirmed

by the FT-IR spectra (Fig. 5). The position of the majorabsorption bands and shoulders corresponding to thedifferent symmetric and asymmetric stretchingvibrations are the same as in the FT-IR spectra for ETS-10 reported by YANG et al.16

In agreement with MIHAILOVA et al.,17 the highfrequency (above 1000 cm–1) is dominated by Si–Obond stretching modes, which generate a stronglyintense band. In the midfrequency range of FT-IR spectra(400–800 cm–1) both Ti–O and Si–O modes contributeto the peaks. The low frequency range (200–400 cm–1)is determined mainly by interactions between Ti and Oatoms in the [TiO6]2– octahedra.

As a consequence of the interactions between thecations (Mn+) and the [TiO6]2– and/or [SiO4] structuralunits the modification of the intensity and the position ofcorresponding peaks on the FT-IR spectra of M-ETS-10samples can be observed (Fig. 5). No correlation betweenthe ionic radius and FT-IR spectral wave numbers wasfound.


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Conclusions

The ETS-10 titanosilicate shows a considerableability to remove 115Cd2+, 204Hg2+, 60Co2+ and 137Cs+from their aqueous solutions, in the absence ofcompeting ions.

For the temperatures higher than 20 °C, the sorptioncapacity (R) of ETS-10 for the above mentionedradioactive ions has an equal value to the ion exchangecapacity of the anhydrous ETS-10 (4.47 meq/g).

The positive values of ∆H° and the negative valuesof ∆G° show that the radioactive cations 115Cd2+,204Hg2+, 60Co2+ and 137Cs+ are preferred in ETS-10phase. The exchange reactions being endothermic, thesorption of radiocations is facilitated by increasingtemperature.

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the sorption of some radiocations on microporous titanosilicate ets-10

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