Adsorption of MoO 4 2- ions on Fe-treated tri-calcium phosphate

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1 Indian Journal of Chemical Technology Vol. 19, May 2012, pp Adsorption of MoO 4 ions on Fe-treated tri-calcium phosphate J Serrano Gómez*, J Bonifacio Martínez & M C López Reyes Instituto Nacional de Investigaciones Nucleares, A P , Col Escandón, Delegación Miguel Hidalgo, C P México, D F México Received 4 May 2011; accepted 24 February 2012 The Fe-treated calcium phosphate (Fe-TPO 4 ) has been prepared with FeCl 3 aqueous solution. The sorption of 99 MoO 4 ions from aqueous solution using Fe-TPO 4 as adsorbent is investigated by batch experiments. Experiments have been performed as a function of shaking time, ph, solute concentration and temperature and the experimental rate data are tested with the pseudo-second-order rate model. The adsorption data as a function of molybdate concentration obey the Freundlich isotherm. Thermodynamic parameters H 0, S 0 and G 0 have been calculated with data of temperature studies. The positive value of H 0 indicates that the adsorption of MoO 4 ions on Fe-TPO 4 is an endothermic process. Keywords: Adsorption isotherms, Calcium phosphate, Kinetics, Molybdate adsorption, Thermodynamic parameters 99 Mo is the parent nuclide of 99m Tc which is the most widely used radioisotope in nuclear medicine. Various methods have been employed to separate 99 Mo from the matrix after a uranium target has been neutron irradiated in a nuclear reactor, and adsorption and solvent extraction are the most frequently used methods. In the production of 99 Mo from neutron irradiated low enriched uranium, Vandegrift et al. 1 have used mixtures of SnO 2 & TiO 2 and ZrO 2 & TiO 2 to adsorb 99 Mo and separate it from concentrated UO 2 (NO 3 ) 2 solutions. They found that the mixture of SnO 2 and TiO 2 has superior Mo(VI) uptake. Lapka et al. 2 have carried out the extraction of molybdenum with diamides of dipicolinic acid from nitric and hydrochloric acids. They found that molybdenum shows no increase in extraction at higher concentrations of nitric acid but shows great extractability from HCl. 99 Mo has also been sorbed in calcined hydrotalcite 3 from aqueous solution through batch experiments, and the molybdate equilibrium sorption was found to be 3.2 mequiv g -1 at ph 5.5 and 20 o C. The authors found that the high molybdate adsorbing capacity of calcined hydrotalcite could be utilized in designing a 99m Tc generator made with low specific activity 99 Mo-molybdate samples. 93m Mo with high specific activity is a potential candidate radionuclide (T 1/2 = 6.9 h) in the field of radiopharmaceutical sciences and can be produced from a natural yttrium * Corresponding author. juan.serrano@inin.gob.mx target using the nuclear reaction 89 Y(Li, 3n) 93m Mo. Complete separation of no-carrier-added molybdenum from the dissolved target 89 Y has been achieved 4 by employing liquid-liquid extraction technique with trioctylamine dissolved in cyclohexane solutions. In another type of study, Goldberg et al. 5 observed the interaction of Mo as anion with iron oxides to establish the mechanism of molybdenum adsorption on soils and soil minerals. They found that molybdate ions form inner-sphere surface complexes with Fe through ligand exchange. On the other hand, to remove toxic anions form aqueous solutions, iron oxides and iron modified materials have been utilized to study the adsorption of chromates 6,7, arsenates and/or arsenites 8,9. Tri-calcium phosphate has been used as an adsorbent material for CrO 4 ions in aqueous solutions 10 but has not been investigated as a Fe-treated material to absorb anions. Therefore, in this work, Fe treated calcium phosphate has been used as a new adsorbent for 99 Mo labeled MoO 4 ions. The sorption of MoO 4 onto Fe-treated calcium phosphate is investigated in terms of variables such as shaking time, initial ph, molybdenum concentration and temperature. Experimental Procedure Treatment of tri-calcium phosphate with FeCl 3 A 10 g sample of tri-calcium phosphate (Fluka) was put in contact with 250 ml of 0.1 N FeCl 3 (Baker) aqueous solution in a 500 ml glass beaker. The suspension was heated until boiling for 1 h,

2 168 INDIAN J. CHEM. TECHNOL, MAY 2012 then cooled to room temperature and finally the supernatant was discarded. These operations were repeated 5 times. To eliminate chloride ions the red-brown solid was washed with distilled water. After centrifuging the red suspension for 30 min, the liquid phase was also discarded. The washing operation was repeated 5 or 6 times until chloride ions were not detected by a AgNO 3 test. The centrifuge tubes containing the red-brown solid were dried in a boiling water bath. Once recovered into a ceramic dish, the red-brown Fe-treated tri-calcium phosphate was thermally treated at 150 o C for 5 h. Tri-calcium phosphate and Fe-treated tri-calcium phosphate were characterized by X-ray diffraction as described elsewhere 7. Transmission infrared spectra of the same two solid samples were obtained with a Nicolet Fourier Transform Infrared Spectrometer (model 550). These two compounds were also analyzed by the Energy Dispersive Spectroscopy (EDS) technique using a DX-4 probe coupled to a Philips XL-30 Scanning Electron Microscope. Kinetic measurement 99 Mo(VI) of high specific activity in aqueous solution was acquired as a fission product from Nordion International. Low specific activity 99 Mo in aqueous solution was prepared by mixing the high specific activity 99 Mo with natural Na 2 MoO 4 (Baker) aqueous solution. The obtained radioactive solution was M with a ph value of 5.5. One hundred miligram of Fe-treated tri-calcium phosphate was shaken with 10 ml of the radioactive solution in glass vials at 20 o C, for the periods ranging from 5 min to 24 h. After shaking, the suspension was centrifuged for phase separation. The liquid was recovered with a pipette and the solid was discarded. All the experiments were performed in duplicate, by running two closed vials simultaneously. The measurement of the 99 Mo activity in the original and in the remaining solutions was made by a gamma-ray Canberra detector (model GC 2019) and a Canberra series 35 plus multichannel analyzer. The 99 Mo activity was calculated from the 740 kev peak by means of a suitable computer program in the multichannel analyzer. The amount of MoO 4 ions adsorbed per unit mass of the adsorbent (q, mmol g -1 ) and the retention per cent of the same ions were calculated by using the following equations: q = [(C _ o C e )V]/m, and %R = [(A _ b A a )100]/A a respectively; where C o and C e are the initial and equilibrium concentration of Mo in the radioactive solution and V is the volume of solution (L) contacted with m gram of adsorbent. A b and A a are the 99 Mo activities before and after adsorption at any shaking time. For the molybdate concentration studies, solutions with the desired concentrations of MoO 4 ions were prepared by successive dilutions of a 0.01 M Na 2 MoO 4 stock solution. Results and Discussión Adsorbent characterization X-ray diffraction study Tri-calcium phosphate X-ray diffraction pattern shows sharp and well defined peaks (Fig. 1a) which corresponds to the whitlockite structure (JCPDS file ), while the X-ray diffraction pattern of the Fe-treated tri-calcium phosphate displays an amorphous structure (Fig. 1b). The amorphous structure of Fe-treated tri-calcium phosphate indicates that the whitlockite structure is destroyed with the FeCl 3 treatment. Fig. 1 (a) XRD patterns of tri-calcium phosphate, and (b) Fe-treated tri-calcium phosphate

3 GOMEZ et al.: ADSORPTION OF MoO 4 IONS ON Fe-TREATED TRI-CALCIUM PHOSPHATE 169 Infrared spectral study As a tetrahedral molecule, tri-calcium phosphate has four normal modes of vibrations but only the frequencies ν 3 and ν 4 are infrared active 11. The infrared spectra of the whitlockite and Fe-treated tri-calcium phosphate show that the fundamental 3- frequencies ν 3 and ν 4 of the PO 4 group appear strongly around 1049 and 564 cm -1 respectively. These results demonstrate that the chemical treatment given to whitlockite with the heated 0.1 N FeCl 3 solution produces no chemical change in the phosphate group. EDS spectral study On the other hand, the EDS spectra of whitlockite sample show the presence of phosphorous, oxygen, calcium, and aluminum as impurity; while for the Fe-treated tri-calcium phosphate sample, in addition to the elements detected in the whitlockite sample, iron is also observed. The semi-quantitative analysis of whitlockite displays the following percentage element composition: O = ± 0.49, Al = 3.06 ± 2.69, P = ± 0.91, and Ca = ± 2.27; and for the Fe-treated tri-calcium phosphate the percentage element composition is found to be: O = ± 1.50, Al = 0.28 ± 0.12, P = ± 0.29, Ca = 4.42 ± 0.22 and Fe = ± From these results, it can be inferred that calcium is displaced by Fe in a high proportion in the original tri-calcium phosphate to form amorphous FePO 4 and only a small amount of the material remains as tri-calcium phosphate with an amorphous structure. Therefore, the final obtained material is a mixture of both amorphous FePO 4 and tri-calcium phosphate. Thus, hereafter this iron-treated mixture will be referred in this work as Fe-TPO 4. Kinetic studies Mo(VI) sorption on Fe-TPO 4 as a function of shaking time was studied using the 99 Mo radioactive solution with ph 5.5 at 20 o C. Molybdate uptake by Fe-TPO 4 is found to be relatively fast at the beginning of the process, then a slow approach to equilibrium is observed and saturation is reached in about 24 h. At equilibrium, the molybdate sorption value is found to be 14.0 mg/g. The kinetics of MoO 4 sorption is also determined for other initial ph values of the 99 Mo-labeled solution, and the results are displayed in Fig. 2. At ph values of 4, 7, 9 and 11 the molybdate equilibrium sorption values on Fe-TPO 4 are found to be 13.7, 13.6, 10.5 and 4.48 mg/g respectively. At ph values of 4 and 5.5, the molybdate anions are adsorbed on the positively charged surface of the Fe-TPO 4, where Fe-OH + 2 groups are formed on the grain surface of the Fe-treated solid, in a similar way as proposed by Goldberg et al. 5 for the adsorption of - HMoO 4 and MoO 4 on iron oxides. At an alkaline ph, the molybdate ions are in competition 12 with the OH - ions in the solution for the OH groups attached to Fe atoms, which leads to a lower molybdate equilibrium sorption. The plots of Fig. 2 show that the amount of Mo(VI) adsorbed at equilibrium decreases with increase in the initial ph value of the Na 2 MoO 4 aqueous solution and the time required to reach saturation (about 24 h) remains practically unchanged Fig. 2 Effect of shaking time on sorption of MoO 4 ions on Fe-TPO 4 at ph values 4 ( ), 5.5 ( ), 7 ( ), 9( ) and 11( ) Fig. 3 Pseudo-second-order kinetic plots for the adsorption of MoO 4 ions from aqueous solutions on Fe-TPO 4

4 170 INDIAN J. CHEM. TECHNOL, MAY 2012 in all the cases. To describe the changes in the adsorption of the molybdate ions with time, the experimental results are fitted to the pseudo-secondorder model. This kinetic model has generally been applied to heterogeneous systems, where the sorption mechanism is attributed to chemical sorption. The pseudo-second-order rate model has the following expression 13 : where k 2 (g/mmol min) is the rate constant of secondorder kinetic model. Figure 3 presents kinetic plots of t/q t versus t for the Mo(VI) removal at different initial ph values. The linear relationship found in these plots and the high values of the correlation coefficient (R 2 ) (Table 1) indicate that the Mo sorption process follows pseudo-second-order kinetics. The initial adsorption rate (h) is given by the product k 2 q 2 e. Table 1 shows that this parameter and the rate constants k 2 decrease when ph is increased. Most of the values of the correlation coefficient are found to be higher than 0.99, and the values of calculated equilibrium adsorption capacity q e are very close to the experimental data. These results indicate that chemisorption plays an important role in the removal of molybdenum by Fe-TPO 4. Adsorption isotherms Mo adsorption data obtained as a function of the molybdate ion concentration were analyzed through Langmuir and Freundlich adsorption isotherms. The data of MoO 4 ions adsorption on Fe-TPO 4 do not fit the Langmuir isotherm but fit the Freundlich adsorption isotherm. In Fig. 4 the logarithm of the amount adsorbed (log q e ) at equilibrium has been plotted versus the logarithm of MoO 4 ion concentration (log C e ) in solution at equilibrium. The obtained straight line and the high correlation coefficient (0.998) show clearly that molybdate ion Table 1 Calculated (c) parameters of the pseudo-second order rate kinetic model for molybdate sorption on Fe-TPO 4. Experimental data [q e (e)] are also included ph q e (e) 10-3 mmol/g q e (c) 10-3 mmol/g h (c) k 2 (c) mmol/g min g/ mmol min R 2 adsorption fits well the Freundlich isotherm in its logarithm form, as shown below: where K and 1/n (0<1/n<1) are the Feundlich constants. The value of K is an indicative of the adsorption capacity of the adsorbent (mmol/g), while the value of 1/n is related to the intensity of the adsorption process. The values of K and 1/n, computed by using the least square technique, are found to be ± 0.01 mmol/g and 0.47 ± 0.02 respectively. The adsorption intensity (n) for MoO 4 ions is found to be This value lies between 1 and 10, indicating a favorable adsorption 14. The value of 1/n (<1) found in this work confirms that the Freundlich isotherm is valid for MoO 4 ion adsorption data and this suggests that the adsorbent surface is heterogeneous in nature with an exponential distribution of the active centers. Thermodynamic studies To survey the effect of temperature on adsorption of MoO 4 ions on Fe-TPO 4, a 0.01 M Na 2 MoO 4 was used (ph 6.0) and the temperature was varied from 293 to 313 K. With the temperature dependence data, the thermodynamic parameters H 0, S 0, G 0 were evaluated utilizing the following thermodynamic expressions 15,16 : where K c is the equilibrium constant given by the expression K c = F e /(1-F e ), and F e is the fraction sorbed at equilibrium 17. H 0, S 0, G 0 are the changes Fig. 4 Freundlich adsorption isotherm of MoO 4 ions on Fe-TPO 4

5 GOMEZ et al.: ADSORPTION OF MoO 4 IONS ON Fe-TREATED TRI-CALCIUM PHOSPHATE 171 Table 2 Thermodynamic parameters for the adsorption of MoO 4 ions on Fe-TPO 4. Equilibrium constants (K) are also depicted Temp., K Equilibrium constant (K C ) H O kj.mol -1 S O kjk -1 mol -1 G O kj.mol Fig. 5 Plot of log K c vs. 1/T for the MoO 4 ions adsorption on Fe-TPO 4 in the standard enthalpy, standard entropy and Gibbs free energy respectively; T is the temperature in Kelvin, and R is the gas constant ( kj. K -1.mol -1 ). The slope of the plot log K c vs. 1/T (Fig. 5) is equal to - H 0 /2.303R, while the intercept is given by S 0 /2.303R, and by simple calculations with R = kj mol -1 K -1 H 0 and S 0 of the adsorption process are found to be kjmol -1 and kj K -1 mol -1 respectively (Table 2). The positive value of H 0 corresponds to an endothermic adsorption process and indicates the amount of energy required to remove the water molecules from molybdate ions prior to their adsorption onto the Fe-TPO 4. The positive value of S 0 depends on the number of molecules set free from the ordered structure of the hydration zone of molybdate ion. The values of K c obtained at different temperatures are given in Table 2. K c increases with the rise in temperature. The calculated values of G 0 are summarized also in Table 2, and the positive values of G 0 demonstrate the non-spontaneous behavior of the sorption process. The numerical values of G 0 decrease as the temperature increases. This decrease with temperature indicates that the adsorption reaction is more favorable at high temperature 18. This can be explained on the basis that at high temperature, the ions are normally desolvated and, hence their adsorption becomes favorable. Conclusion Fe-treated tri-calcium phosphate (Fe-TPO 4 ) has a high capacity to adsorb molybdate ions from aqueous solutions in the ph range from 4 to 7. In this ph interval, the averaged molybdate sorption value, at equilibrium, is found to be ± 0.21 mg/g. At higher ph values, the molybdate adsorption decreases drastically. The adsorption is found to be dependent of initial concentration and temperature. Adsorption data of MoO 4 ions obey the Freundlich isotherm. The temperature studies show that the adsorption of molybdate ions on Fe-TPO 4 is an endothermic and non-spontaneous process. The results obtained show the high efficiency of Fe-treated tri-calcium phosphate to separate MoO 4 ions present in aqueous solutions. References 1 Vandegrift G, Fortner J, Bakel A, Chemerisov S, Gelis A, Jerden J, Stepinski D & Zeigler A, Overview of Argonne progress related to implementation of Mo-99 production by use of a homogeneous reactor, paper presented at 30 th International Meeting on Reduced Enrichment for Research and Test Reactors, Washington, D C, 5-9 October Lapka J L, Paulenova A, Alyapyshev M Yu, Babain V A, Herbst R S & Law J D J, Raidoanal Nucl Chem, 280 (2009) Serrano G J, Bertin V & Bulbulian S, Langmuir, 16 (2000) Nayak D & Lahiri S, Appl Radiat Isotop, 66 (2008) Goldberg S, Johnston C T, Suarez L D & Lesch S M, Mechanism of molybdenum adsorption on soils and soils minerals evaluated using vibrational spectroscopy and surface complexation modeling, Development in Earth & Environmental Sciences 7, edited by Mark O Barnett & Douglas B Kent (Elsevier B V, Holland), Lazaridis N K & Charalambous Ch, Water Res, 39 (2005) Granados-Correa F & Serrano-Gómez J, Sep Sci Technol, 44 (2009) Payne K B & Abdel-Fattah T M, J Environ Sci Health, 40 (2009) Serrano-Gómez J, López-González H, Olguín, M T & Bulbulian S, J Incl Phenom Macrocycl Chem, 67 (2010) Granados-Correa F, Bonifacio-Martínez J & Serrano-Gómez J, J Chil Chem Soc, 54 (2009) 252.

6 172 INDIAN J. CHEM. TECHNOL, MAY Nakamoto K, Infrared Spectra of Inorganic and Coordination Compounds, 1 st edn (John Wiley & Sons Inc, New York), Serrano-Gómez J, Ramírez-Sandoval J L, Bonifacio- Martínez J, Granados-Correa F & Badillo-Almaraz V E, J Mex Chem Soc, 54 (2010) McKay G & Ho Y S, Process Biochem, 34 (1999) McKay G, Blair H S & Garden J R, J Appl Polym Sci, 27 (1982) Qadeer R, Hanif J, Saleem M & Afzal M, Colloid Polym Sci, 271 (1993) Saeed M M, J Radioanal Nucl Chem, 256 (2003) Hasany M S, Saeed M M & Ahmed M, Sep Sci Technol, 35 (2000) Mustafa S, Zaman M I & Khan S, Chem Eng J, 141 (2008) 51.

Thermodynamic parameters of Cs + sorption on natural clays

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