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Thorium adsorption behaviour on mixed ammonium lanthanum oxalate, LAOX
Applied Radiation and Isotopes PERGAMON
Applied Radiation and Isotopes 51 (1999) 21±26
Thorium adsorption behaviour on mixed ammonium lanthanum oxalate, LAOX M.T. Valentini Ganzerli *, L. Maggi, V. Crespi Caramella Dipartimento di Chimica Generale e Centro di Radiochimica e Analisi per attivazione del C.N.R., Universita di Pavia, viale Taramelli 12, 27100 Pavia, Italy Received 4 August 1998; accepted 2 November 1998
Abstract The cation-exchange properties of mixed ammonium lanthanum oxalate, LAOX, were studied by batch equilibration as a function of the concentration of some cations, such as alkaline earths or ammonium and of some anions and acids. The distribution coecients for thorium are high, while U(VI) is not adsorbed over a large acidity range. Thus, the separation of thorium from uranium may be successfully carried out. The experimental conditions of adsorption, elution and recovery of thorium were investigated as well, by using chromatographic columns ®lled with LAOX, in order to set best the separation conditions from uranyl ions. Instrumental neutron activation analysis, ICP emission spectrometry and the UV spectrometry were used to evaluate the thorium, uranium and lanthanum concentrations. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Inorganic adsorbers; Thorium separation; Lanthanum oxalate
1. Introduction Nuclear power plants and the fuel reprocessing need accurate analysis of actinides in order to set the best working conditions to avoid, or at least to moderate, the production of undesired and dangerous isotopes. The possibility of replacing enriched uranium fuel with thorium fuel leads to minimisation of plutonium growth. Therefore the analysis of thorium traces in uranium compounds or uranium traces in thorium compounds, may be of great interest in nuclear technology. This paper deals with the investigation of the main features of lanthanum oxalate as an inorganic cation exchanger, which showed a great anity towards thorium. This adsorber was prepared and characterised by the method of Stella et al. (1995). By precipitating lanthanum oxalate with ammonium oxalate under con-
* Corresponding author. Fax: +39-382-528-544; e-mail: [email protected].
trolled conditions a compound is formed with composition La(NH4)(C2O4)23H2O, named LAOX. This compound showed a great anity towards U(IV), which is partially exchanged with ammonium ions contained, while the uranyl ion is not adsorbed in a large acidity range. From these properties it may be expected that thorium and other actinide ions in the +4 oxidation state, may be strongly adsorbed. So a knowledge of the adsorption behaviour of thorium in dierent media is very important to optimise its separation from uranium and other actinides in the +6 or +5 oxidation state.
2. Experimental 2.1. Reagents . All chemicals were of analytical grade. . The exchanger, LAOX, was prepared as reported by Stella et al. (1995) by adding a 0.2 M ammonium
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M.T. Valentini Ganzerli et al. / Applied Radiation and Isotopes 51 (1999) 21±26
oxalate solution to a 0.1 M lanthanum nitrate solution and then boiling the resulting slurry for 3 h. The suspended compound was then ®ltered and washed with water, dried in an oven for 24 h at 708C and ®nally ground to an about 200 mesh granularity. . A stock solution of uranyl salt, nitrate or chloride, containing 10 mg uranium/mL was prepared by dissolving 1.1793 g of U3O8 in 100 mL of HNO3, or HCl 0.1 M. Other starting uranyl solutions were prepared by appropriate dilution of the stock solution. . A stock solution of thorium nitrate, containing 1 mg of Th/mL, was prepared by dissolving 238 mg of Th(NO3)44H2O in 100 mL of 0.1 M HNO3. Also in this case other starting thorium solutions were prepared by appropriate dilution of the stock solution. 2.2. Apparatus UV±VIS absorption was measured with a spectrophotometer GBC Scienti®c Equipment Pty. Ltd., Australia, mod. 916. Atomic emission measurements were performed with a Perkin-Elmer ICP-400. Gamma ray spectra were recorded and analysed with an Ortec Ge coaxial detector, 25% relative eciency, connected to a computer assisted multichannel analyser. 2.3. Analysis Uranium analyses were performed by the ICP emission method or spectrophotometrically with ammonium thioglycolate (Davemport and Thomason, 1949) according to the concentration level (whether lower than 4 mg/mL or higher). The thorium concentration was evaluated in solutions after equilibration with LAOX. For thorium concentrations higher than 0.8 mg/mL thorium analyses were performed with a spectrophotometric method by aid of arsenzoIII (Onisci and Sekine, 1970), in hydrochloric media, by measuring the absorbance for l at 660 nm after extraction with TTA to eliminate lanthanum interference (Perkins and Kalkwarf, 1956). For more dilute solutions atomic emission ICP was used. In order to evaluate accurately the uptake of thorium in dierent conditions on the exchanger, the thorium and lanthanum content, after equilibration with solutions, was directly measured in the solid phase by the nondestructive neutron activation analysis. The solid phase, after exchange, was repeatedly washed with water, dried at 708C in an oven. An aliquot of about 5 mg was sealed in a polythene vial and then irradiated in the TRIGA MARK II nuclear reactor of the University of Pavia at a neutron thermal ¯ux of 1012 n cm ÿ 2 s ÿ 1 for 3 h with an appropriate reference
standard. After an adequate cooling time the area of the peak at 1596 KeV of 140 La was measured by gamma spectrometry. Thorium content was evaluated after about 15 days cooling by measuring the peak area at 312 KeV of 233 Pa. In presence of lanthanum a long cooling time is necessary owing to very high activity of 140 La formed. 2.4. Kd measurements The adsorption behaviour of thorium was studied as a function of the concentration of EDTA and of many ions, such as sulphate, ammonium, calcium, carbonate. About 200 mg of the ground LAOX were equilibrated with 10 mL of the appropriate solution containing 0.2 mg of 232 Th (corresponding to a starting thorium concentration of 8.6210 ÿ 5 M) by shaking for at least 2 h at room temperature. The pH was adjusted to the desired value and the ionic strength was kept in most cases at 0.2 M with NaCl. After equilibration the pH was again measured, then the samples were submitted to centrifugation to separate the solid phase and the thorium concentration was measured in the solutions. Comparison between concentration before and after adsorption allowed the calculation the distribution coecients, Kd, from the usual following relationship: Kd mmol of adsorbed element=g of LAOX at equilibrium : mmol of element=mL of the solution at equilibrium 2.5. Column experiments Thorium±uranium separation was also studied using chromatographic columns, 0.4 cm i.d.6 cm high, ®lled with known amounts of LAOX, from 0.25 to 1 g. About 20 mL of appropriate solution containing from 0.1 to >20 mg of each element in dierent ratios were percolated through the column, which was then washed with about 20 mL of the same medium. Thorium elution was performed with about 40 mL of eluting solution, warmed to about 908C. The ¯ow rate was kept at about 1 mL/min. The eluates were collected as separate fractions, (usually 10 mL) and the uranium and thorium contents were measured. 3. Results and discussion 3.1. Exchange properties In Fig. 1 the adsorption behaviour of thorium as a function of the HCl or HNO3 concentration is reported. Acid concentrations higher than 0.2 M strongly aect the Kd values, con®rming that the competition between the hydrogen ions and thorium, or
M.T. Valentini Ganzerli et al. / Applied Radiation and Isotopes 51 (1999) 21±26
Fig. 1. The distribution coecients, Kd, for thorium as a function of the acid concentration. The stars refer to the HCl concentration and the crosses to the HNO3 concentration.
thorium hydrolysed ions, is weak at lower acid concentrations. LAOX undergoes to a partial dissolution for more concentrated acid solutions, so the Kd values too are lowered. A slope of about ÿ4 is exhibited by the straight line obtained for an HCl concentration higher than 0.2 M, where thorium hydrolysis is negligible. The competition between Th4+ and hydrogen is indicated by the usual relationship log Kd cost ÿ 4 logH , where cost is the product of the constant of selectivity, between thorium and hydrogen ions and the concentration of the exchangeable sites in the solid phase. By substituting HCl with HNO3 the Kd values are higher, especially at higher acid concentrations, as LAOX is less soluble in this medium, and the complexing ability of nitrate ions towards thorium is lower than that showed by chloride ions (Katzin, 1986). In Fig. 2 the lanthanum concentrations are reported as a function of HCl concentration. For HCl concentrations higher than 0.2 M the lanthanum concentration increases according to a straight line of slope +3. This trend clearly indicates that also lanthanum is in competition with hydrogen ions, as expected. The Kd values and the large acidity range, in which thorium is strongly adsorbed, indicate a great anity of the exchanger towards thorium. By comparing the adsorption behaviour of U(IV) in the same acidity
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Fig. 2. Lanthanum concentrations in solutions equilibrated with LAOX, as a function of the HCl concentration.
range, as reported above, Kd values appear slightly lower. Among the cations ammonium, uranyl, alkaline and alkaline earth ions do not aect thorium adsorption con®rming the higher anity of the exchanger towards thorium. So, once adsorbed, thorium cannot be removed by most cations. This trend is related to behaviour of thorium oxalates, which may carry some alkaline earth or alkaline ions traces during precipitation (Bykhovskii et al., 1972), and therefore the presence of such ions in solution may enhance thorium uptake. Among the complexing anions, sulphates do not aect thorium sorption in acid or neutral media as their complexing ability to form cationic species is poor (Katzin, 1986). Among the complexing species which may be formed by sulphate ions only [Th(SO4)]2+ could be adsorbed by replacing the existing cations, but its size makes the substitution dicult. So the great anity of thorium ion towards the exchanger becomes predominant. EDTA does not strongly aect thorium adsorption in a wide acidity range. Kd values are kept nearly constant in the whole concentration range (10 ÿ 3±0.1 M). Also in this case the same above remarks may be made. The adsorption of thorium is aected, on the contrary, by carbonate ions as shown in Fig. 3, for concentrations higher than 0.1 M. The behaviour of the Kd values vs. the carbonate ions concentration agrees with a straight line of slope of ÿ4. This may indicate
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M.T. Valentini Ganzerli et al. / Applied Radiation and Isotopes 51 (1999) 21±26
Fig. 3. The distribution coecients, Kd, for thorium as a function of carbonate concentration.
the formation of the predominant complex with four carbonate ions for each thorium ion (Dervin and Faucherre, 1973). The plotted Kd are calculated as the ratio between the thorium ion concentration in the solid phase, [Th4+]s, and the concentration of total thorium in solution. In presence of carbonate ions thorium is converted mostly in the carbonate-complex, [Th(CO3)4]4ÿ. The concentration of the complex ion becomes constant for more concentrated solutions of carbonate ions and in turn the concentration of thorium ion in solution, [Th4+]l, is related to the concentration of the carbonate ion. Thus the following equations may be written: Th 4 s =Th 4 l Th 4 s =Th CO3 4 4ÿ 4 b1,4 CO2ÿ 3 ,
where b1,4 refers to the constant of equilibrium of the reaction of complexation of thorium with four carbonate ions and the ratio [Th4+]s/[[Th(CO3)4]4ÿ] represents the Kd measured. The ratio [Th4+]s/[Th4+]l is constant at constant pH and the logarithmic equation related to Kd becomes: log Kd = log cost ÿ4 log[CO2ÿ 3 ]. Thus carbonates appear useful in order to displace thorium from LAOX, once sorbed. Finally the adsorption was studied as a function of the concentration of thorium itself, in 0.02 M HNO3. In Fig. 4 the thorium uptake is reported as a function of the thorium concentration at equilibrium.
Fig. 4. Thorium concentration in the solid phase (mmol adsorbed/g) as a function of the thorium concentration in solution at equilibrium.
Two adsorption steps are clearly shown. The ®rst one is related to a partial substitution of ammonium ion (two third of ammonium ions present), as in the U(IV) case, when the compound with the composition La6Th(NH4)2(C2O4)126H2O is formed. The analysis of the compound after exchange con®rm the trend of the adsorption curve: the ®rst saturation step occurs for thorium concentration lower 0.001 M (0.02 M starting solution), where an in¯ection of the curve corresponding to an uptake of 0.44 mmol of thorium per g of exchanger may be observed. At higher thorium concentrations also lanthanum may be replaced and nearly pure thorium oxalate is formed. A maximum uptake of 2.4 mmol of thorium per g of LAOX was measured. Only the 1.48% of the initial lanthanum amounts is left in the solid phase after equilibration with thorium solutions of concentration higher than 0.04 M (0.07 M starting solution). By precipitating thorium with ammonium oxalate a salt with a 87% thorium content is formed, in which some ammonium traces are still present. So batch experiments agree with precipitation experiments, con®rming that thorium oxalate cannot form mixed salts. The formation of the pure U(IV) oxalate was not observed possibly owing to the formation of polymeric species in more concentrated U(IV) solutions, or also owing to the oxidation extent increasing with concentration (Weigel, 1986).
M.T. Valentini Ganzerli et al. / Applied Radiation and Isotopes 51 (1999) 21±26 Table 1 Separation of 1 mg of thorium from 1 mg of uranium (VI) in dierent media and with 0.5 g of LAOX. The % refers to the percentages passed in solution. The reported values refer to at least two replicate experiments Medium
% Thorium
% Uranium
0.01 M HNO3 0.05 M HNO3 0.05 M HNOa3 0.1 M HNO3 0.05 M HNO3±1 M CH3COOH 0.1 M HCl 0.05 M HCl±1 M CH3COOH
32 2 32 2 42 2 102 4 62 2 202 5 172 4
84 24 90 25 95 25 95 23 95 23 98 23 96 24
a
Adsorption carried out at about 908C.
These experiments also show that the ability of actinides in the +4 oxidation state to form mixed salts is very restricted, perhaps due to large ions size and charge. So a mixed triple salt may be formed only with a low content of the actinide ion. From more concentrated solutions the nearly pure actinide salt may be formed, in which the residual presence of other ions traces may be due to the need of saturation of surface charges. 3.2. Uranium±thorium separation The separation was studied either in batch or in column experiments. Uranyl ion was always used to prepare the starting solutions. U(VI) is not adsorbed so its separation from thorium is easier. Batch experiments were carried out in order to ®nd the best separation conditions. For this purpose dierent nitric or hydrochloric solutions or mixed solutions with acetic acid were used. The starting solutions contained 1 mg of thorium and 1 mg of uranium and 0.5 g of LAOX was employed. All experiments were run at room temperature, with the exception of the one
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Table 2 Separation of 1 mg of thorium from 1 mg of uranium, in 0.05 M HNO3 with chromatographic columns ®lled with dierent amounts of LAOX. Mean values obtained from at least two replicate experiments. The % refers to the percentages left in solution Amount of LAOX used (g)
% Thorium
% Uranium
0.25 0.5 1.0
1522 322 322
9823 9425 9024
marked with the asterisk. The results are summarised in Table 1. The medium was chosen taking into account of the overall adsorption behaviour of the two elements and that acetic acid is a weak complexing agent towards uranyl ions. These experiments showed that thorium uptake is higher from diluted nitric acid solutions, less than 0.1 M, but in weakly acid solutions the uranyl ion is also partially adsorbed, as it undergoes to hydrolysis. Moreover hydrochloric acid may complex both ions lowering thorium uptake. Heating seems not to improve the separation. Then a 0.05 M HNO3 solution showed to be the more suitable medium. Afterwards the separation was checked with chromatographic columns to try to improve the above results. The separation was carried with dierent amounts of LAOX, in order to set the optimum amount, which resulted 0.5 g, according to data reported in Table 2. A lower amount does not allow a complete adsorption of thorium, while an higher amount may adsorb uranium too, as the greater the surface is, the higher is the physical adsorption of colloidal species, arising from uranium hydrolysis. Therefore an amount of 0.5 g of LAOX was always used in the successive experiments, when the separations of dierent uranium and thorium amounts were carried in order to check the procedure. In Table 3,
Table 3 Separation of thorium and uranium in dierent ratios, by means of a chromatographic column ®lled with 0.5 g of LAOX. The reported values refer to at least two replicate experiments U(VI) used (mg)
Th used (mg)
Th adsorbed % from 0.05 M HNO3
U(VI) eluted % from 0.05 M HNO3
Th eluted % from 2 M Na2CO3
0.1 0.1 0.1 5 10 100 200
1 5 20 0.1 0.1 0.1 0.1
r98 r98 r98 962 5 952 5 962 5 972 5
9522 9524 9423 9226 9526 9825 9526
942 3 922 4 952 3 952 4 912 4 922 4 952 4
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the adsorption yields percentages for thorium, from the 0.05 M HNO3 initial solution, are reported together with the simultaneous elution yields percentages for uranium and the recovery yield percentages for thorium with the sequential 2 M Na2CO3 warmed solution, used to thorium recovery. The eluting solution was chosen because carbonate ions are good complexing agents toward thorium ions, as the batch experiments showed. Moreover the basic solution does not elute lanthanum too, owing to the formation of insoluble lanthanum carbonate. Heating, which does not aect the adsorption, is necessary in the elution step to speed up the displacement of thorium from the solid phase by complex formation. The overall elution process is too low at room temperature and consequently higher eluting volumes could be necessary. So by heating most thorium may be recovered in the ®rst eluting fraction (10 mL) and about the 95% of thorium is recovered with 20 mL of eluting solution; on the contrary without heating the elution of the same amount needs at least ®ve or six eluting fractions. Nevertheless thorium recovery is the more dicult step, owing to its great anity towards the exchanger. The recovery yield is good, even if it does not reach the 100%, but uranium-free thorium is always recovered. Thorium elution could be easily accomplished with 1 M HCl, but lanthanum may also be released, which in turn may aect the thorium analysis, or thorium purity. The reported results show that U(VI) and thorium can be successfully separated by the given procedure. The thorium/uranium ratio in solution does not aect
the separation yields. Owing to the high adsorption capacity towards thorium a small LAOX amount may be used also by varying thorium concentration, or, to some extent, thorium amount to be separated. So the procedure may be successfully applied in most cases, where a careful separation of two elements is required.
References Bykhovskii, D.N., Petrova, I.K., Zelenstov, S.S., 1972. Interaction of divalent cations with thorium oxalate. Radiokhimiya 14, 171. Davemport, W.H., Thomason, P.F., 1949. Determination of U(VI) in presence of anions. Anal. Chem. 21, 1093. Dervin, J., Faucherre, J., 1973. Etude des carbonates complexe de thorium et de cerium. II Constitution des complexes in solution. Bull. Soc. Chim. Fr. 11, 2926. Katzin, L.J., 1986. Thorium. In: Katz, J.J., Seaborg, G.T., Morss, L.R. (Eds.), The Chemistry of the Actinides Elements, 2nd ed. Chapman and Hall, London, p. 85. Onisci, H., Sekine, K., 1970. Separation of microgram amounts of thorium and uranium by TTA [thenoyltri¯uoroacetone] extraction: spectrophotometric determination with ArsenazoIII. Jpn. Anal. 19, 473. Perkins, R.W., Kalkwarf, D.R., 1956. Determination of thorium in urine. Anal. Chem. 28, 1989. Stella, R., Ganzerli Valentini, M.T., Maggi, L., 1995. Separation of U(IV) and U(VI) by ion exchange chromatography on lanthanum ammonium oxalate. Appl. Radiat. Isot. 1, 46. Weigel, F., 1986. Uranium. In: Katz, J.J., Seaborg, G.T., Morss, L.R. (Eds.), The Chemistry of the Actinides Elements, 2nd ed. Chapman and Hall, London, p. 338.
Applied Radiation and Isotopes 51 (1999) 21±26
Thorium adsorption behaviour on mixed ammonium lanthanum oxalate, LAOX M.T. Valentini Ganzerli *, L. Maggi, V. Crespi Caramella Dipartimento di Chimica Generale e Centro di Radiochimica e Analisi per attivazione del C.N.R., Universita di Pavia, viale Taramelli 12, 27100 Pavia, Italy Received 4 August 1998; accepted 2 November 1998
Abstract The cation-exchange properties of mixed ammonium lanthanum oxalate, LAOX, were studied by batch equilibration as a function of the concentration of some cations, such as alkaline earths or ammonium and of some anions and acids. The distribution coecients for thorium are high, while U(VI) is not adsorbed over a large acidity range. Thus, the separation of thorium from uranium may be successfully carried out. The experimental conditions of adsorption, elution and recovery of thorium were investigated as well, by using chromatographic columns ®lled with LAOX, in order to set best the separation conditions from uranyl ions. Instrumental neutron activation analysis, ICP emission spectrometry and the UV spectrometry were used to evaluate the thorium, uranium and lanthanum concentrations. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Inorganic adsorbers; Thorium separation; Lanthanum oxalate
1. Introduction Nuclear power plants and the fuel reprocessing need accurate analysis of actinides in order to set the best working conditions to avoid, or at least to moderate, the production of undesired and dangerous isotopes. The possibility of replacing enriched uranium fuel with thorium fuel leads to minimisation of plutonium growth. Therefore the analysis of thorium traces in uranium compounds or uranium traces in thorium compounds, may be of great interest in nuclear technology. This paper deals with the investigation of the main features of lanthanum oxalate as an inorganic cation exchanger, which showed a great anity towards thorium. This adsorber was prepared and characterised by the method of Stella et al. (1995). By precipitating lanthanum oxalate with ammonium oxalate under con-
* Corresponding author. Fax: +39-382-528-544; e-mail: [email protected].
trolled conditions a compound is formed with composition La(NH4)(C2O4)23H2O, named LAOX. This compound showed a great anity towards U(IV), which is partially exchanged with ammonium ions contained, while the uranyl ion is not adsorbed in a large acidity range. From these properties it may be expected that thorium and other actinide ions in the +4 oxidation state, may be strongly adsorbed. So a knowledge of the adsorption behaviour of thorium in dierent media is very important to optimise its separation from uranium and other actinides in the +6 or +5 oxidation state.
2. Experimental 2.1. Reagents . All chemicals were of analytical grade. . The exchanger, LAOX, was prepared as reported by Stella et al. (1995) by adding a 0.2 M ammonium
0969-8043/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 4 3 ( 9 8 ) 0 0 1 8 7 - 0
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oxalate solution to a 0.1 M lanthanum nitrate solution and then boiling the resulting slurry for 3 h. The suspended compound was then ®ltered and washed with water, dried in an oven for 24 h at 708C and ®nally ground to an about 200 mesh granularity. . A stock solution of uranyl salt, nitrate or chloride, containing 10 mg uranium/mL was prepared by dissolving 1.1793 g of U3O8 in 100 mL of HNO3, or HCl 0.1 M. Other starting uranyl solutions were prepared by appropriate dilution of the stock solution. . A stock solution of thorium nitrate, containing 1 mg of Th/mL, was prepared by dissolving 238 mg of Th(NO3)44H2O in 100 mL of 0.1 M HNO3. Also in this case other starting thorium solutions were prepared by appropriate dilution of the stock solution. 2.2. Apparatus UV±VIS absorption was measured with a spectrophotometer GBC Scienti®c Equipment Pty. Ltd., Australia, mod. 916. Atomic emission measurements were performed with a Perkin-Elmer ICP-400. Gamma ray spectra were recorded and analysed with an Ortec Ge coaxial detector, 25% relative eciency, connected to a computer assisted multichannel analyser. 2.3. Analysis Uranium analyses were performed by the ICP emission method or spectrophotometrically with ammonium thioglycolate (Davemport and Thomason, 1949) according to the concentration level (whether lower than 4 mg/mL or higher). The thorium concentration was evaluated in solutions after equilibration with LAOX. For thorium concentrations higher than 0.8 mg/mL thorium analyses were performed with a spectrophotometric method by aid of arsenzoIII (Onisci and Sekine, 1970), in hydrochloric media, by measuring the absorbance for l at 660 nm after extraction with TTA to eliminate lanthanum interference (Perkins and Kalkwarf, 1956). For more dilute solutions atomic emission ICP was used. In order to evaluate accurately the uptake of thorium in dierent conditions on the exchanger, the thorium and lanthanum content, after equilibration with solutions, was directly measured in the solid phase by the nondestructive neutron activation analysis. The solid phase, after exchange, was repeatedly washed with water, dried at 708C in an oven. An aliquot of about 5 mg was sealed in a polythene vial and then irradiated in the TRIGA MARK II nuclear reactor of the University of Pavia at a neutron thermal ¯ux of 1012 n cm ÿ 2 s ÿ 1 for 3 h with an appropriate reference
standard. After an adequate cooling time the area of the peak at 1596 KeV of 140 La was measured by gamma spectrometry. Thorium content was evaluated after about 15 days cooling by measuring the peak area at 312 KeV of 233 Pa. In presence of lanthanum a long cooling time is necessary owing to very high activity of 140 La formed. 2.4. Kd measurements The adsorption behaviour of thorium was studied as a function of the concentration of EDTA and of many ions, such as sulphate, ammonium, calcium, carbonate. About 200 mg of the ground LAOX were equilibrated with 10 mL of the appropriate solution containing 0.2 mg of 232 Th (corresponding to a starting thorium concentration of 8.6210 ÿ 5 M) by shaking for at least 2 h at room temperature. The pH was adjusted to the desired value and the ionic strength was kept in most cases at 0.2 M with NaCl. After equilibration the pH was again measured, then the samples were submitted to centrifugation to separate the solid phase and the thorium concentration was measured in the solutions. Comparison between concentration before and after adsorption allowed the calculation the distribution coecients, Kd, from the usual following relationship: Kd mmol of adsorbed element=g of LAOX at equilibrium : mmol of element=mL of the solution at equilibrium 2.5. Column experiments Thorium±uranium separation was also studied using chromatographic columns, 0.4 cm i.d.6 cm high, ®lled with known amounts of LAOX, from 0.25 to 1 g. About 20 mL of appropriate solution containing from 0.1 to >20 mg of each element in dierent ratios were percolated through the column, which was then washed with about 20 mL of the same medium. Thorium elution was performed with about 40 mL of eluting solution, warmed to about 908C. The ¯ow rate was kept at about 1 mL/min. The eluates were collected as separate fractions, (usually 10 mL) and the uranium and thorium contents were measured. 3. Results and discussion 3.1. Exchange properties In Fig. 1 the adsorption behaviour of thorium as a function of the HCl or HNO3 concentration is reported. Acid concentrations higher than 0.2 M strongly aect the Kd values, con®rming that the competition between the hydrogen ions and thorium, or
M.T. Valentini Ganzerli et al. / Applied Radiation and Isotopes 51 (1999) 21±26
Fig. 1. The distribution coecients, Kd, for thorium as a function of the acid concentration. The stars refer to the HCl concentration and the crosses to the HNO3 concentration.
thorium hydrolysed ions, is weak at lower acid concentrations. LAOX undergoes to a partial dissolution for more concentrated acid solutions, so the Kd values too are lowered. A slope of about ÿ4 is exhibited by the straight line obtained for an HCl concentration higher than 0.2 M, where thorium hydrolysis is negligible. The competition between Th4+ and hydrogen is indicated by the usual relationship log Kd cost ÿ 4 logH , where cost is the product of the constant of selectivity, between thorium and hydrogen ions and the concentration of the exchangeable sites in the solid phase. By substituting HCl with HNO3 the Kd values are higher, especially at higher acid concentrations, as LAOX is less soluble in this medium, and the complexing ability of nitrate ions towards thorium is lower than that showed by chloride ions (Katzin, 1986). In Fig. 2 the lanthanum concentrations are reported as a function of HCl concentration. For HCl concentrations higher than 0.2 M the lanthanum concentration increases according to a straight line of slope +3. This trend clearly indicates that also lanthanum is in competition with hydrogen ions, as expected. The Kd values and the large acidity range, in which thorium is strongly adsorbed, indicate a great anity of the exchanger towards thorium. By comparing the adsorption behaviour of U(IV) in the same acidity
23
Fig. 2. Lanthanum concentrations in solutions equilibrated with LAOX, as a function of the HCl concentration.
range, as reported above, Kd values appear slightly lower. Among the cations ammonium, uranyl, alkaline and alkaline earth ions do not aect thorium adsorption con®rming the higher anity of the exchanger towards thorium. So, once adsorbed, thorium cannot be removed by most cations. This trend is related to behaviour of thorium oxalates, which may carry some alkaline earth or alkaline ions traces during precipitation (Bykhovskii et al., 1972), and therefore the presence of such ions in solution may enhance thorium uptake. Among the complexing anions, sulphates do not aect thorium sorption in acid or neutral media as their complexing ability to form cationic species is poor (Katzin, 1986). Among the complexing species which may be formed by sulphate ions only [Th(SO4)]2+ could be adsorbed by replacing the existing cations, but its size makes the substitution dicult. So the great anity of thorium ion towards the exchanger becomes predominant. EDTA does not strongly aect thorium adsorption in a wide acidity range. Kd values are kept nearly constant in the whole concentration range (10 ÿ 3±0.1 M). Also in this case the same above remarks may be made. The adsorption of thorium is aected, on the contrary, by carbonate ions as shown in Fig. 3, for concentrations higher than 0.1 M. The behaviour of the Kd values vs. the carbonate ions concentration agrees with a straight line of slope of ÿ4. This may indicate
24
M.T. Valentini Ganzerli et al. / Applied Radiation and Isotopes 51 (1999) 21±26
Fig. 3. The distribution coecients, Kd, for thorium as a function of carbonate concentration.
the formation of the predominant complex with four carbonate ions for each thorium ion (Dervin and Faucherre, 1973). The plotted Kd are calculated as the ratio between the thorium ion concentration in the solid phase, [Th4+]s, and the concentration of total thorium in solution. In presence of carbonate ions thorium is converted mostly in the carbonate-complex, [Th(CO3)4]4ÿ. The concentration of the complex ion becomes constant for more concentrated solutions of carbonate ions and in turn the concentration of thorium ion in solution, [Th4+]l, is related to the concentration of the carbonate ion. Thus the following equations may be written: Th 4 s =Th 4 l Th 4 s =Th CO3 4 4ÿ 4 b1,4 CO2ÿ 3 ,
where b1,4 refers to the constant of equilibrium of the reaction of complexation of thorium with four carbonate ions and the ratio [Th4+]s/[[Th(CO3)4]4ÿ] represents the Kd measured. The ratio [Th4+]s/[Th4+]l is constant at constant pH and the logarithmic equation related to Kd becomes: log Kd = log cost ÿ4 log[CO2ÿ 3 ]. Thus carbonates appear useful in order to displace thorium from LAOX, once sorbed. Finally the adsorption was studied as a function of the concentration of thorium itself, in 0.02 M HNO3. In Fig. 4 the thorium uptake is reported as a function of the thorium concentration at equilibrium.
Fig. 4. Thorium concentration in the solid phase (mmol adsorbed/g) as a function of the thorium concentration in solution at equilibrium.
Two adsorption steps are clearly shown. The ®rst one is related to a partial substitution of ammonium ion (two third of ammonium ions present), as in the U(IV) case, when the compound with the composition La6Th(NH4)2(C2O4)126H2O is formed. The analysis of the compound after exchange con®rm the trend of the adsorption curve: the ®rst saturation step occurs for thorium concentration lower 0.001 M (0.02 M starting solution), where an in¯ection of the curve corresponding to an uptake of 0.44 mmol of thorium per g of exchanger may be observed. At higher thorium concentrations also lanthanum may be replaced and nearly pure thorium oxalate is formed. A maximum uptake of 2.4 mmol of thorium per g of LAOX was measured. Only the 1.48% of the initial lanthanum amounts is left in the solid phase after equilibration with thorium solutions of concentration higher than 0.04 M (0.07 M starting solution). By precipitating thorium with ammonium oxalate a salt with a 87% thorium content is formed, in which some ammonium traces are still present. So batch experiments agree with precipitation experiments, con®rming that thorium oxalate cannot form mixed salts. The formation of the pure U(IV) oxalate was not observed possibly owing to the formation of polymeric species in more concentrated U(IV) solutions, or also owing to the oxidation extent increasing with concentration (Weigel, 1986).
M.T. Valentini Ganzerli et al. / Applied Radiation and Isotopes 51 (1999) 21±26 Table 1 Separation of 1 mg of thorium from 1 mg of uranium (VI) in dierent media and with 0.5 g of LAOX. The % refers to the percentages passed in solution. The reported values refer to at least two replicate experiments Medium
% Thorium
% Uranium
0.01 M HNO3 0.05 M HNO3 0.05 M HNOa3 0.1 M HNO3 0.05 M HNO3±1 M CH3COOH 0.1 M HCl 0.05 M HCl±1 M CH3COOH
32 2 32 2 42 2 102 4 62 2 202 5 172 4
84 24 90 25 95 25 95 23 95 23 98 23 96 24
a
Adsorption carried out at about 908C.
These experiments also show that the ability of actinides in the +4 oxidation state to form mixed salts is very restricted, perhaps due to large ions size and charge. So a mixed triple salt may be formed only with a low content of the actinide ion. From more concentrated solutions the nearly pure actinide salt may be formed, in which the residual presence of other ions traces may be due to the need of saturation of surface charges. 3.2. Uranium±thorium separation The separation was studied either in batch or in column experiments. Uranyl ion was always used to prepare the starting solutions. U(VI) is not adsorbed so its separation from thorium is easier. Batch experiments were carried out in order to ®nd the best separation conditions. For this purpose dierent nitric or hydrochloric solutions or mixed solutions with acetic acid were used. The starting solutions contained 1 mg of thorium and 1 mg of uranium and 0.5 g of LAOX was employed. All experiments were run at room temperature, with the exception of the one
25
Table 2 Separation of 1 mg of thorium from 1 mg of uranium, in 0.05 M HNO3 with chromatographic columns ®lled with dierent amounts of LAOX. Mean values obtained from at least two replicate experiments. The % refers to the percentages left in solution Amount of LAOX used (g)
% Thorium
% Uranium
0.25 0.5 1.0
1522 322 322
9823 9425 9024
marked with the asterisk. The results are summarised in Table 1. The medium was chosen taking into account of the overall adsorption behaviour of the two elements and that acetic acid is a weak complexing agent towards uranyl ions. These experiments showed that thorium uptake is higher from diluted nitric acid solutions, less than 0.1 M, but in weakly acid solutions the uranyl ion is also partially adsorbed, as it undergoes to hydrolysis. Moreover hydrochloric acid may complex both ions lowering thorium uptake. Heating seems not to improve the separation. Then a 0.05 M HNO3 solution showed to be the more suitable medium. Afterwards the separation was checked with chromatographic columns to try to improve the above results. The separation was carried with dierent amounts of LAOX, in order to set the optimum amount, which resulted 0.5 g, according to data reported in Table 2. A lower amount does not allow a complete adsorption of thorium, while an higher amount may adsorb uranium too, as the greater the surface is, the higher is the physical adsorption of colloidal species, arising from uranium hydrolysis. Therefore an amount of 0.5 g of LAOX was always used in the successive experiments, when the separations of dierent uranium and thorium amounts were carried in order to check the procedure. In Table 3,
Table 3 Separation of thorium and uranium in dierent ratios, by means of a chromatographic column ®lled with 0.5 g of LAOX. The reported values refer to at least two replicate experiments U(VI) used (mg)
Th used (mg)
Th adsorbed % from 0.05 M HNO3
U(VI) eluted % from 0.05 M HNO3
Th eluted % from 2 M Na2CO3
0.1 0.1 0.1 5 10 100 200
1 5 20 0.1 0.1 0.1 0.1
r98 r98 r98 962 5 952 5 962 5 972 5
9522 9524 9423 9226 9526 9825 9526
942 3 922 4 952 3 952 4 912 4 922 4 952 4
26
M.T. Valentini Ganzerli et al. / Applied Radiation and Isotopes 51 (1999) 21±26
the adsorption yields percentages for thorium, from the 0.05 M HNO3 initial solution, are reported together with the simultaneous elution yields percentages for uranium and the recovery yield percentages for thorium with the sequential 2 M Na2CO3 warmed solution, used to thorium recovery. The eluting solution was chosen because carbonate ions are good complexing agents toward thorium ions, as the batch experiments showed. Moreover the basic solution does not elute lanthanum too, owing to the formation of insoluble lanthanum carbonate. Heating, which does not aect the adsorption, is necessary in the elution step to speed up the displacement of thorium from the solid phase by complex formation. The overall elution process is too low at room temperature and consequently higher eluting volumes could be necessary. So by heating most thorium may be recovered in the ®rst eluting fraction (10 mL) and about the 95% of thorium is recovered with 20 mL of eluting solution; on the contrary without heating the elution of the same amount needs at least ®ve or six eluting fractions. Nevertheless thorium recovery is the more dicult step, owing to its great anity towards the exchanger. The recovery yield is good, even if it does not reach the 100%, but uranium-free thorium is always recovered. Thorium elution could be easily accomplished with 1 M HCl, but lanthanum may also be released, which in turn may aect the thorium analysis, or thorium purity. The reported results show that U(VI) and thorium can be successfully separated by the given procedure. The thorium/uranium ratio in solution does not aect
the separation yields. Owing to the high adsorption capacity towards thorium a small LAOX amount may be used also by varying thorium concentration, or, to some extent, thorium amount to be separated. So the procedure may be successfully applied in most cases, where a careful separation of two elements is required.
References Bykhovskii, D.N., Petrova, I.K., Zelenstov, S.S., 1972. Interaction of divalent cations with thorium oxalate. Radiokhimiya 14, 171. Davemport, W.H., Thomason, P.F., 1949. Determination of U(VI) in presence of anions. Anal. Chem. 21, 1093. Dervin, J., Faucherre, J., 1973. Etude des carbonates complexe de thorium et de cerium. II Constitution des complexes in solution. Bull. Soc. Chim. Fr. 11, 2926. Katzin, L.J., 1986. Thorium. In: Katz, J.J., Seaborg, G.T., Morss, L.R. (Eds.), The Chemistry of the Actinides Elements, 2nd ed. Chapman and Hall, London, p. 85. Onisci, H., Sekine, K., 1970. Separation of microgram amounts of thorium and uranium by TTA [thenoyltri¯uoroacetone] extraction: spectrophotometric determination with ArsenazoIII. Jpn. Anal. 19, 473. Perkins, R.W., Kalkwarf, D.R., 1956. Determination of thorium in urine. Anal. Chem. 28, 1989. Stella, R., Ganzerli Valentini, M.T., Maggi, L., 1995. Separation of U(IV) and U(VI) by ion exchange chromatography on lanthanum ammonium oxalate. Appl. Radiat. Isot. 1, 46. Weigel, F., 1986. Uranium. In: Katz, J.J., Seaborg, G.T., Morss, L.R. (Eds.), The Chemistry of the Actinides Elements, 2nd ed. Chapman and Hall, London, p. 338.