Sorption of iodine on Mg-Al-layered double hydroxide

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1 Clay Minerals (2005) 40, 455±461 Sorption of iodine on Mg-Al-layered double hydroxide H. CURTIUS* AND Z. KATTILPARAMPIL Institute for Safety Research and Reactor Technology, Forschungszentrum JuÈlich, D JuÈlich, Germany (Received 3 August 2004; revised 19 November 2004) ABSTRACT: From leaching experiments with metallic uranium-aluminium material test reactor fuel elements in repository-relevant MgCl 2 -rich salt brines, a Mg-Al-layered double hydroxide with chloride as interlayer anion was identified as a crystalline secondary phase component. In the present study, this Mg-Al-Cl-layered double hydroxide (Mg-Al-Cl-LDH) was synthesized and its ability to adsorb iodine was evaluated. The adsorption of I ÿ was rapid, and equilibrium for an initial concentration of ÿ5 mole/l was obtained within 48 h. Further results indicated that the adsorption isotherm for I ÿ retention could be fitted to Freundlich and Dubinin-Radushkevich equations. The values obtained indicate that iodine is adsorbed on Mg-Al-Cl-LDH by an anionexchange process. The adsorption of I ÿ was also studied as a function of ph. In the ph range between 3.5 and 8.5, the adsorption was independent of the ph. The effects of competiting anions were investigated using an MgCl 2 -solution instead of water, and indicate that chloride anions have a greater affinity towards the Mg-Al-Cl-layered double hydroxide than iodine. KEYWORDS: hydrotalcite, iodine, sorption. Due to the political decision that there should be no reprocessing of reactor fuel elements in Germany, the direct disposal of spent fuel elements has to be investigated. A possible repository under consideration is a salt mine, and a hypothetical water ingress is the accident scenario for long-term safety analysis. Different leaching experiments with material test reactor fuel elements (MTR-FE) 1 in highly-concentrated salt brines at 90ëC showed that the radionuclides were rapidly mobilized, but then trapped by the corrosion products (BruÈcher et al., 2001). Investigations of these secondary corrosion products as a near-field barrier against radionuclide migration indicated that one component is an Mg-Al-Cl-LDH, also referred to as a hydrotalcitelike compound (Mazeina et al., 2003). Hydrotalcite-like compounds can be structurally characterized as containing brucite-like sheets in which some divalent cations have been substituted * h.curtius@fz-juelich.de DOI: / by trivalent ions to form positively charged sheets (Miyata, 1975). The cationic charge created in the layers is compensated by the presence of anions in the interlayer. In the free space of this interlayer, crystalline water is also present. Layered double hydroxides (LDHs) have received considerable attention in recent years because of their unique layered structures and high anion exchange capacities. The exchange capacity of LDHs is similar to that of anion exchange resins, but they have a greater resistance to temperature (Cavani et al., 1991). It is therefore not surprising that many papers, e.g. those by Ulibarri et al. (2001) and Toraishi et al. (2002), describe the 1 The MTR-FE of the DIDO type contains 20 wt.% U-Al alloy with an initial enrichment of 80% in 235 U. The fuel alloy plates with a thickness of 0.6 mm (meat) are covered on both sides with aluminium claddings of 0.38 mm: the total thickness is then 1.36 mm. The spent fuel element will be disposed in a cast iron POLLUX container. # 2005 The Mineralogical Society

2 456 H. Curtius and Z. Kattilparampil sorption of an anionic surfactant by LDHs. In general it was found that the selectivity in the exchange reaction increases with increasing anion charge density, i.e. anionic clays strongly prefer multiply charged anions (Miyata, 1983). With the exception of the LDH-NO 3 ÿ I ÿ system, Miyata (1983) observed the obtained isotherms to be of the L-type (Giles et al., 1960). This type indicates a system where monofunctional molecules or ions adsorb on specific sites of the sorbent, mostly by electrostatic interaction and reaching a saturation value given by a plateau of the isotherm. Iodine-129 is a long-lived radiotoxic fission product that has a half-life of y and is of serious environmental concern because it is poorly retarded by geochemical barriers. Rancon (1988) studied the iodine retention and migration in soils under various conditions using the batch technique, and obtained sorption-desorption isotherms which are linear until soil saturation begins. Among several minerals, chalcopyrite had the best sorption properties. Besides that copper ore, LDH minerals were identified as some of the few inorganic anion exchangers that sorb I ÿ under specific conditions (Miyata, 1983). In the present work, the solid-liquid interactions of I ÿ with a Mg-Al-Cl-LDH as a function of ph, radionuclide concentration and competing anion were investigated. EXPERIMENTAL Deionized water was boiled and stored under an argon atmosphere before use. Chemicals were purchased from Merck and used without further treatment. The radioactive iodine-129 solutions were prepared from a standardized stock solution (mass of solution = g; chemical form = NaI in 0.1 M NaOH; carrier content = 50 mg each of KI, Na 2 SO 3 and LiOH; radionuclide concentration = kbq/g). All experiments were performed under an argon atmosphere. To prepare the Mg-Al-Cl-LDH, 250 ml of water were placed in a three-necked glass flask and a ph of 10 was established using 2 M NaOH. A mixed aqueous solution of MgCl 2 6H 2 O (0.3 M) and AlCl 3 6H 2 O (0.1 M) in 250 ml water was added over a period of 3 h while the ph was maintained at 10 by addition of 2 M NaOH. The temperature was maintained at 70ëC. After the addition was complete, the temperature was raised to 90ëC and stirring continued for 24 h. After cooling to room temperature, the precipitate formed was filtered and then dialysed at 60ëC. For dialysis, a dialysis hose was filled with the substance and placed in a 2 litre vessel containing deionized water. The water was exchanged until it was chloride-free (chloride measurements were performed with the cuvette test LCK 311, Dr Lange). The precipitate was filtered and dried in a desiccator. The substances obtained were characterized by photometry, X-ray diffraction (XRD), differential thermal analysisthermogravimetric analysis (DTA-TGA), Fourier Transform infrared spectroscopy (FTIR) and inductively coupled plasma-mass spectroscopy (ICP-MS. For DTA-TGA, XRD and FTIR measurements, dry samples were milled to powder. For photometric and ICP-MS measurements the solid samples were dissolved in 2 M HNO 3. Sorption experiments were performed using a batch technique. All samples were stored in glass tubes with occasional shaking under an argon atmosphere for 2 days, then filtered (450 nm) and the ph was measured. Aliquots of the solutions were analysed by ICP-MS and the concentration of I ÿ was determined radiometrically by LSC (Packard 2200 Tri-carb). The solids were washed, dried and analysed by XRD and FTIR. Blank experiments that were also performed indicated that adsorption of I ÿ on the glass walls was negligible. The time-dependent adsorption processes were studied as follows: to 10 ml of water, 100 ml ofi ÿ solution ( ÿ5 mole/l) and 0.1 g of Mg-Al- Cl-LDH were added in that order. Samples were taken at different time intervals. Adsorption isotherms were performed as follows: to 10 ml of water, the I ÿ solution (concentration of I ÿ ranged from ÿ6 mole/l to ÿ4 mole/l) and 0.1 g of Mg-Al-Cl-LDH were added in that order. The effect of ph on the adsorption of I ÿ was evaluated as follows: to 10 ml of water, 100 ml of I ÿ solution ( ÿ5 mole/l) were added, the ph was adjusted, and 0.1 g of Mg-Al-Cl-LDH were added. To study the effect of chloride as the competing anion, 0.01 M MgCl 2 and 0.1 M MgCl 2 solutions were used instead of water. Chloride was determined photometrically. DISCUSSION The Mg-Al-Cl-LDH was prepared according to the coprecipitation method as described by Weiss & Toth (1996); with some modifications in relation to the purification and drying steps. For the synthe-

3 Sorption of I by Mg-Al-Cl-LDH 457 sized hydrotalcite, an Mg/Al mole ratio of 3:1 was determined by ICP-MS. In the DTA-TGA thermogram, the first endothermic peak at 137ëC, corresponding to a weight loss of 13.7%, is due to desorption of the interlayer water. The second endothermic peak between 200ëC and 450ëC indicates partial dehydroxylation and elimination of chloride and carbonate. The IR spectra show strong hydroxyl and water stretching and bending bands at 3400 cm ÿ1 and 1635 cm ÿ1. Al-O and Mg-O vibration bands appear in the region 1090 to 550 cm ÿ1. A very weak 2ÿ adsorption band due to adsorbed CO 3 was present at 1352 cm ÿ1. From these results the formula of the Mg-Al-Cl- LDH can be derived as Mg 3 Al(OH) 8 Cl ÿ (CO 3 ) H 2 O. Scanning electron microscopy (SEM) shows the sand rose crystal structure that is typical of hydrotalcites (Fig. 1). X-ray diffraction (Fig. 2) shows the typical sharp 003, 006, 009 and 110 reflections. The lattice parameters obtained, c = A Ê and a =3.06A Ê agree well with published values (Cavani et al., 1991). The sorption experiments were performed using the Mg-Al-Cl-LDH synthesized and characterized thus. For this LDH of molecular weight 314 g/mole, we calculate an anion exchange capacity of ÿ3 mole/g (ignoring the presence of carbonate in the interlayer) for monovalent anions, which corresponds to 2.8 meq/g. That means that at most ÿ3 mole/g chloride in the interlayer can be exchanged by other monovalent anions. FIG. 1. Morphology of the synthesized Mg-Al-Cl-LDH. Figure 3 shows the temporal dependence of I ÿ adsorption with an initial iodine concentration of ÿ5 mole/l in water on the Mg-Al-LDH. Equilibrium was reached within 2 days. About 50% of I ÿ was adsorbed, corresponding to an anion exchange capacity of ~2610 ÿ6 mole/g, while the equilibrium ph was ~8.5. In 0.01 M MgCl 2 solution, only 30% of I ÿ was sorbed (Fig. 4). This corresponds to an anion exchange capacity of ~ ÿ6 mole/g, and indicates that chloride as the competing anion has a greater affinity. This was confirmed by the experiments using a 0.1 M MgCl 2 solution, which showed no sorption of iodine at all. The adsorption of I ÿ on Mg-Al-Cl-LDH was furthermore unaffected by ph in the ph range between 3.5 and 8.5 (Fig. 5). Figure 5 also clearly FIG. 2. XRD patterns of the synthesized Mg-Al-Cl-LDH.

4 458 H. Curtius and Z. Kattilparampil FIG. 3. Percentage adsorption of iodine on Mg-Al-Cl-LDH as a function of time in water at 25ëC. shows that more iodine is adsorbed when chloride anions are absent. X-ray diffraction of the solid, carried out to ensure that no structural changes of the Mg-Al- LDH occur as a consequence of ph treatments, showed the typical 003, 006, 009 and 110 reflections. The lattice parameters, in particular the basal parameter c, had not changed. This was unexpected, because iodine is larger than chloride, and the basal spacing should therefore increase. A possible explanation for the invariant c parameter is that that the exchange reaction between chloride as interlayer anion and iodine takes place only at the edges of the interlayers. For LDHs, possible adsorption sites are located within the interlayer and onto external surfaces (Chatelet et al., 1996). Quantitative information on which sites are occupied can be obtained from the adsorption isotherms, linear adsorption isotherm indicating an ion-exchange mechanism. If the isotherm is no longer linear and reaches a plateau, then this part can be assigned to the adsorption on external sites. In general, isotherms of the L-type or of the H-type according to the classification of Giles et al. (1960) suggest an ionic interaction of the solute-sorbent system, and adsorption takes place on the external sites. In the present work, the adsorption isotherms were obtained by plotting the amount of metal ions adsorbed on Mg-Al-Cl-LDH, C ads, against the metal concentration in solution at equilibrium, C e. Figure 6 shows the adsorption isotherm for I ÿ adsorbed at ph 5.5 and 25ëC on Mg-Al-Cl-LDH in water and in 0.01 M MgCl 2 solution. The isotherms obtained were linear. We therefore assume that the favoured reaction in our system is an ion-exchange process. The sorption data for iodine in the concentration range used were analysed in terms of the Freundlich equation. Freundlich isotherms often describe sorption of trace amounts of sorbing species satisfactorily. The Freundlich equation in the logarithmic form is given by: log C ads = log K + 1/n log C e (1) FIG. 4. Percentage adsorption of iodine on Mg-Al-Cl-LDH as a function of time in 0.01 M MgCl 2 solution at 25ëC.

5 Sorption of I by Mg-Al-Cl-LDH 459 FIG. 5. Percentage adsorption of iodine on Mg-Al-Cl-LDH as a function of ph in water and in 0.01 M MgCl 2 solution at 25ëC. where C ads is the amount adsorbed per gram of the sorbent, C e is the equilibrium concentration, and K and 1/n are constants. A plot of log C ads against log C e gives a straight line, the slope and intercept of which correspond to 1/n and log K, respectively. Freundlich plots for the iodine adsorption on the Mg-Al-Cl-LDH in water and in 0.01 M MgCl 2 solutions are shown in Fig. 7. The Freundlich equation gives satisfactory fits to the experimental results (R 2 = in water and for 0.01 M MgCl 2 ). The linear regressions of the plots give slopes of ~1, indicating that a concentration independent, i.e. a linear adsorption of I ÿ, on Mg-Al-Cl-LDH exists in the concentration range used. As an alternative to the Freundlich isotherm, the data were also fitted using the Dubinin- Radushkevich (D-R) equation (Khan et al., 1995): X = X m exp (ÿke 2 ) (2) where X = amount of solute adsorbed per unit weight of solid (mol/g); X m = sorption capacity of adsorbent per unit weight (mol/g); e = Polanyi potential = RT ln(1+1/c e ); C e = equilibrium concentration of solute in solution (mole/l); R = gas constant (kj/k mol); T = temperature (K); and K = constant related to the sorption energy (mol 2 /kj 2 ). The mean energy of sorption is the free energy change when one mole of ion is transferred to the surface of the solid from infinity in the solution, and is calculated from: E =(ÿ2k) ÿ1/2 (3) The data obtained from the D-R isotherm in Fig. 8 show that the D-R equation also gives satisfactory fits to the experimental results (R 2 = for water and for 0.01 M MgCl 2 ). The values of the mean energies of the sorption in water and in 0.01 M MgCl 2 solution were found to be 8 kj/mol, which is in the range for ion exchange reactions (8ÿ16 kj/mol). The maximum sorption capacity was then calculated by the intercept point of the function with the ordinate. FIG. 6. Adsorption isotherms of iodine on Mg-Al-Cl-LDH in water and in 0.01 M MgCl 2 solution at 25ëC and at ph 5.

6 460 H. Curtius and Z. Kattilparampil FIG. 7. Freundlich plots of iodine sorption on Mg-Al-Cl-LDH at 25ëC in water and in 0.01 M MgCl 2 solution at ph 5. In water, the maximum sorption capacity was ÿ4 mol/g, and in 0.01 M MgCl 2 solution the value decreased to ÿ4 mol/g. Compared to the calculated maximum anion exchange capacity of ÿ3 mol/g, these results predict that it is possible to exchange 29% of the interlayer chloride by iodine in water. In the 0.01 M MgCl 2 solution, where chloride is present as the competing ion, only 7% of the interlayer chloride can be exchanged by iodine. These results are in agreement with those of Miyata (1983), who observed the ion exchange equilibrium constant for monovalent anions to vary in the sequence OH ÿ F ÿ Cl ÿ Br ÿ ÿ NO 3 I ÿ. It should nevertheless be pointed out that for the concentration range studied here, the highest value of exchange capacity for iodine in water was found to be ~ ÿ5 mole/g, corresponding to 0.73%. CONCLUSIONS We synthesized a Mg-Al-Cl-layered double hydroxide and characterized this by XRD, SEM, IR spectroscopy, photometry, DTA-TGA and by chemical analysis using ICP-MS. The formula of the Mg-Al-Cl-LDH can be given as Mg 3 Al(OH) 8 Cl 0.88 (CO 3 2ÿ ) H 2 O. This Mg- Al-Cl-LDH was used to determine the solid-liquid interactions with iodine as a function of time, ph, radionuclide concentration and competing anions. At an initial iodine concentration of ÿ5 mole/l, the adsorption on Mg-Al-Cl-LDH in water at 25ëC reached equilibrium within 2 days. 50% of iodine was adsorbed and the equilibrium ph was ~8. It was furthermore shown that the adsorption in the ph range between 3.5 and 8.5 was not affected by ph. The adsorption behaviour follows both a Freundlich type adsorption isotherm and fits in the FIG. 8. D-R plots of iodine adsorption on Mg-Al-Cl-LDH in water and in 0.01 M MgCl 2 solution.

7 Sorption of I by Mg-Al-Cl-LDH 461 D-R equation. The D-R parameters X m and K were calculated and the X m values obtained clearly showed the sorption capacity for iodine to decrease by increasing the chloride concentration, hence chloride has a higher affinity towards the Mg-Al- Cl-LDH. The adsorption mean energies calculated using the parameter K indicate that iodine is adsorbed on Mg-Al-Cl-LDH by an ion exchange process in water as well as in 0.01 M MgCl 2 solution. In conclusion, the Mg-Al-Cl-LDH synthesized barely retards iodine. In view of a final disposal of irradiated fuel elements in salt formations, the sorption properties of the Mg-Al-Cl-LDH studied are not sufficient to ensure iodine containment. The results obtained further indicate that no immobilization of iodine will take place at all in the presence of large amounts of chloride. Future investigations with different repositoryrelevant radionuclides are in progress. ACKNOWLEDGMENT This work was funded by the BMWi (FoÈrderkennzeichen: 02E9803). REFERENCES BruÈcher H., Curtius H. & Fachinger J. (2001) R&D for back-end options for irradiated research reactor fuel in Germany. Transactions of the 5th Topical Meeting on Research Reactor Fuel Management, April 1ÿ3, 2001, Aachen, Germany, ENS RRFM. Cavani F., Trifiro F. & Vaccari A. (1991) Hydrotalcitetype anionic clays: preparation, properties and applications. Catalysis Today, 11, 173ÿ301. Chatelet L., Bottero J.Y., Yvon J. & Bouchelaghem A. (1996) Competition between monovalent and divalent anions for calcinated and uncalcinated hydrotalcite: anion exchange and adsorption sites. Colloids and Surfaces A. Physicochemical and Engineering Aspects, 111, 167ÿ175. Giles C.H., MacEwan T.H., Nakhwa S.N. & Smith D. (1960) Studies in adsorption. Part XI. A system of classification of solution adsorption isotherms, and its use in diagnosis of adsorption mechanisms and in measurement of specific surface areas of solids. Journal of the Chemical Society, 3973ÿ3993. Khan S.A., Reman R.-U. & Khan M.A. (1995) Adsorption of Cs(I), Sr(II) and Co(II) on Al 2 O 3. Journal of Radioanalytical and Nuclear Chemistry, 190, 81ÿ96. Mazeina L., Curtius H., Fachinger J. & Odoj R. (2003) Characterisation of secondary products of uraniumaluminium material test reactor fuel element corrosion in repository-relevant brine. Journal of Nuclear Material, 323, 1ÿ7. Miyata S. (1975) The synthesis of hydrotalcite-like compounds and their structures and physico-chemical properties. Clays and Clay Minerals, 31, 369ÿ375. Miyata S. (1983) Anion-exchange properties of hydrotalcite-like compounds. Clays and Clay Minerals, 31, 305ÿ311. Rancon D. (1988) Comparative study of radioactive iodine behavior in soils under various experimental and natural conditions. Radiochimica Acta, 44/45, 187ÿ193. Toraishi T., Nagasaki S. & Tanaka S. (2002) Adsorption behavior of IO 3 ÿ by CO3 2ÿ - and NO3 ÿ -hydrotalcite. Applied Clay Science, 22, 17ÿ23. Ulibarri M.A., Pavlovic I., Barriga C., HermosõÂn M.C. & Cornejo J. (2001) Adsorption of anionic species on hydrotalcite-like compounds: effect of interlayer anion and crystallinity. Applied Clay Science, 18, 17ÿ27. Weiss A. & Toth E. (1996) Untersuchungen zur Synthese, Quellungseigenschaften und Anionenaustausch von kristallchemisch modifizierten Doppelhydroxiden vom Hydrotalkit-Typ. Jahrestagung der DTTG-Freiberg, pp. 267ÿ276.

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