Uranium(VI) and Europium(III) - American Chemical Society

ion sorption mechanisms onto three phosphate compounds: ZrP2O7, Zr2O(PO4)2, and Th4(PO4)4P2O7. A ... As part of the storage of nuclear wastes in a dee...
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Uranium(VI) and Europium(III) Speciation at the Phosphate Compounds-Solution Interface Romuald Drot and Eric Simoni* Institut de Physique Nucle´ aire d’Orsay, Groupe de Radiochimie, Universite´ Paris-Sud, Baˆ timent 100, 91406 Orsay Cedex, France Received November 13, 1998. In Final Form: April 7, 1999 Prediction of radionuclides migration through a geosphere is of fundamental interest in order to evaluate the safety of an underground radwastes repository. We have studied uranyl and europium(III) ion sorption mechanisms onto three phosphate compounds: ZrP2O7, Zr2O(PO4)2, and Th4(PO4)4P2O7. A spectroscopic investigation which has been previously reported has allowed one to determine all species involved in the sorption processes under study. This paper presents the modeling of experimental retention data using FITEQLv3.2 code (constant capacitance model). All sorption isotherms were successfully simulated with respect to the structural constraints. We have shown that uranyl sorption onto the solids under study is less influenced by electrostatic interactions between aqueous species and charges on the surface than is that for europium(III) ion. Because many experimental constraints, obtained from independent spectroscopic techniques, have been taken into account for the fitting procedure, the sorption constant values can be determined accurately.

Introduction As part of the storage of nuclear wastes in a deep underground disposal, water could leak into the site and, consequently, radionuclides could be released and migrate through engineered barriers and a geosphere.1-3 Several processes are subject to enhanced or drop retardation as dissolution and/or precipitation phenomena, colloid formation, and sorption processes. To evaluate the safety of an underground disposal, a detailed knowledge of all mechanisms involved in these phenomena appears to be of fundamental interest because laboratory data have to be extrapolated over a geological time scale. In this work, we have chosen to particularly consider cation sorption onto mineral surfaces. The prediction of radionuclides migration needs a quantitative aspect: the sorption constant values (which, of course, depend on the model used). On the one hand, they can be obtained by performing direct simulation of experimental retention data, but several hypotheses which are not experimentally verified have to be assumed. Indeed, in the general case the code used for modeling retention data does not lead to a unique solution (acceptable from both physical and chemical points of view) and some assumptions will allow one to reject some of these, assuming, for example, that any bidentate surface complex can be formed for steric considerations. On the other hand, structural information obtained from spectroscopic study allow one to define clearly sorption equilibria, and then they can be used as experimental constraints for the fitting procedure of these experimental retention data. Then, using structural constraints obtained from several spectroscopic methods, sorption mechanisms and thus sorption constant values can be determined accurately. This is the approach developed in this work. We have considered three phosphate compounds: Th4(PO4)4P2O7, ZrP2O7, and Zr2O(PO4)2. A surface complex* To whom correspondence is addressed. Tel: 01 69 15 73 43. Fax: 01 69 15 71 50. E-mail: [email protected]. (1) Guillaumont, R. Radiochim. Acta 1994, 66/67, 231. (2) Hering, J.; Kraemer, S. Radiochim. Acta 1994, 66/67, 63. (3) De Marsily, G. Radiochim. Acta 1988, 44/45, 159.

ation model was used in order to model experimental retention data. Such a model proposes a precise description of the solid-water interface and then can lead to a detailed knowledge of sorption mechanisms. Phosphate minerals are potential candidates as host materials or engineered barrier additives because of their sparing solubility.4 Moreover, some of them, like apatite and monazite, are stable through a geological scale.4-6 Nevertheless, sorption properties of such compounds were not extensively studied except calcium phosphate7,8 and hydrogen phosphates9,10 which have been widely considered as ions exchangers. Thorium phosphate diphosphate (Th4(PO4)4P2O7) is a very slightly soluble material.11 On the one hand, because it presents simultaneously phosphate and diphosphate groups, the sorption mechanisms involved with this compound are expected to be rather complex. On the other hand, it seemed very interesting to study the influence of each of these groups separately. That is why we have considered zirconium diphosphate (ZrP2O7) and zirconium oxophosphate (Zr2O(PO4)2) compounds as well. The three solids were synthesized and characterized using physicochemical methods.12 Moreover, their surface acidity constants were determined using the constant capacitance model.12 Uranyl and europium(III) ions were chosen in order to model actinide(VI) and -(III) behavior. They allow one to avoid problems arising from radioactivity such as radiolysis phenomena for example. (4) Nriagu, J. O.; Moore, P. B. Phosphate minerals; Springer-Verlag: Heidelberg, 1984. (5) Boatner, L. A.; Sales, B. C. In Radioactive Waste Forms for the Future; Lutze, W., Ewing, R. C., Eds.; Elsevier Science Publishers: Amsterdam, The Netherlands, 1988; Chapter 8. (6) Podor, R. Thesis, Henry Poincare´ University, Nancy I, France, 1994. (7) Jeanjean, J.; Vincent, U.; Fedoroff, M. J. Solid State Chem. 1994, 108, 68. (8) Wu, L.; Forsling, W.; Schindler, P. W. J. Colloid Interface Sci. 1991, 147, 178. (9) Zamin, M.; Shaheen, T.; Dyer, A. J. Radioanal. Nucl. Chem. 1994, 182, 335. (10) Yinjie, S.; Hui, Z.; Qiaoling, Y. J. Radioanal. Nucl. Chem. 1995, 198, 375. (11) Genet, M.; Brandel, V.; Dacheux, N. et al. French Patent B 12050 MDT, FIST 60895, 1995. (12) Drot, R.; Lindecker, C.; Fourest, B.; et al. New J. Chem. 1998, 1105.

10.1021/la981596v CCC: $18.00 © 1999 American Chemical Society Published on Web 06/05/1999

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Table 1. Surface Characteristics for the Three Compounds under Study compound

total surface sites density (sites/nm2)

Th4(PO4)4P2O7

4.0

Zr2O(PO4)2

7.5

ZrP2O7

7.0

groups

log K1

log K2

pHIEP

specific surface area N2-BET (m2‚g-1)

PO4 P2O7 PO4 oxo P2O7

6.5 5.4 4.4 3.3 3.2

-7.8 -6.3 -5.3 -3.9 -4.2

6.8 ( 0.2

1.2 ( 0.2

4.0 ( 0.2

0.9 ( 0.2

3.6 ( 0.2

5.5 ( 0.1

Structural studies carried out using X-ray photoelectron spectroscopy (XPS), optical spectroscopy, and extended X-ray absorption fine structure (EXAFS) have been previously reported.13,14 Nevertheless, we will reiterate the main results in order to lead to an easy understanding of our general sorption processes approach. Then, this structural information will be used as experimental constraints for the fitting procedure of the experimental data (using FITEQLv3.2), which allows one to determine the sorption constant values of U(VI) and Eu(III) ions sorbed onto Th4(PO4)4P2O7, Zr2O(PO4)2, and ZrP2O7. State of the Art The main results obtained from the spectroscopic investigation are the following ones: (i) Optical spectroscopy has shown that the sorbed ions do not diffuse inside the substrate but stay located on the surface.13 This result is very important because a surface complexation model is considered to model retention data. Indeed, the use of such a model implies that no diffusion process occurs after sorption of the cation. Thus, in our experimental conditions, a surface complexation model can be used. (ii) ZrP2O7 admits only one single surface site which corresponds to the surface oxygen atoms of the P2O7 group. Th4(PO4)4P2O7 as well as Zr2O(PO4)2 presents two types of surface sites which correspond respectively to the surface oxygen atoms of the PO4/P2O7 and PO4/oxo groups. These results obtained from XPS experiments have been corroborated by optical spectroscopy measurements.13 (iii) In a KNO3 medium (0.5 M), only an europium(III) nitrate complex has to be taken into account. In contrast, for uranyl ions two species have to be considered: the sorbed uranyl nitrate complex and the sorbed free aqueous species.13 Moreover, recent results obtained by nuclear reaction experiments (14N (d,p) 15N) have clearly shown the presence of nitrogen on these samples and thus support the above spectroscopic conclusions.15 (iv) EXAFS experiments have shown that the uranyl and europium species are sorbed as bidentate inner-sphere complexes.14 (v) The isoelectric points for the three phosphate compounds under study, determined from electrokinetic measurements in a KNO3 medium,12 are respectively 3.6 ( 0.2, 4.0 ( 0.2, and 6.8 ( 0.2 for ZrP2O7, Zr2O(PO4)2, and Th4(PO4)4P2O7. (vi) The surface acidity constants have been determined by modeling potentiometric titration curves using FITEQLv3.2 code, constant capacitance model (C ) 3.08 F‚m-2), in a KNO3 (0.5 M) medium.12 The obtained results are reported in Table 1. Both surface acidity constants are defined as follows, where XO represents a surface sorption site (X is a phosphorus atom for phosphate sites and a (13) Drot, R.; Simoni, E.; Ehrhardt, J. J.; et al. J. Colloid Interface Sci. 1998, 205, 410. (14) Drot, R.; Simoni, E.; Denauwer, C. C.R. Acad. Sci. Paris 1999, 2-IIc, 111. (15) Trocellier, P., private communication.

zirconium atom for oxo sites):

XOH + H+ S XOH2+ XOH S XO- + H+

K1 K2

(vii) Specific surface areas determined using N2-BET adsorption method are respectively 1.2 ( 0.2, 0.9 ( 0.2, and 5.5 ( 0.1 m2‚g-1 for Th4(PO4)4P2O7, Zr2O(PO4)2, and ZrP2O7 compounds.12 With regards to these results, all species involved in sorption processes under study are known and then the corresponding equilibria are clearly defined. Thus, isotherm simulation can be done on the basis of these experimental constraints in order to determine accurate sorption constant values. Experimental Section Cation stock solutions were prepared by dissolving UO2(NO3)2‚ 6H2O (Merck) and Eu(NO3)3‚6H2O (Fluka-Aldrich) solids in a previously acidified (with HNO3) 0.5 M KNO3 solution. This protocol allows one to avoid cation hydrolysis. For all of these solutions, the initial concentration was around 4 × 10-3 M and the initial pH value was close to 1. The potassium nitrate salt was chosen as the supporting electrolyte because it is well accepted that K+ and NO3- ions do not sorb specifically.16,17 Thus, any coadsorption of both ions with UO22+ and Eu3+ can occur. Sorption isotherms were obtained by performing batch experiments in polypropylene tubes. (i) Surface hydration was realized by shaking a weighed amount of solid (200 or 800 mg) for 15 h with 10 mL of a KNO3 solution adjusted at the desired pH with small volumes of HNO3 or KOH solutions. It has already been shown that this time is sufficient to reach equilibrium.12 Then, the suspension is centrifuged at 3500 rpm for 30 min. (ii) Part of the supernatant (250 µL) is removed and replaced by the same volume of a cation stock solution, adjusted to the pH equilibrium value (with HNO3 or KOH solutions). This protocol leads to an initial cation concentration of 9.5 × 10-5 M for UO22+ ion and 6 × 10-5 M for Eu3+ ion. Kinetic study has shown that, whatever the considered system, sorption equilibrium is reached within 5-10 h. Nevertheless, for practical reasons all suspensions were stirred for 24 h before being centrifuged at 3500 rpm for 30 min. The pH value was then measured, and the uptake of uranyl or europium ions was determined using a spectrophotometric method (cation complexation with arsenazo I).18,19 All percentages mentioned in the text refer to the cation concentration in solution. Among parameters which could modify the isotherm curve (position and shape), the total cation concentration/surface sites concentration ratio (R) is particularly influent. If the R value is close to or greater than 1, surface precipitation phenomena can occur.20 Thus, according to the surface site density as well as the specific surface areas previously determined12 and considering (16) Jolivet, J. P. De la solution a` l’oxyde; InterEdition/CNRS Ed.: Paris, 1994. (17) Davis, J. A.; Kent, D. B. In Reviews in Mineralogy: MineralWater Interface Geochemistry; Hochella, M. F., White, A. F., Eds.; Mineralogical Society of America: Washington, DC, 1990; Vol. 23, p 177. (18) Fritz, J. S.; Johnson Rochard, M.; Lane, W. J. Anal. Chem. 1958, 30, 1776. (19) Onishi, H.; Nagai, H.; Toita, Y. Anal. Chim. Acta 1962, 26, 528.

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Table 2. R Values for the Systems under Study system

R value

Eu(III)/Th4(PO4)4P2O7 Eu(III)/Zr2O(PO4)2 Eu(III)/ZrP2O7 U(VI)/Th4(PO4)4P2O7 U(VI)/ZrP2O7

0.8 0.6 0.08 0.30 0.15

the initial cation concentrations, we have chosen to perform sorption experiments with 200 mg for all compounds except in the case of the U(VI)/Th4(PO4)4P2O7 system, for which we have used 800 mg of solid in order to avoid surface site saturation. For such conditions and because EXAFS experiments have shown that both U(VI) and Eu(III) ions are sorbed as bidentate surface complexes, the concentration ratio R has the values reported in Table 2 for the five systems under study. Before experimental retention data are modeled, the sorption reaction reversibility has to be checked. Surface complexation models are based on thermodynamical equilibria, and thus all processes involved need to be described with equilibrated reactions. Nevertheless, it appears rather difficult to check this point. One of the possible ways is to perform desorption experiments and thus the following protocol has been considered: (i) After the sorption step (pHeq), the suspension was centrifuged in the same conditions as described above and the supernatant was entirely removed. (ii) The solid was washed with a small volume of distilled water adjusted at pHeq (with HNO3 or KOH solutions) and dried at room temperature. (iii) The dried solid was immersed in 10 mL of a 0.5 M KNO3 solution (pHeq) and shaken for 24 h. The final concentration of the released cation in solution can then be determined. Note that this procedure does not avoid cation desorption during the washing step and then the final concentration should be lower than the expected one (considering a reversible isotherm). We have chosen this protocol because it represents the most unfavorable case. Thus, if the final concentration of the released cation in solution is not negligible (and roughly the same as the one expected), then we can consider that the sorption reactions under study are reversible. Isotherm modeling was performed using the constant capacitance model (CCM) included in FITEQLv3.2 code.21 Such a model has been used because structural investigation has shown evidences for specific sorption of the cations under study (innersphere complexes). FITEQL is an iterative, gradient-directed nonlinear least-squares optimization program based on the Gauss method. The inner capacitance value was fixed to 3.08 F‚m-2 according to the results previously obtained.12 For analyzing experimental data, the error estimates are (0.05 pH units and 5% for free or bound sorbate measurements. The goodness of fit can be appreciate by the factor WSOS/DF (where WSOS is the weighed sum of squares and DF the total degrees of freedom for the system analyzed). Hydrolysis constants and nitrate and carbonate complex formation constant values considered in this work were those recommended by Grenthe22 for the uranyl ion. For the europium(III) ion, authors are in rather good agreement for hydrolysis constants and carbonate complex formation constants, and we used those reported by Wood23 and Lee.24 However, in contrast, there are a wide range of values corresponding to the formation constants of an aqueous europium nitrate complex. Millero25 reports log β0(EuNO32+) ) 0.83 while Wood23 considers logβ(EuNO32+) ) 1.23 for infinite dilution, and thus the choice of this constant value appears to be difficult. However, it is well established that the europium ion is a surrogate of americium from the chemical point of view, and we have compared the (20) Farley, K. J.; Dzombak, D. A.; Morel, F. M. M. J. Colloid Interface Sci. 1985, 106, 226. (21) Herbelin, A. L.; Westall, J. C. Report 96-01, Department of Chemistry, Oregon University, Corvallis, OR, 1996. (22) Grenthe, I.; Fuger, J.; Konings, R. J. M.; et al. Chemical thermodynamics of uranium; Elsevier Science Publishers: New York, 1992. (23) Wood, S. Chem. Geol. 1990, 82, 159. (24) Lee, J.; Byrne, R. Geochim. Cosmochim. Acta 1992, 56, 1127. (25) Millero, F. J. Geochim. Cosmochim. Acta 1992, 56, 3123.

Figure 1. Sorption isotherm and calculated curves for the U(VI)/ZrP2O7 system (medium), KNO3, 0.5 M; uranium total concentration of 9.5 × 10-5 M). previous values to the one recommended by Silva et al.26 for the formation of AmNO32+ (log β0 ) 1.50). Thus, with regards to this later value, we have used the constant reported by Wood for the europium nitrate complex, which seems better than the one proposed by Millero.

Results 1. Sorption Reaction Reversibility. We have chosen the U(VI)/ZrP2O7 system as an example. For experimental conditions described above, when pH ) 2, the sorption rate for this system is about 50% (Figure 1). Because the initial concentration of uranyl ions is 9.5 × 10-5 M, this means that 4.75 × 10-5 M of uranyl ions are bound to the solid. Then, if the sorption reaction is fully reversible, the uranyl ion concentration after desorption experiments is expected to be close to 2.4 × 10-5 M. The experimentally measured value (2.3 × 10-5 M) is in very good agreement with the expected one. This result clearly demonstrates that the sorption processes which occur can be described with an equilibrium and thus that a surface complexation model can be considered. Sorption reaction reversibility has only been verified for the U(VI)/ZrP2O7 system. Nevertheless, we have already pointed out that only phosphate groups (and oxo groups for the zirconium oxophosphate compound) are involved in sorption processes, and because the three materials present PO4 and/or P2O7 groups, we can consider that the results obtained for the system U(VI)/ZrP2O7 are also verified for the others. Such an assumption is supported by experiments realized with the curium(III) ion sorbed on the same matrixes. For this trivalent ion, it has been shown that sorption reactions are reversible as well.27 2. Europium(III) Sorption Mechanisms. Spectroscopic study has shown that systems involving an europium ion are the simplest because only the EuNO32+ complex sorbs onto the three materials under study. In this section, we will first consider the ZrP2O7 compound because it presents only one single type of surface site and then we will successively simulate results obtained for Zr2O(PO4)2 and Th4(PO4)4P2O7 solids, which are expected to be rather complex systems with regards to the existence of several types of surface sites. Moreover, structural information and modeling results obtained for the Eu(III)/ZrP2O7 system will help with the interpretation (26) Silva, R. J.; Bidoglio, G.; Rand, M. H.; et al. Chemical Thermodynamics of Americium; Elsevier Science Publishers: New York, 1995. (27) Cavellec, R. Thesis, Paris XI Orsay University, Orsay, France, 1998.

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Figure 2. Sorption isotherm for the Eu(III)/ZrP2O7 system (medium KNO3, 0.5 M; europium total concentration of 6 × 10-5 M): comparison of the results obtained with proton release and without proton release.

of the other systems. The main problem for simulating experimental retention data is the lack of information about the number of hydration water molecules or hydroxyl groups linked to the sorbed europium(III). Thus, during the fitting procedure this parameter has to be determined at the same time as sorption reaction constants. 2.1. Eu(III)/ZrP2O7 System. Experimental data corresponding to Eu(III) sorbed onto ZrP2O7 show that the sorption process occurs between pH ) 3 and 4 (Figure 2). The point of zero charge for the zirconium diphosphate compound is around 3.6, and a sorption rate equal to 0.5 is obtained for this pH value. Moreover, the R value is low for this system (R ) 0.08). All of these observations seem to indicate that the trivalent europium is specifically sorbed on ZrP2O7. The zirconium diphosphate compound presents only one single type of surface site which corresponds to P2O7 surface oxygen atoms (noted XOH in the following equations). Moreover, because the spectroscopic investigation has shown as well that only EuNO32+ complexes sorb as bidentate species, we have considered only one sorption reaction in order to model the corresponding isotherm. The best fit of the experimental data has been obtained considering the following equilibrium:

2XOH + Eu3+ + NO3- S (XOH)2EuNO32+

K

where log(K) ) 7.49 ( 0.05 and WSOS/DF ) 1.6. A comparison between the experimental data and calculated curve is presented in Figure 2. We can note that the sorption reaction is not followed by proton release and thus the increase of the sorption rate versus pH results only from the variation of the relative concentrations of positive, neutral, and negative forms of the surface sites with the pH. Such a result has already been obtained for low surface coverage and rareearth ions sorbed onto hematite and alumina.28,29 For comparison, if we take into account proton release, the sorption equilibrium is

2XOH + Eu3+ + NO3- S (XO)2EuNO3 + 2H+

K′

and the fitting procedure gives the results log(K′) ) 0.13 ( 0.06 and WSOS/DF ) 2.8. (28) Marmier, N.; Dumonceau, J.; Chupeau, J.; et al. C.R. Acad. Sci. Paris 1993, 317-II, 311. (29) Marmier, N.; Dumonceau, J.; Chupeau, J.; et al. C.R. Acad. Sci. Paris 1994, 318-II, 177.

Figure 3. Sorption isotherm for the Eu(III)/Zr2O(PO4)2 system (medium KNO3, 0.5 M; europium total concentration of 6 × 10-5 M). (XO)2EuNO3 and (YO)2EuNO3 are respectively for PO4 and oxo groups.

Figure 2 allows one to compare the results obtained for both cases described above. It appears that a sorption mechanism which does not lead to proton release seems to be the best one for modeling the europium(III)/ZrP2O7 system experimental data. Nevertheless, we can note that the sorption isotherm presents a little bump for pH ranging from 2.5 to 3.6 which may indicate the sorption of another surface species. However, according to the spectroscopic results, only one surface species has to be considered as well as one single sorption site. Moreover, as this bump is observed before pH ) 3.6 (pHPZC), this phenomenon could be linked to changes in surface species protonation. Because FITEQL takes into account the surface site charge versus pH, one way was to model this isotherm considering that the cation sorption on the XOH2+ surface site occurs at low pH values, while the increase of the sorption rate after the pHPZC is due to reaction with XOH (or XO-) surface sites. However, this hypothesis did not lead to substantially better results, and up to now, no explanation can be given. 2.2. Eu(III)/Zr2O(PO4)2 System. Eu(III) sorption onto Zr2O(PO4)2 (Figure 3) is observed for a rather large range of pH values (between pH ) 4 and 6). This is consistent with structural study which has shown that there are two surface complexes for this system. Consequently, as the corresponding isotherm is the convolution of two components, it is expected to be spread on a large range of pH values. Moreover, the point of zero charge for this material is close to 4 and, thus, sorption occurs onto a resulting negative surface charge which shows that electrostatic interactions between cations in solution and surface charges play probably an important role in the formation of the inner-sphere surface complex. According to spectroscopic results, we have fitted experimental data for the Eu(III)/Zr2O(PO4)2 system considering two surface complexes corresponding respectively to sorption of europium(III) onto PO4 and oxo groups (respectively XOH and YOH). Then, the best fit was obtained considering the two following equilibria (Figure 3):

2XOH + Eu3+ + NO3- S (XO)2EuNO3 + 2H+

K(PO4)

2YOH + Eu3+ + NO3- S (YO)2EuNO3 + 2H+

K(oxo)

with log(K(PO4)) ) -3.0 ( 0.3, log(K(oxo)) ) 0.31 ( 0.05, and WSOS/DF ) 3.0.

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Figure 4. Sorption isotherm for the Eu(III)/Th4(PO4)4P2O7 system (medium KNO3, 0.5 M; europium total concentration of 6 × 10-5 M): (XO)2EuNO3 and (YO)2EuNO3 are respectively for P2O7 and PO4 groups.

Note that, in contrast with zirconium diphosphate, for zirconium oxophosphate the sorption mechanism of the EuNO32+ complex is followed by proton release. This is not surprising because in this case the concentration ratio R has the value 0.6, and authors have pointed out that, for high R values, the main mechanism is followed by proton release which leads to neutral or less charged surface complexes.30,31 2.3. Eu(III)/Th4(PO4)4P2O7 System. For europium(III) sorption onto Th4(PO4)4P2O7, the process occurs between pH ) 4 and 7 (Figure 4). The point of zero charge for this compound is about 6.8. The sorption process mainly occurs in that case with a resulting positive surface charge, which could indicate that for this system electrostatic interactions between cations in solution and surface charges are less influent than those for the previous case. Moreover, as in the case of the Eu(III)/Zr2O(PO4)2 system, the isotherm is spread on a large range of pH values which is consistent with the formation of two surface complexes resulting from the sorption of EuNO32+ species onto both P2O7 and PO4 surface oxygen atoms (noted respectively as XOH and YOH in the following part). Then, the modeling of the Eu(III)/Th4(PO4)4P2O7 system (Figure 4) was performed considering these two equilibria, and the best simulation of the experimental data was obtained for

2XOH + Eu3+ + NO3- S (XO)2EuNO3 + 2H+

K(P2O7)

2YOH + Eu3+ + NO3- S (YO)2EuNO3 + 2H+

K(PO4)

with log(K(P2O7)) ) 0.94 ( 0.14, log(K(PO4)) ) -2.23 ( 0.13, and WSOS/DF ) 1.1. As in the case of the zirconium oxophosphate compound, the R value (0.8) is rather high for this system and less charged surface complexes are favored. 2.4. Conclusion. The acidity constant values for the three materials under study lead to the surface species speciation diagrams reported in Figure 5. If we compare these diagrams to the sorption isotherms obtained for the europium(III) ion sorbed onto the three solids, the followings remarks can be made: (30) Benjamin, M. M.; Leckie, J. O. Environ. Sci. Technol. 1982, 16, 162. (31) Marmier, N. Thesis, Reims Champagne-Ardenne University, Reims, 1994.

(i) We investigate cation sorption processes, and all europium aqueous complexes are positively charged in our experimental conditions. Then, in the following discussion, we will only consider that surface species under neutral and negative forms are available to react with aqueous entities. (ii) For the zirconium diphosphate compound, for pH values ranging from 3 to 4, surface sites under the XOH form are widely dominant species, and thus we can assume that the sorption process occurs on these forms. (iii) For the zirconium oxophosphate compound, the surface species speciation diagram indicates that, for pH values corresponding to the sorption edge of the europium(III) ion, surface species are mainly under YO- and XOH forms for oxo and PO4 groups, respectively, and the cation in the solution will probably react with these surface species. (iv) For the thorium phosphate diphosphate compound, sorption occurs between pH ) 4 and 7, and in this pH range surface species are mainly under the YOH form for PO4 groups. For P2O7 groups, the negative form is the dominant species for pH values higher than 5. Thus, we can assume that these forms will react with aqueous species. (v) Considering our experimental conditions, the aqueous europium nitrate complex is present in solution and thus the previous sorption constants could be corrected in order to take into account the formation constant of this aqueous species (log β0.5(EuNO32+) ) 0.29). Sorption equilibria and corresponding sorption constants relative to systems involving europium(III) are summarized in Table 3. 3. Uranium(VI) Sorption Mechanisms. Systems involving sorption of the uranyl ion are more complex than those for europium(III). Indeed, spectroscopic investigation has shown that for U(VI) there are two surface complexes for each type of surface sites, in a KNO3 medium. The first one results from sorption of the free uranyl ion and the second from the uranyl nitrate complex. All simulations were performed with respect to these experimental constraints. Because optical spectroscopy has not yet been carried out for the U(VI)/Zr2O(PO4)2 system, we will only present in the following text the modeling of experimental retention data corresponding to both U(VI)/ZrP2O7 and U(VI)/Th4(PO4)4P2O7 systems. 3.1. U(VI)/ZrP2O7 System. For this system, uranyl sorption occurs for pH values ranging from 1 to 3, and then the corresponding isotherm is spread on two pH units (Figure 1). This observation seems to agree with the existence of two surface complexes resulting from a UO22+ aqua ion and UO2NO3+ complex sorption as was already shown from a spectroscopic investigation. On the one hand, the point of zero charge for the zirconium diphosphate compound is 3.6 and, thus, a sorption process occurs for the positively charged surface. On the other hand, for pH values corresponding to this isotherm, only UO22+ and UO2NO3+, which are both positively charged species, exist in solution. Then, we can expect that electrostatic interactions are not very influent for the inner-sphere complex formation for this system. According to the experimental constraints, the fitting procedure was performed considering two equilibria and the best result (Figure 1) was obtained for

2XOH + UO22+ + NO3- S (XO)2UO2NO3- + 2H+ 2XOH + UO22+ S (XO)2UO2 + 2H+

K2

K1

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Figure 5. Amphoteric surface species speciation diagrams for the three phosphate materials. For Zr2O(PO4)2, XOH refers to PO4 groups and YOH refers to oxo groups. For Th4(PO4)4P2O7, XOH refers to P2O7 groups and YOH to PO4 groups. Table 3. Sorption Equilibria for the Europium(III) Ion compound ZrP2O7 Zr2O(PO4)2 Th4(PO4)4P2O7

equilibria -

K

2XOH + + NO3 S (XOH)2EuNO3 2XOH + Eu3+ + NO3- S (XO)2EuNO3 + 2H+ (PO4) 3+ 2YOH + Eu + NO3- S (YO)2EuNO3 + 2H+ (oxo) 2XOH + Eu3+ + NO3- S (XO)2EuNO3 + 2H+ (P2O7) 2YOH + Eu3+ + NO3- S (YO)2EuNO3 + 2H+ (PO4) Eu3+

with log(K1) ) 1.67 ( 0.04, log(K2) ) 1.19 ( 0.95, and WSOS/DF ) 1.5. XOH in the above equations represents the oxygen atoms of surface diphosphate groups. For this system, the ratio R value is not very high (R ) 0.15), but we had to consider proton release in order to get a good correlation between experimental and calculated data. 3.2. U(VI)/Th4(PO4)4P2O7 System. Uranyl ion sorption

2+

107.49

10-3.0 100.31 100.94 10-2.23

onto the thorium phosphate diphosphate compound occurs for pH ranging from 2.5 to 4 (Figure 6). The point of zero charge is about 6.8 for this material, and thus the uranyl ion sorbs onto a resulting positively charged surface which probably indicates that, as for the previous system, electrostatic interactions do not play an important role in this sorption process. According to spectrocopic study, we know that there are four surface uranyl complexes on this

4826 Langmuir, Vol. 15, No. 14, 1999

Drot and Simoni

Table 4. Sorption Equilibria for the Uranyl Ion compound

equilibria -

K -

2H+

2XOH + UO2 + NO3 S (XO)2UO2NO3 2XOH + NO22- S (XO)2UO2 + 2H+ 2XOH + UO22+ + NO3- S (XO)2UO2NO3- + 2H+ (P2O7) 2XOH + UO22+ + H2O S (XO)2UO2OH- + 3H+ (P2O7) 2YOH + UO22+ + NO3- S (YO)2UO2NO3- + 2H+ (PO4) 2YOH + UO22+ + H2O S (YO)2UO2OH- + 3H+ (PO4) 2+

ZrP2O7 Th4(PO4)4P2O7

Figure 6. Sorption isotherm and calculated curves for the U(VI)/Th4(PO4)4P2O7 system (medium KNO3, 0.5 M; uranium total concentration of 0.5 × 10-5 M). XO notation refers to P2O7 groups, and YO refers to PO4 groups.

material and the best simulation of experimental data was obtained considering the four following equilibria (Figure 6):

2XOH + UO22+ + NO3- S (XO)2UO2NO3- + 2H+

K1(P2O7)

2XOH + UO22+ + H2O S (XO)2UO2OH- + 3H+

K2(P2O7)

2YOH + UO22+ + NO3- S (YO)2UO2NO3- + 2H+

K1(PO4)

2YOH + UO22+ + H2O S (YO)2UO2OH- + 3H+

K2(PO4)

with log(K1(P2O7)) ) -1.2 ( 1.1 log(K1(PO4)) ) 0.2 ( 0.2, log(K2(P2O7)) ) -3.45 ( 0.14 log(K2(PO4)) ) -2.82 ( 0.15, and WSOS/DF ) 1.7. The uncertainties corresponding to the sorption constants of the surface nitrate complexes (K1) are very important. This results from the fact that a good simulation of experimental data could be realized without considering these complexes. However, because spectroscopic study has shown that there are two types of surface complexes for each surface site, the fitting procedure was done with respect to these experimental constraints. This problem clearly shows that a direct modeling of experimental retention data can lead to an incorrect result and, thus, that a structural investigation of the sorbed species is needed before performing simulation of sorption isotherms. Note that, for “nonnitrate” complexes and in contrast with all other systems, the best result was obtained by considering the uranyl hydroxide surface complex. The spectroscopic investigation did not allow one to determine the number of OH- groups linked to the surface complex when the solid is in suspension, and then this parameter

101.67 101.19 10-1.2 10-3.45 100.2 10-2.82

had to be determined during the fitting procedure, using the goodness of fit as a criterion. To reduce the degrees of freedom for retention data simulation, it should be necessary to determine experimentally the number of OHgroups linked to the sorbed species, but such a measurement appears to be rather difficult. As for the previous case, proton release had to be taken into account in order to obtain a good correlation between experimental and calculated data (the R value is 0.30 for this system). 3.3. Conclusion. In contrast with the results obtained for the three Eu(III)/phosphate compound systems, for both U(VI)/ZrP2O7 and U(VI)/Th4(PO4)4P2O7 systems the uranyl sorption edge is observed for pH values very lower than the corresponding pHPZC. As indicated in Figure 5, surface species are simultaneously present under positive, neutral, and negative forms, but because all uranyl aqueous species are positively charged in our experimental conditions, only neutral and negatively charged surface sites will be considered as reacting surface forms in the following discussion: (i) For the zirconium diphosphate compound, sorption occurs for pH values ranging from 1 to 3. In this range, surface species are mainly present under the XOH form, and the uranyl ion sorption mechanism probably involves this form. (ii) For the thorium phosphate diphosphate compound and for pH values ranging from 2.5 to 4, the neutral form is widely dominant for diphosphate as well as for phosphate surface sites (respectively XOH and YOH). (iii) The aqueous uranyl nitrate complex does exist considering our experimental conditions, and then the sorption constants determined previously could be corrected from the formation constant of the uranyl nitrate complex (log β0.5(UO2NO3+) ) -0.33). Sorption constants corresponding to the U(VI) systems are summarized in Table 4. Conclusion The different species involved in the sorption process of uranyl and europium(III) ions sorbed on phosphate materials were determined from a previous spectroscopic investigation which has shown, moreover, evidences of inner-sphere complexes. Then, we have used this structural information as a constraint for the fitting procedure of experimental retention data using the constant capacitance model included in the FITEQLv3.2 code. Such an approach of sorption phenomena seems to be very important. Indeed, whatever the code used to simulate retention data, in many cases there are several solutions acceptable from both physical and thermodynamical points of view. Then, spectroscopic techniques appear to be very useful because structural information will allow one to check these potential solutions. Moreover, because the sorption constant value is determined by taking into account many experimental results, it appears more accurate. This study has allowed us to point out that electrostatic interactions between aqueous species and solid surface charges are less influent for the specifically sorbed uranyl

Uranium(VI) and Europium(III) Speciation

ion than in the case of the europium(III) ion (specifically sorbed as well). Nevertheless, we have not yet realized any in-situ measurements and such an investigation appears to be of fundamental interest in order to determine the number of OH- groups or hydration water molecules linked to the sorbed species when the loaded solid is in an aqueous solution. For example, techniques such as Raman diffusion or time-resolved laser-induced fluorescence could be used for such an experimental determination. Moreover, the experimental determination of the inner-layer capacitance

Langmuir, Vol. 15, No. 14, 1999 4827

value is also very important to reduce the degrees of freedom for the systems under consideration, but with regards to the very insulating character of the materials under study, such a determination appears to be rather difficult. Acknowledgment. The authors are very grateful to P. Trocellier from the CEA/DSM/DRECAM/LPS for the nuclear reaction experiments. LA981596V