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Cesium adsorption on soil clay: macroscopic and spectroscopic measurements
Applied Clay Science 29 (2005) 23 – 29 www.elsevier.com/locate/clay
Cesium adsorption on soil clay: macroscopic and spectroscopic measurements L. Bergaouia,*, J.F. Lambertb, R. Prostc b
a Laboratoire de Chimie des Mate´riaux et Catalyse, Faculte´ des Sciences de Tunis, 1060 le Belve´de`re, Tunis, Tunisia Laboratoire de Re´activite´ de Surface, URA 1106 CNRS, Universite´ Pierre et Marie Curie, 4 place Jussieu, 75252 Paris cedex 05, France c Station de Science du Sol, INRA, route de Saint-Cyr, 78000 Versailles, France
Received 22 October 2003; received in revised form 11 August 2004; accepted 21 September 2004 Available online 11 November 2004
Abstract We studied the interaction of cesium cation in the aqueous phase with soil clay by combining microscopic and macroscopic data. Investigations concerning selective sorption of cesium by this soil clay are presented. The sample studied is selective for concentrations b210 3 mol/l in solution. Far-infrared (FIR) shows the presence of two selective adsorption sites. At higher loadings, 133Cs MAS-NMR shows that most cesium is essentially adsorbed on external sites which are not very selective. FIR and adsorption data lead to the conclusion that collapsed illite clay is responsible for the selectivity observed at low concentration. However, cesium cations can diffuse between illite layers only if these layers are involved in a mixed smectite– illite mineral. D 2004 Elsevier B.V. All rights reserved. Keywords: Cesium adsorption; Selectivity; Illite–smectite; Far-infrared;
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1. Introduction The 137Cs nuclide, which is very dangerous, has been introduced in the environment after nuclear testing and accidents. The adsorption behaviour of cesium on soil constituents and minerals has been studied in the past few decades. A large part of these
* Corresponding author. Tel.: +216 71 872 600; fax: +216 71 885 008. E-mail address: [email protected] (L. Bergaoui). 0169-1317/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2004.09.002
Cs MAS-NMR
studies treat the interaction between cesium and clay minerals of soils: they deal only with macroscopic information and are not concerned with molecular level characterisation (Sawhney, 1969, 1970, 1972; Maes et al., 1985; Cornell, 1993; Poinssot et al., 1999). General mechanism of cesium adsorption was discussed by consideration of the effect of cesium concentration, the properties of the mineral and the characteristic of the solution phase. A few studies used spectroscopies to investigate the microscopic environment of the cesium cation adsorbed on pure clay minerals by 133Cs NMR
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(Weiss et al., 1990a,b; Laperche et al., 1990) and by far-infrared (FIR) (Laperche, 1991; Diaz, 1999; Diaz et al., 2002a; Badreddine et al., 2002). 133Cs NMR gives different signals according to the degree of hydration and can inform about the mobility of the cation. Studies of the interactions between compensating cations and the clay lattice by FIR show that the compensating cations are located in bcagesQ whose geometry is related to the structural and chemical properties of the lattice. In this study, we attempt to determine if the mobility of cesium is reduced by adsorption on soil clay. We aim to combine macroscopic (adsorption isotherm) and microscopic (NMR and FIR) studies to determine the structure of the selective sites occupied by cesium on this soil clay.
2. Experimental 2.1. Materials The studied soil comes from the region of Mainfranken in Germany which contains at the surface a loessic limon. It is a sedimentary material, not stratified and formed by clay and limestone (b62.5 Am). This depot can reach 10 m of thickness and deal of good arable earths. The sample comes from 6 m of depth. The b2 Am fraction of the raw soil was separated by sedimentation, exchanged three times with a 1 mol/l NaCl solution and washed thoroughly. Chemical analysis gave an exchange capacity of 56 meq/100 g of calcined clay. Well-known clays were used as references. The used kaolinite is from Saint Austel, the illite clay is from Le Puy en Velais, the vermiculite from Llano and the montmorillonite from Camp Bertaux. Each clay was first saturated with sodium. The Na-clays were exchanged at room temperature with a 210 3 mol/l CsCl solution and washed to remove excess electrolyte. 2.2. Characterisation Adsorption experiments were performed in 20 ml screw cap centrifuges tubes containing 0.4 g of the Naloess clay and 20 ml of bi-ionic NaCl/CsCl solutions with constant ion strength but different Na/Cs proportions. After shaking for 7 days at constant temperature (293 K), suspensions were centrifuged (10,000 rpm for 10 min). Cs+ concentration in the equilibrium solution was determined by atomic absorption using an SSA
Spectra 351. Adsorbed cesium concentrations in the solid phase were determined from the difference between the initial Cs concentration and the equilibrium concentration. The solid phase was washed rapidly with 20 ml of distilled water and dried at room temperature. X-ray diffraction was performed on oriented clay deposits and patterns were recorded on a Siemens D5000 diffractometer using the Cu Ka radiation. Due to the intrinsically lower sensitivity of this technique, 133Cs NMR was attempted on a high Cs loading sample. To prepare this sample, Na-clay fraction was exchanged at room temperature with a 210 3 mol/l CsCl solution and washed to remove excess electrolyte. Spectra were recorded on a Bruker AMX500 spectrometer operating at a Larmor frequency of 65.6 MHz. A short pulse length of 2 As was chosen. The recycle time was 1 s and the spinning frequency was 5 kHz; 2000 scans were collected. Infrared and FIR spectra were recorded on a Bruker 113V controlled by an OPUS data station. All the samples are oriented films, except for vermiculite. Oriented samples are prepared by deposing a suspension on a thin plate glass. After drying at room temperature, the clay film was separated from the plate. For vermiculite, sample was deposited on Millipore filter of 0.45 Am pore size.
3. Results and discussion 3.1. Structural characterisation The raw material contains a large amount of quartz. The diffractogram of the b2 Am fraction shows two peaks at 7.2 and 10.1 2 and a third one at 14 2 which shifts to 12 2 when the clay is saturated with Na+ (Fig. 1). The interlayer spacings of 7.2 and 10 2 correspond, respectively, to kaolinite and T-O-T nonswelling clay. The higher spacing at 14 2 is assigned to an interstratified illite–smectite mineral because this peak broadens with ethylene glycol and its maximum shifts to between 16 and 17 2 (Robert, 1975). Fig. 2 shows the infrared spectra of the b2 Am fraction in the region of the OH stretching vibration. We can distinguish two bands at 3692 and 3621 cm 1 and a shoulder at near 3650 cm 1. These bands may be assigned to kaolinite and dioctahedral clay (Farmer, 1974). The FIR spectrum of the loess clay shows two bands at 196 and 110 cm 1, a shoulder at 90 cm 1 and a weak band at 132 cm 1 (Fig. 3). The FIR spectra of the reference Na-clays show that montmor-
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Fig. 1. Diffractograms of the natural loess clay before (a) and after (b) ethylene glycol adsorption. Fig. 3. Far-infrared spectrum of the natural loess clay.
illonite and vermiculite do not exhibit any intense bands in this region (Fig. 4c and d). On the other hand, reference illite shows the same bands as the soil clay (Fig. 4b), except for the lower intensity of the 196 cm 1 band. So we can propose that the T-O-T non-swelling clay fraction consists of illite. The pure kaolinite clay shows an intense band at 196 cm 1 (Fig. 4a) and therefore, the band at 196 cm 1 in the loess clay spectrum must be a superposition of kaolinite and illite bands. A band in this region is probably due to a lattice vibration (Velde and Couty, 1985; Diaz et al., 2002b).
Fig. 2. Infrared spectrum of the loess clay in the OH vibration region.
Fig. 4. Far-infrared spectrum at room humidity of the kaolinite from Saint Austel (a), the illite from Le Puy (b), the vermiculite from Llano (c) and the montmorillonite from Camp Bertaux (d).
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As regards the other bands at 90, 110 and 132 cm 1 observed on loess clay, they have been reported to be characteristic of the FIR spectra of K+ in dioctahedral mixed layer minerals (Diaz et al., 2002b). Diaz et al. (2002b) have shown that the 110 cm 1 band is the superposition of two vibration modes of K+ parallel to the a and b axes, while the 132 cm 1 is due to the vibration mode parallel to the c axis. They have noticed that the intensity and the position of the lower wavenumber band appear to depend on the number of the illite layers in the sample. Again, according to Diaz et al. (2002b) this band shifts toward lower wave number as the percentage of smectite fraction increases. Laperche (1991) showed that the position of this band remains the same from 90% to 30% of smectite layers and gradually shifts from 85 to 97 cm 1 as the percentage of smectite layers decreases from 30% to 0%. The value observed in our loess mixed layer minerals (90 cm 1) corresponds to less than 20% of smectite layers (Diaz et al., 2002b). This percentage corresponds to two and three layer illites altering with smectite layers. In conclusion, our sample is a mixture of kaolinite, illite and an interstratified illite–smectite with a low percentage of smectite layers.
Fig. 6. Cs adsorption isotherm at 293 K on the Na-loess clay.
selective for cesium over sodium ions at least for an initial concentration b210 3 mol/l. Equilibrium Cs+ adsorption data are presented in Fig. 6. It was first checked that the data are not consistent with a single-site Langmuir model. This is not surprising since our soil sample contains several kinds of clays. A log–log scale is used in Fig. 6; in this type of presentation, a Freundlich-type isotherm would appear as linear. The isotherm actually is only linear for adsorbed cesium less than 3 mmol/100 g. This tendency strongly suggests the existence of more than one adsorption mechanism.
3.2. Adsorption isotherm The square diagram presentation of the ion exchange data (Fig. 5) shows that Na-loess clay is
Fig. 5. Square diagram presentation of the Cs+-adsorption on the Na+-loess clay.
3.3. Spectroscopic characterisation of the cesium adsorption sites Potential adsorption sites on clay minerals are of three kinds: the basal surface, the frayed edges of interlayer and the internal interlayer (Bolt et al., 1986). Cesium adsorbed on basal surfaces is hydrated and easily detected by NMR. The 133Cs MAS-NMR spectrum (Fig. 7) of a sample with high cesium loading (27.9 mmol/100 g) shows an intense peak at 2.58 ppm and a small signal at 50 ppm range. The signal at 2.58 ppm can be attributed to a fully hydrated cesium cation, and the small signal at about 50 ppm has been assigned to Cs+ cations in the collapsed interlayer (Laperche et al., 1990; Weiss et al., 1990a,b). The quantification of these peaks (using CsCl as a solid reference) showed that almost all of the cesium present in the sample is detected by NMR. Therefore, in loess clay, fully hydrated cesium (peak at 2.58 ppm) is predominant with respect to the cesium adsorbed in the collapsed interlayers (peak at
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only collapse to 12.5 2 (Calvet, 1972). We think that in vermiculite the cesium ions are 12-coordinated, bonding to six oxygen atoms from a six-ring in one layer and six oxygen atoms from another ring in the adjacent layer. On the other hand, cesium in montmorillonite is nine-coordinated, bonding six oxygen atoms from one layer and three oxygen atoms from a triad of the adjacent layer. Because of these different molecular environments, the vibration of cesium is only seen in vermiculite. On the other hand, illite shows a modification in the intensity of the bands at 110 and 90 cm 1 after cesium exchange (Figs. 4b and 9b). According to Diaz
Fig. 7.
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Cs MAS-NMR spectrum of the Cs-loess clay.
50 ppm) when the sample is saturated. The fully hydrated cesium could be adsorbed either on basal surfaces or between expanded layers. Since the fraction of smectite layers in loess clay is rather low, the majority of the fully hydrated cesium must be adsorbed on basal surfaces. FIR only allows detecting dehydrated interlayer cations (Laperche, 1991; Diaz, 1999; Diaz et al., 2002a,b; Badreddine et al., 2002). Fig. 8 shows spectra obtained for increasing amounts of adsorbed cesium. When the amount of adsorbed cesium increases, the intensity of the band at 110 cm 1 decreases in comparison with that of the band at 90 cm 1. When the adsorbed cesium amount exceeds 5.48 mmol/100 g (Fig. 8g), a band appears at 60 cm 1. The spectra reference Na-clays and Cs-clays show that neither kaolinite (Figs. 4a and 9a) nor montmorillonite (Figs. 4d and 9d) present any significant modification of their spectrum after cesium adsorption. In contrast, after cesium exchange, vermiculite (Figs. 4c and 9c and Diaz et al., 2002a) exhibits a new band at 58 cm 1. Because it has a low hydration energy, Cs+ may collapse the interlayers of expanding layer silicates. The vermiculite layers, with their greater layer charge, collapse more easily than montmorillonite layers which have a smaller layer charge. Vermiculite layers collapse to 11.5 2 (Laperche et al., 1990) and montmorillonite layers
Fig. 8. Far-infrared spectra of the loess clay with an increasing quantity of adsorbed cesium. The amounts of retained cesium are, respectively, (a) 0.22, (b) 0.49, (c) 1.13, (d) 1.56, (e) 2.22, (f) 5.46, (g) 10.16, (h) 15.33, (i) 16.62 and (j) 27.90 mmol/100 g.
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bulkier than K+. The band shift observed for K+ when the amount of Cs+ in the exchange sites increases is probably due to the fact that K+ then becomes more loosely coordinated as a result of interlayer opening. When the amount of cesium between the layers becomes large enough (5.46 mmol/100 g), Cs+ gives rise to a band at 60 cm 1 like in vermiculite. This band at 60 cm 1 is not observed when a pure illite fraction is exchanged with cesium but the shift of the band at 90 cm 1 is noticed. It seems that the exchange of cesium takes place at the edges of interlayer and the diffusion of Cs+ between illite is difficult when the clay is a pure illite; but if the illite layers are a fraction of interstratified clay, the exchange process can extend from the edges to the centre of the particles. It has already been mentioned that the slope of the isotherm in log–log representation changes when the adsorption amount reaches 3 mmol/100 g. It seems that the Cs+/K+ exchange occurs first in the interlamellar spaces. When the adsorption amount exceeds 3 mmol/100 g, the adsorption takes place also on the external surfaces. Taken together, the spectroscopic data lead us to propose a model of cesium distribution into two main compartments: –
– Fig. 9. Far-infrared spectra at room humidity of Cs-kaolinite from Saint Austel (a), Cs-illite from Le Puy (b), Cs-vermiculite from Llano (c) and Cs-montmorillonite from Camp Bertaux (d).
et al. (2002a), when a large cation (Cs+ or Rb+) replaces K+ in vermiculite, a shift to lower frequency of the adsorption band of K+ is observed and a band characteristic of Rb+ or Cs+ appears. The same seems to occur here with the interstratified illite–smectite minerals. The observed decrease in the intensity of the band at 110 cm 1 in comparison with that of the shoulder at 90 cm 1 can be seen as a consequence of the shift of the band at 90 cm 1 to lower frequency. Thus, our observations probably mean that K+ belonging to the two or three-stacking sequence is progressively exchanged by Cs+. The cesium cations act as wedges to expand the layers because they are
Very selective sites corresponding to adsorption of Cs+ in the internal surface of the illite–smectite fraction. Cesium cations are not hydrated in these sites. Low selectivity sites able to adsorb fully hydrated cesium. The majority of these sites are located on external surfaces.
This model is compatible with what is known on the Cs+ affinity of reference minerals. Kaolinite has a small exchange capacity apparently caused by sorption on basal surfaces of crystallites (Lim et al., 1980). These sites are not particularly selective. Illite is formed by collapsed layers with a smaller cationic exchange capacity than vermiculite. In spite of this low CEC, at low concentrations, illite is more selective for cesium than vermiculite. This selectivity has been attributed to the existence of frayed edges (Comans et al., 1991; Maes et al., 1985) which still have a higher affinity for cesium than the interlayer sites in vermiculite. The concentration of these very selective sites in illite is rather low (Brouwer et al., 1983).
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4. Conclusion The adsorption isotherm shows that loess clay is selective for cesium cations. Cesium adsorption on this soil clay occurs by cation exchange at different sites which possess very different cesium affinities. At low concentrations, far-infrared spectroscopy shows the presence of a very selective adsorption sites which correspond to internal collapsed layer. At high concentrations, 133Cs MAS-NMR shows that cesium is essentially adsorbed on external sites which are not very selective. Thus, we can confirm by using spectroscopies that at low concentration, collapsed clay (illite) is very selective. The interstratification of the illite layers in mixed illite–smectite fractions allows the diffusion of cesium cations between the layers. To conclude, loess clay is a good trap for cesium in dilute solutions but it is not efficient at high concentration. Acknowledgments We would like to take this opportunity to thank Professors K. Czurda and J.-F. Wagner for the very good exchange we had during the French-German cooperation programme on the fate of heavy metals in soils. We thank the French Ministry of Environment for its financial support. References Badreddine, R., Le Dred, R., Prost, R., 2002. Far infrared study of K+, Rb+ and Cs+ during their exchange with Na+ and Ca2+ in vermiculite. Clay Minerals 37, 71 – 81. Bolt, G.H., De Boodt, M.F., Hayes, M.H.B., McBride, M.B., 1986. Interaction at the soil colloid soil solution interface. Serie E: Applied Sciences Interaction at the Soil: Colloid 190, 126 – 127. Brouwer, E., Baeyens, B., Maes, A., Cremers, A., 1983. Cesium and rubidium ion exchange equilibria in illite. Journal of Physical Chemistry 87, 1213 – 1219. Calvet, R., 1972. Hydratation de la montmorillonite et diffusion des cations compensateurs. PhD thesis, Univ. Paris VI. Comans, R.N.J., Haller, M., De Preter, P., 1991. Sorption of cesium on illite: non-equilibrium behaviour and reversibility. Geochimica et Cosmochimica Acta 55, 433 – 440.
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Cornell, R.M., 1993. Adsorption of cesium on minerals: a review. Journal of Radioanalytical and Nuclear Chemistry 171, 483 – 500. Diaz, M., 1999. Etude des interactions cations compensateurs/ feuillets dans les argiles: contribution a` la connaissance des me´canismes de re´tention se´lective. PhD thesis, Univ. Orle´ans, Orle´ans, France. Diaz, M., Huard, E., Prost, R., 2002a. Far infrared analysis of the structural environment of interlayer K+, NH+4 , Rb+ and Cs+ selectivity retained by vermiculite. Clays and Clay Minerals 50, 284 – 293. Diaz, M., Laperche, V., Harsh, J., Prost, R., 2002b. Far infrared analysis of the structural environment of interlayer spectra of K+ in dioctahedral and trioctahedral mixed-layer minerals. American Mineralogist 87, 1207 – 1214. Farmer, V.C., 1974. The Infrared Spectra of Minerals. The Mineralogical Society, pp. 340 – 341. Laperche, V., 1991. Etude de l’e´tat et de la localisation des cations compensateurs dans les phyllosilicates par des me´ thodes spectroscopiques. PhD thesis, Univ. Paris VII, Paris, France. Laperche, V., Lambert, J.F., Prost, R., Fripiat, J.J., 1990. Highresolution solid-state NMR of exchangeable cations in the interlayer surface of a swelling mica: 23Na, 111Cd and 133Cs vermiculites. Journal of Physical Chemistry 94, 8821 – 8831. Lim, C.H., Jackson, M.L., Koon, R.D., Helmke, P.A., 1980. Kaolins: sources of differences in cation-exchange capacity and cesium retention. Clays and Clay Minerals 28, 223 – 229. Maes, A., Verheyden, D., Cremers, A., 1985. Formation of highly selective cesium exchange sites in montmorillonites. Clays and Clay Minerals 33, 251 – 257. Poinssot, C., Baeyens, B., Bradbury, M.H., 1999. Experimental and modelling studies of caesium sorption on illite. Geochimica et Cosmochimica Acta 63, 3217 – 3227. Robert, M., 1975. Principe de de´ termination qualitative des mine´raux argileux a` l’aide des Rayons X. Annales Agronomiques 26, 363 – 399. Sawhney, B.L., 1969. Cesium uptake by layer silicates: effect on interlayer collapse and cation exchange capacity. Proceeding of the International Clay Conference, Tokyo vol. 1. pp. 605 – 611. Sawhney, B.L., 1970. Potassium and cesium ion selectivity in relation to clay mineral structure. Clays and Clay Minerals 18, 47 – 52. Sawhney, B.L., 1972. Selective sorption and fixation of cations by clay minerals: a review. Clays and Clay Minerals 20, 93 – 100. Velde, B., Couty, R., 1985. Far-infrared spectra of hydrous layer silicates. Physics and Chemistry of Minerals 12, 347 – 352. Weiss Jr., C.A., Kirkpatrick, R.J., Altaner, S.P., 1990a. The structural environments of cations adsorbed onto clay: 133Cs variable temperature MAS NMR spectroscopy study of hectorite. Geochimica et Cosmochimica Acta 54, 1655 – 1669. Weiss Jr., C.A., Kirkpatrick, R.J., Altaner, S.P., 1990b. Variation in interlayer cation sites of clay minerals as studied by 133Cs MAS NMR nuclear magnetic resonance spectroscopy. American Mineralogist 75, 970 – 982.
Cesium adsorption on soil clay: macroscopic and spectroscopic measurements L. Bergaouia,*, J.F. Lambertb, R. Prostc b
a Laboratoire de Chimie des Mate´riaux et Catalyse, Faculte´ des Sciences de Tunis, 1060 le Belve´de`re, Tunis, Tunisia Laboratoire de Re´activite´ de Surface, URA 1106 CNRS, Universite´ Pierre et Marie Curie, 4 place Jussieu, 75252 Paris cedex 05, France c Station de Science du Sol, INRA, route de Saint-Cyr, 78000 Versailles, France
Received 22 October 2003; received in revised form 11 August 2004; accepted 21 September 2004 Available online 11 November 2004
Abstract We studied the interaction of cesium cation in the aqueous phase with soil clay by combining microscopic and macroscopic data. Investigations concerning selective sorption of cesium by this soil clay are presented. The sample studied is selective for concentrations b210 3 mol/l in solution. Far-infrared (FIR) shows the presence of two selective adsorption sites. At higher loadings, 133Cs MAS-NMR shows that most cesium is essentially adsorbed on external sites which are not very selective. FIR and adsorption data lead to the conclusion that collapsed illite clay is responsible for the selectivity observed at low concentration. However, cesium cations can diffuse between illite layers only if these layers are involved in a mixed smectite– illite mineral. D 2004 Elsevier B.V. All rights reserved. Keywords: Cesium adsorption; Selectivity; Illite–smectite; Far-infrared;
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1. Introduction The 137Cs nuclide, which is very dangerous, has been introduced in the environment after nuclear testing and accidents. The adsorption behaviour of cesium on soil constituents and minerals has been studied in the past few decades. A large part of these
* Corresponding author. Tel.: +216 71 872 600; fax: +216 71 885 008. E-mail address: [email protected] (L. Bergaoui). 0169-1317/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2004.09.002
Cs MAS-NMR
studies treat the interaction between cesium and clay minerals of soils: they deal only with macroscopic information and are not concerned with molecular level characterisation (Sawhney, 1969, 1970, 1972; Maes et al., 1985; Cornell, 1993; Poinssot et al., 1999). General mechanism of cesium adsorption was discussed by consideration of the effect of cesium concentration, the properties of the mineral and the characteristic of the solution phase. A few studies used spectroscopies to investigate the microscopic environment of the cesium cation adsorbed on pure clay minerals by 133Cs NMR
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(Weiss et al., 1990a,b; Laperche et al., 1990) and by far-infrared (FIR) (Laperche, 1991; Diaz, 1999; Diaz et al., 2002a; Badreddine et al., 2002). 133Cs NMR gives different signals according to the degree of hydration and can inform about the mobility of the cation. Studies of the interactions between compensating cations and the clay lattice by FIR show that the compensating cations are located in bcagesQ whose geometry is related to the structural and chemical properties of the lattice. In this study, we attempt to determine if the mobility of cesium is reduced by adsorption on soil clay. We aim to combine macroscopic (adsorption isotherm) and microscopic (NMR and FIR) studies to determine the structure of the selective sites occupied by cesium on this soil clay.
2. Experimental 2.1. Materials The studied soil comes from the region of Mainfranken in Germany which contains at the surface a loessic limon. It is a sedimentary material, not stratified and formed by clay and limestone (b62.5 Am). This depot can reach 10 m of thickness and deal of good arable earths. The sample comes from 6 m of depth. The b2 Am fraction of the raw soil was separated by sedimentation, exchanged three times with a 1 mol/l NaCl solution and washed thoroughly. Chemical analysis gave an exchange capacity of 56 meq/100 g of calcined clay. Well-known clays were used as references. The used kaolinite is from Saint Austel, the illite clay is from Le Puy en Velais, the vermiculite from Llano and the montmorillonite from Camp Bertaux. Each clay was first saturated with sodium. The Na-clays were exchanged at room temperature with a 210 3 mol/l CsCl solution and washed to remove excess electrolyte. 2.2. Characterisation Adsorption experiments were performed in 20 ml screw cap centrifuges tubes containing 0.4 g of the Naloess clay and 20 ml of bi-ionic NaCl/CsCl solutions with constant ion strength but different Na/Cs proportions. After shaking for 7 days at constant temperature (293 K), suspensions were centrifuged (10,000 rpm for 10 min). Cs+ concentration in the equilibrium solution was determined by atomic absorption using an SSA
Spectra 351. Adsorbed cesium concentrations in the solid phase were determined from the difference between the initial Cs concentration and the equilibrium concentration. The solid phase was washed rapidly with 20 ml of distilled water and dried at room temperature. X-ray diffraction was performed on oriented clay deposits and patterns were recorded on a Siemens D5000 diffractometer using the Cu Ka radiation. Due to the intrinsically lower sensitivity of this technique, 133Cs NMR was attempted on a high Cs loading sample. To prepare this sample, Na-clay fraction was exchanged at room temperature with a 210 3 mol/l CsCl solution and washed to remove excess electrolyte. Spectra were recorded on a Bruker AMX500 spectrometer operating at a Larmor frequency of 65.6 MHz. A short pulse length of 2 As was chosen. The recycle time was 1 s and the spinning frequency was 5 kHz; 2000 scans were collected. Infrared and FIR spectra were recorded on a Bruker 113V controlled by an OPUS data station. All the samples are oriented films, except for vermiculite. Oriented samples are prepared by deposing a suspension on a thin plate glass. After drying at room temperature, the clay film was separated from the plate. For vermiculite, sample was deposited on Millipore filter of 0.45 Am pore size.
3. Results and discussion 3.1. Structural characterisation The raw material contains a large amount of quartz. The diffractogram of the b2 Am fraction shows two peaks at 7.2 and 10.1 2 and a third one at 14 2 which shifts to 12 2 when the clay is saturated with Na+ (Fig. 1). The interlayer spacings of 7.2 and 10 2 correspond, respectively, to kaolinite and T-O-T nonswelling clay. The higher spacing at 14 2 is assigned to an interstratified illite–smectite mineral because this peak broadens with ethylene glycol and its maximum shifts to between 16 and 17 2 (Robert, 1975). Fig. 2 shows the infrared spectra of the b2 Am fraction in the region of the OH stretching vibration. We can distinguish two bands at 3692 and 3621 cm 1 and a shoulder at near 3650 cm 1. These bands may be assigned to kaolinite and dioctahedral clay (Farmer, 1974). The FIR spectrum of the loess clay shows two bands at 196 and 110 cm 1, a shoulder at 90 cm 1 and a weak band at 132 cm 1 (Fig. 3). The FIR spectra of the reference Na-clays show that montmor-
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Fig. 1. Diffractograms of the natural loess clay before (a) and after (b) ethylene glycol adsorption. Fig. 3. Far-infrared spectrum of the natural loess clay.
illonite and vermiculite do not exhibit any intense bands in this region (Fig. 4c and d). On the other hand, reference illite shows the same bands as the soil clay (Fig. 4b), except for the lower intensity of the 196 cm 1 band. So we can propose that the T-O-T non-swelling clay fraction consists of illite. The pure kaolinite clay shows an intense band at 196 cm 1 (Fig. 4a) and therefore, the band at 196 cm 1 in the loess clay spectrum must be a superposition of kaolinite and illite bands. A band in this region is probably due to a lattice vibration (Velde and Couty, 1985; Diaz et al., 2002b).
Fig. 2. Infrared spectrum of the loess clay in the OH vibration region.
Fig. 4. Far-infrared spectrum at room humidity of the kaolinite from Saint Austel (a), the illite from Le Puy (b), the vermiculite from Llano (c) and the montmorillonite from Camp Bertaux (d).
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As regards the other bands at 90, 110 and 132 cm 1 observed on loess clay, they have been reported to be characteristic of the FIR spectra of K+ in dioctahedral mixed layer minerals (Diaz et al., 2002b). Diaz et al. (2002b) have shown that the 110 cm 1 band is the superposition of two vibration modes of K+ parallel to the a and b axes, while the 132 cm 1 is due to the vibration mode parallel to the c axis. They have noticed that the intensity and the position of the lower wavenumber band appear to depend on the number of the illite layers in the sample. Again, according to Diaz et al. (2002b) this band shifts toward lower wave number as the percentage of smectite fraction increases. Laperche (1991) showed that the position of this band remains the same from 90% to 30% of smectite layers and gradually shifts from 85 to 97 cm 1 as the percentage of smectite layers decreases from 30% to 0%. The value observed in our loess mixed layer minerals (90 cm 1) corresponds to less than 20% of smectite layers (Diaz et al., 2002b). This percentage corresponds to two and three layer illites altering with smectite layers. In conclusion, our sample is a mixture of kaolinite, illite and an interstratified illite–smectite with a low percentage of smectite layers.
Fig. 6. Cs adsorption isotherm at 293 K on the Na-loess clay.
selective for cesium over sodium ions at least for an initial concentration b210 3 mol/l. Equilibrium Cs+ adsorption data are presented in Fig. 6. It was first checked that the data are not consistent with a single-site Langmuir model. This is not surprising since our soil sample contains several kinds of clays. A log–log scale is used in Fig. 6; in this type of presentation, a Freundlich-type isotherm would appear as linear. The isotherm actually is only linear for adsorbed cesium less than 3 mmol/100 g. This tendency strongly suggests the existence of more than one adsorption mechanism.
3.2. Adsorption isotherm The square diagram presentation of the ion exchange data (Fig. 5) shows that Na-loess clay is
Fig. 5. Square diagram presentation of the Cs+-adsorption on the Na+-loess clay.
3.3. Spectroscopic characterisation of the cesium adsorption sites Potential adsorption sites on clay minerals are of three kinds: the basal surface, the frayed edges of interlayer and the internal interlayer (Bolt et al., 1986). Cesium adsorbed on basal surfaces is hydrated and easily detected by NMR. The 133Cs MAS-NMR spectrum (Fig. 7) of a sample with high cesium loading (27.9 mmol/100 g) shows an intense peak at 2.58 ppm and a small signal at 50 ppm range. The signal at 2.58 ppm can be attributed to a fully hydrated cesium cation, and the small signal at about 50 ppm has been assigned to Cs+ cations in the collapsed interlayer (Laperche et al., 1990; Weiss et al., 1990a,b). The quantification of these peaks (using CsCl as a solid reference) showed that almost all of the cesium present in the sample is detected by NMR. Therefore, in loess clay, fully hydrated cesium (peak at 2.58 ppm) is predominant with respect to the cesium adsorbed in the collapsed interlayers (peak at
L. Bergaoui et al. / Applied Clay Science 29 (2005) 23–29
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only collapse to 12.5 2 (Calvet, 1972). We think that in vermiculite the cesium ions are 12-coordinated, bonding to six oxygen atoms from a six-ring in one layer and six oxygen atoms from another ring in the adjacent layer. On the other hand, cesium in montmorillonite is nine-coordinated, bonding six oxygen atoms from one layer and three oxygen atoms from a triad of the adjacent layer. Because of these different molecular environments, the vibration of cesium is only seen in vermiculite. On the other hand, illite shows a modification in the intensity of the bands at 110 and 90 cm 1 after cesium exchange (Figs. 4b and 9b). According to Diaz
Fig. 7.
133
Cs MAS-NMR spectrum of the Cs-loess clay.
50 ppm) when the sample is saturated. The fully hydrated cesium could be adsorbed either on basal surfaces or between expanded layers. Since the fraction of smectite layers in loess clay is rather low, the majority of the fully hydrated cesium must be adsorbed on basal surfaces. FIR only allows detecting dehydrated interlayer cations (Laperche, 1991; Diaz, 1999; Diaz et al., 2002a,b; Badreddine et al., 2002). Fig. 8 shows spectra obtained for increasing amounts of adsorbed cesium. When the amount of adsorbed cesium increases, the intensity of the band at 110 cm 1 decreases in comparison with that of the band at 90 cm 1. When the adsorbed cesium amount exceeds 5.48 mmol/100 g (Fig. 8g), a band appears at 60 cm 1. The spectra reference Na-clays and Cs-clays show that neither kaolinite (Figs. 4a and 9a) nor montmorillonite (Figs. 4d and 9d) present any significant modification of their spectrum after cesium adsorption. In contrast, after cesium exchange, vermiculite (Figs. 4c and 9c and Diaz et al., 2002a) exhibits a new band at 58 cm 1. Because it has a low hydration energy, Cs+ may collapse the interlayers of expanding layer silicates. The vermiculite layers, with their greater layer charge, collapse more easily than montmorillonite layers which have a smaller layer charge. Vermiculite layers collapse to 11.5 2 (Laperche et al., 1990) and montmorillonite layers
Fig. 8. Far-infrared spectra of the loess clay with an increasing quantity of adsorbed cesium. The amounts of retained cesium are, respectively, (a) 0.22, (b) 0.49, (c) 1.13, (d) 1.56, (e) 2.22, (f) 5.46, (g) 10.16, (h) 15.33, (i) 16.62 and (j) 27.90 mmol/100 g.
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bulkier than K+. The band shift observed for K+ when the amount of Cs+ in the exchange sites increases is probably due to the fact that K+ then becomes more loosely coordinated as a result of interlayer opening. When the amount of cesium between the layers becomes large enough (5.46 mmol/100 g), Cs+ gives rise to a band at 60 cm 1 like in vermiculite. This band at 60 cm 1 is not observed when a pure illite fraction is exchanged with cesium but the shift of the band at 90 cm 1 is noticed. It seems that the exchange of cesium takes place at the edges of interlayer and the diffusion of Cs+ between illite is difficult when the clay is a pure illite; but if the illite layers are a fraction of interstratified clay, the exchange process can extend from the edges to the centre of the particles. It has already been mentioned that the slope of the isotherm in log–log representation changes when the adsorption amount reaches 3 mmol/100 g. It seems that the Cs+/K+ exchange occurs first in the interlamellar spaces. When the adsorption amount exceeds 3 mmol/100 g, the adsorption takes place also on the external surfaces. Taken together, the spectroscopic data lead us to propose a model of cesium distribution into two main compartments: –
– Fig. 9. Far-infrared spectra at room humidity of Cs-kaolinite from Saint Austel (a), Cs-illite from Le Puy (b), Cs-vermiculite from Llano (c) and Cs-montmorillonite from Camp Bertaux (d).
et al. (2002a), when a large cation (Cs+ or Rb+) replaces K+ in vermiculite, a shift to lower frequency of the adsorption band of K+ is observed and a band characteristic of Rb+ or Cs+ appears. The same seems to occur here with the interstratified illite–smectite minerals. The observed decrease in the intensity of the band at 110 cm 1 in comparison with that of the shoulder at 90 cm 1 can be seen as a consequence of the shift of the band at 90 cm 1 to lower frequency. Thus, our observations probably mean that K+ belonging to the two or three-stacking sequence is progressively exchanged by Cs+. The cesium cations act as wedges to expand the layers because they are
Very selective sites corresponding to adsorption of Cs+ in the internal surface of the illite–smectite fraction. Cesium cations are not hydrated in these sites. Low selectivity sites able to adsorb fully hydrated cesium. The majority of these sites are located on external surfaces.
This model is compatible with what is known on the Cs+ affinity of reference minerals. Kaolinite has a small exchange capacity apparently caused by sorption on basal surfaces of crystallites (Lim et al., 1980). These sites are not particularly selective. Illite is formed by collapsed layers with a smaller cationic exchange capacity than vermiculite. In spite of this low CEC, at low concentrations, illite is more selective for cesium than vermiculite. This selectivity has been attributed to the existence of frayed edges (Comans et al., 1991; Maes et al., 1985) which still have a higher affinity for cesium than the interlayer sites in vermiculite. The concentration of these very selective sites in illite is rather low (Brouwer et al., 1983).
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4. Conclusion The adsorption isotherm shows that loess clay is selective for cesium cations. Cesium adsorption on this soil clay occurs by cation exchange at different sites which possess very different cesium affinities. At low concentrations, far-infrared spectroscopy shows the presence of a very selective adsorption sites which correspond to internal collapsed layer. At high concentrations, 133Cs MAS-NMR shows that cesium is essentially adsorbed on external sites which are not very selective. Thus, we can confirm by using spectroscopies that at low concentration, collapsed clay (illite) is very selective. The interstratification of the illite layers in mixed illite–smectite fractions allows the diffusion of cesium cations between the layers. To conclude, loess clay is a good trap for cesium in dilute solutions but it is not efficient at high concentration. Acknowledgments We would like to take this opportunity to thank Professors K. Czurda and J.-F. Wagner for the very good exchange we had during the French-German cooperation programme on the fate of heavy metals in soils. We thank the French Ministry of Environment for its financial support. References Badreddine, R., Le Dred, R., Prost, R., 2002. Far infrared study of K+, Rb+ and Cs+ during their exchange with Na+ and Ca2+ in vermiculite. Clay Minerals 37, 71 – 81. Bolt, G.H., De Boodt, M.F., Hayes, M.H.B., McBride, M.B., 1986. Interaction at the soil colloid soil solution interface. Serie E: Applied Sciences Interaction at the Soil: Colloid 190, 126 – 127. Brouwer, E., Baeyens, B., Maes, A., Cremers, A., 1983. Cesium and rubidium ion exchange equilibria in illite. Journal of Physical Chemistry 87, 1213 – 1219. Calvet, R., 1972. Hydratation de la montmorillonite et diffusion des cations compensateurs. PhD thesis, Univ. Paris VI. Comans, R.N.J., Haller, M., De Preter, P., 1991. Sorption of cesium on illite: non-equilibrium behaviour and reversibility. Geochimica et Cosmochimica Acta 55, 433 – 440.
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Cornell, R.M., 1993. Adsorption of cesium on minerals: a review. Journal of Radioanalytical and Nuclear Chemistry 171, 483 – 500. Diaz, M., 1999. Etude des interactions cations compensateurs/ feuillets dans les argiles: contribution a` la connaissance des me´canismes de re´tention se´lective. PhD thesis, Univ. Orle´ans, Orle´ans, France. Diaz, M., Huard, E., Prost, R., 2002a. Far infrared analysis of the structural environment of interlayer K+, NH+4 , Rb+ and Cs+ selectivity retained by vermiculite. Clays and Clay Minerals 50, 284 – 293. Diaz, M., Laperche, V., Harsh, J., Prost, R., 2002b. Far infrared analysis of the structural environment of interlayer spectra of K+ in dioctahedral and trioctahedral mixed-layer minerals. American Mineralogist 87, 1207 – 1214. Farmer, V.C., 1974. The Infrared Spectra of Minerals. The Mineralogical Society, pp. 340 – 341. Laperche, V., 1991. Etude de l’e´tat et de la localisation des cations compensateurs dans les phyllosilicates par des me´ thodes spectroscopiques. PhD thesis, Univ. Paris VII, Paris, France. Laperche, V., Lambert, J.F., Prost, R., Fripiat, J.J., 1990. Highresolution solid-state NMR of exchangeable cations in the interlayer surface of a swelling mica: 23Na, 111Cd and 133Cs vermiculites. Journal of Physical Chemistry 94, 8821 – 8831. Lim, C.H., Jackson, M.L., Koon, R.D., Helmke, P.A., 1980. Kaolins: sources of differences in cation-exchange capacity and cesium retention. Clays and Clay Minerals 28, 223 – 229. Maes, A., Verheyden, D., Cremers, A., 1985. Formation of highly selective cesium exchange sites in montmorillonites. Clays and Clay Minerals 33, 251 – 257. Poinssot, C., Baeyens, B., Bradbury, M.H., 1999. Experimental and modelling studies of caesium sorption on illite. Geochimica et Cosmochimica Acta 63, 3217 – 3227. Robert, M., 1975. Principe de de´ termination qualitative des mine´raux argileux a` l’aide des Rayons X. Annales Agronomiques 26, 363 – 399. Sawhney, B.L., 1969. Cesium uptake by layer silicates: effect on interlayer collapse and cation exchange capacity. Proceeding of the International Clay Conference, Tokyo vol. 1. pp. 605 – 611. Sawhney, B.L., 1970. Potassium and cesium ion selectivity in relation to clay mineral structure. Clays and Clay Minerals 18, 47 – 52. Sawhney, B.L., 1972. Selective sorption and fixation of cations by clay minerals: a review. Clays and Clay Minerals 20, 93 – 100. Velde, B., Couty, R., 1985. Far-infrared spectra of hydrous layer silicates. Physics and Chemistry of Minerals 12, 347 – 352. Weiss Jr., C.A., Kirkpatrick, R.J., Altaner, S.P., 1990a. The structural environments of cations adsorbed onto clay: 133Cs variable temperature MAS NMR spectroscopy study of hectorite. Geochimica et Cosmochimica Acta 54, 1655 – 1669. Weiss Jr., C.A., Kirkpatrick, R.J., Altaner, S.P., 1990b. Variation in interlayer cation sites of clay minerals as studied by 133Cs MAS NMR nuclear magnetic resonance spectroscopy. American Mineralogist 75, 970 – 982.