Synthesis and characterization of a novel layered tin(IV) phosphate with ion exchange properties

Materials Research Bulletin, Vol. 34, No. 6, pp. 921–932, 1999 Copyright © 1999 Elsevier Science Ltd Printed in the USA. All rights reserved 0025-5408/99/$–see front matter

PII S0025-5408(99)00076-8

SYNTHESIS AND CHARACTERIZATION OF A NOVEL LAYERED TIN(IV) PHOSPHATE WITH ION EXCHANGE PROPERTIES

Anatoly I. Bortun1, Sergei A. Khainakov1,2, Lyudmila N. Bortun1, Enrique Jaimez2, Jose´ R. Garcı´a2*, and Abraham Clearfield1 1 Department of Chemistry, Texas A&M University, College Station, TX 77842-30012, USA 2 Departamento de Quı´mica Orga´nica e Inorga´nica, Universidad de Oviedo, 33071 Oviedo, Spain (Refereed) (Received February 16, 1998; Accepted August 10, 1998)

ABSTRACT A novel layered tin(IV) phosphate, Sn(NH4PO4)(HPO4)䡠2H2O (␦-SnP–NH4), was synthesized under mild hydrothermal conditions (190°C) in the presence of urea. The treatment of this compound with mineral acids gave a new phase of tin(IV) bis(monohydrogenphosphate), Sn(HPO4)2䡠3H2O (␦-SnP–H). The layered nature of the solid was confirmed from amine intercalation, exfoliation in alkaline media, and the ion exchange behavior towards alkali and alkaline earth ions. High affinity (KdCs ⬃ 5 ⫻ 104 to 2 ⫻ 105 mL g⫺1) and capacity (160 –200 mg Cs⫹ per g of exchanger) for Cs⫹ makes these materials promising for selective radioactive cesium removal from contaminated groundwater and nuclear waste. © 1999 Elsevier Science Ltd KEYWORDS: A. inorganic compounds, A. layered compounds, B. chemical synthesis INTRODUCTION Layered group IV phosphates of general formulae M(IV)(HPO4)2䡠H2O (␣-type) and M(IV)(H2PO4)(PO4)䡠2H2O (␥-type) have received considerable attention in the last several decades [1– 6]. This interest is related to their behavior as solid acids and ion exchangers. The

*To whom correspondence should be addressed. 921

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titanium and zirconium phosphates exhibit unique properties: high thermal, radiation and chemical stability, resistance to oxidation, and selectivity to certain ions and molecules. They have been utilized as ion exchangers and adsorbents, catalysts, proton and ion conductors, and convenient matrices for chemical modification. A quite different situation is observed in the case of tin phosphates. Mainly due to relatively low chemical (hydrolytic) stability, tin phosphates have not received as much attention as their titanium- or zirconium-based analogs. As a result, only one-layered tin phosphate (with the ␣-type structure) is known [7]. However, it has been shown recently [8,9] that hydrothermal syntheses often result in the preparation of new types of polyvalent metal phosphates. A “soft chemistry” approach is of special interest because by such means it is possible to synthesize metastable phases of hydrated materials that may exhibit ion exchange and catalytic properties. In this paper, we report our results on the preparation and characterization of a novel metastable phase of a layered tin(IV) phosphate of formula Sn(NH4PO4)(HPO4)䡠2H2O (␦-SnP–NH4) by a hydrothermal technique. EXPERIMENTAL Synthesis. ␦-Tin(IV) phosphate was prepared as follows. To 180 mL of a 1 M SnCl4 solution in a 1 L Teflon-lined stainless-steel autoclave, 100 g of (NH2)2CO and 225 mL of 85% H3PO4 were added and mixed thoroughly. The reaction mixture was treated hydrothermally (180°C) for 10 days. A precipitate was separated by filtration, washed with distilled water and dried in air at 60°C (␦-SnP–NH4). The hydrogen form (␦-SnP–H) was obtained by treatment of the ␦-SnP–NH4 with an excess of a 0.1 M HNO3 solution and then washed with distilled water. The partially substituted Ca2⫹ ion form (␦-SnP-Ca) of the exchanger was prepared by treatment of ␦-SnP–H with 0.5 M CaCl2 solution at a V:m ratio 100:1 (mL g⫺1) for 2 days at room temperature. Analytical Procedures. The phosphorus and tin content in the solids was determined by using a SpectraSpec spectrometer DCP-AEC after dissolving a weighed amount in HF aqueous solution. The diffractometer used was a Seifert-Scintag PAD-V using Cu K␣ radiation. IR spectra were obtained on a Perkin-Elmer 1720-X FT spectrophotometer, using the KBr pellet technique. 31P NMR spectra at magic angle were obtained on a Bruker MSL-300 spectrometer. 31P NMR spectra in solution were recorded using a Bruker AC-300 spectrometer. Micrographs were recorded using a JEOL JSM-6100 electron microscope operating at 20 kV. A Mettler TA 4000 thermogravimetric unit (under nitrogen at a heating rate 10°C min⫺1) was used to perform thermal analysis. Amine Intercalation. n-Alkylamine intercalation compounds were obtained by exposure of ␦-SnP–H in an atmosphere saturated with amine vapor for 10 –30 days at room temperature. Solution procedures were not used because of potential hydrolytic displacement of phosphate groups. Ion Exchange. Uptake of alkali and alkaline earth metal cations on the ␦-SnP–H was studied using 0.05 N MCln–M(OH)n (M ⫽ Li, Na, K, Cs, Ca, Sr, Ba; n ⫽ 1,2) solutions (V:m ⫽ 200 mL:1 g). A groundwater simulant used in the cesium and strontium uptake study (V:m ⫽ 1–10 L:1 g) contained Ca (100 ppm), Mg (10 ppm), Na (15 ppm), Sr (4.6 – 4.8 ppm), and Cs (5.95 ppm). In all cases, the contact time was 5 days under continuous shaking. The pH of

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FIG. 1 The XRD powder patterns of ␦-SnP–NH4 (a) and ␦-SnP–H (b).

the solutions after equilibration with the exchanger was measured using a Corning 340 pH meter. Initial and final concentrations of alkali and alkaline earth ions in solution were measured using a Varian SpectrAA-250 atomic absorption spectrometer. The extent of phosphorus hydrolysis was determined spectrophotometrically as phosphorovanadiummolybdate heteropolycomplex at ␥ ⫽ 400 nm [10], with a Perkin-Elmer 200 spectrometer. The affinity of the materials for cesium and strontium was expressed through the distribution coefficient (Kd, mL g⫺1) values, which were found according to the formula Kd ⫽ [(Co ⫺ Ce)/Ce]䡠V/m, where Co and Ce are the ion concentrations in the initial solution and in the solution after equilibration with the exchanger, respectively, and V/m is the volume-to-mass ratio. RESULTS AND DISCUSSION Under the given experimental conditions only ␦-SnP–NH4 samples of low crystallinity were prepared. It was found that neither increase in the time of synthesis from 7 to 21 days nor increase of temperature to 230°C enhanced the crystallinity of the product noticeably. The powder XRD pattern of the novel tin(IV) phosphate (Fig. 1a) exhibited a first strong reflection at 15.0 Å. The ammonium ion phase was converted to the proton phase by exhaustive treatment with a 0.1 M HNO3. The proton phase (␦-SnP–H) had a first strong reflection at 15.2 Å (Fig. 1b). It was found by elemental analysis that ␦-SnP–NH4 contains

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FIG. 2 IR spectra of ␦-SnP–NH4 (a) and ␦-SnP–H (b).

32.4% Sn, 16.8% P, and 3.30% N, whereas ␦-SnP–H contains 32.3% Sn and 16.8% P. The molar ratio P:Sn in the solids is 2:1, which is the same as in the ␣-Sn(HPO4)2䡠H2O [7]. This allows assignment to ␦-SnP–NH4 of two different formulae: Sn(NH4PO4)(HPO4)䡠2H2O or SnO(NH4HPO4)(H2PO4)䡠H2O. In order to decide between these formulae, further characterization of the novel tin phosphate was undertaken by IR and 31P MAS NMR spectroscopy, thermogravimetric (TG) analysis, and examination of its ion exchange behavior. The most intense adsorption bands in the IR spectra of ␦-SnP–NH4 are observed at 1113, 1028 and 981 (poorly resolved), 1401, and 3100 –3600 cm⫺1 (Fig. 2). The lack of vibrational bands in the 900 –700 cm⫺1 region, characteristic of condensed phosphates [11], indicates their absence and suggests that ␦-SnP–NH4 is an orthophosphate. Based on the analysis of literature data, we assigned the bands in the 1000 –1200 cm⫺1 region to symmetric and antisymmetric stretching of P–O bonds in PO4 group, respectively [12]. The presence of NH4⫹ ion manifests itself by two bands at 1401 and 3150 cm⫺1. A broad multiple band in the OH stretching region (3300 –3600 cm⫺1) arises from the interlayer water and P–OH groups. The presence of several low and middle intensity bands in the 700 – 400 cm⫺1 region may be related to Sn–O bond vibrations. The IR spectrum of ␦-SnP–H is similar to that of ␦-SnP–NH4, but it lacks bands characteristic to the ammonium groups. The 31P MAS NMR spectra of ␦-SnP–NH4 and ␦-SnP–H are presented in Figure 3. It is seen that both compounds have two peaks: at ⫺6.0 and ⫺7.8 ppm for ␦-SnP–NH4 and at ⫺5.7 and ⫺8.1 ppm for ␦-SnP–H, with intensity ratio 1:2, respectively. This indicates the presence of one type of phosphorus atom existing in two crystallographically inequivalent positions. Given that the crystal structure of the subject compound is unknown, it is difficult

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FIG. 3 The 31P MAS NMR spectra of ␦-SnP–NH4 (a) and ␦-SnP–H (b) and spectra of ␦-SnP–H in aqueous solutions of NaCl–NaOH at pH 7 (c) and pH 8 (d). to make a correct assignment of NMR peaks to the phosphate groups [13]. Based on the combined data of different physical and chemical methods of analysis, including TGA, n-alkylamine intercalation and ion exchange, we assign the NMR peaks in ␦-SnP samples to phosphorus atoms in HPO4 groups, which gives the chemical formulae Sn(NH4PO4)(HPO4)䡠 2H2O for ␦-SnP–NH4 and Sn(HPO4)2䡠3H2O for ␦-SnP–H. The thermogravimetric analysis gave a total mass loss for the ammonium form of 20.2% (calculated 19.52%) and the final product should have the formula SnP2O7. The mass loss occurs in three partially overlapping steps (Fig. 4). In the first step (50 –200°C) 10.9% or 2.2 moles of water is released. We attribute this mass loss to the removal of physically bound and interlayer crystal water. In the next two steps (200 – 490°C and 490 – 800°C) the mass losses amount to 6.4% and 2.9%, respectively. The mass loss to 490°C is attributed to the ammonia (1.00 mol, 4.67%) and structural water release (0.35 mol or 1.73%) due to the partial condensation of HPO4 groups. This attribution was confirmed by IR spectroscopy, which showed the disappearance of the characteristic band for NH4⫹ at 1400 cm⫺1 in the spectrum of ␦-SnP. In the last step, the condensation of HPO4 groups is completed (0.65 mole of structural water is released in the temperature range 490 – 800°C). An X-ray diffraction pattern of the end product of the TGA treatment revealed that it was SnP2O7. The proton phase in the process of thermal transformations to the SnP2O7 yielded a total mass loss of 20.3% (calculated, 19.74%). The mass loss occurs in three overlapping steps. In the first stage, which takes place in the temperature range 50 –280°C, a mass loss of 15.3% was found.

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FIG. 4 TG (—–) and DTG (- - -) curves for ␦-SnP–NH4 (a) and ␦-SnP–H (b). This mass loss was assigned to three molecules of interlayer crystal water release. The mass loss in the second stage (280 – 620°C, 3.7%) is attributed to the release of 0.8 mole of structurally bound water per formula mass, due to the condensation of part of the hydrogenphosphate groups. In the last stage (620 – 810°C) the remaining 0.25 mole (1.3%) of structural water, resulting from condensation of the remaining monohydrogenphosphate groups, is lost. It was found that the ␦-SnP–H is able to react easily with n-alkylamine vapors giving corresponding amine intercalates with interlayer distances larger than that of the initial material (Table 1). The ability to intercalate amines with expansion of the basal spacings confirms the layered structure of ␦-SnP–H. It is notable that the interlayer distances of ␦-SnP–H amine intercalates are close to those for other known layered group IV phosphates. The SEM photographs of the novel tin phosphate (Fig. 5) shows that its particles have a broad size range and an irregular form. The lack of a clearly defined morphology correlates with the low crystallinity of the compounds. The SEM photographs of the compounds intercalated with n-alkylamines was similar to that of the initial material. According to the TGA data, thermal decomposition of the amine intercalates occurs in five main steps in the TABLE 1 Interlayer Distances (Å) of n-Alkylamine Intercalates in ␦-SnP–H n-Alkylamine Butylamine Pentylamine Hexylamine Octylamine Decylamine

␦-SnP

␣-ZrP4

␣-TiP23

␥-TiP23

20.3 22.0 24.2 28.2 32.6

18.8 21.2 22.8 28.0

18.8 21.1 23.1 27.8

20.5 22.5 24.6

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FIG. 5 SEM images of ␦-SnP–NH4. temperature range 40 – 670°C (Fig. 6). This includes the release of water (up to 120 –130°C), splitting out and destruction of the amine molecules as well as release of the structural water due to condensation of HPO4 groups. Quantitative analysis of these data indicates the presence of 1.7–2.0 amine molecules per formula, with the only exception n-pentylamine (Table 2). These values of amine uptake are close to the theoretically possible ones and indicate the absence of steric hindrances for the interaction of n-alkylamines into ␦-SnP–H. The presence in the structure of Sn(HPO4)2䡠3H2O of a monohydrogenphosphate functional group and its layered nature suggests that this compound should behave as a cation exchanger with a theoretical cation exchange capacity (IEC) of 5.48 meq g⫺1. The results of potentiometric titrations carried out with MCl–MOH solutions (M ⫽ alkali metal) are shown in Figure 7 as well as the corresponding hydrolysis curves (phosphate ion release into solution). It is seen that the shape of the curve for Li⫹ ion is different than those of Na⫹, K⫹, and Cs⫹ ions. Lithium exchange starts at pH 2, and this low IEC value of 0.6 – 0.7 meq g⫺1 remains practically constant until pH 7.7. Above this pH, the uptake increases drastically in one step to reach a value of 4.7 meq g⫺1 at pH 11. Ion exchange of all other alkali ions starts at pH ⬍ 2 (IEC 1–2 meq g⫺1) and the following selectivity sequence is observed in acid media: K⫹ ⬎ Cs⫹ ⬎ Na⫹ ⬎⬎ Li⫹. Their equilibrium uptake increases as a function of pH to reach a maximum at pH 4 – 6. These IEC values are (in meq g⫺1): for K⫹ 3.1, Cs⫹ 2.5, and Na⫹ 2.2, or 56%, 46%, and 40% of the theoretical IEC, respectively. With a further increase of pH potassium and sodium ions uptake stays on the same level until pH 8 whereupon the uptake rapidly decreases reaching zero at pH 9.5–10.5. Cesium ion behaves similarly to Na⫹ and K⫹ ions with the only exception that the decrease of its uptake starts at a lower pH [6]. No cesium adsorption is observed at a pH higher than 8. Normally, a decrease of alkali ions adsorption in alkaline media by phosphorus containing inorganic ion exchangers is connected with the beginning of their hydrolytic decomposition leading to a phosphate ion release in a solution. The extent of hydrolysis increases provided

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FIG. 6 TG (—–) and DTG (- - -) curves for n-butylamie (a), n-pentylamine (b), n-hexylamine (c), and n-octylamine (d) intercalates in ␦-SnP–H.

that the equilibrium pH does and depends both on the alkali and on the ion exchanger used [1,3,4,14]. It is known that tin phosphates are the least stable compounds among group IV phosphates [7]. However, hydrolysis curves presented in Figure 4a show that ␦-SnP–H exhibits a rather high hydrolytic stability in acid media (1–3% of phosphate release). The hydrolyzing ability of cations changes in order: Li⫹ ⬎ Na⫹ ⬎ K⫹ ⬎ Cs⫹, corresponding to that found previously for amorphous titanium phosphate [14]. The increase of phosphate release starts at pH 6 – 8, reaching maximum values of only 10 –15% (instead of 60 –100% for ␦-TiP or ␣-SnP) at pH 11. These data suggest that hydrolytic decomposition is not the only reason for the absence of Na⫹, K⫹ and Cs⫹ uptake by ␦-SnP-H in alkaline media. Close observation of alkali ions adsorption by the novel layered tin phosphate shows that the exchanger in contact with MCl–MOH solutions (M ⫽ Na, K, Cs) in neutral media swells easily and transforms gradually into a blue sol and then, with further increase of pH to 8 –10, it dissolves completely. In order to determine what happens with the exchanger, we recorded

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TABLE 2 Thermal Decomposition of n-Alkylamine Intercalates in the ␦-SnP–H: Weight Losses and Temperature Ranges Amine

⌬M1

⌬M2

⌬M3

⌬M4

⌬M5

Butylamine Sn(HPO4)2䡠1.89BuA䡠1.40H2O Pentylamine Sn(HPO4)2䡠0.91PeA䡠1.58H2O Hexylamine Sn(HPO4)2䡠1.68HeA䡠1.31H2O Octylamine Sn(HPO4)2䡠1.99OcA䡠1.41H2O Decylamine Sn(HPO4)2䡠0.71DcA䡠1.89H2O

5.33% (40–120°C) 6.85% (40–120°C) 4.71% (40–120°C) 4.29% (40–130°C) 3.62% (40–130°C)

16.99% (120–260°C) 8.05% (120–270°C) 15.57% (120–250°C) 13.42% (130–250°C) 8.51% (130–250°C)

4.73% (260–340°C) 10.05% (270–500°C) 4.60% (250–320°C) 6.74% (250–290°C) 5.21% (250–290°C)

8.86% (340–480°C) 5.05% (500–660°C) 8.75% (320–470°C) 16.60% (290–470°C) 5.07% (290–420°C)

2.37% (480–650°C)

7.93% (470–650°C) 9.67% (470–670°C) 10.80% (420–670°C)

P NMR spectra of ␦-SnP–H in NaCl–NaOH solutions at pH 7 (sol) and pH 8 (clear solution). In both cases, two NMR peaks (Fig. 2) with practically equal intensity were observed at pH 7 (⫺5.6 ppm, ⫺7.8 ppm) and pH 8 (⫺4.8 ppm, ⫺7.1 ppm). The absence of a 31P NMR peak at 0 ppm, characteristic for free phosphoric acid, and resonance peaks close to those in solid ␦-SnP–H suggest exfoliation of the layered exchanger rather than its chemical decomposition. According to the NMR data, tin phosphate layers easily fall apart when 50 – 60% of its functional groups are ionized. From this point, the anomalous behavior of the Li⫹ ions can be explained by considering their ability to form sparingly soluble lithium phosphates in alkaline media. The formation of insoluble mixed lithium–tin phosphates could be responsible for the stabilization of ␦-SnP structure, preventing its exfoliation. 31

FIG. 7 Uptake of Li⫹ (F), Na⫹ (E), K⫹ (), and Cs⫹ (ƒ) ions by ␦-SnP–H as a function of pH (a) and hydrolysis curves (b).

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FIG. 8 Uptake of Ca2⫹ (F), Sr2⫹ (E), and Ba2⫹ () ions by ␦-SnP–H as a function of pH (a) and hydrolysis curves (b). Potentiometric titration curves of ␦-SnP–H with alkaline earth metal hydroxides and the corresponding hydrolysis curves are presented in Figure 8. Uptake of alkaline earth cations depends on the cation and differs completely from that found for alkali ions. For barium ion it starts at pH ⬍ 2 (IEC 2.2 meq g⫺1) and then increases in one step (pH 3–5) to 5.4 meq g⫺1 at pH 5. Strontium and calcium potentiometric curves are similar and resemble considerably that found for barium. By contrast, their IEC values in acid media are lower (1.5–1.7 meq g⫺1, pH 2) and increase in their uptake occurs over a wider pH range (pH 5– 8). The uptake of all alkaline earth cations with pH increase to 9 –10 does not decrease, as in the case of alkali ions, but remains unchanged at a level of 5.4 –5.6 meq g⫺1, which corresponds closely to the ideal theoretical ion exchange capacity of ␦-SnP–H. The hydrolysis of ␦-SnP–H in MCl2–M(OH)2 solutions (M ⫽ Ca, Sr, Ba) is lower than that found for alkali metal cations. The maximum phosphate release was found in acid media (1.3–1.7%, pH 2), and it decreases gradually with the pH increase. Practically no phosphate release was found at pH ⬎ 7– 8. It is interesting to note that the hydrolyzing ability of alkaline earth cations changes in the order: Sr2⫹ ⬎ Ca2⫹ ⬎ Ba2⫹. It is seen that the novel layered tin phosphate is a sufficiently hydrolytically stable exchanger that shows a rather high uptake of cesium and strontium in mild acid and neutral solutions. By analogy with other layered inorganic phosphorus containing ion exchangers with a large d-spacing (␥-TiP, ␥-ZrP), exhibiting a high affinity for cesium, it is probable that ␦-SnP–H could also be efficient for cesium (and strontium) recovery from moderately acid and neutral nuclear waste solutions, contaminated groundwater and different types of biological liquors. In order to check this point, cesium and strontium removal by ␦-SnP–H and a partially substituted calcium form (␦-SnP–Ca) from ground water simulant was studied (Table 3). It is seen that ␦-SnP–H exhibits an extremely low affinity for strontium, which may be due to the presence of competitive calcium ions. At the same time, 1 g of ␦-SnP–H removes practically all the cesium from 10 L of ground water simulant without any significant

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TABLE 3 Cs⫹ and Sr2⫹ Ions Uptake by ␦-SnP from Groundwater Simulant Material

Number of treatments

pH

KdCs

KdSr

␦-SnP–H

1 2 3 5 7 10 1 2 3 5 7 10

3.3 3.3 3.5 3.7 3.9 4.2 5.2 7 6.9 7.1 6.8 6.7

⬎2 ⫻ 105 1.9 ⫻ 105 1.9 ⫻ 105 2.0 ⫻ 105 1.5 ⫻ 105 8.5 ⫻ 104 9.5 ⫻ 104 1.1 ⫻ 105 7.8 ⫻ 104 5.5 ⫻ 104 4.7 ⫻ 104 3.3 ⫻ 104

1.3 ⫻ 103 500 300 200 190 160 1.0 ⫻ 103 1.5 ⫻ 103 1.2 ⫻ 103 1.2 ⫻ 103 1.3 ⫻ 103 1.1 ⫻ 103

␦-SnP–Ca

decrease of the Kd values. This indicates its extremely high affinity and capacity for cesium despite the presence of the excess of Ca2⫹ and Mg2⫹ ions. Under the same conditions, ␥-TiP, the best known group IV phosphate exchanger for Cs⫹ [15], is able to purify only 1 L of groundwater, and then the KdCs value drops drastically, possibly due to “poisoning” with calcium ions. A high resistance of ␦-SnP–H for calcium poisoning was confirmed by cesium uptake on a tin phosphate sample partially converted into the Ca2⫹ ion form. It is seen (Table 2) that ␦-SnP–Ca exhibits KdCs values of 50,000 –100,000 mL g⫺1 in the first 5 cycles of exchange and only with the increase of the amount of adsorption cycles do they drop gradually to 33,000 mL g⫺1 (10th cycle). The ion exchange capacity of ␦-SnP–H and ␦-SnP–Ca for cesium was determined by using the same groundwater simulant. The only difference found was that ␦-SnP–H contained a higher initial concentration of cesium (26.6 ppm): the ␦-SnP–H cesium adsorption capacity is about 200 mg g⫺1, whereas that of ␦-SnP–Ca is somewhat less than 160 mg g⫺1. It is worth noting that ␦-SnP–H IECCs values are 3– 4 times higher than the IEC capacity of the crystalline sodium titanium silicate, Na2Ti2O3SiO4䡠H2O, which is regarded as one of the best cesium exchangers [16]. Considering that (1) the radiocesium concentration in contaminated ground water is several orders of magnitude less than in the water simulant used, and (2) that the ion exchange capacity of ␦-SnP–H for cesium is extremely high (⬃200 mg g⫺1), it is reasonable to expect the efficient purification of many thousands of column volumes of contaminated ground water by one volume of the novel layered tin(IV) phosphate. ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support of this study by the CICYT (Spain), research project no. MAT97-1185, and the U.S. Department of Energy, Office of Science and Technology’s Efficient Separations and Processing Crosscutting Program, grant no. 198567A-F1.

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