Inorganic cellular monobloc cation-exchange materials, the preparation method thereof, and separation method using same

Abstract

A material in the form of an alveolar monolith consisting of a matrix of an inorganic oxide with a hierarchical and opened porosity comprising macropores, mesopores and micropores, said macropores, mesopores and micropores being interconnected, and nanoparticles of at least one metal cation exchange inorganic solid material being distributed in said porosity. A method for preparing this material and a method for separating a metal cation notably a cation of a radioactive isotope of a metal such as cesium using this material.

Claims

1. A solid material in the form of an alveolar monolith comprising a matrix of an inorganic oxide with a hierarchical and opened porosity comprising macropores, mesopores, and micropores, wherein the macropores, mesopores, and micropores are interconnected, and wherein nanoparticles of at least one metal cation exchange inorganic solid material are distributed in the porosity.

2. The material according to claim 1, wherein the inorganic oxide is at least one oxide of at least one metal or metalloid selected from the group consisting of Si, Ti, Zr, Th, Nb, Ta, V, W, Y, Ca, Mg and Al.

3. The material according to claim 2, wherein the inorganic oxide is silica.

4. The material according to claim 1, wherein the metal cation exchange inorganic solid material is a metal hexa-cyanometallate or a metal octa-cyanometallate.

5. The material according to claim 1, wherein the nanoparticles have the shape of a sphere or of a spheroid.

6. The material according to claim 1, wherein the nanoparticles have an average size from 2 to 300 nm.

7. The material according to claim 1, wherein a content of the nanoparticles of the at least one metal cation exchange inorganic solid material is from 0.5 to 15% by weight.

8. The material according to claim 1, wherein the at least one metal cation exchange inorganic solid material is a metal hexa- or octa-cyanometallate of the following formula:
[Alk.sup.+.sub.x]M.sup.n+.sub.y[M(CN).sub.m].sub.t.sup.z, wherein: Alk is a monovalent cation selected from the group consisting of a cation of at least one alkali metal and the ammonium cation NH.sub.4.sup.+, x is 0, 1, or 2, M is a transition metal, n is 2 or 3, y is 1, 2, or 3, M is a transition metal, m is 6 or 8, z is 3 or 4, and t is 1 or 2.

9. The material according to claim 8, wherein M.sup.n+ is Fe.sup.2+, Ni.sup.2+, Fe.sup.3+, Co.sup.2+, Cu.sup.2+, or Zn.sup.2+.

10. The material according to claim 8, wherein M is Fe.sup.2+, Fe.sup.3+, or Co.sup.3+ and m is 6; or M is Mo.sup.5+ and m is 8.

11. The material according to claim 8, wherein [M(CN).sub.m].sup.z is [Fe(CN).sub.6].sup.3, [Fe(CN).sub.6].sup.4, [Co(CN).sub.6].sup.3 or [Mo(CN).sub.8].sup.3.

12. The material according to claim 8, wherein the at least one metal cation exchange inorganic solid material is represented by the formula [K+.sub.x]Cu.sup.2+.sub.y[Fe(CN).sub.6].sup.z.

13. The material according to claim 12, wherein the at least one metal cation exchange inorganic solid material is represented by the formula K.sub.2CuFe(CN).sub.6.

14. A method for preparing the solid material according to claim 1, the method comprising: (1) mixing a colloidal aqueous suspension of nanoparticles of at least one metal cation exchange inorganic solid material with an aqueous solution comprising an organic surfactant and a precursor of an inorganic oxide, to obtain an aqueous solution; (2) adding an oily phase with mechanical stirring under shearing to the aqueous solution, thereby obtaining an oil-in-water emulsion formed with droplets of the oily phase dispersed in a continuous aqueous phase, wherein the nanoparticles of the at least one metal cation exchange inorganic solid material are present at the interface formed between the continuous aqueous phase and the droplets of the dispersed oily phase; and (3) ripening and mineralizing the oil-in-water emulsion, thereby forming the alveolar monolith and obtaining the solid material.

15. The method according to claim 14, wherein, at the end of the mixing (1) and before the adding (2), when the at least one metal cation exchange inorganic solid material is a metal hexa- or octa-cyanometallate, a pH of the aqueous solution obtained in (1) is adjusted to the vicinity of 2, and (1) is additionally carried out during which an aqueous solution comprising at least a metal fluoride is added to the aqueous solution obtained in (1).

16. A method for separating at least one metal cation from a liquid medium comprising the at least one metal cation, the method comprising contacting the liquid medium with the solid material according to claim 1.

17. The method according to claim 16, wherein the liquid medium is an aqueous liquid medium.

18. The method according to claim 16, wherein the liquid medium is a liquid or effluent obtained from the nuclear industry, from nuclear facilities, or from activities using radionuclides.

19. The method according to claim 16, wherein the at least one metal cation is present at a concentration of 0.1 picogram to 100 mg/L.

20. The method according to claim 16, wherein the at least one metal cation is a cation of an element selected from the group consisting of Cs, Co, Ag, Ru, Fe, Tl and isotopes thereof.

21. The method according to claim 20, wherein the at least one metal cation is a .sup.134Cs cation or .sup.137Cs cation.

22. The method according to claim 20, wherein the isotopes are radioactive isotopes.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a photograph taken with a transmission electron microscope (TEM) of copper ferrocyanide nanoparticles which are found in a colloidal solution prepared like in step 1 of the procedure for preparing monoliths containing nanoparticles.

(2) The scale indicated in FIG. 1 represents 20 nm.

(3) FIG. 2 is a photograph taken with a transmission electron microscope (TEM) of copper ferrocyanide nanoparticles which are found in a colloidal solution prepared like in step 1 of the procedure for preparing monoliths containing nanoparticles.

(4) The scale indicated in FIG. 2 represents 50 nm.

(5) FIG. 3 (A, B, C) shows photographs taken with the scanning electron microscope (SEM) of silica monoliths prepared from emulsions prepared by respectively using a shear rate of 13,000 rpm, (A), a shear rate of 6,500 rpm, (B), and a shear rate of 3,200 rpm (C).

(6) The scales indicated in FIG. 3 (A, B, C) represent 10 m.

(7) FIG. 4 is a graph which gives the average diameter D of the macropores of the monoliths (in m), as a function of shear rate (in min.sup.1) used for preparing the emulsion having been used for preparing these monoliths.

(8) FIG. 5 is a photograph of a silica monolith which does not contain any nanoparticles (0% NP).

(9) FIG. 6 is a photograph of a silica monolith containing 2.50% by weight of nanoparticles (called 2.50@3200; 3,200 being the shear rate).

(10) FIG. 7 is a graph which gives the X-ray diffraction spectra (XRD) of a massive (solid, bulky) block of particles of K.sub.2.07Cu.sub.1.08Fe(CN).sub.6(CuHCF)(curve A), and of monoliths containing these nanoparticles at different concentrations, i.e. the monoliths 2.86@3200(curve B); and 5.10@3200(curve C).

(11) In ordinates is plotted I (in arbitrary units) and in abscissas is plotted 2 (in ).

(12) FIG. 8 is a graph which gives the total adsorbed amount of Cs (Q.sub.ADS) (in %) (in ordinates on the left, points and ), and the normalized adsorbed amount of Cs (in ordinates on the right, points .circle-solid. and .square-solid.) which is the total adsorbed amount of Cs divided by the mass of adsorbent (Q.sub.ADS/m.sub.ADS) (in mmol/g), as a function of the mass fraction of nanoparticles in the monoliths.

(13) The points ( and .circle-solid.) relate to the monoliths prepared from a colloidal solution for which the nanoparticles concentration, [NP], in the colloidal suspension is of 9.0 g/L., and the points ( and .square-solid.) relate to the monoliths prepared from a colloidal solution for which the concentration of nanoparticles, [NP], in the colloidal suspension is of 32.2 g/L.

(14) In FIG. 8 the results obtained with massive ferrocyanides ( and ) are also plotted.

DETAILED DISCUSSION OF PARTICULAR EMBODIMENTS

(15) The invention will now be described with reference to the following examples given as an illustration and not as a limitation.

EXAMPLES

(16) In the following examples, silica monoliths are prepared containing ferrocyanides nanoparticles and these silica monoliths containing ferrocyanides are used as sorbents of cesium ion (Cs.sup.+).

Example 1

(17) In this example, the procedure for preparing silica monoliths containing ferrocyanide nanoparticles is disclosed.

(18) The procedure for preparing silica monoliths containing ferrocyanide nanoparticles comprises the following successive steps: 1. Preparation of colloidal aqueous suspensions of nanoparticles (NP) of [K.sub.x]Cu.sub.yFe(CN).sub.6.

(19) During this step, solutions, colloidal suspensions of nanoparticles (NP) of [K.sub.x]Cu.sub.yFe(CN).sub.6 wherein x ranges from 0.5 to 2.5 and y ranges from 0.5 to 2 are prepared.

(20) The concentration of nanoparticles of these solutions, suspensions may range up to 50 g/L preferably up to 32 g/L.

(21) More specifically, two solutions, colloidal suspensions were prepared with a view to their use in the examples which follow, i.e. a suspension with 9 g/L of [K.sub.1.77]Cu.sub.1.16Fe(CN).sub.6 and a suspension of 32 g/L of [K.sub.2.07]Cu.sub.1.08Fe(CN).sub.6.

(22) To this end, a solution of K.sub.4Fe(CN).sub.6 and a solution of Cu(NO.sub.3).sub.2 are rapidly mixed.

(23) The respective concentrations of K.sub.4Fe(CN).sub.6 and Cu(NO.sub.3).sub.2 in each of the solutions are 5.10.sup.3M and 3.9.10.sup.3M in order to obtain a suspension with 9 g/L of [K.sub.1.77]Cu.sub.1.16Fe(CN).sub.6 and of 1.5.10.sup.2M and 1.1.10.sup.2M for obtaining a suspension with 32 g/L of [K.sub.2.07]Cu.sub.1.08Fe(CN).sub.6.

(24) The stoichiometry of the ferrocyanides was determined on the basis of ICP (Inductively Coupled Plasma, inductive coupling plasma spectrometry) results.

(25) The colloidal suspensions of ferrocyanide nanoparticles obtained are colored in red and are stable for months.

(26) These suspensions are then used as a precursor of the final material, i.e. the silica monolith containing ferrocyanide nanoparticles.

(27) The suspension used depends on the final concentration of nanoparticles (NP), desired in the final material.

(28) The size of the NPs was determined by transmission electron microscopy (TEM).

(29) FIGS. 1 and 2 show photographs taken with a transmission electron microscope (TEM) of copper ferrocyanide nanoparticles prepared during this step: these are nanoparticles of [K.sub.1.77]Cu.sub.1.16Fe(CN).sub.6.

(30) This preparation technique promotes the synthesis of not very monodispersed NPs with average sizes comprised between 10 and 20 nm.

(31) The colloidal solution prepared in this step is called the solution A. 2. Preparation of an aqueous solution of surfactant.

(32) During this step, an aqueous solution with 20% by weight of Pluronic P123 (surfactant marketed by BASF or Sigma-Aldrich) at pH=2, is prepared.

(33) The surfactant solution prepared in this step is called solution B. 3. Preparation of a solution of a silica precursor containing a surfactant.

(34) During this step, a given volume of solution B is sampled and a given volume of tetraethylorthosilicate (TEOS) is added slowly thereto.

(35) One waits for 30 minutes until the solution again becomes clear.

(36) The solution C is thereby obtained. 4. Preparation of an aqueous solution containing ferrocyanide nanoparticles, a silica precursor, and a surfactant.

(37) During this step, a given volume of solution A is mixed with a given volume of solution C.

(38) This mixing is carried out according to a ratio, a volume ratio R defined by the following relation:
R=V colloidal suspension of nanoparticles/V solution of P123 at 20% by weight and at a pH 2(V solution A/V solution C)

(39) This ratio, quotient R may vary between 0 and 2, preferably between 0 and 1. This mixture is called solution D. 5. During this step, a given volume of a solution of sodium fluoride (NaF) at 8 g/L is added into the solution D.

(40) The solution E is thereby obtained. 6. Preparation of an emulsion from solution E.

(41) Rapidly, i.e. within 15 minutes following the addition of the sodium fluoride solution having allowed preparation of the solution E, it is preceded with the emulsification of this solution with a given volume of dodecane (C.sub.12H.sub.26).

(42) For this, a disperser-homogenizer device of the Ultraturrax type is used and dodecane is added slowly into the solution E with shearing.

(43) The shear rate may vary from 0 to 20,000 rpm, and preferably it is 3,200 rpm.

(44) The volume ratios of each constituent used for preparing the emulsion are the following:
V colloidal suspension of nanoparticles(solution/suspension A)+V solution of P123 at 20% by weight and pH 2(solution B)/V TEOS/V Solution of NaF at 8 g/L/V dodecane=1.94/1/9.3.10.sup.3/4.75.

(45) The emulsion F is thereby obtained. 7. Preparation of the monolith.

(46) During this step, a so called ripening step, the monolith containing ferrocyanide nanoparticles is prepared.

(47) For this, the solution F is placed in the oven at 40 C. for 7 days.

(48) At the end of this ripening step, the monolith is formed.

(49) There only remains washing, rinsing/drying steps to be carried out. 8. Washing the monolith.

(50) During this step, the monolith is placed in a cartridge of a Soxhlet extractor and is rinsed for 24 h with refluxed tetrahydrofurane (THF). 9. Drying the monolith.

(51) During this step, the THF is slowly evaporated at room temperature for 7 days. This drying step may also be carried out with the use of supercritical CO.sub.2.

(52) At the end of the drying step, a monolith is obtained, loaded with nanoparticles, ready-to-use.

Example 2

(53) In this example, it is shown that it is possible to control the size of the macroporosity of silica monoliths by acting on the shear rate of the emulsion during step 6 for preparing the emulsion which precedes the step 7 for preparing the monoliths.

(54) This preliminary study dealt with emulsions and materials not containing any nanoparticles.

(55) The goal was to show that it was possible to synthesize silica monoliths for which the size of the macroporosity is controlled, by acting on the shear rate of the emulsion.

(56) For this, several emulsions are prepared from a same solution E, in accordance with step 6, by achieving emulsification of this solution with a given volume of dodecane (C.sub.12H.sub.26).

(57) The volume ratios of each constituent used for preparing the emulsion are the following: V solution P123 at 20% by weight and pH 2/V TEOS/V.sub.NaF.sub._.sub.8g/L/V dodecane=1.94/1/9.3.10.sup.3/4.75.

(58) In order to prepare the emulsions, a disperser-homogenizer device of the Ultraturrax type is used and dodecane is slowly added into the solution E with shearing.

(59) A first emulsion is prepared by using a shear rate of 13,000 rpm, a second emulsion is prepared by using a shear rate of 6,500 rpm, and a third emulsion is prepared by using a shear rate of 3.200 rpm.

(60) A monolith is then prepared from each of the emulsions and then it is washed and dried in accordance with steps 7, 8, and 9.

(61) Each of the thereby prepared monoliths are observed using a Scanning Electron Microscope (SEM).

(62) Scanning electron microscopy allows determination of the statistical size of the macropores of the monolith, the final material.

(63) FIG. 3 (A, B, C) clearly shows the effect of the shear rate of the emulsion on the size of the macroporosity of the prepared monoliths. The size of the macroporosity decreases when the shear rate increases.

(64) It is therefore possible to plot a sort of abacus giving the possibility of predicting the size of the macropores of the monolith according to the shear rate used during the preparation of the emulsion.

(65) FIG. 4 thus represents the average diameter of the macropores of the monolith depending on the shear rate.

Example 3

(66) In this example, silica monoliths are prepared containing nanoparticles (NP) of ferrocyanides [K.sub.x]Cu.sub.yFe(CN).sub.6 wherein x ranges from 0.5 to 2.5 and y ranges from 0.5 to 2.

(67) In order to prepare these monoliths, the procedure described above is used, with concentrations of nanoparticles of the colloidal solution A (step 1 of the procedure) of 9 g/L or of 32 g/L, various volume ratios R (step 4 of the procedure) and a shear rate of 3,200 rpm during the preparation of the emulsion (step 6 of the procedure).

(68) Table I hereafter, gives the NP concentrations of the colloidal solutions, the values of the volume ratios R, the theoretical and measured NP concentrations (by weight) of the monoliths, for the six prepared monoliths containing nanoparticles.

(69) TABLE-US-00001 TABLE I Name 0.21@3200 0.43@3200 0.77@3200 1.83@3200 2.86@3200 5.1@3200 Coll. Sol. 9 g/l 9 g/l 9 g/l 9 g/l 32 g/l 32 g/l R 0.07 0.11 0.24 0.64 0.24 0.64 Theo. wt. 0.3 0.6 1.2 2.5 4.4 8.4 % of NPs. ICP wt. % 0.21 0.43 0.77 1.83 2.86 5.1 of NPs

(70) The weight concentrations of NPs were determined by ICP/AES measurements after having dissolved the monoliths.

(71) It is seen that these measured NP concentration values differ from the theoretical NP concentration values. This is due to the loss of a portion of the NPs during the steps for rinsing the monoliths.

(72) The prepared monoliths all have a hierarchy of porosities with at the same time microporosity, mesoporosity and macroporosity.

(73) Accordingly, the monoliths were also characterized by nitrogen adsorption-desorption measurements for determining their specific BET surface area (S.sub.BET) as well as the size of the mesopores and of the macropores.

(74) The results of these measurements are shown in Table II below.

(75) TABLE-US-00002 TABLE II Name 0.21@3200 0.43@3200 0.77@3200 1.83@3200 2.86@3200 5.1@3200 S.sub.BET 290 278 383 479 547 643 (m.sup.2/g) D.sub.meso 2.6 2.7 2.7 2.7 3.1 6.2 (nm) D.sub.macro 4.2 4.1 4.3 8.6 5.7 5.1 (m)

(76) A silica monolith was also prepared without any nanoparticles (0% NP).

(77) Photographs of this monolith without any nanoparticles and of the monolith designated as 2.50@3200 are respectively shown in FIGS. 5 and 6.

(78) X-ray diffraction analysis (XRD) are then conducted on a massive (bulky, solid) block of particles of K.sub.2.07Cu.sub.1.08Fe(CN).sub.6(CuHCF), and of monoliths containing these nanoparticles at different concentrations, i.e. the monoliths 2.86@3200 and 5.10@3200, the XRD analysis results are shown in FIG. 7.

(79) The XRD analysis gives the possibility of confirming that the structure of the NPs is actually that of a tetragonal structure of K.sub.2CuFe(CN).sub.6.

(80) The diffractograms relative to the monoliths show that the nanoparticles are inserted into the silica network (lattice) and that the structure is not modified very much.

Example 4

(81) In this example, sorption tests of Cs.sup.+ on silica monoliths containing ferrocyanide nanoparticles prepared in Example 3 or else on massive (bulky, solid) copper ferrocyanide are carried out.

(82) The goal of these tests is therefore to compare the exchange capacity of the monoliths loaded with NPs with that of the corresponding massive copper ferrocyanide.

(83) In these tests, the ferrocyanides are used as specific sorbents of cesium ion (Cs.sup.+). Indeed, they contain a potassium ion in the crystalline unit cell which is specifically exchanged with a cesium ion.

(84) The sorption tests summarized hereafter are conducted according to a standardized procedure.

(85) This procedure comprises the following successive steps:

(86) 1. A solution containing 1.10.sup.3M of CH.sub.3COONa and 1.10.sup.4M of CsNO.sub.3 is prepared. This solution is called solution G.

(87) 2. 10 mg of monolith are sampled, or else 10 mg of massive copper ferrocyanide, and they are immersed in 20 mL of the previous solution G.

(88) 3. Stirring is carried out for 24 h.

(89) 4. The supernatant is sampled, filtered and the Cs content is measured by ion chromatography.

(90) This procedure gives the possibility of quantifying the exchange capacity of the monoliths while ensuring the selectivity of the materials for cesium as compared with sodium.

(91) In all the experiments for which the results are summarized in FIG. 8, the sodium concentration was identical before and after immersion of the material in the solution G.

(92) After 24 h of stirring, the adsorption equilibrium is attained, which gives the possibility of comparing the experiments with each other.

(93) FIG. 8 also shows the benefit of using NPs rather than a massive adsorbent. If the extracted amount of Cs is normalized by the mass of adsorbent present in the material, it appears that the materials less concentrated in NPs are the most efficient. This is explained by the fact that when the adsorbent is in a nanometric form, the specific surface area available for the cesium is higher. The increase in the NP mass fraction should promote their aggregation and the adsorption mechanism in the monolith then becomes increasingly similar to what is observed in the case of massive copper ferrocyanides. The observed deviation for both massive ferrocyanides is due to the elementary potassium proportions which are different in both cases.

(94) In the case of the massive ferrocyanide from the colloidal solution at 32.2 g/L, there are on average 2.07 potassium atoms per mole while in the case of the one from the solution at 9.0 g/L, there are only 1.77 moles. The number of exchangeable potassium atoms with cesium is therefore different, which explains this deviation.

Example 5

(95) In this example, sorption tests of .sup.137Cs.sup.+ are conducted on silica monoliths containing ferrocyanide nanoparticles prepared in Example 3, or else on massive (bulky, solid) copper ferrocyanide.

(96) In the case of experiments conducted on radioactive Cs, the concentrations are much lower than in the case of non-radioactive Cs. The parameter allowing an estimation of the decontamination level of a radioactive solution by a sorbent material is then the distribution coefficient K.sub.d. It is expressed in this way:

(97) $K_d=\frac{A_0-A_{\mathrm{eq}}}{A_{\mathrm{eq}}}\frac{V}{m_{\mathrm{mat}}}$
With K.sub.d distribution coefficient (ml/g); A.sub.0, initial activity of the solution to be decontaminated; A.sub.eq activity at equilibrium after 24 h of contact time; V volume of solution used to be decontaminated (ml); and m.sub.mat mass of sorbent material used (in g).

(98) In order to evaluate the selectivity of the monoliths towards other cations competitors of Cs, a complex radioactive solution was prepared with the following composition:
A.sub.0=41.2 kBq/L([Cs]10.sup.10 mol/L)
[Na.sup.+]=0.652 mol/L
[K.sup.+]=0.0015 mol/L
[Mg.sup.2+]=0.002 mol/L
[NO.sub.3.sup.]=0.542 mol/L
[SO.sub.4.sup.2]=0.008 mol/L
[PO.sub.4.sup.3]=0.105 mol/L

(99) The results are noted in the following table III:

(100) TABLE-US-00003 TABLE III Sample 0.21@3200 1.83@3200 5.10@3200 Massive 32 g/L K.sub.d (ml/g) 2.4 10.sup.3 3.0 10.sup.4 1.1 10.sup.5 3.1 10.sup.6

(101) The results show that the monoliths capture .sup.137Cs in a very selective way as compared with the other cations competitors of Cs. The Kds are very high regardless of the NP concentrations of the ferrocyanides immobilized in the monoliths.

(102) The efficiency of the monoliths is therefore validated on radioactive solutions representative of the solutions which have to be processed during a real accidental event, for example the effluents of the Daiichi Fukushima nuclear plant which contains sea water.