WO2016202869A1 - Inorganic cellular monobloc cation-exchange materials, preparation method thereof, and separation method using same - Google Patents

Abstract

The invention relates to a material in the form of a cellular monobloc consisting of a calcinated matrix of an inorganic oxide having open, progressive porosity comprising macropores, mesopores, and micropores, defined by walls, comprising atoms of a transition metal M integrated in and connected to said walls, said macropores, mesopores and micropores being interconnected, in which the nanoparticles of a solid inorganic metal-cation-exchange material selected from the metal hexacyanometallates and octacyanometallates are distributed in said porosity, and the nanoparticles are fixed at an internal surface of said walls by means of said atoms of transition metal M. Method for preparing said material. Method for separating at least one metal cation, such as a radioactive caesium cation, from a liquid medium containing same, in which said liquid medium is placed in contact with said material.

Description

 INVERGANIC MONOLITHIC MATERIALS ALVEOLAR CATIONIC EXCHANGERS, PROCESS FOR THEIR PREPARATION, AND SEPARATION METHOD

 IMPLEMENTING. DESCRIPTION TECHNICAL FIELD

 The invention relates to a cationic exchanger alveolar inorganic monolithic material.

 More specifically, the invention relates to a material in the form of an alveolar monolith constituted by a matrix of an inorganic oxide with an open and interconnected hierarchical porosity, and nanoparticles of a metal hexa- or octacyanometallate. , namely an inorganic solid material exchanger of a metal cation, being distributed in said porosity.

 The invention also relates to the process for preparing said material.

 The present invention also relates to a process for separating metal cations, in particular radioactive cations, contained in a liquid using said material.

 The technical field of the invention may, in general, be defined as that of the treatment of liquid effluents and especially as that of the treatment of radioactive liquid effluents, in particular to eliminate metal cations, such as cesium cations.

STATE OF THE PRIOR ART

 Nuclear facilities such as power reactors, spent nuclear fuel reprocessing plants, laboratories, research centers, and liquid effluent treatment plants generate radioactive liquid effluents.

These effluents, whose volumes are considerable, must be treated and decontaminated before being released into the environment. The pollutants contained in these effluents and which must therefore be eliminated are mainly solid particles and radio-elements essentially present in the form of metal cations in solution.

 Industrial processes for the decontamination of liquid effluents, and in particular radioactive liquid effluents, are, however, few in number because of the complex composition of said effluents, their high ionic strength, and also the wide variety of pH values that they can present. .

 The most common treatments currently used for the decontamination of liquid effluents are evaporation and chemical treatment by coprecipitation.

 Thus, the first step of a process for decontaminating liquid solutions, in particular low-concentration radioactive liquid solutions, on an industrial scale, generally consists in carrying out the evaporation of these solutions in order to concentrate all the ions present in those in the form of a solid waste, which thus becomes a residue of the decontamination process.

 However, this evaporation treatment is not possible for saline effluents because it then occurs scaling of the installation.

 In addition, the presence of certain ions in the liquid effluents causes hot corrosion during the evaporation treatment.

 In the case of effluents, especially highly saline radioactive, another possible treatment is the chemical treatment by coprecipitation or entrainment which is a phase change treatment. It involves transferring the radio-elements present from a liquid phase to a solid phase either by coprecipitation or by entrainment from solid particles.

 These solid particles are then rich in radioactive elements and are then recovered by filtration or decantation before being confined in a suitable matrix.

These methods of coprecipitation (for example by barium sulfate for extraction of ⁹⁰ Sr) or entrainment (from particles of nickel-potassium hexacyanoferrate for ¹³⁷ Cs) have a number of disadvantages.

Firstly, the volumes of sludge formed are substantial and may pose problems of compatibility with the materials currently used to confine the waste, such as glasses or cementitious matrices. In addition, the coprecipitation agents are often sensitive to the chemical composition and the ionic strength of the effluent, which causes a significant drop in the selectivity and therefore an increase in the volume of waste.

 In order to overcome the drawbacks enumerated above of treatment processes, decontamination of liquid effluents, many researchers and industrialists, particularly in the nuclear industry, are currently looking for other ways of treating these effluents.

One of the solutions studied is the use of inorganic ion exchange materials, or more precisely inorganic cation exchange materials which have a high selectivity for the ions to be extracted. Although there is a very large literature on the various inorganic ionic exchange materials selective for ⁹⁰ Sr, ¹³⁷ Cs, or ⁶⁰ Co, the majority of the studies concern discontinuous tests using powder-like exchange materials. .

 Indeed, the inorganic ion exchange materials currently used to sorb the elements to be decontaminated are essentially in the form of relatively fine powders having grain sizes of the order of one micrometer, which are not compatible with a process implemented. continuously.

 These powders, when used in a continuous process, in particular in columns, can cause a high pressure drop in these columns, which can go as far as clogging, clogging, and stopping. installation.

 If the inorganic ion exchange materials are no longer used in the form of loose powders but solid, compact powders, clogging problems are certainly avoided, but the micron size implies a low adsorption capacity because the adsorption occurs in area.

There is therefore, in the light of the foregoing, a need for an inorganic solid ion exchange material, more exactly for an inorganic cation exchange material that is compatible with a continuous implementation in a process for separating a metal cation. from a liquid medium, and which has a high adsorption capacity. In particular, this inorganic solid ion exchange material must be chemically and mechanically stable in order to be packaged in a column allowing continuous operation without the occurrence of clogging or clogging phenomena.

 This inorganic solid ion exchanger material must also have excellent fixing properties, in particular decontamination, that is to say similar, or even superior in particular to that of an inorganic ion exchange material in the form of loose powders.

 The inorganic ion exchange material must also combine good mechanical stability with a high reaction rate as opposed to products in compact form whose low specific surface area leads to slow reaction rates.

 In other words, this inorganic solid ion exchange material must have among others excellent mechanical and chemical stability, a high coefficient of affinity or decontamination, a high reactivity, as well as good selectivity.

 These properties must be obtained with a minimum amount of inorganic solid ion exchange material.

 In addition, particularly in the case of the attachment of radioactive elements, it is necessary that the inorganic solid ion exchange material can be easily immobilized / safely conditioned in known matrices, and / or by known methods.

 Finally, the material must have a perfectly reproducible and controlled composition and properties and must be prepared by a reliable process.

 The object of the present invention is to provide an inorganic solid ion exchange material that meets these needs, among others.

The object of the present invention is still to provide an inorganic solid ion exchange material, which does not have the drawbacks, defects, disadvantages, and limitations of the inorganic solid ion exchange materials of the prior art, especially in the form of loose powders or compact, and which overcomes the problems of the materials of the prior art. The object of the present invention is finally to provide a process for preparing such an inorganic solid ion exchange material.

SUMMARY OF THE INVENTION This and other objects are achieved, according to the invention, by a solid material in the form of an alveolar monolith consisting of a calcined matrix of an inorganic oxide with a hierarchical porosity. and open comprising macropores, mesopores, and micropores, defined by walls, with integrated transition metal atoms M and bonded to said walls, said macropores, mesopores and micropores being interconnected, wherein nanoparticles of a inorganic solid material exchanger of a metal cation selected from metal hexa- and octacyanometallates of formula [Alk ⁺ _x ] M ^{n +} y [M '(CN) _m ] t ^{z "} , where Alk ⁺ is a monovalent cation selected from alkali metal cations and the NH ₄ ⁺ ammonium cation, 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 equal to 6 or 8, z is equal to 3 or 4, and t is 1 or 2, are distributed in said porosity, and the nanoparticles are attached to an inner surface of said walls through said transition metal atoms M.

In the formula given above, Alk ⁺ may therefore represent a monovalent cation of an alkali metal such as Li, Na, or K, K being preferred; or an ammonium NH ₄ ⁺ cation.

The formula given above could possibly be written in a simplified way: [Alk _x ] M [M '(CN) _m ] where M is at oxidation state 2 or 3 and Alk is at oxidation state 1.

 Said nanoparticles may also be called "nanocrystals".

 For the purposes of the present invention, the term "monolith" means a solid object whose average dimension is at least 1 mm.

For the purposes of the present invention, "macropores" are understood to mean pores whose average size, generally defined by the diameter of their cross section -because the pores generally have a circular cross-section, is from 4 μιη to 50 μιη; "mesopores" means pores with an average size of 20 to 500 Å, preferably 20 to 300 Å; and "micropores" means pores with an average size of less than 20 Å, for example 5 to 10 Å.

 The material according to the invention is fundamentally different from the materials of the prior art in that it is in the specific form of an alveolar monolith, in that this monolith consists of a calcined matrix of an inorganic oxide, in that this monolith has a hierarchical porosity associating three types of pores, namely macropores, mesopores, and micropores, in that nanoparticles of a specific material which is an inorganic solid material exchanger of a chosen metal cation among the metal hexa- and octacyanometallates are distributed in this porosity, and finally in that the transition metal atoms M are integrated and bonded to said walls, and the nanoparticles are attached to an inner surface of said walls via said atoms. of transition metal.

 The material according to the invention meets the needs listed above, it does not have the disadvantages of the materials of the prior art and it provides a solution to the problems posed by the materials of the prior art.

 Thus, in the case of a column effluent treatment method, the material according to the invention, which is in the specific form of a hierarchized porous alveolar monolith, has the advantage of greatly reducing the pressure drop. relative to a material consisting of a compact stack of particles.

 The material according to the invention thus makes it possible to limit the risk of clogging of the treatment system, such as a column, due to the production of fine particles within the bed of particles.

 The fact that the material according to the invention is in the form of a monolith gives it high mechanical strength and stability, while the fact that the monolith is mainly constituted by an inorganic oxide gives it a high chemical resistance and ensures hence the treatment of a wide variety of effluents.

In addition according to the invention, the inorganic oxide matrix which constitutes the monolith is calcined, which further increases its mechanical strength and stability. Then, the presence of the inorganic metal cation exchanger solid material in the form of nanoparticles distributed in the porosity of the monolith greatly increases the amount of metal cations that can be absorbed relative to an inorganic solid cation exchange material which is under a massive form.

 This is explained by the fact that when the cation exchanger is in nanometric form, the specific surface area available for metal cations, such as cesium cations, is higher.

 Finally, in the material according to the invention, the transition metal atoms M are integrated and bonded to the walls of the pores and the nanoparticles are attached to the inner surface of said walls, that is to say inside the pores. through said transition metal atoms. As a result, the nanoparticles are firmly bonded to the walls and their training, detachment, will be limited or even non-existent. This constitutes an additional advantage of the material according to the invention with respect to materials in which the nanoparticles are simply distributed in the porosity but without being really fixed to the walls via the transition metal atom M.

 In the material according to the invention, the quantity of active functions, in other words of nanoparticles, immobilized in the monolith is not dependent on the number of groups of the inorganic oxide, such as the silanol groups in the case of silica, which are present on the surface of the inorganic oxide such as silica.

 The degree of functionalization of the materials according to the invention is therefore higher than in the case of post-functionalized porous solids, for example via a graft binding to the groups of the inorganic oxide such as silanols.

 Finally, the materials according to the invention have both excellent mechanical strength especially because the monolith has been calcined, and excellent sorption properties.

 Monoliths with hierarchical porosity are commercially available. It is only silica monoliths.

The incorporation of nanoparticles of cation exchangers in such monoliths has however not been described or suggested, it is the same for their use to eliminate cations of a liquid effluent, in particular for decontaminating complex effluents containing different radio-elements.

 Numerous documents report on the functionalization of inorganic monoliths by chemical functions covalently bonded to the material. It is therefore not about the incorporation of nanoparticles and even fewer nanoparticles of a cation exchanger into the porosity of monoliths.

 For example, WO-A1-2001 / 47855 [1] discloses the functionalization of a silica monolith with a silane in order to more efficiently produce ketene molecules. In this case, the silane is thus bonded to the silica network by post-functionalization, that is to say by reaction with the silanol groups present on the surface of the silica.

 Similarly, the document WO-A2-2008 / 031108 [2] presents a list of silane precursors intended for the functionalization of pre-synthesized porous silicas. The targeted applications are diverse and depend on the type of silane considered.

 Moreover, we know a method of monolith synthesis which was developed by R. BACKOV group of the Paul Pascal Research Center in Bordeaux.

 This process consists of preparing a direct oil-in-water emulsion containing a majority of oil by volume. A precursor of silica previously dissolved in aqueous phase reacts to form an inorganic network surrounding the drops of oil. The oil phase is then removed by rinsing, which releases the macroporosity of the material.

 In order to stabilize the water / oil interface, surfactants are required. These surfactants can be either molecular organic surfactants or colloidal particles.

The process using molecular organic surfactants has been the subject of the application WO-A2-2008129151 [3]. It makes it possible to obtain a material in the form of a solid alveolar monolith consisting of a polymer of an inorganic oxide bearing organic groups, which has a hierarchical porosity with macropores, mesopores and interconnected micropores. More specifically, this process consists in preparing an emulsion by adding an oily phase to an aqueous solution of surfactant, adding to the aqueous solution of surfactant at least one precursor tetraalkoxide of the inorganic oxide polymer, before or after the preparation of the emulsion, to allow the reaction mixture to rest until the precursor is condensed, and then to dry the mixture to obtain a monolith. At least one alkoxide carries an organic group.

 There is no mention or suggestion in this document of the incorporation of nanoparticles, let alone nanoparticles of cation exchangers, in such monoliths. In the same way, the use of these monoliths to remove cations from a liquid effluent, in particular to remove cations of radioactive effluent radioactive elements, is not described or suggested in this document.

 The process employing colloidal solid particles has been the subject of the application WO-A2-2012 / 049412 [4]. It makes it possible to obtain a purely macroporous, monodisperse material and not a hierarchically porous material.

 More specifically, this process comprises at least one step of mineralization of an oil-in-water emulsion, formed by droplets of an oily phase dispersed in a continuous aqueous phase, and in which colloidal solid particles are present at the interface. formed between the continuous aqueous phase and the droplets of the oily phase.

 The colloidal solid particles may be inorganic or organic.

 There is no mention or suggestion in this document that the colloidal solid particles may be cation exchange nanoparticles. Similarly, the use of the monoliths of this document for removing cations from a liquid effluent, particularly for removing cations from radioactive effluent radioactive elements, is neither described nor suggested.

 The role of the colloidal particles in this document is exclusively to stabilize the emulsion, more precisely the water-oil interface of the emulsion, to induce a monodisperse macroporosity of the material.

Indeed, the macroporosity of the material derives directly from the size of the drops of the emulsion. The size of the drops of the emulsion being monodisperse, the macroporosity of the material is consequently also very monodisperse. The role of the particles in this document is in no way to functionalize the monolith in order to confer cation exchange properties, including selective adsorbent certain cations.

 The objective of the material according to the invention which is to functionalize the monoliths with these nanoparticles, is neither mentioned nor suggested in this document where the role of the colloidal particles is only to stabilize the emulsion, and in no way to make the monolith reactive for use in removing cations from a solution.

 According to the invention, the sole and only role of the nanoparticles used is to functionalize the monolith, and not to stabilize the emulsion used during the preparation of the material, as will be seen below in the description of the preparation process of the material according to the invention.

 In fact, cation exchange nanoparticles, in particular ferrocyanides, used alone do not make it possible to stabilize the emulsion and to prepare the monolith. Therefore, in the process according to the invention set out below, a surfactant is needed to stabilize the emulsion.

 Advantageously, the inorganic oxide is selected from oxides of at least one metal or metalloid selected from Si, Ti, Zr, Th, Nb, Ta, V, W, Y, Ca, Mg and Al.

 Preferably, the inorganic oxide is silica.

 According to the invention, the matrix of an inorganic oxide is a calcined matrix.

 By "calcined" is meant that this matrix has undergone a heat treatment at a temperature of 400 ° C to 1000 ° C, for example at a temperature of 500 ° C for a period of 1 to 10 hours, for example 6 hours .

According to the invention, the exchanger material is chosen from hexa and octacyanometallates of metal of formula

given above. This choice is generally made according to the intended application, the nature of the liquid effluent to be treated, and in particular according to the metal cation or cations that are desired to separate.

Advantageously, M ^{n +} is Fe ²⁺ , Ni ²⁺ , Fe ³⁺ , Co ²⁺ , Cu ²⁺ , or Zn ²⁺ . Advantageously, M 'is Fe ²⁺ or Fe ³⁺ or Co ³⁺ and m is 6; or M 'is Mo ⁵⁺ and m is 8.

Advantageously, [M '(CN) _m ] ^z - is [Fe (CN) ₆ ] ^{3 -} , [Fe (CN) ₆ ] ^{4 -} , [Co (CN) ₆ ] ³ or [Mo (CN) ₈ ] ^{3 "}

Preferably, the inorganic solid material exchanger of a metal cation has the formula K ₂ M Fe (CN) ₆ , for example K ₂ Cu Fe (CN) ₆ , K ₂ ZnFe (CN) ₆ , or K ₂ Co Fe (CN) ₆ .

 Indeed, one of the main applications targeted for the materials according to the invention is that of radioactive cesium sorbents for nuclear decontamination purposes.

However, ferrocyanide (and ferricyanide) nanoparticles of copper (FCCu) of general formula [K ⁺ _x ] Cu ²⁺ _y [Fe (CN) 6] ^{z "} , for example K ₂ Cu Fe (CN) 6 are very selective Their crystalline structure is cubic face-centered, and has the advantage of being able to selectively exchange a cesium atom with an unbound potassium atom present in the cell.

 Generally, nanoparticles have a sphere or spheroidal shape.

 Generally, the nanoparticles have an average size, such as a diameter, of 2 to 300 nm, preferably 2 to 100 nm, more preferably 2 to 50 nm.

 Generally, the nanoparticle content of the at least one inorganic solid exchanger material of a metal cation is 0.5% to 15% by weight, preferably 0.5% to 5% by weight.

 The invention further relates to a method for preparing the material according to the invention which comprises at least the following successive stages:

 a) an aqueous solution A of an organic surfactant is prepared, and the pH of this solution is adjusted to a value of 1.5 to 2.5, preferably 1.8 to 2.2, more preferably of 2;

b) adding M ^{n +} ions to solution A, optionally the pH of this solution to which M ^{n +} ions have been added is adjusted to a value of 1.5 to 2.5, preferably of 1.8 to 2 , 2, more preferably 2, whereby an aqueous solution B is obtained;

c) dissolving a precursor of the inorganic oxide in solution B, whereby an aqueous solution C is obtained; d) optionally, the pH of the aqueous solution C is adjusted to a value of 1.5 to 2.5, preferably 1.8 to 2.2, more preferably 2, and an aqueous solution of at least one metal fluoride, preferably at least one alkali metal fluoride such as KF, LiF, or NaF in solution C, whereby an aqueous solution D is obtained;

 e) an oil phase is added rapidly after the end of step d), with mechanical stirring with shear, to the aqueous solution D, whereby an oil-in-water emulsion E formed of droplets of the phase is obtained; oily dispersed in a continuous aqueous phase;

 f) maturation is carried out, mineralization of the emulsion E obtained in step e), whereby the monolith is formed;

 g) washing and / or drying the monolith;

 h) the monolith is calcined;

i) impregnating the monolith with an aqueous solution of [Alk ⁺ _X ] [M '(CN) _m ] t ^{z ~} ; j) washing and drying the impregnated monolith.

It should be noted that the transition metal M contained in the solution B will then serve as a nucleation point for the growth of the nanoparticles of at least one inorganic solid material exchanger of a metal cation at the end of the protocol.

In the emulsion E, the M ^{n +} ions are rather in the vicinity of the micelles of surfactants in the aqueous phase. They are not located at the water / oil interface.

 By "rapidly add" in step e), it is generally meant that the oily phase is added to the aqueous solution D within less than 2 hours after the end of step d).

 The method according to the invention comprises a specific sequence of specific steps which has never been described or suggested in the prior art.

As has already been explained above, according to the invention the only role of the nanoparticles used which are obtained only in the final stage of the process, is to functionalize the monolith and not to stabilize the emulsion implemented during of the preparation of the material, as is the case in the process described in document [4]. In fact, the nanoparticles of cation exchangers, in particular ferrocyanides, are obtained in the process according to the invention after the preparation of the emulsion and can not therefore stabilize the emulsion in order to prepare the monolith. This is why in the process for preparing the material according to the invention, unlike the process described in document [4], an organic, molecular surfactant is required to stabilize the emulsion.

 In the process according to the invention, there is no so-called post-functionalization step, the quantity of active functions, in other words of nanoparticles, immobilized in the monolith is not dependent on the number of groups inorganic oxide, such as silanol groups in the case of silica, which are present on the surface of the inorganic oxide such as silica.

 The degree of functionalization of the materials according to the invention, prepared by the process according to the invention is therefore higher than in the case of post-functionalised porous solids, for example by means of a graft binding to the groups of inorganic oxide such as silanols.

 The process according to the invention makes it possible to prepare materials which have both excellent mechanical strength, in particular because the monolith has been calcined, and excellent sorption properties.

 The process according to the invention makes it possible to prepare a material comprising transition metal hexa- or octacyanometallate nanoparticles such as transition metal ferrocyanides in a calcined monolith while transition metal hexa- or octacyanometallates such as transition metal ferrocyanides can not be calcined since the transition metal hexa- or octacyanometalates such as transition metal ferrocyanides are sensitive to high temperatures and degraded beyond 100 ° C.

The process according to the invention, because of its specific succession of specific steps, makes it possible to prepare a material with a calcined monolith comprising transition metal hexa- or octacyanometallate nanoparticles such as transition metal ferrocyanides, without having to having to post-functionalize this monolith via a graft binding to inorganic oxide groups such as silanols. For this purpose, according to the invention, a solution containing the metal ions M ^{n + is used} as the precursor of the material.

 The monolith, for example the silica monolith, is then grown in this solution (Solution B), and a monolith containing M metals is thus obtained which will act as a growth point for the metal hexa- or octacyanometallate particles of the metal. transitions such as ferrocyanide particles or transition metal ferricyanides.

It then suffices to impregnate the monolith, for example with an acidic aqueous solution of K ₄ Fe (CN) 6, to precipitate the transition metal hexa- or octacyanometallate particles such as transition metal ferrocyanide particles. even within the monolith.

The general formula for transition metal ferrocyanides is K ₂ MFe (CN) ₆ .

 These ferrocyanides are crystalline coordination polymers whose structure is cubic face-centered. These compounds have the particularity of being able to receive in the center of the crystal lattice a potassium atom which can then be exchanged with cesium. This ion exchange is very selective with respect to cesium. That's why ferrocyanides are one of the most efficient cesium exchangers.

 In step a), the aqueous solution A containing the organic surfactant generally has an organic surfactant concentration of 10 to 30% by weight, for example 20% by weight.

The organic surfactant is preferably selected from cationic surfactants and nonionic surfactants such as Pluronics ^® as Pluronic ^® P 123 marketed by BASF or SIGMA-ALDRICH companies.

 Solution A is an aqueous solution.

 By aqueous solution, it is generally meant that the solvent of this solution is selected from water, and mixtures of water and one or more alcohols such as methanol, said mixtures containing a majority of water (more than 50 % water) by volume.

Solution A has a pH generally of 1.5 to 2.5, preferably 1.8 to 2.2, more preferably 2. In order to adjust the pH to this value, solution A contains generally an acid, preferably a mineral acid such as hydrochloric, nitric or sulfuric acid.

Solution B contains the M ^{n +} ion, such as M ²⁺ , generally in the form of a metal salt M.

 This solution B is an aqueous solution.

 Preferably, the water of this aqueous solution is ultra pure water.

 The salt of the metal M contained in this solution B is a salt whose metal M is generally selected from metals capable of giving a cyanometalate of this metal M, such as a hexacyanoferrate of this metal, which is insoluble.

 This metal M may be chosen from all transition metals, for example from copper, cobalt, zinc, nickel and iron, etc.

The ion M ^{n +} may therefore be selected from Fe ²⁺ , Ni ²⁺ , Fe ³⁺ , Co ²⁺ , Cu ²⁺ , and Zn ²⁺ ions.

The salt of the metal M can be, for example, a nitrate, a sulphate, a chloride, an acetate, a tetrafluoroborate, optionally hydrated, of one of these metals M.

 The preferred salts are, for example, nitrates of formula M (NO 3) 2.

 The concentration of the salt of the metal M in solution B is preferably from 0.01 to 1 mol / L, more preferably from 0.01 to 0.05 mol / L.

 The salt concentration of the metal M in the solution B is generally chosen as a function of the final concentration of nanoparticles in the solid material, monolith.

 Solution B has a pH of 1.5 to 2.5, preferably 1.8 to 2.2, more preferably 2. In order to adjust the pH to this value, solution B generally contains an acid, preferably a mineral acid such as hydrochloric, nitric or sulfuric acid.

The aqueous solution C containing the ions M ^{n +} , an organic surfactant, and a precursor of the inorganic oxide is generally prepared by adding, dissolving, a given volume of precursor, generally liquid, to / in the solution B.

 This addition is generally an addition that can be described as slow, preferably this addition is carried out drop by drop.

The inorganic oxide is generally chosen from oxides of metals and metalloids, and the precursor of this oxide is generally chosen from alkoxides of metals or metalloids, and salts of metals or metalloids, such as chlorides and nitrates of metals and metalloids.

 In the case where the inorganic oxide is silica, the precursor (s) of the silica may be chosen from tetramethoxyorthosilane (TMOS), tetraethoxyorthosilane (TEOS), dimethyldiethoxysilane (DMDES), and mixtures thereof.

 The concentration of the surfactant in the aqueous solution C prepared in step c) is generally 10% to 30% by weight, and the concentration of the precursor is generally 1 to 500 g / L.

 The pH of the aqueous solution C prepared in step c) may be 1.5 to 2.5, more preferably 1.8 to 2.2, more preferably 2.

 If the pH of the aqueous solution C prepared in step c) is not 1.5 to 2.5, preferably 1.8 to 2.2, more preferably 2, then the pH of the aqueous solution C is adjusted to a value of 1.5 to 2.5, preferably 1.8 to 2.2, more preferably 2 at the beginning of step d).

 In step d), sodium fluoride is generally used as metal fluoride, especially when the inorganic oxide is silica.

 In step d), the pH is 1.5 to 2.5, preferably 1.8 to 2.2, more preferably 2, especially when the inorganic oxide is silica. because the isoelectric point of the silica is at pH 2. It is indeed sought to decouple the hydrolysis and polycondensation steps of the silica, so that the protocols are "cleaner".

 As isoelectric point of the silica, the polycondensation of the silica is completely inhibited. This is why one needs to add an alkali metal fluoride such as NaF.

 This alkali metal fluoride, such as NaF, will act as a catalyst for the polycondensation reaction of silica. It is the addition of this alkali metal fluoride, such as NaF, which will initiate the reaction which will eventually lead to a silica-based material.

In addition, in the applications [3] and [4] cited above, the pH of the preparation of the materials, or rather the pH of the aqueous phases used, and in particular of the aqueous phase of the emulsion, is close to 0. At such a pH, the nanoparticles of hexa- or metal octacyanometallate of formula [Alk ⁺ _x ] M ^{n +} y [M '(CN) _m ] ^{z "} exposed above, are not stable

 When using such nanoparticles, therefore, it is necessary to place at a pH of the aqueous phase of the emulsion E prepared in the higher step e), generally from 1.5 to 2.5, preferably from 1.8 to 2.2, more preferably 2, and under these conditions, the addition of at least one metal fluoride, such as NaF, is essential, otherwise in step f) following the emulsion remains liquid, the reaction sol-gel evolves only too slowly to the solid, and one does not obtain a monolith.

 The solution of at least one metal fluoride, such as NaF, generally has a concentration of 1 to 40 g / l, for example 8 g / l.

 In step d), a given volume of this solution of at least one metal fluoride in solution C is added.

 The solution D obtained therefore generally has a pH of 1.5 to 2.5, preferably 1.8 to 2.2, more preferably 2, and its concentration of metal fluoride such as NaF is generally from 1 to 100 mg / l, preferably 10 mg / l.

 The oily phase added during step e) is generally constituted by one or more liquid alkanes chosen from linear or branched alkanes having from 5 to 22 carbon atoms, preferably from 7 to 22 carbon atoms, such as dodecane and hexadecane, or from cyclic alkanes of 5 to 22 carbon atoms such as cyclohexane.

The mass ratios of the constituents used to prepare the emulsion can be as follows: mM / m (surfactant solution, for example surfactant solution such as P123, at 20% by weight and pH 2) / m ( precursor, for example TEOS) / m (metal fluoride solution, for example NaF, at 8 g / l) / m (oily phase, for example cyclohexane): 0.02-0.2 / 1-3 / 0 , 5-2 / 9.10 ³ -9.10- ² / 3-9, for example 0.050 / 1.68 / 1 / 9,3.10 ^"3 / 3.98.

 At the end of this step e) a whitish emulsion E is obtained.

The mechanical stirring carried out during step e) is generally carried out using an apparatus intended to emulsify, such as a dispersing-homogenizing apparatus Ultraturrax ^® type. Step e) can be described as an emulsification step of the system constituted by the solution D obtained in step d).

 Mechanical agitation is mechanical stirring with shear.

 The shear rate can range from 1 to 20000 rpm, preferably from 2000 to 15000 rpm, more preferably the shear rate is 3200 rpm.

 It is possible to control the size of the macroporosity of the monoliths by acting on the shear rate of the emulsion. The size of the macroporosity decreases as the shear rate increases.

 The volume fraction of the oily phase of the emulsion E obtained during step e) is generally from 50% to 74%, preferably from 55% to 65% of the volume of the emulsion.

 In step f), the mineralization of the emulsion E obtained in step e) is carried out, whereby the monolith is formed.

 This step can be carried out by leaving the emulsion E obtained in step e) at rest at a temperature of 10 to 60 ° C., for example at a temperature of 40 ° C. for a time sufficient for the monolith with hierarchical porosity. forms. This duration can be for example from 2 hours to 3 weeks, for example 7 days.

 Generally at the end of step f), the monolith is washed and / or dried during step g) -

The washing makes it possible to eliminate the organic residues originating from the oily phase and which are essentially in the macropores.

 This washing can be carried out with an organic solvent such as THF, acetone and their mixtures.

 This washing can be carried out for a period of 12 to 36 hours, for example 24 hours.

 Preferably, this washing is carried out by bringing the organic solvent to reflux.

However, an optimal washing treatment, which makes it possible to obtain a final monolith having a minimum of fractures, is a washing treatment using supercritical CO ₂ , in which case the drying step can be omitted. Drying can be carried out by allowing the organic solvent used for washing at room temperature to evaporate for a period generally of 5 to 10 days, for example 7 days.

Drying can also be performed using a supercritical fluid, such as supercritical CO ₂ .

 It should be noted that if the monolith has been prepared with a volatile alkane such as cyclohexane (the person skilled in the art has no difficulty in identifying, among the alkanes mentioned above which constitute the oily phase, those which are volatile), the steps of washing and then drying of the monolith which are usually carried out, are replaced by a simple drying step which consists in exposing the monolith to the ambient air during which the alkane evaporates and releases the macroporosity of the material .

 In other words, in this case, in step g), it is simply enough to dry the monolith.

 The monolith which is loaded with transition metal is then calcined during step h), to improve its mechanical strength, and then cooled to room temperature.

 This calcination is generally carried out by maintaining the monolith at a temperature, called the calcining temperature, of 500 to 650 ° C. for a period of 1 to 8 hours, for example at a temperature of 500 ° C. for a period of 6 hours.

 Generally, a thermal calcination cycle is carried out in which the temperature of the monolith is raised to the calcination temperature by carrying out one or more temperature ramps. When several temperature ramps are carried out, these ramps can be separated by temperature trays and these ramps can be made at the same rate of rise in temperature or at different rates of rise in temperature.

 Thus, the thermal calcination cycle may comprise, for example, firstly a first temperature ramp during which the temperature of the monolith is raised at a rate of 0.5 ° C./min to reach a temperature of 200 ° C.

A first tray can then be observed at 200 ° C for 2 hours. Then it is possible to carry out a second temperature ramp during which the temperature of the monolith is raised at a rate of 0.2 ° C./min to reach a temperature of 500 ° C., which is the final calcination temperature.

 Finally, a second plate can be observed at the final calcination temperature of 500 ° C. for 6 hours.

 The monolith is then allowed to cool to room temperature.

 At the end of these washing and / or drying steps g), and calcining h), a monolith loaded with a transition metal is obtained.

 The transition metal atoms, for example Zn, Cu or Co, will then serve as growth points for the Prussian blue analog particles during the impregnation stage i).

During this impregnation step i), the monolith is impregnated with an aqueous solution of [Alk ⁺ _X ] [M '(CN) _m ] ^{z "} , for example K ₄ Fe (CN) ₆ .

 The concentration of this solution is generally 0.05 to 0.2 mol / l, preferably 0.1 mol / l.

 This solution may be an acid solution or not.

 In the case where this solution is an acid solution, the acid concentration is generally less than or equal to 1.5 mol / L.

The M ^{n +} ions react with [Alk ⁺ _X ] [M '(CN) _m ] t ^{z ~} to form particles of [Alk ⁺ x] M ^{n +} y [M' (CN) _m ] t ^{z "} . that is, Prussian blue analog particles.

For example, during this step, the monoliths may be impregnated with an acidic or non-aqueous solution of K ₄ Fe (CN) 6, the concentration of which is between 0.05M and 0.2M. Ideally, the concentration of K ₄ Fe (CN) ₆ is 0.1M. The pH of the solution can be maintained using nitric acid, the concentration of which is between 0 and 1.5M.

M ²⁺ ions, for example Co ²⁺ , Zn ²⁺ or Cu ²⁺ , react with Fe (CN) 6 ^{4 -} ions to form Prussian blue K2M Fe (CN) 6 analog particles, for example K ₂ CoFe (CN) 6, K ₂ ZnFe (CN) 6 or K ₂ CuFe (CN) 6. At the end of this impregnation step, the material according to the invention as it has been obtained is obtained. described above. At the end of step i), the impregnated monolith is washed and dried during a step j). The washing can be carried out with water, and the drying can be carried out at a temperature of 40 ° C for 24 hours.

 The material according to the invention can be used in particular, but not exclusively, in a process for separating at least one metal cation from a liquid medium containing it, wherein said liquid medium is brought into contact with the material according to the invention.

 The materials according to the invention, because of their excellent properties such as excellent exchange capacity, excellent selectivity, high reaction rate, are particularly suitable for such use.

 This excellent efficiency is obtained with reduced amounts of inorganic solid material exchanger of a metal cation, such as an insoluble hexacyanoferrate.

 In addition, the excellent properties of strength and mechanical stability of the material according to the invention, resulting from its specific structure allow its packaging in column and the continuous implementation of the separation process, which can thus be easily integrated into an installation existing, for example in a chain or processing line comprising several steps.

 Advantageously, said liquid medium may be an aqueous liquid medium, such as an aqueous solution.

 Said liquid medium may be a process liquid or an industrial effluent.

 Advantageously, said liquid medium may be a liquid medium containing radionuclides. For example, the liquid medium may be selected from liquids and effluents from the nuclear industry, and activities using radionuclides.

 Advantageously, the liquid medium may be an aqueous solution containing, besides said metal cation, salts (of course different from the salts of said metal cation) such as NaCl at a high concentration, for example greater than 30 g / l.

The process according to the invention makes it possible surprisingly to effectively and selectively separate a metal cation such as a cesium cation, even from liquid media, such as aqueous solutions, which are heavily loaded with salts, in particular in NaCl. Such media heavily loaded with NaCl are for example seawater and brackish water.

 Generally, said metal cation may be present at a concentration of 0.1 picogram to 100 mg / L, preferably 0.1 picogram to 10 mg / L.

 The term "metal" also covers the isotopes and in particular the radioactive isotopes of said metal.

 Preferably, the cation is a cation of an element selected from Cs, Co, Ag, Ru, Fe and Ti and the isotopes, especially radioactive thereof.

More preferably, the cation is a ¹³⁴ Cs, or ¹³⁷ Cs cation.

The process according to the invention can be carried out advantageously with a liquid medium which is an aqueous solution, containing as a metal cation a cation of ¹³⁴ Cs, and / or ¹³⁷ Cs, and further containing salts, such as NaCl, at a high concentration, for example greater than 30 g / l.

 The process according to the invention makes it possible to effectively and selectively separate these metal cations from radioactive cesium from such highly saturated liquid media as salts such as NaCl. This effective and selective separation is possible thanks to the selectivity of the material according to the invention vis-à-vis the Cs in the presence of a competing ion such as Na.

 This process has all the advantages intrinsically related to the material according to the invention, implemented in this process, and which have already been described above.

 The invention will be better understood on reading the following detailed description of particular embodiments, particularly in the form of examples, this description being made in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

 - Figure 1 is a photograph of a silica monolith functionalized with cobalt ferrocyanide particles prepared by the method according to the invention.

 Figure 2 is a photograph of a monolith functionalized with zinc ferrocyanide particles prepared by the process according to the invention.

Figure 3 is a photograph of a monolith functionalized with copper ferrocyanide particles prepared by the process according to the invention. Figure 4 is a scanning electron microscope (M EB) photograph of a monolith functionalized with cobalt ferrocyanide particles called ABP particles (i.e. Prussian Blue Analogs. as well as the compounds of the ferro- and ferricyanide family).

 The scale shown in FIG. 4 represents 10 μιη.

 Figure 5 is a scanning electron microscope (M EB) photograph of a monolith functionalized with Zn ferrocyanide particles.

 The scale shown in FIG. 5 represents 10 μιη.

 Figure 6A is a scanning electron microscope (M EB) photograph of a monolith functionalized with copper ferrocyanide particles.

 The scale shown in FIG. 6A represents 10 μιη.

 Figure 6B is a scanning electron microscope (M EB) photograph of a monolith functionalized with copper ferrocyanide particles. This figure is an enlargement of the circled portion of Figure 6A.

 The scale shown in FIG. 6B represents 3 μιη.

 FIG. 7 is a graph which shows the curves obtained during analyzes by the small angle X ray scattering ("SAXS") technique performed on a monolith functionalized with particles of cobalt ferrocyanide, prepared by the process according to the invention (curve A), a monolith functionalized with zinc ferrocyanide particles, prepared by the process according to the invention (curve B), and a monolith functionalized with ferrocyanide particles of copper, prepared by the process according to the invention (curve C).

On the abscissa is the wave vector q (in nnr ¹ ).

 DPC is the distance between two pore centers.

FIG. 8 is a graph which shows the nitrogen adsorption isotherms obtained during nitrogen adsorption measurements carried out on a cobalt ferrocyanide functional monolith ("CoPBA") called monolith - CoPBA (curve A) , and on a monolith not functionalized by K ₄ Fe (CN) 6 and simply loaded in Co called monolith - Co (curve B). On the abscissa is the relative pressure P / Po, and the ordinate is carried the amount of adsorbed nitrogen (in cm ³ / g STP).

 FIG. 9 is a graph which gives the cesium adsorption isotherms carried out on a monolith functionalized with particles of cobalt ferrocyanide, prepared by the process according to the invention (curve A), a monolith functionalized with particles of ferrocyanide of zinc, prepared by the process according to the invention (curve B), and a monolith functionalized with copper ferrocyanide particles, prepared by the process according to the invention (curve C).

These curves represent the amount of cesium sorbed on the solid Q.cs (in mg / g) as a function of the equilibrium concentration of cesium in the solution C _e Cs (in mM).

 FIG. 10 is a graph which gives the sodium adsorption isotherms carried out on a monolith functionalized with particles of cobalt ferrocyanide, prepared by the process according to the invention (curve A), a monolith functionalized with particles of ferrocyanide of zinc, prepared by the process according to the invention (curve B), and a monolith functionalized with copper ferrocyanide particles, prepared by the process according to the invention (curve C).

 These curves represent the amount of sodium sorbed on the Q.Na solid (in mg / g) as a function of the equilibrium concentration of sodium in Ce Na solution (in mM). DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

 The invention will now be described with reference to the following examples given for illustrative and non-limiting.

EXAMPLES.

In the examples which follow, silica monoliths containing nanoparticles of ferrocyanides according to the invention are prepared by the process according to the invention and these silica monoliths containing nanoparticles of ferrocyanides are used as ionic cesium (Cs ⁺ ) sorbents. . Example 1

 In this example, the general preparation protocol according to the invention of silica monoliths containing nanoparticles of transition metal ferrocyanides is prepared, and three silica monoliths containing nanoparticles of cobalt ferrocyanide and ferrocyanide are prepared. of zinc, and copper ferrocyanide by this protocol.

 The protocol for preparing silica monoliths containing ferrocyanide nanoparticles comprises the following successive steps:

1. A solution containing a given weight of Pluronic ^® P123 (surfactant commercially available from BASF ^® or Sigma-Aldrich ^®) so as to obtain a solution with a surfactant concentration of 20% by weight.

The solution prepared in this step is called solution A. 2. To 50 mg of transition metal (M ²⁺ ) is added to 1.68 g of solution A.

The metal source used is a nitrate, M (N 03) 2 with M = Co, Zn, or Cu. The pH is adjusted to 2. The acid used to rectify the pH at 2.0 is hydrochloric acid.

The solution prepared in this step is called solution B.

3. B, lg of tetraethylorthosilicate is dissolved dropwise in solution

(TEOS). Wait 30 minutes until the solution becomes clear again. Solution C is obtained.

4. Add 10 μl of a solution of sodium fluoride (NaF) at 8 g / l in solution C.

The solution prepared in this step is called solution D. 5. Rapidly, that is to say within 15 minutes after the addition of the solution of sodium fluoride which made it possible to prepare solution D, the solution is emulsified with 5.1 ml of cyclohexane. . For this, one uses a disperseur- homogenizer type Ultraturrax ^® T25 device with a S25N probe and cyclohexane are slowly added in the solution D under shear.

 The shear rate can range from 1 (if 0 no shear) to 20000 rpm, and is preferably 3200 rpm.

The mass ratios of the constituents used to prepare the emulsion may be as follows: m of M / m (surfactant solution, for example surfactant solution such as P123, at 20% by weight and pH 2) / m (TEOS) / m (8 g / l NaF solution) / m (cyclohexane): 0.08 / 1.6 / 1 / 9.3 × ³ / 3.98.

 At the end of this step, an off-white emulsion E is obtained.

6. Preparation of the monolith.

 During this step, the so-called ripening step, the monolith is prepared.

 For this, the emulsion E is left standing by placing it in an oven for 7 days, at a temperature between room temperature and 50.degree. C., preferably at 40.degree.

 At the end of this stage of maturation, the monolith is formed.

 All that remains is the washing / drying steps then calcination of the monolith to be carried out before performing the functionalization of the monolith.

7. Washing / drying the monolith.

In this example, the alkane used is cyclohexane. The washing step can therefore be avoided. It is therefore sufficient to leave the monoliths at 25 ° C for 48 hours to evacuate cyclohexane.

8. Calcination of the monolith.

The monolith is then calcined to improve its mechanical strength. The thermal calcination cycle firstly comprises a first temperature ramp during which the temperature of the monolith is raised at a rate of 0.5 ° C./min to reach a temperature of 200 ° C.

 A first plateau is then observed at 200 ° C. for 2 hours.

 Then a second temperature ramp is carried out during which the temperature of the monolith is raised at 0.2 ° C / min to reach a temperature of 500 ° C which is the final calcination temperature.

 Finally, a second plateau is observed at the final temperature of 500 ° C. for 6 hours.

 The monolith is allowed to cool to room temperature.

 At the end of these washing, drying and calcination steps, a monolith loaded with a transition metal is obtained. The transition metal atoms will then serve as growth points for the Prussian Blue analog particles. 9. Impregnation of monoliths.

During this step, the monoliths are impregnated with an aqueous solution of K ₄ Fe (CN) ₆ at 0.1M.

M ²⁺ ions react with Fe (CN) 6 ^{4 -} ions to form Prussian Blue (ABP) K ₂ MFe (CN) ₆ analog particles.

 At the end of this impregnation step, the materials are washed with water and with ethanol.

 After a drying step of 48h at 40 ° C, the material according to the invention is obtained.

The protocol for the preparation of the monoliths described above has been implemented to prepare three materials according to the invention constituted respectively by:

a silica monolith comprising nanoparticles of cobalt ferrocyanide, in other words a silica monolith functionalized with nanoparticles of cobalt ferrocyanide. a silica monolith comprising nanoparticles of Zn ferrocyanide, in other words a silica monolith functionalized with Zn ferrocyanide nanoparticles.

 a silica monolith comprising nanoparticles of copper ferrocyanide, in other words a silica monolith functionalized with copper ferrocyanide nanoparticles.

Example 2

 In this example, a macroscopic observation is made of the three monoliths prepared in Example 1.

 FIGS. 1, 2 and 3 are photographs showing the macroscopic appearance respectively of the silica monolith functionalized with particles of cobalt ferrocyanide, of the monolith functionalized with zinc ferrocyanide particles, and of the monolith functionalized with particles. of copper ferrocyanide prepared by the process according to the invention in Example 1.

 The color of the monolith depends on the type of ferrocyanide that is present in the material.

 We find the classic colors of ferrocyanides cobalt (dark purple), zinc (yellow) and copper (red).

Example 3

 In this example, a scanning electron microscope (SEM) observation of the three monoliths prepared in Example 1 is carried out.

 Figures 4, 5 and 6A, 6B respectively show the morphology of the monolith functionalized with cobalt ferrocyanide particles (Figure 4), the monolith functionalized with Zn ferrocyanide particles (Figure 5), and the monolith functionalized with Copper ferrocyanide particles (FIGS. 6A and 6B).

These pictures show the macroporous structure of these samples. This macroporosity is due to the impression left by the oil drops after the washing step of the material. The average size of macropores is for all three materials around 3 to

4 μιη.

 The average ferrocyanide particle sizes are 80 to 400 nm for cobalt ferrocyanide, 300 nm for zinc ferrocyanide, and 20 to 100 nm for copper ferrocyanide.

 It is also noted that the inner part of the macropores is lined with relatively aggregated ferrocyanide particles. Their sizes and polydispersities depend on the type of metal chosen. It turns out that zinc ferrocyanide particles are the most monodisperse and copper ferrocyanide particles are the smallest.

Example 4

 In this example, an analysis of the three monoliths prepared in Example 1 is carried out by the technique of small angle X-ray scattering ("Small Angle X Ray Scatter Ring" or "SAXS" in English).

 The monoliths according to the invention are macroporous because of their mode of synthesis which uses an emulsion as an imprint. They are also mesoporous because of the use of surfactant in the preparation process. These surfactants are organized in the form of micelles in aqueous phase. The typical size of these micelles reaches a few nanometers. During the monolith washing step, the surfactants are removed and release a cavity of a size close to that of the micelles, that is to say a mesopore.

 The small-angle X-ray scattering technique probes the material at size scales to characterize the organization of these mesopores. The results of the analyzes by this technique are shown in Figure 7.

The curves have an intense peak signifying that there is an average position between two centers of mesopores. The position of this peak at q * makes it possible to evaluate the distance between two centers of pores DPC for each monolith which is equal to 2n / q *. This distance is of the same order of magnitude for each type of monolith. More exactly, the value of DPC is 9.4 nm, respectively; 11.8 nm; and 11.2 nm for monoliths functionalized with ferrocyanides of cobalt, zinc or copper. The variation in the value of this distance can be attributed to the metal-micelle interaction that can vary as the nature of the metal changes.

 The mesopores are therefore weakly structured since an average distance can be determined without the arrangement being crystalline. If this were the case, the SAXS curves would show several peaks of Bragg.

For large wave vector values, namely q> 7 nnr ¹ , several diffraction peaks are visible that can be attributed to the ferrocyanide particles. This shows once again that the monoliths are well functionalized. Example 5

 In this example, the three monoliths prepared in Example 1 are analyzed by the nitrogen adsorption measurement technique. This technique makes it possible to know the specific surface of the materials by applying the BET model.

 The isotherms are shown in Figure 8.

It is noted that the functionalization of the monolith which is carried out during the K ₄ Fe (CN) 6 impregnation stage causes a very sharp reduction in the specific surface area of the material which goes from 710 m ² / g to 406 m ² / boy Wut-

This is consistent with the precipitation of cobalt ferrocyanide particles in the pores.

 Indeed, either the particles are localized in the macropores and block the access to the mesopores located in the silica shells (see Figures 4 to 6), or the particles precipitate directly in the mesopores.

 Whatever the mechanism, this results in a decrease in the specific surface area of the material.

Example 6: Sorption of Cesium

In this example, adsorption tests for Cs ⁺ ionic cesium are carried out on the functionalized silica monoliths containing nanoparticles of copper, zinc or cobalt ferrocyanide prepared in Example 1. In other words, in this example, the ferrocyanides contained in the monoliths are used as specific sorbents for Cs ⁺ ionic cesium.

 Indeed, these ferrocyanides contain a potassium ion in the crystal mesh which is specifically exchanged with a cesium ion.

 The Cs sorption isotherms on monoliths functionalized with ferrocyanides of copper, zinc or cobalt are made according to a standardized protocol.

 This protocol is as follows:

 - Prepare a solution containing [X] mol / l of NaNO3 and [Y] mol / l of CSN O3 with [X] / [Y] = 10. This solution is called solution G.

 - Take 10 mg of monolith and immerse in 20 ml of the previous solution G.

 - shake for 48h.

 - Take the supernatant, filter, and measure the Cs content by ion chromatography.

This protocol makes it possible to quantify the exchange capacity of the monoliths while ensuring the selectivity of materials for cesium vis-à-vis sodium.

 In all the experiments, the results of which are given in FIGS. 9 and 10, the sodium concentration is identical before and after immersion of the monolith in solution G.

 After 48 hours of stirring, the adsorption equilibrium is reached, which makes it possible to compare the experiments with each other.

 FIGS. 9 and 10 show that the functionalized monoliths retain cesium primarily with respect to sodium.

The monolith containing nanoparticles of zinc ferrocyanide is the most efficient, with higher sorption or adsorption capacities than the other monoliths which contain nanoparticles of other ferrocyanides, namely nanoparticles of cobalt ferrocyanide or ferrocyanide. copper. These cesium adsorption capacities are of the order of 22 mg / g, 10 mg / g and 7 mg / g for, respectively, the monoliths which contain ferrocyanide nanoparticles of zinc, copper and cobalt.

 These values are to be compared with those of the literature.

 For example, it was shown in the document by C. Delchet, A. Tokarev, X. Dumail,

G. Toquer, Y. Barre, Y. Guari, C. Guerin, J. Larionova, A. Grandjean, "Extraction of radioactive cesium using innovative functionalized porous materials", RSC Adv., 2, (2012), 5707 [5] that mesoporous silicas or porous glass beads post-functionalized with ferrocyanides had respective sorption capacities of 17 mg / g and 5 mg / g without competitor ion.

 Furthermore, T. Sangvanich, V. Sukwarotwat, RJ Wiacek, RM Grudzien, GE Fryxell, RS Addieman, C. Timchalk, W. Yantasee, T. Selective Capture of Cesium and Thallium from Natural Waters has been shown in the document. [6], that mesoporous silicas functionalised by a different route than that described in the document by C. Delchet et al., published in the Journal of Hazardous Materials, 182, (2010), 225 [6]. had a maximum sorption capacity of 17.1 mg / g in seawater.

 It is therefore noted that the monolithic materials according to the invention, prepared by the process according to the invention have higher performances (22 mg / g for Zinc) in terms of adsorption capacity compared to the materials used in the documents. [5] and [6].

Claims

1. A solid material in the form of an alveolar monolith consisting of a calcined matrix of an inorganic oxide with an open hierarchical porosity comprising macropores, mesopores, and micropores, defined by walls, with atoms of a transition metal M integrated and bonded to said walls, said macropores, mesopores and micropores being interconnected, wherein nanoparticles of an inorganic solid material exchanger of a metal cation selected from hexa and octacyanometallates of metal of formula [Alk ⁺ _x ] M ^{n +} _y [M '(CN) _m ] t ^{z ~} , where Alk ⁺ is a monovalent cation selected from the alkali metal cations and the ammonium cation NH ₄ ⁺ , 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 equal to 1 or 2, are distributed in said porosity, and the nanoparticles are t attached to an inner surface of said walls via said transition metal atoms M.

2. Material according to claim 1, wherein the inorganic oxide is selected from oxides of at least one metal or metalloid selected from Si, Ti, Zr, Th, Nb, Ta, V, W, Y, Ca, Mg. et al.

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

4. Material according to any one of the preceding claims, wherein M ^{n +} is Fe ²⁺ , Ni ²⁺ , Fe ³⁺ , Co ²⁺ , Cu ²⁺ , or Zn ²⁺ .

Material according to any one of the preceding claims, wherein M 'is Fe ²⁺ or Fe ³⁺ or Co ³⁺ and m is 6; or M 'is Mo ⁵⁺ and m is 8.

Material according to any of the preceding claims, wherein [M '(CN) _m ] ^z - is [Fe (CN) ₆ ] ^{3 -} , [Fe (CN) ₆ ] [Co (CN) ₆ ] ³ or [Mo (CN) ₈ ] ^{3 "} .

7. Material according to claim 4, wherein the inorganic solid material exchanger of a metal cation has the formula K2M Fe (CN) 6, for example K ₂ Cu Fe (CN) ₆ , K ₂ ZnFe (CN) ₆ , K ₂ Co Fe (CN) ₆ .

8. Material according to any one of the preceding claims, wherein the nanoparticles have a sphere or spheroidal shape.

9. Material according to any one of the preceding claims, wherein the nanoparticles have an average size, such as a diameter, of 2 to 300 nm, preferably 2 to 100 nm, more preferably 2 to 50 nm.

10. The material as claimed in claim 1, in which the nanoparticle content of the at least one inorganic solid material exchanging a metal cation is from 0.5 to 15% by weight, preferably from 0.5 to 5% by weight. % in weight.

11. Process for preparing the material according to any one of claims 1 to 10 which comprises at least the following successive steps:

 a) an aqueous solution A of an organic surfactant is prepared, and the pH of this solution is adjusted to a value of 1.5 to 2.5, preferably 1.8 to 2.2, more preferably of 2;

b) adding M ^{n +} ions to solution A, optionally the pH of this solution to which M ^{n +} ions have been added is adjusted to a value of 1.5 to 2.5, preferably of 1.8 to 2 , 2, more preferably 2, whereby an aqueous solution B is obtained;

 c) dissolving a precursor of the inorganic oxide in solution B, whereby an aqueous solution C is obtained;

d) optionally, the pH of the aqueous solution C is adjusted to a value of 1.5 to 2.5, preferably 1.8 to 2.2, more preferably 2, and an aqueous solution of at least one metal fluoride, preferably at least one alkali metal fluoride such as KF, LiF, or NaF in solution C, whereby an aqueous solution D is obtained; e) an oil phase is added rapidly after the end of step d), with shearing mechanical stirring, to the aqueous solution D, whereby an oil-in-water emulsion E formed of droplets of the oily phase is obtained; dispersed in a continuous aqueous phase;

 f) maturation is carried out, mineralization of the emulsion E obtained in step e), whereby the monolith is formed;

 g) washing and / or drying the monolith;

 h) the monolith is calcined;

i) the monolith is impregnated with an aqueous solution of [Alk ⁺ _X ]

 j) washing and drying the impregnated monolith.

12. Process for separating at least one metal cation from a liquid medium containing it, wherein said liquid medium is brought into contact with the material according to any one of claims 1 to 10.

The method of claim 12, wherein said liquid medium is an aqueous liquid medium, such as an aqueous solution.

14. A method according to any one of claims 12 or 13, wherein said liquid medium is a liquid medium containing radionuclides, for example the liquid medium is selected from liquids and effluents from the nuclear industry, and activities involving radionuclides.

15. The method according to any one of claims 12 to 14, wherein the liquid medium is an aqueous solution containing in addition to said metal cation, salts such as NaCl at a high concentration, for example greater than 30 g / l.

The method of any one of claims 12 to 15, wherein said metal cation is present at a concentration of 0.1 picogram to 100 mg / L, preferably 0.1 picogram to 10 mg / L.

17. Process according to any one of claims 12 to 16, in which the metal cation is a cation of an element chosen from Cs, Co, Ag, Ru, Fe and Ti and the isotopes, in particular radioactive thereof, preferably, the cation is a ¹³⁴ Cs, or ¹³⁷ Cs cation.

18. A process according to any one of claims 12 to 17, wherein the liquid medium is an aqueous solution, containing as a metal cation a cation of ¹³⁴ Cs, and / or ¹³⁷ Cs, and further containing salts such as as NaCl at a high concentration, for example greater than 30 g / l.