Solid nanocomposite material based on hexa- or octacyanometallates of alkali metals, method for preparing same, and method for extracting metal cations

Abstract

Solid nanocomposite material comprising nanoparticles of a hexacyanometallate or octacyanometallate of an alkali metal and of a transition metal, of formula [Alk.sup.+.sub.x]M.sup.n+[M′(CN).sub.m].sup.z− in which Alk is an alkali metal, x is 1 or 2, M is a transition metal, n is 2 or 3, M′ is a transition metal, m is 6 or 8, z is 3 or 4, attached to at least one surface of a porous inorganic solid support, in which the nanoparticles are attached by adsorption to the at least one surface of the solid support, and in which the surface is a basic surface. Method for preparing this material. Method for extracting at least one metal cation from a liquid medium containing it, wherein the liquid medium is brought into contact with the material.

Claims

1. Solid nanocomposite material comprising nanoparticles of a hexacyanometallate or octacyanometallate of an alkali metal, and of a transition metal, having formula [Alk.sup.+.sub.x]M.sup.n+[M′(CN).sub.m].sup.z− where Alk is an alkali metal, x is 1 or 2, M is a transition metal, n is 2 or 3, M′ is a transition metal, m is 6 or 8, z is 3 or 4, attached to at least one surface of a solid inorganic porous support, wherein the nanoparticles are attached by adsorption to said at least one surface of the solid support, and wherein said surface is a basic surface.

2. The material according to claim 1, wherein the material has a nanoparticles content of 1 mass % to 20 mass % relative to the mass of the solid support.

3. The material according to claim 1, wherein M.sup.n+ is Fe.sup.2+, Ni.sup.2+, Fe.sup.3+, Co.sup.2+, Cu.sup.2+ or Zn.sup.2+.

4. The material according to claim 1, wherein M′ is Fe.sup.2+ or Fe.sup.3+ or Co.sup.3+, and m is 6; or else M′ is Mo.sup.5+, and m is 8.

5. The material according to claim 1, wherein Alk is Li, Na or K.

6. The material according to claim 1, 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.6].sup.3−.

7. The material according to claim 1, wherein the M.sup.n+ cations are Ni.sup.2+, Cu.sup.2+, Fe.sup.2+ or Fe.sup.3+ cations and the [M′(CN).sub.m].sup.z− anions are [Fe(CN).sub.6].sup.3− or [Fe(CN).sub.6].sup.4− anions.

8. The material according to claim 1, wherein the M.sup.n+ cations are Fe.sup.3+ cations and the [M′(CN).sub.m].sup.z− anions are [Mo(CN).sub.8].sup.3− anions.

9. The material according to claim 1, wherein the M.sup.n+ cations are Co.sup.2+ or Ni.sup.2+ cations and the [M′(CN).sub.m].sup.z− anions are [Co(CN).sub.6].sup.3− anions.

10. The material according to claim 1, wherein the nanoparticles have the formula K[Cu.sup.IIFe.sup.III(CN).sub.6] or K.sub.2[Cu.sup.IIFe.sup.II(CN).sub.6] or K[Ni.sup.IIFe.sup.III(CN).sub.6] or K.sub.2[Ni.sup.IIFe.sup.II(CN).sub.6].

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

12. The material according to claim 1, wherein the nanoparticles have a diameter of 3 nm to 30 nm.

13. The material according to claim 1, wherein the support comprises a material selected from metal oxides; metalloid oxides; mixed oxides of metals and/or metalloids; metal aluminosilicates; metal silicates; metal titanates; metal carbides; metalloid carbides; mixtures of metal oxides and/or metalloid oxides, and/or of mixed oxides of metals and/or metalloids; glasses; carbons; and composite materials comprising two or more materials among the aforementioned materials.

14. The material according to claim 13, wherein the metal oxides are selected from the group consisting of transition metal oxides and mixtures thereof; wherein the metalloid oxides are selected from the group consisting of silicon oxides and mixtures thereof; wherein the metal silicates are selected from the group consisting of zirconium silicates, tin silicates, cerium silicates, and compounds of mullite type (aluminium silicate) and cordierite type (aluminous ferromagnesian silicate); wherein the metal titanates are selected from the group consisting of tialite, metalloid titanates and mixtures thereof; wherein the glass is selected from the group consisting of borosilicate glasses; and wherein the carbons are selected from the group consisting of graphite, fullerenes and mesoporous carbons.

15. The material according to claim 1, wherein the support is in a form selected from particles, wherein the particles are selected from the group consisting of granules, beads, fibres, tubes, plates and flakes; membranes; felts; and monoliths.

16. The material according to claim 15, wherein the support is in the form of a powder consisting of particles with a particle size of 0.5 mm to 1 mm.

17. The material according to claim 16, wherein the particles are beads.

18. The material according to claim 1, wherein the support has a BET specific surface area of 5 to 500 m.sup.2/g.

19. Method for preparing the solid nanocomposite material according to claim 1, wherein the following successive steps are carried out: a) a solid support is provided; b) at least one surface of the solid support is made basic; c) the solid support of which at least one surface has been made basic is placed in contact with a solution containing the M.sup.n+ ion, then the solid support obtained is washed one or several times with water and optionally dried; d) the solid support obtained at the end of step c) is placed in contact with a solution containing a salt of [M′(CN).sub.m].sup.z−, and a salt of an alkali metal Alk, then the solid support thus obtained is washed one or several times with water and optionally dried; e) steps c) to d) are optionally repeated; f) if steps c) and d) are the last steps of the method, then during step c) the solid support obtained is washed one or several times with water, and during step d) the solid support thus obtained is washed one or several times and dried.

20. The method according to claim 19, wherein, during step b), at least one surface of the solid support is placed in contact one or several times with at least one basic solution until the pH value of the basic solution in contact with the surface is stabilised, stable, and remains stable at a desired basic value, whereby a solid support is obtained having at least one surface that has been made basic, the solid support is then separated from the basic solution, and the solid support having at least one surface that has been made basic is optionally dried.

21. The method according to claim 19, wherein the solution containing a salt of (M′(CN).sub.m).sup.z− and a salt of an alkali metal Alk, is an aqueous solution.

22. The method according to claim 21, wherein the salt of (M′ (CN).sub.m).sup.z− is a salt of formula [Alk.sub.z] [M′(CN).sub.m].

23. The method according to claim 19, wherein steps c) and d) are performed in static or batch mode, or in dynamic mode.

24. The method according to claim 23, wherein steps c) and d) are performed in a column.

25. The method according to claim 24, wherein when steps c) and d) are performed in the same column.

26. The method according to claim 19, wherein steps c) and d) are repeated 1 to 10 times.

27. The method according to claim 19, wherein the salt of [M′(CN).sub.m].sup.z is a salt of formula [Alk.sub.z] [M′(CN).sub.m].

28. Method for extracting at least one metal cation from a liquid medium in which it is contained, wherein said liquid medium is placed in contact with the material according to claim 1.

29. The method according to claim 28, wherein said liquid medium is an aqueous liquid medium.

30. The method according to claim 29, wherein the aqueous liquid medium is seawater or a brackish water.

31. The method according to claim 28, wherein said liquid medium is a liquid medium containing radionuclides.

32. The method according to claim 28, wherein the liquid medium is an aqueous solution which, additionally to said metal cation, contains salts.

33. The method according to claim 32, wherein said salts are present at a concentration higher than 30 g/L.

34. The method according to claim 28, wherein said metal cation is contained at a concentration of 0.1 picogram to 100 mg/L.

35. The method according to claim 28, wherein the metal cation is a cation of an element selected from among Cs, Co, Ag, Ru, Fe and Tl and their isotopes thereof.

36. The method according to claim 35, wherein the metal cation is a cation of .sup.134Cs or .sup.137Cs.

37. The method according to claim 28, wherein the liquid medium is an aqueous solution containing, as the metal cation, a cation of .sup.134Cs or .sup.137Cs, and additionally containing salts at a concentration higher than 30 g/L.

38. The material according to claim 1, wherein the support consists of a material selected from metal oxides; metalloid oxides; mixed oxides of metals and/or metalloids; metal aluminosilicates; metal silicates; metal titanates; metal carbides; metalloid carbides; mixtures of metal oxides and/or metalloid oxides, and/or of mixed oxides of metals and/or metalloids; glasses; carbons; and composite materials comprising two or more materials among the aforementioned materials.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a graph giving the adsorption capacity Q of caesium (in mg/g) of the material prepared in Example 3, as a function of time (in minutes), during the kinetic tests carried out in Example 5.

(2) FIG. 2 is a graph giving the breakthrough curve obtained in Example 5 with the material prepared in Example 4.

(3) The breakthrough curve gives the concentration of Cs at the column outlet (in mg/L) as a function of the volume having passed through the column (V in mL): this was done with regular sampling at the column outlet and analysis of the cations in solution is then carried out.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

(4) The first step of the method according to the invention consists in providing a solid support.

(5) There are no particular limitations as to the type of constituent material(s) of this solid support.

(6) In general, this support is made of one or more inorganic, mineral, materials.

(7) Preferred materials were cited above.

(8) There is no limitation as to the structure of the support and as to the constituent material thereof.

(9) Thus, the support is generally porous.

(10) It is first specified that the term «porous» such as used in the present invention relating to the support, means that this support contains pore or voids.

(11) Therefore, the density of this porous support is lower than the theoretical density of the same support that is non-porous and termed a solid material.

(12) The pores can be connected or isolated, but in the porous support according to the invention the majority of the pores are connected, communicate. In this case the terms open porosity or interconnected pores are used, but the method of the invention may also be implemented with a porous support not having interconnected pores.

(13) In general, in the support according to the invention, the pores are percolating pores connecting a first surface of said support with a second main surface of said support.

(14) In the meaning of the invention, a support is generally considered to be porous if its density is at most of about 95% of the theoretical density thereof.

(15) The porosity of the support may vary within broad limits, and may generally vary from 25% to 50%.

(16) Porosity is generally measured via nitrogen adsorption-desorption or using a mercury porometer.

(17) The support used in the method of the invention may possibly have only one type of porosity e.g. microporosity, mesoporosity or macroporosity.

(18) Or else the support used in the method of the invention may simultaneously have several types of porosities selected for example from among microporosity (pore size e.g. diameter generally less than 2 nm), mesoporosity (pore size e.g. diameter of 2 to 20 nm), and macroporosity (pore size e.g. diameter larger than 20 nm, e.g. up to 100 nm).

(19) The porosity may or may not be ordered, organized, structured e.g. mesostructured.

(20) There is also no limitation as to the size of the support, and the support size may vary between wide limits.

(21) Thus, the support may be a support of nanoscopic size i.e. of a size (defined by its largest dimension) of 50 nm to 100 nm, a microscopic support i.e. of a size of 100 nm to 1 mm, or a macroscopic support i.e. of a size larger than 1 mm.

(22) The support may have all kinds of forms as already described above.

(23) For example, the support may be in the form of particles such as spheres (beads) or spheroids, fibres, tubes in particular carbon nanotubes, or plates.

(24) However, to allow use of the support in a continuous extraction method implemented in a column in particular, it is generally preferred that the support be in the form of particles forming a powder. It is additionally preferred that this powder has a particle size which reduces potential head losses. An ideal particle size is 0.5 mm to 1 mm.

(25) Particle size is defined by their largest dimension which is the diameter for spheres or spheroids.

(26) Another preferred form for the support and which also allows reduced head loss (for flow operation) is the form of monoliths generally having a size of at least 5 mm and containing macropores.

(27) Advantageously, the support has a BET specific surface area of 50 to 500 m.sup.2/g, preferably of 100 to 200 m.sup.2/g measured by nitrogen adsorption-desorption or mercury porosimetry.

(28) In the prior art methods, such as the method of document [5], the support is generally washed, e.g. with ultra-pure water one or several times and then dried e.g. in an oven at a temperature of 120° C. for 24 hours before performing the other steps of the method. In the method according to the invention, there is no reason for this preparation of the support by washing and drying, and they are not necessary.

(29) In a first step, at least one surface of the support is made basic.

(30) Advantageously, the support is placed in contact with a basic solution, preferably an aqueous basic solution.

(31) This contacting of the support with the basic solution may be carried out in dynamic mode i.e. the solid support is placed in contact with a stream of the solution circulating in an open circuit.

(32) Or else, contacting may be carried out in batch mode under agitation.

(33) The total contacting time is generally of about 2 hours.

(34) For example, it is possible to place the solid support in an aqueous basic solution contained in a vessel. The contacting time (in batch mode under agitation) is very short (instantaneous or near-instantaneous) and is solely used to wet the support.

(35) To this aqueous solution a concentrated basic solution is progressively added, preferably a KOH solution of pH 13. Throughout this addition, the pH of the solution in contact with the support is measured and the addition is halted when the pH of the solution in contact with the support is stable at a set, desired value e.g. between 8 and 10, and when this pH remains stable at this desired value for a time of 10 to 15 minutes, for example 10 minutes.

(36) This step is of short duration e.g. of about 2 hours.

(37) At the end of this step, the solid support is separated from the solution (e.g. by filtration or decanting) and, without any other treatment being required, the solid support is placed in contact with the aqueous solution containing the M.sup.n+ salt.

(38) At the end of this first step, a solid support is therefore obtained having one surface that has been made basic.

(39) Preferably, all the accessible surfaces of the support are made basic.

(40) The steps that will now be described to prepare nanoparticles on a surface made basic of the solid support are substantially similar to those of the method described in document WO-A1-2016/038206 [5]. In particular, according to the invention, during step d) the solid support obtained at the end of step c) is placed in contact with a solution containing a salt of (M′(CN).sub.m).sup.z−, for example a salt of formula [Alk.sub.z] [M′(CN).sub.m], and also a salt of an alkali metal Alk differing from the complex, and not with a solution only containing a complex or salt of (M′(CN).sub.m).sup.z−, e.g. a salt of formula [Alk.sub.z] [M′(CN).sub.m].

(41) Reference may therefore be made to this document in particular with respect to the reagents and operating conditions employed in these steps, but also for the description of the nanoparticles.

(42) In a second step, the nanoparticles are therefore grown on said surface, made basic, of the solid support.

(43) This growth is obtained in two successive steps, optionally repeated.

(44) These are steps c) and d) of the method according to the invention, the succession of which amounts to what may be called an impregnation cycle, said cycle optionally being repeated.

(45) The solid support of which one surface has been made basic is first placed in contact with an aqueous solution containing the M.sup.n+ ion, generally in the form of a metal salt.

(46) The solvent of this solution is therefore water and preferably ultra-pure water.

(47) The metal salt contained in this solution is a salt of a metal generally selected from among metals able to give a cyanometallate of this metal, such as a hexacyanoferrate of this metal, which is insoluble.

(48) This metal may be selected from among all transition metals e.g. from among copper, cobalt, zinc, nickel and iron etc. The M.sup.n+ ion may therefore be selected from among Fe.sup.2+, Ni.sup.2+, Fe.sup.3+, Co.sup.2+, Cu.sup.2+ and Zn.sup.2+ ions.

(49) For example, the metal salt may be a nitrate, a sulfate, a chloride, an acetate, a tetrafluoroborate, optionally hydrated, of one of these metals M.

(50) For example, it may be copper nitrate.

(51) The concentration of the metal salt in the solution is 0.01 to 1 mol/L, more preferably 0.1 to 0.5 mol/L.

(52) The amount of salt used is preferably about 0.1 to 1 mmol/g of treated solid support.

(53) The contacting, that may also be qualified as impregnation of the solid support, is generally conducted at ambient temperature, and generally lasts 4 to 96 hours.

(54) This contacting may be performed in static mode also called batch mode, preferably under agitation, in which case it generally lasts 12 to 96 hours, or else in dynamic mode (in a fixed or fluidized bed) in which case it generally lasts 4 to 24 hours.

(55) At the end of this contacting, a solid support is obtained in which the M.sup.n+ cations are adsorbed on, to the surface made basic of the support.

(56) At the end of contacting, the solid support is washed.

(57) In batch mode, generally the solid support is removed from the solution and washed.

(58) In dynamic mode, the solid support is not removed from the solution but washed directly.

(59) Washing consists in washing one or several times the solid support, for example 1 to 3 times, preferably with water.

(60) This washing operation allows excess metal salt to be removed and a stable product to be obtained having a perfectly defined composition.

(61) Drying is then optionally carried out, which is possible but not necessary.

(62) The solid support that has reacted with the M.sup.n+ metal cation as described above is then placed in contact with a solution of a salt of (M′(CN).sub.m).sup.z−, e.g. a salt of formula Alk.sub.z[M′(CN).sub.m], more preferably a solution of a salt of formula K.sub.z[M′(CN).sub.m]; and also of a salt of an alkali metal Alk.

(63) The alkali metal Alk may be selected from among Li, Na, and K, preferably Alk is K.

(64) The alkali metal salt is different, distinct from the salt of (M′(CN).sub.m).sup.z− that is also contained in the solution.

(65) The alkali metal salt can be selected for example from among the nitrates, sulfates and halides (chlorides, iodides, fluorides) of an alkali metal Alk, such as potassium nitrate.

(66) Advantageously, the solvent of this solution is water and preferably ultra-pure water.

(67) The contacting, which can also be termed impregnation of the solid support, is generally conducted at ambient temperature and generally lasts 2 to 96 hours.

(68) This contacting may be performed in static or batch mode preferably under agitation, in which case it generally lasts from 12 to 96 hours, or else in dynamic mode in which case it generally lasts from 2 to 24 hours.

(69) The (M′(CN).sub.m).sup.z− salt generally meets the following formula:

(70) (Alk).sub.z[M′(CN).sub.m], where M′, m and z have the meaning already given above, and Alk is a monovalent cation selected from among the cations of alkali metals Alk such as K or Na, more preferably the (M′(CN).sub.m).sup.z− salt meets the following formula: K.sub.z[M′(CN).sub.m], for example K.sub.4Fe(CN).sub.6.

(71) The concentration of the salt of an alkali metal in the solution is generally 0.001 to 1 mol/L, preferably 0.001 to 0.05 mol/L.

(72) Advantageously, the concentration of the (M′(CN).sub.m).sup.z− salt, and the concentration of the salt of an alkali metal are the same.

(73) Also, the solution of the [M′(CN).sub.m].sup.z− salt employed is generally prepared so that the weight ratio of the salt to the quantity of impregnation support constituted by the initial solid support is preferably 0.1 to 5 mmol/g of solid support.

(74) In this manner, the fixation, immobilization is obtained of the [M′(CN).sub.m].sup.z− anionic portion, for example [Fe(CN).sub.6].sup.4− of the [M′(CN).sub.m].sup.z salt e.g. of formula Alk.sub.z[M′(CN).sub.m], on the M.sup.n+ cations and simultaneous insertion of the alkali metal in the structure of the crystal. This fixation, immobilization occurs via formation of bonds of covalent type which are relatively strong depending on the medium, and this fixation, immobilization is generally quantitative i.e. all the M.sup.n+ cations react. Fixation, immobilization is therefore not at all random.

(75) Alkali insertion in all the sites of the crystal structure is possible first through the optional presence of an alkali in the salt such as Alk.sub.z[M′(CN).sub.m], but above all through the additional presence of an alkali metal salt added to the impregnation solution.

(76) This is why it is generally preferable that it is the same alkali metal Alk that is contained in the [M′(CN).sub.m]Z salt, such as Alk.sub.z[M′(CN).sub.m], and in the alkali metal salt of the impregnation solution.

(77) At the end of contacting, the solid support is washed.

(78) In batch mode, the solid support is generally removed from the solution and then washed.

(79) In dynamic mode, the solid support is not removed from the solution but is washed directly.

(80) Washing consists in washing one or several times, for example 1 to 3 times, the support, preferably with the same solvent as the solvent of the complex solution, such as ultra-pure water.

(81) The objective of this washing operation is to remove the salts of [M′(CN).sub.m].sup.z and the nanoparticles that have not been attached on the M.sup.n+ cations, and allows a solid support to be obtained in which there no longer exists any free, non-bound [M′(CN).sub.m].sup.z− which could be released.

(82) The succession of contacting steps of the solid support with the M.sup.n+ metal cation and washing steps (one or more times), followed by contacting of the solid support with a solution containing a [M′(CN).sub.m].sup.z− salt, e.g. a salt of [M′(CN).sub.m].sup.3−, and a salt of an alkali metal, then washing (one or several times), may only be performed once, or else it can be repeated, generally 1 to 10 times, for example 1 to 4, 5, 6 or 7 times; it is therefore possible to perfectly adjust the extraction capacity of the material according to the invention.

(83) The weight content of mineral fixer, immobilizing agent e.g. of hexacyanoferrate of insoluble metal and of alkali metal of formula [Alk.sup.+.sub.x]M.sup.n+[M′(CN).sub.m].sup.z− is generally 1 to 10% relative to the weight of the solid support.

(84) The invention will now be described with reference to the following examples given for illustration and nonlimiting.

EXAMPLES

Example 1

(85) In this example, it is shown how the surface of dry, crude silica may be made basic.

(86) (1) Determination of the pH of Dry, Crude Silica

(87) To determine the pH of dry, crude silica, a few grams of dry crude silica not having undergone any pre-treatment, were placed in contact with a few drops of Bromothymol Blue.

(88) The different possible forms of Bromothymol Blue, and the different colours that can be exhibited by Bromothymol Blue depending on pH are indicated in Table 1 below.

(89) TABLE-US-00001 TABLE I Colours of Bromothymol Blue acid colour change acid colour change basic form 1 range form 2 range form magenta about pH 0 yellow pH 6.0 to pH 7.6 blue

(90) It was visually observed that dry crude silica not subjected to any pre-treatment and placed in contact with a few drops of Bromothymol Blue displays a yellow colour.

(91) This yellow colouring shows that the pH of dry crude silica is lower than 6.

(92) (2) Pre-Treatment of Silicas—Preparation of Pre-Treated Silicas Having their Surface Made Basic by Contacting with Basic Solutions of KOH. Pre-treatment of crude silica was carried out to determine whether it is possible to make the surface thereof basic. This pre-treatment step therefore consists in: Preparing a highly concentrated basic solution of KOH at a very high pH of 14. Adding, dropwise, this basic KOH solution of pH 14 thus prepared to three suspensions respectively containing the solid silica, continually monitoring the pH of the mixture of basic solution and dispersion. KOH reacts with the silica.

(93) The adding of the basic solution was halted when the pH of the mixture of basic solution and dispersion reached a required, desired value and is stabilised at a required, desired basic value, namely respectively a value of 9 (pre-treated silica I), 10 (pre-treated silica II) or 11 (pre-treated silica III).

(94) These pH values are called stabilised pH values at 9, 10 and 11 respectively.

(95) (3) Measurement, Verification of the Surface pH of the Pre-Treated Silicas.

(96) To verify the pH of the surface of the three pre-treated silicas I, II, and III, prepared as described above, a few grams of each of these different pre-treated and dried silicas I, II and III were placed in contact with another colour indicator specific to the basic pH range: Phenolphthalein.

(97) It is recalled that Phenolphthalein is of pink colour in the pH range of 9 to 10, and is of a violet colour for a pH higher than 10.

(98) The colours obtained after contacting each of the three pre-treated silicas I, II and III with Phenolphthalein were visually observed.

(99) The different colours obtained show that the pre-treatment applied to the silica leads to an increase in the pH of the material, and hence of the surface.

(100) After contacting with Phenolphthalein, the three pre-treated silicas displayed colours respectively corresponding to a pH of 10.8 (violet) for pre-treated silica III, to a pH of 10 (pink) for pre-treated silica II, to a pH of 9 (pale pink) for pre-treated silica I.

(101) This example shows that it is possible to control the surface pH of silica depending on the pre-treatment conducted with a basic KOH solution, more exactly depending on the pH of the basic KOH solution used for pre-treatment of the silica.

(102) Three silicas having a different surface pH (namely 10.8; 10; 9) depending on the pH of the basic KOH solution used for pre-treatment (namely 11; 10; 9 respectively) were able to be obtained as confirmed by the visual observations made.

Example 2

(103) In this example, a material according to the invention was prepared by inserting nanoparticles of stoichiometric copper-potassium ferrocyanide within a gel of porous silica the surface of which had been made basic by successive contacting of this surface with a basic KOH solution of pH 9.

(104) The caesium extraction capacity of this material was then measured.

(105) Preparation of the Material According to the Invention.

(106) In this example, the preparation method according to the invention was implemented on a support which is a commercial porous silica gel, in the form of granules of particle size 200-500 μm, pore size of 30 nm, and having a BET specific surface area of about 130 m.sup.2/g.

(107) The operating mode for insertion of copper-potassium ferrocyanide (K.sub.2Cu(Fe(CN).sub.6)) within this porous silica gel comprised following steps 1 and 2:

(108) 1. Modifying the silica surface to make it basic through successive contactings of this surface with a basic KOH solution.

(109) 1.1 1.sup.st contacting: About 10 g of silica gel (such as described above) were placed in contact with 250 mL of a KOH solution of pH 9. During this 1.sup.st contacting, the pH of the solution that was initially 9 decreased immediately by two pH units. Agitation for 24 h followed by recovery of the solid by removing the supernatant.

(110) 1.2. 2.sup.nd contacting with 250 mL of KOH solution of pH 9: the pH of the solution, initially of 9, decreased by about 1 pH unit. Agitation for 24 h, followed by recovery of the solid by removing the supernatant.

(111) 1.3. 3.sup.rd contacting with 250 mL of KOH solution of pH 9: the pH of the solution, after 24 h agitation, remained stable, and is pH 9. The solid was recovered by removing the supernatant, and then drying in air for 24 h.

(112) 2. Impregnation step with precursor solutions of copper-potassium ferrocyanides.

(113) 2.1. A first solution of copper nitrate (Cu(NO.sub.3).sub.2) (of concentration 5.Math.10.sup.−2 mol/L in Cu.sup.2+ (i.e. 3.2 g/L)) was prepared. The pH of this solution was 4.5.

(114) 1 g of silica gel modified as described in step 1 was placed in contact with 10 mL of this copper nitrate solution for 2 hours. The solid was then recovered and optionally rinsed with ultra-pure water. No drying step is required here.

(115) At the end of this step, the copper was inserted in the silica in the form of mono- or polynuclear oxo-hydroxo species of hydrated copper, made possible by the basic nature of the silica surface. This synthesis route led to the insertion of a significant quantity of copper species within the silica gel as shown by the blue colouring of the support.

(116) 2.2. A second solution containing potassium ferrocyanide and potassium nitrate each at a concentration of 10.sup.−1 mol/L was prepared.

(117) 1 g of copper-loaded silica gel was placed in contact with 10 mL of this solution for 2 hours. This step led to the formation of stoichiometric copper-potassium ferrocyanide with a potassium ion in the cages of the structure. The solid was recovered, and optionally rinsed with ultra-pure water.

(118) The contacting with (Cu(NO.sub.3).sub.2) and then (K.sub.4Fe(CN).sub.6+KNO.sub.3) is called the «impregnation cycle». This impregnation cycle is repeated twice, whereby the desired material according to the invention is obtained.

(119) Extraction capacity is directly related to the number of impregnation cycles performed (see Example 2), and it is therefore possible to modulate this capacity very easily when synthesising the material.

(120) The material is then rinsed with ultra-pure water and dried in air for 48 hours.

(121) Measurement of the Caesium Extraction Capacity of the Material According to the Invention Prepared Above.

(122) Measurement of the extraction capacity vis à vis Cs of the material according to the invention, prepared as described above with 2 impregnation cycles, was carried out by using the so-called standard test on a 100 mg/g caesium nitrate solution.

(123) 50 mg of the material according to the invention prepared as described above were placed in 50 mL of this 100 mg/g caesium nitrate solution for about twenty hours. Analysis of Cs concentration was performed in the initial and final solutions to evaluate the adsorption capacity of the material. This adsorption capacity is expressed as follows:

(124) $Q_e=\left({\left[\mathrm{Cs}\right]}_i-{\left[\mathrm{Cs}\right]}_f\right)\frac{V}{m}$

(125) where [Cs].sub.i and [Cs].sub.f are the initial and final Cs concentrations respectively, analysed by atomic adsorption; V is the volume of solution used and m is the mass of material used.

(126) The measured capacity in this example for the material according to the invention prepared as described above was 12 mg/g.

Example 3

(127) In this example, a material according to the invention was prepared by inserting nanoparticles of stoichiometric copper-potassium ferrocyanide within a porous silica gel having one surface made basic by contacting this surface with a basic KOH solution the pH of which was continuously monitored and adjusted.

(128) The caesium extraction capacity of this material was then measured.

(129) Preparation of the Material According to the Invention

(130) In this example, the preparation method according to the invention was implemented with the same support as in Example 2, but adjustment of pH at step 1 was controlled. The effect of the number of impregnation cycles on the properties of the material was also studied.

(131) Modification of the Silica Surface to Make it Basic by Controlled Addition of a Basic KOH Solution.

(132) About 5 g of silica gel (such as described above) were placed in contact in a beaker with 250 mL of a KOH solution of pH 10. For agitation a stirring blade was used directly in the beaker. The pH of the solution was monitored in-line using a pH meter and the pH was stabilised at 10 through successive additions of a mother KOH solution of pH 13. After stabilisation of the pH of the solution at 9.3 (hence after the addition of 18 mL of KOH solution of pH 13), corresponding to a test time of about 1.5 h, the supernatant was removed and the impregnation steps (2) performed.

(133) Impregnation Step with Precursor Solutions of Copper-Potassium Ferrocyanides.

(134) A first solution of copper nitrate (Cu(NO.sub.3).sub.2) (of concentration 10.sup.−1 mol/L in Cu.sup.2+ (i.e. 6.4 g/L)) was prepared.

(135) 5 g of modified silica gel such as described in step 1 were placed in contact with 10 mL of this copper nitrate solution for 2 hours. The solid was then recovered and optionally rinsed with ultra-pure water. No drying step is necessary here. At the end of this step, analysis of the copper solution before and after contacting showed incorporation of Cu in the silica gel of approximately 0.5 weight % of Cu (5 mg of Cu per g of solid).

(136) Then, as previously, a second solution containing potassium ferrocyanide and potassium nitrate at concentrations of 10.sup.−1 Mol/L was prepared.

(137) 5 g of copper-loaded silica gel were placed in contact with 10 mL of this solution. This step led to the formation of stoichiometric copper-potassium ferrocyanide with a potassium ion in the cages of the structure.

(138) Contacting with (Cu(NO.sub.3).sub.2) and then (K.sub.4Fe(CN).sub.6+KNO.sub.3) is called the «impregnation cycle». This impregnation cycle is repeated once or twice. After each impregnation cycle, a small amount of material is sampled to measure the Cs extraction capacity and to study the effect of the number of cycles on the properties of the material.

(139) Measurement of the Extraction Capacity Vis a Vis Caesium of the Material According to the Invention Prepared Above, as a Function of the Number of Impregnation Cycles.

(140) This extraction capacity was measured in identical manner to the description in Example 1.

(141) Cycle 1: the measured capacity was 15.76 mg/g

(142) Cycle 2: the measured capacity was 18.14 mg/g

(143) No trace of Cu, Fe was found by ICP-AES analysis of the 2 corresponding final solutions of caesium nitrate (the content of these elements was therefore below the detection limit).

(144) It therefore appears that a single impregnation cycle is sufficient to reach satisfactory extraction capacities. This point is of importance to reduce production costs by limiting steps and hence reducing production time.

Example 4

(145) In this example, a material according to the invention was prepared by inserting nanoparticles of stoichiometric copper-potassium ferrocyanide in a porous silica gel. However, 100 g of silica gel were used instead of 5 g or 10 g to evaluate the effect, on this synthesis route, of the change in scale in the starting quantity of silica gel, with a view to industrialization.

(146) The extraction capacity vis à vis caesium of this material was then measured.

(147) Preparation of the Material According to the Invention.

(148) 100 g of silica gel were placed in contact with 250 mL of a solution having an initial pH of 9 which was adjusted with KOH. The pH was continuously measured, the suspension being agitated with a stirring blade. The pH was then continuously adjusted through the addition of a solution of KOH of pH 13, until the pH became stable at 9.5. This test lasts a time of approximately one to two hours.

(149) At the end of this step, the supernatant was removed.

(150) The impregnation steps were then carried out as described in the preceding examples with 3 impregnation cycles.

(151) Measurement of Extraction Capacity Vis a Vis Caesium of the Material According to the Invention, Prepared Above, as a Function of the Number of Impregnation Cycles.

(152) The extraction capacity was measured between each cycle.

(153) The measured extraction capacities were the following:

(154) Cycle 1: 13.81 mg/g

(155) Cycle 2: 18.01 mg/g

(156) Cycle 3: 34.4 mg/g

(157) No trace of Cu, Fe was found by ICP AES analyses on these 3 corresponding final solutions of caesium nitrate (the content of these elements was therefore lower than the detection limit).

(158) These results validate the possible production of the material according to the invention on a larger scale than in Examples 2 and 3, namely 100 g under «mild» condition only having recourse to water as solvent and with rapid synthesis times, namely about 6 hours for a complete cycle.

Example 5

(159) In this example the caesium extraction and adsorption kinetics by the material according to the invention prepared in Example 4 were studied, and a caesium extraction test by the material according to the invention prepared in Example 4 was carried out in a column.

(160) Kinetic Tests

(161) With a view to use this material in a column, kinetic tests were carried out on the material prepared in Example 4.

(162) These kinetic tests consist in placing 50 mg of material into contact with 50 mL of effluent, under agitation, for different times.

(163) Each kinetic point corresponds to one measurement of the extraction capacity for one contact time.

(164) The initial solution was a solution of ultrapure water in which 100 mg/L of Cs were dissolved in the form of CsNO.sub.3. The Cs in the initial solution and final solutions was analysed by atomic adsorption spectroscopy. The kinetic results are given in FIG. 1.

(165) FIG. 1 shows that the adsorption kinetics are extremely rapid (a few minutes).

(166) In addition, analyses of K in solution show total exchange between the K of the solid and the Cs of the solution, with no release salting-out of Cu, Fe or Si.

(167) Tests in Column

(168) The properties of the material prepared in Example 4 were evaluated by determining a breakthrough curve.

(169) For this purpose, 2 g of material prepared in Example 4 were placed in a column of diameter 1 cm. This amount of material corresponded then to a column height of 5 cm.

(170) A solution containing 70 mg/L of Cs in the form of CsNO.sub.3 was passed through the column at a flow rate of 160 mL/h, corresponding to a linear velocity of 2 m/h, and the concentration at the outlet of the column was measured by atomic adsorption spectroscopy at regular intervals. A breakthrough curve was thus obtained. This breakthrough curve is illustrated in FIG. 2.

(171) The profile of this breakthrough curve is of particular interest since it shows that this material is well suited, in fixed bed form, for implementation in an industrial Cs extraction process.

Example 6

(172) In this example, a material according to the invention was prepared by insertion of nanoparticles of stoichiometric nickel-potassium ferrocyanide in a porous silica gel having its surface made basic through successive contacting of this surface with a basic KOH solution of pH 9.

(173) The extraction capacity vis à vis caesium of this material was then measured.

(174) This objective of this example was to validate the possible extension of the synthesis method to other types of ferrocyanide.

(175) In this example it was therefore sought to obtain a material containing nickel-potassium ferrocyanides, known to have good extraction efficiency vis à vis caesium in an acid medium.

(176) Preparation of the Material According to the Invention.

(177) The step to modify the surface of the silica gel support with a basic solution was the same as described in Example 2.

(178) The impregnation step differed in that the copper salt was replaced by a nickel salt at the first impregnation phase. A first solution of nickel sulfate (Ni(SO.sub.4).sub.2) was therefore prepared (Ni.sup.2+ concentration 10.sup.−1 mol/L).

(179) 1 g of «basified» silica gel was placed in contact with 10 mL of this nickel sulfate solution for 2 hours. The solid was then recovered and rinsed with ultrapure water. No drying was required here. Then, in Example 2, a second solution was prepared containing potassium ferrocyanide and potassium nitrate, each at a concentration of 10.sup.−1 Mol/L. 1 g of nickel-loaded silica was placed in contact with 10 mL of this solution. This step led to the formation of stoichiometric nickel-potassium ferrocyanide with a potassium ion in the cages of the structure. This impregnation cycle was repeated 3 times and the extraction capacity was then measured.

(180) Measurement of the Extraction Capacity Vis a Vis Caesium of the Material According to the Invention Prepared Above, after 3 Impregnation Cycles.

(181) This extraction capacity was measured in the same manner as described in Example 2.

(182) Cycle 3: the measured capacity was 48 mg/g.

(183) Analysis of the potassium released by the material corresponded to pure Cs<->K exchange. The concentration of Ni and Fe was below the detection limit.

(184) The synthesis method of the invention may therefore be used for other types of ferrocyanide, such as nickel-potassium ferrocyanide, leading to a material in which exchange with Cs takes place solely with the potassium of the structure.

REFERENCES

(185) [1] WO-A2-2010/133689. [2] H. Mimura, M. Kimura, K. Akiba, Y. Onodera, «Selective removal of cesium from highly concentrated sodium nitrate neutral solutions by potassium nickel hexacyanoferrate(II)-loaded silica gels», solvent extraction and ion exchange, 17(2), 403-417, (1999). [3] L. Sharigyn, A. Muromskiy, M. Kalyagina, S. Borovkov, “A granular inorganic cation-exchanger selective to cesium”, J. Nuclear Science and Technology, 44 (5), 767-773, (2007). [4] Sanhita Chaudhury, A. K. Pandey; A. Goswami, «Copper ferrocyanide loaded track etched membrane an effective cesium adsorbent», J. Radioanal. Nucl. Chem 204 (2015) 697-703. [5] WO-A1-2016/038206.