Continuous process for separating strontium cations from a liquid medium, with a material comprising a geopolymer and particles of an ion exchanger

Abstract

The present invention relates to a continuous process for separating at least one strontium cation, and in particular at least one cation of a radioactive isotope of strontium such as 90Sr, from a liquid medium containing same, in which process said liquid medium is brought into contact with a solid mesoporous material, comprising an inorganic matrix consisting of a mesoporous geopolymer having an open mesoporosity, said geopolymer having a single porosity scale, and particles of at least one solid inorganic exchanger compound which is selective for the strontium cations and is distinct from the geopolymer, these particles being distributed in the open mesoporosity of the inorganic matrix and being accessible to the strontium cations contained in the liquid medium; said solid mesoporous material being in the form of non-pulverulent grains forming a fixed bed, in particular forming a column packing.

Description

[0001] CONTINUOUS PROCESS FOR SEPARATING STRONTIUM CATIONS FROM A LIQUID MEDIUM, WITH A MATERIAL COMPRISING A GEOPOLYMER AND ION EXCHANGER PARTICLES

[0002] TECHNICAL FIELD

[0003] The invention relates to a continuous process for separating strontium (Sr) cations from a liquid medium, with a material comprising a geopolymer and particles of an ion exchanger, said material being in the form of non-powdery particles forming a fixed bed, for example forming a packing of a column.

[0004] A continuous process means a process that is carried out continuously.

[0005] The invention finds its application more particularly in the field of the treatment of liquid effluents and, in particular, the treatment of radioactive or toxic liquid effluents with a view to eliminating strontium cations therefrom, in particular cations of radioactive isotopes of strontium such as 90 Sr.

[0006] It should be noted that, in this document, the terms "adsorbent", "ion exchanger", "cation exchanger" and "cation exchanger" may be used interchangeably and all designate a "cation exchanger".

[0007] STATE OF THE PRIOR ART

[0008] The materials currently used in columns to decontaminate liquids containing strontium (Sr) cations - and in particular radioactive isotopes of the latter, including strontium 90 - are materials based on powders agglomerated in the form of granules.

[0009] These granules generally include an ion exchanger such as a zeolite or a crystalline silicotitanate for the most selective strontium cations as well as a binder.

[0010] The binder in the granules is generally non-porous or slightly porous, which leads to a loss of efficiency when using these granules in column or fixed-bed treatment processes. Indeed, the interior of the granules is difficult to access, or even not at all, for the effluent to be treated and the specific exchange surface is therefore very low.

[0011] Thus, these materials have slow sorption kinetics (due to the low porosity or even the absence of porosity of the binder) and a very low proportion of the available active sites is used, which leads to a reduction in the Sr decontamination capacities and consequently an increase in the quantity of material required for the decontamination of a given volume.

[0012] Furthermore, numerous publications concerning the use of geopolymers for effluent treatment are available in the literature.

[0013] Geopolymers for Sr cation decontamination are not selective and this induces the use of a large quantity of materials and therefore also the formation of a large quantity of waste.

[0014] Document [1] describes the synthesis of geopolymers and geopolymer-based composite materials including additives. However, this document does not mention any specific application for these composite materials, in particular it does not mention any application of these materials for the treatment of effluents in fixed-bed processes, and even less for the selective extraction of Sr in fixed-bed processes.

[0015] In the context of the specific application of the invention for the selective extraction of Sr in fixed bed processes, the additive must be carefully identified and selected before its integration into the geopolymer, in order to have selective ion exchange properties with respect to Sr depending on the effluents to be treated. This is absolutely not mentioned in document [1].

[0016] Document [1] also does not provide any guidance that could ensure optimal use of the material in this document in a fixed bed, column process. Indeed, such use requires specific shaping of the material filling the column in order to ensure that the properties of this material, such as capacity and selectivity, particularly with respect to Sr, are the best, while limiting pressure losses throughout the installation incorporating the column.

[0017] Document [2] describes a method for preparing a material consisting of a monolithic geopolymer foam comprising nanoparticles of a metal coordination polymer with CN ligands corresponding to the formula [Alk + x]M n+ [M'(CN)m] z- .

[0018] This document also relates to the foam prepared by this process, and the use of this foam for separating a metal or metalloid ion from a stream containing said metal or metalloid ion.

[0019] The process for preparing this foam comprises several steps. During a first step a), a geopolymer foam is brought into contact with a solution containing at least one M ion n+ . Then, during a second step b), the geopolymer foam obtained at the end of step a) is brought into contact with a solution of a salt or complex of [M'(CN) m ] z “ and at least one alkali metal salt, whereby the geopolymer foam comprising nanoparticles of a metal coordination polymer is obtained.

[0020] This multi-step process complicates the overall synthesis of the material, making it difficult to scale up to industrial scale. This process also limits the amount of coordination polymer that can be introduced into the geopolymer.

[0021] Indeed, the coordination polymer nanoparticles are only coated on the surface of the macropores of the foam and are not present in the mesoporous walls of the latter, which limits the contact surface with an effluent and therefore the total capacity of the material.

[0022] Furthermore, the method described in document [2] applies only to metal coordination polymer nanoparticles with CN ligands, which are not suitable in particular for the selective decontamination of Sr.

[0023] Document [3] describes a solid material with open and at least partially interconnected multiple porosity, comprising a matrix of a microporous and mesoporous geopolymer, in which are defined at least partially interconnected open macropores delimited by walls or walls of microporous and mesoporous geopolymer, and particles of at least one solid compound distinct from the geopolymer being distributed in the macropores and / or in the walls or walls.

[0024] This material therefore contains macropores with sometimes thin walls and sharp junctions due to the interconnection of the porosity. These walls therefore have weakened mechanical strength, particularly when an effluent passes through, which can cause their degradation and the formation of fine particles likely to block a column.

[0025] Document [3] also describes the synthesis of this material. To create the macroporous network, the synthesis process involves the formulation of a water-in-oil emulsion containing precursors of the material as well as the removal of the oil phase once the material has hardened. This greatly complicates the synthesis process and makes the material difficult to industrialize.

[0026] Document [4] describes the synthesis of a mesoporous material comprising a geopolymer and zeolite particles, obtained by mixing two suspensions, followed by setting of the geopolymer. This document does not describe the use of this family of materials in column processes, nor for the extraction of Sr.

[0027] There is therefore, in view of the above, a need for a process which makes it possible to improve the performance of fixed-bed Sr decontamination processes, namely which makes it possible to obtain a high Sr fixation capacity and significant selectivity with respect to Sr as well as rapid sorption kinetics and optimized column dynamics.

[0028] The aim of the present invention is, among other things, to meet the needs for such a method, listed above.

[0029] The aim of the present invention is also to provide such a method which does not have the defects, limitations and disadvantages of the methods of the prior art, in particular of the methods described in the documents of the prior art cited above, and which solves the problems of these methods.

[0030] STATEMENT OF THE INVENTION

[0031] This aim, and others, are achieved, in accordance with the invention, by a continuous process for separating at least one cation of strontium, and in particular at least one cation of a radioactive isotope of strontium, such as 90 Sr, from a liquid medium containing it, in which said liquid medium is brought into contact with a mesoporous solid material, comprising an inorganic matrix made of a mesoporous geopolymer having an open mesoporosity, and particles of at least one inorganic solid compound selectively exchanging strontium cations, distinct from the geopolymer, these particles being distributed in the open mesoporosity of the inorganic matrix and accessible to the strontium cations contained in the liquid medium; said mesoporous solid material being in the form of non-powdery grains forming a fixed bed, for example forming a packing of a column.

[0032] By "non-powdery" we mean that the grains obtained are large enough not to be emitted into suspension in the air, nor transported there.

[0033] In the following, particles of the at least one inorganic solid selective strontium cation exchange compound, distinct from the geopolymer, are sometimes referred to as active particles, for the sake of simplification.

[0034] The term "particles" therefore applies to the inorganic solid compound that is a selective exchanger of strontium cations.

[0035] The inorganic matrix in a geopolymer can also be referred to as the "geopolymer binder" or "geopolymer skeleton."

[0036] It should be noted that the term "grains" applies to the mesoporous solid material as a whole, which is a geopolymer / particle composite that constitutes the fixed bed in the form of a plurality of discrete grains such as granules, extrudates or beads.

[0037] The method according to the invention differs from the methods according to the prior art in that it uses a specific material, a combination of a geopolymer with specific porosity and active particles specifically chosen for the purpose of selectively fixing the Sr cations, distributed in the mesoporosity and accessible via the mesoporosity, making it possible to obtain a high Sr fixing capacity, a high selectivity with respect to Sr, very rapid sorption kinetics and optimized column dynamics.

[0038] These capacities, selectivity and kinetics, are in any case higher than those obtained with prior art processes using other materials.

[0039] It has been demonstrated, surprisingly, that the process according to the invention makes it possible to obtain a high capacity for fixing strontium cations, a high selectivity with respect to strontium cations and very rapid sorption kinetics of these strontium cations, as presented in the examples below. These strontium cations are in particular cations of radioactive isotopes of strontium such as 90 Sr.

[0040] The process according to the invention is carried out specifically in a fixed bed, in a column because the material it uses has the robustness and mechanical resistance necessary for this and the size of the grains of the material is adapted to this process in a fixed bed.

[0041] These excellent sorption properties (kinetics, capacity, selectivity) are obtained with reduced quantities of particles of a solid inorganic compound selectively exchanging strontium Sr cations such as a zeolite or a crystalline silicotitanate.

[0042] By "geopolymer" or "matrix" or "geopolymer skeleton" is meant in the context of the present invention a solid and porous material in the dry state, obtained following the hardening of a mixture containing finely ground materials (i.e. generally an aluminosilicate source) and a saline solution (i.e. an activation solution), said mixture being capable of setting and hardening over time. This mixture may also be referred to as "geopolymeric mixture", "geopolymeric composition" or even "geopolymer paste". The hardening of the geopolymer is the result of the dissolution / polycondensation of the finely ground materials of the geopolymeric mixture in a saline solution such as a saline solution of high pH (i.e. the activation solution).

[0043] More specifically, a geopolymer or geopolymer matrix or skeleton is an amorphous alumino-silicate inorganic polymer. Said geopolymer is obtained from a reactive material containing essentially silica and aluminum (i.e. the alumino-silicate source), activated by a strongly alkaline solution (activation solution), the solid / solution mass ratio in the formulation being low. The structure of a geopolymer is composed of a Si-O-Al network formed of silicate (SiO4) and aluminate (AIO4) tetrahedra linked at their vertices by sharing oxygen atoms. Within this network, there are one or more charge compensating cation(s) also called compensation cation(s) which compensate for the negative charge of the AIOT complex.Said compensation cation(s) is (are) advantageously chosen from the group consisting of alkali metals such as lithium (Li), sodium (Na), potassium (K), rubidium (Rb) and cesium (Cs), alkaline earth metals such as magnesium (Mg), calcium (Ca), strontium (Sr) and barium (Ba) and mixtures thereof.

[0044] By "mesoporous geopolymer u" we mean a geopolymer with mesopores, possessing mesoporosity.

[0045] For the purposes of the present invention, the term “mesopores” means pores whose average dimension such as a diameter is 2 to 50 nm.

[0046] The geopolymer of the mesoporous solid material used in the method according to the invention has a single porosity scale, namely a mesoporosity, and does not have a hierarchical porosity like that of the geopolymer of the material described in document [3].

[0047] Because the mesoporous solid material used in the process according to the invention does not have macroporosity with thin and fragile walls, it is much more robust and mechanically resistant than, in particular, the material of document [3].

[0048] This mesoporous solid material, due to its robustness and high mechanical resistance, can therefore be implemented in the form of non-powdery grains, i.e. cannot be emitted in suspension in the air or transported there, forming a fixed bed, for example forming a packing of a column.

[0049] The mesoporous solid material used in the method according to the invention is a solid material with open mesoporosity.

[0050] By "open mesoporosity" is meant that this mesoporosity is accessible for a liquid medium such as an effluent brought into contact with the material. This mesoporosity is also at least partly connected (or interconnected), or even totally interconnected, that is to say that the fluid can pass through the material by passing through the pores connected to each other. This mesoporosity also allows access of a fluid to the active particles. According to the invention, in addition to the simple presence of particles of at least one inorganic solid compound that is a selective exchanger of strontium cations, it is possible to choose the nature and size of these particles with almost no limit and independently of the geopolymer matrix.

[0051] Overall, a geopolymer skeleton has many advantages over a metal oxide skeleton.

[0052] The synthesis of a geopolymer is simpler to master than the synthesis of a metal oxide by sol-gel method.

[0053] The synthesis of a geopolymer requires less expensive precursors than those used to synthesize metal oxides (mainly alkoxides).

[0054] A geopolymer skeleton intrinsically has better mechanical strength than a metal oxide skeleton also obtained by sol-gel method.

[0055] The geopolymer skeleton of the mesoporous solid material used in the process according to the invention contains mesopores, whereas a metal oxide skeleton does not. To create mesoporosity in a metal oxide, it is necessary to add an additional compound to the formulation and therefore to complicate the system.

[0056] Finally, the mesoporous solid material used in the process according to the invention has mesoporosity, is mechanically robust and incorporates particles of at least one inorganic solid compound that is a selective exchanger of strontium cations, for a specific application in a fixed bed, in a column.

[0057] The material used in the process according to the invention ensures improved accessibility of the Sr cations contained in the liquid medium, such as an effluent to be treated, to the particles distributed in the material due in particular to the interconnectivity which exists between the mesopores.

[0058] The grains of the mesoporous solid material are non-powdery grains such as grains, seeds, granules, extrudates, beads, or balls with generally an average size, such as a diameter, of 100 μm to 5 mm, preferably of 300 μm to 5 mm, more preferably of 300 μm to 500 μm. These grains of mesoporous solid material form a fixed bed, for example form a packing of a column. Grains which generally have the size as specified above, can in fact be described as non-powdery, because this size does not define a powder, which can be defined as a size strictly less than 100 μm. These non-powdery particles thus avoid the risks of clogging during implementation in a fixed bed, in a column, and limit pressure losses.

[0059] Advantageously, grains with an average size of 300 pm to 5 mm, preferably 300 pm to 500 pm, are particularly suitable for implementation in a fixed bed by packing a column.

[0060] By "size" is generally meant the largest dimension, such as diameter, of the particles of mesoporous solid material.

[0061] Advantageously, the particles of the at least one inorganic solid compound selectively exchanging strontium cations, distinct from the geopolymer, may be chosen from the group consisting of nanometric particles (generally defined by their diameter of 2 to 100 nm), submicron particles (generally defined by their diameter of 100 nm to 1 pm) and micron particles (generally defined by their diameter of 1 to 10 pm) and may in particular have an average size, such as a diameter, of 2 nm to 50 pm, preferably of 10 nm to 10 pm, more preferably of 20 nm to 1 pm.

[0062] By "size" is also meant here the largest dimension, such as the diameter, of the particles of the at least one inorganic solid selective strontium cation exchange compound distinct from the geopolymer.

[0063] Advantageously, the active particles of the at least one inorganic solid compound selectively exchanging strontium cations have a ball, pearl, sphere or spheroid shape, or even an acicular shape.

[0064] The particles of the at least one inorganic solid compound selectively exchanging strontium cations, distinct from the geopolymer, are preferably inorganic, mineral particles, namely particles consisting only, only (100%) of one or more inorganic, mineral solid compound(s). Such 100% inorganic, mineral particles are particularly advantageous in the context of the treatment of radioactive effluents, due to their compatibility with nuclear waste management systems.

[0065] The material according to the invention, comprising such active particles, 100% mineral, and the geopolymer itself 100% mineral, is then also 100% mineral which makes it entirely compatible with nuclear waste management systems.

[0066] Advantageously, the inorganic, mineral solid compound, selective exchanger of strontium cations, distinct from the geopolymer, can be chosen from the group consisting of zeolites; alkaline silicotitanates, such as sodium silicotitanate; and mixtures thereof.

[0067] The inorganic, mineral, selective strontium cation exchanger solid compound can be chosen according to the properties of the liquid, such as an effluent, and the contaminant to be extracted that it is intended to treat, such as salinity and pH.

[0068] This inorganic solid compound selective exchanger of strontium cations can be for example an ion exchanger based on sodium silicotitanate in the case where the effluent also contains calcium (Ca) cations in order to minimize the competition between Sr cations and Ca cations, or an ion exchanger based on LTA zeolite in the case where the effluent also contains sodium (Na) cations in order to minimize the competition between Sr cations and Na cations.

[0069] There is no limitation on the shape of the active particles of at least one solid compound distinct from the geopolymer.

[0070] Advantageously, the particle content of the at least one inorganic solid compound selectively exchanging strontium cations, distinct from the geopolymer, is from 0.05 to 70% by mass, preferably from 10 to 40% by mass, of the total mass of the mesoporous solid material.

[0071] Advantageously, after having brought the liquid medium into contact in a fixed bed with the mesoporous solid material, the used fixed bed containing the strontium cations fixed on the particles is directly transformed into a conditioning material. This conditioning material can be advantageously obtained by blocking the grains of the mesoporous solid material directly within the fixed bed by incorporating a binder, for example geopolymer or cement, making it possible to form a dense monolithic material incorporating the grains of mesoporous materials.

[0072] In summary, the material used in the process according to the invention:

[0073] - has a high capacity and selectivity towards Sr as well as rapid sorption kinetics, and optimized dynamics for a column process. This makes it possible to reduce treatment time, ensure sustained treatment rates, and limit the size of the columns used and therefore the volume of waste generated by the treatment.

[0074] - can be easily brought into a form suitable for use in a packed column without significant pressure losses throughout the treatment plant.

[0075] - can, in general, be easily prepared by a process that is easy to implement and easily transposable to an industrial scale.

[0076] - has great versatility in terms of composition and shaping geometry which allows the material to be adapted to any type of effluent and any type of fixed bed or column process.

[0077] - generally has a chemical nature compatible with waste management channels specific to the nuclear sector.

[0078] The mesoporous solid material, used in the process according to the invention, as just described, can be prepared by the process described below.

[0079] This process for preparing the mesoporous solid material may comprise at least the following successive steps: a) a first aqueous suspension of particles of at least one inorganic solid compound which is a selective exchanger of strontium cations, in water or in an aqueous solution comprising a surfactant, is mixed with an aqueous activation solution and an aluminosilicate source capable of forming the geopolymer by dissolution / polycondensation, whereby a second aqueous suspension is obtained;b) the second aqueous suspension is allowed to stand, and the geopolymer skeleton is formed by polycondensation, whereby the solid material with open mesoporosity is obtained, comprising a skeleton of a mesoporous geopolymer, having open mesoporosity, and particles of at least one inorganic solid compound selectively exchanging strontium cations, distinct from the geopolymer, being distributed in the mesoporosity and accessible via the mesoporosity.;

[0080] Advantageously, step a) comprises the following successive steps a1), a2) and a3): a1) a first aqueous suspension of particles of at least one inorganic solid compound selectively exchanging strontium cations is prepared, in water or in an aqueous solution comprising a surfactant; a2) an aqueous activation solution is added to the first aqueous suspension of particles of at least one inorganic solid compound selectively exchanging strontium cations obtained at the end of step a1), whereby an aqueous suspension a2 is obtained; a3) an aluminosilicate source capable of forming the geopolymer by dissolution / polycondensation is added to the aqueous suspension a2, whereby said second aqueous suspension is obtained.

[0081] The particle concentration in the second suspension is chosen based on the final particle concentration of at least one inorganic solid strontium cation-selective exchange compound, distinct from the geopolymer, desired in the prepared material.

[0082] Advantageously, at the end of step a2) and before step a3), the suspension a2 is homogenized, for example by mechanical stirring.

[0083] Advantageously, at the end of step a3) and before step b), the second suspension is homogenized, for example by mechanical stirring.

[0084] By "mechanical stirring" is generally meant mechanical stirring which uses a stirring device equipped with a paddle rod or, preferably, a homogenizing or dispersing device (for example of the Ultra-Turrax, IKA® type) which can be equipped with a dispersing rod having a rotor / stator system.

[0085] Advantageously, during step b), the second aqueous solution is allowed to stand, so that the geopolymer skeleton can form by polycondensation in the second aqueous suspension, and it is shaped and formed to obtain a chosen size and shape.

[0086] This preparation process is very simple because the mesoporous solid material is prepared by simple addition of the active solid particles during the synthesis of the geopolymer.

[0087] This preparation process is considerably simpler and therefore more reproducible, less time-consuming and less expensive than processes that require the preparation of an oil-in-water emulsion.

[0088] This preparation process allows the synthesis of the material used in the process according to the invention, that is to say a material with mesoporosity, mechanically robust and integrating active particles with improved accessibility of the active particles to the liquid medium, such as an effluent to be treated.

[0089] The expression "aluminosilicate source" and the expression "reactive material containing essentially silica and aluminum" are, in the present invention, similar and can be used interchangeably.

[0090] The reactive material containing essentially silica and aluminum usable for preparing the geopolymer skeleton of the mesoporous solid material is advantageously a solid source containing amorphous aluminosilicates, known from the prior art for the preparation of geopolymer, such as kaolinite or metakaolin.

[0091] The term "activation solution" means the high pH saline solution well known in the field of geopolymerization. The latter is a highly alkaline aqueous solution which may optionally contain silicate components selected in particular from the group consisting of silica, colloidal silica and vitreous silica. The expressions "activation solution", "high pH saline solution" and "highly alkaline solution" are, in the present invention, similar and can be used interchangeably.

[0092] By "strongly alkaline" or "high pH" is meant a solution whose pH is greater than 9, in particular greater than 10, in particular greater than 11 and, more particularly, greater than 12.

[0093] The activation solution comprises the compensation cation or the mixture of compensation cations as previously defined in the form of an ionic solution or a salt. Thus, the activation solution is in particular chosen from an aqueous solution of sodium silicate (NazSiOs), potassium silicate (KzSiOz), sodium hydroxide (NaOH), potassium hydroxide (KOH), calcium hydroxide (Ca(OH)2), cesium hydroxide (CsOH) and their derivatives etc.

[0094] In summary, the advantages of this process for preparing the mesoporous solid material used in the process according to the invention, namely a mesoporous material comprising active particles are, among others:

[0095] - that the nature of the selective sites of Sr corresponds directly to the nature of the active particles integrated into the formulation of the mesoporous solid material (nanometric, submicron or micron particles),

[0096] - that this preparation process is carried out under mild conditions, at low temperatures, generally at room temperature and atmospheric pressure,

[0097] - that this preparation process uses commercially available, inexpensive and non-toxic reagents, in particular for the activation solution and the aluminosilicate source, the reaction media being essentially aqueous.

[0098] In this respect, this preparation process, unlike processes involving an oil-in-water emulsion, does not use organic solvents but only aqueous phases and is therefore environmentally friendly and economical.

[0099] - that this preparation process is simple, reliable, and easy to implement. It can be carried out with simple installation and equipment. In particular, this preparation process can be a process in which all the steps can be carried out in a single reactor.

[0100] According to a first embodiment, when the solid material with open mesoporosity obtained at the end of step b) is in the form of a monolith, the following successive steps c), d), e) and f) are then carried out at the end of step b): c) grinding the monolith to obtain grains; d) sieving the grains obtained at the end of step c) to obtain non-powdery particles such as grains, seeds, granules, pearls, or balls having a determined average size; preferably an average size, such as a diameter, of 100 μm to 5 mm, more preferably of 300 μm to 5 mm; e) washing the non-powdery particles obtained at the end of step d); f) drying the non-powdery particles.

[0101] Step c) is a coarse grinding step, i.e. a step during which no attempt is made to control the particle size of the grains obtained, which is therefore very wide.

[0102] Step d) is carried out so as to obtain non-powdery grains with a particle size distribution desired for the intended use of the material, for example an average size of 300 pm to 5 mm, which is the particle size most suitable for use in a column treatment process.

[0103] Step e) is necessary to remove fine particles that adhere to the grains following grinding (these fine particles are likely to clog the column) as well as to remove excess cations resulting from geopolymerization.

[0104] One or more washes can be carried out, for example with demineralized water.

[0105] Step f) can be carried out at a temperature of 30°C to 90°C, for a period of 1 to 48 hours.

[0106] According to a second embodiment, during step b), the second aqueous suspension is extruded to form extrudates having a determined, controlled shape and size, which can be chosen with regard to the process in which the extrudates of mesoporous solid material are used.

[0107] According to a third embodiment, the second aqueous suspension can be atomized to form beads or pearls of mesoporous solid material, the size of which is determined according to the application according to the method of the invention.

[0108] The method according to the invention can be used in particular in nuclear decontamination and the depollution of liquid effluents by ion exchange in a fixed bed, in a column, the material being in a granular form.

[0109] Advantageously, said liquid medium may be an aqueous liquid medium, such as an aqueous solution.

[0110] Said liquid medium may be a process liquid or an industrial effluent.

[0111] Advantageously, said liquid medium may be chosen from liquids and effluents from industry and nuclear installations and activities using radionuclides.

[0112] Generally, the Sr cation may be present at a concentration of 0.1 picogram to 500 mg / L, preferably 0.1 picogram to 100 mg / L.

[0113] Zeolites and silicotitanates are particularly well suited for the separation of Sr.

[0114] At the end of the implementation of the separation process of the invention, in particular in the case where the treated liquid medium is a radioactive effluent, the material in the form of grains forming a fixed bed, for example forming a packing of a column, can be densified by adding a suspension of precursors of the geopolymer in the spent column.

[0115] The invention will now be described in more detail in the following, in particular in connection with particular embodiments thereof which are notably the subject of examples.

[0116] This description is given for illustrative and non-limiting purposes, and with reference to the attached drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0117] Figure 1 is a photograph of the Geopo-Zeo-27 grains (300-500 pm) of a selective geopolymer-adsorbent composite material obtained in Example 1.

[0118] Figure 2A shows the diffractograms obtained for the “Geopo-Zéo” materials comprising a geopolymer and the LTA zeolite prepared in Example 1 as well as that of the “Geopo-0” material and the LTA zeolite powder. On the abscissa is plotted 20 (in 0 ) and the intensity (in arbitrary units) is plotted on the ordinate.

[0119] Figure 2B shows the diffractograms obtained for the “Geopo-CST” materials comprising a geopolymer and the crystalline Na silicotitanate CST, prepared in Example 1 as well as that of the “Geopo-0” material and the Na silicotitanate CST powder (“CST powder”). On the abscissa is plotted 20 (in 0 ). The intensity (in arbitrary units) is plotted on the ordinate.

[0120] Figure 3A shows the pore size distributions in the materials “Geopo-0” corresponding to the pure geopolymer and “Geopo-Zéo” comprising a geopolymer and the LTA zeolite prepared in Example 1. The pore diameter (in nm) is plotted on the abscissa, and dV(r) (in cm3 / nm / g) is plotted on the ordinate.

[0121] Figure 3B shows the pore size distributions in “Geopo-CST” materials comprising a geopolymer and the crystalline Na silicotitanate CST, prepared in Example 1 as well as the pore size distribution in the “Geopo-0” material. The abscissa plots the pore diameter (in nm), and the ordinate plots dV(r) (in cm 3 / nm / g).

[0122] Figure 4 represents the evolution of the maximum sorption capacity of the materials prepared in Example 1, as a function of their mass percentage of zeolite (“-zeo”) or silicotitanate (“-CST”), and also shows the maximum sorption capacity of the geopolymer alone (“geopo”) (see Example 3). The square ■ corresponds to the geopolymer alone (“geopo”), the circles • correspond to the “Geopo-Zeo” materials comprising a geopolymer and the LTA zeolite, and the triangles ▲ correspond to the “Geopo-CST” materials comprising a geopolymer and the crystalline Na CST silicotitanate. The mass percentage of adsorbent (zeolite or silicotitanate) is plotted on the abscissa, and Q is plotted on the ordinate (in mg.g -1). Figure 5 shows the evolution of the KD distribution coefficient of the prepared materials with respect to strontium as a function of their mass percentage of adsorbent (zeolite or silicotitanate), and also shows the KD distribution coefficient of the geopolymer alone (see example 3). The square ■ corresponds to the geopolymer alone (“geopo”), the circles • correspond to the “Geopo-Zéo” materials comprising a geopolymer and the LTA zeolite, and the triangles ▲ correspond to the “Geopo-CST” materials comprising a geopolymer and the crystalline Na silicotitanate CST. The mass percentage of adsorbent (zeolite or silicotitanate) is plotted on the abscissa and the KD distribution coefficient with respect to Sr (in mL / g) is plotted on the ordinate.

[0123] Figure 6 shows the evolution of the Sr sorption capacity as a function of the contact time between the material and the solution, for the “Geopo-Zéo-27” material prepared in Example 1, and for the geopolymer alone, namely the “Geopo-0” material. The squares ■ correspond to the geopolymer, and the triangles ▲ correspond to the “Geopo-Zéo-27” materials comprising a geopolymer and the LTA zeolite. The abscissa shows the contact time (in minutes), and the ordinate shows the Sr sorption capacity: Q. (in mg.g -1 ).

[0124] Figure 7 shows the evolution of the distribution coefficient of the materials Géopo-0 and Géopo-Zéo-27 as a function of the equilibrium Sr concentration in the solution. In other words, Figure 7 shows the sorption isotherms carried out on the materials Géopo-0 and Géopo-Zéo-27 in a saline matrix (see example 3). The squares ■ correspond to the geopolymer, and the triangles ▲ correspond to the materials “Géopo-Zéo-27” comprising a geopolymer and the LTA zeolite. The abscissa shows the equilibrium Sr concentration in the solution: [Sr]fin (in mg.L 1 ) and on the ordinate is plotted the distribution coefficient: KD with respect to Sr (in mL.g -1 ).

[0125] Figure 8 shows the breakthrough curves of the materials “Géopo-Zéo Tl”; “Géopo-CST-5” and “Géopo-CST-10”, prepared in example 1, i.e. the evolution of the Sr concentration ([Sr]) (normalized by the initial Sr concentration ([Sr]O) as a function of the volume of effluent having passed through the column (see example 4). The squares ■ correspond to the material “Géopo-Zéo-27”, the circles • correspond to the material “Géopo-CST-5” and the triangles ▲ correspond to the material “Géopo-CST-10”. The abscissa shows the volume of effluent having passed through the column (in mL), and the ordinate shows [Sr] / [Sr]o-

[0126] DETAILED DESCRIPTION OF THE INVENTION

[0127] In the following, a particular embodiment of the method for preparing the material used in the method according to the invention is first described.

[0128] A particular embodiment of the method according to the invention is then described, using this material to separate Sr cations from a liquid medium containing it.

[0129] In this particular embodiment of the method for preparing the material, the solid particles are particles of at least one inorganic solid compound that is a selective exchanger for strontium cations. This particular embodiment of the method for preparing the material implemented in the method according to the invention and then of the separation method according to the invention implementing it comprises the following steps:

[0130] (1) selection of the selective ion exchanger for Sr according to the properties of the effluent to be treated, such as salinity or pH. For example, a sodium silicotitanate-based ion exchanger will be preferred in the case of competition of Sr adsorption with Ca or an LTA zeolite-based ion exchanger in the case of competition with Na;

[0131] (2) synthesis of the ion exchanger (or use of a commercial product) selected in an ideal form for its incorporation into the geopolymer while maintaining good kinetic extraction properties. Thus, submicron-sized particles will preferably be chosen;

[0132] (3) adaptation of the geopolymer composition to present a robust mechanical strength, a mesoporous network as well as a good compatibility with the ion exchanger. Parameters such as, for example, the alkali used (K + , N / A +, etc.), the value of the Si / AI ratio or the quantity of water added in the formulation will be of particular interest; (4) synthesis of the cation exchanger-geopolymer composite by simple addition of the cation exchanger during the synthesis of the geopolymer;

[0133] (5) shaping of the material according to a route adapted to the constraints of the decontamination process: coarse grinding of the composite and sieving to select the particle size best suited to the column processes (generally centered on 300 pm - 5 mm), synthesis of beads or extrudates, etc.

[0134] (6) washing the material to eliminate the presence of fine particles which could clog the column and eliminate excess cations resulting from geopolymerization;

[0135] (7) filling a column with the shaped material;

[0136] (8) percolation of the column by the effluent containing the radioactive Sr to be extracted;

[0137] (9) drying of the column by an air flow then sending the column to the outlets;

[0138] (10) possibly, densification directly of the used column by adding a solution of geopolymer precursors.

[0139] The invention will now be described with reference to the following examples, given for illustrative and non-limiting purposes.

[0140] Examples-

[0141] Example 1: Synthesis of composite materials used in the process according to the invention and of a reference material.

[0142] In this example, the synthesis of grains of “selective adsorbent-geopolymer” composite materials used in the process according to the invention is described, which allow effective and selective decontamination of Sr in aqueous effluents both in a batch process and in a continuous process in a column packed with these grains.

[0143] First, two adsorbents, selective ion exchangers of Sr are chosen:

[0144] - an LTA zeolite with increased selectivity towards Na.

[0145] - a crystalline Na silicotitanate (CST), which is an even more selective adsorbent for Sr than LTA zeolite. In addition to their increased selectivity for Sr, these two adsorbents were also selected because their microstructural properties are compatible with the intended application:

[0146] - their size is small enough that they have rapid sorption kinetics (of the order of a few hundred nm / a few microns). More precisely, the average particle size of LTA Zeolite is 200 to 500 nm, and the average particle size of CST is less than 100 nm.

[0147] - they are completely mineral, which gives them good resistance to irradiation;

[0148] - they are resistant to highly basic environments, which allows their integration into “geopolymer” type matrices.

[0149] The composition of the geopolymer has been optimized to generate the formation of a mesoporous geopolymer binder, which makes the selective adsorbent accessible to the effluent to be treated and mechanically robust enough not to crumble during the decontamination process.

[0150] The synthesis protocol for the “selective adsorbent-geopolymer” composite materials implemented in this example is as follows:

[0151] - Step 1: A predefined quantity of submicron powder of selective adsorbent (either LTA zeolite or CST) is added to 1.77 mL of water and placed for 15 minutes in an ultrasonic bath. The quantities of adsorbent used were calculated so that the particles could represent between 5 and 27% by mass of the final material. These quantities are as follows: 0.420 g of adsorbent for 5% by mass, 0.887 g of adsorbent for 10% by mass, 1.995 g of adsorbent for 20% by mass and 2.952 g of adsorbent for Tl% by mass.

[0152] - Step 2: addition to the suspension obtained at the end of step 1, of 2.12 mL of a solution composed of 81% by mass of a commercial inorganic binder called Betol® K5020T (available from the company Wôllner 8) based on an aqueous solution of modified potassium silicate, and composed of SiO? at 30% by mass, K2O at 18% by mass, and H2O at 52% by mass; and 19% by mass of KOH (at 85%, marketed by Sigma-Aldrich).

[0153] The suspension is then homogenized manually.

[0154] - Step 3: Addition to the suspension obtained at the end of step 2 of 2.64 g of clay powder, namely Metakaolin (Metamax' from BASF).

[0155] Then, the resulting suspension is homogenized again manually or using an Ultra-Turrax' homogenizer equipped with an S25N-18G dispersion head at a shear speed of between 3000 and 5000 rpm.

[0156] After 48 hours of resting at room temperature, a robust monolithic material is obtained.

[0157] - Step 4: The monolithic material is ground, then the grains of material obtained are sieved to finally obtain grains with a size distribution between 300 and 500 pm.

[0158] - Step 5: A washing step is carried out.

[0159] To do this, 3 g of the material grains are stirred manually for 30 seconds in the presence of 100 mL of demineralized water. This step is repeated 4 times, changing the water between each test.

[0160] - Step 6: The material grains are finally dried at 80°C for approximately 12 hours.

[0161] Figure 1 is a photograph of the grains (300-500 pm) of a selective geopolymer-adsorbent composite material obtained from the synthesis described above, called Géopo-Zéo-27.

[0162] It is therefore noted that the process for preparing the materials used in the process according to the invention only requires simple steps and is easily transposable to an industrial scale.

[0163] For comparison, a so-called reference material is also prepared in this example, consisting solely of the geopolymer and containing no active particles, adsorbent, in particular neither zeolite nor silicotitanate. This material is prepared by a process comprising only steps 2, 3, 4, as well as 5 and 6. The materials prepared are named according to the percentage of adsorbent in the material. Table 1 summarizes the materials prepared in Example 1 and their names.

[0164] Table 1

[0165] The composite materials used in the process according to the invention, prepared in example 1, are characterized and used in examples 2, 3, and 4 which follow.

[0166] Example 2: Characterization of the materials prepared in Example 1.

[0167] In this example, the composite materials used in the process according to the invention synthesized in example 1 and the reference material are characterized on the one hand by X-ray diffraction to verify that the selective mineral exchangers are not degraded during the synthesis; and on the other hand by nitrogen adsorption-desorption in order to verify the presence of mesopores in the geopolymer binder.

[0168] The presence of mesopores in the geopolymer binder allows the mineral exchangers to be accessible to the effluent to be treated.

[0169] Analysis of materials by X-ray diffraction.

[0170] Figures 2A and 2B show the diffractograms obtained for the materials described in Example 1.

[0171] Figure 2A shows the diffractograms obtained for the “Geopo-Zéo” materials comprising a geopolymer and a zeolite. Figure 2B shows the diffractograms obtained for the “Geopo-CST” materials comprising a geopolymer and a silicotitanate.

[0172] These figures show that the LTA zeolite and CST silicotitanate particles have been well integrated into the composite material without any degradation of their crystalline structure. Thus, the crystalline structures responsible for the affinity of the materials for Sr are well preserved.

[0173] Analysis of materials by nitrogen adsorption-desorption.

[0174] The different materials prepared in Example 1 are analyzed by nitrogen adsorption-desorption.

[0175] Figure 3A shows the pore size distributions in the “Geopo-Zéo” materials comprising a geopolymer and a zeolite, obtained by the “BJH” method (Barrett, Joyner, Halenda model).

[0176] Figure 3B shows the pore size distributions in the “Geopo-CST” materials comprising a geopolymer and a silicotitanate, obtained by the “BJH” method (Barrett, Joyner, Halenda model).

[0177] It is observed that the synthesized materials are all mesoporous with pore size distributions centered globally between 4 and 40 nm. Since the selective adsorbents zeolite and silicotitanate are not mesoporous, it is the mesopores of the geopolymer binder that are detected.

[0178] Thus, the selective adsorbents dispersed in the geopolymer binder are accessible to the ions of interest to be fixed (Sr in the case of these examples) after the shaping step, via the mesopores of the geopolymer.

[0179] It is noted that the presence of selective adsorbent particles in the formulation of the materials somewhat modifies the size of the mesopores. This point is however not problematic because the measured mesopore size is systematically large enough to allow the rapid diffusion of an ion.

[0180] Thus, the synthesized materials have crystalline sites, crystalline zones, constituted by the selective adsorbents zeolite and silicotitanate which will allow the material to be more selective for Sr, and a mesoporous network which will allow access of ions to these crystalline sites as well as rapid sorption kinetics. Example 3: Sorption performances in “batch” mode of materials prepared in example 1.

[0181] In this example, tests are carried out to evaluate the sorption performance in batch mode of the materials prepared in Example 1.

[0182] Firstly, a study of the sorption capacities and selectivities of the materials is carried out. The objective of these tests is to observe the influence of the presence of selective adsorbents (zeolite and silicotitanate) in the formulation of the materials, in particular on their capacity Q. (mg.g -1 ) and their selectivity, expressed as a function of the selectivity coefficient KD (mL.g -1 ), with respect to strontium.

[0183] In order to estimate the maximum sorption capacity of materials, sorption tests are carried out in a highly concentrated Sr solution.

[0184] The protocol for these sorption tests to estimate the maximum sorption capacity of materials is as follows:

[0185] - 50 mg of material in the form of 300-500 pm grains are placed in 50 mL of a highly saline aqueous matrix (deionized water) comprising 0.25 mol / L of NaNOs, 50 ppm of Ca (Ca(NO3)z salt) and 200 ppm of Sr (SrfNChh salt).

[0186] - stir with a rotary stirrer for 24 hours.

[0187] - after stirring, 15 ml of supernatant is taken with a syringe and filtered with a 0.22 pm syringe filter, then the residual Sr concentration of the solution taken and filtered is analyzed.

[0188] The value of the capacity Q. for extraction or sorption of Sr (quantity of Sr captured per gram of material) is then determined as follows: where [Sr]init and [Sr]fin represent respectively the initial concentration and the final concentration of Sr in solution (mg.L 1 ), V the volume of solution (mL), and m the mass of material (g).

[0189] Figure 4 represents the evolution of the maximum sorption capacity of the materials as a function of their mass percentage of adsorbent. It is observed that, whatever the adsorbent used, the sorption capacity of the material increases with the concentration of adsorbent, thus demonstrating the full interest of the presence of this adsorbent in the material, as well as the accessibility of the adsorbents to the treated solution thanks to the mesoporosity of the geopolymer.

[0190] In order to estimate the value of the distribution coefficient of the materials (ratio between the quantity of Sr captured by the material and the quantity of Sr remaining in solution), sorption tests are carried out in a solution with a low concentration of Sr.

[0191] The protocol for these sorption tests in order to estimate the value of the distribution coefficient of the materials is as follows:

[0192] - 50 mg of material in the form of 300-500 pm grains are placed in 50 mL of a highly saline aqueous matrix (deionized water) comprising 0.25 mol / L of NaNOs, 50 ppm of Ca (Ca(NO3)z salt) and 200 ppm of Sr (SrfNChh salt).

[0193] - stir with a rotary stirrer for 24 hours.

[0194] - after stirring, 15 ml of supernatant is taken with a syringe and filtered with a 0.22 pm syringe filter, then the residual Sr concentration of the solution taken and filtered is analyzed.

[0195] The value of the distribution coefficient KD with respect to Sr is then determined as follows: where [Sr]init and [Sr]fin represent respectively the initial concentration and the final concentration of Sr in solution (mg / L), V the volume of solution (mL) and m the mass of material (g).

[0196] Figure 5 represents the evolution of the KD distribution coefficient of the materials as a function of their mass percentage of adsorbent.

[0197] It is observed that, regardless of the adsorbent used, the distribution coefficient of the composite material with respect to Sr increases with the adsorbent concentration, thus demonstrating the full interest of the presence of the adsorbent in the material. The influence of the two adsorbents is however different. The presence of 27% by mass of LTA zeolite in the material increases the KD, which is 667 mL.g 1 for pure geopolymer, at 1120 mL.g 1 whereas for the same CST concentration, the KD increases to 5648 mL.g -1 .

[0198] It should be noted that these values ​​were obtained with an initial Sr concentration of 2 ppm. It will be shown later that this KD value increases for lower initial Sr concentrations.

[0199] This difference can be explained by the composition of the solution in which the tests are carried out, in particular the Na and Ca concentrations and the nature of the adsorbents used. Indeed, CST is more selective for Sr compared to Ca and Na than LTA zeolite.

[0200] More in-depth batch mode studies were then carried out on two materials, namely the materials called: Géopo-0 and Géopo-Zéo-27.

[0201] These studies are a kinetic study and a complete isotherm.

[0202] Kinetic study.

[0203] The sorption kinetics of materials can be characterized by analyzing the sorption of materials as a function of the contact time between the materials and the effluent to be treated.

[0204] The protocol for these tests to study the sorption kinetics of materials is as follows:

[0205] - 50 mg of material in the form of 300-500 pm grains are placed in 50 mL of a highly saline aqueous matrix comprising 0.25 mol / L of NaNOs, 50 ppm of Ca (Ca(NO3h) salt) and 50 ppm of Sr (Sr(NO3h) salt).

[0206] - several “batches” are prepared and left under rotary stirring for different durations between 5 minutes and 48 hours.

[0207] - after stirring, 15 ml of supernatant is taken with a syringe and filtered with a 0.22 pm syringe filter, then the residual Sr concentration of the sampled and filtered solution is analyzed. Figure 6 shows the evolution of the Sr sorption capacity for the two materials studied (Géopo-27 and Géopo-0) as a function of the contact time between the material and the solution.

[0208] Thus, it has been shown in this example that the process according to the invention, which uses granular materials, and therefore usable in columns, ensures rapid sorption kinetics (equilibria reached in less than 24 hours) due to the mesoporosity of the geopolymer binder which allows easy accessibility of the ions to the particles of selective adsorbents.

[0209] Complete isotherms.

[0210] The objective of these tests is to determine the value of the distribution coefficient at very low Sr concentrations, which are the most representative of the Sr concentrations found in radioactive effluents.

[0211] The protocol for these tests aimed at establishing sorption isotherms is as follows:

[0212] - 50 mg of material in the form of 300-500 pm grains are placed in different batches comprising 50 mL of a highly saline aqueous matrix (deionized water) comprising 0.25 mol.L 1of NaNOs, 50 ppm of Ca (salt Ca(NO3)z), with different concentrations of Sr (salt SrfNChh) between 0.1 and 200 mg.L -1 .

[0213] - the “batches” are left under rotary stirring for 24 hours.

[0214] - after stirring, 15 ml of supernatant is taken with a syringe and filtered with a 0.22 pm syringe filter, then the residual Sr concentration of the solution taken and filtered is analyzed.

[0215] Figure 7 represents the evolution of the distribution coefficient of materials as a function of the Sr concentration at equilibrium in the solution.

[0216] In other words, Figure 7 shows the sorption isotherms carried out on the materials Géopo-0 and Géopo-Zéo-27 in a saline matrix.

[0217] The most representative distribution coefficient of radioactive effluents is located at the plateau obtained at the lowest Sr concentrations. Thus, we observe a significant influence of the presence of selective adsorbents in the material on the selectivity of this material with respect to Sr. In the matrix considered (0.25 mol / L NaNOs and 50 ppm of Ca (salt Ca(NO3)z)), the KD of the Géopo-0 material is around 1030 mL.g 1 while it reaches 2044 mL.g 1 for the Géopo-Zéo-27.

[0218] To conclude, the mesoporous and multiphase microstructure of the materials used in the process of the present invention allows:

[0219] - on the one hand, granular shaping that can be used in a column while maintaining rapid sorption kinetics

[0220] - on the other hand to improve the selectivity of the material with respect to Sr.

[0221] Example 4: Performance of materials in a fixed bed decontamination process

[0222] In this example, the granular composite materials used in the process according to the invention, described in example 1, are tested in a column in order to demonstrate the benefit of their microstructure on their performance in a decontamination process according to the invention, in a fixed bed.

[0223] The tests were carried out according to the following protocol:

[0224] - the grains sieved to 300-500 pm are placed in a glass column of height H = 3 cm and diameter D = 1 cm.

[0225] - the column is placed in an assembly composed of an effluent reserve, a peristaltic pump, and a pressure gauge at the head of the column to measure pressure losses.

[0226] Using this assembly, breakthrough curves (defined below) were produced.

[0227] An aqueous effluent (deionized water), comprising 0.25 mol.L 1 of NaNOs, 50 ppm of Ca (salt Ca(NO3)z) and 100 ppm of Sr (salt SrfNChh) circulates in the packed column at a flow rate of 20 mL.h 1 and the breakthrough curve is obtained by measuring by ICP-OES the Sr concentration in the samples taken at the outlet over time.

[0228] Figure 8 shows the breakthrough curves of the materials Géopo-Zéo Tl; Geopo-CST-5 and Geopo-CST-10, i.e. the evolution of the Sr concentration (normalized by the initial Sr concentration) as a function of the volume of effluent having passed through the column. This figure shows that the breakthrough curves are in the form of sigmoids with vertical slopes, and entirely in line with what could be expected, which demonstrates ideal behavior in a column process.

[0229] The more adsorbents are used, the more volume can be treated with the same amount of material, and the slope of the curve is vertical.

[0230] REFERENCES

[0231] [1] WO-A1-2018 / 015490.

[0232] [2] WO-A1-2016 / 173950. [3] WO-A1-2021 / 152248.

[0233] [4] PAPA ET AL., “Zeolite-geopolymer composite material: Production and characterization”, Journal of Cleaner Production, 17 (2018) 76-84.

Claims

Claims 1. Continuous process for separating at least one cation from strontium, and in particular at least one cation from a radioactive isotope of strontium such as 90 Sr, from a liquid medium containing it, in which said liquid medium is brought into contact with a mesoporous solid material, comprising an inorganic matrix made of a mesoporous geopolymer having an open mesoporosity, said geopolymer having a single porosity scale, and particles of at least one inorganic solid compound selectively exchanging strontium cations, distinct from the geopolymer, these particles being distributed in the open mesoporosity of the inorganic matrix and accessible to the strontium cations contained in the liquid medium; said mesoporous solid material being in the form of non-powdery grains forming a fixed bed, for example forming a packing of a column.

2. The method of claim 1, wherein the grains of the mesoporous solid material, such as grains, seeds, granules, beads, extrudates or balls, have an average size, such as a diameter, of 100 pm to 5 mm, preferably of 300 pm to 5 mm, more preferably of 300 pm to 500 pm.

3. Method according to claim 1 or 2, in which the particles of the at least one inorganic solid compound selectively exchanging strontium cations, distinct from the geopolymer, are chosen from the group consisting of nanometric particles, submicron particles and micron particles, and have in particular an average size, such as a diameter, of 2 nm to 50 pm, preferably of 10 nm to 10 pm, more preferably of 20 nm to 1 pm.

4. Method according to any one of the preceding claims, in which the inorganic solid compound selectively exchanging strontium cations, distinct from the geopolymer, is chosen from the group consisting of zeolites; alkali silicotitanates, such as sodium silicotitanate; and mixtures thereof.

5. Method according to any one of the preceding claims, in which the particle content of the at least one inorganic solid strontium cation-selective exchange compound, distinct from the geopolymer, is from 0.05% to 70% by mass, preferably from 10% to 40% by mass, of the total mass of the mesoporous solid material.

6. Method according to any one of the preceding claims, wherein said liquid medium is an aqueous liquid medium, such as an aqueous solution.

7. Method according to any one of the preceding claims, wherein said liquid medium is chosen from liquids and effluents from industry and nuclear installations and activities using radionuclides.

8. Method according to any one of the preceding claims, in which, after having brought the liquid medium into contact, in a fixed bed, with the mesoporous solid material, the used fixed bed is directly transformed into a conditioning material, in particular by blocking the grains of the mesoporous solid material directly within the fixed bed by incorporating a binder therein.

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