Isabelle Martin

Cédric Patapy

Cédric Boher

Martin Cyr

Investigation of caesium retention by potassium nickel hexacyanoferrate (II) in different pH conditions and potential effect on the selection of storage matrix

Keywords: nuclear waste solution, pH value, chemical stability, potassium nickel

The chemical stability and caesium retention properties of a nonstoichiometric compound K2-xNix/2[NiFe(CN)6].nH2O (KNiFCN) was studied in different pH conditions. Different solutions with controlled pH were prepared. Leaching tests were done with chemical analysis of the solutions. The compound was characterized before and after exposure by chemical analysis, SEM-BSE, X-ray diffraction and infrared spectroscopy. The face-centered cubic (FCC) structure of KNiFCN without caesium was stable even in presence of 1 g/l HNO3 (pH equal 2) or NaOH (pH equal 13). In presence of caesium trapped on KNiFCN structure, it appears that the increase in caesium content change the KNiFCN structure, thus influencing the retention properties in KNiFCN according to the pH of the solution. The KNiFCN structure was decomposed when the NaOH concentration exceeds 1 g/l (pH equal 14), probably due to the hydrolysis of constitutional elements. A selection of cementitious matrix is the proposed to satisfy stability conditions of this compound.

2

with the flood occuring in Fukushima power plant in 2011. Depending on the origin of these wastes, different radioactive elements can be found like uranium ( 238 U), americium ( 241 Am), plutonium ( 240 Pu, 239 Pu, 238 Pu), neptunium ( 237 Np). Gamma radiations which are hazardous are emitted for instance by Cobalt ( 60 Co), Strontium( 90 Sr) and Caesium ( 137 Cs).

Drastic reduction of hazardous elements concentrations in effluents can be acquired through different process with the most used being co-precipitation ([1]- [START_REF] Shakir | Removal of cesium from aqueous solutions and radioactive waste simulants by coprecipitate flotation[END_REF]). This processing operation consists in introducing chemical reagents to the wastewater to form insoluble salts which could be then confined in a solid matrix for long term storage [START_REF] Takahatake | Decontamination of Radioactive Liquid Waste with Hexacyanoferrate(II)[END_REF], [7]. This effluent treatment involves a complex mechanism of ion exchange and chemical adsorption to permit the radionuclide trapping. The selection of trapping sorbent depends on the type of radionuclide present in the wastewater but also on the volume of effluent to be treated in order to optimize the cost of the process. 137 Cs is one of the most prevalent radionuclide where zeolites, tetraphenylborate and especially cyanoferrates sorbents are used to get the selective insolubility of caesium.

Transition metal hexacyanoferrates (II) are added at one step of the treatment of effluents with pH adjustment to allow the trapping of caesium inside insoluble salts [START_REF] Mimura | Ion Exchange of Cesium on Potassium Nickel[END_REF], [START_REF] Iaea | Treatment of Low and Intermediate Level Liquid Radioactive Wastes[END_REF]. For example, the compound of interest in this study is obtained by the in-situ reaction of hexacyanoferrate with nickel sulfate to form a nickel hexacyanoferrate II (Ni-Fe II ). Its structure is based on six cyanure groups (CN) organized in an octahedral configuration with Fe-CN-Ni-NC-Fe bounds between them to form a face-centred cubic cell. Electroneutrality is maintained by potassium cations in interstitial sites. An exchange mechanism between K + and Cs + will be at the origin of caesium trapping. A ratio Ni/Fe of 1-1.5 was shown to be efficient to optimize exchange between caesium (noticed Cs) and potassium in solutions ( [START_REF] Takahatake | Decontamination of Radioactive Liquid Waste with Hexacyanoferrate(II)[END_REF], [START_REF] Mimura | Chemical and thermal stability of potassium nickel hexacyanoferrate(II)[END_REF]). Different authors suggest that sorption capacity depends also on the temperature [START_REF] Mimura | Chemical and Thermal Stability of Potassium Nickel Hexacyanoferrate (II)[END_REF] and on the concentration of Cs in the solution but no data is available to confirm it [START_REF] Ravat | Etude des interactions radionucléides/sels peu solubles du procédé STE3[END_REF].

The addition of nickel sulfate to form the nickel hexacyanoferrate can lead to high value of pH (10.4-11.7) which is then corrected to favor the insolubility of salts containing Cs.

Moreover, the radioactive waste encapsulation is often realized by confinment of the insoluble salts (with residual water) in a solid matrix which can be composed of liquid exhibiting basic conditions (high pH values) up to 13 [START_REF] Mimura | Chemical and Thermal Stability of Potassium Nickel Hexacyanoferrate (II)[END_REF]. Some data are available for potassium/nickel hexacyanoferrate II (K-Ni-Fe II ) in the literature ( [START_REF] Iaea | Treatment of Low and Intermediate Level Liquid Radioactive Wastes[END_REF], [START_REF] Mimura | Chemical and Thermal Stability of Potassium Nickel Hexacyanoferrate (II)[END_REF]). This compound is closed to the nickel hexacyanoferrate with lower Ni/K ratios (K0.87Ni1.57Fe(CN)6). It was exposed in [START_REF] Iaea | Treatment of Low and Intermediate Level Liquid Radioactive Wastes[END_REF], [START_REF] Mimura | Chemical and Thermal Stability of Potassium Nickel Hexacyanoferrate (II)[END_REF] to solutions with different concentrations of HCl, HNO3 and NaOH to reach different pH up to 14. X-Ray diffraction patterns coupled with quantification of Fe in the solution (ICP-OES) showed a degradation of the compound for pH between 13 and 14. On the contrary, no significant degradation of the K-Ni-Fe II was observed in acidic conditions. The ability of retention of Cs was constant up to pH = 13 with a dramatic decrease above.

It is then practically of prime importance to understand the influence of the pH conditions on the ability of Ni-Fe II to keep Cs elements inside its structure. This information can be used to correctly select the operating conditions (time of mixing, concentrations of reactive compounds for pH adjustment,…) and appropriate coating matrix ensuring containment of the radioactive waste without radionuclide release.

The present study deals with the influence of the pH of the solution on the Cs retention properties in Ni-Fe II in order to determine a appropriate cement matrix of radioactive waste encapsulation. The sorbent will be first exposed to three different Cs concentrations to obtain different exchange level inside the structure. It will then be exposed to different pH between 2 and 14 using HNO3 and NaOH. The solutions will be characterized by ICP-OES and ion chromatography to both measure the potential release of Cs and the degradation of the sorbent. X-Ray diffraction (XRD), FTIR and chemical analysis done by SEM-EDS will be used to identify transformation of the sorbent after exposure.

Experimental set-up

Preparation and stoichiometry of potassium nickel hexacyanoferrate (II).

A nonstoichiometric compound K2-xNix/2[NiFe(CN)6].nH2O (KNiFCN) has been prepared by reaction between potassium hexacyanoferrate (II) (K4Fe(CN)6,3H2O) and nickel sulphate hydrate (NiSO4,6H2O), according to equation [START_REF] Takahatake | Decontamination of Radioactive Liquid Waste with Hexacyanoferrate(II)[END_REF].

K Fe(CN) + NiSO → K NiFe(CN) + K SO (1)
The two reactants were introduced in molar ratio Ni Fe

Simulation of Cs trapping

Caesium nitrate (CsNO3 99.8%, Alfa Aesar) was added to the solution to simulate the Cs present without radiological toxicity. Table 1 shows the four concentrations of Cs in the simulated solutions used for the study. Haas [START_REF] Haas | A Review of information on ferrocyanide solids for removal cesium from solutions[END_REF] showed that quantity of Cs trapped inside nickel potassium hexacyanoferrate are generally comprised between 350 and 1900 mmol Cs/g of the sorbent. Then, the highest concentration of Cs in the study (2000 mmol Cs/g) may correspond to a situation of the maximum capacity of fixation of the hexacyannoferrate. Potassium nickel hexacyanoferrate (II) was added to the simulated solutions (total volume = 1 l) and mixed at ambiant temperature during 24 hours to ensure that Cs is trapped into the KNiFCN.

Exposure to different pH

The chemical stability was studied by exposing the simulated solutions to either HNO3 and NaOH (analytical grade) solution in the range of 0.03 to 50.0 g/l at 20°C for 1 day (Table 2).

It can be noticed that the reference pH of the compound in solution is 8. Experiments were done in 100 ml PE containers with stirring using an end-over-end agitator for 24h at 20°C.

After mixing, the simulated solutions containment KNiFCN were filtered with 0.45 µm filter and diluted by 2% of HNO3 addition. Acidification prevents precipitation of stable phases. At the end, the solution was stored in plastic vial at 5°C to avoid any precipitation of solid before analysis. The solution concentrations of Cs, Fe and Ni were analysed by ICP-OES (Optima 7000DV-Perkin Elmer), Atomic Absorption Spectrometry (Aanalyst 400-Perkin Elmer) and ionic chromatography (ICS 3000-Dionex). The residual solid phase after testing was isolated by drying at 20°C for 1 day before crushing into fine powder (< 80 µm) for microstructural characterisation. X-Ray diffraction analyses were done with a Siemens D5000 (CoKα, λ = 1.789 Å) working in Bragg-Brentano geometry with a 2θ range of 5°-70°. Lattices parameters were determined with Fullprof software. The solid phase was also analysed by Fourier transform infrared spectroscopy method (FTIR, Perkin-Elmer Spectrum). The analysis was made by deposition of a low amount of powder directly on the diamond cell after measurement of the background in air medium. The spectrum obtained was an accumulation of 10 measurements made between 4000 cm -1 and 800 cm -1 . For SEM observations, the samples were pressed until 75 MPa during 2 min to obtain pellets of 4-5 mm thickness. The pellets were impregnated under vacuum with an epoxy resin (MECAPREX MA2+) and polished with different polishing pads and diamond spray ranging from 12 to 1 µm.

Influence of Cs concentrations in the solution on the reference product

The potassium nickel hexacyanoferrate product was exposed to different concentrations of Cs (dissolution of caesium nitrate) to reach different quantity of Cs trapped inside the Ni-Fe II sorbent. It will be seen -as discussed later in the paper -(Figure 4) that the concentration of Cs in the solution is very low after the process of fixation by the hexacyanoferrate whatever the initial concentration of Cs in the solution.

Figure 1 presents SEM-BSE observations of residual solid obtained after exposure of the nickel hexacyanoferrate to different concentrations of Cs (0, 0.3, 30 and 2000 mmol/mmol Fe). The BSE mode of the SEM highlighted different grey levels corresponding to heterogeneous atomic numbers (Z). This was slightly visible for Ni-Fe II compounds exposed to low concentrations of Cs (0 and 0,3 mM). The presence of some cracks may be due to the sample preparation (polishing and vacuum stage in the SEM). Elements ratios (Fe/Ni vs K/Ni) obtained by EDS analysis (50 points) are plotted in Figure 2. The high level of dispersion of the chemical compositions seems to show that the composition of KNiFCN was heterogeneous (Fe/Ni between 0.7 and 1.1 and K/Ni between 1.1 and 2.4 for the Ni-FeII exposed to 0.3 mM for example). The spread is more pronounced under variations of K/Ni ratios depending on the localization of the pointing. As K + ions are replaced by Cs + ones during the co-precipitation process, heterogeneities may appear in the Ni-Fe II depending on the spatial distribution of the Cs. The higher the Cs is trapped inside the structure, the lower the K/Ni is. This was confirmed by the narrowing of the heterogeneities of K/Ni ratios for the sample with the highest Cs concentration (K/Ni between 0.6 and 1.2). Fe/Ni ratio are roughly similar whatever the initial concentration of Cs of the solution. Thereafter, plots indicated that the addition of a low amount of Cs (0.3 mM) did not clearly change the chemical composition of KNiFCN. However, the cloud of EDS analyses points moves toward a lower values of K/Ni ratio for the highest amount of Cs (2000 mM). Finally, it is interesting to note that the presence of K + in the sorbent means that the K + ions substitution by Cs + ions was not complete. This is in accordance with the work of Ravat [START_REF] Ravat | Etude des interactions radionucléides/sels peu solubles du procédé STE3[END_REF] which revealed that no more than 22% of the K + can be substituted by Cs. It is possible that higher contents of Cs could be trapped by substitution of K + by Cs + .

(a) 0 mM (b) 0.3 mM (c) 2000 mM Figure 3 shows XRD patterns of the Ni-Fe II samples exposed to 0, 0.3, 30 and 2000 mM of Cs. These diagrams were very similar to the ones obtained by Gelling [START_REF] Gellings | Structure of some Hexacyanoferrates (II) of the Type K2MIIFe(CN)6[END_REF]. They confirmed the presence of the crystalline phase of nickel/ potassium ferrocyanure (K2NiFe(CN)6) with a face-centered cubic (FCC) cristalline structure (database code ICSD 28667). On another hand, these investigations permit to confirm a change in the crystal structure in identifying a left shift of peaks for higher amounts of Cs inside the structure (30 and 2000 mM). According to some authors ( [START_REF] Mimura | Chemical and Thermal Stability of Potassium Nickel Hexacyanoferrate (II)[END_REF], [START_REF] Ćeranić | The Structure Model of an Inorganic Ion Exchanger Cobalt ( II )hexacyanoferrate ( II )[END_REF]), Ni and Fe elements are located at the corners of the elementary cubic lattices, cyano groups (CN) on the edges and the exchangeable cations (K + and Ni 2+ ) at the body center. When K + ions (atomic radius (Ratomic) = 2.77 Å) are replaced by Cs + ions (Ratomic = 3.34 Å), a lattice parameter modification (a, Å) can be observed with a shift to the left of the peak corresponding to larger lattice parameters. XRD patterns were simulated with Fullprof software to evaluate the modification of the "a" lattice parameters (see Table 3). A larger "a" dimension is obtained with increasing Cs concentrations might indicate replacement of K + by Cs + .

Behaviour of KNiFCN under different pH conditions

Evolution of the Cs trapping capacity of KNiFCN-Cs systems as a function of the pH of the solution

The Cs capacity was evaluated by doing quantification of Cs + of the solution in contact with KNiFCN during expositions in solution.

As presented above, reference experiment were done with KNiFCN exposed at pH=8 The overall results show that, at high pH and especially for low Cs concentration of the solution, the amount of Cs released can be significant, notably if the adsorbant (KNiFCN) is stocked in porous matrix allowing the radionucleides diffusion. In order to indentify and follow the evolution of KNiFCN within the KNiFCN-Cs systems, FTIR spectroscopy was used. Figure 5 (a) shows the different spectra of the control solution between 4000 cm -1 and 1200 cm -1 , after chemical treatment at 20°C. On these spectra, six major vibrations bands were identified:

-1385-1410 cm -1 : may be due to the presence of CO3 2-anions due to carbonation of Ni(OH)2 (interaction with air) [START_REF] Motlagh | Synthesis and characterization of Nickel hydroxide / oxide nanoparticles by the complexationprecipitation method[END_REF] and shows the presence of nitrate group [START_REF] Loos-Neskovic | Structure of copper-potassium hexacyanoferrate (II) and sorption mechanisms of cesium[END_REF].

-1420-1450 cm -1 : could be due to the presence of HCO3 2-anions [START_REF] Miller | Infrared Spectra and Characteristic Frequencies of inorganic Ions -Their Use in Qualitative Analysis[END_REF], probably due due to carbonation of Ni(OH)2.

-1640 cm -1 : associated with bonding vibrations of H-OH bonds, related to water [START_REF] El-Didamony | Properties and durability of alkaliactivated slag pastes immersed in sea water[END_REF].

-2085-2100 cm -1 : corresponding to the stretching vibration of CN bonds in the ferrocyanure group (Fe(CN)6 3-) [START_REF] Ćeranić | The Structure Model of an Inorganic Ion Exchanger Cobalt ( II )hexacyanoferrate ( II )[END_REF].

-3400 cm -1 : arises from O-H stretching vibration of water molecules [START_REF] Richard | Diffuse reflectance infrared Fourier transform spectroscopy as a tool to characterise water in adsorption/confinement situations[END_REF] [START_REF] El-Didamony | Properties and durability of alkaliactivated slag pastes immersed in sea water[END_REF].

-3640 cm -1 : could be due to the O-H stretching vibrations, related mainly to Ni(OH)2 [START_REF] Motlagh | Synthesis and characterization of Nickel hydroxide / oxide nanoparticles by the complexationprecipitation method[END_REF] [START_REF] Adekunle | Comparative catalytic properties of Ni(OH)2 and NiO nanoparticles towards the degradation of nitrite (NO2-) and nitric oxide (NO)[END_REF]. Further investigations realized by XRD (Figure 6) and SEM-EDS (Figure 8) confirm the previous observations by showing the nickel oxide (noted N on Figure 6) and iron hydroxide (noted F on Figure 6) formation when the pH value of the solution equal 14. Then the XRD analyses present in Figure 6 (a) show the presence of a secondary phase: nickel ferrocyanure (Ni2Fe(CN)6). The ferrocyanure downgrading leads to a loss of the system crystallinity (strong presence of amorphous product). -1230-1260 cm -1 : may be due to the presence of NO2 -anions [START_REF] Loos-Neskovic | Structure of copper-potassium hexacyanoferrate (II) and sorption mechanisms of cesium[END_REF] [40] associated to the insertion of Cs nitrate in solution (0.3 mM of CsNO3).

The non-detection of the nitrate group by X-ray diffraction seems to suggest a loss in their crystallinity or their low concentration. For the acid condition (Figure 9 (b) (left spectrums)), the intensities of the ferrocyanure compound are not modified. However, according to the previous results, 5% of Cs is detected in solution (Figure 4). Two theories can be used to explain this Cs presence in solution:

-Cs desorption: references found in the literature show that the KNiFCN is more effective for the removal of Cs + from highly concentrated solutions of Na + (from NaOH in solution) and H + (from HNO3 or HCl in solution) [START_REF] Mimura | Chemical and Thermal Stability of Potassium Nickel Hexacyanoferrate (II)[END_REF], [START_REF] Chen | Selective removal of cesium ions from wastewater using copper hexacyanoferrate nanofilms in an electrochemical system[END_REF].

-Downgrading of the news compounds observed at pH equal to 8 (Figure 9 (b)).

In basic condition (Figure 9 (b) (right spectrums)), an important decrease in the intensity of stretching vibration of CN is observed when the pH of the solution increases from 8 to 10.

However, the lack of Cs in solution (see Figure 4) indicates that Cs remains trapped in the solid phase, despite the possible KNiFCN downgrading. A less important decrease in the intensity of ferrocyanure's vibration is observed for pH solution at 11 (violet curve) and 12

(blue curve). A less important decrease in the intensity of ferrocyanure's vibration is observed for pH solution at 11 (violet curve) and 12 (blue curve) without Cs discharge (see Figure 4).

When the pH of the solution increases from 8 to 13 (orange curve), the important decrease in the intensities of ferrocyanure's vibration is correlated to the presence of Cs in solution (5% of Cs detected in solution, see Figure 4). Finally, a new behavior is observed when the pH of the solution reaches a value of 14 (black curve). Infact, a strong decrease in the intensity of ferrocyanure's vibration coupled to the emergence of news vibrations bands at 2047cm -1 and 2010 cm -1 reflects the chemical environment changing of ferrocyanure, leading to the loss of its sorption capacity (15% of Cs detected in solution, see Figure 4). to pH 2), the 'a' value is in the same range of the reference sample (pH8_ref), which is in accordance with FTIR analysis (Figure 9). In basic condition (8 to a greater pH transition by addition of NaOH solution), the 'a' value was in the range of 10.044 to 10.065. The presence of Cs seems to create an important structural modification of KNiFCN-Cs sample according to the pH of the solution. Infact, when the solution increases from:

-8 to 10: the 'a' value increase from 10.055 to 10.065. This increase is in accordance to the FTIR observations (Figure 9).

-8 to 11: the 'a' value decreases from 10.055 to 10.044. The lack of Cs in solution (Figure 4) seems to indicate that this decrease is not due to the Cs desorption.

-8 to 12: the 'a' value is in the same range of the reference sample (pH8_ref) -12 to 13: a decrease of the 'a' value is correlated to the decrease in the intensities of ferrocyanure's vibration (Figure 9) and to the presence of Cs in solution (Figure 4).

The absence of a X-ray diffractogram at pH equal 14 is due to a lack of material to ensure a correct measurement. -1 ) (c) Solution II (30 mM of Cs)

Wavelength (cm

1900
In the case of Solution II (30 mM of Cs), the analysis of the FTIR spectra (presented in Figure 11) revealed the residual presence of ferrocyanure (characteristic band around 2088 cm -1 ) for the entire pH range tested. Also, this analysis showed the precipitation of one or more new compounds (1483 cm -1 , 1392 cm -1 and 1241 cm -1 ) when the pH of the solution is higher than 12 (see orange and black curves). This observations seems to be in adequation with XRD analysis realised in addition (Figure 12 (a)). In fact, following the increase in the pH of the solution, a progressive offset of the characteristics X-ray diffraction lines of KNiFCN is observed. It thus appears that a strong NaOH concentration in the solution leads to a structural ferrocyanure modification due to a possible Na + insertion in the ferrocyanure structure [START_REF] Mimura | Selective Removal of Cesium from Simulated High-level Liquid Wastes by Insoluble Ferrocyanides[END_REF], [START_REF] Martínez-Garcia | On the crystal structures of some nickel hexacyanoferrates (II,III)[END_REF], [START_REF] Mimura | Selective Removal of Cesium from Sodium Nitrate Solutions by Potassium Nickel Hexacyanoferrate-Loaded Chabazites[END_REF], or even the precipitation of new compounds such as Cs2NiFe(CN)6

(represented by triangles) and KNiFe(CN)6 (represnted by squares). The precipitation of these new compounds least soluble seems to preserve the conservation of the Cs sorption capacity in the solid phase (0% of Cs detected in solution, see Figure 4).

The FTIR analysis (Figure 11 For the whole pH range tested (excluding pH 11 and 14), it appears that the degradation of the ferrocyanures is relatively small, which may explain the low Cs content shown in Figure 4.

When the pH of the medium is equivalent to 14, less degradation of the ferrocyanides compared with the previous solutions studied is observed. Similarly, with the exception of the previous solutions, this degradation seems to generate only the formation of iron hydroxide in a small proportion (only detected by XRD, notation N Figure 14 (a)). The lattice parameter show in Figure 14 (b) is in accordance with the previous observations. In fact, the "a" value of 2θ (°) KαCo

Discussion

Sorption of Cs

Previous studies have made it possible to determine the retention capacity of Cs in hexacyanoferrates ( [START_REF] Mimura | Selective Removal of Cesium from Simulated High-level Liquid Wastes by Insoluble Ferrocyanides[END_REF], [START_REF] Chang | Nickel hexacyanoferrate multilayers on functionalized mesoporous silica supports for selective sorption and sensing of cesium[END_REF], [START_REF] Mimura | Chemical and Thermal Stability of Potassium Nickel Hexacyanoferrate (II)[END_REF], [START_REF] Mimura | Selective Removal of Cesium from Sodium Nitrate Solutions by Potassium Nickel Hexacyanoferrate-Loaded Chabazites[END_REF]). The distribution coefficient (noted Kd) for Cs leads to determine the selectivity of hexacyanoferrates using the following equation [START_REF] Chang | Nickel hexacyanoferrate multilayers on functionalized mesoporous silica supports for selective sorption and sensing of cesium[END_REF]:

K (C C ) C V M (2)
where Co is the initial concentration of Cs (mg/L), Cf is the equilibrium concentration of Cs (mg/L), V is the volume of the testing solution (ml), and M is the amount of KNiFCN present in solution (g). Table 4 illustrates the effects of initials concentrations of Cs on the distribution coefficient (Kd) of Cs + . The distribution coefficients (Kd) for Cs were comprise between 2 000 ml/g and 17 000 ml/g for 0.3 mM (Solution I) and 30 mM (Solution II), respectively, which is in accordance with a results observed for loaded chabazites ( [START_REF] Mimura | Separation of Cesium and Strontium by Potassium Nickel Hexacyanoferrate(II)-loaded Zeolite A[END_REF]). For a high concentration of Cs (2000 mM -Solution III), the distribution coefficient decrease to reach the value of 100 ml/g, without causing degradation of sorption capacity of the KNiFCN-Cs product, as observed on Figure 13 and Figure 14. V (ml) = 100 and 1.8 < M (g) < 2.5

The structure effect of KNiFCN compound is similar with zeolites. Therefore, the selectivity for monovalent cation could be explained by the "sieve" effect [START_REF] Mimura | Chemical and thermal stability of potassium nickel hexacyanoferrate(II)[END_REF], [START_REF] Ćeranić | The Structure Model of an Inorganic Ion Exchanger Cobalt ( II )hexacyanoferrate ( II )[END_REF]. Ćeranić [START_REF] Ćeranić | The Structure Model of an Inorganic Ion Exchanger Cobalt ( II )hexacyanoferrate ( II )[END_REF] determined the effective radii of the "cages" (8 cages per unit cell and their entrances. One of the most important result was the evaluation of the effective radii of the windows of KNiFCN, estimated to be about 0.15 nm, according to equation 3.

r a 2 " 2r #,%& (3)
where a is the lattice parameter and re,CN is an equatorial radius (re,CN = 0,178 nm ( [START_REF] Lely | The crystal structure of lithium cyanide[END_REF])).

Therefore it appears that the effective radii of those windows (rw) depend on the lattice parameter of the system. Table 5 presents the effective radii of the windows of KNiFCN-Cs as fonction of the initial concentrations of Cs in the solution. The increase of the initial concentration of Cs does not lead to a significant increase in value, which suggests that the modification of selectivity effect of KNiFCN for Cs + is not only due to the "ion sieve" effect. The analyses conducted on the solid phase (Figure 14) show the presence of new Cs-based compounds, for example Cs2NiFe(CN)6. Therefore, it seems that the presence of a high initial concentration of Cs leads to a possible precipitation of KNiFCN-Cs products, probably less soluble at the surface of grains of KNiFCN. It could also occur that Cs is trapped by occupancy of vacants KNiFCN sites. Solution II (30 mM of Cs) and Solution III (2 000 mM of Cs): the Kd value is almost constantly about 1.10 1 ml/g for 2 000 mM (Solution III) and 1.10 4 ml/g for 0.3 mM (Solution II) over the range of pH.

Stability area and approach on conditionning of the wastes

The influence of the pH of the solution on the KNiFCN and KNiFCN-Cs stability is synthesized into the Figure 16. The light green area correspond to the domain in which the KNiFCN/KNiFCN-Cs is not degraded. In contrast, the orange et red area correspond to the degradation domain of KNiFCN/KNiFCN-Cs products. In the case that any Cs is not present in solution, the KNiFCN stability range is comprise between pH equal to 8 and 12. Beyond this range a significant deterioration, with nickel and iron hydroxide precipitation, is observed. When adding a very low quantity of Cs in the solution (0.3 mM of Cs -Solution I), 1.E+00 This work aimed to determine the stability range of KNiFCN as function of the pH of the solution in order to contain it in a solid matrix for long term storage. Table 6 resumes the low-pH classical cement composed with a mixed between CEM I and mineral addition like SF, FA and S † . The ettringite-based systems are also presented in this table. With the exception of some formulations, all binary or ternary cement are conform with the pH stability range previously defined. It should also be noted the compliance of the ettringitebased systems, making a potential candidate for this specific application. In order to ensure the sludge conditioning for nuclear process by avoiding the discharge of Cs entrapped in the KNiFCN structure, the cementing system developed hereafter will have to satisfy pHs below 12. Binary and ternary ettringite based systems could be potentially good candidates for such applications.

  (CN) ⁄ equal to 1.22. The average chemical composition was estimated as K1,96Ni1,22Fe(CN)6 by the quantification of Ni, Fe and K elements by Atomic Absorption Spectrometry (PERKIN ELMET AAnalyst 400). Particle size distributions obtained by laser granulometry (CILAS/1090 LD model) were D90 = 4.2 µm, D50 = 1.7 µm and D10 = 0.3 µm.

Figure 1 :Figure 2 :

 12 Figure 1: SEM-BSE images of nickel hexacyannoferates powder (solid residual) after exposure to different Cs concentrations samples at pH equal 8.

Figure 3 :

 3 Figure 3: XRD diffractogramms of different KNiFCN-Cs products at pH equal 8.

(

  equilibrium pH) to evaluate the distribution between Cs in liquid and Cs trapped inside the KNiFCN structure. To compare leaching of Cs + at different pH levels, concentrations were presented in Figure 4 on a percentage basis of the reference (Cs + ) in solution at pH=8 for a given initial concentration of Cs (0.3, 30 or 2000 mM). A negative percentage in Cs + ions proportions indicates a potentiel reuptake by KNiFCN or a trap throughout the other phases precipitation and a positive percentage shows potential Cs leaching initially trap in KNiFCN-Cs products. The error associated to these mesurements is about 2%. Likewise trends indicate that the initial Cs concentration introduced in the solution influences the chemical stability of KNiFCN-Cs products. In the case of low Cs concentration (0.3 mM), the proportion of Cs dissolved increases gradually over the pH values range from 8 to 12. In acid condition, the proportion of Cs dissolved reach in total the value of 5%, equal to a solution at pH equal 13. In high alkaline solution (pH equal 14), the proportion of Cs dissolved strongly increased to reach the value of 15%. This high proportion of Cs dissolved indicate a potential downgrading and/or restructuring KNiFCN-Cs products. Further investigations were carried out on the solid phases to give more information over this downgrading/restructuring phenomena (see part 3.2).The increase in Cs concentration sorbed, until an intermediate concentration (30 mM) leads to a modification of a KNiFCN-Cs product downgrading phenomena. In fact, in light of the results presented in Figure4, it appears that the Cs dissolved content in the solution is very low (< 0,1%) and constant until over whole pH range tested. However, in the case of Solution III (2000 mM), a change of the proportion of Cs + ions is observed in the solution. In acid condition, a lack of Cs in solution is observed. Then a decrease of Cs proportion is observed when the pH of the solution increased from 8 (pH reference) to 11. This increase seems to show a reuptake by KNiFCN product (optimal pH condition where the Cs + ions sorption is optimised) or by a news products precipitation. The change of the proportion of Cs + ions could be due to a competition beetween dissolved Cs (chemisorption) and reuptake Cs (physisorption).At pH equal 13 the proportion of Cs + ions in solution achieve 6%. Then this proportion value drop down at pH equal 14. The global evolution of Cs + ions in solution with pH for solution III (2000 mM)) was not completely explained and these measurements should be repeated to confirm this trend.

Figure 4 : 2

 42 Figure 4: Dissolution percentage of Cs from KNiFCN-Cs at different pH values of the aqueous solution *

Figure 5 (

 5 Figure 5 (b) presents the characteristic bands of the CN group (between 2200 cm -1 and 1900 cm -1 ) for the control solution after chemical treatment (acid condition -left spectrums, and basic conditions -rigth spectrums) at 20°C. First, it is noted that the decrease or the increase of the pH values of the solution, until 13 did not affect the chemical environment of the ferrocyanure compound (no shift of the vibration bands). Under acid condition, a decrease in the intensities of the ferrocyanure compound was observed. Similarly, under basic conditions, a gradual decrease in the intensities of the CN bonds vibration over a range of pH values up to 11 was noted. The decrease in the intensities of this vibration mode suggests a probable downgrading of the KNiFCN product. Conversely, above 12, an increase in intensities is observed, reflecting a precipitation or restructuring KNiFCN-Cs products.Beyong the pH value of 12 a new downgrading is observed and even a total disappearance for a extreme pH value (pH equal 14), accompanied by the change of the position of the cyano stretch (from 2085 cm -1 to 2038 cm -1 ) and the nickel hydroxide precipitation (see Figure5

Figure 5 :

 5 Figure 5: FTIR spectra of Control solution (0 mM of Cs) for different pH values of aqueous solution (a) and zooming a section of FTIR spectra around bonding vibrations of KNiFCN products (b).

Figure 6 (Figure 6 : 2 . 7 .

 6627 Figure 6: XRD analyses of the Control solution (0 mM of Cs) after different chemical treatment (a) and lattice parameter according to the pH values of the aqueous solution (b) with N:NiO and F:Fe(OH)2.

Figure 7 :

 7 Figure 7: SEM-BSE images of KNiFCN product at pH equal 8, 12 and 14.

Figure 8 :Figure 9

 89 Figure 8: SEM-EDS analysis of KNiFCN product at pH equal 8, 12 and 14.

Figure 9 :

 9 Figure 9: FTIR spectra of Solution I for different pH values of aqueous solution (a) and zooming a section of FTIR spectra around bonding vibrations of KNiFCN products (b).

Figure 10 :

 10 Figure 10: XRD analyses of the Solution I (0.3 mM of Cs) after different chemical treatment (a) and lattice parameter according to the pH values of the aqueous solution (b).

  (b)) and XRD (Figure 12 (a)) also show that in acid condition (Figure 11 (b) (left spectrum)), ferrocyanure is observed. In basic condition (8 to a greater pH transition by addition of NaOH solution, Figure 11 (b) (right spectrum)), the chemical environment of ferrocyanure compounds does not seem to be affected (lack of vibrations bands offset) until pH of solution equal to 12. Beyond pH equal to 12, the vibration intensity randomly varies, indicating a dissolution/precipitation phenomena of this compound (Figure 11 (b) (right spectrums)). The dissolution/precipitation of the ferrocyanure ensure a trapping of Cs until a pH solution equal 14 (see Figure 4). As mentioned above, the modification of the pH of the solution leads to the lattice parameter modification of the KNiFCN-Cs sample (Figure 12 (b)).

Figure 11 :Figure 12 :

 1112 Figure 11: FTIR spectra of Solution II (30 mM of Cs) for different pH values of aqueous solution (a) and zooming a section of FTIR spectra around bonding vibrations of KNiFCN products (b).

Figure 13 :Figure 14 :

 1314 Figure 13: FTIR spectra of Solution III (2000 mM of Cs) for different pH values of aqueous solution (a) and zooming a section of FTIR spectra around bonding vibrations of KNiFCN products (b).

Figure 15

 15 Figure 15 illustrates the effects of the pH of the solution on the distribution coefficient (Kd) of Cs + for the different initial concentration of caesium.

Figure 15 :

 15 Figure 15: Effects of pH solution on distribution of Cs+

Figure 16 :

 16 Figure 16: Overview of the KNiFCN and KNiFCN-Cs stability study.

(

  Control solution)whole pH range seems to indicate that Cs remains trapped in the solid phase, over a higher pH range(8 < pH < 13). Finally, in cases where the concentration of Cs sorbed is maximum (Solution III), ferrocyanures remains detected at pH equal 14.Further analyses were carried out by XRD, SEM-BSE and FTIR in order to determine the degradation products of the KNiFCN-Cs systems according to the pH values of the aqueous solution. First of all, the chemical composition obtained by SEM-BSE confirmed that the principal Cs sorption capacity into the KNiFCN structure is made by the K + ions replacement.However, it has been found that all of these ions are not substitutable. Despite a higher Cs concentration in the solution, potassium remains in the crystal structure of KNiFCN and the excess Cs precipitates into news compounds on the surface of KNiFCN. Similarly, the results on microstructural evolution as function of the pH of the solution show that the alkalinity and the Cs content sorbed have an impact on the ferrocyanures compositions:In acid condition, no changes were observed on chemical environment of the ferrocyanure compounds and no new compound was detected.In basic condition, under certains conditions, a change of chemical environment of the ferrocyanure compounds was observed. Furthermore, a dissolution/precipitation of new compounds resulting from KNiFCN-Cs products downgrading, such as nickel and/or iron hydroxide (Ni(OH)2, Fe(OH)2 and/or Fe(OH)3.(H2O)), was shown.The observations carried out during this project have put on evidence that the increase in amount of Cs sorbed leads to the precipitation of new compounds based on KNiFCN and Cs. This precipitation is particularly present in cases where Cs concentration is maximum (Solution III). Similarly, the presence of ferrocyanure and the lack of hydroxide confirm that the increase in Cs concentration stabilized the KNiFCN-Cs products until a strong pH values (pH equal 2 and 14) while forming a low soluble compounds such as Cs2NiFe(CN)6.

Table 1 : Concentration of Cs in the simulated solutions.

 1

		Control solution	Solution I	Solution II	Solution III
	[Cs] (mol/mol Fe)	0	3.3.10 -1	2.9.10 1	2.0.10 3

V = 100 ml and 19 < [KNiFCN] (g/l) < 25

Table 2 : Concentration of HNO3 and NaOH used for chemical stability studies.

 2

	Notation	pH2 pH8_reference pH10 pH11 pH12 pH13 pH14
	pH	2	8	10	11	12	13	14
	[HNO3/NaOH] (g/l)	1.22	0.0	0.06	0.13	0.03	1.03	5.08
	3 Results and discussion

Table 3 : Lattice parameter (a) for the differents KNiFCN-Cs samples at pH equal 8.

 3

		a (Å)
	0 mM	10.050
	0.3 mM	10.055
	30 mM	10.062
	2000 mM	10.167

Table 4 : The distribution coefficient (Kd) for Cs for different solutions at pH=8.

 4

	Sample	Co (mg/l) Cf (mg/l) Kd (ml/g)
	Solution I	3.4	4.9.10 -2	0.2.10 4
	Solution II	2.7.10 2	6.3.10 -1	1.7.10 4
	Solution III	1.6.10 4	1.3.10 4	0.1.10 2

Table 5 : Effective radii of cell in the KNiFCN solution at pH equal 8.

 5

	Sample	a (Å)	rw (nm)
	0.3 mM pH8_reference	10.055 0.147
	30 mM pH8_reference	10.062 0.147
	2000 mM pH8_reference 10.167 0.152

  is noted the same stability range as function of pH. The stability range seems to increase with the increase of Cs concentration in the solution. Thus when the Cs concentration is higher than 30 mmol/mmol de Fe (Solution II), KNiFCN-Cs products is detected even at pH equal to or above 13.

		1.E+06
		1.E+05						30 mM
	Kd,Cs (ml/g)	1.E+02 1.E+03 1.E+04						0.3 mM
		1.E+01						2 000 mM
		8	9	10	11	12	13	14
					pH

it

Table 6 :

 6

Low-pH cement summary; CEM I: Portland Cement; SF:Silica fume; CAC: Calcium Alunminate Cement; HH:Hemihydrate; FA: Fly Ash; S: Slag (adapted from [26]) † FS: Silica Fume, CV: Fly Ash and L: Blast Furnace Slag

Acknowledgements

The authors wish to thank Orano Projets for fundings and especially D. Avril (Orano Projets) for the provision of KNiFCN products and his valuable suggestions and discussions on this work.

Data availability

The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

Cement

Composition pH Measurement conditions Reference Binary cement CEM I/SF CEM I 70% SF 30% 12.2 Days of hydration = 180 w/s = 0.55 [START_REF] Bach | Evolution physico-chimique des liants bas pH hydrates -Influence de la température et mécanisme de rétention des alcalins[END_REF] CEM I 60% SF 40% 11.7 Days of hydration = 360 w/s = 0.55 [START_REF] Bach | Evolution physico-chimique des liants bas pH hydrates -Influence de la température et mécanisme de rétention des alcalins[END_REF] 12.2 Days of hydration = 90 w/s = 0.50 [START_REF] García Calvo | Design of low-pH cementitious materials based on functional requirements[END_REF] 11.0 Days of hydration = 58 [START_REF] Vuorinen | Low pH grouting cements -Results of leaching experiments and Modelling[END_REF] 11.3 Days of hydration = 360 [START_REF] Fries | Low pH shotcrete field tests on opalinus clay samples[END_REF] CEM I 50% SF 50% 10.6 Days of hydration = 90 w/s = 1.00 [START_REF] Martino | Low heat high performance concrete used in a full scale tunnel seal[END_REF] Binary cement CAC/HH (ettringite systems) CAC 60% HH 40% 12.0 Days of hydration = 7 w/s = 0.60 [START_REF] Martin | Impact of calcium sulfate type and additions on hydration and properties of ettringite-based systems[END_REF] CAC 50% HH 50% 10.5 Days of hydration = 7 w/s = 0.60 [START_REF] Martin | Impact of calcium sulfate type and additions on hydration and properties of ettringite-based systems[END_REF] Ternary cement CEM I/SF/FA CEM I 37.5% SF 32.5% FA 30%

11.1 Days of hydration = 360 w/s = 0.55 [START_REF] Bach | Evolution physico-chimique des liants bas pH hydrates -Influence de la température et mécanisme de rétention des alcalins[END_REF] 11.4 Days of hydration = 360 w/s = 0.40 [START_REF] Dauzère | Etude expérimentale et modélisation des mécanismes physico-chimiques des interactions béton-argile dans le contexte du stockage géologique des déchets radioactifs[END_REF] CEM I 35% SF 35% FA 35%

10.9 Days of hydration = 90 w/s = 0.50 [START_REF] García Calvo | Design of low-pH cementitious materials based on functional requirements[END_REF] CEM I 40% SF 20% FA 40% Ternary cement CAC/HH/S (ettringite systems) CAC 52% HH 33% S 15% 10.0 Days of hydration = 7 w/s = 0.60 [START_REF] Martin | Parametric study of binary and ternary ettringite-based systems[END_REF] Quaternary cement CEM I/SF/FA/S CEM I 33% SF 40% FA 13.5% S 13.5%

12.1 Days of hydration = 360 w/s = 0.50 [START_REF] Codina | Les bétons bas pH: formulation, caractérisation et étude à long terme[END_REF] CEM I 40% SF 5% FA 25% S 30% 12.7 -

Conclusions

The investigation of Cs retention properties in KNiFCN, according to the pH of the solution, has put forward many aspects. Thus, it was found that the increase in Cs sorbed changed the structure of KNiFCN products, influencing the Cs sorption capacity as function of pH. In fact, without Cs sorbed (Control Solution), the pH range on which KNiFCN is stable is comprised between 8 and 12. When a low amount of Cs is sorbed (Solution I), the pH range in which the Cs remains trapped is sligthly similar to the control solution. Thereafter, when the concentration of Cs is increased (Solution II), the lack of variations in Cs proportions on the