Cs ion exchange by a potassium nickel hexacyanoferrate loaded on a granular support

doi:10.1016/j.ces.2015.07.043

Chemical Engineering Science

Volume 137, 1 December 2015, Pages 904-913

https://doi.org/10.1016/j.ces.2015.07.043 Get rights and content

Highlights

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The sorbent is made of KNiFe nanoparticles distributed over a Zr(OH)₄ porous matrix.
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We identified the ion exchange mechanism to be K/Cs (80%) and Ni/Cs (20%).
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We modeled the exchange K/Cs in a wide range of Cs⁺ concentration.
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The model can be applied for decontamination in saline water.
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The selectivity of this material for Cs is explained by thermodynamic cycle.

Abstract

¹³⁷Cs is considered to be one of the most abundant and hazardous elements due to its presence in many effluents and wastes. We study the cesium sorption by a supported mixed K/Ni ferrocyanide on a porous Zr(OH)₄ matrix from pure water at different pH containing Cs from trace concentration (radioactive solution) to molar concentration. The material was characterized by XRD and SEM, and its chemical composition was deduced by elemental analysis. Isotherms performed at different pH demonstrated that the material had a maximum sorption capacity of 0.22 mmol/g (at pH 7) and that ion exchange was 80% driven by K/Cs exchange and 20% driven by Ni/Cs exchange. Measurement of the pH before and after sorption indicated that protons also participate in the ion exchange process. These experimental experiments allow the construction of thermodynamic database for modeling the different ionic exchanges that occur in this system using the CHESS code. The sorption of cesium from several types of effluents was then correctly modeled by a set of selectivity constants with respect to the K⁺ ion (K/Cs, K/Ni, K/Na and K/H). The present dataset can be extrapolated to other K ferrocyanides and effluents.

Introduction

The ever-increasing pressure to reduce the release of radioactive species into the environment requires constant improvement in the processes and technologies for waste treatment and for dose minimization. Recently, the Fukushima disaster illustrated the need to develop appropriate processes for the removal of radionuclides due to accidental discharge. Various treatments may be applied depending on the effluent composition and targeted radionuclide to be extracted (co-precipitation (Flouret et al., 2012), ion exchange, sorption and others). Ion exchange in a fixed bed column is one of the most common and effective treatment methods for radioactive liquids, which also produces minimal final waste.

Among these radionuclides, ¹³⁷Cs is considered to be the most abundant and hazardous element due to its presence in many wastes and to its relatively long half-life (30 years). A number of studies have already been carried out on the extraction of cesium using ion exchange processes with various inorganic ion exchangers such as zeolites, sodium titanates, silicotitanates and hexacyanoferrates (Haas, 1993, Loos-Neskovic et al., 2004, Merceille, 2012, Merceille et al., 2012, Sachse et al., 2012). Among them, potassium nickel hexacyanoferrate (also called hexaferrocyanide), KNiFC, is employed industrially for cesium removal from radioactive liquid waste. The KNiFC exchanger has a face centered cubic (FCC) structure with Fe²⁺ and Ni²⁺ ions located at the corners of the elementary cubes of the network, cyano groups on the edges and exchangeable K⁺ and Ni²⁺ ions at the body center. Due to small channels in the lattice with diameters of approximately 0.32 nm, small hydrated ions such as Cs⁺ can penetrate into the structure, whereas larger hydrated ions such as Na⁺ are blocked. However, little is known about the ion exchange selectivity and the uptake behavior of Cs⁺ in the presence of other types of cations as well as the competitive effect of Na⁺ or H⁺ ions, which are ions abundant in sea water ([Na⁺]_{sea water}=0.5 mol/L).

The main drawback of these materials is related to their small grain size that prevents their use in fixed bed columns due to pressure loss and clogging. A way to overcome this problem is to immobilize these selective hexacyanoferrate compounds in a solid porous matrix suitable for a column process. Recently, numerous composite solids (organic or inorganic) loaded with hexacyanoferrate particles have been proposed for cesium removal (Delchet et al., 2012, Kawatake and Shigemoto, 2012, Turgis et al., 2013, Vincent et al., 2014). A commercial inorganic sorbent based on nickel potassium hexacyanoferrate distributed over the inorganic carrier zirconium hydroxide is studied in this paper. This adsorbent is produced by the sol–gel method (Sharygin, 2001) thereby obtaining spherical granules having good mechanical properties. The sol–gel method also provides granules with an average size of 600 µm making them a good candidate for decontamination via a column process. Previous work has shown the chemical properties (Sharygin and Muromskii, 2000, Sharygin and Muromskii, 2004) of this adsorbent, such as its selectivity regarding cesium and chemical structure (Sharygin et al., 2007), with the conclusion being reached that nickel potassium hexacyanoferrate is in a cubic form with the chemical formula K₂Ni[Fe^II(CN)₆].

Experiments in radioactive condition are time consuming and scale up experiment for in flow process must be well defined before being implemented with actual radioactive effluents. Then it seems necessary to develop a predictive model based on the description of the different ion exchange that occurs in this system and able to describe with accuracy the distribution coefficient and the selectivity coefficient as a function of the concentration of the studied ions in solution. This chemical modeling is a fruitful approach to obtain a better understanding of ion exchange mechanisms, to derive selectivity constants of ion exchange reactions and to extrapolate experimental data to other configurations (e.g., batch tests to column experiments). Ion exchange is a well-known phenomenon and ion exchange resins have been the subject of several modeling studies to develop the industrial process resulting therefrom Bachet et al. (2014) and Gressier (2008). However, to the authors׳ knowledge, there is no work on the determination of ion exchange reaction constant and selectivity constant in hexacyanoferrates structure. The present study addresses a description of the ion exchange mechanism of this composite material with consideration of the complete solution chemistry including solid precipitation. The sorption properties of the material were tested in synthetic solutions through studying the effect of the pH, the impact of competitive ions, the uptake kinetics, and the sorption isotherms. X-ray diffraction and SEM (scanning electron microscopy) analyses were also carried out to link the physical–chemical properties of the material with its ion exchange behavior. The objective of this work is to obtain a reliable database to transpose modeling to column breakthrough curves in a further step by coupling these thermodynamical data with reactive transport models.

Section snippets

Solution analysis and solid characterization

The concentrations of K, Ni and Cs in an aqueous solution from inactive batch sorption experiments were analyzed by the following three methods: inductively coupled plasma mass spectrometry (ICP-MS Thermo Scientific), cationic chromatography (injected into the column at 40 °C with an elution time of 20 min, Metrohm) and inductively coupled plasma atomic emission spectroscopy (ICP-AES Thermo Scientific). Radiochemical analyses were performed by gamma counting (Eurisys, measured with a germanium

Characterization of solid

The chemical structures of two massive nickel/potassium hexacyanoferrate compounds (K₂Ni[Fe(CN)₆] and Ni₂[Fe(CN)₆]) are shown on Fig. 1A. In the structure of K₂Ni[Fe(CN)₆], eight potassium atoms fill the center of each small cube, while only four nickel atoms randomly occupy the center of the small cube in Ni₂[Fe(CN)₆]. According to the XRD of the solid sample (Fig. 1B), the nickel/potassium hexacyanoferrate present in the zirconium based support is not a pure one with a chemical formula K₂

Maximum exchange capacity

The experimental ion-exchange capacity (CEC=0.22 mmol/g) was directly obtained through the sorption isotherms, as discussed above. CEC is also named the maximum exchange capacity (Q_max).

Selectivity coefficients – Vanselow model

Vanselow selectivity constants were derived from the adsorption isotherms conducted at a pH=7, where the H⁺ concentration and solid degradation were negligible. Vanselow selectivity constants were calculated for each point of the curve by taking into account all of the following exchange reactions: K/Cs, Ni/Cs,

Conclusions

This study investigates the mechanism of cesium sorption by a supported mixed K/Ni ferrocyanide on a porous Zr(OH)₄ matrix by coupling experimental data and modeling.

Cesium sorption mechanisms by a supported mixed K/Ni ferrocyanide on a Zr(OH)₄ matrix were studied by coupling experimental data and modeling. According to the XRD analysis, the composition of the material was a mixture of Ni₂Fe(CN)₆ and K₂NiFe(CN)₆. A mean chemical formula, K_1.34Ni_0.33[NiFe(CN)₆], was calculated from elemental

Acknowledgments

The authors would like to thank J.F Dufrêche for his fruitful discussion as well as Emmanuel Excoffier and Véronique Testud from the LMAC laboratory in CEA Marcoule for the ICP-MS analysis and their contributions to the analytical methods. The authors are grateful to the French “Programme d’Investissements d’Avenir” (ANR-11-RSNR-0005) for the financial support of this work.

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Cs ion exchange by a potassium nickel hexacyanoferrate loaded on a granular support

Highlights

Abstract

Introduction

Section snippets

Solution analysis and solid characterization

Characterization of solid

Maximum exchange capacity

Selectivity coefficients – Vanselow model

Conclusions

Acknowledgments

Chem. Eng. Sci.

J. Solid State Chem.

Sep. Purif. Technol.

Geochim. Cosmochim. Acta

Microporous Mesoporous Mater.

Comput. Geosci.

Chem. Eng. J.

Thermodynamics and ion exchange phenomena

Trans. Kans. Acad. Sci.

Comparison of mass transfer coefficient approach and Nernst–Planck formulation in the reactive transport modeling of Co, Ni, and Ag removal by mixed-bed ion-exchange resins

Ind. Eng. Chem. Res.

Extraction of radioactive cesium using innovative fonctionalized porous materials

RSC Adv.