Facile synthesis of calcium magnesium zirconium phosphate adsorbents transformed into MZr4P6O24 (M: Ca, Mg) ceramic matrix for radionuclides immobilization
Graphical abstract
Introduction
Sodium zirconium phosphates NaZr2P3O12 and calcium zirconium CaZr4P6O24, also known as NZP materials, are characterized by a negative or close to zero coefficient of thermal expansion. These compounds have attracted increased attention since the mid of 1980s [7], [8]. These phosphates form a three-dimensional, highly stable hexagonal structure consisting of connected PO4 tetrahedra and ZrO6 octahedra, forming voids that can be occupied by various cations to compensate for the charge [6], [19], [16]. Due to these features of the crystal chemical structure, the family of NZP phosphates has a number of unique properties, in particular, the stability of the structure towards ion substitution in various lattice sites, high thermal and radiation resistance, which makes them a promising material for the immobilization of long-lived radionuclides for subsequent safe disposal [25], [4], [5].
Using the sol–gel method in the interaction of aqueous solutions of zirconium oxynitrate and phosphoric acid for the immobilization of long-lived radionuclides 137Cs and 90Sr in CsZr2(PO4)3 and SrZr4(PO4)6 matrix, showed that the heat treatment of the resulting gel at 850–1000 °C produces impurity phases ZrP2O7, CsPO3 or Sr(PO3)2 with low resistance to leaching of cesium and strontium [9]. In this connection, the use of crystalline acidic zirconium phosphate HZr2(PO4)3 as a promising ceramic for the subsequent high-temperature (700 °C) immobilization of radionuclides 137Cs and 90Sr was proposed [17], [26], [15]. The choice of acidic zirconium phosphates is due to their high ion exchange properties, as well as selectivity in the adsorption of single and multivalent metal ions and radionuclides [13]. Crystalline zirconium phosphates were obtained by boiling or hydrothermal treatment of sedimentary amorphous zirconium phosphate, or by decomposition of zirconium fluoride complexes with phosphoric acid at elevated temperature. A significant disadvantage of these methods is the high duration of synthesis from 3 to 20 days [11], [21], [14], [30], [20]. In addition, the synthesis was carried out in a large excess of phosphoric acid. In recent years, simpler mechanochemical technologies were developed for the synthesis of ɑ-Zr(HPO4)2·H2O and ɣ-Zr(PO4)(H2PO4)2·2H2O crystalline zirconium phosphates [22], [9], [29], [1].
The above mentioned disadvantages of the crystalline acidic zirconium phosphates synthesis and the formation of undesirable impurity phases when using the sol–gel method determine the relevance of the search for new simpler and more convenient methods for their production, necessary for the immobilization of radionuclides in ceramic NZP matrices. Previously, a heterogeneous synthesis of Ti-Ca-Mg composite phosphates by reacting medium and acidic double phosphates of calcium magnesium with a solution of titanyl-diammonium sulfate was carried out [10], [2]. A significant improvement in the adsorption and selective characteristics of the obtained phosphates was shown in comparison with individual titanium and calcium magnesium phosphates.
In this paper, we attempted to synthesize adsorbents based on calcium magnesium zirconium phosphates using mixed tertiary and acidic calcium magnesium phosphates obtained by phosphating of natural dolomite under various conditions, and zirconium oxynitrate as a zirconium-containing precursor. To the best of our knowledge, there is no data in the literature on the synthesis of NZP phosphate materials using the proposed method. The relevance of the synthesis of these phosphates was due, on the one hand, to their supposed high efficiency as adsorbents of long-lived radionuclides of cesium, strontium and cobalt, and, on the other – to the possibility of immobilization and safe disposal of spent adsorbents by heat treatment and transformation into calcium zirconium CaZr4P6O24 and magnesium zirconium MgZr4P6O24 phosphate ceramic matrices [25].
The aim of the work was to develop a facile and effective method for the synthesis of calcium magnesium zirconium phosphate adsorbents with subsequent transformation into ceramic matrices with an NZP structure. In the work (i) first the use of dolomite phosphate as precursors of available phosphates of calcium and magnesium was proposed, (ii) the influence of synthesis conditions on the chemical composition, structure and textural characteristics of the obtained materials was studied, (iii) the effect of NH4+ substitution of phosphate adsorbents was carried out, (iv) the efficiency of the adsorption of 137Cs, 85Sr, 60Co in a wide pH range was investigated. The obtained results open up significant prospects for the synthesis of materials for the effective radionuclides adsorption and safe immobilization of spent adsorbents.
Section snippets
Adsorbent synthesis
Solutions of phosphoric H3PO4 (85 wt%) and nitric HNO3 acids (65 wt%), zirconium oxynitrate ZrO(NO3)2 were used as reagents for the synthesis of calcium magnesium zirconium phosphates. Samples of phosphatized dolomite Ca0.7Mg0.3HPO4·2H2O (PD-1) and Ca2.65Mg3(NH4)1.3(PO4)4(CO3)0.3·6H2O (PD-2) were obtained from thermally activated dolomite at 800 °C (Ruba deposit, Belarus) with a calcium and magnesium content of 6.74 and 6.55 mmol/g, respectively. Sample PD-1 was synthesized by interaction with
XRD
Analysis of XRD patterns of the PD-1 based phosphates (Fig. 1a) showed that the formed zirconium phosphates were amorphous. At a ratio of ZrO(NO3)2/PD-1 of 2.2 mmol/g, PD-1-2 was represented by the reflexes of calcium hydrophosphate, which is one of the components of PD-1 precursor. The sample PD-1-4 with the maximum content of zirconium did not contain crystalline phases. At the same time, it should be noted that the presence of only calcium hydrophosphate reflexes for PD-1-2 indicated that
Conclusions
Firstly, calcium magnesium zirconium phosphate adsorbents were prepared by heterogeneous interaction of phosphatized dolomites with zirconium oxynitrate solution. The effect of synthesis conditions on phase and chemical composition, texture characteristics and morphology was studied. The obtained materials contained amorphous calcium magnesium zirconium phosphates that was supported by XRD, FTIR, EDX analysis and had developed mesoporous structure, which was proved by N2 adsorption–desorption
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was financially supported by Belarusian Republican Found for Fundamental Research (Grant No. X20MC-022).
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2022, Journal of Cleaner ProductionCitation Excerpt :According to the XRD data, PD-1-2 and PD2-2 phosphates with a lower content of zirconium were composites containing amorphous mixed phosphate and calcium and magnesium hydrogen phosphates ‒ CaHPO4·2H2O and MgHPO4·3H2O. Samples with the maximum content of zirconium (PD-2-2 and PD-2-4) were presented only by an amorphous phase (Ivanets et al., 2021). The chemical composition and textural properties of prepared phosphates are presented in Table 1.