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Article

Investigation of Magnesium-Potassium Phosphates as Potential Nuclear Waste Form for the Immobilization of Minor Actinides

by
Hans-Conrad zur Loye
1,*,
Petr Vecernik
2,
Monika Kiselova
2,
Vlastislav Kašpar
2,
Hana Korenkova
2,
Vlastimil Miller
2,
Petr Bezdicka
3,
Jan Šubrt
3,
Natalija Murafa
3,
Volodymyr Shkuropatenko
2,4 and
Sergey Sayenko
2,4,*
1
Department of Chemistry and Biochemistry, University of South Carolina, 631 Sumter Street, Columbia, SC 29208, USA
2
ÚJV Řež, a. s., Nuclear Research Institute, Hlavní 130, 250 68 Řež, Czech Republic
3
Institute of Inorganic Chemistry of the Czech Academy of Sciences, Husinec-Řež 1001, 250 68 Řež, Czech Republic
4
KIPT, Kharkov Institute of Physics and Technology, 1 Akademichna str., 61108 Kharkiv, Ukraine
*
Authors to whom correspondence should be addressed.
Inorganics 2024, 12(12), 311; https://doi.org/10.3390/inorganics12120311
Submission received: 18 September 2024 / Revised: 26 November 2024 / Accepted: 26 November 2024 / Published: 28 November 2024
(This article belongs to the Section Inorganic Materials)
Browse Figures
Graphical abstract
"> Figure 1
<p>Compressive strength of MKP specimens with different M/P ratios.</p> "> Figure 2
<p>XRD pattern of pure MKP specimen.</p> "> Figure 3
<p>XRD patterns of MKP specimen with Sm<sub>2</sub>O<sub>3</sub> and Nd<sub>2</sub>O<sub>3</sub> additives and pure MKP specimen.</p> "> Figure 4
<p>XRD patterns of MKP specimen with Sm(NO<sub>3</sub>)<sub>3</sub>·6H<sub>2</sub>O and Nd(NO<sub>3</sub>)<sub>3</sub>·6H<sub>2</sub>O additives and pure MKP specimen.</p> "> Figure 5
<p>Raman spectra of obtained MKP-based materials: MKP/Nd—with Nd<sub>2</sub>O<sub>3</sub> additive, MKP/Sm—with Sm<sub>2</sub>O<sub>3</sub> additive, MKP/SmN—with Sm(NO<sub>3</sub>)<sub>3</sub>·6H<sub>2</sub>O additive.</p> "> Figure 6
<p>SEM image of MKP specimen with addition of Nd<sub>2</sub>O<sub>3</sub> (<b>a</b>) and Sm<sub>2</sub>O<sub>3</sub> (<b>b</b>).</p> "> Figure 7
<p>SEM high magnification image and EDS data of MKP specimen with adding of Nd<sub>2</sub>O<sub>3</sub>: (<b>a</b>) grey matrix, (<b>b</b>) light-colored agglomerate.</p> "> Figure 8
<p>SEM elemental mapping for MKP specimen with Nd<sub>2</sub>O<sub>3</sub> additive.</p> "> Figure 9
<p>SEM image for MKP specimen prepared via the addition of an aqueous solution of Sm(NO<sub>3</sub>)<sub>3</sub>·6H<sub>2</sub>O, the yellow box is the area for EDS analysis.</p> "> Figure 10
<p>SEM elemental mapping for MKP specimen prepared via the addition of an aqueous solution of Sm(NO<sub>3</sub>)<sub>3</sub>·6H<sub>2</sub>O additive.</p> "> Figure 11
<p>TEM image of a particle of MKP specimen prepared via the addition of an aqueous solution of Nd(NO<sub>3</sub>)<sub>3</sub>·6H<sub>2</sub>O and EDS spectra for areas (<b>a</b>,<b>b</b>).</p> "> Figure 12
<p>The pH value of hardened (28 d) MKP specimens during leaching test.</p> "> Figure 13
<p>Cumulative concentrations for Nd and Sm.</p> ">
Versions Notes

Abstract

:
Several recent studies have evaluated technologies of spent nuclear fuel processing specifically for solidifying transuranic (TRU) waste as a by-product of fission. Of the TRU group, plutonium and the minor actinides will be responsible for the bulk of the radiotoxicity and heat generation of spent nuclear fuel in the long term (300 to 20,000 years). In this study, we investigated magnesium potassium phosphate (MKP)-based compounds as host waste forms for the encapsulation of inactive trivalent Nd and Sm as analogues of the minor trivalent actinides, Am and Cm. Waste forms were fabricated under ambient atmospheric conditions by adding 5 wt.% of substances containing Nd or Sm via the following two routes: powder oxides and aqueous solutions of nitrate salts. Waste form performance was established using strength and aqueous medium leaching tests of MKP-based specimens. The MKP materials were analyzed by X-ray diffraction (XRD), scanning and transmission electron microscopy (SEM and TEM), energy-dispersive X-ray spectroscopy (EDS), and Raman spectroscopy. The waste forms exhibited a compressive strength of ≥30 MPa and were durable in an aqueous environment. The leachability indices for Nd and Sm, as per the ANS 16.1 procedure, were 19.55–19.78 and 19.74–19.89, respectively, which satisfy the acceptable criteria (>6). The results of the present room temperature leaching study suggest that MKPs can be effectively used as a host material to immobilize actinides (Am and Cm) contained in TRU waste.

Graphical Abstract">
Graphical Abstract

1. Introduction

1.1. Magnesium—Potassium Phosphate Materials

Long-term controlled storage or disposal is one of the key stages of radioactive waste stream management in terms of radiation safety. The preparation of the waste for this stage involves the transfer of waste into a stable solidified form using stable matrices. Cementation has been used extensively in the nuclear industry for radioactive waste (RAW) management of low and intermediate activity levels, despite the disadvantages of the method, especially the relatively low degree of waste salt loading, as well as low hydrolytic stability of the cementitious compounds. Vitrification is currently the only high-level waste (HLW) management technology that is applied in industry [1]. The disadvantages of the method are low chemical and crystallization resistance of the glass at elevated temperatures, as well as the need to use expensive and complex high temperature melters, the disposition of which, after the end of their relatively short operational lifetime, represents an unresolved radioecological problem [2].
Reference [2] notes that ceramic materials [3], and especially synthetic analogues of natural phosphate minerals [4,5,6] are considered alternatives to cement and glass for the immobilization of RAW, primarily obtained after the reprocessing of spent nuclear fuel (SNF) and containing long-lived isotopes of the highly radiotoxic actinides and rare earth elements. One strategy is to take advantage of hierarchical structures [7,8] in phosphate-based materials as potential tailored nuclear waste forms. New waste forms are not expected to replace large scale use of glass or cementitious waste treatment technologies, which are currently the best compromise of performance, processability, predictability, social acceptance, and waste throughput. Alternative waste forms should be considered to augment the existing technologies (e.g., secondary, and off-gas treatment) or utilized for small waste quantities that do not warrant large scale processing facilities.
Referring to [2], the mineral-like phosphate materials, in particular, chemically bonded phosphate ceramics (often referred to as ceramicrete), that were typically obtained at room temperature in aqueous solution by the chemical interaction between metal(II) oxides (MgO, ZnO, FeO, CaO) and orthophosphoric acid (H3PO4) or its derivatives (for example, dihydrogen phosphate of metals or ammonium) are widely developed and studied [9,10]. Magnesium potassium phosphate (MKP)-based materials, particularly MKP cements, are of growing interest as alternatives to Portland-based cements. Note that MKP cement is a binary cement consisting of calcined magnesia and potassium di-hydrogen phosphate. The phosphate-based binders gain their strength via an acid–base reaction (Equation (1)) between MgO, potassium dihydrogen phosphate (KH2PO4), and water, to yield magnesium potassium phosphate hexahydrate—MgKPO4·6H2O, which plays a key role in the strength and durability of the obtained solidified MKP cement mortar.
MgO + KH2PO4 + 5H2O → MgKPO4·6H2O + Qheat
This reaction product, MgKPO4·6H2O, also known as K-struvite, is an analogue of the struvite mineral (NH4MgPO4·6H2O) [11], and is naturally cementitious and often found in guano and kidney stones [12]. Due to the high reactivity and rapid reaction rate, MKP cement sets quickly and the MKP paste exhibits excellent hardness after only room temperature treatment. It was previously reported [10] that to reduce the rate of this reaction and, accordingly, to ensure a technologically acceptable setting time and tight packing of the resulting mixture into containers for subsequent storage, MgO powder should be used in the calcined form after a preliminary heat treatment at 1300–1500 °C. Although boron compounds are sometimes added to the MKP cement to slow the reaction rates, they act only as pH-buffering agents without participating in the MgO–KH2PO4–H2O reaction/network or improving the final strength.
The MKP cements are being actively researched for their use in a wide range of applications, such as rapid repair of damaged concrete structures that require only a short interruption of services [13,14], biocements, e.g., dental [15], solidification of urban river dredged sludge [16], immobilization of galvanic wastes [17], and 3D printing [18,19]. The main factors controlling the performance of concrete products manufactured from MKP cement, e.g., MKP cement mortars, are the following two ratios: W/C—the mass ratio between water and cement (cement includes magnesia and KH2PO4), and M/P—the magnesia-to-phosphate molar ratio (MgO/KH2PO4). Potentially advantageous properties of these MKP cement-based binders include a near-neutral pH, low water demand, low drying shrinkage, and high early compressive strength [20,21,22,23].
Another area of application, largely explored in recent years, is the use of magnesium potassium phosphate matrices for stabilization and solidification of radioactive and hazardous wastes, with pioneering work performed at the Argonne National Laboratory [24] and subsequent work by other groups [25,26,27,28,29,30,31,32,33,34,35,36,37,38]. Wagh et al. [34] reported on a phosphate material with a K-struvite structure, into which inactive Cs was introduced as an isomorphic substitution into the crystalline lattice. This was accomplished by the partial substitution of K with Cs in the crystalline lattice and the formation of the (K, Cs)-struvite mineral Mg(K, Cs)PO4·6H2O.
Vinokurov et al. reported on the use of magnesium–potassium–phosphate compound (MKP) for the immobilization of liquid RAWs in the form of nitric acid solutions containing actinides and rare earth elements [2]. In this reference, it was also established that the compounds are composed of crystalline phases—analogues of natural phosphate minerals (struvite, metaankoleite). Low leaching rates of radionuclides from the compound are established, in particular, for 241Am—5.3 × 10−7g/(cm2∙day).
It was found that the compressive strengths of MKP-based waste forms are approximately two times higher than those of ordinary Portland cement grouts [24], and, as a rule, meet the regulatory national requirements for cement compositions—not less than 5 MPa, that is noted in the reference [28]. According to European WAC (waste acceptance criteria), compressive strength (Fc) should be the following: Fc > 5 MPa at 28 days of curing and Fc > 7 MPa at 90 days [39]. In addition, MKP-based matrices exhibit higher resistance against radioactivity compared to Portland cement products [40], confirming the potential of magnesium—potassium phosphate materials for radioactive waste immobilization. In addition, unlike vitrification or ceramic solidification processes, MKP cements are cost- and energy-efficient and use simple equipment, and, importantly, there is no volatilization of nuclides.

1.2. Minor Actinides

The radioactive waste materials are generally classified and dispositioned according to their hazard level. For example, in the US Department of Energy (DOE), the following three main categories of waste are recognized: low-level waste (LLW), high-level waste (HLW), and transuranic (TRU) waste. These wastes must be processed so that the radioactive and hazardous constituents become permanently stabilized and safely sequestered from the biosphere for millennia [7].
As is well known, elements having atomic numbers greater than that of uranium are called transuranic and are typically man-made and belong to the actinide group in the periodic table of elements. The TRU waste is that which is not classified as HLW but contains alpha-emitting TRU radionuclide elements in concentrations greater than 100 nCi/g (3.7 MBq/kg) and with half-lives greater than 20 years [7,41].
The minor actinide found in spent nuclear fuel include neptunium (Np), americium (Am), curium (Cm), berkelium (Bk), californium (Cf), einsteinium (Es), and fermium (Fm). In accordance with the reference [42], plutonium and the minor actinides (most of all Np, Am, Cm, and Cf) will be responsible for the bulk of the radiotoxicity and heat generation of spent nuclear fuel in the long term (300 to 20,000 years).
Over the past two decades, the DOE Office of Environmental Management has established or is planning industrial-scale processes to remediate the approximately 300 million liters of highly radioactive waste stored in aging underground tanks at the Savannah River and Hanford Sites. Current plans are for these tank wastes to be stabilized as grout (LLW) or glass (HLW) waste forms prior to final disposal, with efforts already underway [42]. Since the decision some decades ago to focus on glass and cementitious materials for tank wastes, significant resources have been expended to design, evaluate, and establish processes to produce vitreous and cementitious waste forms. Nevertheless, there are significant quantities of waste that do not warrant the resources needed to develop industrial-scalable processes, and, thus, for which alternative remediation options could be considered to reduce environmental and financial cost. Finally, there are wastes that are small in volume but troublesome for known technologies (e.g., low solubilities in glass, high mobility, etc.). Actinides may well be classified as this type of waste.
As mentioned, a ceramic material based on an MKP matrix of mixed magnesium oxide and potassium phosphate was created at Argonne National Laboratory for the immobilization of various waste streams [10]; one of the leading scientists of this was Arun Wagh. The work of Russian scientists on the solidification of actinides and REEs is widely known [2,25,26,27,28,29,30]. The Ukrainian authors of this article conducted joint research with A. Wagh from 2013–2016 [34,43]. The purpose of the work documented in this paper is to test the preliminarily optimized MKP-based waste formulations for the immobilization of minor actinide surrogates to demonstrate whether these waste forms will meet requirements for acceptance criteria of storage in controlled sites. Special attention was given to the study of the strength, structure, and chemical resistance, of matrices based on MKP compound containing Nd or Sm, each separately, as minor actinide (Am or Cm) surrogate was carried out and the main results are presented herein. Such waste forms, it is hoped, will isolate targeted radionuclides with increased efficiencies through lower treatment cost and more rapid processing.

2. Materials and Methods

2.1. Materials

The starting materials used in this study were magnesia (MgO, ≥98%, Carl Roth GmbH, Karlsruhe, Germany), potassium dihydrogen phosphate (KH2PO4, ≥98%, Carl Roth GmbH, Karlsruhe, Germany), boric acid (H3BO3, >99%, PENTA Chemicals Unlimited®, Prague, Czech Republic), neodymium oxide (Nd2O3, ≥99%, Thermo Fisher Scientific Inc., Prague, Czech Republic), samarium oxide (Sm2O3, 99.95%, Carl Roth GmbH, Karlsruhe, Germany), neodymium nitrate hexahydrate (Nd(NO3)3·6H2O, 99.9%, Sigma-Aldrich–Merck company, Darmstadt, Germany), and samarium nitrate hexahydrate (Sm(NO3)3·6H2O, 99.9%, Sigma-Aldrich–Merck company, Darmstadt, Germany).

2.2. Specimen Preparation

The synthesis of the MKP matrix was carried out according to Equation (1) using a W/C weight ratio of 1:2 and M/P weight ratios ranging from 1 to 2.7. Boric acid H3BO3, 5 wt.% of the weight of the dry components (MgO + KH2PO4), was added to the initial mixture to decrease the rate of reaction. Prior to use, magnesia MgO was heat-treated at 1200 °C for 2 h in air to reduce reactivity, and then only calcined MgO (fraction < 0.2 mm) was used for all experiments reported herein.

2.2.1. Addition of Nd/Sm as Oxide Powders

The dry mixtures were prepared by mixing MgO powder with KH2PO4 powder and Nd2O3 or Sm2O3 powder using a grinding mill, ALPINE (Alpinetech CZ, Prague, Czech Republic) containing a single pulverizing porcelain bowl with a 400 g capacity, 3.5 cm aluminum oxide balls, speed 50 rpm, mixing time 10 min. Next, boric acid H3BO3 (5 wt.% of the total components, MgO together with KH2PO4), as a reaction retarder, was dissolved in distilled water, and a mixture of dry components was added to this solution and mixed with a mechanical stirrer in a 250 mL plastic container for 3 min until a homogeneous slurry mixture/paste was obtained. Subsequently, the paste was poured into PTFE molds and covered with polyethene film to prevent rapid drying. Cubic specimens of 3 × 3 × 3 cm were demolded after 1 day and kept for at least 28 days to cure indoors under ambient conditions. After 1, 7, and 28 days of exposure, the specimens were tested for compressive strength on a MEGA II-600 DMI-S (FORM + TEST Seidner&Co. GmbH, Riedlingen, Germany universal testing machine at a loading rate of 0.25 MPa/s. Testing of specimens for leaching in distilled water according to the ANS 16.1 method was carried out after 28 days of aging. The concentrations of elements in the leachate were analyzed using an inductively coupled plasma atomic emission spectroscope (ICP–AES, iCAP 6300 Duo, Thermo Fisher Scientific Inc., Waltham, MA, USA).

2.2.2. Addition of Nd/Sm as a Salt Solution

A mixture of dry components (MgO + KH2PO4) was prepared in a lab grinding mill, ALPINE, according to the method described in Section 2.2.1. Then, Nd (or Sm) reagents, i.e., Nd(NO3)3·6H2O or Sm(NO3)3·6H2O, and then H3BO3 were dissolved sequentially in water. After that, the mixture of dry components was added to the resulting solution and mixed in a plastic container until a homogeneous paste was obtained (mechanical stirrer, 3 min). The resulting paste was poured into PTFE molds to create cubic specimens to be used for further tests exactly as described in Section 2.2.1.

2.3. Characterization

2.3.1. Density

The density of the specimens was measured by determining the mass and geometric volume. The density value was determined by averaging 3 density values for each MKP specimen.

2.3.2. Electron Microscopy

The surface morphology and structure of specimens were analyzed using a JSM-6510 with an X-act—10 mm2 SDD Detector (JEOL Ltd., Tokyo, Japan) for scanning electron microscopy (SEM) and X-ray energy dispersive spectroscopy (EDS), and Talos F200X (Thermo Fisher Scientific Inc., Waltham, MA, USA) for high-resolution scanning/transmission electron microscopy (TEM).

2.3.3. XRD Analysis

The phase composition and crystallographic parameters of the materials were studied by X-ray diffraction (XRD) analysis using the following experimental procedure.

Conventional Bragg–Brentano X-Ray Powder Diffraction Measurements

Back-loaded specimen holders were used to minimize preferred orientation. Diffraction patterns were collected with the PANalytical X’Pert PRO diffractometer (Malvern Panalytical Ltd., Malvern, Worcestershire, UK) equipped with a conventional X-ray tube (Cu-Kα radiation, 40 kV, 30 mA) and a linear position sensitive detector PIXcel with an anti-scatter shield. A programmable divergence slit set to a fixed value of 0.5 deg., Soller slit of 0.04 rad, and mask of 15 mm were used in the primary beam. A programmable anti-scatter slit set to a fixed value of 0.5 deg., Soller slit of 0.04 rad, and Ni beta-filter were used in the diffracted beam. Data were collected in the range of 7–90 deg., 2theta with a step of 0.0131 deg., and 500 s/step producing a scan of about 3 h 34 min.

Evaluation of X-Ray Patterns

Qualitative analysis was performed with the HighScore Plus software package (Malvern Panalytical, Almelo, The Netherlands, version 5.2.0) together with the “PDF-5+” database [44]. The line profile analysis was performed using routines implemented in the High Score Plus software [45].
Quantification of the experimental data was performed with the Rietveld method [46]. The Profex 4.3.6/BGMN 4.2.23 code was used for all calculations [47,48,49]. Models of all phases were described as standard Rietveld models [44,50]. Refined parameters comprised unit cell parameters for all phases as well as size and micro-strain broadening parameters. Additional refined non-structural parameters were specimen displacement error and a polynomial for background modelling.

2.3.4. Raman Spectroscopy

Raman spectroscopy was performed on a DXR Raman Spectrometer (Thermo Fisher Scientific Inc., Waltham, MA, USA). The spectral range that was used to characterize MKP materials was between 0 and 3600 cm−1.

2.3.5. Leaching Test

Leaching tests of Nd/Sm doped MKP specimens were carried out according to the ANS 16.1 standard in deionized water for 90 days in a plastic vial at a temperature of 25 °C [51]. The leachate was sampled by ICP–MS measurement, and the fresh leachant was replaced after increasing leaching periods—1, 4, 14, 28, and 43 days. The test lasted 90 days (1 + 4 + 14 + 28 + 43 days). Leachate pH was measured by a pH–Electrode (SenTix 94x(-P) pH 0–14 T 0–30 °C) calibrated using standard buffers between 7.0 and 13.0.
The effective diffusion coefficient (De) and leachability index (LI) were also calculated to quantitatively evaluate whether the MKP matrix can be used for surrogate minor immobilization. The LI value for each element was determined based on the following expressions according to ANS-16.1:
L I = 1 5 ∑ 1 5 log ⁡ β / D e n
where β is a defined constant (1.0 cm2/s).
The De can be calculated using the mass transport equation (i.e., Fick’s second law) as follows:
            D e = π a n A o Δ t n 2 V S 2 T
where De is the effective diffusion coefficient (cm2/s), V is the volume of the specimen (cm3), S is the geometric surface area of the solid specimen (cm2), an is the amount of the element released from the specimen for the n-th leaching interval, Ao is the initial amount of the element in the specimen prior to leaching, Δtn = tn − tn−1 is the duration of the n-th leaching interval, and T is the leaching time representing the mean time (s) of the n-th leaching interval for a semi-infinite medium as follows:
T = 1 2 t n + t n − 1 2

3. Results and Discussion

3.1. Preparing MKP Specimens and Determining Their Compressive Strength

The properties of the cured MKP-based materials are largely dominated by the properties of the binder phase, i.e., the MKP paste. The selection of the mix proportion of an MKP paste normally consists of setting two parameters—the magnesia-to-phosphate molar ratio (M/P) and the water-to-cement mass ratio (W/C). According to Equation (1), the theoretical W/C ratio among the starting raw materials, for an M/P molar ratio of 1/1, can be calculated as 0.51. The effects of W/C on the properties of MKP paste are clear, e.g., higher W/C can lead to longer setting time, better workability, higher porosity, higher permeability, and lower strength [17,37,52,53,54]. It is quite clear that a decrease in W/C will result in a decrease in the paste’s workability and will impede the manufacture of solid products.
The M/P ratio is another key factor influencing the development of physico-mechanical properties of MKP-based compounds. Even though the theoretical molar ratio MgO/KH2PO4 for a complete reaction according to Equation (1) is 1, it was shown that the actual consumption of MgO in the reaction is 20.8% for M/P = 4 and only 8.5% for M/P = 10 [14]. Several studies have shown that this M/P ratio has an optimal effect, since the best mechanical strength is achieved at intermediate M/P ratios. From the literature data [20,52,53], at low M/P ratios (M/P ≤ 1), the lower compressive strength of MKP cement mortar can be explained by the important residual amount of KH2PO4 found in the microstructure. In the opposite case, at high M/P ratios (M/P ≥ 4), the total amount of hydrates formed decreases, leading to decreased cohesion between grains, which also reduces mechanical strength. Thus, as the authors of various articles indicate, optimal properties of the hardened MKP cement-based material are usually achieved with the following intermediate M/P ratios: 1 ≤ M/P ≤ 4.
In this work, MKP specimens with different M/P ratios at constant W/C = 0.5 were obtained and tested. The paste was prepared in a 250 mL plastic container; the basic MKP formulation (M/P = 1) for the starting components included the following: MgO—30.00 g, KH2PO4—101.32 g, H3BO3—6.57 g, H2O—65.66 mL. It was found that the setting time of the paste for all specimens was almost the same and ranged from 9 to 10 min; the maximum temperature reached during reaction (Equation (1)) was 40–42 °C. The results of strength testing of cured specimens are presented in Figure 1.
Research has shown that increasing the air cure time can improve the mechanical strength of MKPs. In other words, the longer the curing time of the MKPs before testing, the better the compression strength was. The MKP specimens with M/P = 2.25 exhibited the highest compressive strength, Fc = 34 MPa (Figure 1), compared to specimens with other M/P ratios. The obtained Fc values of the MKP compounds are in full agreement with the values presented in other references [28,55] and fully satisfy the WAC [39]. A further increase in the amount of MgO (M/P ≥ 2.25) is not advisable due to the fact that there is a decrease in the strength of MKP specimens and unreacted MgO can hydrate with available water to form Mg(OH)2, which is accompanied by a significant increase in volume that can lead to the appearance of cracks in the hardened material and a decrease in its mechanical strength [52].
Taking this into account, when preparing MKP specimens doped with compounds containing Nd/Sm, the ratios M/P = 2.25 and W/C = 0.5 were used. In this case, additives were either Nd2O3 or Sm2O3 oxides in an amount of 5 wt.%, or aqueous solutions of Nd(NO3)3·6H2O or Sm(NO3)3·6H2O (5 wt.%). The density of the pure MKP specimen (ρ) was 1.71–17.2 g/cm3 after 28 days air aging. Upon the addition of 5 wt.% of Nd2O3 or Sm2O3, there is a slight increase in the density of the MKP specimens to ρ = 1.80–1.81 g/cm3. For MKP specimens prepared via the addition of Nd(NO3)3·6H2O or Sm(NO3)3·6H2O solutions, the density did not change significantly compared to the pure MKP specimen and amounted to ρ = (1.69–1.71) g/cm3.
Mechanical tests of MKP specimens after 28 days of curing showed that the compressive strength of specimens with Nd2O3 additives is close to the strength of specimens without additives and the average value was 34 MPa. For MKP specimen with Sm2O3 additive, the average compressive strength slightly increased and was 37 MPa. Similar results were reported for calcium silicate phosphate cement with Sm2O3 additive [56]. On the other hand, the addition of Nd(NO3)3·6H2O or Sm(NO3)3·6H2O reduces the strength to 30 and 32 MPa, respectively. Such changes in strength can be explained by phase differences in the MKP matrix, depending on the method of introducing Nd/Sm, namely, either in the form of oxides or in the form of solutions, which is further confirmed by X-ray diffraction studies. In general, matrices based on MKP containing Nd/Sm additives have a characteristically high strength of ≥30 MPa.

3.2. Microstructural Analysis

3.2.1. XRD Measurements

M/P Effect

In the prepared pure MKP-cured product (M/P = 2.25 and W/C = 0.5), based on XRD analyses (Figure 2), the phases present are K-struvite (MgKPO4·6H2O), with a phase fraction of 77.0%, and periclase (MgO), with a phase fraction of 23.0%. With a decrease in the ratio of MgO to KH2PO4 to M/P = 1.93, the phase fraction of MgKPO4·6H2O and MgO in the cured MKP material remained almost unchanged (76.8% and 22.8%, respectively) and the X-ray diffraction pattern is the same. However, the presence of diffraction lines for residual KH2PO4 (0.4%) was detected, which has a higher solubility and can worsen the stability of the MKP matrix.
Therefore, a ratio of M/P = 2.25 is suitable for preparing MKP materials, similar to what was found from the strength tests (Figure 1).

Specimens Fabricated with Addition of Oxides

The X-ray diffraction patterns of the MKP material fabricated with the addition of 5 wt.% Sm2O3 (Nd2O3) are shown in Figure 3. Regarding the Sm2O3 additive, XRD measurements indicated that the phase fractions of K-struvite and periclase are 80.8% and 14.2%, respectively, while the phase fraction of samarium oxide was 5.0%.
Similar to the MKP containing Sm2O3 the XRD pattern for specimen with Nd2O3 contains the main diffraction lines of K-struvite and periclase with phase fractions of 78.9% and 15.5%, respectively. Diffraction lines of the original neodymium oxide were also detected with a phase fraction of 1.9%. However, it differs due to the presence of diffraction lines due to Nd(OH)3 (2.3%), boric acid H3BO3 (1.0%), and boron phosphate BPO4 (0.4%). It has been reported that the bond strength increases from La to Nd due to the decrease in the ionic size of the lanthanides (Ln3+) from La to Nd. As a result, it is expected that Nd attracts a larger number of OH groups compared to other lanthanides, as reported by Reddy [57]. It is possible that this is the reason for the observed Nd(OH)3 diffraction lines in the X-ray diffraction pattern of the MKP material with the addition of Nd2O3 (Figure 3). X-ray diffraction measurements showed the presence of K-struvite with a slightly lower phase fraction (78.9%) compared to the Sm-doped specimen (80.8%) as well as the presence of H3BO3 and BPO4 (Figure 3).

Specimens Fabricated with Addition of Nitrate Salt Solutions

The XRD measurements for materials fabricated with added aqueous solutions of Nd(NO3)3·6H2O and Sm(NO3)3·6H2O resulted in the identification of the following 3 phases: strong diffraction lines of K-struvite and periclase, as well as weak intensity diffraction lines of KNO3. At the same time, for the material fabricated with the addition of Nd(NO3)3·6H2O, the phase fractions were 72.8% K-struvite, 18.6% periclase, and 8.6% KNO3. In the material fabricated with the addition of Sm(NO3)3·6H2O, the same phases were found, but with the following slightly different phase fractions: 75.9% K-struvite, 17.6% periclase, and 6.5% KNO3. Diffraction patterns of a specimen of pure MKP and a specimen of MKP with the addition of Nd(NO3)3·6H2O are shown in Figure 4. A slight shift in the diffraction lines of K-struvite is observed for the MKP specimen prepared with the addition of aqueous solutions of Nd(NO3)3·6H2O relative to that of the pure MKP specimen.
Characteristic crystalline lattice parameters of the pure MKP specimens and after the introduction of Nd/Sm containing additives are presented in Table 1.
This table provides information on the pure and Nd/Sm-doped MKP specimens, identified as “MKP”, and with additives of Nd2O3 and Sm2O3—“MKP/Nd” and “MKP/Sm”, and with Nd(NO3)3·6H2O and Sm(NO3)3·6H2O—“MKP/NdN” and “MKP/SmN”, respectively. It should be noted that for specimens to which the rare earths were added as aqueous solutions of nitrate salts, a more significant increase in the unit cell parameters of K-struvite was found compared to specimens prepared by adding oxides of neodymium and samarium. Also, comparing the XRD data for the pure K-struvite and the doped K-struvite by neodymium via Nd(NO3)3·6H2O, it can be assumed that a small amount of Nd was incorporated into the MKP K-struvite structure, based on the observed change in the unit cell parameters. Apparently, the same assumption can be applied to the incorporation of samarium.

3.2.2. Raman Spectroscopy Measurements

The Raman spectra of these MKP specimens show the typical phonon vibrational modes around 426, 562, and 943 cm−1 in Figure 5, confirming the formation of the K-struvite, in agreement with the literature [58].
The Raman peak near 295 cm−1 observed in all specimens corresponds to MgO, which is in accordance with data reported in the literature [59]. The presence of MgO in the obtained materials, as mentioned earlier, was also observed in the XRD measurements (Figure 3 and Figure 4). In the Raman spectrum of the MKP/Nd specimen, in addition to the lines related to K-struvite and MgO, there is a series of vibrational modes at 2157, 2380, 2507, and 2622 cm−1, which possibly correspond to presence of the hydration products that were not determined by XRD analysis. As noted earlier, according to XRD measurements, the phase composition of the MKP/Nd specimen consists of K-struvite, MgO, Nd(OH)3, Nd2O3, H3BO3, and BPO4 (Figure 3). If the Raman spectra confirm the presence of K-struvite and MgO for all specimens, then the vibrational modes for Nd(OH)3, Nd2O3, H3BO3, and BPO4 are likely obscured by the characteristic bands of K-struvite and MgO [60,61,62,63].
The vibrational modes of Sm2O3 (MKP/Sm) at approximately 418 and approximately 453 cm−1, [64], are located close to the K-struvite peaks at approximately 426 and approximately 462 cm−1 in the Raman spectrum. The third peak at approximately 325 cm−1, characteristic of Sm2O3, is shifted to 340 cm−1 and has weak intensity.
In the Raman spectrum of the MKP/SmN specimen, in addition to the vibrational modes of K-struvite and MgO, there is a peak at approximately 1042 cm−1, assigned to KNO3 [65], which is completely consistent with the XRD data (Figure 4).
Also, in the spectra of all specimens examined, a wide low-intensity peak is observed in the region from approximately 2800 to approximately 3300 cm−1 corresponding to stretching vibrations of water [58].

3.2.3. SEM Analysis

Specimens Fabricated with Addition of Oxides

The SEM analysis of specimens, both with the addition of Nd2O3 and with the addition of Sm2O3, revealed that in both cases the microstructure corresponds to a two-phase material in the form of a grey matrix that contains light inclusions of various sizes (Figure 6a,b). The obtained MKP products exhibited needle- or prism-like substances that are MKP crystals of K-struvite, which is most clearly visible for the specimen with the addition of Sm2O3 (Figure 6b).
To identify the elemental composition, SEM studies of matrix sections, as well as of the light inclusions, were carried out, illustrated using the example of a MKP specimen with the Nd2O3 additive. In the high magnification SEM photomicrograph and EDS spectrum, strong peaks of Mg, K, P, and O were detected, confirming the formation of a K-struvite-based matrix (grey), which is shown in Figure 7a. The Nd peaks of very low intensity are observed. The most intense Nd signals are observed in the light-colored agglomerates (≤10 μm) embedded in the MKP matrix (Figure 7b).
Figure 8 demonstrates the presence of the main elements that make up K-struvite (K, Mg, P). It should be noted that the distribution of magnesium is uneven, unlike K and P. The presence of areas with increased Mg content is associated with unreacted MgO particles, as indicated by XRD data (Figure 3).
The highlighted area demonstrates that aggregates of Nd2O3 particles are clearly dispersed in the binding MKP matrix (Figure 8). In this way, the phosphate matrix surrounds the Nd2O3 particles and prevents contact of Nd2O3 with the external environment, such as water, as required by the ANS 16.1 standard. This can be used as a basis for producing waste forms to immobilize solid waste pieces containing minor actinides.

Specimens Fabricated with the Addition of Nitrate Salt Solutions

The microstructure of MKP for specimens prepared using salt solutions of Nd or Sm (Figure 9) differs from specimens prepared using oxides of Nd or Sm (Figure 8).
In the case of Sm2O3 or Nd2O3 additives, the material consists of an MKP matrix containing embedded agglomerates of oxide particles. The EDS maps of K, Mg, P, and Sm for the same highlighted area are shown in Figure 10. When neodymium or samarium was introduced in the form of an aqueous solution, SEM analysis indicates their presence at the microlevel throughout the binding phosphate matrix of K-struvite, that is shown in Figure 10 for Sm. In this case, Sm is localized unevenly in the MKP matrix, concentrating in several local areas.

3.2.4. TEM Analysis

The results of TEM observation (200 kV) of a specimen prepared via the addition of an aqueous solution of Nd(NO3)3·6H2O are shown in Figure 11. The MKP material consists of interconnected light (a) and dark (b) formations of various sizes from 100 nm and greater. In turn, formations consist of smaller, densely packed nano scale particles.
For the EDS spectrum of area (a), only strong peaks of Mg, P, K, and O were observed. For area (b), EDS analysis showed strong peaks of Mg, K, P, and O, which are the main elements of K-struvite, along with weak, low intensity peaks of Nd. It can be assumed that some of the Nd signal in the dark crystallites is associated with Nd substituted into the K-struvite crystalline structure. This can also be confirmed by XRD measurements, revealed by an increase in the unit cell parameters of K-struvite in the case of specimens prepared via the addition of an aqueous solutions of Nd(NO3)3·6H2O (Table 1). The struvite mineral family is known to accept a wide range of substituents into the ABC·6H2O structure. These include substitutions of monovalent cations on the A-site (NH4+, K+, Rb+, Cs+, Tl+), divalent cations on the B-site (Mg2+, Ni2+, Zn2+, Co2+, Cd2+, Cr3+, Mn2+, VO2+), and trivalent oxyanions on the C-site (PO43–, AsO43) [23].

3.3. Leaching Test Results

3.3.1. pH Dependence

The initial leachant was characterized as a neutral aqueous fluid at pH = 6.5. To evaluate the water corrosion behavior of Nd/Sm doped MKP cementitious material, the change in the pH value during specimen soaking in deionized water under different times is plotted in Figure 12.
The immersion fluid became alkaline after 1 day, with an increase in the pH value from 6.5 to (8.2–9.42). These high-alkaline conditions in the leachate can be related to active hydration when MKP cementitious material is exposed to water [66]. This may be caused by the hydration of unreacted magnesium oxide (XRD data in Figure 3 and Figure 4) to form magnesium hydroxide [22,67] and the hydrolysis reaction of phosphate anions [37]. Over the period from 1 to 5 days, the pH decreases to 7.55–8.14. The reason for this is believed to be the presence of an unreacted KH2PO4 residue in MKP specimens and its dissolution reducing the pH of the immersion liquid. By the end of the testing period, the pH ceased to change significantly with immersion age. All leachates showed similar pH trends with time.

3.3.2. Leachability Indices

From the results of the leaching tests, the leaching characteristics of the Nd and Sm elements from the hardened MKP specimens were evaluated based on their cumulative concentrations released in the leachates.
The concentrations of the elements Nd and Sm leached from the MKP specimens were analyzed using an inductively coupled plasma atomic emission spectroscope (ICP–AES, iCAP 6300 Duo, Thermo Fisher Scientific Inc., Waltham, MA, USA). The content of elements in the leachate for different periods were summed up, and Figure 13 shows curves of cumulative concentrations for Nd and Sm. On the 90th day of leaching, the total concentration of elements for MKP specimens with these elements introduced through oxides and salt solutions was, respectively, for Nd, as follows: 2.17 × 10−6 g/L and 1.7 × 10−6 g/L, and for Sm as follows: 1.95 × 10−6 g/L and 1.48 × 10−6 g/L.
Interestingly, when introducing Nd and Sm into the MKP matrix as an aqueous solution, the cumulative concentrations of Nd and Sm in the leachate are lower than when Nd and Sm are introduced as oxides. Thus, the introduction of Nd and Sm into the MKP matrix via a solution of the corresponding nitrate salts, Nd(NO3)3·6H2O and Sm(NO3)3·6H2O, provides better specimen stability in the water medium.
The measured concentrations of Nd and Sm in the leachate after fixed time intervals were used to calculate leachability indices, LI, according to Equations (2)–(4). The US Nuclear Regulatory Commission has defined a minimum value of six (6.0) for the leachability index as an acceptance criterion for radioactive waste forms to be disposed of at controlled sites [68], i.e., the leachability index, as calculated in accordance with ANS 16.1, should be greater than 6. Indeed, the leachability indices for Nd and Sm exceed the value of 19, as can be seen from Table 2.
The reliability of these results is consistent with studies [24,69], where the LI value for Ce, as well as an element from the lanthanide group along with neodymium and samarium, ranged from 16.7 to 19.7. To compare the results of LI measurements with those obtained by other authors, the following should be noted. Reference [26] demonstrates the possibility of immobilization of simulated liquid alkaline high-level waste containing actinides, fission, and corrosion products in magnesium–potassium–phosphate matrices. The obtained LI value for 241Am was 14.6 at 44 wt.% waste loading in the MKP matrix. Another reference [43] notes that in the case of immobilization of the Hanford K-Basin waste sludge (KE and KW sludge) simulator in the MKP matrix, the LI value for Nd was 15.4 (KE-waste form) and 15.6 (KW-waste form) and for Sm—11.3 (KE-waste form) and 11.8 (KW-waste form) at 36.5 wt.% waste loading. The LI values for Nd and Sm obtained in this work are higher (>19). This is due to the fact that the higher waste loading (>5 wt.%) typical for the works [26,43] leads to deterioration of the matrix properties, including an increase in leachability, and, therefore, to a decrease in the LI values.
The current leaching experiments were conducted at 25 °C to follow the standard evaluation protocol in ANS 16.1 as already referenced. Depending on the minor actinide loading in the radioactive wastes, higher temperature could be, however, assumed. Therefore, the leaching tests at more extreme conditions would be of interest as one of next research topics.

4. Conclusions

This work demonstrated the possibility of producing magnesium potassium phosphate matrices containing Nd and Sm, as surrogate for the minor actinides Am and Cm. Two methods of introducing Nd and Sm into MKPs were used: as oxides and as aqueous solutions of nitrate salts.
Compressive strength tests have shown that the introduction of Nd and Sm in the form of Nd2O3 (or Sm2O3) as well as Nd(NO3)3·6H2O (or Sm(NO3)3·6H2O) in an amount of 5 wt.% leads to mechanically robust MKP specimens, which have a fairly high strength of ≥30 MPa.
An increase in the unit cell parameters of K-struvite was found for MKP upon the addition of Nd(NO3)3·6H2O (or Sm(NO3)3·6H2O) compared to pure MKP.
It was shown that the microstructure of the MKP material in the case of the addition of Nd2O3 (or Sm2O3) was characterized by a compositional structure with included oxide particles with sizes ≤ 10 μm. When introducing surrogate minor actinides via nitrate salt solutions, for example, Sm(NO3)3·6H2O, the presence of such particles is not observed; a uniform dispersion of Sm is observed throughout the MKP matrix.
The leaching tests of MKPs according to ANS 16.1 standard were carried out, and the results of the performed studies demonstrate the chemical stability of the MKP matrix with Nd (or Sm) additives under water attack.
The results of this research demonstrate the quite acceptable characteristics of MKP matrices on compression strength as well as low leaching of Nd and Sm, indicating that the MKP matrices can function as highly effective insoluble and durable waste forms for the immobilization of minor actinides.

Author Contributions

Conceptualization, H.-C.z.L., P.V. and S.S.; Methodology, V.K.; Formal analysis, V.K., P.B. and S.S.; Investigation, M.K., H.K., V.M., J.Š., N.M. and V.S.; Writing—original draft, S.S.; Writing—review & editing, H.-C.z.L. and P.V. All authors have read and agreed to the published version of the manuscript.

Funding

Research was conducted by the Center for Hierarchical Waste Form Materials (CHWM), an Energy Frontier Research Center (EFRC). Research was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award DE-SC0016574. The research was carried out within the framework of the “UofSC-UJV Subaward No: 23-5123 Supporting Research to Develop TRU-Containing Phosphates for the Immobilization of Radioactive Elements”, 2022–2024. This work was supported by Research Infrastructure NanoEnviCz, supported by the Ministry of Education, Youth and Sports of the Czech Republic under Project LM2023066.

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding authors.

Acknowledgments

Ukrainian authors are grateful to Czech colleagues for the opportunity to work at UJV from mid-2022 since the war with Russia began in Ukraine. The authors thank Petra Ecorchard (Institute of Inorganic Chemistry of the Czech Academy of Sciences) for performing the Raman spectroscopy studies and her assistance in processing the paper.

Conflicts of Interest

The authors declare no conflict of interest.

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Inorganics 12 00311 g001
Figure 1. Compressive strength of MKP specimens with different M/P ratios.
Figure 1. Compressive strength of MKP specimens with different M/P ratios.
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Figure 2. XRD pattern of pure MKP specimen.
Figure 2. XRD pattern of pure MKP specimen.
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Figure 3. XRD patterns of MKP specimen with Sm2O3 and Nd2O3 additives and pure MKP specimen.
Figure 3. XRD patterns of MKP specimen with Sm2O3 and Nd2O3 additives and pure MKP specimen.
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Figure 4. XRD patterns of MKP specimen with Sm(NO3)3·6H2O and Nd(NO3)3·6H2O additives and pure MKP specimen.
Figure 4. XRD patterns of MKP specimen with Sm(NO3)3·6H2O and Nd(NO3)3·6H2O additives and pure MKP specimen.
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Figure 5. Raman spectra of obtained MKP-based materials: MKP/Nd—with Nd2O3 additive, MKP/Sm—with Sm2O3 additive, MKP/SmN—with Sm(NO3)3·6H2O additive.
Figure 5. Raman spectra of obtained MKP-based materials: MKP/Nd—with Nd2O3 additive, MKP/Sm—with Sm2O3 additive, MKP/SmN—with Sm(NO3)3·6H2O additive.
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Figure 6. SEM image of MKP specimen with addition of Nd2O3 (a) and Sm2O3 (b).
Figure 6. SEM image of MKP specimen with addition of Nd2O3 (a) and Sm2O3 (b).
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Figure 7. SEM high magnification image and EDS data of MKP specimen with adding of Nd2O3: (a) grey matrix, (b) light-colored agglomerate.
Figure 7. SEM high magnification image and EDS data of MKP specimen with adding of Nd2O3: (a) grey matrix, (b) light-colored agglomerate.
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Figure 8. SEM elemental mapping for MKP specimen with Nd2O3 additive.
Figure 8. SEM elemental mapping for MKP specimen with Nd2O3 additive.
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Figure 9. SEM image for MKP specimen prepared via the addition of an aqueous solution of Sm(NO3)3·6H2O, the yellow box is the area for EDS analysis.
Figure 9. SEM image for MKP specimen prepared via the addition of an aqueous solution of Sm(NO3)3·6H2O, the yellow box is the area for EDS analysis.
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Figure 10. SEM elemental mapping for MKP specimen prepared via the addition of an aqueous solution of Sm(NO3)3·6H2O additive.
Figure 10. SEM elemental mapping for MKP specimen prepared via the addition of an aqueous solution of Sm(NO3)3·6H2O additive.
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Figure 11. TEM image of a particle of MKP specimen prepared via the addition of an aqueous solution of Nd(NO3)3·6H2O and EDS spectra for areas (a,b).
Figure 11. TEM image of a particle of MKP specimen prepared via the addition of an aqueous solution of Nd(NO3)3·6H2O and EDS spectra for areas (a,b).
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Figure 12. The pH value of hardened (28 d) MKP specimens during leaching test.
Figure 12. The pH value of hardened (28 d) MKP specimens during leaching test.
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Figure 13. Cumulative concentrations for Nd and Sm.
Figure 13. Cumulative concentrations for Nd and Sm.
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Table 1. Unit cell parameters of the MKP specimens.
Table 1. Unit cell parameters of the MKP specimens.
SpecimenUnit Cell Parameters
a/nmb/nmc/nmV/nm3
MKP0.68753(2)0.61629(1)1.10941(3)0.47008
MKP/Nd0.68753(1)0.61630(1)1.10934(2)0.47005
MKP/Sm0.68760(1)0.61631(1)1.10951(1)0.47018
MKP/NdN0.68796(1)0.61651(1)1.10995(2)0.47077
MKP/SmN0.68773(1)0.61641(1)1.10964(1)0.47040
Table 2. Leachability indices.
Table 2. Leachability indices.
ElementSpecimen
MKP/NdMKP/NdNMKP/SmMKP/SmN
Nd19.5519.78--
Sm--19.7419.89
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zur Loye, H.-C.; Vecernik, P.; Kiselova, M.; Kašpar, V.; Korenkova, H.; Miller, V.; Bezdicka, P.; Šubrt, J.; Murafa, N.; Shkuropatenko, V.; et al. Investigation of Magnesium-Potassium Phosphates as Potential Nuclear Waste Form for the Immobilization of Minor Actinides. Inorganics 2024, 12, 311. https://doi.org/10.3390/inorganics12120311

AMA Style

zur Loye H-C, Vecernik P, Kiselova M, Kašpar V, Korenkova H, Miller V, Bezdicka P, Šubrt J, Murafa N, Shkuropatenko V, et al. Investigation of Magnesium-Potassium Phosphates as Potential Nuclear Waste Form for the Immobilization of Minor Actinides. Inorganics. 2024; 12(12):311. https://doi.org/10.3390/inorganics12120311

Chicago/Turabian Style

zur Loye, Hans-Conrad, Petr Vecernik, Monika Kiselova, Vlastislav Kašpar, Hana Korenkova, Vlastimil Miller, Petr Bezdicka, Jan Šubrt, Natalija Murafa, Volodymyr Shkuropatenko, and et al. 2024. "Investigation of Magnesium-Potassium Phosphates as Potential Nuclear Waste Form for the Immobilization of Minor Actinides" Inorganics 12, no. 12: 311. https://doi.org/10.3390/inorganics12120311

APA Style

zur Loye, H.-C., Vecernik, P., Kiselova, M., Kašpar, V., Korenkova, H., Miller, V., Bezdicka, P., Šubrt, J., Murafa, N., Shkuropatenko, V., & Sayenko, S. (2024). Investigation of Magnesium-Potassium Phosphates as Potential Nuclear Waste Form for the Immobilization of Minor Actinides. Inorganics, 12(12), 311. https://doi.org/10.3390/inorganics12120311

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