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Article

Comprehensive Dissolution Study on Two Double Ce(IV) Phosphates with Evidence of Secondary CeO2 Nanoparticle Formation

by
Anastasiia L. Listova
1,
Anastasiia S. Kuzenkova
1,
Mikhail A. Gerasimov
1,
Elizaveta S. Kulikova
2,
Roman D. Svetogorov
1,2,
Daniil A. Novichkov
1,
Alexei A. Averin
3,
Vasiliy O. Yapaskurt
4,
Anna Yu. Romanchuk
1,
Stepan N. Kalmykov
1 and
Tatiana V. Plakhova
1,*
1
Faculty of Chemistry, Lomonosov Moscow State University, Leninskie Gory 1, 119991 Moscow, Russia
2
National Research Centre Kurchatov Institute, Akademika Kurchatova pl. 1, 123182 Moscow, Russia
3
Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Leninsky Prospect 31 bld. 4, 119071 Moscow, Russia
4
Faculty of Geology, Lomonosov Moscow State University, Leninskie Gory 1, 119991 Moscow, Russia
*
Author to whom correspondence should be addressed.
Molecules 2025, 30(10), 2105; https://doi.org/10.3390/molecules30102105
Submission received: 9 April 2025 / Revised: 3 May 2025 / Accepted: 7 May 2025 / Published: 9 May 2025
(This article belongs to the Section Physical Chemistry)
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Abstract

:
Herein, we present a comprehensive study on the dissolution behaviour of two sodium–cerium(IV) phosphate phases synthesised hydrothermally from CeO2 nanoparticles: crystalline Na2Ce(PO4)2 and nanocrystalline NaCe2(PO4)3. For the first time, experimental dissolution data were obtained for both compounds over a wide pH range (1.5–10) under long-term equilibration. The crystalline phase undergoes pH-dependent transformation, including recrystallisation at a near-neutral pH and the formation of secondary CeO2 nanoparticles above pH 7. In contrast, the nanophase NaCe2(PO4)3 exhibits exceptional structural and chemical stability, showing no signs of recrystallisation, phase transformation, or CeO2 formation, even after extended ageing. The experimental results help refine the thermodynamic stability conditions for cerium phosphate and oxide phases, providing insights into the reversible transformation pathways between CeO2 and Ce(IV) phosphates as governed by pH.

1. Introduction

Inorganic phosphates represent a remarkable class of materials widely recognised for their rich structural chemistry, chemical diversity, and unique physicochemical properties [1,2,3,4,5,6]. Among their most valuable characteristics are exceptionally low solubility and high resistance to corrosion by geological fluids [7]. These features make phosphate-based materials particularly appealing for environmental remediation applications, notably as stable matrices for immobilising radioactive waste and selective adsorbents for contaminant removal from wastewater [8,9,10,11,12]. Phosphate-based ceramics such as britholites, monazite/brabantite solid solutions, and thorium phosphate diphosphate (β-TPD), as well as β-TPD/monazite composites, have been extensively studied for their exceptional chemical durability and potential application in actinide immobilisation. Consequently, understanding the dissolution mechanisms, long-term stability, and structural transformations of phosphate materials is essential for assessing their effectiveness in minimising the risks associated with radionuclide leaching into the environment.
Most available studies determined that lanthanide and actinide phosphates are considered soluble only under acidic conditions, reflecting strong binding between f-element cations and phosphate groups [7,13,14,15]. Tetravalent (e.g., Th, Pu) phosphates show significantly lower solubility than trivalent (e.g., La, Nd) ones. Reports indicate that the solubility product (logKsp) values for trivalent phosphates typically range from approximately −25 to −27, varying based on cation radii and trivalent phosphate structures (e.g., rhabdophane, xenotime) [7,16]. Thorium-based phosphates such as Th2(PO4)2(HPO4)·H2O or Pux/2Th2x/2(PO4)2(HPO4) H2O exhibit extremely low solubility products (logKsp~−60) [7]. In many environments, phosphate precipitation can effectively limit actinide mobility [17,18].
Beyond the influence of solution acidity, several other factors critically impact materials’ solubility, including the crystal structure type, temperature, ageing time, or presence of complexing agents. Lanthanide phosphate solubility decreases with increasing temperature (up to 300 °C), a phenomenon referred to as retrograde solubility [19,20]. This behaviour is observed for certain rare-earth element phosphates and is attributed to the weak complexation of trivalent cation in solution, temperature-induced changes in material crystallinity, and modifications to hydration dynamics. Liu and Byrne also highlighted that freshly precipitated lanthanide phosphates are significantly more soluble than aged and well-crystallised counterparts, emphasising the importance of synthesis conditions and prolonged solution–solid interactions [16]. As a result of the dissolution process, the composition of the initial solid phase can change significantly, potentially altering the solubility-control phase and shifting the thermodynamic equilibrium of the system.
The present study focused on cerium phosphates, motivated by the unique redox chemistry of cerium, which can stably exist in both +3 and +4 oxidation states, in contrast to most other lanthanides that predominantly exhibit the +3 state. The Ce3+/Ce4+ redox couple enables redox-tuneable functionality, expanding the application range of cerium-based materials [21,22,23,24,25,26]. Although the dissolution behaviour of Ce(IV) oxide has been extensively studied in different media (including phosphate-rich media) [27,28,29,30,31,32], less attention has been devoted to cerium phosphates. Reliable thermodynamic data are primarily available for rhabdophane-type CePO4, while our knowledge of the solubility behaviour and stability of more complex Ce(IV) phosphate phases in different aqueous media remains limited. At the same time, Ce(IV) phosphates can exhibit greater structural diversity, as additional ions or functional groups (e.g., protons, hydroxyl groups, or other cations) are often incorporated to maintain charge neutrality. This can lead to various complex phosphates, such as hydrophosphates, hydroxyphosphates, and double phosphates [33,34,35,36,37]. Recently, Ce(PO4)(HPO4)0.5(H2O)0.5 was proposed as a promising sorbent for radionuclide removal [24]. Different Ce(IV) and Ce(III) phosphates are under consideration regarding use as components of sunscreens [25,38], further underscoring the importance of exploring the dissolution behaviour of Ce-containing phosphate systems.
Thus, in this study, we systematically investigated the dissolution behaviour of two distinct sodium–cerium phosphate phases: the crystalline Na2Ce(PO4)2 and the nanostructured NaCe2(PO4)3. Both materials merit significant interest from fundamental and practical perspectives. The hydrated phosphate Na1.97Ce1.03(PO4)2·xH2O is a newly discovered crystalline material whose structure was resolved via X-ray diffraction and comprehensively characterised [34]. Our earlier research identified the formation of nanorod-structured NaCe2(PO4)3 during the prolonged transformation of CeO2 nanoparticles in the presence of phosphate species under environmentally relevant conditions [29]. We explored the dissolution process of these phosphates over an extensive pH range through under-saturation experiments, aiming to elucidate cerium concentrations in solution and associated structural transformations. Analyses of the aqueous phase were complemented by comprehensive analyses of the solid phases via synchrotron-based X-ray diffraction (XRD), Raman spectroscopy, and scanning electron microscopy (SEM), to identify the phases that could control solubility under varying environmental conditions.

2. Results

In the present study, sodium–cerium double phosphates were synthesised via a hydrothermal approach using CeO2 nanoparticles as the cerium precursor. The hydrothermal treatment was carried out in a 1 M sodium phosphate-buffer solution at two different pHs: 4.4 and 7.7. The XRD and SEM results for the obtained phosphate samples are presented in Figure 1, confirming the formation of double phosphate phases following the hydrothermal treatment of CeO2 nanoparticles. The structure and morphology of the resulting materials were found to be strongly dependent on the pH of the buffer solution.
The hydrothermal treatment of CeO2 nanoparticles at pH = 7.7 yielded a highly crystalline compound with well-defined, narrow diffraction peaks. The peak positions and relative intensities closely match those of a recently reported crystalline sodium–cerium double phosphate phase dominated by Ce(IV) [34] (Figure 1a). According to SEM analysis, the sodium–cerium double phosphate hydrothermally synthesised at pH = 7.7 exhibits well-developed rhombohedral crystals (Figure 1b). The average vertex-to-vertex diagonal of these crystals is approximately 3 μm. Energy-dispersive X-ray spectroscopy (EDX) performed during SEM analysis confirmed a Na/Ce atomic ratio of 2 (Table S1). Thermogravimetric analysis (TGA) showed negligible mass loss (<1.5%), suggesting that the compound is effectively unhydrated (Figure S1). Therefore, this phase is hereafter referred to as Na2Ce(PO4)2(cr.).
Due to the variability in phosphate polyhedron arrangements and the partial oxidation of Ce3+ to Ce4+, relatively few tetravalent cerium phosphate structures have been refined to date [33,35,39,40]. The structure of Na1.97Ce1.03(PO4)2·xH2O is one of those that have been thoroughly characterised. According to Baranchikov et al. [34], this sodium–cerium phosphate adopts a tunnel-type crystal structure (space group °P21/c) featuring mixed oxidation states of cerium. The structure consists of CeO8 tetragonal antiprisms interconnected with PO4 tetrahedra.
Following the treatment of CeO2 nanoparticles in phosphate buffer at pH = 4.4, a sodium–cerium double phosphate also formed, but with a distinct structure compared to Na2Ce(PO4)2(cr.). The diffraction peaks of this sample are noticeably broader. The average crystallite size in this sample, calculated from the most intensive peaks’ full width at half maximum (FWHM) using the Scherrer equation, is 23 ± 2 nm, confirming its nanocrystalline nature (Figure 1c). SEM images reveal that the product consists of spindle-like aggregates composed of nanorods, clearly showing the anisotropic morphology of the synthesised phosphate phase (Figure 1d). These aggregates are approximately 0.3 μm in width and up to 0.5 μm in length.
The diffraction pattern and unique morphology of this phosphate phase are consistent with NaCe2(PO4)3, which we previously discovered (hereafter referred to as NaCe2(PO4)3(nano)). In our earlier work, this nano sodium–cerium phosphate was identified as a product of the long-term phosphate-induced transformation of CeO2 nanoparticles under mildly acidic conditions (pH ~4) at 25 °C over up to four years. Due to its strong anisotropy and nanoscale nature, the refinement of its crystal structure from XRD data proved difficult; however, the material was extensively characterised using other techniques. X-ray absorption spectroscopy confirmed that cerium predominantly exists in the Ce(IV) oxidation state. Pair distribution function analysis of XRD data further revealed that the local crystal structure is analogous to known Na-Th phosphate, which has a non-centrosymmetric structure (Cc space group). The packing of cerium and phosphate groups forms a tree-dimension framework structure with large channels along the c-axis, containing disordered sodium atoms.
Recently, we demonstrated that a solid phase with structural and morphological characteristics analogous to NaCe2(PO4)3(nano) can form from X-ray amorphous ThO2 under hydrothermal conditions at pH = 4.8 in a 1 M sodium–phosphate buffer [41]. Interestingly, when the hydrothermal treatment of amorphous ThO2 was conducted at pH ~8, the resulting product was a less crystalline analogue of NaTh2(PO4)3. In contrast, CeO2 transforms into a well-crystallised double sodium–cerium phosphate phase at pH = 7.7. Taken together, these findings suggest the greater kinetic stability of cerium in phosphate matrices under near-neutral pH conditions compared to thorium.
As cerium is a redox-sensitive element, XANES spectroscopy was employed to confirm the predominant oxidation state of Ce in the synthesised phosphate samples. Figure 2 presents the XANES spectra near the Ce L3 edge for both the nanocrystalline and crystalline sodium–cerium phosphate phases, alongside reference spectra of Ce(III) and Ce(IV) standards. The Ce L3 edge XANES spectra for various cerium compounds reveal features associated with their oxidation states [42]. Ce(IV) compounds exhibit a double-peaked profile, which corresponds to transitions from the 4f0 configuration to final 4f15d1 states. The second peak after the absorption edge (around 5738 eV) in CeIV spectra is associated with multielectron excitations [42,43,44]. In contrast, the spectrum of CePO4, representing CeIII with a 4f25d1 configuration, shows a single main peak at a lower energy. The oxidation state is typically identified via analysis of the position and the shape of the white line. The Ce L3 edge features of synthesised double Na-Ce phosphates match with the spectra of the Ce(IV) standard and confirm the dominance of tetravalent cerium in the structure. This observation is further supported by the first derivative of the XANES spectra, which reveals a clear alignment between the absorption maxima of the synthesised phosphate samples and the Ce(IV)(OH)PO4 reference compound (Figure S2). The difference in the intensity of the main edge peaks of Na2Ce(PO4)2(cr.) and NaCe2(PO4)3(nano) could be attributed to changes in the phosphate crystal structure, variations in the local coordination environment, and the redistribution of 5d states. The sensitivity of the Ce L3 main edge intensity to structural changes is well documented for oxide, sulphate, and phosphate structures [42,45,46].

Na-Ce Phosphate Dissolution

To evaluate the phase composition stability of the synthesised sodium–cerium phosphates under varying pH conditions, Na2Ce(PO4)2(cr.) and NaCe2(PO4)3(nano) were stored in aqueous solutions (I = 0.01 M) for up to one year at 25 °C. The dependence of the dissolved cerium concentration in the presence of Na2Ce(PO4)2(cr.) as a function of the pH is presented in Figure 3. The dissolution profile can be divided into two distinct regions. In the acidic-to-near-neutral region (1.5 < pH < 7), the cerium concentration decreases significantly with increasing pH, from approximately 3 × 10−5 M to around 3 × 10−10 M (near the detection limit). Additionally, a time-dependent decrease in dissolved cerium concentration can be observed, indicating continued solid–solution interaction and a slow approach to equilibrium. At pH values above 7, the measured cerium concentration increases and stabilises around 10−7 M, with no noticeable dependence on dissolution time.
The phosphorus concentration, measured using ICP-MS alongside cerium, follows a similar pH-dependent trend in the acidic range (1.5 < pH < 7), decreasing in parallel with cerium (Figure S3). Interestingly, the absolute phosphorus concentrations exceed those that would be predicted from the stoichiometry of the Na2Ce(PO4)2(cr.) compound, which can likely be attributed to the surface sorption of loosely bound phosphate groups that are difficult to fully remove during routine post-synthesis washing. More notably, at pH > 7, the phosphorus concentrations in the solution become irregular and show no clear correlation with the cerium concentration, suggesting possible changes in the solubility-controlling phase under alkaline conditions.
Currently, reliable thermodynamic data for cerium phosphates are limited to trivalent cerium phosphate CePO4. The solubility curve for CePO4 was calculated using a solubility product of logKsp = −26.2 and compared with experimental data for Na2Ce(PO4)2(cr.) (Figure S4). Overall, the calculated and experimental pH-dependent dissolved cerium concentrations are in good agreement and exhibit a characteristic V-shaped curve, with the minimum cerium concentration observed at pH 6–7. The experimentally measured cerium concentrations at pH < 7 for Na2Ce(PO4)2(cr.) are slightly lower than the calculated values, which is consistent with the literature reports indicating that tetravalent phosphates generally exhibit lower solubility compared to their trivalent homologues. At pH > 7, notable discrepancies between the calculated and experimental solubilities become apparent.
During the dissolution experiments, both the pH and redox potential (Eh) of the suspensions of Na-Ce(IV) phosphate solids in 0.01 M NaClO4 solution were monitored. A comparison of the experimental redox conditions with the Pourbaix diagram for the Ce4+/Ce3+–PO43−–H2O system shows that, with increasing pH, the system approaches the area of CeO2 stability, indicating the possibility of the formation of CeO2 under alkaline conditions (Figure S5).
Figure 3 compares the experimental dissolution data for Na2Ce(PO4)2(cr.) with the calculated solubility profile of nanosized CeO2, based on the equilibria listed in Table S1 and a literature-derived solubility product logKsp = −59.3. This thermodynamic constant describes the solubility of CeO2 nanoparticles with a size range of 2–5 nm [28,46]. Notably, the calculated cerium concentrations corresponding to CeO2 solubility closely match the experimental values measured at pH > 7 during the dissolution of Na2Ce(PO4)2(cr.). It is suggested that CeO2 may form under alkaline conditions through a dissolution/precipitation mechanism driven by their high thermodynamic oxide stability compared to Na2Ce(PO4)2(cr.).
To reveal the structural transformations during dissolution, Na2Ce(PO4)2(cr.) solids were collected after 11 months of storage in solution at different pHs and then analysed using XRD, SEM, and Raman spectroscopy. XRD analysis showed that at pH 2.8, the initial crystalline phase of Na-Ce(IV) phosphate dominates, with minor impurities arising from recrystallisation products (Figure 4a). At pH 3.8 and 5.0, the primary phase remains stable, reflecting good structural preservation under moderately acidic to neutral conditions. At pH 7.1, in addition to the well-defined narrow diffraction peaks corresponding to Na2Ce(PO4)2(cr.), broad diffraction maxima can also be observed (Figure 4a). Their positions coincide with those characteristic of fluorite-type CeO2, and the significant FWHM values indicate the formation of nanocrystalline CeO2 particles. The crystallite size of the secondarily formed CeO2 nanoparticles, calculated from the FWHM of (111) and (200) diffraction lines using the Scherrer equation, is established as 2.6 ± 0.3 nm.
SEM images of the Na2Ce(PO4)2(cr.) stored in solution at pH 5.0 for 11 months reveal two distinct particle populations: smaller particles with an average size of approximately 2 μm and a morphology similar to the initial material and larger crystallites measuring up to 6 μm (Figure 4b). As no additional crystalline phases were detected via XRD in these samples, it is reasonable to assume that all observed crystallites are of the Na2Ce(PO4)2(cr.) phase. The larger crystallites are likely secondary Na2Ce(PO4)2(cr.) that formed through the coarsening of the original particles during the ageing process. Such crystallisation may also account for the decrease in dissolved cerium concentration at a near-neutral pH over time.
Particle coarsening and the presence of two distinct size-derived generations (1 μm and 6 μm on average) of the Na2Ce(PO4)2(cr.) phase can also be observed after storage and dissolving in solution at pH 7.1 (Figure 4c). However, CeO2 nanoparticles, clearly detected via XRD in this sample, are not observed in the SEM images. This is likely due to the localised nature of the SEM technique and the difficulty in distinguishing nanoscale oxide crystallites from the larger, well-formed phosphate particles. Therefore, Raman spectroscopy measurements were performed to indicate the presence of CeO2 nanoparticles after Na2Ce(PO4)2(cr.) dissolution at pH > 7 to further support the XRD data.
The Raman spectra of all the analysed Na-Ce(IV) phosphate samples exhibit characteristic vibrational modes corresponding to orthophosphate groups (Figure 5). The Raman modes observed in the range of 1022–1100 cm−1 and 977 cm−1 can be assigned to the asymmetric (ν3) and asymmetric (ν1) stretching of the PO43− group. Similarly, the modes observed in the intermediate range of 370–480 cm−1 and 553–650 cm−1 are the symmetric and asymmetric bending modes of the PO43− group (ν2). The characteristic features are consistent with previously reported data for monoclinic double K-Ce(IV) orthophosphate and indicate the absence of polyphosphate species in the structure [37,39].
The spectrum collected after equilibration at pH 7.2, in addition to the phosphate band, displays a broad band centred around 455 cm−1 and 590 cm−1. This feature is absent from the Raman spectrum of the initial Na-Ce(IV) phosphate sample and those stored at pH 4.9, but it matches well with the vibration modes observed for 2 nm CeO2 nanoparticles. The Raman spectrum of CeO2 2 nm nanoparticles recorded under 405 nm excitation is shown along with Na2Ce(PO4)2(cr.). It exhibits the characteristic F2g mode at ~455 cm−1, corresponding to the symmetric vibrational mode of the oxygen atoms around cerium ions (O–Ce–O) in agreement with the literature [47,48]. Additionally, a broad feature around 570 cm−1 is observed, which could be associated with defect-induced modes or second-order phonon processes in nanocrystalline CeO2 [49]. This band typically becomes more pronounced under specific excitation conditions, particularly UV or near-UV Raman, and is often absent or weak in conventional visible Raman spectra. Altogether, the Raman data indicate that after long-term dissolution at pH 7.1, the sample contains a mixture of the original Na2Ce(PO4)2 phase and newly formed CeO2 nanoparticles. These findings fully agree with the XRD results, which also reveal broad diffraction features consistent with fluorite-type CeO2.
The dependence of the cerium concentration in solution in the presence of NaCe2(PO4)3(nano) on pH values is presented in Figure 6. The dissolution behaviour of NaCe2(PO4)3(nano) in 0.01 M NaClO4 reveals notable differences compared to the previously discussed Na2Ce(PO4)2(cr.) phase. In the acidic pH range (pH 1–6), the cerium concentration in solution decreases from approximately 5 × 10−4 M to 1 × 10−8 M. Importantly, no systematic variation in cerium concentration is observed with increasing dissolution time (1 week, 10 months, and 12 months), suggesting that the solid phase remains structurally and chemically stable during dissolution. This contrasts with the behaviour of Na2Ce(PO4)2(cr.), where dissolution is accompanied by particle recrystallisation and structural evolution. At near-neutral and alkaline pH, the measured cerium concentrations align well with the calculated solubility of nanosized CeO2, shown as the cross-hatched area in Figure 6. This correspondence suggests that, as in the case of the dissolution of Na2Ce(PO4)2(cr.), a secondary phase of CeO2 nanoparticles may also form under these conditions.
As in the case of Na2Ce(PO4)2(cr.), the solid phase of NaCe2(PO4)3(nano) was comprehensively analysed after long-term equilibration in solution under different pHs using XRD, SEM, and Raman spectroscopy. The XRD patterns (Figure 7a) show no detectable changes in the crystalline phase across the studied pH range. The diffraction peaks remain broad throughout, indicating the nanocrystalline character of NaCe2(PO4)3(nano) even after prolonged dissolution. This behaviour contrasts with the case of Na2Ce(PO4)2(cr.), where additional crystallisation during ageing is observed. Moreover, no additional diffraction peaks attributable to secondary crystalline phases are detected at pH ~3, whereas in the crystalline Na-Ce phosphate system, such conditions lead to the formation of new crystalline precipitates.
SEM imaging reveals no appreciable morphological changes after dissolution (Figure 7b,c). At pH 2.9, the mean aggregate length and width are measured as 419.7 nm and 261.9 nm, respectively, while at pH 6.8, the corresponding values are 471.1 nm and 256.4 nm. These dimensions agree with the original particle size within the experimental uncertainty. Overall, these XRD and SEM results confirm the high stability of the NaCe2(PO4)3(nano) phase and the preservation of its morphological characteristics under acidic-to-near-neutral conditions.
Attempts to identify the formation of CeO2 nanoparticles following the dissolution of NaCe2(PO4)3(nano) at pH > 7 were limited by the low buffering capacity of aqueous media in the near-neutral range, and reliable solid-phase characterisation was only possible for the sample collected at pH = 6.8. Under these conditions, neither XRD (Figure 7a) nor Raman spectroscopy (Figure S6) provided conclusive evidence for the presence of CeO2. Nonetheless, the possibility of secondary CeO2 nanoparticle formation at higher pH values cannot be entirely excluded, as thermodynamic predictions suggest its increased stability under alkaline conditions. Importantly, under similar pH conditions and over the same ageing period (almost one year), CeO2 nanoparticle formation is clearly observed for the crystalline Na2Ce(PO4)2 phase, but not for the nanophosphate NaCe2(PO4)3. This strongly suggests that the extent of CeO2 formation during the dissolution of the nanophosphate is significantly low.

3. Discussion

In this study, we report for the first time a systematic investigation into the dissolution behaviour of two tetravalent cerium phosphate phases: the crystalline double phosphate Na2Ce(PO4)2(cr.) and the nanocrystalline phase NaCe2(PO4)3(nano). The dissolution data, combined with structural and morphological analyses of the solids, reveal distinct differences in their long-term stability and phase evolution across a wide pH range.
For Na2Ce(PO4)2(cr.), our results demonstrate clear pH-dependent transformations in solution. At a low pH, the material undergoes partial reprecipitation and phase reorganisation, while at a near-neutral pH, the original structure is retained but undergoes crystallisation and coarsening, as evidenced in the SEM data. Under alkaline conditions (pH > 7), the dissolution data, XRD, and Raman spectroscopy suggest the formation of CeO2 nanoparticles. Notably, Fourest et al. investigated the dissolution of the pure thorium phosphate-diphosphate Th4(PO4)4P2O7, which was measured in 0.1 M NaClO4. The results show that the total concentration of thorium in solution is mainly controlled by the precipitation of two compounds: thorium bis(hydrogen phosphate) in acidic media (pH < 4.5) and thorium hydrated oxide in basic and near-neutral media [13]. Our findings for Na2Ce(PO4)2(cr.) exhibit a broadly comparable dissolution behaviour, with a decrease in cerium concentration in acidic media and a plateau or increase at a near-neutral pH, leading to CeO2 nanoparticle formation.
In contrast, the nanocrystalline phase NaCe2(PO4)3(nano) exhibits a higher cerium concentration in solution than the crystalline double Na-Ce(IV) phosphate under near-neutral conditions. At the same time, no evidence of recrystallisation, structural decomposition, or the formation of secondary phases was observed, even after the long-term contact of nano double Ce(IV) phosphate with an aqueous solution. The retention of its low crystallinity and morphology suggests that this phase is highly kinetically stable. This observation, together with the formation of CeO2 nanoparticles as a result of dissolution Na2Ce(PO4)2(cr.) at pH > 7, is particularly significant in light of recent studies highlighting the unusual stability of certain materials in nanocrystalline form, contrary to classical expectations of higher solubility and reactivity in nanoscale solids.
The remarkable stability of the nanocrystalline NaCe2(PO4)3 phase observed in this study aligns with a growing body of evidence suggesting that certain materials exhibit unexpectedly high thermodynamic solubility at the nanoscale. Notably, Navrotsky et al. [50] demonstrated that certain iron oxide polymorphs, such as goethite and maghemite, become thermodynamically stabilised at the nanoscale due to surface energy effects. Their study showed that polymorphs metastable in bulk form can become energetically favoured as nanoparticles, with hydration playing a key role in reducing surface enthalpy [50,51,52,53,54]. Our previous work has demonstrated that the hydrated surfaces of CeO2 nanoparticles significantly reduce the surface energy and promote the long-term stability of 2 nm nanoparticles even across a 4.5-year dissolution timeline [46].
Similarly to oxide-based systems, the exceptional phase stability of nanocrystalline NaCe2(PO4)3 may be attributed to structural features such as hydrated sites, partially associated with sodium within the channels, as demonstrated for the analogous NaTh2(PO4)3 system [41]. This interpretation is supported by TGA data, which indicate a higher hydration of the nanophase compared to crystalline Na2Ce(PO4)2 (Figure S1). On the other hand, the formation of CeO2 as result of the dissolution of the Na2Ce(PO4)2(cr.) phase may be linked to its dissolution mechanism. While nanophase cerium phosphate likely dissolves congruently, the crystalline version appears to undergo incongruent dissolution. A similar phenomenon was reported for crystalline monazite (CePO4), where partial dissolution led to CeO2 formation, while the phosphate framework remained largely intact [55]. This highlights the importance of considering both structural hydration and dissolution mechanisms when evaluating the stability of Ce-phosphate systems.
In our recent work, we demonstrated that the nanocrystalline phase NaCe2(PO4)3(nano) can form spontaneously from CeO2 nanoparticles under prolonged exposure to phosphate-containing solutions [29]. The transformation was strongly pH-dependent: at pH ~4, CeO2 underwent structural changes, while at pH ~8, the oxide nanoparticles remained largely unaltered even after extended contact. However, the driving force behind this transformation remained unclear. In this study, we show that NaCe2(PO4)3(nano) exhibits markedly reduced cerium release into solution compared to CeO2 under acidic conditions (pH < 6). At higher pH levels, however, the concentrations of dissolved cerium from both phases are comparable. This suggests that the transformation of CeO2 into NaCe2(PO4)3(nano) is driven by the lower solubility and greater thermodynamic stability of the phosphate phase under acidic and near-neutral conditions. Both phases may coexist at pH > 6, where the solubilities are close, indicating competitive stability and the potential for equilibrium between oxide and phosphate forms.
In conclusion, the findings establish key thermodynamic stability conditions for Na–Ce(IV) phosphate and CeO2 phases, which is critical in predicting cerium mobility in both natural and engineered systems, including nuclear waste immobilisation and environmental remediation. Given the close similarity in ionic radii and coordination behaviour between Ce(IV), Th(IV), and Pu(IV) (r[Ce4+] = 0.97 Å, r[Th4+] = 1.05 Å, r[Pu4+] = 0.96 Å for coordination number CN = 8 [56]), cerium is commonly used as a non-radioactive surrogate for tetravalent actinides. Several isostructural phosphate compounds have been reported for Ce(IV) and Th(IV), particularly in systems containing potassium, ammonium, and recently sodium as charge-balancing cations [29,35,40,57,58,59]. Structural data on Pu(IV) phosphates remain scarce due to the radioactivity and strong redox sensitivity of plutonium. However, cerium’s ability to reversibly switch between Ce3+ and Ce4+ oxidation states makes it especially relevant in modelling systems where Pu(IV)/Pu(III) redox transitions play a role under reducing conditions. These chemical analogies reinforce the validity of cerium-based phosphates as surrogate systems for probing the dissolution dynamic and long-term evolution of actinide-hosting materials in geochemically relevant contexts.

4. Materials and Methods

4.1. Synthesis of CeO2 Nanoparticles

CeO2 nanoparticles were synthesised via rapid chemical precipitation from a Ce(IV) solution using aqueous ammonia. A 0.1 M aqueous solution of (NH4)2[Ce(NO3)6] was prepared by dissolving 2.74 g of the solid salt in 50 mL of water. Under continuous stirring, 200 mL of deionised water and 50 mL of concentrated aqueous ammonia were mixed, then the prepared (NH4)2[Ce(NO3)6] solution was added dropwise to the ammonia solution. Stirring was maintained for 2 h, resulting in the formation of a light-yellow precipitate of CeO2 nanoparticles. The resulting precipitate was washed three times with MilliQ water to remove synthesis by-products, using centrifugation for phase separation. The particle size distributions obtained from HRTEM data revealed relatively low sample polydispersity, with an average nanoparticle size of 2.4 nm.

4.2. Hydrothermal Treatment

The as-prepared CeO2 nanoparticles were subjected to hydrothermal treatment in sodium phosphate-buffer solutions. The buffers were prepared by mixing 1M Na2HPO4 and 1M NaH2PO4 solutions. The first buffer solution had a Na2HPO4/NaH2PO4 volume ratio of 1:12 and a pH of 4.4 after equilibration under air. The second buffer had a Na2HPO4/NaH2PO4 volume ratio of 14:1 and a final pH of 7.7. The CeO2 nanoparticle suspensions in the phosphate buffers were placed in Teflon-lined autoclaves and heated at 200 °C in a drying oven for 8 h. After treatment, the samples were rinsed thoroughly with MilliQ water.

4.3. Dissolution Experiments

Dissolution experiments were performed under undersaturation conditions [28,60]. For this, double cerium–sodium phosphates in the form of concentrated aqueous suspension were deposited into polypropylene tubes. The pH values were adjusted in the range of 1.5–10 via the successive addition of dilute solutions of NaOH and HClO4. The overall solid concentration in the samples was 70–100 mg/L for samples at pH < 4 and 30–40 mg/L for samples at pH > 4. Dissolution studies were performed at a constant ionic strength (0.01 M NaClO4) at 25.0 ± 0.5 °C. The pH values of the resulting suspensions were determined using an InLab Expert Pro pH electrode (Mettler Toledo, Columbus, OH, USA). The redox potential (E(mV)) was measured using a Pt electrode relative to a Ag/AgCl reference electrode and converted to Eh through the following equation: Eh (mV) = E (mV) + 223.054 − 0.6985∙t (t, the temperature of the sample liquid). Zobell’s standard solution was used for the control.
The pH and the concentrations of cerium and phosphorus in the solutions were monitored throughout the experiments. Sampling was performed within 1 week to 12 months of equilibration. The elemental concentrations in the solution were determined via ICP-MS.

4.4. Inductively Coupled Plasma Mass Spectrometry (ICP-MS) Measurements

The quantitative determination of 31P and 140Ce was performed using ICP-MS with the PlasmaQuant MS Elite (Analytic Jena, Jena, Germany). All samples were diluted with the 1% nitric acid solution to concentrations within the calibration range. The dilution solution was prepared using ultra-high-purity HNO3 and deionized water obtained using a Milli-Q water purification system (resistivity, 18.2 MΩ·cm). The calibration range was 100–1000 µg/L and 0.1–100 µg/L for 31P and 140Ce, respectively. For signal correction and matrix effect compensation, an internal standard with 103Rh and 193Ir was used. The detection limit for cerium measurements via ICP-MS was 0.01 μg/L (7 × 10−11 M). Each data point represents the average of three independent measurements (n = 3), with 10 scans per measurement and 10 replicates per sample. For each measurement, the relative standard deviation was within 10%, corresponding to an uncertainty of approximately ±0.1 in the logarithmic value of the Ce concentration.

4.5. Thermodynamic Modelling

The PHREEQC code [61] was employed to model CeO2 solubility under the experimental redox conditions. The corresponding Eh values used in the calculations are provided in Figure S5. The set of equilibrium reactions and thermodynamic constants used for the modelling is listed in Table S2.

4.6. Characterisation Methods

Synchrotron-based XRD measurements were performed using the X-ray structural analysis beamline of the Kurchatov Synchrotron Radiation Source (NRC «Kurchatov Institute» Moscow) [62]. For synchrotron-based XRD measurements, Na-Ce(IV) phosphate samples were placed in synthetic vacuum oil and mounted on a 200 nm nylon CryoLoop. The measurements were performed in the transmission mode, using a Rayonix SX-165 CCD detector (Rayonix, LLC, Evanston, IL, USA) at a wavelength of 0.75 Å. Raw 2D scattering images were integrated using the Fit2D software (ver. 18 beta). The PDF4+ database was used to identify the crystalline phases.
X-ray absorption near-edge structure (XANES) measurements at the Ce L3 edge were performed using an X-ray laboratory spectrometer located at the Department of Radiochemistry, Moscow State University (Moscow, Russia) [63]. The spectrometer operates based on Johann geometry and is equipped with an X-ray tube with a silver anode and a silicon drift detector (FASTSDD; Amptek Inc., Bedford, MA, USA). A curved Ge crystal monochromator was used for monochromatizing and focusing the X-ray beam, allowing energy scanning in the range of 5710–5780 eV for cerium (reflection 3 3 3). A helium-filled chamber was utilised along the entire optical path of the X-ray beam to prevent signal loss due to air absorption when measuring the Ce L3-edge spectrum. All measurements were conducted at room temperature. The samples were sealed between two 25 μm thick layers of Kapton. Data were collected in transmission mode, with each spectrum representing the sum of 15 scans, which were merged and normalised using the IFEFFIT software package (ver. 0.9.26) [64]. The spectra of well-characterised crystalline Ce(OH)PO4 and CePO4 were used as references for Ce(IV) and Ce(III) [34]. XANES measurements were not performed on solution samples due to the extremely low cerium concentrations, which were below the detection limit for this technique.
The sample morphology was studied using a JSM-IT500 SEM (JEOL Ltd., Tokyo, Japan) with a tungsten thermionic cathode and equipped with an Oxford X-Max-n EDS instrument. Samples deposited on a silicon substrate were covered with a 20–25 nm thick conductive carbon film using a vacuum evaporator. For energy-dispersive X-ray (EDX) analysis, the electron probe current was set at 0.7 nA, and the exposure time was 60 s. The detection thresholds for all analysed elements reached 0.1–0.2 wt%. The obtained spectra were processed using INCA (version 21b) software using the XPP correction model.
Raman spectra were obtained using a Renishaw inVia Raman spectrometer with a 50 mW laser diode at a wavelength of 405 nm. The spectral range was set between 100 and 2000 cm−1. Laser light was focused on the sample through a 50× objective to a spot size of ~2 μm. The power on the sample was <0.1 mW.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/molecules30102105/s1, Table S1: Concentration of elements (in atomic percentages) in Na2Ce(PO4)2(cr.); Figure S1: Thermogravimetric analysis (TGA) of Na2Ce(PO4)2(cr.) and NaCe2(PO4)3 (nano); Figure S2: First derivative of the XANES spectra; Figure S3: Phosphorus concentration in solution in presence of (a) Na2Ce(PO4)2(cr.) and (b) NaCe2(PO4)3(nano) at different pHs; Figure S4: The dependence of cerium concentration in solution on pH for Na2Ce(PO4)2(cr.) sample and the calculated solubility of CePO4; Figure S5: Pourbaix diagram calculated by Hydra/Medusa software (ver. 18 Aug. 2009) with pH/Eh experimental values in solubility experiments; Table S2: Chemical equilibria and corresponding thermodynamic constants used in this work for modelling the solubility of CeO2; Figure S6: Raman spectra of the NaCe2(PO4)3(nano) samples; Figure S7: (a) HRTEM images of CeO2 nanoparticles; (b) particle size distributions obtained from HRTEM analysis.

Author Contributions

Conceptualization and Methodology T.V.P., A.S.K., and A.Y.R.; Investigation A.L.L., A.S.K., and T.V.P.; Data Curation and Investigation M.A.G., V.O.Y., A.A.A., D.A.N., E.S.K., and R.D.S.; Writing—Original Draft Preparation, T.V.P.; Writing—Review and Editing A.S.K. and A.Y.R.; Visualisation, T.V.P.; Supervision, S.N.K.; Funding Acquisition, R.D.S., T.V.P., and S.N.K. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Russian Science Foundation, grant No. 22-73-10056.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Dataset available on request from the authors.

Acknowledgments

The study was supported by the Russian Science Foundation (grant No. 22-73-10056). The Raman measurements was performed using the equipment of SRC FMI IPCE RAS. ICP-MS measurements were carried out on equipment provided by the federal project “Development of Infrastructure for Scientific Research and Personnel Training” of the National Project “Science and Universities” of 29 December 2021, No. 15/34. We thank Tatiana B. Shatalova for performing the thermogravimetric analysis.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
XRDX-ray diffraction
SEMScanning electron microscopy
ICP-MSInductively coupled plasma mass spectrometry
NPsNanoparticles
logKspLogarithmic value of the solubility product constant

References

  1. Pica, M. Treatment of Wastewaters with Zirconium Phosphate Based Materials: A Review on Efficient Systems for the Removal of Heavy Metal and Dye Water Pollutants. Molecules 2021, 26, 2392. [Google Scholar] [CrossRef] [PubMed]
  2. Chen, X.; Li, H.; Ma, Y.; Jiang, Y. Calcium Phosphate-Based Nanomaterials: Preparation, Multifunction, and Application for Bone Tissue Engineering. Molecules 2023, 28, 4790. [Google Scholar] [CrossRef] [PubMed]
  3. Nazaraly, M.; Wallez, G.; Chanéac, C.; Tronc, E.; Ribot, F.; Quarton, M.; Jolivet, J.P. The First Structure of a Cerium(IV) Phosphate: Ab Initio Rietveld Analysis of CeIV(PO4)(HPO4)0.5(H2O)0.5. Angew. Chem. Int. Ed. 2005, 44, 5691–5694. [Google Scholar] [CrossRef] [PubMed]
  4. Brandel, V.; Dacheux, N. Chemistry of Tetravalent Actinide Phosphates—Part I. J. Solid. State Chem. 2004, 177, 4743–4754. [Google Scholar] [CrossRef]
  5. Achary, S.N.; Bevara, S.; Tyagi, A.K. Recent Progress on Synthesis and Structural Aspects of Rare-Earth Phosphates. Coord. Chem. Rev. 2017, 340, 266–297. [Google Scholar] [CrossRef]
  6. Bamberger, C.E.; Haire, R.G.; Begun, G.M.; Hellwege, H.E. The Synthesis and Characterization of Crystalline Phosphates of Thorium, Uranium and Neptunium. J. Less-Common Met. 1984, 102, 179–186. [Google Scholar] [CrossRef]
  7. Du Fou de Kerdaniel, E.; Clavier, N.; Dacheux, N.; Terra, O.; Podor, R. Actinide Solubility-Controlling Phases during the Dissolution of Phosphate Ceramics. J. Nucl. Mater. 2007, 362, 451–458. [Google Scholar] [CrossRef]
  8. Burakov, B.E.; Ojovan, M.I.; Lee, W.; Bill, E. Crystalline Materials for Actinide Immobilisation; Imperial College Press: London, UK, 2010; Volume 1, ISBN 978-1-84816-418-5. [Google Scholar]
  9. Terra, O.; Dacheux, N.; Audubert, F.; Podor, R. Immobilization of Tetravalent Actinides in Phosphate Ceramics. J. Nucl. Mater. 2006, 352, 224–232. [Google Scholar] [CrossRef]
  10. Vinokurov, S.E.; Kulyako, Y.M.; Slyuntchev, O.M.; Rovny, S.I.; Myasoedov, B.F. Low-Temperature Immobilization of Actinides and Other Components of High-Level Waste in Magnesium Potassium Phosphate Matrices. J. Nucl. Mater. 2009, 385, 189–192. [Google Scholar] [CrossRef]
  11. Rafiuddin, M.R.; Donato, G.; McCaugherty, S.; Mesbah, A.; Grosvenor, A.P. Review of Rare-Earth Phosphate Materials for Nuclear Waste Sequestration Applications. ACS Omega 2022, 7, 39482–39490. [Google Scholar] [CrossRef]
  12. Popa, K.; Cologna, M.; Martel, L.; Staicu, D.; Cambriani, A.; Ernstberger, M.; Raison, P.E.; Somers, J. CaTh(PO4)2 Cheralite as a Candidate Ceramic Nuclear Waste Form: Spark Plasma Sintering and Physicochemical Characterisation. J. Eur. Ceram. Soc. 2016, 36, 4115–4121. [Google Scholar] [CrossRef]
  13. Fourest, B.; Rard Lagarde, G.; Perrone, J.; Brandel, V.; Dacheux, N.; Genet, M. Solubility of Thorium Phosphate-Diphosphate. New J. Chem. 1999, 23, 645–649. [Google Scholar] [CrossRef]
  14. Rai, D.; Moore, D.A.; Felmy, A.R.; Rosso, K.M.; Bolton, H. PuPO4(Cr, Hyd.) Solubility Product and Pu3+ Complexes with Phosphate and Ethylenediaminetetraacetic Acid. J. Solut. Chem. 2010, 39, 778–807. [Google Scholar] [CrossRef]
  15. Gausse, C.; Szenknect, S.; Qin, D.W.; Mesbah, A.; Clavier, N.; Neumeier, S.; Bosbach, D.; Dacheux, N. Determination of the Solubility of Rhabdophanes LnPO4·0.667H2O (Ln = La to Dy). Eur. J. Inorg. Chem. 2016, 2016, 4615–4630. [Google Scholar] [CrossRef]
  16. Liu, X.; Byrne, R.H. Rare Earth and Yttrium Phosphate Solubilities in Aqueous Solution. Geochim. Cosmochim. Acta 1997, 61, 1625–1633. [Google Scholar] [CrossRef]
  17. Gorman-Lewis, D.; Shvareva, T.; Kubatko, K.A.; Burns, P.C.; Wellman, D.M.; Mcnamara, B.; Szymanowski, J.E.S.; Navrotsky, A.; Fein, J.B. Thermodynamic Properties of Autunite, Uranyl Hydrogen Phosphate, and Uranyl Orthophosphate from Solubility and Calorimetric Measurements. Environ. Sci. Technol. 2009, 43, 7416–7422. [Google Scholar] [CrossRef]
  18. Romanchuk, A.Y.; Vlasova, I.E.; Kalmykov, S.N. Speciation of Uranium and Plutonium From Nuclear Legacy Sites to the Environment: A Mini Review. Front. Chem. 2020, 8, 630. [Google Scholar] [CrossRef]
  19. Cetiner, Z.S.; Wood, S.A.; Gammons, C.H. The Aqueous Geochemistry of the Rare Earth Elements. Part XIV. The Solubility of Rare Earth Element Phosphates from 23 to 150 °C. Chem. Geol. 2005, 217, 147–169. [Google Scholar] [CrossRef]
  20. Gysi, A.P.; Harlov, D.; Miron, G.D. The Solubility of Monazite (CePO4), SmPO4, and GdPO4 in Aqueous Solutions from 100 to 250 °C. Geochim. Cosmochim. Acta 2018, 242, 143–164. [Google Scholar] [CrossRef]
  21. Hua, K.; Chen, X.; Shui, A.; Xi, X.; Gao, P.; Zheng, Y.; He, C. Preparation and Properties of High Sound-Absorbing Porous Ceramics Reinforced by In Situ Mullite Whisker from Construction Waste. Molecules 2024, 29, 3419. [Google Scholar] [CrossRef]
  22. Kaplin, I.Y.; Lokteva, E.S.; Golubina, E.V.; Lunin, V.V. Template Synthesis of Porous Ceria-Based Catalysts for Environmental Application. Molecules 2020, 25, 4242. [Google Scholar] [CrossRef] [PubMed]
  23. Grzybek, G.; Wójtowicz, A.; Legutko, P.; Greluk, M.; Słowik, G.; Sienkiewicz, A.; Adamski, A.; Kotarba, A. The Role of Synthesis Methods of Ceria-Based Catalysts in Soot Combustion. Molecules 2025, 30, 358. [Google Scholar] [CrossRef] [PubMed]
  24. Romanchuk, A.Y.; Shekunova, T.O.; Larina, A.I.; Ivanova, O.S.; Baranchikov, A.E.; Ivanov, V.K.; Kalmykov, S.N. Sorption of Radionuclides onto Cerium(IV) Hydrogen Phosphate Ce(PO4)(HPO4)0.5(H2O)0.5. Radiochemistry 2019, 61, 719–723. [Google Scholar] [CrossRef]
  25. Kozlova, T.O.; Vasilyeva, D.N.; Kozlov, D.A.; Kolesnik, I.V.; Teplonogova, M.A.; Tronev, I.V.; Sheichenko, E.D.; Protsenko, M.R.; Kolmanovich, D.D.; Ivanova, O.S.; et al. A Comparative Study of Cerium(III) and Cerium(IV) Phosphates for Sunscreens. Molecules 2024, 29, 2157. [Google Scholar] [CrossRef]
  26. Shcherbakov, A.B.; Zholobak, N.M.; Ivanov, V.K. Biological, Biomedical and Pharmaceutical Applications of Cerium Oxide. In Cerium Oxide (CeO2): Synthesis, Properties and Applications; Metal Oxides Series; Elsevier: Amsterdam, The Netherlands, 2019; pp. 279–328. ISBN 9780128156612. [Google Scholar]
  27. Dahle, J.T.; Livi, K.; Arai, Y. Effects of pH and Phosphate on CeO2 Nanoparticle Dissolution. Chemosphere 2015, 119, 1365–1371. [Google Scholar] [CrossRef]
  28. Plakhova, T.V.; Romanchuk, A.Y.; Yakunin, S.N.; Dumas, T.; Demir, S.; Wang, S.; Minasian, S.G.; Shuh, D.K.; Tyliszczak, T.; Shiryaev, A.A.; et al. Solubility of Nanocrystalline Cerium Dioxide: Experimental Data and Thermodynamic Modeling. J. Phys. Chem. C 2016, 120, 22615–22626. [Google Scholar] [CrossRef]
  29. Plakhova, T.V.; Vyshegorodtseva, M.A.; Seregina, I.F.; Svetogorov, R.D.; Trigub, A.L.; Kozlov, D.A.; Egorov, A.V.; Shaulskaya, M.D.; Tsymbarenko, D.M.; Romanchuk, A.Y.; et al. Unexpected Nanoscale CeO2 Structural Transformations Induced by Ecologically Relevant Phosphate Species. Chemosphere 2024, 368, 143664. [Google Scholar] [CrossRef]
  30. Romer, I.; Briffa, S.M.; Dasilva, Y.A.R.; Hapiuk, D.; Trouillet, V.; Palmer, R.E.; Valsami-Jones, E. Impact of Particle Size, Oxidation State and Capping Agent of Different Cerium Dioxide Nanoparticles on the Phosphate-Induced Transformations at Different pH and Concentration. PLoS ONE 2019, 14, e0217483. [Google Scholar] [CrossRef]
  31. Grulke, E.A.; Beck, M.; Yokel, R.A.; Unrine, J.; Graham, U.M.; Hancock, M.L. Surface-Controlled Dissolution Rates: A Case Study of Nanoceria in Carboxylic Acid Solutions. Environ. Sci. Nano 2019, 6, 1478–1492. [Google Scholar] [CrossRef]
  32. Yokel, R.A.; Hancock, M.L.; Grulke, E.A.; Unrine, J.M.; Dozier, A.K.; Graham, U.M. Carboxylic Acids Accelerate Acidic Environment-Mediated Nanoceria Dissolution. Nanotoxicology 2019, 13, 455–475. [Google Scholar] [CrossRef]
  33. Kozlova, T.O.; Mironov, A.V.; Istomin, S.Y.; Birichevskaya, K.V.; Gippius, A.A.; Zhurenko, S.V.; Shatalova, T.B.; Baranchikov, A.E.; Ivanov, V.K. Meet the Cerium(IV) Phosphate Sisters: CeIV(OH)PO4 and CeIV2O(PO4)2. Chem. Eur. J. 2020, 26, 12188–12193. [Google Scholar] [CrossRef] [PubMed]
  34. Baranchikov, A.E.; Kozlova, T.O.; Istomin, S.Y.; Mironov, A.V.; Vasilchikova, T.M.; Gippius, A.A.; Plakhova, T.V.; Vasilyeva, D.N.; Ivanov, V.K. Sodium Cerium Phosphate, (Na,Ce)2Ce(PO4)2⋅xH2O, with Mixed Cerium Oxidation States. ChemistrySelect 2024, 9, e202401010. [Google Scholar] [CrossRef]
  35. Salvadó, M.A.; Pertierra, P.; Trobajo, C.; García, J.R. Crystal Structure of a Cerium(IV) Bis(Phosphate) Derivative. J. Am. Chem. Soc. 2007, 129, 10970–10971. [Google Scholar] [CrossRef] [PubMed]
  36. Shekunova, T.O.; Istomin, S.Y.; Mironov, A.V.; Baranchikov, A.E.; Yapryntsev, A.D.; Galstyan, A.A.; Simonenko, N.P.; Gippius, A.A.; Zhurenko, S.V.; Shatalova, T.B.; et al. Crystallization Pathways of Cerium(IV) Phosphates Under Hydrothermal Conditions: A Search for New Phases with a Tunnel Structure. Eur. J. Inorg. Chem. 2019, 3242–3248. [Google Scholar] [CrossRef]
  37. Bevara, S.; Achary, S.N.; Patwe, S.J.; Sinha, A.K.; Tyagi, A.K. Preparation and Crystal Structure of K2Ce(PO4)2: A New Complex Phosphate of Ce(IV) Having Structure with One-Dimensional Channels. Dalton Trans. 2016, 45, 980–991. [Google Scholar] [CrossRef]
  38. Kurajica, S.; Brleković, F.; Keser, S.; Dražić, G.; Mužina, K.; Mihajlović, V. Evaluation of Mechanochemically Prepared CePO4∙H2O Nanoparticles as UV Filter for Photoprotective Formulations. Molecules 2025, 30, 405. [Google Scholar] [CrossRef]
  39. Bevara, S.; Mishra, K.K.; Patwe, S.J.; Ravindran, T.R.; Gupta, M.K.; Mittal, R.; Krishna, P.S.R.; Sinha, A.K.; Achary, S.N.; Tyagi, A.K. Phase Transformation, Vibrational and K2Ce(PO4)2: A Combined Experimental and Theoretical Study. Inorg. Chem. 2017, 56, 3335–3348. [Google Scholar] [CrossRef]
  40. Kozlova, T.O.; Vasilyeva, D.N.; Kozlov, D.A.; Teplonogova, M.A.; Baranchikov, A.E.; Simonenko, N.P.; Ivanov, V.K. Synthesis and Thermal Behavior of KCe2(PO4)3, a New Full-Member in the AIMIV2(PO4)3 Family. Nanosyst. Phys. Chem. Math. 2023, 14, 112–119. [Google Scholar] [CrossRef]
  41. Kramar, B.V.; Plakhova, T.V.; Kuzenkova, A.S.; Trigub, A.L.; Svetogorov, R.D.; Shiryaev, A.A.; Nevolin, I.M.; Yapryntsev, A.D.; Baranchikov, A.E.; Peters, G.; et al. Formation of a New Hydrated Sodium-Thorium Phosphate from Thorium Dioxide and Its Subsequent Phase Evolution. Dalton Trans. 2025. [Google Scholar] [CrossRef]
  42. Kvashnina, K.O. Electronic-Structure Interpretation: How Much Do We Understand Ce L3 XANES? Chem. Eur. J. 2024, 30, e202400755. [Google Scholar] [CrossRef]
  43. Bianconi, A.; Marcelli, A.; Dexpert, H.; Karnatak, R.; Kotani, A.; Jo, T.; Petiau, J. Specific Intermediate-Valence State of Insulating 4 Compounds Detected by L3 x-Ray Absorption. Phys. Rev. B 1987, 35, 806–812. [Google Scholar] [CrossRef] [PubMed]
  44. Soldatov, A.V.; Ivanchenko, T.S.; Della Longa, S.; Kotani, A.; Iwamoto, Y.; Bianconi, A. Crystal-Structure Effects in the Ce L3 -Edge x-Ray-Absorption Spectrum of CeO2: Multiple-Scattering Resonances and Many-Body Final States. Phy.s Rev. B 1994, 50, 5074–5080. [Google Scholar] [CrossRef] [PubMed]
  45. Yoshida, H.; Yuliati, L.; Hamajima, T.; Hattori, T. Valence of Highly Dispersed Cerium Oxide Species on Silica Quantitatively Estimated by Ce LIII-Edge XANES. Mater. Trans. 2004, 45, 2062–2067. [Google Scholar] [CrossRef]
  46. Plakhova, T.V.; Romanchuk, A.Y.; Konyukhova, A.D.; Seregina, I.F.; Baranchikov, A.E.; Svetogorov, R.D.; Terban, M.W.; Ivanov, V.K.; Kalmykov, S.N. Overlooked Impact of Surface Hydroxylation on the Solubility of Less-Soluble Compounds: A Case Study of CeO2. Environ. Sci. Nano 2024, 11, 3551–3562. [Google Scholar] [CrossRef]
  47. Wu, Z.; Li, M.; Howe, J.; Meyer, H.M.; Overbury, S.H. Probing Defect Sites on CeO2 Nanocrystals with Well-Defined Surface Planes by Raman Spectroscopy and O2 Adsorption. Langmuir 2010, 26, 16595–16606. [Google Scholar] [CrossRef]
  48. Babitha, K.K.; Sreedevi, A.; Priyanka, P.; Sabu, B.; Varghese, T. Structural Characterization and Optical Studies of CeO2 Nanoparticles Synthesised by Chemical Precipitation. Nanosyst. Phys. Chem. Math. 2015, 53, 596–603. [Google Scholar] [CrossRef]
  49. Katta, L.; Sudarsanam, P.; Mallesham, B.; Reddy, B.M. Preparation of Silica Supported Ceria-Lanthana Solid Solutions Useful for Synthesis of 4-Methylpent-1-Ene and Dehydroacetic Acid. Catal. Sci. Technol. 2012, 2, 995–1004. [Google Scholar] [CrossRef]
  50. Navrotsky, A.; Mazeina, L.; Majzlan, J. Size-Driven Structural and Thermodynamic Complexity in Iron Oxides. Science 2008, 319, 1635–1638. [Google Scholar] [CrossRef]
  51. Birkner, N.; Navrotsky, A. Thermodynamics of Manganese Oxides: Effects of Particle Size and Hydration on Oxidation-Reduction Equilibria among Hausmannite, Bixbyite, and Pyrolusite. Am. Mineral. 2012, 97, 1291–1298. [Google Scholar] [CrossRef]
  52. Navrotsky, A. Nanoscale Effects on Thermodynamics and Phase Equilibria in Oxide Systems. ChemPhysChem 2011, 12, 2207–2215. [Google Scholar] [CrossRef]
  53. McHale, J.M.; Auroux, A.; Perrotta, A.J.; Navrotsky, A. Surface Energies and Thermodynamic Phase Stability in Nanocrystalline Aluminas. Science 1997, 277, 788–789. [Google Scholar] [CrossRef]
  54. Guo, X.; Wu, D.; Ushakov, S.V.; Shvareva, T.; Xu, H.; Navrotsky, A. Energetics of Hydration on Uranium Oxide and Peroxide Surfaces. J. Mater. Res. 2019, 34, 3319–3325. [Google Scholar] [CrossRef]
  55. Trail, D. Redox-Controlled Dissolution of Monazite in Fluids and Implications for Phase Stability in the Lithosphere. Am. Mineral. 2018, 103, 453–461. [Google Scholar] [CrossRef]
  56. Shannon, R.; Prewitt, C.T. Effective Ionic Radii in Oxides and Fluorides. Acta Crystallogr. B 1969, 25, 925–946. [Google Scholar] [CrossRef]
  57. Salvadó, M.A.; Pertierra, P.; Bortun, A.I.; Trobajo, C.; García, J.R. Phosphorous Acid and Urea: Valuable Sources of Phosphorus and Nitrogen in the Hydrothermal Synthesis of Ammonium-Thorium Phosphates. Inorg. Chem. 2008, 47, 7207–7210. [Google Scholar] [CrossRef]
  58. Matković, B.; Kojić-Prodić, B.; Šlijukić, M.; Topić, M.; Willett, R.; Pullen, F. The Crystal Structure of a New Ferroelectric Compound, NaTh2(PO4)3. Inorg. Chim. Acta 1970, 4, 571–576. [Google Scholar] [CrossRef]
  59. Matkovic, B.; Prodic, B.; Sljukic, M.; Peterson, S.W. The Crystal Structure of Potassium Dithorium Trisphosphate, KTh2(PO4)3. Chroatica Chem. Acta 1968, 40, 147–161. [Google Scholar]
  60. Plakhova, T.V.; Romanchuk, A.Y.; Seregina, I.F.; Svetogorov, R.D.; Kozlov, D.A.; Teterin, Y.A.; Kuzenkova, A.S.; Egorov, A.V.; Kalmykov, S.N. From X-Ray Amorphous ThO2 to Crystalline Nanoparticles through Long-Term Aging at Room Temperature. J. Phys. Chem. C 2022, 127, 187–195. [Google Scholar] [CrossRef]
  61. Parkhurst, D.; Appelo, T. User’s Guide to PHREEQC (Version 2)—A Computer Program for Speciation, Batch-Reaction, One-Dimensional Transport, and Inverse Geochemical Calculations; U.S. Geological Survey, Techniques of Water-Resources Investigation: Reston, VA, USA, 2000.
  62. Svetogorov, R.D.; Dorovatovskii, P.V.; Lazarenko, V.A. Belok/XSA Diffraction Beamline for Studying Crystalline Samples at Kurchatov Synchrotron Radiation Source. Cryst. Res. Technol. 2020, 55, 1900184. [Google Scholar] [CrossRef]
  63. Novichkov, D.; Trigub, A.; Gerber, E.; Nevolin, I.; Romanchuk, A.; Matveev, P.; Kalmykov, S. Laboratory-Based X-Ray Spectrometer for Actinide Science. J. Synchrotron Radiat. 2023, 30, 1114–1126. [Google Scholar] [CrossRef]
  64. Ravel, B.; Newville, M. ATHENA, ARTEMIS, HEPHAESTUS: Data Analysis for X-Ray Absorption Spectroscopy Using IFEFFIT. J. Synchrotron Radiat. 2005, 12, 537–541. [Google Scholar] [CrossRef]
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Figure 1. (a) XRD pattern and (b) SEM image of crystalline sodium–cerium phosphates (Na2Ce(PO4)2(cr.)) obtained after hydrothermal treatment of CeO2 nanoparticles in phosphate buffer at pH = 7.7. (c) XRD pattern and (d) SEM image of nanocrystalline sodium–cerium phosphates (NaCe2(PO4)3(nano)) obtained after hydrothermal treatment of CeO2 nanoparticles in phosphate buffer at pH = 4.4. ‘p.w.’ refers to experimental XRD patterns obtained at present work. The experimental XRD patterns are compared with literature-derived data for Na1.97Ce1.03(PO4)2·xH2O [34] and NaCe2(PO4)3 [29].
Figure 1. (a) XRD pattern and (b) SEM image of crystalline sodium–cerium phosphates (Na2Ce(PO4)2(cr.)) obtained after hydrothermal treatment of CeO2 nanoparticles in phosphate buffer at pH = 7.7. (c) XRD pattern and (d) SEM image of nanocrystalline sodium–cerium phosphates (NaCe2(PO4)3(nano)) obtained after hydrothermal treatment of CeO2 nanoparticles in phosphate buffer at pH = 4.4. ‘p.w.’ refers to experimental XRD patterns obtained at present work. The experimental XRD patterns are compared with literature-derived data for Na1.97Ce1.03(PO4)2·xH2O [34] and NaCe2(PO4)3 [29].
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Figure 2. XANES spectra at the Ce L3 edge for Na2Ce(PO4)2(cr.) and NaCe2(PO4)3(nano) compared with reference compounds. The spectral features of both synthesised phosphates closely resemble those of the Ce(IV) standard, indicating that cerium is predominantly in the tetravalent oxidation state. Crystalline Ce(OH)PO4 and CePO4 were used as references for Ce(IV) and Ce(III).
Figure 2. XANES spectra at the Ce L3 edge for Na2Ce(PO4)2(cr.) and NaCe2(PO4)3(nano) compared with reference compounds. The spectral features of both synthesised phosphates closely resemble those of the Ce(IV) standard, indicating that cerium is predominantly in the tetravalent oxidation state. Crystalline Ce(OH)PO4 and CePO4 were used as references for Ce(IV) and Ce(III).
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Figure 3. Dissolved cerium concentrations in the presence of Na2Ce(PO4)2(cr.) as a function of the pH at different dissolution times in 0.01 M NaClO4. The orange cross-hatched area shows the model of nanosized CeO2 solubility, calculated using logKsp = −59.3 ± 0.3. ‘NPs’ refers to nanoparticles. The grey area denotes the analytical detection limit of cerium concentration measurements using ICP-MS.
Figure 3. Dissolved cerium concentrations in the presence of Na2Ce(PO4)2(cr.) as a function of the pH at different dissolution times in 0.01 M NaClO4. The orange cross-hatched area shows the model of nanosized CeO2 solubility, calculated using logKsp = −59.3 ± 0.3. ‘NPs’ refers to nanoparticles. The grey area denotes the analytical detection limit of cerium concentration measurements using ICP-MS.
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Figure 4. (a) XRD patterns of Na2Ce(PO4)2(cr.) samples after 11 months of equilibration in aqueous solution at various pH values (2.8–7.1). The reference patterns for Na1.97Ce1.03(PO4)2·xH2O [34] and CeO2 [PDF 81-792] are shown for comparison. SEM images of the solid phase after dissolution at (b) pH = 5.0 and (c) pH = 7.1.
Figure 4. (a) XRD patterns of Na2Ce(PO4)2(cr.) samples after 11 months of equilibration in aqueous solution at various pH values (2.8–7.1). The reference patterns for Na1.97Ce1.03(PO4)2·xH2O [34] and CeO2 [PDF 81-792] are shown for comparison. SEM images of the solid phase after dissolution at (b) pH = 5.0 and (c) pH = 7.1.
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Figure 5. Raman spectra of the Na2Ce(PO4)2(cr.) samples before and after long-term dissolution at pH 4.9 and 7.2, compared with a reference spectrum of CeO2 nanoparticles (2 nm). Spectra obtained at a laser wavelength of 405 nm. The symbols ν1, ν3, and ν2 denote different stretching and bending modes of the PO43− group in double Ce(IV) orthophosphate and F2g to the vibrational mode of the oxygen in the CeO2 structure.
Figure 5. Raman spectra of the Na2Ce(PO4)2(cr.) samples before and after long-term dissolution at pH 4.9 and 7.2, compared with a reference spectrum of CeO2 nanoparticles (2 nm). Spectra obtained at a laser wavelength of 405 nm. The symbols ν1, ν3, and ν2 denote different stretching and bending modes of the PO43− group in double Ce(IV) orthophosphate and F2g to the vibrational mode of the oxygen in the CeO2 structure.
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Figure 6. Dissolved cerium concentrations (log[Ce], M) in the presence of NaCe2(PO4)3(nano) as a function of the pH at different dissolution times in 0.01 M NaClO4. The orange cross-hatched area shows the model of nanosized CeO2 solubility, calculated using logKsp = −59.3 ± 0.3. ‘NPs’ refers to nanoparticles. The grey area denotes the analytical detection limit of cerium concentration measurements using ICP-MS.
Figure 6. Dissolved cerium concentrations (log[Ce], M) in the presence of NaCe2(PO4)3(nano) as a function of the pH at different dissolution times in 0.01 M NaClO4. The orange cross-hatched area shows the model of nanosized CeO2 solubility, calculated using logKsp = −59.3 ± 0.3. ‘NPs’ refers to nanoparticles. The grey area denotes the analytical detection limit of cerium concentration measurements using ICP-MS.
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Figure 7. (a) XRD patterns of NaCe2(PO4)3(nano) samples after one year of equilibration in aqueous solution at various pH values (2.8–7.1). The reference patterns for NaCe2(PO4)3 [29] and CeO2 [PDF 81-792] are shown for comparison [29]. SEM images of the solid phase after dissolution at (b) pH 2.9 and (c) pH 6.8.
Figure 7. (a) XRD patterns of NaCe2(PO4)3(nano) samples after one year of equilibration in aqueous solution at various pH values (2.8–7.1). The reference patterns for NaCe2(PO4)3 [29] and CeO2 [PDF 81-792] are shown for comparison [29]. SEM images of the solid phase after dissolution at (b) pH 2.9 and (c) pH 6.8.
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Listova, A.L.; Kuzenkova, A.S.; Gerasimov, M.A.; Kulikova, E.S.; Svetogorov, R.D.; Novichkov, D.A.; Averin, A.A.; Yapaskurt, V.O.; Romanchuk, A.Y.; Kalmykov, S.N.; et al. Comprehensive Dissolution Study on Two Double Ce(IV) Phosphates with Evidence of Secondary CeO2 Nanoparticle Formation. Molecules 2025, 30, 2105. https://doi.org/10.3390/molecules30102105

AMA Style

Listova AL, Kuzenkova AS, Gerasimov MA, Kulikova ES, Svetogorov RD, Novichkov DA, Averin AA, Yapaskurt VO, Romanchuk AY, Kalmykov SN, et al. Comprehensive Dissolution Study on Two Double Ce(IV) Phosphates with Evidence of Secondary CeO2 Nanoparticle Formation. Molecules. 2025; 30(10):2105. https://doi.org/10.3390/molecules30102105

Chicago/Turabian Style

Listova, Anastasiia L., Anastasiia S. Kuzenkova, Mikhail A. Gerasimov, Elizaveta S. Kulikova, Roman D. Svetogorov, Daniil A. Novichkov, Alexei A. Averin, Vasiliy O. Yapaskurt, Anna Yu. Romanchuk, Stepan N. Kalmykov, and et al. 2025. "Comprehensive Dissolution Study on Two Double Ce(IV) Phosphates with Evidence of Secondary CeO2 Nanoparticle Formation" Molecules 30, no. 10: 2105. https://doi.org/10.3390/molecules30102105

APA Style

Listova, A. L., Kuzenkova, A. S., Gerasimov, M. A., Kulikova, E. S., Svetogorov, R. D., Novichkov, D. A., Averin, A. A., Yapaskurt, V. O., Romanchuk, A. Y., Kalmykov, S. N., & Plakhova, T. V. (2025). Comprehensive Dissolution Study on Two Double Ce(IV) Phosphates with Evidence of Secondary CeO2 Nanoparticle Formation. Molecules, 30(10), 2105. https://doi.org/10.3390/molecules30102105

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