Journal of Hazardous Materials 364 (2019) 309–316

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Journal of Hazardous Materials

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Incorporation of strontium and in crystals of α- T isosaccharinate ⁎ Song Chena, Ahmed F. Abdel-Magieda, Le Fuc, Mats Jonssonb, Kerstin Forsberga, a Department of Chemical Engineering, KTH Royal Institute of Technology, Stockholm, Sweden b Department of Chemistry, KTH Royal Institute of Technology, Stockholm, Sweden c Department of Engineering Sciences, Uppsala University, Uppsala, Sweden

GRAPHICAL ABSTRACT

ARTICLE INFO ABSTRACT

Keywords: The final repository for short-lived, low and intermediate level in Sweden is built toactasa Mobility passive repository. Already within a few years after closure water will penetrate the repository and conditions of Radionuclides high alkalinity (pH 10.5–13.5) and low temperature (< 7 °C) will prevail. The mobility of radionuclides in the Isosaccharinate repository is dependent on the radionuclides distribution between solid and liquid phases. In the present work Precipitation the incorporation of strontium (II) and europium (III) in α-calcium isosaccharinate (ISA) under alkaline con- ditions (pH ∼10) at 5 °C and 50 °C have been studied. The results show that strontium and europium are in-

corporated into α-Ca(ISA)2 when crystallized both at 5 °C and 50 °C. Europium is incorporated to a greater extent than strontium. The highest incorporation of europium and strontium at 5 °C rendered the phase compositions

Ca0.986Eu0.014(ISA)2 (2.4% of Eu(ISA)3 by mass) and Ca0.98Sr0.02(ISA)2 (2.2% of Sr(ISA)2 by mass). XPS spectra show that both trivalent and divalent Eu coexist in the Eu incorporated samples. Strontium ions were found to 2+ 3+ retard the elongated growth of the Ca(ISA)2 crystals. The incorporation of Sr and Eu into the solid phase of

Ca(ISA)2 is expected to contribute to a decreased mobility of these ions in the repository.

1. Introduction radioactive waste in Sweden, SFR1, is designed to retain the radioactive elements during thousands of years [1]. The radioactive material will The final repository for short-lived, low and intermediate level consist of the fission products 152Eu, 151Sm, 166mHo, present as trivalent

⁎ Corresponding author at: KTH Royal Institute of Technology, Dept. of Chemical Engineering, Teknikringen 42, 100 44, Stockholm, Sweden. E-mail address: [email protected] (K. Forsberg). https://doi.org/10.1016/j.jhazmat.2018.10.001 Received 18 April 2018; Received in revised form 10 September 2018; Accepted 1 October 2018 Available online 02 October 2018 0304-3894/ © 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/BY/4.0/). S. Chen et al. Journal of Hazardous Materials 364 (2019) 309–316 ions in SFR, and isotopes of actinides such as Pu, Am and Cm [2]. these elements. The calcium and strontium salts of isosaccharate are Groundwater is expected to penetrate into the repository so that the isostructural and crystallize in the orthorhombic space group P21212, vaults will be completely filled already a few years after closure [3]. with a = 19.609 Å, b = 6.782 Å and c = 5.747 Å for α-Ca(ISA)2 [18] Parts of the engineered barriers in the repository are made of ce- and a = 20.040 Å, b = 6.909 Å and c = 5.738 Å for Sr(ISA)2 [19]. No mentitious materials, which will give a high basicity of the aqueous information about Eu isosaccharinate solid phases could be found in phase. Initially alkali will maintain the pH at about 13.5, literature. then the pH will be kept at about 12.5 in equilibrium with portlandite Stability constants and products for Sr(II), Eu(III) and Ca and finally decalcification of calcium-silica hydrate phases will main- (II) complexes and solid phases of relevance are summarized in Table 1. tain the pH in the range 12.5–10.5. Alkaline and anoxic conditions are Stability constants for Sr(II) and isosaccharinate could not be found but expected to remain for more than 20 000 years and the temperature can be approximated based on the stability constants for Sr(II) and will remain about 5–7 °C for about 50 000 years [4–6]. gluconate as suggested by Ochs et al. [20]. However, the use of glu- The short-lived radioactive waste consists of materials such as ion- conate as a functional analogue for isosaccharinate can be questioned exchange resins, filters, and tissues [1]. The materials will degrade and [9]. Based on the reported stability constants, it is evident that euro- − the composition of degradation products in the different rock vault pium will form complexes with ISA to a much larger extent than compartments will change over time [1]. Many of the degradation strontium and calcium under alkaline conditions. products will form complexes with radionuclides and thereby influence Evans et al. [32] studied the solubilisation of solid phases their mobility. The total concentration of radionuclides in the liquid of Eu(III), Co(III) and Sr(III) by pure isosaccharinate (pH 13.3, phase in the repository will depend on the speciation in the aqueous T = 25 °C). Their results suggested that precipitation of Sr iso- phase and the liquid-solid phase equilibrium distribution of the dif- saccharinate and Eu isosaccharinate occurred. However, no solid ferent species. phases were characterised in the study. One of the main degradation products of the cellulosic materials in Trivalent lanthanides, divalent strontium and calcium ions have saturated solutions is D-isosaccharinate. similar ionic radii and thus lanthanides and/or strontium can exchange Isosaccharinate is expected to be stable under the conditions in the with calcium in different solid phases [33,34]. To the best of our repository [7]. The free concentration of isosaccharinate is limited by knowledge, there are no studies on co-precipitation of calcium and the precipitation of calcium isosaccharinate (Ca(ISA)2). There are no lanthanide or actinide isosaccharinate solid phases. If calcium and a reports of hydrated Ca(ISA)2 phases [8]. The solubility of Ca(ISA)2 has particular radionuclide(s) form a thermodynamically stable phase or an been measured in aqueous solutions in the temperature range from unstable phase with a very slow or limited phase transition, the 2.6 °C to 50.4 °C and the thermodynamic (I = 0) stability constant at radionuclides will be retained in the solid phase and thus be less mo- 20 °C and at 25 °C has been determined, see Table 1 [9–12]. There is a bile. The objective of the present study is to investigate to which extent substantial increase in the solubility of Ca(ISA)2 at 23 °C for pH < 4 Sr and Eu are incorporated in the solid phase of α-Ca(ISA)2. (approx. 2 units of magnitude increase of total Ca conc. per unit de- crease in pH) while the solubility is fairly constant in the pH range 4–12 2. Experimental [12]. The decreased solubility under acidic conditions is due to the fact that isosaccharinate transforms into its form at around pH < 4 The incorporation of Sr(II) and Eu(III) into α-Ca(ISA)2 was studied [13]. Above pH 12 the solubility in terms of total calcium concentration by crystallizing α-Ca(ISA)2 in the presence of Sr(II) and Eu(III). The decrease due to precipitation of calcium hydroxide [12]. supersaturation was generated either by evaporation at 50 °C or by It has been shown that cellulosic degradation products have the cooling a solution from 50 °C to 5 °C within 30 min., after which nu- capability to solubilise radionuclides giving rise to relatively high total cleation and growth occurred at 5 °C. The solid phases were analyzed by concentration of these elements in the aqueous phase [14–17]. This powder X-ray diffraction (Powder-XRD, D8 diffractometer, Bruker could potentially lead to precipitation of solid isosaccharinate phases of Corporation), X-ray fluorescence (XRF, PANalytical, epsilon 3), Fourier transform infrared (FT–IR, Perkin Elmer), scanning electron micro- scopy-energy dispersive X-ray (SEM-EDX, LEO 1550, Zeiss), and ther- Table 1 mogravimetric analysis (TGA-DSC, STA 449 F3, Jupiter thermal ana- Cumulative stability constants and solubility products involving Sr, Eu and Ca lyser). The total concentration of Ca, Eu and Sr in the aqueous phase ions. was determined by using inductively coupled plasma-optical emission Equilibrium reaction a Log K, I = 0 T (°C) Reference spectroscopy (ICP-OES, Thermo Fisher iCAP 7400). X-ray photoelectron spectroscopy (XPS) spectra were obtained by using a Physical 2+ + + Ca +H2O ⬄ CaOH + H −12.83 25 [21] 3+ 2+ + Electronics Quantum 2000 Scanning ESCA microprobe (Al Kα X-ray Eu + H2O ⬄ Eu(OH) + H −7.9 25 [22] 3+ + + source). A powder sample was spread on adhesive C tape. The ion and Eu + 2H2O ⬄ Eu(OH)2 + 2H −16.38 25 [22] 3+ + Eu + 3H2O ⬄ Eu(OH)3 + 3H −25.42 25 [22] electron guns were turned on during measurements to neutralize the 3+ − + Eu + 4H2O ⬄ Eu(OH)4 + 4H −34.53 25 [22] surface charge build-up on the non-conducting sample and the spectral 3+ 4+ + 2Eu + 2H2O ⬄ Eu2(OH)2 + 2H −14.0 25 [23] e 3+ 4+ + energies were calibrated by setting the binding energy of the C C as a 3Eu + 5H2O ⬄ Eu3(OH)5 + 5H −33.0 25 [23] + 2+ reference at 284.8 eV. The solid phase α-Ca(ISA)2 (98% purity), stron- SrOH + H+ ⬄ Sr + H2O 13.2 25 [24] 3+ − + Eu + H4ISA ⬄ EuHISA- + 3H −18.3 ± 0.1 r.t. [25] tium nitrate (Sr(NO3)2, > 99,0%), and europium nitrate hexahydrate 2+ − + Ca + H4ISA ⬄ CaH3ISA + H −10.4 ± 0.2 25 [10] (Eu(NO3)3·6H2O, > 99.9%) were purchased from VWR. The solid 2+ − ⬄ + Ca + H4ISA CaH4ISA 1.80 ± 0.1 22 ± 1 [26] phases Eu(ISA)3 and Sr(ISA)2 are not commercially available. 2+ − + b Sr + H4ISA ⬄ SrH4ISA 1.54 25 Est. [20] − + H4ISAH ⬄ H4ISA + H −3.27 25 [27] 2+ + 2.1. Recrystallization of pure α-Ca(ISA)2 Ca ⬄ Ca(OH)2 (cr) + 2H −22.81 25 [28] 3+ + Eu + 3H2O ⬄ Eu(OH)3 (cr) + 3H −15.1 25 [29] 3+ ⬄ + Eu + 3H2O Eu(OH)3 (cr) + 3H −16.4 25 [30] The α-Ca(ISA)2 (98% purity) was first dissolved in either water or 2+ − ⬄ Ca + 2H4ISA Ca(H4ISA)2 (cr) 6.36 ± 0.1 25 [10] saturated calcium hydroxide at 50 °C by placing the vessel in a tem- Ca2+ + 2H ISA− ⬄ Ca(α-H ISA) (cr) 6.53 ± 0.02 20 ± 1 [11] 4 4 2 perature-controlled water bath. The saturated calcium hydroxide solu- a H4ISA− in these reactions corresponds to ISA− used throughout the text. tion was filtered before dissolving the calcium isosaccharinate. ThepH The 4 hydrogen ions are the protons of the 4 hydroxy-groups in ISA−. in the saturated calcium hydroxide solution was around 12.5 and the b This is an estimation based on reported data for the strontium-gluconate pH in the pure water calcium isosaccharinate solution was measured to system [31]. be around 10. A magnetic stirrer was used to stir the suspension for

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Table 2 Table 4 Initial conditions. Initial conditions and residence times for the experiments at 5 °C (b).

Exp. Aqueous phase Calcium conc. (mmol/L) Exp. Ca(II) Eu(III) Sr(II) Initial molar ratio Residence time (mmol/L) (mmol/L) (mmol/L) at 5 °C (days) A1, A2 Pure water 20.2 Eu/Ca Sr/Ca A3 Saturated Ca(OH)2 48.8 B1b 61.4 0.65 0 1:94 – 7 B2b 66.7 3.0 0 1:23 – 7 24 h, which was enough time to reach saturation. Two methods were B3b 61.6 6.5 0 1:9 – 7 B4b1 – – 0 1:3 – 7 then used in order to recrystallize calcium isosaccharinate, evaporation B5b 79.1 87.2 0 1:0.9 – 7 and cooling. Crystallization by evaporation was achieved by evapor- B6b 101.7 0 5.5 – 1:19 10 ating the aqueous phase at a constant temperature of 50 °C, whereafter B7b 73.8 0 30.1 – 1:2.5 10 the crystals were filtered, washed with ethanol and dried at 50 °C (exps. B8b 96.5 0 58.4 – 1:1.5 7 B9b 71.1 0 110.2 – 2:1 7 A1a, A2a and A3a). The cooling crystallization was performed by B10b 71.7 0 219.6 – 3:1 7 cooling the remaining aqueous phase (after filtration through a 0.2 μm PP filter) from 50 °C to 5 °C at a rate of 1.5 °C/min. No crystals couldbe 1 The initial total concentration of Ca and Eu was not measured in this observed during this time. The samples were then stored under mag- sample but the sample was prepared with an initial ratio of Eu/ Ca equal to 1:3 netic stirring in the water bath for 7 days at 5 °C, during which nu- as reported in the Table. cleation and crystal growth occurred. The crystals were then filtered, washed with ethanol and dried overnight at 50 °C (exps. A2b and A3b) After each experiment the solid phase was separated from the or at room temperature for 2 days (exp. A1b). The solid phases were aqueous phase and the crystals were washed with ethanol and dried at analyzed by powder-XRD and SEM-EDX. The initial conditions of the 50 °C. The concentrations of calcium, europium and strontium in the experiments performed are presented in Table 2. aqueous phases were determined by ICP-OES and the solid phases were The possible influence of washing the solid phase with ethanol was fully characterized. The Cambridge Structural Database (CSD 1119171) investigated in a separate experiment (exp. A4). A sample of α-Ca(ISA)2 was used to identify the solid phases based on the powder XRD data. (0.05 g, 98% purity) crystals were added to 40 mL of ethanol followed The thermal behaviour of the commercial calcium isosaccharinate and by 1 h of stirring at room temperature. The solid phase was then se- calcium isosaccharinate containing europium (exp. B3b) were in- parated from the aqueous phase by centrifugation and was dried at vestigated by TGA-DSC. 50 °C and then weighed. The solid phase was analyzed by powder-XRD There is a possibility that the ions (Sr or Eu) contaminate the solid before and after being in contact with ethanol. phase by being adsorbed to the surface of the crystals. In order to study if a significant part of the Sr(II) is adsorbed to the surface or nottheSr (II) containing Ca(ISA)2 from exp. B8b was investigated by powder-XRD 2.2. Co-crystallization of Eu(III), Sr(II) and Ca(II) isosaccharinate and XRF before and after washing with ethanol.

A specific amount of α-Ca(ISA)2 (98% purity) was added to deio- nized and filtered (Millipore) water. The sealed beaker was placed ina 3. Results and discussion water bath held at 50 °C and a magnetic stirrer was used to stir the suspension for 24 h, which was enough time to reach saturation [11]. 3.1. Recrystallization of pure α-Ca(ISA)2 The saturated aqueous phase was then separated from the remaining solid phase and transferred to a new beaker by using a syringe and a PP The SEM-EDX data from experiment A1a is presented in Fig. 1. The syringe filter (0.2 μm). The aqueous phase was spiked with either Eu crystals had an elongated shape and Ca, O and C could be detected by (III) or Sr(II) ions by adding a nitrate solution of the respective ion. Two EDX. The Au and Pd were from the sputter coating during the SEM different methodologies to generate supersaturation, evaporation and sample preparation. The solid phase obtained was identified to be cooling, were then applied to crystallize calcium isosaccharinate, in crystalline Ca(ISA)2 by powder-XRD as shown in Fig. 2. As it is not clear whether the drying conditions affect the phases of recrystallized cal- accordance with the methodology to prepare the pure α-Ca(ISA)2 crystals (see section 2.1). The initial conditions in the evaporation ex- cium isosaccharinate, we compared the results obtained by drying the periments are presented in Table 3. For the cooling experiments the crystals at room temperature and at 50 °C. The XRD results showed the temperature was lowered from 50 °C to 5 °C with a rate of 1.5 °C/min. obtained powders were pure calcium isosaccharinate (CSD 1,119,171). No crystals could be observed during this time. The samples were then No phase change can be observed comparing the samples obtained at stored for 7 or 10 days at 5 °C under magnetic stirring during which 5 °C and then dried at room temperature (exp. A1b) and at 50 °C (exp. nucleation and crystal growth occurred (see Table 4). A2b) respectively. The powder-XRD patterns of the solid phase obtained at 50 °C (A3a) Table 3 and 5 °C (A3b) respectively in saturated calcium hydroxide is shown in Initial conditions for the evaporation experiments at 50 °C (a). Fig. 3. The solid phase obtained after evaporation was mainly amor- phous while the solid mass obtained after cooling was identified as Exp. Ca(II) Eu(III) Sr(II) Molar ratio (aq) crystalline calcium isosaccharinate. No other solid phases, such as (mmol/L) (mmol/L) (mmol/L) Eu/Ca Sr/Ca calcium hydroxide or calcium carbonate, could be detected. Ethanol was used to wash the powder throughout the study, but B1a 22.2 0.23 0 1:96 – there are no reports regarding the solubility of calcium isosaccharinate B2a 21.6 0.90 0 1:24 – in ethanol and the reaction between calcium isosaccharinate and B3a 19.0 1.86 0 1:10 – B4a 12.3 4.10 0 1:3 – ethanol is unknown. Therefore an experiment was conducted to in- B5a 21.2 16.3 0 1:1 – vestigate whether the calcium isosaccharinate could be washed by B6a 21.6 0 1.6 – 1:14 ethanol. The powder XRD patterns before and after contacting the solid B7a 22.3 0 9.0 – 1:2 phase with ethanol for 1 h (exp. A4) are shown in Fig. 4. The solid phase B8a 21.3 0 18.2 – 1:1 was identified as calcium isosaccharinate both before and after storage B9a 21.8 0 33.7 – 2:1 B10a 21.3 0 65.7 – 3:1 in ethanol and no additional peaks appeared after storage. However, the crystallinity decreased slightly after being in contact with ethanol

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Fig. 1. SEM image and EDX data for Ca(ISA)2 obtained by evaporation at 50 °C (exp. A1a).

Fig. 2. Powder XRD patterns of Ca(ISA)2 obtained at 5 °C and dried at r.t. (exp. A1b, upper black) or 50 °C (exp. A2b, lower red) respectively from pure water solution (For interpretation of the references to colour in this figure legend, the Fig. 4. Powder-XRD patterns of Ca(ISA)2 before (lower black) and after (upper reader is referred to the web version of this article). red) storage in ethanol for 1 h (exp. A4) (For interpretation of the references to colour in this figure legend, the reader is referred to the web version ofthis article).

Fig. 3. Powder XRD patterns of powders obtained at 50 °C (exp. A3a, lower black) and at 5 °C (exp. A3b, upper red) respectively from saturated calcium hydroxide solution (For interpretation of the references to colour in this figure Fig. 5. XRD patterns of Ca(ISA)2 obtained by evaporation at 50 °C (exps. A1a legend, the reader is referred to the web version of this article). (black), B3a (green), B4a (blue) and B8a(red)) (For interpretation of the re- ferences to colour in this figure legend, the reader is referred to the web version for 1 h. The weight of the solid phase decreased by 0.0037 g, which of this article). indicate that the solubility of calcium isosaccharinate in ethanol (0.092 g/L) is lower than the solubility in water (3 g/L) at room tem- 3.2. Co-crystallization of Eu(III), Sr(II) and Ca(II) isosaccharinate perature [11]. Although it is not established that equilibrium conditions were reached after 1 h of residence time. We can conclude that it is a 3.2.1. Crystallization at 50 °C good option to wash the crystals with ethanol, but the crystals should The powder-XRD patterns of the crystals obtained in experiments not be left in ethanol for longer durations. A1a, B3a, B4a and B8a are presented in Fig. 5. All samples had the same diffraction patterns, showing that the crystal phases in all the samples

were Ca(ISA)2. For the Sr/Ca = 1:1 system (exp. B8a), EDX showed the existence of Sr in the solid phase together with calcium (see Fig. 6).

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Fig. 6. SEM image and EDX data for Ca(ISA)2 obtained by evaporation at 50 °C (exp. B8a, Ca/Sr = 1:1, Srinitial = 18 mmol/L).

Table 5 Molar ratio in the aqueous phase before and after growth at 5 °C.

Exp. Initial molar Final molar Exp. Initial molar Final molar ratio Eu/Ca ratio Eu/Ca ratio Sr/Ca ratio Sr/Ca

B1b 1:94 1:64 B6b 1:19 1:12 B2b 1:23 1:15 B7b 1:2.5 1:2 B3b 1:9 1:10 B8b 1:1.5 1:1 B4b 1:3 – B9b 2:1 2:1 B5b 1:0.9 – B10b 3:1 3:1

Together these results indicate that Sr(II) is incorporated into the crystal lattice of Ca(ISA)2 substituting for calcium due to molecular resemblance. The SEM analysis is shown in Fig. 6, and it is clear that the crystals were less elongated than the crystals obtained with no Sr pre- Fig. 8. Powder XRD patterns of Eu-containing Ca(ISA)2 (exps. B1b (lower), B2b sent (exp. A1a, see Fig. 1). It is possible that Sr ions or complexes of Sr (middle) and B3b (upper)). ions are capable of retarding the growth of the faces perpendicular to the direction of elongation of the crystals. The amount of powder in Table 6 exp. B8a was not enough to perform a XRF analysis. The solid phases Crystallization of Ca(ISA)2 in the presence of Eu(III). obtained in experiment B3a (Eu/Ca = 1:10) and B4a (Eu/Ca = 1:3) Exp. Initial Phase composition Molar Δc (Ca)tot Δc were analyzed by XRF. The results showed that Ca0.98Eu0.02(ISA)2 was + molar ratio (mmol/ (CaH4ISA )free obtained when Eu/Ca = 1:10 and that Ca0.94Eu0.06(ISA)2 was obtained ratio Eu/ Eu/Ca L) (mmol/L) when Eu/Ca = 1:3. The results show that Sr(II) and Eu(III) are in- Ca (aq) (s) corporated in crystalline Ca(ISA)2 under the applied conditions and that B1b 1:94 Ca0.995Eu0.005(ISA)2 0.005 26 15 the incorporation of Sr (Sr/Ca = 1:1) and Eu (Eu/Ca = 1:3, Eu/ (0.01)

Ca = 1:10) do not change the phase compositions of the products. B2b 1:23 Ca0.986Eu0.014(ISA)2 0.01 32 21 (0.04)

B3b 1:9 (0.1) Ca0.987Eu0.013(ISA)2 0.01 8 4 3.2.2. Crystallization at 5 °C The molar ratios of Sr/Ca and Eu/Ca in the aqueous phase before and after crystal growth at 5 °C in experiments B1b to B10b are pre- obtained when the initial Eu/Ca ratio was 1:94 (exp. B1b), 1:23 (exp. sented in Table 5. The results show that when the initial fraction of Sr B2b) and 1:9 (exp. B3b) has similar morphology and size, as shown in and Eu in the aqueous phase decreases so does the fraction in the solid Fig. 7. phase. The XRD patterns are presented in Fig. 8. The crystals were iden- No crystals were obtained in the experiments where the initial Eu/ tified as Ca(ISA)2 in all experiments. The elemental composition of the Ca was 1:3 (exp. B4b) and 1:0.9 (exp. B5b). This can be explained by crystals from experiment B1b–B3b, determined by XRF, are presented the complexation of isosaccharinate with europium. The crystals

Fig. 7. SEM images of Eu-containing Ca(ISA)2. (a) B3b, (b) B2b, (c) B1b.

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crystalline and that the only phase detected in all experiments is Ca (ISA)2. The SEM images of the crystals are presented in Fig. 10. The crystals are in the micrometer range implying that the mixing will have little effect on the impurity incorporation. The presence of Sr affected the

morphology of the Ca(ISA)2 crystals (see Fig. 10). The Sr ions seem to retard the growth of the crystals in the elongated direction more than the growth of the other faces of the crystals. The crystals in the sample with the highest concentration of Sr (exp. B9b) are larger than in the other samples. This could be due to a lower supersaturation driving − force to nucleate Ca(ISA)2 since a larger part of ISA is in complex with Sr. Due to the lower supersaturation, the number of nuclei formed are lower and thus the crystals become larger. The elemental composition of the crystals from experiments B6b–B9b, determined by XRF, are presented in Table 7. The XRF results

confirm that Sr co-precipitate with Ca(ISA)2. Since the Sr concentration in the aqueous phase changes during the growth of the crystals the Fig. 9. Powder XRD of Sr-containing Ca(ISA)2. (exps. B6b (green), B7b (blue), composition might not be the same throughout the crystals. The pro- B8b (red) and B9b (black)). (For interpretation of the references to colour in portion of the concentration of Sr in the aqueous phase and in the solid this figure legend, the reader is referred to the web version of this article). phase is not the same for the different experiments. The effective dis- tribution ratio (impurity content in the solid versus in the liquid phase) in Table 6. The XRF results confirmed that Eu co-precipitate with Ca of an impurity in the aqueous phase and in the solid phase can depend

(ISA)2. The chemical speciation in the aqueous phase before nucleation both on thermodynamics and kinetics. Typically, as the crystal growth and after growth was estimated by using the software Medusa [35] and rate decreases so does the effective distribution ratio leading to a higher the stability constants reported in Table 1. The supersaturation reported purity of the crystals. The amount of Sr in experiment B9b is lower than in Table 6 is expressed both in terms of total calcium concentration, as in experiment B8b although the impurity concentration in the aqueous measured by ICP-OES, and in terms of the concentration of the complex phase is highest in experiment B9b. The lower content of Sr in experi- + CaH4ISA before nucleation and after growth. The mass % of Eu in the ment B9b could be due to a lower supersaturation driving force in this solid phase from experiment B2b and B3b is similar although the con- experiment compared to experiment B8b and thus a slower growth rate centration of Eu in the aqueous phase is higher in experiment B3b. of the crystals with a higher degree of impurity rejection. Since the However, the initial supersaturation driving force for crystallizing cal- initial concentration of calcium is higher in experiment B8b (see cium isosaccharinate is lower in experiment B3b than in experiment Table 4) the initial supersaturation, before nucleation, is higher than in B2b. The phase composition of the crystals obtained in experiment B3b experiment B9b. In addition, the concentration of Sr is higher in ex-

(Ca0.987Eu0.013(ISA)2) at 5 °C is similar to the composition of the crystals periment B9b than in experiment B8b and thus the concentration of obtained at 50 °C in experiment B3a (Ca0.98Eu0.02(ISA)2), with a similar isosaccharinate in complex with calcium is also lower in experiment ratio of Eu/Ca (1:10). Bb9. In addition, the crystal surface coverage of the impurity is po- No crystals were obtained in the experiment with the highest con- tentially larger in sample Bb9 than in sample B8b, which also could centration of Sr (exp. B10b, Sr/Ca = 3:1). The strontium ions are ex- impede the growth rate. pected to form complexes with isosaccharinate and thus at high con- By comparing the data in Tables 6 and 7 it can be seen that Eu(III) is centrations of strontium ions the amount of isosaccharinate and incorporated into Ca(ISA)2 to a larger extent than Sr(II) under com- calcium might not be enough to surpass the solubility limit of calcium parable conditions. A possible explanation for this could be that the isosaccharinate (or of strontium isosaccharinate). The powder-XRD concentration of Eu in complex with isosaccharinate is higher than that patterns of the Sr containing crystals obtained in experiment B6b–B9b of Sr in complex with isosaccharinate under identical conditions (log +2 + are presented in Fig. 9. The results show that the powders obtained are KEuISA < logK1,SrISA , see Table 1).

Fig. 10. SEM images of Sr-containing Ca(ISA)2. (a) B6b, (b) B7b, (c) B8b, (d) B9b.

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Table 7

Crystallization of Ca(ISA)2 in the presence of Sr (II).

+ Exp. Initial molar ratio Sr/Ca (aq.) Phase composition Molar ratio Sr/Ca (s) Δc (Ca)tot (mmol/L) Δc (CaH4ISA )free (mmol/L)

B6b 1:19 (0.05) Ca0.999Sr0.001(ISA)2 0.001 37 29

B7b 1:2.5 (0.4) Ca0.995Sr0.005(ISA)2 0.005 7 4

B8b 1:1.5 (0.7) Ca0.98Sr0.02(ISA)2 0.02 44 26

B9b 2:1 (2) Ca0.99Sr0.01(ISA)2 0.01 12 7

Fig. 13. Thermogravimetric analysis of commercial Ca(ISA) (upper black) and Fig. 11. Powder XRD of unwashed (lower black) and washed (upper red) Ca 2 Ca(ISA) containing with Eu (lower red) from experiment B3b (For inter- (ISA) (For interpretation of the references to colour in this figure legend, the 2 2 pretation of the references to colour in this figure legend, the reader is referred reader is referred to the web version of this article). to the web version of this article).

Table 8 (ISA)2 belong to P21212 space group and both of Ca and Sr ions are XRF results of unwashed and washed Ca(ISA)2. coordinated by eight oxygen atoms [8]. In addition, Ca(ISA)2 and Sr Exp. B8b (unwashed) Exp. B8b (washed) (ISA)2 are also isostructural and they have closely related cell para- meters. Eu and Ca have similar ionic radius. Therefore the Sr and Eu are Sr/Ca (mol ratio) 0.017 0.021 more likely to substitute Ca ions than to be incorporated at an inter- stitial position.

The FT-IR spectra of the crystals from experiments A1b (Ca(ISA)2) and B3b (Ca0.987Eu0.013(ISA)2) are presented in Fig. 12. The spectra for the two samples are identical. The peak at 1590 cm−1 is due to the C] O vibration. The peaks between 400–1500 cm−1 result from the CeH and OeH vibrations.

The TGA analysis for the commercial Ca(ISA)2 and Ca0.987Eu0.013(ISA)2 (exp. B3b) is presented in Fig. 13. Heating samples to 800 °C resulted in a continuous loss in weight. The weight loss of commercial Ca(ISA)2 before 200 °C was presumably the loss of free and crystallization water. The Ca(ISA)2 started to melt at 238 °C and showed a great weight loss between 238 °C and 256 °C, which is related to the

decomposition of Ca(ISA)2. At temperatures ranging from 400 °C to 530 °C, the weight loss is probably due to the loss of carbon monoxide and the loss of carbon dioxide was observed at temperatures ranging from 600 °C to 720 °C. The final mass obtained at 720 °C was 27% ofthe

initial mass. The TGA curve of Ca0.987Eu0.013(ISA)2 was similar to that of commercial Ca(ISA)2. However, the remaining mass of the Eu con- Fig. 12. FT-IR spectrum of Ca(ISA) samples, A1b (lower), B3b (upper). 2 taining Ca(ISA)2 after thermal analysis was different than for the commercial Ca(ISA)2. The reason might be that the commercial Ca

The powder-XRD pattern of Sr containing Ca(ISA)2 (exp. B8b) be- (ISA)2 contains other organic impurities and the concentration of these fore and after washing with ethanol are shown in Fig. 11. No additional impurities in the solid phase is reduced during the recrystallization phase besides calcium isosaccharinate was identified before or after process. Therefore the Eu containing Ca(ISA)2 contains less impurities washing with ethanol. The XRF results are presented in Table 8. The Sr/ than the commercial sample. Ca molar ratio in the solid phase does not decrease after washing. In XPS spectra showed the existence of Ca, P, C, O and Eu in the Eu fact, the ratio is even higher after the wash step. The results indicate incorporated sample, see Fig. 14 (a). Moreover, the Eu 3d5/2 peaks at 3+ that the Sr ions are not adsorbed to the surface of the crystals to a large 1133 eV and 1123 eV were observed, which correspond to Eu and 2+ extent. Whether the incorporation is substitutional or interstitial de- Eu respectively [36], see Fig. 14 (b). pends on chemical similarity of the incorporated ions. Ca(ISA)2 and Sr

315 S. Chen et al. Journal of Hazardous Materials 364 (2019) 309–316

Fig. 14. (a) XPS of the Eu incorporated sample with the range between 0 eV and 1200 eV. (b) XPS signals showing the coexistence of Eu(II) and Eu(III).

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