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Article

Recrystallization and Uptake of 226Ra into Ba-Rich (Ba,Sr)SO4 Solid Solutions

by
Felix Brandt
*,
Martina Klinkenberg
,
Jenna Poonoosamy
and
Dirk Bosbach
Institute of Energy and Climate Research (IEK-6)-Nuclear Waste Management and Reactor Safety, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany
*
Author to whom correspondence should be addressed.
Minerals 2020, 10(9), 812; https://doi.org/10.3390/min10090812
Submission received: 24 August 2020 / Revised: 10 September 2020 / Accepted: 12 September 2020 / Published: 15 September 2020
(This article belongs to the Special Issue Formation of Sulfate Minerals in Natural and Industrial Environments)
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Abstract

:
226Ra is an important contributor to naturally occurring radioactive materials (NORM) and also considered in safety cases related to the disposal of spent nuclear fuel in a deep geological repository. Recrystallization and solid solution formation with sulfates is regarded as an important retention mechanism for 226Ra. In natural systems sulfates often occur as (Ba,Sr)SO4. Therefore, we have chosen this solid solution at the Ba-rich end for investigations of the 226Ra uptake. The resulting 226Ra-solubility in aqueous solution was assessed in comparison with a thermodynamic model of the solid solution-aqueous solution system (Ba,Sr,Ra)SO4 + H2O. The temperature and composition of the initial (Ba,Sr)SO4 solid solution were varied. Measurements of the solution composition were combined with microscopic observations of the solid and thermodynamic modeling. A complex recrystallization behavior of the solid was observed, including the dissolution of significant amounts of the solid and formation of metastable phases. The re-equilibration of Ba-rich (Ba,Sr)SO4 to (Ba,Sr,Ra)SO4 leads to a major reconstruction of the solid. Already trace amounts of Sr in the solid solution can have a significant impact on the 226Ra solubility, depending on the temperature. The experimental findings confirm the thermodynamic model, although not all solids reached equilibrium with respect to all cations.

1. Introduction

The fate of 226Ra is relevant to a number of environmental questions, mainly due to the fact that it is one of the main contributors to naturally occurring radioactive materials (NORM). 226Ra containing NORM appears in many raw material production processes e.g., phosphate industry, unconventional gas production, geothermal energy production, and oil extraction [1,2,3,4,5,6]. 226Ra is also considered as a relevant radionuclide in safety cases that are prepared for the deep geological disposal of high-level nuclear waste [7,8,9]. There, it will occur as a fission product of the 238U decay chain and may dominate the dose after about 100,000 years.
The migration of radionuclides in the geosphere is, to a large extent, controlled by sorption processes onto minerals and colloids. On a molecular level, sorption phenomena involve surface complexation, ion exchange as well as co-precipitation reactions. Co-precipitation can lead to the formation of solid solutions in which the radionuclides are structurally incorporated in a host structure [6,8,10]. Such solid solutions are ubiquitous in natural systems—most minerals in nature are mixtures of elements on the molecular scale rather than pure compounds. Recent studies [11,12,13,14,15,16,17,18] have shown that the formation of a (Ba,Ra)SO4 solid solution significantly reduces the solubility of 226Ra in aqueous systems. Rapid uptake via co-precipitation [11,12,13] as well as the slower recrystallization process can lead to this solid solution [14,15,16,17,18]. Recrystallization of BaSO4 in the presence of 226Ra has been considered to be relevant with respect to nuclear waste disposal [19] as well as to ore processing [20,21], where barite was also observed to take up 226Ra from the process solutions.
In order to predict the resulting solubility of 226Ra in such a system, Vinograd et al., 2013 [22] combined theoretical approaches and experimental data. They derived a thermodynamic model for the solid solution-aqueous solution (SS-AS) system (Ba,Ra)SO4 + H2O [22]. In natural systems sulfates often occur as (Ba,Sr)SO4 rather than pure barite. Hence, this thermodynamic model was later on extended to the ternary SS-AS system (Ba,Sr,Ra)SO4 + H2O, and experimentally confirmed for the Sr-rich corner [23,24,25].
However, the most interesting feature of the predicted system behavior, a minimum of the 226Ra solubility in the Ba-rich corner of the SS-AS system (mole fraction of SrSO4, XSrSO4 < 10 mol%) still remained experimentally unconfirmed. Here, we have carried out extended long-term recrystallization experiments of more than 660 days in the Ba-rich region of the SS-AS (Ba,Sr,Ra)SO4 + H2O and assessed the 226Ra-uptake into the solid as well as the resulting 226Ra-solubility. Macroscopic observations of the solution composition and thermodynamic considerations were combined with microscopic observations to follow in detail the process of solid solution formation due to recrystallization of Ba-rich (Ba,Sr)SO4 put into contact with 226Ra as a function of temperature and initial solid solution composition.

2. Materials and Methods

Homogeneous (Ba,Sr)SO4 solid solutions of a defined composition were synthesized according to the flux method already applied in Klinkenberg et al. (2018) [25]. This method was adapted from procedures of Patel and Koshy (1968) and Patel & Bhat (1971) [26,27]. A detailed characterization of the chemical and morphological homogeneity was carried out by scanning electron microscopy combined with energy-dispersive X-ray spectrometry (SEM-EDX) (Quanta 200F, FEI, Eindhoven, Netherlands; EDAX, Weiterstadt, Germany). The synthesized solids are summarized in Table 1.
The grain size was adjusted to 20–63 µm by grinding and sieving. The chemical homogeneity and morphology of the initial solid solution particles is shown in the back-scatter electron (BSE) image of Figure 1. In order to allow for comparison, the preparation of the solids as well as the general set-up of the recrystallization experiments were adopted from earlier studies (e.g., [17,25]). 0.01 or 0.1 g of solid were added to 10 mL of a 0.2 mol/kg NaCl solution in 25 mL glass vessels. The particles were pre-equilibrated for four weeks at 23 ± 2 °C before the start of the actual experiments to avoid high energy surface sites and ultrafine particles.
Long-term batch recrystallization experiments running 664 days were performed at 90 °C, 70 °C and ambient conditions (23 ± 2 °C). 10 mL of tracer solution were added to 10 mL of the pre-equilibrated suspension, resulting in solid/liquid ratios (S/L) of 0.5 g/kg and 5.0 g/kg, respectively, and an ionic strength I = 0.1 mol/kg NaCl. For the same type of glass vessels, in earlier experiments no measurable wall adsorption of 226Ra was detected. All recrystallization experiments were started from a concentration of c(Ra) = 5.5 ± 0.5 × 10−6 mol/kg 226RaBr2. A summary of the experiments is provided in Table 2.
After a settling time of 1 h, samples of 500 µL of the aqueous solution were taken at the same time intervals for all experiments. The settling time was required for cooling and handling of the radioactive solutions at 70 °C and 90 °C. Based on the experience of Klinkenberg et al., 2018 [25], this is a much shorter time than required for barite and 226Ra to re-equilibrate to the lower temperature. The solution samples were filtered through Advantec ultrafilters (Molecular weight cut-off (MWCO) = 10,000 Da) to avoid possible colloids or fine particles without measurable adsorption of 226Ra at the given filtered solution amount. This procedure was tested in earlier studies [14,15]. Parallel recrystallization experiments without 226Ra were carried out as reference.
A N2 cooled high-purity (HP) Ge-detector was used for the quantification of the 226Ra concentration at the characteristic 186 keV γ- peak of 226Ra. The Sr and Ba concentrations in solution were quantified using an ICP-MS ELAN 6100 DRC (PerkinElmer SCIEX, Waltham, MA, USA) instrument. The filtered solution was diluted in 0.1 m HNO3 by 1:1000 for Ba and 1:10,000 for Sr-measurements.
Small amounts of solid (10 µL of the suspension) were sampled at selected sampling times from the settled particles of the recrystallization experiments. The evolution of the crystal morphology and chemical composition were studied using SEM combined with EDX. In order to avoid artefacts due to precipitation of e.g., NaCl, SrSO4 or RaSO4, the samples were separated from their solution by two washing steps in iso-propanol. The samples were then prepared as a suspension on a Cu holder and subsequently dried.
Thermodynamic calculations were carried out to compare theoretical predictions based on a thermodynamic model for the SS-AS system (Ba,Sr,Ra)SO4 + H2O with the experimental results. The thermodynamic model derived in Vinograd et al. (2018) [23] and refined in Klinkenberg (2019) [25] was used for the calculation of the total equilibrium between the solid and aqueous phase.
In the case of SS-AS systems, not only do the activities of ions in solution but also of the components of the solid need to be considered. In contrast to pure phases, in SS-AS systems the solution composition is not independent of the amount of solid. For SS-AS equilibria, the solution composition is also linked to the composition of the solid. Gibbs energy minimization approaches implemented in the GEMS3K solver (http://gems.web.psi.ch/GEMS3K) and described in Kulik et al. (2013) [28] were applied to calculate the solid solution composition as well as the aqueous solution equilibria at 23 °C, 70 °C and 90 °C. The equilibria were calculated assuming full equilibration of all (Ba,Sr)SO4 with 226Ra in solution. The activity coefficients for all dissolved species (γj) were calculated according to the extended Debye–Hückel equation [29]. Thermodynamic data for aqueous species were taken from the PSI-Nagra database [30] integrated in GEMS that inherits temperature and pressure dependencies for most aqueous ions and complexes from the Helgeson-Kirkham-flowers equation of state (HKF EoS) [29] as given in the SUPCRT92 database (http://gems.web.psi.ch/TDB). Interaction parameters for the ternary (Ba,Sr,Ra)SO4 + H2O SS-AS system were taken from Klinkenberg et al. (2018) [25].

3. Results

3.1. The Evolution of the 226Ra Concentration over Time

Distinct differences with respect to the evolution of the 226Ra concentrations in solution were observed, depending on the composition of the original solid solution (Figure 2). Qualitatively, all experiments follow the trend predicted by the thermodynamic modelling, i.e., the highest uptake of 226Ra is observed at XSrSO4 = 29 mol% in the initial solid solution. The kinetics of the 226Ra uptake also follow a trend according to XSrSO4 of the initial solid solution, with a slower uptake at low initial XSrSO4 and an increasing uptake rate from 17 mol% to 29 mol%.
In particular, the combination of low temperature (23 °C) and a low initial XSrSO4 keeps the 226Ra concentration in solution almost on the original level for more than 100 days. Compared to pure BaSO4, the 226Ra uptake is slightly slower in the case of XSrSO4 = 5 mol% and faster at higher XSrSO4 of the original solid solution (Figure 2). At 70 °C and 90 °C, the 226Ra concentration in solution has a minimum below the predicted equilibrium concentration before equilibrium is approached at later stages of the experiment. This is likely to be a kinetic effect which leads to the metastable “entrapment” of a surplus of 226Ra due to a relatively high uptake rate. This effect was also observed with 226Ra uptake into pure barite in earlier studies [31]. In addition to temperature, the S/L has an effect on the uptake kinetics, resulting in higher 226Ra uptake rate at 90 °C and S/L = 5 g/kg (Figure 3) in comparison to 0.5 g/kg.
Within 100 days, the majority of the recrystallization experiments reach a plateau of the 226Ra concentrations which is close to the predicted equilibrium (lines in Figure 2). At 23 °C and S/L = 0.5 g/kg, the effect of Sr added to the SS-AS system results in a significant decrease of the Ra solubility. Compared to pure BaSO4 this decrease can be up to one order of magnitude. The predicted 226Ra-solubilities for the respective experiments become more similar with increasing temperatures of 70 °C and 90 °C. This is also experimentally observed for the final 226Ra concentrations in this study. At 90 °C, the observed and predicted differences of the 226Ra solubility between the different solid solutions and pure BaSO4 are small and within the experimental error (Figure 2). At 90 °C and S/L = 5 g/kg, the predicted results of the 226Ra solubility as well as the experimental results are almost independent from the original XSrSO4 of the solid solution (Figure 3).

3.2. The Evolution of Ba and Sr Concentrations over Time

As shown in Figure 2, the presence of 226Ra has a rather small impact on the calculated equilibrium concentrations of Ba and Sr in solution. The predicted Ba-solubility at all temperatures decreases in the order from Ba-rich to Ba-poor original solid solutions whereas the Sr-solubility increases from Sr-rich to Sr-poor original solid solutions. Therefore, the final predicted solids are much more similar to each other in composition than the original solid solutions before re-equilibration (Table 3). Due to the high proportion of total (Ba + Sr) compared to the amount of 226Ra added to the respective experiment, the predictions for the Ba and Sr solubility after recrystallization are very similar for corresponding 226Ra-free reference experiments and the 226Ra-recystallization experiments.
A comparison of the experimental results and predicted equilibrium indicates Ba to be supersaturated in the aqueous solution at the beginning of all experiments. The concentration of Sr in solution starts from values well below the predicted equilibrium and usually approaches equilibrium later than Ba. After 200 to 400 days, in most of the experiments the concentrations of Sr and Ba are close to or at the predicted equilibrium (Figure 2). The kinetic behavior of the (Ba,Sr)SO4 recrystallization is more or less independent of the presence of 226Ra. In the series of 226Ra free reference experiments, the experiment with XSrO4 = 5 mol% is an exception because the concentration of Sr in solution in particular stays well below the predicted equilibrium, and at 23 °C the Ba concentration stays higher than predicted-similar to the corresponding 226Ra recrystallization experiment.

3.3. Chemical and Microstructural Evolution of the Solid

The solid composition corresponding to the respective aqueous solution of each experiment at a given time is accessible in two independent ways, (1) via mass balance between original solid composition and solution at a given time and (2) via microchemical (SEM-EDX) analyses of individual particles. While (1) indicates the general evolution of the system, (2) can be used to evaluate the variation of particle morphology, composition and homogeneity during the approach to equilibrium. Based on the results in 3.2, three extreme examples are discussed here:
(1)
XSrSO4 = 5 mol%, 23 °C, S/L = 0.5 g/kg, slow macroscopic recrystallization kinetics;
(2)
XSrSO4 = 29 mol%, 23 °C, S/L = 0.5 g/kg, fast macroscopic recrystallization kinetics and 226Ra entrapment;
(3)
XSrSO4 = 5 mol%, 90 °C, 5 g/kg, fast macroscopic recrystallization kinetics, no entrapment of 226Ra.
The evolution of the average particle composition (mass balance) versus the 226Ra concentration in solution for the three examples is depicted in Figure 4. Starting at the initial 226Ra concentration of ca. 5.5 × 10−6 mol/kg (broken line in Figure 4), 226Ra in solution drops up to three orders of magnitude while XSrSO4 stays more or less constant. At 23 °C, the calculated average XSrSO4 stays constant for the (Ba0.95Sr0.05)SO4 solid solution during the complete experiment, and more than 42 days for (Ba0.71Sr0.29)SO4 (arrows in Figure 4a). In the recrystallization experiment with 5 g/kg (Ba0.95Sr0.05)SO4 and 90 °C, already after 42 days the concentration of 226Ra in solution is close to the final value. The average XSrSO4 only changes from 5 mol% to 4.1 mol% after 42 days. Only minor adjustments of the 226Ra and Sr concentrations in solution are observed later on (Figure 4b).
For the solids of this study, mainly Sr and Ba can be quantified by EDX whereas 226Ra can only be quantified with this method at local concentrations of more than 0.5 at%. Depending on the chemical and morphological variability, between 5 and 25 EDX spot measurements were carried out on each powder sample. The best match between the average compositions obtained via EDX and mass balance for a given sampling time were observed at the end of the experiments. Here, the average XSrSO4 and XBaSO4 obtained by both methods agree within experimental error (Table 4). However, the XSrSO4 still deviates significantly from the calculated equilibrium. A trend in the temporal evolution towards the equilibrium composition is visible in Table 4 and Figure 4 and discussed in more detail in Section 4.
The XSrSO4 of individual particles as well as their morphology were analyzed as a function of time. For experiment (Ba0.95Sr0.05)SO4_0.5 g/kg_RT, during the first 98 days the grain morphology remained almost unchanged. Steps on the surface due to cleavage during sample preparation were still visible at day 98 (Figure 5). The chemical composition of the particles at a given sampling time in this series was quite variable until the end of this experiment, with a range of XSrSO4 between 0.4 and 9.8 mol% (Table A21).
The morphology of the particles taken from experiment (Ba0.71Sr0.29)SO4_0.5 g/kg_RT changed after day 1 as large cavities occurred. At day 42 and 98, new smooth surfaces were visible in some areas whereas the cavities appeared to become smaller (Figure 5). Coatings were typical on some surfaces whereas other surfaces were interrupted by cavities. Some particles still contained almost 2/3 of the original SrSO4. The early morphological evolution over time of the particles taken from experiment (Ba0.71Sr0.29)SO4_5 g/kg_90 was similar to (Ba0.71Sr0.29)SO4_0.5 g/kg_RT, just faster. The grains lost their cavities and developed smooth, well defined surfaces with time. Simultaneously to the morphological evolution, the XSrSO4 shifted towards lower values. However, even at the end of the experiments, the particles were not homogeneous but Sr-rich and Sr-poor zones in individual particles were observed (Figure 5, spots 4 and 5).
In addition to the differences in the morphological evolution with time, also the chemical homogeneity and local enrichment of 226Ra varied among the experimental series. The 226Ra uptake for experiment (Ba0.95Sr0.05)SO4_0.5 g/kg_RT was mainly homogenous—only a small number of EDX spectra detected an enrichment of 226Ra. Only at the end of this experiment, some areas showed a significant 226Ra enrichment (spot 1 of Figure 5, Table A21). 226Ra-rich areas were detected on some particles, often small particles associated with the surfaces of larger particles. At higher XSrSO4 and 23 °C, already at the beginning 226Ra-rich areas in some particles were observed (Table A22; spot 2 in Figure 5). The surfaces appeared to be covered by Ba–Ra-rich coatings (spot 3 in Figure 5). In experiment (Ba0.71Sr0.29)SO4_5 g/kg_90, between day 1 and day 98 226Ra was detected in significant amounts in the solid phase, usually associated with higher XBaSO4 as well. A complete homogenization of the solid was not observed in any experiment.

4. Discussion

4.1. Effect of XSrSO4 upon the Solubility of 226Ra

The theoretically derived thermodynamic model for the SS-AS system (Ba,Sr,Ra)SO4 of Vinograd et al. (2018) [23] predicts a significant impact of the mole fraction XSrSO4 upon the solubility of 226Ra. Depending on temperature, the 226Ra solubility is expected to vary up to several orders of magnitude in the range of XSrSO4 between 0 and 10 mol%. According to the model, the re-equilibration of Ba-rich (Ba,Sr)SO4 to (Ba,Sr,Ra)SO4 requires a major reconstruction of the solid. In order to reach equilibrium, a large fraction of more than 95 mol% of the Sr formerly present in the solid needs to be released into the aqueous solution while 226Ra is taken up. At the same time they indicate that already trace amounts of Sr in the solid solution can have a significant impact on the 226Ra solubility if the solid solution is in full equilibrium with the aqueous solution. According to these calculations, this impact depends on temperature as well, i.e., at 23 °C the differences between the 226Ra solubilities are more pronounced than at 70 °C or 90 °C.
On the macroscopic side, the experimental findings are coherent with the thermodynamic model. In particular, the plateau of the final c(226Ra) in solution was close to the predicted equilibrium. The final Ba and Sr concentrations in solution approached equilibrium, but especially Sr in solution deviated significantly from the prediction in some of the experiments, indicating that these were still not at equilibrium (Figure 6). Within the duration of the experiments at 23 °C, XSrSO4 was not completely adjusted to equilibrium in any solid. In particular, the experiments with only 5 mol% SrSO4 in the initial solid solution didn’t reach equilibrium, but at high temperature and high S/L the deviation for the same initial solid solution composition became small, close to the experimental error (Figure 6b).

4.2. Kinetics of the Recrystallization from (Ba,Sr)SO4 to (Ba,Sr,Ra)SO4

The SS-AS system is dominated by the re-equilibration of (Ba,Sr)SO4. All three original (Ba,Sr)SO4 solid solutions need to release only a very low proportion of total BaSO4 from the solid to reach the predicted equilibrium solution composition, i.e., dissolution at the surface is sufficient to fulfill this condition. On the other hand, more than 97 mol% of the SrSO4 originally present in the solid solutions of this study would need to be released from the solid into the aqueous solution in order to reach equilibrium. Taking into account the amount to be released from the solid, SrSO4 may be more accessible to dissolution at the particle surfaces in the case of higher XSrSO4, and therefore equilibrium may be reached earlier.
Brandt et al., 2018 [32] have shown that in certain combinations of S/L and temperature, Sraq can inhibit the recrystallization of BaSO4 and the uptake of 226Ra. Therefore, at 23 °C and low XSrSO4 the system may behave similar to pure BaSO4, and the presence of Sr in solution may thus slow down the kinetics of recrystallization. The solid solution with initially only XSrSO4 = 5%, recrystallized at 23 °C, appeared to undergo very little change of XSrSO4 with time (Figure 6a). A higher XSrSO4 of the original solid solution lead to faster 226Ra uptake kinetics, and in some cases even to a minimum of the 226Ra concentration, which was attributed to a kinetic “entrapment” effect. The faster re-equilibration correlated with the higher solubility of SrSO4 compared to the other two sulfates.
For a given composition, 226Ra appeared to be adjusted more or less independent of Sr. As soon as 226Ra was structurally taken up, the concentration in solution dropped by several orders of magnitude whereas the re-structuring of the solid towards a full equilibrium required several steps of dissolution and re-precipitation as microscopically observed. Microscopically, the recrystallization of the binary (Ba,Sr)SO4 solid solution to (Ba,Sr,Ra)SO4 is a complex process that is clearly different from the replacement reaction observed for the formation of (Ba,Ra)SO4 from barite [31]. Instead, the re-equilibration lead to similar features as observed in earlier studies on the reaction of SrSO4 with Ba in solution [33] and on the recrystallization of Sr-rich (Sr,Ba)SO4 in the presence of 226Ra [25]. Rims of newly formed phases were observed on the original particles. The original particles dissolved partially, leaving large cavities in the original grains of some experiments presented here. Already, at the beginning of the experiments with high XSrSO4 in the original solid solution, or at high temperature and solid/liquid ratio, theses cavities indicate a significant dissolution. Later on, the particles of the experiment with higher XSrSO4 changed in their morphology and new smooth surfaces became visible in some areas whereas the cavities appeared to become smaller. In some areas, an idiomorphic habitus occurred. However, even at day 664 the morphology and also XSrSO4 were still not at equilibrium (Figure 6). In many cases, the particles remained chemically heterogeneous. Simultaneously to the morphological evolution, the XSrSO4 changed, with XSrSO4 in some measurements even below the predicted equilibrium. Therefore, the grain morphology apparently followed the macroscopically observed recrystallization kinetics.
At slow recrystallization rates as observed for experiment (Ba0.95Sr0.05)SO4_0.5 g/kg_RT, during the first 98 days the grain morphology remained almost unchanged. Here, the cavities which were observed early on in the other experiments occurred at the end of the experiment.

5. Conclusions

The newly derived thermodynamic model for the SS-AS system (Ba,Sr,Ra)SO4 + H2O [23,25] was tested in recrystallization experiments at the Ba-rich corner. In contrast to pure barite, in the ternary system significant dissolution and neo-formation of particles with a more ideal particle morphology occurs. A simultaneous evolution of the grain morphology and the XSrSO4 was observed. After 664 days, many experiments reach a partial equilibrium with c(226Ra) already close to the predicted values. Most experiments approach the predicted equilibrium concentrations of Ba and Sr, but only the experiments with high XSrSO4 in the original solid reached the predicted equilibrium within the duration of the experiments.
In conclusion, the trends predicted by the thermodynamic model of Vinograd et al. (2018) [23] and a favorable role of small amounts of Sr in the (Ba,Sr)SO4 solid solution with respect to the uptake of 226Ra can be confirmed by this study.

Author Contributions

Conceptualization, F.B. and M.K.; validation, F.B.; investigation, F.B., M.K. and J.P.; data curation, M.K. and J.P.; writing—original draft preparation, F.B., J.P. and M.K.; writing—review and editing, D.B. and J.P.; visualization, M.K. and J.P.; project administration, F.B.; funding acquisition, F.B. and D.B. All authors have read and agreed to the published version of the manuscript.

Funding

The research leading to these results has received partial funding from the German Federal Ministry of Education and Research (BMBF) ThermAc3 project (project number 02NUK039D).

Acknowledgments

We are grateful to Murat Güngör, Dimitri Schneider, Fabian Kreft, Andreas Wilden, Ralf König and Giuseppe Modolo for their support.

Conflicts of Interest

The authors declare no conflict of interest.

Appendix A

Table
Table A1. 226Ra concentrations in solution of the experiments at 23 °C.
Table A1. 226Ra concentrations in solution of the experiments at 23 °C.
DayRa Concentration in Solution (10−8 mol/kg)
(Ba0.95Sr0.05)SO4_0.5 g/kg_RT(Ba0.83Sr0.17)SO4_0.5 g/kg_RT(Ba0.71Sr0.29)SO4_0.5 g/kg_RT
0.5536523559
1542442355
3551433324
7550406280
14568431268
21569404223
30580357148
425713022.30
566002470.18
7062579.40.14
985560.610.35
1336050.470.35
1615440.510.37
2245680.630.41
2944380.340.41
4062640.650.49
52512.50.680.51
66420.90.720.61
Equilibrium (GEMS)3.241.130.76
Table
Table A2. 226Ra concentrations in solution of the experiments at 70 °C.
Table A2. 226Ra concentrations in solution of the experiments at 70 °C.
DayRa Concentration (10−8 mol/kg)
(Ba0.95Sr0.05)SO4_0.5 g/kg_70(Ba0.83Sr0.17)SO4_0.5 g/kg_70(Ba0.71Sr0.29)SO4_0.5 g/kg_70
0.5546534531
1552441222
353940280.0
75592810.45
145761.66
215190.61
303780.45
4258.30.470.37
5623.10.45
7021.50.47
9820.90.290.61
13321.30.721.06
16121.70.761.51
22422.50.852.25
29419.21.463.27
40626.44.304.70
52524.76.955.32
66423.50.117.16
Equilibrium (GEMS)16.26.554.59
Table
Table A3. 226Ra concentrations in solution of the experiments at 90 °C.
Table A3. 226Ra concentrations in solution of the experiments at 90 °C.
DayRa Concentration (10−8 mol/kg)
(Ba0.95Sr0.05)SO4_0.5 g/kg_90(Ba0.83Sr0.17)SO4_0.5 g/kg_90(Ba0.71Sr0.29)SO4_0.5 g/kg_90
0.5546541532
154730830.5
357672.20.41
7522
143440.55
2180.8
3036.2
4219.80.220.45
5617.0
7017.6
9814.10.410.51
13314.00.38
16115.70.362.64
22415.80.673.35
29416.11.703.11
40624.23.356.95
52525.63.357.43
66431.64.319.10
Equilibrium (GEMS)24.910.67.78
Table
Table A4. 226Ra concentrations in solution of the experiments at 90 °C and solid/liquid ratio (S/L) = 5 g/kg.
Table A4. 226Ra concentrations in solution of the experiments at 90 °C and solid/liquid ratio (S/L) = 5 g/kg.
DayRa Concentration (10−8 mol/kg)
(Ba0.95Sr0.05)SO4_5 g/kg_90(Ba0.83Sr0.17)SO4_5 g/kg_90(Ba0.71Sr0.29)SO4_5 g/kg_90
0.5507503490
155.41.020.86
420.430.410.37
980.410.450.51
4060.430.430.51
6641.100.790.89
Equilibrium (GEMS)0.620.490.57
Table
Table A5. Ba concentrations in solution of the experiments at 23 °C.
Table A5. Ba concentrations in solution of the experiments at 23 °C.
DayBa Concentration (10−6 mol/kg)
(Ba0.95Sr0.05)SO4_0.5 g/kg_RT(Ba0.83Sr0.17)SO4_0.5 g/kg_RT(Ba0.71Sr0.29)SO4_0.5 g/kg_RT
1 20.912.9
327.415.719.1
726.613.19.83
1426.512.29.00
2127.612.58.21
3027.111.47.45
4230.010.97.41
5631.911.06.06
7031.111.14.62
9832.38.403.27
13332.35.913.35
16133.15.693.53
22432.24.512.79
29432.23.812.52
40631.93.692.32
52520.22.791.92
66420.93.262.35
Equilibrium (GEMS)9.402.901.70
Table
Table A6. Ba concentrations in solution of the reference experiments without 226Ra, 23 °C.
Table A6. Ba concentrations in solution of the reference experiments without 226Ra, 23 °C.
DayBa Concentration (10−6 mol/kg)
Reference (Ba0.95Sr0.05)SO4_0.5 g/kg_RTReference (Ba0.83Sr0.17)SO4_0.5 g/kg_RTReference (Ba0.71Sr0.29)SO4_0.5 g/kg_RT
122.77.066.39
326.36.706.11
723.36.415.10
141.825.834.59
2123.15.900.08
3029.95.170.17
4223.64.953.75
5622.74.813.88
7026.24.953.60
9824.24.002.77
13322.64.084.35
16124.03.863.02
22422.43.502.53
29425.03.132.41
40625.03.272.18
52522.42.771.74
66424.42.921.94
Equilibrium (GEMS)9.052.871.69
Table
Table A7. Ba concentration of experiments at 70 °C.
Table A7. Ba concentration of experiments at 70 °C.
DayBa Concentration (10−6 mol/kg)
(Ba0.95Sr0.05)SO4_0.5 g/kg_70(Ba0.83Sr0.17)SO4_0.5 g/kg_70(Ba0.71Sr0.29)SO4_0.5 g/kg_70
150.022.721.3
349.326.714.5
7 13.99.69
1450.016.47.88
2147.212.76.53
3052.09.636.11
4242.58.375.64
5648.89.745.85
7037.39.455.68
9837.37.195.28
13356.35.677.41
16137.57.815.40
22427.36.905.10
29435.5 4.66
40636.16.835.19
52529.35.264.26
66431.76.084.98
Equilibrium (GEMS)26.69.425.62
Table
Table A8. Ba concentrations in solution of the reference experiments without 226Ra, 70 °C.
Table A8. Ba concentrations in solution of the reference experiments without 226Ra, 70 °C.
DayBa Concentration (10−6 mol/kg)
Reference (Ba0.95Sr0.05)SO4_0.5 g/kg_70Reference (Ba0.83Sr0.17)SO4_0.5 g/kg_70Reference (Ba0.71Sr0.29)SO4_0.5 g/kg_70
138.014.313.2
352.715.67.88
734.013.75.42
1434.114.65.91
2133.512.65.57
3031.711.85.04
4232.111.32.41
5630.410.54.66
7032.810.65.32
9832.09.544.56
13333.19.325.08
16131.88.815.24
22431.37.724.91
29432.06.630.85
40633.15.964.66
52529.15.034.07
66430.85.854.52
Equilibrium (GEMS)2.589.295.58
Table
Table A9. Ba concentration of experiments at 90 °C.
Table A9. Ba concentration of experiments at 90 °C.
DayBa Concentration (10−6 mol/kg)
(Ba0.95Sr0.05)SO4_0.5 g/kg_90(Ba0.83Sr0.17)SO4_0.5 g/kg_90(Ba0.71Sr0.29)SO4_0.5 g/kg_90
156.290.317.2
351.825.810.7
752.516.88.60
1451.212.67.86
2146.49.176.07
3044.610.46.74
4240.77.735.50
5638.98.525.80
7043.09.406.69
9842.28.696.15
13343.18.746.29
16143.38.966.79
22442.98.726.66
29436.86.734.70
40641.87.986.40
52540.47.965.73
66442.68.726.29
Equilibrium (GEMS)32.912.27.39
Table
Table A10. Ba concentrations in solution of the reference experiments without 226Ra, 90 °C.
Table A10. Ba concentrations in solution of the reference experiments without 226Ra, 90 °C.
DayBa Concentration (10−6 mol/kg)
Reference (Ba0.95Sr0.05)SO4_0.5 g/kg_90Reference (Ba0.83Sr0.17)SO4_0.5 g/kg_90Reference (Ba0.71Sr0.29)SO4_0.5 g/kg_90
143.515.68.69
343.312.16.74
744.111.05.94
1445.210.45.87
2142.07.994.23
3043.18.695.07
4241.87.094.20
5642.27.72
7042.08.615.34
9842.28.265.04
13343.48.394.98
16144.19.205.57
22446.39.115.29
29443.97.324.51
40642.18.495.20
52540.58.7611.6
66438.29.3013.2
Equilibrium (GEMS)32.012.10.32
Table
Table A11. Ba concentration of experiments at 90 °C and S/L = 5 g/kg.
Table A11. Ba concentration of experiments at 90 °C and S/L = 5 g/kg.
DayBa concentration (10−6 mol/kg)
(Ba0.95Sr0.05)SO4_5 g/kg_90(Ba0.83Sr0.17)SO4_5 g/kg_90(Ba0.71Sr0.29)SO4_5 g/kg_90
130.97.817.59
327.28.338.32
720.77.146.45
1418.66.816.28
2113.85.044.70
3015.85.935.46
4212.15.724.67
569.665.123.60
7013.66.115.77
9812.50.004.82
13312.64.955.15
16112.65.595.72
22411.65.194.75
2947.723.603.63
4068.153.563.33
5257.643.513.35
6648.973.993.60
Equilibrium (GEMS)7.374.164.16
Table
Table A12. Ba concentration of the reference experiments without 226Ra at 90 °C and S/L = 5 g/kg.
Table A12. Ba concentration of the reference experiments without 226Ra at 90 °C and S/L = 5 g/kg.
DayBa concentration (10−6 mol/kg)
Reference (Ba0.95Sr0.05)SO4_5 g/kg_90Reference (Ba0.83Sr0.17)SO4_5 g/kg_90Reference (Ba0.71Sr0.29)SO4_5 g/kg_90
125.25.465.25
316.76.305.80
714.05.265.09
1414.25.185.10
2111.13.864.09
3012.94.344.85
4210.24.173.97
569.223.603.63
7011.64.814.70
9811.94.464.38
13311.74.464.63
16113.84.965.00
22412.34.524.43
29410.93.803.41
40611.23.914.09
52511.64.464.31
66413.26.055.57
Equilibrium (GEMS)6.974.194.19
Table
Table A13. Sr concentrations of the experiments at 23 °C.
Table A13. Sr concentrations of the experiments at 23 °C.
DaySr Concentration (10−5 mol/kg)
(Ba0.95Sr0.05)SO4_0.5 g/kg_RT(Ba0.83Sr0.17)SO4_0.5 g/kg_RT(Ba0.71Sr0.29)SO4_0.5 g/kg_RT
10.9215.717.5
30.8112.229.7
70.8510.617.3
140.7910.318.9
210.7710.917.9
300.8211.218.3
420.7811.219.6
560.9011.123.7
700.8711.128.9
980.8413.933.2
1330.8418.137.7
1610.9019.840.2
2240.8522.342.3
2940.8625.344.2
4061.1326.644.0
5252.5126.645.1
6643.1628.249.3
Equilibrium (GEMS)10.837.465.2
Table
Table A14. Sr concentrations of reference experiments without Ra at 23 °C.
Table A14. Sr concentrations of reference experiments without Ra at 23 °C.
DaySr Concentration (10−5 mol/kg)
Reference (Ba0.95Sr0.05)SO4_0.5 g/kg_RTReference (Ba0.83Sr0.17)SO4_0.5 g/kg_RTReference (Ba0.71Sr0.29)SO4_0.5 g/kg_RT
10.9510.721.4
30.9810.122.0
70.1210.822.1
140.6740.621.9
210.8812.715.9
30 13.414.6
421.1713.933.6
560.7714.432.4
700.9015.334.2
984.2016.635.0
1330.5118.956.1
1610.7417.939.9
2240.8519.342.3
2940.7520.243.5
4061.0824.443.7
5250.9021.942.7
6640.8824.244.6
Equilibrium (GEMS)10.837.565.4
Table
Table A15. Sr concentration of experiments at 70 °C.
Table A15. Sr concentration of experiments at 70 °C.
DaySr Concentration (10−5 mol/kg)
(Ba0.95Sr0.05)SO4_0.5 g/kg_70(Ba0.83Sr0.17)SO4_0.5 g/kg_70(Ba0.71Sr0.29)SO4_0.5 g/kg_70
11.0011.623.7
31.1014.4523.1
72.3212.130.7
141.0515.044.3
211.0921.647.1
301.4525.850.5
422.5227.352.7
563.7333.254.3
703.4236.651.9
983.6534.455.1
1336.2236.368.0
1614.1336.855.2
2244.6139.557.6
2944.4939.755.8
4064.7574.050.4
5255.3242.848.5
6646.2136.250.9
Equilibrium (GEMS)10.737.064.0
Table
Table A16. Sr concentration of reference experiments without Ra at 70 °C.
Table A16. Sr concentration of reference experiments without Ra at 70 °C.
DaySr Concentration (10−5 mol/kg)
Reference (Ba0.95Sr0.05)SO4_0.5 g/kg_70Reference (Ba0.83Sr0.17)SO4_0.5 g/kg_70Reference (Ba0.71Sr0.29)SO4_0.5 g/kg_70
11.5911.839.4
32.2713.534.1
71.7014.538.0
141.5016.942.0
211.7216.047.1
301.7816.348.4
422.6616.749.6
562.0417.651.4
702.2018.454.1
982.2720.052.4
1332.3621.759.7
1612.3522.956.1
2242.5322.154.8
2942.4126.619.8
4062.7531.448.7
5251.6328.946.0
6641.7728.356.6
Equilibrium (GEMS)10.737.164.4
Table
Table A17. Sr concentration of experiments at 90 °C.
Table A17. Sr concentration of experiments at 90 °C.
DaySr Concentration (10−5 mol/kg)
(Ba0.95Sr0.05)SO4_0.5 g/kg_90(Ba0.83Sr0.17)SO4_0.5 g/kg_90(Ba0.71Sr0.29)SO4_0.5 g/kg_90
10.6943.222.6
30.9913.528.0
71.2016.235.7
141.3521.039.8
212.0024.142.5
302.5726.443.6
422.9528.2
563.1929.652.5
703.3531.149.9
983.3231.247.9
1333.5331.447.4
1613.5731.143.3
2243.7232.947.5
2943.7432.947.2
4063.9837.056.1
5254.6033.748.7
6645.4037.965.3
Equilibrium (GEMS)10.736.762.7
Table
Table A18. Sr concentration of reference experiments without Ra at 90 °C.
Table A18. Sr concentration of reference experiments without Ra at 90 °C.
DaySr Concentration (10−5 mol/kg)
Reference (Ba0.95Sr0.05)SO4_0.5 g/kg_90Reference (Ba0.83Sr0.17)SO4_0.5 g/kg_90Reference (Ba0.71Sr0.29)SO4_0.5 g/kg_90
11.4011.926.0
31.2713.937.6
71.3416.835.2
141.3518.938.7
211.4320.441.7
301.2521.041.8
421.4522.143.4
561.4823.4
701.4024.547.4
981.6013.348.6
1331.5426.046.1
1611.7525.645.6
2241.6626.848.4
2941.9127.048.7
4061.9031.150.6
5251.6328.946.0
6641.7728.356.6
Equilibrium (GEMS)10.736.763.3
Table
Table A19. Sr concentration of experiments at 90 °C and S/L = 5 g/kg.
Table A19. Sr concentration of experiments at 90 °C and S/L = 5 g/kg.
DaySr Concentration (10−5 mol/kg)
(Ba0.95Sr0.05)SO4_5 g/kg_90(Ba0.83Sr0.17)SO4_5 g/kg_90(Ba0.71Sr0.29)SO4_5 g/kg_90
18.8137.342.1
310.645.051.7
712.748.255.2
1416.051.060.7
2117.552.659.9
3018.554.961.5
4220.364.661.5
5621.377.764.1
7021.956.968.3
9822.2 66.6
13323.058.168.0
16123.359.072.4
22423.760.571.4
29423.360.168.2
40628.474.985.0
52535.774.777.3
66436.270.977.4
Equilibrium (GEMS)63.1113113
Table
Table A20. Sr concentration of reference experiments without Ra at 90 °C and S/L = 5 g/kg.
Table A20. Sr concentration of reference experiments without Ra at 90 °C and S/L = 5 g/kg.
DaySr Concentration (10−5 mol/kg)
Reference (Ba0.95Sr0.05)SO4_5 g/kg_90Reference (Ba0.83Sr0.17)SO4_5 g/kg_90Reference (Ba0.71Sr0.29)SO4_5 g/kg_90
19.6038.940.1
311.340.646.3
712.847.150.8
1414.044.952.0
2114.751.255.3
3016.153.558.2
4216.155.055.7
5616.650.957.5
7017.555.159.2
9842.757.059.3
13318.060.460.1
16117.956.860.0
22418.358.760.0
29418.450.760.8
40620.866.171.7
52518.765.985.9
66418.261.371.1
Equilibrium (GEMS)66.8114114
Table
Table A21. Temporal evolution of the solid composition analyzed by EDX of (Ba0.95Sr0.05)SO4_0.5 g/kg_RT. Superscript numbers indicate spot measurements in Figure 5.
Table A21. Temporal evolution of the solid composition analyzed by EDX of (Ba0.95Sr0.05)SO4_0.5 g/kg_RT. Superscript numbers indicate spot measurements in Figure 5.
DayParticleXSrSO4XRaSO4XBaSO4
(%)(%)(%)
(Ba0.95Sr0.05)SO4_0.5 g/kg_RT
1 P14.6<0.595.1
P24.50.894.7
P34.8<0.595.0
P43.50.695.9
P59.8n.d.90.2
Average EDX5.4 94.2
Average mass balance5.60.0495.4
42P17.3<0.592.5
P13.9<0.595.8
P23.9<0.595.9
P33.50.596.0
P38.20.591.3
P46.0<0.593.8
P52.9<0.596.9
Average EDX5.1 94.6
Average mass balance4.70.0395.3
98P13.7n.d.96.3
P13.2n.d.96.8
P210.1<0.589.8
P210.3<0.589.4
P35.0n.d.95.0
P43.3<0.596.5
P54.7<0.595.0
Average EDX5.8 94.1
Average mass balance4.70.0395.3
664P14.1<0.595.7
P14.2<0.595.7
P23.6<0.596.2
P24.4<0.595.2
P35.7<0.593.9
P37.0<0.592.8
P43.8<0.595.9
P44.1<0.595.6
P55.0<0.594.7
P54.50.794.9
P52.81.295.8
P65.5<0.593.8
P65.8<0.593.9
P73.4<0.596.4
P74.1<0.595.7
P72.00.597.5
P80.40.798.9
P81.31.297.5
P81.22.795.3
P91.30.897.9
P96.60.793.4
Average EDX3.8 95.6
Average mass balance3.60.0396.1
Calculated equilibrium0.30.0299.7
n.d. not detected.
Table
Table A22. Temporal evolution of the solid composition analyzed by EDX of (Ba0.71Sr0.29)SO4_0.5 g/kg_RT. Superscript numbers indicate spot measurements in Figure 5.
Table A22. Temporal evolution of the solid composition analyzed by EDX of (Ba0.71Sr0.29)SO4_0.5 g/kg_RT. Superscript numbers indicate spot measurements in Figure 5.
DayParticleXSrSO4XRaSO4XBaSO4
(%)(%)(%)
(Ba0.71Sr0.29)SO4_0.5 g/kg_RT
1P127.4n.d.72.6
P15.8<0.593.8
P120.8<0.578.9
P29.2<0.590.7
P28.10.591.5
P330.5<0.569.3
P327.4n.d.72.6
P322.3<0.577.4
P49.0<0.590.5
P415.81.5982.6
P523.9<0.575.8
P64.3<0.595.4
Average EDX17.0 82.6
Average mass balance23.20.176.6
42P16.51.192.8
P121.6<0.578.0
P28.4<0.591.1
P37.32.889.9
P35.42.492.2
P411.21.187.8
P427.9<0.571.7
P523.1<0.576.7
P59.10.690.3
Average EDX13.4 85.6
Average mass balance22.30.377.4
98P17.90.991.2
P222.7<0.576.9
P217.6<0.582.2
P26.9<0.592.8
P311.8<0.587.8
P315.4<0.584.5
P417.30.782.0
P512.91.086.1
P611.90.987.3
P624.3<0.575.3
P614.8<0.584.8
Average EDX14.1 85.4
Average mass balance16.90.382.8
664P19.90.689.6
P19.80.589.7
P211.60.587.9
P210.5<0.589.3
P37.8<0.591.9
P47.2<0.592.5
P48.10.591.4
P48.9n.d.91.1
P521.4<0.578.4
P55.0<0.594.6
P59.0<0.590.9
P511.70.787.6
P69.0<0.590.7
P65.2<0.594.5
P68.4<0.591.3
P75.30.594.2
P710.3<0.589.4
P74.7<0.595.0
P88.2<0.591.5
P810.01.488.6
P87.4<0.592.2
P921.3<0.578.3
P910.00.589.5
P109.60.889.6
P1017.4<0.582.3
Average EDX9.9 89.7
Average mass balance9.50.490.2
Calculated equilibrium0.60.399.1
n.d. not detected.
Table
Table A23. Temporal evolution of the solid composition analyzed by EDX of (Ba0.95Sr0.05)SO4_5 g/kg_90. Superscript numbers indicate spot measurements in Figure 5.
Table A23. Temporal evolution of the solid composition analyzed by EDX of (Ba0.95Sr0.05)SO4_5 g/kg_90. Superscript numbers indicate spot measurements in Figure 5.
DayParticleXSrSO4XRaSO4XBaSO4
(%)(%)(%)
(Ba0.95Sr0.05)SO4_5 g/kg_90
1P110.3<0.589.5
P112.8n.d87.2
P113.6<0.586.5
P210.1n.d89.9
P23.91.794.4
P34.0<0.595.6
P44.9<0.594.6
Average EDX8.5 91.1
Average mass balance4.60.0395.4
42P14.10.595.4
P11.50.797.1
P22.0<0.597.8
P34.0<0.595.9
P41.0<0.598.8
Average EDX2.5 97.0
Average mass balance4.10.0395.9
98P12.6<0.597.2
P21.60.9897.4
P31.5<0.598.2
P47.0<0.591.8
P50.8<0.599.0
P64.4<0.595.5
Average EDX3.0 96.5
Average mass balance4.00.0396.0
664P13.4<0.596.3
P26.3n.d93.7
P26.0<0.593.8
P33.1<0.596.5
P32.30.697.1
P44.4<0.595.2
P45.0<0.594.9
P51.6<0.598.0
P52.7<0.597.2
P65.8<0.594.0
P63.8<0.595.9
P72.20.597.3
P84.8<0.595.0
P93.20.696.2
Average EDX3.9 95.8
Average mass balance3.40.0396.6
Calculated equilibrium2.20.0297.8

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Minerals 10 00812 g001 550
Figure 1. Scanning electron microscope (SEM) images of initial Ba-rich (Ba,Sr)SO4 solid solution particles used for recrystallization experiments. (a) (Ba0.95Sr0.05)SO4; (b) (Ba0.83Sr0.17)SO4; (c) (Ba0.71Sr0.29)SO4.
Figure 1. Scanning electron microscope (SEM) images of initial Ba-rich (Ba,Sr)SO4 solid solution particles used for recrystallization experiments. (a) (Ba0.95Sr0.05)SO4; (b) (Ba0.83Sr0.17)SO4; (c) (Ba0.71Sr0.29)SO4.
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Minerals 10 00812 g002 550
Figure 2. Temporal evolution of the 226Ra, Ba and Sr concentrations in solution for experiments with 0.5 g/kg. Data for pure BaSO4 recrystallization with 226Ra (grey symbols) are taken from experiments with identical solid/liquid ratios, ionic strength and temperature as published in [24,31]. The grey dotted line in the c(Sr) vs. time plot refer to the solubility of pure SrSO4, and to pure BaSO4 in the other two plots. Data are given in the Appendix A Table A1, Table A2, Table A3, Table A4, Table A5, Table A6, Table A7, Table A8, Table A9, Table A10, Table A11, Table A12, Table A13, Table A14, Table A15, Table A16, Table A17, Table A18, Table A19 and Table A20. The thermodynamic predictions (lines) are based on [23,25].
Figure 2. Temporal evolution of the 226Ra, Ba and Sr concentrations in solution for experiments with 0.5 g/kg. Data for pure BaSO4 recrystallization with 226Ra (grey symbols) are taken from experiments with identical solid/liquid ratios, ionic strength and temperature as published in [24,31]. The grey dotted line in the c(Sr) vs. time plot refer to the solubility of pure SrSO4, and to pure BaSO4 in the other two plots. Data are given in the Appendix A Table A1, Table A2, Table A3, Table A4, Table A5, Table A6, Table A7, Table A8, Table A9, Table A10, Table A11, Table A12, Table A13, Table A14, Table A15, Table A16, Table A17, Table A18, Table A19 and Table A20. The thermodynamic predictions (lines) are based on [23,25].
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Minerals 10 00812 g003 550
Figure 3. Temporal evolution of the 226Ra, Ba and Sr concentrations in solution for experiments with 5 g/kg and 90 °C. Data are given in the Appendix A Table A1, Table A2, Table A3, Table A4, Table A5, Table A6, Table A7, Table A8, Table A9, Table A10, Table A11, Table A12, Table A13, Table A14, Table A15, Table A16, Table A17, Table A18, Table A19 and Table A20. The thermodynamic predictions (lines) are based on [23,25].
Figure 3. Temporal evolution of the 226Ra, Ba and Sr concentrations in solution for experiments with 5 g/kg and 90 °C. Data are given in the Appendix A Table A1, Table A2, Table A3, Table A4, Table A5, Table A6, Table A7, Table A8, Table A9, Table A10, Table A11, Table A12, Table A13, Table A14, Table A15, Table A16, Table A17, Table A18, Table A19 and Table A20. The thermodynamic predictions (lines) are based on [23,25].
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Figure 4. Temporal evolution of the 226Ra concentration in solution and the average mole fraction XSrSO4 of the corresponding solid during the recrystallization experiments; (a) green symbols are XSrSO4 = 5 mol%, 23 °C, 0.5 g/kg, black symbols are XSrSO4 = 29 mol%, 23 °C, 0.5 g/kg, (b) blue symbols are XSrSO4 = 5 mol%, 90 °C, 5 g/kg; the corresponding predicted equilibria are symbolized by the crosshairs in the respective color. The black dashed line indicates the initial 226Ra concentration.
Figure 4. Temporal evolution of the 226Ra concentration in solution and the average mole fraction XSrSO4 of the corresponding solid during the recrystallization experiments; (a) green symbols are XSrSO4 = 5 mol%, 23 °C, 0.5 g/kg, black symbols are XSrSO4 = 29 mol%, 23 °C, 0.5 g/kg, (b) blue symbols are XSrSO4 = 5 mol%, 90 °C, 5 g/kg; the corresponding predicted equilibria are symbolized by the crosshairs in the respective color. The black dashed line indicates the initial 226Ra concentration.
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Figure 5. SEM micrographs of representative particles taken from recrystallization experiments with XSrSO4 = 5 mol%, 23 °C, 0.5 g/kg, XSrSO4 = 29 mol%, 23 °C, 0.5 g/kg, and XSrSO4 = 5 mol%, 90 °C, 5 g/kg. The numbered spots marked with (*1, *2, *3, *4, *5) represent the areas where EDX analyses were taken (Table A21, Table A22 and Table A23).
Figure 5. SEM micrographs of representative particles taken from recrystallization experiments with XSrSO4 = 5 mol%, 23 °C, 0.5 g/kg, XSrSO4 = 29 mol%, 23 °C, 0.5 g/kg, and XSrSO4 = 5 mol%, 90 °C, 5 g/kg. The numbered spots marked with (*1, *2, *3, *4, *5) represent the areas where EDX analyses were taken (Table A21, Table A22 and Table A23).
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Figure 6. Calculated 226Ra solubility curves as a function of XSrSO4 in the solid solution in equilibrium with the aqueous solution. Experimental end points (stars) are compared with theoretical equilibria (crosshairs). (a) experiments with a solid/liquid ratio of 0.5 g/kg; (b) 5 g/kg.
Figure 6. Calculated 226Ra solubility curves as a function of XSrSO4 in the solid solution in equilibrium with the aqueous solution. Experimental end points (stars) are compared with theoretical equilibria (crosshairs). (a) experiments with a solid/liquid ratio of 0.5 g/kg; (b) 5 g/kg.
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Table
Table 1. Overview of synthesized Ba-rich (Ba,Sr)SO4 solid solutions.
Table 1. Overview of synthesized Ba-rich (Ba,Sr)SO4 solid solutions.
Solid SolutionXBaSO4XSrSO4
(Ba0.95Sr0.05)SO40.950.05 ± 20%
(Ba0.83Sr0.17)SO40.830.17 ± 20%
(Ba0.71Sr0.29)SO40.710.29 ± 20%
Table
Table 2. Overview of the recrystallization experiments with Ba-rich (Ba,Sr)SO4 solid solutions.
Table 2. Overview of the recrystallization experiments with Ba-rich (Ba,Sr)SO4 solid solutions.
NameSolid/Liquidc(Ra)Temperature
(g/kg)(10−6 mol/kg)(°C)
(Ba0.95Sr0.05)SO4_0.5 g/kg_RT0.5523 ± 2
(Ba0.83Sr0.17)SO4_0.5 g/kg_RT0.5523 ± 2
(Ba0.71Sr0.29)SO4_0.5 g/kg_RT0.5523 ± 2
Reference (Ba0.95Sr0.05)SO4_0.5 g/kg_RT0.5023 ± 2
Reference (Ba0.83Sr0.17)SO4_0.5 g/kg_RT0.5023 ± 2
Reference (Ba0.71Sr0.29)SO4_0.5 g/kg_RT0.5023 ± 2
(Ba0.95Sr0.05)SO4_0.5 g/kg_700.5570
(Ba0.83Sr0.17)SO4_0.5 g/kg_700.5570
(Ba0.71Sr0.29)SO4_0.5 g/kg_700.5570
Reference (Ba0.95Sr0.05)SO4_0.5 g/kg_700.5070
Reference (Ba0.83Sr0.17)SO4_0.5 g/kg_700.5070
Reference (Ba0.71Sr0.29)SO4_0.5 g/L_700.5070
(Ba0.95Sr0.05)SO4_0.5 g/kg_900.5590
(Ba0.83Sr0.17)SO4_0.5 g/kg_900.5590
(Ba0.71Sr0.29)SO4_0.5 g/kg_900.5590
Reference (Ba0.95Sr0.05)SO4_0.5 g/kg_900.5090
Reference (Ba0.83Sr0.17)SO4_0.5 g/kg_900.5090
Reference (Ba0.71Sr0.29)SO4_0.5 g/kg_900.5090
(Ba0.95Sr0.05)SO4_5 g/kg_905.0590
(Ba0.83Sr0.17)SO4_5 g/kg_905.0590
(Ba0.71Sr0.29)SO4_5 g/kg_905.0590
Reference (Ba0.95Sr0.05)SO4_5 g/kg_905.0090
Reference (Ba0.83Sr0.17)SO4_5 g/kg_905.0090
Reference (Ba0.71Sr0.29)SO4_5 g/kg_905.0090
Table
Table 3. Calculated equilibrium compositions of solid solutions after total equilibration of the system (X for mole fraction).
Table 3. Calculated equilibrium compositions of solid solutions after total equilibration of the system (X for mole fraction).
ExperimentComposition of Solid Solution Present at Equilibrium
XBaSO4XRaSO4 (%)XSrSO4
(Ba0.95Sr0.05)SO4_0.5 g/kg_RT99.740.240.02
(Ba0.83Sr0.17)SO4_0.5 g/kg_RT99.520.270.21
(Ba0.71Sr0.29)SO4_0.5 g/kg_RT99.060.310.63
Reference (Ba0.95Sr0.05)SO4_0.5 g/kg_RT99.99-0.01
Reference (Ba0.83Sr0.17)SO4_0.5 g/kg_RT99.83-0.17
Reference (Ba0.71Sr0.29)SO4_0.5 g/kg_RT99.49-0.51
(Ba0.95Sr0.05)SO4_0.5 g/kg_7099.720.240.04
(Ba0.83Sr0.17)SO4_0.5 g/kg_7099.290.270.44
(Ba0.71Sr0.29)SO4_0.5 g/kg_7098.340.301.36
Reference (Ba0.95Sr0.05)SO4_0.5 g/kg_7099.96-0.04
Reference (Ba0.83Sr0.17)SO4_0.5 g/kg_7099.63-0.37
Reference (Ba0.71Sr0.29)SO4_0.5 g/L_7098.88-1.12
(Ba0.95Sr0.05)SO4_0.5 g/kg_9099.690.230.07
(Ba0.83Sr0.17)SO4_0.5 g/kg_9099.170.260.57
(Ba0.71Sr0.29)SO4_0.5 g/kg_9097.600.302.10
Reference (Ba0.95Sr0.05)SO4_0.5 g/kg_9099.94-0.06
Reference (Ba0.83Sr0.17)SO4_0.5 g/kg_9099.42-0.58
Reference (Ba0.71Sr0.29)SO4_0.5 g/kg_9098.24-1.76
(Ba0.95Sr0.05)SO4_5 g/kg_9097.830.022.15
(Ba0.83Sr0.17)SO4_5 g/kg_9090.210.029.77
(Ba0.71Sr0.29)SO4_5 g/kg_9090.200.039.77
Reference (Ba0.95Sr0.05)SO4_5 g/kg_9098.02-1.98
Reference (Ba0.83Sr0.17)SO4_5 g/kg_9092.55-7.45
Reference (Ba0.71Sr0.29)SO4_5 g/kg_9092.55-7.45
Table
Table 4. Average solid compositions (X = mole fractions) after day 1, 42, 98 and 664 compared to calculated composition from solution and equilibrium composition calculated by GEMS. All measured energy-dispersive X-ray spectrometry (EDX) data are given in the Appendix A in Table A21, Table A22 and Table A23.
Table 4. Average solid compositions (X = mole fractions) after day 1, 42, 98 and 664 compared to calculated composition from solution and equilibrium composition calculated by GEMS. All measured energy-dispersive X-ray spectrometry (EDX) data are given in the Appendix A in Table A21, Table A22 and Table A23.
DayMethodNumber of Particles EDS-AnalysesXSrSO4 MinimumXSrSO4 MaximumXSrSO4 Average
(%)(%)(%)
(Ba0.95Sr0.05)SO4_0.5 g/kg_RT
1EDX553.59.86.2
Mass balance 4.6
42EDX572.98.25.0
Mass balance 4.7
98EDX573.210.35.8
Mass balance 4.7
664EDX9210.49.84.1
Mass balance 3.6
Calculated equilibrium 0.3
(Ba0.71Sr0.29)SO4_0.5 g/kg_RT
1EDX6124.330.517.0
Mass balance 23.2
42EDX595.427.913.4
Mass balance 22.3
98EDX6116.924.314.1
Mass balance 16.9
664EDX10254.721.49.9
Mass balance 9.5
Calculated equilibrium 0.6
(Ba0.95Sr0.05)SO4_5 g/kg_90
1EDX473.913.68.5
Mass balance 4.6
42EDX4514.12.5
Mass balance 4.1
98EDX660.873.0
Mass balance 4.0
664EDX9141.66.33.9
Mass balance 3.4
Calculated equilibrium 2.2

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Brandt, F.; Klinkenberg, M.; Poonoosamy, J.; Bosbach, D. Recrystallization and Uptake of 226Ra into Ba-Rich (Ba,Sr)SO4 Solid Solutions. Minerals 2020, 10, 812. https://doi.org/10.3390/min10090812

AMA Style

Brandt F, Klinkenberg M, Poonoosamy J, Bosbach D. Recrystallization and Uptake of 226Ra into Ba-Rich (Ba,Sr)SO4 Solid Solutions. Minerals. 2020; 10(9):812. https://doi.org/10.3390/min10090812

Chicago/Turabian Style

Brandt, Felix, Martina Klinkenberg, Jenna Poonoosamy, and Dirk Bosbach. 2020. "Recrystallization and Uptake of 226Ra into Ba-Rich (Ba,Sr)SO4 Solid Solutions" Minerals 10, no. 9: 812. https://doi.org/10.3390/min10090812

APA Style

Brandt, F., Klinkenberg, M., Poonoosamy, J., & Bosbach, D. (2020). Recrystallization and Uptake of 226Ra into Ba-Rich (Ba,Sr)SO4 Solid Solutions. Minerals, 10(9), 812. https://doi.org/10.3390/min10090812

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