Challenging the thorium-immobility paradigm

Thorium is the most abundant actinide in the Earth’s crust and has universally been considered one of the most immobile elements in natural aqueous systems. This view, however, is based almost exclusively on solubility data obtained at low temperature and their theoretical extrapolation to elevated temperature. The occurrence of hydrothermal deposits with high concentrations of Th challenges the Th immobility paradigm and strongly suggests that Th may be mobilized by some aqueous fluids. Here, we demonstrate experimentally that Th, indeed, is highly mobile at temperatures between 175 and 250 °C in sulfate-bearing aqueous fluids due to the formation of the highly stable Th(SO4)2 aqueous complex. The results of this study indicate that current models grossly underestimate the mobility of Th in hydrothermal fluids, and thus the behavior of Th in ore-forming systems and the nuclear fuel cycle needs to be re-evaluated.


Results and data treatment
Phase characterization of the reference solid (ThO2) was performed by X-ray Diffraction (XRD) in order to ensure no phase change occurred (i.e., Th(SO4)2(solid)) during the experiments. An example of the XRD spectra obtained from solids taken at the end of multiple experiments is illustrated in Supplementary Fig. S2, and shows that, indeed, the solid remained ThO2 and that there are no peaks for Th(SO4)2.
The results of the solubility experiments are presented in Supplementary Table S1, which lists the experimental parameters for each solution, the logarithm of the molality of Th measured, the activity of sulfate calculated at the experimental conditions, the pH measured after completion of the experiments (pH25°C), and the pH extrapolated to the experimental temperature (pHT). The pH at the experimental temperature (pHT) differs from the pH measured at ambient conditions (pH25°C) owing to changes in the dissociation constant of water and the dissolved species. In order to determine the pHT, the thermodynamic modeling software HCh was used 1 . The thermodynamic model employed for these calculations involved the following species: H2O, H + , OH -, O2, H2, Na + , NaOH°, NaSO4 -, NaCl°, SO4 2-, HSO4 -, Cl -, and HCl° and thermodynamic data for modeling the aqueous solutions at each experimental temperature were taken from Refs. [2][3][4] . The dissociation constant and thermodynamic properties of water were calculated using the Marshall and Frank model 5 and the Haar-Gallagher-Kell model 6 , respectively. Initially, the composition of the experimental solution was modeled to determine the concentration of HCl at 25°C corresponding to the experimentally measured pH25°C. Subsequently, the pH was recalculated at the experimental temperature (pHT) using the measured concentrations of HCl. The activity model used in these and all subsequent calculations (see below) was the Debye-Hückel model modified by Refs. [7][8][9] , recommended for NaCldominated solutions up to I=6 and temperatures up to 600°C: where A and B are the Debye-Hückel solvent parameters, , and ̇ are the individual molal activity coefficient, the charge, and the distance of closest approach of an ion i, respectively. The effective ionic strength calculated using the molal scale is , Γ is a molarity to molality conversion factor, and is the extended-term parameter for NaCl from Refs. 8 and 9 .

Calculation of equilibrium constants
As reported in Supplementary Table S1, the pH of the experimental solutions varied within 0.5 log units for the range of conditions of our experimental solutions. In order to identify the Th-sulfate species from the stoichiometric slope of our experimental data, we normalized the pH based on the two species that have been observed in sulfate-bearing solutions at acidic to moderately acidic conditions (ThSO4 2+ and Th(SO4)2): As shown in Fig. 1 12 . The contribution of polynuclear species to the solubility of Th was not considered as it can be assumed that these species become unstable at high temperature due to the large increase of electrostatic repulsion associated with the decrease of the dielectric constant of water 13 , as demonstrated in Ref. 12 .

Extrapolation to low temperature and comparison to previous studies
To the best of our knowledge, this is the first study to investigate Th-sulfate speciation at hydrothermal conditions. The other thermodynamic data for Th(SO4)2 are restricted to ambient conditions, and are reported in an extensive review performed by the Nuclear Energy Agency (NEA) 14 . This report cites several studies that have derived equilibrium constants at ambient conditions for Th(SO4)2 and ThSO4 2+ ; the species expected to predominate in an aqueous solution at low temperature [15][16][17][18] . These studies 11.10 ± 0.10, respectively. In order to compare our data with those reported in previous studies, the formation constants obtained from this study were extrapolated to low temperature by fitting the log 2 for each temperature to the Ryzhenko-Bryzgalin model (MRB) 21 modified by Ref. 22 as described in Ref. 23 . This model fits the temperature and pressure dependence of the dissociation constant for ion pairs through the following equation: where K is the dissociation constant of the ion pair, Tr and Pr are the reference temperature and pressure, B(T,P) accounts for the property of water at the temperature and pressure calculated from data contained in Ref. 5 , and Azz/a and Bzz/a are the fitting parameters. The parameters derived from this model for Th(SO4)2 are found in Supplementary Table S2.
Supplementary Fig. S4 shows the thermodynamic formation constants from this study extrapolated to 10°C for comparison with the selected constant from the NEA review 14 , and values derived from Ref. 20 . As shown in this figure, the formation constants calculated in this study systematically increase with temperature, and when back-extrapolated to low temperature, show excellent agreement with the previously reported values.

Modeling
The model presented in this contribution simulates progressive hydrothermal alteration and re-distribution of REE and Th by an acidic solution in a one-dimensional column of a rock containing 0.5 wt. % apatite-OH (Ca-hydroxy-phosphate, to allow for the formation of REE phosphates), which was evaluated using a step-flow reactor approach, similar to Refs. 24  ions REE 3+ and Th 4+ were taken from Ref. 10 . The thermodynamic data for REE and Th aqueous species incorporated in this model are identical to those described in Ref. 25 .
Exceptions are the REE-sulfate complexes, REESO4 + and REE(SO4)2 -30 and the data derived in this contribution for Th(SO4)2. For more detail on the sources and selection of thermodynamic data, readers are referred to Refs. 25 34 . For details on data selection, readers are referred to Ref. 25 .
To Supplementary Figure S4: Extrapolation and comparison of calculated thermodynamic formation constants from this study to low temperature. Formation constants (log 2 ) for Th(SO4)2 plotted as a function of temperature. The experimental data obtained in this study (blue squares) were fitted to the Bryzgalin-Ryzhenko model (dashed line) to compare with the selected data from the Nuclear Energy Agency 14 (green square) and data collected from Ref. 20 (orange squares). The error (SD) associated with the individual points is smaller than the size of the markers. Our data shows excellent agreement with the low temperature values.