The solubility of thorium in carbonate-bearing solutions at hydrothermal conditions
Introduction
Thorium, the most abundant actinide in the Earth’s crust, is generally considered to be immobile in nature, based on data collected at ambient conditions (Rand et al., 2008). This interpretation, however, is in conflict with observations of hydrothermal Th mineralization, including that of REE ore deposits produced by hydrothermal fluids (Castor, 2008, Sheard et al., 2012, Cook et al., 2013). Owing to its radioactivity, Th is viewed as an unfavorable contaminant, and its enrichment therefore impacts negatively on the feasibility of mining endeavors. There has been growing interest in the past few decades, however, in the possibility of exploiting deposits of Th to substitute for or enhance U nuclear fuels. Consequently, it is necessary to determine the mechanisms responsible for the mobilization, enrichment, and depletion of Th in hydrothermal fluids.
The main impediment to evaluating the transport and deposition of Th by hydrothermal solutions is a lack of information on the behavior and properties of Th-bearing aqueous species at elevated temperature. The published data on this speciation are restricted almost entirely to temperatures below 100 °C, with most of the data being for ambient conditions (Rand et al., 2008). We recently launched a research program to address this knowledge gap by conducting solubility experiments involving crystalline ThO2 at temperatures > 175 °C in chloride- and sulfate-bearing systems, i.e., systems containing ligands expected to be important in natural hydrothermal systems (Nisbet et al., 2018, Nisbet et al., 2019). These experiments indicated that SO42− has a major impact on the solubility and mobility of Th at elevated temperature (175–250 °C), even if the concentrations of SO42− (>0.5 m) are modest (Nisbet et al., 2019). A notable gap in our current knowledge is a lack of understanding of the behavior of Th in carbonate-bearing systems. Among the possible complexing ligands, the carbonate anion has been shown to form very stable complexes with actinides, such as Th, at ambient conditions (Rand et al., 2008), and it is logical to speculate, as many researchers have (Wood, 1990, Haas et al., 1995), that this complex may also play a significant role in Th mobility at elevated temperature. Indeed, it is noteworthy that, in nature, thorium tends to concentrate in highly evolved systems that have elevated carbonate concentrations including carbonatites (>50 % carbonate) (Ault et al., 2015). Many of these systems were formed and/or altered by hydrothermal fluids (e.g., Mountain Pass, Bear Lodge; Castor, 2008, Andersen et al., 2017); yet, to date, there have been no investigations of Th solubility and speciation in carbonate-bearing fluids at high temperature. Consequently, the role of carbonate-species in the mobilization of Th in these systems cannot be evaluated.
The thermodynamic data for Th-carbonate speciation have been summarized in a thorough review by the Nuclear Energy Agency (NEA) (Rand et al., 2008). Results of 19 sets of experiments have been reported for aqueous solutions at ambient conditions, all of which show that Th has a strong affinity for carbonate anions. Indeed, reactions involving the complexation of actinides with carbonate have been considered “some of the most important reactions in aqueous systems” (Altmaier et al., 2005). However, there have been serious inconsistencies in the identification of the dominant Th-carbonate species. Altmaier et al., 2005, Altmaier et al., 2006) identified the ternary complexes Th(OH)(CO3)45− and Th(OH)2(CO3)22− as the dominant species with minor contributions from Th(OH)2(CO3)(aq), Th(OH)3(CO3)− and Th(OH)4(CO3)2− in solutions with an ionic strength of 0.5 M. In contrast, Osthols et al., 1994, Felmy et al., 1997, Felmy and Rai, 1999 proposed that Th(CO3)65− is the dominant complex in solutions containing 0.1–2.0 M CO32− and Th(OH)3(CO3)− to dominate at lower carbonate concentrations and near-neutral pH conditions. It should be noted that the above mentioned studies were all conducted using an amorphous ThO2 reference phase, which has been shown to lead to discrepancies of several orders of magnitude in the measured concentrations of dissolved Th relative to those using crystalline ThO2, owing to the higher solubility of amorphous solids (Rand et al., 2008).
Evidently, even at ambient conditions, there is uncertainty regarding Th-carbonate speciation, and at elevated temperature, thermodynamic data are simply non-existent. The purpose of this study is to investigate the speciation of Th in carbonate-bearing fluids at hydrothermal conditions and to derive the thermodynamic properties of the dominant complexes to permit forward modeling of Th mobility in natural, carbonate-bearing hydrothermal systems.
Section snippets
Methods
To investigate the speciation of Th in carbonate-bearing solutions, solubility experiments were performed in solutions of varying carbonate concentration (0.05–0.5 m NaHCO3/Na2CO3, pHT 7.83–9.82) at elevated temperature (175–250 °C) and the pressure of saturated water vapor, with crystalline thorium dioxide (ThO2) as the solid reactant (particle size > 10 μm). The experimental solutions were contained in Teflon-lined titanium autoclaves. Before each experiment, small Teflon tubes containing
Results and data treatment
The results of the solubility experiments are reported in Table 1. This table lists the concentration of NaHCO3 and Na2CO3 in the solution added to each autoclave, the logarithm of the concentration of Th measured in the solutions after each experiment, the calculated activity of HCO3− and the pH calculated for the experimental temperature (pHT). In order to determine the pHT and the activity of HCO3−, we used the thermodynamic modeling software, HCh (Shvarov and Bastrakov, 1999). The model
Discussion
The relative proportions of the various Th-hydroxyl species as a function of pH at the temperature of our experiments is illustrated in Fig. 9. As is evident from this figure, there is a progressive shift in the predominance of the complexes from Th(OH)22+ at low pH to Th(OH)40 and Th(OH)5− at high pH. This increase in the degree of the hydrolysis of ThO2 with increasing pH is a predictable consequence of the increase in the concentration of OH− ions available for complexation with Th4+ in the
Conclusion
The experimental data collected in this study demonstrate that there is negligible Th-carbonate complexation at elevated temperature (>175 °C) in carbonate-bearing solutions (0.05–0.5 m NaHCO3/Na2CO3). This result is in sharp contrast to the results of experiments performed at ambient conditions for which Th-carbonate species are reported to be highly stable. Instead, the hydrolysis of Th is the major control on the solubility of ThO2, with Th(OH)40 predominating at 175 and 200 °C and Th(OH)5−
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
We would like to thank Joshua White for conducting the XRD analyses, and Oana Marina and Chelsea Neil for performing the ICP-MS analyses. We thank three anonymous reviewers for their comments and suggestions.
Funding: This work was supported by the Laboratory Directed Research and Development (LDRD) program of Los Alamos National Laboratory (LANL), New Mexico, USA (project number 20180007DR), and by LANL's Center for Space and Earth Sciences (CSES). CSES is funded by LANL’s LDRD program under
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