Yttrium complexation and hydration in chloride-rich hydrothermal fluids: A combined ab initio molecular dynamics and in situ X-ray absorption spectroscopy study

Abstract

Accurate knowledge of rare earth elements (REE) speciation in high pressurehigh temperature fluids is required to model REE transport and precipitation in subduction zones and magmatic-hydrothermal environments, and the formation of rare metal deposits. Recent experiments (lanthanum, ytterbium, erbium) have demonstrated that REE chloride complexes are the main REE form in many hydrothermal fluids (Migdisov, et al., 2016). However, the speciation of yttrium *Manuscript (Y(III)), a cation with an ionic radius similar to that of Ho(III), remains poorly constrained in chloride-rich hydrothermal solutions.We used ab initio molecular dynamics (MD) simulations to calculate the nature of Y(III)-Cl complexes and the thermodynamic properties of these species at temperatures up to 500 ºC and pressures of 800 bar and 1000 bar. The MD results were complemented by in-situ X-ray absorption spectroscopy (XAS) measurements.Our results indicate that at temperature below 200 ºC, chloro-complexes do not form readily, even in highly concentrated brines. At ambient condition, the Y(III) aqua ion binds to eight water molecules in a square antiprism geometry, which is consistent with previous ab initio studies (Ikeda, et al., 2005). The thermodynamic integration method was employed to calculate the formation constants (log) of Y(III)-Clcomplexes in two simulation boxes containing different Y:Cl ratios; we obtained very consistent results of the standard log of same complexes from the two independent calculations, which confirms that the thermodynamic integration method is reliable and not significantly affected by technical limitations in box size, box composition, or simulation times.Based on the derived formation constants, we fit Modified Ryzhenko-Bryzgalin (MRB) equation of state parameters, which enable extrapolation of the formation constants at elevated temperature and pressure. The results are consistent with the XAS data, and show that the stability of Y(III)-Cl complexes increases with increasing temperature, Y(III) forming high order Clcomplexes (up to YCl 4 -) in high salinity solutions at high temperature and pH = 3. We also compare the extrapolated log with the available data for other REE at 150 ˚C, 200 ˚C and 250 ˚C. At 200 ˚C, yttrium behaves more like a heavy REE, but from 200 ˚C to ˚C, the log of Y(III)-Cl complexes increases dramatically and behave more like the light REE. The difference of Cldominant species between Ho(III) (HoCl 2+ ) and Y(III) (YCl 2 + ) may account for the formation of anomalous Y/Ho ratios in some hydrothermal environments.

Carbonate complexation enhances hydrothermal transport of rare earth elements in alkaline fluids

“…However, temperature affects LREE and HREE differently (Fig. 2): Gd and Yb display a retrograde solubility from 100 °C onwards, similar to what we previously reported for Eu, Yb, or Y in acidic Cl-and S-bearing solutions 26,27,44 ; on the contrary, La solubility increases suddenly from below detection limit (~10 ppm) at T < 300 °C (e.g., absorption edge is barely visible on Fig. 1A) to more than 10 times the detection limit at 400-500 °C (Figs.…”

“…1A and 2A). This is the first time we observe prograde solubility for an REE in our in situ experiments 26,27,44 .…”

“…The aqueous solutions were left to equilibrate for a few minutes after each heating step and spectra were then collected for 1-3 h at each temperature depending on the aim of the measurements (solubility or speciation experimentssee below). The reader is referred to previous publications on Cu, Eu, Pb, Pd, Y, or Yb hydrothermal speciation for further details on the autoclave and beamline capacities at extreme P-T conditions 26,27,44,[80][81][82] .…”

The role of sulfur in molybdenum transport in hydrothermal fluids: Insight from in situ synchrotron XAS experiments and molecular dynamics simulations

“…The BLYP functional generally provides a good description of the structural properties of water including O-O interactions, H-bond statistics, angular distributions, coordination numbers, and radial distribution functions when compared with experiments (Gillan et al, 2016;Lin et al, 2012). However, as discussed in our previous studies using the BLYP exchange correlation functional (Mei et al, 2018;Guan et al, 2020), we note that there is a well-known tendency for most functionals, including BLYP, to overstructure liquid water (Lee and Tuckerman, 2006;Lin et al, 2012). For example, Bankura et al (2014) showed that BLYP, like most other functionals, best reproduces the hydrogenbonded network of water at ambient conditions when using an elevated temperature (80 °C in their case), and it could be argued that there is a temperature offset of approximately 50-55 °C between the computed data as compared to the experimental structure of the solvent at room temperature.…”

Molecular Dynamic Simulations to Understand the Hydrothermal Deposits of Rare Earth Elements

“…Ab initio molecular dynamic simulations (MD) are an effective method to predict metal complexes and their thermodynamic properties [1] and offer the opportunity to achieve conditions that cannot be reached via the experimental approach. Our group has obtained new properties for the Y(III)-Cl - [2], Y(III)-SO 4 2and La(III)-Clcomplexes at temperature up to 500 ˚C, 600 to 1000 bar via MD studies. The predicted geometries are supported by in-situ XAS measurements conducted over similar P-T-X range.…”