Speciation in Aqueous MgSO₄Fluids at High Pressures and High Temperatures from ab Initio Molecular Dynamics and Raman Spectroscopy

Abstract

Ab initio molecular dynamics simulations and in situ Raman spectroscopy are used to study the speciation in two molal aqueous MgSO 4 solutions at high pressures, P, and temperatures, T. While at ambient conditions the fluid is dominated by dissociated SO 4 2-(aq) ions and solvent-separated ion pairs, ion association strongly increases with increasing temperature and pressure along a 1.33 g/cm 3 isochore. At T ) 450 °C and P ) 1.4 GPa, the ν 1 (SO 4 2-) Raman band is well described by three Gaussian + Lorentzian components of about equal intensity with peaks at about 980, 995, and 1005 cm -1 . Analysis of the simulations, however, indicates the coexistence of more than three species, including dissociated SO 4 2-(aq) ions, and contact and triple ion pairs as well as larger complexes. In addition, the sulfate groups may be bonded to Mg as monodentate or bidentate ligands. The frequencies of the associated species seem to depend mainly on the type and number of Mg-SO 4 bonds. We interpret the two rather broad high-frequency Raman components as a single "Mg-SO 4 contact" component with variable frequency distribution. As a consequence, the ν 1 (SO 4 2-) Raman band provides only information on the molecular environment of the sulfate group; i.e., individual species cannot be resolved. At fluid densities less than about 1.2 g/cm 3 and temperatures above 400 °C, the formation of HSO 4 -(aq)containing species is observed in both simulations and experiments, which may be accompanied by a change in pH and electrical conductivity.

In situ observations of liquid–liquid phase separation in aqueous ZnSO4 solutions at temperatures up to 400 °C: Implications for Zn2+–SO42− association and evolution of submarine hydrothermal fluids

“…1015 and 1028 cm -1 . It has been reported that the v1(SO4 2-) band becomes broader and the wavenumber increases with the increase of the length of inner-sphere ion pairs in aqueous MgSO4 solution [21,23,[35][36]. Considering the obvious higher-wavenumber shift of the v1(SO4 2-) band (Fig.…”

“…980 cm -1 [34]. Both v1(UO2 2+ ) and v1(SO4 2-) bands can serve as indictors to investigate the complexation in the UO2SO4-H2O system because these two Raman bands are sensitive to changes in coordination environment [13][14][17][18][19][20][21][22][23].…”

“…980 cm -1 should arise from unassociated SO4 2-, whereas the v1(SO4 2-) components at ca. 992 cm -1 and 1002 cm -1 may represent the contribution from contact ion pair and triple ion pair, respectively [20][21][22][23][24]. However, it's difficult to make clear assignments for the v1(SO4 2-) components at ca.…”

“…In fact, Raman spectra of the v1(SO4 2-) band can also be used to investigate the ion interaction between UO2 2+ and SO4 2-in aqueous solution because the complexation will influence the S-O vibrations of sulfate and cause a shift in the v1(SO4 2-) band, which has been well documented in aqueous MgSO4 solutions [20][21][22][23][24]. However, nearly all of the documented Raman spectroscopic descriptions of aqueous UO2SO4 solutions were conducted at ambient temperature and focused on the v1(UO2 2+ ) mode, which limits our knowledge on the UO2 2+ -SO4 2-association at high temperatures as well as the factors inducing the liquid-liquid immiscibility.…”

Spectroscopic and X-ray diffraction investigation of the behavior of hanksite and tychite at high pressures, and a model for the compressibility of sulfate minerals

“…In addition, much study has gone into MgSO 4 , BaSO 4 , various lithium sulfates, and sulfate salts (Lemos et al 1991;Sakuntala andArora 1999, 2000;Chen et al 2009Chen et al , 2010Brand et al 2010;Machon et al 2010;Crichton et al 2011;Jahn and Schmidt 2010;Zhang and Sekine 2007;Santamaría-Pérez et al 2011;Antao 2012). Hydrated sulfates like gypsum undergo several structural changes below 10 GPa, while others like BaSO 4 exhibit few signs of transformation up to 20 GPa.…”