Enthalpies of formation of polyhalite: A mineral relevant to salt repository

Guo, Xiaofeng; Xu, Hongwu

doi:10.1016/j.jct.2017.05.031

Enthalpies of formation of polyhalite: A mineral relevant to salt repository

Journal Article · Fri Jun 02 00:00:00 EDT 2017 · Journal of Chemical Thermodynamics

DOI:https://doi.org/10.1016/j.jct.2017.05.031· OSTI ID:1364549

Guo, Xiaofeng ^[1]; Xu, Hongwu ^[1]

Los Alamos National Lab. (LANL), Los Alamos, NM (United States)

Polyhalite is an important coexisting mineral with halite in salt repositories for nuclear waste disposal, such as Waste Isolation Pilot Plant (WIPP) in Carlsbad, New Mexico. The thermal stability of this mineral is a key knowledge in evaluating the integrity of a salt repository in the long term, as water may release due to thermal decomposition of polyhalite. Previous studies on structural evolution of polyhalite at elevated temperatures laid the basis for detailed calorimetric measurements. Using high-temperature oxide-melt drop-solution calorimetry at 975 K with sodium molybdate as the solvent, we have determined the standard enthalpies of formation from constituent sulfates (ΔH°_f,sul), oxides (ΔH°_f,ox) and elements (ΔH°_f,ele) of a polyhalite sample with the composition of K₂Ca₂Mg(SO₄)₄·1.95H₂O from the Salado formation at the WIPP site. The obtained results are: ΔH°_f,sul = -152.5 ± 5.3 kJ/mol, ΔH°_f,ox = -1926.1 ± 10.5 kJ/mol, and ΔH°_f,ele = -6301.2 ± 9.9 kJ/mol. Furthermore, based on the estimated formation entropies of polyhalite, its standard Gibbs free energy of formation has been derived to be in the range of -5715.3 ± 9.9 kJ/mol to -5739.3 ± 9.9 kJ/mol. In conclusion, these determined thermodynamic properties provide fundamental parameters for modeling the stability behavior of polyhalite in salt repositories.

View Accepted Manuscript (DOE)

Research Organization:: Los Alamos National Laboratory (LANL)

Sponsoring Organization:: USDOE Laboratory Directed Research and Development (LDRD) Program

Grant/Contract Number:: AC52-06NA25396

OSTI ID:: 1364549

Alternate ID(s):: OSTI ID: 1550156

Report Number(s):: LA-UR-17-20042

Journal Information:: Journal of Chemical Thermodynamics, Journal Name: Journal of Chemical Thermodynamics Vol. 114; ISSN 0021-9614

Publisher:: ElsevierCopyright Statement

Country of Publication:: United States

Language:: English

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Sample seal-and-drop device and methodology for high temperature oxide melt solution calorimetric measurements of PuO ₂ Guo, Xiaofeng; Boukhalfa, Hakim; Mitchell, Jeremy N. Review of Scientific Instruments, Vol. 90, Issue 4 https://doi.org/10.1063/1.5093567	journal	April 2019
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