Hydrothermal solution calorimetry in acidic aqueous solutions and revisiting the standard partial molal thermodynamic properties of Nd3+ from 25 to 300 °C
Journal Article
·
· Geochimica et Cosmochimica Acta
- New Mexico Institute of Mining and Technology, Socorro, NM (United States)
The mobility of rare earth elements (REE) can be predicted in aqueous fluids using geochemical modeling but the accuracy of these models strongly depends on the availability of robust thermodynamic properties for the REE aqueous species. The REE3+ aqua ions are important in the derivation of the formation constants of all the major REE complexes including the chloride, sulfate, and fluoride species which predominate in many hydrothermal-magmatic systems. However, the thermodynamic properties of the REE3+ aqua ions are still commonly derived from the Helgeson-Kirkham-Flowers (HKF) equation of state parameters tabulated several decades ago. The standard state thermodynamic properties at reference conditions (25 °C and 1bar) and their extrapolations to high temperature need to be verified, if not revised, based on hydrothermal experiments. In this study, the enthalpy of solution was measured for synthetic Nd hydroxide from 25 to 150 ºC to retrieve the standard partial molal thermodynamic properties of Nd3+ as a function of temperature. The experiments were conducted in aqueous perchloric acid based solutions with starting pH of 2 and varying ionic strength (0.01 to 0.09 mol/kg NaClO4). The standard partial molal enthalpy of formation (ΔfH°) of Nd3+ derived from the experimental study displays differences of up to 10 kJ/mol compared to the enthalpy values derived from the HKF equation of state in the studied temperature range. These inaccuracies are resolved by adjusting the standard partial molal Gibbs energy of formation (ΔfG°) of Nd3+ at 25 °C and 1 bar from -672.0 to -679.7 kJ/mol. The heat capacity function (Cp°) derived between 25 and 150 ºC can be described by: Cp° = a0 + a1·T + a2·T-2, with a0 = 1256, a1 = -2.68, a2 = -55.56·106 and T in Kelvin. A set of recommended thermodynamic properties is provided for the Nd3+ aqua ions and corrections are provided for the chloride and fluoride species to remain internally consistent with the experimentally derived properties. These results allow predicting accurately the solubility of monazite between 25 and 300 ºC. Before these corrections, the properties for the Nd3+ aqua ions derived from the HKF parameters resulted in up to ~1.5 orders of magnitude lower monazite solubility than determined experimentally. Here, a revision of the REE+3 aqua ions properties is necessary to accurately predict the mobility of REE in hydrothermal acidic solutions.
- Research Organization:
- New Mexico Institute of Mining and Technology, Socorro, NM (United States)
- Sponsoring Organization:
- National Science Foundation (NSF); USDOE Office of Science (SC), Basic Energy Sciences (BES)
- Grant/Contract Number:
- SC0021106
- OSTI ID:
- 2522073
- Report Number(s):
- DOE-NMT--0021106
- Journal Information:
- Geochimica et Cosmochimica Acta, Journal Name: Geochimica et Cosmochimica Acta Vol. 406; ISSN 0016-7037
- Publisher:
- Elsevier; The Geochemical Society; The Meteoritical SocietyCopyright Statement
- Country of Publication:
- United States
- Language:
- English
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