The solubility of ErPO4 and Er speciation in hydrothermal fluids at varying pH and salinity between 350 and 450 °C.
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| Title: | The solubility of ErPO4 and Er speciation in hydrothermal fluids at varying pH and salinity between 350 and 450 °C. |
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| Authors: | Kershaw, Charles T.1 (AUTHOR) charles.kershaw@student.nmt.edu, Hurtig, Nicole C.1 (AUTHOR), Gysi, Alexander P.1,2 (AUTHOR), Migdisov, Artas A.3 (AUTHOR), Waters, Laura E.1 (AUTHOR), Harlov, Daniel4,5,6 (AUTHOR) |
| Source: | Geochimica et Cosmochimica Acta. Apr2026, Vol. 419, p96-113. 18p. |
| Subjects: | Erbium compounds, pH effect, High temperature chemistry, Geothermal brines, Rare earth metals, Geochemical modeling, Salinity |
| Abstract: | The rare earth elements (REE) are important for the green-energy transition and can be incorporated into the REE phosphates, such as xenotime-(Y), which also hosts heavy REE (Tb–Lu). Xenotime-(Y) is a common accessory mineral in metamorphic rocks and a range of mineral deposits where it controls the mobility of heavy REE, however, the impact of high temperature aqueous fluids on the behavior of heavy REE is largely unknown. Thermodynamic modeling can be utilized as a tool to predict the mobility of REE in hydrothermal aqueous fluids, but must be supported by accurate experimental data. Here, we measured the solubility of endmember synthetic xenotime-structured ErPO 4 in NaCl-HCl-NaOH-bearing aqueous solutions at 350 °C and water vapor saturation pressure, at 400 and at 450 °C and 500 bar using batch-type Inconel reactors. Erbium speciation was investigated as a function of pH from 2.8 to 8, where Er chloride species are predominant at acidic conditions (pH <3) and Er hydroxyl complexes are predominant at near-neutral to alkaline conditions (pH >3). At pH 7–9, the measured ErPO 4 solubility (−9.8 to −7.5 log m Er) is up to 2.5 orders of magnitude lower than thermodynamic predictions (−9.4 to −6.7 log m Er) using existing thermodynamic databases. At pH 2–3, the predicted ErPO 4 solubility is ∼0.5 orders of magnitude higher at 350 °C and ∼1 order of magnitude lower at 450 °C compared to experimentally measured Er concentrations. The thermodynamic properties of aqueous Er species were therefore revised in this study. The partial molal Gibbs energy of formation (Δ f G 0 T,P) for aqueous Er hydroxyl and chloride species are optimized using GEMSFITS and the logarithmic formation constants (log β n (Cl,OH)) were derived at each experimental temperature and pressure: ▪ The updated thermodynamic properties for Er hydroxyl species (Er(OH)+2, Er(OH) 2 +, and Er(OH) 3 0) show that their stability shifts to more acidic conditions at and below 400 °C. The Er chloride species (ErCl+2 and ErCl 2 +) show increased stability compared to Er hydroxyl species at temperatures of 450 °C and 0.01 mol/kg NaCl. The updated thermodynamic properties are implemented into the GEM-Selektor modeling package to investigate the mobility of Er in saline hydrothermal fluids in equilibrium with alkaline rocks. Importantly, the updated properties for Er hydroxyl species result in low Er solubility at rock equilibrated pH conditions due to an expanded hydroxyl predominance zone, but lower aqueous complex stability overall, whereas previous models suggest greater stability for aqueous Er species. Furthermore, ErPO 4 solubility increases with decreasing temperature due to the deprotonation of HCl, which increases the acidity of hydrothermal fluids and the availability of Cl− to complex with the REE. These simulations highlight how fluid-rock reaction and temperature affect the mobility of REE in hydrothermal ore-forming systems. [ABSTRACT FROM AUTHOR] |
| Copyright of Geochimica et Cosmochimica Acta is the property of Pergamon Press - An Imprint of Elsevier Science and its content may not be copied or emailed to multiple sites without the copyright holder's express written permission. Additionally, content may not be used with any artificial intelligence tools or machine learning technologies. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.) | |
| Database: | Engineering Source |
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| Header | DbId: egs DbLabel: Engineering Source An: 192589611 AccessLevel: 6 PubType: Academic Journal PubTypeId: academicJournal PreciseRelevancyScore: 0 |
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| Items | – Name: Title Label: Title Group: Ti Data: The solubility of ErPO4 and Er speciation in hydrothermal fluids at varying pH and salinity between 350 and 450 °C. – Name: Author Label: Authors Group: Au Data: <searchLink fieldCode="AR" term="%22Kershaw%2C+Charles+T%2E%22">Kershaw, Charles T.</searchLink><relatesTo>1</relatesTo> (AUTHOR)<i> charles.kershaw@student.nmt.edu</i><br /><searchLink fieldCode="AR" term="%22Hurtig%2C+Nicole+C%2E%22">Hurtig, Nicole C.</searchLink><relatesTo>1</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Gysi%2C+Alexander+P%2E%22">Gysi, Alexander P.</searchLink><relatesTo>1,2</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Migdisov%2C+Artas+A%2E%22">Migdisov, Artas A.</searchLink><relatesTo>3</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Waters%2C+Laura+E%2E%22">Waters, Laura E.</searchLink><relatesTo>1</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Harlov%2C+Daniel%22">Harlov, Daniel</searchLink><relatesTo>4,5,6</relatesTo> (AUTHOR) – Name: TitleSource Label: Source Group: Src Data: <searchLink fieldCode="JN" term="%22Geochimica+et+Cosmochimica+Acta%22">Geochimica et Cosmochimica Acta</searchLink>. Apr2026, Vol. 419, p96-113. 18p. – Name: Subject Label: Subjects Group: Su Data: <searchLink fieldCode="DE" term="%22Erbium+compounds%22">Erbium compounds</searchLink><br /><searchLink fieldCode="DE" term="%22pH+effect%22">pH effect</searchLink><br /><searchLink fieldCode="DE" term="%22High+temperature+chemistry%22">High temperature chemistry</searchLink><br /><searchLink fieldCode="DE" term="%22Geothermal+brines%22">Geothermal brines</searchLink><br /><searchLink fieldCode="DE" term="%22Rare+earth+metals%22">Rare earth metals</searchLink><br /><searchLink fieldCode="DE" term="%22Geochemical+modeling%22">Geochemical modeling</searchLink><br /><searchLink fieldCode="DE" term="%22Salinity%22">Salinity</searchLink> – Name: Abstract Label: Abstract Group: Ab Data: The rare earth elements (REE) are important for the green-energy transition and can be incorporated into the REE phosphates, such as xenotime-(Y), which also hosts heavy REE (Tb–Lu). Xenotime-(Y) is a common accessory mineral in metamorphic rocks and a range of mineral deposits where it controls the mobility of heavy REE, however, the impact of high temperature aqueous fluids on the behavior of heavy REE is largely unknown. Thermodynamic modeling can be utilized as a tool to predict the mobility of REE in hydrothermal aqueous fluids, but must be supported by accurate experimental data. Here, we measured the solubility of endmember synthetic xenotime-structured ErPO 4 in NaCl-HCl-NaOH-bearing aqueous solutions at 350 °C and water vapor saturation pressure, at 400 and at 450 °C and 500 bar using batch-type Inconel reactors. Erbium speciation was investigated as a function of pH from 2.8 to 8, where Er chloride species are predominant at acidic conditions (pH <3) and Er hydroxyl complexes are predominant at near-neutral to alkaline conditions (pH >3). At pH 7–9, the measured ErPO 4 solubility (−9.8 to −7.5 log m Er) is up to 2.5 orders of magnitude lower than thermodynamic predictions (−9.4 to −6.7 log m Er) using existing thermodynamic databases. At pH 2–3, the predicted ErPO 4 solubility is ∼0.5 orders of magnitude higher at 350 °C and ∼1 order of magnitude lower at 450 °C compared to experimentally measured Er concentrations. The thermodynamic properties of aqueous Er species were therefore revised in this study. The partial molal Gibbs energy of formation (Δ f G 0 T,P) for aqueous Er hydroxyl and chloride species are optimized using GEMSFITS and the logarithmic formation constants (log β n (Cl,OH)) were derived at each experimental temperature and pressure: ▪ The updated thermodynamic properties for Er hydroxyl species (Er(OH)+2, Er(OH) 2 +, and Er(OH) 3 0) show that their stability shifts to more acidic conditions at and below 400 °C. The Er chloride species (ErCl+2 and ErCl 2 +) show increased stability compared to Er hydroxyl species at temperatures of 450 °C and 0.01 mol/kg NaCl. The updated thermodynamic properties are implemented into the GEM-Selektor modeling package to investigate the mobility of Er in saline hydrothermal fluids in equilibrium with alkaline rocks. Importantly, the updated properties for Er hydroxyl species result in low Er solubility at rock equilibrated pH conditions due to an expanded hydroxyl predominance zone, but lower aqueous complex stability overall, whereas previous models suggest greater stability for aqueous Er species. Furthermore, ErPO 4 solubility increases with decreasing temperature due to the deprotonation of HCl, which increases the acidity of hydrothermal fluids and the availability of Cl− to complex with the REE. These simulations highlight how fluid-rock reaction and temperature affect the mobility of REE in hydrothermal ore-forming systems. [ABSTRACT FROM AUTHOR] – Name: AbstractSuppliedCopyright Label: Group: Ab Data: <i>Copyright of Geochimica et Cosmochimica Acta is the property of Pergamon Press - An Imprint of Elsevier Science and its content may not be copied or emailed to multiple sites without the copyright holder's express written permission. Additionally, content may not be used with any artificial intelligence tools or machine learning technologies. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract.</i> (Copyright applies to all Abstracts.) |
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| RecordInfo | BibRecord: BibEntity: Identifiers: – Type: doi Value: 10.1016/j.gca.2026.02.010 Languages: – Code: eng Text: English PhysicalDescription: Pagination: PageCount: 18 StartPage: 96 Subjects: – SubjectFull: Erbium compounds Type: general – SubjectFull: pH effect Type: general – SubjectFull: High temperature chemistry Type: general – SubjectFull: Geothermal brines Type: general – SubjectFull: Rare earth metals Type: general – SubjectFull: Geochemical modeling Type: general – SubjectFull: Salinity Type: general Titles: – TitleFull: The solubility of ErPO4 and Er speciation in hydrothermal fluids at varying pH and salinity between 350 and 450 °C. Type: main BibRelationships: HasContributorRelationships: – PersonEntity: Name: NameFull: Kershaw, Charles T. – PersonEntity: Name: NameFull: Hurtig, Nicole C. – PersonEntity: Name: NameFull: Gysi, Alexander P. – PersonEntity: Name: NameFull: Migdisov, Artas A. – PersonEntity: Name: NameFull: Waters, Laura E. – PersonEntity: Name: NameFull: Harlov, Daniel IsPartOfRelationships: – BibEntity: Dates: – D: 15 M: 04 Text: Apr2026 Type: published Y: 2026 Identifiers: – Type: issn-print Value: 00167037 Numbering: – Type: volume Value: 419 Titles: – TitleFull: Geochimica et Cosmochimica Acta Type: main |
| ResultId | 1 |
