Constraining silicon isotope exchange kinetics and fractionation between aqueous and amorphous Si at room temperature.
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| Title: | Constraining silicon isotope exchange kinetics and fractionation between aqueous and amorphous Si at room temperature. |
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| Authors: | Zheng, Xin-Yuan1 (AUTHOR) xzheng75@wisc.edu, Beard, Brian L. (AUTHOR), Johnson, Clark M. (AUTHOR) |
| Source: | Geochimica et Cosmochimica Acta. 5/15/2019, Vol. 253, p267-289. 23p. |
| Subjects: | Silicon isotopes, Isotope exchange reactions, Chemical equilibrium, Ionic solutions, Ionic strength |
| Abstract: | Silicon (Si) isotopes are useful tracers for the modern and ancient Si cycle, but their interpretation is limited by inadequate understanding of Si isotope exchange kinetics and fractionation factors at low temperature. This study investigated Si isotope exchange and fractionation between aqueous and amorphous Si at circumneutral pH and room temperature through a series of 29Si-spiked isotope-exchange experiments. Four different amorphous Si solids with varied surface areas were reacted with aqueous Si solutions of high ionic strength similar to seawater, or low ionic strength typical of freshwater, under conditions close to chemical equilibrium with respect to amorphous Si solubility. In contrast to the common perception of negligible Si isotope exchange at low temperature, ∼50–85% isotope exchange was achieved between aqueous and amorphous Si within ∼60 days. Larger solid surface areas and higher aqueous ionic strength generally promoted Si isotope exchange. Drying/aging of Si gel, however, impedes Si isotope exchange between amorphous and aqueous Si relative to freshly prepared Si gels. Excluding the experiments that used the aged Si gel, temporal trajectories of Si isotope evolution of the two phases from all other experiments showed significant curvature in three-isotope space (29Si/28Si and 30Si/28Si). These results can be best explained by a model that comprises two Si isotope exchange processes with different exchange rates and fractionation factors during the interactions between aqueous and amorphous Si towards isotope equilibrium. The faster exchange is associated with surface sites, and slower exchange occurs between exterior and interior Si atoms of the solid. Exchange with surface sites tends to partition heavy Si isotopes in the aqueous phase relative to the solid surface, whereas exchange between surface and interior sites in the solid tends to enrich heavy Si isotopes in the interior. Two experiments that achieved >80% isotope exchange provided the best estimates of equilibrium Si isotope fractionation factors between bulk amorphous Si solid and aqueous monomeric silicic acid H 4 SiO 4 (Δ30Si amorphous–aqueous) at 23 °C: +0.52‰ (±0.15‰, 1sd) at seawater ionic strength, and −0.98‰ (±0.12‰) at freshwater ionic strength. The observed "salt effect" on Si isotope exchange kinetics and fractionation factor is interpreted to reflect an influence of cations on Si speciation of solid surfaces. This work highlights the value of three-isotope method in studying both reaction kinetics and isotope fractionation mechanisms. The observed Si isotope exchange between amorphous and aqueous Si at low temperature implies that Si isotope re-equilibration, a previously neglected process, may be important in controlling Si isotope compositions of natural samples. [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 |
| FullText | Text: Availability: 0 |
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| Header | DbId: egs DbLabel: Engineering Source An: 136179300 AccessLevel: 6 PubType: Academic Journal PubTypeId: academicJournal PreciseRelevancyScore: 0 |
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| Items | – Name: Title Label: Title Group: Ti Data: Constraining silicon isotope exchange kinetics and fractionation between aqueous and amorphous Si at room temperature. – Name: Author Label: Authors Group: Au Data: <searchLink fieldCode="AR" term="%22Zheng%2C+Xin-Yuan%22">Zheng, Xin-Yuan</searchLink><relatesTo>1</relatesTo> (AUTHOR)<i> xzheng75@wisc.edu</i><br /><searchLink fieldCode="AR" term="%22Beard%2C+Brian+L%2E%22">Beard, Brian L.</searchLink> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Johnson%2C+Clark+M%2E%22">Johnson, Clark M.</searchLink> (AUTHOR) – Name: TitleSource Label: Source Group: Src Data: <searchLink fieldCode="JN" term="%22Geochimica+et+Cosmochimica+Acta%22">Geochimica et Cosmochimica Acta</searchLink>. 5/15/2019, Vol. 253, p267-289. 23p. – Name: Subject Label: Subjects Group: Su Data: <searchLink fieldCode="DE" term="%22Silicon+isotopes%22">Silicon isotopes</searchLink><br /><searchLink fieldCode="DE" term="%22Isotope+exchange+reactions%22">Isotope exchange reactions</searchLink><br /><searchLink fieldCode="DE" term="%22Chemical+equilibrium%22">Chemical equilibrium</searchLink><br /><searchLink fieldCode="DE" term="%22Ionic+solutions%22">Ionic solutions</searchLink><br /><searchLink fieldCode="DE" term="%22Ionic+strength%22">Ionic strength</searchLink> – Name: Abstract Label: Abstract Group: Ab Data: Silicon (Si) isotopes are useful tracers for the modern and ancient Si cycle, but their interpretation is limited by inadequate understanding of Si isotope exchange kinetics and fractionation factors at low temperature. This study investigated Si isotope exchange and fractionation between aqueous and amorphous Si at circumneutral pH and room temperature through a series of 29Si-spiked isotope-exchange experiments. Four different amorphous Si solids with varied surface areas were reacted with aqueous Si solutions of high ionic strength similar to seawater, or low ionic strength typical of freshwater, under conditions close to chemical equilibrium with respect to amorphous Si solubility. In contrast to the common perception of negligible Si isotope exchange at low temperature, ∼50–85% isotope exchange was achieved between aqueous and amorphous Si within ∼60 days. Larger solid surface areas and higher aqueous ionic strength generally promoted Si isotope exchange. Drying/aging of Si gel, however, impedes Si isotope exchange between amorphous and aqueous Si relative to freshly prepared Si gels. Excluding the experiments that used the aged Si gel, temporal trajectories of Si isotope evolution of the two phases from all other experiments showed significant curvature in three-isotope space (29Si/28Si and 30Si/28Si). These results can be best explained by a model that comprises two Si isotope exchange processes with different exchange rates and fractionation factors during the interactions between aqueous and amorphous Si towards isotope equilibrium. The faster exchange is associated with surface sites, and slower exchange occurs between exterior and interior Si atoms of the solid. Exchange with surface sites tends to partition heavy Si isotopes in the aqueous phase relative to the solid surface, whereas exchange between surface and interior sites in the solid tends to enrich heavy Si isotopes in the interior. Two experiments that achieved >80% isotope exchange provided the best estimates of equilibrium Si isotope fractionation factors between bulk amorphous Si solid and aqueous monomeric silicic acid H 4 SiO 4 (Δ30Si amorphous–aqueous) at 23 °C: +0.52‰ (±0.15‰, 1sd) at seawater ionic strength, and −0.98‰ (±0.12‰) at freshwater ionic strength. The observed "salt effect" on Si isotope exchange kinetics and fractionation factor is interpreted to reflect an influence of cations on Si speciation of solid surfaces. This work highlights the value of three-isotope method in studying both reaction kinetics and isotope fractionation mechanisms. The observed Si isotope exchange between amorphous and aqueous Si at low temperature implies that Si isotope re-equilibration, a previously neglected process, may be important in controlling Si isotope compositions of natural samples. [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.2019.03.031 Languages: – Code: eng Text: English PhysicalDescription: Pagination: PageCount: 23 StartPage: 267 Subjects: – SubjectFull: Silicon isotopes Type: general – SubjectFull: Isotope exchange reactions Type: general – SubjectFull: Chemical equilibrium Type: general – SubjectFull: Ionic solutions Type: general – SubjectFull: Ionic strength Type: general Titles: – TitleFull: Constraining silicon isotope exchange kinetics and fractionation between aqueous and amorphous Si at room temperature. Type: main BibRelationships: HasContributorRelationships: – PersonEntity: Name: NameFull: Zheng, Xin-Yuan – PersonEntity: Name: NameFull: Beard, Brian L. – PersonEntity: Name: NameFull: Johnson, Clark M. IsPartOfRelationships: – BibEntity: Dates: – D: 15 M: 05 Text: 5/15/2019 Type: published Y: 2019 Identifiers: – Type: issn-print Value: 00167037 Numbering: – Type: volume Value: 253 Titles: – TitleFull: Geochimica et Cosmochimica Acta Type: main |
| ResultId | 1 |
