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Calcite-mediated uranium isotope fractionation and U(IV) sequestration during U(VI) reduction by sulphide.

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Title: Calcite-mediated uranium isotope fractionation and U(IV) sequestration during U(VI) reduction by sulphide.
Authors: Yuan, Yan1 (AUTHOR), Chen, Tianyu1 (AUTHOR) tianyuchen@nju.edu.cn, Luo, Rengan2 (AUTHOR), Liu, Yuanyuan3 (AUTHOR)
Source: Geochimica et Cosmochimica Acta. May2026, Vol. 421, p403-413. 11p.
Subjects: Calcite, Oxidation-reduction reaction, Carbonate rocks, Sulfides, Biogeochemical cycles, Isotopic fractionation
Abstract: The reduction of hexavalent uranium (U(VI)) to tetravalent uranium (U(IV)) and its associated isotope fractionation in natural systems are critical for understanding Earth's U isotopic cycling. When U isotopes are applied to carbonate archives, authigenic enrichment of isotopically heavy U(IV) has been found to obscure accurate reconstruction of marine U isotopic signatures. However, the sequestration of U(IV) and the associated isotope fractionation in carbonate sediments remain poorly understood. Through systematically designed laboratory experiments, we investigated U(VI) reduction kinetics and associated fractionation effects with aqueous sulfide (S2−) in two systems: calcite-saturated pure solutions (pur system) and calcite suspensions (cal system). Despite similar [Ca], pH, and temperature, calcite suspensions exhibited significantly slower pseudo-first-order reduction rates (k cal = 0.003–0.004 h−1 vs. k pur = 0.013–0.015 h−1) and larger U isotope fractionation (α cal = 1.00049) compared to pur experiments (α pur = 1.00024–1.00026), pointing toward the kinetic isotope effect associated with electron transfer reaction rates. Remarkably, we found U isotope enrichment fractionation (ε) in pur and cal experiments aligned with a universal scaling relationship between normalized reaction rate constant (k) and ε that could be applied to biotic and abiotic isotope fractionation experiments reported previously. Quantitative modeling through an autocatalytic reaction framework, as well as microscopic characterization, indicates that S species adsorbed to calcite particles might lose their capacity to catalyze electron transfer between U(VI) and S2−. Our work thus demonstrates a previously unrecognized inhibitory role of mineral surfaces in retarding U(VI) reduction and amplifying U isotope fractionation in carbonate diagenetic environments. Therefore, considering heterogeneous reactions in models for biogeochemical U cycling will enhance the understanding of U isotopic behaviors in the Earth' s surficial environment. [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: 193806315
AccessLevel: 6
PubType: Academic Journal
PubTypeId: academicJournal
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Items – Name: Title
  Label: Title
  Group: Ti
  Data: Calcite-mediated uranium isotope fractionation and U(IV) sequestration during U(VI) reduction by sulphide.
– Name: Author
  Label: Authors
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  Data: <searchLink fieldCode="AR" term="%22Yuan%2C+Yan%22">Yuan, Yan</searchLink><relatesTo>1</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Chen%2C+Tianyu%22">Chen, Tianyu</searchLink><relatesTo>1</relatesTo> (AUTHOR)<i> tianyuchen@nju.edu.cn</i><br /><searchLink fieldCode="AR" term="%22Luo%2C+Rengan%22">Luo, Rengan</searchLink><relatesTo>2</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Liu%2C+Yuanyuan%22">Liu, Yuanyuan</searchLink><relatesTo>3</relatesTo> (AUTHOR)
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  Data: <searchLink fieldCode="JN" term="%22Geochimica+et+Cosmochimica+Acta%22">Geochimica et Cosmochimica Acta</searchLink>. May2026, Vol. 421, p403-413. 11p.
– Name: Subject
  Label: Subjects
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  Data: <searchLink fieldCode="DE" term="%22Calcite%22">Calcite</searchLink><br /><searchLink fieldCode="DE" term="%22Oxidation-reduction+reaction%22">Oxidation-reduction reaction</searchLink><br /><searchLink fieldCode="DE" term="%22Carbonate+rocks%22">Carbonate rocks</searchLink><br /><searchLink fieldCode="DE" term="%22Sulfides%22">Sulfides</searchLink><br /><searchLink fieldCode="DE" term="%22Biogeochemical+cycles%22">Biogeochemical cycles</searchLink><br /><searchLink fieldCode="DE" term="%22Isotopic+fractionation%22">Isotopic fractionation</searchLink>
– Name: Abstract
  Label: Abstract
  Group: Ab
  Data: The reduction of hexavalent uranium (U(VI)) to tetravalent uranium (U(IV)) and its associated isotope fractionation in natural systems are critical for understanding Earth's U isotopic cycling. When U isotopes are applied to carbonate archives, authigenic enrichment of isotopically heavy U(IV) has been found to obscure accurate reconstruction of marine U isotopic signatures. However, the sequestration of U(IV) and the associated isotope fractionation in carbonate sediments remain poorly understood. Through systematically designed laboratory experiments, we investigated U(VI) reduction kinetics and associated fractionation effects with aqueous sulfide (S2−) in two systems: calcite-saturated pure solutions (pur system) and calcite suspensions (cal system). Despite similar [Ca], pH, and temperature, calcite suspensions exhibited significantly slower pseudo-first-order reduction rates (k cal = 0.003–0.004 h−1 vs. k pur = 0.013–0.015 h−1) and larger U isotope fractionation (α cal = 1.00049) compared to pur experiments (α pur = 1.00024–1.00026), pointing toward the kinetic isotope effect associated with electron transfer reaction rates. Remarkably, we found U isotope enrichment fractionation (ε) in pur and cal experiments aligned with a universal scaling relationship between normalized reaction rate constant (k) and ε that could be applied to biotic and abiotic isotope fractionation experiments reported previously. Quantitative modeling through an autocatalytic reaction framework, as well as microscopic characterization, indicates that S species adsorbed to calcite particles might lose their capacity to catalyze electron transfer between U(VI) and S2−. Our work thus demonstrates a previously unrecognized inhibitory role of mineral surfaces in retarding U(VI) reduction and amplifying U isotope fractionation in carbonate diagenetic environments. Therefore, considering heterogeneous reactions in models for biogeochemical U cycling will enhance the understanding of U isotopic behaviors in the Earth' s surficial environment. [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:
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    Identifiers:
      – Type: doi
        Value: 10.1016/j.gca.2026.01.049
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      – Code: eng
        Text: English
    PhysicalDescription:
      Pagination:
        PageCount: 11
        StartPage: 403
    Subjects:
      – SubjectFull: Calcite
        Type: general
      – SubjectFull: Oxidation-reduction reaction
        Type: general
      – SubjectFull: Carbonate rocks
        Type: general
      – SubjectFull: Sulfides
        Type: general
      – SubjectFull: Biogeochemical cycles
        Type: general
      – SubjectFull: Isotopic fractionation
        Type: general
    Titles:
      – TitleFull: Calcite-mediated uranium isotope fractionation and U(IV) sequestration during U(VI) reduction by sulphide.
        Type: main
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      – PersonEntity:
          Name:
            NameFull: Yuan, Yan
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            NameFull: Chen, Tianyu
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            NameFull: Luo, Rengan
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            NameFull: Liu, Yuanyuan
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            – D: 15
              M: 05
              Text: May2026
              Type: published
              Y: 2026
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              Value: 421
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            – TitleFull: Geochimica et Cosmochimica Acta
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