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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]
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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]
ISSN:00167037
DOI:10.1016/j.gca.2026.01.049