Wang, Z, Kwon, KD, Peacock, C orcid.org/0000-0003-3754-9294 et al. (4 more authors) (2022) Zn stable isotope fractionation during adsorption onto todorokite: A molecular perspective from X-ray absorption spectroscopy and density functional theory. Geochimica et Cosmochimica Acta, 327. pp. 116-136. ISSN 0016-7037
Abstract
Mineral-solution interface reactions control the mobility and fate of trace metals (e.g., Zn) and may drive their associated isotopic fractionation in the natural environment, but understanding the coupling between interfacial reactions and isotopic behavior requires a molecular-level understanding of these processes. In this study, we investigate Zn stable isotope fractionation during adsorption to todorokite as a function of reaction time, pH and Zn concentrations. We show that Zn stable isotope fractionation reaches equilibrium at ∼12 h reaction time at pH 6 and Zn concentrations of 0.05 mM. For Zn concentrations of 0.05 and 0.2 mM, Zn isotopic fractionation between adsorbed and aqueous Zn (Δ66Znadsorbed-aqueous) is approximately −0.1 ± 0.04‰ at pH 3–5, and Δ66Znadsorbed-aqueous gradually increases from −0.1 ± 0.04‰ to 0.05 ± 0.05‰ at higher pH 6–8. Extended X-ray absorption fine structure (EXAFS) spectroscopy shows that Zn adsorbs to the todorokite surface as an outer-sphere octahedral complex with an average Zn–O interatomic distance of 2.06 Å at pH 3. In contrast, Zn is predominantly present as a tetrahedral coordinated structure with a shorter average Zn–O interatomic distance of ∼2.00–2.05 Å at pH 6 and 8, suggesting the presence of a mixture of octahedral outer-sphere and tetrahedral inner-sphere surface complexes. Density functional theory calculations suggest that outer-sphere surface complexes exist in the center of the structural tunnels of todorokite, yielding a theoretical Zn isotopic fractionation (Δ66Znadsorbed-aqueous) of −0.2‰ to −0.3‰, whereas the tetrahedral inner-sphere surface complex results in a large Δ66Znadsorbed-aqueous of +0.5‰ to +0.8‰. Combined laboratory experiments and theoretical calculations demonstrate that different magnitudes of Zn isotopic fractionation are controlled by structural changes (e.g., coordination and bond distance) in the Zn surface complexes formed on todorokite relative to its aqueous form (i.e., aqua Zn(H2O)62+). These results provide important new constraints for understanding Zn isotope signatures in natural Mn-rich sediments and lead to a more complete understanding of Zn isotopes in the ocean.
Metadata
Item Type: | Article |
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Authors/Creators: |
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Copyright, Publisher and Additional Information: | © 2022 Elsevier Ltd. All rights reserved. This is an author produced version of an article published in Geochimica et Cosmochimica Acta. Uploaded in accordance with the publisher's self-archiving policy. |
Keywords: | Mn oxide; Zinc; Adsorption; Sorption; Stable isotope; Fractionation; Surface precipitation; EXAFS; DFT |
Dates: |
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Institution: | The University of Leeds |
Academic Units: | The University of Leeds > Faculty of Environment (Leeds) > School of Earth and Environment (Leeds) > Earth Surface Science Institute (ESSI) (Leeds) |
Funding Information: | Funder Grant number Royal Society IEC\NSFC\191423 |
Depositing User: | Symplectic Publications |
Date Deposited: | 31 May 2022 14:33 |
Last Modified: | 23 Apr 2023 00:13 |
Status: | Published |
Publisher: | Elsevier |
Identification Number: | 10.1016/j.gca.2022.04.016 |
Open Archives Initiative ID (OAI ID): | oai:eprints.whiterose.ac.uk:187500 |