Sherman, DM and Peacock, CL orcid.org/0000-0003-3754-9294 (2010) Surface complexation of Cu on birnessite (d-MnO2): Controls on Cu in the deep ocean. Geochimica et Cosmochimica Acta, 74 (23). pp. 6721-6730. ISSN 0016-7037
Abstract
Hexagonal birnessite (d-MnO2) is a close analogue to the dominant phase in hydrogenetic marine ferromanganese crusts and nodules. These deposits contain 0.25 wt.% Cu which is believed to be scavenged from the overlying water column where Cu concentrations are near 0.1 lg/L. Here, we measured the sorption of Cu on d-MnO2 as a function of pH and surface loading. We characterized the nature of the Cu sorption complex at pH 4 and 8 using EXAFS spectroscopy and find that, at pH 4, Cu sorbs to birnessite by inner-sphere complexation on the {0 0 1} surface at sites above Mn vacancies to give a three to four fold coordinated complex with 6 Mn neighbors at 3.4 A ̊ . At pH 8, however, we find that some Cu has become structurally incorporated into the MnO2 layer by occupying the vacancy sites to give 6 Mn neighbors at 2.91 A ̊ . Density functional calculations on CuMn O ðOHÞ ðH OÞ 4 and CuMn O ðOHÞ ðH OÞ 1 clusters predict a threefold coordinated surface 18 24 30 2 3 18 21 33 2 3 complex and show that the change from surface complexation to structural incorporation is a response to protonation of oxy- gens surrounding the vacancy site. Consequently, we propose that the transformation between sorption via surface complex and vacancy site occupancy should be reversible. By fitting the Cu sorption as a function of surface loading and pH to the formation of the observed and predicted surface complex, we developed a surface complexation model (in the basic Stern approximation) for the sorption of Cu onto birnessite. Using this model, we demonstrate that the concentration of inorganic Cu in the deep ocean should be several orders of magnitude lower than the observed total dissolved Cu. We propose that the observed total dissolved Cu concentration in the oceans reflects solubilization of Cu by microbially generated ligands.
Metadata
Item Type: | Article |
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Authors/Creators: |
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Copyright, Publisher and Additional Information: | Copyright © 2010 Elsevier Ltd. All rights reserved. This is an author produced version of a paper published in Geochimica et Cosmochimica Acta. Uploaded in accordance with the publisher's self-archiving policy. |
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) |
Depositing User: | Symplectic Publications |
Date Deposited: | 11 Jul 2019 10:27 |
Last Modified: | 11 Jul 2019 10:35 |
Status: | Published |
Publisher: | Elsevier |
Identification Number: | 10.1016/j.gca.2010.08.042 |
Open Archives Initiative ID (OAI ID): | oai:eprints.whiterose.ac.uk:85778 |