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Atom exchange between aqueous Fe(II) and structural Fe in clay minerals

Lookup NU author(s): Dr Anke Neumann

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This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License (CC BY-NC 4.0).

Abstract

Due to their stability towards reductive dissolution, Fe-bearing clay minerals are viewed as a renewable source of Fe redox activity in diverse environments. Recent findings of interfacial electron transfer between aqueous Fe(II) and structural Fe in clay minerals and electron conduction in octahedral sheets of nontronite, however, raise the question whether Fe interaction with clay minerals is more dynamic than previously thought. Here, we use an enriched isotope tracer approach to simultaneously trace Fe atom movement from the aqueous phase to the solid (⁵⁷Fe) and from the solid into the aqueous phase (⁵⁶Fe). Over 6 months, we observed a significant decrease in aqueous ⁵⁷Fe isotope fraction, with a fast initial decrease which slowed after 3 days and stabilized after about 50 days. For the aqueous ⁵⁶Fe isotope fraction, we observed a similar but opposite trend, indicating that Fe atom movement had occurred in both directions: from the aqueous phase into the solid and from the solid into aqueous phase. We calculated that 5-20% of structural Fe in clay minerals NAu-1, NAu-2, and SWa-1 exchanged with aqueous Fe(II), which significantly exceeds the Fe atom layer exposed directly to solution. Calculations based on electron-hopping rates in nontronite suggest that the bulk conduction mechanism previously demonstrated for hematite¹ and suggested as an explanation for the significant Fe atom exchange observed in goethite² may be a plausible mechanism for Fe atom exchange in Fe-bearing clay minerals. Our finding of 5-20% Fe atom exchange in clay minerals indicates that we need to rethink how Fe mobility affects the macroscopic properties of Fe-bearing phyllosilicates and its role in Fe biogeochemical cycling, as well as its use in a variety of engineered applications, such as landfill liners and nuclear repositories.

Publication metadata

Author(s): Neumann A, Wu L, Li W, Beard BL, Johnson CM, Rosso KM, Frierdich AJ, Scherer MM

Publication type: Article

Publication status: Published

Journal: Environmental Science and Technology

Year: 2015

Volume: 49

Issue: 5

Pages: 2786-2795

Print publication date: 03/03/2015

Online publication date: 11/02/2015

Acceptance date: 20/01/2015

Date deposited: 11/02/2015

ISSN (print): 0013-936X

ISSN (electronic): 1520-5851

Publisher: American Chemical Society

URL: http://dx.doi.org/10.1021/es504984q

DOI: 10.1021/es504984q

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