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Proton Binding and Ion Exchange at the Akaganéite/Water Interface
Umeå University, Faculty of Science and Technology, Department of Chemistry.
Umeå University, Faculty of Science and Technology, Department of Chemistry.
2013 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 117, no 12, 6409-6419 p.Article in journal (Refereed) Published
Abstract [en]

Proton exchange in nanosized synthetic akaganéite particles suspended in aqueous media and in ionic strengths of 3–100 mM NaCl and NaClO4 was monitored by high precision potentiometry at 25 °C. Proton budgets in the pH 3–10 range pertain to simultaneously occurring surface complexation and bulk ionic exchange reactions. Surface complexation reactions involve proton binding to (hydr)oxo groups of the dominant crystallographic planes of the particles. These are responsible for the colloidal attributes of the akaganéite particles, as confirmed by electrophoretic mobility measurements. Bulk ionic exchange involves the codiffusion of protons and chloride ions through the tunnel structure of the hollandite-type akaganéite bulk. Chloride ions migrate to bulk complexation sites that are ideally defined by eight surrounding hydroxyl groups, ≡(OH)8. Protons are in turn considered to be bound to neighboring oxo groups, ≡O. Collectively, the complexes are referred as [≡(OH)8···Cl······HO≡]. A thermodynamic model accounting for these two processes was developed to predict the pH (3–9), ionic strength (3–100 mM), and ionic medium (NaCl, NaClO4) dependence of the potentiometric data. This model is supported by new zeta potential data pointing to an isoelectric point of 9.6–10.3 for pristine akaganéite particles and by Fourier transform infrared spectra showing the impact of pH and ionic medium on bulk proton-chloride loadings. Our proposed stoichiometry for a chloride-rich solid of β-FeOOH·(HCl)0.192 corresponds to a maximal occupancy of 75% for chloride ions in the [≡(OH)8···Cl······HO≡] bulk complexation sites. Samples equilibrated in pure aqueous solutions should have a composition of β-FeOOH·(HCl)0.151, corresponding to a 60% occupancy for chloride ions due to a partial exchange of HCl. Our model can be used to predict compositional changes in the akaganéite bulk and surfaces upon any variations in pH and ionic media considered in this work.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2013. Vol. 117, no 12, 6409-6419 p.
National Category
Chemical Sciences
URN: urn:nbn:se:umu:diva-68054DOI: 10.1021/jp3101046OAI: diva2:615638
Available from: 2013-04-11 Created: 2013-04-11 Last updated: 2014-01-29Bibliographically approved
In thesis
1. Charge Development at Iron Oxyhydroxide Surfaces: The Interplay between Surface Structure, Particle Morphology and Counterion Identity
Open this publication in new window or tab >>Charge Development at Iron Oxyhydroxide Surfaces: The Interplay between Surface Structure, Particle Morphology and Counterion Identity
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Iron (oxyhydr)oxide (FeOOH) minerals play important roles in various natural, technological and societal settings. The widespread abundance of these minerals has prompted numerous studies on their surface reactivity in aqueous media. Surface charge development, one that namely takes place through the adsorption of potential determining ions (p.d.i.; H+, OH-) and coadsorption of counterions (e.g. Cl-, ClO4-, Na+), is particularly interesting in this regard. Mineral surface charge development is determined by numerous factors related to the interplay of mineral surface structure, particle morphology and counterion identity.

In this thesis the interplay between these factors is resolved by monitoring charge development on submicron-sized synthetic iron oxyhydroxide particles of different structures and sizes in aqueous media with counteranions of contrasting charge-to-size ratio (i.e. NaCl, NaClO4). This work, which is summarized in an introductory chapter and detailed in five appendices, is focused on three types of synthetic lepidocrocite (ã- FeOOH) of different shapes and surface roughness, three types of goethite (á-FeOOH) of different levels of surface roughness, and finally akaganéite (â-FeOOH), a mineral representing unique ion exchange properties due to its hollandite-type structure. While charge development was chiefly monitored by high precisition potentiometric titrations, these efforts were supported by a range of techniques including electrolyte ion uptake by cryogenic X-ray photoelectron spectroscopy, particle imaging by (high resolution) transmission electron microscopy, porosity analysis by N2 adsorption/desorption, surface potential development by electrokinetics, as well as thermodynamic adsorption modeling.

These efforts showed that lepidocrocite particles of contrasting morphology and surface roughness acquired highly comparable pH and ionic strength p.d.i. loadings. Equilibriation times required to develop these loadings were however altered when particles became aggregated by aging.

Goethite particles of contrasting surface roughness also acquired incongruent p.d.i. loadings, which were predominantly explained by the different charge-neutralizing capabilities of these surfaces, some of which were related to pore size distributions controlling the entrance of ions of contrasting sizes. Such size exclusion effects were also noted for the case of akaganéite where its bulk 0.4×0.4 nm wide channels permitted chloride diffusion but blocked perchlorate. Charge development at goethite surfaces in binary mixtures of NaCl and NaClO4 solutions also showed that the larger size-to-charge ratio chloride ion exerted a strong effect on these results even when present as a minor species. Many of these aforementioned effects were also modeled using variable, counterion- and loading-specific, Stern layer capacitance values.

The findings summarized in this thesis are providing a better understanding of surface processes occurring at iron oxyhydroxide surfaces. They should impact our ability in designing uses of such particles, for example, effective sorption in aquatic media, as well as to understand how they behave in natural systems.

Place, publisher, year, edition, pages
Umeå: Umeå universitet, 2014. 90 p.
Iron oxyhydroxides, adsorption, mineral-water interface, electrolyte, surface roughness, pores
National Category
Inorganic Chemistry Physical Chemistry
urn:nbn:se:umu:diva-85195 (URN)978-91-7459-797-4 (ISBN)
Public defence
2014-02-20, KBC-huset, KB3B1, Umeå universitet, Umeå, 10:00 (English)
Available from: 2014-01-30 Created: 2014-01-29 Last updated: 2014-01-30Bibliographically approved

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Kozin, Philipp ABoily, Jean-François
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