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Particle Morphological and Roughness Controls on Mineral Surface Charge Development
Umeå University, Faculty of Science and Technology, Department of Chemistry.
Umeå University, Faculty of Science and Technology, Department of Chemistry.
2014 (English)In: Geochimica et Cosmochimica Acta, ISSN 0016-7037, E-ISSN 0016-1258, Vol. 141, no 15 September, 567-578 p.Article in journal (Other academic) Published
Abstract [en]

Effects of mineral particle morphology and roughness on potential determining ion (p.d.i.; H+, OH) loadings achieved at synthetic lepidocrocite (γ-FeOOH) surfaces were predominantly investigated by potentiometry and thermodynamic modeling. Nanosized rod- (RL) and lath-shaped (LL) particles exhibiting different proportions of the same predominant crystallographic faces acquired largely comparable pH, ionic strength and counterion (NaCl, NaClO4) dependencies on p.d.i. loadings. These results supported previous claims that faces ideally containing proton silent sites only, are likely populated by additional proton active sites. This concept was supported further by results of roughened LL-like particles (LLR) also showing highly congruent pH-, ionic strength- and composition-dependent p.d.i. loadings with those of LL and RL. These loadings thereby correspond to maximal levels allowed by net attractive and repulsive forces at each solution composition, irrespective of particle morphology. Contrasting equilibration times required to achieve these loadings revealed considerably slower exchange of p.d.i. and electrolyte ions near the point of zero charge in the rough LLR than in the more idealized LL and RL particles.

Thermodynamic modeling was used to test various concepts accounting for these results. The model made use of a novel framework capable of isolating electrostatic contributions from different faces, and of accounting for ion-specific double-layer properties within a single crystallographic face. These efforts made use of capacitance values for each electrolyte ions within the framework of a recently developed Variable Capacitance Model. Attempts at modeling all three particle types were used to suggest that the (0 1 0) face contains ∼0.9 site nm−2 of proton active sites, a value notably constrained by recently published Na+, Cl, and ClO4 loadings derived by cryogenic X-ray photoelectron spectroscopy. The model presented in this work thus provides a means to predict p.d.i. loadings on multifaceted mineral particle surfaces, and can therefore be used to constrain further our understanding of mineral/water interface reactivity.

Place, publisher, year, edition, pages
Elsevier, 2014. Vol. 141, no 15 September, 567-578 p.
National Category
Inorganic Chemistry
URN: urn:nbn:se:umu:diva-85193DOI: 10.1016/j.gca.2014.06.016ISI: 000341926100034OAI: diva2:692046
Swedish Research Council, VR 2012-2976
Available from: 2014-01-29 Created: 2014-01-29 Last updated: 2014-10-29Bibliographically 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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Boily, Jean-FrancoisKozin, Philipp A.
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