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Water film-driven Mn (oxy)(hydr)oxide nanocoating growth on rhodochrosite
Umeå University, Faculty of Science and Technology, Department of Chemistry.ORCID iD: 0000-0002-0118-8207
Pacific Northwest National Laboratory, WA, Richland, United States.
Umeå University, Faculty of Science and Technology, Department of Chemistry.ORCID iD: 0000-0002-4766-2672
Umeå University, Faculty of Science and Technology, Department of Chemistry.ORCID iD: 0000-0003-4954-6461
2022 (English)In: Geochimica et Cosmochimica Acta, ISSN 0016-7037, E-ISSN 1872-9533, Vol. 329, p. 87-105Article in journal (Refereed) Published
Abstract [en]

Minerals exposed to moist air stabilize thin water films that drive a score of chemical reactions of great importance to water-unsaturated terrestrial environments. In this study, we identified Mn (oxy)(hydr)oxide nanocoatings formed by the dissolution, oxidation and precipitation of Mn in oxygenated water films grown on rhodochrosite (MnCO3) microparticles. Nanocoatings that could be identified by vibrational spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and (scanning and transmission) electron microscopy formed in water films containing the equivalent of at least 7 monolayers (∼84 H2O/nm2). These films were formed by exposing microparticles to moist air with at least 50% relative humidity (RH). Films of neutral pH reacted up to 14% of the MnII located in the topmost ∼5 nm region of the microparticles in atmospheres of up to 90% RH for 7 d. These reactions produced MnOOH, birnessite (MnO2) and hausmannite (Mn3O4) nanoparticles of low crystallinity, while exposure to atmospheric air for 1 yr. converted only 2% of MnII in this region to MnOOH. In contrast, reactions in alkaline water films converted up to ∼75% of the MnII but only after 16 d of reaction. These films produced MnOOH and MnO2 of low crystallinity, as well as crystalline hausmannite. Kinetic modeling of the time-resolved growth of the Mn[sbnd]O stretching vibrational bands of these nanocoatings revealed two concurrent reaction processes. A 1rst-order process was assigned to nucleation events terminating only after a few hours, and a 0-order process was assigned to the sustained growth of nanocoatings from these nuclei over longer reaction time. By identifying nanocoatings formed by water film-driven reactions on rhodochrosite, our study adds new insight into mineralogical transformations relevant to anoxic–oxic boundaries in water-unsaturated environments.

Place, publisher, year, edition, pages
Elsevier, 2022. Vol. 329, p. 87-105
Keywords [en]
Oxidation, Precipitation, Rhodochrosite, Water films
National Category
Inorganic Chemistry
Identifiers
URN: urn:nbn:se:umu:diva-203210DOI: 10.1016/j.gca.2022.05.019ISI: 000818523300006Scopus ID: 2-s2.0-85132406337OAI: oai:DiVA.org:umu-203210DiVA, id: diva2:1728458
Part of project
Chemistry within the confines of mineral-bound thin water films, Swedish Research Council
Funder
Swedish Research Council, 2020-04853Swedish Research Council, 2016-03808Umeå UniversityAvailable from: 2023-01-18 Created: 2023-01-18 Last updated: 2023-09-04Bibliographically approved
In thesis
1. Water film-mediated mineralogical transformations and photocatalytic reactions
Open this publication in new window or tab >>Water film-mediated mineralogical transformations and photocatalytic reactions
2023 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Mineral particles capture water vapor in the atmosphere in the form of water films that are only few monolayers thick. Water films form nanoscale hydration environments that mediate a wide range of important reactions in nature and technology. This thesis explored two important phenomena that commonly occur within the confines of water films: mineralogical transformations (Topic 1) and photocatalytic decomposition of organics (Topic 2). These transformations were chiefly identified by vibrational spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy and (Transmission and Scanning) electron microscopy. Interpretations of reaction mechanisms were partially supported by chemometrics, kinetic and thermodynamic modeling, as well as molecular simulations.

Mineralogical transformations (Topic 1) resolved in this thesis involved the hydroxylation (Papers I, II) and carbonation (Paper III) of periclase (MgO), and the oxidation of rhodochrosite (MnCO3) (Paper IV). Two types of MgO nanocubes with contrasting physical properties were used to resolve nucleation- and diffusion-limited hydroxylation reactions to brucite and carbonation reactions to amorphous magnesium carbonate (AMC). While nucleation-limited reactions completely transformed (8 nm) small and aggregated MgO nanocubes to brucite, the reactions became diffusion-limited in larger (32 nm) monodispersed MgO nanocubes because of brucite surface nanocoatings (Paper I). Additionally, brucite nanosheets grew under (GPa-level) crystallization pressures because of the important volumetric expansion of the reaction, which took place in a complex network of microporosity between the small and within the larger MgO nanocubes. Brucite stacking mechanisms, explored in Paper II, focused on the early stages of MgO-water interaction in water films of different thicknesses. These were suggested to involve the stacking and (epitaxial-like) growth of precursor Mg(OH)2 nanosheets in water films. Carbonation reactions explored in Paper III completely hampered hydroxylation reactions studied in Papers I and II, and produced AMC nanocoatings grown over an unreacted MgO core. Finally, oxidation-driven reactions involving rhodochrosite in Paper IV produced MnO2, Mn3O4, and MnOOH nanocoatings with growth rates being scaled with water loadings.

Photocatalytic decomposition reactions of organics (Topic 2) were focused on the case of oxalate bound to TiO2 nanoparticles (Paper V). Photodecomposition rates scaled with humidity in oxygenated water films, and were explained by the combination of hole transfer (HT), ligand-to-metal charge transfer (LMCT), and the formation of hydroxyl radicals and reactive oxygen species. Decreasing rates in oxygen-free water films were, on the other hand, explained by water-driven charge localization, which eventually limited radical production and charge transfers via HT and LMCT. The reactions involved limited HT and LMCT processes which also competed with a charge recombination process across all humidity ranges.

This thesis provides new insight into two key types of transformations mediated by water films on minerals. This knowledge can be used to understand the reactivity of mineral (nano)particles exposed to variations in atmospheric humidity and oxygen content, which are both highly relevant to a wide range of settings in nature and technology. It can also advance new ideas in the study of mineral growth, especially within the confines of nanometer-thick water films.

Place, publisher, year, edition, pages
Umeå: Umeå University, 2023. p. 70
Keywords
mineral, water films, carbon dioxide, MgO, MnCO3, TiO2, hydroxylation, carbonation, oxidation, transformation, photocatalysis
National Category
Materials Chemistry Inorganic Chemistry Physical Chemistry Geochemistry
Research subject
Inorganic Chemistry; nanomaterials; Physical Chemistry
Identifiers
urn:nbn:se:umu:diva-213817 (URN)9789180701501 (ISBN)9789180701518 (ISBN)
Public defence
2023-09-29, Lilla hörsalen, KB.E3.01, KBC building, Linnaeus väg 10, Umeå, 09:00 (English)
Opponent
Supervisors
Funder
Swedish Research Council, 2016-03808Swedish Research Council, 2020-05853Swedish Research Council Formas, 2022-01246
Available from: 2023-09-08 Created: 2023-09-01 Last updated: 2023-09-04Bibliographically approved

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Luong, N. TanShchukarev, AndreyBoily, Jean-Francois

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