Iron-oxide crystallinity increases during soil redox oscillations

An Inceptisol A-horizon from Hawaii was subjected to a series of reduction-oxidation cycles—14 d cycle length over a 56 d duration—across the “soil-Fe” [Fe(OH) 3.Fe 2+ (aq), log K o = 15.74] equilibrium in triplicate redox-stat reactors. Each reducing event simulated the flush of organic C and dimin...

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Bibliographic Details
Published in:Geochimica et cosmochimica acta 2006-01, Vol.70 (7), p.1710-1727
Main Authors: Thompson, Aaron, Chadwick, Oliver A., Rancourt, Denis G., Chorover, Jon
Format: Article
Language:English
Subjects:
Iron oxides
Minerals
Nanocomposites
Nanomaterials
Nanostructure
Oscillations
Short range order
Soil (material)
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Summary:An Inceptisol A-horizon from Hawaii was subjected to a series of reduction-oxidation cycles—14 d cycle length over a 56 d duration—across the “soil-Fe” [Fe(OH) 3.Fe 2+ (aq), log K o = 15.74] equilibrium in triplicate redox-stat reactors. Each reducing event simulated the flush of organic C and diminished O 2 that accompanies a rainfall-induced leaching of bioavailable reductants from the forest floor into mineral soil. The soil contained considerable amounts of short-range ordered (SRO) minerals (e.g., nano-goethite and allophane) and organic matter (11% org-C). Room temperature and cryogenic 57Fe Mössbauer spectroscopy showed that the iron-bearing minerals were dominated by nano- to micro-scale goethite, and that ferrihydrite was not present. Over the four full cycles, fluctuations in E h (from 200 to 700 mV) and pFe 2+ (from 2.5 to 5.5) were inversely correlated with those of pH (5.5 to 4). Here, we focus on the solubility dynamics of the framework elements (Si, Fe, Ti, and Al) that constitute 35% of the oxygen-free soil dry mass. Intra-cycle oscillations in dissolved (
ISSN:0016-7037
1872-9533
DOI:10.1016/j.gca.2005.12.005