Investigating the effect of ascorbate on the Fe(II)-catalyzed transformation of the poorly crystalline iron mineral ferrihydrite
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Ferrihydrite
Lepidocrocite
Oxidizing agent
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Abiotic Fe(II) oxidation by O2 commonly occurs in the presence of mineral sorbents and organic matter (OM) in soils and sediments; however, this tertiary system has rarely been studied. Therefore, we examined the impacts of mineral surfaces (goethite and γ-Al2O3) and organic matter [Suwannee River fulvic acid (SRFA)] on Fe(II) oxidation rates and the resulting Fe(III) (oxyhydr)oxides under 21 and 1% pO2 at pH 6. We tracked Fe dynamics by adding 57Fe(II) to 56Fe-labeled goethite and γ-Al2O3 and characterized the resulting solids using 57Fe Mössbauer spectroscopy. We found Fe(II) oxidation was slower at low pO2 and resulted in higher-crystallinity Fe(III) phases. Relative to oxidation of Fe(II)(aq) alone, both goethite and γ-Al2O3 surfaces increased Fe(II) oxidation rates regardless of pO2 levels, with goethite being the stronger catalyst. Goethite surfaces promoted the formation of crystalline goethite, while γ-Al2O3 favored nano/small particle or disordered goethite and some lepidocrocite; oxidation of Fe(II)aq alone favored lepidocrocite. SRFA reduced oxidation rates in all treatments except the mineral-free systems at 21% pO2, and SRFA decreased Fe(III) phase crystallinity, facilitating low-crystalline ferrihydrite in the absence of mineral sorbents, low-crystalline lepidocrocite in the presence of γ-Al2O3, but either crystalline goethite or ferrihydrite when goethite was present. This work highlights that the oxidation rate, the types of mineral surfaces, and OM control Fe(III) precipitate composition.
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Abstract Interactions between mugineic acid (MA) purified from root washings of Fe‐deficient barley ( Hordeum vulgare L., cv. Minorimugi) and synthetically produced Fe oxides (goethite, hematite, lepidocrocite, and ferrihydrite) were studied in the equilibrium pH range of 3 to 11. The amount of MA adsorbed on Fe oxides was related to their specific surface area and followed the order: ferrihydrite > > goethite ≥ lepidocrocite ≥ hematite. The adsorption of MA on Fe oxides also decreased with increasing equilibrium pH and increased with increasing MA concentration. The MAFe complexes were also adsorbed on Fe oxides, especially on ferrihydrite and goethite, at pH 3 to 7. At pH > 10, however, MAFe complexes were decomposed to MA and Fe(OH) 3 colloids. The amount of Fe dissolved from Fe oxides by MA was in the following order: ferrihydrite > > lepidocrocite ≥ hematite = goethite. The Fe dissolution from Fe oxides by MA was related to their crystallinity and the maximum amount of Fe dissolved by MA was in the pH range of 7 to 8. The amounts of MA adsorbed on Fe oxides and Fe dissolved by MA from Fe oxides depended on MA concentration, pH, and the type and the amount of Fe oxides added in the system. The Fe dissolution processes from Fe oxides by MA could involve two factors, namely (i) the complexation of MA with Fe exposed on the surface of Fe oxides by ligand exchange; and (ii) the release of MAFe complexes from adsorption sites on Fe oxides by nucleophilic substitution. Our data further clarify the chemistry of Fe nutrition of graminaceous plants in Fe‐deficiency‐causing soils.
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Aqueous Fe(II) is known to catalyze the abiotic transformation of ferrihydrite to more stable Fe minerals. However, little is known about the impacts of coprecipitated OM on Fe(II)-catalyzed ferrihydrite transformation and its consequences for C dynamics. Accordingly, we investigated the extent and pathway of Fe(II)-induced transformation of OM-ferrihydrite coprecipitates as a function of C/Fe ratios and aqueous Fe(II) concentrations, and its implications for subsequent C dynamics. The coprecipitated OM resulted in a linear decrease in ferrihydrite transformation with increasing C/Fe ratios. The secondary mineral profiles upon Fe(II) reaction with OM-ferrihydrite coprecipitates depend on Fe(II) concentrations At 0.2 mM Fe(II), OM completely inhibited goethite formation and stimulated lepidocrocite formation. At 2 mM Fe(II), whereas goethite was formed in the presence of OM, OM reduced the amount of goethite and magnetite formation and increased the formation of lepidocrocite. The solid-phase C content remained unchanged after reaction, suggesting that OM remains associated with Fe minerals following ferrihydrite transformation to more stable Fe minerals. However, C desorbability by H2PO4– from the resulting Fe minerals following reaction was enhanced. The study indicates a "lepidocrocite favoring effect" by OM and suggests that Fe(II)-catalyzed transformation of ferrihydrite may decrease OM stability in natural environments under moderately reducing conditions.
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Aqueous ferrous iron (Fe(II)) accelerates the transformation of ferrihydrite into secondary, more crystalline minerals however the factors controlling the rate and, indeed, the underlying mechanism of this transformation process remain unclear. Here, we present the first detailed study of the kinetics of the Fe(II)-accelerated transformation of ferrihydrite to goethite, via lepidocrocite, for a range of pH and Fe(II) concentrations and, from the results obtained, provide insight into the factors controlling the transformation rate and the processes responsible for transformation. A reaction scheme for the Fe(II)-accelerated secondary mineralization of ferrihydrite is developed in which an Fe(II) atom attaches to the ferrihydrite surface where it is immediately oxidized to Fe(III) with the resultant electron transferred, sequentially, to other iron oxyhydroxide Fe(III) atoms before release to solution as Fe(II). This freshly precipitated Fe(III) forms the nuclei for the formation of secondary minerals and also facilitates the ongoing uptake of Fe(II) from solution by creation of fresh surface sites. The concentration of solid-associated Fe(II) and the rate of transport of Fe(II) to the oxyhydroxide surface appear to determine which particular secondary minerals form and their rates of formation. Lepidocrocite growth is enhanced at lower solid-associated Fe(II) concentrations while conditions leading to more rapid uptake of Fe(II) from solution lead to higher goethite growth rates.
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Akaganéite
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Abstract Iron oxide coatings from ped surfaces and pores of three rice paddy soils and one non‐paddy soil near Beaumont, TX, provided samples for studying Fe oxide mineralogy and the relationship of extractable Fe to extractable Al, Si, and P. Analyses of Fe oxide coatings, concentrated by a combination of scraping, sonication, centrifugation, and magnetic separation, showed that these Fe oxide coatings cement clay minerals and quartz particles together. Iron oxide coatings in all soils contained lepidocrocite and smaller amounts of goethite. An improved differential x‐ray diffraction (DXRD) method, in situ DXRD on a nonreflecting quartz plate, allowed the identification of ferrihydrite, which is otherwise difficult to identify because of its poor crystallinity and low concentration. Most of the ferrihydrite was dissolved by 10 min in pH 3 ammonium oxalate in the dark (AOD). Sequential 10‐, 50‐, and 180‐min AOD treatments extracted lepidocrocite of progressively increasing particle size. The 180‐min AOD treatment extracted minimal amounts of goethite. The DXRD and chemical data indicate that the paddy soils have more ferrihydrite in relation to total Fe oxides than the non‐paddy soil. The P/Fe ratio decreased with each AOD treatment step for all soils. This study demonstrates that physical separation and in situ AOD treatment can be used with the DXRD method to identify ferrihydrite, lepidocrocite, and goethite in soil clays containing extractable Fe concentrations as low as 10 to 44 g/kg. We concluded that a short incremental AOD treatment (≈10–30 min) is a better approach to DXRD of ferrihydrite in <2‐µm fractions of seasonally reduced soils than the much longer treatment recommended in the past for soils.
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