Influence of Fe2+-catalysed iron oxide recrystallization on metal cycling

Abstract Recent work has indicated that iron (oxyhydr-)oxides are capable of structurally incorporating and releasing metals and nutrients as a result of Fe 2 + -induced iron oxide recrystallization. In the present paper, we briefly review the current literature examining the mechanisms by which iron oxides recrystallize and summarize how recrystallization affects metal incorporation and release. We also provide new experimental evidence for the Fe 2 + -induced release of structural manganese from manganese-doped goethite. Currently, the exact mechanism(s) for Fe 2 + -induced recrystallization remain elusive, although they are likely to be both oxideand metal-dependent. We conclude by discussing some future research directions for Fe 2 + -catalysed iron oxide recrystallization. Metal incorporation in iron oxides Natural iron (oxyhydr-)oxides are rarely pure. Instead, they often contain structural trace metal impurities (e.g. Key words: goethite, haematite, iron oxide, magnetite, metal cycling, recrystallization. Abbreviations used: XAS, X-ray absorption spectroscopy. 1 To whom correspondence should be addressed (email [email protected]). In the present paper, we provide a brief overview of the evidence and possible mechanisms of Fe 2 + -catalysed iron oxide recrystallization in the absence of secondary transformations and summarize recent findings on metal incorporation and/or release during recrystallization. We also present some new results demonstrating Mn 2 + release from goethite in the presence of aqueous Fe 2 + and provide some closing remarks on future research directions for Fe 2 + -catalysed iron oxide recrystallization. Fe 2 + -catalysed iron oxide recrystallization of goethite, haematite and magnetite There were some clear early indications in the literature that the reaction of aqueous Fe 2 + with the more stable iron oxides, such as goethite, haematite and magnetite, was more dynamic than a simple adsorption reaction. For example, Tronc et al. [25

Influence of time and ageing conditions on the properties of ferrihydrite

Natural conversion of ferrihydrite (Fh), a widespread Fe(iii)-oxyhydroxide mineral at the Earth's surface, to thermodynamically more stable iron oxides such as goethite (Gt) and hematite (Hm) is a slow process that spans months to years. Here we examined the effects of synthesis and storage conditions on the hydration, the ratio of tetrahedral to octahedral iron sites, and the transformation of naturally aged 2-line Fh at room temperature and mildly acidic pH over an ageing period of 5 years. Fh samples synthesized and aged in either aerobic or anaerobic conditions were characterized over time by XRD, SEM, thermogravimetric analysis - mass spectroscopy (TGA-MS), and X-ray absorption spectroscopies (XANES and XMCD). The findings show that the ratio of tetrahedral to octahedral Fe(iii) sites in Fh is correlated to its extent of hydration, with fresher Fh samples exhibiting a higher ratio and more bound water. Fresh Fh aged in aerobic conditions has similar bound inorganic carbon, is more hydrated, and has less tetrahedral Fe(iii) than that aged in anaerobic conditions. Hence, for relatively fresh Fh there is a link between Fh properties and storage conditions. However, the long-term ageing characteristics, such as the transformation rate and relative phase fraction of Gt and Hm products, are not noticeably impacted by storage conditions. TGA-MS measurements coupled with O K-edge XANES spectra confirm that Fh tends to lose its hydration as it ages, as expected. Corresponding Fe L2,3-edge XMCD spectra reveal that this dehydration is coupled to a steady decrease in the ratio of tetrahedral to octahedral Fe(iii) sites. In addition to the obvious constraints these findings place on making comparisons across Fh samples of different age and environmental settings, they also highlight that Fh structure, and consequently magnetism, are linked to its bound water content

Prenormative verification and validation of a protocol for measuring magnetite–maghemite ratios in magnetic nanoparticles

An important step in establishing any new metrological method is a prenormative interlaboratory study, designed to verify and validate the method against its stated aims. Here, the 57Fe Mössbauer spectrometric ‘centre of gravity’ (COG) method was tested as a means of quantifying the magnetite/maghemite (Fe3O4/γ-Fe2O3) composition ratio in biphasic magnetic nanoparticles. The study involved seven laboratories across Europe and North and South America, and six samples—a verification set of three microcrystalline mixtures of known composition, and a validation set of three nanoparticle samples of unknown composition. The spectra were analysed by each participant using in-house fitting packages, and ex post facto by a single operator using an independent package. Repeatability analysis was performed using Mandel’s h statistic and modified Youden plots. It is shown that almost all (83/84) of the Mandel h statistic values fall within the 0.5% significance level, with the one exception being borderline. Youden-based pairwise analysis indicates the dominance of random uncertainties; and in almost all cases the data analysis phase is only a minor contributor to the overall measurement uncertainty. It is concluded that the COG method is a robust and promising candidate for its intended purpose

Stable U(IV) Complexes Form at High-Affinity Mineral Surface Sites

Uranium (U) poses a significant contamination hazard to soils, sediments, and groundwater due to its extensive use for energy production. Despite advances in modeling the risks of this toxic and radioactive element, lack of information about the mechanisms controlling U transport hinders further improvements, particularly in reducing environments where UIV predominates. Here we establish that mineral surfaces can stabilize the majority of U as adsorbed UIV species following reduction of UVI. Using X-ray absorption spectroscopy and electron imaging analysis, we find that at low surface loading, UIV forms inner-sphere complexes with two metal oxides, TiO2 (rutile) and Fe3O4 (magnetite) (at <1.3 U nm–2 and <0.037 U nm–2, respectively). The uraninite (UO2) form of UIV predominates only at higher surface loading. UIV–TiO2 complexes remain stable for at least 12 months, and UIV–Fe3O4 complexes remain stable for at least 4 months, under anoxic conditions. Adsorbed UIV results from UVI reduction by FeII or by the reduced electron shuttle AH2QDS, suggesting that both abiotic and biotic reduction pathways can produce stable UIV–mineral complexes in the subsurface. The observed control of high-affinity mineral surface sites on UIV speciation helps explain the presence of nonuraninite UIV in sediments and has important implications for U transport modeling

Fe Electron Transfer and Atom Exchange in Goethite: Influence of Al-Substitution and Anion Sorption

The reaction of Fe­(II) with Fe­(III) oxides and hydroxides is complex and includes sorption of Fe­(II) to the oxide, electron transfer between sorbed Fe­(II) and structural Fe­(III), reductive dissolution coupled to Fe atom exchange, and, in some cases mineral phase transformation. Much of the work investigating electron transfer and atom exchange between aqueous Fe­(II) and Fe­(III) oxides has been done under relatively simple aqueous conditions in organic buffers to control pH and background electrolytes to control ionic strength. Here, we investigate whether electron transfer is influenced by cation substitution of Al­(III) in goethite and the presence of anions such as phosphate, carbonate, silicate, and natural organic matter. Results from ⁵⁷Fe Mössbauer spectroscopy indicate that both Al-substitution (up to 9%) and the presence of common anions (PO₄^3‑, CO₃^2‑, SiO₄^4‑, and humic acid) does not inhibit electron transfer between aqueous Fe­(II) and Fe­(III) in goethite under the conditions we studied. In contrast, sorption of a long-chain phospholipid completely shuts down electron transfer. Using an enriched isotope tracer method, we found that Al-substitution in goethite (10%), does, however, significantly decrease the extent of atom exchange between Fe­(II) and goethite (from 43 to 12%) over a month’s time. Phosphate, somewhat surprisingly, appears to have little effect on the rate and extent of atom exchange between aqueous Fe­(II) and goethite. Our results show that electron transfer between aqueous Fe­(II) and solid Fe­(III) in goethite can occur under wide range of geochemical conditions, but that the extent of redox-driven Fe atom exchange may be dependent on the presence of substituting cations such as Al

Stable U(IV) Complexes Form at High-Affinity Mineral Surface Sites

Uranium (U) poses a significant contamination hazard to soils, sediments, and groundwater due to its extensive use for energy production. Despite advances in modeling the risks of this toxic and radioactive element, lack of information about the mechanisms controlling U transport hinders further improvements, particularly in reducing environments where U^IV predominates. Here we establish that mineral surfaces can stabilize the majority of U as adsorbed U^IV species following reduction of U^VI. Using X-ray absorption spectroscopy and electron imaging analysis, we find that at low surface loading, U^IV forms inner-sphere complexes with two metal oxides, TiO₂ (rutile) and Fe₃O₄ (magnetite) (at <1.3 U nm^–2 and <0.037 U nm^–2, respectively). The uraninite (UO₂) form of U^IV predominates only at higher surface loading. U^IV–TiO₂ complexes remain stable for at least 12 months, and U^IV–Fe₃O₄ complexes remain stable for at least 4 months, under anoxic conditions. Adsorbed U^IV results from U^VI reduction by Fe^II or by the reduced electron shuttle AH₂QDS, suggesting that both abiotic and biotic reduction pathways can produce stable U^IV–mineral complexes in the subsurface. The observed control of high-affinity mineral surface sites on U^IV speciation helps explain the presence of nonuraninite U^IV in sediments and has important implications for U transport modeling