Tracking Initial Fe(II)-Driven Ferrihydrite Transformations: A Mössbauer Spectroscopy and Isotope Investigation

Transformation of nanocrystalline ferrihydrite to more stable microcrystalline Fe(III) oxides is rapidly accelerated under reducing conditions with aqueous Fe(II) present. While the major steps of Fe(II)-catalyzed ferrihydrite transformation are known, processes in the initial phase that lead to nucleation and the growth of product minerals remain unclear. To track ferrihydrite-Fe(II) interactions during this initial phase, we used Fe isotopes, Mössbauer spectroscopy, and extractions to monitor the structural, magnetic, and isotope composition changes of ferrihydrite within ∼30 min of Fe(II) exposure. We observed rapid isotope mixing between aqueous Fe(II) and ferrihydrite during this initial lag phase. Our findings from Mössbauer spectroscopy indicate that a more magnetically ordered Fe(III) phase initially forms that is distinct from ferrihydrite and bulk crystalline transformation products. The signature of this phase is consistent with the early stage emergence of lepidocrocite-like lamellae observed in previous transmission electron microscopy studies. Its signature is furthermore removed by xylenol extraction of Fe(III), the same approach used to identify a chemically labile form of Fe(III) resulting from Fe(II) contact that is correlated to the ultimate emergence of crystalline product phases detectable by X-ray diffraction. Our work indicates that the mineralogical changes in the initial lag phase of Fh transformation initiated by Fe(II)-Fh electron transfer are critical to understanding ferrihydrite behavior in soils and sediments, particularly with regard to metal uptake and release.

Electron Transfer, Atom Exchange, and Transformation of Iron Minerals in Soils: The Influence of Soil Organic Matter

Despite substantial experimental evidence of electron transfer, atom exchange, and mineralogical transformation during the reaction of Fe(II)_aq with synthetic Fe(III) minerals, these processes are rarely investigated in natural soils. Here, we used an enriched Fe isotope approach and Mössbauer spectroscopy to evaluate how soil organic matter (OM) influences Fe(II)/Fe(III) electron transfer and atom exchange in surface soils collected from Luquillo and Calhoun Experimental Forests and how this reaction might affect Fe mineral composition. Following the reaction of ⁵⁷Fe-enriched Fe(II)_aq with soils for 33 days, Mössbauer spectra demonstrated marked electron transfer between sorbed Fe(II) and the underlying Fe(III) oxides in soils. Comparing the untreated and OM-removed soils indicates that soil OM largely attenuated Fe(II)/Fe(III) electron transfer in goethite, whereas electron transfer to ferrihydrite was unaffected. Soil OM also reduced the extent of Fe atom exchange. Following reaction with Fe(II)_aq for 33 days, no measurable mineralogical changes were found for the Calhoun soils enriched with high-crystallinity goethite, while Fe(II) did drive an increase in Fe oxide crystallinity in OM-removed LCZO soils having low-crystallinity ferrihydrite and goethite. However, the presence of soil OM largely inhibited Fe(II)-catalyzed increases in Fe mineral crystallinity in the LCZO soil. Fe atom exchange appears to be commonplace in soils exposed to anoxic conditions, but its resulting Fe(II)-induced recrystallization and mineral transformation depend strongly on soil OM content and the existing soil Fe phases.

Characterization of an active ingredient made of nanoscale iron(oxyhydr)oxide for the treatment of hyperphosphatemia

Kidney disease is one of the main non-communicable diseases. Every year millions of people worldwide die from kidney dysfunction. One cause is disturbances in the mineral metabolism, such as abnormally high phosphate concentrations in the blood, medically referred to as hyperphosphatemia. A new active ingredient based on nanoscale iron(oxyhydr)oxide with particle sizes below 3 nm surrounded by an organic coating has been developed for a more effective treatment. The examination of the structural properties of these particles within this study promises to gain further insights into this improved effectiveness. More than half of the active ingredient consists of organic substances, the rest is mostly iron(oxyhydr)oxide. Analyzes by transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS), and dynamic light scattering (DLS) show that the organic molecules act as stabilizers and lead to ultrasmall iron(oxyhydr)oxide cores with a size of 1.0–2.8 nm. The nanoparticles coated with the organic molecules have an average size of 11.7 nm. At 4.2 K, the nanoparticles display a magnetic hyperfine field of 45.5 T in the Mössbauer spectrum, which is unusually low for iron(oxyhydr)oxide. The material is also not ferrimagnetic. Combining these results and taking into account the composition of the nanoparticles, we identify low crystalline ferrihydrite as the most likely phase in the iron(oxyhydr)oxide nuclei. At the same time, we want to emphasize that a final identification of the crystal structure in iron(oxyhydr)oxides can be impeded by ultrasmall particle sizes. In summary, by a combinatorial characterization, we are able to observe extraordinary properties of the ultrasmall nanomaterial, which is the basis for the investigation of the high phosphate-binding efficacy of this active ingredient.

The combination of different analytical methods, supported by TEM, DLS, SAXS, Mössbauer spectroscopy, and SQUID, allows more accurate characterization of a new nanoscale active ingredient based on iron(oxyhydr)oxide against hyperphosphatemia.

Fe(II)-Catalyzed Transformation of Organic Matter-Ferrihydrite Coprecipitates: A Closer Look Using Fe Isotopes

Ferrihydrite is a common Fe mineral in soils and sediments that rapidly transforms to secondary minerals in the presence of Fe(II). Both the rate and products of Fe(II)-catalyzed ferrihydrite transformation have been shown to be significantly influenced by natural organic matter (NOM). Here, we used enriched Fe isotope experiments and ⁵⁷Fe Mössbauer spectroscopy to track the formation of secondary minerals, as well as electron transfer and Fe mixing between aqueous Fe(II) and ferrihydrite coprecipitated with several types of NOM. Ferrihydrite coprecipitated with humic acids transformed primarily to goethite after reaction with Fe(II). In contrast, ferrihydrite coprecipitated with fulvic acids and Suwannee River NOM (SRNOM) resulted in no measurable formation of secondary minerals. Despite no secondary mineral transformation, Mössbauer spectra indicated electron transfer still occurred between Fe(II) and ferrihydrite coprecipitated with fulvic acid and SRNOM. In addition, isotope tracer experiments revealed that a significant fraction of structural Fe in the ferrihydrite mixed with the aqueous phase Fe(II) (∼85%). After reaction with Fe(II), Mössbauer spectroscopy indicated some subtle changes in the crystallinity, particle size, or particle interactions in the coprecipitate. Our observations suggest that ferrihydrite coprecipitated with fulvic acid and SRNOM remains a highly dynamic phase even without ferrihydrite transformation.

Inhibition in binding between fullerene and bisporphyrin in presence of silver nanoparticles: a new physicochemical insight into fullerene-bisporphyrin complexation in solution

The present paper reports non-covalent interaction between fullerenes (C(60) and C(70)) and a designed bisporphyrin (1) in p-xylene in absence and presence of silver nanoparticles (AgNp) having diameter ∼5-15 nm. Both UV-vis and fluorescence experiments reveal reduction in the average binding constant (K(av)) value for C(60)-1 (K(C60-)(1)=27,980 dm(3)mol(-1)) and C(70)-1 systems (K(C70-)(1)=115,020 dm(3)mol(-1)) in presence of AgNp, i.e., K(C60-)(1)(-AgNp)=22,465 dm(3)mol(-1) and K(C70-)(1)(-AgNp)=30,830 dm(3)mol(-1). Dynamic light scattering experiment gives clear indication regarding formation of larger aggregates as a result of electrostatic attraction between porphyrin-based supramolecule and AgNp in solution. Scanning electron microscope measurement reveals formation of molecular cluster comprising C(70)-1-AgNp composite.

Ordered ferrimagnetic form of ferrihydrite reveals links among structure, composition, and magnetism

The natural nanomineral ferrihydrite is an important component of many environmental and soil systems and has been implicated as the inorganic core of ferritin in biological systems. Knowledge of its basic structure, composition, and extent of structural disorder is essential for understanding its reactivity, stability, and magnetic behavior, as well as changes in these properties during aging. Here we investigate compositional, structural, and magnetic changes that occur upon aging of "2-line" ferrihydrite in the presence of adsorbed citrate at elevated temperature. Whereas aging under these conditions ultimately results in the formation of hematite, analysis of the atomic pair distribution function and complementary physicochemical and magnetic data indicate formation of an intermediate ferrihydrite phase of larger particle size with few defects, more structural relaxation and electron spin ordering, and pronounced ferrimagnetism relative to its disordered ferrihydrite precursor. Our results represent an important conceptual advance in understanding the nature of structural disorder in ferrihydrite and its relation to the magnetic structure and also serve to validate a controversial, recently proposed structural model for this phase. In addition, the pathway we identify for forming ferrimagnetic ferrihydrite potentially explains the magnetic enhancement that typically precedes formation of hematite in aerobic soil and weathering environments. Such magnetic enhancement has been attributed to the formation of poorly understood, nano-sized ferrimagnets from a ferrihydrite precursor. Whereas elevated temperatures drive the transformation on timescales feasible for laboratory studies, our results also suggest that ferrimagnetic ferrihydrite could form naturally at ambient temperature given sufficient time.

Effects of magnetic interactions in antiferromagnetic ferrihydrite particles

The effects of magnetic interactions in the magnetic properties of six-line ferrihydrite particles were investigated by studying the behavior of aggregated versus coated particles. Four different coating agents (sugar, alginate, lactate and ascorbate) were employed in order to obtain dispersed particles and prevent particle agglomeration; one sub-sample was allowed to dry with no coating agent. The five sets of ferrihydrite particles were from the same batch and the size was estimated as 3.6 ± 0.4 nm in length. Low temperature magnetization, ac susceptibility and Mössbauer spectroscopy data showed contrasting blocking temperatures for uncoated and coated samples with a decrease of T(P) from about 50 K to 12 K, respectively. The contributions from magnetic interactions were recognized in magnetic measurements and the effective anisotropy constant for non-interacting ferrihydrite was estimated as (100 ± 10) × 10(3) J m(-3). Overall, employing sugar and alginate as coating agents was more successful in preventing particle aggregation and magnetic interactions. In contrast, ascorbate and lactate were unsuitable due to the chemical reaction between the coating agent and ferrihydrite surface.

Arsenic abundance and variation in golf course lakes

Monosodium methanearsonate (MSMA) is a commonly used herbicide on golf courses. To investigate the variation in abundance of arsenic (As) after MSMA application, 28 golf course lakes were monitored monthly for one year. The As concentrations varied substantially in and between individual lakes with values up to 124 microg/L (mean=10.9 microg/L, n=336). This is considerably higher than the As concentrations in comparable "non-golf course" lakes (up to 100-times) in the study area. The highest values of As in the lakes were generally observed in the late spring and early summer and corresponded to the intensity of MSMA applications. Arsenic seems to be sequestered by the golf course lake sediments with concentrations as high as 302 mg/kg, which were significantly higher than the 0.1 to 3 mg/kg expected for comparable sediments in central Florida. Arsenic correlates well with Fe in the top 15 cm of the lake sediments suggesting that As is sorbed by hydrous ferric oxides (HFO). As long as conditions are in favour of HFO stability, As is retained in the lake sediments preventing its migration into the Floridan aquifer systems. However, once the loading capacity of the sediment is reached or when as a result of changing physico-chemical conditions HFO may become unstable, As may enter the Floridan aquifer.