Synthetic Ferrihydrite Research Articles

Speciation of iodine on biogenic iron oxyhydroxides by I K-edge and LIII-edge XANES

Abstract The host phase of iodine (I) in biogenic iron oxyhydroxides (BIOS) was determined using X-ray absorption near-edge structure (XANES). I K-edge and LIII-edge XANES analyses of BIOS collected from deep-sea hydrothermal environments revealed that the primary form of I in the BIOS is an organic species. Furthermore, LIII-edge XANES fitting implied the dominance of aromatic-bound I among BIOS. Our first observation of organic–trace element association in natural BIOS can explain the enhanced adsorption of I on BIOS compared to synthetic ferrihydrite. This further suggests the importance of various chemistry on this organic–mineral composite for understanding the geochemical cycling of trace elements.

Interparticle magnetic interactions and magnetic field dependence of superparamagnetic blocking temperature in ferrihydrite nanoparticle powder systems

In this study, nanoparticles of initial synthetic ferrihydrite have been coated with arabinogalactan. The synthesized series of samples with different degrees of coverage of particles has been characterized by X-ray photoelectron spectroscopy, Mössbauer spectroscopy, transmission electron microscopy and magnetometry. The superparamagnetic blocking temperature decreases monotonically with an increase in the degree of coverage of ferrihydrite particles, which is unambiguously related to the different role of the interparticle magnetic interactions in the investigated powder systems. Analysis of the field dependence of the blocking temperature within the random anisotropy model has shown that an increase in the degree of coverage of ferrihydrite particles leads to a decrease in the size of a cluster in which the behaviors of the nanoparticle magnetic moments are correlated. The results obtained have shown the possibility of effective control of the strength of magnetic interparticle interactions in powder ferrihydrite systems by coating nanoparticles with arabinogalactan.

Seasonal Fluctuations in Iron Cycling in Thawing Permafrost Peatlands.

In permafrost peatlands, up to 20% of total organic carbon (OC) is bound to reactive iron (Fe) minerals in the active layer overlying intact permafrost, potentially protecting OC from microbial degradation and transformation into greenhouse gases (GHG) such as CO2 and CH4. During the summer, shifts in runoff and soil moisture influence redox conditions and therefore the balance of Fe oxidation and reduction. Whether reactive iron minerals could act as a stable sink for carbon or whether they are continuously dissolved and reprecipitated during redox shifts remains unknown. We deployed bags of synthetic ferrihydrite (FH)-coated sand in the active layer along a permafrost thaw gradient in Stordalen mire (Abisko, Sweden) over the summer (June to September) to capture changes in redox conditions and quantify the formation and dissolution of reactive Fe(III) (oxyhydr)oxides. We found that the bags accumulated Fe(III) under constant oxic conditions in areas overlying intact permafrost over the full summer season. In contrast, in fully thawed areas, conditions were continuously anoxic, and by late summer, 50.4 ± 12.8% of the original Fe(III) (oxyhydr)oxides were lost via dissolution. Periodic redox shifts (from 0 to +300 mV) were observed over the summer season in the partially thawed areas. This resulted in the dissolution and loss of 47.2 ± 20.3% of initial Fe(III) (oxyhydr)oxides when conditions are wetter and more reduced, and new formation of Fe(III) minerals (33.7 ± 8.6% gain in comparison to initial Fe) in the late summer under more dry and oxic conditions, which also led to the sequestration of Fe-bound organic carbon. Our data suggest that there is seasonal turnover of iron minerals in partially thawed permafrost peatlands, but that a fraction of the Fe pool remains stable even under continuously anoxic conditions.

Interparticle magnetic interactions in synthetic ferrihydrite: Mössbauer spectroscopy and magnetometry study of the dynamic and static manifestations

Samples of synthetic ferrihydrite with an average nanoparticle size of 2.7 nm have been examined by magnetometry and Mössbauer spectroscopy. Ferrihydrite is characterized by the antiferromagnetic interactions between the magnetic moments of iron atoms. In ferrihydrite nanoparticles, as in any other antiferromagnetic ones, structural defects induce the formation of an uncompensated magnetic moment, which determines the magnetic properties typical of single-domain ferro- and ferrimagnetic particles. The manifestation of the magnetic interactions between ferrihydrite nanoparticles in the magnetic properties of the material and in the temperature evolution of Mössbauer spectra has been in focus. The results obtained on synthetic ferrihydrite have been compared with the data for the biogenic ferrihydrite sample with a similar average size of particles surrounded by a polysaccharide shell, which weakens and screens the interparticle magnetic interactions. A clear manifestation of the effect of the interparticle magnetic interactions on the transition to the blocked state is the presence of a significant contribution of the relaxation component in the Mössbauer spectra at temperatures of the transition from the superparamagnetic to blocked state. The temperature dependence of the particle relaxation time obtained from the Mössbauer spectra points out the collective effect of freezing of the magnetic moments of particles due to the magnetic interactions between them.

Thermal Stability and Decomposition Products of P-Doped Ferrihydrite.

This work aimed to determine the effect of various amounts of P admixtures in synthetic ferrihydrite on its thermal stability, transformation processes, and the properties of the products, at a broad range of temperatures up to 1000 °C. A detailed study was conducted using a series of synthetic ferrihydrites Fe5HO8·4H2O doped with phosphates at P/Fe molar ratios of 0.2, 0.5, and 1.0. Ferrihydrite was synthesized by a reaction of Fe2(SO4)3 with 1 M KOH at room temperature in the presence of K2HPO4 at pH 8.2. The products of the synthesis and the products of heating were characterized at various stages of transformation by using differential thermal analysis accompanied with X-ray diffraction, Fourier transform infrared spectroscopy, and scanning electron microscopy-energy dispersive X-ray spectroscopy. Coprecipitation of P with ferrihydrite results in the formation of P-doped 2-line ferrihydrite. A high P content reduces crystallinity. Phosphate significantly inhibits the thermal transformation processes. The temperature of thermal transformation increases from below 550 to 710–750 °C. Formation of intermediate maghemite and Fe-phosphates, is observed. The product of heating up to 1000 °C contains hematite associated with rodolicoite FePO4 and grattarolaite Fe3PO7. Higher P content greatly increases the thermal stability and transformation temperature of rodolicoite as well.

Interactions between Escherichia coli survival and manganese and iron oxides in water under freeze-thaw

Pathogenic survivals were dramatically affected by Fe3+ and Mn2+ under freeze-thaw (FT), and the dissolutions of manganese and iron oxides (MIOs) were also accelerated under FT. But the mutual influences of pathogenic bacterial survival and MIOs under FT have not been profoundly explored yet. In this work, aqueous systems containing Escherichia coli as well as synthetic ferrihydrite (Fh) and manganese dioxide (MnO2) were experimented under simulated FT cycles to study the mutual influences of metal oxides and bacteria survival while oxide dissolutions and appearances, bacterial morphology and activities (survival number, cell surface hydrophobicity (CSH) and superoxide dismutase (SOD)) were obtained. The results showed that broken E. coli cells by ice growth were observed, but both oxides promoted E. coli survival under FT stress and prolonged bacterial survival time by 1.2–2.9 times, which were mainly attributed to the release of Fe3+ and Mn2+ caused by FT. The dissolutions of Fh and MnO2 under FT, which took place at a low level in absence of E. coli cells, were markedly enhanced with bacterial interferences by 2–8 times and higher dissolved manganese concentrations were detected than iron. This was probably because that concentrated organic matters which were released from broken cells, rejected into unfrozen liquid layer and acted as electron donors and ligands to oxide dissolution. Compared with Fh system, more significant promotion of E. coli survival under FT in MnO2 systems were found because of more SOD generations associated with high dissolved manganese concentrations and the stronger cellular protection by MnO2 aggregations. The results suggested that FT significantly influenced the interactions between metal oxides and bacterial in water, resulting to changes in pathogen activity and metal element cycling.

Simulation of Natural Iron Oxide Alteration in Soil: Conversion of Synthetic Ferrihydrite to Hematite Without Artificial Dopants, Observed With Magnetic Methods

AbstractWe present new results on the conversion of pure, undoped synthetic ferrihydrite, wet‐annealed at pH 6.56 and 90°C without stabilizing ligands, to nanophase goethite, hematite, and an intermediate magnetic phase, nanophase maghemite. Our analyses included magnetic field and temperature‐dependent properties and characterization by powder X‐ray diffraction, Mössbauer spectra, and high‐resolution transmission electron microscopy. We sampled alteration products after 0.5 hr, and then in a geometric progression to 32 hr, yielding a detailed examination of the earliest alteration phases. There are many similarities to the latest studies of pure ferrihydrite alteration but with a significant difference: We observe early appearance of oriented nanophase goethite along with a soft magnetic contribution, while rhombohedral hematite crystals form later, as reported in previous studies. Our observations attest to the non‐uniqueness of the magnetic enhancement process and to its strong dependence on environmental conditions, with important implications for use of the hematite/goethite ratio as a paleoprecipitation proxy.

Fate of organic compounds during transformation of ferrihydrite in iron formations

The absence of organic compounds from Precambrian iron formations (IF) challenges the hypothesis of the biogenic origin of IF. Here we address the fate of adsorbed organic compounds during transformation from ferrihydrite to hematite. We determined the binding energy between hematite and common molecular terminations found in extracellular polymeric substances and biofilms: carboxylic, hydroxyl and phosphate functional groups. We found that the bond between hematite and hydroxyl groups is approximately 2 times stronger than the bond between hematite-carboxyl and -phosphate groups. We transformed synthetic ferrihydrite to hematite at 90 oC in presence of glycerol, which has a high density of hydroxyl groups and measured the amount of mineral associated glycerol before and after the transformation. We show that the transformation releases glycerol highlighting that organic compounds adsorbed at precursor ferrihydrite could be desorbed during the process of IF sedimentation and diagenesis. Our results suggest that the absence of organic compounds in IF should not be used as evidence against their biogenic origin.

Variable surface charge of humic acid-ferrihydrite composite: Influence of electrolytes on ciprofloxacin adsorption

Antimicrobial compounds are found in a range of environments as pollutants. Here, we evaluated the influence of two common anions, NO3− and PO43-, on ciprofloxacin adsorption on humic acid/ferrihydrite composite (HA-DIG/Fh), synthetic ferrihydrite (Fh), and humic acid (HA-DIG) under controlled pH (7.0), ionic strength (0.1 M) and temperature (25 °C). All materials were characterized by isoelectric point (IEP), while the composite and the iron oxide were characterized by Mössbauer spectroscopy. Kinetic and isotherm adsorption studies were carried out using cyclic voltammetry (in KH2PO4) and square wave voltammetry (in KNO3). The application of kinetic models for both anions revealed Fh to fit to a pseudo second order model (R2 = 0.941); while HA-DIG (R2 = 0.950) and HA-DIG/Fh (R2 = 0.993) were fitted to pseudo first order models. The adsorption results showed a high dependency electrolyte, especially in Fh, where different shape curves (H-type in KNO3 and C-type in KH2PO4) and maximum experimental adsorbed amount Cm were observed. This finding is supported by the distinct IEP values and change in sign of surface charge between the two ions. Finally, results suggest that HA-DIG could be potentially used in environmental remediation to remove antibiotics from natural matrices, though the risk of antibiotic transportation increased with depth in the soil profile.

Enrichment mechanisms of antimony and arsenic in marine ferromanganese oxides: Insights from the structural similarity

Marine ferromanganese crusts and nodules as potential mineral deposits have received increasing attention. However, much less knowledge is available concerning the incorporation and enrichment mechanisms for antimony (Sb) and arsenic (As) in marine ferromanganese oxides. In this study, the surface complexations of Sb(V) and As(V) on synthetic ferrihydrite and Mn oxides (δ-MnO2 and birnessite) were investigated by a combination of adsorption experiments, extended X-ray absorption fine structure (EXAFS) analyses, and quantum chemical calculations. The speciation, distribution, and local structure of Sb and As in different types of natural marine ferromanganese oxides were determined by X-ray absorption near edge structure (XANES) and EXAFS analyses to reveal the enrichment mechanisms for the two elements in ferromanganese oxides at the molecular level. To the best of our knowledge, the Sb EXAFS analyses for different types of marine ferromanganese oxides are herein reported for the first time. Results showed that Sb(V) is preferentially adsorbed on Mn oxides through energetically favorable bidentate–mononuclear complexation because of the structural similarity between the octahedron SbV(OH)6− and MnO6 unit, although bidentate–binuclear (corner-sharing) and bidentate–mononuclear (edge-sharing) complexes can be formed on ferrihydrite and Mn oxides for the adsorption of Sb(V). By contrast, tetrahedral AsVO43− is mostly adsorbed on ferrihydrite and Mn oxides with the formation of bidentate–binuclear complexes. In natural marine ferromanganese oxides, Sb and As can be retained by Fe and Mn (oxyhydr)oxide components, and the disparate distribution of the two elements to Mn oxides may largely depend on the Mn/Fe ratio and constituent minerals. The larger enrichment factor of Sb than that of As in marine ferromanganese oxides may result from their preferential attachment modes onto the Fe and Mn phases and different inhibition effects from coexisting anions in seawater. Compared with As, a part of Sb may be strongly associated with the lateral sites in Mn oxides via the formation of bidentate edge-sharing complexes, with which anions such as sulfate in seawater do not significantly compete. The findings from this study provide the molecular-scale insights into the enrichment processes and mechanisms of Sb and As in marine ferromanganese oxides. Our study also helps elucidate the incorporation mechanisms and geochemical behaviors of other oxyanions in marine and surface environments.

Direct Fe(III) Reduction from Synthetic Ferrihydrite by Haloalkaliphilic Lithotrophic Sulfidogens

Ability to reduce insoluble Fe(III) compounds has not been shown for alkaliphilic lithotrophic sulfate and sulfur reducers. Detection of this metabolic process in sulfidogenic prokaryotes could significantly expand the present knowledge on physicochemical range of their growth and physiological activity, which is now limited by low negative ambient redox potential. Capacity for direct reduction of Fe(III) from chemically synthesized ferrihydrite was tested for eight species of hydrogenotrophic haloalkaliphilic sulfidogens grown with formate or H2 as electron donors in the absence of sulfur compounds in the medium. Out of eight tested species, six reduced iron with formate and five, with hydrogen as the electron donor. Iron reduction correlated with stimulation of growth on formate or hydrogen only in two sulfidogenic species. Analysis of available genomes of five tested species revealed that only Dethiobacter alkaliphilus and Desulfuribacillus alkaliarsenatis possess the gene sets of multiheme cytochromes c required for typical dissimilatory iron reduction. The presence of these genes in two strains with high iron-reducing activity indicates the capacity of some haloalkaliphilic sulfidogenic bacteria for carrying out direct dissimilatory reduction of insoluble Fe(III) forms in the absence of sulfur-containing electron acceptors, i.e., without using sulfide as a soluble mediator of iron reduction. In other studied microorganisms, the ability to reduce iron is probably caused by nonspecific metabolic activity and is not directly linked to energy generation for growth, although the rates of Fe(III) reduction determined in our experiments make it possible to suggest significant role of sulfidogenic microorganisms (normally reducing sulfur and sulfate) in the iron cycle in haloalkaline ecosystems upon decreased content of sulfur compounds.

Adsorption Characteristics of Phosphorus Wastewater on the Synthetic Ferrihydrite

The synthetic ferrihydrite, FerrorMox (FM), was used as adsorbent for removing phosphorus from wastewater. SEM, EDS, XRD, FTIR and Raman were used to characterize FM, and the results indicated that FM was amorphous 2 L ferrihydrite and was composed of Fe, O, Ca and Si, etc. Afterwards, FM was applied to adsorb phosphorus from wastewater, and the adsorption performance, influence factors and adsorption mechanism were investigated. The phosphorus removal rate reached 99.14% under the condition of adsorption time of 60 min, initial pH phosphate solution of 2, relative dosing quantity of 7 g·L-1, reaction temperature of 25℃, initial concentration of 10 mg·L-1, and solution volume of 50 mL. Adsorption isotherms were well fitted with the Langmuir isothermal adsorption model at different temperature with the correlation coefficient reaching above 0.95. The thermodynamic parameters showed that the phosphorus adsorption by FM was a spontaneous endothermic reaction. The phosphate removal kinetics well followed both pseudo-first-order model and pseudo-second-order model. About 99% of phosphate adsorbed on FM could be desorbed in 0.1 mol·L-1 NaOH solution. Therefore, FM was a promising absorbent material for the removal of phosphate from waste water.

Effect of synthetic amorphous and crystalline iron oxides on redox potential and exchangeable cations in wetland soils of Maputaland Kwazulu-Natal, South Africa

Some grey-coloured soils of the Maputaland Coastal Plains in northern KwaZulu-Natal do not exhibit the redox morphology normally associated with seasonal water saturation, resulting in serious limitations in using soil morphology for wetland delineation. One of the hypotheses to explain this phenomenon is that these soils simply do not contain enough iron minerals that can be reduced and re-oxidised to form redox accumulations and depletions. Alternatively, if iron minerals are present, they are too crystalline to be readily reduced as a result of the lower iron activity associated with these oxides and can only be reduced at a slow rate. This investigation aimed to determine if a deficiency in iron oxides could explain the absence of redoximorphic features in the wetland soils. A split-plot experimental design with 78 cores was set up to investigate the effect that varying iron crystallinity had on soil redox potential. The experiment was conducted in a controlled environment in the laboratory with the temperature set at 23°C with fluctuations of ±2°C. Synthetic ferrihydrite and goethite were mixed with soils (0.84 g.kg −1 ) and packed to a bulk density of 1.4 Mg m −3 . The cores were then saturated to 73% of porosity and the pH, Eh, Ca, Mg, Fe, and Mn measured bi-weekly. The ferrihydrite and goethite treatments did not affect ammonium acetate extractable Ca and Mg. The treatments did, however, increase significantly the ammonium acetate extractable Fe, Mn, and pH, while both treatments significantly decreased pe over the 42 day extraction period. The goethite treatment reduced within five days, while ferrihydrite only reduced after 20 days. The Fe o /Fe d ratios for the treatments suggested that ferrihydrite started as less crystalline and increased sharply after saturation, while the crystallinity of the goethite treatment only increased slightly. Results presented here indicate that the absence of redoximorphic features in the soils of Manguzi could be attributed to the low iron oxide content of the soils. Other indicators, such as vegetation or long-term water measurements, therefore need to be engaged to delineate wetlands in the soils of the Maputaland Coastal Plain.

Reactivity of Uranium and Ferrous Iron with Natural Iron Oxyhydroxides.

Determining key reaction pathways involving uranium and iron oxyhydroxides under oxic and anoxic conditions is essential for understanding uranium mobility as well as other iron oxyhydroxide mediated processes, particularly near redox boundaries where redox conditions change rapidly in time and space. Here we examine the reactivity of a ferrihydrite-rich sediment from a surface seep adjacent to a redox boundary at the Rifle, Colorado field site. Iron(II)-sediment incubation experiments indicate that the natural ferrihydrite fraction of the sediment is not susceptible to reductive transformation under conditions that trigger significant mineralogical transformations of synthetic ferrihydrite. No measurable Fe(II)-promoted transformation was observed when the Rifle sediment was exposed to 30 mM Fe(II) for up to 2 weeks. Incubation of the Rifle sediment with 3 mM Fe(II) and 0.2 mM U(VI) for 15 days shows no measurable incorporation of U(VI) into the mineral structure or reduction of U(VI) to U(IV). Results indicate a significantly decreased reactivity of naturally occurring Fe oxyhydroxides as compared to synthetic minerals, likely due to the association of impurities (e.g., Si, organic matter), with implications for the mobility and bioavailability of uranium and other associated species in field environments.

Arsenic in Porewaters of the Unsaturated Zone of an Argentinean Watershed: Adsorption and Competition with Carbonate as Important Processes that Regulate its Concentration

The arsenic (As) concentration in porewaters of the unsaturated (vadose) zone of a watershed located in the Chaco-Pampean Plain of Argentina was investigated. A water displacement method using carbon tetrachloride was applied to the sediments in order to obtain the water samples, which could not be obtained by a simple high-speed centrifugation method. The CD-MUSIC surface complexation model was applied to calculate arsenate adsorption on sediments, arsenate concentration in porewaters in contact with the sediments and effects of carbonate. Ferrihydrite was considered to represent the active adsorbing material in the sediments. Therefore, proton adsorption (surface charge) data and arsenate adsorption isotherms obtained with a synthetic ferrihydrite were used to calibrate the CD-MUSIC model. Arsenate and carbonate concentrations in the studied porewaters were positively correlated. The model was able to predict As concentration within a factor of two in most samples. Carbonate affects As concentration by competing with arsenate species for adsorption sites on the mineral surface. As it occurs with groundwater samples of the saturated zone in many aquifers, this article shows for the first time that adsorption–desorption processes also seem to control As concentration in oxic porewaters of the unsaturated zone.

Ni isotope fractionation during sorption to ferrihydrite: Implications for Ni in banded iron formations

A major decline in marine methanogen populations during the Late Archaean Eon has been proposed as a driver for the Great Oxidation Event (GOE) at ~2.4Ga. Konhauser et al. (2009, Nature 458, 750–753) recently hypothesized that a dramatic decrease between 2.7 and 2.4Ga in the concentration of dissolved Ni, an essential nutrient for methanogens, may have been responsible for the decline. To test that hypothesis, Ni stable isotopes, a novel proxy for methanogen Ni limitation, will be applied to the Late Archaean–Paleoproterozoic rock record in the future, possibly through analysis of banded iron formations (BIFs). In order to facilitate interpretation of those data, we investigated the fractionation of Ni stable isotopes during sorption to synthetic ferrihydrite, which was likely the primary precipitating phase during BIF deposition, via bench-top experiments. The observed fractionations for adsorption and coprecipitation experiments were indistinguishable and averaged Δ60/58Nidissolved-sorbed=+0.35±0.10‰ (1 sd; light Ni sorbed). Parallel, linear trends in isotope compositions of dissolved and sorbed Ni as a function of fraction of Ni sorbed indicate an equilibrium isotope effect with rapid and continuous exchange between dissolved and sorbed Ni. Additionally, we observed fractionation of Δ60/58Nidissolved–sorbed=+0.23±0.07‰ (1 sd) in experiments in which ferrihydrite had been aged for up to 30days and had partially transformed to goethite and hematite; thus the fractionations in all of our experiments are the same within uncertainties. Although our simple system differs from Archaean seawater, our results suggest that ferrihydrite precipitating to form BIFs may always have been offset by a constant fractionation from coexisting dissolved Ni in seawater.

Effect of Biofilm Constituents on Cd<sup>2+</sup> Adsorption onto Sediments from Yellow River

Biofilm main constituents of the humic acid and synthetic ferrihydrite were used as sorbent on the Wuhai sediment of Yellow River, comparative analysis the Wuhai section sediment of the Yellow River and after doping humic acid and ferrihydrite of Cd2+ adsorption characteristics. Discusses the adsorption time, various initial concentrations on Cd2+ adsorption onto sediments from Wuhai of Yellow River.The adsorption isotherms and adsorption kinetics were fitted and distribution coefficients were calculated. The results illustrated that the adsorption processes of sediments with different proportions of biofilm main constituents to Cd2+ were consistent with Langmuir adsorption isotherm curves (R2≧0.99,p≦0.02,n=7); Doping of humic acid and ferrihydrite on sediment, the distribution coefficients of Cd2+ were 6.2959 and 2.711, which were higher than the original (0.8626).

Properties of impurity-bearing ferrihydrite III. Effects of Si on the structure of 2-line ferrihydrite

Siliceous ferrihydrites are abundant nanoparticles in natural environments. Although it is well known that the physical properties of ferrihydrite are affected when formed in the presence of silicate oxoanions (SiO44−), the structure of siliceous ferrihydrites (SiFh), and the speciation of Si within these nanosolids are not well understood. In this study we evaluate the effects of Si (at concentrations ranging from 5 to 40mol% Si) on synthetic ferrihydrite precipitates using structural data derived from synchrotron-based high energy X-ray scattering and pair distribution function (PDF) analysis, in combination with X-ray absorption near edge structure (XANES) spectroscopy, and transmission electron microscopy (TEM). Silicate oxoanions have a major effect on Fe(O,OH)x polyhedral polymerization and ferrihydrite particle growth, illustrated by the formation of smaller, poorly crystalline, structurally disordered/strained ferrihydrite nanocrystallites. Variation in Fh unit-cell parameters is suggested to arise from substantial particle size-induced structural disorder. As a result of this significant size-dependent structural disorder, it was not possible to identify evidence for Si4+ for Fe3+ substitution in these samples based on unit cell parameter variations or refinement of different structural models. Principal component analyses (PCA) and linear combination fits carried out on the PDFs suggest that iron partitions between several phases (e.g., ferrihydrite and an Fe-bearing amorphous silica phase (Amorph. SiO2+Fe)) in these co-precipitates. A mechanism of co-precipitation is proposed, in which silicate binds to Fe polymers and Fh particles, thus inhibiting particle growth at low Si content. At higher Si content, SiO44− polymerization traps significant Fe, and we suggest that the occurrence of this second Fe pool limits further the availability of Fe required for ferrihydrite particle development. Such Si-ferrihydrite co-precipitates are expected to be more stable in natural environments with respect to reductive dissolution or transformation, and to impact the bioavailability of Fe(III).

Adsorption behavior of cadmium ion onto synthetic ferrihydrite: effects of pH and natural seawater ligands

Batch experiments were conducted to study the adsorption behavior of cadmium ion onto the synthetic ferrihydrite. The adsorbent was characterized using X-ray diffraction and N2 adsorption–desorption measurements. The effects of pH value and the major seawater anions (nitrate, carbonate, chloride and sulphate) at their natural concentrations on the adsorption of cadmium ion were investigated. The results showed that the adsorption of cadmium ion was highly pH-dependent and suppressed in the presence of chloride, sulphate and nitrate, while carbonate was found to enhance the adsorption of cadmium ion over the examined range of 4.0–7.5. The maximum percentage of cadmium ion adsorbed can reach 89.96 % at pH 7.5. It is proposed that the competition for binding sites played a key role in the reduction of cadmium ion adsorption by nitrate, chloride and sulphate. X-ray diffraction spectroscopic data for cadmium adsorbed on ferrihydrite showed that CdCO3(s) was formed on ferrihydrite; the formation of CdCO3 precipitation or surface precipitation in carbonate system may be the reason for the increase of cadmium adsorption.

A nano-disperse ferritin-core mimetic that efficiently corrects anemia without luminal iron redox activity

The 2-5nm Fe(III) oxo-hydroxide core of ferritin is less ordered and readily bioavailable compared to its pure synthetic analogue, ferrihydrite. We report the facile synthesis of tartrate-modified, nano-disperse ferrihydrite of small primary particle size, but with enlarged or strained lattice structure (~2.7Å for the main Bragg peak versus 2.6Å for synthetic ferrihydrite). Analysis indicated that co-precipitation conditions can be achieved for tartrate inclusion into the developing ferrihydrite particles, retarding both growth and crystallization and favoring stabilization of the cross-linked polymeric structure. In murine models, gastrointestinal uptake was independent of luminal Fe(III) reduction to Fe(II) and, yet, absorption was equivalent to that of ferrous sulphate, efficiently correcting the induced anemia. This process may model dietary Fe(III) absorption and potentially provide a side effect-free form of cheap supplemental iron. From the Clinical EditorSmall size tartrate-modified, nano-disperse ferrihydrite was used for efficient gastrointestinal delivery of soluble Fe(III) without the risk for free radical generation in murine models. This method may provide a potentially side effect-free form iron supplementation.