Natural ferrihydrites in surface deposits from Finland and their association with silica

Young ochreous precipitations from Fe-bearing spring waters in Finland consist mainly of ferrihydrite. a poorly ordered Fe-oxide with a layer structure and the bulk composition 5 Fe₂O₃ ·9 H₂O Crystallinity ranges from a reasonably well developed structure to a highly disordered one with only two prismatic reflections at 2.5 and 1.5 Å. In contrast to other Fe-oxides. ferrihydrite is highly soluble in oxalate. Electron microscopy shows spherical particles 2–5 nm in diameter forming aggregates of 100–300 nm. The specific surface ranges from 220 to 560 m²/g. During their formation, the ferrihydrites adsorb large quantities of silica, part of which is unpolymerized as indicated by Si-O-Fe bonds (i.r.), and part of which is polymerized. NaOH preferentially extracts polymerized silica causing a shift in the i.r. absorption band. Silica also causes a shift in the temperature at which ferrihydrite converts to hematite. ‘Hydrous Fe(III)-oxides’ with 0–15mol% Si prepared from Si containing Fe(III) salt solutions showed similar properties: Si-O-Fe bonds are shown by i.r. and increasing temperatures of transformation to hematite with increasing amount of Si. Adsorbed Si may also retard the transformation of ferrihydrite to the more stable goethite in nature.     

Intermobility of barium,strontium, and lead in chloride and sulfate leach solutions

Iron(III)-precipitates formed by the oxidation of dissolved Fe(II) are important sorbents for major and trace elements in aquatic and terrestrial systems. Their reductive dissolution in turn may result in the release of associated elements. We examined the reductive dissolution kinetics of an environmentally relevant set of Fe(II)-derived arsenate-containing Fe(III)-precipitates whose structure as function of phosphate (P) and silicate (Si) content varied between poorly-crystalline lepidocrocite, amorphous Fe(III)-phosphate, and Si-containing ferrihydrite. The experiments were performed with 0.2–0.5 mM precipitate-Fe(III) using 10 mM Na-ascorbate as reductant, 5 mM bipyridine as Fe(II)-complexing ligand, and 10 mM MOPS/5 mM NaOH as pH 7.0 buffer. Times required for the dissolution of half of the precipitate (t_50%) ranged from 1.5 to 39 h; spanning a factor 25 range. At loadings up to ~ 0.2 P/Fe (molar ratio), phosphate decreased the t_50% of Si-free precipitates, probably by reducing the crystallinity of lepidocrocite. The reductive dissolution of Fe(III)-phosphates formed at higher P/Fe ratios was again slower, possibly due to P-inhibited ascorbate binding to precipitate-Fe(III). The slowest reductive dissolution was observed for P-free Si-ferrihydrite with ~ 0.1 Si/Fe, suggesting that silicate binding and polymerization may reduce surface accessibility. The inhibiting effect of Si was reduced by phosphate. Dried-resuspended precipitates dissolved 1.0 to 1.8-times more slowly than precipitates that were kept wet after synthesis, most probably because drying enhanced nanoparticle aggregation. Variations in the reductive dissolution kinetics of Fe(II) oxidation products as reported from this study should be taken into account when addressing the impact of such precipitates on the environmental cycling of co-transformed nutrients and contaminants.

     

The effect of silica and natural organic matter on the Fe(II)-catalysed transformation and reactivity of Fe(III) minerals

The Fe(II)-catalysed transformation of synthetic schwertmannite, ferrihydrite, jarosite and lepidocrocite to more stable, crystalline Fe(III) oxyhydroxides is prevented by high, natural concentrations of Si and natural organic matter (NOM). Adsorption isotherms demonstrate that Si adsorbs to the iron minerals investigated and that increasing amounts of adsorbed Si results in a decrease in isotope exchange between aqueous Fe(II) and the Fe(III) mineral. This suggests that the adsorption of Si inhibits the direct adsorption of Fe(II) onto the mineral surface, providing an explanation for the inhibitory effect of Si on the Fe(II)-catalysed transformation of Fe(III) minerals. During the synthesis of lepidocrocite and ferrihydrite, the presence of equimolar concentrations of Si and Fe resulted in the formation of 2-line ferrihydrite containing co-precipitated Si in both cases. Isotope exchange experiments conducted with this freeze-dried Si co-precipitated ferrihydrite species (Si-ferrihydrite) demonstrated that the rate and extent of isotope exchange between aqueous Fe(II) and solid ⁵⁵Fe(III) was very similar to that of 2-line ferrihydrite formed in the absence of Si and which had not been allowed to dry. In contrast to un-dried ferrihydrite formed in the absence of Si, Si-ferrihydrite did not transform into a more crystalline Fe(III) mineral phase over the 7-day period of investigation. Reductive dissolution studies using ascorbic acid demonstrated that both dried Si-ferrihydrite and un-dried 2-line ferrihydrite were very reactive, suggesting these species may be major contributors to the rapid release of dissolved iron following flooding and the onset of conditions conducive to reductive dissolution in acid sulphate soil environments.     

The influence of Fe-rich coatings on the dissolution of anorthite at pH 2.6

Surface coatings are very common on mineral grains in soils but most laboratory dissolution experiments are carried out on pristine, uncoated mineral grains. An experiment designed to unambiguously isolate the effect of surface coatings on mineral dissolution from any influence of solution saturation state is reported. Two aliquots of 53 to 63 μm anorthite feldspar powder were used. One was dissolved in pH 2.6 HCl, the other in pH 2.6 FeCl₃ solution, both for ∼6000 h in flow-through reactors. An amorphous Fe-rich, Al-, Ca- and Si-free orange precipitate coated the anorthite dissolved in the FeCl₃ solution. BET surface area of the anorthite increased from 0.16 to 1.65 m² g⁻¹ in the HCl experiment and to 3.89 m² g⁻¹ in the FeCl₃ experiment. The increase in surface area in the HCl experiment was due to the formation of etch pits on the anorthite grain surface whilst the additional increase in the FeCl₃ experiment was due to the micro- and meso-porous nature of the orange precipitate. This precipitate did not inhibit or slow the dissolution of the anorthite. Steady state dissolution rates for the anorthite dissolved in the HCl and FeCl₃ were ∼2.5 and 3.2 × 10⁻¹⁰ mol_feldspar m⁻² s⁻¹ respectively. These rates are not significantly different after the cumulative uncertainty of 17% in their value due to uncertainty in the inputs parameters used in their calculation is taken into account. Results from this experiment support previous theoretical and inference-based conclusions that porous coatings should not inhibit mineral dissolution.     

Equilibrium and kinetic Si isotope fractionation factors and their implications for Si isotope distributions in the Earth’s surface environments

Several important equilibrium Si isotope fractionation factors among minerals, organic molecules and the H₄SiO₄ solution are complemented to facilitate the explanation of the distributions of Si isotopes in Earth’s surface environments. The results reveal that, in comparison to aqueous H₄SiO₄, heavy Si isotopes will be significantly enriched in secondary silicate minerals. On the contrary, quadra-coordinated organosilicon complexes are enriched in light silicon isotope relative to the solution. The extent of ²⁸Si-enrichment in hyper-coordinated organosilicon complexes was found to be the largest. In addition, the large kinetic isotope effect associated with the polymerization of monosilicic acid and dimer was calculated, and the results support the previous statement that highly ²⁸Si-enrichment in the formation of amorphous quartz precursor contributes to the discrepancy between theoretical calculations and field observations. With the equilibrium Si isotope fractionation factors provided here, Si isotope distributions in many of Earth’s surface systems can be explained. For example, the change of bulk soil δ³⁰Si can be predicted as a concave pattern with respect to the weathering degree, with the minimum value where allophane completely dissolves and the total amount of sesqui-oxides and poorly crystalline minerals reaches their maximum. When, under equilibrium conditions, the well-crystallized clays start to precipitate from the pore solutions, the bulk soil δ³⁰Si will increase again and reach a constant value. Similarly, the precipitation of crystalline smectite and the dissolution of poorly crystalline kaolinite may explain the δ³⁰Si variations in the ground water profile. The equilibrium Si isotope fractionations among the quadra-coordinated organosilicon complexes and the H₄SiO₄ solution may also shed light on the Si isotope distributions in the Si-accumulating plants.     

Ferrihydrite dissolution by pyridine-2,6-bis(monothiocarboxylic acid) and hydrolysis products

Pyridine-2,6-bis(monothiocarboxylate) (pdtc), a metabolic product of microorganisms, including Pseudomonas putida and Pseudomonas stutzeri was investigated for its ability of dissolve Fe(III)(hydr)oxides at pH 7.5. Concentration dependent dissolution of ferrihydrite under anaerobic environment showed saturation of the dissolution rate at the higher concentration of pdtc. The surface controlled ferrihydrite dissolution rate was determined to be 1.2 × 10⁻⁶ mol m⁻² h⁻¹. Anaerobic dissolution of ferrihydrite by pyridine-2,6-dicarboxylic acid or dipicolinic acid (dpa), a hydrolysis product of pdtc, was investigated to study the mechanism(s) involved in the pdtc facilitated ferrihydrite dissolution. These studies suggest that pdtc dissolved ferrihydrite using a reduction step, where dpa chelates the Fe reduced by a second hydrolysis product, H₂S. Dpa facilitated dissolution of ferrihydrite showed very small increase in the Fe dissolution when the concentration of external reductant, ascorbate, was doubled, suggesting the surface dynamics being dominated by the interactions between dpa and ferrihydrite. Greater than stoichiometric amounts of Fe were mobilized during dpa dissolution of ferrihydrite assisted by ascorbate and cysteine. This is attributed to the catalytic dissolution of Fe(III)(hydr)oxides by the in situ generated Fe(II) in the presence of a complex former, dpa.     

Synergistic effect of calcium and bicarbonate in enhancing arsenate release from ferrihydrite

Many groundwater systems contain anomalously high arsenic concentrations, associated with less than expected retention of As by adsorption to iron (hydr)oxides. Although carbonates are ubiquitous in aquifers, their relationship to arsenate mobilization is not well characterized. This research examines arsenate release from poorly crystalline iron hydroxides in abiotic systems containing calcium and magnesium with bicarbonate under conditions of static and dynamic flow (pH 7.5-8). Aqueous arsenic levels remained low when arsenate-bearing ferrihydrite was equilibrated with artificial groundwater solution containing Ca, Mg, and HCO₃⁻. In batch titrations in which a solution of Ca and HCO₃⁻ was added repeatedly, the ferrihydrite surface became saturated with adsorbed Ca and HCO₃⁻, and aqueous As levels increased by 1-2 orders of magnitude. In columns containing Ca or Mg and HCO₃⁻, As solubility initially mimicked titrations, but then rapidly increased by an additional order of magnitude (reaching 12 μM As). Separately, calcium chloride and other simple salts did not induce As release, although sodium bicarbonate and lactate facilitated minor As release under flow. Results indicate that adsorption of calcium or magnesium with bicarbonate leads to As desorption from ferrihydrite, to a degree greater than expected from competitive effects alone, especially under dynamic flow. This desorption may be an important mechanism of As mobilization in As-impacted, circumneutral aquifers, especially those undergoing rapid mineralization of organic matter, which induces calcite dissolution and the production of dissolved calcium and bicarbonate.     

Association of dissolved aluminum with silica: Connecting molecular structure to surface reactivity using NMR

We studied uptake mechanisms for dissolved Al on amorphous silica by combining bulk-solution chemistry experiments with solid-state Nuclear Magnetic Resonance techniques (²⁷Al magic-angle spinning (MAS) NMR, ²⁷Al{¹H} cross-polarization (CP) MAS NMR and ²⁹Si{¹H} CP-MAS NMR). We find that reaction of Al (1 mM) with amorphous silica consists of at least three reaction pathways; (1) adsorption of Al to surface silanol sites, (2) surface-enhanced precipitation of an aluminum hydroxide, and (3) bulk precipitation of an aluminosilicate phase. From the NMR speciation and water chemistry data, we calculate that 0.20 (±0.04) tetrahedral Al atoms nm⁻² sorb to the silica surface. Once the surface has sorbed roughly half of the total dissolved Al (∼8% site coverage), aluminum hydroxides and aluminosilicates precipitate from solution. These precipitation reactions are dependent upon solution pH and total dissolved silica concentration. We find that the Si:Al stoichiometry of the aluminosilicate precipitate is roughly 1:1 and suggest a chemical formula of NaAlSiO₄ in which Na⁺ acts as the charge compensating cation. For the adsorption of Al, we propose a surface-controlled reaction mechanism where Al sorbs as an inner-sphere coordination complex at the silica surface. Analogous to the hydrolysis of , we suggest that rapid deprotonation by surface hydroxyls followed by dehydration of ligated waters results in four-coordinate (>SiOH)₂Al(OH)₂ sites at the surface of amorphous silica.     

A new preparation process of coal-based magnetically activated carbon

Fe/C-based magnetically activated carbon （MAC） was obtained by carbonizing and activating its precursor, that was prepared by co-precipitation of anthracite coal impregnated in ferric chloride solution. The effect of the concentrations of FeC13 and pH of solution on BET surface area, pore volume and magnetic properties of the MAC was studied by BET N2 adsorption and VSM method. The results indicated that the magnetization of MAC gradually increases with increasing concentration of FeC13 and pH value of solution, and BET surface area was inclined to fluctuation. The largest BET surface area and magnetization of MAC were 1327.5 m2/g and 35.56 emu/g, respec- tively. The form of magnetic matter in the magnetically activated carbon was mainly Fe3C by X-ray powder diffrac- tion （XRD） and magnetic attraction test.     

Magnetite biomineralization induced by Shewanella oneidensis

Shewanella oneidensis is a dissimilatory iron reducing bacterium capable of inducing the extracellular precipitation of magnetite. This precipitation requires a combination of passive and active mechanisms. Precipitation occurs as a consequence of active production of Fe²⁺_(aq) when bacteria utilize ferrihydrite as a terminal electron acceptor, and the pH rise probably due to the bacterial metabolism of amino acids. As for passive mechanisms, the localized concentration of Fe²⁺_(aq) and Fe³⁺_(aq) at the net negatively charged cell wall, cell structures and/or cell debris induces a local rise of supersaturation of the system with respect to magnetite, triggering the precipitation of such a phase.These biologically induced magnetites are morphologically identical to those formed inorganically in free-drift experiments (closed system; 25 °C, 1 atm total pressure), both from aqueous solutions containing Fe(ClO₄)₂, FeCl₃, NaHCO₃, NaCO₃ and NaOH, and also from sterile culture medium added with FeCl₂. However, organic material becomes incorporated in substantial amounts into the crystal structure of S. oneidensis-induced magnetites, modifying such a structure compared to that of inorganic magnetites. This structural change and the presence of organic matter are detected by Raman and FT-IR spectroscopic analyses and may be used as a biomarker to recognize the biogenic origin of natural magnetites.     

Ni isotope fractionation during coprecipitation of Fe(III)(oxyhydr)oxides in Si solutions

The dramatic decline in aqueous Ni concentrations in the Archean oceans during the Great Oxygenation Event is evident in declining solid phase Ni concentrations in Banded Iron Formations (BIFs) at the time. Several experiments have been performed to identify the main removal mechanisms of Ni from seawater into BIFs, whereby adsorption of Ni onto ferrihydrites has shown to be an efficient process. Ni isotopic measurements have shown limited isotopic fraction during this process, however, most experiments have been conducted in simple solutions containing varying proportions of dissolved Fe and Ni as NO₃ salts, as opposed to Cl salts which are dominant in seawater. Further, Archean oceans were, before the advent of siliceous eukaryotes, likely saturated with amorphous Si as seen in the interlayered chert layers within BIFs. Despite Si being shown to greatly affect the Ni elemental partitioning onto ferrihydrite solids, no studies have been made on the effects of Si on the Ni isotope fractionation. Here we report results of multiple coprecipitation experiments where ferrihydrite precipitated in mixed solutions with Ni and Si. Ni concentrations in the experiments ranged between 200 and 4000 nM for fixed concentrations of Si at either 0, 0.67 or 2.2 mM. The results show that Si at these concentrations has a limited effect on the Ni isotope fractionation during coprecipitation of ferrihydrite. At 0.67 mM, the saturation concentration of cristobalite, the isotopic fractionation factors between the precipitating solid and experimental fluid are identical to experiments not containing Si (0.34 ± 0.17‰). At 2.2 mM Si, and the saturation concentration of amorphous silica, however, the Ni isotopic composition of the ferrihydrite solids deviate to more negative values and show a larger variation than at low or no Si, and some samples show fractionation of up to 0.5‰. Despite this seemingly more unstable fractionation behaviour, the combined results indicate that even at as high concentrations of Si as 2.2 mM, the δ⁶⁰Ni values of the forming ferrihydrites does not change much. The results of our study implicate that Si may not be a major factor in fractionating stable Ni isotopes, which would make it easier to interpret future BIF record and reconstruct Archean ocean chemistry.     

Sequestration of arsenate from aqueous solution using 2-line ferrihydrite: equilibria,kinetics, and X-ray absorption spectroscopic analysis

Arsenic(V), as the arsenate (AsO₄ ^3?) ion and its conjugate acids, has a strong affinity on Fe, Mn, and Al (oxyhydr)oxides and clay minerals. Removal of arsenate from aqueous solution by poorly crystalline ferrihydrite (hydrous ferric oxide) via a combination of macroscopic (equilibria and kinetics of sorption) and X-ray absorption spectroscopic studies was investigated. The removal of arsenate significantly decreased with increasing pH and sorption maxima of approximately 1.994 mmol/g (0.192 mol_As/mol_Fe) were achieved at pH 2.0. The Langmuir isotherm is most appropriate for arsenate sorption over the wide range of pH, indicating that arsenate sorption preferentially takes place at relatively homogenous and monolayer sites rather than heterogeneous and multilayer surfaces. The kinetic study demonstrated that arsenate sorption onto 2-line ferrihydrite is considerably fast, and sorption equilibrium was achieved within the reaction time of 2 h. X-ray absorption near-edge structure spectroscopy indicates no change in oxidation state of arsenate following interaction with the ferrihydrite surfaces. Extended X-ray absorption fine structure spectroscopy supports the efficient removal of arsenate by the 2-line ferrihydrite through the formation of highly stable inner-sphere surface complexes, such as bidentate binuclear corner-sharing (²C) and bidentate mononuclear edge-sharing (²E) complexes.     

Biotransformation of two-line silica-ferrihydrite by a dissimilatory Fe(III)-reducing bacterium: formation of carbonate green rust in the presence of phosphate

The reductive biotransformation of two Si-ferrihydrite coprecipitates (1 and 5 mole % Si) by Shewanella putrefaciens, strain CN32, was investigated in 1,4-piperazinediethanesulfonic acid-buffered media (pH ∼7) with lactate as the electron donor. Anthraquinone-2,6-disulfonate, an electron shuttle, was present in the media. Experiments were performed without and with PO₄³⁻ (P) (1 to 20 mmol/L) in media containing 50 mmol/L Fe. Our objectives were to define the combined effects of SiO₄⁴⁻ (Si) and P on the bioreducibility and biomineralization of ferrihydrites under anoxic conditions. Iron reduction was measured as a function of time, solids were characterized by powder X-ray diffraction and Mössbauer spectroscopy, and aqueous solutions were analyzed for Si, P, Cl⁻ and inorganic carbon. Both of the ferrihydrites were rapidly reduced regardless of the Si and P content. Si concentration had no effect on the reduction rate or mineralization products. Magnetite was formed in the absence of P whereas carbonate green rust GR(CO₃²⁻) ([Fe_(6−x)^IIFe^III_x(OH)₁₂]^x+(CO₃²⁻)_0.5x · yH₂O) and vivianite [Fe₃(PO₄)₂ · 8H₂O], were formed when P was present. GR(CO₃²⁻) dominated as a mineral product in samples with <4 mmol/L P. The Fe(II)/Fe(III) ratio of GR(CO₃²⁻) varied with P concentration; the ratio was 2 in 1 mmol/L P and approached 1 with 4- and 10 mmol/L P. Green rust appeared to form by solid-state transformation of ferrihydrite. Media P and Si concentration dictated the mechanism of transformation. In the 1 mole % Si coprecipitate with 1 mmol/L P, an intermediate Fe(II)/Fe(III) phase with structural Fe(II) slowly transformed to GR with time. In contrast, when ferrihydrite contained more Si (5 mole %) and/or contained higher P (4 mmol/L), sorbed Fe(II) and residual ferrihydrite together transformed to GR. Despite similar chemistries, P was shown to have a profound effect on extent of ferrihydrite reduction and biotransformations while that of Si was minimal.     

Effect of adsorbed and substituted Al on Fe(II)-induced mineralization pathways of ferrihydrite

The poorly crystalline Fe(III) hydroxide ferrihydrite is considered one of the most important sinks for (in)organic contaminants and nutrients within soils, sediments, and waters. The ripening of ferrihydrite to more stable and hence less reactive phases such as goethite is catalyzed by surface reaction with aqueous Fe(II). While ferrihydrite within most natural environments contains high concentrations of adsorbed or co-precipitated cations (particularly Al), little is known regarding the impact of these cations on Fe(II)-induced transformation of ferrihydrite to secondary phases. Accordingly, we explored the extent, rates, and pathways of Fe(II)-induced secondary mineralization of Al-ferrihydrites by reacting aqueous Fe(II) (0.2 and 2.0 mM) with 2-line ferrihydrite containing a range of Al levels substituted within (6-24 mol% Al) or adsorbed on the surface (0.1-27% Γ_max). Here, we show that regardless of the Fe(II) concentration, Al substituted within or adsorbed on ferrihydrite results in diminished secondary mineralization and preservation of ferrihydrite. In contrast to pure ferrihydrite, the concentration of Fe(II) may not in fact influence the mineralization products of Al-compromised ferrihydrites. Furthermore, the secondary mineral profiles upon Fe(II) reaction with ferrihydrite are not only a function of Al concentration but also the mode of Al incorporation. While Al substitution impedes lepidocrocite formation and magnetite nucleation, Al adsorption completely inhibits goethite formation and appears to have a lesser impact on magnetite nucleation. When normalized to total Al content associated with ferrihydrite, Al adsorption results in greater degree of ferrihydrite preservation relative to Al substitution. These findings provide insight into mechanisms that may be responsible for ferrihydrite preservation and low levels of secondary magnetite typically found in sedimentary environments. Considering the preponderance of cation substitution within and adsorption on ferrihydrite in soils and sediments, the reactivity of natural (compromised) ferrihydrites and the subsequent impact on mineral evolution needs to be more fully explored.     

An experimental study on the process of zeolite formation

Results are reported of an experimental study which examined the effect of solution composition on the composition, structure, and crystallization path of phillipsite and merlinoite in the system Na₂OK₂O-Al₂O₃-SiO₂H₂O at 80°C and $\text{pH = 13.34–13.71}$.At a fixed 3.5 M total dissolved silica concentration, zeolite Si/Al ratio was found to be a linear function of pH within the pH range of the experiments.²⁹Si NMR spectra of the initial solutions show that pH determines the distribution of aqueous aluminosilicate species and, as a result, the precipitated zeolite Si/Al ratio. SEM observations reveal that zeolites may precipitate with or without the presence of an intermediary gel phase, depending on solution composition. The growth rate of the zeolites was found to be dependent upon solution pH and total dissolved aluminum concentration. These observations are discussed in terms of their possible applications to natural zeolite paragenesis and serve to delineate the framework of a comprehensive theory for the mechanism of zeolite crystallization from highly alkaline solutions.     

Preparation of iron phosphate from sludge sourced from the electrocoagulation treatment of source-separated urine

In order to prepare iron phosphate by waste sludge, we report a method for effective utilization of the sludge obtained from the electrocoagulation treatment of source-separated urine. The sludge was dissolved with hydrochloric acid and pretreated with H₂O₂ and Na₃PO₄; finally, NaOH was added to precipitate iron phosphate from the solution. Thermal treatment of the precipitate at 750 °C in air yielded crystalline quartz-like anhydrous FePO₄. The precipitate was characterized by a number of thermal techniques such as thermogravimetry/differential thermal analysis, scanning electron microscopy, and X-ray powder diffraction.     

Composition and structural aspects of naturally occurring ferrihydrite

A series of naturally occurring ferrihydrites sampled from an acid mine drainage environment were characterized and compared with synthetic 2-line ferrihydrite using high energy X-ray total scattering and pair distribution function analysis, Scanning Transmission X-ray Microscopy (STXM), Transmission Electron Microscopy (TEM), BET N₂ surface area measurements, and chemical extractions in order to place constraints on their structural and physical properties as a function of composition. Overall, the short- and intermediate-range ordering of the natural samples is comparable to synthetic ferrihydrite. However, with increasing Al, Si, and organic matter contents, a decrease in particle size and an increase in structural disorder were observed. Silica is suspected to have a pronounced effect on the crystallinity of ferrihydrite as a result of its inhibitory effect on Fe polymerization and particle growth, and it is likely complexed at the surfaces of ferrihydrite nanoparticles. Aluminum, on the other hand may substitute for Fe³⁺ in natural ferrihydrite. Organic matter is pervasive and intimately associated with ferrihydrite aggregates, and its presence during ferrihydrite precipitation may have contributed to additional structural disorder. The increase in impurity content affects not only the particle size and structural order of ferrihydrite but may also have a significant effect on its surface reactivity.     

Formation of iron-containing colloids by the weathering of phyllite

The formation of colloids during the weathering of phyllite was investigated by exposing ground phyllite to Milli-Q water. Secondary mineral colloids of 10¹–10² nm were detected in significant concentrations. At pH of about 8.5, the solution concentration of these colloids reached up to 10 mg/L (however, acidification to pH 4.0 prevented the formation of the colloids). The mineralogical composition of the secondary mineral colloids is assumed to be a mixture of ferrihydrite, manganese oxyhydroxides, aluminosilicates, amorphous Al(OH)₃ and gibbsite with possible additions of iron silicates and␣iron-alumino silicates. The colloids were stable over longer periods of time (at least several weeks), even in the presence of suspended ground rock. Direct formation of iron-containing secondary mineral colloids at the rock–water interface by the weathering of rock material is an alternative to the well-known mechanism of iron colloid formation in the bulk of water bodies by mixing of different waters or by aeration of anoxic waters. This direct mechanism is of relevance for colloid production during the weathering of freshly crushed rock in the unsaturated zone as for instance crushed rock in mine waste rock piles. Colloids produced by this mechanism, too, can influence the transport of contaminants such as actinides because these colloids have a large specific surface area and a high sorption affinity.     

Structure vs. composition: A solid-state H and Si NMR study of quenched glasses along the Na2O-SiO2-H2O join

A suite of six hydrous (7 wt.% H₂O) sodium silicate glasses spanning sodium octasilicate to sodium disilicate in composition were analyzed using ²⁹Si single pulse (SP) magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy, ¹H-²⁹Si cross polarization (CP) MAS NMR, and fast MAS ¹H-NMR. From the ²⁹Si SPMAS data it is observed that at low sodium compositions dissolved water significantly depolymerizes the silicate network. At higher sodium contents, however, dissolved H₂O does not affect a significant increase in depolymerization over that predicted based on the Na/Si ratio alone. The fast MAS ¹H-NMR data reveal considerable complexity in proton environments in each of the glasses studied. The fast MAS ¹H-NMR spectra of the highest sodium concentration glasses do not exhibit evidence of signficantly greater fractions of dissolved water as molecular H₂O than the lower sodium concentration glasses requiring that the decrease in polymerization at high sodium contents involves a change in sodium solution mechanism. Variable contact time ¹H-²⁹Si cross polarization (CP) MAS NMR data reveal an increase in the rotating frame spin lattice relaxation rate constant (T_1ρ*) for various Qⁿ species with increasing sodium content that correlates with a reduction in the average ¹H-²⁹Si coupling strength. At the highest sodium concentration, however, T_1ρ* drops significantly, consistent with a change in the Na₂O solution mechanism.     

Evidence for a simple pathway to maghemite in Earth and Mars soils

Soil magnetism is greatly influenced by maghemite (γ-Fe₂O₃), the presence of which is usually attributed to the following: (1) heating of goethite in the presence of organic matter; (2) oxidation of magnetite (Fe₃O₄); or (3) dehydroxylation of lepidocrocite (γ-FeOOH). Formation of the latter two minerals in turn requires the presence of Fe(II) in the system. No laboratory experiment or soil study to date has shown whether maghemite can form from ferrihydrite, a poorly crystalline Fe(III) oxide [∼Fe_4.5(O,OH,H₂O)_13.5], below 250°C. However, ferrihydrite is the usual precursor of goethite (α-FeOOH) and hematite (α-Fe₂O₃), the most frequently occurring crystalline Fe(III) oxides in soils. Here is presented in vitro evidence that ferryhidrite can partly transform into maghemite at 150°C. This transformation occurs upon aging of ferrihydrite precipitated in the presence of phosphate or other ligands capable of ligand exchange with Fe-OH surface groups. This maghemite coexists with hematite and is a transient phase in the transformation of ferrihydrite to hematite, which is apparently stabilized by the adsorbed ligands. Its particle size is small (10 to 30 nm), and its X-ray diffraction pattern exhibits superstructure reflections. The possible formation of maghemite in Mars and in different Earth soils can partly be explained in the light of this pathway with minimal ad hoc assumptions.