Lepidocrocite Structure Research Articles

Solvent-Driven Self-Assembly of One-Dimensional Lepidocrocite Titanium-Oxide-Based Nanofilaments.

Herein, the self-assembly of one-dimensional titanium oxide lepidocrocite nanofilaments in 10 different water miscible organic solvents was investigated. The nanofilament snippets, with minimal cross sections of ∼5 × 7 Å2 and lengths around 30 nm, begin as an aqueous colloidal suspension. Upon addition, and brief mixing, of the colloidal suspension into a given solvent, a multitude of morphologies─seemingly based on the hydrophilicity and polarity of the solvent─emerge. These morphologies vary between sheets, highly networked webs, and discrete fibers, all with no apparent change in the lepidocrocite structure. On the micro- and nanoscale, the morphologies are reminiscent of biological, rather than inorganic, materials. The results of this work give insight into the self-assembly of these materials and offer new pathways for novel macrostructures/morphologies assembled from these highly adsorbent and catalytically active low-dimensional materials.

DFT-guided flux synthesis of a family of layered titanates crystallizing in the lepidocrocite structure type

K0.72Fe0.72Ti1.28O4, a layered mixed-metal oxide belonging to the lepidocrocite structure type, was grown out of a molten KCl-KF mixture at 800 °C. The compound crystallizes in orthorhombic space group Cmcm and exhibits two-dimensional layers, composed of mixed Fe/Ti octahedra, separated by K ions that occupy the interlayer layer space and maintain charge balance. The targeted flux synthesis of the Ni, Cu, and Zn analogs was carried out after density functional theory (DFT) calculations predicted their formation to be energetically favorable, yielding K0.77Ni0.38Ti1.62O4, K0.77Cu0.38Ti1.62O4, and K0.73Zn0.36Ti1.64O4. All four compositions can be prepared via conventional solid-state technique by mixing and heating stoichiometric amounts of KNO3, appropriate transition metal oxide precursors, and TiO2.

Evolutional solid phase and solid-liquid interface uranium immobilization mechanisms by nanoscale zero-valent iron and enhanced uranium stability control strategy

Uranium is a carbon dioxide free nuclear energy, and uranium-contained wastewater poses serious chronic health effects toward human. Nanoscale zero-valent iron (nZVI) separates uranium from wastewater with ultrafast kinetic, high capacity and selectivity. However, the dynamic interfacial uranium binding mechanisms and its stability, controlled by the phase transformation of nZVI, is important but poorly understood. After 120 h reaction, the fresh nZVI was oxidized and transformed to ferrihydrite and then lepidocrocite and hematite. Analysis for the structures and valence states of U, Fe species indicated that the dominate U(VI) uptake mechanism changed from Fe0 induced reduction to adsorption as nZVI transformed to iron (oxyhydr)oxides. Density functional theory calculation revealed that uranyl formed corner-sharing configurations on the surface of iron nanoparticles, and some uranium ions prefer to incorporate into the structure of lepidocrocite than hematite during the crystallization processes of ferrihydrite. Meanwhile, 100 ∼ 10000 μg/L uranyl ions were quickly captured, the residual uranium could be maintained at ∼9.31 μg/L and the content of uranium in reacted iron nanoparticles reached ∼24.16 wt%. Interestingly, the oxidization reduction potential (Eh) was a potential parameter to control uranium immobilization in the CSTR system, and the Eh control strategy could be used to increase the fraction of U(IV)/U(V), sparingly soluble and highly stable, higher than 90 %. The findings augment our understanding of U-nZVI reactions and guide nZVI technology to remedy real uranium-contained wastewater.

Synchronous sequestration of cadmium and fulvic acid by secondary minerals from Fe(II)-catalyzed ferrihydrite transformation

As consequence of the dual demands for pollution control and carbon (C) fixation in soils, Fe(II)-catalyzed mineral transformation may be a promising method to simultaneously immobilize heavy metals or organic matter (OM), but the underlying mechanisms remain unclear. Here, the synchronous sequestration mechanism of cadmium (Cd) and fulvic acid (FA) during Fe(II)-catalyzed the transformation ferrihydrite with C/Fe molar ratio of 0.21 were examined. Mineral phase analysis revealed that increasing the Fe(II) concentration (1–5 mM) favored the transformation of lepidocrocite and goethite to magnetite, and ferrihydrite transformation rate increased with increasing Fe(II) concentration. Color overlays and line profiles of elements depicted that Cd was dominantly adsorbed on the lepidocrocite and goethite surfaces. A positive correlation between the quantity of nonextractable Cd and magnetite further indicated that Cd may be sequestered by magnetite. Meanwhile, FA molecules were adsorbed on goethite surfaces and magnetite aggregates, and incomplete structure of lepidocrocite provide spaces for immobilizing C. Newly formed iron (Fe) (oxyhydr)oxides may immobilize Cd through surface binding, structural substitution, and physical encapsulation. The OM bound to the newly formed Fe (oxyhydr)oxides was rich in aromatic and carboxyl functional groups, which was beneficial for binding Cd, whereas the presence of Cd promoted the generation of nano pore spaces or defects and consequently enhanced FA sequestration. Therefore, Cd immobilization and FA sequestration can be synchronously achieved during the phase transformation. The findings provide a profound insight into various nanoscale mechanisms accounting for the fate of Cd and FA coupled with mineral transformation. The findings also are very helpful for developing strategies for simultaneously immobilizing heavy metals and C in soils.

Synthesis and Electrochemical Properties of Lepidocrocite Titanate- Graphene Heterostructures

Layered titanates with lepidocrocite structure can be used as anodes for lithium, sodium or potassium batteries in conventional or flow cell configurations. Although they are attractive owing to their low cost, low toxicity, and high abundancy, their low conductivity and poor cycling performance are major hurdles for these materials. Here, we used a self-assembly approach to overcome electronic transport limitations and to build in the conductivity by incorporating a conductive component such as reduced graphene oxide (rGO) or into nitrogen doped graphene (NDG) in the van der Waals gaps to make 2D heterostructures.In this presentation, solution-phase synthesis of K0.8[Ti1.73Li0.27]O4 (lepidocrocite titanate, LT) heterostructures with graphene is demonstrated. After synthesis by conventional solid-state routes, K0.8[Ti1.73Li0.27]O4 is proton exchanged and ion-exchanged with bulky cations, causing expansion of the structure, and exfoliated by mechanical shaking. Exfoliated layered titania sheets are self-assembled with either reduced graphene oxide (rGO) or dopamine into heterostructures through flocculation via counter cations (e.g. Mg2+, Na+) or in situ conversion of dopamine into nitrogen doped graphene (NDG), respectively. Both coagulated LT-rGO and LT-NDG heterostructures exhibit capacities which are more than two times higher than the capacities of pristine lepidocrocite titanate (K0.8[Ti1.73Li0.27]O4), and display good capacity retention after 50 cycles in sodium half-cells configurations.

Effects of Al substitution on local structure and morphology of lepidocrocite and its phosphate adsorption kinetics

Aluminum (Al) substitution into the lepidocrocite structure commonly occurs in redoximorphic soils. However, little is known about its effects on the local structure, morphology, and surface properties of lepidocrocite due to its metastability in the environment. In this study, a series of Al-substituted lepidocrocite samples was synthesized and the local structure and surface properties were characterized by X-ray diffraction (XRD), extended X-ray absorption fine structure (EXAFS) spectroscopy, high resolution transmission electron microscopy (HRTEM), zeta potential and phosphate (P) adsorption kinetics. With the substitution of Al into the lepidocrocite structure, the lepidocrocite unit cell volume decreased due to the smaller ionic radius of Al3+ (r = 0.535 Å) than Fe3+ (r = 0.645 Å), and both the corner-sharing (along c-axis) and edge-sharing (along a-axis) Me-Me (Me = Al, Fe) distances decreased, with a more significant decrease in corner-sharing Me-Me distances. In addition, hydrogen bonding in the interlayer regions of the mineral along the b-axis was weakened with Al substitution. These changes of the local structure led to a gradual evolution of the lepidocrocite morphology: the aspect ratio (length to width) decreased with increasing Al substitution, and the Al-substituted lepidocrocite crystals became thinner, resulting in a decrease in the final ratios of coherent scattering domain (CSD) sizes of face (020) to those of terminating face (200) and (002). Theorfore, with increasing Al substitution, the relative proportion of face (020) with doubly coordinated oxygens per specific surface area decreased, leading to an increase of active hydroxyls and P adsorption density at the surface. These results shed new insights on the evolution mechanism of local structure and morphology of Al-substituted lepidocrocite and provided important information for evaluating the surface reactivity and environmental behaviors of lepidocrocite in soils.

First Principles Investigation of Sodium Intercalation Mechanisms into Corrugated Titanate Structures for Sodium-Ion Battery Anodes

Sodium titanates have emerged as attractive anode materials for sodium ion batteries due to their ability to reversibly intercalate sodium ions at low voltages, while exhibiting a rich compositional range with varying crystal structures. In this work we investigate two types of titanates that have been recently synthesized and tested exhibiting similar sloping voltage profiles. The first set of materials is based on Na1+xTi3O6(OH)×2H2O, also known as sodium nonatitanate1,2 (NNT), and the second is based on a family of lepidocrocite structures such as Na0.8Ti1.73Li0.27O4 and Na0.8Ti1.4Mg0.6O4 3. Lepidocrocite titanates consist of corrugated metal oxide layers between which large cations such as potassium or sodium and, optionally, water are inserted. The layers can be stacked in-phase (P-type) or anti-phase (C-type) with one another. NNT shares some structural features with the lepidocrocite type titanates, having a stepped layered structure consisting of edge-sharing Ti6O14 units linked by corners to form steps, with sodium ions and water between the layers. A dehydration process irreversibly removes the water while maintaining the gross structural features of NNT. Using density functional theory (DFT) we investigated the structural changes occurring during sodium intercalation, and calculated voltage profiles for the different materials. We also calculated changes to the sodium diffusion energy barriers at different sodium concentrations. Structural results indicate that sodium intercalation is a site-limited process, with the NNT having higher capacity for sodium insertion compared to the lepidocrocite structures. Our results also indicate that the layers in the lepidocrocite Na0.8Ti1.73Li0.27O4 structure in space group Cmmm are able to accommodate more sodium cations by shifting layers, compared to the same composition in space group Pmmm. This is corroborated by the experimental evidence on samples with the different stacking arrangements, prepared by different heating regimes. The ability for layers to shift is also attributed to the presence of the lower valence ions Li or Mg in the metal oxide layers, which have different abilities to transition from the titanium layer to sodium sites, depending on charge density. The energy landscape for the lepidocrocite structure indicates strong M-M interactions with secondary ions such as Li or Mg preferring to distribute among different titanium layers. The presence of lower valence cations also reduces electrostatic repulsion forces leading to sodium binding energies between 0.2-0.3 eV, which dominate sodium diffusion pathways in the lepidocrocite structure. Sodium diffusion is also found to have preferential orientation perpendicular to the corrugated titanium layers. Energy barriers are also examined at different sodium concentrations in order to determine changes at different states of charge. References Shirpour M., Cabana J., Doeff M. “New materials based on a layered sodium titanate for dual electrochemical Na and Li intercalation systems”, Energy Environ. Sci., 6, 2538 (2013).Seshadri D., Shirpour M., Doeff M. “Electrochemical Properties of Electrodes Derived from NaTi3O6OH·2H2O in Sodium and Lithium Cells” J. Electrochem. Soc. 2015Vol 162, issue 1, A52-A59.Shirpour M., Cabana J., Doeff M. “Lepidocrocite-type Layered Titanate Structures: New Lithium and Sodium Ion Intercalation Anode Materials.” Chemistry of Materials 2014 26 (8), 2502-2512.

Giant humidity dependence of conductivity in a single exfoliated titania nanosheet

We have investigated an in-plane electrical transport in an individual titania nanosheet, which forms a lepidocrocite structure with a few atomic layer thickness. The conductivity in the titania nanosheet varied five orders for the change of the relative humidity from 45% to 95%. The drastic change in the conductivity is due to a sensitive response with respect to water molecules adsorbed on the surface of the two-dimensional nanosheet. The results may open an avenue for nanodevices built from various oxide nanosheets.

Raman Investigation of Sodium Titanate Nanotubes under Hydrostatic Pressures up to 26.9 GPa

High pressure behavior of sodium titanate nanotubes (Na2Ti2O5) is investigated by Raman spectroscopy in a diamond anvil cell (DAC) at room temperature. The two pressure-induced irreversible phase transitions are observed under the given pressure. One occurs at about 4.2 GPa accompanied with a new Raman peak emerging at 834 cm−1 which results from the lattice distortion of the Ti-O network in titanate nanotubes. It can be can be assigned to Ti-O lattice vibrations within lepidocrocite-type (H0.7Ti1.825V0.175O 4·H2O)TiO6 octahedral host layers with V being vacancy. The structure of the nanotubes transforms to orthorhombic lepidocrocite structure. Another amorphous phase transition occurs at 16.7 GPa. This phase transition is induced by the collapse of titanate nanotubes. All the Raman bands shift toward higher wavenumbers with a pressure dependence ranging from 1.58–5.6 cm−1/GPa.

Lepidocrocite-like structure of the TiO2 monolayer grown on Ag(100)

Titanium oxide ultrathin films were grown on Ag(100) by evaporation of titanium in an O(2) atmosphere. The growth of the oxide films was monitored by means of XPS. The Ti2p XPS spectra indicate the formation of films with a TiO(2) stoichiometry in the whole range of coverages studied here. The STM results show that titania films appear to grow as islands of uniform thickness up to the completion of the first layer. Up to the formation of one complete monolayer, a (5 × 1) LEED pattern is observed. This pattern can be interpreted as a coincidence mesh and the lattice parameters of the oxide layer are very close to those of TiO(2) films with a lepidocrocite-like structure. However, the STM images show a long-range coincidence between the periodicity of the oxide film and that of the substrate along the short side of the oxide unit cell revealing that this lattice parameter is not exactly equal to that of the substrate. Above the monolayer coverage, additional spots become visible in the LEED pattern which can be interpreted in terms of the unit cell of rutile (110). The structural determination was carried out for the monolayer of titania by means of XPD and LEED intensity analysis. The results of these investigations demonstrate that the titania monolayer has a lepidocrocite-like structure. The DFT calculations carried out for the titania monolayer show the higher stability of the lepidocrocite structure with respect to other structures derived from crystallographic planes of bulk TiO(2) phases. Moreover, these calculations allow us to determine the oxide-substrate interface energy as well as to clarify the effect of the strain on the stability of the oxide layer.

Heterogeneous photodegradation of bisphenol A with iron oxides and oxalate in aqueous solution

To understand the degradation of endocrine disrupting chemicals (EDCs) with existence of iron oxides and polycarboxylic acids in the natural environment, the photodegradation of bisphenol A (BPA) at the interface of iron oxides under UV illumination was conducted. Four iron oxides were prepared by a hydrothermal process and then sintered at different temperatures of 65, 280, 310, and 420 °C named “IO-65,” “IO-280,” “IO-310,” and “IO-420,” respectively. The prepared iron oxides were characterized by X-ray diffraction (XRD) and Brunauer–Emmett–Teller (BET) methods. The XRD pattern of IO-65 showed a crystal structure of lepidocrocite ( γ-FeOOH) and that of IO-420 demonstrated a crystal structure of hematite ( α-Fe 2O 3), while IO-280 and IO-310 have the mixed crystal structures of maghemite ( γ-Fe 2O 3) and hematite. The BET results revealed that the specific surface areas decreased with the increase of sintering temperature. The results demonstrated that the photodegradation of BPA depends strongly on the properties of iron oxides and oxalate, and pH. The properties of iron oxides influenced strongly the dependence of the BPA degradation on the oxalate concentration. The optimal initial concentrations of oxalate for BPA degradation under UV illumination were determined to be 2.0, 2.0, 2.4, and 2.0 mM for IO-65, IO-280, IO-310, and IO-420, respectively. The first-order kinetic constants k for BPA degradation under UV illumination in the presence of oxalate with the optimal initial concentration are ranked as IO-280 > IO-310 > IO-65 > IO-420. The experiments demonstrated that the optimal pH value should be in the range of 3–4. Furthermore, the dependence of BPA degradation should be also attributable to the formation of the dissolved Fe–oxalate in the solution and the adsorbed Fe–oxalate on the surface of iron oxides, and also the formation of hydrogen peroxide.

Chemical Bonding Character and Physicochemical Properties of Mesoporous Zinc Oxide-Layered Titanate Nanocomposites

We have investigated the chemical bonding character and physicochemical properties of mesoporous zinc oxide-layered titanate nanocomposites synthesized by an exfoliation−restacking route. According to our analyses using X-ray diffraction, high resolution transmission electron microscopy, and N2 adsorption−desorption isotherm measurements, nanocrystalline zinc oxides are stabilized not only in the interlayer space of the host titanate but also in the mesopores of the nanocomposites formed by the house-of-card type stacking of the crystallites. X-ray absorption spectroscopic analyses at Ti K- and Zn K-edges clearly demonstrate that nanocrystalline zinc oxides crystallized with a Wurzite type structure, and the lepidocrocite structure of the host titanate remains unchanged after hybridization and postcalcination. Upon hybridization with zinc oxide nanoparticles, the photocatalytic activity of layered titanate is enhanced with respect to the oxidative photodegradation of phenol and dichloroacetate. But of gre...

Bottom-Up Assembly of Single-Domain Titania Nanosheets on(1×2)−Pt(110)

A bottom-up route towards the synthesis of titania nanosheets is explored, alternative to the exfoliation of layered titanates. Nanosheets are assembled from the constituent elements and epitaxially matched to a suitable substrate: (1 x 2)-Pt(110). Their basic lepidocrocite structure is modulated at the nanoscale due to coincidence with the substrate. Density functional calculations reveal the structure details of the nanosheet, which is also shown to be in close relationship with a (001)-oriented anatase bilayer.

Adsorption of Cr(VI) ion on synthetic hydrated oxides of iron

The adsorption of Cr(VI) ion on synthetic hydrated oxides of iron (goethite (α-FeOOH) and lepidocrocite (γ-FeOOH)) with different crystallinities was studied in order to remove the Cr(VI) ion from polluted water. At pH 4.5, the amounts of the adsorbed Cr(V1) ion on goethite and lepidocrocite were 51.9 and 62.2 mmol g−, respectively. Probably, this is due to the fact that the structure of lepidocrocite is more open than that of goethite. Lepidocrocite, therefore, is more effective for the removal of Cr( V1) ion from Cr(Vl)-contaminated and acidic groundwater and sewage.

57Fe Mössbauer Effect Study of Al-Substituted Lepidocrocites

AbstractSeven Al-containing lepidocrocite samples, γ-Fe1−xAlxOOH, prepared from FeCl2/Al(N03)3 solutions with initial Al/(Al + Fe) mole ratios Ci of 0.0025, 0.01, 0.025, 0.05, 0.075, 0.10 and 0.15 mol/mol, were examined by means of Mössbauer spectroscopy at room temperature (RT) and at various temperatures in the range of 8 to 80 K. The spectra at RT and 80°K consist of broadened quadrupole doublets and were analyzed in terms of a single doublet and of a model-independent quadrupole-splitting distribution, the latter yielding the best fit. The observed variations of the quadrupole-splitting parameters with increasing Ci are inconclusive as to whether the Al cations are substituting into the structure. The temperature at which the onset of magnetic ordering is reflected in the spectra, was measured by the thermoscan method with zero source velocity. A gradual shift from 50 K for Ci = 0.0025 mol/mol to 44 K for Ci = 0.10 mol/mol was observed for that temperature. As compared to earlier studies of Al-free γ-FeOOH samples with similar morphological characteristics, the fractional doublet area in the mixed sextet-doublet spectra at 35 K is significantly higher for the present lepidocrocites. This observation is ascribed to the substitution of Al cations into the lepidocrocite structure. A similar conclusion is inferred from the variation with Ci of the maximum-probability hyperfine field derived from the spectra recorded at 8 K and fitted with a model-independent hyperfine-field distribution. The magnetic results suggest that for the sample corresponding to Ci = 0.15 mol/mol, not all of the initially present Al has been incorporated into the structure.

The Transformation of Lepidocrocite During Heating: A Magnetic and Spectroscopic Study

AbstractInfrared (IR) spectroscopy, in combination with magnetic methods, was used to study the thermally induced transformation of synthetic lepidocrocite (γ-FeOOH) to maghemite (γ-Fe2O3). Magnetic analyses showed that the thermal conversion began at about 175°C with the formation of superparamagnetic maghemite clusters. The overall structural transformation to ferrimagnetic γ-Fe2O3 occurred at 200°C and was complete around 300°C. At higher temperatures, the maghemite converted into hematite (α-Fe2O3). Observation of the transformation from γ-FeOOH to γ-Fe2O3 using variable-temperature IR spectroscopy indicated that dehydroxilation on a molecular level was initiated between 145°C and 155°C. The lag time between the onset of the breaking of OH bonds and the release of H2O from lepidocrocite around 175°C can be explained by diffusive processes. Overall dehydroxilation and the subsequent breakdown of the lepidocrocite structure was complete below 219°C. The comparison of the magnetic and IR data provides evidence that the dehydroxilation may precede the structural conversion to maghemite.

The influence of concentration on the oxidation of ferrous hydroxide in acidic sulphated aqueous medium: Particle size analysis of lepidocrocite

The evolution of the oxidation end products of ferrous hydroxide in the acidic sulphated aqueous medium at R = {[FeSO 4]/[NaOH]} initial = 0.5833 has been investigated at various initial concentrations of ferrous ions. The oxidation end products consist of lepidocrocite and goethite at relatively higher initial concentration and mainly of lepidocrocite at lower concentrations. The crystal structure of lepidocrocite is not modified as the initial concentration is decreased, contrary to what has been noticed for goethite in the basic medium.

Activated state in the lepidocrocite structure during thermal treatment

change of AMP against ATP and of calcium-flee calmodulin against calcium-loaded calmodulin the ATPase complex is workable again. Obviously, the energy transfer via calcium has the advantage that in case of emergency needs great amounts of free energy can be made available in a short time independently of the ATP supply. This is done by. flushing calcium-free calmodulin produced by continuous pumping and stored in the nearly calcium-flee cytosol with calcium coming from the periphery or any other calcium source of the cell. Thus the calcium/calmodulin system is part of a storage power station of the cell. In the pumping process based on the calcium transfer by calmodulin the lowest calcium concentration is set by the spontaneous dissociation of the calcium/calmodulin complex. This is consistent with the assumption made above to evaluate the upper limit of the free energy production by the calcium-loading of calmodulin. If a sustained stimulation results in a sustained reaction of the cell, the detour, via the cytosol, in the transport of calcium-loaded and calcium-flee calmodulin between the adenylate cyclase complex and the ATPase complex may be avoided. So the calcium cycling in the membrane region, described in [4] will be established. An increase in the concentration of calcium-loaded calmodulin in the cytosol not only starts the pumping process mentioned above but also activates phosphodiesterase which, as is well known, destroys cAMP by hydrolysis [2]. The main difference in the new concept as compared to the former models is seen in the fact that it attributes to calcium not only the function of a messenger but also the function of a transporter for the free energy needed to start endergonic reactions. The transfer of free energy by calcium does not need to be restricted to the production of cAMP for the initiation of chemical reactions. It can be expected that also for other endergonic processes in the cell, e.g., for the generation of mechanical work the transfer of free energy by calcium is of importance.

The Influence of Aluminum on Iron Oxides. VII. Substitution of Al for Fe in Synthetic Lepidocrocite

AbstractThe presence of Al hydroxy species in solution during the synthesis of lepidocrocite had been previously found to influence the reaction towards goethite formation. However, under certain conditions, which are not unrealistic in terms of the natural soil environment, this influence does not occur, and Al appears to substitute for Fe(III) in the lepidocrocite structure. This substitution causes the unit-cell dimensions to decrease along the “a” direction and to increase along the “b.” From the differential line broadening of X-ray powder diffraction peaks, the incorporation of Al was found to inhibit crystal growth preferentially in the b-axis direction, the hkl peaks being more broadened the higher the value of k relative to h and l. Al-substituted lepidocrocites have been suggested to occur in soils, and although they can be synthesized under conditions approaching those expected in soils, it is considered that their formation in nature is unlikely or restricted to unusual environments.

The Influence of [Fe(II)], [Si], and pH on the formation of lepidocrocite and ferrihydrite during oxidation of aqueous FeCl2 solutions

AbstractLepidocrocite was synthesized from the oxidation of FeCl2 solutions with and without soluble Si and at various Fe(II) concentrations and pH. Pure well crystallized lepidocrocite was only obtained if CO2 was excluded (to prevent goethite formation) and if pH was kept between 5 and 7 (to prevent maghemite and ferrihydrite formation). Si in solution prevented crystallization of lepidocrocite with increasing Si/Fe atomic ratio (varied between 0 and 0·156) and at the same ratio with decreasing pH. Most of the Si added is probably included in the lepidocrocite and ferrihydrite structure (up to 4·4% Si) and these Si contents are linearly correlated with the initial Si/Fe ratio. No indication from XRD and i.r. spectroscopy was found for changes in the lepidocrocite structure due to Si uptake. The ferrihydrite containing 4·35% Si showed a broad Si-O-stretching vibration which in contrast to pure silica (1080 cm−1) is shifted to lower frequencies (940 cm−1) indicating some Si-O-Fe bonds.