Colloidal Hematite Research Articles

Influence of Sodium Halides (NaF, NaCl, NaBr, NaI) on the Photocatalytic Performance of Hydrothermally Synthesized Hematite Photoanodes

It has been suggested that a high concentration of Fe(3+) in solution, a low pH, and noncomplexing ions of high ionic strength are all essential for developing a high-quality hematite array. Our curiosity was piqued regarding the role of the electrolyte ions in the hydrothermal synthesis of hematite photoanodes. In this study, we prepared hematite photoanodes hydrothermally from precursor solutions of 0.1 M FeCl3 at pH 1.55 with a background electrolyte of 1.0 M sodium halide (NaF, NaCl, NaBr, or NaI). We compared the structures and properties of the as-obtained hematite photoanodes with those of the material prepared in 1.0 M NaNO3, the most widely adopted electrolyte in previous studies. Among our studied systems, we found that the hematite photoanode prepared in NaCl solution was the only one possessing properties similar to those of the sample obtained from the NaNO3 solution-most importantly in terms of photoelectrochemical performance (ca. 0.2 mA/cm(2) with +0.4 V vs SCE). The hematites obtained from the NaF, NaBr, and NaI solutions exhibited much lower (by approximately 2 orders of magnitude) photocurrent densities under the same conditions, possibly because of their relatively less ordered crystallinity and the absence of rodlike morphologies. Because the synthetic protocol was identical in each case, we believe that these two distinct features reflect the environments in which these hematite photoanodes were formed. Consistent with the latest studies reported in the literature of the X-ray photoelectron spectra of fast-frozen hematite colloids in aqueous solutions, it appears that the degree of surface ion loading at the electrolyte-hematite interface (Stern layer) is critical during the development of hematite photoanodes. We suspect that a lower ion surface loading benefits the hematite developing relatively higher-order and a rodlike texture, thereby improving the photoelectrochemical activity.

Formation of crystalline PuO2+x·nH2O nanoparticles upon sorption of Pu(V,VI) onto hematite

It has been recognized that natural aquatic colloids can readily sorb actinide elements, including plutonium, whose behavior is complicated by its multiple valence states and the possibility of redox reactions under environmental conditions. In this paper, the sorption and surface-mediated redox transformations of hexavalent plutonium on synthetic well-characterized hematite colloids are studied in a series of batch sorption experiments. The variation in the kinetics of the Pu–hematite interactions, Pu-L3-XAFS, and HRTEM over a broad range of total concentrations of Pu have been studied in an attempt to define the molecular-level speciation of Pu. The surface-mediated slow reduction of Pu(V/VI) results in the formation of crystalline nanoparticles of PuO2+x·nH2O approximately 1.5nm in size at [Pu]tot⩾10−9M. This result is confirmed independently by HRTEM images of Pu-containing particles and through the identification in the EXAFS of a Pu neighbor shell at 3.8Å. The formation of such nanoparticles potentially influences the colloid-mediated transport of Pu in the subsurface environment because of the very slow leaching of Pu from the hematite colloids.

Dispersion Stability and Electrokinetic Properties of Intrinsic Plutonium Colloids: Implications for Subsurface Transport

Subsurface transport of plutonium (Pu) may be facilitated by the formation of intrinsic Pu colloids. While this colloid-facilitated transport is largely governed by the electrokinetic properties and dispersion stability (resistance to aggregation) of the colloids, reported experimental data is scarce. Here, we quantify the dependence of ζ-potential of intrinsic Pu(IV) colloids on pH and their aggregation rate on ionic strength. Results indicate an isoelectric point of pH 8.6 and a critical coagulation concentration of 0.1 M of 1:1 electrolyte at pH 11.4. The ζ-potential/pH dependence of the Pu(IV) colloids is similar to that of goethite and hematite colloids. Colloid interaction energy calculations using these values reveal an effective Hamaker constant of the intrinsic Pu(IV) colloids in water of 1.85 × 10(-19) J, corresponding to a relative permittivity of 6.21 and refractive index of 2.33, in agreement with first principles calculations. This relatively high Hamaker constant combined with the positive charge of Pu(IV) colloids under typical groundwater aquifer conditions led to two contradicting hypotheses: (a) the Pu(IV) colloids will exhibit significant aggregation and deposition, leading to a negligible subsurface transport or (b) the Pu(IV) colloids will associate with the relatively stable native groundwater colloids, leading to a considerable subsurface transport. Packed column transport experiments supported the second hypothesis.

Self-assembly of colloidal hematite cubes: a microradian X-ray diffraction exploration of sedimentary crystals

The structure of spontaneously formed crystals in the sediments of colloidal hematite (α-Fe2O3) cubes is studied using microradian X-ray diffraction. The hematite cubes possess a superball shape and a permanent magnetic dipole moment, both influencing the self-assembly behaviour and inducing directionality in different ways. To control the inter-cube interactions, the cubes were dispersed in different solvents and/or coated with a silica layer and allowed to form sediments under the simultaneous effect of the gravitational field and an applied magnetic field. The microradian X-ray diffraction revealed that changing the double layer repulsion that opposes the magnetic and van der Waals attractions can induce short to long-range ordering in the fast sedimenting systems of the cubes. The presence of an external magnetic field during sedimentation induced the formation of a single crystal with long-range order that showed different symmetries, rectangular or hexagonal, close to the fluid–solid interface. The identified 3D crystal structure of both was found to have a body centred monoclinic lattice periodicity that rotates deeper in the sediment due to gravitational compression. This experimental structure differs from the expected crystal lattices determined by simulations and is most likely due to the effects of long-range repulsion and dipole–dipole interactions present here.

In situ hard X-ray microscopy of self-assembly in colloidal suspensions

An experimental setup combining full-field transmission X-ray microscopy (TXM) and microradian X-ray diffraction (μradXRD) is tested for the in situ study of self-organization of colloidal dispersions of anisotropic particles. Averaged structural information of the samples is determined by μradXRD and local morphology is determined by TXM. Utilization of hard X-rays (12.2 keV) and the ease of switching from diffraction mode to microscopy mode makes such a setup a unique tool, especially for the study of opaque colloidal systems. We demonstrate diffraction patterns together with real space images of the following morphologies: smectic structures in the sediment of colloidal silica rods, the reorientation of a smectic phase of goethite particles in an elevated magnetic field and an interface region between isotropic and ordered phases in a dispersion of colloidal hematite cubes.

Polymerization of Silicate on Hematite Surfaces and Its Influence on Arsenic Sorption

Iron oxides and oxyhydroxides are important sorbents for arsenic in soils, sediments, and water treatment systems, but their long-term potential for arsenic retention may be diminished by the formation of polymeric silicate on their surfaces. To study these interactions, we first investigated the sorption of silicate to colloidal hematite (α-Fe(2)O(3)) in short-term (48 h) and long-term (210 days) batch experiments. The polymerization of silicate on the hematite surface was monitored by attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. The pH dependence of silicate sorption exhibited a maximum between pH 9.0 and 9.5. The condensation of silicate on hematite surfaces adsorbed from monomeric silicate solutions steadily continued over the 210 day period, whereby surface polymerization was slower at pH 3 than at pH 6. The effect of silicate surface polymerization on arsenate and arsenite sorption was studied by use of hematite pre-equilibrated with silicate for different time periods of up to 210 days. The competitive effect of silicate on arsenate and arsenite sorption increased with increasing silicate pre-equilibration time. Only under strongly acidic conditions (pH 3), where silicate sorption was weakest and surface polymerization was slowest, was arsenate and arsenite sorption not affected by the presence of silicate. We conclude that the long-term exposure to dissolved silicate can decrease the potential of natural iron (oxyhydr)oxides for adsorbing inorganic arsenic.

Fractionation and kinetic processes of humic acid upon adsorption on colloidal hematite in aqueous solution with phosphate

Batch experiments were carried out to investigate fractionation and kinetics of humic acid (HA) during adsorption onto hematite and the effect of phosphate. The concentrations and weight-average molecular weight (Mw) of HA in solution were determined by total organic carbon (TOC) analyzer and high-performance size exclusion chromatography (HPSEC). Addition of phosphate (simultaneously or beforehand) decreased HA adsorption due to competition. Fractions of HA with Mw values larger than 4000Da were preferably adsorbed. The adsorption kinetics can be described with Pseudo-second order model or Elovich kinetic model. Fractions of HA with relatively low Mw (3000–4000Da) were quickly adsorbed in the first hour, and then were replaced slowly by larger ones (>5000Da). This phenomenon could be explained by fast diffusion of relatively small HA particles from solution to mineral surface, but the overall binding affinity is higher for the bigger particles as a result of more reactive groups present on bigger particles compared to smaller particles. At last, molecular weight distribution results in adsorption isotherms and kinetics indicated that the replacement between relatively small and larger fractions of HA could last more than 24h.

X-ray photoelectron spectroscopy of fast-frozen hematite colloids in aqueous solutions. 4. Coexistence of alkali metal (Na+, K+, Rb+, Cs+) and chloride ions

Colloidal suspensions of hematite in contact with aqueous solutions of 50mM alkali metal chloride electrolytes (NaCl, KCl, RbCl, CsCl) were investigated by cryogenic X-ray photoelectron spectroscopy (XPS) and electrophoretic mobility. Suspension pH values were varied from 2 to 11 in order to evaluate effects of positively- and negatively-charged hematite surfaces. XPS revealed coexisting cations and chloride ions both below and above the point of zero charge. Concentration profiles of adsorbed cations point to a Hofmeister series in the order of Na+>K+>Rb+≈Cs+. Binding energies of photoelectrons emitted from electrolyte ions increased with pH at roughly 0.04eV per pH unit. This shift was attributed to variations in the surface electric potential of hematite. This effect, compounded by rises in aliphatic carbon signals with pH, called for referencing of all spectra to the 530.0eV oxide component of the hematite O1s spectrum. This departure from the traditional use of the external C 1s 285.0eV peak is hereby proposed for cryogenic XPS studies of interfacial reactions involving hematite.

Small-angle scattering from spindle-shaped colloidal hematite particles in external magnetic fields

In the presence of phosphate anions, spindle-shaped mesoscale hematite particles can be preparedviacontrolled precipitation of iron(III) chloride. The aspect ratio of the particles is determined by the concentration of phosphate anions selectively covering specific crystal surfaces and thus enabling anisotropic growth of the particles. The scattering function for suspensions of polydisperse spindles is derived and used to analyse the small-angle scattering resulting from these particles. In the presence of an external magnetic field, the particles align perpendicular to the field direction as a result of the negative anisotropy of their magnetic susceptibility \Delta\chi. Hereby, an isotropic–nematic phase transition can be induced in external magnetic fields.

X-ray Photoelectron Spectroscopy of Fast-Frozen Hematite Colloids in Aqueous Solutions. 3. Stabilization of Ammonium Species by Surface (Hydr)oxo Groups

The speciation of ammonium at the hematite/water interface was probed by cryogenic X-ray photoelectron spectroscopy. Wet pastes of colloidal hematite spheroids equilibrated in aqueous solutions of 50 mM NH4Cl exhibit distinctive pH-sensitive N 1s peaks for both NH4+ (401.7 eV) and NH3 (400.1 eV), yet total N/Fe ratios remain relatively invariant (0.029 ± 0.006) throughout the pH 2.2−10.5 range. Both NH4+ and NH3 species coexist throughout most of the tested pH range. NH4+ is most likely stabilized at the interface by hydrogen bonding with surface (hydr)oxo groups. A cationic sorption edge for NH3 is driven by proton abstraction of NH4+ by (hydr)oxo groups, forming surface complexes of the type ≡Fe−OH···NH3. These interactions shift the NH4+/NH3 equilibrium from pKa = 9.3 in water to 8.4 at the interface. Removal of excess water by vacuum dehydration induces, on the other hand, formation of NH2 directly bound to surface Fe atoms. These results underscore distinct ammonium species in contact with mineral su...

Plutonium sorption onto hematite colloids at femto- and nanomolar concentrations

Abstract The surface-mediated reduction of Pu(VI) upon sorption onto hematite colloids is addressed in the paper that bring some new light on the mechanisms of interaction of Pu in high valence states with different Fe(III) oxides. We study the sorption of Pu taken at two different total concentrations around 10−9 and 10−14 M using 239Pu and short-lived 237Pu tracer. In order to conclude on the mechanisms of Pu sorption onto hematite, the kinetics of sorption of Pu(IV) and Pu(VI) at two concenrations is studied as well as its leching behavior. The rate limiting step in Pu(VI) sorption is its surface-mediated reduction. The experimental evidence of the polymerization of Pu(IV) on the surface taken at Pu total concentration of ∼10−9 M is presented based on the kinetics of sorption and leaching behavior.

Self-assembly behavior of hematite nanoparticles with controllable anisotropic morphology

In this work, complex structures such as long chain, “semi-flexible” chain, “threefold junction” and network were formed by self-assembly of colloidal hematite nanoparticles. Morphology of these colloidal nanoparticles used as building blocks transformed from truncated rhombohedra, hexagon to pseudo-hexagon by altering reaction time and surfactants. By further observation using HRTEM, these nanoparticles were confirmed to grow along the c-axis. It was found that the molecular structures of surfactants make great influence on the transformation of bonding modes between carboxyl and iron atom on the surface. Then crystal growth rate was changed. It led to two opposite growth trends along the c-axis. More interestingly, the chains formed by these colloidal nanoparticles were also assembled along the c-axis. Meanwhile, configuration diversity seemed related to the morphological anisotropy along the c-axis. It was believed that two main forces between the nanoparticles were responsible for the various configurations, magnetic dipole–dipole and exchange-coupling interaction. The morphological anisotropy was considered to play a key role in the coordination of the two interactions which led to different complex structures by self-assembly. Discussion was taken to explain the formation of these interesting configurations.

The influence of dicarboxylic acid structure on the stability of colloidal hematite

Low molecular weight organic acids comprise an important pool of reactive ligands in aquatic systems. These acids readily bind to nano-sized mineral particles and thereby strongly influence a particle’s physicochemical behavior. Predicting this influence requires the integration of molecular-level details that control surface complexation mechanisms and structures with macro-scale observations of mineral colloid behavior. We report on the aggregation kinetics of nano-sized hematite in the presence of fumaric acid and maleic acid, which are naturally occurring dicarboxylic acids of similar size and structure. Our results indicate that the structure and orientation of the adsorbed dianion at the hematite surface, not the adsorption mechanism, defines the resulting effect. Maleate, which directs both carboxyl groups to the surface in the form of inner- and outer-sphere surface complexes, enhances colloidal stability. Fumarate, however, which binds to the hematite surface as an outer-sphere complex with just one carboxyl group only slightly influenced particle stability. This outcome suggests that subtle differences in the structure of adsorbed acids produce important differences in the physicochemical behavior of particles in dilute aquatic systems.

X-ray Photoelectron Spectroscopy of Fast-Frozen Hematite Colloids in Aqueous Solutions. 2. Tracing the Relationship between Surface Charge and Electrolyte Adsorption

Colloidal-sized hematite spheroids exposed to aqueous NaCl solutions were investigated by X-ray photoelectron spectroscopy using the fast-frozen technique. The O1s region provided evidence for (de)protonation reactions of surface (hydr)oxo groups of OH-enriched/O-depleted hematite surfaces. These results were also correlated to changes in sodium (Na 1s) and chloride (Cl 2p) contents with pH. Electrolyte ion surface loadings were successfully predicted using a classic thermodynamic adsorption model normalized for surface site density. These efforts pointed to ion-specific inner-Helmholtz plane capacitances.

XPS of Fast-Frozen Hematite Colloids in NaCl Aqueous Solutions: I. Evidence for the Formation of Multiple Layers of Hydrated Sodium and Chloride Ions Induced by the {001} Basal Plane

The composition of fast-frozen wet pastes of hematite particles of different morphologies equilibrated in NaCl aqueous solutions was investigated by X-ray photoelectron spectroscopy. Two hematite p ...

Effect of humic acid on sorption of neptunium on hematite colloids

Sorption of neptunium on hematite colloids has been studied at varying pH values (3–10) in absence and presence of humic acid using239Np as a tracer. The experiments were carried out under aerobic and anaerobic conditions. The results show negligible sorption of Np(V) on hematite colloids at lower pH<5, beyond which it was found to increase sharply reaching ∼90% at pH 10. Presence of humic acid (2mg/L) was found to decrease the sorption of Np(V) on hematite at higher pH values while at lower pH the effect was found to be negligible. The stability constant of Np(V)-humate complex (logβ) was determined to be 4.2±0.2 by solvent extraction method. Sorption of humic acid on hematite as a function of pH was also measured by UV-visible spectrophotometry. Under nitrogen atmosphere and in presence of sodium dithionite, Np(V) was reduced to Np(IV) as evident by the enhanced sorption at all pH values, which was further enhanced in presence of humic acid at lower pH. However at higher pH values, humic acid does not affect the sorption of neptunium under reducing conditions, which could be due to competing reduction process involving ferric ions obtained from dissolved hematite.

Oil-in-Water Emulsification Induced by Ellipsoidal Hematite Colloids: Evidence for Hydrolysis-Mediated Self-Assembly

We report on the preparation of a novel type of particle-stabilized oil-in-water emulsions. The emulsification mechanism comprises partial hydrolysis of the oil phase promoted by the alkaline surface of ellipsoidal hematite colloids stabilized by tetramethylammonium hydroxide. This mechanism yields monodisperse oil droplets with embedded single ellipsoidal particles. The emulsions, which are stable for at least several months, can be polymerized by radical initiation, to yield latex-like particles with interesting optical and magnetic properties due to their anisotropic hematite cores. Moreover, we show that complex composite core-shell colloids can be prepared by PMMA growth and silica deposition on polymerized emulsion droplets. Finally, as an example of a possible application for our system, we have measured translational and rotational diffusion coefficients of hematite-stabilized oil droplets by depolarized dynamic light scattering. The latter technique can also be employed to monitor the spontaneous emulsification in time.

Effect of humic acid on the sorption of technetium on hematite colloids using 95mTc and 96Tc as tracers

Sorption of technetium on hematite colloids, at varying pH (3–10), has been studied in absence and presence of humic acid using 95mTc-96Tc radiotracers. Technetium was found to be weakly sorbed on hematite at lower pH (<5) values, while no sorption was observed at higher pH values. Humic acid was found to have no effect on the sorption of technetium on hematite under aerobic conditions, while at lower pH values small reduction was observed which was attributed to the reduced zeta potential of the hematite colloids owing to the strong sorption of humic acid.

Geology and geochemical aspects of lode gold mineralization at Dimako–Mboscorro, SE Cameroon

The Dimako–Mboscorro lode gold deposit has two styles of mineralization: quartz veins (4.5–54 ppm Au) and altered wallrock (up to 2.6 ppm Au). The veins occur in steeply dipping brittle–ductile shear zones in a foliated biotite metagranite. The alteration envelop around the gold-bearing quartz veins shows lateral zoning comprising a silicified/ ferruginized and mylonitic zone (SFZ) at the centre, through a transitional sericite zone (SZ), to an outer, slightly sheared, but mineralogically less altered, though bleached, metagranite zone (BZ) at the shear zone boundaries. Hydrothermal alteration in these zones is characterized by silicification (SFZ, SZ), hematite formation (SFZ), alkali metasomatism with muscovite/sericite formation (SFZ, SZ, to a much lesser extent BZ), and pyritization (SFZ, SZ). The auriferous quartz veins are concordant to the host structure but discordant to the foliation in the metagranite. The main ore minerals in the veins and in the SFZ and SZ zones are gold, hydrothermal hematite, sphalerite, galena, pyrite, chalcopyrite, arsenopyrite, chalcocite, covellite and colloidal supergene hematite. Gold occurs as inclusions in sphalerite (15.7±0.4 wt% Ag; 83.6±0.2 wt% Au), as coarse grains associated with euhedral hematite pseudomorphs (18.5±1.0 wt% Ag; 82.0±1.1 wt% Au), as stringers within fractures in quartz (13.3±0.1 wt% Ag; 87.1±0.5 wt% Au), as well as fine to coarse grains related to limonite, goethite and colloidal hematite (8.1±1.8 wt% Ag; 92.1±2.2 wt% Au). A hydrothermal metal association of Ag, Au, As, Ba, Pb, V, W, and Zn is identified in the wallrock and quartz veins. These metals, together with K-enrichment, are potential pathfinder elements for wider lithogeochemical exploration in this region, especially when combined with the lateral wallrock zonation pattern. Preliminary fluid inclusion data from the auriferous quartz vein samples indicate that gold was probably deposited from low salinity (&lt;1 to 8 wt% eq. NaCl, with mean at 5.52 wt% eq. NaCl) H 2 O–CO 2 fluids that have total homogenization temperatures of 263–335°C.

Ferricoxychloride and Hematite Nanoparticles: Synthesis and Phase Transformation

A synthetic procedure based on thermal hydrolysis of iron(III) chloride solutions for the preparation of hematite (α-Fe2O3) and ferricoxychloride (FeOCl) colloids consisting of nanoparticles (NPs) is described. Transmission electron microscopy indicated that both colloids consisted of particles smaller than 10 nm. X-ray diffraction measurements revealed transformation of FeOCl NPs into α-Fe2O3 NPs after a few months. The phase transformation was explained in terms of redissolution – recrystallization process. UV-vis spectroscopy was used for precise determination of the band gap of α-Fe2O3.