Arsenic adsorption onto hematite and goethite

Surface complexation reactions on mineral affect the fate and the transport of arsenic in environmental systems and the global cycle of this element. In this work, the sorption of As(V) on two commercial iron oxides (hematite and goethite) was studied as a function of different physico-chemical parameters such as pH and ionic strength. The main trend observed in the variation of the arsenic sorbed with the pH is a strong retention in acidic pH and the decrease of the sorption on both sorbents at alkaline pH values. The sorption experiments for these iron oxides show that there is no effect of the ionic strength on arsenate adsorption suggesting the formation of an inner sphere surface complex. At pH values corresponding to natural pH water, both hematite and goethite are able to adsorb more than 80% of arsenic, whatever the initial concentration may be. The iron oxides used in this work should be suitable candidates as sorbents for As(V) removal technologies.     

Infrared spectroscopy of goethite dehydroxylation. II. Effect of aluminium substitution on the behaviour of hydroxyl units

Dehydroxylation of goethite as affected by aluminium substitution was investigated using Fourier transform infrared spectroscopy (FT-IR) in conjunction with X-ray diffraction (XRD), thermogravimetric analysis (TGA) and differential thermogravimetric analysis (DTGA). The band intensities of hydroxyl vibrations were indicative of the degree of dehydroxylation and the changes in band parameters due to aluminium substitution were observed. The effect of aluminium substitution on band parameters of FT-IR spectra of goethite and its partially and fully dehydroxylated products, the mixture of goethite/hematite and hematite, were interpreted. The results of this study have confirmed that aluminium substituted goethite is thermally more stable than non-substituted goethite and is in harmony with the results of XRD and DTGA. A larger amount of non-stoichiometric hydroxyl units is associated with a higher aluminium substitution. A shift to a higher wavenumber of bending and hydroxyl stretching vibrations is attributed to the effects of aluminium substitution associated with non-stoichiometric hydroxyl units on the a-b plane relative to the b-c plane of goethite. The results provide information for the characterisation of activated bauxite containing hematite and goethite.     

The transformation of ferrihydrite into goethite or hematite, revisited

During the oxidation of iron, poorly crystallized phases are firstly formed: 2- and 6-line ferrihydrite, which presents for the last phase, a similarity with wustite FeO but also with hematite α-Fe₂O₃. Crystallization increases with time and the solid phase obtained is dependent on temperature and pH. Obviously, high temperature favours the formation of the oxide hematite α-Fe₂O₃. As for the pH factor, it is more complicated. Low and high values of pH (2-5 and 10-14) favour the formation of goethite α-FeOOH, while obtaining hematite is favoured at neutral pH (values around 7). Goethite or hematite are obtained either through a dissolution-crystallization process or in the solid state, through a topotactic transformation.Given the structural relationships observed between ferrihydrite and wustite and hematite, it is allowed to think that a structural continuity could exist between wustite Fe_(1−x)O and hematite via ferrihydrite.     

Adsorption of sulfur dioxide on hematite and goethite particle surfaces

The adsorption of sulfur dioxide (SO(2)) on iron oxide particle surfaces at 296 K has been investigated using X-ray photoelectron spectroscopy (XPS). A custom-designed XPS ultra-high vacuum chamber was coupled to an environmental reaction chamber so that the effects of adsorbed water and molecular oxygen on the reaction of SO(2) with iron oxide surfaces could be followed at atmospherically relevant pressures. In the absence of H(2)O and O(2), exposure of hematite (alpha-Fe(2)O(3)) and goethite (alpha-FeOOH) to SO(2) resulted predominantly in the formation of adsorbed sulfite (SO(3)(2-)), although evidence for adsorbed sulfate (SO(4)(2-)) was also found. At saturation, the coverage of adsorbed sulfur species was the same on both alpha-Fe(2)O(3) and alpha-FeOOH as determined from the S2p : Fe2p ratio. Equivalent saturation coverages and product ratios of sulfite to sulfate were observed on these oxide surfaces in the presence of water vapor at pressures between 6 and 18 Torr, corresponding to 28 to 85% relative humidity (RH), suggesting that water had no effect on the adsorption of SO(2). In contrast, molecular oxygen substantially influenced the interactions of SO(2) with iron oxide surfaces, albeit to a much larger extent on alpha-Fe(2)O(3) relative to alpha-FeOOH. For alpha-Fe(2)O(3), adsorption of SO(2) in the presence of molecular oxygen resulted in the quantitative formation of SO(4)(2-) with no detectable SO(3)(2-). Furthermore, molecular oxygen significantly enhanced the extent of SO(2) uptake on alpha-Fe(2)O(3), as indicated by the greater than two-fold increase in the S2p : Fe2p ratio. Although SO(2) uptake is still enhanced on alpha-Fe(2)O(3) in the presence of molecular oxygen and water, the enhancement factor decreases with increasing RH. In the case of alpha-FeOOH, there is an increase in the amount of SO(4)(2-) in the presence of molecular oxygen, however, the predominant surface species remained SO(3)(2-) and there is no enhancement in SO(2) uptake as measured by the S2p : Fe2p ratio. A mechanism involving molecular oxygen activation on oxygen vacancy sites is proposed as a possible explanation for the non-photochemical oxidation of sulfur dioxide on iron oxide surfaces. The concentration of these sites depends on the exact environmental conditions of RH.     

Morphology of synthetic goethite particles

The specific surface area of synthetic goethite depends on the preparation: the Fe(III):OH ratio, the rate of base titration of Fe salt, and the temperature and time of crystallization. The crystals also have different morphologies as determined by SEM or TEM. Carbon coating is used to improve the quality of SEM images of nonconducting specimens. We show here that needle-like goethite particles become substantially thicker in the course of standard carbon coating, and the length-to-width ratio obtained for carbon-coated particles is lower than that for the original goethite particles. The morphology of the goethite particles was also studied by tapping mode AFM.     

Infrared spectroscopy of goethite dehydroxylation: III. FT-IR microscopy of in situ study of the thermal transformation of goethite to hematite

Fourier transform infrared microscopy has been used to investigate in situ dehydroxylation of goethite to form hematite. The characterisation was based on the behaviour of hydroxyl units, which were observed in the hydroxyl stretching and hydroxyl deformation and water bending regions, and the Fe-O vibrations of the newly formed hematite during the thermal dehydroxylation process. Two hydroxyl stretching modes (v1 and v2), and three bending (V(bending-1, 2, 3)) and two deformation (V(deformation-1, 2)) modes were observed for goethite. The characteristic vibration at 916 cm(-1) was observed together with the residuals of the v1 and v2 bands in hematite spectrum. The structural transformation between goethite and hematite through thermal dehydroxylation was interpreted in order to provide criteria that can be used for the characterisation of thermally activated bauxite and their conversion to activated alumina phases.     

Phosphate adsorption on synthetic goethite and akaganeite

Low crystalline iron hydroxides such as goethite (alpha-FeOOH) and akaganeite (beta-FeOOH) were synthesized, and the selective adsorption of phosphate ions from phosphate-enriched seawater was examined. The results of the distribution coefficients (K(d)) of oxoanions in mixed anion solutions at pH 8 follow the selectivity order Cl-, NO3-, SO4(2-) < CO3(2-), HPO4(2-) for goethite, and Cl-, CO3(2-) < NO3- < SO4(2) < HPO4(2-) for akaganeite. In seawater, both adsorbents show high selectivity for phosphate ions despite the presence of large amounts of major cations and anions in seawater. The adsorption isotherms fitted better with the Freundlich equation and the maximum uptake of phosphate from phosphate-enriched seawater was 10 mg P/g at an equilibrium phosphate concentration of 0.3 mg P/L on both adsorbents. The phosphate adsorption/desorption cycles show that akaganeite is an excellent adsorbent even after 10 cycles and its chemical stability is good.     

Adsorption of trimethyl phosphate on maghemite, hematite, and goethite nanoparticles

Adsorption of trimethyl phosphate (TMP) on well-characterized hematite, maghemite and goethite nanoparticles was studied by in situ DRIFT spectroscopy as a model system for adsorption of organophosphorous (OP) compounds on iron minerals. The iron minerals were characterized by X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), specific surface area, and pore size distribution. The minerals were found to consist of stoichimetrically and morphologically well-defined maghemite, hematite, and goethite nanoparticles. Analysis of in situ diffuse reflectance Fourier transform (DRIFT) spectroscopy shows that TMP bonds mainly to Lewis acid Fe sites through the O phosphoryl atom (-P═O-Fe) on hematite and maghemite. On goethite most TMP molecules bond to Br?nstedt acid surface OH groups and form hydrogen bonded surface complexes. The vibrational mode analysis and uptake kinetics suggest two main reasons for the observed trend of reactivity toward TMP (hematite > maghemite > goethite): (i) larger number of accessible Lewis acid adsorption sites on hematite; (ii) stronger interaction between the Lewis acid Fe sites and the phosphoryl O atom on TMP for hematite and maghemite compared to goethite with concomitant formation of surface coordinated TMP and dimethyl phosphate intermediates. As a result, on the oxides a surface oxidation pathway dominates during the initial adsorption, which results in the formation of surface methoxy and formate. In contrast, on goethite a slower hydrolysis pathway is identified, which eventually yields phosphoric acid. The observed trends of the reactivity and analysis of the corresponding surface structure and particle morphology suggest an intimate relation between the surface chemistry of exposed crystal facets on the iron minerals. These results are important to understand OP surface chemistry on iron minerals.     

Electron microscopic study of dehydration transformations. I. Twin formation and mosaic structure in hematite derived from goethite

A study by means of electron microscopy at high resolution has enabled the analysis of the microstructure of hematite as produced by dehydration from goethite. The apparently “polycrystalline” structure is found to consist of aggregates of well-oriented twin-related hematite crystals, separated by regularly spaced H(001) walls of voids resulting from the loss of water. The material is thus broken up into blocks with sizes smaller than 50 Å. The crystallography of the twinned structure can be analyzed by electron diffraction data and dark-field imaging. These aggregates of hematite crystals provide an ideal mosaic structure. In particular the effect of this mosaic structure on the individual reflections of the diffraction patterns has been analyzed and discussed.     

Sorption of silicates on goethite, hematite, and magnetite: experiments and modelling

Sorption of H(4)SiO(4) (including experiments as a function of time, K(d) measurement with different m/v ratios and sorption edges) onto different iron (hydro)oxides as goethite (alpha-FeOOH), hematite (alpha-Fe(2)O(3)), and magnetite (Fe(3)O(4)) has been studied with concentration of silicates under solubility limit. A surface complexation model has been used to account for sorption edge of silicates onto these iron oxide surfaces. It reveals that two types of surface complex namely FeH(3)SiO(4) and FeH(2)SiO(4)(-), are needed to describe properly the experimental observations.     

Porous texture in hematite derived from goethite: Mechanism of thermal decomposition of geothite

The thermal decomposition of mineral goethite is investigated at various temperatures between 25 and 700°C in vacuo. The mineral used in the present work consists of the agglomerates in which primary particles of goethite are tightly bound to each other. At temperatures below 200°C, the water molecules that blocked the interstice within the primary particles are removed and, as a result, the slit-shaped voids of 3.7 nm in width are opened between the primary particles. The thermal decomposition of mineral goethite starts at the temperatures above 200°C. At 230–300°C, the silt-shaped micropores of uniform size (0.8 nm in width) are progressively opened in the course of decomposition reaction. The microporous texture obtained in the present work completely coincides with that obtained by decomposition of the synthetic goethite fine particles reported previously. The process of formation of the microporous texture has been examined in the light of the structural relationship of topotaxy between goethite and hematite. On the basis of the results of micropore formation, the process of nuclei formation of hematite in the goethite matrix is discussed.     

Mechanism of the formation of micropores in the thermal decomposition of goethite to hematite

The mechanisms of the formation of micropores in the thermal decomposition of goethite to hematite have been studied through the measurements of X‐ray diffraction, Brunauer–Emmett–Teller adsorption and scanning electronic microscopy, as well as the estimation of crystal cell parameters in this work. The results have shown that the thermal decomposition led to the crystal cells of goethite to be contracted along the [010] and [001] directions and to be elongated along the [100] direction, forming nano‐scale and rod‐like hematite. It was the contractions along the two directions that led to the formation of the micropores in the hematite particles. Copyright © 2015 John Wiley & Sons, Ltd.     

Liquid crystal phase behavior of sterically-stabilized goethite

The liquid crystalline phase behavior of sterically-stabilized goethite particles in toluene was studied using small-angle X-ray scattering. The results were compared with those from charged particles in water, with and without magnetic field: similarly rich phase behavior was found. Furthermore, the special magnetic properties were retained after coating the particles with amino-functionalized polyisobutylene chains. A remarkable difference between the aqueous and toluene samples is the latter's tendency to form gels. Smaller domains of the different liquid crystalline phases were observed and the columnar phase does not fully develop, furthermore a higher field is needed to align the full sample.     

Influence of adsorbed anions on the dehydroxylation of synthetic goethite

The influence of surface structure on the dehydroxylation of synthetic goethite has been studied using heat-flux differential scanning calorimetry. Adsorption of various anions by goethite causes a progressive change in both the amount and environment of surface hydroxyls and this is reflected by changes in the DSC curves. These alterations indicate the importance of surface features in determining the course of dehydroxylation and further suggest that, in studies of this type of reaction, the use of synthetic products known to be free of surface contaminants could prove to be of great value in understanding dehydroxylation mechanisms.

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Zusammenfassung Der Einfluß von Oberflächenstrukturen auf die Dehydroxylierung synthetischer Goethite wurde unter Anwendung der Wärmestrom-Differential-Abtastkalorimetrie untersucht. Die Adsorption verschiedener Anione durch Goethit verursacht eine progressive Änderung sowohl der Menge als auch der Umgebung der Oberflächenhydroxyle und dies wird durch die Änderungen der DSC-Kurven wiedergespiegelt. Diese Änderungen zeigen die Bedeutung der Oberflächenbeschaffenheiten bei der Bestimmung des Verlaufs der Dehydroxylierung und weisen ferner darauf hin, daß bei Studien dieses Reaktionstyps der Einsatz synthetischer Produkte, die bekanntlich frei von Oberflächenverunreinigungen sind, sich hinsichtlich der Klärung der Dehydroxylierungsmechanismen als sehr wertvoll erweisen könnte.

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Résumé On a étudié par analyse calorimétrique différentielle (DSC) l'influence de la structure de surface sur la déshydroxylation de goethites synthétiques. L'adsorption d'anions divers par la goethite entraîne une variation progressive à la fois de la quantité et de l'environnement des hydroxyles de surface, ce qui se reflète dans les variation des courbes DSC. Ces altérations indiquent l'importance des propriétés de surface quand il s'agit de déterminer le cours de la déshydroxylation et suggèrent en outre que l'emploi de produits synthétiques connus comme étant exempts de contaminants de surface, pourrait être de grande valeur dans des études de ce type de réactions, pour comprendre les mécanismes de déshydroxylation.

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Differential thermal analysis - evolved gas analysis of synthetic goethite

Samples of synthetic goethite have been studied by combined DTA-EGA. The total CO₂ content of the samples ranged between 1.0 and 2.7 %wt. The origin of the evolved CO₂ is discussed and it is proposed that the samples contain CO₂ as surface complexes and incorporated in the crystal structure.     

DTA study of the dehydration of synthetic goethite αFeOOH

The shape of DTA endothermic peak, associated with the dehydration of goethite FeOOH into hematite, depends strongly on the size of the goethite particles. With samples of low specific surface, two distinct peaks were observed. The dehydration process of a low-specific surface goethite was followed by X-ray diffraction and lowtemperature nitrogen adsorption. The results are in good agreement with the hypothesis of the formation of an impermeable hematite layer on each grain which temporarily stops the dehydration process of the internal part of the grain.

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Zusammenfassung Die Form des mit der Entwässerung von Goethit FeOOH zu Hämatit verbundenen endothermen DTA-Peaks hängt stark von der Grösse der Goethit-Partikel ab. Bei Proben von kleiner spezifischer Oberfläche wurden zwei scharfe Peaks beobachtet. Der Entwässerungsprozess eines Goethits von kleiner spezifischer Oberfläche wurde durch Röntgendiffraktion und Stickstoff-Adsorption bei niedrigen Temperaturen verfolgt. Die Ergebnisse sind in guter Übereinstimmung mit der Hypothese nach welcher eine undurchdringliche Hämatitschicht auf jedem Körnchen gebildet wird, die den Entwässerungs vorgang im inneren des Körnchens vorübergehend stoppt.

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Résumé La déshydratation de la goethite de synthèse se marque en ATD par un phénomène endothermique plus ou moins dédoublé, ce dédoublement étant fonction de la dimension des particules de l'échantillon. La déshydratation d'une goethite de faible surface spécifique a été suivie par diffraction de rayons X et par adsorption d'azote à basse température sur des échantillons refroidis brutalement en cours d'ATD. C'est l'hypothèse de la formation sur chaque grain d'une couche externe d'hématite, bloquant temporairement la diffusion de la vapeur d'eau vers l'extérieur, qui se trouve le plus en accord avec les résultats expérimentaux.

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Photoelectrochemical behavior of molybdenum-modified nanoparticulate hematite electrodes

The preparation of molybdenum-modified hematite electrodes by means of chemical bath deposition and their photoelectrochemical behavior toward water oxidation are reported in this work. The addition of a molybdenum precursor to the bath solution for hematite deposition induces a remarkable change of morphology in the resulting film from (110)-oriented nanorods to polyhedral nanoparticles. Despite the resulting loss of order, by controlling the Mo/Fe molar ratio in the bath solution, a significant improvement of the water oxidation photocurrent is achieved compared to nanorod pristine hematite electrodes. Such a (photo)electrochemical enhancement is mainly explained by an effective surface state passivation in Mo-modified hematite films. FE-SEM, TEM, XRD, and XPS were employed for electrode structural and morphological characterization.

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Surface complex characteristics of synthetic maghemite and hematite in aqueous suspensions

The acid-base properties of the maghemite (gamma-Fe2O3)/water and hematite (alpha-Fe2O3)/water interfaces have been studied by means of high precision potentiometric titrations and the experimental results are evaluated as surface complexation reactions. Synthetic maghemite and hematite were prepared and characterized using a combination of SEM, FT-IR and XRD. The specific surface area of the minerals was determined by the BET method. The titrations were performed at 25.0+/-0.2 degrees C within the range 2.8相似文献   

Characterization of colloidal hematite particle shape and dispersion behavior

An elliptically polarized light scattering (EPLS) technique for the size and shape characterization of ellipsoidal hematite particles is presented. The hematite particles are synthesized by aging aqueous FeCl(3) or Fe(NO(3))(3) solutions at 100 degrees C. Different reaction conditions create particles of aspect ratios between 1 (spherical) and 8 (elongated). Cross-sectional diameter and aspect ratio are observed as a function of reaction (aging) time. Growth of the elongated particles, as well as their fractal aggregation behavior, is characterized using EPLS and comparisons are made with photon correlation spectroscopy and TEM measurements.     

Equilibrium sorption of hexavalent chromium from aqueous solution using synthetic hematite

Hematite sample prepared by transformation of ferrihydrite was used to remove Cr(VI) from aqueous solution. Effects of initial concentration, contact time, pH, ionic strength, and temperature were investigated. The data were fitted to Langmuir, Freundlich, Dubinin-Kaganer-Radushkevich and Temkin models of adsorption. The results showed the best fit to the Langmuir isotherm. The increase of pH as well as that of increasing ionic strength resulted in the decrease of adsoiption of Cr(VI). Two kinetic models namely pseudo-first order and pseudo-second order were used to fit the experimental data. The thermodynamic parameters were found, and the results indicated spontaneous and exothermic nature of the process.