Fe(II)-induced transformation from ferrihydrite to lepidocrocite and goethite

The transformation of Fe(II)-adsorbed ferrihydrite was studied. Data tracking the formation of products as a function of pH, temperature and time is presented. The results indicate that trace of Fe(II) adsorbed on ferrihydrite can accelerate its transformation obviously. The products are lepidocrocite and/or goethite and/or hematite, which is different from those without Fe(II). That is, Fe(II) not only accelerates the transformation of ferrihydrite but also leads to the formation of lepidocrocite by a new path. The behavior of Fe(II) is shown in two aspects—catalytic dissolution-reprecipitation and catalytic solid-state transformation. The results indicate that a high temperature and a high pH(in the range from 5 to 9) are favorable to solid-state transformation and the formation of hematite, while a low temperature and a low pH are favorable to dissolution-reprecipitation mechanism and the formation of lepidocrocite. Special attentions were given to the formation mechanism of lepidocrocite and goethite.     

The transformation of ferrihydrite in the presence of trace Fe(II): The effect of the ammonia, amine and the coordination ions of Fe(III)

This work examined Fe(II)-induced transformation of ferrihydrite in the presence of ammonia, amine and the coordination ions of Fe(III). Our earlier results showed that ferrihydrite transformed into the mixture of lepidocrocite, goethite and/or hematite in the presence of trace Fe(II) and absence of ammonia and similar species. However, the formation of lepidocrocite was restrained when using ammonia as precipitants. When introducing some amines (e.g. ethanolamine and diethanolamine) and some coordination ions (e.g. F⁻ and ions) into the reaction system, a similar effect on the transformation of ferrihydrite was found. Probably, the complexes formed between Fe(III) and those additives favor the formation of goethite. At the same time, the introduction of these additives hinders Fe(II) from interacting with ferrihydrite, which makes the catalytic dissolution of ferrihydrite be limited, thus, the formation of lepidocrocite be restrained.     

Studies on the controllable transformation of ferrihydrite

Ferrihydrite was prepared by two different procedures. Ferrihydrite-1 was prepared by dropping NaOH solution into Fe(III) solution. Ferrihydrite-2 was prepared by adding Fe(III) and NaOH solutions into a certain volume of water simultaneously. Our earlier results obtained at ∼100 °C have shown that the structure of ferrihydrite-2 favors its solid state transformation mechanism. Further research reveals that the structure of ferrihydrite-2 favors its dissolution re-crystallization mechanism at a temperature of ≤60 °C. Based on the transformation mechanism of ferrihydrite at different temperatures, the controllable transformation from ferrihydrite to various iron (hydr)oxides such as lepidocrocite, goethite, hematite and magnetite can be achieved by adjusting the pH, transformation temperature, transformation time, the amount of Fe(II) as well as the preparation procedures of ferrihydrite. The results in the present paper give a nice example that the transformation of a precursor can be controlled with the help of mechanism.     

The transformation of ferrihydrite in the presence of trace Fe(II): The effect of the anionic media

The transformation from ferrihydrite to various iron oxides and iron oxyhydroxides has been given much attention not only in environmental science and geochemistry but also in biology and material science. This laboratory study attempted to investigate Fe(II)-induced transformation of ferrihydrite in sulfate-rich medium. The results indicate that the transformation in sulfate-rich medium differs from that in Cl⁻ medium in the species, the amount and the morphology of products and transformation rate. Lepidocrocite is a main ingredient in the product in Cl⁻ medium at room temperature (RT), while goethite is the only product in SO₄²⁻ medium at RT. Goethite particles obtained in Cl⁻ medium are star-like but rod-like in SO₄²⁻ medium. The transformation rate in the latter medium is obviously slower than that in the former medium. The formation of lepidocrocite depends on both the ionic strength of the system and the dissolution rate of ferrihydrite.     

不同晶化度γ-FeOOH的合成及其液相转化研究

进行了空气氧化Fe(OH)2悬浮液, EDTA作用下可见光诱导, 不同温度(14～20 ℃)制备不同晶化度γ-FeOOH的研究, 进行了其沸腾回流液相转化历程的探讨. 结果表明: 随温度的升高, γ-FeOOH的晶化程度变差; 而γ-FeOOH的晶化程度越差, 在液相沸腾回流时就越易转化成α-Fe2O3, 反之则易转化为α-FeOOH. 在pH近中性, 微量Fe(II)存在条件下低晶态的γ-FeOOH可以快速转化为均匀球形的α-Fe2O3.     

Transformation of ferrihydrite in the presence or absence of trace Fe(II): The effect of preparation procedures of ferrihydrite

Two-line ferrihydrite was prepared by two different procedures. In procedure 1, which is widely used, ferrihydrite (named as ferrihydrite-1) was prepared by droping NaOH solution into Fe(III) solution. In procedure 2, which is rarely reported, ferrihydrite (named as ferrihydrite-2) was prepared by adding Fe(III) and NaOH solutions into a certain volume of water simultaneously. The results showed that mixing procedures of Fe(III) and alkaline were critical in the sub-microstructures and the conversion mechanisms of ferrihydrites in the presence or absence of trace Fe(II). The sub-microstructure of ferrihydrite-1 favored the mechanism of its dissolution re-crystallization and hematite nanoparticles with rough surface were obtained. The sub-microstructure of ferrihydrite-2 favored the solid state transformation from ferrihydrite to hematite and hematite nanoparticles with smooth surface were formed. These research results will be helpful for us to control the synthesis of hematite nanoparticles with different surface state.     

Enhancement of the reductive transformation of pentachlorophenol by polycarboxylic acids at the iron oxide-water interface

The enhancement effect of polycarboxylic acids on reductive dechlorination transformation of pentachlorophenol (PCP) reacting with iron oxides was studied in anoxic suspension. Batch experiments were performed with three species of iron oxides (goethite, lepidocrocite and hematite) and four species of polycarboxylic acids (oxalate, citrate, succinate, and tartrate) through anoxic abiotic reactors. The chemical analyses and morphological observation from scanning and transmission electron microscopy showed that different combinations between polycarboxylic acids and iron oxides produced distinct contents of Fe(II)-polycarboxylic ligand complexes, which significantly enhanced PCP transformation. Generation of the surface-bound Fe(II) depended on concentration of polycarboxylic acids. The optimal concentration for the enhancement was 2.0 mM oxalic acid. The dechlorination mechanism was further demonstrated by generation of chloride ions. The results suggest that surface-bound Fe(II) formed on the iron oxides surface appears to be a key factor in enhancing PCP transformation, and the mole ratio of oxalate to surface-bound Fe(II) (oxalate/Fe(II)) acted as an indicator of the enhancement effect. The enhancement mechanism attributes to strong nucleophilic ability and low reductive potential of the equivalent Fe(II)-polycarboxylate complexes. Therefore, the enhancement effects might be helpful for understanding the natural attenuation of reducible organic pollutants at the interface of contaminated soil in anoxic condition.     

Sorption of two aromatic acids onto iron oxides: experimental study and modeling

The transport of aromatic carboxylate compounds in the environment can be strongly influenced by adsorption onto certain minerals, such as iron oxides and hydroxides, found in ground water and soils. Batch experiments with five iron oxides were conducted to quantify the contributions to adsorption from different iron mineral surfaces and compare adsorption characteristics of selected organic acids (gentisic acid (GA) and 1-hydroxy-2-naphthoic acid (HNA)). Because of their widespread abundance in soils and sediments, goethite, lepidocrocite, ferrihydrite, hematite, and magnetite were investigated. Sorption of two organic acids onto iron oxides was examined over a wide range of conditions (pH, ionic strength, and sorbate concentration). Specific surface area and mineral surface charge proved be important for the adsorption of these compounds. The sorption isotherm was described well by the Tempkin equation for both organic acids, with the adsorption constant higher for HNA than GA. For modeling the sorption edges of ferrihydrite and hematite, surface reactions involving the formation of mononuclear (1:1) surface species were proposed. These results indicate that the generalized two-layer model, with the assumption of homogeneous surface sites, could predict sorption on iron oxides over a range of pH conditions. The results of this study suggest that the mineralogy of the iron oxides and the pH value should be considered when predicting sorption of aromatic acids onto iron oxides and their fate in the soil and the environment.     

One-pot aqueous solution syntheses of iron oxide nanostructures with controlled crystal phases through a microbial-mineralization-inspired approach

Iron-oxidizing bacteria produce trivalent iron oxides with the controlled crystal phases outside of their cells. Herein we have synthesized iron oxides with controlled oxidation states and crystal phases through a microbial-mineralization-inspired approach in an aqueous solution at low temperature. Trivalent iron oxides, such as lepidocrocite, ferrihydrite, goethite, and hematite, are selectively obtained from an aqueous solution containing divalent iron ions below 90 °C. The presence of a chelating agent facilitates the control of the oxidation states through the ligand-controlled approach because the precipitation of the divalent iron species is inhibited by the complexation between divalent iron ion and a chelating agent. The control of the crystal phases is achieved by the tuning of the synthetic conditions, such as the initial pH, the concentration of a chelating agent, and the reaction temperature. Furthermore, the resultant iron oxides have hierarchically organized structures consisting of nanoscale objects. The microbial-mineralization-inspired approach by using a chelating agent has potentials for the further morphological control of iron oxides and the further application to aqueous-solution syntheses of other metal oxides.     

Formation of protein-metal oxide nanostructures by the sonochemical method: observation of nanofibers and nanoneedles

The sonochemical reaction of iron pentacarbonyl is explored in water and in water with the protein BSA (bovine serum albumen). In water, the reaction is found to produce spherical nanoparticles of magnetite (Fe3O4) with a particle size distribution of <10 to approximately 60 nm. In water with BSA, the reaction produces either nanofibers or nanoneedles, depending on the concentration of BSA. The nanofiber and nanoneedle samples are found to be mixtures of goethite, lepidocrocite, and hematite (alpha-FeOOH, gamma-FeOOH, and alpha-Fe2O3, respectively). The sonochemical reaction of iron pentacarbonyl with BSA in water is thought to proceed through the thermal decomposition mechanism for iron pentacarbonyl with BSA acting as a templating agent.     

FT-IR、XPS和DFT研究水杨酸钠在针铁矿或赤铁矿上的吸附机理

采用傅里叶变换红外(FT-IR)光谱、X射线光电子能谱(XPS)以及基于周期平面波的密度泛函理论(DFT)分别研究了水杨酸钠在针铁矿或赤铁矿表面上的吸附结构,并将计算得到的光电子能谱移动(CLS)和电荷转移与实验得到的XPS结果进行对比。FT-IR结果表明,水杨酸钠可能以双齿双核(V)和双齿单核(IV)的形式分别吸附于针铁矿或赤铁矿表面。由DFT计算结果可知,水杨酸钠在针铁矿(101)晶面上形成双齿双核化合物(V)的吸附能为-5.46 eV。而水杨酸钠在针铁矿(101)晶面上形成双齿单核化合物(IV)的吸附能为3.80 eV,因此水杨酸钠在针铁矿上基本不以双齿单核化合物(IV)构型存在。水杨酸钠在赤铁矿(001)晶面上形成双齿单核化合物(IV)时吸附能为-4.07 eV,说明水杨酸钠在赤铁矿(001)晶面上形成了双齿单核化合物(IV)。另外,理论计算的针铁矿(101)晶面上吸附位点铁原子的Fe 2p的CLS值(-0.68 eV)与实验观察到的Fe 2p的CLS值(-0.5 eV)吻合。理论计算的赤铁矿(001)晶面上吸附位点铁原子的Fe 2p的CLS值(-0.80 eV)与实验观察到的Fe 2p的CLS值(-0.8 eV)吻合。因此,水杨酸钠吸附在针铁矿表面时能够通过羧酸基团上一个氧原子和酚羟基上的氧原子与针铁矿(101)表面上的两个铁原子形成双齿双核(V)结构,而在赤铁矿(001)表面上,水杨酸钠中羧酸基团上一个氧原子和酚羟基上的氧原子与赤铁矿(001)表面上的一个铁原子形成了双齿单核(IV)结构。     

Mechanisms and efficiency of the simultaneous removal of metals and cyanides by using ferrate(VI): crucial roles of nanocrystalline iron(III) oxyhydroxides and metal carbonates

The reaction of potassium ferrate(VI), K₂FeO₄, with weak‐acid dissociable cyanides—namely, K₂[Zn(CN)₄], K₂[Cd(CN)₄], K₂[Ni(CN)₄], and K₃[Cu(CN)₄]—results in the formation of iron(III) oxyhydroxide nanoparticles that differ in size, crystal structure, and surface area. During cyanide oxidation and the simultaneous reduction of iron(VI), zinc(II), copper(II), and cadmium(II), metallic ions are almost completely removed from solution due to their coprecipitation with the iron(III) oxyhydroxides including 2‐line ferrihydrite, 7‐line ferrihydrite, and/or goethite. Based on the results of XRD, Mössbauer and IR spectroscopies, as well as TEM, X‐ray photoelectron emission spectroscopy, and Brunauer–Emmett–Teller measurements, we suggest three scavenging mechanisms for the removal of metals including their incorporation into the ferrihydrite crystal structure, the formation of a separate phase, and their adsorption onto the precipitate surface. Zn and Cu are preferentially and almost completely incorporated into the crystal structure of the iron(III) oxyhydroxides; the formation of the Cd‐bearing, X‐ray amorphous phase, together with Cd carbonate is the principal mechanism of Cd removal. Interestingly, Ni remains predominantly in solution due to the key role of nickel(II) carbonate, which exhibits a solubility product constant several orders of magnitude higher than the carbonates of the other metals. Traces of Ni, identified in the iron(III) precipitate, are exclusively adsorbed onto the large surface area of nanoparticles. We discuss the relationship between the crystal structure of iron(III) oxyhydroxides and the mechanism of metal removal, as well as the linear relationship observed between the rate constant and the surface area of precipitates.     

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.     

Regularities of the formation of iron(III) nanocrystalline particles during the oxidation of iron(II) compounds in a neutral medium

The regularities of the formation of iron(III) oxide hydroxides as nanocrystalline particles via oxidation of iron(II) compounds in a near-neutral pH region were studied by potentiometric titration, electron microscopy, chemical analysis, and X-ray diffraction. The oxidation process comprises two steps. The first step produces Fe(II)-Fe(III) hydroxo salts having a “green rust” structure in the form of nanocrystalline particles shaped as hexagons. The second step produces anisotropic nanocrystalline particles of iron(III) oxide hydroxides via the dissolution-oxidation-precipitation mechanism and via solid-phase oxidation. The oxidation of chlorine-containing suspensions helps the formation of single-phase nanocrystalline lepidocrocite, while oxidation in the presence of sulfate ions yields nanocrystalline goethite.     

Bet nitrogen adsorption studies of iron oxides from natural and synthetic sources.

A vacuum microbalance technique has been used to obtain the surface areas and porosities of iron oxyhydroxides derived from Fe(III) and Fe(II) sources. The pore shape of the Fe(III)-derived material was of the wide-bodied, narrow-necked type, whereas the Fe(II)-derived precipitates were composed of parallel plates or slit-shaped pores. The surface area of the Fe(II)-derived material was found to be influenced by the temperature of the precipitate formation. These differences in surface character are discussed in relation to the adsorption of trace constituents at the iron oxyhydroxide-water interface.     

Nanostructured pure gamma-Fe2O3 via forced precipitation in an organic solvent

Pure maghemite, gamma-Fe(2)O(3), was prepared as ultra fine particles in the nanometer-sized range via the forced precipitation method in an organic solvent. The precipitation of iron(III) ions, from iron(III) chloride in 2-propanol led selectively to highly dispersed particles of ferrihydrite, which upon treatment with temperatures higher than 200 degrees C under dynamic vacuum resulted in high-surface-area particles of gamma-Fe(2)O(3). Precipitation in water also led to ferrihydrite, but the final product, after heating at 300 degrees C, contained a mixture of gamma-Fe(2)O(3) and alpha-Fe(2)O(3) (hematite). The precipitation from iron(III) nitrate in water resulted in goethite which was converted to hematite upon heating. On the other hand, the final product in 2-propanol was a mixture of maghemite and hematite. The products were characterized by FTIR, TGA, XRD, and gas sorption analysis. Nitrogen gas adsorption studies for the pure gamma-Fe(2)O(3) samples revealed mesoporous particles with high surface areas in the range of 70-120 m(2) g(-1) after heat treatment at 300 degrees C. The gamma-Fe(2)O(3) particles retained their gamma-phase as well as their mesoporous structure at relatively high temperatures, as high as 400 degrees C.     

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.     

Sorption of selenite onto chlorite considering mineral dissolution

In this study we investigated the sorption of selenite (SeO₃ ^2?) onto chlorite as a function of Se(IV) concentration, pH, and ionic strength. The sorption isotherm of Se(IV) onto chlorite was successfully presented by both the Langmuir isotherm and Tempkin equation although the Langmuir isotherm is somewhat better than the Tempkin equation. The sorption of Se(IV) onto chlorite was maintained to be constant at an acidic pH region, while the sorption decreased with an increasing pH at neutral and alkaline pH regions. However, the Se(IV) sorption onto chlorite was independent of the ionic strength of NaClO₄ solution. The amount of Se(IV) sorbed onto chlorite was significantly low compared to those of iron oxides such as apatite, goethite, hematite, and magnetite because of the lower content of Fe. We also investigated the effect of Fe(II) ions dissolved from chlorite on the Se(IV) sorption as a function of contact time. The chemical oxidation states of selenium sorbed onto chlorite surface were identified using X-ray absorption near edge structure (XANES) at the Pohang synchrotron light source. The amount of Fe(II) dissolved was increased by the contact time of 28 days but decreased after 28–56 days although the amount of dissolved Fe(II) ions was significantly small. This decrease of the dissolved Fe(II) may be due to the formation of Fe-oxyhydroxides such as ferrihydrite. The results of XANES measurements also showed that the Se(IV) sorbed onto chlorite was not reduced into Se(0) or Se(-II) even in the presence of Fe(II) ions in the solution because of the low Fe content of the chlorite although the mechanism was not clearly understood.     

Transformation of synthetic γ-FeOOH (lepidocrocite) in aqueous solutions of ferrous sulphate

The course of the transformation of synthetic γ-FeOOH (lepidocrocite) samples, prepared by different methods was investigated and the effect of a preliminary heat treatment of the initial material on the product phase composition and morphology studied. Both the goethite-tohematite ratio and the particle size of the products depend on the preparation method and on the preliminary heat treatment of the initial lepidocrocite. The results are discussed in terms of the dissolution of the less stable lepidocrocite followed by the crystallization of the more stable phases α-FeOOH and α-Fe₂O₃ from the solution. The final product phase composition and the particle size are affected considerably by a presence of goethite and/or hematite germs in the initial γ-FeOOH.     

XRD study of the goethite-hematite transformation: Application to the identification of heated prehistoric pigments

When heated, yellow goethite dehydrates and transforms to red hematite. Both iron oxides were used by the Palaeolithic artists as pigments, one question being whether those people took advantage of the phase transformation. To answer this question, the dehydration of synthetic goethite was studied by XRD coupled to Rietveld refinement. It was shown that no hydroxylated hematite is formed during the early stages of dehydration, the presence of hydroxyl ions in materials treated at high temperatures being explained by trapped water inside porous microstructure (TEM). Archaeological samples from the south of France were investigated. Some of them exhibit distinctive features of heating which, supports the idea that Palaeolithic people used both natural and ex-goethite hematite.