Formation Of Akaganeite Research Articles

Corrosion mechanism of press-hardened steel with zinc coating in controlled atmospheric conditions: A laboratory investigation

The formation and evolution of corrosion products on zinc galvanized press-hardened steel during various atmospheric tests were studied. A high protective ability of the coating containing Zn-Fe intermetallics was shown. The composition models of the soluble and stable corrosion products were calculated. A significant effect of barrier corrosion products on corrosion resistance, especially simonkolleite, hydrozincite and dawsonite, was demonstrated. Exposure to constant high relative humidity resulted in the largest mass loss due to the accelerated formation of akaganeite. On the contrary, the presence of sulphates caused formation of additional barrier corrosion products and almost a threefold drop in mass loss.

Corrosion failure analysis of engineering structural steels in tropical marine atmospheres: A comparative study of ordinary and new weathering steels

Reliable applications for weathering steels face challenges in harsh tropical marine atmospheric environments. The development of new weather steel suitable for tropical marine atmospheric environments is of great significance. In the paper, the corrosion failure behavior of ordinary and newly designed weathering steels exposed to tropical marine environments for two years was studied by employing various analytical methods. The results indicate that the extensive formation of akaganeite and lepidocrocite occurs deep inside the abnormally thick corrosion products of ordinary weathering steel with the accumulation of Cl−, which is laborious to be affected by the rinsing effect of rainfall. Based on the study of the composition, phase, and structure of abnormally thick corrosion products, a chain evolution mechanism of corrosion failure is proposed. By optimizing the content of Cu, Mo, and Ni, the newly designed weathering steel can effectively suppress the chain failure process and increase the corrosion resistance by approximately doubling. On the whole, the newly designed weathering steel can be well applied to tropical marine atmospheric environments.

Impacts of coexisting mineral on crystallinity and stability of Fe(II) oxidation products: Implications for neutralization treatment of acid mine drainage.

The neutralization treatment of acid mine drainage involves the oxidation of Fe(II), but little is known about the effects of co-existing minerals on the oxidation and hydrolysis of Fe(II) to iron oxides. Here we investigated the transformation of fresh and heated Fe(II) oxidation coprecipitates, which were synthesized in the presence and the absence of five co-existing minerals (montmorillonite, kaolin, quartz (SiO2), aluminium oxide (Al2O3) and calcium carbonate (CaCO3)). In the FeSO4 system with montmorillonite or kaolin, the formation of lepidocrocite was inhibited with the increase of clay mineral contents. In the same system, heated coprecipitates of montmorillonite were mainly comprised of amorphous ferrihydrite and its transformation was retarded by the excess montmorillonite. In the FeCl2 system with SiO2, Al2O3 or CaCO3, akaganeite formation was inhibited with the increase in the corresponding mineral contents. In the same system, goethite formation was blocked by either CaCO3 or Al2O3 and the growth of lepidocrocite was inhibited by CaCO3 or SiO2. However, magnetite formation was enhanced by addition of CaCO3. These findings are important for predicting products of abiotic Fe(II) oxidation during the neutralization of acid mine drainage and for better understanding the transformation of amorphous iron oxides in the complicated environmental matrix.

Formation of Fe(III) (Hydr)oxides from Fe(II) Sulfides: Implications for Akaganeite Detection on Mars

Akaganeite on Mars could form from Fe(II) sulfides, but formation conditions remain unconstrained. We investigated akageneite formation by oxidative alteration of natural pyrrhotites exposed to HCl and oxidation–hydrolysis of Fe(II) HCl-leached from pyrrhotites at initial pH0 1.5, 2, 3, and 4. X-ray diffraction and Mössbauer analyses revealed the formation of poorly crystallized akageneite in oxidative alteration experiments. Air exposure of the HCl-reacted dry pyrrhotites led to an increase in akageneite formation and precipitation of Fe(II) hydrated sulfates, goethite, and hydronium jarosite. Iron(II) oxidation–hydrolysis was sensitive to Si dissolved from phyllosilicates in one pyrrhotite sample. Akaganeite and goethite formed at pH0 1.5 and 2 with akageneite more abundant at a dissolved Si/Fe ratio of 0.08 and goethite more abundant at a Si/Fe of 0.01. Akaganeite formed together with hematite, ferrihydrite, and goethite at pH0 3, and formation was suppressed at pH0 4. Well-crystallized akageneite precipitated at pH0 1.5, while akaganeite of poorer crystallinity formed at pH0 2 and 3. Akageneite on Mars could form from sulfides by both mechanisms during late diagenetic events triggered by interactions of acidic Cl-bearing groundwater with Fe(II) sulfides. Akaganeite in Yellowknife Bay, Gale Crater, could have formed by Fe(II) oxidation–hydrolysis either as a sole Fe(III) (hydr)oxide at pH < 2 or along with ferrihydrite and hematite at 2 < pH < 4 under Si-enriched conditions. Akaganeite formation at the Vera Rubin ridge, Gale Crater, could have occurred through oxidative alteration of sulfides in Cl-bearing pH 1.2–1.5 solutions. The presence of well-crystallized akageneite in the Rock Hall site at the Vera Rubin ridge indicates that Fe(II) oxidation–hydrolysis contributed to akageneite formation.

Performance Evaluation of Fe-Al Bimetallic Particles for the Removal of Potentially Toxic Elements from Combined Acid Mine Drainage-Effluents from Refractory Gold Ore Processing

Acid mine drainage (AMD) is a serious environmental issue associated with mining due to its acidic pH and potentially toxic elements (PTE) content. This study investigated the performance of the Fe-Al bimetallic particles for the treatment of combined AMD-gold processing effluents. Batch experiments were conducted in order to eliminate potentially toxic elements (including Hg, As, Cu, Pb, Ni, Zn, and Mn) from a simulated waste solution at various bimetal dosages (5, 10, and 20 g/L) and time intervals (0 to 90 min). The findings show that metal ions with greater electrode potentials than Fe and Al have higher affinities for electrons released from the bimetal. Therefore, a high removal (&gt;95%) was obtained for Hg, As, Cu, and Pb using 20 g/L bimetal in 90 min. Higher uptakes of Hg, As, Cu, and Pb than Ni, Zn, and Mn also suggest that electrochemical reduction and adsorption by Fe-Al (oxy) hydroxides as the primary and secondary removal mechanisms, respectively. The total Al3+ dissolution in the experiments with a higher bimetal content (10 and 20 g/L) were insignificant, while a high release of Fe ions was recorded for various bimetal dosages. Although the secondary Fe pollution can be considered as a drawback of using the Fe-Al bimetal, this issue can be tackled by a simple neutralization and Fe precipitation process. A rapid increase in the solution pH (initial pH 2 to &gt;5 in 90 min) was also observed, which means that bimetallic particles can act as a neutralizing agent in AMD treatment system and promote the precipitation of the dissolved metals. The presence of chloride ions in the system may cause akaganeite formation, which has shown a high removal capacity for PTE. Moreover, nitrate ions may affect the process by competing for the released electrons from the bimetal owing to their higher electrode potential than the metals. Finally, the Fe-Al bimetallic material showed promising results for AMD remediation by electrochemical reduction of PTE content, as well as acid-neutralization/metal precipitation.

Development of a novel method for the in-situ dechlorination of immovable iron elements: optimization of Cl\u2212 extraction yield through experimental design

The conservation of iron objects exposed to marine aerosol is threatened by the formation of akaganeite, a highly unstable Cl-bearing corrosion phase. As akaganeite formation is responsible of the exfoliation of the rust layer, chlorides trigger a cyclic alteration phenomenon that often ends with the total consumption of the iron core. To prevent this degradation process, movable iron elements (e.g. archaeometallurgical artefacts) are generally immersed in alkaline dechlorination baths. Aiming to transfer this successful method to the treatment of immovable iron objects, we propose the in-situ application of alkaline solutions through the use of highly absorbent wraps. As first step of this novel research line, the present work defines the best desalination solution to be used and optimizes its extraction yield. After literature review, a screening experimental design was performed to understand the single and synergic effects of common additives used for NaOH baths. Once the most effective variables were selected, an optimization design was carried out to determine the optimal conditions to be set during treatment. According to the experimental work here presented, the use of 0.7 M NaOH solutions applied at high temperatures (above 50 °C) is recommended. Indeed, these conditions enhance chloride extraction and iron leaching inhibition, while promoting corrosion stabilization.

(Invited) Structural Analysis of Rust Layer Formed on Carbon Steel Exposed to Atmospheric Corrosion Environments Using Synchrotron Radiation X-Rays

Carbon steel has been a beneficial material to humankind, but unfortunately is easily oxidized back to iron ore if the steel touches with an atmospheric environment. This phenomenon is well-known as atmospheric corrosion which proceeds under an electrolyte film containing oxygen and water covering the steel surface. Through atmospheric corrosion, surface of a carbon steel becomes covered with rust layer which largely affects the corrosion process of the steel. In this study, structure of rust layer formed on carbon steel exposed to atmospheric environment is explored using synchrotron radiation X-rays.The X-ray diffraction (XRD) and X-ray adsorption fine structure (XAFS) analyses for the rust layer containing Cr revealed that Cr is incorporated in the corrosion product with goethite (alpha-FeOOH) structure and resultantly the particle size of which becomes less than ten nanometer. This ultrafine Cr-goethite 1) works as a strong protective barrier for the corrosives in the atmosphere. XAFS spectra at Cr K-edge explain the probable site of Cr3+ in goethite and this Cr3+ gives cation selectivity to goethite in the following way. It was shown by the extended X-ray absorption fine structure analysis that the incorporated Cr3+ is not located at a definite distance from Fe3+ and is coordinated with O2-; therefore Cr3+ forms CrOx 3-2x complex ion having negative charge. Namely, CrOx 3-2x complex ion is adsorbed to the goethite crystal surface and its negative charge gives cation selectivity which reduces penetration of corrosive anion such as Cl- through the rust layer.In-situ XRD observation of the formation process of rust layer directly indicates the Fe2+ - Fe3+ change during the corrosion processes of a carbon steel. An addition of metallic salt to the electrolyte film covering the steel surface changes the rust formation process. For instance, Cl- promotes the formation of akaganeite (beta-FeOOH) and some other metallic salts help goethite formation 2).The addition of nickel sulfate to the surface of carbon steel in the chloride and sulfuric acid mist environment promoted the formation of goethite and akaganeite 3). It was assumed that this akaganeite was not conventional β-FeOOH with a tetragonal crystal structure, but monoclinic akaganeite containing Ni2+. This monoclinic akaganeite might reduce the corrosion rate of the carbon steel since the monoclinic akaganeite incorporates corrosive Cl- in the crystal structure of the akaganeite.Based on the arguments mentioned above, it is said that the rust structure is greatly affected by the coexisting ions on the steel surface and resultantly controls corrosion process of a carbon steel.References1) M. Yamashita, H. Konishi, J. Mizuki and H. Uchida, Materials Transactions, 45, No.6, 1920-1924(2004).2) M. Yamashita, H. Konishi, T. Kozakura, J. Mizuki and H. Uchida, Materials Transactions, 46, No.5, 1004-1009 (2005).3) S. Hayashida, M. Takahashi, H. Deguchi, H. Tsuchiya, K. Hanaki, M. Yamashita and S. Fujimoto, Zairyo-to-Kankyo, 69, No.5, 148-153(2020).

Corrosion of Carbon Steel in Marine Environments: Role of the Corrosion Product Layer

This article presents a synthesis of recent studies focused on the corrosion product layers forming on carbon steel in natural seawater and the link between the composition of these layers and the corrosion mechanisms. Additional new experimental results are also presented to enlighten some important points. First, the composition and stratification of the layers produced by uniform corrosion are described. A focus is made on the mechanism of formation of the sulfate green rust because this compound is the first solid phase to precipitate from the dissolved species produced by the corrosion of the steel surface. Secondly, localized corrosion processes are discussed. In any case, they involve galvanic couplings between anodic and cathodic zones of the metal surface and are often associated with heterogeneous corrosion product layers. The variations of the composition of these layers with the anodic/cathodic character of the underlying metal surface, and in particular the changes in magnetite content, are thoroughly described and analyzed to enlighten the self-sustaining ability of the process. Finally, corrosion product layers formed on permanently immersed steel surfaces were exposed to air. Their drying and oxidation induced the formation of akaganeite, a common product of marine atmospheric corrosion that was, however, not detected on the steel surface after the permanent immersion period.

In-situ formation of durable akaganeite (β-FeOOH) nanorods on sulfonate-modified poly(ethylene terephthalate) fabric for dual-functional wastewater treatment

Industrial oily wastewater with dye-pollution is a critical environmental issue for water purification. However, the fabrication of effective and stable materials for oil-water separation and simultaneous degradation of organic contaminants remains a critical challenge. Herein, we report a new process for in-situ formation of akaganeite (β-FeOOH) nanorods layer on the surface of poly(ethylene terephthalate) (PET) fabric via radiation-induced graft polymerization of glycidyl methacrylate, which is then sulfonated and mineralized. The resultant product is labeled as β-FeOOH@PET, which exhibits dual purification by demonstrating an effective oil-water separation and organic dyes photodegradation under the illumination of visible light. It provides stable performance even after 1000 wash cycles. The sulfonated layer acts as not only an electronic transport layer to prevent electron-hole recombination, but also as an anchored interface for immobilizing β-FeOOH nanorods on the sulfonated layer via strong covalent bonds. Overall, the superhydrophilicity and underwater superoleophobicity of β-FeOOH@PET fabric, along with its robustness with a flexible organic substrate and its dual purification function, provide a new insight toward oily wastewater remediation and water purification in large-scale applications.

Mineral characterization of the biogenic Fe(III)(hydr)oxides produced during Fe(II)-driven denitrification with Cu, Ni and Zn

The recovery of iron and other heavy metals by the formation of Fe(III) (hydr)oxides is an important application of microbially-driven processes. The mineral characterization of the precipitates formed during Fe(II)-mediated autotrophic denitrification with and without the addition of Cu, Ni, and Zn by four different microbial cultures was investigated by X-ray fluorescence (XRF), Raman spectroscopy, scanning electron microscopy equipped with energy dispersive X-Ray analyzer (SEM-EDX), Fourier transform infrared spectroscopy (FTIR) and X-ray Powder Diffraction (XRD) analyses. Fe(II)-mediated autotrophic denitrification resulted in the formation of a mixture of Fe(III) (hydr)oxides composed of amorphous phase, poorly crystalline (ferrihydrite) and crystalline phases (hematite, akaganeite and maghemite). The use of a Thiobacillus-dominated mixed culture enhanced the formation of akaganeite, while activated sludge enrichment and the two pure cultures of T. denitrificans and Pseudogulbenkiania strain 2002 mainly resulted in the formation of maghemite. The addition of Cu, Ni and Zn led to similar Fe(III) (hydr)oxides precipitates, probably due to the low metal concentrations. However, supplementing Ni and Zn slightly stimulated the formation of maghemite. A thermal post-treatment performed at 650 °C enhanced the crystallinity of the precipitates and favored the formation of hematite and some other crystalline forms of Fe associated with P, Na and Ca.

Effect of Solution pH and Chloride Concentration on Akaganeite Precipitation: Implications for Akaganeite Formation on Mars

AbstractAkaganeite, a chloride‐bearing Fe(III) (hydr)oxide, has been reported on Mars in Yellowknife Bay in Gale crater by X‐ray diffraction (XRD), in Robert Sharp crater by Compact Reconnaissance Imaging Spectrometer for Mars, and possibly at the Gusev crater and Meridiani Planum by combined Mössbauer and bulk chemistry analyses. Because formation conditions remain unknown, we investigated akaganeite precipitation as a function of solution pH and chloride concentration. Akaganeite was synthesized through hydrolysis of Fe(III) perchlorate in the presence of 0.02, 0.05, and 0.1 M chloride and at initial solution pH of 1.6, 4, 6, and 8 at 90 °C. Mineralogy of the precipitated Fe(III) (hydr)oxides was characterized by XRD, infrared spectroscopy, and Mössbauer spectroscopy. Total chloride and perchlorate contents were determined by ion chromatography. XRD revealed that akaganeite formed alone or in mixtures with ferrihydrite, hematite, and/or goethite at initial pH 1.6 with 0.02, 0.05, and 0.1 M Cl− and at initial pH from 4 to 8 with 0.05 and 0.1 M Cl−. Infrared analysis showed that akaganeite bands at ~2 and ~2.45 μm were sensitive to amount of akaganeite, total chloride content, and presence of other Fe(III) (hydr)oxides. The results indicate that akaganeite in Yellowknife Bay in Gale crater likely formed under acidic to alkaline (1.6 &lt; pH &lt; 8) oxidizing conditions in solutions containing &gt;0.05 M Cl− by the dissolution of basaltic minerals. Akaganeite in Robert Sharp crater may have formed in acidic (pH &lt; 4) solutions with ~0.1 M Cl− through oxidative dissolution of Fe(II) sulfides and/or acidic oxidizing dissolution of basalts.

Evolution of Akaganeite in Rust Layers Formed on Steel Submitted to Wet/Dry Cyclic Tests.

The evolution of akaganeite in rust layers strongly impacts the atmospheric corrosion behavior of steel during long-term exposure; however, the factors affecting the evolution of akaganeite and its mechanism of formation are vague. In this work, wet-dry cyclic corrosion tests were conducted to simulate long-term exposure. Quantitative X-ray diffraction analysis was employed to analyze variations in the relative amounts of akaganeite; scanning electron microscopy and electron probe microanalysis were used to study the migration of relevant elements in the rust layer, which could help elucidate the mechanism of akaganeite evolution. The results indicate that the fraction of akaganeite tends to decrease as the corrosion process proceeded, which is a result of the decrease in the amount of soluble chloride available and the ability of the thick rust layer to block the migration of relevant ions. This work also explores the location of akaganeite formation within the rust layer.

Determination of the key parameters involved in the formation process of akaganeite in a laboratory-simulated wet-dry cyclic process

To determine the key parameters of akaganeite formation during a wet-dry cyclic process, cyclic immersion tests and simulated experiments were employed under laboratory conditions. The constituents of rust were characterized using XRD and FTIR. According to the results, green rust (FeII4FeIII2Cl2−x(OH)12+x) was a crucial intermediate product during the formation of akaganeite. The evaporation rate of the electrolyte and the amount of Cl− per area played a key role in the formation of the green rust. Additionally, the required amounts of salt for akaganeite formation were determined from simulation experiments in different relative humidity environments.

Preparation of molybdenum-doped akaganeite nano-rods and their catalytic effect on the viscosity reduction of extra heavy crude oil

Abstract Molybdenum-doped akaganeite nano-rods were prepared by one step facile synthesis. The formation of akaganeite depends on the molar ratio of hexaammonium molybdate to ferric chloride hexahydrate. The nano-rods have an average particle size of ca. 30 nm and disperse uniformly. As a catalyst, the nano-rods can effectively increase the viscosity reduction rate of Shengli Oilfield extra heavy crude oil (175000 mPa s at 50 °C), especially combined with tetralin (a hydrogen donor). The results of SARA analysis, elemental analysis, and 1H NMR spectra indicate that the increase of light component caused by the denitrification and desulfuration of the catalyst plus tetralin as well as the loosening of heavy component molecules can be responsible for the viscosity reduction of the heavy oil.

Formation process of akaganeite in the simulated wet-dry cycles atmospheric environment

In order to clarify the formation mechanism and conditions for akaganeite in long-term exposure, the influence of the former corrosion results on akaganeite formation was investigated by simulated experiments in laboratory. The combination of XRD, FTIR, SEM and EPMA enabled the identification of the rust layer formed on the surface. Accordingly, the nature of the rust layer and the amount of the corrosive species in the rust layer varied with the extension of the exposure. Among them, comparing with the corrosion condition in initial stage, the structure of rust layer after repeated wet-dry cycles was disadvantage for akaganeite formation. Element Cl aggregated at the interface between rust and substrate in the thick part can participate in the formation of akaganeite after the rust layer covered removed. The accumulation effect of salt deposited contributed to akaganeite formation under the condition that salt deposition rate was relatively low.

Synthesis of akaganeite in the presence of sulfate: Implications for akaganeite formation in Yellowknife Bay, Gale Crater, Mars

Akaganeite, a Cl-bearing Fe(III) (hydr)oxide, has been recently discovered in Yellowknife Bay in Gale crater on Mars by the Mars Science Laboratory (MSL) Curiosity Rover. Akaganeite was associated with sulfate and sulfide minerals at Yellowknife Bay indicating that sulfate ions could be present in solution during akaganeite formation. The mechanism and conditions of akaganeite formation in the Yellowknife Bay mudstone are unknown. We investigated formation of akaganeite through hydrolysis of ferric chloride solution in the presence of 0, 0.01, 0.05, 0.1 and 0.2M sulfate and at initial pH of 1.5, 2 and 4 at 90°C. Mineralogy of the precipitated Fe(III) phases was characterized by X-ray diffraction and infrared spectroscopy. The precipitates were also acid digested to determine total sulfate and chloride contents. Akaganeite and natrojarosite formed at initial solution pH of 1.5; akaganeite, goethite and natrojarosite precipitated in initial pH 2 solutions and goethite, hematite and 2-line ferrihydrite precipitated at initial solution pH of 4. Sulfate addition did not inhibit akaganeite formation. Increasing initial solution sulfate concentrations resulted in increasing sulfate to chloride ratio in the precipitated akaganeite. Infrared spectroscopy revealed akaganeite bands at ∼2μm (H2O combination band) and at ∼2.46μm (OH combination band). The H2O combination band position linearly correlated with total chloride content in akaganeite. Overall, laboratory studies demonstrated formation of akaganeite at initial sulfate concentration ⩽0.2M (sulfate to chloride molar ratio ⩽0.3) and pH⩽2, implying that those conditions might prevail (perhaps as micro-environments) during akaganeite formation in Yellowknife Bay mudstone. The occurrence of Fe(II) sulfides (pyrite and pyrrhotite) in Yellowknife Bay mudstone is a potential acidity source. Dissolution of sulfide minerals might occur under localized oxidizing water-limiting Cl-rich conditions creating favorable environments for akaganeite formation.

Environmental Conditions for Akaganeite Formation in Marine Atmosphere Mild Steel Corrosion Products and Its Characterization

The corrosion of mild steel in chloride-rich atmospheres is a highly topical issue. The formation of the oxyhydroxide akaganeite (β-FeOOH) in this type of atmosphere leads to a notable acceleration of the steel corrosion process. The scientific literature contains many references to outdoor marine atmospheric tests, but so far has failed to clarify two basic matters in relation to akaganeite: the environmental conditions necessary for its formation, and its morphological characterization. Research has been performed at three atmospheric corrosion stations located at Cabo Vilano wind farm (Camariñas, Spain) at different distances from the shoreline (332, 590, and 2,400 m), with chloride deposition rates of 390, 74, and 29 mg/m2/day, respectively, with the exposure of mild steel specimens for 1 year. This paper reports the environmental conditions that generally led to the formation of akaganeite: an annual average relative humidity of around 80% or higher, and simultaneously, an annual average chloride deposition rate of approximately 60 mg/m2/day or higher. Rigorous characterization of akaganeite was performed by x-ray diffraction, scanning electron microscopy/energy dispersive spectroscopy, and transmission electron microscopy/selected area electron diffraction.

Enhanced phosphate removal from aqueous solution by ferric-modified laterites: Equilibrium, kinetics and thermodynamic studies

The phosphate removal performances of a series of ferric-modified laterites (ML) were tested and compared with raw laterite (RL) in this study. After the modification with 0.5M FeCl3 solution, the resulting adsorbent ML-C exhibited 90.12% of phosphate removal, which was 37.47% higher than that of RL under the same experimental condition. This may be attributed to the significant increase of BET surface area and total pore volume for ML-C, arising from the formation of akaganeite. The effects of contact time, initial phosphate concentration, temperature, pH, and co-existing ions on the adsorption capacity of ML-C were investigated in detail. The equilibrium data of ML-C were fitted better by the Freundlich model than the Langmuir model, suggesting the heterogeneity of the adsorbent surface. The maximum adsorption capacity was estimated to be 31.53mg P/g at 25°C, which decreased with increasing temperatures. The negative change in free energy (ΔG°) and in enthalpy (ΔH°) indicated that the adsorption was a spontaneous and exothermic process. The phosphate adsorption kinetics was better described by the pseudo-second-order model, which indicated the adsorption process was chemisorption. Phosphate removal capacities decreased with increasing pH. The presence of 0.001M and 0.01M completive ions, e.g. Cl−, NO3-, SO42-and HCO3-, had neglectable effects on the phosphate adsorption. In the reusability study, the adsorbent showed no significant loss in their adsorption performance after four adsorption–desorption cycles, indicating that ML-C was able to be utilized as a potential cost-effective phosphate adsorbent for practical applications.

57Fe Mössbauer study of β-FeOOH obtained in presence of Al 3+ and Ti 4+ ions

This paper deals with the influence of Al 3+ and Ti 4+ ions on the formation of akaganeite when prepared by hydrolysis of FeCl 3 solutions. The products were characterized by X-ray diffraction (XRD) using Rietveld refinement, Infrared spectroscopy (FTIR) and 300 and 77 K Mössbauer spectrometry (MS). Akaganeite is the only phase being formed independent upon the type and concentration of the metallic cations. No significant variations in the hyperfine and crystallographic parameters on the akaganeite prepared in the presence of Al 3+ ions with concentrations lower than or equal to 10 mol% were observed. One does conclude that the present chemical route allows only rather small amounts of Al to enter in the structure of akaganeite. On the other hand, Ti 4+ affects the physical properties of akaganeite: its presence favors the decrease of the average grain sizes, the reduction of both the unit cells and the magnitude of all average hyperfine parameters and the formation of structural defects.

Energetics of Bulk and Nano-Akaganeite, β-FeOOH: Enthalpy of Formation, Surface Enthalpy, and Enthalpy of Water Adsorption

Akaganeite, β-FeOOH, is a commonly occurring ferric mineral in the environment and is a sorbent, ion exchanger, and catalyst. It is often fine-grained (nanophase) and frequently contains excess water. Its enthalpy of formation was studied by solution calorimetry in aqueous HCl. The enthalpy of water adsorption was studied by a new calorimetric technique combining a Calvet microcalorimeter and an automated gas dosing system, used for surface adsorption measurements. Akaganeite samples with surface areas of 30−280 m2/g were used. Sample characterization was performed by X-ray powder diffraction, Fourier transform infrared spectroscopy, thermogravimetric analysis, Brunauer−Emmett−Teller method, scanning electron microscopy, and transmission electron microscopy. Surface enthalpy and enthalpy of water adsorption are reported for the first time. By adsorbing water, akaganeite decreases its effective surface enthalpy from 0.44 J/m2 to 0.34 J/m2. The enthalpy of formation of akaganeite can vary by 10−12 kJ/mol as a function of the surface area. The standard enthalpy of formation of akaganeite with zero surface area was refined and is −554.7 ± 1.9 kJ/mol. Thus, the standard enthalpy of formation and surface enthalpy of akaganeite are between those of goethite and lepidocrocite. The more metastable the polymorph, the lower its surface energy.