Overview
Characterization of the Incorporation and Adsorption of Arsenate and Phosphate Ions into Iron Oxides in Aqueous Solutions
2023 Volume 64 Issue 2 Pages 307-317
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2023 Volume 64 Issue 2 Pages 307-317
This article provides an overview of the incorporation and adsorption of arsenate and phosphate into iron oxides formed in aqueous solutions. Although arsenic (As) and phosphorus (P) and belong to the same group in the periodic table, As is usually toxic, whereas P is a biological element. Arsenate and phosphate ions can be incorporated into iron oxides through different routes, depending on the solution conditions. In aqueous solutions, the chemical state of Fe in iron oxides is FeII (ferrous) or FeIII (ferric), that of As is AsIII (arsenite) and AsV (arsenate), and that of phosphorus is PV (phosphate). The composition and structure of iron oxides, including ferric oxyhydroxides and hydroxides formed in aqueous solutions, are affected by solution conditions such as the electrochemical potential, pH, temperature, and the chemical state and composition of the relevant elements. Moreover, arsenate and phosphate ions are usually adsorbed on the surface of iron oxides in aqueous solutions, but the incorporation and adsorption of arsenate and phosphate ions in iron oxides cannot be easily explained using simple thermodynamic models. This is because these characteristics are accompanied by different stages of coprecipitation, incongruent dissolution of multicomponent iron oxides, and so on. Since hydrated iron-oxide particles are often very fine and poorly crystallized, they are difficult to filter. However, it has been shown that coarse polyhedral particles of hydrated iron oxide can be synthesized by oxidizing FeII ions. In addition, the coarse particles of hydrated iron oxide are transformed into agglomerates of fine FeIII oxides with a coarse outer shell by alkaline treatment, and these coarse porous iron-oxide particles exhibit good anion-adsorption capacity in aqueous solutions. The formation mechanisms of these different iron-oxide particles are discussed based on the results of the incorporation and adsorption of arsenate and phosphate ions in iron oxides in aqueous solutions.
Fig. 16 TEM image of the porous iron oxides obtained from the hydrated iron oxide particle scorodite, which was treated in alkaline solution.
Iron oxides, including ferric oxyhydroxides and hydroxides, are generally composed of ferrous (FeII or Fe2+) and ferric (FeIII or Fe3+) ions, together with foreign anions such as oxyanions. The formation of multicomponent iron-based oxides is influenced by the solution conditions and electrochemical properties of the relevant elements.1,2) In the atmospheric corrosion of iron and steel, FeII and FeIII ions are formed by the oxidation of metallic Fe, whereas hydroxyl ions are formed by the reduction of oxygen gas in air. Consequently, corrosion products of different iron oxides are formed on steel surfaces.3) A schematic of the major pathways involved in forming oxides from FeII and FeIII cations in aqueous solutions was proposed by Misawa, as shown in Fig. 1.3,4,8) This diagram was constructed primarily based on the electrochemical reactions of iron species and provides important information on the corrosion products of iron and steel.5–8) A number of studies on the atmospheric corrosion of various steels have been carried out, taking into account the electrochemical properties of elements using various analytical methods.9–13)
Schematic diagram of formation processes of different intermediate iron oxides, including oxyhydroxides and hydroxides in aqueous solution at room temperature. The oxides are plotted against the pH value and the fraction of FeIII. For simplicity, it is modified from the original diagram.3,8) Solid and broken circles, and rectangular denote solid phases and solution phases, respectively. a: air oxidation (aS: slow, aR: rapid), GCI: green complex I, GCII: green complex II, and D.R.C: dark red complex.
As shown in Fig. 1, the stability and structure of green rusts (GRs) containing FeII and FeIII were not well established in the 1970s owing to the high reactivity of FeII with oxygen. Since then, careful synthesis and analysis has allowed clarification of the structure and thermodynamic properties of GRs.14–22) Figure 2 shows a diagram of the electrochemical potential Eh vs. pH of the Fe–H2O system with sulphate-containing GR2(SO42−).21) In this figure, the number n represents the negative exponent of the FeII concentration in the equation [FeII] = 10n.
Eh vs. pH diagram of GR2(SO42−) at [SO42−] = 0.1 M. The dotted line defines the region of H2 gas evolution. The number, −n, in the figure is the exponent of the FeII (Fe2+) concentration expressed as [FeII] = 10−n.21)
Initial studies on GRs showed that GR2(SO42−) was formed by the oxidation of Fe(OH)2 by slow air bubbling and subsequent ferric oxyhydroxide transformation.14) During oxidation, successive reactions of Fe(OH)2 → GR2(SO42−) → α-FeOOH (goethite) were observed in the sulfate medium. Although GR2(SO42−) is unstable in sulfate-free solutions, the composition and formation enthalpy of GR2(SO42−) were experimentally inferred.
In addition to corrosion studies on iron and steel, the formation of iron oxides is important in fields such as hydrometallurgy and mineralogy, where the immobilization of specific elements from deposits is essential.23) These investigations suggest that the difference in the reactivity between FeII and FeIII influences the properties of the iron-oxide particles. For example, it is known that fine goethite (α-FeOOH) particles are formed from FeIII ions in the following reaction:
| \begin{equation} \text{Fe$^{\text{III}}$} + \text{2H$_{2}$O} \to \text{$\alpha$-FeOOH} + \text{3H$^{+}$} \end{equation} | (1) |
During the precipitation or coprecipitation of solid particles in multicomponent solutions, several reaction factors or stages should be considered, as the process is influenced by the chemical composition, crystallinity, particle size, and morphology of the precipitated particles. Therefore, it was pointed out that the correlation between precipitation conditions and particle properties should be investigated, considering solid–liquid equilibria, crystallization kinetics such as nucleation and growth, particle aggregation, and so on.24) This indicates that the coprecipitation processes of the particles in solution consist of different stages, although it is difficult to describe these stages using a simple model.
This article provides an overview of the stability and structure of multicomponent FeII and FeIII oxide species formed in solution in order to understand the fundamentals of coprecipitation of multicomponent particles and their applications in the immobilization or fixation of specific elements. In particular, it is of great interest to fix arsenic (As) and phosphorus (P) using iron oxide, as these elements are toxic and biological nutrients, respectively. Although they belong to the same group in the periodic table and their chemical properties are fundamentally similar, their incorporation into iron oxides and adsorption on oxides in aqueous solutions differ; the filterability of iron oxide particles in water is important in industrial processes. Therefore, in this article, we review the literature on the incorporation and adsorption of arsenate and phosphate ions, with a focus on the size and morphology of the oxide particles.
To understand the electrochemical or geochemical properties of toxic elements such as As and Sb, Eh–pH diagrams of these elements under standard conditions were investigated in the 1980s–1990s.25–32) These diagrams are useful for discussing the general characteristics of these elements, although there are minor differences in the thermodynamic data. The immobilization of As by the coprecipitation of scorodite particles from AsV and FeIII ions in aqueous solution was studied using different methods.29–35) The chemical formula of scorodite is FeAsO4·2H2O, and these particles are formed from byproducts of nonferrous metal processes, typically copper smelting. For example, scorodite particles were synthesized by precipitation from FeIII and AsV solution at temperatures higher than 125°C to obtain relatively high scorodite crystallinity.29) Furthermore, different Fe/As precipitates were prepared from acidic solutions containing FeIII and AsV ions by processing at 80°C and treatment with alkaline solutions.29) After filtering a slurry of the precipitates, the cake of the particles was repulped in distilled water, filtered, and washed. X-ray diffraction (XRD) patterns of the precipitates revealed relatively high crystallinity, and the solubility of crystalline scorodite was approximately two orders of magnitude lower than that reported for apparently amorphous scorodite. These results indicate that crystalline scorodite particles are suitable for the safe storage of arsenic.
As for the chemical state of scorodite, it is known that As exists as the arsenate (HnAsO4n−3) complex of AsV and the arsenite (HnAsO3n−3) complex of AsIII in aqueous solutions. Eh–pH diagrams have been used to visualize the equilibrium stabilities of arsenic species, as shown in Fig. 3.35) Although stability of arsenic species has some relevance in discussing the structure of arsenite polymorphs, the adsorption of arsenic species on iron oxides is the main focus of this study.
Schematic Eh–pH diagram showing the equilibrium stability fields for arsenic species in aqueous solution.35)
The crystal structures of the scorodite particles were investigated using XRD,36,37) in which the atomic positions of Fe, As, O, and H in the structure were determined. Figure 4 shows the atomic-scale packing of the FeO6 octahedra and AsO4 tetrahedra in the unit cell of scorodite.37) The orthorhombic structure of ideal scorodite is described using FeO6 octahedral and AsO4 tetrahedral units. In the FeO6 unit, Fe is coordinated by six O atoms, which are shared by four AsO4 tetrahedra and two water molecules. The two water molecules surrounding Fe formed hydrogen bonds with the O atoms in AsO4. It should be noted that the structure of scorodite is distorted, as scorodite particles precipitate from multicomponent solutions and undergo incongruent dissolution.38,39)
Packing of FeO6 octahedra and AsO4 tetrahedra in the unit cell of scorodite.37)
The solubility products of amorphous and crystalline scorodite particles coprecipitated in aqueous solution depend on the solution conditions, even under standard conditions, implying that the structure and morphology of scorodite as ferric arsenate compounds may be changed by their synthesis methods. To synthesize stable ferric arsenate compounds, the precipitation processes of scorodite in aqueous solution have been studied by analyzing the precipitates and solution.32–43) In these studies, scorodite particles were precipitated by the reaction of FeIII with AsV in an aqueous solution. During the coprecipitation of scorodite, the following reaction is considered to occur:
| \begin{equation} \text{Fe$^{\text{III}}$} + \text{H$_{3}$AsO$_{4}$} + \text{2H$_{2}$O} \to \text{Fe$^{\text{III}}$AsO$_{4}{\cdot}$2H$_{2}$O(s)} + \text{3H$^{+}$} \end{equation} | (2) |
The pressure in this reaction is assumed to be the ambient pressure. The size of scorodite particles formed by this reaction is typically in the sub-micrometer range owing to their high precipitation density. Therefore, the solubility of As in water appears to be relatively high, and fine particles are aggregated with low dewaterability.
Since FeIII-based compounds containing arsenate ions, typically scorodite, appear to be effective for the immobilization or disposal of As, Eh–pH diagrams of arsenic-relevant systems and the adsorption characteristics of arsenate on ferric compounds have also been studied.43) Figure 5 shows the solubility curves of the total FeIII content (m) in contact with the precipitates, estimated based on the four models. In Models 1 and 2, congruent dissolution of scorodite and congruent dissolution followed by the precipitation of ferrihydrite were assumed, respectively. The difference between Models 1 and 2 is the additional precipitation reaction of ferrihydrite. The congruent dissolution of scorodite and precipitation of arsenate-enriched amorphous hydrous ferric oxide (HFO), which is similar to ferrihydrite, were assumed in Model 3. Finally, the congruent dissolution of arsenate-enriched HFO and precipitation of scorodite were assumed in 4. The dominant reaction in Model 3 is different from that in Model 4. In these models, the solid–liquid equilibria curves were calculated using PHREEQC and database,44,45) although nucleation and growth of the crystals and the aggregation of particles strictly contribute to the formation of particles by coprecipitation. However, these models cannot accurately reproduce the real solubility curves of FeIII and AsV in solutions with different pH values because of a few assumptions.
The equilibrium concentration curves show that scorodite is minimally soluble at a pH of ≈5. However, the solubility products of Fe and As are not necessarily clear in solutions of higher pH because As is easily dissolved in higher-pH solutions, and its solubility is sensitive to the surface area of particles.32)
Furthermore, in order to improve the filtration capability of particles, coarse polyhedral particles of scorodite were coprecipitated from FeII and AsV solutions by injecting oxygen gas at ≈95°C.46–52) The diameter of the scorodite particles synthesized under these conditions is larger than 10 µm, which is much larger than the size of the scorodite particles obtained from the FeIII and AsV solutions. Experimentally, the coprecipitation of scorodite particles in this process occurs as follows:
| \begin{align} &\text{4H$_{3}$AsO$_{4}$} + \text{4Fe$^{\text{II}}$SO$_{4}$} + \text{O$_{2}$} + \text{6H$_{2}$O} \to\\ &\quad\text{4Fe$^{\text{III}}$AsO$_{4}{\cdot}$2H$_{2}$O(s)} + \text{4H$_{2}$SO$_{4}$} \end{align} | (3) |
In the coarse particles of well-crystallized scorodite synthesized by this process, the surface area was reduced, and the filterability of water was improved. In this process, an aqueous solution containing AsV and FeII sulfate with a given Fe/As molar ratio was heated and stirred until it reached the reaction temperature. A suspension containing solid particles was coprecipitated in the solution by injecting oxygen gas into the solution. The suspension was filtered, and the solid particles were rinsed with water and dried.
The concentration of As dissolved from these particles in the aqueous solution was lower than that dissolved from the particles obtained from the FeIII and AsV solutions. The characteristics of scorodite particles synthesized by this method are amenable to the disposal and storage of As. The conditions of the coprecipitation reaction are not the same as those in eq. (2), and eq. (3) can occur under atmospheric conditions.46,50)
Scanning electron microscopy (SEM) and in some cases transmission electron microscopy (TEM) have been used to observe the morphology of the scorodite particles. The crystal structure and surface composition of the particles were analyzed using XRD with a CuKα radiation source and X-ray photoelectron spectroscopy (XPS) using a AlKα. Figure 6 shows SEM images of scorodite particles synthesized in solution containing FeII and AsV ions by injection of oxygen gas.37) Scorodite particles synthesized under these conditions were coarse and polyhedral, indicating good filtration capability.
SEM images of scorodite particles synthesized in solution from ferrous and arsenate ions by injection of oxygen gas for 1 h.37)
To understand the coprecipitation processes of the scorodite particles, their surface composition was analyzed using XPS. Figure 7 shows the relative surface atomic composition of Fe and As on the surface of scorodite particles coprecipitated by injecting oxygen gas vs. the reaction time. A reaction time of zero represents the starting point of injecting oxygen gas, and then the coprecipitation of scorodite or its precursor particles begins. The results suggest that after coprecipitation, excess FeIII ions precipitated on scorodite particles, and AsV ions were adsorbed on the particles.37) Changes in the morphology of the scorodite particles with reaction time were observed in the SEM images. Scorodite exhibits relatively good crystallinity, as electron diffraction patterns of a scorodite particle obtained by transmission electron microscopy reveal the same crystallographic orientation despite the limited number of observations.51)
Relative atomic compositions of Fe and As vs. reaction time. The compositions were determined using XPS in the surface layer of the scorodite particles coprecipitated by injecting oxygen gas. It is suggested that excess FeIII ions are further precipitated after coprecipitation, on which AsV ions are adsorbed.37)
Hydrated iron oxides containing arsenate ions, including scorodite, have been synthesized from solutions containing FeIII or FeII ions and AsV ions under different conditions.53–66) In these investigations, the formation of hydrated iron oxides containing arsenate ions at a given temperature was studied in order to describe the thermodynamic equilibrium of the chemical species. Realistically, incorporation and adsorption may not occur homogeneously in industrial processes because the concentrations of chemical species in different phases change dynamically. Nevertheless, the equilibrium relationships of the chemical species may provide rough indications for practical processes.
The synthesis of different hydrated iron phosphates has been investigated in the research field of cathode materials for lithium-ion batteries.67–70) For example, hydrated iron phosphates such as strengite and metastrengite, with the chemical formula FePO4·2H2O, were synthesized as crystalline powders by precipitation from an aqueous solution containing FeIII and PVO43− ions. The structure of the unit cell of these hydrated iron phosphates is fundamentally described using FeO6 octahedra and PO4 tetrahedra, as shown for scorodite in Fig. 4. However, the solubility of these hydrated iron phosphates differs from that of scorodite.
Figure 8 shows the solubility curves of aqueous FeIII ions (m) in contact with strengite and phosphate-enriched HFO, which were calculated using models.43) In these models, the curves were calculated under the assumptions of solid–liquid equilibria.
Solubility curves of total FeIII (mFeIII) in aqueous solutions in contact with phosphate-enriched HFO, strengite and ferrihydrite as a function of pH in aqueous solution.43)
In general, hydrated iron phosphate particles prepared from FeIII and PV solutions are fine, similar to scorodite. Therefore, it would be interesting to synthesize coarse hydrated iron phosphate particles from FeII and PV solutions by oxidation. To obtain coarse particles of well-crystallized, hydrated iron phosphate (FePO4·2H2O), a mixture of FeII and PV solutions ([FeII]/[PV] = 1 to 1.5) was investigated. A certain concentration of hydrated iron phosphate particles were synthesized at 95°C by following literature procedure for scorodite synthesis.21,37) Hydrated iron-phosphate particles were synthesized from ferrous sulfate heptahydrate and phosphoric acid solutions, in which phosphoric acid was added to an aqueous solution of iron sulfate.73) After heating the solution to ≈95°C, particles of hydrated iron phosphate (phosphosiderite) were precipitated by injecting oxygen gas into the solution. After the reaction was completed, a centrifuge was used for solid–liquid separation to collect the precipitates.
The SEM images of the particles synthesized at different concentrations are shown in Fig. 9. The values in Fig. 9 are the concentrations of ferrous sulfate heptahydrate in the solution. Notably, the particles produced from the 0.01 mol/L solution (Fig. 9(a)) have an aggregated and spherical structure (≈0.5 µm), while the particles prepared using the 0.5 mol/L solution (Fig. 9(c)) are coarser than those prepared with the 0.1 mol/L solution (Fig. 9(b)). At a concentration of 1 mol/L (Fig. 9(d)), the primary particles of the prepared sample (≈1 µm) aggregated to form secondary particles with a size >10 µm. These results suggest that the concentration of the solution has a substantial impact on the morphology of the produced particles.
SEM images of solid particles precipitated at different concentrations.
Figure 10 shows XRD patterns of particles precipitated at different concentrations, which correspond to the samples shown in Fig. 9. It can be seen that phosphosiderite was formed at concentrations of (b) 0.1 and (c) 0.5 mol/L solutions, and similar results were obtained previously.73) On the other hand, strengite was formed from (d) a solution with a concentration of 1 mol/L, indicating that the solution concentration affects the crystalline structure of the hydrated iron phosphate. The broad diffraction peaks in the XRD patterns of the synthesized phosphosiderite indicate that the crystallinity was relatively low.
XRD patterns of solid particles precipitated at different concentrations, which correspond to the samples show in Fig. 9.
It is important to determine the optimal reaction conditions to obtain coarse and well-crystallized particles of hydrated iron phosphates. These conditions ensured that coarse particles were precipitated without the aggregation of fine particles to maintain a stable shape. The results shown in Fig. 10 suggest that different solutions can be used to obtain different hydrated iron phosphates. Since strengite is formed in solutions with slightly higher pH than the synthesis of phophosiderite, coarse particles of well-crystallized strengite are formed by adjusting the pH of the solution. It was also expected that adjusting the pH would be effective in controlling the structure and morphology of the particles, even in a reaction system that uses FeII, considering that the solubility of iron oxides in aqueous solutions may decrease within an appropriate pH range.73)
Figure 11 shows the SEM images of the particles precipitated from solutions with initial pHs of (a) 1.2 and (b) 3.7. The particles formed in solutions with a pH of 1.2 were a few micrometers, where the majority of particles were ≈1 µm. The images show that with an initial pH of ≈3.7, coarse polyhedral particles with relatively smooth surfaces (≈5–10 µm) were precipitated in a well-crystallized state. In solutions with a high pH > ca. 3.5, the particles were well-dispersed, without aggregation.
SEM images of solid particles precipitated at initial pH values of 1.2 and 3.7.
Since pH is also an important factor governing precipitation in aqueous solutions, the influence of pH on the coprecipitation of hydrated iron phosphates was investigated. Figure 12 shows the XRD patterns of particles precipitated from solutions with initial pH values of 1.2 and 3.7. The higher pH was selected for this experiment because the solubility of hydrated iron phosphate decreases as the solution pH increases. The results show that phosphosiderite and strengite particles precipitated at initial pH values of 1.2 and 3.7, respectively. The crystalline particles appear to be higher in strengite content, which is consistent with the particle shape, as shown in Fig. 11.
XRD patterns of solid particles precipitated at initial pH values of 1.2 and 3.7.
As mentioned above, hydrated iron arsenate and phosphate particles can be synthesized at adequate Fe concentrations and pH values to obtain coarse particles with good filterability from aqueous FeII solutions. The coprecipitation processes of coarse particles are sometimes accompanied by the precipitation of fine particles of excess ferric ions owing to incongruent dissolution. Fine particles of iron oxide act as adsorbents for arsenate and phosphate ions. The adsorption characteristics of arsenate and phosphate ions onto iron oxides have been studied by many groups.71–84)
For example, the adsorption characteristics of phosphate on iron oxides have been widely investigated.43,85–92) Hydrated iron oxides or hydrous ferric oxide (HFO) are amorphous compounds with a high affinity for anions under strongly or mildly acidic conditions,43) and the complicated structure may be related with the affinity of anions.93–95) Owing to the small particle size of HFO, its adsorption capacity is high and may influence its thermodynamic properties. By experimentally determining the enthalpies of formation, it was shown that the adsorption of phosphate stabilizes HFO. At lower pH values, phosphate-doped HFO is less soluble than the ferrihydrite and crystalline FeOOH polymorphs. Phosphate availability in soils can be controlled by enriching HFO with phosphate; the phosphate-enriched HFO is a few orders of magnitude less soluble than crystalline FeIII phosphates (typically strengite), as shown in Fig. 8. Thermodynamic dissolution models showed that under mildly acidic conditions, a substantial amount of PV was released into the aqueous solution at pH 7. These data can be used to determine the equilibrium concentrations of FeIII and PV in the solutions.
In addition, arsenate ions are adsorbed on fine iron-oxide particles in aqueous solution.71–81) Hydrated iron oxides, including scorodite and siderite, are unstable or dissolve in high-pH solutions, as shown in Fig. 5 and Fig. 8. Therefore, we attempted to prepare porous iron-oxide particles from coarse particles through treatment with alkaline solution.73) These experiments showed that arsenate ions in water were easily adsorbed on porous iron oxides with large surface areas, and the adsorption characteristics depended on the solution conditions. In these investigations, scorodite dissolution was measured over an environmental range of pH and temperatures,73) and it was shown that dissolution rates of As were slowest at a pH of ≈3 and increased with increasing and decreasing pH.
Coarse particles of porous iron oxides were synthesized from particles of siderite and phosphosiderite, which were prepared using the aforementioned procedures, and were further treated with an alkaline solution, as in the previous work.73) The siderite- and phosphosiderite-derived samples are hereafter referred to as S′ and P′, respectively. Figure 13 shows the XRD patterns of the S′ and P′ particles after treatment with alkaline solution. Despite the broadness of the peaks at ≈35° and 62°, these patterns can be assigned to the crystal structure of fine maghemite (γ-Fe2O3) particles, which have a spinel structure. Owing to the oxidation of the FeII solution, maghemite composed of FeIII was formed.21)
XRD patterns of samples S′ and P′, which were obtained from strengite and phosphosiderite particles by alkaline solution treatment.
Figure 14 shows the SEM images of porous iron oxides in Samples S′ (Figs. 14(a-1) and (a-2)) and P′ (Figs. 14(b-1) and (b-2)) at different magnifications. Samples S′ and P′ were treated in alkaline solutions after coprecipitation from solutions with initial pH values of 1.2 and 3.7, respectively. In Sample P′, fine particles were aggregated. A part of the particles was intentionally broken to observe the inside of the sample. Further, the surface of a coarse particle was observed using SEM, and aggregation was found inside of the particle in Sample P′. The morphology of the exterior was identical to that before alkaline treatment, as shown in Fig. 11. However, the coarse particles resulted from the re-precipitation of the very fine oxide particles inside the outer shell. The morphology of the apparent coarse iron oxides consisting of very fine iron oxides were obtained from phosphosiderite in the previous study.73) Thus, since these apparent coarse iron oxides are porous iron-oxide particles with a large surface area, they exhibit superior adsorption properties of anions, such as arsenate ions.
SEM images of porous iron oxides in Samples S′ ((a-1) and (a-2)) and P′ ((b-1) and (b-2)) at different magnifications.
To synthesize hydrated iron phosphate with the desired structure and morphology, it is important to refer to the thermodynamic Eh–pH diagram for the relevant elements. It is useful to consider the reaction conditions for synthesizing hydrated iron phosphates, which are influenced by the electrochemical potential and pH of the solutions, although they are also influenced by the temperature and activities of the elements. Furthermore, the actual reaction rate of coprecipitation is determined by the transfer of chemical species to the reaction sites and oxidation of FeII to FeIII. Practically, the present results stress that conditions such as the concentration of the chemical species and pH of the aqueous solutions influence the structure and morphology of the synthesized particles. In particular, the morphology of the resultant particles in inhomogeneous reactions involving different solid particles is sensitive to changes in precursor particles.
An alkaline solution was used to obtain porous particles with aggregated maghemite fine particles from the hydrated iron-phosphate particles. It is sometimes desirable to immobilize nutritional P in aqueous solution. To stabilize P in aqueous solutions, for example, in the synthesis of coarse porous iron oxide particles, maghemite nanoparticles and coarse particles of phosphosiderite were prepared by injecting oxygen.73) Porous iron-oxide particles were obtained in an alkaline solution, by which maghemite nanoparticles were re-precipitated as aggregates of iron oxide nanoparticles in the coarse particles. In this study, X-ray absorption spectroscopy (XAS) in the range of Fe K-edge X-ray absorption near-edge structure spectra of the products during the reaction was used to analyze the local structure of iron in the particles. Similarly, the XAFS spectra of samples P′ and S′ precipitated at pH values of 1.2 and 3.7 were measured. Figure 15 shows the XAFS spectra of the P′ and S′ samples. The spectra are similar to those of coarse particles of phosphosiderite,73) suggesting that the coarse particles are composed of aggregates of fine particles of maghemite.
Fe K-edge EXAFS spectra of porous iron oxides comprising samples P′ and S′, and reference materials α-Fe2O3 and γ-Fe2O3.
Figure 16 shows the TEM image of a cross-sectional particle obtained from coarse scorodite particles by alkaline treatment. Although these porous particles have a large specific surface area, they are generally fragile and less filterable owing to their underdeveloped morphology. Thus, it is necessary to prepare well-crystallized hydrated iron arsenate or phosphate, in which the starting particles have coarse outer shapes. During the synthesis of apparent coarse particles consisting of porous iron oxides, the rate of dissolution of scorodite seems to be balanced with the reprecipitation rate of fine particles of iron oxide, although these microscopic reactions are likely dynamic and inhomogeneous. Moreover, note that reaction conditions, such as the pH of the solution, which is likely to be an important factor of the reactions, are changed during the dissolution of scorodite particles and reprecipitation of fine iron oxides. As a result, the outer shell of the aggregated iron oxides likely reflects the morphology of the starting scorodite particles after alkaline treatment.
TEM image of the porous iron oxides obtained from the hydrated iron oxide particle scorodite, which was treated in alkaline solution.
The formation and structure of hydrated iron arsenate and phosphate are also important when considering the biological effects on the incorporation and adsorption of oxyanions in iron oxides.96–104) For example, it has been shown that scorodite is formed in the absence of primary minerals with the help of thermoacidophilic iron-oxidizing archaeon Acidianus sulfidivorans.102) In this study, XRD measurements identified scorodite, which were grown on FeII at 80°C and pH 1 in the presence of arsenate ions. The simultaneous biologically induced crystallization of ferric iron FeIII and arsenic to scorodite prevented the accumulation of ferric iron.
Furthermore, to understand the effect of inhomogeneous mineral seeding on biogenic scorodite formation, the oxidation of toxic AsIII from acidic effluents from metal refineries was also studied using the thermostable acidophilic archaeon Acidianus brierleyi.103) The thermostable archaeon Acidianus brierleyi used in this study was an FeII/AsIII-oxidizing Thiomonas cuprina. The results demonstrated that hematite seeding was effective in a low-temperature scorodite crystallization reaction using the mesophilic acidophilic bacterium Acidimicrobium. Therefore, hematite absorbed anionic AsV and SO42−, which shortened the time required for the biogenic scorodite crystallization process. Thus, these results indicated that crystal growth of scorodite was favored over nucleation, by which crystalline biogenic scorodite formed similarly to mineral scorodite.
Since phosphorus is an important biological nutrient but a limiting mineral, the recovery of phosphorus using the adsorption of iron oxides is of great interest.96–99) Adsorption properties of phosphate ions on iron oxides are also affected by biological atmospheres, as the adsorption of these ions takes place under biological conditions.99–105) Iron oxides, typically FeIII oxyhydroxides, exhibit superior adsorption properties to anions, and iron oxide is of interest for controlling the presence of trace elements in the earth’s environment. FeIII oxyhydroxide has been used as an adsorbent in laboratory experiments, and there are many biogenic FeIII oxyhydroxides in the environment. Although the mechanisms of the adsorbents on iron oxides have not been well elucidated, the effects of the structure on the adsorption properties of trace elements on FeIII oxyhydroxide have been studied.99)
5.2 Role of oxyanions on global environmentThe formation and structure of iron oxide containing the oxyanions of arsenate and phosphate ions were discussed in the above section. The other oxyanions, carbonate and silicate, play an important role in the global environment owing to their abundance. Global warming is currently a major issue in not only the natural sciences but also the social sciences. Global warming is believed to be primarily caused by an increase in greenhouse gases (mainly carbon dioxide) emitted from the combustion of fossil fuels. In recent years, carbon neutrality has been strongly sought after to curb global warming. In the beginning, earth was covered with a large amount of carbon dioxide gas, which was absorbed by ocean water.105) The absorption of carbon dioxide from the atmosphere is associated with chemical changes and the oxidation of FeII ions in seawater, and seawater tends to become more acidic as it absorbs carbon dioxide.105,106) In recent years, it has been reported that atmospheric carbon dioxide is absorbed by seawater, which tends to acidify water.107) Carbonate ions react with FeII ions in water to form GR1(CO32−) and interact with other cations and anions in seawater. Although the properties of FeII and FeIII ions are complex, and their parameters are unclear, new knowledge is accumulating.
Thermodynamic laws and equilibrium equations are sometimes used to describe natural phenomena and environmental reactions, but they often deviate from true phenomena and reactions. This is mainly because the actual reaction is influenced by a variety of factors, including the chemical state and composition of the relevant chemical species in each phase, and the reaction rate (deviation from equilibrium) at each temperature. As an example, a model phase diagram of a nanoparticle system showing the contribution of surface energy and bulk energy to the total particle free-energy as a function of particle size and surface area is schematically shown in Fig. 17. The model phase diagram of the nanoparticle system qualitatively suggests that the surface area of fine particles cannot be neglected at the total free-energy.93) The contribution of surface energy to the total energy increases with decreasing particle size, favoring forms with lower surface energies per unit area, as shown by the formation of scorodite particles with different shapes. Such fundamental and reliable knowledge on the formation and structure of various iron oxides is important when considering environmental issues.
Model phase diagram showing the effect of surface area, where surface and bulk energy contribute to the total particle free-energy with respect to particle size.
The characteristics of the incorporation and adsorption of arsenate and phosphate in iron oxides formed in aqueous solutions were summarized in this paper. Electrochemical potential vs. pH diagrams are generally used to represent the speciation of iron and relevant elements in aqueous solutions at room temperature. To understand the formation conditions of the different hydrated iron-oxide particles, they were analyzed along with their solutions using various methods. Under specific solution conditions and at relatively high temperatures, coarse hydrated iron-oxide particles with good filterability were formed from FeII ions containing arsenate or phosphate ions in the solutions, unlike the formation of FeIII ions. Coarse polyhedral iron-oxide particles, such as scorodite and strengite, transform into coarse polyhedral particles composed of fine porous iron oxide upon alkali solution treatment. The morphology of the starting multicomponent iron-oxide particles was shown to influence the aggregation morphology of the final porous iron-oxide particles. The importance of biological effects on the formation of multicomponent iron oxides and the roles of various oxyanions in iron oxide formation in the global environment are also discussed.
The authors acknowledge Grant-in-Aids for Scientific Research by the Japan Society for the Promotion of Science, Japan Oil, Gas, and Metals National Corporation, and New Energy and Industrial Technology Development Organization for supporting a series of studies. The authors would like to express their sincere gratitude to Prof. Y. Waseda, Prof. T. Nakamura, Prof. E. Shibata, Prof. K. Sugiyama, Dr. T. Fujita and Dr. R. Murao for helpful discussions. They also thank Mr. K. Sakata, Dr. S. Fujieda, Mr. T. Kandani, and Mr. M. Fukuoka for assistance with the experiments. Part of this article was presented by one of the authors (S.S.) at a plenary lecture at the Fall Meeting of the Japan Institute of Metals and Materials (JIM) held in 2020. He also appreciates the appointment of a fellow by JIM for human resource development.
