Figure 1 SEM images of (a) as-precipitated particles and (b) alkaline-treated particles.
To obtain large porous iron oxide particles for application to arsenic adsorbents, hydrated iron phosphate particles of strengite (FePO4·2H2O) were synthesized from a solution containing ferrous (Fe(II)) ions and then treated using an alkaline solution. As the size of each alkaline-treated particle is several tens of micrometers, the water filterability of these particles is high. In addition, alkaline-treated particles have a large specific surface, because each particle consists of agglomerated fine maghemite (γ-Fe2O3) particles of several nanometers in diameter. It is concluded that hydrated iron phosphate particles synthesized from a solution containing Fe(II) ions are appropriate precursors.
Arsenic contamination of drinking water is a serious problem because of its toxicity.1 The ion adsorption technique is useful to remove arsenic from contaminated water,2 and great effort has been dedicated to the development of arsenic adsorbents based on iron oxides and iron oxihydrooxides.3–5 The synthesis of these fine particles with large specific surface is effective for obtaining a large arsenic adsorption capacity. However, their filterability is likely low. It is required to obtain arsenic adsorbents with both large specific surface and high water filterability.
Recently, porous iron oxide particles of about 20 μm in size have been obtained by a novel method using an alkaline solution.6 In this method, hydrated iron phosphate particles of about 20 μm in size are synthesized as precursors from a solution containing ferrous (Fe(II)) ions by the injection of oxygen gas at 368 K. Then, these particles are immersed in the alkaline solution. As the phosphorous in hydrated iron phosphate particles is removed to the alkaline solution, porous iron oxide particles are obtained. The particle size after immersion is almost the same as that before immersion. Thus, such particles exhibit a large arsenic adsorption capacity and high water filterability.6 The investigation mentioned above focused on the hydrated iron phosphate of metastrengite with the space group P21/n. However, it has been reported that hydrated iron phosphate of strengite with the space group Pbca is synthesized from an aqueous solution containing ferric (Fe(III)) ions by adjusting reaction conditions.7 In this investigation, hydrated iron phosphate particles of strengite were synthesized from a solution containing Fe(II) ions and these particles were treated using an alkaline solution. The cross section of an alkaline-treated particle was observed. In addition, the adsorption property in aqueous solution containing arsenic was investigated for the application to arsenic adsorbents.
Hydrated iron phosphate particles were synthesized from ferrous sulfate (Fe(II)SO4) hydrate and aqueous phosphoric acid (H3PO4). First, an iron sulfate solution was prepared by using deaerated water. Subsequently, aqueous phosphoric acid was added to the iron sulfate solution in a reaction vessel with continuous bubbling of nitrogen gas. The iron concentration in the solution was 1 mol/L. The initial molar ratio of Fe(II) and P was set at 1.5 to 1. Oxygen gas was injected into the solution at approximately 368 K for 3 hours. A suspension containing precipitated particles was obtained by the above-mentioned procedure. The precipitated particles were separated by filtering and then washed with distilled water several times. In order to obtain porous iron oxide particles without phosphorus, as-precipitated particles were immersed in 1 M NaOH solution for 1 hour. The remaining particles, that is, alkaline-treated particles were separated from the solution by the filtering and then washed with distilled water several times.
The morphology of as-precipitated particles and alkaline-treated particles was observed by a scanning electron microscopy (SEM). The crystal structure of these particles was identified by X-ray diffraction measurements. The cross section of an alkaline-treated particle was observed by transmission electron microscopy (TEM). An electron beam 1 nm in diameter was used for a electron diffraction measurement. To investigate the adsorption property, 200 ml of arsenic solution with about 100 As-mg/L was prepared using aqueous arsenic acid (H3AsO4), and then 40 mg of alkaline-treated particles was immersed in such solution for 240 minutes. The arsenic concentration change in the solution after immersion of alkaline-treated particles was analyzed using inductively coupled plasma atomic emission spectroscopy (ICP-AES).
Figure 1(a) shows an SEM image of as-precipitated particles obtained from a solution containing Fe(II) ions by the injection of oxygen gas. The size of each as-precipitated particle is several tens of micrometers. Such large particles size is maintained after the treatment using an alkaline solution, as shown in Fig. 1(b). Therefore, the water filterability of alkaline-treated paticles is high.
Figure 1 SEM images of (a) as-precipitated particles and (b) alkaline-treated particles.
To evaluate the phosphorous concentration, about 50 mg of alkaline-treated particles was dissolved in 50 ml hydrochloric acid and the solution was analyzed by ICP-AES. The result of ICP-AES analysis showed that the phosphorous concentration in the solution was less than the detection limit under the present experimental conditions, indicating that alkaline-treated particles are iron oxides or iron oxyhydroxides.
X-ray diffraction patterns of as-precipitated particles and alkaline-treated particles are presented in Fig. 2. The reference diffraction patterns of strengite, metastrengite, maghemite and magnetite are also given in the same figure. The diffraction peaks of as-precipitated particles are assigned to those of strengite, although small peaks assigned to those of metastrengite are also observed. Note that such particles were synthesized by the reaction for 180 minutes, though hydrated iron phosphate particles of strengite have been synthesized from a solution contain Fe(III) ions by the reaction for 2 days at boiling temperature.7 On the other hand, alkaline-treated particles exhibit broad diffraction peaks. A similar X-ray diffraction pattern has been observed in porous iron oxide particles obtained from metastrengite.6 These peaks seem to be assigned to a spinel-type structure such as magnetite (Fe3O4), composed of Fe(II) and Fe(III) ions, and maghemite (γ-Fe2O3), composed of Fe(III) ions. It is likely that alkaline-treated particles are composed of Fe(III) ions, because strengite and metastrengite are composed of such ions. In addition, porous iron oxide particles obtained from metastrengite have been identified as maghemite by extended X-ray adsorption fine structure (EXAFS) measurement at the Fe K adsorption edge.6 Hence, alkaline-treated particles are reasonably identified as maghemite.
Figure 2 X-ray diffraction patterns of as-precipitated particles and alkaline-treated particles. The reference diffraction patterns of strengite, metastrengite, maghemite and magnetite are also indicated.
In order to characterize the inside of an alkaline-treated particle, it was cut by a focused ion beam (FIB) apparatus with gallium ions. The cross section images of an alkaline-treated particle are presented in Fig. 3. As shown in Fig. 3(a), a certain amount of pores is observed among particles with diameters of several tens of nanometers. In addition, each particle is composed of agglomerated fine particles of a few nanometers in diameter as shown in Fig. 3(b). It is likely that iron porous particles obtained from iron phosphate particles of strengite have both high filterability and a large specific surface.
Figure 3 (a) TEM image of the cross section of an alkaline-treated particle and (b) a high magnification image of the same particle.
The electron diffraction pattern of alkaline-treated particles is presented in Fig. 4. A clear diffraction pattern is observed in Fig. 4(a). All diffraction spots can be assigned to those of maghemite, as shown in Fig. 4(b). The result of electron diffraction is consistent with that of X-ray diffraction as shown in Fig. 2. In the crystal structure of strengite, Fe ions form FeO6 octahedral units, which are linked with PO4 tetrahedral units. On the other hand, magnetite is composed of both FeO6 octahedral units and FeO4 tetrahedral units. Thus, the local structure of Fe and the network of Fe-O units in alkaline-treated particles are different from those of as-precipitated particles. It is suggested that iron porous particles are formed by the dissolution of strengite in the alkaline solution and the precipitation of maghemite. Such reactions initially occur on the surface of as-precipitated particles in the alkaline solution. Hence, the surface part of as-precipitated particles changes from strengite to porous iron oxide identified as maghemite. As the alkaline solution can penetrate into pores, the interface between strengite and maghemite proceeds to the inside of as-precipitated particles. It is highly probable that the reaction process described above leads to the formation of large porous iron oxide particles.
Figure 4 (a) Electron diffraction pattern of an alkaline-treated particle and (b) a schematic of indexed diffraction spots.
To evaluate the arsenic adsorption property, alkaline-treated particles obtained from strengite were immersed in an aqueous solution containing arsenic. Figure 5 shows the arsenic concentration in an aqueous solution at pH = 3 as a function of the immersion time when alkaline-treated particles were immersed. For comparison, the data of alkaline-treated particles obtained from metastrengite are also indicated in the same figure. The arsenic concentration decreases with increasing immersion time, indicating that alkaline-treated particles adsorb arsenic in the aqueous solution. The arsenic adsorption capacity of alkaline-treated particles obtained from strengite is estimated to be about 79 As-mg/g, which is larger than that of alkaline-treated particles obtained from metastrengite. Though further investigation and development are necessary, it is concluded that porous iron oxide particles obtained from hydrated iron phosphate particles of strengite are promising for use as arsenic adsorbents.
Figure 5 The arsenic concentration in an aqueous solution at pH = 3 as a function of the immersion time when alkaline-treated particles obtained from hydrated iron phosphate particles of (a) strengite and (b) metastrengite were immersed.
Hydrated iron phosphate particles of strengite (FePO4·2H2O) were synthesized from a solution containing ferrous (Fe(II)) ions by the injection of oxygen gas for 180 minutes, and then these were treated using an alkaline solution. The size of each alkaline-treated particle was several tens of micrometers, and it was composed of agglomerated fine particles of maghemite (γ-Fe2O3). Therefore, alkaline-treated particles exhibited a large arsenic adsorption capacity of about 79 As-mg/g at pH = 3. Furthermore, their water filterability was high. It is concluded that hydrated iron phosphate particles synthesized from a solution containing Fe(II) ions are appropriate precursors to synthesize porous iron oxide particles for application as arsenic adsorbents.
This research was partially supported by Grants-in-Aid for Scientific Research Funds from the Japan Society for Promotion of Science (No. 23360276 and No. 24656452).
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