Bromate removal by Fe(II)–akaganeite (β-FeOOH) modified red mud granule material

Abstract

A multifunction red mud granule material was prepared and modified by Fe(II)–akaganeite (β-FeOOH), which was applied to remove bromate from aqueous solution. According to the zeta-potential, scanning electron microscopy (SEM) and Brunauer–Emmett–Teller (BET) analysis, Fe(II) and akaganeite changed the zeta potentials and structural properties of the red mud granule material (RMGM), which caused Fe(II)–akaganeite/RMGM to effectively remove bromate. The results of the experiments showed that the mechanism for bromate removal contained four actions: electrostatic adsorption and ligand exchange of Fe(II)–akaganeite/RMGM with bromate, ion-exchange of akaganeite with bromate and reduction of bromate by Fe(II). The RMGM with 5.3% Fe loading exhibited the highest removal capacity.

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1. Introduction

Bromate is the by-product of ozonation or chlorination in bromide-containing water treatment.^1,2 Bromate is a potential carcinogenic inorganic substance, and the maximum acceptable contaminant level in drinking water has been defined at 0.01 mg L⁻¹ by the World Health Organization (WHO) and the United States Environmental Protection Agency (USEPA).³ Bromate was classified by the International Agency for Research on Cancer (IARC) as a Group 2B carcinogen.⁴ Bromate is difficult to remove by using conventional water treatment technologies such as coagulation, filtration and chlorination, because of its high solubility and stability in water. In response to this issue, a number of approaches have been explored to eliminate bromate, which are classified as three aspects: removing the bromate precursors, controlling the bromate formation and using physical and chemical methods to remove bromate after formation.⁵

Up to now, a variety of techniques have been researched to remove excessive bromate, such as adsorption, chemical reduction, coagulation, ultraviolet irradiation, electrochemical reduction, membrane processes, photocatalytic decomposition, biological remediation or ion exchange.^6–8 Among these techniques, adsorption is a common practice for bromate removal, such as granulated activated carbon (GAC)⁹ and organo-montmorillonite etc.¹⁰

Nevertheless, some limitations were presented during the process of the treatment, such as high doses, high cost and low efficiency of the materials. Recently, several researchers devoted different kinds of low-cost materials for removing some contaminants in water, which included alum sludge,¹¹ wastewater sludge,¹² red mud,¹³ saw dust¹⁴ and other waste materials.¹⁵ Red mud, as a bauxite residue, is a by-product from the process of alumina production.¹⁶ Most researches of environmental applications showed that red mud is an effective absorbent in water treatment.¹⁷ For instance, use of red mud as adsorbent for remove nitrate, phosphate and congo red.¹⁸ The disposal problem and environmental pollution caused by red mud could be resolved by comprehensive utilization of red mud. While, using red mud for making granular material is the sustainable way for red mud disposal and could make the material separated from water easily. In most references, specific surface area and isoelectric point of red mud granular are not sufficient enough for adsorption. Therefore, metal and metallic oxides could be used as the modifiers for improving the surface performance of red mud granular. Bromate reduction by ruthenium (Ru), palladium (Pd) or iron oxides^18–20 is promisingly sorbent due to their cost effective and high efficiency. Ferrous ion (Fe²⁺)²¹ and akaganeite (β-FeOOH)²² have already been tested as an adsorbent for bromate removal and they were highly efficient. But is difficult to recycle and likely to caused highly iron content in water. Load akaganeite and Fe(II) to granule material can reduce the akaganeite dosage and easy to recycle. Therefore, it has great significance to use Fe(II) and akaganeite (β-FeOOH) for modifying the surface of red mud granular, which applies to remove bromate from aqueous solution.

In this paper, red mud granule material was modified by Fe(II) and akaganeite (β-FeOOH), which aimed to investigate the surface property of red mud granule material for bromate removal. In addition, the reaction mechanisms of bromate with Fe(II)–akaganeite/RMGM was necessary to be discussed.

2. Material and methods

2.1 Chemicals and materials

Raw red mud was obtained from China Shandong Aluminum Industry Company. Ferric chloride (FeCl₃·6H₂O), urea, ferrous sulfate (FeSO₄·7H₂O), polyvinyl alcohol (PVA), bentonite, hydrochloric acid (HCl) and KBrO₃ were obtained from Tianjin Kermel Reagent Co., which were of analytical grade. All solutions were prepared with deionized water.

2.2 Sample preparation

2.2.1 Preparation of red mud granule material. The red mud granule material was prepared with red mud, bentonite and pulverized coal. All of the raw materials were sieved with a 200 mesh screen and then mixed at the mass ratio of 90 : 4 : 6. 2 g mixed powder was added into 2 mL of polyvinyl alcohol (5%) at 75 °C to become a gunk form. Then the gunk was under 20 mesh sieve to manufacture spherical particles with a diameter of 0.8 mm. After dried, the spherical granule material was placed in a muffle furnace and preheated at 400 °C for a duration of 50 min and letter sintered at 1030 °C for 40 min. During sintering process, the heating rate was 2 °C min⁻¹. Finally, the sample was cooled down to room temperature.

2.2.2 Preparation of modified red mud granule material by akaganeite. Red mud granule material was modified by akaganeite. (0.40, 0.60, 0.80, 1.00 and 1.20 g) FeCl₃·6H₂O and (0.53, 0.80, 1.07, 1.33 and 1.60 g) urea were dissolved in (4, 6, 8, 10 and 12 mL) water and then 0.5 g red mud granule material was added to this solution, with being into a water bath at 95 °C for a duration 4 h. Afterward, the samples were took out from the solution and washed thoroughly with water. After that, the samples were dried at 95 °C for 24 h, and then were naturally cooled down to the room temperature. The iron content of different supported samples were determined by o-phenanthroline spectrophotometric. 0.5 g of akaganeite/RMGM solids was added to 10 mL (1 + 1) HCl solution in a flask to stew for 24 h. Thereafter, the iron content in the solution was determined. Results showed that the iron loading amounts of akaganeite/RMGM prepared with 0.40, 0.60, 0.80, 1.00 and 1.20 g of FeCl₃·6H₂O were 10.02, 24.17, 25.42, 40.17 and 35.42 mg g⁻¹, corresponding to 1%, 2.4%, 2.5%, 4% and 3.5% relative to the RMGM mass, respectively. In this study, the akaganeite/RMGM materials were designated as M-akaganeite/RMGM (M = 1%, 2.4%, 2.5%, 4% and 3.5%).

2.2.3 Preparation of modified red mud granule material by Fe(II)–akaganeite. Red mud granule material was modified by Fe(II) and akaganeite. 5 mL of (0.1, 0.2, 0.3, 0.4 and 0.5 mol L⁻¹) FeSO₄·7H₂O and 0.5 g 4%-akaganeite/RMGM in a 25 mL beaker at a stirring rate of 100 rpm for 4 h and then the samples were placed at 110 °C for 24 h. The specimens were washed repeatedly with water and dried to attain constant weight. The Fe(II)–akaganeite/RMGM samples were appointed as X-Fe(II)–akaganeite/RMGM (X = 4.1%, 4.4%, 4.9%, 5.3% and 5.0%).

2.2.4 Bromate removal experiments. In order to determine bromate removal capacity by red mud granule material, batch experiments were carried out. Briefly, 0.5 g of the granule material was introduced into the 25 mL beaker, which was contained 5 mL of (0.8 mg L⁻¹)²³ bromate solution at pH 6.8. Experiments were performed on thermostatic oscillator at 100 rpm at room temperature. After reacting for 4 h, the specimens were filtered through 0.45 μm filter, and the bromate concentration of the filtrates were measured by an ion chromatography system equipped with an AS23 chromatographic column.

2.3 Analytical methods

The chemical composition of red mud was determined by X-ray fluorescence analysis (PANalytical, AXIOS-PW4400, Netherlands). The X-ray diffraction (XRD) patterns of samples were obtained on a Rigaku D/max-IIIB X-ray diffractometer with a Cu Kα radiations (λ = 1.5406 Å) generated at 40 kV and 20 mA. Micromorphological characteristics of samples were characterized using Hitachi S-4800 scanning electron microscopy (SEM) at an accelerating voltage of 5.0 kV. The Brunauer–Emmett–Teller (BET) surface areas of the samples were determined using N₂ adsorption on a Micromeritics ASAP2420 instrument and the plot of the pore-diameter distribution was determined by using the Barrett–Joyner–Halenda (BJH) method from the desorption branch of the isotherm. The loading amounts of Fe was determined by using a phenanthroline spectrophotometric method. The surface charges of supported sample at pH 1.0–9.0 were determined using a zeta-potential analyzer (Horiba, SZ-100Z, France).

3. Results and discussion

3.1 Bromate removal by supported samples

The bromate ions removal capacity of RMGM and supported samples is shown in Fig. 1. As can be seen, pure RMGM had almost no capacity on the bromate removal. For both akaganeite/RMGM and Fe(II)–akaganeite/RMGM samples, the decreased concentration of bromate was depended on the iron loading amounts of RMGM. For akaganeite/RMGM (Fig. 1a), when the iron loading amounts varied from 0 to 4%, the bromate concentration decreased from 800 to 290 μg L⁻¹. A similar observation occurred for Fe(II)–akaganeite/RMGM (Fig. 1b), the concentration of residual bromate was 248, 208, 176, 69 and 168 μg L⁻¹ for of 4.1, 4.4, 4.9, 5.3 and 5.0% (iron loading amounts), respectively. These results clearly indicated that the higher is amount of Fe in the adsorbent, the more efficient is bromates removal. The bromate removal rate of Fe(II)–akaganeite/RMGM are much higher than that of akaganeite/RMGM.

Fig. 1 Bromate removal by akaganeite/RMGM (a) and Fe(II)–akaganeite/RMGM (b).

3.2 The effect of surface charge on bromate removal

In the experiment above, it is noted from Fig. 1 that the pure RMGM has almost no capacity on bromate removal. Ying Zhao²⁴ studied the phosphate removal by red mud and observed the main mechanism for phosphate removal was charge neutralization. Therefore, this phenomenon was mainly caused by the surface electrical properties. Fig. 2 shows the zeta potentials of RMGM in an aqueous solution of various pH values. It can be seen, the zeta potential was decreased from 20.73 to −31.11 mV with the increasing pH from 1.0 to 9.0. The zeta potentials of RMGM are all decreased with increased pH values. The isoelectric point of RMGM was around 3.0, which the SiO₂ played a major role here (the isoelectric point of SiO₂ was around 2.0 (ref. 25)). This is consistent with result of XRD. It is seen from Fig. 3 that the XRD patterns of raw red mud and sintered red mud granule material were obvious. It can be seen that the three major phases, quartz SiO₂, gibbsite Al(OH)₃ and hematite Fe₂O₃, exist in air-dried red mud. After sintering, the peaks belonging to gibbsite disappeared from the system, and quartz SiO₂ is still the major phases exist in sintered red mud. Hence, when pH > 3.0 zeta potentials of RMGM was negative. Therefore, during the reaction process, the negative charges on the surface of RMGM produced electrostatic repulsive interaction with bromate, which caused that the reaction of the raw RMGM with bromate was suppressed. As shown in Table 1, after modified, the zeta potentials were enhanced by Fe(II)–akaganeite compare with that of RMGM at pH 7.0, which may improve bromate adsorption through electrostatic attractive interaction.

Fig. 2 Zeta potentials of raw RMGM at various pH.

Fig. 3 XRD patterns of raw red mud (a) and sintered (b) red mud granule material.

Table 1 Zeta potentials of RMGM β-FeOOH/RMGM and Fe²⁺–β-FeOOH/RMGM

	RMGM	Akaganeite/RMGM	Fe(II)–akaganeite/RMGM
Zeta potentials	−10.97	2.56	2.93

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3.3 The effect of chemical composition of red mud on bromate removal

Table 2 listed the chemical composition of red mud, it can be seen that there are many different metal oxides contained in the red mud. In an aqueous solution, these oxides can be hydroxylated on the surface of RMGM. Sequentially, bromate ion could react with metal oxide by ligand exchange (as shown in eqn (1) and (2)).²⁶

≡MOH + H⁺ ⇔ MOH₂⁺ (1)

≡MOH₂⁺ + BrO₃⁻ ⇔ ≡MOH₂ − BrO₃ (or MBrO₃ + H₂O) (2)

where M represents the metal ions of RMGA surface.

Table 2 The composition of red mud (wt%)

Composition	SiO2	Fe2O3	Al2O3	Na2O	TiO2	CaO
Red mud	36.338	28.030	22.846	8.864	1.781	1.078

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3.4 The effect of modification on the removal of bromate

Morphology of RMGM and supported samples were investigated by SEM images, as shown in Fig. 4a, a number of rounded shape aggregate particles and very few pores were existed in the surface of raw red mud granule. After sintered, the porous structure and the uneven pore size of granule surface were obtained (Fig. 4b), which provided a high specific surface area for loading. Fig. 4c showed that RMGM was loaded with akaganeite. Most of the synthesized spindle-shaped akaganeite were immobilized on the macropore of RMGM, because the size of akaganeite is in the range of 50–400 nm, which was more likely to get into macropore. From Fig. 4d, it can be seen that the akaganeite was crosswise arranged in the macropore, which can formed more mesopore and higher BET surface areas. This is in agreement with the BET result shown in Fig. 5 and Table 3. It can be seen that the surface area of the supported samples were all enlarged noticeably compared with that of RMGM. The isotherm of RMGM and supported samples were of classical IV, which stated that the surface morphology of the samples were mesoporous structure. From Fig. 5a, before modification, pore size distribution of RMGM had a wide range. After modification, the main concentration of pore size distribution is mesoporous range. Meanwhile, Wang, L.²⁷ researched proved that the richly mesoporous had a high and an excellent bromate adsorption efficiency. Hence, compared with RMGM, the bromate removal capacity of the akaganeite/RMGM was higher. The surface morphology of red mud granule could be changed by akaganeite, which showed the better selectivity toward bromate removal. Consequently, bromate removal efficiency increases after loading akaganeite.

Fig. 4 SEM images of raw RMGM (a), sintered RMGM (b), akaganeite/RMGM (c and d).

Fig. 5 (a) The corresponding pore size distribution curves and (b) N₂ adsorption–desorption isotherms of RMGM and supported samples.

Table 3 Textural properties of RMGM, akaganeite/RMGM and Fe(II)–akaganeite/RMGM

Samples	BET surface areas (m^2 g^−1)	Average pore width (nm)
RMGM	3.3	14.8
Akaganeite/RMGM	14.9	13.2
Fe(II)–akaganeite/RMGM	19.9	9.8

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The synthesized Fe(II)–akaganeite/RMGM were analyzed by XPS to determine the oxidation state of iron on their surface. Fig. 6a illustrated the XPS spectrum of Fe 2p, the binding energies of Fe 2p_3/2 and Fe 2p_1/2 for the Fe(II)–akaganeite/RMGM sample were 710 and 723 eV, respectively. The presence of the satellite peak for Fe 2p_3/2 was at 715 eV. Meanwhile, the Fe 2p_3/2 peak with the full width at half maximum was broader (FWHM = 4.2),²⁸ which was the clear evidenced for the existence of both Fe³⁺ and Fe²⁺ on the surface. As seen in Fig. 6b, the XPS O 1s can be fitted by four peaks at binding energies of around 529.9, 530.2, 531.2 and 532.2 eV, which corresponding to O²⁻, OH⁻ and chemically or physically adsorbed water.²⁹ These values indicated that the Fe 2p peak, together with the presence of OH⁻ and O²⁻ group were clearly confirmed for the existence of akaganeite. Based on these results, the Fe²⁺ and the akaganeite had come together, resulting in the bromate removal.

Fig. 6 XPS spectra of Fe(II)–akaganeite/RMGM. (a) Fe 2p, (b) O 1s.

3.5 Mechanism of Fe(II)–akaganeite/RMGM for the removal of bromate

Akaganeite structure has been reported by Dr. M. Nambu et al.³⁰ The crystal structure of akaganeite was shown in Fig. 7. Akaganeite has a (2 × 2) tunnel structure and the chloride anions resides in the channels. Bromate can react with akaganeite by the ion-exchange of Cl⁻ and BrO₃⁻,³¹ and the reaction equilibrium was attained in 60 min. pH had a great influence on bromate removal. The optimal pH around 4 for bromate reduction. While, BrO₃⁻ removal capacity has decreased at the pH range of 5–10, which was due to the OH⁻ suffered competitive adsorption with BrO₃⁻. The reaction of bromate removal by akaganeite was as following eqn (3)

Fig. 7 Crystal structure of akaganeite.

Bromate reaction with Fe²⁺ was studied by M. Siddiqui,³² who observed that the bromate was reduced to the bromide and the Fe²⁺ transferred to the Fe³⁺. The equation for reduction of BrO₃⁻ by Fe²⁺ is as follows eqn (4) and (5):

Fe²⁺BrO₃⁻ + H⁺ → Fe³⁺ + Br⁻ + H₂O (4)

Fe²⁺ + BrO₃⁻ + H₂O → Fe³⁺ + Br⁻ + OH⁻ (5)

Gordon has reported that bromate removal by Fe²⁺ as a function of pH.³³ Then the [FeOH]⁺ played a major role in bromate removal by FeSO₄. Fe²⁺ in the form of [FeOH]⁺ has a dominant position when the pH was around 8.0, which indicated that the optimal pH was around 8.0 for bromate removal by Fe²⁺. Hence, akaganeite played a principal role in the reaction at acidic conditions and Fe²⁺ played a main role at alkaline conditions.

According to the above discussion, the bromate removal behavior by Fe(II)–akaganeite/RMGM has been investigated particularly, which could be divided into four kinds of functions, as is shown in Fig. 8: firstly, the positive charges on the surface of RMGM after modified by Fe(II)–akaganeite, through electrostatic adsorption, BrO₃⁻ can attach on the particle surface. Basis on the result of zeta potentials, it is observed that the optimum pH for the electrostatic adsorption is in low or neutral pH values. After that, BrO₃⁻ reacted with akaganeite through ion-exchange with Cl⁻. From the foregoing, the optimum conditions of ion-exchange is the acidic conditions. Similarly, BrO₃⁻ can reduced by Fe²⁺ of RMGM surface. On the basis of the discussion, the optimum conditions alkaline is the acidic conditions. BrO₃⁻ also may reacted with metal oxide by ligand exchange. Since the ligand exchange reactions is BrO₃⁻ coupled with OH⁻ ions, so the adsorption is favored in low or neutral pH values. Fe(II)–akaganeite/RMGM, which have the function of electrostatic adsorption, ion-exchange, ligand exchange and reduction simultaneously, was applied to remove bromate.

Fig. 8 Schematic representation of bromate removal by Fe(II)–akaganeite/RMGM.

4. Conclusion

The multifunction surface of red mud granule material was realized by the loading of Fe(II) and akaganeite. The results showed that Fe(II)–akaganeite/RMGM exhibits higher bromate removal capacity than raw red mud granule material due to its higher IEP, richly mesoporous, and multifunction surface. When the initial BrO₃⁻ concentration was 800 μg L⁻¹, the removal rate of bromate reached 91% in 4 hour. Bromate removal capacity was enhanced by Fe(II) and akaganeite on RMGM, whose reaction mechanism was discussed. It included four effects: through electrostatic adsorption, bromate could be attached on the surface of Fe(II)–akaganeite/RMGM. Through ligand exchange, bromate could react with metal oxide of Fe(II)–akaganeite/RMGM. Through ion-exchange, BrO₃⁻ could exchange with the Cl⁻, which was in the akaganeite channel. Through redox, the electrons from Fe(II) could transfer to bromate and get bromate reduced to bromide. In addition, for the acid reaction condition, akaganeite play a principal role in the reaction. For the alkaline conditions, Fe(II) play a main role.

Conflict of interest

The authors declare no competing financial interest.

Acknowledgements

This work was supported by the Natural Science Foundation of Heilongjiang Province (E201456), 2015 college students' innovative entrepreneurial training project plan of Heilongjiang Province (201510212949), Natural Science Foundation for young scholars of China (51302054).

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