Cr(VI) removal by mesoporous FeOOH polymorphs: performance and mechanism

Abstract

The mesoporous FeOOH polymorphs, i.e., goethite (α-FeOOH), akaganeite (β-FeOOH), lepidocrocite (γ-FeOOH), and feroxyhyte (δ-FeOOH) were synthesized and characterized before and after reaction with Cr(VI) by scanning electron microscopy (SEM), N₂ adsorption–desorption isotherms, X-ray diffraction (XRD), saturation magnetization measurements, and X-ray photoelectron spectroscopy (XPS). The Cr(VI) removal efficiencies by these four FeOOH polymorphs were investigated. After reaction for 120 min, the Cr(VI) removal efficiencies by α-FeOOH, β-FeOOH, γ-FeOOH, and δ-FeOOH were 94.5%, 52.0%, 100%, and 84.6%, respectively. The Cr(VI) removal efficiencies by these four FeOOH polymorphs were influenced by their specific surface area. The XPS analysis indicated that the removal of Cr(VI) by these FeOOH polymorphs was a process of electrostatic attraction and ligand exchange. The study of this paper can reveal the reaction mechanism of FeOOH with Cr(VI) and the different behaviors of the four FeOOH polymorphs in the removal of Cr(VI).

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1. Introduction

Recently, there is an increasing interest in the study of iron oxyhydroxide (FeOOH) polymorphs that have important applications in adsorption and catalysis.^1–3 The FeOOH polymorphs naturally occur in several crystal forms, including goethite (α-FeOOH), akaganeite (β-FeOOH), lepidocrocite (γ-FeOOH), and feroxyhyte (δ-FeOOH), which are categorized by their phase structures.^1,4,5 They always appear as nanoparticles with a high specific surface area in soil and sediments.^4,6

The physical and chemical properties of these four FeOOH polymorphs are different. Among these FeOOH polymorphs, α-FeOOH is the most common and most stable FeOOH phase with the lowest Gibbs energy,^6–8 and it is widespread in soil and sediments.^9,10 β-FeOOH follows α-FeOOH in the stability,⁶ however, the synthesis of β-FeOOH was always conducted in acidic solution, which would lead to the extra protonation of FeOOH within iron octahedral and resulting in the destabilization of structures.^11,12 γ-FeOOH is metastable compared with α-FeOOH and β-FeOOH.^2,4,6 The other polymorph, δ-FeOOH, perhaps less known, is the unique FeOOH polymorph because it is ferromagnetic,^13,14 thus it can be easily recovered after use with a simple magnet.

FeOOH has been used as a catalyst to enhance the degradation efficiency of oxalic acid by ozone.³ FeOOH was also used as a Fenton-like catalyst for the oxidation of organics.^14,15 With the variation of pH, the surface hydroxyl groups of FeOOH changed and formed –OH₂⁺, –OH, or –O⁻.^13,16–18 Since the contaminants such as As(III), As(V), Se(IV), and Cr(VI) exist as anions, they can be attracted by the positively charged functional groups of –OH₂⁺ in FeOOH through electrostatic attraction. FeOOH polymorphs have been used as adsorbents in removing As(III)/As(V),^{2,13,17,19,20} Se(IV),²¹ and Cr(VI).^2,22 However, to date, the comparison studies of the four FeOOH polymorphs in removing contaminants have not been reported. One could not predict which polymorphs under what conditions are more favorable to remove contaminants. Therefore, it is very important to study the reaction mechanism and contaminants removal efficiency by different FeOOH polymorphs.

The Cr(VI) is well known for its carcinogenicity and high mobility in the environment.^16,23–26 In aqueous solutions, Cr(VI) exists in several stable forms including CrO₄²⁻, HCrO₄⁻, and Cr₂O₇²⁻ depending on the solution pH and Cr(VI) concentration, as shown in eqn (1) and (2).^27–30 Thus, HCrO₄⁻ is the predominant form of Cr(VI) in an acidic medium, while CrO₄²⁻ is the predominant form of Cr(VI) in neutral or basic mediums.^28,31,32

Cr₂O₇²⁻ + H₂O ⇌ 2HCrO₄⁻, pK_a = 2.2 (1)

HCrO₄⁻ ⇌ CrO₄²⁻ + H⁺, pK_a = 5.9 (2)

In the present work, α-FeOOH, β-FeOOH, γ-FeOOH, and δ-FeOOH were prepared successfully according to several previous reports with some modifications. FeOOH polymorphs were characterized by scanning electron microscopy (SEM), N₂ adsorption–desorption isotherms, X-ray diffraction (XRD), and saturation magnetization measurements. The potential of these four FeOOH polymorphs used for removing Cr(VI) from aqueous solution was evaluated. The Cr(VI) removal mechanism by these FeOOH polymorphs was also studied by X-ray photoelectron spectroscopy (XPS) analysis. The study of this paper can reveal the reaction mechanism of FeOOH with Cr(VI) and different behaviors of the four FeOOH polymorphs in the removal of Cr(VI).

2. Experimental

2.1. Materials

All chemicals were of analytical grade and used without further purification. The water used was deionized water. Fe₂(SO₄)₃, FeSO₄·7H₂O, FeCl₃·6H₂O, urea (CH₄N₂O), Fe(NO₃)₃·9H₂O, hydrazine monohydrate (N₂H₄·H₂O), (NH₄)₂Fe(SO₄)₂·6H₂O, and H₂O₂ were used in the synthesis of the FeOOH polymorphs. A stock solution of Cr(VI) was prepared by dissolving K₂Cr₂O₇ to a final concentration of 0.1 g L⁻¹ of Cr(VI). Cr(VI) solutions used during the experiment were prepared by diluting the stock solution to the desired concentration daily.

2.2. Preparation of the FeOOH polymorphs

2.2.1. Preparation of α-FeOOH. The synthesis of α-FeOOH was carried out as described by Wang et al.²² with some modifications. 1.0 g Fe₂(SO₄)₃ and 0.2 g FeSO₄·7H₂O were dissolved in 150 mL deionized water. After stirring for 5 min, a transparent solution was obtained and then transferred into a 200 mL crucible and heated at 160 °C for 3.0 h. After cooling down to room temperature, the brownish yellow precipitates were washed with deionized water followed by absolute ethanol and filtered, and finally dried at 60 °C for 3.0 h.

2.2.2. Preparation of β-FeOOH. The synthesis of β-FeOOH was carried out as described by Yue et al.⁵ with some modification. 5.4 g FeCl₃·6H₂O was added into a beaker containing 200 mL of deionized water with 5.0 g urea. The reaction mixture was heated at 80 °C under water bath for 24.0 h. After cooled to room temperature, the brownish yellow precipitates were separated by centrifugation, washed with deionized water, and dried at 60 °C.

2.2.3. Preparation of γ-FeOOH. The synthesis of γ-FeOOH was referred to Maiti et al.⁸ with some modification. 2.8 g Fe(NO₃)₃·9H₂O was dissolved in 300 mL deionized water. Then, N₂H₄·H₂O was added dropwise to bring the pH around 7.0. The brownish black precipitates were washed with water followed by ethanol, filtered, and dried at 70 °C for 3.0 h.

2.2.4. Preparation of δ-FeOOH. The preparation of δ-FeOOH was referred to Faria et al.¹³ and Chagas et al.³³ 5.6 g (NH₄)₂Fe(SO₄)₂·6H₂O was dissolved in 50 mL deionized water. Then, 50 mL 2.0 M NaOH alcoholic solution was added. After the formation of green rust precipitates, 10 mL 30% H₂O₂ was immediately added with stirring. The precipitates turned reddish brown within a few seconds. The precipitates were washed with deionized water several times, filtered, and dried in a vacuum desiccator at room temperature.

2.3. Analytical methods

2.3.1. SEM. The morphologies of the synthesized FeOOH polymorphs were observed under a scanning electron microscope (LEO1530VP, Zeiss Co., Germany), using an operating voltage of 10.0 kV.

2.3.2. The N₂ adsorption–desorption isotherms. The N₂ adsorption–desorption isotherms of the synthesized FeOOH polymorphs were performed by a Micromeritics instrument (TriStar II 3020, US) with a degassing temperature of 150 °C for 6.0 h. The Barrett–Emmett–Teller (BET) method was used for specific surface area calculation at P/P₀ = 0.05–0.30. The total pore volume was estimated from the amount of N₂ adsorbed at P/P₀ = 0.97–0.99, and the pore size distribution was calculated based on the Barret–Joyner–Halender (BJH) theory.

2.3.3. XRD. The XRD patterns of the synthesized FeOOH polymorphs were recorded on an X-ray powder diffractometer (XD-2, Purkinje General Instrument Co., Ltd., Beijing, China) that employed Cu Kα radiation. The accelerating voltage and applied current were 36.0 kV and 20.0 mA, respectively.

2.3.4. Saturation magnetization measurements. The magnetic measurements of the synthesized FeOOH polymorphs were carried out using a vibrating sample magnetometer (VSM) (Model-735, Lakeshore Co., US).

2.3.5. The surface charges measurements. The surface charges of the synthesized FeOOH polymorphs were measured following a potentiometric titration method. It was preformed with an ionic strength of 0.005 M KNO₃ and a solid-to-liquid ratio of 2.5 g L⁻¹. Standard buffer pH solutions (pH 4.00, 6.86, and 9.18) were used to calibrate the electrode. 0.1 M HNO₃ and NaOH solutions were used as titrants. A small amount of HNO₃ was added to protonate a significant part of the surface sites, rendering the surface positive. After 15–20 min, the new equilibration pH value was recorded. The suspension was then titrated by adding small volumes of 0.1 M NaOH and the pH was recorded. The surface charges of the four FeOOH polymorphs were calculated from the potentiometric data using the following relationship.³⁴

δ₀ = (C_a − C_b + [OH⁻] − [H⁺])/C_s (3)

Where δ₀ is surface charge (μmol g⁻¹), C_a and C_b are concentrations (M) of acid and base after addition to the FeOOH polymorphs suspension solution, C_s is the concentration of the FeOOH polymorphs (g L⁻¹), and [H⁺] and [OH⁻] are the concentrations of H⁺ ions and OH⁻ ions determined from the pH of the solution.

2.3.6. Fourier transform infrared spectroscopy (FTIR). FTIR spectra were carried out by a spectrometer (Nicolet 6700, Thermo fisher, USA) with KBr pellets in the range of 4000–400 cm⁻¹.

2.3.7. XPS. The XPS spectra were conducted with an Amicus (Shimadzu Co., Japan) X-ray photoelectron spectrometer, using Al Kα radiation (1486.8 eV). Data analysis involved smoothing, non-linear Shirley-type background subtraction and curve fitting using mixed Gaussian–Lorentzian functions. Spectral bands were deconvoluted into peaks (sum function of 90% Gaussian and 10% Lorentzian) with the software XPSPEAK from RCSMS lab using an integrated background subtraction.

2.4. Batch experiments of Cr(VI) removal by FeOOH

Batch experiments were conducted to examine the Cr(VI) removal by the FeOOH polymorphs. The FeOOH powder was added into 100 mL 20.0 mg L⁻¹ Cr(VI) solutions to obtain FeOOH concentration of 2.5 g L⁻¹. The solution was periodically sampled and filtered immediately. Then, the residual Cr(VI) concentration in the filtrate was determined with the 1,5-diphenylcarbazide method at 540 nm,³⁵ using a UV-2400 spectrophotometer (Shanghai, China). To investigate the effect of pH values on the Cr(VI) removal, initial pH values of the solutions were adjusted to 3.0, 5.0, 7.0, and 9.0 at room temperature using 1.0 M H₂SO₄ or NaOH solutions.

To study the reusability of FeOOH polymorphs, recycling tests were also conducted. After 2.5 g L⁻¹ FeOOH was firstly used to remove 20 mg L⁻¹ Cr(VI) at pH 3.0, the used FeOOH was filtered and washed with deionized water for several times. Then the FeOOH was dried in a desiccator for reuse in the next cycle.

2.5. Adsorption isotherm

The adsorption isotherms studies were conducted at the initial Cr(VI) concentration of 20, 40, 80, 120, 180, 240, and 300 mg L⁻¹ 0.25 g FeOOH polymorphs was added in 100 mL of Cr(VI) solution at initial pH 3.0. The mixture was continuously shaken at 110 rpm on a thermostatic shaker for 5 h under controlled temperature condition (25 °C). The equilibrium adsorption capacity (q_e) (mg g⁻¹) for Cr(VI) was calculated according to the following equation:where C₀ and C_e (mg L⁻¹) are the initial and equilibrium Cr(VI) concentrations, respectively, V (L) is the volume of Cr(VI) solution, and m (g) is the mass of FeOOH polymorphs.

3. Results and discussion

3.1. Characterization of the FeOOH polymorphs

3.1.1. SEM analysis of FeOOH polymorphs. The SEM images of α-FeOOH, β-FeOOH, γ-FeOOH, and δ-FeOOH are shown in Fig. 1. α-FeOOH (Fig. 1(a1)) with the uniform 3D urchin-like structure was clearly observed. Such microstructure can provide necessary mechanical robustness against wear and tear while sustaining water flow,²² which can be confirmed according to their rarely undamaged microstructures after reaction with Cr(VI) in Fig. 1(a2). Fig. 1(b1) shows that β-FeOOH was rod-like in shape with a length of 200–300 nm. The shape of β-FeOOH did not change after reaction with Cr(VI) (Fig. 1(b2)). However, it was clear that the SEM images of γ-FeOOH and δ-FeOOH changed before and after reaction with Cr(VI). According to Fig. 1(c1), γ-FeOOH consisted of fine-grained irregular particles along with some flakes, however, the agglomeration of these particles can be clearly observed. In Fig. 1(d1), δ-FeOOH consisted of many irregular nanoflakes, and these nanoflakes were assembled together to form particles with many apertures. The unconsolidated surface of γ-FeOOH and δ-FeOOH implies their good porous properties. After Cr(VI) removal, the loose surface became caked and lumpish (Fig. 1(c2) and Fig. 1(d2)), indicating that the mechanical robustness of γ-FeOOH and δ-FeOOH particles was poor.

Fig. 1 SEM images of (a₁) α-FeOOH (×30 000), (a₂) α-FeOOH after reaction with Cr(VI) (×30 000); (b₁) β-FeOOH (×50 000), (b₂) β-FeOOH after reaction with Cr(VI) (×50 000); (c₁) γ-FeOOH (×50 000), (c₂) γ-FeOOH after reaction with Cr(VI) (×50 000); and (d₁) δ-FeOOH (×50 000), (d₂) δ-FeOOH after reaction with Cr(VI) (×50 000).

3.1.2. The N₂ adsorption–desorption isotherms of FeOOH polymorphs. Fig. 2 shows the N₂ adsorption–desorption isotherms and BJH pore size distributions of the four FeOOH polymorphs. The special surface area and porosity of the four FeOOH polymorphs are also presented in Table S1.† According to the IUPAC classification, the isotherms of α-FeOOH (Fig. 2(a)), and β-FeOOH (Fig. 2(b)) are type IV with H1 hysteresis loops, which are characteristic of mesoporous materials.^13,19 Meanwhile, the isotherm of γ-FeOOH (Fig. 2(c)) is type IV with H2 hysteresis loops. The isotherm of δ-FeOOH (Fig. 2(d)) is type IV with a combination of H1 and H2 hysteresis loops. The average pore size of γ-FeOOH and δ-FeOOH are 3 and 9 nm, respectively, so they are also mesoporous materials (pore size 2–50 nm).^36,37 According to the Insets of Fig. 2, the pore size distributions of γ-FeOOH and δ-FeOOH are steeper than that of α-FeOOH and β-FeOOH, which means that the pore distributions of γ-FeOOH and δ-FeOOH are more uniform. In addition, δ-FeOOH has the largest pore volume.

Fig. 2 N₂ adsorption–desorption isotherms of (a) α-FeOOH, (b) β-FeOOH, (c) γ-FeOOH, and (d) δ-FeOOH (inset: the corresponding BJH pore size distribution curve).

The specific surface area of the four FeOOH polymorphs differed for the microstructure of the materials. β-FeOOH with micron sized exhibited a specific surface area of 47.9 m² g⁻¹. Meanwhile, the specific surface area of α-FeOOH, γ-FeOOH, and δ-FeOOH were much higher than that of the reported α-FeOOH (100 m² g⁻¹),³⁸ γ-FeOOH (133 m² g⁻¹),² and δ-FeOOH (135 m² g⁻¹),¹³ respectively. The high specific surface area is an important characteristic of these materials for application as adsorbents. As is known, high specific surface area is beneficial for these materials to remove contaminants in aqueous solution. Among the four FeOOH polymorphs, γ-FeOOH exhibited the highest specific surface area, and hence we could expect its superior capability for Cr(VI) removal.

3.1.3. XRD analysis of the FeOOH polymorphs. Fig. 3 shows the XRD diffractograms of the four FeOOH polymorphs before and after Cr(VI) removal. The crystalline structure of these FeOOH polymorphs did not change after Cr(VI) removal. The diffraction peaks observed in Fig. 3(a) and (b) were unambiguously assigned to α-FeOOH (JCPDS no. 29-0713) and β-FeOOH (JCPDS no. 34-1266), respectively. The pattern peaks in Fig. 3(c) can be assigned to the γ-FeOOH phase (JCPDS no. 74-1877), however, the diffraction lines were broadened, perhaps indicating the presence of very fine particles. The XRD patterns in Fig. 3(d) were consistent with the δ-FeOOH phase (JCPDS no. 13-0087). The diffraction peaks relevant to Cr can not be identified by the XRD patterns perhaps due to its small crystalline size or small amount.

Fig. 3 XRD patterns of (a) α-FeOOH, (b) β-FeOOH, (c) γ-FeOOH, and (d) δ-FeOOH before and after Cr(VI) removal.

3.1.4. Magnetic measurements of the FeOOH polymorphs. The M–H curves of the FeOOH polymorphs measured at room temperature are presented in Fig. 4. It can be seen that the M–H curves of α-FeOOH, β-FeOOH, and γ-FeOOH are straight lines without saturation magnetization, exhibiting paramagnetism of these three FeOOH polymorphs.^39,40 In addition, the small slope of the lines can be attributed to the poor magnetism of α-FeOOH, β-FeOOH, and γ-FeOOH. However, other than the former three materials, the saturation magnetization of δ-FeOOH is about 7.6 emu g⁻¹. The values of the coercivity (18.4 Oe) and the remnant magnetization (0.125 emu g⁻¹) of δ-FeOOH are small, indicating that the δ-FeOOH is a ferromagnetism material.^13,14 So δ-FeOOH can be easily recovered after use with a simple magnet. This characteristic makes δ-FeOOH a good candidate for removing contaminants in aqueous medium.

Fig. 4 The magnetization versus field (M–H) curves of the FeOOH polymorphs (inset: the zoom of the region of low magnetic fields).

3.2. Surface charges of the FeOOH polymorphs

The dependence of the surface charges on pH of these FeOOH polymorphs is shown in Fig. 5. Significant differences were observed due to their unique surface properties. The pH_pzc (pH of the point of zero charge) of α-FeOOH, γ-FeOOH, and δ-FeOOH were about 3.6, 7.1, and 8.0, respectively. However, the surface charges of β-FeOOH were always negative and the pH_pzc can not be obtained.

Fig. 5 Potentiometric titration curves of the FeOOH polymorphs (FeOOH concentration 2.5 g L⁻¹, I = 0.005 M).

As is reported,^13,22,23,41 when the solution pH was below the pH_pzc of FeOOH, most surface hydroxyl groups on the FeOOH were presented as protonated species. The deprotonated functional groups were predominant at pH above pH_pzc of FeOOH, as shown in the following equations:

FeO–OH + H⁺ → FeO–OH₂⁺ (pH < pH_pzc) (5)

FeO–OH + OH⁻ → FeO–O⁻ + H₂O (pH > pH_pzc) (6)

In general, when the solution pH < pH_pzc, the positively charged surface (FeO–OH₂⁺) favored the electrostatic attraction with Cr(VI) oxyanions, thus improving the Cr(VI) removal. When the solution pH > pH_pzc, repulsion between negatively charged surface (FeO–O⁻) and Cr(VI) species readily decreased the removal for Cr(VI).

3.3. Cr(VI) removal by FeOOH polymorphs

3.3.1. Comparison of Cr(VI) removal by the four FeOOH polymorphs. The Cr(VI) removal by α-FeOOH, β-FeOOH, γ-FeOOH, and δ-FeOOH is shown in Fig. 6. The Cr(VI) removal efficiencies by these FeOOH polymorphs decreased in the sequence of γ-FeOOH > α-FeOOH ≈ δ-FeOOH > β-FeOOH. The specific surface area of these FeOOH polymorphs followed the order of γ-FeOOH > δ-FeOOH > α-FeOOH > β-FeOOH. As is known, the high special surface area is beneficial to remove contaminants in aqueous solution, which is in good agreement with the above results.

Fig. 6 Comparison of Cr(VI) removal by the four FeOOH polymorphs ([Cr(VI)]₀ = 20.0 mg L⁻¹, FeOOH concentration 2.5 g L⁻¹, [pH]₀ = 3.0).

At 60 min, the Cr(VI) removal efficiencies by α-FeOOH, β-FeOOH, γ-FeOOH, and δ-FeOOH were about 85.5%, 33.5%, 99.3%, and 82.2%, respectively. After 60 min, the Cr(VI) removal efficiencies remained almost constant. The short equilibrium time of 60 min indicated that the removal of Cr(VI) was very fast, which can be attributed to most of the active sites in the exterior of the FeOOH polymorphs being easily accessible by the Cr(VI) species.⁴²

3.3.2. Effect of pH on Cr(VI) removal by FeOOH polymorphs. The effect of solution pH on Cr(VI) removal is shown in Fig. 7. From Fig. 7(a), the Cr(VI) removal efficiencies by α-FeOOH were all over 85.0% in the pH range from 3.0 to 9.0. The highest Cr(VI) removal efficiency by β-FeOOH was only 52.0% (Fig. 7(b)), which can be attributed to its lowest specific surface area and its non-porous surface morphology (Table S1†). According to the insets in Fig. 7(a) and (b), it was clearly that the solution pH decreased immediately and remained acidic after the addition of α-FeOOH and β-FeOOH, perhaps due to their acidity.

Fig. 7 Effect of initial pH on Cr(VI) removal by (a) α-FeOOH, (b) β-FeOOH, (c) γ-FeOOH, and (d) δ-FeOOH ([Cr(VI)]₀ = 20.0 mg L⁻¹, FeOOH concentration 2.5 g L⁻¹). Insets: the pH changes during the Cr(VI) removal.

Fig. 7(c) and (d) indicated that the Cr(VI) removal efficiencies of γ-FeOOH and δ-FeOOH decreased with increasing pH. This can be attributed to the decrease in positive surface charges and the increase in negative surface charges of γ-FeOOH and δ-FeOOH. The negatively charged surface sites of γ-FeOOH and δ-FeOOH were not favorable for the Cr(VI) removal due to the electrostatic repulsion between the negative surface charges and Cr(VI) species (HCrO₄⁻/CrO₄²⁻). Thus, the electrostatic interactions played an important role in the Cr(VI) removal process and lower pH was favorable for Cr(VI) removal by γ-FeOOH and δ-FeOOH. By comparison, at the given pH, γ-FeOOH always showed better Cr(VI) removal efficiency than δ-FeOOH, which may be attributed to the low final pH values of γ-FeOOH.

The summary of the data of the Cr(VI) removal are also presented in Table S2.† It can be seen that the final pH values have a tendency to get close to the pH_pzc values for α-FeOOH, γ-FeOOH, and δ-FeOOH. The Cr(VI) removal can be explained, at least in part, from the pH-dependent electrostatic force existing between the FeOOH surface and Cr(VI) species.

Overall, after reaction for 120 min, in the optimum pH conditions, the highest Cr(VI) removal efficiencies by α-FeOOH, γ-FeOOH, and δ-FeOOH were 94.5%, 100.0%, and 84.6%, respectively, indicating that the three polymorphs were better materials for Cr(VI) removal. The β-FeOOH with low specific surface area was distinctly the most unsuitable material for Cr(VI) removal. The high specific surface area was the major determinant for their superior Cr(VI) removal capacity.

3.3.3. Adsorption isotherms. Three adsorption isotherm models, three adsorption isotherm models, the Langmuir (eqn (7)), Freundlich (eqn (8)), and Redlich–Peterson (eqn (9)) isotherm equations were employed in the study.Where q_m (mg g⁻¹) and K_L are Langmuir constants related to the maximum adsorption capacity of adsorbents and the energy of adsorption, respectively; K_F and n are Freundlich constants and represent adsorption capacity and adsorption intensity, respectively; a (L mg⁻¹) and b (L mg⁻¹) are Redlich–Peterson constants and affinity coefficients, and m is the exponent which lies between 0 and 1. Fitted adsorption isotherms of Cr(VI) on FeOOH polymorphs are shown in Fig. 8.

Fig. 8 Fitted adsorption isotherms of Cr(VI) on FeOOH polymorphs, (a) α-FeOOH, (b) β-FeOOH, (c) γ-FeOOH, and (d) δ-FeOOH.

The isotherm parameters are listed in Table S3.† As shown in Table S3,† the maximum adsorption capacity (q_m) of Cr(VI) followed the order of δ-FeOOH ≈ γ-FeOOH > α-FeOOH > β-FeOOH. The Freundlich and Redlich-Peterson models are better than the Langmuir model in evaluating the adsorption process for Cr(VI) on FeOOH polymorphs.

3.3.4. Reuse of FeOOH polymorphs. To explore the practical applicability of the FeOOH polymorphs, the Cr(VI) removal by FeOOH polymorphs in the successive sorption cycles was studied (Fig. 9). As shown in the figures, the increase of the residual Cr(VI) concentration were observed in the 3 cycles for α-FeOOH, β-FeOOH, and δ-FeOOH. This phenomenon may be the result of the loss of some active sites on the surface of α-FeOOH, β-FeOOH, and δ-FeOOH. However, slightly increase of Cr(VI) residual concentration were observed in the 3 cycles for γ-FeOOH, and the residual concentration of Cr(VI) was 0.56 mg L⁻¹ in the third cycle and the Cr(VI) removal efficiency still remained more than 97%. Since the γ-FeOOH has a highest surface area (294.5 m² g⁻¹), it is possible that there are still enough reaction sites available for further Cr(VI) removal.

Fig. 9 Recycled time at solution pH 3.0: (a) α-FeOOH, (b) β-FeOOH, (c) γ-FeOOH, and (d) δ-FeOOH.

3.4. FTIR analysis

The FIIR spectra of FeOOH polymorphs before and after removal of Cr(VI) were shown in Fig. 10. The two peaks at 1630–1633 and 3442–3452 cm⁻¹ were assigned to the vibration bending and stretching modes of O–H group, respectively, indicating the presence of adsorbed water on the surface of FeOOH polymorphs.^8,18,43–45 For α-FeOOH (Fig. 10(a)), the peaks at 794 and 889 cm⁻¹ were caused by the in-plane bending of surface hydroxyl of Fe–OH–Fe.⁴⁴ The band at around 1132 cm⁻¹ was related to the Fe–O vibration mode in FeOOH.⁴⁶ In addition, the band at 3150 cm⁻¹ was related to the stretching mode of Fe–O–OH in α-FeOOH.⁸ For β-FeOOH (Fig. 10(b)), the band at 854 cm⁻¹ was assigned to the in-plane bending vibrations of hydroxyl groups of O–H⋯OH₂ and O–H⋯Cl hydrogen bonds.¹⁹ Peaks at 484 and 679 cm⁻¹ could be attributed to the vibration mode of the FeO₆ coordination octahedron.⁴⁵ For γ-FeOOH (Fig. 10(c)), the peaks appeared at 1019 and 1076 cm⁻¹ were assigned to the in-plane bending vibration of Fe–OH.⁸ In Fig. 10(d), the peaks at 479, 619, and 3444 cm⁻¹ were indexed to δ-FeOOH, and the peak at 3444 cm⁻¹ could be also assigned to the stretching vibration of –O–H in δ-FeOOH.⁴³ After reaction with Cr(VI) for the four FeOOH polymorphs, the new peak appeared at 926 cm⁻¹ belonged to Cr species⁴³ (Fig. 10), indicating that the Cr was transferred onto the FeOOH polymorphs.

Fig. 10 FIIR spectra of FeOOH polymorphs before and after reaction with Cr(VI): (a) α-FeOOH, (b) β-FeOOH, (c) γ-FeOOH, and (d) δ-FeOOH.

3.5. XPS analysis of the FeOOH polymorphs

To further confirm the elemental composition and chemical oxidation states of the samples, XPS analysis for α-FeOOH and γ-FeOOH for which processed better Cr(VI) removal efficiencies were carried out. All spectra were calibrated using C 1s (284.6 eV) as the reference.

Fig. 11(a) shows the XPS wide scan spectra of α-FeOOH before and after Cr(VI) removal, and significant changes can be seen after Cr(VI) removal. The peaks at the binding energies of 568–594 eV for Cr 2p appeared, indicating that chromium was transferred onto α-FeOOH from the aqueous solution. Fig. 11(b) presents the Fe 2p spectrum of α-FeOOH. The peaks of Fe 2p1/2 and Fe 2p3/2 can be observed at 724.7 and 710.9 eV, respectively.^40,47 In addition, the satellite peak at 732.7 eV is the satellite peak of Fe 2p1/2. Another satellite peak at 718.9 eV is the satellite peak of Fe 2p3/2. The results indicated that the Fe³⁺ is present in α-FeOOH.⁴⁰ As shown in Fig. 11(c), the O 1s peaks are fitted with Fe–O at 529.6 eV and Fe–OH at 531.1 eV for α-FeOOH.^10,47 After Cr(VI) removal, the Cr 2p spectra in α-FeOOH were investigated in Fig. 11(d). Results show that the Cr 2p1/2 peak centered at 587.2 eV and Cr 2p3/2 peak centered at 577.9 eV,⁴³ and the data matched well with Cr(VI), which testified that Cr(VI) ions were adsorbed on α-FeOOH and no Cr(III) occurred during the Cr(VI) removal process.

Fig. 11 (a) XPS spectra of α-FeOOH before and after reaction with Cr(VI); XPS patterns of (b) Fe 2p, (c) O 1s, and (d) Cr 2p of α-FeOOH after Cr(VI) removal.

Fig. 12(a) illustrates the wide scan spectrum of γ-FeOOH before and after Cr(VI) removal. The new chromium peaks are also observed after Cr(VI) removal. Fig. 12(b) presents the Fe 2p spectrum of γ-FeOOH. The binding energies of the Fe 2p1/2 and Fe 2p3/2 peaks are located at 724.5 and 710.8 eV, respectively. The Fe 2p3/2 peak has an associated satellite peak, which is located approximately 8 eV higher than the main Fe 2p3/2 peak. Another satellite peak at 732.8 eV is the satellite peak of Fe 2p1/2. These satellite peaks can be attributed to the presence of Fe³⁺ ions in γ-FeOOH.² As shown in Fig. 12(c), the O 1s peaks in the spectra of γ-FeOOH at 529.9 and 531.6 eV could be assigned to Fe–O and Fe–OH bonds in γ-FeOOH structure, respectively. Cr 2p spectra from γ-FeOOH after Cr(VI) removal were shown in Fig. 12(d), and the Cr 2p1/2 peak centered at 587.2 eV and Cr 2p3/2 peak centered at 578.0 eV were attributed to the Cr(VI).^43,48 From the above analysis, the existence of the Fe–OH reactive group was confirmed, and the valence states of Cr(VI) did not change during the Cr(VI) removal process by α-FeOOH and γ-FeOOH.

Fig. 12 (a) XPS spectra of γ-FeOOH before and after reaction with Cr(VI); XPS patterns of (b) Fe 2p, (c) O 1s, and (d) Cr 2p of γ-FeOOH after Cr(VI) removal.

3.6. Mechanism of Cr(VI) removal

In this study, the four FeOOH polymorphs were applied to remove Cr(VI) from the aqueous solution. From the above results, the Cr(VI) removal efficiencies by these FeOOH polymorphs decreased in the sequence of γ-FeOOH > α-FeOOH ≈ δ-FeOOH > β-FeOOH, which was consistent with the order of the specific surface area of these FeOOH polymorphs. Thus, the adsorption might be a major role during the removal of Cr(VI) by the four FeOOH polymorphs. According to the XPS analysis, the valence state of Cr(VI) did not change during the Cr(VI) removal process, and no Cr(III) occurred, which indicated that it was an adsorption process for Cr(VI) removal by these FeOOH polymorphs.

During the adsorption process, the Cr(VI) oxyanions came close to the surface of FeOOH due to the electrostatic attraction between the surface functional groups (–OH₂⁺) and Cr(VI) species. Then, the hydroxyl ions were replaced by HCrO₄⁻ through ligand exchange, and the Cr(VI) removal would be achieved. The Cr(VI) (take HCrO₄⁻ for example) removal by the four FeOOH polymorphs was speculated as follows:

FeO–OH₂⁺ + HCrO₄⁻ → FeO–HCrO₄ + H₂O (10)

That is, the Cr(VI) removal was simultaneously controlled by electrostatic attraction and ligand exchange. The schematic of Cr(VI) removal by the four FeOOH polymorphs is shown in Fig. 13. According to the reference,¹⁸ the surface hydroxyl groups of the FeOOH polymorphs were firstly protonated and formed –FeOH₂⁺ in aqueous solution (eqn (5)). The Cr(VI) oxyanions came close to the protonated surface of FeOOH rapidly due to the electrostatic attraction, which was physical adsorption. Then the hydroxyl ions were replaced by Cr(VI) oxyanions through ligand exchange, which was chemical adsorption, as shown in eqn (10). In this process, electrostatic attraction facilitated the ligand exchange between hydroxyl ions and Cr(VI) oxyanions.

Fig. 13 Schematic of Cr(VI) removal mechanism by the four FeOOH polymorphs: (1) FeOOH was positively charged through protonation in aqueous solution, (2) Cr(VI) oxyanions can be adsorbed to the protonated surface of FeOOH through electrostatic attraction and ligand exchange according to the equation: FeO–OH₂⁺ + HCrO₄⁻ → FeO–HCrO₄ + H₂O.

4. Conclusions

In the present study, α-FeOOH, β-FeOOH, γ-FeOOH, and δ-FeOOH were prepared and characterized by SEM, N₂ adsorption–desorption isotherms, XRD, and saturation magnetization measurement. The Cr(VI) removal by these FeOOH polymorphs was investigated. After reaction for 120 min, the Cr(VI) removal efficiencies by α-FeOOH were all over 85.0% at pH from 3.0 to 9.0, and α-FeOOH can achieve the efficiency of 94.5% at pH 9.0. β-FeOOH was the most unsuitable material for Cr(VI) removal. The low Cr(VI) removal efficiency by β-FeOOH can be attributed to its low specific surface area and its non-porous surface morphology. The Cr(VI) removal efficiency of 100% was obtained by γ-FeOOH at the initial pH 3.0. δ-FeOOH can achieve a Cr(VI) removal efficiency of 84.6% at initial pH 3.0, and δ-FeOOH was easily recovered due to its feature of ferromagnetic. The XPS analysis revealed that the Cr(VI) removal by the four FeOOH polymorphs was a process of adsorption, which was simultaneously controlled by electrostatic attraction (physical adsorption) and ligand exchange (chemical adsorption) between hydroxyl ions and Cr(VI) oxyanions.

Acknowledgements

This research was supported by the National Natural Science Foundation of China (No. 51008084), Development Program for Outstanding Young Teachers in Guangdong Province (No. Yq2013055), and Science and Technology Planning Project of Guangdong Province (No. 2016A020221032).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra14522a

‡ Shijiao Wu and Jianwei Lu contributed equally to this work and should be considered co-first authors.

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