Calcined ZnAl- and Fe₃O₄/ZnAl–layered double hydroxides for efficient removal of Cr(VI) from aqueous solution

Abstract

Calcined ZnAl- and Fe₃O₄/ZnAl–layered double hydroxides (ZnAl–CLDH and Fe₃O₄/ZnAl–CLDH) were used as adsorbents to evaluate the uptake properties for Cr(VI) removal from aqueous solution by batch equilibrium experiments. The XRD, FTIR and TEM results showed that the calcined ZnAl–LDH and Fe₃O₄/ZnAl–LDH recovered layered structures according to the memory effect of hydrotalcite. Optimized conditions of Cr(VI) adsorption were obtained: 2.0 g L⁻¹ adsorbent, initial solution pH of 3.0 and contact time of 60 min. The kinetic data was described better by a pseudo-second-order kinetic equation. The adsorption isotherms had a good fit with Langmuir and Freundlich models. Thermodynamic analyses indicated that the adsorption process was endothermic and spontaneous in nature. The adsorption mechanisms involved the reconstruction of LDHs, surface complexation, anion exchange and physical adsorption. The higher adsorption capacities of the calcined products suggested that the ZnAl–CLDH and Fe₃O₄/ZnAl–CLDH were potential adsorbents for Cr(VI) removal from water and wastewater. Moreover, the magnetic Fe₃O₄/ZnAl–CLDH can be easily separated using a magnet after the adsorption process.

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Introduction

Chromium has been widely used in various industrial processes, such as electroplating, tanning, textiles, pigments, chemical manufacturing, catalysis, mining and metallurgy.^1–4 Chromium exists in two stable oxidation states, Cr(III) and Cr(VI). The hexavalent form Cr(VI) is generally considered more toxic than the trivalent form Cr(III) because it is very soluble and can be easily absorbed and accumulated in the body, especially in the kidney, stomach and liver.^1,5 Because of its high toxicity, it is necessary to remove Cr(VI) from industrial wastewater before discharging it into inland surface water in order to prevent its deleterious impact on ecosystem and public health.

There are various methods to remove Cr(VI) from wastewater, including chemical precipitation,⁶ reverse osmosis,^7,8 biological method,^9,10 electrolysis,¹¹ anion exchange^12,13 and adsorption.^14,15 Compared to these methods, the adsorption treatment proves to be more effective due to its low cost, higher uptake capacity, greater selectivity, faster regeneration, less production of sludge and easy operation. A variety of materials such as fertilizer industry waste,¹⁵ modified chitosan,¹⁶ iron oxides,¹⁷ calcined magnesium aluminum hydrotalcite,¹⁸ activated alumina and activated charcoal,¹⁹ have been used as adsorbents for removal of Cr(VI) from aqueous solution. Among these adsorbents, layered double hydroxides (LDHs) and their calcined products have attracted increasing attention for their low cost, large adsorption capacity and unique structure.

LDHs, also known as hydrotalcites, have received wide attention as an effective adsorbent in recent years. The general formula of LDHs is [M_1−x²⁺M_x³⁺(OH)₂][Aⁿ⁻]_x/n·yH₂O, where M²⁺ and M³⁺ are divalent and trivalent metal cations, Aⁿ⁻ is the incorporated anions in the interlayer space along with water molecules for charge neutrality and structure stability, and x normally ranges from 0.17 to 0.33.²⁰ Due to their high charge density of the sheets and the exchangeability of the interlayer anions, LDHs have been employed for removing different anions by both surface adsorption and anion exchange.^21–24 Some studies of hexavalent chromium adsorption by MgAl–LDH,^23,25 Mg_3−xCa_xFe–LDH,²⁴ MgAlFe–LDH,²⁶ ZnAl₄–LDH,²⁷ CoBi–LDH²⁸ and MgAl-, NiAl-, ZnAl–LDHs²⁹ were investigated in detail.

The calcination of LDHs (CLDH) can markedly improve the Cr(VI) uptake from aqueous solution.^18,30–33 The improvement was extensively explained by the rehydration of the calcined LDHs in water, known as “memory effect”. Reconstruction occurs only in a given temperature range where the thermal decomposition is performed. Calcination temperature strongly influenced the microstructure characteristics and adsorption properties. The maximum surface area, pore volume and pH_zpc of calcined MgAl–LDH were observed at 500 °C, corresponding to the highest adsorption capacity of Cr(VI).³²

To solve the separation of the adsorbent from suspension after adsorption, magnetic separation technology has attracted much attention for its easy separation procedure and high separation efficiency by applying an external magnetic field.^34–36 For LDHs, combination of Fe₃O₄ nanoparticles can enhance the separation and re-dispersion of solid from aqueous solution.^37–41 A magnetic alginate–LDH composite for Cr(VI) removal were prepared through immobilization of magnetic iron oxide and calcined MgAl–LDH powders into an alginate matrix.⁴² The magnetic CoFe₂O₄/MgAl–LDH⁴³ and spinel type CoFe oxide porous nanosheet transformed from CoFe–LDH⁴⁴ can also adsorb Cr(VI) with facile separation. Xiang et al. reported a new methodology to synthesize MgFe₂O₄ film with enhanced magnetization and superparamagnetic characteristics based on MgFe–LDH.⁴⁵

While extensive studies on the adsorption behavior of Cr(VI) onto LDHs and their calcined products, little information is available on the use of uncalcined and calcined magnetic LDHs. In this study, the feasibility of applying ZnAl-, Fe₃O₄/ZnAl–LDHs and their calcined products at 500 °C for effective removal of Cr(VI) was examined. The adsorption kinetic, isotherm and thermodynamics of calcined ZnAl- and Fe₃O₄/ZnAl–LDHs were evaluated by batch equilibrium methods.

Materials and methods

Preparation of adsorbents

The chemicals used in this work were all analytical grade reagents and ZnAl–LDH was synthesized by a co-precipitation method. Briefly, a mixed-metal nitrate solution (0.25 mol L⁻¹ Zn²⁺ and 0.25 mol L⁻¹ Al³⁺) was adjusted to pH of 9–10 with another solution containing 2.0 mol L⁻¹ NaOH and 0.5 mol L⁻¹ Na₂CO₃ under stirring. The precipitate was maintained at 80 °C in a water thermostat for 18 h. Then it was filtered and rinsed with de-ionized water. After drying at 85 °C, the solid was gently ground to obtain ZnAl–LDH. The product was calcined at 500 °C for 3 h and signed as ZnAl–CLDH.

The Fe₃O₄ microspheres were prepared via a modified solvothermal route.²⁴ The FeCl₃·6H₂O (8.0 g) was dissolved in ethylene glycol (160 mL) under stirring and CH₃COONa·3H₂O (24.0 g), tetra-2-propanol (80 mL) were added. The mixture was stirred for 30 min and sealed in a stainless steel with polytetrafluoroethylene liner, then heated for 8 h at 200 °C. The resultant solid was sequentially washed several times with de-ionized water and dried at 60 °C giving the powder of Fe₃O₄.

The as-prepared Fe₃O₄ (0.23 g) was ultrasonically dispersed into 150 mL de-ionized water for 15 min and then transferred into a 500 mL flask at 60 °C in a water thermostat with vigorous stirring. An alkaline solution containing 0.72 g NaOH and 0.56 g Na₂CO₃ was added dropwise into the suspension to adjust suspension pH of 10. Another mixed metal nitrate solution of 1.78 g Zn(NO₃)₂·6H₂O and 0.75 g Al(NO₃)₃·9H₂O was simultaneously added maintaining the pH of 9–10 for 8 h. Then the solid was separated using a permanent magnet and thoroughly washed with de-ionized water until the effluent solution pH was neutral. The wet solid was dried at 80 °C giving product of Fe₃O₄/ZnAl–LDH and then calcined for 3 h at 500 °C to obtain the calcined magnetic LDH signed as Fe₃O₄/ZnAl–CLDH.

Characterization methods

The prepared LDHs and CLDHs were characterized by a D8 ADVANCE X-ray diffractometer (Bruker, Germany) using CuKα radiation and a Vertex 70 Fourier transform infrared (FTIR) spectrophotometer (Bruker, Germany) using transmission model. The zeta potentials of adsorbent suspension before and after Cr(VI) adsorption were measured by a Nano ZS90 Zetasizer analyzer (Malvern Instruments Ltd, UK). The morphology was examined by TEM (Tecnai G20, FEI Corporation, USA) and SEM (NoVa NanoSEM 250, FEI Corporation, USA) with energy dispersive spectroscopy (EDS). X-ray photoelectron spectroscopy (XPS) was performed on multifunctional imaging electron spectrometer (Thermo, ESCALAB 250XI, USA).

Adsorption experiments

A stock solution containing 1000 mg L⁻¹ Cr(VI) was prepared by dissolve K₂Cr₂O₇ in de-ionized water. The batch experiments were carried out in 40 mL centrifuge tubes under constant shaking (200 rpm) in a thermostat shaker. Effect of adsorbent dosage was conducted with fixed Cr(VI) concentration (20 mg L⁻¹) in 25 mL aqueous solution and the adsorbents amount varied from 0.01 to 0.12 g for shaking time of 60 min. The solution pH was adjusted with 0.1 mol L⁻¹ HNO₃ and NaOH solution within the range of 3–10. The effect of contact time was carried out at specific time intervals of 5–120 min with Cr(VI) concentration of 20 mg L⁻¹ and adsorbents dosage of 0.05 g. Adsorption isotherms were conducted at 30, 40 and 50 °C with different initial Cr(VI) concentrations, initial solution pH of 3.0, adsorbents amount of 0.05 g, and contact time of 60 min. After adsorption, the suspensions of Fe₃O₄/ZnAl–LDH or Fe₃O₄/ZnAl–CLDH and Cr(VI) were separated using a magnet for 5 min. The mixture of ZnAl–LDH or ZnAl–CLDH and Cr(VI) aqueous solution were separated by centrifugation at 2500 rpm for 25 min. The colorimetric method with 1,5-diphenylcarbazide as the complexing agent was used for the determination of initial and finial Cr(VI) concentrations.¹⁹ The adsorption capacity q_e (1) or removal ratio R (%) (2) were given as the following:where C₀ and C_e are the initial and equilibrium concentration of Cr(VI) in solution (mg L⁻¹), q_e is the equilibrium adsorption capacity (mg g⁻¹), m is the adsorbent dry weight (g), and V is the suspension volume (L).

Results and discussion

Characterization of adsorbents

The XRD patterns of LDHs and CLDHs were shown in Fig. 1. It could be seen from Fig. 1(A) that a series of peaks appeared as sharp and intense symmetric lines at low 2θ values and clear reflections at high 2θ values, indicating the characteristic basal reflections of hydrotalcite-like ZnAl–LDH materials.⁴⁶ Strong peaks corresponding to (003) and (006) faces were observed and the angle of the peak of (003) indicated large distance between inter layers.²⁰ In Fig. 1(B), the diffraction pattern of Fe₃O₄/ZnAl–LDH exhibited the superimposition of reflections of the Fe₃O₄ phase and ZnAl–LDH phase.⁴⁷ There was a series of characteristic peaks at (220), (311), (400), (511), and (440), which was similar to the pattern of Fe₃O₄. The (003) and (006) reflected of a typical ZnAl–LDH material were also clearly observed.

Fig. 1 The XRD patterns of LDHs and CLDHs.

After calcination, the characteristic peaks at (003) and (006) of ZnAl–LDH disappeared regardless of ZnAl–CLDH and Fe₃O₄/ZnAl–CLDH. This indicated that the hydrotalcite structure was seriously destroyed and there was disordering in the stacking of the layers. However, when ZnAl–CLDH and Fe₃O₄/ZnAl–CLDH adsorbed Cr(VI) from aqueous solution, the characteristic peaks (003) and (006) of ZnAl–LDH reappeared. The reconstruction of layered structure was likely due to the memory effect of LDHs. This can also be identified by the TEM images (Fig. 2). The ZnAl–CLDH and Fe₃O₄/ZnAl–CLDH were nano sized and aggregated together. After adsorbed Cr(VI) from aqueous solution, the hydrotalcite nanoplate structure produced due to the memory effect of ZnAl–LDH. In addition, the characteristic peaks of Fe₃O₄ existed in the process of calcination (Fig. 1(B)) suggesting that the Fe₃O₄/ZnAl–CLDH was still magnetic.

Fig. 2 The TEM images of ZnAl–CLDH (A and B) and Fe₃O₄/ZnAl–CLDH (C and D) before (A and C) and after (B and D) Cr(VI) adsorption.

The FTIR spectroscopy (Fig. 3) was also used to characterize the samples. The strong and broad bands centered around 3448 cm⁻¹ were associated with the O–H stretching vibrations of the hydroxyl groups in the layers and interlayer water molecules. Another absorption band corresponding to a water deformation was recorded around 1632 cm⁻¹. The peak around 1380 cm⁻¹ could be attributed to the interaction between the CO₃²⁻ and the OH group, indicating that some CO₃²⁻ ions existed in the gallery of LDHs.⁴⁸ However, the peaks intensity of CO₃²⁻ ions were decreased after calcination, indicating there was disordering in the stacking of the layers. The peaks from 400 to 900 cm⁻¹ were usually assigned to the stretching vibration and bending vibration of M–O and M–OH.^47,49 The intense absorption at 585 cm⁻¹ was observed from the Fe–O lattice vibration, indicating good magnetism of the obtained Fe₃O₄/ZnAl–LDH and Fe₃O₄/ZnAl–CLDH.

Fig. 3 The FTIR spectroscopy of LDHs and CLDHs.

Effect of adsorbent dosage

The effect of adsorbent dosage on the adsorption of Cr(VI) was studied and shown in Fig. 4.

Fig. 4 Adsorption of Cr(VI) onto ZnAl–LDH, ZnAl–CLDH (A), Fe₃O₄/ZnAl–LDH and Fe₃O₄/ZnAl–CLDH (B) as a function of adsorbent dosage.

The adsorption capacity of the calcined adsorbents increased rapidly with the augment in adsorbent dosage from 0.01 to 0.05 g and reached a plateau at the adsorbent dosage of 0.05 g. With further increase of dosage, the removal ratio of Cr(VI) increased unobviously. Moreover, the removal ratio of Cr(VI) by ZnAl–CLDH and Fe₃O₄/ZnAl–CLDH was higher than by ZnAl–LDH and Fe₃O₄/ZnAl–LDH. This may due to the removal of CO₃²⁻ during calcination and the followed intercalation of Cr(VI) during reconstruction of LDHs.

From the economical point of view, 0.05 g ZnAl–CLDH and Fe₃O₄/ZnAl–CLDH were selected as optimum parameter. For the hardly adsorbed of Cr(VI) by ZnAl–LDH and Fe₃O₄/ZnAl–LDH, the adsorption properties of ZnAl–LDH and Fe₃O₄/ZnAl–LDH were not discussed further.

Effect of initial solution pH

The adsorption experiments were conducted in the initial solution pH range of 3.0–10.0, and the results were shown in Fig. 5. It was evident that the removal ratio of Cr(VI) tended to decrease with the increase of solution pH. This could be attributed to the Cr(VI) species in solution and the acid–base properties of adsorbent surface at different pH values. At the experimental conditions (20 mg L⁻¹ Cr(VI) and solution pH of 3–10), the HCrO₄⁻ and CrO₄²⁻ are the most predominant species. At pH 3–6.8, HCrO₄⁻ is the dominant species of hexavalent chromium, and at 6.8–10, only CrO₄²⁻ is stable.⁵⁰ The zeta potential determination was also conducted and the results were shown in Fig. 6. The isoelectric points (pH_zpc) of ZnAl–CLDH and Fe₃O₄/ZnAl–CLDH determined by the pH location where zeta potential equals zero were 7.90 and 6.70. After Cr(VI) adsorption, the zeta potentials increased significantly at different pH values and the pH_zpc shifted to even higher pH values of 8.98 and 8.11, respectively.

Fig. 5 Adsorption of Cr(VI) onto ZnAl–CLDH and Fe₃O₄/ZnAl–CLDH as a function of initial Cr(VI) solution pH.

Fig. 6 Zeta potentials of ZnAl–CLDH (A) and Fe₃O₄/ZnAl–CLDH (B) before and after Cr(VI) adsorption.

Considering the properties of the adsorbent, the LDH colloidal particles possess not only permanent positive charges because of isomorphous replacement, but also variable charges due to the adsorption of H⁺ or OH⁻ in solution. In acid solution, the hydrated surfaces of ZnAl–LDH and Fe₃O₄/ZnAl–LDH are protonated and therefore have positive charges, and the degree of protonation reduces with the increase of pH. Therefore, at values below the pH_zpc, there may be two possible mechanisms for Cr(VI) adsorption onto ZnAl–LDHs. The main mechanism in LDHs was anion exchange, where the OH⁻ ions that are associated with the surface or interlayer of ZnAl–LDHs exchanged with the HCrO₄⁻ molecules in solution, resulting the increased pH_zpc (Fig. 6). Another mechanism may be surface complexation, which was suggested for anion adsorption onto the hydrous solids. Thus, acidic conditions (pH 3–6.8) were favored for Cr(VI) adsorption.^50,51 As the pH increased, CrO₄²⁻ is now the dominant species, and the positive charge on the surface of ZnAl–LDHs decreased due to the deprotonation. The CrO₄²⁻ must compete with OH⁻ in the solution for anion exchange sites that are associated with the surface or interlayer spaces. Consequently, the amount adsorbed decreased. Considering the adsorption capacity, initial solution pH of 3.0 was used in following experiments.

Adsorption kinetic

Cr(VI) adsorption ratio onto ZnAl–CLDH and Fe₃O₄/ZnAl–CLDH as a function of contact time was shown in Fig. 7. The Cr(VI) adsorption showed a fast initial adsorption rate followed by a relatively slow adsorption. The Cr(VI) adsorption capacity increased rapidly in the first 60 min. After that, it increased slowly. The adsorption equilibrium was achieved within 60 min, and the Cr(VI) adsorption remained almost steady until the end of the experiment.

Fig. 7 Adsorption of Cr(VI) onto ZnAl–CLDH and Fe₃O₄/ZnAl–CLDH as a function of contact time.

In order to clarify the adsorption kinetic, the pseudo-first-order and pseudo-second-order kinetic models were often used. However, it was clearly found that the pseudo-second-order equation, which agrees with chemisorption as the rate-control mechanism, was able to better describe the adsorption of Cr(VI) onto different LDHs.^{18,23,26,28,29,32} In addition, some of the kinetic data were also fit the pseudo-first-order equation simultaneously with R² > 0.95.^18,28,29,32

In this study, the linear form of the pseudo-first-order (eqn (3)) and pseudo-second-order equation (eqn (4)) as following were used to analyze the kinetic data of Cr(VI) adsorption by ZnAl–CLDH and Fe₃O₄/ZnAl–CLDH.

where q_e and q_t (mg g⁻¹) are the amount of Cr(VI) adsorbed on the CLDHs at equilibrium and at time t (min). k₁ (min⁻¹) and k₂ (g (mg⁻¹ min⁻¹)) are the rate constant of pseudo-first-order and pseudo-second-order kinetic models.

The obtained kinetic model parameters by linear regressions were given in Table 1. It could be seen that the correlation coefficients R² of pseudo-second-order equation were higher than that of pseudo-first-order equation. So the adsorption of Cr(VI) onto ZnAl–CLDH and Fe₃O₄/ZnAl–CLDH was governed by the pseudo-second-order kinetic model. This indicated that chemisorption or chemical bonding between adsorbent active sites and Cr(VI) might dominate the adsorption process.

Table 1 Adsorption rate constants and correlation coefficients of pseudo-first-order and pseudo-second-order kinetic models

Adsorbents	Pseudo-first-order			Pseudo-second-order
	qe	k1	R^2	qe	k2	R^2
ZnAl–CLDH	3.57	0.017	0.75	9.87	0.014	1.00
Fe3O4/ZnAl–CLDH	3.84	0.026	0.94	7.94	0.013	0.99

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Adsorption isotherm

Adsorption isotherms of Cr(VI) onto ZnAl–CLDH and Fe₃O₄/ZnAl–CLDH at various temperatures were shown in Fig. 8. The adsorption isotherm data were further fit by two equilibrium models, Langmuir (eqn (5)) and Freundlich equation (eqn (6)):where C_e (mg L⁻¹) and q_e (mg g⁻¹) are the equilibrium adsorbate concentration in the aqueous and solid phases. q_m (mg g⁻¹) is the maximum adsorption capacity and b (L mg⁻¹) is the Langmuir adsorption equilibrium constant. n is a constant indicating the Freundlich isotherm curvature and k_f ((mg g⁻¹) (mg L⁻¹)⁻ⁿ) is the Freundlich equilibrium constant.

Fig. 8 Adsorption isotherms of Cr(VI) onto ZnAl–CLDH (A) and Fe₃O₄/ZnAl–CLDH (B) at 303 K, 313 K and 323 K by nonlinear regression of Freundlich (solid lines) and Langmuir equation (dot lines).

The fitted constants for the two isotherm models along with regression coefficients were summarized in Table 2. The R² values obtained for the Langmuir isotherms were all above 0.98, indicating a very good mathematical fit. Based on the values of calculated q_m, Cr(VI) adsorption capacity for ZnAl–CLDH was slightly higher than Fe₃O₄/ZnAl–CLDH. The data were also fit Freundlich model well (R² > 0.93). The calculated value of n (>1) indicated a favorable adsorption process. The similar results have been reported for the adsorption of Cr(VI) by MgAlFe–LDH²⁶ and CoBi–LDH.²⁸

Table 2 Freundlich and Langmuir constants and correlation coefficients for Cr(VI) adsorption onto ZnAl–CLDH and Fe₃O₄/ZnAl–CLDH

Adsorbents	T	Langmuir equation			Freundlich equation
		qm	b	R^2	kf	1/n	R^2
ZnAl–CLDH	303	21.3	0.18	0.99	5.67	0.34	0.93
	313	22.2	0.23	0.99	6.62	0.32	0.95
	323	23.6	0.28	0.99	7.75	0.31	0.94
Fe3O4/ZnAl–CLDH	303	19.6	0.12	0.99	4.45	0.36	0.95
	313	20.8	0.13	0.98	4.86	0.36	0.97
	323	21.5	0.16	0.98	5.44	0.35	0.96

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Adsorption thermodynamics

The influence of temperature on Cr(VI) adsorption was carried out at temperatures ranging from 303 to 323 K. The thermodynamic equilibrium constants (K_d) of the adsorption process were computed using the method of Lyubchik et al.⁵² by plotting ln(q_e/C_e) versus q_e and extrapolating q_e to zero. The change in Gibbs free energies (ΔG) was then calculated with eqn (7). ΔH and ΔS were calculated from the slope and intercept of the plot of ln K_d versus 1/T using eqn (8) which was shown in Fig. 9.

ΔG = −RT ln K_d (7)

Fig. 9 Plots of ln K_d versus 1/T for the Cr(VI) adsorption by ZnAl–CLDH and Fe₃O₄/ZnAl–CLDH.

All the thermodynamic parameters were listed in Table 3. The negative ΔG values indicated that the adsorption process of Cr(VI) was spontaneous under the experimental condition. The value of enthalpy change ΔH was positive for Cr(VI) adsorption, suggesting the endothermic nature of the reaction. The positive entropy change ΔS implied that the degree of disorder increased with the increase of species number at the solid/liquid interface when Cr(VI) from the hydrous phase moved to the surface of CLDHs. The endothermic and spontaneous process was consistent with the adsorption of Cr(VI) by calcined MgAl- and MgFeAl–hydrotalcite.⁵¹

Table 3 Thermodynamic data for Cr(VI) adsorption onto ZnAl–CLDH and Fe₃O₄/ZnAl–CLDH

Adsorbents	T	Kd	ΔG	ΔH	ΔS
ZnAl–CLDH	303	1.86	−1.56	15.29	55.78
	313	2.39	−2.23
	323	2.70	−2.66
Fe3O4/ZnAl–CLDH	303	1.50	−1.02	11.12	40.07
	313	1.72	−1.41
	323	1.97	−1.83

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Adsorption mechanisms

Generally, the adsorption of Cr(VI) by uncalcined LDHs was occurred via two different mechanisms: (i) surface complexation between Cr(VI) and external surface of LDH, and (ii) intercalation by anion exchange between Cr(VI) and interlayer anions.^21,26,50 For calcined LDHs, the intercalation accounts for a large proportion during reconstruction process due to the memory effect.^18,23,51 In our previous work, the interlayer anion of ZnAl–LDH and Fe₃O₄/ZnAl–LDH was suggested to be CO₃²⁻.^47,53 However, the existence of CO₃²⁻ made the anion exchange difficult due to its strong electrostatic interaction with the layers. During calcination at 500 °C, the ZnAl–LDH and Fe₃O₄/ZnAl–LDH containing carbonates as the interlayer anion was decomposed into mixed zinc and aluminum oxides. Some of the CO₃²⁻ was removed. When the calcined products were used to remove Cr(VI) from aqueous solution, they can rehydrate and incorporate the anions to rebuild the initial hydrotalcite structure (Fig. 1 and 3). Then the adsorption of Cr(VI) may involve the rehydration of mixed metal oxides and concurrent intercalation of HCrO₄⁻ or CrO₄²⁻ into the interlayer to reconstruct the LDHs.

To further clarify the adsorption mechanisms, X-ray photoelectron spectroscopy (XPS) measurements of the CLDH were conducted. Fig. 10 shows the high-resolution XPS (HR-XPS) spectrum of Cr 2p in ZnAl–CLDH and Fe₃O₄/ZnAl–CLDH after Cr(VI) adsorption. The peaks of Cr 2p_3/2 and Cr 2p_1/2 at E_B (binding energy) of 579.4 and 586.6 eV, respectively, were very closed to the E_B of Cr 2p peaks in K₂Cr₂O₇.²⁶ This suggested the Cr(VI), existing as HCrO₄⁻ or CrO₄²⁻, was physical bound with the ZnAl–CLDH and Fe₃O₄/ZnAl–CLDH. Another peak at 576.8 eV can be attributed to the formation of chemical complex (Zn-, Al- or Fe–O–Cr). The surface complexation also involved in the adsorption of Cr(VI) by CLDH.

Fig. 10 HR-XPS spectrum of Cr 2p of ZnAl–CLDH (A) and Fe₃O₄/ZnAl–CLDH (B) after Cr(VI) adsorption.

For investigation of the element distribution of CLDH after adsorption of Cr(VI), the elemental mapping of Zn, Al, C, O, Fe and Cr were elaborated, as shown in Fig. 11. The distribution of Cr was similar to that of Zn, Al, C, O for ZnAl–CLDH and Zn, Al, C, O, Fe for Fe₃O₄/ZnAl–CLDH, which supported the evidence of Cr entering into the layered structure from another point of view.

Fig. 11 Elemental mapping of ZnAl–CLDH (A) and Fe₃O₄/ZnAl–CLDH (B) after Cr(VI) adsorption.

Therefore, the adsorption mechanisms of Cr(VI) on the CLDH involved the reconstruction of the LDHs, anion exchange, surface complexation and physical adsorption. Similar results have been reported for the adsorption of Cr(VI) by calcined MgAl–CO₃ hydrotalcite,¹⁸ calcined flower-like MgAl–LDH,²³ and calcined nano-Mg/Al hydrotalcite (CH–Mg/Al and CH–Mg/Al/Fe).⁵¹

Conclusions

In this study, the calcined ZnAl–LDH and Fe₃O₄/ZnAl–LDH were prepared, characterized and performed for the removal of Cr(VI) from aqueous solution. Structural characterization of XRD, FTIR and TEM showed that the memory effect of LDHs occurred after Cr(VI) adsorption. The removal ratio of Cr(VI) by ZnAl–CLDH and Fe₃O₄/ZnAl–CLDH tended to decrease with the increase of solution pH. The kinetic experiments indicated the adsorption process could reach equilibrium quickly within 60 min and the kinetic data were accurately described by pseudo-second-order kinetic model. The adsorption isotherm data agreed well with the Langmuir and Freundlich equation. Adsorption thermodynamic illustrated that the adsorption of Cr(VI) was endothermic and spontaneous in nature. The adsorption mechanisms involved the reconstruction of the LDHs by intercalating of HCrO₄⁻ or CrO₄²⁻ into the interlayer, surface complexation, anion exchange and physical adsorption. The removal ratio of Cr(VI) onto the calcined LDHs were higher than the uncalcined materials, suggesting that ZnAl–CLDH and Fe₃O₄/ZnAl–CLDH were more potential for Cr(VI) removal from wastewater. Furthermore, the magnetic Fe₃O₄/ZlAl–CLDH can be easily separated using a magnet after the adsorption process.

Acknowledgements

This work was funded by the Natural Science Foundation of China (21577048, 21377046), the Natural Science Foundation of Shandong Province (ZR2014BL033), the Key R & D Program of Shandong Province (2015GSF117015), the Science & Technology Development Project of Jinan (201303081) and the Special Project for Independent Innovation and Achievements Transformation of Shandong Province (2014ZZCX05101).

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