Efficient and fast removal of Pb(II) by facile prepared magnetic vermiculite from aqueous solution

Abstract

Magnetic modified vermiculite was prepared by a simple one-pot solvothermal method to remove lead from aqueous solution. The physiochemical properties of vermiculite and magnetic vermiculite were analyzed by X-ray diffraction (XRD), surface-enhanced Raman scattering (SERS), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscope (SEM) with energy dispersive spectroscopy (EDS), transmission electron microscope (TEM), specific surface area determination, and magnetic property and zeta potential measurement. The results proved that the vermiculite was successfully functionalized by nanoparticles of Fe₃O₄. The magnetic vermiculite had superparamagnetic property and higher specific surface areas (52.6 cm³ g⁻¹). Batch adsorption experiments of Pb(II) by vermiculite and magnetic vermiculite were carried out. The results indicated that the adsorption isotherm and kinetic data followed the Langmuir model and the pseudo-second-order equation, respectively. The adsorption process of vermiculite and magnetic vermiculite was fast and the maximum adsorption capacities were 37.0 mg g⁻¹ and 70.4 mg g⁻¹, respectively. In addition, the magnetic vermiculite can be fleetly and easily separated using a magnet after adsorption. It has good potential for cost-effective treatment of lead-contaminated wastewater.

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Introduction

As one of the most hazardous heavy metals, Pb(II) is being directly or indirectly discharged into the water system, causing high toxicity and long-term permanence in aquatic bodies at low concentrations.^1–3 Pb(II) could cause severe saturnine poisoning, such as lesions in the central nervous system, brain damage, mental deficiency and malaise, because it participates in the biological cycles and accumulates in the body.^4–6 It harms the health of human beings, via food-chain structure, and then exerts toxic effects.⁷ There is an imperative demand for a highly effective, specific and economical technique for Pb(II) removal from aqueous solution.

Many researchers concentrated on wastewater treatment, including chemical precipitation, ion-exchange, membrane separation, solvent extraction, conventional coagulation, etc.^4,5 However, these methods are limited by their low price-to-performance ratio, adverse efficiency and complex steps.^8,9 In recent years, adsorption method has boomed into the frequently used and promising technique for the disposition of heavy metals. The most commonly used adsorbent for the adsorption process in industrial wastewater treatment systems is activated carbon due to its large specific surface area. On the other hand, the use of this adsorbent is limited because of the high cost of activated carbon.^10,11 Some of low-cost sorbents including peat, bark, lignin, agricultural wastes, fly ash and clay minerals are often used as adsorbents.¹² Compared with different kinds of clay minerals, vermiculite is very abundant and much cheaper compared with other clays. Owing to its remarkable features, vermiculite is commonly used in agricultural, industrial and environmental applications.^13,14 Vermiculite is a class of typical 2 : 1 cationic layered silicate including two silicon oxygen tetrahedrons and an alumina oxygen octahedral with potassium as the interlayer cation. The alumina ion can be replaced by magnesium and iron ion in the place of octahedral, and that silicon ion is replaced by alumina ion in the place of tetrahedron, which endowed the vermiculite with high cation exchange capacity, widely available, eco-friendly properties.¹⁵ Moreover, isomorphic substitution endows vermiculite with homogeneous negative charges that make it possible for the effective adsorption of heavy metals.¹⁶ EI-Bayaa et al. investigated the effect of ionic strength on the adsorption of copper and chromium ions by vermiculite.¹⁷ Saleh et al. prepared the chitosan-modified vermiculite for arsenite removal from aqueous solution.¹⁸ Alexandre-Franco et al. used vermiculites and micas to adsorb metal ions from aqueous solution.¹⁹

The common adsorbents used in adsorption field are usually powder. In general, centrifugation and filtration methods are used to separate the adsorbents from aqueous solution. However, these separation methods are time-consuming.²⁰ The blockage of filters and the loss of adsorbents may occur in filtration method. The use of magnetic separation method offers an alternative way to solve this problem.²¹ In our previous study, the magnetic diatomite and illite clay had been prepared to remove phosphate from aqueous solution.²² A magnetic Fe₃O₄/Mg–Al–CO₃-layered double hydroxides were synthesized to adsorb Cd(II) from solution.²³ Odio et al.²⁴ reported the adsorption of heavy metal ions onto a novel poly-thiolated magnetic nano-platform, in which thiol and carboxyl groups played the functional role. Xie et al.²⁵ used magnetic microspheres based on chitosan/organic rectorite to adsorb low-concentration heavy metal ions from solution. Under the addition of a magnetic field, powdered adsorbent could be separated from solution fleetly, compared to common centrifugation, filtration and gravity sedimentation technology.

The aim of this work was to prepare a magnetic-functionalized vermiculite via a facile one-pot solvothermal method as adsorbent to remove Pb(II) from aqueous solution. The batch adsorption performance was investigated by using the effects of adsorbents dosage, contact time, solution pH, and adsorption kinetics and equilibrium isotherms. Furthermore, the structure and surface properties of the adsorbents were characterized by X-ray diffraction (XRD), surface-enhanced Raman scattering (SERS), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscope (SEM) with energy dispersive spectroscopy (EDS), transmission electron microscope (TEM), specific surface area determination, and magnetic property and zeta potential measurement.

Materials and methods

Reagents

The chemicals, Pb(NO₃)₂, FeCl₃·6H₂O and NaAc, offered by Kermel Chemical Reagent Co, Ltd. (Tianjin, China), were all analytical grade reagents and used without further purification. Ethyleneglycol and ethylenediamine were also analytical reagent grade and received from Tianjin Fuyu Fine Chemical Co., Ltd. (Tianjin, China). The vermiculite (VER), a natural clay material that milling fine particles, was industrial grade and supplied by Lingshou County Xinnuo Mineral Co. Ltd. (Hebei, China).

Preparation of magnetic vermiculite

Magnetic vermiculite (MVER) was synthesized by the solvothermal method, similar to our previous work of magnetic diatomite and illite.²² Briefly, FeCl₃·6H₂O (2.0 g) was suspended in ethyleneglycol (40 mL). The solution was combined and vigorously stirred to form a mixed transparent solution. NaAc (6.0 g) and ethylenediamine (20 mL) were then added into the mixed solution. Subsequently, the vermiculite VER (2.0 g) was added into the above solution slowly. After being stirred to obtain a homogeneous mixture, it was transferred into a polytetrafluoroethylene liner (100 mL), and then put in a stainless steel autoclave and maintained at 200 °C for 8 h at a drying oven. The obtained product was washed repeatedly by distilled water for several times and dried overnight at 60 °C. The collected solid was ground to pass through a 100 mesh sieves before use.

Characterization methods

The XRD patterns were recorded in the 2θ range of 5–70° at a scan speed of 0.03° s⁻¹ using a diffractometer (D8 Advance, Bruker Corporation, Germany) with Cu Kα radiation (40 mA, 45 kV). The SERS (100–1500 cm⁻¹) of the samples were acquired using a Laser Confocal Micro-Raman Spectroscopy (LabRAM HR800, Horiba Jobin Yvon Corporation, France) coupled with a 633 nm He–Ne laser source. The FTIR spectra were collected on a Vertex 70 FTIR spectrometer (Bruker Corporation, Germany) using transmission mode. The SEM images with EDS was detected with the scanning electron microscope (S4800, Hitachi Corporation, Japan), all samples were prepared on a graphite carrier and coated with a 3 nm layer of gold-platinum. The TEM images were obtained on a transmission electron microscope (Tecnai G20, FEI Corporation, USA). The samples were dispersed in ultrasonic bath and one drop of each sample was placed on the copper grids. The magnetic measurement was carried out by a magnetic property measurement system (MPMS-XL-7, Quantum Design Corporation, USA) at room temperature with an applied field of plus-minus 20 kOe. The specific surface area of the samples (heated at 200 °C for 6 h before use) was determined by N₂ adsorption/desorption at 77 K on a surface area and porosity analyzer (3Flex, Micromeritics Corporation, USA). The Brunauer–Emmet–Teller (BET) equation and Barrett–Joyner–Halenda (BJH) method were used to calculate the specific surface area and pore size distribution, respectively. The zeta potentials of the adsorbent suspension were measured by a Nano ZS90 Zetasizer analyzer (Malvern Instruments Ltd, UK). The pH values of solutions were determined by a precision pH meter (Starter 3100, Ohaus Instruments (Shanghai) Co., Ltd, China).

Batch adsorption studies

A series of batch tests were conducted to study the Pb(II) adsorption performance onto the VER and MVER at 25 ± 1 °C. A certain amount of VER or MVER composites were transferred to the polypropylene centrifuge tubes with 20 mL Pb(II) solution. The mixtures were then placed in the constant temperature oscillator at 200 rpm for a certain contact time. The suspension of VER was separated by centrifugation at 5000 rpm for 15 min and the suspension of MVER was separated by magnetic separation using a magnet. Then these two supernatants were both filtered through a syringe water-membrane filter (0.45 μm), and the surplus Pb(II) concentration was analyzed by the atomic absorption spectroscopy method, using an AA-7000 atomic absorption spectrometer (Shimadzu, Japan) with an air-acetylene burner. The amounts of Pb(II) adsorbed onto the samples and removal ratio of Pb(II), are defined in the following equations:where q_e (mg g⁻¹) is the capacity of Pb(II) adsorbed; R (%) is the removal ratio of Pb(II) by adsorbent; C_Pb (mg L⁻¹) and C_Pb,t (mg L⁻¹) are the concentration of Pb(II) at initial and fixed contact time or equilibrium time, respectively; V (L) is the volume of Pb(II) solution, and m (g) is the weight of adsorbent.

To be specific, for the effect of adsorbent dosage, different VER and MVER amounts (0.01–0.15 g) were added into 20 mL solution containing 100 mg L⁻¹ Pb(II) and shaken for 60 min at 25 °C with original pH. For the contact time studies, 0.05 g adsorbent was added into the solution of 100 mg L⁻¹ Pb(II) with original pH and shaking at a predetermined time (1–150 min). Besides, for the effect of pH, the pH values of 100 mg L⁻¹ Pb(II) solution were adjusted around 2–4.5 by 0.1 mol L⁻¹ HCl and NaOH. Then the interaction between 0.05 g adsorbent and Pb(II) solutions was conducted at 25 °C for 60 min. For the adsorption isotherm, 0.05 g of the adsorbent was mixed with 20 mL Pb(II) solution at concentrations of 20–300 mg L⁻¹ and the initial pH for 60 min. The experiments of adsorption/desorption cycles for VER and MVER were also carried out. After 100 mg L⁻¹ Pb(II) was adsorbed by VER and MVER, 20 mL EDTA solution (0.05 mol L⁻¹), using as desorbing agents, was added. The mixture was shaken for 60 min at 25 °C. Then the solids were separated from the solution and used to the next adsorption/desorption cycle.

Results and discussion

Material characterization

Fig. 1 shows the SEM pictures of VER and MVER. Both VER and MVER exhibited representative lamellar structure and the surface of VER was more smoother than that of MVER. The MVER became irregular in shape and was coated with a large number of tiny crystal spheres in aggregation form. This suggested that the nano-Fe₃O₄ spheres may be loaded to the surface of VER. In addition, there were some small flakes on the surface of MVER, and the round-edged particles with rough surface, this was attributed to the solvothermal process of higher temperature and pressure. Fig. 1e and f shows the EDS results of VER and MVER. The VER mainly consisted of SiO₂, Al₂O₃, Fe₂O₃ and MgO. Compared to VER, the elements of MVER were very similar. Nonetheless, it can be seen clearly that the percentage of Fe was promoted, further indicating the combination of iron oxides with VER.

Fig. 1 SEM images (a–d) and EDS results (e and f) of VER (a, b and e) and MVER (c, d and f).

Fig. 2 shows the TEM pictures of VER and MVER at different magnification levels. The surface of VER was decorated with some blackspots because the natural vermiculite contained Fe originally. After the solvothermal process, the crystal lattices with black ball shape attached to the surface of vermiculite (Fig. 2d and e). Furthermore, from the high-resolution TEM (HRTEM) images of MVER (Fig. 2f), the lattice spacing was 0.148 nm and 0.250 nm, which was equal to the spacings calculated from XRD patterns of MVER (2θ = 62.46°, d = 0.148 nm and 2θ = 35.81°, d = 0.250 nm).

Fig. 2 TEM (a–e) and HRTEM (f) images of VER (a and b) and MVER (c–f).

Fig. 3a illustrates the XRD patterns of VER and MVER. There were relatively weak characteristic peaks at 26.54°, 34.16°, corresponding to the quartz (SiO₂), mullite (Al₂O(SiO₄)), illite and fewer amounts of chlorite.²⁶ Furthermore, the angle 2θ of 8.84°, 19.20°, 35.81° and 54.92° were related to octahedral iron,²⁷ proving that the natural vermiculite contains the element of Fe. After solvothermal process, the typical diffraction peaks at the angle 2θ of 8.84°, 19.28°, 27.0°, 34.13°, 54.91° still remained sharp and strong. This suggested that the basic crystal structure of vermiculite has not been destroyed during the solvothermal process, and MVER remained high crystallinity. Because the XRD picture of γ-Fe₂O₃ was similar to that of Fe₃O₄, it was difficult to accurately distinguish these two kinds of iron oxides by XRD. To further confirm the iron oxide species of MVER, VER and MVER were measured by SERS, as shown in Fig. 3b. There appeared a new band at lower frequency of 668 cm⁻¹, attributing to the Fe–O lattice mode of Fe₃O₄.²⁸ This proved that the loaded particles on VER surface is pure Fe₃O₄, which was in accordance with the XRD analysis.

Fig. 3 XRD patterns (a), SERS spectra (b), FTIR spectra (c), N₂ adsorption/desorption isotherms (d), hysteresis loop curves (e) and zeta potentials (f) of VER and MVER.

The FTIR spectra of the VER and MVER are demonstrated in Fig. 3c. The spectra of 3710 cm⁻¹ band belonged to the stretching vibration of structural –OH groups, the broad band between 3600 and 3000 cm⁻¹ was related to the –OH stretching vibration of absorbed water molecules.²⁹ The absorption at 1637 cm⁻¹ was primarily due to water directly coordinated to the exchangeable cation of the clay.³⁰ The strong band at 1010 cm⁻¹ ascribed to the Si–O–Si groups of the silicate layers.³¹ The peaks at 437, 671 and 721 cm⁻¹ were not obvious, belonging to the deformation and bending modes of the Si–O.³² By comparison, the MVER possessed the characteristic peaks that existed on the VER as well. This indicated the solvothermal process would not change the crystal structure of VER.

The N₂ adsorption/desorption isotherms of VER and MVER are presented in Fig. 3d. Both MVER and VER showed type IV isotherm with type H₃ hysteresis loop according to IUPAC classification, indicating the presence of mesopore structure. The BET specific surface area, pore diameters and total pore volume of VER were 34.1 m² g⁻¹, 4.1 nm and 0.071 cm³ g⁻¹, and that of MVER were 52.6 m² g⁻¹, 6.0 nm and 0.159 cm³ g⁻¹, respectively.

Magnetic property is vital to nano-materials containing iron oxide for their applications in a magnetic separation system. Fig. 3e shows the hysteresis loops of the MVER and Fe₃O₄ recorded at room temperature. As illustrated in Fig. 3e, the typical hysteresis loops of MVER showed superparamagnetic property, without any remnant magnetization or coercivity. The saturation magnetization (M_s) of MVER was about 14.2 emu g⁻¹, which accounted for 14.3% of the Fe₃O₄ with the M_s of 99.0 emu g⁻¹, indicating the nanosized Fe₃O₄ was successfully loaded onto VER. Moreover, MVER responded well to the magnetic field, which favoured the magnetic separation before and after adsorption process.

Zeta potential is an important parameter to analyze the surface charge of solid and it determines the electrophoretic mobility where the net total particle charge is zero. Fig. 3f presents the zeta potential of MVER and VER as a function of solution pH. The zeta potential of both samples ranged from about −45 mV to 45 mV and the pH_zpc of VER and MVER were 4.2 and 3.3, respectively.

Effect of varying conditions on the adsorption capacities of VER and MVER

Batch tests were used to study the adsorption properties of VER and MVER for Pb(II) at different conditions, including adsorbent dosage, contact time and initial solution pH (Fig. 4). The Pb(II) removal efficiency increased with the amount from 0.01 g to 0.15 g and achieved equilibrium at the adsorbent dosage of 0.05 g for MVER and 0.12 g for VER (Fig. 4a). It is clearly that MVER possessed higher adsorption efficiency compared with VER at the same adsorbent dosage. When the dosage increased to 0.05 g, the removal ratio was almost 100% for MVER, suggesting that nearly all Pb(II) ions in solution were absorbed. From the economical and practical point of view, the optimum adsorbents dosage of 0.05 g was chosen for the next steps.

Fig. 4 Adsorption of Pb(II) by VER and MVER as functions of adsorbent dosage (a), contact time (b) and initial solution pH (c).

The adsorption of Pb(II) on VER and MVER was investigated as a function of contact time in the range of 1–150 min. As shown in Fig. 4b, the adsorption capacity increased from 27.8 mg g⁻¹ to 29.2 mg g⁻¹ for VER and 39.0 mg g⁻¹ to 39.5 mg g⁻¹ for MVER. It is obvious that the Pb(II) adsorption onto VER and MVER increased fleetly within 20 min and reached a plateau with the increasing of contact time. The rapid adsorption by VER and MVER observed within 20 min may be put down to the direct electrostatic attraction and complexation reaction on the external surface and the plentiful active sites of the adsorbents.³³

Solution pH is one of the most significant factors influencing the surface charge of adsorbent and the speciation of adsorbate³⁴ Fig. 4c shows the Pb(II) adsorption capacity of VER and MVER by varying initial pH values from 2 to 4.5. At lower solution pH, the surface of VER and MVER possess positive charge, and the adsorption capacity of Pb(II) was lower due to the electrostatic repulsion. With the increasing of initial solution pH, the surface charge changed from positive to negative. This caused the adsorption capacities of Pb(II) had an upward tendency, and then attained a plateau and fluctuated at about 24.0 and 39.8 mg g⁻¹ for VER and MVER, respectively.

Adsorption kinetics

In order to analyze the mechanisms of the adsorption process, the kinetic data were investigated using pseudo-first-order and pseudo-second-order models. The pseudo-first-order and pseudo-second-order models are described as follows:^35,36where q_t (mg g⁻¹) and q_e (mg g⁻¹) are the amount of Pb(II) adsorbed over a given period of time t and at equilibrium, respectively; t (min) is the adsorption time; k₁ (min⁻¹), k₂ (mg (g min)⁻¹) are the adsorption rate constant of the pseudo-first-order and the pseudo-second-order model, respectively.

The parameters are listed in Table 1, high correlation coefficients are derived from fitting experimental data into the pseudo-second-order model, compared with the pseudo-first-order equation. The same results were reported by Tran et al.,³⁷ in which mercaptoethylamine and mercaptopropyltrimethoxysilane functionalized vermiculites adsorbed Hg(II) from aqueous solution. This suggested that the adsorption of Pb(II) was chemisorption and the process was a rate-limiting step.³⁶ As can be seen from Table 1, the k₂ and q_e values of Pb(II) adsorption on MVER (3.81 g (mg min)⁻¹ and 39.5 mg g⁻¹) were significantly greater than that of VER (0.11 g (mg min)⁻¹ and 29.2 mg g⁻¹).

Table 1 The adsorption kinetic constants and correlation coefficients of Pb(II) adsorption onto VER and MVER

Adsorbent	Pseudo-first-order			Pseudo-second-order
	qe (mg g^−1)	k1 (min^−1)	R^2	qe (mg g^−1)	k2 g (mg min)^−1	R^2
VER	1.30	0.011	0.84	29.2	0.011	0.99
MVER	0.305	0.029	0.23	39.5	3.81	1.00

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Adsorption isotherm

Batch equilibrium adsorption experiments were carried out at the following optimized conditions: 0.05 g of VER and MVER, reaction time of 120 min and the initial solution pH without adjustment. The results are shown in Fig. 5 and the removal amounts of Pb(II) by MVER and VER increased with increasing initial Pb(II) concentration until the saturation of available reactive sites. The Freundlich and Langmuir model were employed to analyze the Pb(II) adsorption on VER and MVER. These adsorption isotherms can be represented mathematically as follows:where c_e (mg L⁻¹) is the equilibrium concentration of Pb(II) in aqueous solution; q_e (mg g⁻¹) is the amount of Pb(II) adsorbed at equilibrium; k_F and 1/n are the Freundlich equilibrium constant; q_m (mg g⁻¹) is the maximum adsorption capacity and b is the Langmuir adsorption equilibrium constant.

Fig. 5 Adsorption isotherms of Pb(II) onto VER and MVER.

The obtained parameters of fitting Freundlich and Langmuir adsorption isotherms are summarized in Table 2. As shown in Fig. 5 and Table 2, the R² for the Langmuir equation were found to be 0.99 for VER and MVER, better than for the Freundlich model. This illustrated that the adsorption of Pb(II) onto VER and MVER both followed the Langmuir adsorption isotherm model. The Pb(II) adsorption took place at specific homogeneous reactive sites on VER and MVER was probably a monolayer process.³⁸

Table 2 The adsorption isotherm constants and correlation coefficients of Pb(II) adsorption onto VER and MVER

Adsorbent	Langmuir equation			Freundlich equation
	qm (mg g^−1)	b (L mg^−1)	R^2	kF ((mg g^−1) (mg L^−1)^−n)	1/n	R^2
VER	37.0	0.971	0.99	14.7	0.21	0.84
MVER	70.4	0.916	0.99	28.3	0.23	0.32

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The Langmuir isotherm model proved that the adsorption process was irreversible, favourable, linear or unfavourable by means of an equilibrium dimensionless parameter, which could further reveal the adsorption nature and as defined in the following equation:³⁹

where C₀ (mg L⁻¹) is the initial Pb(II) concentration. The adsorption process is considered favourable when 0 < R_L < 1.³⁹ Based on this work, the R_L value were varied from 0.0490 to 0.00342 and 0.0518 to 0.0363 for VER and MVER as the initial concentration of Pb(II) increased from 20 to 300 mg L⁻¹, respectively. This indicated that the adsorption of Pb(II) onto VER and MVER occurred favourably.

Fig. 6 shows the adsorption efficiency of VER and MVER for five adsorption/desorption cycles. The removal ratio of MVER and VER reached 95.8% and 92.9% after the first cycle, respectively, and still kept higher than 90% and 85% after five cycles. This suggested that the VER and MVER had relatively good reusability for Pb(II) removal from aqueous solution.

Fig. 6 The adsorption/desorption cycles of Pb(II) by VER and MVER.

Table 3 summarizes the comparison of the maximum adsorption capacity of various adsorbents for Pb(II) removal from aqueous solution.^40–47 It was clear that the adsorption capacities of VER and MVER were similar or superior to the other sorbent materials. Furthermore, the magnetic vermiculite can be easily separated from the mixture of adsorbent and adsorbate by the external magnetic field in a short time after adsorption process. This indicated that the prepared magnetic vermiculite composite was an effective and easily separated adsorbent for Pb(II).

Table 3 Comparison of the adsorption capacities of Pb(II) by various sorbents

Adsorbent	T (°C)	pH	qmax (mg g^−1)	Reference
Activated carbon	30 ± 1	6.0	21.8	40
Oxidized MWCNTs	25 ± 1	10.0	2.10	41
Iron oxide	55	5.5	36.0	42
Sawdust	30	6.0	34.3	43
Na-bentonite	65	10.0	47.8	44
Hazelnut shell	30	6.0	28.2	45
Oak bark char	25	5.0	13.0	46
Chitosan–pectin pellets	35	6.0	12.4	47
Vermiculite	25 ± 1	4.5	37.0	This work
Vermiculite/magnetic	25 ± 1	4.5	70.4	This work

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Conclusion

In this study, the magnetic vermiculite was prepared via a facile one-pot solvothermal method for the removal of Pb(II) from aqueous solution. Physicochemical properties of VER and functionalized MVER were conducted by XRD, SERS, FTIR, SEM, EDS, TEM, VSM, BET measurements and zeta potential determination. The XRD, SERS and FTIR analysis showed that VER and MVER all existed their characteristic peak. The TEM and SEM analysis showed that nano-Fe₃O₄ was loaded on the surface of vermiculite successfully. MVER had superparamagnetic property and specific surface area of 52.6 cm³ g⁻¹. In addition, the effects of adsorbent dosage, solution pH, contact time and concentration of Pb(II) were studied, the adsorption of Pb(II) on VER and MVER increased with the increasing of pH and time, reached equilibrium within 30 min. The adsorption process conformed to the pseudo-second-order equation well, and the adsorption isotherm accorded well with the Langmuir model. The maximum adsorption capacity for VER and MVER was 37.0 mg g⁻¹ and 70.4 mg g⁻¹, respectively. Furthermore, the magnetic vermiculite can be separated easily and quickly by the application of magnetic field before and after adsorption.

Acknowledgements

This work was funded by the Natural Science Foundation of China (21577048, 21377046 and 41472216), the Graduate Innovation Foundation of University of Jinan (YCXS15017), the Natural Science Foundation of Shandong Province (ZR2014BL033), the Key R & D Program of Shandong Province (2015GSF117015) and the Special Project for Independent Innovation and Achievements Transformation of Shandong Province (2014ZZCX05101).

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