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Enhanced Removal of Pb2+, Cu2+, and Cd2+ by Amino-Functionalized Magnetite/Kaolin Clay Lilu Qin, Liangguo Yan,* Jian Chen, Tiantian Liu, Haiqin Yu, and Bin Du School of Resources and Environment, Shandong Provincial Engineering Technology Research Center for Groundwater Numerical Simulation and Contamination Control, University of Jinan, Jinan 250022, P. R. China S Supporting Information *

ABSTRACT: The amino-functionalized magnetite/kaolin clay (MKC) was synthesized via a simple solvothermal method and used to remove Pb2+, Cu2+, and Cd2+ from aqueous solutions. In comparison, the kaolin clay (KC) and MKC were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, and scanning and transmission electron microscopy with energy dispersive spectrometry. The results indicated amino-functionalized MKC was formed with Fe3O4 particles adhering to the surface by interactions with negatively charged KC. The optimal experimental conditions were evaluated, and the adsorption performance of MKC for Pb2+, Cu2+, and Cd2+ was far better than that of KC. This was mainly caused by adding an amino group, in which the amino group displayed complexing ability toward metal ions. In kinetic data representation, the pseudo-first-order, pseudo-second-order, and Elovich models were employed, and the second one gave the better fitting. Langmuir, Freundlich, and Dubinin−Radushkevich models were chosen for isotherm data correlation, of which the first one showed better suitability. The X-ray photoelectron spectroscopy analysis of MKC before and after adsorption further revealed that the adsorption mechanisms of Pb2+, Cu2+, and Cd2+ could be a combined reaction of complexation between functional groups and metal ions and electrostatic attraction. In addition, MKC can be rapidly separated using only a magnet after the adsorption process. Al2O3·2SiO2·2H2O. The structure is formed by one tetrahedral sheet (SiO4) and one octahedral sheet of alumina octahedral (AlO6), linked through oxygen atoms. It is a layered silicate composed of platelets which are connected by hydrogen bonding. Over the past years, KC has been applied to many industrial processes because of its good bonding ability and thermal stability. There are few, if any, reports on the adsorption properties of KC because of its low cation exchange capacity and small specific surface.10−12 Unuabonah et al.13 studied the adsorption of Pb2+ on phosphate-modified and unmodified kaolinite clay. The adsorption rate of phosphatemodified kaolinite clay is much faster than that of raw kaolinite, and the mechanisms can be attributed to chemisorption. Yavuz et al.14 investigated the removal of heavy metals from aqueous

1. INTRODUCTION Heavy metal ions, such as lead, copper, and cadmium can pose a hazardous threat to ecological systems including human, animal, and plant health, especially in surface or subsurface water even at low concentrations.1−4 According to the World Health Organization (WHO), the maximum acceptable concentrations recommended for lead, copper, and cadmium in drinking water are 0.01, 2.00, and 0.003 mg/L, respectively.5 It is imperative to lower the concentration of heavy metals in the environment so as to control the level of water pollution. Among many techniques such as coagulation and flocculation, membrane separation, electrodialysis, ion exchange, reverse osmosis, adsorption, and precipitation, adsorption stands out because of its simplicity and easy operational conditions.6−9 Clay minerals, such as bentonite, kaolin, and diatomite, are often used as adsorbents to remove heavy metals from water or wastewater.1,3,4,6,7 Kaolin clay (KC), which mainly consists of metal oxides Al2O3 and SiO2, is available and abundant in nature. The chemical composition could be described as Al2Si2O5(OH)4 or © XXXX American Chemical Society

Received: February 17, 2016 Revised: June 16, 2016 Accepted: June 20, 2016

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DOI: 10.1021/acs.iecr.6b00657 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

electron microscopy (SEM, Hitachi S570, Japan) and transmission electron microscopy (TEM) images with energy dispersive spectrometry (EDS), which were acquired from a JEM-2100 microscope (JEOL, Japan). Zeta potentials of adsorbent suspensions were tested by a Nano ZS90 Zetasizer analyzer (Malvern, United Kingdom). X-ray photoelectron spectroscopy (XPS) was performed on a multifunctional imaging electron spectrometer (Thermo, ESCALAB 250XI, United States). 2.4. Adsorption Studies. The adsorption experiments were conducted through batch equilibrium method. Heavy metals (Pb2+, Cu2+, and Cd2+) stock solutions of 1000 mg/L were prepared in deionized water. Adsorption processes for single-factor experiments were carried out in airtight centrifuge tubes, which contained 20 mL of heavy metal solutions with a fixed amount of KC or MKC. According to the preliminary experiment, the solution concentrations for adsorption of Pb2+, Cu2+, and Cd2+ were performed at 100, 30, and 30 mg/L, respectively. For the effect of adsorbent dosage, different adsorbent dosages were added into aqueous solutions containing Pb2+, Cu2+, and Cd2+ ions at 25 °C with original pH of the solution for 60 min of shaking. As for pH studies, the pH of Pb2+, Cu2+, and Cd2+ solutions were adjusted to the required values (pH 2.0−4.5 for Pb2+, 2.5−6.3 for Cu2+, 2.5− 8.3 for Cd2+ because higher pH causes metal hydroxide according to their different Ksp) with 1.0% HCl and 0.1% NaOH. Then, 0.08 g of KC or MKC for Pb2+ and Cu2+, 0.05 g for Cd2+ were put into the solutions at 25 °C for 60 min. In the next procedure of kinetic adsorption experiments, the samples (concentration, 100 mg/L for Pb2+ and 30 mg/L for Cu2+ and Cd2+; dosage, 0.08 g for Pb2+ and Cu2+ and 0.05 g for Cd2+; pH, original pH) were allowed to contact at fixed time intervals (1− 180 min). The adsorption isotherm data were collected by adding 0.08 g of adsorbent for Pb2+ (20−1000 mg/L) and Cu2+ (20−200 mg/L), 0.05 g for Cd2+ (20−200 mg/L) into 20 mL solutions at 25 °C in its original pH value. The suspensions were shaken for 60 min until equilibrium was achieved. Then the tubes were shaken on an orbital shaker at 200 rpm for a given time and centrifuged at 6000 rpm for 15 min (KC) or separated using a magnet for 5 min (MKC). The supernatant solution was filtered using a syringe filter with a 0.45 μm water membrane after the adsorbent was removed. The concentration of Pb2+, Cu2+, and Cd2+ was measured by an AA-7000 atomic absorption spectrophotometer (AAS, Shimadzu, Japan) with an air-acetylene burner. The removal rate R (%) (eq 1) and adsorption capacity q (mg/g) (eq 2) were calculated using the following formulas:

solution using a raw kaolinite and found that kaolinite showed higher absorption affinity for Cu2+ than Mn2+, Co2+, and Ni2+. However, the adsorbent materials are usually powders, which make it harder to separate the solid from a liquid phase. To solve this disadvantage, magnetic separation technology with an external magnetic field has aroused great interest in recent years because it features high separation efficiency, less energy consumption, shorter time, and other advantages under an external magnetic field.15−17 In addition, it is known that adsorbents with specific active groups like amino (−NH2), sulfonic (−SO3H), and acylamino (−CONH2) demonstrate complexing capacity toward metal ions.18,19 For instance, amino groups show high affinity for Pb2+ and Cd2+.20,21 It would be excellent to add suitable active groups into adsorbents which interact with target metal ions while preparing magnetic particles. In this work, amino-functionalized magnetite/kaolin (MKC) was synthesized via a solvothermal reaction. During the solvothermal process, KC acted as the porous matrix and the Fe3O4 precipitation by the reaction of Fe2+ + 2Fe3+ + 8OH− → Fe3O4 + 4H2O can exist on the surface of KC; the ethylenediamine provided amino group simultaneously. While extensive work has been performed on magnetic separation method and KC adsorption, few reports focus on the adsorption of metal ions by magnetic kaolin composite. Therefore, this work aims to prepare MKC through the simple solvothermal method and to evaluate the adsorption performance of KC and MKC for heavy metal ions (Pb2+, Cu2+, and Cd2+) in aqueous solution. The effects of several parameters, viz., adsorbent dosage, contact time, initial solution pH, and concentration of heavy metal ions, were tested. In addition, mechanisms for the removal of heavy metal ions by MKC were also discussed.

2. MATERIALS AND METHODS 2.1. Materials. Kaolin (Al2Si2O9H4) was purchased from Aladdin Chemistry Co. Ltd. (Shanghai, China). The other reagents were all of analytical grade and were used without further purification. The main chemicals used in the experiment were FeCl3·6H2O, ethylene glycol (C2H6O2), CH3COONa· 3H2O, ethylenediamine (C2H8N2), Pb(NO3)2, Cu(NO3)2· 3H2O, Cd(NO3)2·4H2O, HNO3, and NaOH. 2.2. Preparation of Amino-Functionalized Magnetite/ Kaolin Clay. The magnetite (Fe3O4) was synthesized by solvothermal method as reported in our previous study.2 The MKC was also prepared through the simple solvothermal method. First, 2.0 g of FeCl3·6H2O was dispersed in 40 mL of ethylene glycol solvent. Then 6.0 g of CH3COONa·3H2O and 20 mL of ethylenediamine were added into the solution successively, and the reaction mixture was stirred for 30 min. Afterward, 0.8 g of KC powder was put into the mixture, which was then sealed in a stainless steel vessel with a polytetrafluoroethylene liner. The vessel was heated for 8 h at 200 °C. The products were washed with deionized water several times and separated using a magnet, dried at 60 °C, and finally yielded the MKC composite. 2.3. Characterization Methods. X-ray diffraction (XRD) patterns of KC and MKC were determined by D8 Focus X-ray diffractometer (Bruker, Germany) with Cu Kα radiation (40 kV, 30 mA, λ = 0.154 nm), and the data were collected in a scan range from 5° to 70° (2θ) with an increment of 0.03°. Fourier transform infrared (FTIR) spectra were obtained on a Vertex 70 FTIR spectrometer (Bruker, Germany) in the spectral range of 4000−450 cm−1. The morphology was taken on scanning

R=

c0 − ct × 100% c0

(1)

q=

(c 0 − c t ) × V m

(2)

where c0 (mg/L) is the initial concentration of metal ion in the solution, ct (mg/L) the final concentration after adsorption, V (L) the volume of the solution, and m (g) the mass of adsorbent. 2.5. Adsorption Model Fitting. The experimental data of the kinetic study were fitted to the following well-known models: (1) Pseudo-first-order kinetic equation: B


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Industrial & Engineering Chemistry Research lg(qe − qt) = lg qe −

k1 t 2.303

(3)

where qt (mg/g) is the amount of metal ion adsorbed at time t (min), qe (mg/g) the equilibrium adsorption capacity, and k1 (1/min) the rate constant. (2) Pseudo-second-order kinetic equation: t 1 1 = + t qt qe k 2qe 2

(4)

where qt (mg/g) is the amount of metal ion adsorbed at time t (min), qe (mg/g) the equilibrium adsorption capacity, and k2 (g/(mg·min)) the constant rate. (3) Elovich equation: 1 1 qt = + ln t β ln(αβ) β

Figure 1. XRD patterns of KC (a), MKC (b), and Fe3O4 (c).

(5)

namely, 2θ = 12.30° (d = 0.719 nm) and 24.86° (d = 0.359 nm) for KC and 2θ = 30.12° (d = 0.296 nm), 35.44° (d = 0.253 nm), 43.06° (d = 0.154 nm), 56.94° (d = 0.162 nm), and 62.52° (d = 0.148 nm) for magnetite, and the intensities of the characteristic reflections were not significantly changed. Besides, the main characteristic peaks of MKC (2θ = 12.30°, d = 0.719 nm; 24.86°, d = 0.359 nm) did not change too much compared to KC (2θ = 12.30°, 24.84°). This signified the completeness of the crystal structure of MKC, indicating that the Fe3O4 phase may exist on the surface of MKC. The results of FTIR also confirmed the similar KC structure of MKC. As shown in Figure S1, the bands at 3695, 3620, and 914 cm−1 are attributed to Al−OH stretching, and bands at 3437 and 1631 cm−1 correspond to OH-stretching. In the lower-frequency region of the spectra (1500−500 cm−1), bands at 1105 and 1032 cm−1 correspond to Si−O stretching vibration, and bands at 690 and 540 cm−1 correspond to Si−O−Al stretching vibration.25−27 Broad bands between 690 and 540 cm−1 were observed, which were associated with Fe−O bending vibration of iron oxide.28 The characteristic absorption band of N−H stretching vibration at 3437 cm−1 and N−H bending vibration at 1633 cm−1, overlapped by the bands of KC, were heightened after the modification.29,30 The morphology and iron particle distribution on MKC were analyzed by SEM, TEM, and the corresponding EDS spectrum (Figure 2). The KC presented a characteristic structure of layered hexagonal platelets. There existed dark dots with uneven size on the surface of MKC (Figure 2b), which indicate that the ball-like and agglomerate magnetite had been successfully generated on the surface of KC.23,31 Similar results also can be found in the TEM images. Plenty of tiny spherical morphology can be seen on the surface of MKC with average diameter about 20 nm. The high-resolution TEM images of KC and MKC (Figure 2e,f) displayed many lattice planes with perfect crystallinity. As for KC, the lattice spacing 0.353 nm was close to the calculated XRD spacing value 0.358 nm (2θ = 24.84°). For MKC, the lattice fringe spacing of 0.253 nm belongs to the (311) reflection of Fe3O4 particle (0.250 nm). The other lattice spacing of 0.295 and 0.515 nm also agreed well with the XRD analysis of MKC in 2θ = 30.12° (d = 0.296 nm) and 2θ = 18.26° (d = 0.486 nm). The SEM and TEM observations corresponding with the results of XRD analysis may deduce the magnetite particles adhered to KC surface by interactions with negatively charged surface of KC. According to the EDS analysis (Figure 2g,h), the percentage of element Fe on MKC reached 39.48%, compared with that of KC. Assuming that most of the oxygen dated from clay and magnetite, the

where qt (mg/g) is the amount of metal ion adsorbed at time t (min) and α (mg/(g·min)) is the constant of initial adsorption rate; β (g/mg) denotes the extent of surface coverage and activation energy for chemisorption. The adsorption isotherm data were fitted to the following models: (1) Langmuir model: ce c 1 = e + qe qm KLqm (6) where ce (mg/L) is the equilibrium concentration of metal ion in solution, qe (mg/g) the amount adsorbed at equilibrium, qm (mg/g) the maximum adsorption capacity, and KL a Langmuir adsorption constant. (2) Freundlich model: ln qe =

1 ln ce + ln KF n

(7)

where ce (mg/L) is the equilibrium concentration of metal ion in solution, qe (mg/g) the amount adsorbed at equilibrium, KF a Freundlich constant, and 1/n the Freundlich coefficient. (3) Dubinin−Radushkevich model: ln qe = ln qm − KDR ε 2

(8)

⎛ 1⎞ ε = RT ln⎜1 + ⎟ ce ⎠ ⎝

(9)

where ce (mg/L) is the equilibrium concentration of metal ion in solution, qe (mg/g) the amount adsorbed at equilibrium, qm (mg/g) the maximum adsorption capacity, KDR (mol2/kJ2) the constant related to the mean free energy of adsorption, ε the Polanyi potential, R (J/(mol·K)) the gas constant (8.314), and T (K) the absolute temperature.

3. RESULTS AND DISCUSSION 3.1. Characterization of MKC. The XRD patterns of KC and MKC are shown in Figure 1. The main diffraction reflections of KC were at 2θ = 12.30°, 35.92° (Al2O3), 2θ = 24.84°, 38.34°, and 62.3° (SiO2), which are the characteristic peaks of KC.22,23 For magnetite, the diffraction angles appeared at 2θ = 18.26°, 30.12°, 35.46°, 43.08°, 56.96°, and 62.58°, corresponding to crystal surface of the Fe3O4 pure phase (111), (220), (311), (400), (511), and (440).24 The XRD patterns of MKC exhibited the reflections of both KC and magnetite, C


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Figure 2. SEM (a, b), TEM (c, d), and HRTEM (e, f) images and EDS element composition analysis (g, h) of KC (a, c, e, g) and MKC (b, d, f, h).

peaks of Al 2p, Si 2p, Fe 2p, and N 1s in MKC spectra are also presented. As seen in Figure 3b−e, the peaks of Al 2p, Si 2p, and Fe 2p at binding energy (BE) of 74.3, 103 (103.5 eV), and 710.5 eV were ascribed to Al2O3 (or Al2O3·nH2O), SiO2 (SiO2· nH2O), and Fe3O4, respectively, according to the NIST X-ray photoelectron spectroscopy database. The peak of N 1s at BE about 399.8 eV may result from −NH2 (−NH− or −NH3+ under different pH values) in MKC. Other literature contributions also reported similar results.27,33

weight ratio of KC on MKC was close to 45.44%. Consequently, the ratio of Fe element to KC was 1.15:1, indicating the coating of KC surface by nanoparticle Fe3O4. In addition, the presence of N in the MKC justified the presence of amino-functionalized magnetite in KC, even at a low ratio (2.02 wt %). This can be further confirmed by XPS test. XPS spectra are extensively applied to discriminate the different forms of the same element and determine the existence of a particular element in a material.32,33 Figure 3 shows the XPS spectra for MKC before adsorption. The typical D


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Figure 3. XPS spectra of MKC: full scan (a), Al 2p (b), Si 2p (c), Fe 2p (d), and N 1s (e).

3.2. Effect of Varying Conditions on the Adsorption of Pb2+, Cu2+, and Cd2+. The effects of experimental conditions such as adsorbent dosage, initial solution pH, and contact time on the removal of metal ions were investigated. Figure 4a presents the effects of KC and MKC dosage on the removal efficiency of Pb2+, Cu2+, and Cd2+. It can be observed that MKC has a significant advantage over KC for the heavy metal ion adsorption. The removal efficiency increases greatly as the amounts of MKC increase from 0.01 g (0.5 g/L) to 0.05 g (2.5 g/L) for Pb2+, Cu2+, and Cd2+. Then it reaches a plateau with the dosages increasing to 10.0 g/L; however, the adsorption trend for KC continues to increase slowly. Considering all the aspects, the dosages of 4.0 g/L for Pb2+ and Cu2+ and 2.5 g/L for Cd2+ were selected as the optimum adsorbent dosages for the following experiments. The effects of initial pH on the adsorption capacity of Pb2+, Cu2+, and Cd2+ onto KC and MKC are depicted in Figure 4b.

The initial heavy metal solution pH values were set below 5.0 for Pb2+, 6.5 for Cu2+, and 8.5 for Cd2+ for the probability of the formation of metal hydroxide precipitates. As seen in Figure 4b, the adsorption capacity kept increasing with the increasing of solution pH and then reached a plateau, and the adsorption capacity of Pb2+ was apparently higher than that of the other two ions, as illustrated in previous studies.21,32,34,35 When the pH increased to about 3.0 for Pb2+ and 4.0 for Cu2+ and Cd2+, the removal efficiency of MKC was all above 90%, and the initial pH of heavy metal ion solutions were 4.5 for Pb2+, 5.5 for Cu2+, and 7.0 for Cd2+; therefore, there is no need to adjust the solution pH during the experimental process. The effect of contact time on the adsorption capacity of KC and MKC are shown in Figure 4c. The adsorption ratio increased significantly at first and then reached equilibrium at 60 and 30 min for KC and MKC, respectively. Therefore, it is enough to pick 60 min of contact time for KC and MKC in the E


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adsorption of heavy metal ions, which was also selected in the following experiments. 3.3. Adsorption Kinetics. The kinetic data were analyzed by the pseudo-first-order, pseudo-second-order, and Elovich models. The parameters of the kinetic models are presented in Table 1. It can be observed that the adsorption of Pb2+, Cu2+, and Cd2+ onto KC and MKC fit the pseudo-second-order model well, because all of its correlation coefficients R2 were beyond 0.99. The R2 obtained for Elovich were lower than those of the pseudo-second-order equation.36 Moreover, the theoretical calculated qe,cal values obtained from the pseudosecond-order model were in good agreement with the experimental values (qe,exp). This indicated that the rate-limiting step of the adsorption mechanism for KC and MKC was chemical adsorption involving valence forces through the sharing or exchange of electrons between sorbent and sorbate.32,37−39 Similar kinetic results were found for the adsorption of Pb2+, Cd2+, and Cr3+ ions by magnetic chrysotile nanotubes;40 Pb2+ onto phosphate-modified kaolinite;13 and Cd2+, Cr3+, Cu2+, Pb2+, and Zn2+ by iron-coated zeolite.41 3.4. Adsorption Isotherm. To further investigate the interactive behaviors between the heavy metal ions and adsorbents, the Langmuir, Freundlich, and Dubinin−Radushkevich isotherm models were employed to simulate the experimental data. The obtained values for Langmuir, Freundlich, and Dubinin−Radushkevich isotherm constants and correlation coefficients are listed in Table 2. The Langmuir model fit better to the experimental data (Figure 5), with higher correlation coefficients (>0.95), and the maximum adsorption capacity of MKC for heavy metal ions was much higher than that of KC. Moreover, the adsorption capacity followed the order Pb2+ > Cd2+ > Cu2+. This may be explained by the size of the hydrated ions, free energy of hydration, and activity of metal ions. The smaller the hydrated radius is, the greater the affinity can be, and the order of the hydrated radius is as follows: Cd2+ > Cu2+ > Pb2+.35 When metals have a higher level of free energy of hydration, they tend to remain in the aqueous phase and reduce the chance for heavy metal ions to go into the adsorbent structure. The order of the free energy of hydration is as follows: Cu2+ > Cd2+ > Pb2+.32,35 As a result, the Pb2+ ion has a higher tendency to adsorb on the adsorbent. The degree of suitability of adsorbent toward heavy metal ions can be evaluated by the values of the separation factor constant, RL, in the Langmuir and free energy, Es (kJ/mol), from the Dubinin−Radushkevich isotherm models, which were applied to further indicate the adsorption property.42−44

Figure 4. Effect of adsorbent dosage (a), initial solution pH (b), and contact time (c) on the adsorption of Pb2+, Cu2+, and Cd2+ by KC and MKC.

Table 1. Calculated Parameters of Pseudo-First-Order, Pseudo-Second-Order, and Elovich Models for the Adsorption of Pb2+, Cu2+, and Cd2+ on KC and MKC pseudo-first-order adsorbent/metal ion KC Pb2+ Cu2+ Cd2+ MKC Pb2+ Cu2+ Cd2+

pseudo-second-order

Elovich model

R2

qe,cal (mg/g)

k2 g/(mg·min)

R2

α (mg/(g·min))

β (g/mg)

R2

0.0193 0.00216 0.0195

0.919 0.230 0.924

8.55 1.96 3.21

0.0708 0.461 0.328

0.999 0.998 0.999

6.24 −0.023 0.106

2.38 −41.2 11.9

0.965 0.149 0.981

0.0182 0.0167 0.0296

0.639 0.302 0.607

24.9 6.61 11.8

1.88 0.403 0.145

1.000 0.999 0.999

0.270 1.55 210

16.5 3.25 1.55

0.689 0.703 0.579

c0 (mol/L)

qe,exp (mg/g)

qe,cal (mg/g)

k1 (1/ min)

100 30 30

8.45 1.95 3.19

1.36 0.121 0.302

100 30 30

24.9 6.61 11.7

0.0714 0.302 1.203

F


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Table 2. Parameters of Langmuir, Freundlich, and Dubinin−Radushkevich Isotherm for the Adsorption of Pb2+, Cu2+, and Cd2+ on KC and MKC Langmuir equation adsorbent/metal ion KC Pb2+ Cu2+ Cd2+ MKC Pb2+ Cu2+ Cd2+

Freundlich equation

Dubinin−Radushkevich equation

KL (L/mg)

qm (mg/g)

RL

R2

KF ((mg/g)/ (mg/L)n)

n

R2

KDR (mol2/ kJ2)

qm (mg/g)

ES (kJ/mol)

R2

0.0158 0.0908 0.0148

26.2 2.69 13.1

0.0733−0.760 0.0522−0.355 0.253−0.772

0.972 0.997 0.995

3.51 1.06 0.611

3.39 5.49 1.83

0.925 0.850 0.985

0.00265 0.00208 0.00582

36.3 4.20 44.3

13.7 15.5 9.27

0.866 0.836 0.994

0.0925 0.117 0.460

86.1 16.5 22.1

0.0133−0.351 0.0410−0.299 0.0108−0.098

0.984 0.968 0.996

3.99 5.49 7.61

0.397 0.916 0.866

0.00205 0.00164 0.00109

15.6 17.5 21.4

0.373 0.896 0.879

18.3 6.31 11.9

123 21.7 30.2

and MKC, respectively. The values of 1/n were all between 0 and 1, which also suggested that the adsorption of heavy metal ions by KC and MKC was favorable. At the same time, the Es values (Table 2) for Pb2+ were found to be 13.7 and 15.6 kJ/ mol, in the energy range of adsorption reactions 8−16 kJ/mol, which means the adsorption process followed ion-exchange. The magnitude of Es for Cd2+ and Cu2+ on MKC were 17.5 and 21.4 kJ/mol, between 16 and 40 kJ/mol, indicating chemisorptions.44,45 As a whole, the adsorption of Pb2+, Cu2+, and Cd2+ onto KC and MKC can be described as chemical adsorption, which also agrees with the results of adsorption kinetics. To display the application advantages of MKC adsorption, a number of adsorbents reported recently in other research are presented for comparison (Table 3). The maximum adsorption Table 3. Reported Adsorption Capacities of Different Adsorbents for Pb2+, Cu2+, and Cd2+ adsorbent MKC

sulfhydryl functionalized hydrogel with magnetism chitosan/SiO2/Fe3O4

Figure 5. Linear fit of Langmuir isotherm model for Pb2+, Cu2+, and Cd2+ adsorption on KC (a) and MKC (b).

1 RL = 1 + KLc0 Es =

1 2KDR

amine-functionalized SBA-15 amino-functionalized Fe3O4 magnetic nanoparticles dimercaptosuccinic acid (DMSA) anchored Fe3O4 chitosan/sulfydryl-functionalized graphene oxide composites

(10)

polyaniline thiacalix[4]arene

(11)

where KL and Es are the Langmuir and Dubinin−Radushkevich constants and c0 (mg/L) is the initial concentration of metal ions. The value of RL suggests the type of isotherm: irreversible (RL = 0), favorable (0 < RL < 1), linear (RL = 1), or unfavorable (RL > 1).43 The calculated RL values were all within the range of 0 < RL < 1 (Table 2), indicating the metal ion adsorption on KC and MKC was favorable. In addition, the values of the Freundlich adsorption isotherm constant n were 3.39, 5.49, and 1.83 and 3.99, 5.49, and 7.61 for Pb2+, Cu2+, and Cd2+ on KC

adsorbate

max adsorption capacity (mg/g)

source

Pb2+

86.1

this work

Cu2+ Cd2+ Pb2+ Cu2+ Cd2+ Pb2+ Cu2+ Cd2+ Pb2+ Cd2+ Pb2+

16.5 22.1 36.7 15.6 27.4 9.32 31.7 4.48 39.0 41.0 40.1

Pb2+

46.2

Pb2+ Cu2+ Cu2+ Cd2+

447 425 313 286

32

46

21 42 46 47 48

capacity values in this work were 86.1 mg/g for Pb2+, 16.5 mg/g for Cu2+, and 22.1 mg/g for Cd2+. As presented in Table 3, some low-cost adsorbents (such as hydrogel,32 SBA-1521) showed relatively lower adsorption capacities, even after being modified by amino or magnetite (Fe3O4). At the same time, some polymer adsorbents, like chitosan/sulfydryl-graphene47 and polyaniline thiacalix[4]arene composite,48 exhibited G


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Figure 6. XPS spectra of MKC after adsorption: Pb2+ (a, b), Cu2+ (c, d), Cd2+ (e, f), N 1s (a, c, e), Pb 4f (b), Cu 2p (d), and Cd 3d (f).

adsorption capacities much higher than that of MKC, but their cost was higher too. Considering both the economical and practical sides, MKC has great potential for application in metal ion uptake from water. In addition, it is much easier to separate the adsorbent from solution using a magnet after adsorption. 3.5. Adsorption Mechanisms of MKC. Generally, the adsorption of a heavy metal on clay mineral occurs via ion exchange and electrostatic attraction.49 For kaolin clay, the absence of exchangeable ions between sheets and the presence of strong hydrogen bonds between galleries made it difficult for other molecules to enter into the interlayer. There existed lattice defect in the bottom of KC, and the fracture of alumina octahedral and silica tetrahedron made the functional group such as silanols and aluminols exposed at the edge of the sheets,10,49 which can interact with other target pollutants. Then, the adsorption of heavy metals on KC can be summed up as two mechanisms: (i) chemical bonding between heavy

metals and functional hydroxyl group on the surface of KC, forming the inner complexes,50 and (ii) electrostatic attraction between heavy metals and the negatively charged structure (or negatively charged −O− groups on the surface), forming the outer complexes.51 To further confirm the adsorption mechanisms of Pb2+, Cu2+, and Cd2+ by MKC, we conducted XPS measurement and zeta potential determination. Figure S2 and Figure 6 show the highresolution XPS (HRXPS) spectrum of full scan, N 1s, and the other three new peaks of the heavy metal. Compared with the N 1s spectrum in Figure 3e, new peaks were tested at relative higher binding energies after the adsorption of heavy metal ions, namely, peaks at 400.6 eV for Pb2+, 400.5 eV for Cu2+, and 400.3 eV for Cd2+. This could be caused by the formation of −NH2M2+ (M represents Pb, Cu, or Cd) complexes. Nonamino MKC had also been prepared and measured, but amino-functionalized H


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Industrial & Engineering Chemistry Research Scheme 1. Proposed Mechanisms for the Adsorption of Pb2+, Cu2+, and Cd2+ on MKC

Furthermore, Si−O− at the edge of KC could also attract heavy metal ions. The HRXPS survey of Al 2p, Si 2p, and Fe 2p after metal ion adsorption was also analyzed (Figure S4). The spectra for Al 2p and Si 2p did not change too much after adsorption, while the spectra of Fe 2p emerged another peak in the BE of 711.5 eV, namely, the formula of Fe(OH)O. This phenomenon indicated that during the adsorption process, Fe3O4 took part in it as well, with the change of pH values. In addition, O 1s XPS spectra did not clearly show any changes after metal ion adsorption, in which the BE of 531.5 eV represented AlO(OH) and 532.5 eV symbolized SiO2. It is likely that the contribution of Fen+/M2+−O interaction to the adsorption may mainly occur through a nonspecific physical interaction (electrostatic attraction). As a whole, the FTIR spectrum presented the bands of N−H stretching vibration (3437 cm−1) and N−H bending vibration (1633 cm−1). The kinetics fit the pseudo-second-order model rather than the pseudo-first-order and Elovich models, signifying that chemical adsorption influenced the rate-limiting step of the adsorption for KC and MKC. The values of RL in Langmuir and Es in Dubinin−Radushkevich isotherm models indicate that the adsorption of Pb2+, Cu2+, and Cd2+ was a monolayer adsorption to some degree, and it can be described as chemical adsorption, respectively. Above all, the adsorption of Pb2+, Cu2+, and Cd2+ can be reasonably speculated to occur in two ways: (1) complexation between functional groups −NH2, −OH on the surface of MKC, and metal ions and (2) electrostatic attraction between negatively charged MKC, Si−O− groups, and positively charged metal ions. The proposed adsorption mechanisms for Pb2+, Cu2+, and Cd2+ on MKC are illustrated in Scheme 1.

MKC presented adsorption capacity higher than that of both nonamino MKC and KC (data not shown). A lone pair of electrons in the N atom was given to the shared bond between nitrogen atom and M2+, and the electron cloud density of the nitrogen atom was reduced,52 resulting in the appearance of a higher binding energy peak. The XPS spectra of Pb 4f, Cu 2p, and Cd 3d were also measured, as shown in Figure 6. The species distribution of Pb ions in solution at pH < 6.0 was mainly Pb2+ (>98%). The binding energy for Pb 4f 7/2 was 138.3 eV, signifying the formation of PbCO3 for the similar BE of 139.0 eV. As for the spectra of Cu 2p, there emerged four new peaks after accurate positioning, and the binding energies were 952.5 and 955.7 for Cu 2p 1/2, 933.9 and 934.4 for Cu 2p 3/2, denoting a complexation of CuO or Cu(OH)2 according to NIST X-ray photoelectron spectroscopy database. Precipitation of Cd ions also starts to occur at about pH 8.96, with the transformation of species from Cd2+ to Cd+. The BE for Cd 3d 5/2 was 405.3 eV, which is similar to value reported in the studies of metal ions forming bidentate complexity on sulfhydryl containing adsorbents.53 Therefore, the XPS spectra of N 1s, Pb 4f, Cu 2p, and Cd 3d effectively confirmed the complexation mechanisms between heavy metals and functional groups on the surface of MKC. The zeta potential (pHpzc) of MKC before and after Pb2+, Cu2+, and Cd2+ adsorption are presented in Figure S3. The isoelectric point (pHzpc) of MKC determined by the pH location where zeta potential equals zero was 5.19, and the pH of Pb2+, Cu2+, and Cd2+ aqueous solution used for adsorption were 5.41, 5.73, and 7.34, respectively. After metal ion adsorption, all the pHzpc for Pb2+, Cu2+, and Cd2+ aqueous solution were almost 5.67. Therefore, the negatively charged surface of KC may react with positively charged metal ions. I


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4. CONCLUSIONS The amino-functionalized magnetite/kaolin clay was successfully prepared by simple solvothermal method for effective removal of Pb2+, Cu2+, and Cd2+ from aqueous solutions. XRD results certified the crystal structure of MKC. The SEM and TEM results further confirmed the existence of Fe3O4 on the surfaces of KC. XPS analysis proved the complexation between heavy metals and functional groups (−NH2, −OH) on the surface of KC. The adsorption kinetic data of Pb2+, Cu2+, and Cd2+ were well-represented with the pseudo-second-order kinetic model. The Es values in the Dubinin−Radushkevich equation indicated the adsorption was mainly chemical. The equilibrium isotherm data fit best to the Langmuir equation. The adsorption capacity of MKC was in the order Pb2+ > Cd2+ > Cu2+. In addition, XPS analysis speculated that the adsorption mechanisms of Pb2+, Cu2+, and Cd2+ could be a conjunction reaction of complexation and electrostatic attraction between heavy metal ions and functional groups. With external magnetic field, the mixed suspension of MKC and heavy metal ions can achieve quick separation. Because MKC had adsorption capacity higher than that of KC, is facile to prepare, and is easy to separate from water, the magnetite kaolin nanocomposite may provide a simple and effective treatment choice for the removal of heavy metals from contaminated water in industrial applications.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b00657. FTIR spectra of KC and MKC; XPS spectra and zeta potential of MKC before and after adsorption (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel: + 86-531-82767617. E-mail: [email protected], chm_ [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by the Natural Science Foundation of China (21577048, 21377046), 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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