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Highly Efficient Adsorption of Heavy Metals onto Novel Magnetic Porous Composites Modified with Amino Groups Chunrong Ren,† Xingeng Ding,*,†,‡ Wenqi Li,† Huating Wu,† and Hui Yang†,‡ †

Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China Zhejiang California International Nano Systems Institute, Hangzhou 310027, China

‡

S Supporting Information *

ABSTRACT: A novel porous magnetic composite CoFe2O4−SiO2 (CF−S) was prepared by a simple template method. After facile amino-functionalization, it exhibited bigger adsorption capacity and higher removal efficiency (>90%) for all the heavy metal ions (Cu(II), Mn(II), Cd(II), and Pb(II)) in single or mixed metal ions solution. The adsorption affinity for heavy metal ions was not much impacted by the presence of competitive ions. Moreover, effects of experimental parameters, including adsorbent dosage, solution pH, initial concentration, reaction temperature and time, were discussed. The isothermal adsorption results were well described by the Langmuir model, and the maximum adsorption capacities were 429.18 mg·g−1 (25 °C), 500.00 mg·g−1 (35 °C), 523.56 mg·g−1 (45 °C), and 555.51 mg·g−1 (55 °C) for Cu(II) ions. The adsorption kinetics fitted well with pseudo-second order kinetic model. Analysis of thermodynamic study for Cu(II) showed that the process of adsorption was spontaneous and endothermic in nature. The results indicated that the excellent adsorption was attributed to chemical adsorption through strong surface complexation, as well as the special structure of the CF−S−N composite.

1. INTRODUCTION Heavy metal pollutants cause a severe threat to human health and ecological systems.1−3 The effective techniques to eliminate heavy metals from wastewater are urgently required due to the toxicity associated with nonbiodegradability of heavy metal ions. Several methods can be used to remove heavy metals,4−6 such as chemical precipitation, membrane filtration, ionexchange, adsorption, and reverse osmosis. Among them, adsorption is a promising and economical strategy owing to its simplicity and high efficiency, and without yielding harmful byproducts.7−10 Therefore, it is important to discover a novel adsorbent with easy preparation, high yield, low cost, and great adsorption efficiency. The silicas and silica-based adsorbents with porous structure have been extensively investigated for removal of heavy metals.11−16 In the past decade, mesoporous silicas, such as MCM, KIT, SBA, etc., have been widely used as adsorbents for wastewater purification because of their high surface area, welldefined pores, nontoxicity, and biocompatibility. More importantly, the silicas are easy to be functionalized, giving rise to various silica-based composites. N-Propylsalicylaldiminofunctionalized SBA-15 mesoporous silica shows large adsorption capacity and high selectivity for copper ions.17 Aminofunctionalized mesoporous and nanomesoporous silica (MCM41) was applied for the removal of Ni(II), Cd(II), and Pb(II) ions from aqueous solution.18 Ligand modified mesoporous silica monoliths can be used effectively for sensitive and efficient Cu(II) sensing and removal in water and wastewater treatment.19 However, these adsorbents show separation © XXXX American Chemical Society

limitations from aqueous solution. To overcome this problem, recently a number of reports have been published about composites that combined magnetic properties and mesoporous silica structures.14−17,20−22 For example, Anbia et al.20 synthesized a novel modified magnetic mesoporous MCM-48 silica, which was used to rapidly remove Pb(II), Cu(II), Cr(VI), and Cd(II) metal ions from aqueous media. Yuan et al.21 fabricated a multifunctional microsphere with a large pore size mesoporous silica shell and a magnetic core (Fe3O4) to effectively adsorb and remove heavy metal ions. Barakat et al.22 proposed a porous magnetic silica composite (Fe3O4@nSiO2@ mSiO2) and studied the adsorption affinity for Pb(II) and Cd(II) ions in single and binary metal adsorption systems. Nevertheless, these composites were usually prepared in N2 atomosphere and other experimental conditions which might be costly. Cobalt ferrite (CoFe2O4) is a remarkable magnetic material, which has high chemical and corrosive stability, moderate saturation magnetization,10,23,24 easy preparation, and rapid separation.25,26 In this study, we synthesize a porous magnetic composite CoFe2O4−SiO2 (CF−S) with high yield, and it is functionalized by 3-aminopropyltriethoxysilane (APTES) to improve the adsorption affinity for heavy metal ions. As far as we know, the novel magnetic composite having both special structure and excellent adsorption property has not been Received: February 21, 2017 Accepted: April 19, 2017

A

DOI: 10.1021/acs.jced.7b00198 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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reported yet. The amino-functionalized CoFe 2 O 4 −SiO 2 composite (CF−S−N) exhibited excellent adsorption performance and was used as an adsorbent to remove heavy metal ions (Cu(II), Cd(II), Mn(II), and Pb(II)) from single and mixed heavy metals solutions. Effects of adsorbent dosage, solution pH, reaction temperature, reaction time, and initial concentration of Cu(II) on adsorption performance were examined. In addition, the adsorption isotherms, kinetics, and thermodynamics of the adsorbents were also investigated. Finally, the adsorption mechanism of CF−S−N was also illustrated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analysis. The novel magnetic composite shows a great application potential used as an excellent, economic adsorbent for the removal of heavy metal ions.

collected by a magnet, washed with deionized water to neutral (pH = 7), and dried under 80 °C for 12 h. Finally, the powder was sintered at 350 °C for 4 h to remove carbon and form stable magnetic porous composite (CF−S). Lastly, CF−S (2.0 g) was dispersed in 150 mL of o-xylene, and the mixture was stirred at 50 °C for 1 h. Subsequently, APTES (8 mL) was dropwise added to the above suspension. The mixture was refluxed at 100 °C for 24 h under mechanical stirring. The resulting product was isolated by magnetic separation, washed with ethanol, and dried in vacuum under 80 °C for 12 h. 2.4. Batch Adsorption Experiments. To test the heavy metal ions removal ability of CF−S−N, batch adsorption experiments were carried out to examine the effect of adsorbent dosage, solution pH, initial concentration, reaction time, and temperature on the adsorption performance. The adsorption of Cu(II) was studied using CF−S−N (5, 10, 15, 20, 30, 40, 50, 60, and 70 mg) in 50 mL aqueous solution of 80 mg·L−1 Cu(II). The pH values were adjusted from 4 to 7 using NaOH (0.1 M) or HCl (0.1 M). Meanwhile, the concentration of Cu (NO3)2 (80 mg·L−1) and adsorbent (400 mg·L−1) was kept constant. In the sorption isotherm experiments, the initial Cu(II) concentrations in the solutions varied from 20 to 320 mg·L−1, and the experiment was taken at pH 7 and 35 °C. To study the effect of reaction temperature, CF−S−N (20 mg) was added to 50 mL of 80 mg·L−1 Cu(NO3)2 solution (pH 7) at different contact temperature (25, 35, 45, and 55 °C). In the adsorption kinetic experiments, 20 mg of the CF−S−N was added into a flask (100 mL) containing 50 mL of Cu, Cd, Mn, and Pb solution with an initial metal concentration at 80 mg· L−1, and at an initial solution pH of ∼7. Moreover, ion competitive adsorption studies were performed by treating 50 mL of a mixed metal solution with equivalent concentrations (20 mg·L−1 of Cu(II), Cd(II), Mn(II), and Pb(II) ions) with 20 mg of adsorbent. Finally, the solution was separated by magnet, and the metal concentration was analyzed using AAS. All the sorption experiments were conducted in duplicate, and the mean values were reported. The removal efficiency (RE (%)) and the adsorption capacity (mg·g−1) of CF−S−N for heavy metal ions were calculated by the following equations:

2. EXPERIMENTAL SECTION 2.1. Chemicals. 3-Aminopropyltriethoxysilane (APTES) was obtained from Aladdin Chemical Reagent Co., Ltd. Fe(NO3)3·9H2O, Co(NO3)2·6H2O, Cu(NO3)2·3H2O, Cd(NO3)2·4H2O, Pb(NO3)2, Mn(NO3)2·4H2O, sodium hydroxide (NaOH), nitric acid (HNO3), polyethylene glycol (PEG), ammonium hydroxide (NH3·H2O, 25%), urea, formaldehyde, o-xylene, citric acid, ethanol, and tetraethyl orthosilicate (TEOS) were purchased from Sinopharm Chemical Reagent Co., Ltd. All reagents were analytical grade without further purification. Deionized water was used in all the experiments. 2.2. Instruments. The structure and morphology of CF−S, CF−S−N and CF−S−N−Cu were investigated by SEM (SU70). The distribution of element of CF−S−N was determined via energy-dispersive spectroscopy (EDS). The crystal structure of the samples was characterized by powder X-ray diffraction (XRD; Panalytical X’pert PRO) with Cu Kα radiation (λ = 1.5418 Å). The diffraction data were collected over the angle range 10−80° with a step size of 0.02°. Fourier transform infrared spectra (FT-IR) were recorded in KBr pellets with a Nicolet 5700 spectrometer. XPS data were obtained with an electron spectrometer (ESCALAB−250Xi) using monochromatized Al Kα radiation (hν = 1486.6 eV). The specific magnetization of the powders was also measured as a function of the applied magnetic field at room temperature using a vibrating-sample magnetometer (VSM; 7407). The concentrations of metal ions were determined by atomic adsorption spectroscopy (AAS; AAanlyst 800). 2.3. Synthesis of Amino-Functionalized Magnetic Composites (CF−S−N). First, CoFe2O4 NPs were synthesized via an improved reverse coprecipitation method, which was reported by our previous work.27,28 To obtain a stable magnetic fluid, the prepared magnetic NPs (2.0 g) were dispersed via sonication in 200 mL of citric acid solution (0.1 M). Citric acid-modified magnetic NPs were collected by a magnet, washed three times with ethanol, and were dispersed in deionized water (250 mL) by sonication for 2 h, and the obtained magnetic fluid was stored in a vial (500 mL) until further use. Second, 50 mL of tetraethyl orthosilicate (TEOS) was added into 100 mL mixed solution (V (ethanol/water) = 3:2), and then a certain amount of HNO3 (1.0 M) was added into the above solution drop by drop to promote the hydrolysis of TEOS. After SiO2 sol was obtained, 20 g of urea and 100 mL of magnetic fluid was dispersed in it sequentially by sonication for 3 h. Then, the solution pH was adjusted to ∼2, and formaldehyde (30 mL) was quickly added into the mixture and stirred mechanically for 24 h. The resulting powder was

REt (%) =

qt =

(C0 − Ct )V m

REe(%) =

qe =

C0 − Ct × 100 C0

(1)

(2)

C0 − Ce × 100 C0

(C0 − Ce)V m

(3)

(4) −1

where C0, Ct, and Ce (mg·L ) are the concentrations of ions initially, at time t, and equilibrium, respectively. REt (%) and REe (%) are the removal efficiency at time t and equilibrium, respectively. qt and qe are the adsorption capacity at time t and equilibrium, respectively. V (L) is the volume of the adsorbate, and m (g) is the mass of adsorbent.

3. RESULTS AND DISCUSSION 3.1. Characterization of CF−S−N. The morphologies of CF−S and CF−S−N were determined via SEM, and the results B



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characteristic peaks in 3428 cm−1 (−OH/H2O), 1633 cm−1(−OH), 793 cm−1 (Si−O−Si), 602 cm−1 (Fe−O), and 468 cm−1 (Si−O), three new peaks with wavenumber of 1569, 1498, and 702 cm−1 appear, corresponding to the bending vibrations of amino groups.21,29,30 In addition, the broad band near 3428 cm−1 refers to the vibration of the −OH groups, and the contribution of − NH2 to it also exists.31 Moreover, the free silanol group at 946 cm−1 disappears. A strong peak at 1086 cm−1 broadened and divided into two peaks at 1131 and 1045 cm−1, which is due to the overlapped stretching vibration of C− N32 and Si−O bonds, respectively. The results verified that the amino-group has been grafted onto CF−S. The microstructure of the CF−S−N composites was also ascertained by using the elemental mapping. EDS mapping images corresponding to the elemental distribution of C, Si, O, Fe, Co, and N are shown in Figure 3. The distributions of all the elements are relatively uniform. In contrast, the concentration of Fe and Co is lower than that of C, Si, O, and N. One it indicates that CoFe2O4 nanoparticles were located evenly in the interior of CF−S−N, another, it declares that the surface of CF−S was successfully amino-functionalized. XPS analysis had been performed to further understand the composition of the particle surface. The survey XPS spectra of the products are presented in Figure 4a. The peaks at 532.08, 397.07, 283.06, 153.12, and 103.08 eV are assigned to O 1s, N 1s, C 1s, Si 2s, and Si 2p, respectively. In the XPS spectrum of the CF−S−N composite, the intensity of peaks for N 1s and C 1s was increased apparently, confirming the deposition of APTES on the CF−S surface. High-resolution spectra of Si 2p and O 1s regions are shown in Figure 4 panels b and c, respectively. As shown in Figure 4b, Si 2p spectrum of CF−S was divided into three peaks positioned at 103.9, 103.4, and 102.8 eV, which could be assigned to Fe−O−Si, Si−OH, and O−Si−O. This result indicates that the surface Fe−OH groups within CoFe2O4 interacted with the surface Si−OH groups within SiO2 via Fe−O−Si covalent bonds in their surface. After amino-functionalization, the weak Fe−O−Si groups disappeared, Si−OH interacted with −CH3 from APTES, and Si−C bonds (101.3 eV) were formed. The Si 2p peak centered at 101.9 eV is typically assigned to C−Si−O groups within APTES. It is obvious that the surface of CF−S is likely to be amino-functionalized successfully. The results of the O 1s binding energy region can support the high-resolution XPS analysis of Si 2p described above. As shown in Figure 4c, for

are presented in Figure 1. As shown in Figure 1a,b, the surface of CF−S block is ravined and rough, and the inner part of CF−

Figure 1. SEM images of CF−S (a, b) and CF−S−N (c, d).

S is loose and rugged (inset of Figure 1a). According to N2 adsorption−desorption isotherms (Figure S1, Supporting Information), the relative pressure range is 0.5−0.9, suggesting the presence of mesopores. Also, the BET surface area of the CF−S composite is estimated at 167.513 m2·g−1. The porous structure would significantly increase the effective surface area and available amino-modified sites, and thus improve adsorption performance. After amino-functionalization, the surface of CF−S−N becomes smooth. It might be because the mesopore on the surface of CF−S is filled by APTES. Moreover, there exists some holes and small openings (Figure 1d), which may facilitate the pore diffusion during adsorption. X-ray diffraction patterns (Figure 2a) of all samples show that the diffraction peaks matched well with the Jade PDF card (01-1121) for CoFe2O4. The small broad peaks at 20°−28° in the XRD patterns of CF−S and CF−S−N indicate that SiO2 is amorphous. Furthermore, after being modified with APTES, the intensity of the amorphous SiO2 peak shows a slight increase (blue line). FTIR spectra of the three samples in the range of 4000−400 cm−1 are shown in Figure 2b. In the spectrum of CF−S−N, in addition of CoFe2O4 and SiO2

Figure 2. X-ray diffraction patterns (a) and FTIR spectra (b) of CF, CF−S, and CF−S−N. C



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Figure 3. EDS mapping results of CF−S−N.

Figure 4. Wide (a), Si 2p (b), and O 1s (c) XPS spectra of CF−S and CF−S−N.

35.742 and 28.834 emu·g−1 for CF−S and CF−S−N, respectively. The decreased SM is due to the increase of modified layer on the surface of CF−S. The inset displays the result of the colloidal stability of aqueous solution of CF−S−N. It is evident that CF−S−N powders can be dispersed in water to form a black dispersion and remain suspended in aqueous solution. Once a magnet is placed beside the vial, the CF−S−N could be moved quickly to the sidewall from the solution. 3.2. Effect of Experimental Parameters on the Adsorption of Heavy Metals onto CF−S−N. 3.2.1. Comparation of CF−S and CF−S−N on Adsorption. CF−S and CF−S−N (20 mg) were used to adsorb 50 mL of 80 mg·L−1 of Cu(II). From Figure 6a, it is obvious that CF−S−N shows relatively higher Cu(II) removal efficiency than the unmodified

CF−S composite, the O 1s peaks centered at 533.8, 533.2, 532.7, and 532.2 eV are assigned to Fe−O−Si (Fe−OH interacted with Si−OH), C−O (residual from process of preparation), O−Si−OH, and Si−O−Si (SiO2). For CF−S−N composite, the O 1s spectrum is also divided into four peaks positioned at 532.2, 531.8, 531.6, and 530.55 eV, which could be assigned to Si−O−Si (SiO2), Si−O−C (APTES), O−Si−C (O−Si−OH interacted with −CH3 from APTES) and −OH bonds, respectively. This was consistent with the results of XRD, FTIR, and EDS analysis. Magnetization curves, as shown in Figure 5, were measured on powder samples of CF−S and CF−S−N at room temperature. The samples exhibit small coercivity and remanence, and their saturation magnetizations (SM) are D



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number of adsorption sites, and further increase in the adsorbent dosage has no effect on the metal removal. The corresponding adsorption capacity of Cu(II) decreases with the adsorbent dose increasing. Hence it could be concluded the optimum dosage of CF−S−N composite for the Cu(II) removal is 400 mg·L−1. 3.2.3. Effect of Initial pH on Adsorption. It is well-known that solution pH is also an important parameter affecting the adsorption of heavy metal ions. The pH not only affects metal species in solution, but also influences the surface properties of the adsorbents. The influence of pH on the adsorption of Cu(II) by CF−S−N is shown in Figure 6c. The adsorption efficiency gradually increases from 20.5 to 98.72% when the solution pH value is increased from 3 to 7, and the largest removal is over 98% at pH = 7. Smaller adsorption at low pH can be due to the protonation of the amino groups, the number of binding sites available for Cu(II) ions uptake are then decreased. Further increasing pH, proton concentration is low, and more and more active amino groups on the surface of the adsorbent can complex with heavy metals ions. So, a neutral condition is favorable for heavy metal ions adsorption. 3.2.4. Effect of reaction temperature on adsorption. Figure 6d shows the effect of temperature on adsorption of Cu(II) by CF−S−N. At the beginning time, the adsorption rate improves by increasing the reaction temperature. The adsorption capacity increases from 196.08 mg·g−1 (25 °C) to 199.20 mg·g−1 (55 °C) (Figure S3), and the removal efficiency accordingly rises from 96.86% (25 °C) to 99.12% (55 °C). It can be explained

Figure 5. Magnetization curves of CF−S and CF−S−N.

composite (CF−S), although CF−S has a bigger surface area and mesoporous structure. The bigger adsorption capacity of Cu(II) on CF−S−N (197.44 mg·g−1) than CF−S (151.9 mg· g−1) was shown in Figure S2. It may be due to the chemical complexation between metal ion and −NH2 on the surface of CF−S−N. 3.2.2. Effect of Adsorbent Dosage. The adsorbent dosage is an important variable to evaluate the metal adsorption ability of an adsorbent. The plots of removal efficiency and adsorption capacity versus adsorbent dosage are shown in Figure 6b. The removal efficiency of Cu(II) ions increases with the increase of adsorbent dose, and reaches about 98% at the appropriate dose of 400 mg·L−1 of CF−S−N which could be due to greater

Figure 6. (a) The removal efficiency of CF−S and CF−S−N on the adsorption of Cu(II); effect of adsorbent dose (b), initial pH (c), and reaction temperature (d) on the adsorption of Cu(II) onto CF−S−N. E



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Figure 7. (a) Effect of initial concentration on the adsorption of Cu(II) by CF−S−N at different reaction temperatures. (b) Adsorption isotherms of Cu(II) on the CF−S−N composite (80 mg/L) at different initial concentrations (5−320 ppm). Fitting of isotherms data with (c) linear Langmuir models and (d) linear Freundlich models for the removal of Cu(II).

Table 1. Isotherm Adsorption Parameters Obtained Using Langmuir and Freundlich Models Langmuir model reaction temperature 25 35 45 55

°C °C °C °C

Freundlich model

2

qm

kl

R

429.18 500.00 524.16 555.55

0.3319 0.6042 0.7022 0.7563

0.99960 0.99989 0.99993 0.99988

RL

kf

n

R2

0.0093−0.1309 0.0051−0.0764 0.0044−0.0665 0.0041−0.0620

130.28 163.84 175.71 190.45

3.4385 3.2805 3.1957 3.1014

0.90590 0.83924 0.78064 0.83491

Freundlich equations expressed in eqs 5 and 6, respectively, are chosen to model these adsorption isotherm data.

that adsorbed ions move much faster at high temperature. Then, the contact area between adsorbent and metal ions increases, resulting in the increase of the adsorption capacity and removal efficiency of adsorbent for metal ions. 3.3. Adsorption Isotherms. Figure 7a shows the adsorption of Cu(II) on CF−S−N as a function of its initial concentration (from 20 to 320 mg·L−1). As the concentration increases, the adsorption capacity of CF−S−N shows a continuous improvement. When the initial concentration is increased to ∼240 mg·L−1, there is almost no further capacity enhancement. This fact indicates that Cu(II) ions fill the surface space of CF−S−N at the critical concentration. The maximum adsorption capacity for Cu(II) was 424.80 mg·g−1 (25 °C), 497.15 mg·g−1 (35 °C), 520.33 mg·g−1 (45 °C), and 552.13 mg·g−1 (55 °C), respectively. The analysis of the isotherm data is important to describe how the adsorbates interact with adsorbents. The Langmuir and

Ce C 1 = e + qe qm klqm log(qe) = RL =

1 log(Ce) + log(k f ) n

1 1 + klC0

(5)

(6)

(7)

where qe (mg·g−1) is the adsorption capacity at equilibrium, C0 and Ce (mg·L−1) are the initial and equilibrium concentration of the heavy metals, respectively. qm (mg·g−1) is the Langmuir monolayer adsorption capacity. kl (L·mg−1) is the Langmuir constant. kf (mg·g−1) is the Freundlich parameter, and 1/n is F



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parameters (ΔG°, ΔH°, and ΔS°) are shown in Table S1. The value of ΔH° (38.372 kJ·mol−1) is positive and between 30 and 70 kJ·mol−1, which indicates that the adsorption process of Cu(II) on CF−S−N nanocomposite is endothermically driven and constitutes chemical adsorption. In addition, the value of ΔS° is also positive due to the complexation of amino groups on CF−S−N with more mobile metal ions, which would cause an increase in the entropy during the adsorption process. Furthermore, ΔG° at 298, 308, 318, and 328 K is −4.3612, −5.7952, −7.2292, and −8.6632 kJ·mol−1, respectively. The negative value of ΔG° indicates the adsorption process is spontaneous. It also reveals the fact that a more negative ΔG° reflects a greater driving force of adsorption, resulting in faster adsorption rate and higher adsorption capacity. 3.5. Adsorption Kinetics. 3.5.1. Adsorption in Single Solutions. The contact time plays a vital role in the adsorption process. Figure 9a shows the adsorption kinetics of CF−S−N toward Cu(II), Cd(II), Mn(II), and Pb(II) (initial concentration of 80 mg·L−1 for each) in single solution, respectively. The results reveal that the adsorption of the four metals is accompanied by a two-stage kinetic behavior: the initial rapid adsorption within the first 20−50 min, and a second stage with a much lower adsorption rate during 30−150 min. This can be related to fast diffusion of M2+ ions from the solution to the adsorbent surfaces with a large number of vacant mesoporegrafted amino sites, which are available for adsorption of heavy metal ion during the initial adsorption stage. The later low removal efficiency corresponds to the slow diffusion of heavy metal ions. It is attributed to the longer diffusion range of M2+ ions from the adsorbent surfaces to the inner of CF−S−N.35,36 After that, the uptake of the heavy metal ions remains almost unchanged with increasing contact time, which indicates that all adsorption sites have been almost saturated. The fast adsorption equilibrium could provide high efficiency and low cost for a real-world water treatment system. In addition, the order of the adsorption rates was Pb(II) > Cu(II) > Cd(II) > Mn(II), indicating the selective adsorption of CF−S−N. Dynamic models are of great significance to well-study the adsorption kinetic mechanism. In general, two mathematical models are used most commonly, Lagergren pseudo-first-order equation and pseudo-second-order equation, as shown in eqs 11 and 12, respectively.

the adsorption intensity. RL is a contant separation factor and calculated using eq 7. The adsorption isotherms of Cu(II) on the CF−S−N composite at different reaction temperatures are given in Figure 7b. The Langmuir and Freundlich isotherm parameters, calculated from the linear plots (Figure 7c,d), are given in Table 1. The high R2 value of the Langmuir isotherms (>99%) suggests that the adsorption performance of Cu(II) onto the CF−S−N composite could be explained reasonably by the Langmuir model. The Langmuir adsorption model is based on the assumptions that the adsorption takes place at specific homogeneous surface sites within the adsorbent as well as monolayer adsorption.33,34 Therefore, the adsorptions of heavy metal ions onto CF−S−N could be regarded as monolayer adsorption processes. The calculated maximum adsorption capacity for Cu(II) is 429.18 mg·g−1 (25 °C), 500.00 mg·g−1 (35 °C), 523.56 mg·g−1 (45 °C), and 555.51 mg·g−1 (55 °C), respectively, which approximates to the experimental values. The essential characteristic of the Langmuir isotherm can be expressed in terms of a constant separation factor RL. The value of RL indicates the shape of the isotherm to be either unfavorable (RL > 1), linear (RL = 1), favorable reaction (0 < RL < 1), or irreversible case (RL = 0). All the values of RL for the adsorbent are between 0 and 1 (Table 1), confirming that the adsorption is a favorable process. 3.4. Adsorption Thermodynamics. Thermodynamic parameters were estimated to investigate in-depth information on inherent energetic changes that are associated with adsorption. The Gibbs energy (ΔG°), enthalpy (ΔH°), and entropy (ΔS°) changes can be calculated from the following equations: q Kd = e Ce (8)

ΔS° ΔH ° − R RT

(9)

ΔG° = ΔH ° − T ΔS°

(10)

ln Kd =

where Kd is the distribution coefficient, T (K) is the absolute temperature, and R is the universal gas constant (8.314 J·mol−1· K−1). The value of ΔH° (kJ·mol−1) and ΔS° (kJ·mol−1) can be calculated from the slope and intercept of the plot of ln Kd versus 1/T, as shown in Figure 8. The thermodynamic

⎛ k ⎞ log(qe − qt ) = log(qe) − ⎜ 1 ⎟ ⎝ 2.303 ⎠

(11)

t 1 t = + 2 qt qe k 2qe

(12)

where k1 is the rate constant of pseudo-first-order adsorption (min−1), k2 is the rate constant of pseudo-second-order adsorption (g/mg/min), t is the adsorption time (min), qe and qt (mg·g−1) are adsorption capacity at equilibrium and time t, respectively. The linear form of pseudo-first-order and pseudo-secondorder kinetics is respectively shown in Figure 9 panels b and c, and the corresponding kinetic adsorption parameters are listed in Table 2. As seen in Table 2, compared with the correlation coefficient R2 of the two models, the value of R2 of pseudosecond-order model is more close to 1, suggesting that the heavy metal ions adsorption onto CF−S−N was controlled by a chemical process,37,38 which is further supported by the adsorption thermodynamic. Furthermore, as shown in Table

Figure 8. Linear dependence of ln Kd on 1/T based on the adsorption thermodynamic. G



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Figure 9. (a) Adsorption kinetics, (b) pseudo-first-kinetic and (c) pseudo-second-kinetic of Cu(II), Cd(II), Mn(II), and Pb(II) on CF−S−N, each metal separately (80 ppm); (d) adsorption kinetics, (e) pseudo-first-kinetic and (f) pseudo-second-kinetic of the mixture of the four metals on CF− S−N (20 ppm per metal).

Table 2. Kinetic Adsorption Parameters Obtained Using Pseudo-first-order and Pseudo-second-order Models; Metals in Single and Mixed Solution Are Marked as M (II)S and M (II)M, Respectively pseudo-first-order

pseudo-second-order

heavy metal

Re (%)

qe

k1

qe,cal

R2

k2

qe,cal

R2

Cu(II)S Cd(II)S Mn(II)S Pb(II)S Cu(II)M Cd(II)M Mn(II)M Pb(II)M

98.72 93.72 97.67 91.63 97.15 91.82 95.59 89.83

197.44 187.43 195.33 183.26 48.57 45.91 47.80 44.91

0.0300 0.0568 0.0292 0.0195 0.0255 0.0152 0.0351 0.0179

44.99 161.59 60.28 10.99 11.61 18.34 99.32 6.37

0.97766 0.45979 0.98379 0.95886 0.90496 0.81147 0.89194 0.79677

0.00130 0.00229 0.00856 0.00404 0.003459 0.000984 0.000179 0.004304

198.41 187.97 197.24 183.49 48.99 47.17 54.98 45.21

0.99999 0.99999 0.99998 1.00000 0.99997 0.99973 0.97001 0.99993

mg·g−1 (Pb(II)), respectively. There exists a bit of difference with the calculated values of the equilibrium adsorption capacity (qe, cal) according the pseudo-second-order model

2 and Figure S4a, the adsorption capacity calculated using the data rooting in the experiment, is 197.44 mg·g−1 (Cu(II)), 195.33 mg·g−1 (Mn(II)), 187.43 mg·g−1 (Cd(II)), and 183.26 H



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Figure 10. (a, b) SEM image of CF−S−N−Cu, (c) XPS survey scans, and (d) XPS spectra of N 1s region of CF−S−N and CF−S−N−Cu.

(Table 2). The adsorption capacity of Cu(II), Mn(II), Cd(II), and Pb(II) on CF−S−N is different, although they are all divalent cations. It may be affected by many factors, such as ionic and hydrated ionic radius of metal ions, their hydration energies or charge. The ionic radius of the four metal ions is Cu(II) < Mn(II) < Cd(II) < Pb(II), which is inversely proportional to the adsorption capacity. Therefore, ionic radius of metal ions may be the main influence on the different adsorption of Cu(II), Mn(II), Cd(II), and Pb(II). 3.5.2. Adsorption in Mixed Solutions. In wastewater, there always exists some other heavy metal ions. To test the binding of each metal ion in the presence of others, synthetic solutions mixed with the four metal ions (20 ppm per metal) were tested. Results are gathered in Figure 9d, Figure S4b, and Table 2, and the corresponding pseudo-first-kinetic and pseudo-secondkinetic of the mixture of the four metals on CF−S−N is shown in Figure 9 panels e and f, respectively. The adsorption rates increase in the following order: Pb(II) > Cu(II) > Cd(II) > Mn(II). The removal efficiency and adsorption capacities decrease in the order Pb(II) < Cd(II) < Mn(II) < Cu(II). It shows the same rules with the adsorption in single solution. In other words, the adsorption of the four metals in the mixture is relatively unaffected by other metal ions. 3.6. Adsorption Mechanism. To confirm the mechanism of the CF−S−N for adsorption of M2+ ions in aqueous solutions, we chose Cu(II) adsorption as a representative process, measuring the SEM of the CF−S−N composite after adsorption of Cu(II). According to Figure 10, the surface of CF−S−N−Cu is porous and connected, which is different with the CF−S−N before adsorption (Figure 1 panels c and d). As shown in Figure 11, we postulate that once the Cu(II) is transferred on to the CF−S−N, complexation begins at the interface between the CF−S−N surface and the Cu(II) aqueous solution. After the amino sites on the surface are

Figure 11. Adsorption mechanism of CF−S−N composite.

occupied, Cu(II) ions traverse farther and deeper into the inner parts to interact with the amino groups. Thus, pore diffusion may happen during the adsorption process. XPS was also employed to determine the surface composition of CF−S−N after adsorbing Cu(II) ions. As shown in Figure 10c, Cu 2p appears in CF−S−N-Cu, which gives direct evidence of adsorption of Cu(II) on the surface of the CF−S−N. The high-resolution N 1s XPS spectra of CF− S−N and CF−S−N-Cu are recorded and presented in Figure 10d. Compared with the spectrum of adsorbent, N 1s line of Cu-loaded adsorbent shifts toward higher binding energy, which is due to the complexation between amino groups on the CF−S−N and Cu(II) ions. The same results are obtained in other heavy metal ions.

4. CONCLUSION In summary, a novel amino-functionalized CoFe2O4−SiO2 (CF−S−N) composite with high yield was synthesized for the adsorptive removal of heavy metals from aqueous solutions. I



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The composite showed big adsorption capacity and high removal efficiency for all heavy metal ions in single metal ions solutions (Cu(II), 197.44 mg·g−1, 98.72%; Mn(II), 195.22 mg· g−1, 97.67%; Cd(II), 187.43 mg·g−1, 93.72%; Pb(II), 183.26 mg·g−1, 91.63%), resulting from complexation of the metal ions by amino groups. The adsorption performance of CF−S−N in mixed metal ions solution shows that the adsorption affinity for the heavy metal ions is not much impacted by the presence of competitive ions. The maximum adsorption capacity was obtained at pH 7.0 and the optimum dosage of CF−S−N was 400 mg·L−1. The adsorption process was well described by the Langmuir model and pseudo-second-order kinetics model, respectively. Thermodynamic studies illustrated that the adsorption process of Cu(II) with CF−S−N was spontaneous and endothermic in nature. The excellent adsorption was attributed to chemical adsorption through strong surface complexation, as well as the special structure of CF−S−N nanocomposite. This finding demonstrates the potential of the magnetic nanocomposite for effective removal of heavy metal ions from aqueous solutions.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.7b00198. BET analysis; figures showing the comparison of uptake capacity of CF−S and CF−S−N on the adsorption of Cu(II) and adsorption capacity of Cu(II) by CF−S−N at different reaction temperatures; measured the adsorption of four metal ions (Cu(II), Cd(II), Mn(II) and Pb(II)) on CF−S−N in a single metal ions solution and in a mixed metal ions solution; providing the thermodynamic parameters for the adsorption of Cu(II) on CF−S−N magnetic composite (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Funding

This work was supported by the National High-Tech Research and Development Program of China (863 Program) (No. 2015AA034701) and the Fundamental Research Funds for the Central Universities (No. 2008QNA4008). Notes

The authors declare no competing financial interest.

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