# Fe3O4 Nanoparticles Embedded Sodium Alginate/PVP/Calcium Gel Composite for Removal of Cd2+

Authors: Long Jiao; Peishi Qi; Yunzhi Liu; Bo Wang; Lili Shan
Journal: Journal of Nanomaterials
 (2015)

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## Abstract

Magnetic ferroferric oxide (Fe3O4) nanoparticles embedded sodium alginate (SA)/polyvinylpyrrolidone (PVP)/calciumgel composites (MSPC) were synthesized for the removal of Cd2+. The physicochemical properties of the composite gel ball were characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), power X-ray diffraction patterns (XRD), and vibrating sample magnetometer (VSM). Quantitative adsorptions experiments were performed in view of sorbent dosage, pH, contact time, and temperature. Results showed that the optimum quantity of particles was 1.2 g·L−1, when the Cd2+ concentration was fixed at 150 mg/L. The optimum pH for adsorption is 6.2. When the initial Cd2+ concentration was from 50 to 200 mg·L−1, the adsorption equilibrium time was 80–120 minutes. The isotherms results showed the maximum sorption was 97.8 mg/g calculated from the Langmuir isotherm. The pseudo-second-order kinetic model could be well described by the sorption of Cd2+ in the sediment. The correlation coefficients (R2) were all higher than 0.96. Results showed that MSPC was a practical and low-cost material for heavy metal removal from the river sediment and simultaneously solves the problems in convenient separation under additional magnetic field.


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## Body

## 1. Introduction

Nowadays, river sediment contamination by heavy metals has become an important problem for environmental considerations. Emission of the wastewater effluents containing heavy metal ions has significantly toxic influence on human beings and ecological systems [1, 2]. The heavy metal ions were deposited in river sediment and then released with the change of the surrounding environmental conditions. The situ and ex situ remediation are traditional improvement for heavy metal pollution. These two methods are based on physical, chemical, and biological treatments in the process of ecological restoration.In the traditional remediation, conventional chemical methods contain extraction and filtration [3], redox, fixed adsorption, ion exchange, and electrodialysis [4]. Among these methods, fixed adsorbent has been accepted for the removal of low content of heavy metals ions [5]. Numerous adsorbents, such as activated carbon, minerals, calcium nitrate, resins, and aluminum hydroxide, were available for heavy metal removal and recovery. Adsorbents can efficiently decrease the content of heavy metals complexed by fixed site. Some of these methods can obtain good separation efficiency for contaminated sediment when metal ions are at high concentration levels. Magnetic techniques and materials using Fe3O4 nanoparticles for the removal of heavy metals have been reported [6, 7]. Fe3O4 particles that possess superparamagnetic feature [8], convenient separation characteristics, and independent recycling properties under a magnetic field have been increasingly used in the fields of bioscience and medicine [9, 10]. Therefore, immobilization of Fe3O4 into some biological polymer materials seems to be a promising alternative [11].As a kind of natural polysaccharide, sodium alginate (SA) has the stability, solubility, viscosity, and security properties. Its unique biocompatibility, biodegradability, immunogenicity, nontoxicity, and abundant carboxyl groups in the network make alginate beads an effective candidate for pollutant removal [12, 13]. The synthesis of alginate-goethite beads and the application in Cr(VI) and Cr(III) removal have been reported [14, 15]. Immobilization of Fe3O4 nanoparticles into polymeric biomaterials, especially by using polysaccharide and crosslinking agent, was studied [16].Immobilization of magnetic Fe3O4 nanoparticles within biological materials, such as agar, agarose, chitosan, collagen, and calcium [17, 18], is an essential step for industrial scale-up of biomass sorption by providing adequate size, density, and mechanical strength required by continuous systems [19]. Alginate can form a reticulated structure when it contacts with calcium and polymer, and then it has been used to immobilize activated carbon and metal hydroxides for removal of organic compounds [20].This work aims to immobilize magnetic Fe3O4 nanoparticles into sodium alginate (SA)/polyvinylpyrrolidone (PVP)/calcium gel composite (MSPC) for removal of Cd2+ in the river sediment. The MSPC was characterized using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction patterns (XRD), and vibrating sample magnetometer (VSM). The effects of pH and contact time on adsorption capacity were tested, and the isotherms and kinetics data were obtained by the experiment.


## 2. Materials and Methods

### 2.1. Materials and Instruments

Sodium alginate (biochemical grade) and polyvinylpyrrolidone (a molecular weight of ca. 1,300,000, chemical grade) were purchased from Sinopharm Chemical Reagent Co. Ltd, China. Magnetic Fe3O4 nanoparticles were prepared according to the reported procedure [21]. Calcium nitrate anhydrous (CaNO3) was served as the cross-linker for PVP and SA. Cadmium acetate dihydrate (C4H6CdO4·2H2O) and ammonium hydroxide (NH3·H2O) were from Aladdin Co. Ltd. All purchased chemicals were received without any purification.The sediment was obtained from Songhua River by heavy metals pollution in Harbin China. In this study, the objective of main heavy metal was Cd2+. So cadmium acetate dihydrate (C4H6CdO4·2H2O) was added to the river sediment in order to reach different experiment concentrations of Cd2+ ions. As listed in Table 1, other metal ions content was so low that cannot affect the overall removal efficiency.Table 1
The concentration of metal ions from the river sediment.
Sediment
Cd
As
Cr
Cu
Pb
Ni
Sample 1
0.125
—
0.723
0.307
0.916
0.382
Sample 2
0.036
—
1.096
0.632
0.41
0.556
Sample 3
0.034
—
1.079
0.614
0.415
0.561Scanning electron microscopy (SEM, HITACHI S-4800) was employed to analyze the surface morphologies of the products and the elemental composition. Energy-dispersive X-ray spectroscopy (EDX, Quanta-200, USA) was used to characterize the samples after gold plating at an accelerating voltage of 20 kV. X-ray diffraction patterns (XRD, D/Max-2550pc, Rigaku inc., Japan) were used with Cu-Kαradiation (λ = 0.1542 nm). The magnetic properties were assessed with VSM (JDM-14D, Jilin University, China). All the wastewater and sediment parameters were determined by inductively coupled plasma mass spectrometry (ICP-MS, Thermo Electron X Series II ICP apparatus).


### 2.2. Preparation of Sorbent

Adsorption of particles preparation process is performed as follows.2 g SA (dry weight) was dissolved in 100 mL water and 0.5 g PVP was added to the sodium alginate solution and stirred for 0.5 h at 25°C. Then the magnetic Fe3O4 particles were added into SA/PVP mixture and stirred until completely mixed. Subsequently, the blended mixture was added to 20 mL Ca(NO3)2 (20 g/L) and dried in a vacuum oven for 24 h at 60°C. Finally, MSPC was obtained and stored at a sealed bottle for further use, and preparation of sodium alginate/PVP/calcium gel composite (SPC) was same to MSPC except for adding Fe3O4 particles.


### 2.3. Adsorption Experiments

All the following adsorption experiments were performed by a batch method in which the experimental conditions, the ratio of the mud mixture containing Cd2+, and MSPC were prepared according to their different needs.The influence of pH ranging from 1 to 7.5 was examined. The pH values of the solutions were adjusted by 0.1 M HCl or 0.1 M NaOH. The optimized pH for the adsorption derived from the experimental results was adopted for all the experiments. Adsorption kinetics and the effect of contact time were investigated at three temperatures (293 K, 303 K, and 313 K). The MSPC were put into 100 mL of Cd2+ solution (50–200 mg/L) at 293 K at pH value of 6.2, and the concentration of Cd2+ in the solution was detected at fixed time.MSPC adsorption capacityQe and pollutant removal rate R(%) were calculated by the following formula:(1)Qe=C0-CeVW,R%=C0-CeC0×100%,where C0 (mg/L) is the initial concentration of Cd2+, Ce (mg/L) is the concentration of Cd2+ at equilibrium, W (g) is the mass of the adsorbent (dry), and V (L) is the volume of the solution.


## 2.1. Materials and Instruments

Sodium alginate (biochemical grade) and polyvinylpyrrolidone (a molecular weight of ca. 1,300,000, chemical grade) were purchased from Sinopharm Chemical Reagent Co. Ltd, China. Magnetic Fe3O4 nanoparticles were prepared according to the reported procedure [21]. Calcium nitrate anhydrous (CaNO3) was served as the cross-linker for PVP and SA. Cadmium acetate dihydrate (C4H6CdO4·2H2O) and ammonium hydroxide (NH3·H2O) were from Aladdin Co. Ltd. All purchased chemicals were received without any purification.The sediment was obtained from Songhua River by heavy metals pollution in Harbin China. In this study, the objective of main heavy metal was Cd2+. So cadmium acetate dihydrate (C4H6CdO4·2H2O) was added to the river sediment in order to reach different experiment concentrations of Cd2+ ions. As listed in Table 1, other metal ions content was so low that cannot affect the overall removal efficiency.Table 1
The concentration of metal ions from the river sediment.
Sediment
Cd
As
Cr
Cu
Pb
Ni
Sample 1
0.125
—
0.723
0.307
0.916
0.382
Sample 2
0.036
—
1.096
0.632
0.41
0.556
Sample 3
0.034
—
1.079
0.614
0.415
0.561Scanning electron microscopy (SEM, HITACHI S-4800) was employed to analyze the surface morphologies of the products and the elemental composition. Energy-dispersive X-ray spectroscopy (EDX, Quanta-200, USA) was used to characterize the samples after gold plating at an accelerating voltage of 20 kV. X-ray diffraction patterns (XRD, D/Max-2550pc, Rigaku inc., Japan) were used with Cu-Kαradiation (λ = 0.1542 nm). The magnetic properties were assessed with VSM (JDM-14D, Jilin University, China). All the wastewater and sediment parameters were determined by inductively coupled plasma mass spectrometry (ICP-MS, Thermo Electron X Series II ICP apparatus).


## 2.2. Preparation of Sorbent

Adsorption of particles preparation process is performed as follows.2 g SA (dry weight) was dissolved in 100 mL water and 0.5 g PVP was added to the sodium alginate solution and stirred for 0.5 h at 25°C. Then the magnetic Fe3O4 particles were added into SA/PVP mixture and stirred until completely mixed. Subsequently, the blended mixture was added to 20 mL Ca(NO3)2 (20 g/L) and dried in a vacuum oven for 24 h at 60°C. Finally, MSPC was obtained and stored at a sealed bottle for further use, and preparation of sodium alginate/PVP/calcium gel composite (SPC) was same to MSPC except for adding Fe3O4 particles.


## 2.3. Adsorption Experiments

All the following adsorption experiments were performed by a batch method in which the experimental conditions, the ratio of the mud mixture containing Cd2+, and MSPC were prepared according to their different needs.The influence of pH ranging from 1 to 7.5 was examined. The pH values of the solutions were adjusted by 0.1 M HCl or 0.1 M NaOH. The optimized pH for the adsorption derived from the experimental results was adopted for all the experiments. Adsorption kinetics and the effect of contact time were investigated at three temperatures (293 K, 303 K, and 313 K). The MSPC were put into 100 mL of Cd2+ solution (50–200 mg/L) at 293 K at pH value of 6.2, and the concentration of Cd2+ in the solution was detected at fixed time.MSPC adsorption capacityQe and pollutant removal rate R(%) were calculated by the following formula:(1)Qe=C0-CeVW,R%=C0-CeC0×100%,where C0 (mg/L) is the initial concentration of Cd2+, Ce (mg/L) is the concentration of Cd2+ at equilibrium, W (g) is the mass of the adsorbent (dry), and V (L) is the volume of the solution.


## 3. Results and Discussion

### 3.1. Particle Size and Structure of the MSPC

Figure1 shows the photographs of MSPC. Immobilization of Fe3O4 particles made MSPC gel ball partly black in Figure 1(a). The size of the synthesized gel balls is asymmetric with a mean diameter of 4.0 mm. Figure 1(b) shows the MSPC gel balls were absorbed by the super magnet. And the MSPC gel balls were agglomerate material synthesized by immobilization. After that, the MSPC gel balls were put into the sediment in Figures 1(c) and 1(d).Figure 1
Photographs of MSPC (a, b) and photographs of MSPC mixed in the sediment (c, d).
(a)
(b)
(c)
(d)The SEM images of the inner structure of the SPC and MSPC are given in Figures2(a) and 2(b), respectively. As shown in Figure 2(a), the sodium alginate surface was loose and smooth. Meanwhile, lots of tiny interspace structures can be adsorbed easily by the adsorbents [22]. SEM images in Figure 2(b) show the inner structure of the MSPC where porous microstructure and Fe3O4 nanoparticles spread over their surface. Figures 2(c) and 2(d) present the SEM image of SPC and MSPC that have absorbed Cd2+, the differences are more obvious, and Fe3O4 nanoparticles are visible on the MSPC surface.Figure 2
SEM image of material surface: SPC (a), MSPC (b), SPC after absorption (c), and MSPC after absorption (d).
(a)
(b)
(c)
(d)The EDX spectra are given in Figure3. The signal of Au could be due to the gold plating for measurement. In Figure 3, carbon, oxygen, sodium, chlorine, and calcium elements are detected for SPC, which are the basic components of SPC, while for MSPC, a large amount of iron element has been detected, confirming that Fe3O4 was on the adsorbent surface and had been embedded by sodium alginate and PVP. And there are small amounts of aluminum and silicon in the Fe3O4. The presence of these elements does not affect the adsorbent. Thus, a kind of adsorption characteristic composite has been achieved in this work.Figure 3
EDX spectra of the SPC gel balls (a) and the MSPC gel balls (b).
(a)
(b)


### 3.2. XRD Analysis

Figure4 showed the XRD pattern of MSPC. Six characteristic peaks at 30.5(220), 35.6(311), 43.3(400), 53.7(422), 57.3(511), and 62.8(440) were corresponded well to the planes of spinel structured Fe3O4. However, the intensities were much lower than those of peaks in the database, which could be explained by the cross-link reaction between Fe3O4 and the sodium alginate, while the reaction process did not give rise to the phase change of Fe3O4, which was well consistent with the literature [20].Figure 4
XRD pattern of the MSPC gel balls.


### 3.3. Magnetism Analysis

Magnetic properties of MSPC were characterized by measuring the hysteresis and remanence curves, as shown in Figure5. The shape of the magnetization curves was smooth and parabolic, and the saturation magnetization (Ms) was measured to be 7.92 emu/g. The MSPC exhibited nearly super-paramagnetic properties. It indicated the MSPC was characteristic of magnetic response under the added magnetic field, which made it easy to remove metal ions absorbed by the MSPC.Figure 5
Magnetic hysteresis loop of the MSPC.


### 3.4. Effect of Sorbent Dosage

The adsorption capacity of MSPC and the removal rate of Cd2+ could be affected by the sorbent dosage. Figure 6 shows the effect of MSPC dosage ranging from 0.2 g to 2.0 g on adsorption capacity. The concentration Cd2+ of 150 mg/L could be representative of the high levels pollution of heavy metals in the river sediment. As shown in Figure 6, the removal rate of Cd2+ increases with the concentration of MSPC. However, the adsorption capacity of MSPC is close to the saturation at the same situations. Keeping Cd2+ concentration of 150 mg/L, adsorbent adsorption capacity changes a little at the range of 80–100 mg/g. It showed that adapted holding MSPC mass content was positive for Cd2+ removal. As seen from Figure 6(b), the optimal quantity of particles is 1.2 g/L, in which the removal efficiency of Cd2+ (100 mL, 150 mg/L) in the river sediment is about 87%.Figure 6
Effect of sorbent dosage on theQe (a) and sorbent dosage on the removal efficiency of Cd2+ (b).
(a)
(b)


### 3.5. Effect of pH Value

pH value, which affects the formation distribution of heavy metals in sediment solution by influencing the other components of sediment the same time, also indirectly affects the removal efficiency of the heavy metals. The effect of pH on the adsorption properties is shown in Figure7. Clearly, the adsorption capacity was enhanced quickly with pH increase from 1 to 2 but slowly with pH from 2 to 6. At pH value of 6.2, maximum adsorption quantity is obtained. When the pH value exceeds 7, the uptake decreases because the cadmium ions start to precipitate as Cd(OH)2, which is also a kind of sediment. At lower pH values, the functional groups for coordination including H+ and Cd2+ groups compete for adsorption sites, which do not favor the adsorption. When the solution pH values is increased, the competitive adsorption is reduced, so more negatively charged adsorption sites can positively charge the Cd2+.Figure 7
Effect of pH value on the adsorption of Cd2+ by MSPC.


### 3.6. Effect of Contact Time

The effect of contact time on the adsorption process is shown in Figure8. With the increase of contact time, the adsorption of volume cadmium ions is increased. When the adsorption time is 120 min, adsorption capacity reaches the maximum. When the initial concentration of Cd2+ is 50 and 100 mg/L, adsorption equilibrium time is 80–90 min. SA and Fe3O4 nanoparticles form a mesh with granular structure through the macromolecular cross-linking together. The initial concentration of Cd2+ is changed to 150 and 200 mg/L, and adsorption equilibrium time is 90–120 min. When the active sites are limited, more competition Cd2+ ions limit combining site resulted in the added adsorption equilibrium time. Therefore, the MSPC with open structure and three-dimensional polymer network reinforce Cd2+ adsorption into active sites, which could increase the adsorption rate and adsorption capacity.Figure 8
Effect of contact time on the adsorption of Cd2+ by MSPC.


### 3.7. Sorption Isotherms

Langmuir, Freundlich, and D-R isotherm models were employed for adsorption data correlation to quantify the adsorption capacity of MSPC in Figure9. The linear forms of these three models are listed as follows [23, 24]:(2)Langmuir  model:CeQe=1BQm+CeQm,Freundlich  model:ln⁡⁡Qe=ln⁡⁡Kf+1nln⁡⁡Ce,D-R  model:ln⁡⁡Qe=ln⁡⁡Qm+Kε2,ε=RTln⁡⁡1+ln⁡⁡Ce,E=12K,where Qe (mg/g) is the amount of adsorbed Cd2+ at equilibrium and Ce (mg/L) is the concentration of Cd2+ at equilibrium. Kf and n are associated with the adsorption capacity and adsorption intensity, respectively. B (L/mg) is the Langmuir constants. ε is polanyi potential; E (kJ/mol) is the mean free energy of adsorption.Figure 9
Sorption isotherms fit of Cd2+: Langmuir isotherm fit (a), Freundlich isotherm fit (b), and D-R isotherm fit (c).
(a)
(b)
(c)As listed in Table2, because of the relatively high correlation parameters, the experimental data can be well described by the three models, in which the Langmuir model gives the best fit (R2>0.96). Freundlich parameter, the Kf value, supports the order of cadmium uptake that is 293 K > 303 K > 313 K, which was confirmed by the corresponding 1/n values. Langmuir isotherm model is based on the assumption that the dynamic adsorption-desorption processes occur on homogeneous surface, taking no account of the interaction between the MSPC and the solute in Figure 9(a). The maximum sorption calculated from the Langmuir isotherm is 97.8 mg/g. It is slightly greater than the experimental values regarding that there are still unoccupied sorption sites. Obviously, plots of ln⁡⁡Qe versus ln⁡⁡Ce yield a straight line of slope 1/n and intercept ln⁡⁡Kf [25]. The plot isotherm fit is shown in Figure 9(b). The adsorption mechanism of Cd2+ is predicted by D-R isotherm model in Figure 9(c). E values relating to the mean free energy of adsorption are 14.35 kJ/mol, 13.22 kJ/mol, and 9.98 kJ/mol at 293 K, 303 K, and 313 K, respectively, which is indicative of the ion-exchange adsorption.Table 2
Adsorption isotherm parameters of MSPC gel balls for Cd2+.
T (K)
Langmuir
Freundlich
D-R isotherm
Q
m (mg/g)
B (L/mg)
R
2
K
f
1
/
n
R
2
Q
m (mg/g)
E (kJ/mol)
R
2
293
97.8
0.28
0.984
20.13
1.25
0.702
98.7
14.35
0.960
303
94.3
0.19
0.953
19.36
1.11
0.404
92.8
13.22
0.809
313
84.4
0.12
0.964
13.41
1.02
0.882
86.2
9.98
0.989


### 3.8. Adsorption Kinetics

In order to reveal the adsorption mechanism, the kinetic data under these three systems are fitted to pseudo-first-order equation, pseudo-second-order equation, and intraparticular diffusion model in Figure10. The linear form of these three models can be written as follows [26]:(3)First-order  equation:ln⁡⁡Qe-Qt=ln⁡⁡Qe-K1t,Second-order  equation:tQt=1k2Qe2+tQe,Intraparticular diffusion model:Qt=kidt1/2,where Qe and Qt are the amounts of adsorbed Cd2+ (mg/cm2) at equilibrium and at time t, respectively. k1 (min−1), k2 (g/(mg min)), and kid (mg/(g min1/2)) are the rate constants.Figure 10
Adsorption kinetics fit of Cd2+: pseudo-first-order kinetic fit (a), pseudo-second-order kinetic fit (b), and intraparticular diffusion fit (c).
(a)
(b)
(c)The parameters of the three models are summarized in Table3. Because of low correlation coefficient, first-order reaction kinetics model is not enough to describe Cd2+ adsorption on MSPC. The correlation coefficient (R2>0.96) indicates that pseudo-second-order model could be representative of the reaction mechanism, pointing to a chemical sorption character in Figure 10(b). SA provide plenty of carboxylic acid and hydroxyl sites, and Fe3O4 provides surface hydroxyl group, which can be used as active absorption sites with Cd2+ ion exchange.Table 3
Adsorption kinetic parameters of MSPC gel balls for Cd2+.
C (mg/L)
Pseudo-first-order
Pseudo-second-order
Intraparticular diffusion model
K
1
·
10
-
2 (g/mg min−1)
Q
e
(mg/g)
R
2
K
2
·
10
-
2 (g/mg min−1)
Q
e (mg/g)
R
2
K
1
d (mg/g min1/2)
R
2
K
2
d (mg/g min1/2)
R
2
50
2.37
13.22
0.895
4.95
26.73
0.986
1.46
0.983
0.03
100
3.11
17.82
0.872
3.03
53.16
0.969
1.45
0.974
0.08
150
2.15
24.31
0.978
1.93
78.88
0.976
2.72
0.987
0.12
0.979
200
1.74
32.85
0.954
1.07
108.41
0.979
3.60
0.987
0.2
0.986The initial adsorption rate increases with the increase of initial concentration of Cd2+. However, when concentration of Cd2+ is increased greatly, the initial adsorption rate drops, and the pseudo-second-order model reaction rate constant K2 also shows the same trend. This phenomenon is attributed to the low initial concentration of metal and the abundant adsorption sites of MSPC. But after the initial metal concentration is increased to a certain value, the adsorption quantity reached the saturation, which reveals that most of the active adsorption sites on the adsorbent have been dominated by metal ions [27].Intraparticular diffusion is more linear, indicating that intraparticular diffusion is not the limiting step. Adsorption process of Cd2+ is the combination of surface adsorption point and spreading reticulated structure inside the tunnel, and the process of joint surface adsorption and intraparticular diffusion control. Intraparticular diffusion equation, which can identify the diffusion mechanism, confirms that two stages occur as mentioned above. When the initial concentration of Cd2+ is from 50 to 200 mg/L, K1d is from 1.46 to 3.60 mg/g min1/2. The rapid stage is due to MSPC diffusion where the intraparticular diffusion is the rate-limiting step. Comparatively, the slow stage is the equilibrium step where intraparticular diffusion starts to slow down.


## 3.1. Particle Size and Structure of the MSPC

Figure1 shows the photographs of MSPC. Immobilization of Fe3O4 particles made MSPC gel ball partly black in Figure 1(a). The size of the synthesized gel balls is asymmetric with a mean diameter of 4.0 mm. Figure 1(b) shows the MSPC gel balls were absorbed by the super magnet. And the MSPC gel balls were agglomerate material synthesized by immobilization. After that, the MSPC gel balls were put into the sediment in Figures 1(c) and 1(d).Figure 1
Photographs of MSPC (a, b) and photographs of MSPC mixed in the sediment (c, d).
(a)
(b)
(c)
(d)The SEM images of the inner structure of the SPC and MSPC are given in Figures2(a) and 2(b), respectively. As shown in Figure 2(a), the sodium alginate surface was loose and smooth. Meanwhile, lots of tiny interspace structures can be adsorbed easily by the adsorbents [22]. SEM images in Figure 2(b) show the inner structure of the MSPC where porous microstructure and Fe3O4 nanoparticles spread over their surface. Figures 2(c) and 2(d) present the SEM image of SPC and MSPC that have absorbed Cd2+, the differences are more obvious, and Fe3O4 nanoparticles are visible on the MSPC surface.Figure 2
SEM image of material surface: SPC (a), MSPC (b), SPC after absorption (c), and MSPC after absorption (d).
(a)
(b)
(c)
(d)The EDX spectra are given in Figure3. The signal of Au could be due to the gold plating for measurement. In Figure 3, carbon, oxygen, sodium, chlorine, and calcium elements are detected for SPC, which are the basic components of SPC, while for MSPC, a large amount of iron element has been detected, confirming that Fe3O4 was on the adsorbent surface and had been embedded by sodium alginate and PVP. And there are small amounts of aluminum and silicon in the Fe3O4. The presence of these elements does not affect the adsorbent. Thus, a kind of adsorption characteristic composite has been achieved in this work.Figure 3
EDX spectra of the SPC gel balls (a) and the MSPC gel balls (b).
(a)
(b)


## 3.2. XRD Analysis

Figure4 showed the XRD pattern of MSPC. Six characteristic peaks at 30.5(220), 35.6(311), 43.3(400), 53.7(422), 57.3(511), and 62.8(440) were corresponded well to the planes of spinel structured Fe3O4. However, the intensities were much lower than those of peaks in the database, which could be explained by the cross-link reaction between Fe3O4 and the sodium alginate, while the reaction process did not give rise to the phase change of Fe3O4, which was well consistent with the literature [20].Figure 4
XRD pattern of the MSPC gel balls.


## 3.3. Magnetism Analysis

Magnetic properties of MSPC were characterized by measuring the hysteresis and remanence curves, as shown in Figure5. The shape of the magnetization curves was smooth and parabolic, and the saturation magnetization (Ms) was measured to be 7.92 emu/g. The MSPC exhibited nearly super-paramagnetic properties. It indicated the MSPC was characteristic of magnetic response under the added magnetic field, which made it easy to remove metal ions absorbed by the MSPC.Figure 5
Magnetic hysteresis loop of the MSPC.


## 3.4. Effect of Sorbent Dosage

The adsorption capacity of MSPC and the removal rate of Cd2+ could be affected by the sorbent dosage. Figure 6 shows the effect of MSPC dosage ranging from 0.2 g to 2.0 g on adsorption capacity. The concentration Cd2+ of 150 mg/L could be representative of the high levels pollution of heavy metals in the river sediment. As shown in Figure 6, the removal rate of Cd2+ increases with the concentration of MSPC. However, the adsorption capacity of MSPC is close to the saturation at the same situations. Keeping Cd2+ concentration of 150 mg/L, adsorbent adsorption capacity changes a little at the range of 80–100 mg/g. It showed that adapted holding MSPC mass content was positive for Cd2+ removal. As seen from Figure 6(b), the optimal quantity of particles is 1.2 g/L, in which the removal efficiency of Cd2+ (100 mL, 150 mg/L) in the river sediment is about 87%.Figure 6
Effect of sorbent dosage on theQe (a) and sorbent dosage on the removal efficiency of Cd2+ (b).
(a)
(b)


## 3.5. Effect of pH Value

pH value, which affects the formation distribution of heavy metals in sediment solution by influencing the other components of sediment the same time, also indirectly affects the removal efficiency of the heavy metals. The effect of pH on the adsorption properties is shown in Figure7. Clearly, the adsorption capacity was enhanced quickly with pH increase from 1 to 2 but slowly with pH from 2 to 6. At pH value of 6.2, maximum adsorption quantity is obtained. When the pH value exceeds 7, the uptake decreases because the cadmium ions start to precipitate as Cd(OH)2, which is also a kind of sediment. At lower pH values, the functional groups for coordination including H+ and Cd2+ groups compete for adsorption sites, which do not favor the adsorption. When the solution pH values is increased, the competitive adsorption is reduced, so more negatively charged adsorption sites can positively charge the Cd2+.Figure 7
Effect of pH value on the adsorption of Cd2+ by MSPC.


## 3.6. Effect of Contact Time

The effect of contact time on the adsorption process is shown in Figure8. With the increase of contact time, the adsorption of volume cadmium ions is increased. When the adsorption time is 120 min, adsorption capacity reaches the maximum. When the initial concentration of Cd2+ is 50 and 100 mg/L, adsorption equilibrium time is 80–90 min. SA and Fe3O4 nanoparticles form a mesh with granular structure through the macromolecular cross-linking together. The initial concentration of Cd2+ is changed to 150 and 200 mg/L, and adsorption equilibrium time is 90–120 min. When the active sites are limited, more competition Cd2+ ions limit combining site resulted in the added adsorption equilibrium time. Therefore, the MSPC with open structure and three-dimensional polymer network reinforce Cd2+ adsorption into active sites, which could increase the adsorption rate and adsorption capacity.Figure 8
Effect of contact time on the adsorption of Cd2+ by MSPC.


## 3.7. Sorption Isotherms

Langmuir, Freundlich, and D-R isotherm models were employed for adsorption data correlation to quantify the adsorption capacity of MSPC in Figure9. The linear forms of these three models are listed as follows [23, 24]:(2)Langmuir  model:CeQe=1BQm+CeQm,Freundlich  model:ln⁡⁡Qe=ln⁡⁡Kf+1nln⁡⁡Ce,D-R  model:ln⁡⁡Qe=ln⁡⁡Qm+Kε2,ε=RTln⁡⁡1+ln⁡⁡Ce,E=12K,where Qe (mg/g) is the amount of adsorbed Cd2+ at equilibrium and Ce (mg/L) is the concentration of Cd2+ at equilibrium. Kf and n are associated with the adsorption capacity and adsorption intensity, respectively. B (L/mg) is the Langmuir constants. ε is polanyi potential; E (kJ/mol) is the mean free energy of adsorption.Figure 9
Sorption isotherms fit of Cd2+: Langmuir isotherm fit (a), Freundlich isotherm fit (b), and D-R isotherm fit (c).
(a)
(b)
(c)As listed in Table2, because of the relatively high correlation parameters, the experimental data can be well described by the three models, in which the Langmuir model gives the best fit (R2>0.96). Freundlich parameter, the Kf value, supports the order of cadmium uptake that is 293 K > 303 K > 313 K, which was confirmed by the corresponding 1/n values. Langmuir isotherm model is based on the assumption that the dynamic adsorption-desorption processes occur on homogeneous surface, taking no account of the interaction between the MSPC and the solute in Figure 9(a). The maximum sorption calculated from the Langmuir isotherm is 97.8 mg/g. It is slightly greater than the experimental values regarding that there are still unoccupied sorption sites. Obviously, plots of ln⁡⁡Qe versus ln⁡⁡Ce yield a straight line of slope 1/n and intercept ln⁡⁡Kf [25]. The plot isotherm fit is shown in Figure 9(b). The adsorption mechanism of Cd2+ is predicted by D-R isotherm model in Figure 9(c). E values relating to the mean free energy of adsorption are 14.35 kJ/mol, 13.22 kJ/mol, and 9.98 kJ/mol at 293 K, 303 K, and 313 K, respectively, which is indicative of the ion-exchange adsorption.Table 2
Adsorption isotherm parameters of MSPC gel balls for Cd2+.
T (K)
Langmuir
Freundlich
D-R isotherm
Q
m (mg/g)
B (L/mg)
R
2
K
f
1
/
n
R
2
Q
m (mg/g)
E (kJ/mol)
R
2
293
97.8
0.28
0.984
20.13
1.25
0.702
98.7
14.35
0.960
303
94.3
0.19
0.953
19.36
1.11
0.404
92.8
13.22
0.809
313
84.4
0.12
0.964
13.41
1.02
0.882
86.2
9.98
0.989


## 3.8. Adsorption Kinetics

In order to reveal the adsorption mechanism, the kinetic data under these three systems are fitted to pseudo-first-order equation, pseudo-second-order equation, and intraparticular diffusion model in Figure10. The linear form of these three models can be written as follows [26]:(3)First-order  equation:ln⁡⁡Qe-Qt=ln⁡⁡Qe-K1t,Second-order  equation:tQt=1k2Qe2+tQe,Intraparticular diffusion model:Qt=kidt1/2,where Qe and Qt are the amounts of adsorbed Cd2+ (mg/cm2) at equilibrium and at time t, respectively. k1 (min−1), k2 (g/(mg min)), and kid (mg/(g min1/2)) are the rate constants.Figure 10
Adsorption kinetics fit of Cd2+: pseudo-first-order kinetic fit (a), pseudo-second-order kinetic fit (b), and intraparticular diffusion fit (c).
(a)
(b)
(c)The parameters of the three models are summarized in Table3. Because of low correlation coefficient, first-order reaction kinetics model is not enough to describe Cd2+ adsorption on MSPC. The correlation coefficient (R2>0.96) indicates that pseudo-second-order model could be representative of the reaction mechanism, pointing to a chemical sorption character in Figure 10(b). SA provide plenty of carboxylic acid and hydroxyl sites, and Fe3O4 provides surface hydroxyl group, which can be used as active absorption sites with Cd2+ ion exchange.Table 3
Adsorption kinetic parameters of MSPC gel balls for Cd2+.
C (mg/L)
Pseudo-first-order
Pseudo-second-order
Intraparticular diffusion model
K
1
·
10
-
2 (g/mg min−1)
Q
e
(mg/g)
R
2
K
2
·
10
-
2 (g/mg min−1)
Q
e (mg/g)
R
2
K
1
d (mg/g min1/2)
R
2
K
2
d (mg/g min1/2)
R
2
50
2.37
13.22
0.895
4.95
26.73
0.986
1.46
0.983
0.03
100
3.11
17.82
0.872
3.03
53.16
0.969
1.45
0.974
0.08
150
2.15
24.31
0.978
1.93
78.88
0.976
2.72
0.987
0.12
0.979
200
1.74
32.85
0.954
1.07
108.41
0.979
3.60
0.987
0.2
0.986The initial adsorption rate increases with the increase of initial concentration of Cd2+. However, when concentration of Cd2+ is increased greatly, the initial adsorption rate drops, and the pseudo-second-order model reaction rate constant K2 also shows the same trend. This phenomenon is attributed to the low initial concentration of metal and the abundant adsorption sites of MSPC. But after the initial metal concentration is increased to a certain value, the adsorption quantity reached the saturation, which reveals that most of the active adsorption sites on the adsorbent have been dominated by metal ions [27].Intraparticular diffusion is more linear, indicating that intraparticular diffusion is not the limiting step. Adsorption process of Cd2+ is the combination of surface adsorption point and spreading reticulated structure inside the tunnel, and the process of joint surface adsorption and intraparticular diffusion control. Intraparticular diffusion equation, which can identify the diffusion mechanism, confirms that two stages occur as mentioned above. When the initial concentration of Cd2+ is from 50 to 200 mg/L, K1d is from 1.46 to 3.60 mg/g min1/2. The rapid stage is due to MSPC diffusion where the intraparticular diffusion is the rate-limiting step. Comparatively, the slow stage is the equilibrium step where intraparticular diffusion starts to slow down.


## 4. Conclusions

The MSPC were synthesized under different mass fraction of Fe3O4 nanoparticles, SA and PVP, subsequently treated by immobilization, and finally used for removal of Cd2+ from river sediment. Thus, this new adsorbent not only possesses the high adsorption capacity but also is easy to separate from the mud mixture. The conclusions are shown below.(1)
The character analyses indicated that the MSPC had a diameter of about 4 mm, with many tiny interspace structures on the surface and contained amide and hydroxide groups which could absorb metal ions.(2)
SEM elucidated that magnetic Fe3O4 nanoparticles were embedded successfully and might be distributed asymmetrically. The spinel structure of Fe3O4 did not change during the synthesized process. The MSPC show 7.92 emu/g Ms by VSM.(3)
The optimum pH for adsorption was 6.2 and the contact time required to achieve equilibrium was 80–120 min, when the initial Cd2+ concentration varied from 50 to 200 mg/L. The removal rate of Cd2+ is 87%, when the concentration of Cd2+ was fixed at 150 mg/L.(4)
The Langmuir adsorption isotherm fits well with the adsorption equilibrium data. The maximum sorption calculated from the Langmuir isotherm was 97.8 mg/g for MSPC in sediment. A pseudo-second-order rate mechanism can be used to illustrate the sorption of MSPC. The correlation coefficients were allR2>0.96.



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*Source: 940985-2015-06-04.xml*