Fe 3 O 4 Nanoparticles Embedded Sodium Alginate / PVP / Calcium Gel Composite for Removal of Cd 2 +

Magnetic ferroferric oxide (Fe 3 O 4 ) nanoparticles embedded sodium alginate (SA)/polyvinylpyrrolidone (PVP)/calciumgel composites (MSPC) were synthesized for the removal of Cd. 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, when the Cd concentration was fixed at 150mg/L. The optimum pH for adsorption is 6.2. When the initial Cd concentration was from 50 to 200mg⋅L, the adsorption equilibrium time was 80–120 minutes. The isotherms results showed the maximum sorption was 97.8mg/g calculated from the Langmuir isotherm.The pseudo-second-order kinetic model could be well described by the sorption of Cd in the sediment. The correlation coefficients (R) were all higher than 0.96. Results showed that MSPC was a practical and low-costmaterial for heavymetal removal from the river sediment and simultaneously solves the problems in convenient separation under additional magnetic field.


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 Fe 3 O 4 nanoparticles for the removal of heavy metals have been reported [6,7].Fe 3 O 4 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 Fe 3 O 4 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 Fe 3 O 4 nanoparticles into polymeric biomaterials, especially by using polysaccharide and crosslinking agent, was studied [16].Immobilization of magnetic Fe 3 O 4 nanoparticles within biological materials, such as agar, agarose, chitosan, collagen, and calcium [17,18], is an essential step for industrial scaleup 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 Fe 3 O 4 nanoparticles into sodium alginate (SA)/polyvinylpyrrolidone (PVP)/calcium gel composite (MSPC) for removal of Cd 2+ in the river sediment.The MSPC was characterized using scanning electron microscopy (SEM), energy-dispersive Xray 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.1, other metal ions content was so low that cannot affect the overall removal efficiency.

Materials and Methods
Scanning 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-Kradiation ( = 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).

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 Fe 3 O 4 particles were added into SA/PVP mixture and stirred until completely mixed.Subsequently, the blended mixture was added to 20 mL Ca(NO 3 ) 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 Fe 3 O 4 particles.

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 Cd 2+ , 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 Cd 2+ solution (50-200 mg/L) at 293 K at pH value of 6.2, and the concentration of Cd 2+ in the solution was detected at fixed time.
MSPC adsorption capacity   and pollutant removal rate (%) were calculated by the following formula: where  0 (mg/L) is the initial concentration of Cd 2+ ,   (mg/L) is the concentration of Cd 2+ at equilibrium,  (g) is the mass of the adsorbent (dry), and  (L) is the volume of the solution.After that, the MSPC gel balls were put into the sediment in Figures 1(c) and 1(d).

Results and Discussion
The SEM images of the inner structure of the SPC and MSPC are given in Figures 2(a    The EDX spectra are given in Figure 3.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 Fe 3 O 4 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 Fe 3 O 4 .The presence of these elements does not affect the adsorbent.Thus, a kind of adsorption characteristic composite has been achieved in this work.However, the intensities were much lower than those of peaks in the database, which could be explained by the cross-link reaction between Fe 3 O 4 and the sodium alginate, while the reaction process did not give rise to the phase change of Fe 3 O 4 , which was well consistent with the literature [20].

Magnetism Analysis.
Magnetic properties of MSPC were characterized by measuring the hysteresis and remanence curves, as shown in Figure 5.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.6 shows the effect of MSPC dosage ranging from 0.2 g to 2.0 g on adsorption capacity.The concentration Cd 2+ 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 Cd 2+ increases with the concentration of MSPC.However, the adsorption capacity of MSPC is close to the saturation at the same situations.Keeping Cd 2+ 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 Cd 2+ removal.As seen from Figure 6(b), the optimal quantity of particles is 1.2 g/L, in which the removal efficiency of Cd 2+ (100 mL, 150 mg/L) in the river sediment is about 87%.

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 Figure 7. 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 Cd 2+ 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 Cd 2+ .

Effect of Contact Time.
The effect of contact time on the adsorption process is shown in Figure 8.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 Cd 2+ is 50 and 100 mg/L, adsorption equilibrium time is 80-90 min.SA and Fe 3 O 4 nanoparticles form a mesh with granular structure through the macromolecular cross-linking together.The initial concentration of Cd 2+ is changed to 150 and 200 mg/L, and adsorption equilibrium time is 90-120 min.When the active sites are limited, more competition Cd 2+ ions limit combining site resulted in the added adsorption equilibrium time.Therefore, the MSPC with open structure and three-dimensional polymer network reinforce Cd 2+ adsorption into active sites, which could increase the adsorption rate and adsorption capacity.

Sorption Isotherms.
Langmuir, Freundlich, and D-R isotherm models were employed for adsorption data correlation to quantify the adsorption capacity of MSPC in Figure 9.The linear forms of these three models are listed as follows [23,24]: where   (mg/g) is the amount of adsorbed Cd 2+ at equilibrium and   (mg/L) is the concentration of Cd 2+ at equilibrium.  and  are associated with the adsorption capacity and adsorption intensity, respectively. (L/mg) is the Langmuir constants. is polanyi potential;  (kJ/mol) is the mean free energy of adsorption.
As listed in Table 2, 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 ( 2 > 0.96).Freundlich parameter, the   value, supports the order of cadmium uptake that is 293 K > 303 K > 313 K, which was confirmed by the corresponding 1/ 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   versus ln   yield a straight line of slope 1/ and intercept ln   [25].The plot isotherm fit is shown in Figure 9(b).The adsorption mechanism of Cd 2+ is predicted by D-R isotherm model in Figure 9(c). 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.

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 Figure 10.The linear form of these three models can be written as follows [26]: First-order equation: ln (  −   ) = ln   −  1 , Second-order equation: where   and   are the amounts of adsorbed Cd 2+ (mg/cm 2 ) at equilibrium and at time , respectively. 1 (min −1 ),  2 (g/(mg min)), and   (mg/(g min 1/2 )) are the rate constants.The parameters of the three models are summarized in Table 3.Because of low correlation coefficient, first-order reaction kinetics model is not enough to describe Cd 2+ adsorption on MSPC.The correlation coefficient ( 2 > 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 Fe 3 O 4 provides surface hydroxyl group, which can be used as active absorption sites with Cd 2+ ion exchange.
The initial adsorption rate increases with the increase of initial concentration of Cd 2+ .However, when concentration of Cd 2+ is increased greatly, the initial adsorption rate drops, and the pseudo-second-order model reaction rate constant  2 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 Cd 2+ 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 Cd 2+ is from 50 to 200 mg/L,  1 is from 1.46 to 3.60 mg/g min 1/2 .The rapid stage is due to MSPC diffusion where the intraparticular diffusion is the ratelimiting step.Comparatively, the slow stage is the equilibrium step where intraparticular diffusion starts to slow down.

Conclusions
The MSPC were synthesized under different mass fraction of Fe 3 O 4 nanoparticles, SA and PVP, subsequently treated by immobilization, and finally used for removal of Cd 2+ 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 Fe 3 O 4 nanoparticles were embedded successfully and might be distributed asymmetrically.The spinel structure of Fe 3 O 4 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 Cd 2+ concentration varied from 50 to 200 mg/L.The removal rate of Cd 2+ is 87%, when the concentration of Cd 2+ 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 all  2 > 0.96.

3. 1 .
Particle Size and Structure of the MSPC. Figure 1 shows the photographs of MSPC.Immobilization of Fe 3 O 4 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.

Figure 1 :
Figure 1: Photographs of MSPC (a, b) and photographs of MSPC mixed in the sediment (c, d).

Figure 3 :Figure 4 :
Figure 3: EDX spectra of the SPC gel balls (a) and the MSPC gel balls (b).

Figure 5 :
Figure 5: Magnetic hysteresis loop of the MSPC.

Figure 6 :
Figure 6: Effect of sorbent dosage on the   (a) and sorbent dosage on the removal efficiency of Cd 2+ (b).

Figure 7 :
Figure 7: Effect of pH value on the adsorption of Cd 2+ by MSPC.

Figure 8 :
Figure 8: Effect of contact time on the adsorption of Cd 2+ by MSPC.

Table 1 :
The concentration of metal ions from the river sediment.

4 nanoparticles were prepared according to the reported procedure [21]. Calcium nitrate anhydrous (CaNO 3 ) was served as the cross-linker for PVP and SA. Cadmium acetate dihydrate (C 4 H 6 CdO 4 ⋅2H 2 O) and ammonium hydroxide (NH 3 ⋅H 2 O) 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 Cd 2+ .So cadmium acetate dihydrate (C 4 H 6 CdO 4 ⋅2H 2 O) was added to the river sediment in order to reach different experiment concentrations of Cd 2+ ions.As listed in Table

Table 2 :
Adsorption isotherm parameters of MSPC gel balls for Cd 2+ .

Table 3 :
Adsorption kinetic parameters of MSPC gel balls for Cd 2+ .