Abstract

Carbon-coated Fe3O4 porous particles were synthesized with phenolic resin as a carbon source using impregnating pyrolysis. The magnetic property, phase structure, pore structure, and surface morphology of the pyrolysis products were characterized by vibrating specimen magnetometer, X-ray diffraction, Brunauer–Emmett–Teller (BET), and scanning electron microscopy, respectively. Also, the adsorption properties of iron (III) ions on Fe3O4@C as well as pure carbon, including adsorption isothermal, kinetics, and pH effect, were investigated. The results showed that the size of synthesized Fe3O4@C particles ranged from 10 to 50 μm with micro-meso pores sized below 5 nm. The main phases of Fe3O4@C were magnetite, graphite and amorphous carbon. The adsorption kinetics of iron (III) ions on Fe3O4@C could be expressed by the pseudo-second-order kinetic model and the adsorption isotherm was fitted by a Freundlich model. Nano-Fe3O4 had synergism to porous carbon on the absorption of iron (III) ions.

INTRODUCTION

The presence of iron ions, as one of the heavy metals in ground and industrial water, becomes toxic at high levels and then may cause environmental and human health problems (Sarin et al. 2004; Chaturvedi & Dave 2012). Iron ions have attracted wide and increasing research attention since they have been found in many manufacturing industries such as the coatings, car, aeronautic and steel industries.

Various technological methods, such as precipitation, cementation, sedimentation, filtration, coagulation, flotation, complexing, solvent extraction, membrane separation, electrochemical techniques, biological processes, reverse osmosis, and ion exchange and adsorption, have been used for the removal of toxic heavy metals from wastewater (De et al. 2009; Delaire et al. 2016; Habiba et al. 2016; Hao & Wang 2017). Among these methods, adsorption is cost-effective and simple to operate (Abas et al. 2013). Activated carbon is one of the most common absorbents reported for removal of iron ions, because of its high specific surface area and good adsorption capacity for pollutants (Li et al. 2015; Xie et al. 2016). Fe3O4 nano-particles are also effective in adsorbing iron ions, whereas activated carbon particles cannot be recycled after treating sewage. Meanwhile, nano particles of Fe3O4 are easily agglomerated and unstable when they are exposed to air (Zhang et al. 2012). Carbon coated nano-Fe3O4 (Fe3O4@C) is a new type of magnetized adsorbent, due to the fact that the carbon shell can protect nano-Fe3O4 and the adsorbent is also easily recycled using external magnetic fields (Fan et al. 2012; Yuan et al. 2013; Bao et al. 2014; Ranjithkumar et al. 2014; Maddah et al. 2016).

At present, the main preparation methods for Fe3O4@C are pyrolysis, gas carburizing, self-propagating high temperature synthesis, biological material carbonization method, polyimide matrix synthesis, plasma deposition, hydrothermal method and thermal reduction method (Xuan et al. 2007; Hojati-Talemi et al. 2010; Zheng et al. 2012).

In this study, Fe3O4@C was prepared by a convenient impregnating pyrolysis method, with phenolic resin as the carbon precursor. Phenolic resin was selected as the carbon source in this study because it can polymerize and form complex carbon molecules (Zhou et al. 2015). At first, phenolic resin was blended with nano-Fe3O4 and then carbonized at 800 °C. After grinding, carbon-coated nano-Fe3O4 porous particles were obtained. The magnetization, carbonization progress, crystalline phase, morphologies, microstructures and pore structure of Fe3O4@C were detected by VSM, TG, XRD, SEM and BET, respectively. Also, we presented the property of Fe3O4@C for removal of iron ions from aqueous solution. The influencing factors such as contact time, initial concentration and pH have been systematically investigated. Moreover, the removal mechanism of iron (III) ions by Fe3O4@C was discussed.

EXPERIMENTAL

Chemicals

Phenolic resin (2130) was provided by Jintong Letai Chemical Products Co., Ltd (Beijing). Fe3O4 nanoparticles (with a diameter of 100 nm), p-toluene sulfonic acid, and a type of silane coupling agent, KH-550, were purchased from Jingwen Glass Reagent Company (Beijing).

Preparation of Fe3O4@C

Fe3O4@C particles were synthesized by a modified impregnating pyrolysis method using phenolic resin as the carbon source (Schaper et al. 2004; Wang et al. 2016). For preparing each sample, 10 g phenolic resin was added into a paper vessel, followed by 0.77 g silane coupling agent KH-550 and 2.31 g Fe3O4 nanoparticles, stirring for 10 minutes. Then 2.31 g p-toluene sulfonic acid was added into the mixture and stirred again for 10 min. After natural curing, the cured product was taken out of the vessel and carbonized in a tube furnace in a nitrogen atmosphere. The temperature was increased to 800 °C at a heat rate of 2 °C/min and held at the final temperature for 2 hours (Harris & Tsang 1998). After carbonization, the block product was crushed to Fe3O4@C particles.

Characterization of Fe3O4@C

The thermal property of phenolic resin was determined by thermogravimetric (TG) analysis to analyze the process of carbonization in a nitrogen atmosphere. Hysteresis loops of Fe3O4@C were measured by a vibrating specimen magnetometer (VSM, BKT-4500). Powder X-ray diffraction (XRD) patterns of Fe3O4@C were analyzed on a Rigaku D/max-2200 XRD at 25°, using Cu K radiation (λ = 1.5406 Å) with a scanning angle of 3–80° and step width 0.02°. The chemical composition and surface morphology of Fe3O4@C were determined with scanning electron microscopy (SEM). The specific surface area, pore volume, and pore size of prepared samples were measured by Brunauer–Emmett–Teller (BET).

Adsorption experiments

The concentration of iron ions in solution used in the adsorption experiments were 0.2, 0.4, 0.6, 0.8, 1, and 1.2 mg/L respectively, as shown in Table 1. The absorbance was measured by UV VIS spectrophotometer at the maximum absorption wavelength of 466 nm (Pozdnyakov et al. 2006), and then the standard curve and regression equation were obtained.

Table 1

Amount of chemicals in different iron (III) ion solutions

No.Standard iron solution (C = 20 mg/L) (mL)10% sulfonic acid (mL)Pure water (mL)Iron ions concentration (mg/L)
0.25 19.5 0.2 
0.5 19 0.4 
0.75 18.5 0.6 
18 0.8 
1.25 17.5 
1.5 17 1.2 
No.Standard iron solution (C = 20 mg/L) (mL)10% sulfonic acid (mL)Pure water (mL)Iron ions concentration (mg/L)
0.25 19.5 0.2 
0.5 19 0.4 
0.75 18.5 0.6 
18 0.8 
1.25 17.5 
1.5 17 1.2 

To determine the adsorption kinetics of iron (III) ion on Fe3O4@C, batch experiments were performed in six glass conical flasks. In each flask, 0.3 g of 15 wt% Fe3O4@C powder and 25 mL iron (III) ion solution (C = 2,500 mg/L) were added. The six flasks were shaken in an oscillator at 250 r/min and 25 ± 1° for 1, 5, 10, 30, 60, 120, and 180 min, respectively.

The adsorption isotherms experiments were carried out with different initial concentrations from 500 to 5,000 mg/L and the same shaking time of 180 min. Other parameters were the same as those of the experiments of adsorption kinetics.

To investigate the effect of pH of Fe3O4@C on the adsorption properties, experiments were performed in three glass conical flasks. In each flask, 0.3 g of 15 wt% Fe3O4@C powder was added at an initial concentration of 5,000 mg/L. The pH of the working solutions was adjusted from 1 to 3 with HCl or NaOH solution. The three flasks were then shaken in an oscillator at 250 r/min and 25 ± 1°C for 180 min.

In batch adsorption experiments, the solution was centrifuged at 4,500 r/min for 2 min and the upper clear liquid was filtrated. The remaining concentration of iron (III) ion was measured by absorption spectrophotometry. Furthermore, the blank sample of carbon particles which was carbonized from phenolic resin was also obtained for comparison.

RESULTS AND DISCUSSION

The magnetic property of Fe3O4@C

Figure 1 shows the hysteresis loops of the prepared Fe3O4@C particles. As we know, the magnetic property of Fe3O4@C can affect its separability from the working solution. It can be seen in Figure 1 that the saturation magnetization of Fe3O4@C was 19.9 emu/g. Although the saturation magnetization of Fe3O4@C was 59.1 emu/g lower than that of Fe3O4 (79.0 emu/g) because of its carbon shell, the remaining magnetism was still sufficient to achieve a fast separation by an externally applied magnetic field from working solution. Figure 2 shows that Fe3O4@C can be separated from the iron (III) ion solution by a magnet.

Figure 1

The hysteresis loops of Fe3O4 and Fe3O4@C.

Figure 1

The hysteresis loops of Fe3O4 and Fe3O4@C.

Figure 2

Fe3O4@C separation of iron (III) ion solution by magnet.

Figure 2

Fe3O4@C separation of iron (III) ion solution by magnet.

TG analysis of the precursor

Figure 3 shows the TG curve of phenolic resin capsulating Fe3O4. Ten per cent mass was lost when the temperature increased to 300 °C because of the escape of water and non-crosslink small molecules in phenolic resin. The major mass loss occurred from 400 to 800 °C because of polymerization of phenolic resin. Small molecule substances were released (i.e. H2, CO, CH4 and CO2), which formed porous carbon (Cui et al. 2006). After 900 °C, mass loss almost stopped and the carbon residue rate was 61.8%.

Figure 3

TG curves of phenolic resin capsulating Fe3O4.

Figure 3

TG curves of phenolic resin capsulating Fe3O4.

However, the micropores gradually disappeared when heating up to 800 °C. The number of macropores increased and the specific surface area decreased, which has a negative effect on the property of adsorption. As a result, 800 °C was a suitable carbonization temperature in this experiment.

Phase structure of Fe3O4@C

Figure 4 shows the XRD pattern of samples of 15 wt% Fe3O4@C. Peaks at 2θ = 30.3, 35.7, 43.4, 57.1 and 62.8° matched well with PDF card 89-688 (iron oxide), which matched the Fe3O4 crystal face (220), (311), (400), (511) and (440) respectively. Peaks at 2θ = 26.24 and 44.4° matched with PDF card 75-1621 (carbon), the broad peaks were also obvious. This result indicated that the main phases in Fe3O4@C were magnetite, graphite and amorphous carbon.

Figure 4

XRD pattern of particle samples of 15 wt% Fe3O4@C.

Figure 4

XRD pattern of particle samples of 15 wt% Fe3O4@C.

Figure 5 shows XRD patterns of different mass fraction of Fe3O4@C. It can be seen that the broad peaks were obvious in the XRD patterns of 5 and 10 wt% samples, which means that the main phase of carbon was amorphous carbon. In the patterns of 15 and 20 wt%, graphite peaks were obvious, indicating that with the increasing mass fraction of Fe3O4, the peaks became sharper and the half-high-width decreased. Moreover, the Fe3O4 acted as a catalyst to the graphitization process of carbon in this experiment.

Figure 5

XRD patterns of samples of Fe3O4@C with different concentrations of Fe3O4: (a) 5 wt%, (b) 10 wt%, (c) 15 wt% and (d) 20 wt%.

Figure 5

XRD patterns of samples of Fe3O4@C with different concentrations of Fe3O4: (a) 5 wt%, (b) 10 wt%, (c) 15 wt% and (d) 20 wt%.

Specific surface area and pore size of Fe3O4@C

The major structural properties of Fe3O4@C were determined by N2 adsorption- desorption isotherms, as shown in Figure 6. Isotherms increased immediately at low relative pressure. In the area of middle to high relative pressure, the loop was similar to type IV isotherm. The hysteresis was similar to that of type H4, which means that it was a micro-mesoporous structure and had narrow fracture.

Figure 6

N2 adsorption-desorption isotherms of Fe3O4@C: (a) 5 wt%, (b) 10 wt%, (c) 15 wt% and (d) 20 wt%.

Figure 6

N2 adsorption-desorption isotherms of Fe3O4@C: (a) 5 wt%, (b) 10 wt%, (c) 15 wt% and (d) 20 wt%.

The structural properties, including pore volume, specific surface area, and average pore diameter, of Fe3O4@C with different mass fraction of Fe3O4 are summarized in Table 2.

Table 2

The structural properties of Fe3O4@C with different mass fraction of Fe3O4

Mass fraction (%)Pore volume (mL·g–1)Specific surface area (m2/g)Average pore diameter (nm)Relative pressure (p/p0)
0.1 230 3.2 0.00498–0.01001 
10 0.4 200 4.1 0.04942–0.25335 
15 0.2 209 3.6 0.04997–0.24646 
20 0.2 183 4.0 0.04955–0.24717 
Mass fraction (%)Pore volume (mL·g–1)Specific surface area (m2/g)Average pore diameter (nm)Relative pressure (p/p0)
0.1 230 3.2 0.00498–0.01001 
10 0.4 200 4.1 0.04942–0.25335 
15 0.2 209 3.6 0.04997–0.24646 
20 0.2 183 4.0 0.04955–0.24717 

Figure 7 displays the pore size distribution of Fe3O4@C with different concentrations of Fe3O4. The size of most pores was smaller than 5 nm and there were a small number of mesopores and macropores. Combined with isotherm and pore size distribution, it can be determined that Fe3O4@C was of a core/shell structure with a mesoporous carbon shell.

Figure 7

Pore size distribution of Fe3O4@C: (a) 5 wt%, (b) 10 wt%, (c) 15 wt% and (d) 20 wt%.

Figure 7

Pore size distribution of Fe3O4@C: (a) 5 wt%, (b) 10 wt%, (c) 15 wt% and (d) 20 wt%.

Surface morphology of Fe3O4@C

Figure 8 displays the SEM micrographs of the synthesized Fe3O4@C particles. The diameter was approximately 10–50 μm and the surface was rough. There were many pores (∼100 nm) on the surface. A small part of Fe3O4 particles showed agglomeration while most of them had good dispersion.

Figure 8

SEM micrographs of prepared Fe3O4@C particles.

Figure 8

SEM micrographs of prepared Fe3O4@C particles.

Figure 9 shows the energy spectrum result of Fe3O4@C. It can be seen that the white particles on the surface of samples mainly contained elements of C, Fe, and O, (Table 3). Combined with the result of the energy spectrum of nano-Fe3O4 (Figure 10) and the XRD pattern (Figure 4), it can be determined that the white particles should be Fe3O4.

Table 3

Percentage of each element in nano-Fe3O4 particles and Fe3O4@C

ElementFe3O4@C
nano-Fe3O4 particles
Weight/%Atom/%Weight/%Atom/%
72.4 85.3 – – 
12.2 10.8 32.8 63.0 
Fe 15.4 3.9 67.2 37.0 
ElementFe3O4@C
nano-Fe3O4 particles
Weight/%Atom/%Weight/%Atom/%
72.4 85.3 – – 
12.2 10.8 32.8 63.0 
Fe 15.4 3.9 67.2 37.0 
Figure 9

EDS result of a part on the surface of Fe3O4@C.

Figure 9

EDS result of a part on the surface of Fe3O4@C.

Figure 10

EDS result of nano-Fe3O4 particles.

Figure 10

EDS result of nano-Fe3O4 particles.

Adsorption performance of Fe3O4@C

Standard curve of iron (III) ion solution

Figure 11 shows the standard curve of the iron (III) ion solution as well as the linear regression equation. The solution concentration and the absorbance had a good linear relationship with R2 at 0.9997. The linear equation (y = 0.2662x – 0.0038) was in good agreement with the Lambert Bill law. According to the absorbance value and linear equation, the concentration of iron (III) ion in the solution can be calculated, thus the adsorption capacity of iron (III) ion on Fe3O4@C can also be obtained.

Figure 11

The standard curve of iron (III) ion solution used in adsorption experiments.

Figure 11

The standard curve of iron (III) ion solution used in adsorption experiments.

The calculation formula of the adsorption capacity and adsorption rate are as follows:  
formula
(1)
 
formula
(2)
where qe (mg/g) is the equilibrium adsorption quantity; V (L) is the volume of the iron (III) ion solution; M (g) is the quantity of the adsorbent.

Adsorption kinetics

The kinetic curves of iron (III) ion absorbed on the prepared Fe3O4@C and carbon from phenolic resin are compared in Figure 12. It can be seen that there were two steps in the adsorption process of iron (III) ion on Fe3O4@C. In the first 10 min, a large amount of iron (III) ion was adsorbed rapidly by the exterior surface of Fe3O4@C (129.9 mg/g). When the adsorption of the exterior surface reached saturation, iron (III) ion entered into the pores and was absorbed by the interior surface of the meso-pores (Ge et al. 2012). The adsorption rate constant decreased afterwards and the adsorption equilibrium was nearly reached at 180 min. The maximum adsorption quantity was 163.0 mg/g and the maximum adsorption rate was 78.3%. As for the carbon particles from pure phenolic resin, the curve showed the same trend. The adsorption equilibrium was also nearly reached at 180 min (120.1 mg/g). Compared with Fe3O4@C, the pure carbon particles had a slightly superior adsorption in the first 10 min. However, the adsorption equilibrium of Fe3O4@C was far higher than that of pure carbon particles from phenolic resin.

Figure 12

Iron (III) ion adsorption kinetics on Fe3O4@C and pure carbon particles (initial concentration of iron (III) ion: 2,500 mg/L; pH: 1).

Figure 12

Iron (III) ion adsorption kinetics on Fe3O4@C and pure carbon particles (initial concentration of iron (III) ion: 2,500 mg/L; pH: 1).

In order to investigate the adsorption mechanism of iron (III) ion adsorption on Fe3O4@C, the experimental data were modeled using the pseudo-first-order rate equation and the pseudo-second-order rate equation as shown below by Equations (3) and (4) respectively.  
formula
(3)
 
formula
(4)
where k1, k2 are the rate constants of pseudo-first-order and pseudo-second-order adsorption respectively; qt (mg/g) and qe (mg/g) represent the adsorbed concentrations of iron (III) ion at time t (min) and at equilibrium respectively (Liu et al. 2011).

The results showed that the pseudo-second-order fitted better than the pseudo-first-order (Table 4). The correlation coefficient of pseudo-second-order reaction kinetics is higher than that of the pseudo-first-order reaction kinetic. Moreover, the maximum adsorption quantity of iron (III) ion onto Fe3O4@C and pure carbon obtained from experience (163.0 and 120.1 mg/g) were approximately identical to those calculated from the pseudo-second-order fitting results (164.1 and 115.3 mg/g). Thus, iron (III) ion adsorption processes on both Fe3O4@C and pure carbon obeyed pseudo-second-order kinetics (Khalili et al. 2016). In this model, the adsorption rate is controlled by the chemical reaction. The kinetic results suggested that the process of iron (III) ion adsorption by Fe3O4@C and pure carbon were both two-speed processes involving both rapid and slow adsorption (Lu et al. 2014).

Table 4

Kinetic parameters for the adsorption of iron (III) ion on pure carbon and Fe3O4@C particles

SamplePseudo-first-order
Pseudo-second-order
qeK1R2qeK2R2
Pure carbon 112.7 1.4 0.8040 115.3 2.4 × 10–2 0.9349 
Fe3O4@C 156.7 0.2 0.8882 164.1 2.4 × 10–3 0.9779 
SamplePseudo-first-order
Pseudo-second-order
qeK1R2qeK2R2
Pure carbon 112.7 1.4 0.8040 115.3 2.4 × 10–2 0.9349 
Fe3O4@C 156.7 0.2 0.8882 164.1 2.4 × 10–3 0.9779 

Adsorption isotherms

The adsorption isotherms of iron (III) ion on Fe3O4@C and pure carbon were compared in Figure 13. The adsorption amount of iron (III) ion on Fe3O4@C was greater than that on pure carbon at the concentration range from 500 to 5,000 mg/L. This indicated that Fe3O4 had synergism to pure carbon particles, which can increase the adsorption amount of iron (III) ion.

Figure 13

Adsorption isotherms of Iron (III) on Fe3O4@C and pure carbon particles (pH:1; contact time: 180 min).

Figure 13

Adsorption isotherms of Iron (III) on Fe3O4@C and pure carbon particles (pH:1; contact time: 180 min).

Freundlich and Langmuir isotherm models were tested to investigate the adsorption of iron (III) ion by Fe3O4@C and pure carbon. The Langmuir model was the first adsorption isotherm with a theoretical basis. It assumed that the adsorbent surface is uniform with limited adsorption sites, only a monolayer is formed during adsorption (Shaker & Albishri 2014). Freundlich was the empirical formula describing the adsorption isotherm. It was used for adsorbents with an irregular surface or single solute systems within a specific concentration range and it supposed that different adsorption sites existing on the solid phase surface have different adsorption energy.

The equilibrium data were fitted by the Langmuir (Equation (5)) and Freundlich (Equation (6)) models as follows:  
formula
(5)
 
formula
(6)
where qm (mg/g) is the theoretical maximum adsorption amount of iron (III) ion; qe (mg/g) is the equilibrium adsorption amount at equilibrium concentration Ce (mg/L); kL, kF, are the Langmuir and Freundlich relative adsorption capacity respectively; nF is the heterogeneity factor.

The results showed that the Freundlich model fitted better than the Langmuir model (Table 5). The correlation coefficient of the Freundlich isotherm was higher than that of the Langmuir isotherm. KF values were higher than kL, which also confirmed that the Freundlich model fitted better than the Langmuir model. The adsorption of iron (III) ion on pure carbon was a favorable adsorption situation (nF > 1), whereas the adsorption on Fe3O4@C was unfavorable (nF < 1). The iron (III) ion adsorption on Fe3O4@C can be considered to be a heterogeneous surface adsorption process.

Table 5

Isotherm parameters for the adsorption of iron (III) ion on pure carbon particles and Fe3O4@C

SampleLangmuir model
Freundlich model
qmkLR2nFkFR2
Pure carbon 243.0 5.0 × 10–4 0.7173 1.1 2.0 × 10–2 0.9669 
Fe3O4@C 331.2 5.8 × 10–4 0.7765 0.9 1.3 × 10–1 0.9919 
SampleLangmuir model
Freundlich model
qmkLR2nFkFR2
Pure carbon 243.0 5.0 × 10–4 0.7173 1.1 2.0 × 10–2 0.9669 
Fe3O4@C 331.2 5.8 × 10–4 0.7765 0.9 1.3 × 10–1 0.9919 

The effect of pH

According to the calculation result of Fe(OH)3 solubility, iron (III) ion begin to be hydrolyzed and formed brown Fe(OH)3 precipitation at pH = 2.9. Figure 14 displays the precipitation phenomenon. The precipitate will lead to inaccurate experimental results. Thus the instance of the solution with pH <2.9 was only discussed.

Figure 14

Effect of pH on hydrolysis of iron (III) ion: (a) pH = 1, no hydrolysis; (b) pH = 3, hydrolyzed.

Figure 14

Effect of pH on hydrolysis of iron (III) ion: (a) pH = 1, no hydrolysis; (b) pH = 3, hydrolyzed.

Figure 15 shows the relationship between pH and adsorption of iron (III) ion on Fe3O4@C. It can be seen that Fe3O4@C had a larger quantity of iron (III) ion adsorption and the reason could mainly be that when the pH value increased, Fe3O4@C adsorbed more H+, thus hindering the adsorption of iron (III) ion.

Figure 15

Effect of pH on the adsorption of iron (III) ion on Fe3O4@C (initial concentration of iron (III) ion: 5,000 mg/L; contact time: 180 min).

Figure 15

Effect of pH on the adsorption of iron (III) ion on Fe3O4@C (initial concentration of iron (III) ion: 5,000 mg/L; contact time: 180 min).

Regeneration of Fe3O4@C

After the adsorption of iron (III) ion, Fe3O4@C was collected by external magnet. The used Fe3O4@C was washed using 1 M HCl solution for 1 hour to remove the remnant iron (III) ion, then washed with deionized water several times (Yuan et al. 2013). After drying at 100 °C for 12 hours, the regenerated Fe3O4@C could be used for adsorption again. Figure 16 shows three cycles of the same adsorbent for iron (III) ion. As can be seen, although the percentage of iron (III) ion removal dropped slightly during every cycle, it still maintained a high adsorption capacity (72.4%).

Figure 16

Effect of different cycles on the adsorption rate of iron (III) ion on Fe3O4@C (initial concentration of iron (III) ion: 2,500 mg/L; contact time: 180 min).

Figure 16

Effect of different cycles on the adsorption rate of iron (III) ion on Fe3O4@C (initial concentration of iron (III) ion: 2,500 mg/L; contact time: 180 min).

Comparison study

A comparison of the removal rates of different adsorbents, including this study, for the removal of iron ion is presented in Table 6. The removal rate will depend on the physical nature and chemical composition of the materials used (Indah et al. 2016). The comparison showed that the adsorption capacity of Fe3O4@C for iron removal was passable compared to other adsorbents reported in the bibliography. The low cost, high efficiency and ease of recycling make Fe3O4@C very promising in future applications.

Table 6

Comparison of adsorption capacity of different adsorbents for iron

AdsorbentsRemoval rate (%)References
Pretreated orange peel 78 Lugo-Lugo et al. (2012)  
Activated carbon from coconut shells 87 Moreno-Piraján et al. (2011)  
Untreated activated carbon 52 Üçer et al. (2006)  
Tannic acid immobilized activated carbon 70 Üçer et al. (2006)  
Fe3O4@C 80 Present study 
AdsorbentsRemoval rate (%)References
Pretreated orange peel 78 Lugo-Lugo et al. (2012)  
Activated carbon from coconut shells 87 Moreno-Piraján et al. (2011)  
Untreated activated carbon 52 Üçer et al. (2006)  
Tannic acid immobilized activated carbon 70 Üçer et al. (2006)  
Fe3O4@C 80 Present study 

CONCLUSIONS

In this study, carbon-coated nano-Fe3O4 was synthesized and then the microstructure as well the adsorb property of iron (III) ion was investigated. The result showed that the size of synthesized Fe3O4@C particles ranged from 10 to 50 μm. The surface of composite particles was rough, with a large number of circular holes in it, and the nano-Fe3O4 was randomly distributed in the interior and surface with good dispersion. The porous structure was due to polymerization of phenolic resin when heating up to 400 °C. Fe3O4@C was a mesoporous material and the average pore diameter was 3–4 nm. The main phases of Fe3O4@C were magnetite, graphite and amorphous carbon. Also, it was found that Fe3O4 can enhance the crystallinity of the product, indicating that this additive acted as a catalytic in the graphitization process. The adsorption kinetics of iron (III) ion on Fe3O4@C could be expressed by the pseudo-second-order kinetic model and the adsorption isotherm was fitted by the Freundlich model. It indicated that the adsorption of iron (III) ion on Fe3O4@C was controlled by the chemical reaction and occurred on the heterogeneous surface. In the acidic solution, Fe3O4@C exhibited considerable superiority to pure carbon from phenolic resin, which indicated that nano-Fe3O4 had synergism to porous carbon for the absorption of iron (III) ion. When the equilibrium iron (III) ion concentration was 2,500 mg/L, the maximum adsorption capacity of Fe3O4@C was 163.0308 mg/g. The composite particles of Fe3O4@C were verified as an efficient adsorbent to iron (III) ions, with a fast separation rate from working solution. Accordingly, from an application point of view, it is believed that Fe3O4@C particles can be used for iron (III) ion wastewater treatment.

ACKNOWLEDGEMENTS

This work was financially supported by the National Natural Science Foundation of China (Grant No. 51372232) and the Fundamental Research Funds for the Central Universities (Grant No. 2652015106).

REFERENCES

REFERENCES
Abas
,
S. N. A.
,
Ismail
,
M. H. S.
,
Kamal
,
M. L.
&
Izhar
,
S.
2013
Adsorption process of heavy metals by low-cost adsorbent: a review
.
World Appl. Sci. J.
28
(
11
),
1518
1530
.
Chaturvedi
,
S.
&
Dave
,
P. N.
2012
Removal of iron for safe drinking water
.
Desalination
303
(
10
),
1
11
.
Cui
,
Z. Z.
,
Li
,
D. C.
,
Qiao
,
G. J.
&
Li
,
J. S.
2006
Preparation of porous carbon template with controllable channels by phenolic resin pyrolyzing
.
J. Inorgan. Mater.
21
(
4
),
848
854
.
Fan
,
W.
,
Gao
,
W.
,
Zhang
,
C.
,
Weng
,
W. T.
,
Pan
,
J.
&
Liu
,
T.
2012
Hybridization of graphene sheets and carbon-coated Fe3O4 nanoparticles as a synergistic adsorbent of organic dyes
.
J. Mater. Chem.
22
(
48
),
25108
25115
.
Hao
,
Y. N.
&
Wang
,
X. M.
2017
Structure characterization and adsorption performance for Cu2+ of activated carbon derived from Xanthoceras sorbifolia bunge shell
.
Appl. Chem. Ind.
46
(
1
),
81
85
.
Harris
,
P. J. F.
&
Tsang
,
S. C.
1998
Encapsulating uranium in carbon nanoparticles using a new technique
.
Carbon
36
(
12
),
1859
1861
.
Hojati-Talemi
,
P.
,
Azadmanjiri
,
J.
&
Simon
,
G. P.
2010
A simple microwave-based method for preparation of Fe3O4/carbon composite nanoparticles
.
Mater. Lett.
64
(
15
),
1684
1687
.
Khalili
,
F. I.
,
Khalifa
,
A.
&
Al-Banna
,
G.
2016
Removal of uranium(VI) and thorium(IV) by insolubilized humic acid originated from azraq soil in Jordan
.
J. Radioanal. Nucl. Chem.
311
(
2
),
1375
1392
.
Lugo-Lugo
,
V.
,
Barrera-Díaz
,
C.
,
Ureña-Núñez
,
F.
,
Bilyeu
,
B.
&
Linares-Hernández
,
I.
2012
Biosorption of Cr(III) and Fe(III) in single and binary systems onto pretreated orange peel
.
J. Environ. Manage.
112
(
24
),
120
127
.
Moreno-Piraján
,
J. C.
,
Garcia-Cuello
,
V. S.
&
Giraldo
,
L.
2011
The removal and kinetic study of Mn, Fe, Ni and Cu ions from wastewater onto activated carbon from coconut shells
.
Adsorption
17
(
3
),
505
514
.
Pozdnyakov
,
I. P.
,
Plyusnin
,
V. F.
,
Grivin
,
V. P.
,
Vorobyev
,
D. Y.
,
Bazhin
,
N. M.
,
Pagés
,
S.
&
Vautheyb
,
E.
2006
Photochemistry of Fe(III) and sulfosalicylic acid aqueous solutions
.
J. Photochem. Photobiol. A Chem.
182
(
1
),
75
81
.
Ranjithkumar
,
V.
,
Hazeen
,
A. N.
,
Thamilselvan
,
M.
&
Vairam
,
S.
2014
Magnetic activated carbon-Fe3O4 nanocomposites-synthesis and applications in the removal of acid yellow dye 17 from water
.
J. Nanosci. Nanotechnol.
14
(
7
),
4949
4959
.
Sarin
,
P.
,
Snoeyink
,
V. L.
,
Bebee
,
J.
,
Jim
,
K. K.
,
Beckett
,
M. A.
,
Kriven
,
W. M.
&
Clement
,
J. A.
2004
Iron release from corroded iron pipes in drinking water distribution systems: effect of dissolved oxygen
.
Water Res.
38
(
5
),
1259
1269
.
Schaper
,
A. K.
,
Hou
,
H.
,
Greiner
,
A.
,
Schneider
,
R.
&
Phillipp
,
F.
2004
Copper nanoparticles encapsulated in multi-shell carbon cages
.
Appl. Phys. A
78
(
1
),
73
77
.
Wang
,
L.
,
Gan
,
K.
,
Lu
,
D.
&
Zhang
,
J.
2016
Hydrophilic Fe3O4@C for high capacity adsorption of 2,4-dichlorophenol
.
Eur. J. Inorgan. Chem.
2016
(
6
),
890
896
.
Xuan
,
S.
,
Hao
,
L.
,
Jiang
,
W.
,
Gong
,
X.
,
Hu
,
Y.
&
Chen
,
Z.
2007
A facile method to fabricate carbon-encapsulated Fe3O4 core/shell composites
.
Nanotechnology
18
(
3
),
035602
.
Yuan
,
Q.
,
Nan
,
L.
,
Yue
,
C.
,
Geng
,
W.
,
Yan
,
W.
,
Ying
,
Z.
,
Xiotian
,
L.
&
Bin
,
D.
2013
Effect of large pore size of multifunctional mesoporous microsphere on removal of heavy metal ions
.
J. Hazard Mater.
254–255
(
1
),
157
165
.
Zheng
,
J.
,
Liu
,
Z. Q.
,
Zhao
,
X. S.
,
Liu
,
M.
,
Liu
,
X.
&
Chu
,
W.
2012
One-step solvothermal synthesis of Fe3O4@C core-shell nanoparticles with tunable sizes
.
Nanotechnology
23
(
16
),
165601
.
Zhou
,
J.
,
Qiu
,
Z.
,
Zhou
,
J.
,
Si
,
W.
,
Cui
,
H.
&
Zhuo
,
S.
2015
Hierarchical porous carbons from alkaline poplar bark extractive-based phenolic resins for supercapacitors
.
Electrochim. Acta
180
,
1007
1013
.