Magnetic Nanohybrid Materials for Water-Pollutant Removal

CHAPTER 1

Magnetic Nanohybrid Materials for Water-Pollutant Removal Ya Pang1, Jiangfang Yu2,3, Lin Tang2,3, Guangming Zeng2,3, Chao Zhu2,3 and Xue Wei2,3 1

Department of Biology and Environmental Engineering, Changsha University, Changsha, P.R. China College of Environmental Science and Engineering, Hunan University, Changsha, P.R. China Key Laboratory of Environmental Biology and Pollution Control (Hunan University), Ministry of Education, Changsha, P.R. China

2 3

Contents 1.1 Adsorption of Heavy Metals Using Magnetic Nanohybrid Materials 1.1.1 Preparation of Magnetic Nanohybrid Materials 1.1.2 Application of Magnetic Nanohybrid as Adsorbent 1.1.3 Factors That Influence Adsorption Effect of Magnetic Nanohybrid Adsorbent 1.1.4 Evaluation of the Magnetic Nanohybrid Adsorbent 1.2 Removal of Water Pollutants Based on Magnetic Nanohybrid Catalysts 1.2.1 Carbon-Based Magnetic Nanohybrid Adsorbent 1.2.2 Multimetals Based Magnetic Nanohybrid Catalyst 1.3 Future Perspectives and Expectations References

4 4 6 11 15 19 20 25 25 26

Nanomaterial is defined as material with any external dimension in the scale of 1
100 nm or having internal structure in the nanoscale (1
100 nm). Natural source, incidental forming, and artificial preparation are the main sources of nanomaterial. Since its report, nanomaterial has been receiving great attentions due to its unique size, high surface area, novel physical, and chemical property. It likes a bridge between bulk materials and atom or molecular size materials and shows different properties compared to bulk or atom materials. For example, copper nanoparticles smaller than 50 nm are considered to be super hard materials and possess super ductility. Gold nanoparticles appear deep red to black in solution, and nanozero valent iron Fe0 can ignite spontaneously in air. And they have been applied in various fields, such as sensor, environmental remediation, advanced materials preparation, medicine, and energy. Nanomaterials can be divided into different types. Based on the chemical Nanohybrid and Nanoporous Materials for Aquatic Pollution Control DOI: https://doi.org/10.1016/B978-0-12-814154-0.00001-3

© 2019 Elsevier Inc. All rights reserved.

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Nanohybrid and Nanoporous Materials for Aquatic Pollution Control

compositions, it was categorized as metal nanomaterial such as nano-Fe0 or iron oxides, Pt, Ag, Au, nano-TiO2, nonmetal nanomaterial, like nanocarbon-based materials, nano-SiO2, nanosemiconductor materials, high molecular nanomaterial as well as nanohybrid materials. Among which, nanohybrid materials, combined with two or more compositions together, could possess the advantages of different materials and even show new properties, endowing them with more diversity and unlimited possibility. In the past decades, magnetic nanoparticles such as Fe0, Fe3O4, γ-Fe2O3, CuFe2O4 have been widely studied and applied as catalyst [1], adsorbent [2], and carrier [3], since they are low cost, nontoxic, easy for separation, and modification. However, they have their own drawbacks in water remediation when used alone. First, in aqueous solutions, some of these magnetic nanoparticles are unstable and easily tend to aggregate, which greatly weaken their availability for environmental application. Second, these original magnetic nanoparticles are very active and can be easily oxidized when in air and aquatic environment. Third, other species such as phosphates or bicarbonate can compete with heavy-metal ions for sorption sites due to their high concentrations in groundwater. Fourth, the natural hydroxyl and carboxyl groups could be easily formed on the surface of iron oxide nanoparticles; thus, nonheavy metal ions, such as Mg21 and Ca21 (common coexisting ions in real samples), can strongly interact with binding sites and occupy them. These factors limited its practical extensive use for environmental remediation. Therefore, developing magnetic nanohybrid materials is necessary to improve the performances and practicability of magnetic nanoparticles. Magnetic nanohybrid material not only possesses the property of nanomaterials but also contains magnetic property, which makes it an excellent adsorbent or catalyst in water-pollution remediation. Hybridization process could introduce new composition and change the chemical structure of the original materials, bring new features to the materials. As magnetic nanohybrid substance, the distinctive feature is magnetism. When using magnet to approach the solution, magnetic nanohybrid materials accumulate to the side of magnet quickly, and the suspension become clear within 1
3 min. After removing the magnet and shaking the sample for seconds, the magnetic nanohybrid materials dispersed well again. Separation and dispersion process can be easily switched by simply applying an external magnetic field, which

Magnetic Nanohybrid Materials for Water-Pollutant Removal

3

demonstrated satisfying water dispersion and magnetic separation ability for application. Using magnetic nanohybrid materials as adsorbent, the introduction of other organic or inorganic composition could improve the adsorption capacity greatly. Professor Ahmad Umar’s group published a review paper about using magnetic iron oxide nanoparticles for water contaminant removal, such as heavy metals, dye, and other organic pollutants [4]. As summarized in that review, the adsorption capacity for heavy metals by pristine magnetic nano-Fe3O4 was commonly low. For example, the pristine Fe3O4 nanoparticles have a maximum adsorption capacity of 96.8 mg/g for Pb under the optimal operation conditions [5]. After modification with thiosalicylhydrazide, the maximum adsorption capacities of Pb21, Cd21, Cu21, Zn21, and Co21 were found to be 188.7, 107.5, 76.9, 51.3, and 27.7 mg/g, respectively [6]. And Zarei et al. reported that a novel magnetic nanohyrid adsorbent showed a high adsorption capacity of 476 mg/g for Cd [7]. For preparation of this adsorbent, magnetic nano-Fe3O4 cluster@SiO2 magnetic nanoparticles were first functionalized with N-(2-aminoethyl)-3-aminopropyltrimethoxy silane to obtain NH2-modified magnetic nanoparticles (NH2
MNP), then, recombinant form of a rice metallothionein isoform was immobilized onto the surface of the synthesized NH2
MNP to obtain the final adsorbent. Besides, the aggregation of magnetic nanohybrid materials was weaker than pristine magnetic nanoparticles as proved by Liu’s research [8]. The acidic
basic stability was also improved after modification of magnetic nanoparticles due to the protection of functional substances. Magnetic nanoparticles heterogeneous catalyst can activate H2O2 or persulfate (peroxymonosulfate and peroxydisulfate) to produce radicals for organic-pollutants removal. For example, 75% of 10 mg/L acetaminophen was degraded in the presence of 0.8 g/L nano-Fe3O4 and 0.2 mmol/L peroxymonosulfate [9]. Carbon materials can also act as catalyst to activate persulfate or peroxymonosulfate. And a complete removal of 9.4 mg/L phenol was obtained in the presence of 0.1 g/L carbon nanotubes (CNTs) and 1 mM peroxymonosulfate [10]. Single composition may suffer the drawbacks of low stability, high cost, inconvenient for reuse. Combination of two or more materials together to develop hybrid materials could improve the performances. And using magnetic multi-walled carbon nanotubes (MWCNTs) (CuFe2O4/MWCNTs) to activate persulfate proved to be an effective way to remove organic pollutants [11]. Magnetic ordered mesoporous carbon was an efficient catalyst in Suzuki
Miyaura reactions, and this catalyst was easy for quantitative

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Nanohybrid and Nanoporous Materials for Aquatic Pollution Control

recover. Therefore, developing magnetic nanohybrid catalyst becomes a feasible way to improve the catalytic activity and practicability. In this chapter, we mainly focus on environmental application of magnetic nanohybrid materials—specifically, using magnetic nanohybrid materials as highly effect adsorbent for heavy-metal removal and as catalyst in sulfate radical-based advanced oxidation technology for organicpollutants removal. The preparation and application of magnetic nanohybrid materials would be discussed and some representative work by our groups would be presented.

1.1 ADSORPTION OF HEAVY METALS USING MAGNETIC NANOHYBRID MATERIALS 1.1.1 Preparation of Magnetic Nanohybrid Materials Nano-Fe3O4 particles or Fe
M (M 5 Co, Mn, Ni, Zn) complex are the most common used materials for the source of magnetic property [12]. Chemical coprecipitation method is convenient for preparation of spherical nano-Fe3O4 with relative uniform size. And the particle size could be controlled by adjusting the reaction time, temperature, solution pH, and mechanical rotational speed. For example, using this method, we synthesized magnetic nano-Fe3O4 with relative even particle size [13]. Besides, Pereira synthesized superparamagnetic ferrite nanoparticles (MFe2O4, M 5 Fe, Co, Mn) through a novel one-step chemical coprecipitation method by use of alkanolamines isopropanolamine and diisopropanolamine as alkaline agent [14]. Hydrothermal method has its advantages to prepare magnetic nanoparticles since the relative high temperature (100
200˚C) and pressure ( . 10 MPa) conditions are good for the enhancement of purity and magnetic property. For instance, Chen reported a hydrothermal technique for rapid preparation of CoxNi12xFe2O4 (x 5 0, 0.3, 0.7, 0.9, 1.0) magnetic nanoparticles with excellent magnetic properties and high adsorption capacity for Congo Red [15]. In addition, sol
gel method [16] and pyrolysis method [17] are also often applied for preparation of magnetic nanoparticles. The method for preparation of magnetic nanoparticles is relative mature and diversity. Choice could be made based on the purpose and subsequent modification, Fig. 1.1 presented several typical characterization images of Fe3O4 nanoparticles. For synthesizing of magnetic nanohybrid materials, inorganic or organic functional compounds, or microbes are hybridized with the magnetic nanosubstance through chemical or physical modifications, which

Magnetic Nanohybrid Materials for Water-Pollutant Removal

5

Figure 1.1 (A) Scanning electron microscopy (SEM) image of nano-Fe3O4 particles, (B) High resolution transmission electron microscopy (HRTEM) image, and (C) Selected area electron diffraction (SAED) patterns of the mesoporous Fe3O4 nanoparticles. Reprinted with permission from S. Asuhan, H.L.Wan, S. Zhao, W. Deligeer, H.Y. Wu, L. Song, O.Tegus, Water-soluble, mesoporousFe3O4: Synthesis,characterization, and properties, Ceramics International 38 (2012) 6579
6584.

greatly increase the diversity and application of magnetic nanomaterials. Thousands of magnetic nanohybrid materials were prepared and applied. Based on the structure and constitute, they can be divided into three categories. One is using magnetic nanoparticles as host, which was modified with functional compositions, such as polyethylenimine (PEI) or Polyacrylic acid (PAA)-modified magnetic nanohybrid materials. Another one is using magnetic nanoparticles as doping or guest composition to modify the main composition, like magnetic nanoparticles dispersed on nanotubes or microbes. The third one is to evenly combine the magnetic nanoparticles and hybrid substance, including some magnetic biochar and ploymetallic nanohybrid materials. And magnetic nanohybrid materials could be produced through one pot method [18,19]. For instances, CuO
Fe3O4 nanohybrid material can be easily prepared by mixing CuSO4  5H2O and FeSO4  7H2O in pH of 8, with the continuous air purging (flow rate 5 3 L/min) at 80˚C. 3Fe21 1 6HO2 1 0:5O2 -Fe3 O4 1 3H2 O

(1.1)

Cu21 1 2HO2 -CuO 1 H2 O

(1.2)

For preparation of Fe3O4/reduced graphene oxide (rGO), graphene oxide (GO), and Fe21 and Fe31 were mixed well, then adjusting the solution pH to 9
11 and stirring for 30 min under temperature of 80˚C. Similarly, Fe3O4/CNTs or Fe3O4/biochar can also be prepared by this method. This synthesis strategy avoids the separation and purification of

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Nanohybrid and Nanoporous Materials for Aquatic Pollution Control

intermediates, which could improve the yield and decrease secondary pollution. But its applicability is limited since some compositions could not be formed in the same reaction conditions. Multisteps method is another universal way to prepare magnetic nanohybrid materials [20,21]. For example, using Fe(NO3)3  9H2O and melamine as precursors, followed by ultrasonication and drying process, the resulting solid sample was further carbonized at 800˚C under N2 atmosphere to obtain nitrogen-doped magnetic carbon materials [19]. And our group prepared nano-Fe3O4 particles by chemical coprecipitation method, and then PEI was modified to its surface by using 2% glutaric dialdehyde as cross linking agent [13]. For specific application, target production, the precursor, cost and operation conditions should be taken into consideration when choosing suitable method. After preparation, characterization of the magnetic nanohybrid materials is necessary to explore morphology and physical
chemical properties. SEM, TEM, X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) are the most used tools. SEM and TEM are used to observe the morphology and physical structure. XRD is used to describe the crystal structure. FTIR is used to detect the possible functional groups on materials. And XPS is a useful tool to check the surface chemical state of materials from atomic scale, which could provide valuable quantitative information of the material being studied. Typical representative imagines (Figs. 1.2
1.4) showed the characterization results of some magnetic nanohybrid materials [6, 11, 22].

1.1.2 Application of Magnetic Nanohybrid as Adsorbent Using different functional groups, such as amino, carboxyl, hydroxyl, and thiol groups to modify the pristine nanomaterials to increase the effective adsorption sites, is a strategy to develop magnetic nanohybrid adsorbent. Our group developed a series of PEI-modified magnetic nanohybrid adsorbent for heavy-metal removal. For example, PEI-grafted magnetic porous adsorbent was synthesized to uptake Cu, Zn, and Cd from water with maximum adsorption capacities of 157.8, 138.8, and 105.2 mg/g, respectively, based on Langmuir adsorption model [23]. What is more, the adsorbent showed a preferential adsorption of Cu21 . Zn21 . Cd21, as presented in Fig. 1.5. And the adsorption preference among them was attributed the radius of the heavy metals and stability constant between heavy metals and amino, as shown in Fig. 1.6, which indicated that the

Figure 1.2 (A) SEM and (B) TEM images of CuFe2O4/MWCNTs and (C) FTIR spectra of samples (CuFe2O4, CuFe2O4/MWCNTs, and recycle CuFe2O4/MWCNTs) calcined at 400°C. Reprinted with permission from X. Zhang, M. Feng, R. Qu, H. Liu, L. Wang, Catalytic degradation of diethyl phthalate in aqueous solution by persulfate activated with nano-scaled magnetic CuFe2O4/MWCNTs, Chem. Eng. J. 301 (2016) 111.

Figure 1.3 The XPS spectra for Fe 2p of fresh and used CuO
Fe3O4 magnetic nanohybrid material. XPS, X-ray photoelectron spectroscopy. Reprinted with permission from Y. Lei, C.S. Chen, Y.J. Tu, Y.H. Huang, H. Zhang, Heterogeneous degradation of organic pollutants by persulfate activated by CuO
Fe3O4mechanism, stability, effects of pH and bicarbonate ions, Environ. Sci. Technol., 49 (11) (2015) 6838
6845. Copyright (2015) American Chemical Society.

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Nanohybrid and Nanoporous Materials for Aquatic Pollution Control

(3 1 1)

(1 1 1) Intensity (a.u.)

(4 4 0) (5 1 1)

(2 2 0)

(C)

(4 0 0)

(5 3 3)

(4 2 2)

(B)

(A) 5

15

25

35

45

55

65

75

85

2θ(°)

Figure 1.4 The XRD patterns of the synthesized MNPs: (A) Fe3O4, (B) Fe3O4@PAA, and (C) Fe3O4@PAA@TSH. Reprinted with permission from K. Zargoosh, H. Abedini, A. Abdolmaleki, M.R. Molavian, Effective removal of heavy metal ions from industrial wastes using thiosalicylhydrazide-modified magnetic nanoparticles, Ind. Eng. Chem. Res., 52(42) (2013) 14944
14954. Copyright (2013) American Chemical Society.

(A)

(B)

160 140

80

40 20 0

100

200

300

400

500

60

2+

40 20

Cu

60

+

Cu2+ Zn2+ Cd2+ Langmuir model for Cu2+ Langmuir model for Zn2+ Langmuir model for Cd2+

Zn 2

80

0 Cd 2+

qe (mg/g)

100

Trimetal solution Cu2++Cd2+ solution Cu2++Zn2+ solution Cd2++Zn2+ solution

100

efficienc Removal

y (%)

120

120

Ce (mg/L)

Figure 1.5 Adsorption isotherms and modeled results using Langmuir equation and competitive adsorption of multimetal solution (adsorbent dose: 0.05 g, pH: 6.5, contact time: 10 min). Reprinted with permission from Y. Pang, G.M. Zeng, L. Tang, Y. Zhang, Y.Y. Liu, X.X. Lei, et al., PEI-grafted magnetic porous powder for highly effective adsorption of heavy metal ions, Desalination 281 (2011) 278
284.

intrinsic properties of heavy metals determined the adsorption behaviors. During the operation process, by applying an external magnetic field, the adsorbent could be easily separated and the liquid become clear within several minutes.

Magnetic Nanohybrid Materials for Water-Pollutant Removal

9

Figure 1.6 Relationships between qm and atomic radius Ra (A); between v0 and stability constant of amino complex lg β 3 (B). Reprinted with permission from Y. Pang, G.M. Zeng, L. Tang, Y. Zhang, Y.Y. Liu, X.X. Lei, et al., PEI-grafted magnetic porous powder for highly effective adsorption of heavy metal ions, Desalination 281 (2011) 278
284. (A)

(B) 6 5

80

4

60 100 mg/L 200 mg/L 400 mg/L 500 mg/L

40 20

t/qt

Removal efficiency (%)

100

3

100 mg/L 200 mg/L 400 mg/L 500 mg/L t/qt = 0.0403t + 0.0143 (100 mg/L) t/qt = 0.0213t + 0.0107 (200 mg/L) t/qt = 0.0134t + 0.0163 (400 mg/L) t/qt = 0.0121t + 0.0115 (500 mg/L)

2 1

0 0

20

80 40 60 Time (min)

100 120

0 0

20

40

60 80 t (min)

100 120

Figure 1.7 Effect of contact time on removal of different initial concentrations Cr(VI) and linear fit of experimental data using pseudo-second-order kinetic model (adsorbent dose 0.08 g, pH value 2.2, temperature 25°C). Reprinted with permission from Y. Pang, G.M. Zeng, L. Tang, Y. Zhang, Y.Y. Liu, X.X. Lei, et al., Preparation and application of stability enhanced magnetic nanoparticles for rapid removal of Cr(VI), Chem. Eng. J. 175 (2011) 222
227.

Besides, using stability-enhanced magnetic nanohybrid (PEI-coated γ-Fe2O3@Fe3O4) adsorbent to remove Cr(VI) was carried out by our group [13]. The adsorbent was able to effectively remove anionic Cr(VI) by electrostatic adsorption in the pH range of 2
3 due to the large amount of protonated imine groups on its surface. Almost 10 min is enough to obtain adsorption equilibrium independent of initial Cr(VI) concentration, and pseudo-second-order model could well describe the process as shown in Fig. 1.7. Competition from common coexisting ions

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Nanohybrid and Nanoporous Materials for Aquatic Pollution Control

Table 1.1 Leached Fe rate and Cr(VI) removal efficiency of PEI-modified magnetic nanohybrid materials after treated by different concentrations HCl and NaOH solution [13] HCl (mol/L) NaOH (mol/L) Untreated

Concentration 0.2 0.5 1 2 4 0.2 0.5 1 Leaching rate (%) 2.74 4.68 21.6 58.9 100 0.15 0.23 0.18 0 Removal rate (%) 88.4 91.0 92.1 88.6 87.0 85.4 88.6 86.8

(K1, Na1, Ca21, Cu21, Cl2, and NO2 3 ) was found to be insignificant. The adsorbent had satisfying acid
alkali stability and could be regenerated by 0.02 mol/L NaOH solution. Leaching of Fe ions was also investigated, as presented in Table 1.1. Besides using PEI, tetraethylenepentamine [24], polyacrylic acid [25], γ-mercaptopropyltriethoxysilane [26] are effective functional polymers for magnetic nanoparticles modification. Biomass-based magnetic nanohybrid adsorbent is another promising strategy to develop economic adsorbent [27,28]. This kind of adsorbent was mainly prepared by doping or depositing magnetic nanoparticles on biomass. The feedstock of biomass, including microbe, fungal, algae, and agricultural wastes, often have abundant functional groups, which are satisfied for developing cheap and efficient magnetic nanohybrid adsorbent. For example, chitosan-coated magnetic nanoparticles were proved to be an economic and efficient nanoadsorbent for Hg removal from industrial wastewater [29]. Coconut shell derived magnetic carbon nanoadsorbent was effective for toxic dye removal [30]. One of the advantages of nanoadsorbent is the high specific surface area, which greatly increases the uptake capacity for pollutants. Therefore, developing magnetic nanohybrid adsorbent with higher surface area becomes another strategy. Magnetic metal organic frameworks (MOFs), magnetic activated carbon, and magnetic mesoporous carbon were typical adsorbent with this feature. It was reported that the surface area of MOFs could be as high as 7000 m2/g, which was applied to adsorb gas, heavy metals, and organic pollutants widely [31,32]. Some specific examples about developing magnetic nanohybrid adsorbent for removal of heavy metals from water were listed above. It is worth noting that magnetic nanohybrid materials were not the simple mixture of different materials. The aim, target pollutant, cost, and preparation technology should be taken into consideration for design, preparation, and application of the magnetic nanohybrid materials.

Magnetic Nanohybrid Materials for Water-Pollutant Removal

11

1.1.3 Factors That Influence Adsorption Effect of Magnetic Nanohybrid Adsorbent Specific surface area and particle size are two of the most important parameters for magnetic nanohybrid materials, since they are related to the adsorption capacity and speed. The smaller the size of the material, the higher the surface area obtained. It is reported that the adsorption capacity of 8 nm Fe3O4 nanoparticles was 7 times higher of 50 mm Fe3O4 particles [33]. And adsorption of Cu21 could achieve equilibrium within 1 min using monodisperse chitosan-bound Fe3O4 nanoparticles [34]. Nanomaterials generally have high surface area; for pristine magnetic nanoparticles, the surface area based on the specific surface area (BET) calculation is often no more than 100 m2/g, and this value could be improved to several times larger after introducing new composition or pore structure [35]. The magnetic nanohybrid materials are inclined to aggregation in aquatic environment due to the high free energy. By diluting the concentration [36] or surface modification [37,38] may decrease the aggregation. Therefore, in practice, a balance between high surface area and good dispersion need to be considered when using magnetic nanohybrid materials as adsorbent. The kind of functional groups and structure is also crucial to the adsorption performances of magnetic nanohybrid materials. Different from magnetic nanoparticles, magnetic nanohybrid materials could possess lots of functional groups due to the hybridization of new substance. For adsorption of heavy metals, NH
, NH2
, COOH
, OH
, and SH
are the widely used groups, and the adsorption capacity could increase greatly after introduction of functional groups to the magnetic nanohybrid materials [39
43]. For instance, amine-functionalized mesoporous Fe3O4 nanomaterial showed high adsorption capacity of 523.6, 446.4, and 369.0 mg/g for Cu(II), Cd(II), and Pb(II), respectively [44], which were larger than activated carbon. Besides, modification of activated carbon, carbon nanotubes, GOs, halloysite, hydrotalcite, chitosan, and so on with magnetic nanoparticles was also an effective strategy to develop magnetic nanohybrid adsorbent. And some of the magnetic nanohybrid materials could reduce the adsorbed heavy metals to low toxic species. For example, magnetic Fe3O4/halloysite nanohybrid materials showed an adsorption capacity of 132.2 mg/g for Cr(VI), and the adsorbed Cr(VI) could be reduced to low toxicity Cr(III) because of the reduction effect of Fe3O4 nanoparticles and hydroxyl groups [45]. And if Fe0 was the source of magnetism, the reduction ability of magnetic nanohybrid adsorbent is expected [46].

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Nanohybrid and Nanoporous Materials for Aquatic Pollution Control

The porous structure was also beneficial for the adsorption performance of magnetic nanohybrid materials, since its existence reduced the contacting resistance between adsorbent and pollutant in liquid, increased the contacting area of adsorbent. An amino-functionalized magnetic Fe3O4 nanoadsorbent owned the maximum adsorption capacity of 40.10 mg/g for Pb21 [47], where the adsorption capacity of Pb increased to 369 mg/g when using amine-functionalized mesoporous Fe3O4 nanoparticles [44]. Several strategies can achieve this goal. One is using porous materials as carriers, such as mesoporous silicon, mesoporous carbon, and metal organic frame, where pristine magnetic nanoparticles or functionalized magnetic nanoparticles could be introduced on subsequently. As for another strategy, after the preparation of magnetic nanohybrid materials, the porous structure could be brought in by calcination or washing treatment. Isoelectric point, which reflects the surface charge of material, also influences the adsorption effect of magnetic nanohybrid materials. For pristine magnetic Fe3O4 nanoparticles, the pHZPC was about 6. The pHzpc changed greatly after forming new magnetic nanohybrid materials. For example, the pHzpc of humic acid coated Fe3O4 nanoparticles decreased to 2.3 [8]. On the contrary, PEI-modified Fe3O4 nanoparticles increased to 11.4 [13]. It is well known that the materials are positive charged when the solution pH is lower than pHzpc, and negative charged when the solution pH is larger than pHzpc. Most of the heavy metals exist in acid or neutral aquatic environment with positive charged species and form metal hydroxide precipitation under alkali solution. From the point of electrostatic adsorption, the negative charged magnetic nanohybrid materials was good for adsorption of most of the heavy metals. It is worthy to note that Cr(VI) exists in water mainly with the negative charged 22 2 form, such as CrO22 4 ; Cr2 O7 ; and HCrO4 , which is different from Cu, Zn, Cd, and Pb. And positive charged magnetic nanohybrid adsorbent is beneficial for Cr(VI) removal. In practice, the properties of target pollutant should be taken into consideration for design of magnetic nanohybrid adsorbent. Besides electrostatic adsorption, chemical binding is an important mechanism to remove heavy metals from water. The modification of different functional groups not only change the pHzpc of magnetic nanohybrid materials, which can be applied to absorb heavy metals through electrostatic effect, at the same time, supply abundant active sites for binding heavy metals through chemical bonds. And the different adsorption mechanism could be used to uptake heavy metals selectively in binary-

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Magnetic Nanohybrid Materials for Water-Pollutant Removal

(A)

(B)

100 0.5 80

qe (mmol/g)

0.4 2–

60 %

PbEDTA 2+ Pb 2– ZnEDTA 2+ Zn

40

0.3 0.2 0.1

20 0

Zn Pb

0.0 0

1

2

EDTA (mmol/L)

3

4

0.4

0.8

1.2

1.6

2.0

EDTA (mmol/L)

Figure 1.8 Speciation diagrams for equimolar zinc and lead at pH 6 as a function of EDTA concentration (A); adsorption of Zn and Pb in binary components solution as a function of EDTA concentration at pH of 6 (B). Reprinted with permission from G.M. Zeng, Y. Pang, Z.T. Zeng, L. Tang, Y. Zhang, Y.Y. Liu, et al., Removal and recovery of Zn21 and Pb21 by imine-functionalized with tunable selectivity, Langmuir 28 (2012) 468
473. Copyright (2012) American Chemical Society.

metal solution. In our research, an interesting study was done to elucidate the selective adsorption Pb and Zn binary-metal solution based on different adsorption mechanism. Specifically, the pHzpc of PEI-modified magnetic nanoparticles material was 11. By adjusting the solution pH and adding different concentration of ethylene diamine tetraacetic acid (EDTA), the Pb and Zn existed with different species, as presented in Fig. 1.8. The Pb existed with the species of PbEDTA22 and Zn existed mainly with the species of Zn21, when 1.2 or 1.4 mM EDTA was added to the Pb (1 mM) and Zn (1 mM) binary-metal solution. Based on this species distribution, Zn could be selectively removed through chemical binding between imine groups and Zn ions, when the solution condition was [EDTA]/[M21] 5 0.7 with pH of 6. And the species distribution results were calculated by the software of Visual MINTEQ2 [48]. By adjusting the solution pH, Zn and Pb species were shown in Fig. 1.9. For selective adsorption of Pb, the solution condition was of [EDTA]/[M21] 5 0.7 with pH of 2, and Pb was removed mainly through the electrostatic adsorption between negative charged magnetic nanohybrid materials and positive Pb species; the result was presented in Fig. 1.10. Additionally, using magnetic nanohybrid materials to simultaneously remove heavy metals and organic pollutants such as dye, phenol were also widely applied [49
52], where some of them facilitate the adsorption of each other whereas others may play negative effect on another removal.

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Nanohybrid and Nanoporous Materials for Aquatic Pollution Control

(A)

(B) 100

100 Zn2+

60

ZnEDTA2–

80

ZnHEDTA1–

60

%

ZnH2EDTA

Pb2+

%

80

40

40

20

20

PbEDTA2– PbHEDTA1–

(A)

0

(B)

0 2

3

5

4

6

2

3

pH

4

5

6

pH

Figure 1.9 Speciation diagrams for zinc (A) and lead (B) in the presence of EDTA as a function of pH. Reprinted with permission from G.M. Zeng, Y. Pang, Z.T. Zeng, L. Tang, Y. Zhang, Y.Y. Liu, et al., Removal and recovery of Zn21 and Pb21 by iminefunctionalized with tunable selectivity, Langmuir 28 (2012) 468
473. Copyright (2012) American Chemical Society.

1.4 1.2

qe (mmol/g)

1.0 0.8 0.6

Pb Zn Fit curve for Pb

0.4 0.2 0.0 0.0

0.5

1.0

1.5 2.0 Ce Pb (mmol/L)

2.5

3.0

3.5

Figure 1.10 Selective adsorption of Pb in binary component solution (equimolar metal ions, adsorbent dosage: 0.05 g, contact time: 12 h, pH 2). Reprinted with permission from G.M. Zeng, Y. Pang, Z.T. Zeng, L. Tang, Y. Zhang, Y.Y. Liu, et al., Removal and recovery of Zn21 and Pb21 by imine-functionalized with tunable selectivity, Langmuir 28 (2012) 468
473. Copyright (2012) American Chemical Society.

The abovementioned parameters (surface area, size, structure, isoelectric point) are the inherent properties of magnetic nanohybrid materials. The external factors, which include solution pH, temperature, coexisting ions, natural organic matters (NOMs), also influence the

Magnetic Nanohybrid Materials for Water-Pollutant Removal

15

adsorption performance greatly. As mentioned above, adsorbent has its own isoelectric point, and solution pH influences the surface charge of magnetic nanohybrid materials, leading to the change of electrostatic adsorption. What is more, the species of heavy metals pollutant also change under different solution pH, and the H1 or OH2 may also compete with pollutants for the active sites. It is possible that the metal composition leaks out from the materials under strong acidic conditions, resulting in the poor stability and secondary pollution. Therefore, study of the effect of pH on pollutants removal is necessary for research and practical application. And developing magnetic nanohybrid materials with wide pH adaptability could improve its performances. Most of the adsorption is exothermic process, which means that increasing the temperature is beneficial for pollutants removal. And adsorption isothermal and thermodynamics could well describe the process. Lots of common anions or cations coexist in aquatic environment with the target contaminant. They may prohibit or promote the removal of the pollutants. Therefore, their effects need to be investigated during operation. For practical application, the effect of NOMs is complicated, and humic acid is often chosen as the model NOMs to study. On the one hand, humic acid could occupy the adsorption sites of magnetic nanohybrid adsorbent, reducing the adsorption capacity. On the other hand, humic acid could combine heavy metals due to lots of functional groups on it, contributing the removal of pollutants [53]. Some of research is even using humic acid to modify magnetic nanoparticles for high efficient removal of heavy metals [8]. In practice, these parameters should be taken into consideration.

1.1.4 Evaluation of the Magnetic Nanohybrid Adsorbent Adsorption isothermal is one of the most important factors to evaluate the performances of artificial magnetic nanohybrid adsorbent, since adsorption capacity, adsorption stability, and adsorption state can be expressed through adsorption isothermal. Traditional Freundlich and Langmuir models, as well as Temkin and Dubinin
Redushckevich (D
R) models are often applied to describe it. Freundlich sorption model assumes that different sites with several adsorption energies are involved, which is widely used to describe the adsorption behaviors occurring on heterogeneous surface sites. It is expressed as qe 5 KF Ce1=n

(1.3)

16

Nanohybrid and Nanoporous Materials for Aquatic Pollution Control

Table 1.2 Langmuir and Freundlich model parameters for Cr(VI) adsorption by PEImodified γ-Fe2O3@Fe3O4 were at different temperatures [13] Langmuir model Freundlich model

35˚C 25˚C 15˚C

qm (mg/g)

KL (L/mg)

KL (L/mol)

R2

KF

n

R2

74.07 78.13 83.33

0.0684 0.111 0.125

3.56 5.77 6.50

0.991 0.989 0.988

11.74 18.13 20.85

3.27 3.46 3.75

0.925 0.969 0.934

where KF and n are the Freundlich constants related to adsorption capacity and adsorption intensity, respectively. The Langmuir adsorption model is based on the assumptions that the adsorption occurs on monolayer and all sites are equal, which is often applicable for modeling the adsorption on homogeneous surface sites. Its model is qe 5

qm bCe ð1 1 bCe Þ

(1.4)

where Ce (mg/L) is the equilibrium solute concentration, qe (mg/g) is the amount of heavy metals adsorbed at equilibrium, b (L/mg) is the equilibrium constant related to adsorption energy, and qm (mg/g) is the maximum adsorption capacity. For example, we prepared PEI-modified magnetic nano-γ-Fe2O3@Fe3O4 as adsorbent to adsorb Cr(VI), and used Freundlich and Langmuir models to describe the adsorption isothermals, where the results were presented in Table 1.2. Besides the above two classical models, Temkin model was also used for the further analysis. This model is developed based on the assumption that adsorption energy decreases linearly with the surface coverage and is more applicable to chemical adsorption process. The corresponding model is qe 5 KT lnCe 1 KT lnf

(1.5)

where qe (mg/g) is the amount of adsorbate after adsorption equilibrium, Ce (mg/L) is the equilibrium solute concentration, KT (L/mg) is the Temkin constant related to the heat of adsorption, and f is the maximum binding energy constant. Additionally, the D
R model, which does not assume a homogeneous surface, agrees more with the actual situation, and has been used

Magnetic Nanohybrid Materials for Water-Pollutant Removal

17

Table 1.3 The obtained results of isotherm models of TC adsorption on SNMS-800 at pH 5 7 [54] Temperature Langmuir model Freundlich model Temkin model D
R model (K)

298

R2 qm KL RL

0.988 187.266 0.044 0.236

R2 KF 1/n

0.989 69.949 0.147

R2 0.909 R 0.784 KT 14.754 qm 154.965 f 167.340 Kd 4.272 E 0.342

308

R2 qm KL RL

0.986 238.095 0.052 0.376

R2 KF 1/n

0.989 78.933 0.173

R2 KT f

0.922 R 0.837 20.383 qm 192.235 78.634 Kd 0.768 E 0.807

318

R2 qm KL RL

0.995 253.807 0.085 0.291

R2 KF 1/n

0.996 84.867 0.184

R2 KT f

0.934 R 0.921 22.680 qm 210.304 79.292 Kd 1.204 E 0.6445

for adsorption mechanism distinguish, generally, which has been presented as below: qe 5 qm expð2 Kd ε2 Þ

(1.6)

1 Þ Ce

(1.7)

ε 5 RT lnð1 1

E 5 ð2kd Þ2ð1=2Þ

(1.8)

where qe (mg/g) is the amount of adsorbate after adsorption equilibrium, Ce (mg/L) is the equilibrium solute concentration, Kd (mol2/kJ2) is the constant related to the adsorption energy, ε (J/mol) refers to the Polanyi potential, and E (kJ/mol) presents the mean free energy of adsorption. Generally, according the above formulas, when the obtained mean free energy (E kJ/mol) value is less than 8, the adsorption is considered to be dominated by physical forces. Correspondingly, the E (kJ/mol) value referring to chemical adsorption is ranging from 8 to 16. For instance, our laboratory prepared magnetic sludge biochar and applied it to adsorption tetracycline hydrochloride [54], the adsorption isothermals using the four models were presented in Table 1.3.

18

Nanohybrid and Nanoporous Materials for Aquatic Pollution Control

Table 1.4 Express and characters of different kinetics models [54]

Kinetics Nonlinear pseudomodel first-order

Nonlinear pseudosecond-order

logðqe 2 qt Þ 5

qe (mg/g): equilibrium adsorption capacity; logqe 2 k1 =2:303 t qt (mg/g): the sorption amount at time t; k1 (1/min): the adsorption rate constant
k2 (g/mg min): the rate t=qt 5

constant determined t=qe 1 1=k2 qe 2 by the plots of t/qt versus t


Weber
Morris

qt 5 kip t 1=2 1 ci

kip (mg/g min0.5): the rate constant; ci: the intercept related to the thickness of the boundary layer

Bangham equation

lg qt 5
1=m lg t 1 lg k

m and k: the constant determined by the plots of lg qt versus lg t

Evolich equation

qt 5 α 1 ke ln t

α and ke: the adsorption constants calculated by the linear fitting equation about qt versus ln t

Adsorption kinetics is another important factor to evaluate magnetic nanohybrid materials. As reported in our researches [13,23], only 5
15 min are needed for PEI-modified magnetic nanomaterials to achieve adsorption equilibrium. The fast speed is due to the good dispersity and small size of nanomaterials. And different kinetics models, such as pseudo-first-order, pseudo-second-order, Bangham model, Elovich model, and Weber
Morris model could be used to describe the kinetic process and are summarized in Table 1.4, among which, the first two models are the most widely used. And the pseudo-first-order model was based on the membrane diffusion theory. The pseudo-second-order adsorption model was based on the assumption that the rate-controlling step of chemisorption involved valence forces through sharing or exchange of electrons between adsorbent and adsorbate. If the magnetic nanohybrid adsorbent has porous structure, then Bangham model is a

Magnetic Nanohybrid Materials for Water-Pollutant Removal

19

good choice to describe its adsorption kinetics. Besides, Weber
Morris model could use to explain the kind of adsorption controlled the adsorption speed. Actually, besides the above adsorption kinetics models, Elovich model could well reflect the adsorption kinetics, which occurred on the unsmooth adsorbent. The stability of application magnetic nanohybrid materials as adsorbent to remove water pollutant is important for the usability. The stability includes the integrality of nanohybrid and the metal element leaking in solution and effective lifetime. Chloride acid and sodium hydrogen are often chosen to investigate the acid
base stability of the magnetic nanohybrid materials by immersing the materials in the solution for a while and then determine the possible metal ions concentration such as Fe, Co, or Mn. Besides, the change of saturation magnetization of the magnetic nanohybrid materials could also reflect its stability. XPS, inductive coupled plasma emission spectrometer (ICP), atomic absorption spectroscopy (AAS), and vibrating sample magnetometer (VSM) are the common used tools to check the stability. Specifically, XPS was applied to check the elements change before and after use, especially the atomic percentage and binding energy of Fe. ICP or AAS or elemental analyzer were used to determine the concentration of leaking composition, such as Fe, Co, N, S, and so on. VSM was used to determine the saturation magnetization. Regeneration of adsorbent removes the adsorbed pollutants and recovers the active sites again for reuse. It is directly related to the cost and also influences the recovery of valuable heavy metals. The regeneration method is chosen based on the adsorption mechanism. Acid or base solution washing is the most widely used method, since acid or base could supply abundant H1 or OH2 to compete and replace the original target pollutant. Since Cd, Zn, Pb, and Cd are mainly existed in water with positive charged form, HCl or acetic acid (HAC) are often used as regeneration solution. On the contrary, Cr and As existed mainly with the negative charged form; therefore, sodium hydrogen is often used to regenerate the adsorbent. Besides, heat treatment and adding chelation agent are also useful for regeneration.

1.2 REMOVAL OF WATER POLLUTANTS BASED ON MAGNETIC NANOHYBRID CATALYSTS Apart from being used as adsorbent, magnetic nanohybrid materials also act as catalyst in advance oxidation technology to remove water

20

Nanohybrid and Nanoporous Materials for Aquatic Pollution Control

pollutants. Advanced oxidation technology is a chemical treatment procedure that is designed to remove pollutants by oxidation through reaction with high active radicals such as hydroxyl radicals or sulfate radicals. And in this part, we have mainly discussed the sulfate radical-based advance oxidation technology. Sulfate radicals have a standard redox potential of 2.6 eV and life time of t1/2 5 30
40 μs [55]. And the reactivity of sulfate radical is pH independent, its reaction with organic pollutants is very fast (105
106 M21 S21). Sulfate radical are produced by activation of persulfate or peroxymonosulfate. Different methods were applied to activate persulfate, which included UV light, O3, heat, ultrasound, and base [56,57]; transit metal Co, Fe, Cu, Mn, Ag, and its metal oxide as well as metal complex [58,59]; carbon-based materials, like activated carbon, carbon nanotubes, mesoporous carbon, GO [10,60]; and organic compounds, such as humic acid, ketones, low carbon chain aldehydes, quinones [61,62]. Among which, magnetic nanoparticles and nanohybrid materials are gaining wide attentions. Fe has three valence states [Fe0, Fe (II), and Fe(III)], which means it can serve as electron donor or acceptor. It has been reported that using magnetic nanoparticles such Fe3O4, Fe2O3, or Co2FeO4 as catalyst to activate hydrogen peroxide or persulfate for hydroxyl or sulfate radical production was effective for organicpollutants removal [63
65]. Although they are able to activate oxidant to produce active radicals, the pristine magnetic iron oxides have the drawbacks, such as low efficiency, poor stability, easy to aggregation. Magnetic nanohybrid materials based on iron oxides nanoparticles may solve the problems. They are divided into two categories to discuss in this chapter. One is carbon-based magnetic nanohybrid materials, the other is magnetic binary or ternary metals nanohybrid catalyst.

1.2.1 Carbon-Based Magnetic Nanohybrid Adsorbent Carbon materials such as carbon nanotubes, mesoporous carbon, GO, fullerene, carbon fiber have been getting wide attention due to the structure and physical or chemical properties. Some of them have been used alone in advanced oxidation technology. For example, 3D cubic mesoporous carbon was proved to effectively activate persulfate for phenol compounds degradation, and the efficiency was even better than Fe21 or Ag1 based homogeneous systems [66]. And single- or multiwalled nanotubes could also activate persulfate to degrade phenolic compounds and certain pharmaceuticals through nonradical pathways [10]. Recently, rGO was also

Magnetic Nanohybrid Materials for Water-Pollutant Removal

21

applied for persulfate activation [67]. But these carbon catalysts are not convenient for separation and recycle. What is more, they are expensive compared to magnetic nanoparticles. Therefore, developing carbon-based magnetic nanohybrid materials could combine the advantages of carbon materials and magnetic nanoparticles, improving the catalyzing efficiency. Actually, different kinds of carbon-based magnetic nanohybrid materials are prepared. Using Fe3O4 as source of magnetism, magnetic core/shell nanospheres supported Mn catalyst Fe3O4@C/Mn, was synthesized using redox (R), hydrothermal (H), and impregnation (I) methods, respectively [68].The hybrid materials were used to activate peroxymonosulfate for phenol degradation. It is found that Fe3O4@C/Mn prepared by redox method had the best catalytic activity, due to the relative high loading of Mn. And the activity of magnetic nanohybrid materials was better than single carbon nanosphere, Fe3O4, Fe3O4@C and MnO2. The magnetic nanoparticles mainly acted as a carrier and separation measure, and Mn took the main role in activation of peroxymonosulfate. In some researches, the primary catalytic source comes from the magnetic nanoparticles, and modification of other composition could improve the whole performances. For instances, nanoscaled magnetic CuFe2O4/MWCNTs was prepared by sol
gel combustion method, and this magnetic nanohybrid catalyst was able to activate persulfate for degradation of endocrine disruptor diethyl phthalate [11]. And the results showed that the nanohybrid catalyst was much better than single CuFe2O4 or MWCNTs. XPS and EPR analyses indicated that Cu21/Cu31 is the critical catalytic center for persulfate activation. As for the functions of MWCNTs, it not only acts as framework for loading of CuFe2O4 nanoparticles but also facilitates the electron transfer for pollutant degradation and metal ions regeneration. Magnetic carbon xerogels, which were composed of interconnected carbon microspheres and iron and cobalt microparticles, could activate sodium persulfate for degradation of bisphenol A (BPA) [69]. Another interesting research was using magnetic activated carbon (CuFe2O4/AC) as catalyst for activation of peroxymonosulfate [70]. CuFe2O4 distributed on porous-activated carbon by coprecipitation calcination method. And the mass ratio between CuFe2O4 and activated carbon of 1.5 possessed the best catalytic activity for methylene blue, due to the appropriate adsorption capacity for target pollutants and dispersion of CuFe2O4. Peroxymonosulfate was not only used to produce sulfate radicals but also acts as regeneration agent for CuFe2O4/AC. Ascorbic acid coated

22

Nanohybrid and Nanoporous Materials for Aquatic Pollution Control

magnetic nano-Fe3O4 was also proved to be effective catalyst for activation of persulfate, and the catalytic efficiency of magnetic nanohybrid catalyst was about 60% higher than pure nano-Fe3O4 in degradation of 2,4DCP [71]. These researches indicated that although magnetic nanoparticles played the key effect for activation of persulfate, the hybridization of another composition is very important for the enhancement of the whole performances of catalyst. On design and preparation of magnetic nanohybrid catalyst, the host materials and variable materials should be carefully chosen. Besides, GO-based magnetic nanohybrid catalysts are also widely studied. GO is a kind of new carbon materials, which has two-dimensional flat structure with sp2-hybridized carbon configurations, and distributed with abundant functional groups (hydroxyl, carboxyl, epoxyl, carbonyl) [72]. Its unique structure, high surface area, and π bonds make it to be an excellent adsorbent or catalyst or carrier [73]. Pristine GO has no ability to activate persulfate for radical production. But rGO is able to activate peroxymonosulfate for degradation of 2,4-DCP and methylene blue, its activity is better than activated carbon and Co3O4 nanoparticles [67]. Oxygen groups and enriched defects on rGO were the main reasons for its catalysis activity, but the reusability and separation operation of rGO was not satisfactory. Actually, the catalytic activity of rGO could be improved by doping N or S or B. For instance, nitrogen-modified rGO showed an apparent degradation rate constant of BPA of 0.71 min21, being about 700 times that (0.001 min21) by N-free rGO in persulfate system [74]. And codoping of S and N further increased the activity for peroxymonosulfate because of the significant change of surface charge distribution and electrostatic potential of graphene [75]. But the stability and reusability of GO- or rGO-based catalyst was poor compared to conventional metal-based catalyst. Because the surface property and structure of carbon materials are easily affected by the relative high concentration of oxidant, and target pollutant and intermediates could occupy the surface sites. Besides, the separation of carbon materials was not convenient compared to magnetic separation. Therefore, developing GO- or rGObased magnetic nanohybrid catalyst becomes a good strategy to improve the comprehensive catalysis performances. For instances, a simple nanosized Fe0/rGO composite was applied to activate persulfate for degradation of trichloroethylene, and the catalytic activity of this magnetic nanohybrid catalyst was higher than nano-Fe0, due to the better electron transfer rate and lower aggregation [76]. Similarly, rGO-supported magnetite nanoparticle (nFe3O4) composite was also synthesized and used for

Magnetic Nanohybrid Materials for Water-Pollutant Removal

23

Figure 1.11 (A) Kinetic adsorption of trichloroethylene (TCE) on (1) Fe3O4, (2) rGO, (3) nFe3O4/rGO with mass ratio of 1:1, (4) nFe3O4/rGO with mass ratio of 1:2, (5) nFe3O4/rGO with mass ratio of 1:4 and (6) nFe3O4/rGO with mass ratio of 1:6; (B) kinetic data of TCE degradation in the systems of (1) Na2S2O8, (2) nFe3O4
Na2S2O8, (3) rGO
Na2S2O8, (4) nFe3O4/rGO (mass ratio of 1:1)
Na2S2O8, (5) nFe3O4/rGO (mass ratio of 1:2)
Na2S2O8, (6) nFe3O4/rGO (mass ratio of 1:4)
Na2S2O8, and (7) nFe3O4/ rGO (mass ratio of 1:6)
Na2S2O8; A proposed mechanism for the degradation of TCE by magnetic nano-Fe3O4/rGO of persulfate. Reprinted with permission from J. Yan, W. Gao, M. Dong, L. Han, L. Qian, Degradation of trichloroethylene by activated persulfate using a reduced graphene oxide supported magnetite nanoparticle, Chem. Eng. J. 295 (2016) 309
316.

activation of persulfate [77]. Both nano-Fe3O4 and rGO could activate persulfate, and the hybridization of them facilitates the electron transfer and dispersion of catalyst and exhibited satisfying effect as shown in Fig. 1.11A and B. What is more, the excellent magnetically separable nanocatalyst is good for practical application. The mechanism about using rGO/Fe3O4 for persulfate activation was proposed and shown in Fig. 1.11. rGO
Ag0/Fe3O4 nanohybrid material was prepared using in situ nucleation and crystallization method [78]. It was found that deposition of Ag0 and Fe3O4 nanoparticles on rGO nanosheet enhanced the catalytic activity greatly. And pharmaceuticals and endocrine disrupting compounds (phenol, acetaminophen, ibuprofen, naproxen, bisphenol A,

24

Nanohybrid and Nanoporous Materials for Aquatic Pollution Control

β-estradiol, and α-ethinylestradiol) could be well removed from water after activation of persulfate. The adsorption and oxidative degradation of the magnetic carbon-based materials was better than single component (Ag0, Fe3O4, rGO). The synergetic effect between adsorption and catalysis was the main mechanism for pollutants removal. Biochar is a carbon-rich solid derived by pyrolyzing biomass with little or no oxygen. Recently, biochar has been receiving great attentions due to its environment-friendly, wide availability of feedstocks, low cost, and favorable physical/chemical surface characteristics. Apart from being used as adsorbent for heavy-metal organic-pollutants removal, magnetic biochar nanohybrid materials are also prepared as catalysts. Different from the abovementioned relative expensive CNTs or GO, biochar was prepared by pyrolysis or carbonization of crop residues, wood biomass, animal litters. And hybridization of magnetic nanoparticles could be achieved by pretreating biomass using iron ion and chemical coprecipitation of iron oxides or Fe0 onto biochar. For example, Fe0 nanoparticles formed by reduction of FeSO4  7H2O were deposited on rice-hall original biochar to develop magnetic biochar nanohybrid catalyst [79]. This material was able to activate persulfate for degradation of trichloroethylene, and the catalytic efficiency was about 40% and 60% higher than Fe0/persulfate and biochar/ persulfate system under the same operation conditions. And the mass ratio between Fe0 and biochar (1:1, 1:3, 1:5, 1:7) was investigated where the optimal ratio was 1:5. The mechanism study showed that both Fe21/Fe31 redox action and oxygen groups on biochar activated persulfate, where Fe21/Fe31 played the main role. And biochar acted as a mediator to accelerate the electron transfer of Fe0, which also indicated the hybridization between different materials. Similarly, nano-Fe3O4/biochar was prepared for activation of persulfate for degradation of polycyclic aromatic hydrocarbons in marine sediments [80]. Besides, magnetic sludge-derived biochar was synthesized by high temperature coprecipitation method and used as an effective heterogeneous catalyst for persulfate activation [81]. The magnetic nanohybrid catalyst has good stability with negligible iron ion leaking from pH 5.2 to 9.1. In preparation of this sludge-derived magnetic biochar, the magnetic source comes from the adding of iron salts with subsequent chemical coprecipitation. Actually, flocculants polyacrylamide (PAM) and polymeric ferric sulfate (PFS) [[Fe2(OH)n(SO4)32n/2]m (in which, n ,2) were widely used in civil sludge dehydration process. PFS could be the source of iron for preparation of magnetic sludge biochar, and this was confirmed in our laboratory [54]. It is necessary to

Magnetic Nanohybrid Materials for Water-Pollutant Removal

25

mention that different feedstock of biochar greatly affects its property (surface area, pore volume, functional groups, trace elements), and the biochar-based magnetic nanohybrid catalyst also present different performances.

1.2.2 Multimetals Based Magnetic Nanohybrid Catalyst Single metal or metal oxide such as Fe0, iron oxides, MFe2O4 (M 5 Cu, Ni, Co, Mn) have been developed for activation of persulfate or peroxymonosulfate And multimetals based magnetic nanohybrid catalyst was also used to activate persulfate, magnetic CuO@Fe3O4 nanocatalyst is an example Lei presented a research about using CuO
Fe3O4/PS for degradation of phenol [22]. It was found that removal of phenol was mainly attributed to the radicals and Cu(II) resulting from the reaction between PS and Cu(II). The binary metal catalyst had good stability and reusability, and the presence of bicarbonate ions is beneficial for the removal of phenol, which is helpful for practical application of this magnetic nanohybrid catalyst. Similarly, magnetic nano-CoFe2O4/titanate nanotubes (CoFe2O4/TNTs) catalyst was prepared by impregnation
calcination method [82]. And it was proved to be highly effective for peroxymonosulfate activation. And the Co leaking was much smaller than conventional CoFe2O4 or CoFe2O4/SBA-15, which could be attributed to the unique ion-exchange ability between Co21 and TNTs support.

1.3 FUTURE PERSPECTIVES AND EXPECTATIONS Here we discussed the preparation, adsorption, and catalysis application of magnetic nanohybrid materials. Different magnetic nanohybrid adsorbents have different adsorption performances for heavy metals due to their inner property such as surface area, functional groups, zeta potential and pore structure, as well as the adsorption conditions such as solution pH, temperature, contacting time. These factors are discussed specifically. Acting as catalyst in sulfate radical-based advance oxidation technology, the activity of magnetic nanohybrid catalyst is the most important property. We summarized three kinds of magnetic nanohybrid adsorbents, especially the carbon-based magnetic nanohybrid catalyst. These results indicated that using magnetic nanohybrid materials as adsorbent or catalyst are both theory and practical feasible. However, most of the studies are mainly limited in laboratory. Further enhancement

26

Nanohybrid and Nanoporous Materials for Aquatic Pollution Control

the adsorption capacity and catalytic activity, as well as the reusability of magnetic nanohybrid materials, at the same time, decreasing the cost are necessary and need more researches. Besides, adsorption and catalysis are almost simultaneous in the advanced oxidation process, where the synergetic effect need to be further elucidated. As a kind of nanomaterials, the environmental impact evaluation of the magnetic nanohybrid materials also needs to be taken into study.

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