Functional group-rich hyperbranched magnetic material for simultaneous efficient removal of heavy metal ions from aqueous solution

Journal Pre-proof Functional group-rich hyperbranched magnetic material for simultaneous efficient removal of heavy metal ions from aqueous solution Huicai Wang, Zhenwen Wang, Ruirui Yue, Feng Gao, Ruili Ren, Junfu Wei, Xiaolei Wang, Zhiyun Kong

PII:

S0304-3894(19)31242-7

DOI:

https://doi.org/10.1016/j.jhazmat.2019.121288

Reference:

HAZMAT 121288

To appear in:

Journal of Hazardous Materials

Received Date:

29 July 2019

Revised Date:

6 September 2019

Accepted Date:

22 September 2019

Please cite this article as: Wang H, Wang Z, Yue R, Gao F, Ren R, Wei J, Wang X, Kong Z, Functional group-rich hyperbranched magnetic material for simultaneous efficient removal of heavy metal ions from aqueous solution, Journal of Hazardous Materials (2019), doi: https://doi.org/10.1016/j.jhazmat.2019.121288

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Functional group-rich hyperbranched magnetic material for simultaneous efficient removal of heavy metal ions from aqueous solution Huicai Wanga,c*, Zhenwen Wangb,c, Ruirui Yuea,c, Feng Gaob,c, Ruili Rena,c, Junfu Weia,c, Xiaolei Wanga,c, Zhiyun Konga,c a

School of Chemistry and Chemical Engineering, Tianjin Polytechnic University,

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Tianjin 300387, China

School of Environmental Science and Engineering, Tianjin Polytechnic University,

Tianjin 300387, China

State Key Laboratory of Separation Membranes and Membrane Processes, Tianjin

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Polytechnic University, Tianjin 300387, China

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c

Corresponding author

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Graphical abstract

Huicai Wang

Affiliation: School of Environmental and Chemical Engineering, Tianjin PolytechnicUniversity, Tianjin 300387, China Tel: +862283955859 E-mail address: [email protected]

1

HIGHLIGHTS 

High density multifunctional group adsorbent was synthesized, called Fe3O4HBPA-ASA.



Simultaneous efficient removal of Cd(II), Cu(II) and Pb(II) over a wide concentration range. The competitive effects among the metal ions were discussed in the multiple

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

system. 

XPS and DFT calculations reveal the adsorption mechanism towards heavy metal

Fe3O4-HBPA-ASA showed excellent recycling ability in desorption experiment.

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

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ions.

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Abstract:

In order to achieve the purpose of simultaneous removal of coexisting heavy metal

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ions, in this work, functionalized magnetic mesoprous nanomaterials (Fe3O4-HBPA-

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ASA) with high density and multiple adsorption sites were designed and prepared. The obtained Fe3O4-HBPA-ASA was characterized by SEM, FTIR, VSM, TGA and zeta potential. Cu(II), Pb(II) and Cd(II) were chosen as the model heavy metal ions, the adsorption experiments showed that Fe3O4-HBPA-ASA showed hightheoretical adsorption capacitiesin individual system, and the maximum adsorption capacity was 136.66 mg/g, 88.36 mg/g and 165.46 mg/g, respectively. In the binary and ternary 2

systems, the competitive adsorption leads to a decrease in the adsorption capacity of Cu(II), Pb(II) and Cd(II). However, in the ternary system with a concentration lower than 15 mg/L, the simultaneous removal rate was still higher than 90%. The adsorption isotherms and kineticswere well fitted by Langmuir and pseudo-second-order models, respectively. The XPS and density functional theory (DFT) analysis have confirmed that the adsorption of metal ions was related to various types of functional groups on

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the surface of Fe3O4-HBPA-ASA, while the adsorption mechanisms of Cu(II), Cd(II) and Pb(II) were different. Keywords:

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Simultaneous removal; Heavy metal ions; Magnetic mesoprous nanomaterials;

Introduction

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Multiple adsorption sites; Competitive adsorption.

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Recently, with the rapid development of industrialization, the contamination of heavy metal ions has received extensive attention. It is well known that the toxicity and

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non-biodegradability of heavy metal can cause serious environmental problems that endanger human health [1, 2]. As the research progressed, it was found that water

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contaminated by heavy metals usually contained not only single heavy metal ion [3]. Bing et al. found that six heavy metal ions (Cd, Cr, Cu, Ni, Pb and Zn) coexisted in the sediments along the main stream of the Three Gorges Reservoir [4]. The coexistence of different heavy metals might result in a more toxic composite pollution through intermolecular interaction or complexation. In addition, the simultaneous removal

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behavior of coexisting heavy metal ions is more complicated than that of single heavy metal pollution [5]. However, research on the removal of coexisting heavy metal ions is still in its infancy. Various approaches such as adsorption,chemical precipitation, solvent extraction, nanofiltration, membrane separation, and ultra filtration have been utilized to treat heavy metal pollution in wastewater [6, 7]. Among these technologies, the adsorption

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process is considered to be an effective method for removing heavy metal composite

pollutants due to the flexible design of the adsorbent material, simple operation, and

easy popularization [2, 8]. However, some problems in the adsorption and removal of

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heavy metal ions still limit its development, such as the adsorption material lacks

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sufficient binding sites, and the diffusion limitation in the matrix may lead to a decrease in adsorption rate and available capacity, and some adsorption materials showed slow

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adsorption speed and poor adaptability [9]. Moreover, much of the work on removal of

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heavy metals has focused on the single system, little attention has been paid on the coexisted heavy metal system, which are closer to the actual situation, and the

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competitive and antagonistic adsorption in the coexisting systems leads to poor removal. According to reports, the adsorption mechanisms of certain heavy metal ions such

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as copper, cadmium and lead, include interaction with oxygen or other electronegative functional groups, electrostatic interactions, and adsorption at defect sites. Usually chemical adsorption (complexation) coexists with physical adsorption (electrostatic attraction and ion exchange). The dominant adsorption mechanism is different in different situations [10, 11]. In the adsorption process dominated by the chemical

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adsorption mechanism, the active functional groups (such as -COOH and -OH) on the surface of the adsorbentproduce a negative surface charge in aqueous solution, which will cooperate with heavy metal cations to greatly enhance the adsorption and facilitate the removal of heavy metals [12]. Mojgan et al. Prepared Fe3O4/NaP/NH2 nanocomposites to remove 95% of Pb (II) and Cd (II) in aqueous solutions [13]. Shang et al. achieved simultaneous removal of Pb(II), Cd(II) and Hg(II) by a low-cost

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mercapto-modified coal gangue (GC-SH) [14]. Wu et al. synthesized high molecular weight poly (arylene ether sulfone) with side chain carboxyl groups to remove Cu(II),

Pb(II) and Cd(II) in water. It was found that increasing the amount of carboxyl

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functional groups could improve the removal efficiency[7]. Therefore, in the design of

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the adsorbent material, it should be sufficiently considered to improve the adsorption performance of the adsorbent by increasing the density of the functional groups.

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Previously, our group synthesized Fe3O4-HBPA magnetic nanomaterials containing

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high density amino groups, and this study showed that simultaneous removal of Pb(II), Cd(II) and Cu(II) was not possible with the single amino functional group [15].

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Although increasing the density of single functional groupexhibited high adsorption capacity for individual or multiple metal ions, competitive adsorption between different

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heavy metal ions inevitably leads to a low simultaneous removal efficiency. Sun et al. proposed that nanoscale zero-valent iron (nZVI) can remove a variety of metal ions in water, but the agglomeration of nZVI iron in solution needs to be fixed in a specific carrier, and the operation process was cumbersome [3, 16, 17]. It was difficult to remove heavy metal composite pollution by only single group. Different metals might

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require certain adsorption sites for adsorption, and it is expected that providing specific sites for different heavy metals would reduce competitive adsorption and improve the simultaneous removal efficiency of coexisted heavy metal ions. Therefore, it is reasonable to assume that the design of adsorption materials with high density and multiple functional groups could achieve high removal efficient of coexisted heavy metal ions.

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Currently, many studies have been carried out on the use of multifunctional groups

such as nitrogen-containing groups and oxygen-containing groups to modify conventional adsorbent materials to obtain high performance adsorbent materials [18,

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19]. For the Fe3O4-HBPA previously studied by our group [15], in order to improve the

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removal efficiency of Cd, an active site capable of adsorbing Cd(II) should be introduced on the surface of the adsorbent. It has been proved that the introduction of

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groups with high affinity for Cd(II) on the surface of adsorbent materials could improve

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its adsorption capacity. For example, An et al. grafted aminosalicylic acid onto the surface of the polymer (PGMA/SiO2) to synthesize ASA-PGMA/SiO2, which increased

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the adsorption capacity of Cd(II) [20]. The magnetic nanocomposite generally consists of a central portion of a magnetic

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substance and functional groups (-NH2, -COOH, and -OH, etc.) coated on its surface [21]. By the external magnetic field, the magnetic adsorption material can be quickly separated from the liquid phase and exhibits excellent reproducibility [22]. Up to now, many techniques have been used to prepare magnetic nanocomposite adsorbents to remove heavy metal ions. For example, Wang et al. prepared an amino functionalized

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Fe3O4@SiO2 nanocomposite. The composite material is made of surface functionalized magnetic nanoparticles to adsorb heavy metal ions (Pb(II), Cd(II) and Cu(II)) [23]. Liu et al. used urea as a modifier and soft template to prepare functionalized magnetic microspheres NiFe2O4 by simple one-pot solvothermal method for removing metal ions (copper, cadmium, chromium and zinc ions) and coexisting contaminants of three fluoroquinolones (ciprofloxacin, enrofloxacin and norfloxacin) [22].

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In order to confirm our assumption that high density and multifunctional adsorption materials could achieve sufficient high removal rate in coexisting heavy metal ions

system. In this study, Fe3O4-HBPA-ASA with high density of amino, imine and

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salicylic acid groups were designed and prepared for the simultaneous removal of the

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coexisted Pb(II), Cd(II) and Cu(II), and the competitive adsorption behavior was investigated thoroughly. In order to better describe the adsorption mechanism, the

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Langmurir and Freundlich isotherm adsorption models were established. Influencing

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factors such as adsorption kinetics, adsorption thermodynamics, pH, initial concentration and recyclability were also studied. The adsorption mechanism was also

Experiments

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2

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studied by XPS technology and DFT theoretical calculations.

2.1

Chemicals and materials

All chemicals are of analytical grade and are available from commercial sources and can be used without further purification. Namely sodium acetate trihydrate,ferric chloride hexahydrate (FeCl3∙6H2O, >99%), ethylene glycol (EG), 2-Aminoethanol,

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methyl acrylate, methyl alcohol (CH3OH), diethylenetriamine, petroleum ether, glutaric dialdehyde(GA, 50% in water), amidinothiourea, 5-aminosalicylic acid (5-AS)and sodium hydroxide (NaOH, 40.01 g/mol , >96%) are purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd. These national standard solution of Pb(II) (1000mg/L), Cu(II) (1000mg/L) and Cd(II) (1000mg/L) were obtained from National Nonferrous Metals and Electronic Materials Analysis and Testing Center. The standard solution of

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various heavy metals was used to dilute to the concentration required for the experiment, and the pH of the solution was adjusted by sodium hydroxide or nitric acid (HNO3,

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63.01 g/mol, 65.0-68%). Double distilled water was used in all experiments.

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2.2 Preparation of amino-salicylic acid-modified amino-hyperbranched functionalized magnetic magnetite nanomaterials (Fe3O4-HBPA-ASA)

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Firstly, the terminal amino hyperbranched polymer (HBPA) functionalized

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magnetic nanomaterials (Fe3O4-HBPA) were prepared using our reported method [15]. Then the amino-salicylic acid-modified amino-hyperbranched functionalized magnetic

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magnetite nanomaterials (Fe3O4-HBPA-ASA) were fabricated as follow: 0.3g Fe3O4HBPA was dispersed in 50 ml of NaOH (0.01mol/L) followed by the addition of 100

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mL of sodium hydroxide solution (0.02 mol/L) containing 0.3g of 5-aminosalicylic acid. After thoroughly mixing the above system, 3 mL of GA was added and stirred at 25℃ for 8 h. The final product was washed with double-distilled water for 4 times and dried under vacuum at 60 ℃ to give a pale yellow product of Fe3O4-HBPA-ASA. The preparation process of the adsorbent material and the structure of the polymer are shown

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in Scheme 1.

2.3 Characterization In this study, scanning electron microscope (SEM) images were recorded by JSM6380 LV microscope to characterize the morphology of the magnetic nanomaterials. Xray photoelectron spectra (XPS) were carried out on a K-alpha (Thermo Fisher

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Scientific, USA). Fourier-transform infrared (FTIR) analysis was obtained on a Nicolet 6700 (Thermo Scientific, USA) in the range of 4000-450 cm-1 using the KBr pellets

technique. The magnetic properties were performed using a vibrating sample magne-

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tometer (VSM, LAKESHORE-7304, USA). Powder XRD patterns of the nanoparticles

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were obtained on a Holland PAN-alyticalX'Pert PRO X-ray diffractometer. The thermal gravimetric analysis (TGA) curves were measured using a STA409PC DTA/TGA

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instrument (Netzsch, Germany) in the temperature range of 25℃ to 1000℃ under N2 at

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a heating rate of 20℃ min-1. Zetasizer nano ZS90 (Malvern, UK) was used to measure the surface zeta potentials of nanomaterials at various pH values. The specific surface

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area of nanoparticles was measured by N2 adsorption/desorption on a JW-BK surface area measurement instrument following the BET method. The concentration of ions in

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solution was determined by an inductive coupled plasma mass spectrometry (ICP-MS) (iCAP-Q, Thermal Fisher Scientific, USA).

2.4 Adsorption experiments In order to explore the adsorption property of Fe3O4-HBPA-ASA, copper, lead and

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cadmium were selected as adsorbates in this experiment. Batch experiments were carried out in 150 mL Erlenmeyer flask containing a certain amount of adsorbent and 50 mL metal ions. The solution was continuously agitated using a water bath shaker (140 rpm) during the adsorption process, which enhanced the interaction between the metal ions and the adsorbent surface. After adsorption, the adsorbent was separated from the solution by a magnet, and the concentration of heavy metal ions in the solution

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was measured by ICP-MS. The detailed parameters of various batch experiments are shown in Table 1.

Adsorption experiments were carried out in mono- and multi-target systems. In a

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single system, the adsorption capacity of Fe3O4-HBPA-ASA for single metal ions was

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investigated separately. Isothermal adsorption experiments were performed with different initial concentrations of Cu(II), Cd(II) and Pb(II), which determined mono-

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target adsorption isotherms model. Then, in a multi-target system (Cu/Cd/Pb, Cu/Cd,

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Cu/Pb, Cd/Pb), the co-adsorption experiments of heavy metal ions were conducted on Fe3O4-HBPA-ASA, and the adsorption equilibrium concentrations of various metal

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ions were determined to explore the adsorption capacity on magnetic materials. Simultaneously, adsorption isothermal, kinetic and thermodynamic models were

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established based on the measured data to illustrate the adsorption mechanism. The adsorption removal rate and the adsorption capacity of metal ions were calculated according to the following equations:

R(%) 

C0  Ce 100 C0

(1)

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qe 

(C0  Ce )  V m

(2)

Where C0 and Ce represent the initial and equilibrium metal ion concentrations (mg/L), respectively. V is the volume (mL) of the heavy metal solution. m is the amount of the adsorbent (mg). qe represents the equilibrium adsorption capacity of the metal ion (mg/g) [24]. Table 1 Adsorption experimental conditions for metal ions.

No.

Initial metal ions Evaluated parameter

concentration

pH

(mg/L)

(g/L)

temperature

(min)

(℃)

120

25

120

25

1-120

25

Adsorbentdosage

5

2

pH

5

3

Time

5

7

0.8

4

Temperature

5-50

7

0.8

120

25-55

2-150

7

0.8

120

25

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concentration

0.8

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metalions

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Initial

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0.1-2

time

1

5

7

Adsorbbent dosage

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S.

2.5 Density functional theory (DFT) calculations

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Theoretical calculations were used to better understand the adsorption mechanism and configuration of Fe3O4-HBPA-ASA toward heavy metal ions. The hybrid density

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functional theory (B3LYP) have been widely used to investigate the interaction between adsorbent and metal ions [25, 26]. Considering the large molecular structure of HBPA

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and HBPA-ASA, the initial structure of HBPA-ASA for DFT calculation needs to be simplified to improve the calculation efficiency, and thus the initial structure used a fragment containing four amino groups, two of which were modified with aminosalicylic acid. For geometry optimization and energies calculation, the 6-31G(d) basis set was employed for carbon, hydrogen, oxygen and nitrogen, and LANL2DZ

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pseudopotential function was used as the basis set for the heavy metal ions. Furthermore, the solvent effects were also taken into consideration by using SMD model [27]. All calculations were carried out using the Gaussian 16 package [28]. The adsorption energies were calculated according to Eq(3).: ΔE  E complex  E metal ion  E adsorbent (3)

metal ions and the selected fragment, respectively.

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where Ecomplex, Emetalion and Eadsorbent are the calculated total energies of metal complex,

2.6 Desorption regeneration-reuse performance of Fe3O4-HBPA-ASA

To evaluate the reusability of Fe3O4-HBPA-ASA, adsorption of heavy metals

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(Cu(II), Cd(II) and Pb(II)) and regeneration of sorbent were conducted in five

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successive cycles under the same conditions.20mg of samples loaded with heavy metal ions was put in 20ml of saturated EDTA aqueous solution for 3h for desorption. Then,

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the Fe3O4-HBPA-ASA was separated magnetically and thoroughly washed with

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deionized water, and dried under vacuum at 70 °C. Meantime, the concentration of metal ions in the solution were recorded. During this process, the adsorbed metal ions

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are desorbed and subsequently released from the solid adsorbent into the desorption

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medium. The regenerated adsorbent was reused in the next adsorption cycle.

3 Results and discussion 3.1 Characterization of adsorbents SEM was undertaken to characterize the surface morphology of nanomaterials. Fig. 1 shows the SEM images ofmagnetic Fe3O4, Fe3O4-HBPA and Fe3O4-HBPA-ASA. As

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shown in Fig. 1A, the magnetic Fe3O4 exhibited relatively regular spheres with good monodispersity. After varying degrees of modification, the Fe3O4-HBPA (Fig. 1B) and Fe3O4-HBPA-ASA (Fig.1C and D) were observed loose in structure and low dispersibility, irregular in spherical shape and rough in particle surface. This phenomenon is because glutaraldehyde cross-links aminosalicylic acid and HBPA molecules on the magnetic materials to form a thick and dense organic molecular layer,

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which is not easily separated from the magnetic carrier. This results also illustrated the high functional density of the modified material.

The FTIR spectra of magnetic Fe3O4, Fe3O4-HBPA and Fe3O4-HBPA-ASA

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nanoparticles are compared in Fig. 2A. It can be observed that all three nanoparticles

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have absorption peaks at 570 cm-1 and should be generated by Fe-O vibration, which was consistent with the characteristic peak of Fe3O4 [23]. The spectra of magnetic Fe3O4

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and Fe3O4-HBPA are basically consistent with the reported references [15]. The peaks

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around 3450 cm-1 and 1629 cm-1were related to the characteristic peak of the amino group of Fe3O4; for Fe3O4-HBPA, a new adsorption peaks at 1727 cm-1was attributed

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to the vibration of C=O, and the two characteristic peaks at 1599 cm-1 and 3450 cm-1 were caused by the vibration of the amino group, indicating that HBPA has been

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successfully reacted with Fe3O4. As for Fe3O4-HBPA-ASA, the peaks were very similar to Fe3O4-HBPA, and it is difficult to prove that aminosalicylic acid has reacted successfully. Therefore, new evidence was needed to support this conclusion. XRD measurements were used to characterize the crystal structure of magnetic Fe3O4, Fe3O4-HBPA and Fe3O4-HBPA-ASA (Fig. 2B). All three nanomaterials

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exhibited six sharp diffraction peaks at 2θ=30.1˚, 35.5˚, 43.3˚, 53.4˚, 57.2˚ and 62.5˚, corresponding to the (220), (311), (400), (422), (511), and (440) planes. The position and relative intensity of all diffraction peaks were closely related to those of Fe3O4 (JCPDS01-085-1463), which indicated that the functionalization of HBPA andaminosalicylic acid did not affect the crystal form of Fe3O4. The thermal stability of the magnetic nanoparticleswas further determined using

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TGA (Fig. 2C). The total decomposed quantities of the magnetic Fe3O4, the Fe3O4-

HBPA and the Fe3O4-HBPA-ASA samples were calculated to be 8.72 wt%, 40.13 wt% and 58.13 wt%, respectively. For all the samples, there are two decomposing interval

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including weight losses below 350 ℃ and 350 to 900 ℃. In the firstweight loss step,

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the weight losses of 2.24 wt%, 7.73 wt% and 16.10 wt% were detected corresponding to the magnetic Fe3O4, Fe3O4-HBPA, and Fe3O4-HBPA-ASA nanoparticles,

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respectively, which could be attributed to adsorbed water on the samples. In second

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weight loss step, magnetic Fe3O4 had a weight loss of about 6.48 wt%, which was assigned to the 2-aminoethanol grafted onto the surface of magnetic Fe3O4. For Fe3O4-

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HBPA, the weight loss corresponding to the decomposition of HBPA molecules was 32.4%. Compared to Fe3O4-HBPA, the Fe3O4-HBPA-ASA also revealed extra mass loss

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step of 18 wt% due to the decomposition of 5-aminosalicylic acid on the surface of the sample, and it also proved that functional 5-aminosalicylic acid in Fe3O4-HBPA-ASA nanocomposite was about 18 wt%. The molar ratio of aminosalicylic acid to the remaining amino groups on the surface of Fe3O4-HBPA-ASA was approximately calculated to be about 1:1 based on the mass decay. The results also indicated that the

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density of functional groups of Fe3O4-HBPA-ASA was much higher. Different functional groups on the surface of the adsorbent were analyzed by XPS technique. Fig. S1A shows that all three magnetic materials contain C, N, O and Fe, and the contents of the corresponding atom are listed in Table S1. As the degree of modification deepened, the ratio of C and N atom increased, while that of O and Fe decreased significantly, indicating an increase in the content of functional groups on the

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surface of the material. Table S1 shows that the ratio of N on the surface of magnetic

Fe3O4 is only 3.34%, and that of Fe3O4-HBPA is 8.1%, demonstrating that the

crosslinking of HBPA greatly increased the density of the amino functional groups, and

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the functional group density was higher than other studies[23, 29-33]. As shown in Fig.

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S1B, Fe3O4-HBPA contained a variety of nitrogen-containing groups, wherein the amino group content was 35.0%, and the amide group and C=N were only 14.7% and

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6.2%, respectively. For Fe3O4-HBPA-ASA, the N content change was attributed to the

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successful reaction of 5-aminosalicylic acid. Fig. S1B shows the relative amounts of different nitrogen-containing functional groups on Fe3O4-HBPA-ASA. Compared to

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Fe3O4-HBPA, the C-N content (16.5%) of Fe3O4-HBPA-ASA was most significantly reduced, which was related to the fact that a part of the terminal amino groups was

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occupied by salicylic acid groups. At the same time, these amino groups were converted to C=N, and resulted in a significant increase in the content of C=N (26.7%) on the surface of Fe3O4-HBPA-ASA. The results indicated that aminosalicylic acid modified hyperbranched magnetite nanomaterials have been successfully prepared. The charge on the surface of the nanoparticles was studied by the zeta potential and

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the surface charge distribution of the three materials at a pH range of 2.0-10.0 is shown in Fig. 2D. The isoelectric point (IEP) of magnetic Fe3O4 was 4.0, and that of Fe3O4HBPA was determined to be 8.5. The zeta potential of a material with a weak electrolyte on the surface was greatly affected by the pH of the solution [34]. Compared with magnetic Fe3O4, the Zeta potential of Fe3O4-HBPA increases at the same pH due to the introduction of abundant amino and imino functional groups on the surface of the

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material. The IEP of Fe3O4-HBPA-ASA was 3.6, which was much lower than that of Fe3O4-HBPA. This result was due to the fact that the introduced aminosalicylic acid

replaced a part of terminal amino groups of HBPA while increasing the -COOH content,

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resulting in a decrease in the IEP.

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VSM technology was performed to measure the magnetic properties of different materials, and the magnetization curve indicated that all three materials were soft

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magnets as shown in Fig.3A. The saturation magnetization value was found to be 78.57

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emu/g for magnetic Fe3O4, 66.69 emu/g for Fe3O4-HBPA and 60.07 emu/g for Fe3O4HBPA-ASA. Although the HBPA molecular layer wrapped on the surface of the

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magnetic nanospheres caused a decrease in the magnetic properties, and the magnetic saturation strength of Fe3O4-HBPA-ASA was lower than that of Fe3O4-HBPA, all

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materials still exhibited strong magnetic properties, which met the basic requirements for magnetic separation ( Fig. S2). As shown in Fig. 3B-D，the nitrogen adsorption-desorption isotherms and pore size distribute curves were determined by BET technology for magnetic Fe3O4, Fe3O4HBPA and Fe3O4-HBPA-ASA. For magnetic Fe3O4 and Fe3O4-HBPA, isotherms

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showed a typically III type curve, which suggested very low specific surface area and no pores on the surface of magnetic Fe3O4 and Fe3O4-HBPA. As for Fe3O4-HBPA-ASA, it showed IV type curve with an H3-type hysteretic loop, illustrating the existence of mesopores in Fe3O4-HBPA-ASA [15]. In addition, the calculated results indicated that the surface area of magnetic Fe3O4, Fe3O4-HBPA and Fe3O4-HBPA-ASA were 14.23,

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17.24 and 32.57 m2/g, respectively (Table S2).

Sorption studies

3.2.1 Comparison of removal efficiencies of heavy metals on magnetic Fe3O4, Fe3O4-

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HBPA and Fe3O4-HBPA-ASA

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The adsorption capacity of magnetic Fe3O4, Fe3O4-HBPA and Fe3O4-HBPA-ASA adsorbents in multi-target system for Cu(II), Cd(II) and Pb(II) solution (5 mg/L) is

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shown in Fig. 4A. It can be seen from the figure that the removal rate of Pb(II) by

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magnetic Fe3O4 is higher than that of Cu(II) and Cd(II), indicating that there was competitive adsorption among Pb(II), Cu(II) and Cd(II). After HBPA modification, the

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removal of Cu(II) and Pb(II) was greatly increased, indicating that increasing the density of functional groups could improve the adsorption performance, however, the

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removal rate of Cd(II) was still very low. After the introduction of salicylic acid structure, the removal rate of Cd(II) was significantly improved, indicating that simply increasing the density of functional groups could not achieve the purpose of simultaneous removal of several heavy metals, and a group with strong affinity for specific heavy metals should be introduced. Furthermore, compared with Fe3O4-HBPA,

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the removal rate of Pb(II) on Fe3O4-HBPA-ASA was reduced by 3.03%, which may be attributed to the decrease in the amino groups on the surface of the material after introduce of aminosalicylic acid. Those results indicated that the two type functional groups should have an appropriate ratio for the purpose of simultaneous removal. The data showed that the amino group and the salicylic acid group worked better at a ratio of 1:1. All those results provided a good proof for our assumption.

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3.2.2 Effect of sorbent dosage and initial pH

The pH of the solution is considered to be the most important controlling factor in

the heavy metal adsorption process because it strongly affects the surface charge of the

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adsorbent and the valence and ion form of heavy metals in the solution [7]. This study

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systematically studied the effect of solution pH on the adsorption process. The initial pH was researched over the range of 2-9 for Cu(II), Cd(II) and Pb(II) to prevent the

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possible hydrolysis at higher pH value. As shown in Fig. 4B, the adsorption capacity of

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the Fe3O4-HBPA-ASA had been increasing in the pH range of 2-7, and gradually stabilized in the pH range of 7-8. The surface charges of adsorbents are positive as pH

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< IEP due to the functional groups (-NH2, -OH and -COOH) on the surface of the adsorbent were protonated. As shown in Fig. S3, in this pH range, there heavy metals

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were mainly present as divalent cations state (Cd2+, Cu2+and Pb2+). Adsorption of the cations was inhibited by electrostatic repulsion. As the pH value increases, the functional groups became deprotonated and the zeta potential transferred from positive to negative, which easily bound to positively charged metal ions [35]. Therefore, in this pH range the adsorption capacity increased with increasing pH. From Fig. S3, it can be

18

seen that heavy metals readily combined with hydroxide in alkaline solution to form complex ions or precipitates. Although various metal complexes were formed in the solution of Cu(II) and Pb(II) in the pH range of 6-7 (Fig. S3B-C), divalent cations were still dominant in the system. The adsorption still increased with increasing pH. However, after pH was greater than 7, the adsorption capacity was slightly reduced, especially for Pb(II). This is because, on the one hand, all the three ions formed hydroxide species,

ro of

which affected the interaction between the adsorption active sites and ions. On the other

hand, the Pb(II) solution of high pH was slightly turbid, suggesting that a precipitate formed, resulting in a decrease in adsorption capacity. In addition, complex interaction

-p

between metal ions also contributed to the adsorption capacity. pH 7 was selected for

re

all the adsorption experiments,where the total maximum removal efficiency of heavy metals could be achieved in multi-target systems.

lP

To fully optimize the adsorption conditions, the effects of the adsorbent does on

na

the adsorption and removal efficiency of Cu(II), Cd(II) and Pb(II) were studied. As shown in the Fig.4C, as the mass of the adsorbent increased from 5 mg to 100 mg, the

ur

removal rate continuously increased, and the average adsorption capacity decreased rapidly, which clarified that the active sites on the surface of absorbents were not fully

Jo

utilized at the higher adsorbent dosage [35]. In addition, the removal rate of Cu(II) and Pb(II) by Fe3O4-HBPA-ASA remained almost stable when the adsorption dose reached 30 mg, while that of Cd(II) continued to increase rapidly, which indicated the competitive adsorption occurred between Cu(II), Pb(II) and Cd(II). In order to maximize the interaction between active sites of adsorbents and metal ions and achieve

19

simultaneous removal of Cu(II), Cd(II) and Pb(II), 0.04 g of adsorbent dose was selected for the following experiments. 3.2.3 Adsorption isotherms of single and multi-target system In order to understand the distribution of heavy metal ions in aqueous solution and on the surface of adsorbents at equilibrium, and to understand the competitive effect between heavy metal ions in multi-target systems, two adsorption isotherm models

ro of

were established [36, 37]. The Langmuir isotherm [Eq. 4] and the Freundlich isotherm [Eq. 5] were applied:

K L Q m Ce 1  K L Ce

(4)

q e  K FC1/n e

-p

qe 

(5)

re

where Qm is the Langmuir parameter indicating the maximum monolayer absorption of

lP

metal ions (mg/g), KL is the Langmuir constant associated with the affinity of the binding site (L/mg), qe is the equilibrium adsorption capacity of heavy metals (mg/g),

na

Ce is the residual metal ion concentration (mg/L) in the solution at equilibrium, KF and n are the Freundlich constants related to the adsorption capacity of targets (L/g) and the

ur

adsorption capacity of adsorbents [6]. The Langmuir model is based on the assumption

Jo

of the monolayer, homogenous chemisorption, finite active sites and no interaction between the adsorbents [38, 39]. As an empirical formula, the Freundlich isotherm assumes that the adsorption reaction occurs on a heterogeneous surface and the number of sites is not constant [35]. As shown in Fig. 5A-C and Fig. S4A-C, Langmuir and Freundlich isotherm were applied to the experimental data of Cu(II), Cd(II) and Pb(II) to explain the adsorption 20

mechanism, respectively. All models parameters and constants for the single and multiple systemwere estimated and reported in Table S3. It was observed that the high value of correlation coefficients (0.980 < R2 < 0.992 for single system, 0.977< R2 < 0.990 for binary system, and 0.965 < R2 < 0.975 for ternary system ) were obtained based on the Langmuir isotherm modelfor Cu(II), Cd(II) and Pb(II). And the adsorption capacity of the adsorbent calculated by Langmuir model was very close to the

ro of

experimental data. As for the Freundlich isotherm model, the R2 values ranged from

0.858 to 0.948 for single system, from 0.961 to 0.981 for binary system, and the R2 values ranged from 0.836 to 0.946 for the ternary system. The results showed that the

-p

adsorption conformed to the Langmuir isothermal adsorption model. Moreover,

re

regardless of the single or multiple system, the KL values for Cu(II) on Fe3O4-HBPAASA are greater than that of Cd(II) and Pb(II), possibly revealing that a strong affinity

lP

between Cu(II) and adsorbents.

na

The maximum adsorption capacity (qm) of Cu(II), Cd(II) and Pb(II) on PP/PDA/ASA in all system calculated according to the Langmuir model was presented

ur

in Table S3. It can be seen that the maximum adsorption capacities of Cu(II), Cd(II) and Pb(II) were in individual system 136.66, 88.36 and 165.46mg/g, respectively. The

Jo

comparison of the qm values of Cu(II), Cd(II) and Pb(II) with other magnetic adsorbents reported previously was shown in Table 2. The maximum adsorption capacity of Cu(II), Cd(II) and Pb(II) on Fe3O4-HBPA-ASA were higher than other magnetic adsorbents. In the binary system, the adsorption capacity of each metal ion was decreased compared to the single system. The maximum adsorption capacity of Cu(II) calculated by the

21

Langmuir model were 77.53 and 67.20 mg/g in the system of Cu(II)/Cd(II) and Cu(II)/Pb(II), respectively; and the maximum adsorption capacity of Cd(II) in the system of Cd(II)/Cu(II) and Cd(II)/Pb(II) were 46.16 and 62.25 mg/g, respectively; and the maximum adsorption capacity of Pb(II) were 81.68 and 95.08 mg/g corresponding to the system of Pb(II)/Cu(II) and Pb(II)/Cd(II). Moreover, the theoretical maximum adsorption capacity was reduced to 45.35, 35.18 and 52.05 mg/g for Cu(II), Cd(II) and

ro of

Pb(II) in the ternary system. These results indicated that the coexistence of the three metal ions produced a strong competitive effect in multi-component system.

Table 2 Comparison of adsorption capacities of Fe3O4-HBPA-ASA to magnetic

qm (mg g−1)

CMNPs

Cu(II)

74.63

45.66

44.84

pH6, 25℃

[22]

27.95

27.83

-

pH6-7, 25℃

[40]

57.74

18.25

-

pH5, 25℃

[41]

Fe3O4@C

81.94

-

-

pH3, 30℃

[42]

Fe3O4-Si-COOH BC/FM

ur

KIT-6-SH

na

rGO-PDTC/Fe3O4

147.06

116.28

113.64

pH6, 25℃

[43]

86.2

-

-

pH4, 50℃

[44]

155

93

-

pH6, 25℃

[45]

154

127

-

pH5, 25℃

[46]

-

85

-

pH6, 25℃

[47]

Fe3O4-SH

-

73.85

-

pH6, 25℃

[47]

165.46

88.36

136.66

pH8, 25℃

This work

Jo

Fe3O4-HBPA-ASA

lP

NH2 -MCM-41 nanoparticle

Refs.

Cd(II)

NH2 functionalized magnetic graphene composite

Conditions

Pb(II)

re

Adsorbent

ECCSB@Fe3O4

-p

nanoparticles for Cu(II), Cd(II) and Pb(II) removal.

3.2.4 Sorption kinetics of mono-and multi-target system The contact time between the adsorbate and the adsorbent is an important factor affecting the adsorption performance of the adsorbent. Fig. 6A-C and Fig. S5A-C show the adsorption capacity of Fe3O4-HBPA-ASA towards Cu(II), Cd(II) and Pb(II) in 22

mono-and multi-target system as a function of contact time. The same tendency was observed for all metal ions in the adsorption process, in which the adsorption capacity increased rapidly in the first 20 min, and the increase was slower between 20-40 min. After that, the adsorption capacity remained basically constant, even if the adsorption time was extended to 120 min. However, compared with a single system, the adsorption capacity of each metal ion

ro of

in the multi-target system was relatively reduced, indicating that the coexistence of the three metal ions produced a strong competitive effect, thereby inhibiting the interaction of the contaminants with the functional groups on the surface of the adsorbent.

-p

The initial rapid removal was due to the fast diffusion of contaminants from the

re

aqueous solution onto the surface of Fe3O4-HBPA-ASA. The good dispersing property allowed the adsorbent to be uniformly dispersed in the aqueous solution, and the targets

lP

easily diffused to the active site. The hyperbranched structure provided a large number

na

of adsorption sites for the metal ions, and the adsorbent had a high affinity for metal ions.

ur

Herein, two of the most common adsorption kinetic models of pseudo-first-order (PFO, Eq. 6) and pseudo-second-order models (PSO, Eq. 7) were used to analyze the

Jo

control mechanism of the adsorption process. q t  Qe  Qe e  k1t

qt 

k 2Q e2 t 1  k 2Qe t

(6) (7)

where qe (mg/g) and qt represent the equilibrium adsorption capacity of Fe3O4-HBPAASA at equilibrium an and at time t (min), respectively, k1 (min-1 ) and k2 (g mg-1 min23

1

) are the adsorption rate constant of pseudo-first and second-order models, respectively. Fig. 6A-C and Fig. S5A-C display the fitting curves from the pseudo-first and

second-order kinetic models, and the parameters obtained from the above models are listed in Table S4. Obviously, the correlation coefficient (R2 > 0.995 for single system, R2 > 0.986 for binary system, and R2 > 0.993 for ternary system) of the pseudo-secondorder kinetic model, for the removal of Cu(II), Cd(II) and Pb(II) on Fe3O4-HBPA-ASA,

ro of

were higher than that of the pseudo first-order kinetic model. And the calculated

equilibrium adsorption capacity (qe, calc) from the pseudo-second-order kinetic model deviated less from the experimental data (qe, exp), illustrating that this adsorption

-p

system was a chemical process that conformed to the pseudo-second-order model.

re

In order to clarify the diffusion mechanism and ratecontrol steps in the adsorption process, the intraparticle diffusion (IPD) kinetic model proposed by Weber and Morris

lP

was applied: q t  k 3 t1/2  C

na

(8)

where k3 is the IPD kinetic model rate constant (mg g-1 min-1/2 ), and C is a constant

ur

related to the thickness of the boundary layer. As illustrated in Fig. 6D and Fig. S6A-C, the multi-linear fitting of qt versus t1/2 displayed three different regions. All the

Jo

parameters obtained by fitting are given in Table S5. The first linear region referred to the collision between the outside of the particles, and the metal ions were combined with the active sites on the outer surface of the adsorbent. The first linear section was a rapid adsorption stage, which represented the process in which metal ions collided with each other in the bulk solution and rapidly diffused to and combined with the surface

24

of the adsorbent. The second section was the diffusion of metal ions in the pores of the magnetic nanospheres, and the diffusion rate was relatively lower than the first stage. The final process was the adsorption equilibrium stage, in which the diffusion rate was reduced, which was mainly caused by the decrease in both the concentration of the adsorbate in the solution and the effective active sites of the adsorbent [35]. 3.2.5 Competitive adsorption

ro of

Fig. 7 shows the adsorption capacity and removal rate of Fe3O4-HBPA-ASA for

Cu(II), Cd(II) and Pb(II) in the mixed solution. As shown in Fig. 7A, the adsorption efficiency of metal ions in binary and multiplemetal ions solutions was particularly

-p

lower than that of a single system, suggesting that the heavy metal ions competed with

re

each other. The Fe3O4-HBPA-ASA could efficiently remove all three metal ions in single component solution. In binary system, the adsorption capacity of Cd(II) was

lP

greatly affected by the presence of Cu(II) in the solution, which indicated that Cu(II)

na

efficiently occupied the active sites on the adsorbent, leading to the inhibition of Cd(II). The presence of Pb(II) in the system also had a little effect on the adsorption of Cd(II).

ur

In present of Cd(II), the adsorption capacity of Pb(II) did not reduce significantly, indicating that Cd(II) had little inhibitory effect on Pb(II). However, when Cu(II) was

Jo

present, the amount of Pb(II) adsorbed was decreased and the adsorption capacity of Cu(II) was close to that of Cu(II) and Cd(II) coexisting system. The results indicated that the adsorbent has a higher affinity for Cu(II) than Pb(II). In the ternary system, as the concentration of heavy metals increased from 2 mg/L to 100 mg/L, the adsorption capacity of the three metal ions increased first and then stabilized, however, the removal

25

rate showed an overall downward trend in Fig. 7B. At low concentration, the adsorbent had sufficient adsorption sites, Cu(II), Cd(II) and Pb(II) exhibited high removal efficiency. The order of the adsorption capacity of three metal ions on Fe3O4-HBPAASA was Cu(II) > Pb(II) > Cd(II), further indicating that the group on the surface of the adsorbent exhibited a strong binding ability to Cu(II). However, at high concentration, the order of adsorption capacity became Pb(II) > Cu(II) > Cd(II).

ro of

Although such a result seems contradictory, it is not the case. It is well known that the

interaction between heavy metal ions and adsorbent follows stoichiometry. More importantly, the atomic weights of Cu, Cd and Pb are 63.55, 112.4 and 207.2,

-p

respectively. For a specific adsorbent, the active sites are the same, and the heavy metal

re

ions with a large atomic weight requires fewer active sites and the corresponding adsorption capacity is lager. Consequently, the order of affinity of the adsorbent for the

lP

three heavy metal ions might be Cu(II) > Cd(II) > Pb(II). For Cd(II), the initial removal

na

efficiency decreased rapidly when the concentrations of metal ions were higher than 15 mg/L, which implied that there was a strong competitive effect between Cu(II)/Pb(II)

ur

and Cd(II). In summary, the competition at high concentrations resulted in a low adsorption capacity for the adsorbent. However, at lower concentrations, the adsorbent

Jo

exhibited high removal rate for the Cu(II), Pb(II) and Cd(II), and the simultaneous removal of Cu(II), Pb(II) and Cd(II) could be achieved in wastewater at low concentrations. There are many factors that affect the heavy metal ions compete for the effective active sites of adsorbents. The ion radius is one of the important factors affecting

26

competitive adsorption. The order of the hydration radius of the three metal ions is Pb(II) (4.01Ǻ) < Cu(II) (4.19Ǻ) < Cd(II) (4.26Ǻ) [48, 49]. Metal ions with smaller radius exhibited a higher adsorption capacity because smaller particles are less subject to the diffusion resistance andmore likely diffuse to the surface and pores of the adsorbent. Moreover, the electronegativity of heavy metal contaminants is another important factor. The electronegativity of Pb(II) is significantly stronger than those of Cu(II) and Cd(II):

ro of

Pb(II) (2.33) > Cu(II) (1.90) > Cd(II) (1.69), therefore Pb(II) is more easily complexed

with -NH2 groups on the surface of the material than Cu(II) and Cd(II) [45, 49]. In addition, Pb(II) is paramagnetic, which also makes it more likely to be absorbed by the

-p

paramagnetic Fe3O4-HBPA-ASA [45, 50, 51]. The ability of metal ions to bind to

re

adsorption sites was also a major factor affecting competitive adsorption. Aminosalicylic acid had a stronger chelation ability with Cu(II) than Cd(II) [20], and

lP

the binding ability of -NH2 to Pb(II)was strong with Cu(II) and Cd(II) [15]. The removal

na

of heavy metal ions by adsorbent was the comprehensive result of the above factors. 3.2.6 Sorption thermodynamics

ur

Thermodynamic studies were conducted to investigate the spontaneity and thermodynamic properties of the adsorption process. The three main important

Jo

parameters (Gibbs free energy change (ΔG°), standard enthalpy change (ΔH°) and standard entropy change (ΔS°)) were obtained by the Van't Hoff equation [Eq. 9] and Gibbs-Helmholtz equation [Eq. 10].

lnK d 

ΔS ΔΗ  R Τ

(9)

G      S  -RTlnKd (10) 27

where R is universal gas constant (8.314 J/mol·K), T is the temperature (K) and Kd is equilibrium constant calculated by the following equation[52]:

Kd 

C0  Ce Ce

(11)

where C0 is the concentration of metal ions in the solution before adsorption(mg/L),

ro of

Cerepresents the equilibrium concentration of metal ions(mg/L). The thermodynamic curves of Fe3O4-HBPA-ASAwere shown in Fig. S7A-C. The value of ∆H° and ∆S° could be obtained using the slope and intercept of the plot of

-p

Hoff plot ( ln Kd and 1/T). The calculated value ∆H°, ∆G° and ∆S° for Pb(II), Cu(II)

and Cd(II) removal are given in Table 3. As shown in the table, the value of ∆G° of

re

Cu(II), Cd(II) and Pb(II) on Fe3O4-HBPA-ASA were negative, indicating that the

lP

adsorption process was spontaneous. As the temperature increases, the value of ∆G° gradually increased, which also supported that the elevated temperature would reduce

na

the adsorption capacity of metal ions. It was confirmed that the heavy metal adsorption was an exothermic process by the negative value of ∆H° obtained. The negative value

ur

of ∆S° supported that heavy metal adsorption was a process of transition from

Jo

randomness in solution to relative order on solid surface. Table 3 Sorption thermodynamics model constants for the sorption of Cu(II), Cd(II) and Pb(II) onFe3O4-HBPA-ASA in multi-component systems. Cu(II) Concentration

5mg/L

Cd(II)

Pb(II)

T

∆G

∆H

∆S

∆G

∆H

∆S

∆G

∆H

∆S

298

-9.18

-34.41

-85.53

-5.23

-21.15

-53.51

-6.57

-28.06

-71.78

308

-7.76

-4.73

-6.19

318

-7.09

-4.01

-5.17

28

20 mg/L

50 mg/L

-6.55

-3.65

298

-6.93

308

-6.27

-3.60

-5.45

318

-6.07

-3.40

-4.88

328

-5.62

-3.00

-4.07

298

-2.76

308

-2.48

-0.50

-3.20

318

-2.36

-0.23

-2.73

328

-2.15

-0.10

-2.41

-18.94

-8.31

-40.65

-18.79

-3.74

-0.73

-4.44 -10.69

-7.07

-23.18

-21.34

-5.50

-3.42

-19.60

-46.77

-13.73

-34.50

Adsorption mechanism

ro of

3.2.7

328

In order to further explore the adsorption mechanism, this study performed XPS

analysis on samples before and after adsorption. The XPS spectra of magnetic Fe3O4-

-p

HBPA-ASA before and after adsorption are shown in Fig. 8A. After adsorption, three

re

new binding energies on the XPS spectra of the adsorbent were observed at 140 eV, 415 eV and 933 eV, respectively, indicating that Pb(II), Cd(II) and Cu(II) were adsorbed on

lP

Fe3O4-HBPA-ASA. As shown in Fig. 8, the C1s and O1s energy peaks of Fe3O4-HBPAASA before and after adsorption of Cd(II) were displayed and the high-resolution of

na

Cd 3d peaks was shown in Fig. 8E. Before adsorption, the C1s spectrum (Fig. 8B) consisted of 5 peaks, corresponding to C-C (284.4eV), C-N (285.4eV), C-O and N-

ur

C=O (286.7eV), C=N (287.5eV), and O-C=O (288.4eV), respectively. The O1s

Jo

spectrum (Fig. 8C) was also composed of 5 peaks, attributing to Fe-O (529.5eV), C-O (530.7eV), O-H (532.0eV), C=O (532.6eV), and O-C=O (533.3eV), respectively. The N1s spectra (Fig. 8D) of Fe3O4-HBPA-ASA was deconvoluted into three peaks, which corresponded to C-N-C and C=N (399.1eV), N-C=O (400.2eV), and N-H (401.3eV), respectively [53, 54]. After binding to Cd(II), As shown in Fig. 8F-H, the binding energy of C-C 29

(284.4eV), C-N (285.3eV), C-N-C and C=N (399.1eV), N-C=O (400.2eV), and N-H (401.3eV) were almost unchanged, but that of C-O (286.5eV) and O-C=O (287.8eV) were decreased. Conversely, thebinding energy of O-H (532.4eV), C=O (532.9 eV), and O-C=O (533.7 eV) increased. This result indicated that the carboxyl and hydroxyl group of aminosalicylic acid were complexed with Cd(II), and the oxygen in the carboxyl group and the hydroxyl group shares electrons with Cd(II), so that the

ro of

electrons and density of adjacent carbon atoms increase caused a decrease in binding energy [46]. Meantime, the increase in the binding energy of the corresponding oxygen

atoms was attributed to the decrease in electron cloud density [1]. At the same time, the

-p

peak area ratios of C-O and O-H before and after adsorption were observed to change

re

significantly, which indicated that the material also had ion exchange effect on Cd(II) adsorption [45]. Notably, the binding energy of C=N (287.3 eV) in the C 1s spectrum

lP

decreased, and the new peak at binding energy of 399.6 eV in the N 1s spectrum was

na

observed, indicating that C=N and NH2 also interacted with Cd(II). However, considering the low removal efficiency of Cd(II) on Fe3O4-HBPA, it might be

ur

considered that C=N and NH2 were not the main force for removing Cd(II), which was probably due to the weak affinity of C=N and NH2 on Fe3O4-HBPA-ASA to Cd(II).

Jo

Therefore, the high removal efficiency of Cd(II) on Fe3O4-HBPA-ASA was mainly attributed to the coordination with salicylic acid groups. Fig. S8 provided the high-resolution C 1s, N 1s and O 1s spectra on Fe3O4-HBPAASA after adsorption of Cu(II) and Pb(II). After adsorption of Cu(II) and Pb(II), as shown in Fig. S8C and G, new peaks in the N 1s spectrum for Cu(II) and Pb(II) were

30

observed at 406.5 eV and 406.6 eV, respectively. Moreover the binding energy of C-N in the C 1s spectrum (Fig. S8B and F) shifted from 285.3 eV to 285.2 eV. These phenomena suggested that the adsorption mechanism of Cu(II) and Pb(II) was the interaction of metal ions with amino groups [2]. In addition, it can be found that the binding energy of O-H (532.2eV), C=O (532.7 eV), and O-C=O (533.5 eV) in the O 1s spectrumincreased significantly from Fig. S8C, which indicated that the carboxyl and

ro of

hydroxyl group of aminosalicylic acid also complexed with Cu(II) [45]. However, it

can be observed from Fig. S8F that the C-O (286.5eV) and O-C=O (287.8eV) binding

energies of the C 1s spectrum did not change significantly after the Fe3O4-HBPA-ASA

-p

adsorbed Pb(II), indicating that Pb(II) did not coordinate with the salicylic acid group.

re

It was proved by XPS technology that the adsorption of metal ions was mainly attributed to the strong chelation between groups (-NH2, C=N, -COOH and -OH) and

lP

metal ions. Generally, at lower functional group densities, the distance between

na

adjacent groups was larger, and the adsorption depended on the carboxyl group and the hydroxyl groups in the aminosalicylic acid molecule to form a stable six-membered

ur

ring structure with the metal ion. The introduction of hyperbranched polymers greatly increase the functional group density on the surface of the materials, resulting in a tight

Jo

distance between functional groups. The formation of heavy metal multi-ligand chelate might not only be in salicylic acid groups on the same microsphere (Fig. S9A), but also the salicylic acid groups on adjacent microspheres(Fig. S9B) might be involved in chelation [55]. Therefore, the main adsorption of Cd(II) onFe3O4-HBPA-ASAmight be the mechanism shown in Fig. S9A and B, and the adsorption of Pb(II) might be the

31

mechanism shown in Fig. S9C. As for the adsorption of Cu(II), the three adsorption mechanisms might be coexisted. 3.3 DFT analysis The interaction between adsorbent and heavy metal ions was further studied by DTF theoretical simulation to elucidate the adsorption mechanism. Studying the interaction of heavy metal ions with various groups in the adsorbent is useful for

ro of

interpreting competitive adsorption. Therefore, it is important to choose appropriate

structure for DFT simulation. Due to the complex structure of Fe3O4-HBPA-ASA, it is not realistic to select the adsorbent as a whole, and the conventional method of selecting

-p

the structure unit cannot represent the characteristics of the adsorbent. Herein, a quarter

re

of the HBPA-ASA structure (Scheme 1), which is relatively large and contains all of the functional groups on the surface of the adsorbent, was selected for DFT calculation

lP

to achieve closer to actual results and improve computational efficiency. Six possible

na

structures (HBPA-ATA-1~6) were optimized based on the results of TGA that the half terminal amino group was crosslinked by aminosalcylic acid. The optimized structures

ur

were shown in Fig.S10 and the energies were listed in Table S6. The result exhibited that the HBPA-ATA-1 had the lowest energy, thus it was selected for subsequent

Jo

calculation. The highest occupied molecular orbital (HOMO) and the lowest unoccupied orbital (LUMO) of HBPA-ATA-1 was calculated which was usually used to confine the adsorption sites of heavy metal ions. As shown in Fig.S11, the HOMO and LUMO were mainly situated at the nitrogen groups (-NH2, -C=N), oxygen group (-COO-) and Benzene ring, which would be the active sites for the adsorption of heavy

32

metal ions. The heavy metal ions were placed over different spatial locations near those sites to study the possible complex formed. The optimized complex structures of the HBPA-ATA-1 with Cd(II) were shown in Fig. 9. There were four complex structures in which the Cd(II) was mainly located on the -NH2, -C=N, -COO- and -C=O groups. The optimized complex structures of HBPA-ATA-1 with Cu(II) and Pb(II) were shown in Fig. S12, S13, respectively. The complex of Cu(II) and adsorbent also exhibited four

ro of

structures, and its binding sites is similar to Cd(II). However, the complex of Pb(II) and

adsorbent had only two structures which mainly situated at the NH2 and -C=O, indicating that the adsorption sites for Pb(II) was less than that of Cd(II) and Cu(II).

-p

The results agreed well with that of the XPS analysis. The adsorption energies (ΔE)

re

were listed in Table 4. The more negative the value is, the stronger is the interaction between the metal-adsorbent complex [56, 57]. It has been reported that the value of

lP

ΔE can be used to determine whether adsorption was controlled by physical adsorption

na

or chamisorption. In general, ΔE is more than 0 eV means that the metal ion cannot be absorbed onto this site; ΔE between 0 and -0.2 eV represents the physical adsorption.

ur

When ΔE is less than -0.5 eV, the chemical adsorption takes place [58]. From Table 4, all of the ΔE, calculated based on optimized geometries, were less than -0.5 eV,

Jo

indicating that the adsorption was chemisorption, which was consistent with the experimental results. The solution effect was assumed by performing single-point calculations with a SMD model, and the results showed that all binding energies were decreased, indicating that the three heavy metal ions were still stably adsorbed onto the adsorbent when it was in water. Furthermore, the order of affinity between the

33

adsorbent and the heavy metal ions calculated by DFT simulation was Cu(II) > Cd(II) > Pb(II), which was the same as the experimental results. The Cu(II) ions showed the largest negative ΔE value, suggesting the strongest chelating interaction between Cu(II) and the absorbent. It should be note that the adsorption sites of Cu(II) and Cd(II) were basically the same, and the adsorption of Cu(II) might cause changes in the conformation of the two salicylic acid groups, especially for HBPA-ATA-Cu-2 and

ro of

HBPA-ATA-Cu-4 displayed in Fig. S12, at the end of the adsorbent, thereby affecting

its interaction with Cd(II). Consequently, there was strong competition between Cu(II) and Cd(II), which was consistent with the experimental results. The ΔE of Cd(II) was

-p

slightly lower than that of Pb(II), and Cd(II) possessed two more adsorption sites than

re

Pb(II), which might be the reason why the adsorbent could improve the removal efficiency of Cd(II) after salicylic acid groups functionalized.

ions

Adsorption sites

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Table 4 The energies ( E) and adsorption energy (ΔE) of different metal complexes.

HBPA-ATA-1

Complexes

KJ/mol

eV

HBPA-ATA-1-Cd-1

-2969.750

-889.399

-9.218

HBPA-ATA-1-Cd-2

-2969.741

-864.622

-8.961

-2969.763

-922.438

-9.560

-2969.799

-1017.452

-10.545

-3117.906

-1537.905

-15.938

-3117.949

-1650.082

-17.101

-3117.902

-1527.820

-15.834

-3117.892

-1500.234

-15.548

-2925.237

-859.686

-8.910

-2925.198

-757.499

-7.851

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ions

Cd

HBPA-ATA-1-Cd-3

-47.156

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HBPA-ATA-1-Cd-4 HBPA-ATA-1-Cu-1 HBPA-ATA-1-Cu-2 HBPA-ATA-1-Cu-3

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Cu

-2922.256 -195.065

HBPA-ATA-1-Cu-4

Pb

Adsorption energy (ΔE)

Energies (E) (Hartree)

HBPA-ATA-1-Pb-1 HBPA-ATA-1-Pb-2

-2.653

3.4 Desorption and reusability Regeneration of adsorbent, also called recovery of adsorption capacity, is a 34

particularly important indicator for assessing the application potential of the prepared adsorbent. At low pH, the observed adsorption of heavy metals was inhibited, meaning that acid treatment was a viable method of regenerating heavy metal-loaded adsorbents. However, the acid might exhibit etching effect on the magnetic adsorbent. Therefore, a series of regeneration experiments were carried out by using EDTA as eluent. As shown in Fig.10, the removal rates of Fe3O4-HBPA-ASA for Pb(II) and Cu(II) still remained

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above 81.64% after five consecutive adsorption/resolution cycles. And the removal rate

of Cd(II) by Fe3O4-HBPA-ASA were 72.34%. This result indicated that the Fe3O4HBPA-ASA showed good regeneration performance after EDTA treatment. It also

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reflected that the adsorbent was stable and the functional groups on the surface of the

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4 Conclusion

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material did not fall off during the elution process.

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In this study, 5-aminosalicylic acid was crosslinked with Fe3O4-HBPA to obtain multifunctional adsorbent Fe3O4-HBPA-ASA, which achieved simultaneous removal

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of heavy metal ions. The modification of aminosalicylic acid was proved by infrared spectroscopy, scanning electron microscopy and TGA analysis etc. Compared with

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Fe3O4-HBPA, the adsorption capacity of modified adsorbent increased by introducing different types of functional groups. Kinetic studies showed that adsorption process of metal ions on Fe3O4-HBPA-ASA were more likely to be modeled by pseudo-secondorder equation. Thermodynamic studies indicated that the adsorption process was an exothermic reaction. Considering atomic weights of Cu, Cd and Pb, the order of

35

adsorption capacity of metal ions in the single and multi-target systemssystem was Cu(II) > Cd(II) > Pb(II). The adsorption results showed that the adsorption mechanism of Pb(II), Cu(II) and Cd(II) on Fe3O4-HBPA-ASA was different. The main adsorption of Cd(II) was mainly attributed to the complexation of aminosalicylic acid; the removal of Pb(II) was related to the remained amino groups on the surface of Fe3O4-HBPAASA; and the aminosalicylic acid and terminal amino groups were combined with Cu(II)

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to remove Cu(II) from the solution. In addition, it exhibited good regeneration

performance after five adsorption/desorption cycles. These result indicated that high density, multi-functional groups modified adsorbent could efficiently treat wastewater

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with multiple heavy metal ions.

Associated content

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Supplementary material. (A) The survey scan XPS spectra of three magnetic

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adsorbents; (B) N 1s XPS spectra of Fe3O4-HBPA; (C) N 1s XPS spectra of Fe3O4HBPA-ASA (Fig. S1). Image before and after magnetic separation of Fe3O4-HBPA (a)

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and Fe3O4-HBPA-ASA (b) (Fig. S2). The effect of pH on the ion form of Cd(II) (A), Cu(II) (B) and Pb(II) (C) (Fig. S3). Adsorption isotherms of Cu(II) (A), Cd(II) (B) and

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Pb(II) (C) in binary system(Fig. S4). Pseudo-first and -second-order models of (A) Cu(II) and Cd(II) in the system of Cu(II)/Cd(II), (B) Cu(II) and Pb(II) in the system of Cu(II)/Pb(II), (C) Cd(II) and Pb(II) in the system of Cd(II)/Pb(II) (Fig. S5). Intraparticle diffusion kinetic model of (A) Cu(II) and Cd(II) in the system of Cu(II)/Cd(II), (B) Cu(II) and Pb(II) in the system of Cu(II)/Pb(II), (C) Cd(II) and Pb(II) in the system of

36

Cd(II)/Pb(II) (Fig. S6). Thermodynamic fitting curve for sorption of (A) Cu(II), (B) Cd(II) and (C) Pb(II) on Fe3O4-HBPA-ASA (Fig. S7). (A) Cu 2p (A), C 1s (B), N 1s (C) and O 1s (D) XPS spectra of Fe3O4-HBPA-ASA after adsorption of Cu(II); (E) Pb 4f (A), C 1s (F) and N 1s (G) spectra of Fe3O4-HBPA-ASA after adsorption of Pb(II) (Fig. S8). Adsorption mechanism of Fe3O4-HBPA-ASA resins for metal ions (Fig. S9). The optimized six possible structures of HBPA-ATA (Fig. S10). HOMO (a) and LUMO

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(b) plots of HBPA-ATA-1(Fig. S11). The optimized structures of HBPA-ATA-1 with

Cu(II) (Fig. S12). The optimized structures of HBPA-ATA-1 with Pb(II) (Fig. S13). XPS determined surface elemental composition of three magnetic adsorbents (Table

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S1). Specific surface area of three magnetic adsorbents measured by BET (Table S2).

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Adsorption isotherm model constants by nonlinear regression method for the sorption of Cu(II), Cd(II) and Pb(II) on Fe3O4-HBPA-ASA in single- and multi-component

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systems (Table S3). Kinetic parameters of PFO and PSO models obtained by nonlinear

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regression method for the sorption of Cu(II), Cd(II) and Pb(II) onto Fe3O4-HBPA-ASA in single- and multi-component systems (Table S4). Kinetic parameters for intraparticle

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diffusion model of Cu(II), Cd(II) and Pb(II) on Fe3O4-HBPA-ASA in single systems (Table S5). The energies (E) and energy (KJ/mol) of six structures of HBPA-ATA (Table

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S6).

Acknowledgments This work was supported byScience and Technology Plan Projects of Tianjin (No.16PTGCCX00070), and the Science and Technology Plans of Tianjin (No.

37

18PTSYJC00180).

Notes The authors declare no competing financial interest.

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Synergistic antibacterial activity and molecular simulated facet specific adsorption studies, J Photoch Photobio B, 162 (2016) 199-207. [58] H.H. Zeng, L. Wang, D. Zhang, P. Yan, J. Nie, V.K. Sharma, C.Y. Wang, Highly efficient and selective removal of mercury ions using hyperbranched polyethylenimine functionalized carboxymethyl chitosan composite adsorbent, Chemical Engineering

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Journal, 358 (2019) 253-263.

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Scheme 1 Schematic diagram of the synthesis process of Fe3O4-HBPA-ASA magnetic

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material.

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Fig. 1. SEM micrographs of magnetic Fe3O4 (50000x) (A), Fe3O4-HBPA (50000x) (B),

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na

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Fe3O4-HBPA-ASA (50000x) (C) and Fe3O4-HBPA-ASA (100000x) (D).

49

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Fig. 2. (A) FT-IR spectra of magnetic Fe3O4 (a), Fe3O4-HBPA (b), and Fe3O4-

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HBPA-ASA (c); (B) XRD pattern of magnetic Fe3O4 (a), Fe3O4-HBPA (b), and Fe3O4HBPA-ASA (c); (C) thermogravimetric analysis (TGA) curves of magnetic Fe3O4 (a),

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materials.

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Fe3O4-HBPA (b), and Fe3O4-HBPA-ASA (c); (D) Zeta potential of three magnetic

50

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Fig. 3. (A) magnetization curves of three magnetic adsorbents; BET of magnetic Fe3O4

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(B), Fe3O4-HBPA (C) and Fe3O4-HBPA-ASA (D).

51

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Fig. 4. (A) Selective removal of heavy metal ions on magnetic Fe3O4 (a), Fe3O4-HBPA (b) and Fe3O4-HBPA-ASA(c) (C0 = 5 mg/L, pH = 7.00 ± 0.01); (B)effect of pH on

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of three metal ions.

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Fe3O4-HBPA-ASA of three metal ions; (C) effect of sorbent dose on Fe3O4-HBPA-ASA

52

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Fig. 5. Adsorption isotherms of Cu(II) (A), Cd(II) (B) and Pb(II) (C) in single and

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ternary system. (m=20mg in single system; m=40mg in multi system; pH=7.0±0.02)

53

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Fig. 6. Pseudo-first and -second-order models of Cu(II) (A), Cd(II) (B) and Pb(II) (C) in single and multilevel system, respectively; intraparticle diffusion kinetic model of

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Cu(II), Cd(II) and Pb(II) on Fe3O4-HBPA-ASA (D).(C0=5 mg/L, m=20mg in single

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system; C0=5 mg/L, m=40mg in multi system; pH=7.0±0.02)

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Fig. 7. (A) Removal rate of three metal ions corresponding to single, binary, multilevel system(C0=50 mg/L, m=40mg, pH=7.0±0.02); The effect of the initial concentration on

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the adsorption capacity (B) and removal rate (C) of the three metal ions ( m=40mg,

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pH=7.0±0.02).

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Fig. 8. (A)The survey scan XPS spectra of Fe3O4-HBPA-ASA before (a) and after

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adsorption of Cu(II) (b), Pb(II) (c) and Cd(II) (d); C 1s (B), O 1s (C)and N 1s (D) XPS

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spectra of Fe3O4-HBPA-ASA; (E) high-resolutionscan XPS spectra of Cd 3d; C 1s (F),

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O 1s (G) and N 1s (H) XPS spectra of Fe3O4-HBPA-ASA after adsorption of Cd(II).

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Fig. 9. The optimized structures of HBPA-ATA-1 with Cd(II) (The H, C, N, O and Cd

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are depicted by white, grey, blue, red color and yellow, respectively).

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Fig. 10. Influence of the five sorption/desorption cycles on the removal rate of Fe3O4-

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HBPA-ASA for Cu(II), Cd(II) and Pb(II).

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