CoreâShell Magnetic Amino-Functionalized Microporous Organic

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Core-Shell Magnetic Amino-Functionalized Microporous Organic Network Nanospheres for the Removal of Tetrabromobisphenol A from Aqueous Solution Zong-Da Du, Yuan-Yuan Cui, Cheng-Xiong Yang, and Xiu-Ping Yan ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b02119 • Publication Date (Web): 21 Feb 2019 Downloaded from http://pubs.acs.org on February 22, 2019

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ACS Applied Nano Materials

1

Core-Shell Magnetic Amino-Functionalized

2

Microporous Organic Network Nanospheres for the

3

Removal of Tetrabromobisphenol A from Aqueous

4

Solution

5

Zong-Da Du,† Yuan-Yuan Cui,† Cheng-Xiong Yang*,†, and Xiu-Ping Yan‡

6

†

7

of Molecular Recognition and Biosensing, Nankai University, Tianjin 300071, China

8

‡

9

on Food Safety, Institute of Analytical Food Safety, School of Food Science and

10

College of Chemistry, Research Center for Analytical Science, Tianjin Key Laboratory

State Key Laboratory of Food Science and Technology, International Joint Laboratory

Technology, Jiangnan University, Wuxi 214122, China

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ABSTRACT: Development of functional porous materials for efficient elimination of

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environmental pollutants is of great importance in green chemistry and environmental

3

protection. Here we report the design, synthesis and application of a core-shell

4

magnetic

5

(Fe3O4@MON-NH2) for efficient magnetic adsorption of a typical brominated flame

6

retardant tetrabromobisphenol A (TBBPA) from aqueous solution. By integrating the

7

hydrophobic networks and hydrogen binding sites within MON-NH2, the synthesized

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core-shell Fe3O4@MON-NH2 nanospheres gave good adsorption for TBBPA. The

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adsorption equilibrium of TBBPA (50 mg L-1) on Fe3O4@MON-NH2 was achieved within

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1 min, showing ultrafast adsorption kinetics of Fe3O4@MON-NH2 for TBBPA. The

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adsorption of TBBPA on Fe3O4@MON-NH2 followed the pseudo-second-order kinetics

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and Langmuir adsorption models, giving a maximum adsorption capacity of 135.9 mg g-

13

1

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endothermic process controlled by positive entropy. Encouraged by large adsorption

15

capacity, rapid adsorption kinetics and the little effect of pH (5-10), dissolved organic

16

matter and ionic strength on the adsorption, the application of Fe3O4@MON-NH2 for

amino-functionalized

microporous

organic

network

nanosphere

at 25 oC. The adsorption of TBBPA on Fe3O4@MON-NH2 was a spontaneous and


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TBBPA removal in real water samples was achieved. The used Fe3O4@MON-NH2 can

2

be easily regenerated and reused at least 4 times without significant reduction of the

3

adsorption capacity. These results revealed the potential of Fe3O4@MON-NH2 for

4

adsorption and removal of environmental pollutants from aqueous solution.

5

KEYWORDS:

6

tetrabromobisphenol A, magnetic adsorption, core-shell, aqueous solution

microporous

organic

network,

7


magnetic

nanosphere,


1

INTRODUCTION

2

Brominated flame retardants (BFRs) are organobromine compounds with inhibitory

3

effect on combustion chemistry and widely used in diverse domains. About 390,000

4

tons of BFRs were purchased in 2011, which comprise approximately 20% of the flame

5

retardants market. Tetrabromobisphenol A (TBBPA) is the most widely used BFR,1,2

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which has been profoundly utilized in plastic polymers, electronic circuit boards and

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many other electronic products. Overuse of TBBPA has also caused many harmful

8

effects on both humans and environment. TBBPA can be easily combined with humus

9

due to hydrophobic interaction, and then transferred into underground water bodies.

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Recently, TBBPA has been considered as potential persistent organic contaminants

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due to its toxicity such as potential thyroid hormone interference activity,

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immunotoxicity, and neurotoxicity and wide distribution in environment. TBBPA can be

13

detected in a variety of environmental media such as soil, air, sediment, water, aquatic

14

organisms, humans and breast milk, which have caused serious pollution problems.3,4

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Therefore, it is imperative to eliminate TBBPA from the environment.


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Conventional TBBPA elimination methods include photoradiation degradation,5

2

microbial degradation,6 high temperature incineration, oxidative degradation7,8 and

3

adsorption.9-17 TBBPA can be completely degraded by photoradiation and microbial

4

degradation.5,6 However, the significant defect of light radiation and microbial

5

degradation is the long degradation time. Under different conditions, the degradation

6

half-life (the time required for the concentration of TBBPA to reduce to half its initial

7

value) can last several days to several months.18 The mechanism of oxidative

8

degradation is simple, but it is difficult to operate. While the operation of high

9

temperature incineration is easy, but the co-produced brominated dioxins and furans

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are highly carcinogenic.5 Adsorption is an effective and rapid way to eliminate TBBPA

11

from aqueous solution as its simple operation and high degradation rate.9 To date,

12

diverse kinds of sorbents including carbon-based adsorbents,10-12 graphene oxide

13

(GO),13,14 and natural materials17 have been used to remove TBBPA from water.

14

Microporous organic networks (MONs) are a novel class of microporous materials

15

constructed via Sonogashira coupling of rigid organic monomers.19-35 The porous,

16

aromatic and robust MONs’ networks make them extensively explored in gas storage,19 5 ACS Paragon Plus Environment


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sensing,20,21 battery,22-25 drug delivery26 and catalysis.27-30 The large surface area, good

2

stability and hydrophobic networks also make MONs potential in adsorption and

3

removal of aromatic environmental pollutants.31-35 For example, Seung Uk Son’s group

4

reported the synthesis of MOF@MON hybrids for the adsorption of toluene from

5

water.33 Two aromatic benzene rings and four hydrophobic bromine atoms are involved

6

within the TBBPA structure, suggesting the hydrophobic MONs should be a good

7

candidate for TBBPA removal. In addition, two hydroxyl groups are included within the

8

TBBPA structure. Therefore, incorporation of hydrogen bonding sites into MONs’

9

networks to synthesize functionalized MONs should be an efficient way to enhance the

10

TBBPA removal efficiency on MONs.

11

Cumbersome steps such as centrifugation or filtration were required during

12

adsorption. Magnetic separation has obtained great attention due to its fast separation

13

from matrix and easy operation under magnetic field.36-38 Seung Uk Son’s group also

14

reported the synthesis of magnetic Fe3O4@C and Co@C from MONs’ composites to

15

enhanced the anode performances in lithium ion batteries and to remove aromatic


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pollutants from water,25,32 revealing the great potential of magnetic MON nanoparticles

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in diverse areas.

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Herein, we report the fabrication of a core-shell magnetic amino-functionalized MON

4

nanospheres (Fe3O4@MON-NH2) for efficient magnetic adsorption and fast removal of

5

TBBPA from aqueous solution. Fe3O4@MON-NH2 with magnetic Fe3O4 core allows its

6

easy separation from aqueous solution under external magnetic fields. The incorporated

7

amino groups within Fe3O4@MON-NH2 can provide hydrogen bonding sites for TBBPA

8

to improve the adsorption performance of Fe3O4@MON without amino groups for

9

TBBPA. The adsorption kinetics, thermodynamics, effects of pH, humid acid (HA) and

10

ionic strength, and regeneration of Fe3O4@MON-NH2 for TBBPA removal were studied

11

in detail. Fe3O4@MON-NH2 also gave higher adsorption capacity than many other

12

reported adsorbents for TBBPA. In addition, Fe3O4@MON-NH2 shows promising for

13

adsorption and removal of other bisphenols. Adsorption and removal of TBBPA from

14

real river and lake water samples on Fe3O4@MON-NH2 was also achieved. The large

15

adsorption capacity, rapid adsorption kinetics, excellent solvent and chemical stability,



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and good reusability make Fe3O4@MON-NH2 great potential for TBBPA removal from

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aqueous solution.

3

EXPERIMENTAL SECTION

4

Materials

and

Reagents.

The

poly(4-styrenesulfonic

acid-co-maleic

acid,

5

SS:MA=3:1)sodium salt (PSSMA 3:1, Mw 20000), FeCl3·6H2O, bisphenol A (BPA,

6

99.0%), bisphenol F (BPF, 99.0%), bisphenol AF (BPAF, 98.0%), TBBPA (98.0%) and

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ethylene glycol (EG, 99.0%) were supplied by Aladdin Chemistry Co. (Shanghai,

8

China). Toluene (99.5%) was purchased from Chemical Reagent Sixth Factory (Tianjin,

9

China).

Tetrakis(4-ethynylphenyl)methane

was

obtained

from

Chengdu

10

TongChuangYuan Pharmaceutical Co. (Chengdu, China). 2,5-Dibromobenzenamine

11

(98.0%) was purchased from Energy Chemical Technology Co. (Shanghai, China).

12

Bis(triphenylphosphine)palladium(II) chloride (98.0%) was bought from TCI Chemical

13

Industry Development Co. (Shanghai, China). Copper (I) iodide (99.9%) was purchased

14

from Sigma Aldrich Trading Co. (Shanghai, China). Pure water was obtained from

15

Wahaha Foods Co. Ltd. (Tianjin, China). HA and anhydrous sodium acetate (99.0%)

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were obtained from Guangfu Fine Chemical Research Institute (Tianjin, China). 8 ACS Paragon Plus Environment

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Methanol (HPLC grade), ethanol (99.7%), isopropanol (99.7%), triethylamine (99.0%),

2

acetonitrile (99.7%) and dichloromethane (99.0%) were purchased from Concord Fine

3

Chemical Research Institute (Tianjin, China).

4

Instrumentation. The N2 adsorption experiments were recorded on a NOVA 2000e

5

surface area and pore size analyzer (Quantachrome, USA) using N2 at 77 K. Thermo

6

gravimetric analysis (TGA) was recorded on a PTC-10A thermal gravimetric analyzer

7

(Rigaku, Japan) from room temperature to 700 oC. The X-ray diffraction spectrometry

8

(XRD) patterns were recorded on a D/max-2500 diffractometer (Rigaku, Japan). Fourier

9

transform infrared (FT-IR) spectra were measured on a Magna-560 spectrometer

10

(Nicolet, Madison, WI). High angle annular dark field scanning transmission electron

11

microscopy (HAADF-STEM) images and energy-dispersive X-ray spectroscopy (EDX)

12

elemental mapping were measured on FEI Tecnai G2 F20 S-TWIN at 200 kV. The

13

transmission electron microscopy (TEM) images were recorded on Tecnai G2 F20

14

(Philips, Holland) at 200 kV. The magnetic hysteresis loops were recorded on a LDJ

15

9600-1 vibrating sample magnetometer (VSM) (LDJ Electronics Inc., USA). The HPLC

16

analysis of TBBPA was measured on a Baseline C18 column (25 cm long × 4.6 mm i.d., 9 ACS Paragon Plus Environment


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China). The methanol/water (90/10, 1.0 mL min-1) was used as the mobile phase. The

2

UV detector was set at 227 nm. The ion chromatography was performed on an ion

3

chromatograph (ICS-1100) fitted with an ION PACTM AS14 column (25 cm long × 4 mm

4

i.d., Dionex, USA). The ultraviolet absorption (UV) spectra were measured on a UV-

5

3600 spectrophotometer (Shimadzu, Japan). The X-ray photoelectron spectroscopy

6

(XPS) experiments were conducted on an Axis Ultra DLD (Kratos Analytical Ltd. Britain).

7

Synthesis of Fe3O4 Magnetic Nanospheres. Magnetic Fe3O4 nanospheres were

8

prepared via a solvothermal method similar to Gao et al.39 Briefly, 0.7 g of PSSMA was

9

dissolved with 10 mL EG, followed by adding 10 mL EG solution of 0.81 g FeCl3·6H2O

10

and 1.8 g of sodium acetate. The mixture was then transferred into a Teflon stainless

11

steel autoclave and reacted at 200 oC for 10 h. After cooling down to the room

12

temperature, the product was separated by a magnet, thoroughly washed with pure

13

water and ethanol, and then dried under vacuum overnight.

14

Synthesis of Fe3O4@MON-NH2 Nanospheres. Fe3O4 nanospheres (0.10 g), CuI (0.50

15

mg, 2.6 μmol) and Pd(PPh3)2Cl2 (1.7 mg, 2.4 μmol) were dispersed with triethylamine

16

(7.5 mL) and toluene (7.5 mL) in a 100 mL three-necked flask. The suspension was 10 ACS Paragon Plus Environment

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sonicated for 30 min and mechanically stirred (500 rpm) at 90 oC for 30 min. 2,5-

2

Dibromobenzenamine (30.1 mg, 0.12 mmol) and tetrakis (4-ethynylphenyl) methane (25

3

mg, 0.06 mmol) were then added. The mixture was reacted at 90 oC for 5 h. After

4

cooling down to the room temperature, the obtained Fe3O4@MON-NH2 was separated

5

by a magnet, washed three times with dichloromethane and methanol, and dried under

6

vacuum overnight. [email protected], Fe3O4@MON-2NH2, and Fe3O4@MON-4NH2

7

were

8

ethynylphenyl)methane and 2,5-dibromobenzenamine concentration to 0.03 and 0.06,

9

0.12 and 0.24, 0.24 and 0.48 mmol, respectively. MON-NH2 was prepared parallelly

10

synthesized

under

the

same

conditions

by

changing

the

tetrakis(4-

without the addition of Fe3O4 for comparison.

11

Preparation of TBBPA Solution. The stock solution of TBBPA (1 mg mL-1) was

12

prepared with ultrapure water at pH 8 and stored at 4 oC in the dark.40 The working

13

solution was prepared from the stock solution by stepwise dilution with ultrapure water

14

at pH 8 just before use. The pH was adjusted with 0.1 M HCl or 0.1 M NaOH.

15

Adsorption Kinetics. For adsorption kinetics study, 3.0 mg of Fe3O4@MON-NH2 was

16

dispersed with 3 mL of TBBPA solution (50, 100 and 200 mg L-1) at 25 oC. The mixture 11 ACS Paragon Plus Environment


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was shaken (200 rpm) and then separated by a magnet after contacting for a fixed time

2

(1 to 240 min). The residual TBBPA in the supernatant was then analyzed by HPLC.

3

The adsorption kinetics of TBBPA (50, 100 and 200 mg L-1) on MON-NH2 was

4

performed parallelly for comparison.

5

Adsorption

Isotherms

and

Thermodynamics.

For

adsorption

isotherms

and

6

thermodynamics studies, 3.0 mg of Fe3O4@MON-NH2 was dispersed with 3 mL of

7

TBBPA solution (50-500 mg L-1). The mixture was shaken (200 rpm) for 2 h in a water

8

bath at predetermined temperature (25-55 oC). The mixture was then separated by a

9

magnet and the residual TBBPA in the supernatant was then analyzed by HPLC. The

10

adsorption isotherm of TBBPA at 25 oC on MON-NH2 was evaluated parallelly for

11

comparison.

12

Effects of HA, Ionic Strength, and pH. To evaluate ionic strength and HA effects, the

13

TBBPA solution (200 mg L-1) was prepared with ultrapure water including various

14

concentrations of NaCl and HA, respectively. To investigate the effect of pH, the pH (5-

15

10) values of TBBPA solution (200 mg L-1) was adjusted with 0.1 M HCl or 0.1 M NaOH.

16

Although TBBPA is poor soluble in acidic pH range, it is still possible to study the pH 12 ACS Paragon Plus Environment

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1

effect in a weak acidic solution.11,13,14 We found that the TBBPA is insoluble at pH 4.

2

Therefore, the pH effect was study in the range of 5-10. The working solution of TBBPA

3

with different pH values was prepared via the following method. The stock TBBPA

4

solution (1 mg mL-1) was first prepared by diluting proper amount of TBBPA in water at

5

pH=8. The water with different pH values was separately prepared with 0.1 M HCl or 0.1

6

M NaOH. The working solution (200 mg L-1) was prepared by diluting the stocking

7

solution with pure water of different pH. The concentration of TBBPA solution was

8

chosen to be 200 mg L-1 for these studies, as almost all the TBBPA was fully adsorbed

9

on Fe3O4@MON-NH2 at concentrations below 100 mg L-1. 3.0 mg of Fe3O4@MON-NH2

10

was dispersed with 3 mL of TBBPA solution (200 mg L-1) with different pH values or

11

ionic strengths or HA concentrations. The mixture was shaken (200 rpm) in a water bath

12

at 25 oC for 2 h. The mixture was then separated by a magnet and the residual TBBPA

13

in the supernatant was then analyzed by HPLC.

14

Desorption Experiments. The pre-adsorbed Fe3O4@MON-NH2 was prepared for

15

desorption experiments. 3.0 mg of Fe3O4@MON-NH2 was dispersed with 3 mL of

16

TBBPA solution (200 mg L-1) under shaking (200 rpm) for 2 h. The suspension was then 13 ACS Paragon Plus Environment


1

separated by a magnet and collected as pre-adsorbed Fe3O4@MON-NH2. The TBBPA

2

was desorbed from the pre-adsorbed Fe3O4@MON-NH2 using methanol, acetonitrile

3

and isopropanol as the desorption solvents. 0.5 mL of desorption solution was mixed

4

with 3.0 mg of pre-adsorbed Fe3O4@MON-NH2. After sonication for 3 min, the mixture

5

was then magnetically separated and the concentration of TBBPA in the supernatant

6

was measured by HPLC. Such desorption process was repeated for 4 times.

7

Reusability of Fe3O4@MON-NH2 for TBBPA. To investigate the reusability of

8

Fe3O4@MON-NH2 for TBBPA, 3 mL of TBBPA solution (200 mg L-1) was mixed with 3.0

9

mg of regenerated Fe3O4@MON-NH2 and kept at 25 oC in a water bath for 2 h. The

10

mixture was separated by a magnet and the supernatant was analyzed with HPLC.

11

Such procedures were repeated for another three times to study the reusability of

12

Fe3O4@MON-NH2 for TBBPA.

13

Comparison of Fe3O4@MON-NH2 with Other Adsorbents for the Adsorption of TBBPA.

14

The adsorption and removal of TBBPA on Fe3O4 and Fe3O4@MON was performed for

15

comparison. 3 mL of TBBPA solution (200 mg L-1) was mixed with 3.0 mg of Fe3O4 or


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1

Fe3O4@MON and kept at 25 oC in a water bath for 2 h. The mixture was separated by a

2

magnet and the residual TBBPA in the supernatant was then analyzed by HPLC.

3

Adsorption of TBBPA in Real Water Samples on Fe3O4@MON-NH2. The lake and

4

river water samples were obtained from local XinKai Lake and Weijin River (Tianjin,

5

China). The water samples were filtered with 0.22 μm membranes before use. 3.0 mg of

6

Fe3O4@MON-NH2 was mixed with 3 mL of lake water or river water spiked with TBBPA

7

(50 mg L-1). The mixture was kept at 25 oC in a water bath for 2 h. The mixture was then

8

separated by a magnet and the supernatant was analyzed with HPLC.

9

RESULTS AND DISCUSSION

10

Preparation of Fe3O4@MON-NH2. Scheme 1 illustrates the in-situ preparation of core-

11

shell Fe3O4@MON-NH2 nanospheres. The Fe3O4 nanospheres were synthesized

12

according to the literature procedure.39 The tetrakis (4-ethynylphenyl) methane, 2,5-

13

dibromobenzenamine and catalysts were then added for in-situ growth of MON-NH2

14

shell on Fe3O4 core. The effect of MON-NH2 monomer concentration on morphology

15

and superparamagnetism of Fe3O4@MON-NH2 was first studied (Figures 1a-1c and

16

Figure S1). The thickness of external MON-NH2 shell of [email protected], 15 ACS Paragon Plus Environment


1

Fe3O4@MON-NH2 and Fe3O4@MON-2NH2 was about 30, 50 and 55 nm, respectively

2

(Figure S1), showing the significant roles of initial MON-NH2 monomer concentration to

3

controllable synthesis of Fe3O4@MON-NH2 nanospheres. However, when the initial

4

MON-NH2 monomer concentration is four times than that for Fe3O4@MON-NH2, thick

5

and uneven MON-NH2 shell was obtained (Figure S1), revealing much higher MON-NH2

6

monomer concentration is not favorable to form uniform MON-NH2 shell on Fe3O4. The

7

uniform MON-NH2 shell with the thickness of 50 nm can be formed at the concentration

8

of 0.06 mmol for tetrakis (4-ethynylphenyl)methane and 0.12 mmol for 2,5-

9

dibromobenzenamine (Figures 1b and 1c). With the increase of MON-NH2 monomer

10

concentration, the Ms values are gradually decreased from 52, 45, 42 to 24 emu g-1 for

11

[email protected], Fe3O4@MON-NH2, Fe3O4@MON-2NH2, and Fe3O4@MON-4NH2,

12

respectively (Figure 1d), showing that too much higher MON-NH2 monomer

13

concentration is not favorable to obtain good superparamagnetism nanospheres.

14

Considering the morphology, the thickness of the MON-NH2 shell and the

15

superparamagnetism, the Fe3O4@MON-NH2 was selected as the adsorbent for

16

subsequent experiments. 16 ACS Paragon Plus Environment

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1


Scheme 1. Illustration for the Fabrication of Fe3O4@MON-NH2

2 3

Characterization of Fe3O4@MON-NH2. The prepared Fe3O4@MON-NH2 was

4

characterized with TEM, EDX, HAADF-STEM, XRD, TGA, N2 adsorption-desorption and

5

water contact angle experiments. The HAADF-STEM, TEM and EDX elemental

6

mapping data revealed the uniform and core-shell structure of the obtained

7

Fe3O4@MON-NH2 nanospheres (Figures 1a-1c). The thickness of MON-NH2 shell on

8

Fe3O4 core was about 50 nm (Figure 1b). The XRD pattern of Fe3O4@MON-NH2

9

showed the characteristic face center cubic peaks of Fe3O4 in the range of 20o-70o

10

(JCPDS

no.19-0629)

(Figure

1d).41

Fe3O4@MON-NH2

11

superparamagnetism with Ms value of 45 emu g-1 (Figure 1e).41 The appearance of the

12

characteristic peaks for C-N at 1498 cm-1 and for Fe-O at 590 cm-1 in the FT-IR spectra

13

of Fe3O4@MON-NH2 revealed the successful formation of MON-NH2 shell on Fe3O4 17 ACS Paragon Plus Environment

also

showed

good


1

(Figure 1f). The TGA curve showed the Fe3O4@MON-NH2 was stable up to 300 oC

2

(Figure S2).

3

The N2 adsorption-desorption results (Figure 1g and Table S1) revealed the

4

Brunauer-Emmett-Teller (BET) surface areas of Fe3O4, MON-NH2 and Fe3O4@MON-

5

NH2 were 18.6, 1038.9, and 186.8 m2 g-1, respectively. The BET surface area of

6

Fe3O4@MON-NH2 increased due to the larger surface area of MON-NH2 than Fe3O4.

7

The pore size and pore volume of the prepared Fe3O4@MON-NH2 was 0.9 nm and

8

0.091 cm3 g−1, respectively (Figure S3, Table S1). The above results demonstrated the

9

successful synthesis of Fe3O4@MON-NH2 and the enhanced surface area in

10

Fe3O4@MON-NH2.


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1 2

Figure 1. TEM images of (a) bare Fe3O4 and (b) Fe3O4@MON-NH2. (c) HAADF-STEM

3

image and EDX elemental mapping of Fe3O4@MON-NH2. (d) XRD patterns of MON-

4

NH2, Fe3O4 and Fe3O4@MON-NH2. (e) Magnetic hysteresis loops of Fe3O4 and



Page 20 of 52

1

Fe3O4@MON-NH2 with different monomer concentration. (f) FT-IR spectra and (g) N2

2

adsorption-desorption isotherms of MON-NH2, Fe3O4 and Fe3O4@MON-NH2.

3

The

water

contact

angle

experiments

are

performed

to

characterize

the

4

hydrophobicity of synthesized Fe3O4, MON-NH2, and Fe3O4@MON-NH2 (Figure 2). The

5

water contact angles of Fe3O4, MON-NH2 and Fe3O4@MON-NH2 are 24o, 143o and

6

142o, respectively, showing the good hydrophobicity of MON-NH2 and Fe3O4@MON-

7

NH2. The water contact angle of Fe3O4@MON-NH2 increased due to the good

8

hydrophobicity of MON-NH2 than Fe3O4. The results also reveal the incorporation of

9

amino groups into the MON-NH2 networks gave little effect on the hydrophobicity of

10

MON (141o). Although the Fe3O4@MON-NH2 is hydrophobic, the larger density of

11

Fe3O4@MON-NH2 than water and the well dispersion during the adsorption gives the

12

opportunity of Fe3O4@MON-NH2 to contact with TBBPA in aqueous solution (Figure S4).

13


Page 21 of 52 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1


Figure 2. Water contact angles of (a) Fe3O4, (b) MON-NH2, and (c) Fe3O4@MON-NH2.

2

Kinetics for the Adsorption of TBBPA on Fe3O4@MON-NH2. Time-dependent

3

adsorption of TBBPA (50, 100, and 200 mg L-1) on Fe3O4@MON-NH2 was studied. The

4

adsorption equilibrium was achieved within 1 min at TBBPA initial concentration of 50

5

mg L-1, showing the ultrafast adsorption kinetics of Fe3O4@MON-NH2 for TBBPA

6

(Figure 3a). Even at a high initial concentration of 200 mg L-1, the adsorption equilibrium

7

of TBBPA on Fe3O4@MON-NH2 was still achieved within 1 h. The adsorption capacity

8

of TBBPA increased as the initial concentration increased (Table 1), suggesting the

9

adsorption sites of Fe3O4@MON-NH2 do not reach the saturation.



1 2

Figure 3. (a) Time-dependent adsorption of TBBPA and (b) the plots of pseudo-second-

3

order kinetics for the adsorption of TBBPA at diﬀerent initial concentrations on

4

Fe3O4@MON-NH2 at 25

5

corresponding Langmuir plots for the adsorption of TBBPA on Fe3O4@MON-NH2 at the

6

temperature range of 25-55 oC.

oC

and pH 8.0. (c) Adsorption isotherms and (d) their


Page 22 of 52

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1

The pseudo-first-order (eq 1) and pseudo-second-order (eq 2) kinetic models are

2

applied to study the adsorption kinetics of Fe3O4@MON-NH2 for TBBPA (Figure 3b,

3

Figure S5a).41

4

ln (𝑞e ― 𝑞t) = ln𝑞e ―

5

𝑡 1 1 = + 𝑡 𝑞t 𝑘2𝑞2e 𝑞e

𝑘1 2.303

𝑡

(1) (2)

6

where qe and qt are the adsorption capacities (mg g-1) at equilibrium and fixed time t

7

(min), respectively. The k is the adsorption rate constants (g mg-1 min-1). The good

8

linearity (R2 > 0.990) for the curve of t / qt versus t reveals that the adsorption kinetic of

9

TBBPA on Fe3O4@MON-NH2 is better correlated with the pseudo-second-order kinetic

10

model than pseudo-first-order kinetic model (Figure 3b cf. Figure S5a; Table 1 cf. Table

11

S2). The value of k2 decreases as the initial concentration increases, revealing the

12

chemisorption dominates the rate-limiting step, including the ability to share or

13

exchange electrons between TBBPA and [email protected]

14

Table 1. Kinetic Parameters for the Adsorption of TBBPA on Fe3O4@MON-NH2 at 25 oC

pseudo-second-order kinetic model C0

qe(cal)

qe(exp) 23 ACS Paragon Plus Environment

K2

R2


Page 24 of 52

50

50.0

50.0

-

0.999

100

100.0

100.0

0.029

0.999

200

131.8

131.9

0.006

0.999

1

Adsorption Isotherms and Thermodynamics Studies. The adsorption isotherms were

2

then studied at 25-55 oC in diverse concentrations (50-500 mg L-1) (Figure 3c). The

3

TBBPA adsorption capacity increased with increasing initial concentration, indicating

4

that higher concentration is favorable for TBBPA adsorption on Fe3O4@MON-NH2. To

5

further evaluate the maximum adsorption capacity of Fe3O4@MON-NH2 for TBBPA, the

6

adsorption isotherms were fitted with Freundlich and Langmuir adsorption models (eqs

7

3-4).41

8

9

𝐶e 𝑞𝑒

=

𝐶e 𝑄0

+

1 𝑄0𝑏

(3)

1 ln𝑞e = ln𝐾F + ln𝐶e 𝑛

(4)

10

where qe (mg g-1) is the equilibrium adsorption capacity, Ce (mg L-1) is the equilibrium

11

concentration of TBBPA, b is the Langmuir constant (L mg-1), and Q0 is the maximum

12

adsorption capacity (mg g-1).42 n and KF are Freundlich constants relating to adsorption

13

intensity and capacity, respectively.42 24 ACS Paragon Plus Environment

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1

The results reveal that Langmuir model is better than Freundlich model to fit the

2

adsorption process of TBBPA on Fe3O4@MON-NH2 (Table 2, Table S3, Figure 3d and

3

Figure S5b). As the adsorption temperature increased from 25 oC to 55 oC, the

4

maximum adsorption capacity of TBBPA increased from 135.9 to 161.1 mg g-1,

5

indicating the adsorption of TBBPA on Fe3O4@MON-NH2 is endothermic.

6

To study the adsorption mechanism for TBBPA on Fe3O4@MON-NH2, the enthalpy

7

change (ΔH, kJ mol-1), the entropy change (ΔS, J mol-1 K-1) and the free energy change

8

(ΔG, kJ mol-1) are calculated according to eqs 5 and 6 (Figure S6).

9

10

∆𝐺 = ―𝑅𝑇ln𝑏

ln𝑏 =

∆𝑆 ∆𝐻 ― 𝑅 𝑅𝑇

(5)

(6)

11

The negative ΔG values indicate spontaneous adsorption of TBBPA on Fe3O4@MON-

12

NH2 (Table 2). A positive ΔS value suggests that the randomness of the increase in

13

adsorption of TBBPA on Fe3O4@MON-NH2 may be due to the fact that the amount of

14

desorbed water molecules is greater than the number of adsorbed TBBPA molecules.42



Page 26 of 52

1

A positive ΔH value reveals that the adsorption of TBBPA is an endothermic process,

2

consistent with an increase in adsorption capacity with temperature.

3

Table 2. Langmuir and Thermodynamic Parameters for the Adsorption of TBBPA on

4

Fe3O4@MON-NH2 Langmuir parameters

Thermodynamic parameters

T (K)

Q0 (mg g-1)

b (L mol-1)

R2

ΔG (kJ mol-1)

298

135.9

4.77 × 105

0.999

-32.4 ± 0.1

308

148.6

4.80 × 105

0.999

-33.5 ± 0.2

318

152.0

4.93 × 105

0.999

-34.7 ± 0.1

328

161.1

5.01 × 105

0.998

-35.8 ± 0.1

ΔH (kJ mol-1)

ΔS (J mol-1 K-1)

1.4 ± 0.2

113.4 ± 0.8

5

Effect of pH on the Adsorption of TBBPA. The pH of natural water varies with its

6

environment and affects the adsorption process.43 The effect of pH on the adsorption of

7

TBBPA on Fe3O4@MON-NH2 was investigated in pH 5-10 (Figure 4a). The TBBPA

8

adsorption capacity on Fe3O4@MON-NH2 changed little as pH increased from 5 to 10,

9

showing the good adsorption stability of Fe3O4@MON-NH2 for TBBPA. The dissociation

10

constants of TBBPA are pKa1=7.5 and pKa2=8.5, respectively.44 The TBBPA could be

11

positively charged at pH < pKa1, negatively charged at pH > pKa2, or zwitterion at pKa1
pKa2, the hydroxyl groups are easily deprotonated and the

8

main form is anionic (TBBPA- and TBBPA2-),44 which make them difficult to form

9

hydrogen bonding with Fe3O4@MON-NH2, leading to the decrease of adsorption

10

capacity. Increase of the pH higher than 7 may also lead to the increase of dissolution

11

of TBBPA in water, which is unfavorable for the adsorption of TBBPA on Fe3O4@MON-

12

NH2. Therefore, the hydrophobic interaction between the TBBPA and Fe3O4@MON-NH2

13

should be the main adsorption mechanism of Fe3O4@MON-NH2 for TBBPA due to the

14

good hydrophobicity of Fe3O4@MON-NH2 (Figure 2) and the large octanol-water

15

partition coefficient value of TBBPA (log Kow = 9.7).44



1

2

Figure 4. Eﬀects of (a) pH, (b) HA and (c) ionic strength on the adsorption of TBBPA on

3

Fe3O4@MON-NH2.

4

Effects of HA and Ionic Strength on the Adsorption of TBBPA. HA is the typical model

5

dissolved organic matter in natural water.10 Influence of HA on TBBPA adsorption on

6

Fe3O4@MON-NH2 was performed to evaluate the practical application of Fe3O4@MON-


Page 28 of 52

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1

NH2 for TBBPA adsorption and removal from natural water (Figure 4b). Increase of the

2

HA concentration gave little effect on the adsorption capacity of Fe3O4@MON-NH2 for

3

TBBPA, showing the good prospect of Fe3O4@MON-NH2 in natural water. Commonly,

4

HA contains numerous functional groups including carboxyl, amine and hydroxyl

5

groups.12 The good anti-interference of Fe3O4@MON-NH2 for HA likely resulted from

6

the good hydrophobic interaction between TBBPA and Fe3O4@MON-NH2. The

7

influence of ionic strength on TBBPA adsorption on Fe3O4@MON-NH2 was further

8

studied at different NaCl concentrations (Figure 4c). The Fe3O4@MON-NH2 still gave

9

stable adsorption capacity at the concentrations of NaCl lower than 100 mmol L-1,

10

further revealing the good adsorption of Fe3O4@MON-NH2 for TBBPA.

11

Desorption and Regeneration of Fe3O4@MON-NH2. From a sustainable point of view,

12

the easy desorption of pollutants from the adsorbent and the easy regeneration of the

13

adsorbent are critical to the reuse of adsorbents. Methanol, acetonitrile and isopropanol

14

were chosen as the desorption solvents to desorb TBBPA from Fe3O4@MON-NH2

15

(Figure 5a). The results show that methanol had the best desorption of TBBPA from

16

Fe3O4@MON-NH2. The methanol gave larger solubility of TBBPA than those for 29 ACS Paragon Plus Environment


1

acetonitrile and isopropanol,45 resulting in the good desorption of TBBPA from

2

Fe3O4@MON-NH2. In addition, the adsorbed TBBPA can be desorbed using methanol

3

under ultrasonication within 3 min (Figure S8). Furthermore, the adsorption capacity of

4

TBBPA on Fe3O4@MON-NH2 decreased only 8.7% even after four adsorption-

5

desorption cycles (Figure 5b), indicating the good reusability of Fe3O4@MON-NH2 for

6

TBBPA. There were no significant changes for the TEM, magnetic hysteresis loops,

7

XRD and TGA data of Fe3O4@MON-NH2 before and after adsorption, showing the good

8

stability of Fe3O4@MON-NH2 during the adsorption (Figure S9).


Page 30 of 52

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1 2

Figure 5. (a) Effect of desorption solvent volume on the desorption efficiency of TBBPA

3

from Fe3O4@MON-NH2. (b) Reusability of Fe3O4@MON-NH2 for TBBPA. (c) Adsorption

4

of TBBPA (200 mg L-1) on Fe3O4, Fe3O4@MON and Fe3O4@MON-NH2 at 25 oC. (d)

5

Adsorption of BPA, BPF, BPAF and TBBPA on Fe3O4@MON-NH2. The initial

6

concentration of each analytes is 200 mg L-1.

7

8

Comparison of Fe3O4@MON-NH2 with Other Adsorbents for the Adsorption of TBBPA. To show the advantages of Fe3O4@MON-NH2 for TBBPA, the adsorption of TBBPA on



1

Fe3O4, MON-NH2 and Fe3O4@MON was performed for comparison (Figures 5c, S10

2

and S11). Fe3O4@MON-NH2 demonstrated significantly higher adsorption capacity for

3

TBBPA than Fe3O4 (Figure 5c), revealing the crucial role of the MON-NH2 for TBBPA

4

adsorption. The pure MON-NH2 showed larger adsorption capacity (149.2 mg g-1) than

5

that on Fe3O4@MON-NH2 (135.9 mg g-1), revealing the dominant role of MON-NH2 for

6

TBBPA adsorption (Figure S10 and Table S4). However, Fe3O4@MON-NH2 gave faster

7

adsorption kinetics than those on MON-NH2, showing ultrafast adsorption kinetics for

8

TBBPA removal (Figure S11, Tables S5-6). In addition, Fe3O4@MON-NH2 gave larger

9

adsorption capacity than Fe3O4@MON without amino group, further demonstrating the

10

significant roles of hydrogen bonding interaction between the amino groups on

11

Fe3O4@MON-NH2 and hydroxyl groups on TBBPA during the adsorption. The larger

12

adsorption capacity of Fe3O4@MON than bare Fe3O4 also suggests the good

13

hydrophobic interaction between MON and TBBPA. The large water contact angle of

14

Fe3O4@MON-NH2 (142o) also reveals the possibility of hydrophobic interaction between

15

Fe3O4@MON-NH2 and TBBPA (Figure 2). By integrating the hydrophobic and hydrogen

16

bonding interaction within Fe3O4@MON-NH2 nanosphere, Fe3O4@MON-NH2 gave 32 ACS Paragon Plus Environment

Page 32 of 52

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1

better

adsorption

and

faster

kinetics

for

TBBPA

than

MWCNTs@Fe3O4,

2

MWCNTs@Fe3O4-NH2, GO and many other reported adsorbents (Table S7). Although

3

the adsorption capacity of Fe3O4@MON-NH2 (135.9 mg L-1 at 25 oC) is lower than the

4

pure MWCNTs (162.8 mg L-1 at 30 oC), the Fe3O4@MON-NH2 gave much larger

5

adsorption capacity (135.9 mg L-1) than those for magnetic MWCNTs composites such

6

as MWCNTs@Fe3O4 (22.0 mg g-1) and MWCNTs/Fe3O4-NH2 (33.7 mg g-1), revealing

7

the potential of Fe3O4@MON-NH2 for efficient adsorption of TBBPA from aqueous

8

solution.

9

To further evaluate the adsorption mechanisms and the potential of Fe3O4@MON-

10

NH2 for other bisphenol pollutants, the adsorption of BPA, BPF and BPAF on

11

Fe3O4@MON-NH2 was studied as they are endocrine disruptors with cytotoxicity,

12

genotoxicity and reproductive toxicity.41 Removal of these bisphenol pollutants are of

13

great importance. Their similar structures to TBBPA are also of importance to study the

14

possible adsorption mechanisms. The results demonstrate the better adsorption

15

efficiency of Fe3O4@MON-NH2 for BPA (0.39 mmol g-1), BPF (0.42 mmol g-1) and BPAF



1

(0.27 mmol g-1) than TBBPA (0.25 mmol g-1, Figure 5d and Table S8), showing the

2

potential of Fe3O4@MON-NH2 for adsorption and removal of other bisphenol pollutants.

3

4

Figure 6. The C1s and N1s XPS spectra of Fe3O4@MON-NH2 before (a), (c) and after

5

(b), (d) adsorption of TBBPA.

6

The FT-IR spectra of Fe3O4@MON-NH2 before and after TBBPA adsorption were

7

performed to study the adsorption mechanism (Figure S12a). The characteristic amino

8

group peak at 3282 cm-1 on Fe3O4@MON-NH2 was shifted to 3290 cm-1, suggesting the


Page 34 of 52

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1

hydrogen bonding interaction between the amino groups on Fe3O4@MON-NH2 and

2

hydroxyl groups on TBBPA during the adsorption.46

3

The XPS data were further used to study the adsorption mechanism (Figure 6).47,48

4

The C1s XPS spectra can be divided into three peaks of 285.18, 284.57 and 286.40 eV,

5

corresponding to C-O-Fe, C=C and C-N groups, respectively.12,49,50 The formation of the

6

C-O-Fe

7

nanoparticles,12,51 which is agreed with the TEM results (Figure 1b-c). The peak of C=C

8

was shifted from 284.57 eV to 284.44 eV after TBBPA adsorption (Figures 6a-6b),

9

indicating the π-π interaction between TBBPA and [email protected] The UV-vis

10

spectra also showed the π-π interaction between TBBPA and Fe3O4@MON-NH2

11

(Figure S12b).42 The peaks of amine nitrogen (-NH and -NH2) located at 397.06 and

12

400.18 eV were shifted to 397.46 eV and 402.25 eV after the adsorption of TBBPA,

13

respectively, suggesting the hydrogen bonding (N⋯H⋯O) interaction between N sites

14

on Fe3O4@MON-NH2 and -OH groups on TBBPA played significant roles for TBBPA

15

adsorption.10,53 Therefore, we speculate that the hydrogen bond, π-π and hydrophobic

16

interaction result in the good adsorption of Fe3O4@MON-NH2 for TBBPA.53

bond

indicated

the

interaction

between


the

MON-NH2

and

Fe3O4


1

2

Figure 7. Adsorption of spiked TBBPA (50 mg L-1) from river and lake water samples on

3

Fe3O4@MON-NH2.

4

Adsorption of TBBPA in Real Water Samples on Fe3O4@MON-NH2. The adsorption

5

of TBBPA in river and lake water samples was then performed to demonstrate the

6

feasibility of Fe3O4@MON-NH2 for TBBPA adsorption and removal in natural water

7

samples (Figure 7). The composition of lake and river water samples was analyzed with

8

ion chromatography (Table S9). The concentration of Cl-, SO42- and HCO3- changed

9

little during the adsorption, showing the good anti-interference ability of Fe3O4@MON-

10

NH2 for TBBPA (Table S9). The results show the adsorption capacity of Fe3O4@MON-

11

NH2 for TBBPA in control, river and lake water samples did not significantly changed


Page 36 of 52

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1

(Figure 7), indicating the great potential of Fe3O4@MON-NH2 for TBBPA adsorption and

2

removal in real water samples.

3

CONCLUSION

4

In summary, we have reported a facile and controllable solvent refluxing method to

5

synthesize core-shell Fe3O4@MON-NH2 nanosphere for efficient magnetic adsorption

6

and removal of TBBPA from water with large adsorption capacity and ultrafast

7

adsorption kinetics. The synthesized Fe3O4@MON-NH2 shows good anti-interference

8

ability to pH (5-10), coexisting NaCl and HA. By integrating the hydrophobic and

9

hydrogen binding sites within MON-NH2 shell, the Fe3O4@MON-NH2 gave higher

10

adsorption capacity than most of the reported porous adsorbents. With the aid of

11

magnetic Fe3O4 core and MON-NH2 shell, application of Fe3O4@MON-NH2 for other

12

bisphenol pollutants adsorption and for TBBPA removal from real water samples was

13

realized. These results revealed the feasibility to design, synthesis and application of

14

functionalized MONs composites for environmental pollutants adsorption and removal.

15

ASSOCIATED CONTENT

16

Supporting Information. 37 ACS Paragon Plus Environment


1

The Supporting Information is available free of charge on the ACS Publications website.

2

Additional Figures S1-S12, and Tables S1-S9.

3

AUTHOR INFORMATION

4

Corresponding Author

5

* E-mail: [email protected].

6

ORCID

7

Cheng-Xiong Yang: 0000-0002-0817-2232

8

Xiu-Ping Yan: 0000-0001-9953-7681

9

Notes

10

The authors declare no competing financial interest.

11

ACKNOWLEDGMENT

12

This work was supported by the National Natural Science Foundation of China (Nos.

13

21777074 and 21775056), and the National Basic Research Program of China (No.

14

2015CB932001), Tianjin Natural Science Foundation (No. 18JCQNJC05700), NFFTBS

15

(No. J1103306).


Page 38 of 52

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1

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Interfacial

99TcO -Remediation 4

Thermodynamics

of

by a Cationic Polymeric Network.

The


pH-related

Sorption

of


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