Insights into the glyphosate adsorption behavior and mechanism by a

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Insights into the glyphosate adsorption behavior and mechanism by a MnFe2O4@cellulose activated carbon magnetic hybrid Quan Chen, Jiewei Zheng, Qian Yang, Zhi Dang, and Lijuan Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b22386 • Publication Date (Web): 05 Apr 2019 Downloaded from http://pubs.acs.org on April 5, 2019

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Insights into the glyphosate adsorption behavior and mechanism

2

by a MnFe2O4@cellulose activated carbon magnetic hybrid

3

Quan Chen,† Jiewei Zheng,† Qian Yang,† Zhi Dang,‡ Lijuan Zhang *,†

4 5

†

6

Chemistry and Chemical Engineering, South China University of Technology,

7

Guangzhou 510640, P R China. E-mail: [email protected]. Telephone/Fax: +86-20-

8

87112046.

9

‡

10

Guangdong Provincial Key Lab of Green Chemical Product Technology, School of

School of Environment and Energy, South China University of Technology,

Guangzhou 510006, P R China.

11 12

Abstract:

13

To enhance the removal of the negatively charged organophosphorus pesticide (OPP)

14

Glyphosate (GLY), we prepared a positively charged MnFe2O4@ cellulose activated

15

carbon (CAC) hybrid by immobilizing MnFe2O4 nanoparticles on the CAC surface via

16

a simple one-pot solvothermal method. SEM, BET, TEM, IR, Raman, XRD and XPS

17

analysis proved the successful synthesis of MnFe2O4 with a particle size of 100~300

18

nm. The particles were distributed on the surface of CAC to form the MnFe2O4@CAC

19

hybrid. MnFe2O4@CAC exhibited a positive charge at pH below 6 values and had good

20

magnetic properties and dispersion stability. The maximum GLY adsorption capacity

21

of MnFe2O4@ CAC (167.2 mg/g) was much higher than that of CAC (61.44 mg/g) and

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MnFe2O4 nanoparticles (93.48 mg/g). The adsorption process was dominated by

23

chemisorption, and the formation of new chemical bonds between GLY and MnFe2O4

24

was confirmed by simulations. The newly formed chemical bonds were attributed to

25

the conjugation between p electrons of the adsorbent and the d electrons of the

26

adsorbate. Collectively, the results indicate that the as-prepared MnFe2O4@CAC is

27

promising for anionic pollutant adsorption and the removal of OPPs, and our 1 ACS Paragon Plus Environment

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mechanistic results are of guiding significance in environmental cleanup.

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Keywords: MnFe2O4; Glyphosate; Adsorption; Quantum chemical simulations;

30

Electronic transfer

31 32

1. Introduction

33

Glyphosate (GLY), an organophosphorus pesticide (OPP), has been widely

34

applied as a postemergence, broad-spectrum, nonselective, and low-cost herbicide in

35

agricultural production to increase crop yields.1 Due to its abundant and inappropriate

36

use worldwide, GLY has severely polluted water and farmland soil. Moreover, GLY

37

has been detected in vegetables, fruits, and food, which directly or indirectly poses a

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great threat to the ecological environment and human safety.2-3 Therefore, the removal

39

of residual OPPs is of great significance in reducing their negative impacts on

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environmental and food safety. In this regard, substantial work so far has focused on

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removal technologies, among which the adsorption method is favored in terms of its

42

low-cost adsorbents, simple operation, high efficiency, and safety.4-7

43

Metal oxide nanoparticles (MONPs) have emerged as adsorbents for the efficient

44

removal of OPPs owing to their relatively good OPP adsorption affinity. However, most

45

MONPs agglomerate easily due to their high surface energy, and have potential

46

ecotoxic effects on the environment, which limit their further application.8-10 Therefore,

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biocompatibility, easy separation and dispersion stability should be considered during

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the selection of MONPs.11 Among various MONPs, MnFe2O4 is an attractive candidate

49

due to its positive electric characteristics, strong magnetism, high natural abundance

50

and low ecotoxicity.12 For instance, Lian et al.13 synthesized magnetic MnFe2O4

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microspheres for the selective enrichment and effective isolation of phosphopeptides;

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the results showed that MnFe2O4 was highly selective for phosphopeptides because of

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the strong coordination interaction, and exhibited rapid magnetic separation with 15 s.

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However, the matter of dispersion stability remains unresolved.

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The most efficient and convenient method to improve the dispersion stability of 2 ACS Paragon Plus Environment


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MONPs is to compound the MONPs with substrate materials. For example, Sood’s

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group14 prepared a graphene oxide- MnFe2O4 nanohybrid for the efficient removal of

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Pb(II), As(III), and As(V) from contaminated water, the maximum adsorption capacity

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was 673 mg/g, 146 mg/g, 207 mg/g, respectively, and the adsorbents enabled easy

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magnetic separation. However, the graphene oxide is difficult to mass produce, which

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limits its application. Therefore, it is necessary to load MONPs on a mass-produced

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substrate. Among substrates, activated carbon, is often mentioned as a standalone

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material due to its remarkably high surface area, porous structure and good adsorption

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capacities towards various substances.15 However, when activated carbon is used alone

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as the adsorbent, the adsorption process is hindered by the electrostatic repulsion

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between negatively charged activated carbon and GLY.16-17 Therefore, we attemptted

67

to load MnFe2O4 onto cellulose-activated carbon (CAC). This operation not only can

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impart positive electrical properties to the CAC, but also improve the dispersion

69

stability of MnFe2O4, as well as reduce the ecotoxicity and cost of the composite

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adsorbent.

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On the other hand, adsorption is commonly driven by electron sharing or transfer,

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and there is an urgent need to elucidate the electronic mechanism. However, the

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electronic phenomenon is difficult to prove through experimental analysis. Therefore,

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it is necessary to investigate the possible electronic mechanism with simulation

75

methods. Quantum chemical simulations can accurately predict the lattice constant of

76

the crystal configuration, calculate the steady-state energy, and analyze the electronic

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behavior during the adsorption process.18 Density functional theory (DFT) simulation

78

has been employed to study the selective adsorption of heavy metal ions on Mn-doped

79

α-Fe2O3. The research revealed the selective adsorption mechanism of heavy crystal

80

ion adsorption onto nanocrystals with different crystal faces and Mn doping amounts.19

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Moreover, MnFe2O4 is a member of unique class of spinel-structured compounds with

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stable crystal configuration and is therefore a good candidate for the analysis of the

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electronic mechanism by quantum chemical simulations.20 3 ACS Paragon Plus Environment

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Taking the above mentioned defects into cosideration, we herein report a novel

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positively charged, strong magnetic hybrid adsorbent (MnFe2O4@CAC) formed by

86

loading MnFe2O4 nanoparticles onto CAC. The OPP GLY was selected as the model

87

adsorbate to evaluate the adsorption capacity and ability of MnFe2O4@CAC with batch

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adsorption experiments. The influences of pH value on the adsorption capacity were

89

explored. Classical adsorption theory models were adopted to describe the adsorption

90

process and behavior, and to predict the adsorption mechanism. Furthermore, quantum

91

chemical simulations (DFT/frontier orbital theory (FOT)) were carried out

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systematically and deeply to investigate the electronic mechanism during the adsorption

93

process. The objective of this work is to provide a highly efficient adsorbent for OPP

94

removal and to clarify the electronic mechanism during the adsorption process.

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2. Materials and methods

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2.1 Materials

98

Cellulose (powder), polyethylene GLYcol (PEG, MW 2000), and GLYphosate

99

(GLY, 99%) were purchased from Sigma-Aldrich, . Ethylene GLYcol (EG, AR),

100

ethanol (C2H6O, AR), and nitric acid (HNO3, AR) were obtained from Sinopharm

101

Group. Ferric chloride hexahydrate, manganese chloride tetrahydrate, sodium

102

hydroxide, sodium acetate, hydrochloric acid, potassium hydroxide, potassium nitrate,

103

sodium sulfate, sodium carbonate, sodium chlorate, disodium phosphate, and

104

sodium dihydrogen phosphate (FeCl3·6H2O, MnCl2·4H2O, NaOH, NaAc, HCl, KOH,

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KNO3, Na2SO4, Na2CO3, NaClO3, Na2HPO4 and NaH2PO4, respectively; AR,

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Guangzhou Chemical Reagent Factory) were used directly.

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2.2 Preparation of CAC and MnFe2O4@CAC

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CAC was prepared with cellulose as carbon source and activated with KOH.21

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Briefly, dried cellulose powder was evenly spread in a crucible and placed in the center

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of a tube furnace. The heating rate was 5 ºC/min, and the nitrogen flow rate was 50 4 ACS Paragon Plus Environment


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mL/min; the temperature was raised from room temperature to 300 ºC, maintained for

112

1 h and then cooled to room temperature to obtain cellulose char (designated CC). The

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CC and KOH solids were mixed at a mass ratio of 1:3, deionized water was added, and

114

the mixture was stirred (KOH concentration: 30%) and then placed in an 80 °C oven to

115

obtain a cellulose alkali (named CC-KOH). The CC-KOH was calcined in the tube

116

furnace. The heating rate was set at 5 ºC/min and the nitrogen flow rate was 50 mL/min;

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the temperature was raised from room temperature to 700 ºC, and held for 1h to ensure

118

the formation of aromatic structures, before cooling to room temperature. The samples

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were then washed with a large amount of deionized water to until neutral. Finally, the

120

product was dried in a vacuum oven and named CAC.

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Subsequently, we doped MnFe2O4 nanoparticles on the CAC surface via a simple

122

one-pot solvothermal method. In detail, 0.5 g of CAC was added to 70 mL of ethylene

123

GLYcol and ultrasonically dispersed for 10 min; then, 2 g of FeCl3·6H2O and 0.752 g

124

of MnCl2·4H2O were added to the above solution and ultrasonically dispersed for

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another 3 h. Soon after, 5 g of NaAc and 3 g of PEG were added to the above solution,

126

and stirred at room temperature for 1 h. The as-prepared solution was transferred into a

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hydrothermal reaction kettle, reacted at 200 °C for 10 h, cooled to room temperature,

128

and washed with ethanol and deionized water. The solid products were dried in a

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vacuum oven.22 At the same time, bare MnFe2O4 was prepared under the same

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hydrothermal procedures but without adding CAC.

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2.3 Material characterization and test methods

132

Fourier transform infrared (FTIR) spectroscopy (Vector 33-IR, Bruker) was

133

employed to analyze chemical structure in the samples by the KBr pellet technique. The

134

spectra were collected between 400 and 4000 cm−1. The surface area and pore size

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distribution were determined by N2 adsorption-desorption isotherms with an ASAP

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2010 analyzer (Micromeritics) at 77 K. The surface morphology, microstructure and

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elemental composition of the samples were observed by scanning electron microscopy

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(SEM, Merlin, Zeiss) and high-resolution transmission electron microscopy (HR-TEM, 5 ACS Paragon Plus Environment

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JEM-2100, Hitachi). X-ray diffraction spectroscopy (XRD, D8 Advance X, Bruker)

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with Cu Kα radiation generated at 45 kV and 40 mA was used to identify the crystalline

141

structure of the samples, and the spectra were obtained from 5~80°. The qualitative and

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quantitative determination of surface elements were characterized by X-ray

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photoelectron spectroscopy (XPS, K-Alpha, Thermo Fisher Scientific) and analyzed

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with XPS PEAK 4.1 software. Raman spectroscopy (Raman, LabRAM Aramis, H. J.

145

Y.) was employed to analyze the degree of graphitization at a wavelength of 632 nm,

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and a scan range of 0 to 3500 cm-1. Dynamic light scattering (DLS, Malvern Zetasizer

147

Nano S) was utilized to determine the surface zeta potential of the samples at various

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pH values, which were adjusted by the addition of 0.1 M NaOH and HCl. The

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dispersion stability was measured by UV-VIS spectrophotometer (UV-2450, Shimadzu,

150

Japan) through transmittance tests.

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The concentration of GLY was determined by a high-performance liquid

152

chromatography (HPLC) instrument (C18 column, Kromasil 100-5, 150×4.6 mm, 5 um)

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with a fluorescence detector (wavelength of 264 nm). Before entering the HPLC, the

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GLY solutions needed to be derivatized. The procedure was as follows: 0.12 mL of

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0.05 mol/L sodium tetraborate solution and 0.2 mL of 1.0 g/L of ruthenium

156

methoxycarbonyl chloride (FMOC-Cl)-acetonitrile solution were added to 1 mL GLY

157

solution, vortexed and mixed thoroughly, and derivatized at room temperature for 4 h.

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The solutions were filtered through a 0.22 μm filter, and the filtrate was injected into

159

the HPLC.23 The mobile phase was a mixture of phosphoric acid aqueous solution (0.2%

160

v/v) and acetonitrile with a flow rate of 0.5 mL/min. The gradient elution procedures

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are shown in Table S1, brief statement in listing the contents of the material supplied

162

as Supporting Information. The injection volume was 20 μL, the column temperature

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was 308 K, and the excitation and emission wavelengths were 254 nm and 301.5 nm,

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respectively. External standards of GLY (1~100 mg/L) were adopted to generate a

165

linear calibration curve (Figure S1), and the sample concentrations were obtained from

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the integrated peak areas. 6 ACS Paragon Plus Environment


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2.4 Batch adsorption tests

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The effects of variables on GLY adsorption onto MnFe2O4@CAC, CAC and

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MnFe2O4 were investigated by a static method combined with single-factor tests.

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Kinetic adsorption experiments were conducted to measure the equilibrium adsorption

171

capacity of GLY adsorption by the adsorbents. The experiments were carried out using

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100 mg/mL (C0) GLY for 2 to 1140 min at 298 K. The adsorption isotherm experiments

173

were conducted with C0 values of 5 to 200 mg/mL at 288, 298 and 308 K for 12 h. In

174

the study of the effect of solution pH on GLY adsorption, the initial GLY concentration

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was 100 mg/mL, the pH ranged from 2.0 to 11.3 (the solution pH was adjusted with 0.1

176

M NaOH and 0.1 M HNO3), the adsorption time was 12 h. The adsorbent concentration

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in all the adsorption experiments was 0.5 mg/mL. After adsorption, the solutions were

178

filtered through a 0.45 μm filter membrane, and the GLY concentration of the filtrates

179

was determined by HPLC after derivatization. The adsorption capacity (qe) is the ratio

180

of the adsorbed GLY to the adsorbent dose, and the removal percentage (Pe) is the ratio

181

of the residual GLY concentration to the initial GLY concentration. All adsorption

182

experiments were conducted in three parallel groups and are expressed with standard

183

deviation.

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2.5 Quantum chemical calculations

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The

molecular

and

electronic

mechanism

of

GLY

adsorption

onto

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MnFe2O4@CAC was investigated by computer simulations. Density functional

187

calculations were conducted using the Dmol3 package in Materials Studio 2017 R2.24

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An 8.5×8.5×8.5 Å MnFe2O4 unit cell was constructed and is presented in Figure S2A.

189

A geometrically optimized Mn-terminated (1,0,0) plane (Figure S2B) was selected and

190

treated as a 2×1 supercell.25 An atom-centered grid was used for the atomic basis

191

function. The dual numerical polarization (DNP 4.4) all-electron basis set was selected

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as the electronic basis set. The self-consistent field convergence value was 1.0×10-6.

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The “DFT semi-core pseudopods (DSPP)” method was implemented as the core 7 ACS Paragon Plus Environment

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treatment. The dispersion was corrected by the Grimme scheme to avoid the limitations

195

in handling weak interactions. The conductor-like screening model (COSMO) with a

196

permittivity of 78.54 (water) was used to mimic structures encased by an aqueous layer,

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and the temperature was set as 298 K. The size of the base set was similar to that of the

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Gaussian function type 6-31G (d, p) basis set but more accurate. This high-precision

199

numerical basis set reduced the basis set superposition error, and the system was

200

accurately described. Exchange-correlation functions were described by the

201

generalized gradient correction (GGA)-Perdew-Burke-Ernzerhof (PBE) functional.26 A

202

simple graphene structure composed of seven aromatic rings was selected as the model

203

compound for CAC,27 and the k-points were set to 4×4×1 after convergence. Integration

204

was performed in the Brillouin zone with a 15 Å vacuum layer in reciprocal space.28

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The adsorbate structure was the optimized GLY.29

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The adsorption binding energy is the difference between the energy of the steady

207

adsorption state and the energy of isolated adsorbent and adsorbate state. The energy

208

gap (Eg) is the difference in energy between the highest occupied molecular orbital

209

(HOMO) and the lowest unoccupied molecular orbital (LUMO). The electronic cloud

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overlapping between the adsorbent and GLY and the electronic densities of state (DOSs)

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were analyzed to clarify the electronic transfer in the adsorption process.

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3. Results and discussion

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3.1 Analysis of the chemical structure

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To confirm the successful formation of the MnFe2O4@CAC hybrid, IR, Raman,

216

XRD, and XPS analysis were employed to characterize the chemical structure of

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MnFe2O4@CAC, CAC, and MnFe2O4. Figure 1A shows IR spectra of the samples

218

collected in the range of 4000~400 cm-1. The spectra of MnFe2O4@CAC and MnFe2O4

219

were nearly the same, and the characteristic peaks at 557 cm-1 and 460 cm-1

220

corresponded to the formation of Fe-O and Mn-O bonds, indicating the successful 8 ACS Paragon Plus Environment


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synthesis of MnFe2O4.14 The broad band at 3400 cm-1 belonged to the stretching and

222

bending vibration of O-H groups of crystal water and adsorbed water. The characteristic

223

peaks at 2921 and 2850 cm-1 in the pattern of CAC were attributed to the stretching

224

vibration of C-H, while those at 1741 cm-1 and 1560 cm-1 were allocated to C=O and

225

C-C stretching vibrations. The characteristic peaks of aromatic rings appeared at

226

1200~1400 cm-1, showing the graphitization of CAC. The spectra of MnFe2O4@CAC

227

and CAC were quite different, and many characteristic peaks of CAC weakened

228

considerably or even disappeared in the pattern of MnFe2O4@CAC beacuse the long-

229

term solvothermal reaction destroyed the aldehyde, ketone and phenol structures in

230

CAC.

231

Raman was conducted to determine the reduction degree of the MnFe2O4@CAC,

232

and the results are shown in Figure 1B. The spectrum of MnFe2O4@CAC retained the

233

characteristic peaks of both CAC and MnFe2O4, indicating that MnFe2O4 particles were

234

distributed on the CAC surface. The patterns of MnFe2O4@CAC and MnFe2O4 showed

235

a strong peak at 600 cm-1, which could be assigned to the presence of Fe-O bonds. The

236

peaks at 1342 cm-1 (D peak) and 1605 cm-1 (G peak) were allocated to disordered or

237

defective carbon atoms and the graphitized carbon atom formed in the sp2 hybrid,

238

respectively. In addition, both the D peak and G peak emerged in the patterns of

239

MnFe2O4@CAC and CAC. A weak G peak appeared in the pattern of MnFe2O4 due to

240

the reduction of the raw materials during the solvothermal reaction.30 On the other hand,

241

the intensities of the G peak and D peak were defined as IG and ID, respectively, and the

242

ratio of IG and ID was applied to reflect the reduction degree. The IG/ID value of

243

MnFe2O4@CAC (1.063) was larger than that of CAC (0.989), suggesting that

244

MnFe2O4@CAC had a higher degree of reduction and ratio of graphite carbon structure.

245

The reason might be that the hydrothermal reaction process promoted the further

246

reduction of CAC.

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The crystallinity and phase purity of the samples were tested using XRD analysis

248

(Figure 1C). The XRD patterns of MnFe2O4@CAC and MnFe2O4 were almost the 9 ACS Paragon Plus Environment

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249

same. Two broad peaks at 20°~30° and 40°~45° emerged in the CAC pattern and were

250

indexed to the (002) and (100) crystal planes of activated carbon, respectively. The

251

diffraction peaks of CAC disappeared in the pattern of MnFe2O4@CAC, which was

252

ascribed to the high coverage of MnFe2O4 on the CAC. Almost no impurity peaks were

253

observed in the patterns of MnFe2O4@CAC and MnFe2O4, and the characteristic peaks

254

were sharp and intense, indicating that the loaded MnFe2O4 in the hybrid also had good

255

purity, crystallinity and crystal form. The characteristic peaks at 17.78°, 30.04°, 35.50°,

256

42.98°, 53.32°, 56.74°, 62.56°, and 73.46° corresponded to the (111), (220), (311),

257

(400), (422) (511), (440), and (533) crystal planes, respectively, this result was

258

essentially consistent with the MnFe2O4 standard database.31

259

To further investigate the chemical structure and surface characteristics of the

260

samples, XPS measurements were performed and the corresponding results are

261

presented in Figure 1D, Figure S3, Figure S4 and Figure 2. As shown in the XPS

262

survey spectra (Figure 1D), MnFe2O4@CAC and MnFe2O4 were composed of C, O,

263

Mn, and Fe, while CAC was only composed of C and O. A small amount of C was

264

detected in the MnFe2O4, due to the carbonaceous impurities produced in the

265

solvothermal reaction. The deconvoluted elemental spectra in MnFe2O4@CAC are

266

presented in Figure 2. The existence of C (Figure 2A) was consistent with that in CAC,

267

and the forms of O, Fe and Mn (Figure 2B, C, D) were in agreement with those of

268

MnFe2O4. Figure S3 shows the deconvoluted elemental spectra in CAC. The C1s

269

spectrum of CAC presented 3 peaks centered at 284.8, 285.7, and 288.4 eV associated

270

with C-C, C-O, and C=O bonds, respectively. The O1s spectrum showed only one peak

271

at 532.9 eV, which was assigned to O-C and O=C bonds. The deconvoluted elemental

272

spectra of MnFe2O4 are displayed in Figure S4. O existed in two forms, metal-O and

273

H-O, with binding energies of 530.2 eV and 531.5 eV, respectively. The typical binding

274

energies at 711.2 and 724.0 eV were allocated to the characteristic doublets of Fe 2p3/2

275

and Fe 2p1/2, respectively, indicating that Fe existed in trivalent form. The

276

characteristic peaks of Mn 2p3/2 and Mn 2p1/2 clearly appeared at 641.2 eV and 652.8 10 ACS Paragon Plus Environment


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eV, suggesting that Mn existed in divalent form.32 The binding energies of the

278

characteristic peaks and corresponding deconvoluted peaks in the hybrid were nearly

279

the same as those in the literature.32 Moreover, the calculated SFe/SMn was 1.98 in

280

manganese iron, which closely corresponds to the Fe/Mn atomic ratio in MnFe2O4.

281

Based on the above data and analysis, the XPS results further confirmed the fluky

282

formation of the MnFe2O4@CAC hybrid.

283 284

Figure 1 FTIR spectra (A), Raman spectra (B), XRD patterns (C) and XPS survey

285

spectra (D) of MnFe2O4@CAC, CAC and MnFe2O4

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Figure 2 XPS C1s (A), O1s (B), Fe2p (C) and Mn2p (D) spectra of MnFe2O4@CAC

288

3.2 Analysis of morphology and physical structure

289

The morphology, size, and composition of the CAC, MnFe2O4 and

290

MnFe2O4@CAC were examined by SEM, energy-dispersive X-ray analysis (EDX),

291

and TEM, and the images are displayed in Figure 3. As seen from the SEM and TEM

292

images (Figure 3A and D) of MnFe2O4@CAC, the MnFe2O4 nanoparticles were

293

successfully loaded onto the CAC surface. The pore structure of MnFe2O4 could still be

294

observed; such a structure increases the probability of contact between the contaminants

295

and adsorbent. CAC showed a rugged honeycomb shape with a rough surface and a

296

large number of pores (Figure 3B), and a graphite sheet structure was observed in the

297

TEM image (Figure 3E). A batch of MnFe2O4 spherical particles with a size of

298

approximately 100~600 nm was observed in Figure 3C and F, as well as a distinct pore

299

structure. In addition, the EDX results (Figure 3I and Table 1) showed that Mn and Fe 12 ACS Paragon Plus Environment


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were decorated on the MnFe2O4@CAC, and their contents agreed well with the XPS

301

results.

302 303

Figure 3 SEM (I) and TEM (II) images of MnFe2O4@CAC (A, D), CAC (B, E), and

304

MnFe2O4 (C, F). The corresponding SEM-EDX images are shown as insets in A, B,

305

and C

306

Nitrogen adsorption-desorption was employed to investigate the pore structure of

307

MnFe2O4@CAC, CAC and MnFe2O4; the results are shown in Figure 4 and Table 1.

308

The specific surface areas of MnFe2O4@CAC, CAC and MnFe2O4 were 265.4 m2/g,

309

912.3 m2/g and 83.0 m2/g, the pore volumes were 0.238 cm3/g, 0.536 cm3/g and 0.083

310

cm3/g, and the pore sizes were 2.56 nm, 2.35 nm and 4.02 nm, respectively. The specific

311

surface area and pore volume of MnFe2O4@CAC were much smaller than those of

312

CAC, owing to the plugging effect of MnFe2O4 nanoparticles on the CAC surface. In

313

contrast, the pore size of MnFe2O4@CAC was larger than that of CAC, because the

314

MnFe2O4 nanoparticles introduced a greater number of large pores into the hybrid. The

315

specific surface area and pore volume were reduced after loading MnFe2O4, which is

316

an normal phenomenon in the preparation of hybrid adsorbents.13 Moreover, magnetic

317

and positive charge properties were imparted to the hybrid after loading MnFe2O4; these

318

properties benefit adsorbent recovery and anionic pollutant adsorption. 13 ACS Paragon Plus Environment

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319 320

Figure 4 Nitrogen adsorption-desorption isotherms (A) and pore size distributions (B)

321

of MnFe2O4@CAC, CAC and MnFe2O4

322 323

Table 1 Surface elemental composition and BET data for MnFe2O4@CAC, CAC and

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MnFe2O4 XPS (wt%) Sample

SEM-EDX (wt%)

BET surface

Pore

Pore

volume

size

C

O

Fe

Mn

C

O

Fe

Mn

MnFe2O4@CAC

60.34

26.32

10.04

3.30

67.1

19.1

12.0

1.8

265.4

0.238

2.56

CAC

85.65

14.35

85.7

14.3

--

--

912.3

0.536

2.35

MnFe2O4

--

63.01

--

53.6

38.0

8.4

83.0

0.083

4.02

30.12

6.87

325

Notes：BET surface area (m2/g), pore volume (cm3/g), pore size (nm).

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3.3 Surface potential and dispersion stability

area

327

Environmental pH affects the charge properties and strength of adsorbents. Figure

328

5A shows the surface potential of MnFe2O4@CAC, CAC and MnFe2O4 as a function

329

of pH. The surface potentials of MnFe2O4@CAC and MnFe2O4 showed the same

330

tendency, gradually decreasing from positive to negative with increasing pH.

331

Regardless, the hybrid was positively charged at pH

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