Synthesis of CoreâShell Magnetic Nanocomposite Fe3O4@ Microbial

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Synthesis of core-shell magnetic nano-composite Fe3O4@ microbial extracellular polymeric substances for simultaneous redox sorption and recovery of silver ions as silver nanoparticles Wei Wei, Ang Li, Shanshan Pi, Qilin Wang, Lu Zhou, Jixian Yang, Fang Ma, and Bing-Jie Ni ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03075 • Publication Date (Web): 09 Nov 2017 Downloaded from http://pubs.acs.org on November 11, 2017

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ACS Sustainable Chemistry & Engineering

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Synthesis of core-shell magnetic nano-composite Fe3O4@ microbial extracellular

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polymeric substances for simultaneous redox sorption and recovery of silver ions

3

as silver nanoparticles

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Wei Wei,† Ang Li, *,† Shanshan Pi,† Qilin Wang,‡ Lu Zhou,† Jixian Yang, *,† Fang Ma,†

6

Bing-Jie Ni§

7 †

8

State Key Laboratory of Urban Water Resource and Environment, School of Environment, Harbin Institute of Technology, Harbin 150090, China

9 10

‡

Griffith School of Engineering & Centre for Clean Environment and Energy, Griffith University, QLD 4111, Australia

11 12 13

§

State Key Laboratory of Pollution Control and Resources Reuse, College of

Environmental Science and Engineering, Tongji University, Shanghai 200092, China

14 15

*Corresponding authors:

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E-mail: [email protected] (A. Li); Phone: +86 451 8628 3787

17

E-mail: [email protected] (J. Yang); Phone: +86 451 8628 3088

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1

ACS Paragon Plus Environment


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ABSTRACT: Microbial extracellular polymeric substance (EPS) is a complex high

20

molecular compound secreted from many organisms. In this work, magnetic

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nano-composite Fe3O4@EPS of Klebsiella sp. J1 were firstly synthesized for silver

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ions (Ag+) wastewater remediation, which synergistically combined the advantages of

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the easy separation property of magnetic Fe3O4 nanoparticles and the superior

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adsorption capacity of EPS of Klebsiella sp. J1. The physical and chemical properties

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of Fe3O4@EPS were analyzed comprehensively. Fe3O4@EPS exhibited the

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well-defined core-shell structure (size 50 nm) with high magnetic (79.01emu g-1).

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Batch adsorption experiments revealed that Fe3O4@EPS achieved high Ag+

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adsorption capacity (48mg g-1), which was also much higher than many reported

29

adsorbents. The optimal solution pH for Ag+ adsorption is around 6.0, with the

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sorption process followed pseudo second-order kinetics. Ag+ adsorption on

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Fe3O4@EPS was mainly attributed to the reduction of Ag+ to silver nanoparticles

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(AgNPs) by benzenoid amine (–NH–), accompanied by the chelation between Ag+

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and hydroxyl groups, ion exchange between Ag+ and Mg2+ and K+, and physical

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electrostatic sorption. The repeated adsorption-desorption experiments showed a good

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recycle performance of Fe3O4@EPS. This study has great importance for

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demonstrating magnetic Fe3O4@EPS as potential adsorbent to remove Ag+ from

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contaminated aquatic systems.

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KEYWORDS: Microbial extracellular polymeric substance, Redox sorption,

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Magnetic nano-composite, Magnetic Fe3O4, Silver ions, Synthesis

40 2


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INTRODUCTION

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The silver ions (Ag+) have been extensively used in a variety of fields due to their

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outstanding antimicrobial properties.1,2 However, the inevitable release of silver ions

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into water sources through wastewater discharge (e.g., varied from 10 to 500 mg L−1)

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poses serious risks to the environment and human health.1,3 Thus, treatment of silver

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ions pollution attracts increasing concerns in environmental protection areas. The

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adsorption-reduction technology is a beneficial strategy, which can not only remove

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Ag+ from contaminated water but also transform waste (Ag+) into assets (AgNPs),4,5

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creating new businesses with recycle waste material. Great efforts have been

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contributed to the development of new adsorption-reduction materials including

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activated carbon,6

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biomass.10 However, most of these materials suffer from various drawbacks.11

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Therefore, the green, high efficient and biocompatible alternative materials are

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desired.

coal,7 chemical polymer,1 microorganisms8,9 and waste

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Microbial extracellular polymeric substances (EPS), which derived from

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metabolism of microorganisms, lead the way due to the unique superiority.12 As

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inspired by the discovery that EPS could mitigate the antibacterial activity of Ag+ via

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constituting a permeability barrier with reducing constituents,2 EPS may provide

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unprecedented opportunities for the reduction of Ag+ and sequentially removal from

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contaminated aquatic systems, although the mechanism of Ag+ reduction by EPS has

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not been elucidated. Recently the EPS from Klebsiella sp. J1 that existed widely in

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natural water and grain was characterized.13 The results showed that EPS from 3



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Klebsiella sp. J1 achieved stronger reactivity with metal ions than other EPS due to

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their more abundant functional groups and higher protein content.14,15 These findings

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suggested that EPS from Klebsiella sp. J1 might be a promising biomaterial for

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adsorption-reduction of Ag+ from contaminated aquatic systems.

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However, the application of EPS suffers from its poor separability, which greatly

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limits its recycle and regeneration as well as precious metals recovery. Iron magnetic

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nanoparticles (Fe3O4) have aroused considerable interest for their facile separation

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property by simply applying an external magnetic field, but they still exhibit low

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adsorption/reduction performance due to limited functional groups and tendency of

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aggregation.16,17 Recently, organic substrates (i.e. humic acid and chitosan)-bound

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Fe3O4 magnetic nanoparticles with facile separation property were developed for the

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removal of toxic metals such as Cu2+, Pb2+, Cd2+ and Hg2+.18,19 As inspired by these

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researches, magnetic nano-composite (Fe3O4@EPS) by coating EPS on Fe3O4

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magnetic nanoparticles may potentially combine the respective advantages of iron

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magnetic nanoparticles and EPS to overcome their application limitations ultimately.

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In addition, coating EPS on magnetic Fe3O4 nanoparticles could prevent Fe3O4

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aggregation and air oxidation and eliminate their toxicity.20-22 Nevertheless, the

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synthesis method is another challenge. Due to the limited functional groups on

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magnetic Fe3O4 nanoparticles, their pre-modification is generally required for coating

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procedure, such as SiO2 coating and amino functionalization as linking agent.23 The

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pre-modification greatly complicates synthesis process, and even damages the

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structure and magnetic property of Fe3O4 nanoparticles. 4


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Therefore, the aim of this study is to synthesize magnetic Fe3O4@EPS by coating

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EPS on magnetic Fe3O4 nanoparticles under the facile oxidative copolymerization of

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Na2S2O8 for the first time. In this way, directly coating route without pre-modification

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was realized. The physical and chemical characterization of the synthesized magnetic

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Fe3O4@EPS were conducted, and the applicability of magnetic Fe3O4@EPS in Ag+

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wastewater remediation and AgNPs recovery was assessed in view of the adsorption

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capacity, the effects of initial pH, as well as the adsorption isotherm and kinetics. The

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sorption mechanism was also proposed. Finally, the regeneration of magnetic

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Fe3O4@EPS after repetitive use was evaluated.

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EXPERIMENTAL SECTION

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Materials. Ferric chloride hexahydrate (FeCl3·6H2O), Ferrous Sulfate heptahydrate

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(FeSO4·7H2O), ammonium hydroxide (NH3·H2O), sodium persulfate (Na2S2O8), and

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silver nitrate (AgNO3) were purchased from Sigma. Other chemicals used to produce

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EPS from Klebsiella sp. J1 were purchased from the Sinopharm Chemical Reagent

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Co., Ltd.. All reagents were analytical grade and used without further purification.

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AgNO3 was used to prepare all Ag+ containing solutions. Ultrapure water (18 MΩ cm

102

1

−

) was obtained from a Milli-Q gradient system for all experiments.

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Synthesis of Fe3O4@EPS of Klebsiella sp. J1. Magnetic Fe3O4 nanoparticles were

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prepared via the co-precipitation method detailed in Liu et al.18 Briefly, FeCl3·6H2O

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(6.1 g) and FeSO4·7H2O (4.2 g) were dissolved in 100 mL ultrapure water and heated

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to 90oC. Then 10 mL of NH3·H2O (25%) were added rapidly. The mixed solution was 5



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sequentially stirred at 90°C for 30 min and then cooled to room temperature. The

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black precipitates were collected by filtration, washing and drying. The obtained

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precipitates were magnetic Fe3O4 nanoparticles and were stored for use. EPS of

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Klebsiella sp. J1 (CGMCC No. 6243) were prepared via the method developed in our

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previous work.15

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The prepared magnetic Fe3O4 particles (0.5 g) were fully dispersed in 500 mL

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ultrapure water under ultrasonification. EPS from Klebsiella sp. J1 (0.5 g) and

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Na2S2O8 (0.05 g) were added sequentially into Fe3O4 dispersed solution. The reaction

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system was kept stirring under ice−water bath (0°C) for 5 h. Subsequently, the black

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solid products were collected via magnetic separation followed by washing and

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drying procedures. The obtained black particles were denoted as magnetic

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nano-composite (Fe3O4@ EPS extracted from Klebsiella sp. J1).

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Characterization of Fe3O4@EPS of Klebsiella sp. J1. The morphology and size

120

of the Fe3O4@EPS particles were characterized by transmission electron microscopy

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(TEM, XDT-10) equipped with energy-dispersive spectroscopy (EDS, Oxford-INCA).

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The component and molecular structure of the biomaterials were analyzed by Fourier

123

transform infrared spectroscope (FT-IR, Nexus 870) between 4000 and 400 cm−1 with

124

KBr disc technique. The crystal structure was recorded by X-ray diffraction meter

125

(XRD, Bruker D8) with Cu Kα radiation over the 2θ range from 10o to 90o. X-ray

126

photoelectron spectroscopy (XPS) measurements were performed on a Thermo

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Scientific Escalab 250Xi with Al Kα X-ray as the excitation source to analyze the

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major elemental contents on the surface of Fe3O4@EPS. Magnetic properties were 6


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also evaluated using a vibrating sample magnetometer (VSM, Lake Shore 7307) with

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an applied field between in the range of -10000 ~ 10000 Oe at room temperature.

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Batch adsorption experiments. The stock Ag+ solution (200 mg L-1) was prepared

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by dissolving AgNO3 in ultrapure water. The desired working solutions were obtained

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by appropriate dilution of the stock solutions with ultrapure water as well as pH

134

adjustment using 1 mol L−1 HNO3 or 1 mol L−1 of NaOH. In each batch adsorption

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experiment, 1 g L-1 Fe3O4@EPS was added to 20 mL Ag+ aqueous solution (50 mg

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L-1, pH 5.0) and stirred for 24 h at room temperature (22oC). After sorption, the solid

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and liquid were magnetic separated using a hand-held magnet. The concentrations of

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initial and residual Ag+ in the aqueous solution were then measured by inductively

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coupled plasma optical emission spectrometry (ICP-OES; Optima 5300 DV, PE, USA)

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with the detection limit of 10 ug L-1. The removal percentage and the adsorption

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capacity of Ag+ by Fe3O4@EPS (qe) were calculated as follows:1

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Removal percentage(%) =

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qe = ( C0 − Ce ) V / W

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where C0 and Ce are the initial and equilibrium concentrations of Ag+ (mg L−1), V is

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the volume of Ag+ working solution (L), and W is the dosage of Fe3O4@EPS (g). All

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the experimental data were the average of triplicate measurements with standard

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errors less than within 5%.

C0 − Ce ×100 C0

(1) (2)

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To evaluate the adsorption performance of the Fe3O4@EPS regarding Ag+ removal,

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the adsorption capacity of Fe3O4@EPS during 24-h adsorption test was compared to

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those of bare Fe3O4 and sole EPS, with their dosage being equivalent to the amount of 7



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Fe3O4 and EPS on Fe3O4@EPS, respectively. In addition, the effect of solution initial

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pH on Ag+ Removal of Fe3O4@EPS was also investigated by adjusting solution pH to

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the required value (1.0-6.0) with 1 mol L−1 HNO3 or 1 mol L−1 of NaOH at the

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beginning of each experiment.

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In the isotherm experiments, the initial concentration of Ag+ was varied from 10 to

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80 mg L−1. Langmuir, Freundlich and Redlich-Peterson models were applied to

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determine the sorption equilibrium. For sorption kinetic experiment of Fe3O4@EPS,

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the sorption time was selected from 5 min to 1440 min. The experimental data were

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analyzed using pseudo first-order, pseudo second-order and intraparticle diffusion

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kinetic models. All models and the key parameters were presented in Table S1 of

161

Supporting Information (SI).

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Furthermore, to further clarify the adsorption mechanism of Ag+ onto Fe3O4@EPS,

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the samples of Fe3O4@EPS before and after adsorption were measured by XPS, XRD,

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and FTIR. The adsorption test was carried out at the optimal pH 6.0 for 24 h at 22oC

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for 50 mg L-1 Ag+ contaminated water.

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Desorption and recycle experiments. Desorption experiment was performed by

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NaOH as eluent. Briefly, the samples of Fe3O4@EPS after Ag+ adsorption (1 g L-1)

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were mixed in the 0.2 mol L−1 NaOH solution for 2 h at room temperature (22oC),

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followed by magnetic separation, washing and drying. After desorption, recycle

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experiments of Fe3O4@EPS were carried out by applying them into the Ag+

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adsorption tests again, as detailed in the previous section, for 5 cycles in total.

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RESULTS AND DISCUSSION

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Synthesis of Fe3O4@EPS of Klebsiella sp. J1. Figure 1A,B illustrated the TEM

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images of Fe3O4 and Fe3O4@EPS, respectively. Clearly, the morphologies of bare

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Fe3O4 were spheroidal nanoparticles with a diameter around 30 nm. After Na2S2O8

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initiated polymerization reaction, Fe3O4@EPS exhibited well defined core-shell

178

configuration. The light-colored shell layer of EPS (10 nm thick) were successful

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coated on the black core of magnetic Fe3O4 nanoparticles, and formed spheroidal

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nanoparticles with a diameter around 50 nm. Figure 1C,D showed the elements

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distribution (i.e. Fe, O, C, and N) of Fe3O4@EPS nanoparticles via TEM-EDS line

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scanning across its diameter. The results demonstrated that Fe and O elements were

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mainly enriched in the core while C and N elements mainly distributed in the shell

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layer, which further supported the well-defined Fe3O4 core-EPS shell structure of

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Fe3O4@EPS, in agreement with the above TEM image results.

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Figure 1. TEM images of Fe3O4 (A), Fe3O4@EPS (B); STEM mode image of

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Fe3O4@ EPS (C), EDS line scanning of Fe, O, C, N elements of Fe3O4@ EPS (D).

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Characterization of Fe3O4@EPS of Klebsiella sp. J1. The FT-IR spectra of Fe3O4,

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EPS and Fe3O4@EPS were shown in Figure 2A. All characteristic bands of EPS

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appeared in the FT-IR spectra of Fe3O4@EPS, such as −OH groups (3386 cm−1), C–H

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groups (2928 cm−1), –N= groups (1625 cm−1), –NH– groups (1521 cm−1), C=N of

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quinoid rings (1390 cm−1) and C–N of benzenoid rings (1085 cm−1).24 In particular,

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Fe–O vibration (586 cm 1),25 the unique group of Fe3O4, was also present in the

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spectrum of Fe3O4@EPS, suggesting that EPS were successfully immobilized onto

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the Fe3O4 nanoparticles. Furthermore, to provide insight into the mechanism of

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core-shell configuration, the FT-IR spectra of Fe3O4@EPS was compared to those of

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Fe3O4 and EPS. After coating EPS on Fe3O4, Fe−O peak were shifted obviously and

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C=N of quinoid rings peak intensity decreased significantly, indicating that EPS

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bonded on Fe3O4 due to the interaction between Fe–O and C=N of EPS.

−

10


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6000

70

400

3000

422 511

4000 311

80

5000

400

90

111

100

Intensity (a.u.)

% Transmittance

Fe3O4 EPS Fe3O4@EPS

7000 B

220

Fe3O4 EPS Fe3O4@EPS

A

110

C=N

120

2000 1000

60

0

50 4000

3500

3000

2500

2000

1500

1000

10

500

20

-1

Wavenumbers (cm )

200000

C

160

Fe3O4@EPS Magnetization (emu/g)

O1s O

150000

100000

C1sO N1sO

50000

30

40

50

60

70

80

90

Bragg angle, 2θ (degree)

D

120

Intensity (a.u.)

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


Fe-O

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Fe3O4 Fe3O4@EPS

80 40 0 -40 -80 -120

0

-160

1400 1200 1000

202

800

600

400

200

-10000

0

-5000

0

5000

10000

Applied magnetic field (Oe)

Binding energy (ev)

203

Figure 2. FTIR spectra of Fe3O4, EPS and Fe3O4@EPS (A); XRD patterns of

204

Fe3O4, EPS and Fe3O4@EPS (B); Full range XPS of Fe3O4@EPS (C);

205

Magnetization curves of Fe3O4 and Fe3O4@EPS (D).

206

The XRD patterns in Figure 2B showed that Fe3O4 and Fe3O4@EPS displayed

207

similar locations of diffraction peak. All the diffraction peaks at 18.3, 30.1, 35.5, 43.2,

208

53.4, 57.1, and 62.6°assigned to the indices (220), (311), (400), (422), (511) and (440)

209

of cubic Fe3O4 phase.26 The similar XRD patterns of Fe3O4 and Fe3O4@EPS

210

suggested that the modification process with EPS coating did not change the cubic

211

phase of Fe3O4 cores.

212

XPS spectra of Fe3O4@EPS (Figure 2C) revealed that the major elemental contents

213

on the surface are O, N, and C. However, the shake-up satellite peaks at 724.1 and

214

710.2 eV for Fe 2p27 did not appear. These results further supported the fact that the 11



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215

Fe3O4 core in the composite was confined within a shell of EPS.

216

The magnetic properties of Fe3O4 and Fe3O4@EPS were characterized using the

217

magnetization hysteresis loops. As shown in Figure 2D, the saturation magnetization

218

(Ms) of Fe3O4 and Fe3O4@EPS was 134.62 and 79.01emu/g, respectively. Although

219

Ms of Fe3O4@EPS is slightly lower than that of Fe3O4 due to the coating of EPS, it is

220

higher than many reported magnetic materials (29.7-68.1 emu/g).17,18,26,27 The

221

undetectable hysteresis and coercivity in the magnetization curve of Fe3O4@EPS

222

suggesting its satisfactory superparamagnetic property, that is, no magnetism

223

remained after the removal of the external magnetic field. Therefore, the synthesized

224

Fe3O4@EPS nanoparticles were easily separated from their aqueous dispersions

225

within 10 s using a hand-held magnet owing to their good magnetic response.

226

Moreover, bare Fe3O4 nanoparticles were easy to lose magnetic property due to the air

227

oxidation, whereas no significant change of Ms was observed after storing

228

Fe3O4@EPS in water for one month, indicating coating EPS on Fe3O4 could maintain

229

their magnetic property by prohibiting oxidation.

230

In sum, these results demonstrated that high magnetic Fe3O4@EPS nanomaterials

231

with well-defined Fe3O4 core-EPS shell structure were synthesized exploiting the

232

polymerization reaction of EPS on Fe3O4 particles with the interaction between Fe–O

233

and C=N of EPS, which avoid complicated pretreatment procedures.

234

Ag+ adsorption capacity of Fe3O4@EPS. Figure 3A presented the Ag+ adsorption

235

capacity on Fe3O4, EPS and Fe3O4@EPS at different adsorption time. Fe3O4 showed

236

extremely low even undetectable Ag+ adsorption capacity during 24-h adsorption test 12


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237

due to the limited functional groups,16 indicating that Fe3O4 did not participate in Ag+

238

adsorption. In contrast, EPS exhibited superior Ag+ adsorption capacity during the

239

adsorption process. The adsorption capacity of EPS for Ag+ rose rapidly during the

240

initial 0.5 h, and increased gradually until adsorption saturation at 24 h with the

241

highest adsorption capacity of 45.2 mg g-1. Interestingly, Fe3O4@EPS achieved even

242

higher Ag+ adsorption capacity (maximum 47.6 mg g-1) compared to the same amount

243

of EPS during 24-h adsorption test, although Fe3O4 had no adsorption capacity.

244

Moreover, no iron leaching was observed after Fe3O4@EPS adsorption. A comparison

245

study of adsorption capacity for Ag+ between Fe3O4@EPS and other reported sorbents

246

under similar conditions was also carried out. The data in SI Table S2 revealed that

247

Fe3O4@EPS have higher adsorption capacity than other sorbents. Therefore,

248

Fe3O4@EPS has a great potential in Ag+ removal from contaminated aquatic systems. 100 B

Adsorption Capacity (mg/g)

Adsorption Capacity (mg/g)

50 A 40 30

Fe3O4 EPS Fe3O4@EPS

20 10 0

249

5

10

15

20

25

-8

60

-4

40

0

Adsorption Capacity Zeta Potential

20 0

0

-12

80

0

1

2

3

4

5

6

4

7

Zeta Potential (mv)

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


8

pH

Adsorption Time (h)

250

Figure 3. Adsorption capacity of Ag+ on Fe3O4, EPS and Fe3O4@EPS during 24-h

251

adsorption experiment (A); Effect of initial pH on the adsorption capacity of Ag+

252

and the corresponding zeta potential of Fe3O4@EPS (B).

253

The high Ag+ adsorption capacity of Fe3O4@EPS might be due to the fact that the

254

core-shell configuration by coating EPS on Fe3O4 decreased the overall size to

255

nanometer, which resulted in remarkably increase of the adsorption areas, thereby 13



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256

enhancing Ag+ adsorption capacity.28 Therefore, the synthesis of Fe3O4@EPS by

257

coating EPS on Fe3O4 not only conferred upon its magnetic property but also

258

synergistically improved its Ag+ adsorption capacity. In addition, the sorption process

259

of Fe3O4@EPS went through the stage from a rapid but instant step (0-0.5 h) to a slow

260

but primary stage (0.5-20 h) till adsorption saturation (20-24 h). The initial fast step

261

may be attributed to physical sorption of Ag+ onto Fe3O4@EPS via electrostatic forces.

262

In contrast, the subsequent slow step may relate to chemical interactions (e.g.

263

chelation and redox) between Ag+ and Fe3O4@EPS over the long-term sorption

264

period.

265

Impacts of initial solution pH. The pH condition governs the speciation of

266

metallic ions and chemical/biological reactions in water system. To avoid the possible

267

error result from AgOH precipitation formation due to higher pH value, the effect of

268

initial solution pH on adsorption capacity of Ag+ by Fe3O4@EPS was investigated in

269

the range of 1.0–6.0. As shown in Figure 3B, the adsorption capacity of Ag+ was

270

positively correlated with elevated pH. The low Ag+ adsorption capacity at low pH

271

values could be attributted to a competitive adsorption between H+ and Ag+, that is,

272

the abundant competitive H+ occupied the adsorption sites of Fe3O4@EPS. At the

273

same time, the activity of functional groups of Fe3O4@EPS (e.g., hydroxyl, carboxyl,

274

amine and imine group) was affected by pH and reflected in surface charge change.1

275

The zeta potential of Fe3O4@EPS was measured as positive when pH ≤ 2.0 and the

276

overall surface of Fe3O4@EPS was negatively charged at the pH between 3.0 and 6.0

277

(Figure 3B). At low pH values (pH 1-2), functional groups of Fe3O4@EPS gained 14


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278

protons and the zeta potentials of Fe3O4@EPS were shown as positive values.

279

Increased electrostatic repulsion restricted the interaction between Ag+ and

280

Fe3O4@EPS. However, the competition from H+ decreased with the increasing pH,

281

and the surface of Fe3O4@EPS carried negative charge via deprotonation of

282

functional groups, enhancing Ag+ adsorbability. The minimum negative zeta potential

283

value (zP =−13.57 mV) was observed at pH 6.0, which corresponded to the maximum

284

adsorption capacity (49.6 mg g-1) of Ag+ onto Fe3O4@EPS. It should be noted that the

285

Ag+ concentration after treatment at pH 6.0 indeed meet the EPA standard (＜0.5 mg

286

L-1) for wastewater discharge.29

287

Adsorption

isotherm

and

kinetics.

The

Langmuir,

Freundlich

and

288

Redlich-Peterson isotherm were used for analyzing the adsorption equilibrium of Ag+

289

onto Fe3O4@EPS. The plots for the Langmuir and Freundlich isotherm models were

290

presented in SI Figure S1A with the model parameters being listed in SI Table S3. The

291

Langmuir isotherm is an empirical model to describe a monolayer adsorption on

292

homogeneous surface without interaction between adsorption sites, while the

293

Freundlich isotherm assumes a homogeneous adsorption process with interaction.30 It

294

was observed that the adsorption isotherm behavior of Ag+ onto Fe3O4@EPS could

295

not be well described by these two models. In comparison, the Redlich−Peterson

296

isotherm model that combines the Langmuir and Freundlich models can accurately

297

describe the experimental data (SI Figure S1B) , indicating the adsorption behavior of

298

Ag+ onto Fe3O4@EPS was a hybrid and concurrent process rather than ideal

299

monolayer adsorption.1 15



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

300

The pseudo first-order and pseudo second-order kinetic models were applied to

301

analyze the adsorption kinetics of Ag+ onto Fe3O4@EPS. As shown in SI Figure S1C

302

and Table S3, the adsorption process fitted well with the pseudo second-order model

303

compared to the pseudo first-order model, which also failed to estimate the

304

equilibrium adsorption capacity (qe) correctly. The pseudo second-order model

305

assumes that the chemical adsorption is the main adsorption mechanism over the

306

whole adsorption process, which means that the adsorption of Ag+ onto Fe3O4@EPS

307

mainly depended on chemical reaction (e.g. chelation and redox).17 Predicting the

308

rate-limiting step is another important factor to be considered in the adsorption kinetic

309

process. For a solid-liquid adsorption process, the adsorption dynamics can be

310

described by the following three consecutive steps: 1) instantaneous or external

311

surface adsorption, 2) diffusion from the surface to the intra-particular sites and 3)

312

solute adsorption by chemical reaction (e.g. chelation and redox).31 It is generally

313

accepted that the early step (1) is quite rapid and does not represent the

314

rate-determining step. Thus, the intraparticle diffusion kinetic was employed and the

315

results were shown in SI Figure S1D and Table S3. The plots qt vs. t0.5 exhibited a

316

piecewise-linear pattern with three slopes, which were corresponding to three

317

consecutive steps of adsorption process. However, the intraparticle diffusion kinetic

318

model can not accurately simulate the adsorption experiments, suggesting that the

319

intra-particle diffusion was not the rate-limiting step, and chemical reaction (e.g.

320

chelation and redox) might be involved.

321

Elucidation of sorption-reduction mechanism. To further clarify the adsorption 16


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322

mechanism of Ag+ onto Fe3O4@EPS, several typical techniques such as XPS, XRD,

323

and FTIR were applied together. As shown in Figure 4A, typical XPS peak of Ag0

324

appeared on Fe3O4@EPS after Ag+ adsorption. The deconvolution of Ag 3d XPS peak

325

were assigned to the peak at 367.4 ev for Ag+ and the peak at 367.9 ev for Ag0 and

326

80.6% of Ag+ were reduced to Ag0 by [email protected] The presence of Ag0 suggested

327

redox sorption did occur during the adsorption of Ag+, whereby the Fe3O4@EPS acted

328

as a reductant and Ag+ as an oxidant. Compared to the X-ray diffraction pattern of

329

Fe3O4@EPS (Figure 4B), Ag+ adsorbing Fe3O4@EPS emerged three additional sharp

330

peaks at 38.1°, 44.5°, and 64.8°, which correspond to the diffraction of (111), (200),

331

and (220) lattice planes of AgNPs, respectively. These results verified the redox

332

sorption of Ag+ by Fe3O4@EPS.

333

In order to provide insights into the redox sorption process of Ag+ onto

334

Fe3O4@EPS, the N 1s and O 1s XPS spectra of Fe3O4@EPS before and after Ag+

335

adsorption were also analyzed and shown in Figure 4C,D. Before Ag+ adsorption, N

336

1s peak of Fe3O4@EPS was deconvoluted into two peaks (Figure 4C), benzenoid

337

amine (–NH–) at 398.9 ev and quinoid imine (–N=) at 397.9 ev, and their molar ratios

338

were 83.9% and 16.1%, respectively. After adsorption, the molar ratio of –NH–

339

deceased to 41.9% while that of –N= increased significantly, implying benzenoid

340

amine (–NH–) on Fe3O4@EPS as reduction components contributed to the reduction

341

of Ag+ into AgNPs. Meanwhile, a new peak appeared at 400.5 ev for the protonated

342

quinoid imine (–N=+), which may be due to the doping of H+ on quinoid imine.

343

Actually, a slight pH increase after Ag+ adsorption was observed, indicating that a 17



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Page 18 of 28

344

certain amount of H+ were consumed due to protonation, accompanying with Ag+

345

reduction by Fe3O4@EPS. Figure 4D showed the O1s XPS peaks of Fe3O4@EPS

346

before and after Ag+ adsorption. For original Fe3O4@EPS, two peaks occurred at

347

533.1 ev and 532.3 ev, which correspond respectively to C–O and –OH of phenols or

348

alcohols.32 After Ag+ adsorption, the molar ratio of these two peaks decreased

349

accompanied by the presence of two new peaks at 531.6 ev and 530.7 ev for the O

350

binding with metal ions,33 indicating that hydroxyl participated in the chelation with

351

Ag+.

352

The functional groups responsible for the sorption of Ag+ and the reductive

353

formation of AgNPs can be further confirmed by obvious change of FTIR spectra of

354

Fe3O4@EPS before and after Ag+ sorption. Figure 4E revealed that

355

3392 cm-1 corresponding to −OH groups weakened or even disappeared after

356

adsorbing Ag+, which further proved the interaction between hydroxyl and Ag+. The

357

peak of benzenoid amine (–NH–) (1521 cm−1) disappeared while the peak of quinoid

358

imine (–N=) (1625 cm−1) became stronger after adsorption. At the same time, the

359

relative intensity of C–N of benzenoid rings (1085 cm−1) decreased but that of C=N of

360

quinoid rings (1390 cm−1) increased. It was thus concluded that the benzenoid amine

361

(–NH–)

the shark

peak at

on Fe3O4@EPS as a reductant was oxidized to quinoid imine (–N=) by Ag+.

362

To ascertain the occurrence of ion exchange sorption, the element composition of

363

Fe3O4@EPS before and after Ag+ adsorption was characterized by XPS. It was

364

observed in Figure 4F,G that the disappearance of both Mg signal peak at 1302.8 ev

365

and K at 292.5 ev after Ag+ adsorption occureed, suggesting that ion exchange also 18


Page 19 of 28

366

existed in Ag+ adsorption process via exchange with Mg2+ and K+ on Fe3O4@EPS. 2800

A

2700

B

Fe3O4@EPS Fe3O4@EPS-Ag

2600 2500 2400

Intensity (a.u.)

Intensity (a.u.)

(80.6%)Ag0

2300 111

2200 2100

AgNPs

200

2000

220

1900

(19.4%)Ag+

1800 1700 1600 1500 1400 1300

364

365

366

367

368

369

370

371

10

372

20

Binding energy（ ev）

N1s

Intensity (a.u.)

Intensity (a.u.)

(41.9%) –NH– After

397

398

399

400

401

402

403

After (52.2%) Ag-O


40 4000

Before adsorption After adsorption 3500

3000

2500

2000

1500

1000

1296

500

Wavenumbers (cm )

368

1300

1302

1304

1306

1308

Before adsorption After adsorption K2p

298

367

1298

Binding energy (ev)

-1

G

Mg1s

Before adsorption After adsorption

Intensity (a.u.)

C-N

50 45

(36.2%) C–O

527 528 529 530 531 532 533 534 535 536 537 538

404

C=N -N= -NH-

O-H

% Transmittance

55

90

(18.4%) Ag–OH

F

60

80

(15.5 %) –OH (13.9 %) C–O

E

65

70

Before


70

60

O1s

(12.8%) –N=+

396

50

(63.8%) –OH

(45.3%) –N=

395

40

D

Before

(83.9%) –NH–

(16.1%) –N=

394

30

Bragg angle, 2θ (degree)

C

Intensity (a.u.)

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


296

294

292

290

288

Binding energy (ev)

Figure 4. The spectra of Fe3O4@EPS before and after Ag+ adsorption. Ag 3d XPS 19



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

369

(A); XRD (B); N 1s XPS (C); O 1s XPS (D); FT-IR (E); Mg 1s XPS (F); K 2p

370

XPS (G).

371

Based on these comprehensive results, the sorption mechanism of Ag+ onto

372

Fe3O4@EPS was proposed as follows: in a short-term sorption, physical electrostatic

373

sorption between Ag+ and negatively charged Fe3O4@EPS rapidly happen.

374

Simultaneously, Ag+ was successively exchanged with Mg2+ and K+ on Fe3O4@EPS.

375

Hydroxyl groups participate in the chelation with Ag+. With the sorption proceeding,

376

Ag+ ions are reduced to AgNPs by benzenoid amine (–NH–) on Fe3O4@EPS. Redox

377

sorption is the major mechanism of Ag+ onto Fe3O4@EPS, and the other three kinds

378

of sorption (i.e., physical electrostatic sorption, ion exchange sorption and chelation

379

sorption) are minor contributors.

380

Desorption and recycle performance. The recycle performance of materials is of

381

very importance for their widespread application. To investigate the recycle property

382

of the synthesized magnetic Fe3O4@EPS nanomaterial, the repeated adsorption-

383

desorption experiments were conducted for five cycles. After desorption of Ag+

384

adsorbing Fe3O4@EPS by NaOH, it was found that 88% of the total adsorbed Ag was

385

desorbed. As seen in SI Figure S2, Fe3O4@EPS exhibited good adsorption capacity in

386

each cycle and adsorption capacity remained 85% after five cycles. Moreover,

387

magnetic property of Fe3O4@EPS was not appreciably deteriorated. These results

388

confirmed the application feasibility of the new magnetic Fe3O4@EPS as a cost

389

effective and high efficient nanomaterial for Ag+ wastewater remediation.

390 20


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391

CONCLUSIONS

392

Magnetic nano-composite (Fe3O4@ EPS extracted from Klebsiella sp. J1) were

393

synthesized by coating of EPS on magnetic Fe3O4 particles. The Fe3O4@ EPS had a

394

well-defined core-shell structure with a size of around 50 nm. The EPS bonded on

395

Fe3O4 due to the interaction between Fe–O and C=N of EPS. The modification

396

process with EPS coating did not change the cubic phase of Fe3O4 cores. The Fe3O4@

397

EPS nanoparticles were effective in Ag+ removal with the maximum adsorption

398

capacity of 49.6 mg g-1 at pH 6.0 for 24h. Chemical reaction (e.g. chelation and redox)

399

was revealed to be the rate-limiting step for the sorption process. The Ag+ adsorption

400

on Fe3O4@EPS was mainly attributed to the reduction of Ag+ to AgNPs by benzenoid

401

amine (–NH–), accompanied by the chelation between Ag+ and hydroxyl groups, ion

402

exchange between Ag+ and Mg2+ and K+, and physical electrostatic sorption. The

403

adsorption capacity of Fe3O4@EPS remained 85% after five recycle experiments,

404

indicating the great potential of Fe3O4@EPS in Ag+ wastewater remediation.

405 406

AUTHOR INFORMATION

407

Corresponding Authors

408

*

409

*

410

Notes

411

The authors declare no competing financial interest.

Phone: +86 451 8628 6837; fax: +86 451 8628 3787; e-mail: [email protected]. Phone: +86 451 8628 3088; fax: +86 451 8628 3088; e-mail: [email protected].

412 21



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

413

ACKNOWLEDGMENTS

414

We acknowledge the financial support by Natural Science Foundation of China (No.

415

51578179), HIT Environment and Ecology Innovation Special Funds (No.

416

HSCJ201604), HIT State Key Lab of Urban Water Resource and Environment

417

Foundation (No. 2016TS01) and the Fundamental Research Foundation for the

418

Central Universities (No. HIT. NSRIF. 2015095).

419 420

SUPPORTING INFORMATION

421

Two figures and three tables in Supporting Information, isotherm and kinetic plots for

422

the adsorption of Ag+ onto Fe3O4@EPS (Fig. S1); Ag+ adsorption capacities of

423

magnetic Fe3O4@EPS at different regeneration cycles (Fig. S2); the models and

424

parameters used for the adsorption of Ag+ by Fe3O4@EPS (Table S1); comparison of

425

Ag+ sorption capacity of Fe3O4@EPS with other reported adsorbents under similar

426

experimental conditions (Table S2); isotherm and kinetic constants for the adsorption

427

of Ag+ onto Fe3O4@EPS (Table S3).

428 429

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(32) Swiatkowski, A.; Pakula, M.; Biniak, S.; Walczyk, M. Influence of the surface

537

chemistry of modified activated carbon on its electrochemical behaviour in the

538

presence of lead (II) ions. Carbon, 2004, 42(15), 3057-3069. DOI: 10.1016/

539

j.carbon.2004.06.043.

540

(33) Lim, S.F.; Zheng, Y.M.; Zou, S.W.; Chen, J.P. Characterization of copper

541

adsorption onto an alginate encapsulated magnetic sorbent by a combined FT-IR,

542

XPS, and mathematical modeling study. Environ. Sci. Technol. 2008, 42(7):

543

2551-2556. DOI: 10.1021/es7021889.

544

27



TOC graphic

545 C N1s

Before

(83.9%) –NH–

(16.1%) –N=

Intensity (a.u.)

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

(45.3%) –N=

(41.9%) –NH– After (12.8%) –N=+

394

395 396

Ag + wastewater

Step II Purified water

Adsorption

397 398 399 400 401 402 403 404


Nano-Fe3 O4

Nano-biosorbent

Fe3 O4 @EPS-Ag AgNPs

Step I

+ e- Ag

Oxidant

Synthesis Regeneration

EPS

Step III

546

Concentrated Ag species

NaOH eluent

547

Synopsis: Fe3O4@EPS were firstly synthesized as recycled material for silver

548

ions (Ag+) wastewater remediation by simultaneous redox sorption

549

28


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Synthesis of CoreâShell Magnetic Nanocomposite Fe3O4@ Microbial

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