Highly Efficient Water Decontamination by Using Sub-10 nm FeOOH

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Highly efficient water decontamination by using sub-10 nm FeOOH confined within millimeter-sized mesoporous polystyrene beads Xiaolin Zhang, Cheng Cheng, Jieshu Qian, Zhenda Lu, Siyuan Pan, and Bing-Cai Pan Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b01608 • Publication Date (Web): 18 Jul 2017 Downloaded from http://pubs.acs.org on July 19, 2017

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Environmental Science & Technology

Highly efficient water decontamination by using sub-10 nm FeOOH

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confined within millimeter-sized mesoporous polystyrene beads

2 3 4

Xiaolin Zhang,† ‡ Cheng Cheng,† Jieshu Qian,§ Zhenda Lu, ‡‖ Siyuan Pan,† Bingcai

5

Pan*†‡

6 7 8 9 10 11 12 13 14

†

State Key Laboratory of Pollution Control and Resource Reuse, School of the

Environment, Nanjing University, Nanjing 210023, China ‡

Research Center for Environmental Nanotechnology (ReCENT), Nanjing University,

Nanjing 210023, China §

School of Environmental and Biological Engineering, Nanjing University of Science

and Technology, Xiaolingwei 200, Nanjing, 210094, China ‖

College of Engineering and Applied Science, Nanjing University, Nanjing 210023,

China

15 16 17 18

* To whom correspondence should be addressed

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E-mail: [email protected] (B. Pan)

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Tel: +86-25-8968-0390

ACS Paragon Plus Environment


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ABSTRACT

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Millimeter-sized polymer-based FeOOH nanoparticles (NPs) provide a promising

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option to overcome the bottlenecks of direct use of NPs in scaled-up water

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purification, and decreasing the NP size below 10 nm is expected to improve the

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decontamination efficiency of the polymeric nanocomposites due to the size and

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surface effect. However, it is still challenging to control the dwelled FeOOH NP sizes

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to sub-10 nm, mainly due to the wide pore size distribution of the currently available

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polymeric hosts. Herein, we synthesized mesoporous polystyrene beads (MesoPS) via

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flash freezing to assemble FeOOH NPs. The embedded NPs feature with α-crystal

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form, tunable size ranging from 7.3 to 2.0 nm and narrow size distribution.

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Adsorption of As(III/V) by the resultant nanocomposites was greatly enhanced over

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the α-FeOOH NPs of 18×60 nm, with the iron mass normalized capacity of As(V)

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increasing to 10.3 to 14.8 fold over the bulky NPs. Higher density of the surface

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hydroxyl groups of the embedded NPs as well as their stronger affinity toward As(V)

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was proved to contribute to such favorable effect. Additionally, the as-obtained

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nanocomposites could be efficiently regenerated for cyclic runs. We believe this study

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will shed new light on how to fabricate highly efficient nanocomposites for water

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decontamination.


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INTRODUCTION

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Iron oxide nanopartilces (NPs) have been extensively used in water decontamination

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for various inorganic pollutants, owing to their promising properties such as high

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reactivity, abundant active surface sites, low cost and environmental friendliness.1-3

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As a typical class of amphoteric compounds,3 iron oxides exhibit desirable adsorption

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reactivities toward both oxyanions (e.g., arsenic, phosphorus and chromate)4 and

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heavy metal cations (e.g., lead, cadmium and copper).5 For instance, magnetite NPs

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were utilized to remove chromate from water in the presence of fulvic acid, natural

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organic matters and isolated landfill leachate, demonstrating that chromate adsorption

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by hematite NPs was still significant even when high concentration natural organics

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(100 mg/L) are present.6 Common anions including sulfate, chloride, nitrate and

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carbonate exert ignorable influence on the specific adsorption of arsenic onto iron

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oxide NPs,7 and the capacities of >0.20 mmol/g toward either arsenite (As(III)) or

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arsenate (As(V)) were observed for magnetite NPs of 34 nm.8 Moreover, the

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exhausted iron oxide NPs could be regenerated by using alkaline solutions for

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oxyanions7 or using EDTA solution for heavy metal cations9 without significant

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capacity loss, thereby realizing the cyclic utilization of these NPs. However, the

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scaled-up utilization of NPs in water treatment is intensively hindered by their

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intrinsic drawbacks such as self-aggregation, difficult operation and risk of possible

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leakage into water.10 To overcome the above defects, metal oxide NPs were often

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supported in porous solids of larger size such as activated carbon,11 mesoporous

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silicates,12 biomass,13 sand,14 diatomite15. Among these host materials, the crosslinked



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porous polystyrene (PS) is one of the most attractive options taking advantage of their

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robustness, chemical stability, tunable structure, and ease of operation of the polymer

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hosts,10, 16-19 and some of them have been successfully applied in field application

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with exceptional stability and satisfied efficiency.20, 21

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Adsorption of pollutants from water onto iron oxide NPs is highly dependent

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upon their size. Such size dependece of NPs is mainly associated with higher specific

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surface area as well as higher surface atom proportion of the smaller NPs.22, 23 In the

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past years, people were particularly interested in the amazing decontamination

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reactivities of the sub-10 nm iron oxide NPs due to their significant size and surface

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effect in this size range.6, 24, 25 For instance, Yavuz et al.25 demonstrated that reducing

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Fe3O4 NPs size from 300 to 20 nm would result in an increase in the maximum

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adsorption capacity (Qm) from near zero to several or 25 mg/g for As(V) and As(III),

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respectively. Further drop of the NPs size to ~10 nm would dramatically increase the

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Qm values to 200 or 160 mg/g for As(V) and As(III), respectively. Similarly, Madden

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et al.24 observed that the hematite NPs of 7 nm exhibited much stronger affinity

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towards Cu2+ than those of 25 and 88 nm. Swindle et al.6 utilized magnetite NPs of 90

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nm and 6 nm for the removal of chromate, suggesting that the adsorption capacity

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rised dramatically from around 0.038 mmol/g for hematite of 90 nm to around 0.248

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mmol/g for those of 6 nm.

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To date, some polymeric nanocomposite adsorbents are commercially available

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for advanced water treatment.20, 21 However, the growth of sub-10 nm NPs inside

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polymer hosts still remains a great challenge. This is mainly because the widely used


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polymer hosts of crosslinking structure feature a wide pore size distribution from 100 nm.26-29 Correspondingly, the particle sizes of the embedded iron oxide

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NPs exhibited a broad distribution from several to dozens of nanometers, and most

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NPs were higher than 10 nm in size.26-29 Besides, the stability of the iron-based

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nanocomposites at acidic pHs is another issue during their practical utilization in

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water/wastewater treatment.30 Generally, the dissolution of iron oxide NPs turned

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more intensive for those of smaller size.22, 23 Thereby, it still remains attractive but

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challenging to enhance adsorption of target pollutants onto the embedded iron oxide

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NPs via reducing their size below 10 nm without compromising their stability.

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In this study, we describe an in situ method to prepare sub-10 nm FeOOH

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embedded in mesoporous PS (denoted as Fe@MesoPS) for highly efficient water

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decontamination by using As(V) and As(III) as the target pollutants. Arsenic is

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usually observed in natural water and listed as the priority pollutant by US and China

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EPA, with the maximum allowable concentration in drinking water of 10 μg/L. The

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MesoPS beads of ca. 2 mm in diameter were fabricated via flash freezing, a recently

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emerging strategy for prepartion of highly ordered mesoporous polymers.31 The

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morphology and surface chemistry of the embedded FeOOH NPs were also

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investigated to elucidate the underlying mechanism for the enhanced arsenic

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adsorption.

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EXPERIMENTAL SECTION

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Chemicals and reagents. All the chemicals used in this study, including ferric

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chloride hexahydrate, sodium hydroxide, N, N’-dimethylformamide (DMF), methanol,

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ethanol and tert-butyl alcohol, are of analytical grade unless specified otherwise. The

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stock solutions of As(V) (1000 mg/L) were prepared by dissolving Na2HAsO4·7H2O

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(Sigma-Aldrich, China) in ultrapure water (18 MΩ cm). Polystyrene (PS, MW= 192

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kDa) and humic acid were purchased from Aldrich-Sigma (Shanghai, China). The

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commercial porous PS (CPPS) was available from Zhengguang Resin Co. Ltd.

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(Hangzhou, China).

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Preparation of Fe@MesoPS nanocomposite adsorbents. The fabrication of

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Fe@MesoPS is schematically illustrated in Figure 1. MesoPS beads were prepared

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through a flash freezing route modified from a previous study.31 In brief, PS was

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dissolved in DMF to obtain concentrated polymer solution (20~40 wt%), and the

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resultant solution was injected dropwise into liquid nitrogen containing excessive

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methanol (in the form of frozen solid, >10 times of the DMF volume). Upon the

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gradual volatilization of liquid nitrogen, the frozen methanol turned into the liquid

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form eventually, simultaneously extracting glassy DMF nanocrystals from PS matrix

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to form nanopores. The samples were then freeze dried to obtain MesoPS. In this

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study, we prepared four MesoPS samples by adjusting the concentration of PS

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solution at 20 wt%, 30 wt%, 35 wt% and 40 wt%, respectively, and the MesoPS

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samples were denoted as MesoPS-1, 2, 3, and 4 in sequence.


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Afterward, 1.0 g MesoPS beads were added into a 20-mL solution containing 3.0

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M FeCl3. The mixture was magnetically stirred for 24 h. The beads were then filtered

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out, followed by mixing with 20 mL NaOH solution (5 wt %) and stirring for another

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24 h. Finally, the beads were filtered out and subjected to thermal treatment at 323 K

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for 5 h to obtain the final product Fe@MesoPS-X (X=1, 2, 3, and 4).

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Batch adsorption experiments. To investigate the batch adsorption of Fe@MesoPS,

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0.25 g adsorbent was introduced into 250-mL As(V) solution of preset concentration,

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followed by stirring at 298 K for 24 h. Sodium hydroxide (0.10 M) and hydrochloride

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acid (0.10 M) were used to maintain pH at 5.0 at very short intervals unless otherwise

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noted. Atomic fluorescence spectrometer (AF-640A, Rayleigh Instruction Co. Beijing,

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China) was used to determine As(V) concentration of the detection limit at 0.20 μg

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L-1. According to a mass balance between the initial and final concentration, the

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amount of As(V) uptake was calculated. The As(V)-preloaded nanocomposites were

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shaken with 5 wt% NaOH solution at 298 K for 24 h to regenerate their adsorption

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capacity. The concentration of As(V) in the alkaline solutions were determined to

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calculate the amount of the desorbed As(V). The Fe@MesoPS beads were then

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utilized

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adsorption-regeneration assays. All the above experiments were carried out in

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triplicate unless stated otherwise.

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Characterization and analysis. The specific surface area and pore size distribution

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of the MesoPS and Fe@MesoPS samples were determined using N2 adsorption-

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desorption test at 77 K (NOVA3000, Quantachrome, Boynton Beach, FL, USA). The

for

treatment

of

fresh

As(V)

solution


to

initiate

the

cyclic


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FeOOH NPs in Fe@MesoPS were observed with high resolution transmission

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electron microscope (HRTEM, JEM-200CX, JEOL, Japan) working at 200 kV. To

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preprare the grids, each sample was ground and dispersed in methanol uniformly. A

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few drops of the mixture were dried on a copper grid covered with a holey carbon

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film. Afterwards, the grids were air-dried and then mounted on the sample holder for

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observation. For each sample, at least five TEM images were obtained to investigate

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the microscopic strucutre of the embedded NPs. As well, at least 200 particles from

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each image were measured to statistically analyze their size. The crystalline structures

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of the samples were investigated by X-ray diffraction (XRD, X’TRA, ARL,

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Switzerland) with Cu Kɑ radiation. After acidic digestion, Fe contents in

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Fe@MesoPS were examined by atomic absorption spectrophotometer (AA-7000,

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Shimadzu, Japan). To investigate the distribution of Fe on the cross section of

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Fe@MesoPS, scanning electron microscope (SEM, S-3400 II, Hitachi, Japan) coupled

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with energy dispersive spectroscopy (EDS) was utilized. Potentiometric titration and

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zeta potential (Nano ZS90, Malvern Instruments, U.K.) were employed to analyze

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surface chemistry of the confined FeOOH NPs. Potentiometric titration was

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accomplished by an automatic titration system (T50, Mettler Toledo, Switzerland)

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equipped with a combined glass pH electrode (DGi115-SC, Mettler Toledo,

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Switzerland).

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RESULTS AND DISCUSSION

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Basic properties of the materials. We prepared four MesoPS hosts according to

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Figure 1 by changing PS concentrations (20%, 30%, 35% and 40% in mass).


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Accordingly, the FeOOH-loaded nanocomposites were referred to Fe@MesoPS-1 to

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Fe@MesoPS-4 in sequence. The macroscopic appearances of the MesoPSs before and

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after FeOOH NPs encapsulation are shown in Figure 2a, b. All the MesoPS samples

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were present as white spherical beads of uniform size (ca. 2 mm in diameter), while

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the corresponding nanocomposites, i.e., Fe@MesoPS-X, remained similar shape and

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size but developed reddish brown color. The surface areas and Fe-loading contents of

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all the samples are summarized in Table S1. No substantial difference in FeOOH NPs

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contents (ca. 6 wt % in Fe mass) were observed among the four Fe@MesoPS samples.

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The elemental Fe distribution of an individual Fe@MesoPS-4 bead is illustrated in

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Figure 2c. As seen, the FeOOH NPs mainly located in the periphery region (ca. 0.10

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mm) of the bead, which is expected to facilitate fast diffusion of the target pollutants

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during adsorption.

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The pore structure of the samples was determined by nitrogen adsorption-

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desorption experiments at 77 K, and the isotherms are illustrated in Figure S1 in

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supporting information (SI). The type IV adsorption isotherms with H1 loop (IUPAC

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classification) for all the samples suggest the presence of uniform and cylindrical

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mesopores.32 The pore size distribution of all the MesoPS and Fe@MesoPS samples

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based on the BJH model is available in Figure S2. All the MesoPS samples possessed

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narrow size distribution with the mean pore size of 26, 12, 9.5, and 7.9 nm for

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MesoPS-1 to MesoPS-4 respectively. As illustrated in Figure 1, when the PS-DMF

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solution was cooled by liquid nitrogen, DMF was crystallized in the nano-sized

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domains, resulting in a microphase separation process. Then, the solvent nanocrystals



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acted as porogenic agents to generate permanent pore structures during extraction by

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the melted methanol.31 Figure S2 shows the mean pore size of each Fe@MesoPS

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sample decreased sharply to 1-5 nm after the deposition of FeOOH NPs. Obviously,

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the growth of FeOOH NPs inside MesoPS blocked the pore regions to a considerable

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content and created new pores in size of 1-5 nm simultaneously.

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Crystalline structure of the embedded FeOOH NPs. XRD spectra of the four

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Fe@MesoPS samples are present in Figure 3. The broad diffraction peak of the four

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samples at 2θ value of 19.0o is ascribed to PS matrix, while those at 2θ values of 21.0,

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33.0, 34.8, 35.3, 36.7, 53.2 and 59.0o correspond to the diffraction of the (110), (130),

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(021), (101), (111), (221) and (160) planes of the crystalline α-FeOOH (JCPDS No.

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29-0713). Also, some crystal defects were observed in the HR-TEM image of

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Fe@MesoPS (Figure 4d), and the possible presence of amorphous ferrihydrite should

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not be ruled out. Surprisingly, α-FeOOH could be formed under such mild conditions,

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since the formation of α-FeOOH normally requires relatively high temperature (e.g.,

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90 oC) and/or aging over several days.33 To preliminarily probe what favors the

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formation of α-FeOOH, we employed CPPS, a commercial porous PS with broader

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pore size distribution than MesoPS (see Figure S3), to support nano iron oxide under

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otherwise identical conditions. As observed in Figure S4, CPPS of unordered pores

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resulted in the formation of amorphous ferrihydrite instead of α-FeOOH, which is

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well consistent with previous studies.34-36 The results imply that the mesoporous

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structure of PS facilitates the formation of α-FeOOH NPs. Possibly, the following

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three reasons should be considered for the distinct crystallization of FeOOH NPs


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inside mesopores. Firstly, due to the size exclusion of the host mesopores,

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considerable amount of impurities cannot diffuse inside nanopores, thus avoiding the

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undesirable interruption of the homogeneous necleation process and facilitating the

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formation of crystallized α-FeOOH NPs.37-40 Secondly, different solution chemistry

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under nanoconfinement from the surrounding solution, e.g., both the H+ activity and

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the polarity of water molecules decreases in nanopores,41, 42 also favors α-FeOOH

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NPs formation. In addition, the slower exchange of the nano-confined molecules than

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in bulk solution may also contribute the crystallization. Obviously, more solid

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evidences are still required to reveal the underlying mechanism. Fortunately, the

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formation of α-FeOOH enables Fe@MesoPS to work at more acidic pHs than

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Fe@CPPS, as discussed below.

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Microscopic structure of the confined FeOOH NPs. The morphology and structure

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of the FeOOH NPs embedded in four MesoPS were characterized. For simplification,

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we provided TEM images of Fe@MesoPS-1 and Fe@MesoPS-4 in Figure 4a, b. As

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observed, the FeOOH NPs were well dispersed in the PS matrix, suggesting the

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efficient inhibition of NPs aggregation by the robust MesoPS host. Figure 4d shows

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the HR-TEM images of Fe@MesoPS, and the marked lattice space of 0.269 nm

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corresponds to the (130) crystal facet of α-FeOOH. Also, considerable crystal defects

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were observed in Figure 4d, implying the presence of amorphous ferrihydrite phase.

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In order to obtain the size distribution of the FeOOH NPs, we carried out a

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statistic analysis on 200 particles randomly selected from several TEM images of each

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sample. As shown in Figure 4c, the mean values of the NP diameter are 7.3±1.4,



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4.8±1.3, 3.3±0.9 and 2.0±0.6 nm for Fe@MesoPS-1, 2, 3, and 4, respectively. More

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importantly, all the FeOOH NPs possess narrow size distribution with a maximum

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poly-dispersity index (PDI) of 1.06, as obtained by dividing the value of the

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weight-averaged diameter by that of the number-averaged diameter. On the contrary,

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the FeOOH NPs formed within CPPS were characterized by irregular morphologies

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and wide size distirbution (PDI=1.31), ranging from several to even more than one

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hundred nanometers (Figure S4). These results render us to believe that mesoporous

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PS, i.e., MesoPS, enables the preparation of sub-10 nm FeOOH NPs of tunable

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diameters and narrow size distribution.

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Surface chemistry of the confined FeOOH NPs. Surface electrochemical properties

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of the confined α-FeOOH NPs was characterized via potentiometric titration on both

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Fe@MesoPS and MesoPS samples. To demonstrate the distinct surface chemistry of

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sub-10 nm α-FeOOH NPs from those of larger size, α-FeOOH NPs of 18×60 nm (see

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Figure 5) were prepared for comparison according to the reference43. The results are

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shown in Figure 6, where the net surface charge QH represents the net amount of H+

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(QH>0) or OH- (QH

Highly Efficient Water Decontamination by Using Sub-10 nm FeOOH

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