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Rational Design of 3D Urchin-like FeMnO@FeOOH for Water Purification and Energy Storage Lu-Bin Zhong, Qing Liu, Jun-Qiu Zhu, Yue-San Yang, Jian Weng, Peng Wu, and Yu-Ming Zheng ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02689 • Publication Date (Web): 29 Jan 2018 Downloaded from http://pubs.acs.org on January 30, 2018

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ACS Sustainable Chemistry & Engineering

Rational Design of 3D Urchin-like FeMnxOy@FeOOH for Water Purification and Energy Storage

Lu-Bin Zhong,† Qing Liu,† Jun-Qiu Zhu,‡ Yue-San Yang,† Jian Weng,§ Peng Wu,† and Yu-Ming Zheng*,†

†

CAS Key Laboratory of Urban Pollutant Conversion, Institute of Urban

Environment, Chinese Academy of Sciences, 1799 Jimei Road, Xiamen 361021, P. R. China. ‡

College of Chemical Engineering and Materials Science, Quanzhou Normal

University, 398 Donghai Road, Quanzhou 362000, P. R. China. §

Department of Biomaterials, College of Materials, Xiamen University, 422 South

Siming Road, Xiamen 361005, P. R. China.

* Corresponding Author: E-mail: [email protected] .

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ABSTRACT: Three-dimensional (3D) urchin-like Fe-Mn binary oxide (UFMO), was designed and successfully prepared by a facile one-pot template-free method. The as-synthesized UFMO, having a high specific surface area of 142 m2 g-1, was consisted of an amorphous Fe-Mn binary oxide (FeMnxOy) core and a well-aligned α-FeOOH nanorod shell growing radially outwards. Based on the systematic analysis of products formed at different synthetic stages, a plausible two-stage growth process, fast nucleation followed by slow crystallization, was proposed to illustrate the formation of the 3D hierarchical nanostructure. The internal structure of UFMO can be manipulated by tuning the molar ratio of Fe/Mn. Owing to its 3D hierarchical morphology and heterogeneous surface chemical composition, UFMO can serve as a promising adsorbent for the removal of heavy metals from water and a potential electrode material for supercapacitor in the field of energy storage. UFMO was able to reduce the concentration of both As(III) and Cd(II) from 100 ppb to 1 ppb at neutral pH with a dosage of 0.1 g L-1. The maximum adsorption capacities for As(III) and Cd(II) are 147.2 and 79.06 mg g-1, respectively. Furthermore, the fast gravimetrical separation from solution and good regenerability of UFMO, implying a green, economic and sustainable approach for large-scale water purification applications. UFMO also exhibited a high specific capacitance of 158 F g-1 at a current density of 0.5 A g-1 (20 A m-2) and good reversibility with a cycling efficiency of 90% after 1000 cycles in the electrochemical performance tests. In general, UFMO has great potentials in water treatment and energy storage applications, owing to its high performance, low cost, and environmentally benign nature. KEYWORDS:

Adsorption,

As(III),

Cd(II),

Hierarchical

nanostructure,

Supercapacitor


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INTRODUCTION Problems concerning water pollution and energy crisis are prominent at a global level in the 21st century. Hence, development of new functional materials for water pollution control and energy storage has been a hot topic recently.1-2 Nanomaterials have attracted ever-increasing attention due to their stimulating physicochemical properties, such as surface effect, quantum size effect, macroscopic quantum tunnelling effect, and so on.3 Recently, many efforts have been focused on the fine-tuning of surface morphologies and chemical compositions of nanomaterials.4-5 Three-dimensional (3D) hierarchical nanostructures are an important class of nanomaterials with special morphologies and unique properties, such as high specific surface area, outstanding framework stability, and etc.6-8 It has been widely recognized that 3D hierarchical nanostructured materials possess superior properties compared to their building blocks.9 Recently, many researchers have been concentrating on fabricating 3D nanostructures with desired properties and explore the potential applications in energy storage and conversion,5-6,10-12 water and wastewater treatment,8-9,13-15 catalysts,9,16-17 and sensors.18 For example, 3D hierarchical twin-sphere Co3O4 was fabricated and exhibited excellent electrochemical performances in the constructed supercapacitors for energy storage.11 When used as gas sensor, 3D hierarchical ZnO showed significantly improved sensitivity and fast response to acetone compared to other mono-morphological ZnO.18 Among various 3D hierarchical nanostructured materials, 3D nanostructured iron oxides play important roles due to the low cost, natural abundance and environmentally friendly nature of Fe element. Up to now, various 3D nanostructured iron oxides with different morphologies, such as box-like,19 chrysanthemum-like,14 hollow nest-like15 and urchin-like,13 have been synthesized and used in water treatment, as well as energy storage and conversion. Generally, strategies for fabricating 3D hierarchical nanostructured materials can be of the following routes: deposition, template, and self-assembly methods.20 Among the three methods, self-assembly method, which self-assembles the low-dimension building blocks into 3 ACS Paragon Plus Environment


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regular higher level structures, is simplist and cheapist. However, this method usually requires

extensive

use

of

toxic/expensive

inducing

reagents,

such

as

tetrabutylammonium bromide (TBAB), polyvinyl-pyrrolidone (PVP) or ethylene glycol (EG), which serve as the soft template for the formation 3D hierarchically structures.8,13,21 Besides, high temperature and pressure (hydrothermal treatment) are often needed in the process.9,17,22 On the other hand, compared to single metal oxides, hybrid or mixed composites often have superior properties.12,23-27 For instance, manganese doping of iron oxides can effectively improve the adsorption capacity for heavy metals.23-25 Many reports indicate that hybrid metal oxides have enhanced electrochemical properties compared to monometallic oxides.12,26-27 However, current preparation methods for hybrid metal oxides with 3D hierarchical nanostructure are fairly tedious, while there is lack of a facile one-pot method so far. Inspired by the above research development, we managed to fabricate a 3D hierarchical nanostructured urchin-like Fe-Mn binary oxide (UFMO) by a facile one-pot template-free method. In our method, nucleation centres consisting of Fe-Mn binary oxide (FeMnxOy) were formed rapidly by the redoxidation between KMnO4 and FeSO4, while needle-like iron oxides (α-FeOOH) grew radially from nucleation centres following the addition of NaOH. Compared to conventional hydrothermal methods which require high temperature, high pressure, and often involve surfactant or template, our method can be conducted at atmospheric pressure and a relatively low temperature of 100 °C with no introduction of other compounds. Besides, by simply varying the operating parameters, the architecture can switch from solid to hollow. Series of batch adsorption experiments and electrochemical tests were conducted, demonstrating that the UFMO could effectively remove extremely toxic heavy metals, As(III) and Cd(II), from water, and exhibit high specific capacitance and good cycling stability.

EXPERIMENTAL SECTION 4 ACS Paragon Plus Environment

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Materials. Ferrous sulfate heptahydrate (FeSO4·7H2O), potassium permanganate (KMnO4), sodium hydroxide (NaOH), and nitric acid (HNO3) were of analytical grade and obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Sodium arsenite (NaAsO2) and tetrahydrated cadmium nitrate (Cd(NO3)2·4H2O) with purity of 99.0% were received from Maya Reagent (Jiaxing, China) and Tingxin Chemical Reagent Factory (Shanghai, China), respectively. Commercially available Fe3O4 (50-100 nm, Sigma-Aldrich, USA), α-Fe2O3 (30 nm, Aladdin Chemical Reagent Co. Ltd., China) and MnO2 (Aladdin Chemical Reagent Co. Ltd., China) were employed for comparative purpose. All chemicals were used without further purification. Preparation of the UFMO. The UFMO was prepared via a facile one-pot template-free method. First, 6.25 g FeSO4·7H2O and 1.19 g KMnO4 were separately dissolved in 100 mL of deionized water (DI). The prepared KMnO4 solution was heated to boiling (at 100 ºC), then, FeSO4 solution was gradually added into the KMnO4 solution under vigorous magnetic stirring. NaOH solution (13 mL, 5 mol L-1) was then added dropwise to the boiling mixture. After that, the mixed solution was cooled down to 25 °C, and the precipitates were collected and washed several times with deionized water to remove the impurities. After washing, the product was dried in an oven at 80 °C to a constant weight. Finally, the dried product was crushed and stored in a desiccator for further use. In order to investigate the formation mechanism of UFMO, we have analyzed the Fe-Mn binary oxide core formed before NaOH addition, and the resultant supernatant solution. Once the core was formed, heating was turned off, and the solution was immediately cooled in the ice-bath to stop further reaction. It was then retrieved from the solution after settling down, while the supernatant was filtered through 0.22 µm Millipore membrane filter and Fe and Mn contents in solution were determined. Characterization. The surface morphology and element composition of the UFMO were acquired by a field emission scanning electron microscopy (FESEM, 5 ACS Paragon Plus Environment


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Hitachi S-4800, Japan) with an energy dispersive X-ray spectrometer (EDX) (Genesis, XM2, USA). Transmission electron microscopy (TEM) image and EDX elemental mapping of the sample were obtained using a Philips Tecnai F30 (FEI, USA) operated at 300 kV. X-ray diffraction (XRD) patterns were collected on a PANalytical X’ Pert PRO X-ray diffractometer (Almelo, The Netherlands) with a Ni filter, Cu Kα radiation source, and angular variation of 10-80° operated at a tube voltage of 40 kV and a tube current of 40 mA. The sample was thoroughly dried and grinded in an agate mortar before testing. Nitrogen adsorption-desorption isotherms were conducted at 77 K with a Micromeritics ASAP2020 M+C surface area and porosity analyzer (GA, USA). X-ray photoelectron spectroscopy (XPS) spectra were collected using a PHI Quantum-2000 electron spectrometer (Ulvac-Phi, Japan) with 150 W monochromatized Al Kα radiation (1486.6 eV). Ion concentration was determined by inductively coupled plasma optical emission spectroscopy (ICP-OES, PerkinElmer, Optima 7000DV, USA) or inductively coupled plasma mass spectroscopy (ICP-MS, Agilent 7500cx, CA, USA). As(III) and As(V) concentrations was determined by a high

performance

liquid

chromatography-inductively

coupled

plasma-mass

spectroscopy (HPLC-ICP-MS) (Text S1, Supplementary Information). Batch adsorption experiments for heavy metals removal. For batch adsorption experiments, UFMO with a dosage of 0.05 g L-1 were added to a 150 mL flask containing 100 mL of As(III) or Cd(II) solution with a concentration of 100 ppb unless otherwise stated, while pH for As and Cd solution were kept at 7.0 ± 0.1 and 6.5 ± 0.1, respectively. All samples were shaken at 25 °C and 200 rpm to achieve adsorption equilibrium. All samples were filtered through Millipore membrane filters (0.22 µm) before testing. In the pH effect study, the pH value of mixed solution was adjusted by 0.1 M HNO3 or 0.1 M NaOH solution. In the ionic strength effect study, NaNO3 with concentrations ranging from 0 to 500 mM was used as a common electrolyte background.


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In the kinetics experiments, 0.025 g of UFMO were dosed into 500 mL of 100 ppb As(III) and Cd(II) solution. 5 mL aliquot was withdrawn at predetermined time interval. Concentrations of all samples were measured, and the adsorption capacity qt (mg g-1) at a specific time t was calculated by equation 1: qt =

( Ci -Ct )V

(1)

M

where Ci and Ct are the concentrations of initially and at time t, respectively; V (L) is the volume of the solution and M (g) is the weight of the adsorbent. The adsorption isotherm study was performed with initial As(III) concentrations from 0.1 to 50 mg L-1. The adsorption capacity qe (mg g-1) was calculated by equation 2: qe =

( Ci -Ce )V

(2)

M

where Ce is the equilibrium concentration. To evaluate the sustainable use of UFMO, regeneration was also investigated. First, UFMO was added to As(III) or Cd(II) solution and shaken for 6 h to ensure adsorption equilibrium was reached. The adsorbent was then separated from the solution by gravity in less than 2 h. In the desorption cycle, the separated UFMO were shaken in 10 mM NaOH (for As(III)) or 1 mM HNO3 (for Cd(II)) for 4 h, and then washed with DI water to until neutralization and dried in an oven for 8 h. Finally, the regenerated UFMO were used for adsorption and desorption in the succeeding cycles.

Electrochemical performance tests. All electrochemical experiments were carried out with a electrochemical workstation (CHI660a, Shanghai Chenhua, China) at 25 °C. The working electrode was prepared by loading a slurry containing 80 wt% UFMO (about 2 mg), 10 wt% polytetrafluoroethylene (PTFE) in N-methylpyrrolidone and 10 wt% acetylene black on a nickel foam. After the electrode materials were loaded, the working electrode was pressed and vacuum-dried at 80 °C for 2 h. In a three electrode system, the above loaded nickel foam as working electrode was investigated with a Pt counter electrode and Ag/AgCl reference electrode in 6 M KOH solution as the 7 ACS Paragon Plus Environment


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electrolyte. Cyclic voltammetry (CV) curves were obtained in the potential range of -0.05 to 0.35 V vs. Ag/AgCl by varying the scan rate from 1 to 50 mV s-1. Charge/discharge measurements were carried out at a constant current of 0.5 to 5 A g-1 (2 to 20 A m-2) with a potential window of -0.05 to 0.35 V.

RESULTS AND DISCUSSION

Figure 1. (a) SEM (inset: magnified image), (b) TEM and (c) STEM images of UFMO, (d-f) EDX elemental mappings of Fe, Mn and O, respectively, showing Fe-Mn binary oxide core and iron oxide shell. Characterization of the UFMO. UFMO was synthesized via a one-pot heating method without introducing any surfactant. Surface morphology of UFMO was examined by both SEM and TEM. As shown, UFMO is composed of uniform urchin-like architectures with an average diameter of 500 nm (Figure 1a). The detailed morphology (Figure 1a inset and Figure 1b) reveals that the architecture is built by a spherical core and numerous nanorods, which grow radially from the core. The spherical core is about 250 nm in diameter, while the nanorods are close to 125 nm long and 15 nm wide. STEM image and corresponding elemental mappings of an individual 3D urchin-like architecture (Figure 1c-f) suggest that UFMO is composed 8 ACS Paragon Plus Environment

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of three elements: Fe, Mn, and O. Noteworthy, Fe and O exist in the whole 3D urchin-like architecture, while Mn is only present in the core, implying a likely combination of Fe-Mn binary oxide core and iron oxide shell in the architecture. However, the shell is not an enclosed shell, but rather represents many needle-like nanorods which attach to the core.

Figure 2. (a) XPS wide-scan spectrum, high resolution (b) Fe 2p and (c) Mn 2p XPS spectra of UFMO, (d) high resolution Mn 2p XPS spectrum of the Fe-Mn binary oxide core (before NaOH addition). XPS was used to investigate the oxidation states of Fe and Mn in the UFMO as illustrated in Figure 2. High resolution XPS spectrum of Fe 2p displays two broad peaks positioning around 724.8 and 711.1 eV (Figure 2b), which are the two characteristic positions of Fe (III).28 As manganese only presents in the core while the iron oxide nanorods are about 125 nm long, Mn 2p peak is not so significant due to the limited detectable sample depth of XPS (Figure 2c).29 Thus, to assess the oxidation state of Mn, the Fe-Mn binary oxide core before the nanorod growth (SEM image as shown in Figure 3a), that is, before NaOH addition, was also tested. As shown in Figure 2d, two binding energies at 654.2 and 642.8 eV are assigned to Mn 9 ACS Paragon Plus Environment


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2p1/2 and Mn 2p3/2, respectively, corresponding to Mn (IV).30 The nitrogen adsorption and desorption isotherms of UFMO exhibits a typical type IV isotherm with H3 hysteresis loop (Figure S1, Supporting Information), indicating a characteristic distribution of slit-shaped mesopores.31 The as-synthesized UFMO possesses a specific surface area of 142 m2 g-1, relatively higher compared to many other 3D nanomaterials reported (Table 1). The higher specific surface area suggests the presence of more active sites, which are conducive to water treatment and energy storage applications. Table 1. The BET specific surface areas (SBET) of UFMO and other 3D hierarchical nanostructures reported in literature. Sample

SBET (m2 g-1)

Size (µm)

Ref.

α-Fe2O3 hollow spheres

103.3

∼1

32

3D hierarchical flowerlike α-Fe2O3

107

5-6

33

Hollow core/shell γ- Fe2O3

101.7

~5

34

Hollow core/shell γ-Fe3O4

113.0

~5

34

MnO2-coated Fe3O4

118

0.218

25

Chrysanthemum-like α-FeOOH

120.8

∼1

14

Flowerlike α-Fe2O3

130

0.8-1

35

Chestnutlike Fe2O3

143.12

1.5

36

Hollow nestlike α-Fe2O3 spheres

152.42

∼0.4

15

Novel Fe2O3 hollow spheres

42.6

0.15-0.2

37

Cage-like Fe2O3 hollow spheres

30.5

0.6

38

Urchin-like α-FeOOH hollow spheres

96.9

0.5-1

13

Fe2O3 microboxes

52.2

1

19

α-Fe2O3 hollow microspheres

43.2

1.2

39

UFMO

142

0.5

This study

Formation mechanism of the UFMO. The evolution of Fe-Mn binary oxides was investigated via SEM observation, EDX and XRD analysis of the precipitates collected at different time during the formation of UFMO. Before NaOH addition, the 10 ACS Paragon Plus Environment

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nanostructured particles are smooth, and the diameters are between 200 and 300 nm (Figure 3a). The corresponding EDX analysis (Figure S2a, Supporting Information) reveals that these nanoparticles are composed of Fe, Mn and O elements, and their molar ratio (Fe:Mn:O) is about 2.3:1:11.1, which is lower than the Fe/Mn molar ratio in the initial dose (3:1), indicating that some of Fe ions may still be present in the supernatant. The residual ion concentration in the supernatant measured with ICP-OES also confirmed that much higher content of Fe (2871.6 ppm) was present compared to Mn ions (2.6 ppm). The oxidation state of Fe ions in the supernatant was furthered verified to be +III based on the chromogenic reaction analysis (Figure S3, Supporting Information). XRD pattern shows that no obvious diffraction peak is detected (Figure S2b, Supporting Information), suggesting that the Fe-Mn binary oxide nanoparticles are probably amorphous (presented by FeMnxOy later on). Sample collected at 20 min after NaOH addition shows that the surface of the nanoparticles become rougher due to the growth of needle-like nanorods (Figure 3b), while the Fe/Mn molar ratio increases (Figure S2a) with no clear diffraction peak formed (Figure S2b). As the reaction proceeded, these nanoparticles eventually evolve into 3D urchin-like nanostructures in 60 min after NaOH addition (Figure 3c). The corresponding EDX analysis (Figure S2a, Supporting Information) displays that these 3D hierarchical urchin-like nanostructures consist of Fe, Mn and O, and their molar ratio is about 3.1:1:12.7, respectively. The molar ratio of Fe/Mn matches well with the initial dose, and is higher than that in the early evolution stage of UFMO (Figure S2a, Supporting Information), indicating that the nanorods grown on the nanoparticle surface are likely to be iron oxides. This also agrees with the elemental mapping results (Figure 1d and 1f). The XRD pattern shows several clear diffraction peaks (Figure S2b, Supporting Information) matching with the orthorhombic phase of α-FeOOH (JCPDS: 029-0713),19 differing from that of the sample collected before NaOH addition. Thus, the nanorods in UFMO were probably α-FeOOH.



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Figure 3. SEM images of samples collected at different time intervals: (a) before NaOH addition; after NaOH addition: (b) 20 min, and (c) 60 min. (d) Schematic illustration of the morphological evolution process of 3D urchin-like UFMO (FeMnxOy@FeOOH). Based on the obtained experimental results and above analysis, a plausible formation mechanism of the UFMO was proposed (Figure 3d). Initially, redox reaction took place rapidly between KMnO4 and FeSO4 under heating, and numerous primary Fe-Mn binary oxide nanonuclei were formed. These primary Fe-Mn binary oxide nanonuclei gradually aggregated into microspheres due to their high surface energy,14 while some unreacted Fe(III) was still present in the solution. With the NaOH addition, Fe(III) ions took the agglomerates as cores for crystallization, and the nanorods consisting of α-FeOOH gradually grew on the agglomerates surface, finally forming the 3D urchin-like structure. In general, the formation of UFMO is a two-stage growth process similar to previous studies,13, 21 that is, a fast nucleation of Fe-Mn binary oxide nanoparticles, and a slow crystallization of urchin-like iron oxide nanorods. Noteworthy, heating (energy) promotes the rapid nucleation and aggregation of the nanoparticles, which is critical to the formation of 3D 12 ACS Paragon Plus Environment

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nanostructure (Figure S4, Supporting Information). In addition, 3D architecture can switch from solid to hollow when Fe/Mn molar ratio increases from 3:1 to 6:1 (Figure S5, Supporting Information). The in-depth growth mechanism of hollow 3D architecture and its adsorption/electrochemical performance should be studied and discussed in the future work. Adsorption performance for heavy metal removal. Heavy metal contamination in natural water is a worldwide threat to human health and ecosystems. Among the various heavy metals, anionic As(III) and cationic Cd(II) are extremely toxic and persistent in aqueous environment. Hence, these two heavy metals with opposite charges were chosen as model ions to provide a wide representation.

Figure 4. (a) Removal performance of As(III) and Cd(II) from aqueous solution by UFMO at different adsorbent dosages. (b) Removal capabilities of As(III) and Cd(II) using UFMO at varying pH. (c) Effect of ionic strength on As(III) and Cd(II) adsorption on UFMO. (d) Comparison of As(III) and Cd(II) removal efficiencies by UFMO, spherical Fe-Mn oxide (Figure S6, Supporting Information), commercial Fe3O4, α-Fe2O3 and MnO2. Experimental conditions: initial concentration = 100 ppb, 13 ACS Paragon Plus Environment


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pHAs(III) = 7.0 ± 0.1, pHCd(II) = 6.5 ± 0.1, dose rate = 0.05 g L-1. Error bars represent the standard deviation of triplicate experiments. Figure 4a shows the adsorption efficiency of UFMO for As(III) and Cd(II) at different adsorbent dosages. As shown, UFMO can effectively reduce both As(III) and Cd(II) concentration from 100 ppb to less than 1 ppb, separately. Only 0.05 g L-1 of UFMO is required to treat the contaminated water in order to meet the recommendation by World Health Organization (WHO) on the maximum contaminant levels (MCL) for arsenic (10 ppb) and cadmium (5 ppb) in drinking water. Besides, the removal capability of UFMO was comparable with or better than other reported nanomaterials in literature (Table S1, Supporting Information). Moreover, leaching of Fe and Mn from UFMO is negligible (< 3 ppb) (Figure 4a), indicating a high stability of UFMO during the adsorption process and no occurrence of secondary pollution. Solution pH is one of the most important factors in the adsorption process, which may affect the surface charge property and degree of ionization of the adsorbent. Thus, the adsorption capabilities of UFMO at different pH were studied. As(III) adsorption performance was stable at pH 4 to 7, while Cd(II) adsorption capacity increased with pH, and the highest adsorption capacity was found at pH 6 to 7 (Figure 4b). The effect of ionic strength was also studied, and its impact on both As(III) and Cd(II) adsorption was fairly low (Figure 4c). To better distinguish the adsorption capabilities, As(III) and Cd(II) removal efficiencies by UFMO were also compared to those by spherical Fe-Mn oxide, commercial Fe3O4, α-Fe2O3 and MnO2. As shown in Figure 4d, UFMO is able to achieve nearly 100% removal of As(III) and Cd(II) at 0.05 g L-1 dose rate, which is similar to spherical Fe-Mn oxide, while significantly better than commercial Fe3O4, α-Fe2O3 and MnO2.


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Figure 5. (a) Residual As (total As, As(III), and As(V)) concentration in solution and its removal rate by UFMO. (b) Adsorption kinetics of As(III) on UFMO. (c) Residual Cd(II) concentration in solution and its removal rate by UFMO. (d) Adsorption kinetics of Cd(II) on UFMO. Experimental conditions: initial concentration = 100 ppb, pHAs(III) = 7.0 ± 0.1, pHCd(II) = 6.5 ± 0.1, dose rate = 0.05 g L-1. Error bars represent the standard deviation of triplicate experiments. The adsorption kinetics, oxidation state change and removal rate of As(III) during its adsorption process on UFMO were also investigated (Figure 5a and 5b). It can be seen that total As concentration reduced from 100 ppb to below 10 ppb within 60 min, and the adsorption equilibrium could be obtained in 200 min. The final As concentration was less than 1 ppb, and the removal rate was higher than 99%. As oxidation state change in solution during the adsorption process was given in Figure 5a. Less than 1 ppb of As(V) was detected when the As(III) concentration sharply decreased from 100 ppb to 34.8 ppb in 8 min, indicating that oxidation from As(III) to As(V) occurred and the oxidation rate was much slower than adsorption at the beginning of adsorption process. Then As(V) concentration gradually increased and reached a peak value of 2.4 ppb in 60 min. After that, As(V) concentration declined slowly and disappeared. As As(V) concentration is very low (

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