Sea-urchin-like iron oxide nanostructures for water treatment

Journal of Hazardous Materials 262 (2013) 130–136

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Sea-urchin-like iron oxide nanostructures for water treatment Hyun Uk Lee a,∗ , Soon Chang Lee b , Young-Chul Lee c , Stane Vrtnik a , Changsoo Kim a , SangGap Lee a , Young Boo Lee d , Bora Nam d , Jae Won Lee e , So Young Park a , Sang Moon Lee a , Jouhahn Lee a,∗∗ a

Division of Materials Science, Korea Basic Science Institute, Daejeon 305-333, Republic of Korea Department of Fine Chemical Engineering and Applied Chemistry, Chungnam National University, Daejeon 305-764, Republic of Korea Department of Biological Engineering, College of Engineering, Inha University, Incheon 402-751, Republic of Korea d Jeonju Center, Korea Basic Science Institute, Jeonju 561-756, Republic of Korea e Department of Energy Engineering, Dankook University, Cheonan 330-714, Republic of Korea b c

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• The u-MFN were synthesized via a ultrasound irradiation and/or calcinations process. • The u-MFN exhibited excellent adsorption capacities. • The u-MFN also displayed excellent adsorption of organic polluent after recycling. • The u-MFN has the potential to be used as an efficient adsorbent material.

a r t i c l e

i n f o

Article history: Received 8 May 2013 Received in revised form 1 August 2013 Accepted 2 August 2013 Available online 23 August 2013 Keywords: Metal oxide Nanostructure Nanorod Synthesis

a b s t r a c t To obtain adsorbents with high capacities for removing heavy metals and organic pollutants capable of quick magnetic separation, we fabricated unique sea-urchin-like magnetic iron oxide (mixed ␥Fe2 O3 /Fe3 O4 phase) nanostructures (called u-MFN) with large surface areas (94.1 m2 g−1 ) and strong magnetic properties (57.9 emu g−1 ) using a simple growth process and investigated their potential applications in water treatment. The u-MFN had excellent removal capabilities for the heavy metals As(V) (39.6 mg g−1 ) and Cr(VI) (35.0 mg g−1 ) and the organic pollutant Congo red (109.2 mg g−1 ). The u-MFN also displays excellent adsorption of Congo red after recycling. Because of its high adsorption capacity, fast adsorption rate, and quick magnetic separation from treated water, the u-MFN developed in the present study is expected to be an efficient magnetic adsorbent for heavy metals and organic pollutants in aqueous solutions. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Heavy metals and organic contaminants in natural water are a major global problem because of their adverse effects on the environment and human health, even at very low levels [1–6].

∗ Corresponding author. Tel.: +82 42 865 3637; fax: +82 42 865 3610. ∗∗ Corresponding author. Tel.: +82 42 865 3613; fax: +82 42 865 3610. E-mail addresses: [email protected], [email protected] (H.U. Lee), [email protected] (J. Lee). 0304-3894/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhazmat.2013.08.014

Nanomaterials are attracting worldwide attention for environmental remediation and pollution control applications, because nanostructured surfaces offer large surface areas and a rich variety of valence states, which provide enhanced affinity and adsorption capabilities for these pollutants [7,8]. It has recently been reported that magnetic iron oxide [maghemite (␥-Fe2 O3 ) and magnetite (Fe3 O4 )] nanoparticles are very effective in the removal of toxic heavy metal ions and organic pollutants from wastewater because of their strong adsorption capacities and because they can be easily separated, collected, and reused using an external magnetic field [9–12]. The removal of adsorbents from

H.U. Lee et al. / Journal of Hazardous Materials 262 (2013) 130–136

Fig. 1. Schematic illustration of preparation of u-FN and u-MFN.

wastewater using a magnetic field is also more selective and efficient than centrifugation, filtration, or gravitational separation [13,14]. However, magnetic iron oxide nanoparticles, which show promising heavy metals removal capacities, are difficult to separate from water, even under a high magnetic field of ∼5 kOe [14–16]. This is because the desired magnetic response of the magnetic adsorbent decreases in water as its size decreases [16]. Three-dimensional (3D) magnetic iron oxide nanostructures that are constructed from one-dimensional (1D) nanostructures such as nanoparticles [17], nanoplates [18], or nanorods [19] exhibit high specific surface areas because of the abundant interparticle spaces and/or intraparticle pores in their complex structures. They are also more easily separated because of their larger size, weaker Brownian motion, and better magnetic properties compared with nanosized powder adsorbents [20,21]. However, the magnetic 3D ␥-Fe2 O3 and Fe3 O4 materials reported previously have had relatively small surface areas and show low adsorption capacities for heavy metals [As(V) and Cr(VI)] and organic pollutants (Orange II) [22]. In this study, we successfully fabricated sea-urchin-like magnetic iron oxide (mixed ␥-Fe2 O3 /Fe3 O4 phase) nanostructures (called u-MFN) with large surface areas (94.1 m2 g−1 ) and strong magnetic properties (57.9 emu g−1 ) using a simple growth process (ultrasound irradiation/calcination process). With maximum removal capacities of 39.6, 35.0, and 109.2 mg g−1 for As(V), Cr(VI), and Congo red, respectively, u-MFN has the potential to be an efficient adsorbent material for the removal of toxic heavy metal ions and organic pollutants from water. 2. Experimental

in the u-MFN. The morphologies, distributions, and crystallinities of u-MFN samples were investigated using field emission scanning electron microscopy (FE-SEM; S-4700, Hitachi, Japan) and high-resolution transmission electron microscopy (HR-TEM; JEM 2200, JEOL, Japan). Before the analyses, the samples were placed on the surfaces of Cu grids and dried under ambient conditions. The Brunauer–Emmett–Teller (BET) surface areas, pore volumes, and pore diameters of the u-MFN samples were determined using a BET analyzer (ASAP 2020, Micromeritics, USA). Magnetic measurements were carried out on powdered samples using a model 4HF vibrating sample magnetometer (VSM, ADE Co., Ltd., USA) with a maximum magnetic field of 20 kOe. 2.3. Measurement of adsorption of heavy metals and organic pollutants As(V) is considered to be a highly toxic pollutant in water resources, and its efficient removal from water is very important. To evaluate the adsorption abilities of our samples, heavy metal adsorption tests (contain real samples tests) were carried out according to Standard Methods [24–26]. Na2 HAsO4 ·7H2 O and K2 Cr2 O7 were used as the sources of As(V) and Cr(VI), respectively [2,14,16,24]. Standards for calibration were prepared from As(V) standard reference sodium (meta) arsenic. Stock solutions of As(V) were prepared by dissolving Na2 HAsO4 ·7H2 O in deionized water. Stock solution (1000 mg/L−1 ) was frozen to prevent oxidation [2,14]. Arsenic working solutions were freshly prepared by diluting arsenic solutions with deionized water. The pH values of the solutions were adjusted using NaOH or HCl. In a typical removal procedure, u-MFN samples (60 mg L−1 ) were added to 25 mL of As(V) and Cr(VI) solutions (0.1 mg L−1 ), and the containers were sealed and shaken at 45 rpm for 10 h at room temperature. After shaking, the mixtures were placed under an external magnetic field and the u-MFN samples were separated from the solutions. The As(V) concentration in the supernatant solution was measured using inductively coupled plasma atomic emission spectroscopy (ICP-AES; Thermo-Fisher Scientific, USA) [27–29]. Every experiment was performed in triplicate, and average values are given in the graphs. The minimum detection limit of ICP-AES for As(V) and Cr(VI) were 0.003 and 0.015 mg L−1 , respectively. The adsorption capacity was roughly estimated using Eq. (1) [14]:

2.1. Synthesis of u-FN and u-MFN qe = All reagents were analytical grade (Sigma–Aldrich Co., USA) and used without further purification. In a typical synthesis, FeCl3 (2 mg) and NaH2 PO4 (3 mg) were dissolved in distilled water (100 mL), and TiO2 nanopowder (1 g, particle size: 45 nm) was added (Fig. 1). High-intensity 20 kHz ultrasound was then applied from the top of a glass reactor (with a volume of ∼50 mL) using a Sonics and Materials VC750 ultrasonic generator [21,23]. Ultrasound irradiation was applied for 40 min, and the electrical energy input was maintained at 100 W cm−2 . After completion of the reaction, the product was isolated by centrifugation for 10 min at 4000 rpm and then washed three times with ethanol. It was then dried overnight at room temperature. The as-synthesized material (urchin-like iron oxide nanostructure, u-FN) was then calcined at 350 ◦ C for 1 h under nitrogen (to form u-MFN). 2.2. Characterization The crystalline structures of the u-MFN samples were investigated by X-ray diffraction (XRD; Rigaku RDA-cA X-ray diffractometer, Japan) using Cu K␣ radiation and a nickel filter. At liquid helium temperature, we obtained a 57 Fe3+ nuclear magnetic resonance spectrum (NMR spectrum; MRS400, Josef Stefan Institute, Slovenia) to confirm the coexistence of ␥-Fe2 O3 and Fe3 O4

131

(C0 − C)V W

(1)

where qe (mg g−1 ) is the adsorption capacity, C0 (mg L−1 ) is the initial concentration of the As(V) or Cr(VI) solution, C (mg L−1 ) is the equilibrium concentration of As ions, V (L) is the initial volume of the As(V) or Cr(VI) solution, and W (g) is the weight of the adsorbent. The Freundlich isotherm is derived by assuming a heterogeneous surface with a non-uniform distribution over surface [30]. 1/n

qe = KF C0

(2)

In this equation, KF is the adsorption capacity at the unit concentration and 1/n is the adsorption intensity. 1/n values indicate the type of isotherm-whether it is irreversible (1/n = 0), favorable (0 < 1/n < 1), or unfavorable (1/n > 1). The Langmuir equation can be applied for the monolayer sorption onto a surface of a finite number of identical sites. The Langmuir isotherm is presented by the following equation [2]: qe =

qmax KL C0 1 + KL C0

(3)

Here, C0 is the initial concentration (mg L−1 ), qe is the adsorption capacity at equilibrium (mg g−1 ), KL denotes the Langmuir adsorption capacity at equilibrium (L mg−1 ) and qmax is the theoretical maximum adsorption capacity (mg g−1 ).

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Fig. 2. (a) X-ray diffraction patterns and (b–d) scanning electron microscopy images of TiO2 , u-FN, and u-MFN. The peaks are identified as anatase (A), rutile (R), ␣-Fe2 O3 (␣), and ␥-Fe2 O3 (␥).

Congo red (C32 H22 N6 O6 S2 Na2 ), which is an azo dye commonly used in the textile industry, was selected as a model organic water pollutant. Various amounts of u-MFN samples were mixed with 20 mL of Congo red solution (100 mg L−1 ). After 12 h of stirring, the u-MFN samples were separated, and the supernatant solutions were analyzed using ultraviolet–visible–infrared (UV-VIS-IR) spectroscopy (Cary 5000, Varian, Australia). The residual concentration of Congo red was obtained by integrating the area of the absorbance bands in the wavelength range 400–600 nm using a linear calibration curve over 1–100 mg L−1 . To demonstrate the stability of our sample, we recycled the used u-MFN. In the recycling tests, the u-MFN samples with Congo red were recovered by magnetic separation with a permanent magnet made of Nd–Fe–B. The recovered samples were subsequently washed with distilled water/ethanol (70:30 w%) several times, and calcined at 350 ◦ C for 10 min under nitrogen after every cycle. For the adsorption tests with heavy metals and organic pollutants, each test was repeated up to four times. The data were averaged and are expressed as the mean ± standard deviation. 3. Results and discussion 3.1. Physicochemical and structural properties of u-FN and u-MFN The XRD patterns of TiO2 , u-FN, and u-MFN samples are shown in Fig. 2a. The XRD pattern of u-FN has TiO2 peaks and one broad peak from iron oxide, because of the TiO2 (JCPDS 73-1764)/␣-phase (hematite, JCPDS 33-0664) core/shell nanostructures. From the uFN results, it is evident that ␣-Fe2 O3 appeared after the growth of rod-like crystals on the TiO2 due to ultrasound irradiation (Fig. 2b and c). The extreme reaction conditions induced by ultrasound irradiation enhance the chemical reactivity of the precursor to provide an appropriate environment for the formation of a nanostructure with a unique morphology through kinetic control in a short period of time [21,31,32]. We therefore suggest that the formation of u-FN proceeds as follows [31–36]. (i) Ultrasound irradiation promotes the initial precipitation of kinetically accessible ␤-FeOOH nanoparticles and nanorods, as well as a few ␣-Fe2 O3 nanoparticles. (ii) Further nucleation of ␣-Fe2 O3 nanoparticles occurs with

increasing irradiation time. (iii) ␤-FeOOH dissolution begins to occur in response to the Fe3+ concentration gradient established by the additional nucleation of the more thermodynamically stable ␣-Fe2 O3 nanoparticles. A further increase in irradiation also provides a driving force for the oriented attachment of ␣-Fe2 O3 nanoparticles, which is mediated by the strong adsorption of bidentate (bridging) phosphate inner sphere complexes to faces parallel to the ␣-Fe2 O3 c-axis. (iv) The ␤-FeOOH nanorods are degraded to a size similar to that of the ␣-Fe2 O3 nanoparticles through dissolution of loosely packed c-planes. The ␣-Fe2 O3 nanorods then grow and coarsen further during ultrasound irradiation through the oriented attachment mechanism, until the supply of primary ␣-Fe2 O3 nanoparticles is exhausted. Fig. 2c shows that the u-FN has an urchin-like morphology and is nearly uniform, with particle diameters of about 400–500 nm. After calcination of u-FN at 300 ◦ C for 1 h under nitrogen, a red-brown product (u-MFN) was obtained. FE-SEM images at different magnifications showed that u-MFN featured sea-urchin-like structures comprising many rod-like crystals with diameters in the range of 480–650 nm (Fig. 2d). The XRD pattern of the red-brown product was very close to that of ␥-Fe2 O3 (maghemite, JCPDS 39-1346) and agreed well with the standard XRD pattern of Fe3 O4 (magnetite, JCPDS 79-0418). The mean crystal sizes determined from the XRD data using the Debye–Scherrer equation were ∼10 nm for ␥-Fe2 O3 and/or Fe3 O4 [extracted from the (3 1 1) reflection] [26,33]. The broad XRD peaks of the u-FN were transformed into sharp ␥-Fe2 O3 and/or Fe3 O4 peaks after calcination as a result of crystallization of the ␣-phase Fe2 O3 shell structures. NMR spectroscopy was used to show that there exist the two distinct magnetic phases of the spinel iron oxides mixed in u-MFN [37–39]. Fig. S1 shows the 57 Fe3+ NMR spectra of commercial bulk maghemite (␥-Fe2 O3 ; #107927, Kojundo Chemical Lab.), magnetite (Fe3 O4 ; #518158, Sigma–Aldrich), and u-MFN; all the spectra were recorded at 4 K in zero external field. In Fig. S1, the overall shape of the u-MFN spectrum is similar to that of the bulk ␥-Fe2 O3 . However, the signal in the region of 68–70 MHz could not be explained by the spectrum of ␥-Fe2 O3 solely. As mentioned above, Fe3 O4 can exist in u-MFN, and it shows an NMR signal at 70 ± 1 MHz in the bulk state. Therefore, the signal at the lower end of the u-MFN spectrum would arise from the Fe3 O4 in the u-MFN.

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Fig. 3. (a) Low- and (b) high-resolution transmission electron microscopy images of u-MFN. Inset image is energy-dispersive X-ray pattern of u-MFN.

HR-TEM/energy-dispersive X-ray spectroscopy (EDS) images showed that the u-MFN consisted of Ti and Fe elements (Fig. S2). The HR-TEM images clearly showed that sea-urchin-like structures were assembled from rod-like polycrystals with stone-pillar shapes (Fig. 3a). The electron diffraction patterns of the rod-like polycrystals with the ␥-Fe2 O3 (and/or Fe3 O4 ) phase were observed in the selected area electron diffraction patterns (Fig. 3b). Lattice fringes with interplanar distances of 0.25 nm, which is in good agreement with the d-spacing value of the ␥-Fe2 O3 (3 1 1) planes [40], were clearly observed in the HR-TEM image. An investigation of the magnetic properties of u-FN and u-MFN shows that both are ferromagnetic at room temperature (Fig. 4). The room-temperature magnetic hysteresis loops of u-FN and uMFN are shown in Fig. 4. The nonlinear hysteresis loops with nonzero remnant magnetization and coercivity show that u-MFN has pronounced ferromagnetic properties. The maximum saturation magnetization of u-MFN (57.9 emu g−1 ) is about 115.8 times higher than that of u-FN (0.5 emu g−1 ). This indicates that u-MFN is superparamagnetic [18,24]. It can therefore be seen that the uMFN samples obtained via the synthesis route used here possess high saturation magnetization. This feature is beneficial for their application as adsorbents [12,18,24]. The BET surface areas of u-FN and u-MFN were calculated to be 126.4 m2 g−1 and 94.1 m2 g−1 , respectively, which are notably high values for iron oxide nanostructures (Table 1). The average pore diameters and pore volumes of u-MFN were estimated to be 15.71 nm and 0.15 cm3 g−1 , respectively. These pores were probably formed from the void spaces between the interwoven nanorods

that constitute the u-MFN. It has been shown that 3D nanostructured materials with large specific surface areas and ideal pore size distributions usually possess desirable adsorption properties for the removal of pollutants from water [14,17,41]. 3.2. Adsorption of heavy metals and organic pollutants by u-FN and u-MFN As(V) and Cr(VI) are considered to be primary highly toxic pollutants in water resources, and their efficient removal from water is of great importance [2,12,14,22]. In this study, we investigated the use of u-FN and u-MFN for water treatment. In particular, we tested the effects of the adsorbent concentration on As(V) and Cr(VI) removal capacities. The adsorptions of As(V) and Cr(VI) at different adsorbent concentrations are shown in Fig. 5a. In this test, the total concentration of the aqueous As(V) solution was kept at 0.10 mg L−1 . Fig. 5 clearly shows that the removal efficiency of As(V) increases with increasing u-FN and u-MFN concentrations. The percentage removal increases significantly from 13.0% to 97.0% when the concentration of u-FN is increased from 2 to 400 mg L−1 . Actually, the arsenic equilibrium concentration drops to well below 0.015 mg L−1 at the absorbent concentration of 60 mg L−1 . The effect of contact time on As(V) adsorption was also investigated. For this experiment, 25 mL of 0.1 mg L−1 As(V) solution and 60 mg L−1 of u-FN or u-MFN were mixed at pH 4.0 without pH control to see feasibility of adsorption pattern [42]. The adsorption of As(V) and Cr(VI) on u-FN ([k] = ln(C0 /C) = 2.386) was 2.1 times faster than that on u-MFN ([k] = 1.136). The enhanced adsorption of As(V) and Cr(VI) appears to be attributable to the properties of the adsorbent, such as its large surface area. In general, u-FN, with its large surface area, induces faster adsorption of water pollutants than other adsorbents, because the 3D nanostructure provides larger numbers of active sites to adsorb water pollutants. However, uFN has no magnetic properties, so it is not possible reuse the material. The effects of contact time on the uptake of As(V) and Cr(VI) ions are shown in Figs. 5b and S3. It can be clearly seen that in both cases, the removal mainly involves rapid As(V) and Cr(VI) uptake within 10 min of contact time, followed by a slower phase. The rapid process is attributed to external surface adsorption and pH effect. At pH 4.0, the predominantly stable species of Table 1 Physicochemical properties of TiO2 , u-FN, and u-MFN.

Fig. 4. Room-temperature hysteresis loops for u-FN and u-MFN.

Sample

BET surface area (m2 /g−1 )

Pore diameter (nm)

Pore volume (cm3 /g−1 )

TiO2 u-FN u-MFN

34.1 126.4 94.1

15.68 10.17 15.71

0.08 0.11 0.15

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Fig. 5. (a) Effects of u-FN and u-MFN adsorbent dosages on adsorption of total As and Cr ions. (b) Adsorption rate curves and (c) adsorption isotherms of As(V) and Cr(VI) using u-FN and u-MFN.

As(V) and Cr(VI) were H2 AsO4 − and HCrO4 − /Cr2 O7 2− , respectively [43], this adsorbent has a positive charge at this pH because of 6.5, point of zero charge (PZC). Due to electrostatic attraction between them, fast adsorption of pollutants onto adsorbent occurred. As a result, as increase >pH 7.0, the adsorption capacity of this adsorbent would be decreased. In addition, the removal efficiency of As(V) is

higher than that of Cr(VI) at the same initial As concentration of 0.1 mg L−1 . About 56.5% of the As(V) was removed by u-MFN during the initial 25 min of the adsorption process, but for Cr(VI), only 50.5% was removed within the same time. The reason is probably their different charges. It is proposed that the mechanism for the removal of metal and/or anionic contaminants involves surface complexation and ion exchange between the iron oxide surface and the toxic ions in the aqueous solution [12,14,22]. No significant change also was observed in heavy metal adsorption of the real samples, and As(V) and Cr(VI) solution (Figs. 5b and S3). Figs. 5 and S4 shows that the experimental data fit well with Langmuir adsorption and Freundlich isotherms. Based on these isotherms, we calculated the maximum adsorption capacities of u-MFN as ∼39.6 mg g−1 for As(V) and ∼35.0 mg g−1 for Cr(VI). These values are much higher than those of previously reported nanomaterials [22,23,28,29,44–46], as shown by the data in Table 2. The adsorption data for As(V) and Cr(VI) onto u-MFN samples were fitted to the Langmuir adsorption and Freundlich isotherms, as shown in Fig. S4. These results indicate that u-MFN has large adsorption capacities for As(V) and Cr(VI) and could potentially be used as an excellent adsorbent of As(V) and Cr(VI) for water treatment. To further investigate the advantages of using u-MFN for water treatment, we studied its capabilities for adsorbing Congo red, a representative organic water pollutant. Congo red is typically adsorbed onto the surfaces of metal oxides through coordination between metal ions and the terminal amine groups of Congo red molecules [2]. Fig. 6a shows the adsorption results for Congo red solutions (of concentration 100 mg L−1 ) after treatment with different dosages of u-FN and u-MFN for 10 h. The Congo red was almost completely removed from the water at a u-FN dosage of 0.8 g L−1 (Fig. 6a). A series of experiments were conducted using this dosage to determine the adsorption rates of Congo red on u-FN and u-MFN. As shown in Fig. 6b, u-FN removed ∼90% of the Congo red within 50 min, and with increasing time, the final efficiency of Congo red removal was up to 95% within 300 min. The maximum adsorption capacities of u-FN and u-MFN for Congo red were calculated to be 125.4 mg g−1 and 109.2 mg g−1 , respectively. This high removal efficiency was largely attributed to the large surface area of the 3D nanostructure, as well as electrostatic attraction between the u-FN (or u-MFN) surfaces and Congo red. It appears that the adsorption capacities of our samples for Congo red are much higher than those of the majority of similar materials (Table 2). It is worth noting that the typical adsorption results reported in the literature are obtained under optimized conditions, including optimum pH, temperature, and adsorption time [47–50]. It is therefore conceivable that the reported high removal capacities and efficiencies cannot be achieved under practical conditions. Because of the superior adsorption performance achieved here without adjusting the process conditions, and because of the facile regeneration method, u-FN and u-MFN are highly promising materials for use in water treatment systems.

Fig. 6. (a) Effects of u-FN and u-MFN adsorbent dosages on adsorption of Congo red. (b) Adsorption rate curves for Congo red using u-FN and u-MFN.

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Table 2 As(V), Cr(VI), and Congo red adsorption capacities of various adsorbents. Sample

Magnetic property

Maximum As (V) removal capacity (mg g−1 )

Maximum Cr (VI) removal capacity (mg g−1 )

u-MFN u-FN 3D flowerlike magnetic ␥-Fe2 O3 3D flowerlike magnetic Fe3 O4 Fe3 O4 nanoparticles ␥-Fe2 O3 nanoparticles Fe3 O4 /BN Flower like ␣-Fe2 O3 Hollow CeO2 nanospheres Flower like CeO2 Hollow boehmite microspheres Hierarchical hollow MnO2 nanostructure Hollow urchin like ␣-FeOOH nanostructure Hierarchical spindle like ␥-Al2 O3

O X O O O O O X X X X X X X

39.6 45.3 4.8 4.7 46.7 50.0 32.18 51.0 22.4 14.4

35.0 40.9

Maximum adsorption capacity for Congo red (mg g−1 ) 109.2 125.4

30.0 15.4 5.9 111.3 80 239 90

References

This study This study [18] [18] [19] [20] [21] [32] [33] [34] [35] [36] [37] [38]

Note: The presence/absence of magnetic properties for samples are marked by symbols of O and X in where it shows magnetic property as O while no magnetic property as X.

Acknowledgments This research was supported by the Converging Research Center Program through the Ministry of Science, ICT and Future Planning, Korea (2013K000163).

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jhazmat. 2013.08.014.

References

Fig. 7. Regeneration studies of u-MFN over six cycles.

In practical applications, it is desirable that a material can be fully recovered and reused so that it can be used cyclically in a cost-effective manner. Fig. 7 shows the removal efficiency of Congo red by u-MFN during six adsorption–regeneration cycles. It was observed that in the first cycle, the removal efficiency decreased significantly, and in subsequent cycles, the removal efficiency decreased slowly. The removal efficiency after six cycles still reached 64.3%. The decrease in adsorption at each cycle was attributed to loss and adsorbed heavy metals of the u-MFN.

4. Conclusions In summary, u-MFN was synthesized using a simple growth method (ultrasound irradiation/calcination processing). u-MFN, with its large surface area (94.1 m2 g−1 ) and strong magnetic properties (57.9 emu g−1 ), exhibits excellent adsorption capacities toward As(V) (39.6 mg g−1 ), Cr(VI) (35 mg g−1 ), and Congo red (109.2 mg g−1 ). In fact, these values are much higher than those of most reported nanomaterials. u-MFN also displays excellent adsorption of Congo red after recycling. As a result of its high adsorption capacity, rapid adsorption rate, and quick magnetic separation from treated water, the u-MFN developed in the present study is an ideal magnetic adsorbent for removal of heavy metals and organic pollutants from aqueous solutions.

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