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Low-Field Dynamic Magnetic Separation by Self-Fabricated Magnetic Meshes for Efficient Heavy Metal Removal Xiangxia Wei, Pon Janani Sugumaran, Erwin Peng, Xiao Li Liu, and Jun Ding ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10549 • Publication Date (Web): 03 Oct 2017 Downloaded from http://pubs.acs.org on October 5, 2017

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ACS Applied Materials & Interfaces

Low-Field Dynamic Magnetic Separation by SelfFabricated Magnetic Meshes for Efficient Heavy Metal Removal Xiangxia Wei†‡, Pon Janani Sugumaran†‡, Erwin Peng†, Xiao Li Liu†§, and Jun Ding*,† †Department of Materials Science and Engineering, National University of Singapore, 117575, Singapore *E-mail: [email protected] (J. Ding)

KEYWORDS: Heavy metal removal, low-field dynamic separation, magnetic filter, quasisuperparamagnetic Fe3O4 nanoparticles, without by-products.

ACS Paragon Plus Environment

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ABSTRACT: Wastewater contaminated with heavy metals is a worldwide concern due to the toxicity to human and animals. Herein, current study presents an incorporation of adsorption and low-field dynamic magnetic separation technique for the treatment of heavy metal contaminated water. The key components are the eco-fabricated magnetic filter with mesh architectures (constituted of a soft magnetic material (Ni,Zn)Fe2O4) and poly(acrylic acid)-coated quasisuperparamagnetic Fe3O4 nanoparticles. Poly(acrylic acid)-coated Fe3O4 nanoparticles possess high adsorption capacity of heavy metal ions including Pb, Ni, Co and Cu, and can be easily regenerated after adjustment of pH. Moreover, magnetic mesh filter has shown excellent collection ability of quasi-superparamagnetic particles under a magnetic field as low as 0.7 kOe (0.07 Tesla), and can easily release these particles during ultrasonic washing when small magnets are removed. In the end, after one filtration process, the heavy metal concentration can be significantly decreased from 1.0 mg L-1 to below the drinking water standard recommended by World Health Organization (for example less than 0.01 mg L-1 for Pb). Overall, a proof-ofconcept adsorption and subsequent low-field dynamic separation technique is demonstrated as an economical and efficient route for heavy metal removal from wastewater.

1. INTRODUCTION

Water contamination of heavy metal has become a serious issue due to various industrial activities such as electroplating, battery manufacturing, mining & mineral processing and so on. The pollutants are concentrated up the food chain and eventually cause serious damages of tissues and organs.1 Hence, various processes have been developed for heavy metal removal, including chemical precipitation, ion exchange, electrochemical treatment, adsorption particularly biosorption and membrane filtration.2-6 However, for the chemical routes among


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these techniques, the relatively high operation cost may limit their practical applications, including sludge treatment, expensive regeneration and energy consumption. In contrast, secondary pollution may happen if conventional absorbents and membranes after fouling are not properly stored. Therefore, an efficient and economic technique for heavy metal removal without secondary pollution is highly desirable. It will also have a great impact on other environmental and biomedical issues, for example, antibiotics contamination which raise the danger of super bacteria.7-9 Additionally, it is to note that dynamic water treatment systems are much more favourable for industrial applications compared to static counterparts, because dynamic ones are more mobile and flexible in terms of volume and other geographic limitations. Nowadays, numerous reports utilized magnetic nanoparticles as effective adsorbents for heavy metal removal because of their great advantages, such as large specific surface area, convenient separation from water and easy functionalization with different chemical groups.10-18 However, the adsorption and magnetic separation processes mainly work for a small volume in a static batch mode.19 For a dynamic process, a high gradient magnetic separator (HGMS) with packed magnetically susceptible wires was usually applied to selectively capture the magnetic materials under dynamic flow conditions.20-22 But, the recovery of magnetic nanoparticles may not be an easy task. In addition, the high operation cost with a strong magnetic field (in order of 2 T (20 kOe)) makes this technique less competitive compared to other existing techniques.23 Therefore, we believe that a breakthrough could be achieved by the development of a method, which enables efficient capture of magnetic nanoparticles under a relatively low magnetic field and simultaneously possesses easy recovery ability to avoid contaminated by-products. Recently, we have successfully fabricated high-quality structures of functional oxides (high-Tc superconductor YBCO and hard and soft magnetic ferrites NiFe2O4 and BaFe12O19 by simple


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additive manufacturing technique—extrusion free-forming.24-26 These works have demonstrated the great potentials of fabrication of functional structures of complicated geometry by additive manufacturing or 3Dprinting. In the present research work, (Ni, Zn)Fe2O4 was chosen because of its large saturation magnetization and soft magnetic properties, which allows magnetization under low magnetic field. Meanwhile, the magnetic filter had a multilayered mesh structure with optimized parameters such as diameter, distance and number of layers. The soft-magnetic mesh has following functions: (i) ability to be held under a relatively low magnetic field (for example two small pieces of NdFeB permanent magnets without energy assumption) as a magnetic filter, and can be removed if magnetic field is removed; (ii) enhance magnetic flux and increase magnetic field gradient; (iii) its rigid surface can be used to capture magnetic nanoparticles for magnetic separation. In addition, the magnetic filter can work even under harsh environment because the ferrite possesses excellent chemical stability and mechanical hardness, instead of nylon and stainless steel mesh.27-30 In order to improve capture rate and adsorption capacity, magnetite nanoparticles with different average sizes of 24 nm and 10 nm were synthesized by thermal decomposition reaction according to our previous studies on nanoparticles for biomedical applications.31-34 Subsequently, as-synthesized Fe3O4 nanoparticles were modified by a surfactant—poly(acrylic acid) (PAA), which not only provided well dispersion stability but also had favorable surface adsorption tailored for efficient heavy metal removal,35 including Pb(II), Ni(II), Cu(II) and Co(II). Meanwhile, nanoparticles with small particle size showed high adsorption rate and excellent capacity, whereas large particles did not exhibit much deteriorated performance due to the welldispersed properties rather than forming aggregates. For these large particles with size close to


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the critical size of superparamagnetism, the quasi-paramagnetic properties lead to large magnetic response and thus result in high collection efficiency by the aforementioned magnetic filter. Lastly, an efficient and economical technique is developed by a combination of magnetic nanoparticles adsorption and the followed low-field dynamic separation as schematically illustrated in Figure 1. The heavy metal, for instance, Pb(II) can be reduced from 1 to 0.01 mg L1

, satisfied the standard suggested by WHO. It is also demonstrated that this technique also

works for the large volume, which can continuously treat 200 mL of wastewater in flow conditions. Besides, the magnetic filter and nanoparticles can be well recovered and regenerated by simple washing and pH adjustment, allowing for further decontamination processes without fading efficiency. Overall, this work presents an effective and economic heavy metal removal technique for water remediation without secondary pollution.


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Figure 1 Schematic illustration of heavy metal removal combined by adsorption and low-field and dynamic separation technique. (a) Decontamination of Pb(II) from wastewater using Fe3O4 nanoparticles and low field magnetic filtration. (b) Recovery and regeneration of the magnetic filter and Fe3O4 nanoparticles (NPs). Note: in this work, two pieces of small NdFeB magnets were used. The magnetic field generated is only 0.7 kOe (0.07 T). 2. EXPERIMENTAL SECTION Materials. The starting powder for the self-designed magnetic filter was NiO, ZnO and Fe2O3 (Sigma-Aldrich, 99.99% pure). Binder polyvinyl alcohol (PVA, Mowiol®4-88) and plasticizer ploy(ethylene glycol) (PEG-400) were purchased from Sigma-Aldrich. Dispersant Solsperse 20000 was from Lubriol. For the synthesis of Fe3O4 nanoparticles, Iron(III) acetylacetonate (Fe(acac)3, 97.9%) was obtained from Fluka. Oleic acid (OA), oleylamine (OAm), and poly(acrylic acid) (PAA, Mw = 1800) were purchased from Aldrich. Analytical grade Pb(NO3)2, Cu(NO3)2, Ni(NO3)2, Co(NO3)2 were employed as source materials for heavy metal ions. The chemicals were used directly as received. All solutions were prepared using deionized (DI) water. 3D printing of magnetic filter. The self-designed magnetic filter was fabricated by a 3D printer (3Dison Multi) with the programmable patterning built layer-by-layer. During the printing, a plastic nozzle was used and the moving speed was 5 mm s-1. The dimensions of self-designed objects were input into the system. Subsequently, the printed structures were dried in air to remove the water solvent and maintain the 3D structures. Lastly, heat treatment at 1300 oC was undertaken for crystal phase formation by the solid state reaction.


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Synthesis of iron oxide nanoparticles. The different sized Fe3O4 nanoparticles were synthesized using a thermal decomposition reaction (see details in the Supporting Information). Then, the nanoparticles were surface modified by PAA through the ligand exchange reaction. Materials characterization. To characterize the fabricated filter, rheological study of pastes was conducted on a Discovery HR-2 rheometer (TA Instruments). The shear rate was in a range from 0.01 to 100 s−1 at a fixed temperature of 25 oC. Moreover, the weight loss of printable pastes was analyzed using a SDT Q600 thermal gravimetric analyzer (TGA, TA Instruments) in a nitrogen environment. In addition, the surface morphology and microstructure of calcined samples were characterized using a field-emission scanning electron microscope (FESEM, Zeiss Supra 40). Elemental analysis was examined by energy dispersive X-ray spectroscopy attachment of the SEM. The analysis of elements state was performed on X-ray photoelectron spectroscopy (XPS, Kratos AXIS Ultra DLD). XRD was carried out on a Bruker D8 Advanced System. Finally, magnetic properties were tested using a superconducting quantum interference device (SQUID, MPMS 3). For the as-synthesized Fe3O4 nanoparticles (NPs), the shape and size were observed by transmission electron microscopy (TEM, JEOL 3010). To confirm the successful surface modification by a surfactant PAA through ligand exchange, the FT-IR spectra by a Varian 3100 Excalibur Series FT-IR Spectrophotometer and zeta potential by a Malvern Zetasizer Nano-ZS were conducted. The XRD profile was also used to character the crystal structures. Magnetic properties were recorded on a vibrating sample magnetometer (VEM, Lake Shore Model 7407). Batch adsorption study. The kinetics was studied to investigate of heavy metal ions adsorption rate on PAA coated Fe3O4 nano-adsorbents. The initial heavy metal concentration is 10 mg L-1 at neutral conditions with an adsorbent dose of 2 mg. At a fixed period between 5 and 200 min, 1 mL mixture was collected and analyzed for the residual concentration by the inductively coupled


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plasma optical emission spectroscopy (ICP-OES, Perkin Elmer Optima 5300DV). Then, the heavy metal adsorption capacity is calculated in equation (1): =

( − ) (1)

Where qt (mg g-1) is the adsorption capacity of heavy metal per gram of adsorbent, C0 (mg L-1) represents the initial heavy metal concentration, Ct (mg L-1) is the concentration at different adsorption time. V (L) and m (g) are the volume of solution and adsorbent dosage, respectively. For further explanation, the pseudo-second-order kinetic model was employed to simulate the experimental data, which can be expressed as equation (2): 1

= + (2) where k (g mg-1 min-1) is the rate constant of pseudo-second-order, qe is the equilibrium adsorption capacity, which can be determined by plotting t/qt versus t. Adsorption isotherm experiment was carried out with initial heavy metal concentration ranged from 10 to 450 mg L-1 at room temperature by adding 1 mg adsorbent. The adsorption data were fitted with Langmuir isotherm model which assumes a homogeneous monolayer of adsorption surface, and the model is given in linear form as equation (3): 1 = + (3) The qe (mg g-1) is the adsorption capacity at equilibrium, qm (mg g-1) is the theoretical maximum adsorption capacity, Ce (mg L-1) represents the equilibrium concentration and b (L mg1

) measures the energy of adsorption. The constants including qm and b can be deduced from the

slope and intercept of the linear plot of Ce/qe versus Ce.


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Assembly of low-field and dynamic separation device. A low-field and dynamic magnetic separation was performed for wastewater treatment. Initially, wastewater was mixed with 2 mg Fe3O4 nano-adsorbents after contact for 0.5 h to remove heavy metal lead ions, for instance. The succeeding self-assembled magnetic and dynamic separation was carried out in low magnetic field (~0.7 kOe or 0.07 T, measured by a Gaussmeter with a Hall probe), which was produced by two small commercially permanent magnets. When the mesh filter was put between these two magnets, the actual magnetic field at the center increased from 0.07 T to 0.09 T due to enhancement of magnetic flux of the soft magnetic filter. Lastly, the wastewater after adsorption was fed to with a controlled flow rate at 5 mL min-1, leaving the purified water as the effluent with the treated volume up to 200 mL. Continual dynamic capture and recover study. The self-assembled separation device was applied to capture the water-soluble PAA coated Fe3O4 nanoparticles (NPs) from aqueous solution. The removal efficiency of NPs by the fabricated magnetic filter in dynamic flow conditions is calculated in equation (4): (%) = 1 −

, ! " × 100 (4) , !

C0,NPs (100 mg L-1) is the initial concentration of NPs, and Ce,NPs (mg L-1) is the concentration of NPs in effluent after filtration (determined by iron contents using ICP-OES). We assume the volume does not change after passing through the magnetic filter. Subsequently, Fe3O4 nanoparticles were recovered by ultrasonic washing with a small amount of clean water when the magnetic field is removed. The volume of rinsed water was the same


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with that of the effluent, thus, the recovery efficiency of Fe3O4 nano-composites is simplified defined as equation (5): &'((%) = (

), ! ) × 100 (5) , ! − , !

Cr,NPs (mg L-1) is the concentration of recovered NPs in washing water. For multiple recycling processes, the rinsed Fe3O4 solution was used to flow through the filter in the succeeding cycling test. The dynamic collection and recovery of Fe3O4 nano-composites were tested over six cycles to evaluate the reusability of magnetic filter. Regeneration of Fe3O4 nano-adsorbents. A recyclable agent for the heavy metal removal should involve the reversible desorption of adsorbents. For desorption studies, the collected Fe3O4 nano-composites were dispersed in 0.1 M HCl acidic solution. The released concentration of heavy metal ions was measured by ICP-OES. Moreover, the regenerated adsorbentwas reused for consecutive adsorption cycle under the same conditions. This adsorption/desorption process was repeated to verify the reusability of Fe3O4 nano-adsorbents. 3. RESULTS AND DISCUSSION 3D-printed magnetic filter. Magnetic filter with mesh architectures in this work is comprised of (Ni,Zn)Fe2O4 because of its excellent chemical stability, good mechanical hardness and remarkable soft magnetic properties with widely application.28,36,37 However, this functional ceramic was only limited to be processed into thick films or simple shapes by traditional diepressing, slip-casting or tape-casting methods.38-40 In order to obtain complex geometries for extensive applications as magnetic filter, additive manufacturing has been considered.41 Herein, powder based, extrusion 3D printing followed by solid state reaction was used to fabricate


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multilayered ferrites with desirable mesh structures via a layer-by-layer fashion controlled by the computer. The printable pastes were essentially high volume fractions of powder dispersed in an aqueous binder solution containing the dissolved binder polyvinyl alcohol (PVA), plasticizer ploy(ethylene glycol) (PEG-400) and few amount of dispersant Solsperse 20000 with formulations summarized in Table S1. This process is eco-friendly without any toxic solvents. In addition, extrusion was typically based on a rather cheap printer ( Ni(II) > Cu(II) > Co(II). Overall, it is evident that these two Fe3O4 samples have good


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performance on heavy metal adsorption rate and capacity. The PAA-coated quasisuperparamagnetic nanoparticles (Sample 1 with particle size of 24 nm) have comparable performance to Sample 2 with a particle size of 10 nm. The result is important for our realization of a novel technique–low-field dynamic magnetic separation process for effective heavy metal ions removal, as Sample 1 with a large particle size should be much more easily to be captured.

Figure 4 Kinetics and isotherms of Pb(II) decontamination by PAA coated Fe3O4 nanoadsorbents with different sized Fe3O4. (a) Time profile for Pb(II) adsorption by two samples. (b) Pseudo-second-kinetic model for Pb(II) adsorption. (c) Adsorption isotherm by two samples at room temperature. (d) Langmuir isotherm model for adsorption.


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Low-field dynamic collection and recovery Fe3O4. As presented previously in Figure 1, we have designed a simple system for low-field magnetic separation. Two small pieces of NdFeB permanent magnets were placed on the both sites of a magnetic mesh filter with a square shape of a size of 17x17 mm2. Magnetic field at the center was measured to be 0.7 kOe (0.07 T). With the system described above, we have tested the most critical part of this research work–the capability of capture of magnetic particles after filtration under a low magnetic field of 0.7 kOe, and recovery of magnetic nanoparticles from the filter after a simple ultrasonic washing. Figure 5a shows the results of capture capacity of magnetic nanoparticles after filtration. PAAcoated Fe3O4 nanoparticles were dissolved in 200 mL aqueous solution with a starting nanoparticle concentration of 100 mg L-1. Removal efficiency (in %) was estimated through ICP measurement after filtration with a flow rate of 5 mL min-1 under a careful control. For Sample 1 (Fe3O4 nanoparticles with particle size of 24 nm), around 85% of nanoparticles can be captured when a soft ferrite mesh of 6 layers was used. As magnetic nanoparticles should be reused after recovery, we have also tested removal efficiency of released nanoparticles after ultrasonic washing (more detail is found later in the main text when recovery efficiency is discussed as shown in Figure 5c). Up to 6 cycles, removal efficiency remains approximately constant around 85%. The removal efficiency of approximately 85% means that 15% of Fe3O4 nanoparticles were left over in the aqueous solution after filtration. The result is certainly not satisfactory for heavy metal ion removing purpose. In order to enhance removal efficiency, we have increased the layer numbers. When we used a ferrite mesh of 20 layers, all nanoparticles could be well captured, confirming from the near transparent solution in the effluent (Video 1 in the Supporting Information). No noticeable signal of iron could be measured by ICP, showing that the removal efficiency is well above 99%. It is


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interesting to study removal efficiency of Sample 2 with a particle size of 10 nm. As shown in Figure 6a, the removal efficiency was below 80% when a magnetic mesh filter of 20 layers was used. The results confirm that quasi-superparamagnetic nanoparticles are more favorable for low-field dynamic magnetic separation. Lower magnetization and smaller particle size of Sample 2 are not suitable for low-field capture, as schematically shown in Figure 5b. Moreover, to estimate the concentration of sample 1 in the mesh, a highly concentrated Fe3O4 solution about 500 mg L-1 was flowed through the magnetic filter (20 layers) with volume varied from 10 mL to 200 mL. The removal efficiency remained above 99% even when the treated volume up to 200 mL as shown in Figure S7. Based on a rough evaluation of total surface area of the mesh filter, the corresponding concentration is approximately 2 mg cm-2 (Fe3O4 nanoparticles on the surface of magnetic filter). And a total of approximately 0.5 mg of Fe3O4 particles with a particle size of 24 nm can form a single uniform layer on this magnetic mesh filter after simple evaluation. Another important issue is how much magnetic nanoparticles can be recovered for reuse. After collection of magnetic nanoparticles under magnetic field of 0.7 kOe, the magnetic filter was washed in an ultrasonic both after permanent magnets were removed (Video 2 in the Supporting Information). As shown in Figure 5c, the recovery efficiency was approximately 97%. The value remained unchanged after recycling of 6 times. The results have demonstrated that quasisuperparamagnetic particles can be easily recycled for reuse. In this work, we have also fabricated a non-magnetic Fe2O3 mesh with 20 layers based on antiferomagnetic α-Fe2O3, which has no noticeable magnetic response at room temperature (Figure S8 in the Supporting Information). It can be clearly seen from Figure 5d, only


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approximately 40% of nanoparticles can be captured by the rigid mesh structure possibly due to Van der Waals force between particle/surfactant and Fe2O3 surface. It is to note that the removal efficiency of approximately 40% was independent if permanent magnets were present or not.

Figure 5 Removal and recovery of Fe3O4 nanoparticles. (a) The removal efficiency by magnetic filters with various layers during multiple cycles. (b) Schematic illustration of collection ability on different sized Fe3O4 nanoparticles. (c) The recovery efficiency by the magnetic filter (20 layers) during 6 cycles, and inset photographs from left to right show sample 1 in original solution, in effluent and in recovery solution for cycle 1 and cycle 6, respectively. (d) The removal and recovery efficiency by non-magnetic α-Fe2O3 filter, and similarly, inset photographs are sample 1 in original solution, in effluent after filtration and in recovery solution.


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Heavy metal removal and adsorbents regeneration. The above study has shown that quasisuperparamagnetic Fe3O4 nanoparticles (Sample 1 with a particle size of 24 nm) can be well recovered after filtration under a low magnetic field of 0.7 kOe. Meanwhile, as described previously, PAA-coated Fe3O4 nanoparticles have high adsorption capacity and fast kinetics for heavy metal removal. That is to say, it is relatively easy to reduce heavy metal concentration to low values. Therefore, we have paid particular attention to decrease heavy metal ions concentration from 1 mg L-1 to the level of WHO recommended standard for drinking water. In a container of 200 mL water, an initial heavy metal content was 1.0 mg L-1 (in this work, Pb(II) and Ni(II) were studied). After injection of PAA-coated Fe3O4 nanoparticles into the contaminated water, magnetic filtration was taken place after adsorption for 30 min. Furthermore, the heavy metal concentration after filtration was measured as a function of the weight of Fe3O4 nanoparticles. It was found that the reduction from 1.0 mg L-1 of heavy metals to WHO standard (0.01 mg L-1 for Pb(II) and 0.02 mg L-1 for Ni(II), Figure 6) required a weight of PAA-coated Fe3O4 nanoparticles to be 18 mg and 15 mg for Pb(II) and Ni(II) respectively, only after single filtration. To avoid by-products which could result in secondary pollution, we have studied whether PAA-coated Fe3O4 nanoparticles can be recovered after adsorption of heavy metal ions and whether the regenerated PAA-coated Fe3O4 can still perform well. In this case, after the low magnetic field filtration, magnetic filter was initially separated from nanoparticles using a simple ultrasonic washing as aforementioned. Similar recovery efficiency was obtained as shown in Figure 5c previously. Subsequently, the pH value was adjusted to 1-2 by adding acidic solution after removing of the magnetic filter. ICP measurements demonstrate that heavy metal ions and PAA-coated Fe3O4 nanoparticles can be well separated.16 And no leaching of Fe ions was


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detected due to the successful surface coating. Meanwhile, these regenerated Fe3O4 nanoparticles could also be well suspended in aqueous solution. More importantly, our study found that there was no apparent difference in the performance of heavy metal ion removal between freshly prepared and regenerated PAA-coated quasi-superparamagnetic Fe3O4 nanoparticles (Sample 1 with a particle size of 24 nm).

Figure 6 Pb(II) and Ni(II) concentration when the Fe3O4 solution (Sample 1 after adsorption for 30 min) flowing through the magnetic filter. 4. CONCLUSIONS In the present research work, we have successfully developed a low-field dynamic magnetic separation system for effective and economic removal of heavy metal ions in wastewater. Our strategy was to use a soft-magnetic ferrite mesh as magnetic filter, and PAA-coated quasisuperparamagnetic Fe3O4 nanoparticles as adsorbent. The major findings are: (i) PAA-coated Fe3O4 nanoparticles have high adsorption capacity and fast kinetics. (ii) Using soft-magnetic ferrite mesh of 20 layers, >99% of nanoparticles can be captured under a low magnetic field of 0.7 kOe (two NdFeB permanent magnets). Magnetic mesh and nanoparticles could be recovered for reuse after a simple ultrasonic washing. (iii) With only 15-20 mg of Fe3O4 nanoparticles, the heavy metal concentration can be reduced from 1 mg L-1 to meet WHO drinking water standard


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(0.01 mg L-1 for Pb(II) and 0.02 mg L-1 for Ni(II)) just after a single filtration process. Moreover, these magnetic nanoparticles could be well regenerated for reuse. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available. Magnetic filter fabrication by 3D printing and characterization by EDX and XPS analysis; iron oxide nanoparticles synthesis and adsorption behaviors; effect of pH, temperature and the amount of adsorbents; adsorption mechanism; the concentration of Fe3O4 nanoparticles in the filter; non-magnetic filter (α-Fe2O3) fabrication and characterization by VSM and XRD analysis. (PDF) Supporting Video 1 shows the low-field dynamic separation of Fe3O4 nanoparticles for heavy metal removal. (avi) Supporting Video 2 shows the simple recovery of Fe3O4 nanoparticles and magnetic filter. (avi) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions ‡

X. W and Pon J. S contributed equally to this work.


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Present Address §

Xiao Li Liu: National Center for Nanoscience and Technology (NCNST), 100190 Beijing, P. R.

China Notes The authors declare no competing financial interests. ACKNOWLEDGMENT The project is financially supported by NUS Strategic Research Fund R261509001646 & R261509001733 and NRF-CRP16-2015-01 (R284000159281). The first author is also grateful to the scholarship from China Scholarship Council (CSC No. 201406320189). REFERENCES (1) McNutt, M. Mercury and Health. Science 2013, 341, 1430. (2) Fu, F.; Wang, Q. Removal of Heavy Metal Ions from Wastewaters: A Review. J. Environ. Manage. 2011, 92, 407-418. (3) Chen, X.; Lam, K. F.; Mak, S. F.; Yeung, K. L. Precious Metal Recovery by Selective Adsorption Using Biosorbents. J. Hazard. Mater. 2011, 186, 902-910. (4) Tian, Y.; Wu, M.; Liu, R.; Li, Y.; Wang, D.; Tan, J.; Wu, R.; Huang, Y. Electrospun Membrane of Cellulose Acetate for Heavy Metal Ion Adsorption in Water Treatment. Carbohydr. Polym. 2011, 83, 743-748. (5) Xin, S.; Zeng, Z.; Zhou, X.; Luo, W.; Shi, X.; Wang, Q.; Deng, H.; Du, Y. Recyclable Saccharomyces Cerevisiae Loaded Nanofibrous Mats with Sandwich Structure Constructing via Bio-Electrospraying for Heavy Metal Removal. J. Hazard. Mater. 2017, 324, 365-372. (6) Pelit, L.; Ertas, F. N.; Eroglu, A. E.; Shahwan, T.; Tural, H. Biosorption of Cu(II) and Pb(II) Ions from Aqueous Solution by Natural Spider Silk. Bioresour. Technol. 2011, 102, 8807-8813. (7) Vilela, D.; Parmar, J.; Zeng, Y.; Zhao, Y.; Sanchez, S. Graphene-Based Microbots for Toxic Heavy Metal Removal and Recovery from Water. Nano Lett. 2016, 16, 2860-2866. (8) Yang, J.; Zhang, H.; Yu, M.; Emmanuelawati, I.; Zou, J.; Yuan, Z.; Yu, C. High-Content, Well-Dispersed γ-Fe2O3 Nanoparticles Encapsulated in Macroporous Silica with Superior Arsenic Removal Performance. Adv. Funct. Mater. 2014, 24, 1354-1363. (9) Martinez, J. L. Antibiotics and Antibiotic Resistance Genes in Natural Environments. Science 2008, 321, 365-367. (10) Chandra, V.; Park, J.; Chun, Y.; Lee, J. W.; Hwang, I. C.; Kim, K. S. Water-Dispersible Magnetite-Reduced Graphene Oxide Composites for Arsenic Removal. ACS Nano 2010, 4, 3979-3986.


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The Table of Contents: TOC Figure


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