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Cite This: ACS Appl. Mater. Interfaces 2017, 9, 36772-36782

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* Department of Materials Science and Engineering, National University of Singapore, 117575, Singapore

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S Supporting Information *

ABSTRACT: Wastewater contaminated with heavy metals is a worldwide concern due to the toxicity to human and animals. The 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) (PAA)-coated quasi-superparamagnetic Fe3O4 nanoparticles (NPs). PAA-coated Fe3O4 NPs possess high adsorption capacity of heavy metal ions including Pb, Ni, Co, and Cu and can be easily regenerated after the 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 T) 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 the World Health Organization (e.g., less than 0.01 mg L−1 for Pb). Overall, a proof-of-concept adsorption and subsequent low-field dynamic separation technique is demonstrated as an economical and efficient route for heavy metal removal from wastewater. KEYWORDS: heavy metal removal, low-field dynamic separation, magnetic filter, quasi-superparamagnetic Fe3O4 nanoparticles, without byproducts

1. INTRODUCTION

Nowadays, there are numerous reports of magnetic nanoparticles (NPs) 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 with packed magnetically susceptible wires was usually applied to selectively capture the magnetic materials under dynamic flow conditions.20−22 However, the recovery of magnetic nanoparticles may not be an easy task. In addition, the high operation cost with a strong magnetic field (in the 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 that enables efficient capture of magnetic nanoparticles under a relatively low magnetic field and simultaneously possesses easy recovery ability to avoid contaminated byproducts. Recently, we have successfully fabricated high-quality structures of functional oxides (high-Tc superconductor

Water contamination of heavy metal has become a serious issue due to various industrial activities such as electroplating, battery manufacturing, mining and mineral processing, and so on. The pollutants are concentrated up the food chain and eventually cause serious damages to 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 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 economical 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 raises the danger of super bacteria.7−9 Additionally, it worth noting that dynamic water treatment systems are much more favorable for industrial applications compared to static counterparts because dynamic ones are more mobile and flexible in terms of volume and other geographic limitations. © 2017 American Chemical Society

Received: July 19, 2017 Accepted: October 3, 2017 Published: October 3, 2017 36772

DOI: 10.1021/acsami.7b10549 ACS Appl. Mater. Interfaces 2017, 9, 36772−36782

Research Article

ACS Applied Materials & Interfaces

Figure 1. Schematic illustration of heavy metal removal combined with 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).

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 well-dispersed properties rather than forming aggregates. For these large particles with size close to 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 L−1, satisfying 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 the efficiency. Overall, this work presents an effective and economical heavy metal removal technique for water remediation without secondary pollution.

yttrium barium copper oxide and hard and soft magnetic ferrites NiFe2O4 and BaFe12O19) by simple additive manufacturing technique of extrusion free forming.24−26 These works have demonstrated great potentials for the fabrication of functional structures of complicated geometry by additive manufacturing or three-dimensional (3D) printing. 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 (e.g., two small pieces of NdFeB permanent magnets without energy assumption) as a magnetic filter, and can be removed if the magnetic field is removed; (ii) enhance magnetic flux and increase magnetic field gradient; and (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 To improve the capture rate and adsorption capacity, magnetite nanoparticles with different average sizes of 24 and 10 nm were synthesized by thermal decomposition reaction according to our previous studies on nanoparticles for biomedical applications.31−34 Subsequently, the 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

2. EXPERIMENTAL SECTION 2.1. Materials. The starting powder for the self-designed magnetic filter was NiO, ZnO, and Fe2O3 (Sigma-Aldrich, 99.99% pure). Binder poly(vinyl alcohol) (PVA, Mowiol 4-88) and plasticizer poly(ethylene glycol) (PEG-400) were purchased from Sigma-Aldrich. Dispersant Solsperse 20000 was from Lubriol. For the synthesis of Fe3O4 36773


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where k (g mg−1 min−1) is the rate constant of pseudo-second-order and qe is the equilibrium adsorption capacity, which can be determined by plotting t/qt versus t. Adsorption isotherm experiment was carried out with the initial heavy metal concentration in the range 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 eq 3

nanoparticles, iron(III) acetylacetonate (Fe(acac)3, 97.9%) was obtained from Fluka. Oleic acid, oleylamine, and poly(acrylic acid) (PAA, Mw = 1800) were purchased from Aldrich. Analytical grade Pb(NO3)2, Cu(NO3)2, Ni(NO3)2, and Co(NO3)2 were employed as source materials for heavy metal ions. The chemicals were used directly as received. All of the solutions were prepared using deionized water. 2.2. Three-Dimensional (3D) Printing of Magnetic Filter. The self-designed magnetic filter was fabricated by a 3D printer (3 Dison Multi) with a programmable patterning built layer by layer. During 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 °C was undertaken for crystal phase formation by the solid-state reaction. 2.3. 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 ligandexchange reaction. 2.4. 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 0.01−100 s−1 at a fixed temperature of 25 °C. Moreover, the weight loss of printable pastes was analyzed using a SDT Q600 thermogravimetric analyzer (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 (Zeiss Supra 40). Elemental analysis was performed by energydispersive X-ray (EDX) spectroscopy attachment of the scanning electron microscopy (SEM). The analysis of elements state was performed by X-ray photoelectron spectroscopy (XPS, Kratos AXIS Ultra DLD). X-ray diffraction (XRD) was carried out on a Bruker D8 Advanced System. Finally, magnetic properties were tested using a superconducting quantum interference device (MPMS 3). For the assynthesized 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 Fourier transform infrared (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 analyze the crystal structures. Magnetic properties were recorded on a vibrating sample magnetometer (VSM, Lake Shore Model 7407). 2.5. Batch Adsorption Study. The kinetics was studied to investigate the adsorption rate of heavy metal ions on PAA-coated Fe3O4 nanoadsorbents. 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 plasma optical emission spectroscopy (ICP-OES, PerkinElmer Optima 5300 DV). Then, the heavy metal adsorption capacity is calculated in eq 1 (C − Ct )V qt = 0 m

Ce C 1 = + e qe bqm qm

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 mg−1) 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. 2.6. 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 nanoadsorbents 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 a low magnetic field (∼0.7 kOe or 0.07 T, measured by a Gaussmeter with a Hall probe), which was produced by two small commercial permanent magnets. When the mesh filter was put between these two magnets, the actual magnetic field at the center increased from 0.07 to 0.09 T due to the 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. 2.7. Continual Dynamic Capture and Recover Study. The selfassembled separation device was applied to capture the water-soluble PAA-coated Fe3O4 nanoparticles (NPs) from the aqueous solution. The removal efficiency of NPs by the fabricated magnetic filter in dynamic flow conditions is calculated in eq 4

⎛ Ce,NPs ⎞ ⎟⎟ × 100 Removal (%) = ⎜⎜1 − C0,NPs ⎠ ⎝

(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 as that of the effluent; thus, the recovery efficiency of Fe3O4 nanocomposites is simply defined as eq 5 ⎛ ⎞ Cr,NPs ⎟⎟ × 100 Recovery (%) = ⎜⎜ ⎝ C0,NPs − Ce,NPs ⎠

(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 nanocomposites were tested over six cycles to evaluate the reusability of magnetic filter. 2.8. Regeneration of Fe3O4 Nanoadsorbents. A recyclable agent for the heavy metal removal should involve the reversible desorption of adsorbents. For desorption studies, the collected Fe3O4 nanocomposites were dispersed in 0.1 M HCl acidic solution. The released concentration of heavy metal ions was measured by ICP-OES. Moreover, the regenerated adsorbent was reused for consecutive adsorption cycle under the same conditions. This adsorption/ desorption process was repeated to verify the reusability of Fe3O4 nanoadsorbents.

(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, and Ct (mg L−1) is the concentration at different adsorption times. V (L) and m (g) are the volume of solution and the adsorbent dosage, respectively. For further explanation, the pseudo-second-order kinetic model was employed to simulate the experimental data, which can be expressed as eq 2

t 1 t = + 2 qt q kqe e

(3)

(2) 36774


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Figure 2. Characterization of the fabricated magnetic filter. (a) Digital photographs of the as-printed and sintered body of ferrite filter with mesh structures (inset shows the width and distance). (b) SEM microstructures of the magnetic filter. (c) XRD pattern of the fabricated filter. (d) Magnetic hysteresis loops at 10 and 300 K, and inset is magnified loops from −50 to 50 Oe.

3. RESULTS AND DISCUSSION 3.1. Three-Dimensional (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 wide applications.28,36,37 However, this functional ceramic was only limited to be processed into thick films or simple shapes by traditional die-pressing, slipcasting, or tape-casting methods.38−40 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 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 poly(vinyl alcohol) (PVA), plasticizer poly(ethylene glycol) (PEG-400), and low 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 performance on heavy metal adsorption rate and capacity. The PAA-coated quasi-superparamagnetic 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 of low-field 36778


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

were left over in the aqueous solution after filtration. The result is certainly not satisfactory for heavy metal ion removing purpose. To enhance the removal efficiency, we increased the number of layers. When we used a ferrite mesh of 20 layers, all of the nanoparticles could be well captured, confirmed by the neartransparent 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 interesting to study the 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 of about 500 mg L−1 was flowed through the magnetic filter (20 layers) with volume varying from 10 to 200 mL. The removal efficiency remained above 99% even when the treated volume was up to 200 mL, as shown in Figure S7. On the basis of a rough evaluation of the 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

Figure 6. Pb(II) and Ni(II) concentration when the Fe3O4 solution (sample 1 after adsorption for 30 min) flowing through the magnetic filter.

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 the collection of magnetic nanoparticles under magnetic field of 0.7 kOe, the magnetic filter was washed in an ultrasonic after both permanent magnets had been 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. 36779


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PAA-coated Fe3O4 nanoparticles have a high adsorption capacity and fast kinetics. (ii) Soft-magnetic ferrite mesh of 20 layers can be used to capture >99% of the nanoparticles 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 (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.

The results have demonstrated that quasi-superparamagnetic particles can be easily recycled for reuse. In this work, we have also fabricated a nonmagnetic 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 that only approximately 40% of the nanoparticles can be captured by the rigid mesh structure possibly due to van der Waals force between particle/surfactant and Fe2O3 surface. Of note, the removal efficiency of approximately 40% was independent of the presence of permanent magnets. 3.5. Heavy Metal Removal and Adsorbents Regeneration. The above study has shown that quasi-superparamagnetic 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, PAAcoated Fe3O4 nanoparticles have a 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 the decrease in concentration of heavy metal ions from 1 mg L−1 to the level recommended by WHO 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 the injection of PAA-coated Fe3O4 nanoparticles into the contaminated water, magnetic filtration was carried out 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 of heavy metals from 1.0 mg L−1 to the WHO standard (0.01 mg L−1 for Pb(II) and 0.02 mg L−1 for Ni(II); Figure 6) required the weight of PAA-coated Fe3O4 nanoparticles to be 18 and 15 mg for Pb(II) and Ni(II), respectively, only after single filtration. To avoid byproducts, which could result in secondary pollution, we studied whether the 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 mentioned earlier in the article. Similar recovery efficiency was obtained, as shown in Figure 5c. Subsequently, the pH value was adjusted to 1−2 by adding acidic solution after the removal of the magnetic filter. ICP measurements demonstrate that heavy metal ions and PAAcoated Fe3O4 nanoparticles can be well separated.16 And no leaching of Fe ions was detected due to the successful surface coating. Meanwhile, these regenerated Fe3O4 nanoparticles could also be well suspended in the 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 quasisuperparamagnetic Fe3O4 nanoparticles (sample 1 with a particle size of 24 nm).

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b10549. 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; nonmagnetic filter (α-Fe2O3) fabrication and characterization by VSM and XRD analysis (PDF) Low-field dynamic separation of Fe3O4 nanoparticles for heavy metal removal (Video S1) (AVI) Simple recovery of Fe3O4 nanoparticles and magnetic filter (Video S2) (AVI)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Xiangxia Wei: 0000-0001-6898-1907 Present Address ‡

National Center for Nanoscience and Technology (NCNST), 100190 Beijing, P. R. China (X.L.L.). Author Contributions †

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

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The project is financially supported by NUS Strategic Research Fund R261509001646 and R261509001733 and NRF-CRP162015-01 (R284000159281). The first author is also grateful to the scholarship from China Scholarship Council (CSC No. 201406320189).

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REFERENCES

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4. CONCLUSIONS In the present research work, we have successfully developed a low-field dynamic magnetic separation system for effective and economical removal of heavy metal ions in wastewater. Our strategy was to use a soft-magnetic ferrite mesh as magnetic filter and PAA-coated quasi-superparamagnetic Fe3O4 nanoparticles as adsorbent. The major findings are as follows: (i) 36780


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