Sustainable Wastewater Treatment Using Microsized Magnetic

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Sustainable Wastewater Treatment Using Microsized Magnetic Hydrogel with Magnetic Separation Technology Samuel C. N. Tang,†,‡ Dickson Y. S. Yan,†,§ and Irene M. C. Lo*,† †

Department of Civil and Environmental Engineering, The Hong Kong University of Science and Technology, Hong Kong, China Hong Kong Green Building Council, 1/F, Jockey Club Environmental Building, 77 Tat Chee Avenue, Kowloon Tong, Hong Kong, China § Faculty of Science and Technology, The Technological and Higher Education Institute of Hong Kong, Hong Kong, China ‡

ABSTRACT: A novel magnetic polymeric adsorbent, namely, magnetic hydrogel, was used to investigate its reusability and applicability in Cr(VI)-bearing wastewater treatment using magnetic separation. Different concentrations and amounts of NaCl solution and a stepwise approach were used for the regeneration experiment. A stepwise adsorption process followed by stepwise 3.0 M NaCl regeneration with a 40:1 wastewater-to-recovery volume ratio was found to be the most applicable working condition. The Cr concentration in the recovery solution was increased 25−30 times to 500−600 mg/L. The Cr(VI) removal and recovery performance of magnetic hydrogel was maintained for 20 cycles. An industrial wastewater treatment prototype, including a magnetic separation unit, was developed. The magnetic separation unit was designed to provide a magnetic field at the bottom with a zigzag pathway feature for maximizing the chance of capturing magnetic hydrogel. The separation efficiency for the magnetic hydrogel was above 97% throughout the 20 cycles of treatment. another option for Cr(VI) removal. Membrane filtration can remove suspended particles, as well as inorganic pollutants like heavy metals. Ultrafiltration, nanofiltration, and a reverse osmosis process can effectively remove heavy metals and concentrate metal ions in the retentate stream;11,12 however, membrane filtration is usually pressure driven requiring highpressure pumping.13 This could increase the operation cost due to high energy consumption. The overall cost of using membrane separation is also relatively high, compared to other current treatment technologies. The fouling problem and chemical stability of the membrane are the challenges for applying membrane filtration for industrial wastewater treatment, particularly when industrial wastewater possesses high ionic strength and extreme pH.14 Therefore, an innovative and sustainable treatment technology, which is simple to operate, to recover Cr from industrial wastewater is necessary. A novel magnetic polymeric adsorbent, namely, magnetic hydrogel, has been recently developed by Tang et al.15 It shows its advantages in fast Cr(VI) removal kinetics, reaching equilibrium in 5 min, and a high removal capacity of around 200 mg/g. The adsorbed Cr(VI) can be easily recovered through regeneration with NaCl solution. In addition, magnetic hydrogel can be separated magnetically within a few minutes due to its magnetic properties provided by the embedded γFe2O3 nanoparticles. These characteristics of magnetic hydrogel can be applied for developing an efficient and sustainable industrial wastewater treatment system, when coupled with magnetic separation. However, at this stage, only batch studies

1. INTRODUCTION Heavy metals are always featured in the treatment priority list and discharge standards, in regard to the impact on human health. Pollution is mainly caused by spillage, accidental leakage, or improper discharge of wastewater from various industrial processes.1−3 Considering that heavy metals are nonbiodegradable and possibly accumulate in organisms through the food chain, the discharge and release of heavy metals should be carefully handled and monitored. Particularly, chromium has been of great public concern for decades due to the highly toxic and carcinogenic properties of Cr(VI).4 Chromium is a widely used heavy metal, involved in many industrial processes, such as electroplating, stainless steel production, wood preservation, and paint and dye manufacture.1 Since chromium is useful in a wide range of industries and is a nonrenewable resource, chromium in industrial wastewater should be recovered and recycled for sustainable operations.5 Among a suite of industrial wastewater treatment technologies, chemical reduction and precipitation is one of the most widely adopted technologies for Cr(VI) removal.6,7 Cr(VI) is a strong oxidizing agent which reacts with various reducing chemicals such as Fe(II) and sulfite. During wastewater treatment, Cr(VI) is reduced to Cr(III) and then precipitated out by pH adjustment due to the low solubility of Cr(III) in alkaline conditions. Although the process is simple in operation and equipment requirement, excess chemicals are usually required to achieve the regulatory discharge standards. Consequently, a massive amount of chemical sludge is generated. Improper disposal of the chemical sludge can lead to secondary contamination of the soil and groundwater, since Cr(III) can be oxidized to Cr(VI) in natural environments.8,9 While it is difficult to recover chromium in the form of chemical sludge,10 a membrane separation process offers © 2014 American Chemical Society

Received: Revised: Accepted: Published: 15718

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have been performed due to the limited application of magnetic separation from batch to prototype or large-scale studies. Magnetic separation is a widely adopted method for collecting magnetic substances from flowing streams in industrial processes.16 High gradient magnetic separation (HGMS) is one of the common techniques for magnetic separation.17−20 In an HGMS, a pile of magnetically susceptible metal wires is installed inside an electromagnetic system, like a filter. To generate high magnetic field gradients around the wires, a magnetic field from the electromagnetic system is applied across the wires. The magnetic particles in the stream are captured when the flow crosses the magnetic field around the wires. The generation of a high magnetic field gradient is one of crucial factors for particle collection. To achieve efficient magnetic separation, the magnetic force generated from the magnetic field gradient should override other forces acting on the magnetic particles. The magnetic force acting on the magnetic particles is proportional to the product of the gradient and the intensity of the magnetic field and is inversely proportional to the size of the magnetic particles and the distance away from the magnetic field source. The recovery of magnetic particles after capture depends on the magnetic properties of the particles and the magnetic field strength. Ferromagnetic materials are magnetized after being captured by the magnetic field and are tightly held with the magnetic surface, even though the external magnetic field is removed. It was reported that magnetic nanoparticles were effectively removed by magnetic field but were hardly recovered and held tightly on the magnetic wires.21 In addition, the reusability of the recovered magnetic particles has not been reported, although numerous results showed the magnetic particles can be separated with hand-held magnets at the batch scale. In this study, different concentrations and amounts of regeneration solution were applied to investigate the most applicable regeneration condition for magnetic hydrogel. Two different regeneration approaches, single and stepwise treatment, were also tested. The removal and recovery performance of the magnetic hydrogel was studied in the most applicable regeneration condition for repeated cycles. To investigate the applicability of magnetic separation for magnetic hydrogel and its Cr(VI) removal and recovery performance, a wastewater treatment prototype with a magnetic separation unit was developed and used for adsorption-regeneration treatment cycles with the magnetic hydrogel.

Figure 1. Schematic of magnetic hydrogel synthesis.

hydrogel with a capacity of 205 mg/g, achieving equilibrium in 5 min. The Cr(VI) removal mechanism was found to be ion exchange.23 After Cr(VI) adsorbed, the spent magnetic hydrogel can be regenerated by NaCl solution. To mimic the Cr(VI) concentration in actual electroplating wastewater, the synthetic electroplating wastewater was prepared by a known quantity of K2Cr2O7 in ultrapure water to obtain a Cr(VI) concentration of 20 mg/L. For the adsorption process, the batch experiment was conducted by mixing 1 g/L magnetic hydrogel with 80 mL of synthetic electroplating wastewater by magnetic stirring for 15 min, followed by different adsorption or regeneration processes to investigate the treatment process performance. All batch experiments were performed in duplicate. The Cr(VI) concentration in the collected effluent and recovery sample was measured using a flame atomic absorption spectrometer (AAS, Varian 220FS). 2.2.1. Magnetic Hydrogel Regeneration Efficiency. In order to enhance the applicability of the magnetic hydrogel for industrial wastewater treatment, reducing the amount recovery solution and increasing the Cr(VI) concentration in the recovery solution are required. This was investigated by applying a higher concentration and a smaller amount of NaCl solution. After the adsorption process, effluent samples were collected by separating the magnetic hydrogel. Various concentrations (2.0, 3.0, and 4.0 M) and amounts (2, 4, and 8 mL) of NaCl were then used for the regeneration process. The recovery solution was collected by separating the magnetic hydrogel after stirring for 15 min. A set of regeneration tests was conducted with stepwise additions of NaCl solution. The recovery solution was collected after 15 min, followed by another addition of the NaCl solution. The details of the experimental condition are shown in Table 1. The recovery samples from the stepwise addition were mixed before measurement. The regeneration efficiency was calculated by

2. MATERIALS AND METHODS 2.1. Materials and Chemicals. (3-Acrylamidopropyl)trimethylammonium chloride (APTMCl) (75 wt % solution in water), N,N′-methylenebisa-crylamide (MBA), N,N,N′,N′tetramethylethylenediamine (TEMED), and potassium persulfate (KPS) were purchased from the Aldrich Chemical Co., Inc. for hydrogel synthesis. The γ-Fe2O3 nanoparticles (10 nm) were laboratory made as described by Wang and Lo22 and were imbedded into the hydrogel to provide the hydrogel with magnetic properties. Magnetic hydrogel was synthesized, as illustrated in Figure 1, via radical polymerization of APTMCl as the monomer and MBA as the cross-linker, as described by Tang et al.15 The chemical stock solutions were prepared by dissolving laboratory grade chemicals from the Aldrich Chemical Co., including K2Cr2O7 and NaCl, and then diluting to the desired concentrations using ultrapure water. 2.2. Batch Experiments. It was reported by Tang et al.15 that Cr(VI) can be effectively removed by the magnetic

R × Vre × 100% (C0 − Cf ) × Vw

where C0 is the initial Cr(VI) concentration of the synthetic wastewater, Cf is the Cr(VI) concentration of the effluent, Vw is the volume of the treated synthetic wastewater, R is the Cr(VI) concentration of the recovery solution, and Vre is the volume of the recovery solution. To further reduce the amount and concentrate the recovered Cr(VI) solution, a set of experiments was performed with another approach by a stepwise adsorption process, followed by the most applicable regeneration condition based on the results of experiments mentioned above. In brief, after the first 15719

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Table 1. Experimental Conditions of AdsorptionRegeneration Batch Experiment process

applied solutions

vol.a (mL)

wastewater-to-recovery ratio (v/v)

adsorption regeneration

20 mg/L Cr(VI) 2 M NaCl

80 2 4 8 2/(1 + 1) 4/(2 + 2) 8 2 4/(2 + 2)

n/a 40:1 20:1 10:1 40:1 20:1 10:1 40:1 20:1

3 M NaCl

4 M NaCl a

The numbers in brackets indicate stepwise addition of NaCl for sequential regeneration. Figure 2. Schematic explanation of the wastewater treatment prototype with a magnetic separation unit.

adsorption process treating 80 mL of synthetic wastewater, followed by separation, another 80 mL of synthetic wastewater was applied for the second adsorption process. Step-wise regeneration was then performed with NaCl solution. The details of the experimental condition are shown in Table 2.

setup. For the magnetic separation unit, an electromagnetic system was incorporated in the bottom of the separation unit for providing the magnetic field. The magnetic field strength of the electromagnetic system was ∼200 mT (measured by a PHYWE digital teslameter with a Hall probe). Details of the magnetic separation unit are referred to in the Chinese Patent Application No. 201310049470.2(2) and the Hong Kong Patent Application No. 13112634.2. For each adsorption-regeneration cycle, the magnetic hydrogel suspension was introduced into the stirring tank reactor by a vacuum pump and mixed with 5 L of synthetic electroplating wastewater containing 20 mg/L Cr(VI), where the final concentration of the magnetic hydrogel in the wastewater was 1 g/L. The mixture was stirred for 15 min to ensure adsorption equilibrium and then transferred to the magnetic separation unit. The mixture flowed along a zigzag path and was kept in the magnetic separation unit for 5 min to separate the magnetic hydrogel. The treated wastewater was then discharged. The effluent was collected for Cr(VI) measurement to determine the Cr(VI) removal efficiency of each cycle. The separation efficiency of the magnetic separation unit was determined by turbidity measurement according to

Table 2. Experimental Conditions of a Step-Wise Adsorption and Regeneration Batch Experiment process

applied solutions

vol.a (mL)

wastewater-to-recovery ratio (v/v)

adsorption desorption

20 mg/L Cr(VI) 3 M NaCl

(80 + 80) (2 + 2)

n/a 40:1

a

The numbers in bracket indicate stepwise addition of solutions for sequential processes.

2.2.2. Adsorption-Regeneration Cycle. To investigate the Cr(VI) removal efficiency and the magnetic hydrogel regeneration performance in the long run, a 20-cycle adsorption-regeneration experiment was performed using the most applicable adsorption-regeneration condition, on the basis of the findings of the previous section. After each cycle of adsorption-regeneration, the magnetic hydrogel was thoroughly washed to neutrality with ultrapure water and then used in the succeeding cycle. The details of the experimental condition are also shown in Table 2. The regeneration efficiency of each cycle was calculated by

TU0 − TUi × 100% TU0

where TU0 is the turbidity of the magnetic hydrogel in 1 g/L and TUi is the turbidity of the effluent collected after magnetic separation in the ith cycle. It has been found that the concentration of the magnetic hydrogel was in a linear relationship of turbidity (Figure 3). After discharging the treated wastewater and switching off the magnetic field of the magnetic separation unit, the magnetic hydrogel was recovered for the regeneration process by flushing with 100 mL of 3.0 M NaCl which was the wastewater-to-recovery volume ratio determined in the previous batch study. The regeneration efficiency was determined using the same method as for the batch study. The regenerated magnetic hydrogel was then washed to neutrality with ultrapure water and then used in the succeeding cycle.

R i + 1 × Vre × 100% (C0 − Ci + 1) × Vw + (C0 − Ci) × Vw − (R i × Vre)

where C0 is the initial Cr(VI) concentration of the synthetic wastewater, Ci is the Cr(VI) concentration of the effluent of the ith cycle, Vw is the volume of the treated synthetic wastewater, Ri is the Cr(VI) concentration of the recovery solution of the ith cycle, and Vre is the volume of the recovery solution. (C0 − Ci+1) × Vw is the amount of Cr(VI) adsorbed onto the magnetic hydrogel. (C0 − Ci) × Vw − (Ri × Vre) is the remaining Cr(VI) in the magnetic hydrogel after the previous regeneration step. 2.3. Prototype Experiments on Industrial Wastewater Treatment with Magnetic Separation. A magnetic separation study was performed in a wastewater treatment prototype with an electromagnetic system. The wastewater treatment prototype included a 5 L stirring tank reactor and a 5 L magnetic separation unit. The units were connected with pipes and valves. Figure 2 shows the schematic experimental

3. RESULTS AND DISCUSSION 3.1. Magnetic Hydrogel Regeneration Efficiency. The results of the regeneration test are shown in Table 3, where the numbers in brackets indicate the results of the stepwise 15720

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regeneration at 20:1 wastewater-to-recovery volume ratio was selected as the most applicable condition. To further reduce the amount of the recovery solution and increase the Cr(VI) concentration in the recovery solution, another approach of adsorption-regeneration was studied. The regeneration process was the same as that mentioned above, but the adsorption process was changed to stepwise treatment. The removal efficiency was maintained at around 97%, even though the magnetic hydrogel was not regenerated after the first adsorption. This is probably due to the large removal capacity of Cr(VI), which was not saturated after the first adsorption and not affected by the adsorbed Cr(VI). After the two adsorption processes, a stepwise regeneration process was applied, accounting for a 40:1 wastewater-to-recovery volume ratio, since double the amount of wastewater was treated. The regeneration efficiency was found to be around 60%. Although the regeneration efficiency was lower than that for one step adsorption (70%), the Cr(VI) concentration in the recovery solution was higher, which resulted from accumulated Cr(VI) from the two steps of adsorption. 3.2. Adsorption-Regeneration Cycle. A stepwise adsorption process followed by a stepwise 3.0 M NaCl regeneration with a 40:1 wastewater-to-recovery volume ratio was selected as the most applicable working condition. The adsorptionregeneration process was carried out for 20 cycles to study the performance of the magnetic hydrogel in a relatively long run. Regarding the Cr(VI) removal performance, the effluent Cr(VI) concentration was maintained at around 0.45 and 0.60 mg/L after the first and second adsorption, respectively, showing 98% and 97% Cr(VI) removal efficiencies (Figure 4).

Figure 3. Relationship between turbidity and concentration of magnetic hydrogel.

Table 3. Regeneration Efficiency Using Various Dosages and Concentrations of NaCl wastewater-to-recovery ratioa (v/v) conc. of regeneration solution

10:1 (%)

20:1 (%)

40:1 (%)

2 M NaCl 3 M NaCl

80 ± 1.3 83 ± 1.4

55 ± 1.4 66 ± 1.6 (70 ± 1.3) 65 ± 1.5 (69 ± 1.2)

32 ± 2.5 39 ± 2.4 (40 ± 2.8) 39 ± 2.6

4 M NaCl a

The numbers in brackets indicate stepwise addition of NaCl for sequential regeneration.

regeneration. The Cr(VI) removal efficiency for all conditions was around 98% (data not shown), which is consistent with the previous study. The regeneration efficiency increases with increasing concentration of NaCl, because more Cl− is available for Cr(VI) exchange. With the increased amount of regeneration solution applied, a higher concentration of NaCl provides a higher concentration gradient between the aqueous phase and the solid phase; thus, more Cr(VI) can be exchanged. However, when the concentration of NaCl increased to 4.0 M, no significant improvement in the regeneration efficiency was observed. This is probably due to the high concentration of Cr(VI) present in the recovery solution after regeneration. Since the regeneration process is a reversible reaction, achieving equilibrium of ion concentration between the aqueous and solid phases, the high concentration of Cr(VI) in the recovery solution could limit further recovery. This is supported by the increase of regeneration efficiency with the increase in the amount of applied recovery solution (i.e, wastewater-to-recovery volume ratio). The regeneration efficiency improved from 39% to 66% on average for doubling the amount of applied regeneration solution (the wastewaterto-recovery volume ratio decreased from 40:1 to 20:1). The concentration of Cr(VI) in the recovery solution was lower when more regeneration solution was applied. To maintain a high concentration gradient and minimize the possible inhibitory effect of the Cr(VI) concentration in the recovery solution, stepwise regeneration was also studied. The recovery solution was separated after the first application of regeneration solution, followed by the second application of the fresh regeneration solution. The regeneration efficiency of the stepwise regeneration was slightly higher than the one step regeneration for the same wastewater-to-recovery ratio. In order to strike a balance between amount of recovery solution and the regeneration efficiency, 3.0 M NaCl with stepwise

Figure 4. Concentration of Cr(VI) in the effluent and Cr(VI) removal efficiency of adsorption steps 1 and 2 in 20 cycles of the adsorptionregeneration test.

During the regeneration process, the Cr(VI) concentration in the recovery solution was higher than 500 mg/L (Figure 5). The Cr(VI) concentration was concentrated more than 25 times, from 20 mg/L in the synthetic wastewater to higher than 500−600 mg/L in the recovery solution. The concentrated Cr(VI) solution can be recycled for industrial applications. 15721

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switching off the electromagnetic system and removing the partition walls. Concentrated magnetic hydrogel suspension is collected from the discharge port, followed by the regeneration process. This magnetic separation unit design facilitates the recovery of magnetic particles and maintains a high throughput. However, the magnetized surface is limited to the bottom of the magnetic separation unit. It should be noted that dimensions of the magnetic separation unit are case specific, depending on the dosage, magnetic properties, and particle size of the applied magnetic particles. 3.3.2. Prototype Experiments for Industrial Wastewater Treatment with Magnetic Separation. In order to investigate the treatment performance of magnetic hydrogel, experiments on industrial wastewater treatment using the magnetic hydrogel with the designed treatment system prototype coupled with the magnetic separation unit was conducted by treating 5 L of synthetic wastewater. Twenty cycles of adsorption-regeneration process were performed. After the adsorption process, the magnetic hydrogel was separated and recovered by the magnetic separation unit. A magnetic separation efficiency of 97% or even higher was achieved in a 5 min separation (Figure 6). The slightly lower separation efficiency in the first 4 cycles

Figure 5. Concentration of Cr(VI) in the recovery solution, Cr(VI) regeneration efficiency, and accumulation in the magnetic hydrogel in 20 cycles of the adsorption-regeneration test.

However, after the regeneration process, a portion of the adsorbed Cr(VI) remained in the magnetic hydrogel, since the regeneration efficiency was around 65% in the first cycle. For every cycle, Cr(VI) was adsorbed and accumulated in the magnetic hydrogel from cycle to cycle due to incomplete regeneration. Despite Cr(VI) accumulation, the Cr(VI) removal performance was maintained for 20 cycles and only slightly altered in the last 2 cycles. The amount of accumulated Cr(VI) almost reached 200 mg/g in the last treatment cycle. To lengthen the operation of the magnetic hydrogel, a thorough regeneration with a larger amount of NaCl should be undertaken before the 20th cycle to regain the removal capacity. This thorough cleanup of magnetic hydrogel depends on the industrial wastewater characteristics, the Cr(VI) concentration in the wastewater and effluent. 3.3. Prototype Experiments with Magnetic Separation. 3.3.1. Magnetic Separation Unit Design. A 5 L industrial wastewater treatment prototype, coupled with a magnetic separation unit, was developed to investigate how the magnetic hydrogel performs in an industrial wastewater treatment process. The magnetic field was provided by an electromagnetic system which was installed at the bottom of the magnetic separation unit. There are several partition walls of various heights in the magnetic separation unit. These partition walls divide the separation unit into several chambers, which can also direct the flow of wastewater in a zigzag path. When the treated wastewater is introduced into the magnetic separation unit, the first chamber is gradually filled up and then overflows into the next chamber. Almost the whole separation unit, except the outlet part, is covered by the magnetic field. This design enhances the capture of the magnetic hydrogel by providing more time for the magnetic hydrogel to stay in the magnetic field. The magnetic hydrogel is attracted and captured by the magnetic field at the bottom of the magnetic separation unit. To recover the magnetic hydrogel, clear effluent is discharged, when the magnetic hydrogel is retained at the bottom of the magnetic separation unit. Recovery of the magnetic hydrogel can be easily achieved by flushing with NaCl solution after

Figure 6. Magnetic hydrogel separation efficiency in 20 cycles of the prototype experiment.

was probably due to the presence of fine magnetic hydrogel particles, produced during the synthesis of the magnetic hydrogel powder. These fine hydrogel particles may contain an insufficient amount of embedded magnetic nanoparticles, leading to lower capture by the magnetic field. The separation efficiency was maintained at around 98%. For the Cr(VI) removal performance, the Cr(VI) concentration in the effluent gradually increased from around 0.5 to 0.7 mg/L in the first 3 cycles and then remained constant at around 0.7 mg/L until the 20th cycle (Figure 7). The corresponding removal efficiency was 96% or higher for 20 treatment cycles. The slight variation in removal performance was probably due to the gradual accumulation of Cr(VI) in the magnetic hydrogel. Efficient Cr(VI) removal can be achieved for 20 cycles. The magnetic hydrogel was regenerated with 3.0 M NaCl, and the Cr(VI) concentration in the recovery solution was around 180 mg/L (Figure 8). Since the adsorbed Cr(VI) cannot be thoroughly recovered, accumulation of Cr(VI) occurred in the magnetic hydrogel (Figure 8), which was also observed in the batch study. After 20 cycles, the Cr(VI) accumulated in the magnetic hydrogel was about 130 mg/g, which was around 65% of the total removal capacity of the magnetic hydrogel. It is predicted that 15 more cycles can be performed before reaching removal capacity saturation. 15722

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remove Cr(VI) and can also be separated and recovered by a magnetic separation unit in 20 cycles.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +852 2358 7157. Fax: +852 2358 1534. E-mail: cemclo@ ust.hk. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors wish to thank the Research Grants Council of the HKSAR Government for providing financial support under General Research Fund 617309 for this research study.

Figure 7. Concentration of Cr(VI) in the effluent and Cr(VI) removal efficiency of the magnetic hydrogel in 20 cycles of the prototype experiment.

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REFERENCES

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Figure 8. Concentration of Cr(VI) in the recovery solution, Cr(VI) regeneration efficiency, and Cr(VI) accumulation in the magnetic hydrogel in 20 cycles of the prototype experiment.

4. CONCLUSIONS Magnetic hydrogel can remove Cr(VI) effectively and can be regenerated using NaCl solution. To enhance the applicability with concentrating and reducing the amount of recovery solution, regeneration by applying various concentrations and amounts of NaCl was studied. A stepwise adsorption process was followed by stepwise 3.0 M NaCl regeneration, with 40:1 wastewater-to-recovery volume ratio being selected as the most applicable working condition. The Cr concentration in the recovery solution reached 500−600 mg/L with the stepwise adsorption and regeneration process, applying a 40:1 wastewater-to-recovery volume ratio. The Cr(VI) removal and recovery performance of magnetic hydrogel was maintained for 20 cycles with a 97−98% Cr(VI) removal efficiency. An industrial wastewater treatment prototype was developed, which consisted of a stirring tank reactor and a magnetic separation unit. The magnetic separation unit was designed with a zigzag pathway feature to maximize the magnetic surface contact. The magnetic field source was provided at the bottom of the magnetic separation unit. The results of the prototype experiment indicate that the magnetic hydrogel can effectively 15723

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(21) Yavuz, C. T.; Mayo, J. T.; Yu, W. W.; Prakash, A.; Falkner, J. C.; Yean, S.; Cong, L.; Shipley, H. J.; Kan, A.; Tomson, M.; Natelson, D.; Colvin, V. L. Low-field magnetic separation of monodisperse Fe3O4 nanocrystals. Science 2006, 314, 964−967. (22) Wang, P.; Lo, I. M. C. Synthesis of mesoporous magnetic γFe2O3 and its application to Cr(VI) removal from contaminated water. Water Res. 2009, 43, 3727−3734. (23) Lo, I. M. C.; Yin, K.; Tang, S. C. N. Combining material characterization with single and multi-oxyanion adsorption for mechanistic study of chromate removal by cationic hydrogel. J. Environ. Sci. 2011, 23, 1004−1010.

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