sustainable wastewater treatment using microsized magnetic hydrogel with magnetic separation...

Sustainable Wastewater Treatment Using Microsized MagneticHydrogel with Magnetic Separation TechnologySamuel 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 andapplicability in Cr(VI)-bearing wastewater treatment using magnetic separation. Different concentrations and amounts of NaClsolution and a stepwise approach were used for the regeneration experiment. A stepwise adsorption process followed by stepwise3.0 M NaCl regeneration with a 40:1 wastewater-to-recovery volume ratio was found to be the most applicable workingcondition. The Cr concentration in the recovery solution was increased 25−30 times to 500−600 mg/L. The Cr(VI) removaland 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 atthe bottom with a zigzag pathway feature for maximizing the chance of capturing magnetic hydrogel. The separation efficiency forthe magnetic hydrogel was above 97% throughout the 20 cycles of treatment.

1. INTRODUCTION

Heavy metals are always featured in the treatment priority listand discharge standards, in regard to the impact on humanhealth. Pollution is mainly caused by spillage, accidentalleakage, or improper discharge of wastewater from variousindustrial processes.1−3 Considering that heavy metals arenonbiodegradable and possibly accumulate in organismsthrough the food chain, the discharge and release of heavymetals should be carefully handled and monitored. Particularly,chromium has been of great public concern for decades due tothe highly toxic and carcinogenic properties of Cr(VI).4

Chromium is a widely used heavy metal, involved in manyindustrial processes, such as electroplating, stainless steelproduction, wood preservation, and paint and dye manufac-ture.1 Since chromium is useful in a wide range of industriesand is a nonrenewable resource, chromium in industrialwastewater should be recovered and recycled for sustainableoperations.5

Among a suite of industrial wastewater treatment technol-ogies, chemical reduction and precipitation is one of the mostwidely adopted technologies for Cr(VI) removal.6,7 Cr(VI) is astrong oxidizing agent which reacts with various reducingchemicals such as Fe(II) and sulfite. During wastewatertreatment, Cr(VI) is reduced to Cr(III) and then precipitatedout by pH adjustment due to the low solubility of Cr(III) inalkaline conditions. Although the process is simple in operationand equipment requirement, excess chemicals are usuallyrequired to achieve the regulatory discharge standards.Consequently, a massive amount of chemical sludge isgenerated. Improper disposal of the chemical sludge can leadto secondary contamination of the soil and groundwater, sinceCr(III) can be oxidized to Cr(VI) in natural environments.8,9

While it is difficult to recover chromium in the form ofchemical sludge,10 a membrane separation process offers

another option for Cr(VI) removal. Membrane filtration canremove suspended particles, as well as inorganic pollutants likeheavy metals. Ultrafiltration, nanofiltration, and a reverseosmosis process can effectively remove heavy metals andconcentrate metal ions in the retentate stream;11,12 however,membrane filtration is usually pressure driven requiring high-pressure pumping.13 This could increase the operation cost dueto high energy consumption. The overall cost of usingmembrane separation is also relatively high, compared toother current treatment technologies. The fouling problem andchemical stability of the membrane are the challenges forapplying membrane filtration for industrial wastewater treat-ment, particularly when industrial wastewater possesses highionic strength and extreme pH.14 Therefore, an innovative andsustainable treatment technology, which is simple to operate, torecover Cr from industrial wastewater is necessary.A novel magnetic polymeric adsorbent, namely, magnetic

hydrogel, has been recently developed by Tang et al.15 It showsits advantages in fast Cr(VI) removal kinetics, reachingequilibrium in 5 min, and a high removal capacity of around200 mg/g. The adsorbed Cr(VI) can be easily recoveredthrough regeneration with NaCl solution. In addition, magnetichydrogel can be separated magnetically within a few minutesdue to its magnetic properties provided by the embedded γ-Fe2O3 nanoparticles. These characteristics of magnetic hydrogelcan be applied for developing an efficient and sustainableindustrial wastewater treatment system, when coupled withmagnetic separation. However, at this stage, only batch studies

Received: June 22, 2014Revised: August 31, 2014Accepted: September 9, 2014Published: September 9, 2014

Article

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© 2014 American Chemical Society 15718 dx.doi.org/10.1021/ie502512h | Ind. Eng. Chem. Res. 2014, 53, 15718−15724

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have been performed due to the limited application of magneticseparation from batch to prototype or large-scale studies.Magnetic separation is a widely adopted method for

collecting magnetic substances from flowing streams inindustrial processes.16 High gradient magnetic separation(HGMS) is one of the common techniques for magneticseparation.17−20 In an HGMS, a pile of magnetically susceptiblemetal wires is installed inside an electromagnetic system, like afilter. To generate high magnetic field gradients around thewires, a magnetic field from the electromagnetic system isapplied across the wires. The magnetic particles in the streamare captured when the flow crosses the magnetic field aroundthe wires. The generation of a high magnetic field gradient isone of crucial factors for particle collection. To achieve efficientmagnetic separation, the magnetic force generated from themagnetic field gradient should override other forces acting onthe magnetic particles. The magnetic force acting on themagnetic particles is proportional to the product of the gradientand the intensity of the magnetic field and is inverselyproportional to the size of the magnetic particles and thedistance away from the magnetic field source. The recovery ofmagnetic particles after capture depends on the magneticproperties of the particles and the magnetic field strength.Ferromagnetic materials are magnetized after being captured bythe magnetic field and are tightly held with the magneticsurface, even though the external magnetic field is removed. Itwas reported that magnetic nanoparticles were effectivelyremoved by magnetic field but were hardly recovered and heldtightly on the magnetic wires.21 In addition, the reusability ofthe recovered magnetic particles has not been reported,although numerous results showed the magnetic particles canbe separated with hand-held magnets at the batch scale.In this study, different concentrations and amounts of

regeneration solution were applied to investigate the mostapplicable regeneration condition for magnetic hydrogel. Twodifferent regeneration approaches, single and stepwise treat-ment, were also tested. The removal and recovery performanceof the magnetic hydrogel was studied in the most applicableregeneration condition for repeated cycles. To investigate theapplicability of magnetic separation for magnetic hydrogel andits Cr(VI) removal and recovery performance, a wastewatertreatment prototype with a magnetic separation unit wasdeveloped and used for adsorption-regeneration treatmentcycles with the magnetic hydrogel.

2. MATERIALS AND METHODS2.1. Materials and Chemicals. (3-Acrylamidopropyl)-

trimethylammonium chloride (APTMCl) (75 wt % solutionin water), N,N′-methylenebisa-crylamide (MBA), N,N,N′,N′-tetramethylethylenediamine (TEMED), and potassium persul-fate (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 wereimbedded into the hydrogel to provide the hydrogel withmagnetic properties. Magnetic hydrogel was synthesized, asillustrated in Figure 1, via radical polymerization of APTMCl asthe monomer and MBA as the cross-linker, as described byTang et al.15 The chemical stock solutions were prepared bydissolving laboratory grade chemicals from the AldrichChemical Co., including K2Cr2O7 and NaCl, and then dilutingto 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

hydrogel with a capacity of 205 mg/g, achieving equilibrium in5 min. The Cr(VI) removal mechanism was found to be ionexchange.23 After Cr(VI) adsorbed, the spent magnetichydrogel can be regenerated by NaCl solution. To mimic theCr(VI) concentration in actual electroplating wastewater, thesynthetic electroplating wastewater was prepared by a knownquantity of K2Cr2O7 in ultrapure water to obtain a Cr(VI)concentration of 20 mg/L. For the adsorption process, thebatch experiment was conducted by mixing 1 g/L magnetichydrogel with 80 mL of synthetic electroplating wastewater bymagnetic stirring for 15 min, followed by different adsorptionor regeneration processes to investigate the treatment processperformance. All batch experiments were performed induplicate. The Cr(VI) concentration in the collected effluentand recovery sample was measured using a flame atomicabsorption spectrometer (AAS, Varian 220FS).

2.2.1. Magnetic Hydrogel Regeneration Efficiency. In orderto enhance the applicability of the magnetic hydrogel forindustrial wastewater treatment, reducing the amount recoverysolution and increasing the Cr(VI) concentration in therecovery solution are required. This was investigated byapplying a higher concentration and a smaller amount ofNaCl solution. After the adsorption process, effluent sampleswere collected by separating the magnetic hydrogel. Variousconcentrations (2.0, 3.0, and 4.0 M) and amounts (2, 4, and 8mL) of NaCl were then used for the regeneration process. Therecovery solution was collected by separating the magnetichydrogel after stirring for 15 min. A set of regeneration testswas conducted with stepwise additions of NaCl solution. Therecovery solution was collected after 15 min, followed byanother addition of the NaCl solution. The details of theexperimental condition are shown in Table 1. The recoverysamples from the stepwise addition were mixed beforemeasurement. The regeneration efficiency was calculated by

×− ×

×R V

C C V( )100%re

0 f w

where C0 is the initial Cr(VI) concentration of the syntheticwastewater, Cf is the Cr(VI) concentration of the effluent, Vw isthe volume of the treated synthetic wastewater, R is the Cr(VI)concentration of the recovery solution, and Vre is the volume ofthe recovery solution.To further reduce the amount and concentrate the recovered

Cr(VI) solution, a set of experiments was performed withanother approach by a stepwise adsorption process, followed bythe most applicable regeneration condition based on the resultsof experiments mentioned above. In brief, after the first

Figure 1. Schematic of magnetic hydrogel synthesis.

Industrial & Engineering Chemistry Research Article

dx.doi.org/10.1021/ie502512h | Ind. Eng. Chem. Res. 2014, 53, 15718−1572415719

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

2.2.2. Adsorption-Regeneration Cycle. To investigate theCr(VI) removal efficiency and the magnetic hydrogelregeneration performance in the long run, a 20-cycleadsorption-regeneration experiment was performed using themost applicable adsorption-regeneration condition, on the basisof the findings of the previous section. After each cycle ofadsorption-regeneration, the magnetic hydrogel was thoroughlywashed to neutrality with ultrapure water and then used in thesucceeding cycle. The details of the experimental condition arealso shown in Table 2. The regeneration efficiency of each cyclewas calculated by

×− × + − × − ×

×+

+

R VC C V C C V R V( ) ( ) ( )

100%i

i i i

1 re

0 1 w 0 w re

where C0 is the initial Cr(VI) concentration of the syntheticwastewater, Ci is the Cr(VI) concentration of the effluent of theith cycle, Vw is the volume of the treated synthetic wastewater, Riis 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 themagnetic hydrogel. (C0 − Ci) × Vw − (Ri × Vre) is theremaining Cr(VI) in the magnetic hydrogel after the previousregeneration step.2.3. Prototype Experiments on Industrial Wastewater

Treatment with Magnetic Separation. A magneticseparation study was performed in a wastewater treatmentprototype with an electromagnetic system. The wastewatertreatment prototype included a 5 L stirring tank reactor and a 5L magnetic separation unit. The units were connected withpipes and valves. Figure 2 shows the schematic experimental

setup. For the magnetic separation unit, an electromagneticsystem was incorporated in the bottom of the separation unitfor providing the magnetic field. The magnetic field strength ofthe electromagnetic system was ∼200 mT (measured by aPHYWE digital teslameter with a Hall probe). Details of themagnetic separation unit are referred to in the Chinese PatentApplication No. 201310049470.2(2) and the Hong KongPatent Application No. 13112634.2.For each adsorption-regeneration cycle, the magnetic

hydrogel suspension was introduced into the stirring tankreactor by a vacuum pump and mixed with 5 L of syntheticelectroplating wastewater containing 20 mg/L Cr(VI), wherethe final concentration of the magnetic hydrogel in thewastewater was 1 g/L. The mixture was stirred for 15 min toensure adsorption equilibrium and then transferred to themagnetic separation unit. The mixture flowed along a zigzagpath and was kept in the magnetic separation unit for 5 min toseparate the magnetic hydrogel. The treated wastewater wasthen discharged. The effluent was collected for Cr(VI)measurement to determine the Cr(VI) removal efficiency ofeach cycle. The separation efficiency of the magnetic separationunit was determined by turbidity measurement according to

−×

TU TUTU

100%i0

0

where TU0 is the turbidity of the magnetic hydrogel in 1 g/Land TUi is the turbidity of the effluent collected after magneticseparation in the ith cycle. It has been found that theconcentration of the magnetic hydrogel was in a linearrelationship of turbidity (Figure 3). After discharging thetreated wastewater and switching off the magnetic field of themagnetic separation unit, the magnetic hydrogel was recoveredfor the regeneration process by flushing with 100 mL of 3.0 MNaCl which was the wastewater-to-recovery volume ratiodetermined in the previous batch study. The regenerationefficiency was determined using the same method as for thebatch study. The regenerated magnetic hydrogel was thenwashed to neutrality with ultrapure water and then used in thesucceeding cycle.

3. RESULTS AND DISCUSSION3.1. Magnetic Hydrogel Regeneration Efficiency. The

results of the regeneration test are shown in Table 3, where thenumbers in brackets indicate the results of the stepwise

Table 1. Experimental Conditions of Adsorption-Regeneration Batch Experiment

process applied solutions vol.a (mL)wastewater-to-recovery

ratio (v/v)

adsorption 20 mg/L Cr(VI) 80 n/aregeneration 2 M NaCl 2 40:1

4 20:18 10:1

3 M NaCl 2/(1 + 1) 40:14/(2 + 2) 20:18 10:1

4 M NaCl 2 40:14/(2 + 2) 20:1

aThe numbers in brackets indicate stepwise addition of NaCl forsequential regeneration.

Table 2. Experimental Conditions of a Step-Wise Adsorptionand Regeneration Batch Experiment

process applied solutions vol.a (mL)wastewater-to-recovery

ratio (v/v)

adsorption 20 mg/L Cr(VI) (80 + 80) n/adesorption 3 M NaCl (2 + 2) 40:1

aThe numbers in bracket indicate stepwise addition of solutions forsequential processes.

Figure 2. Schematic explanation of the wastewater treatmentprototype with a magnetic separation unit.



regeneration. The Cr(VI) removal efficiency for all conditionswas around 98% (data not shown), which is consistent with theprevious study. The regeneration efficiency increases withincreasing concentration of NaCl, because more Cl− is availablefor Cr(VI) exchange. With the increased amount ofregeneration solution applied, a higher concentration of NaClprovides a higher concentration gradient between the aqueousphase and the solid phase; thus, more Cr(VI) can beexchanged. However, when the concentration of NaClincreased to 4.0 M, no significant improvement in theregeneration efficiency was observed. This is probably due tothe high concentration of Cr(VI) present in the recoverysolution after regeneration. Since the regeneration process is areversible reaction, achieving equilibrium of ion concentrationbetween the aqueous and solid phases, the high concentrationof Cr(VI) in the recovery solution could limit further recovery.This is supported by the increase of regeneration efficiency withthe increase in the amount of applied recovery solution (i.e,wastewater-to-recovery volume ratio). The regenerationefficiency improved from 39% to 66% on average for doublingthe amount of applied regeneration solution (the wastewater-to-recovery volume ratio decreased from 40:1 to 20:1). Theconcentration of Cr(VI) in the recovery solution was lowerwhen more regeneration solution was applied. To maintain ahigh concentration gradient and minimize the possibleinhibitory effect of the Cr(VI) concentration in the recoverysolution, stepwise regeneration was also studied. The recoverysolution was separated after the first application of regenerationsolution, followed by the second application of the freshregeneration solution. The regeneration efficiency of thestepwise regeneration was slightly higher than the one stepregeneration for the same wastewater-to-recovery ratio. Inorder to strike a balance between amount of recovery solutionand the regeneration efficiency, 3.0 M NaCl with stepwise

regeneration at 20:1 wastewater-to-recovery volume ratio wasselected 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. Theregeneration 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%, eventhough the magnetic hydrogel was not regenerated after thefirst adsorption. This is probably due to the large removalcapacity of Cr(VI), which was not saturated after the firstadsorption and not affected by the adsorbed Cr(VI). After thetwo adsorption processes, a stepwise regeneration process wasapplied, accounting for a 40:1 wastewater-to-recovery volumeratio, since double the amount of wastewater was treated. Theregeneration efficiency was found to be around 60%. Althoughthe regeneration efficiency was lower than that for one stepadsorption (70%), the Cr(VI) concentration in the recoverysolution was higher, which resulted from accumulated Cr(VI)from the two steps of adsorption.

3.2. Adsorption-Regeneration Cycle. A stepwise adsorp-tion process followed by a stepwise 3.0 M NaCl regenerationwith a 40:1 wastewater-to-recovery volume ratio was selected asthe most applicable working condition. The adsorption-regeneration process was carried out for 20 cycles to studythe performance of the magnetic hydrogel in a relatively longrun. Regarding the Cr(VI) removal performance, the effluentCr(VI) concentration was maintained at around 0.45 and 0.60mg/L after the first and second adsorption, respectively,showing 98% and 97% Cr(VI) removal efficiencies (Figure 4).

During the regeneration process, the Cr(VI) concentration inthe recovery solution was higher than 500 mg/L (Figure 5).The Cr(VI) concentration was concentrated more than 25times, from 20 mg/L in the synthetic wastewater to higher than500−600 mg/L in the recovery solution. The concentratedCr(VI) solution can be recycled for industrial applications.

Figure 3. Relationship between turbidity and concentration ofmagnetic hydrogel.

Table 3. Regeneration Efficiency Using Various Dosages andConcentrations of NaCl

wastewater-to-recovery ratioa (v/v)

conc. of regenerationsolution 10:1 (%) 20:1 (%) 40:1 (%)

2 M NaCl 80 ± 1.3 55 ± 1.4 32 ± 2.53 M NaCl 83 ± 1.4 66 ± 1.6

(70 ± 1.3)39 ± 2.4(40 ± 2.8)

4 M NaCl 65 ± 1.5(69 ± 1.2)

39 ± 2.6

aThe numbers in brackets indicate stepwise addition of NaCl forsequential regeneration.

Figure 4. Concentration of Cr(VI) in the effluent and Cr(VI) removalefficiency of adsorption steps 1 and 2 in 20 cycles of the adsorption-regeneration test.



However, after the regeneration process, a portion of theadsorbed Cr(VI) remained in the magnetic hydrogel, since theregeneration efficiency was around 65% in the first cycle. Forevery cycle, Cr(VI) was adsorbed and accumulated in themagnetic hydrogel from cycle to cycle due to incompleteregeneration. Despite Cr(VI) accumulation, the Cr(VI)removal performance was maintained for 20 cycles and onlyslightly altered in the last 2 cycles. The amount of accumulatedCr(VI) almost reached 200 mg/g in the last treatment cycle.To lengthen the operation of the magnetic hydrogel, athorough regeneration with a larger amount of NaCl shouldbe undertaken before the 20th cycle to regain the removalcapacity. This thorough cleanup of magnetic hydrogel dependson the industrial wastewater characteristics, the Cr(VI)concentration in the wastewater and effluent.3.3. Prototype Experiments with Magnetic Separa-

tion. 3.3.1. Magnetic Separation Unit Design. A 5 L industrialwastewater treatment prototype, coupled with a magneticseparation unit, was developed to investigate how the magnetichydrogel performs in an industrial wastewater treatmentprocess. The magnetic field was provided by an electromagneticsystem which was installed at the bottom of the magneticseparation unit. There are several partition walls of variousheights in the magnetic separation unit. These partition wallsdivide the separation unit into several chambers, which can alsodirect the flow of wastewater in a zigzag path. When the treatedwastewater is introduced into the magnetic separation unit, thefirst chamber is gradually filled up and then overflows into thenext chamber. Almost the whole separation unit, except theoutlet part, is covered by the magnetic field. This designenhances the capture of the magnetic hydrogel by providingmore time for the magnetic hydrogel to stay in the magneticfield. The magnetic hydrogel is attracted and captured by themagnetic 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 themagnetic separation unit. Recovery of the magnetic hydrogelcan be easily achieved by flushing with NaCl solution after

switching off the electromagnetic system and removing thepartition walls. Concentrated magnetic hydrogel suspension iscollected from the discharge port, followed by the regenerationprocess. This magnetic separation unit design facilitates therecovery of magnetic particles and maintains a high throughput.However, the magnetized surface is limited to the bottom ofthe magnetic separation unit. It should be noted thatdimensions of the magnetic separation unit are case specific,depending on the dosage, magnetic properties, and particle sizeof the applied magnetic particles.

3.3.2. Prototype Experiments for Industrial WastewaterTreatment with Magnetic Separation. In order to investigatethe treatment performance of magnetic hydrogel, experimentson industrial wastewater treatment using the magnetic hydrogelwith the designed treatment system prototype coupled with themagnetic separation unit was conducted by treating 5 L ofsynthetic wastewater. Twenty cycles of adsorption-regenerationprocess were performed. After the adsorption process, themagnetic hydrogel was separated and recovered by themagnetic separation unit. A magnetic separation efficiency of97% or even higher was achieved in a 5 min separation (Figure6). The slightly lower separation efficiency in the first 4 cycles

was probably due to the presence of fine magnetic hydrogelparticles, produced during the synthesis of the magnetichydrogel powder. These fine hydrogel particles may containan insufficient amount of embedded magnetic nanoparticles,leading to lower capture by the magnetic field. The separationefficiency was maintained at around 98%.For the Cr(VI) removal performance, the Cr(VI) concen-

tration in the effluent gradually increased from around 0.5 to0.7 mg/L in the first 3 cycles and then remained constant ataround 0.7 mg/L until the 20th cycle (Figure 7). Thecorresponding removal efficiency was 96% or higher for 20treatment cycles. The slight variation in removal performancewas probably due to the gradual accumulation of Cr(VI) in themagnetic hydrogel. Efficient Cr(VI) removal can be achievedfor 20 cycles. The magnetic hydrogel was regenerated with 3.0M NaCl, and the Cr(VI) concentration in the recovery solutionwas 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 alsoobserved 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 themagnetic hydrogel. It is predicted that 15 more cycles can beperformed before reaching removal capacity saturation.

Figure 5. Concentration of Cr(VI) in the recovery solution, Cr(VI)regeneration efficiency, and accumulation in the magnetic hydrogel in20 cycles of the adsorption-regeneration test.

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



4. CONCLUSIONSMagnetic hydrogel can remove Cr(VI) effectively and can beregenerated using NaCl solution. To enhance the applicabilitywith concentrating and reducing the amount of recoverysolution, regeneration by applying various concentrations andamounts of NaCl was studied. A stepwise adsorption processwas followed by stepwise 3.0 M NaCl regeneration, with 40:1wastewater-to-recovery volume ratio being selected as the mostapplicable working condition. The Cr concentration in therecovery solution reached 500−600 mg/L with the stepwiseadsorption and regeneration process, applying a 40:1 waste-water-to-recovery volume ratio. The Cr(VI) removal andrecovery performance of magnetic hydrogel was maintainedfor 20 cycles with a 97−98% Cr(VI) removal efficiency. Anindustrial wastewater treatment prototype was developed,which consisted of a stirring tank reactor and a magneticseparation unit. The magnetic separation unit was designedwith a zigzag pathway feature to maximize the magnetic surfacecontact. The magnetic field source was provided at the bottomof the magnetic separation unit. The results of the prototypeexperiment indicate that the magnetic hydrogel can effectively

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

■ AUTHOR INFORMATION

Corresponding Author*Tel.: +852 2358 7157. Fax: +852 2358 1534. E-mail: [email protected].

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe authors wish to thank the Research Grants Council of theHKSAR Government for providing financial support underGeneral Research Fund 617309 for this research study.

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Figure 7. Concentration of Cr(VI) in the effluent and Cr(VI) removalefficiency of the magnetic hydrogel in 20 cycles of the prototypeexperiment.

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



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