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Separation and Purification Technology 56 (2007) 249–256

Comparative study of various magneticnanoparticles for Cr(VI) removal

Jing Hu, Irene M.C. Lo, Guohua Chen ∗Environmental Engineering Program, School of Engineering, Hong Kong University of Science and Technology,

Clear Water Bay, Kowloon, Hong Kong, PR China

Received 22 October 2006; accepted 8 February 2007

bstract

In this study, various magnetic nanoparticles were prepared by chemical co-precipitation method and used for the removal of Cr(VI) fromynthetic electroplating wastewater. The size of these magnetic nanoparticles was measured using transmission electron microscopy (TEM) andound to be about 20 nm. Their magnetic properties were characterized by vibrating sample magnetometer (VSM). The technical feasibilityf magnetic nanoparticles for Cr(VI) removal was investigated in batch studies in acidic synthetic solution using 5 g/L of different magneticanoparticles and 100 mg/L of Cr(VI). The Cr(VI) removal performances were compared and the adsorption capacities followed the order:nFe2O4 > MgFe2O4 > ZnFe2O4 > CuFe2O4 > NiFe2O4 > CoFe2O4. Contact time required by all types of ferrite particles was relatively short,

anging from 5 to 60 min. Factors affecting adsorption including solution pH, shaking rate, and magnetic properties were further investigated. In

ddition, the desorption of Cr(VI)-adsorbed magnetic nanoparticles were conducted using 0.01 M NaOH as eluent and the desorption efficiencyas found to be more than 90%. While much lower amount of Cr(VI) desorbed from Cr-loaded MnFe2O4 nanoparticles using the same eluent due

o a chemical redox occurred during adsorption. 2007 Elsevier B.V. All rights reserved.

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eywords: Adsorption; Nanoparticle; Chromium; Desorption; Magnetic prope

. Introduction

Chromium and its compounds are widely used in plating,eather tanning, metal finishing, photography, and nuclear powerlants [1,2]. The effluents from these industries can containexavalent chromium in concentrations ranging from tens toundreds of mg/L [3]. Chromium has been put on the top prior-ty list of toxic pollutants by the USEPA and is present in aqueousystem in both the trivalent form (Cr3+) and the hexavalent formCr6+). Hexavalent chromium in wastewaters is primarily in theorms of oxyanions, such as chromates (CrO4

2−), dichromatesCr2O7

2−), and bichromates (HCrO4−). The discharge limit for

r(VI) for releasing into inland surface waters is 0.1 mg/L andn potable water is 0.05 mg/L [4]. With its high solubility, Cr(VI)

s very toxic to living organisms compared to Cr(III). If Cr isngested beyond the maximum concentration (0.1 mg/L), it canead to liver damage, pulmonary congestion, oedema and cause

∗ Corresponding author. Tel.: +852 23587138; fax: +852 23580054.E-mail address: kechengh@ust.hk (G. Chen).

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383-5866/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.seppur.2007.02.009

kin irritation resulting in ulcer formation. Therefore, it is neces-ary for these industries to reduce the chromium concentrationn their effluents to an acceptable level before discharging into

unicipal sewers. Besides, the recovery of chromium presentn wastewater is an attractive option for environmental and eco-omical reasons.

Among the available treatment methods, chemical redox fol-owed by precipitation has been considered the most commonechnique for the removal of Cr(VI) from wastewater [5]. How-ver, this system possesses the following disadvantages: highaste treatment equipment costs; large consumption of reagents;

ignificantly high volume of sludge generation; potential hazardso the environment due to landfill leaching; inefficient recoveryf treated metals for reuse [6]. Especially for the small-scalendustries in Hong Kong, the conventional method seems unsuit-ble for treating the metal-bearing wastewater due to the lack ofpace. In view of the growing concern of environmental issues,

lternative techniques must be found to handle the problem.rom various studies, adsorption processes have the potential tovercome many of the limitations associated with precipitation-ased treatment methods. For instance, high treatment efficiency

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ithout discharging any harmful by-products to treated waternd the effective recovery of adsorbed heavy metals can bexpected during adsorption system [7].

Magnetic separation has been gradually regarded as a rapidnd effective technique for separating magnetic particles [8,9].t has been used for many applications in biochemistry, celliology, analytical chemistry, mining, and environmental tech-ology [10]. The advantages of this separation technologyre that the harmful ingredients together with the mag-etic particles can be eliminated from the polluted systemy a simple magnetic field. After magnetic separation, thearmful components can be easily removed from the mag-etite particles, and the recovered magnetic particles can beeused.

Adsorption process combined with magnetic separation haseen used extensively in the processing of minerals and moreecently water treatment and environmental application [11].

agnetite or iron ferrite had been used to separate a wideariety of substances such as dissolved metal species, partic-late matter, organic and biological materials. Magnetite hashe ability to remove heavy metals as reported by Kochen andavratil [12] who used a magnetic polymer resin for the removalf actinides and other heavy metals from contaminated water.erromagnetic microparticles coated with octyl-N,N-diisobutylarbamoylmethylphosphine oxide (CMPO) were used to removehe transuranic nuclides from aqueous wastes [13]. The removalf lead and copper ions from diluted solution by sorption intolinoptilolite, together with magnetite, as reported by Fengt al. [14] showed that the process was very fast, effectivend yielded clear solutions with low residual concentrationsf heavy metals. Nanotechnology as a powerful platformor the 21st century technologies could substantially enhancenvironmental quality and sustainability through pollution pre-ention, treatment and remediation. In particular, applicationf magnetic nanoparticle technology to solve environmentalroblems had received considerable attention in recent years.ron sulphide nanoparticles produced by sulphate-reducing bac-eria were applied to remove Cd and Cu from wastewatery Watson and Cressey [15], and high removal capacity wasound. Watson and Croudace [16] investigated the adsorptionf radioactive metals which illustrated selective adsorption ofadioactive pertechnetate ion onto iron sulphide nanoparticles.he supported nanoscale zero-valent iron rapidly separated and

mmobilized Cr(VI) and Pb(II) from aqueous solution [17].his technology used extra-small superparamagnetic compositearticles, which were embedded with iron oxide nanoparticleshat did not lose their magnetic properties in the presence of a

agnetic field. Elliott and Zhang [18] performed a field demon-tration in which nanoscale bimetallic (Fe/Pd) particles wereravity-fed into groundwater contaminated by trichloroethenend other chlorinated aliphatic hydrocarbons at a manufacturingite.

Among all the magnetic materials abovementioned, most of

hem inevitably have the drawback of small adsorption capac-ty, and especially inefficient regeneration of the adsorbents,hich limits their practical application. To circumvent these

imitations, several kinds of magnetic nanoparticles with com-

moaw

Technology 56 (2007) 249–256

aratively high surface area have been prepared in our laboratorynd used for the Cr(VI) removal from aqueous solution. Ourarlier study reported the adsorption/desorption of Cr(VI) usinganoscale maghemite and the results demonstrated the possibil-ty of recovery of metal and regeneration of adsorbent particles19,20]. To look for a cheaper and more effective magneticdsorbent for Cr removal, another series of nanoscale ferritesere considered. Such group of magnetic materials has been ofreat importance for many potential technological applications,anging from information storage and electronic devices to mag-etic fluid and magnetically guided drug delivery [21]. However,hey have rarely been considered a potential adsorbent for heavy

etal removal from wastewater, which may be due to their com-aratively large size. In this study, six kinds of ferrites, MeFe2O4Me = Mn, Co, Cu, Mg, Zn, Ni) were synthesized in nanoscalesing a simple method and tested for the adsorption/desorptionf Cr(VI) from synthetic wastewater. Furthermore, the effectsf magnetic properties on the removal of Cr(VI) werenvestigated.

. Experimental

.1. Materials

In this study, all chemicals are reagent grade. Ferric chlo-ide hexahydrate and ferrous nitrate tetrahydrate were obtainedrom Sigma–Aldrich (USA). Manganese(II) nitrate tetradydratend nickel(II) nitrate tetradydrate were obtained from Riedel-eHaen (Germany). Zinc(II) nitrate tetrahydrate, magnesium(II)itrate tetrahydrate, cobalt(II) nitrate tetrahydrate, and cop-er(II) nitrate trihydrate were obtained from Nacalai-TesqueJapan). Milli-Q ultrapure water was used for the preparationf all kinds of solutions.

.2. Preparation of magnetic nanoparticles

All the magnetic nanoparticles studied were synthesizedsing chemical co-precipitation method in our laboratory. First,00 mL of purified, deoxygenated water was bubbled by nitro-en gas for 30 min, and the Me(NO3)2 (Me = Mn, Co, Cu, Mg,n, Ni) and Fe(NO3)3 salts with a molar ratio of 1:2 were suc-essively dissolved in ultrapure water with vigorous mechanicaltirring. Under the protection of nitrogen gas, the mixture waseated up to 70 ◦C in a water bath and then 2 M NaOH was addedrop-wise into the above solution till pH 11. To ensure completerowth of the nanoparticle crystals, the reaction was kept con-inually at 70 ◦C for 2 h. After that, the stirrer was switchedff and magnetic particles settled gradually. The precipitate wassolated by an external magnetic field and the supernatant wasecanted. To obtain pure and neutral pH products, synthesizedaterials were rinsed again with ultrapure water and the rinseas discarded as before. The rinsing was repeated for a third

ime and the magnetic nanogel was then freeze-dried. Finally,

agnetic nanoparticles such as CoFe2O4 and MnFe2O4 were

btained. To produce nanoscale MgFe2O4, ZnFe2O4, CuFe2O4nd NiFe2O4 particles, subsequent calcination at 400 ◦C for 2 has needed to assure the complete crystallization. The reaction

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J. Hu et al. / Separation and Pu

ccurred is as follows:

FeCl3 + MeCl2 + 8NaOH = MeFe2O4 + 8NaCl + 4H2O

(1)

To produce MgxCu1−xFe2O4 (0 < x < 1) particles, Mg(NO3)2,u(NO3)2 and Fe(NO3)3 salts with a molar ratio of x:(1 − x):2ere first dissolved in 200 mL ultrapurified deoxygenated waterith vigorous mechanical stirring, and the subsequent proce-ures followed those of MgFe2O4 synthesizing. The reactionccurred is as Eq. (2):

xMgCl2 + (1 − x)CuCl2 + 2FeCl3 + 8NaOH

= MgxCu1−xFe2O4 + 8NaCl + 4H2O (2)

.3. Analysis methods

Characterizations of the magnetic nanoparticles were carriedut using X-ray diffractometer (XRD) for crystal identifica-ion, TEM for size investigation, BET method for surfacerea measurement, VSM for magnetic behavior analysis, and-ray photoelectron spectroscopy (XPS) for the analysis of ele-ental composition and chemical oxidation state of surface

nd near-surface species. XRD (PW-1830) was used for thenvestigation of elemental information and structure of synthe-ized materials at ambient temperature. The, instrument wasquipped with a copper anode generating Cu K� radiationλ = 1.5406 A). A bright-field TEM (JEOL-2010) was used forhe size measurement of the magnetic nanoparticles. To preparehe sample for TEM measurement, a copper grid (200 meshnd covered with carbon) was coated with a thin layer ofiluted magnetic particle suspension. The copper film was thenried in a desiccator for 24 h before the measurement. Surfacerea measurements were performed by BET method at liquiditrogen temperatures using conventional gas adsorption appa-atus. Wet magnetic particles were freeze-dried for 24 h andhen used for VSM (7037/9509-P) measurement. XPS (PHI-600) measurements were made with an Mg K� X-ray source1253.6 eV) at a constant retard ratio of 40. The zeta poten-ial of various magnetic nanoparticles obtained at different pHas determined using a zeta potential analyzer (ZETA PLUS,rookhaven Instruments Corporation). The concentration ofhromium was measured by an inductively coupled plasma-tomic emission spectrometer (ICP-AMS, OPTIMA 3000XL,ERKIN ELMER).

.4. Batch adsorption/desorption studies

A chromium sample was prepared by dissolving a knownuantity of potassium dichromate (K2CrO4) in ultrapure waternd used as a stock solution. The batch adsorption experimentsere carried out on a rotary shaker using 125 mL conical flasks

nder standard conditions (pH of 2.0, shaking rate of 400 rpm,emperature of 298 K and 5 g/L magnetic nanoparticles) excepttherwise stated. The mixture can be separated via external mag-etic field and the final concentration of chromium was measured

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tion Technology 56 (2007) 249–256 251

y ICP. All experiments were performed in duplicate and theveraged values were presented.

Adsorption kinetic studies were carried out by shaking 50 mLf 100 mg/L Cr(VI) solution with different kinds of magneticanoparticles individually. Volumes of 5 mL of samples wereaken at certain time interval for Cr measurement in solu-ion. Since Cr(VI) exists as oxyanions (CrO4

2−, Cr2O72− and

CrO4−) in the solution, acidic to basic pH was tested to find

he optimal Cr(VI) adsorption. Standard acid 0.1 M HNO3 andase 0.1 M NaOH solutions were used for pH adjustment. About0 mL of 100 mg/L Cr(VI) solutions with different pH rangingrom 2.0 to 9.0 were shaken with 0.1 g magnetic nanoparticles,imultaneously. The mixing rate affects the mass transfer inhe bulk; the shaking rates of the rotary shaker ranging from00 to 800 rpm were tested to establish the value when massransfer effect is insignificant. The effect of mixing rate wasonducted and ambient temperature (22.5 ◦C) at pH of 2.0.he effect of magnetic properties was investigated by mix-

ng 20 mL of 100 mg/L Cr(VI) solution with 5 g/L of variousgxCu1−xFe2O4 or MgFe2O4 nanoparticles under optimal con-

itions. After the separation of the particles, the Cr(VI) contentn the supernatant was measured.

For desorption studies, various Cr-loaded ferrite nanopar-icles were first washed by ultrapure water to remove thenadsorbed Cr attaching to the vial and adsorbent. The treatedarticles were then shaken with 5 mL of 0.01 M NaOH for 24 h.fter desorption equilibrium was reached, the particles were

eparated and the supernatant was diluted and acidified using.2% HNO3 for metal concentration measurements.

. Results and discussion

.1. Characterization of magnetic nanoparticles

Images of the prepared particles using TEM were obtaineds shown in Fig. 1. MeFe2O4 surface morphology analysisemonstrated the agglomeration of many ultrafine particlesith diameter of about 20 nm. XRD diffraction characterization

howed broad diffraction peaks from the powder sample thatrecipitated out of the solution. As for CoFe2O4 and MnFe2O4ynthesized at 70 ◦C, the sample was identified as pure compo-ent; while the other samples produced at 400 ◦C were identifieds a mixture of MeO, Fe2O3 and MeFe2O4 in crystalline state.ake MgFe2O4 as an example, XRD image, Fig. 2, showed

hat MgFe2O4 particles produced at relatively lower tempera-ure (e.g. 400 ◦C) contain less MgO and Fe2O3 impurities; aftereating the sample in air at high temperature (e.g. 800 ◦C), theixture was continually transformed into crystalline MgFe2O4

anoparticles that contained Fe2O3 impurity of less than 1%y weight. All the surface physical characteristics of variousagnetic nanoparticles are summarized in Table 1. The ratio

f cation distribution equals to 1:2 deduced from XPS investi-ation suggested the high purity of the synthesized materials,

hile the ratio of less than 1:2 revealed the presence of moree2O3 impurity on the surface of synthesized ferrite particles,hich is due to the incomplete dehydration and crystallization at

omparatively lower calcination temperature during the synthe-

252 J. Hu et al. / Separation and Purification Technology 56 (2007) 249–256

Fig. 1. TEM images of various magnetic nanoparticles.

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Fig. 2. X-ray diffraction patterns of MgFe2O4 nanoparticles.

is process. The points of zero charge pHpzc for various magneticanoparticles were measured to be around 7.0–8.0. BET resultsuggested the comparatively higher surface area of nanoscale

articles, especially for nanoscale MnFe2O4.

It is known that magnetic particles of less than 30 nmill exhibit superparamagnetism [22]. Therefore, the preparedagnetic nanoparticles have superparamagnetic properties and

able 1agnetization, BET surface area and Fe content for the prepared particles

errite Saturationmoment (emu/g)

pHzpc Me:Fe (fromXPS)

BET surfacearea (m2/g)

oFe2O4 3.7 6.9 1:2.0 55.1gFe2O4 1.1 7.3 1:2.5 70.3

nFe2O4 1.2 8.2 1:2.4 79.6iFe2O4 2.2 8.0 1:2.1 101.2uFe2O4 3.2 6.5 1:2.2 93.8nFe2O4 4.6 6.8 1:2.1 180.0

itiosFctctdniMa

ig. 3. Magnetic moment vs. applied magnetic field for: (a) MgxCu1 − xFe2O4

ynthesized at 400 ◦C; (b) MgFe2O4 synthesized at different temperatures.

re expected to respond well to magnetic fields without anyermanent magnetization. The superparamagnetic propertiesf the magnetic particles were verified by the magnetizationurve measured by VSM. Hysteresis loop as applied mag-etic field changes strength and direction, which were observedn the results of VSM, presents the saturation magnetizationnd coercivity. It is observed from Table 1 that the magneticroperties of the MnFe2O4 nanoparticles were the highest com-ared to the other ferrites, while the MgFe2O4 nanoparticleshowed the lowest magnetic properties. The large saturationagnetization of magnetic particles makes them very suscep-

ible to magnetic field, and therefore makes the solid andiquid phases separate easily. These measurements indicatedhat the co-precipitation method produced MeFe2O4 particleshat have magnetic properties very similar to bulk values. Tonvestigate the effect of the ratio of metals and calcinationemperature on the magnetic properties, the magnetism of var-ous MgxCu1−xFe2O4 particles were measured. Typical plotsf magnetization versus applied magnetic field (M–H loop) forynthesized MgxCu1−xFe2O4 particles at 400 ◦C are shown inig. 3(a). The magnetization curve exhibits zero remanence andoercivity, and follows the Langevin function [23], which proveshat magnetic particles have superparamagnetic properties. Byomparing their saturation moments, Mg0.2Cu0.8Fe2O4 illus-rated the highest magnetic properties; while Mg0.9Cu0.1Fe2O4emonstrated the lowest magnetic properties. Hence, the mag-

etic properties for various MgxCu1−xFe2O4 increased with thencrease in the content of Cu. The hysteresis loops of different

gFe2O4 particles synthesized at 400, 600, 800, and 900 ◦Cre shown in Fig. 3(b). It is found that the magnetic properties

rification Technology 56 (2007) 249–256 253

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J. Hu et al. / Separation and Pu

f MgFe2O4 particles increased with increasing the calcinationemperature.

.2. Adsorption kinetics

The Cr(VI) removal as a function of contact time wastudied at the optimum conditions and the results are presentedn Fig. 4. It is observed that the percentage adsorption ofr(VI) increased with an increase in contact time and grad-ally reached constant within less than 60 min. The Cr(VI) rem-val efficiency followed the decreasing order: MnFe2O4 >gFe2O4 > ZnFe2O4 > CuFe2O4 > NiFe2O4 > CoFe2O4. The

quilibrium time for Cr uptake by MnFe2O4, MgFe2O4,nFe2O4, CuFe2O4, NiFe2O4 and CoFe2O4 nanoparticles is, 45, 30, 20, 15 and 60 min, respectively. It is evident that Crdsorption onto MnFe2O4 particles reached equilibrium in thehortest time compared to the other ferrites, which may be dueo the rapid redox reaction occurred between the Cr speciesnd the external adsorbent surface. The second strongestagnetic particles, CoFe2O4 indicated the longest adsorption

ime for reaching equilibrium, since longer contact time maye needed to separate the aggregated nanoparticles and thenix themselves with the Cr anions. On the other hand, when

omparing the BET surface area data from Table 1 with thedsorption time listed above, it is not difficult to find that theigher the surface area, the shorter the adsorption equilibriumime. Since the available active sites for nanoparticles are

ostly present outside of the surface, higher surface areaeans more adsorption sites for Cr(VI). For a fixed number

f adsorbate Cr ions and nanoparticles with relatively higherurface area, the ratio of the initial number of Cr(VI) to thedsorption sites of adsorbent becomes lower. Therefore, mostf the Cr ions can be adsorbed onto the exposed active sitesaster and thereby shortening the equilibrium time. On thehole, the contact time required by all types of ferrite particlesas relatively short, ranging from 5 to 60 min, implying that

he most surface active sites reside on particles external surface.he result is encouraging, as equilibrium time is one of the

mportant parameters for an economical wastewater treatment

lant. It is also noticed that nanoscale MnFe2O4 and NiFe2O4urpassed our previous studied nanoparticles such as Fe3O4nd �-Fe2O3 on the adsorption time. Furthermore, the Cr(VI)dsorption efficiency of MnFe2O4 is much higher than that of

Fig. 4. Effect of contact time on Cr removal by various ferrites.

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Fig. 5. Effect of pH on Cr removal by various ferrites.

e3O4 and �-Fe2O3 nanoparticles [19,20]. The study of thisype of nanoparticles was carried out in more details and theesults were reported elsewhere [24].

.3. Effect of pH

Knowledge of the optimum pH is very important since pHot only affects the surface charge of adsorption, but also theegree of ionization and speciation of adsorbate during reac-ion. To examine the effect of pH on the adsorption efficiencyf Cr(VI), the pH of the solution was varied from 2.0 to 9.3.ig. 5 shows that the Cr removal by various ferrite nanoparti-les decreased sharply as the pH increased from 2 to 9.3. Theffect of increasing pH on anion adsorption can be explainedy the following reasons: (1) the surface charge is neutral atHpzc, which is 7.0–8.0 for all these ferrites. Below the pHpzc, allf the adsorbent ferrite surface is positively charged (MeOH2+

nd MeOH groups), and anion Cr(VI) existing as HCrO4− and

rO42− adsorption occurred; (2) at pH higher than pHpzc, the

dsorption surface is negatively charged, increasing electrostaticepulsion between negatively charged Cr(VI) species and neg-tively charged adsorbent particles would result in a release ofhe adsorbed HCrO4

− and CrO42−. When electrostatics govern

he adsorption (i.e. non-specific adsorption), the surface mustave an overall positive charge in order for Cr(VI) adsorptiono take place. In contrast, where specific adsorption is involvede.g. ligand exchange), an overall positive surface charge is notequired, which explains why adsorption can occur at pH abovehe pHpzc. By comparison with previous data, it is found thathe Cr(VI) removal by Fe3O4 and �-Fe2O3 nanoparticles alsoollowed this trend with solution pH. Finally, it should be notedhat acidic pH was especially favored for the occurrence of thehemical redox reaction between Cr(VI) and Mn(II) during thedsorption process, thus the adsorption of Cr(VI) onto MnFe2O4ecreased more sharply with higher solution pH compared to thether ferrites.

.4. Effect of shaking rate

Since an optimum shaking rate is essentially required to max-mize the interactions between metal ions and adsorption sitesf adsorbent in the solution, the effect of shaking rate on Crdsorption was investigated in this part. At pH 2.0, the per-

254 J. Hu et al. / Separation and Purification

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ig. 6. Effect of shaking rate on Cr removal by MgxCu1−xFe2O4 nanoparticles.

entage removal of Cr(VI) from 20 mL of 100 mg/L K2CrO4olution by 0.1 g magnetic nanoparticles under various shak-ng rates are shown in Fig. 6. It is found that Cr(VI) removalfficiency gradually increased with an increase in shaking ratesrom 100 to 400 rpm and then remained almost constant with fur-her increasing the shaking rate till 800 rpm. This phenomenonan be explained by the fact that, for a relatively lower shakingate, the system is incompletely mixed; hence the poor disper-ion of nanoparticles in solution resulted in only a small portionf surface area of adsorbent being exposed and reacted with ther(VI) ions. With further increasing the shaking rate from 400 to00 rpm, the effect of shaking rate on the Cr adsorption became

omparatively insignificant; since the system was well mixednder a comparatively higher shaking rate, say 400 rpm. As aonsequence, a shaking rate of 400 rpm was selected as the opti-um value for all types of ferrites mentioned here and used for

mC(p

Fig. 7. Effect of magnetic properties on Cr(VI) removal

Technology 56 (2007) 249–256

he remaining adsorption studies. While for Fe3O4 and �-Fe2O3anoparticles we studied before, a relatively lower shakingate, 200 rpm is enough for a good mixing between the parti-les and the bulk fluid. As most of the ferrites studied showedtronger magnetic properties than Fe3O4 and �-Fe2O3 particles,higher mechanical shaking force may be needed to disperse

he magnetic particles through destroying their mutual magneticorce.

.5. Effect of magnetic properties

Combining the data from Table 1 and Fig. 4, the Cr(VI)dsorption efficiency by different kinds of magnetic nanopar-icles did not show any trend with the change of their magneticroperties. Take CoFe2O4 particles as an example, the adsorp-ion efficiency of Cr(VI) onto them is the lowest comparedo those of the other magnetic particles, although their mag-etic properties are the second highest. The possible explanationor such phenomenon is that the other factors such as surfacerea, particle morphology, and available activate sites, etc. maylso influence the Cr adsorption, apart from magnetic proper-ies. On the other hand, the effect of magnetic properties ofhe same kind of ferrites on the removal of Cr(VI) was alsonvestigated. Take MgxCu1−xFe2O4 (0 ≤ x ≤ 1) as an example,he magnetic properties can be varied by changing the ratiof Mg/Cu, as can be seen from Fig. 3. Since Mg(II) is non-

agnetic while Cu(II) is magnetic, the higher the content ofu, the stronger the magnetic properties of MgxCu1−xFe2O4

Fig. 7(a)). Besides, calcination temperature during synthesisrocess can influence the magnetism of certain ferrites (Fig. 3).

by: (a), (b) MgxCu1 − xFe2O4; (c), (d) MgFe2O4.

rification Technology 56 (2007) 249–256 255

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J. Hu et al. / Separation and Pu

s regards for MgFe2O4 (MgxCu1−xFe2O4, when x = 1), theagnetic properties increased with increasing calcination tem-

erature (Fig. 7(b)). To examine the relationship betweenagnetic properties and Cr adsorption efficiency of nanoscalegxCu1−xFe2O4, 100 mg/L Cr(VI) solution was shaken withg/L of various synthesized MgxCu1−xFe2O4 particles underptimal conditions until equilibrium and the results are shown inig. 7. It is observed that the Cr adsorption efficiency decreasedith increasing magnetic properties, through increasing the

ontent of Cu or calcination temperature. Furthermore, linearelationship exists between the magnetic properties and thedsorption efficiency. This phenomenon can be explained byhe following three reasons: (1) the increasing magnetic prop-rties resulted in the enhancing agglomeration of magneticanoparticles and thereby lowering the surface area, leadingo a lower Cr(VI) removal percentage; (2) the Cr(VI) removalecreased with an increase in copper content since CuFe2O4howed the lower adsorption capacity than MgFe2O4 (Fig. 4);3) higher Fe2O3 impurity among synthesized ferrite causedhe higher Cr(VI) removal, as Fe2O3 showed much strongerffinity for Cr(VI) than ferrite [25]. The content of impuritye2O3 decreased with raising calcination temperature, due to

he complete crystallization state at higher heating conditions26].

To directly investigate the effect of magnetic properties onhe separation of ferrite nanoparticles from solution, the mix-ure of various ferrite particles and Cr(VI) was separated withn external magnet. It is observed that the ferrites with high mag-etic properties could be separated from solution more rapidlyhan those with weaker magnetism. As an example, MnFe2O4as completely isolated from the mixture within 0.5 min, whilegFe2O4 was completely isolated from solution within around

0 min. Taken together, the comparatively high magnetic prop-rties could enhance the separation of magnetic nanoparticles;n the contrary impairing their adsorption capacity. It is verymportant to strike a balance between magnetic properties anddsorption capacity of ferrites.

.6. Desorption studies

Since Cr(VI) adsorption is an reversible physical process, its possible for the regeneration or activation of the adsorbent,hich can be considered for reuse. The adsorption of Cr(VI)nto MeFe2O4 nanoparticles is highly pH-dependent; hence theesorption of Cr(VI) can be accomplished by increasing theolution pH. An 0.01 M of NaOH revealed the highest desorp-ion efficiency compared to other concentrations of NaOH or theame concentration of other eluents such as NaHCO3, Na2CO3nd Na3PO4 [19]. The desorption efficiency of Cr-adsorbederrite using 0.01 M NaOH is illustrated in Fig. 8. By compar-son, desorption of adsorbed-Cr from MnFe2O4 particles usinghe same eluent showed the lowest desorption efficiency, basedn the chemical reaction occurred between reducing Mn(II)

nd oxidizing Cr(VI). The other nanoscale ferrites, however,xhibited higher desorption efficiency by more than 90%. Therimary objective of desorption is to restore the adsorptionapacity of exhausted adsorbent while the secondary objective

refM

Fig. 8. Desorption efficiency of Cr-adsorbed ferrites.

s to recover valuable metals present in the adsorbed phase,f any. It should be noticed that when undergoing desorptionrocess, the initial 100 mg/L of Cr(VI) ions were concentratednto a smaller volume and thereby the final concentration of Creached as high as 500 mg/L. The high concentration of Cr(VI)olutions can be thus considered for recycling to industrialurposes.

. Conclusion

Ferrite nanoparticles had been successfully produced by theo-precipitation method. The formation of MeFe2O4 nanopar-icles was confirmed by the XRD. The TEM studies confirmedhat the ferrite particles prepared have fairly uniform structurend a mean particle size of around 20 nm. The magnetic prop-rties of all the synthesized magnetic particles were verifiedsing VSM. By comparison of saturation moment, it is observedhat the magnetic properties of MgxCu1−xFe2O4 increasedith increasing ratio of Cu/Mg, as well as the calcination

emperature.It is conclusively evident from batch adsorption studies

hat the use of some magnetic nanoparticles (e.g. MnFe2O4nd MgFe2O4) for the Cr(VI) removal is technically feasi-le, environmental-friendly, and economically attractive for thereatment of Cr-contaminated wastewater. Comparing to con-entional separation, the advantages of adsorption followed byagnetic separation are attributed to its rapidness, efficiency,

nd simplicity. Adsorption of Cr(VI) by all types of nanoscaleerrites reached equilibrium within 1 h. The removal efficiencyas highly pH-dependent and the optimal adsorption occurred

t pH 2. Cr(VI) adsorption efficiency increased with the shak-ng rates from 100 to 400 rpm but remained almost constanthereafter. At the same initial Cr concentration (100 mg/L),mong all types of ferrite nanoparticles, MnFe2O4 nanoparti-les stands out for having the highest Cr adsorption efficiency99.5%) and the shortest adsorption time. Thus, MnFe2O4 haseen considered the most potential magnetic adsorbent for theapid removal of Cr(VI). When considering reuse of ferrite and

ecovery of Cr(VI), the recovery efficiency of MnFe2O4 is, how-ver, the lowest compared to other ferrites. This is due to theact that chemical redox reaction occurred between Cr(VI) and

n(II).

2 cation

A

K

R

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56 J. Hu et al. / Separation and Purifi

cknowledgement

The authors are grateful to the financial support from Hongong Research Grant Council with project RGC600606.

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