Journal of Chemical Technology and Biotechnology J Chem Technol Biotechnol 80:20–27 (2005)DOI: 10.1002/jctb.1142

Removal of water-soluble azo dyeby the magnetic material MnFe2O4Rongcheng Wu and Jiuhui Qu∗SKLEAC, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China

Abstract: Magnetic ferrite material, MnFe2O4, as a novel adsorbent was prepared and characterized.Adsorption tests indicated that it is an excellent adsorbent for the removal of the azo dye Acid Red B(ARB) from water. After adsorbing ARB and recovery by the magnetic separation method, it can beregenerated by Fenton’s reagent. The pseudo-first-order and second-order kinetic models were used todescribe the kinetic data and the rate constants were evaluated. The adsorption capacity was highly affectedby the pH of the solution, and pH 3.8 was optimal. After regeneration, the adsorption capacity of MnFe2O4

increased significantly, which was the result of a decrease in average pore diameter, an increase in surfacearea of the adsorbent and the adsorption of ferric hydroxide produced in the regeneration reaction. Theadsorption can be described with the Langmuir model and the maximum adsorption capacity for ARB was53.8 mg g−1 adsorbent. FTIR study for ARB on MnFe2O4 indicated that the adsorption of ARB occurredvia the azo group and the sulfonic group of the dye through the formation of a complex with the adsorbentsurface. 2004 Society of Chemical Industry

Keywords: adsorption; regeneration; Acid Red B; MnFe2O4; magnetic separation

INTRODUCTIONThe effluents of various wastewaters from differentsources vary significantly in quantity and composi-tion, and some of them are highly toxic and non-biodegradable. These kinds of wastewater cannot betreated with conventional microbiological technolo-gies, so several chemical and physical processes areneeded. Adsorption and chemical catalytic oxidationhave been among the promising methods for removinghazardous and environmentally undesirable chemicalssuch as chlorinated hydrocarbons and azo dyes.1–4

Although adsorption can occur on a variety ofsurfaces, few materials are known to possess adsorptiveefficiencies or reactive surfaces sufficiently favorablefor adsorbing these organic compounds. These includeactivated carbon, zeolites, clays and some metaloxides.4–6 It is well known that the smaller the particlesof adsorbent or catalyst, the better its propertiesare, and the more difficult the solid/liquid separationis. In the form of powder, zeolites and clays havebetter adsorption properties, but one encounters theproblems of solid/liquid separation and regeneration.Currently, the most widely used material as adsorbentfor water treatment is high-surface-area activatedcarbon. Activated carbon, especially in the powderform possesses some unique adsorption properties,but it also presents some problems including the fact

that it is expensive. The adsorbate molecules are oftennot destroyed or decomposed but instead are only heldat the surface so that the powdered activated carbonbecomes hazardous waste that should be treated,but its recovery is difficult and the regeneration iscomplicated.

There is a growing interest in inexpensive high-surface-area materials, especially metal oxides, andin their unique applications including adsorption andchemical catalysis.

It is known that, in natural environments, iron andmanganese oxides, which have relatively high surfacearea and surface charge, often regulate free metal andorganic matter concentration in soil or water throughadsorption reactions.7 Many researchers have appliediron and manganese oxides to the treatment of heavymetals and organic matter from water.8–11 However,when these oxides are used as fine powders orhydroxide flocculation or colloid gel, they retain theirdesirable adsorptive properties of high capacity andrapid adsorption rate, but the solid/liquid separationis fairly difficult.

Recently, some researchers have developed tech-niques for coating iron oxide onto a sand surface inorder to overcome the difficulties of using iron oxidepowders in the water treatment process, and showedsome good results.12,13 But the adsorption capacity of

∗ Correspondence to: Jiuhui Qu, SKLEAC, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing100085, ChinaE-mail: jhqu@mail.rcees.ac.cnContract/grant sponsor: National Science Fund for Distinguished Young Scholars; contract/grant number: 50225824(Received 2 July 2003; revised version received 16 June 2004; accepted 21 June 2004)Published online 27 September 2004

2004 Society of Chemical Industry. J Chem Technol Biotechnol 0268–2575/2004/$30.00 20

Removal of water-soluble azo dye by MnFe2O4

iron-coated sand was fairly low compared with thatof fine powders of iron oxides, and therefore a largevolume adsorption unit was needed to achieve highremoval efficiency.

So the development of new adsorbents andseparation technologies to overcome the difficulty ofseparating fine particles from water to enhance theapplication of powder adsorbents in water treatmentis attractive.

The magnetic separation technique is a high speedand effective method for separating magnetic particlesand has been used for many applications in biochem-istry, microbiology, cell biology, analytical chemistry,mining ores and environmental technology.14,15 Thus,if the powder adsorbent is magnetic, it could be recov-ered by magnetic separation technology. There is someinformation in the literature on the application ofFe3O4 as magnetic adsorbent particles for removingheavy metal ions and some organic pollutants,14,16

but its adsorption capacity is low, and Fe3O4 can beoxidized and the preparation is complicated.

The MnO–Fe2O3 composite, manganese ferrite,possesses some good magnetic properties, and has thespecific saturation magnetism of about 40 emu g−1,which means it can be conveniently separatedand recovered by magnetic separation technology.Furthermore, it is stable and easy to prepare withcheap reagents. Thus, the use of powder MnO–Fe2O3

composite as adsorbent may combine the excellentadsorption properties of iron and manganese oxidespowders and the high effective recovery of manganeseferrite by the magnetic separation process in watertreatment.

In addition, advanced oxidation processes (AOPs)are often used directly in a raw water matrix fordecomposing organic compounds,17 but they oftenhave the disadvantage of incomplete mineralization ofsome contaminants and some more toxic substancesmay be produced, resulting in secondary pollution ofthe treated water. Furthermore, it seems that the lowerthe concentration of contaminants is, the higher thecost for the mineralization of a defined amount oforganic compounds.

Herein, we report the results of our recent studieson the removal of the azo dye Acid Red B (ARB), as arepresentative, from water by powder MnFe2O4 andpre-concentrating on the magnetic powder adsorbent;after solid/liquid magnetic separation, the contaminanton the adsorbent was mineralized by Fenton’s reagentin a smaller volume unit and the adsorbent was

regenerated at the same time. To evaluate theeffectiveness of the magnetic powder MnFe2O4 inremoving pollutants from water, powder MnFe2O4

was prepared and characterized, and the adsorptionproperties and reusability for the removal of the azodye Acid Red B (ARB) from water were investigated.It can be used for water treatment, as described inFig 1.

EXPERIMENTALMaterialsMnFe2O4 was prepared by a co-precipitationmethod. Mn(II) sulfate (0.01 mol) and Fe(III) sul-fate (0.02 mol) were dissolved in 100 cm3 of distilledwater. Under vigorous magnetic-stirring, the pH wasslowly raised to about 10 by adding 10% NaOHsolution, stirring was continued for 30 min, and thenstopped. The suspension was heated at 95–100 ◦C for2h. After cooling, the prepared magnetic adsorbentwas repeatedly washed with distilled water. By mag-netic separation, the solid was separated from waterand dried at 50 ◦C for 2h and at 110 ◦C for 3h. Thedry material was crushed and then calcined at 300 ◦Cfor 1h.

Acid Red B was purchased from Beijing ChemicalsCo (Beijing, China) and used without furtherpurification. Its structure is shown in Scheme 1.Deionized water was used throughout this study.

MethodsThe crystalline structure of MnFe2O4 was determinedusing the X-ray powder diffraction method with aRigaku III/B MAX diffractometer using Ni-filteredCuKα radiation.

Specific saturation magnetization (σs), which is ameasure of a particle’s magnetism, was determinedusing a VSM model 155 magnetic meter.

Scanning electron microscopy/energy dispersive X-ray (SEM/EDAX) was carried out by means ofHitachi S-3500N scanning electron microscope withan EDAX KEVEX Level 4 instrument.

N N

OH

SO3Na

SO3Na

Scheme 1.

Recovery of Adsorbent byMagnetic Separation

Regenerated Adsorbent

Regeneration of Adsorbentby Advanced Oxidation

ARB/Adsorbent

RawWater Adsorption by Adsorbent Magnetic Separation

Product Water

Figure 1. Use of MnFe2O4 in water treatment.

J Chem Technol Biotechnol 80:20–27 (2005) 21

R Wu, J Qu

FTIR infrared diffuse reflectance infrared Fouriertransform spectra of the azo dye ARB and magneticadsorbent were determined by a Nicolet 670 FTIRspectrophotometer.

BET area, pore diameter and pore volumewere determined by an ASAP2000 surface analyzer(Micromeritics Co, USA) with N2 as the adsorbate.The particle size of the adsorbent was determined bya Mastersizer 2000 (Malvern Co) instrument.

The concentration of dissolved Fe3+ and Mn2+ wasdetermined by atomic absorption spectrophotometry(Hitachi, Z-6100).

Adsorption studies were carried out in glass vesselswith agitation provided by a shaker. The temperaturewas controlled at 25 ◦C by an air bath. The pHwas adjusted by addition of HCl or NaOH. Thesuspension containing 0.200 g adsorbent and varyingamounts of ARB was shaken on an orbit shakerat 140 rpm. Samples were taken at different timeintervals. The concentration of ARB was determinedby a spectrophotometer (U-3010, Hitachi Co) at516 nm and by a total organic carbon (TOC) analyzer(Phoenix 8000, Tekmar Dohrmann Co, USA) afterfiltration using a 0.45 µm membrane.

For the adsorption rate experiments, the solutionpH was at 3.8. For the experiments investigatingthe adsorption properties at different pH values, theconcentration of ARB was 100 mg dm−3. Adsorptionisotherm experiments were conducted using thecompletely mixed batch reactor bottle-point method,and the solution pH was at 3.8.

Adsorbent was recovered by a magnetic separationprocess. After adsorption or regeneration, the adsor-bent was separated and recovered from water witha permanent magnet of 40 mm diameter and 10 mmthickness, made with Nd–Fe–B (2300 Gauss).

After magnetic separation, the collected adsorbentloaded with ARB was regenerated by H2O2/Fe2+ in10 cm3 solution of molar ratio H2O2/Fe2+ = 50/1,and reacted for 3 h at pH values ranging from 2.0to 3.0 at 25 ◦C. The amount of H2O2 was excessive.The regenerated adsorbent was recovered by magneticseparation and followed by washing with 10 cm3 waterfor further tests.

RESULTS AND DISCUSSIONCharacterization of MnFe2O4

The XRD analysis indicated the spinel structure ofMnFe2O4, and the surface morphology analysis bySEM showed the agglomeration of many microfineparticles with diameter of about 200 nm, which ledto a rough surface and the presence of a porousstructure. The main characteristics of the MnFe2O4

adsorbent are presented in Table 1. It can be seen thatMnFe2O4 adsorbent has a surface area of 68.6 m2 g−1,a mesopore structure with average pore diameter9.02 nm and a fine particle size of 24.7 µm. Allthese characteristics are in favor of adsorption. Thespecific saturation magnetization, σs, is 42.8 emu g−1,

Table 1. Characteristics of MnFe2O4 adsorbent

Particlesize(µm)

Specificsaturation

magnetization(emu g−1)

Surfacearea

(m2 g−1)

Averagepore

diameter(nm)

Averagepore

volume(cm3 g−1)

24.7 42.8 68.6 9.02 0.12

and in the batch experiment, recovery of above99% for the magnetic adsorbent MnFe2O4 could beachieved with a permanent magnet (2300 Gauss).Therefore, the magnetic adsorbent MnFe2O4 wasable to be recovered efficiently by magnetic separationtechnology after adsorption or regeneration.

Adsorption propertiesAdsorption kinetics of ARB by MnFe2O4

The adsorption kinetic property of ARB by MnFe2O4

is important since it will be used to optimize themixing time of wastewater and adsorbent. The resultis illustrated in Fig 2. The plot represents the amountsof ARB adsorbed, qt, onto the MnFe2O4 versus time.It can be seen that the adsorption rate is very rapidduring the first 20 min, and the equilibrium wasachieved within 30 min. This may be due to the highcomplexation rate between the dye molecules andadsorbent, and it is also related to the fine particlesize of the adsorbent, average size 24.7 µm, and itsmesopore structure, as presented in Table 1. Thesmaller particle size was favorable for both the diffusionof dye molecules onto the active site of the adsorbentand the complexation between the dye molecules andadsorbent. As a result, a higher adsorption capacity wasrealized in a shorter adsorbing time. Thus, the contacttime of 120 min is adequate in the later adsorptionstudies.

The pseudo-first-order adsorption and the pseudo-second-order adsorption model were used to describethe kinetics of ARB uptake. The pseudo-first-orderrate expression is given as:18

dqt/dt = k1(qe − qt) (1)

0 20 40 60 80 100 120 140 160 180 2000

5

10

15

20

25

___

----- pseudo-2nd-order model

q t (

mg

g-1)

Time (min)

pseudo-1st-order model

Figure 2. Kinetics of ARB adsorption onto MnFe2O4 (the initialconcentration of ARB: 100 mg dm−3, 50 cm3; MnFe2O4: 0.200 g).

22 J Chem Technol Biotechnol 80:20–27 (2005)

Removal of water-soluble azo dye by MnFe2O4

where qe and qt (mg g−1) are the amounts of dyeadsorbed on adsorbent at equilibrium and at time trespectively, and k1 (min−1) is the rate constant of first-order adsorption. By applying the boundary conditionqt = 0 at t = 0, eqn (1) becomes:

ln(qe − qt) = ln qe − k1t (2)

The pseudo-second-order rate expression is givenas:18,19

dqt/dt = k2(qe − qt)2 (3)

where k2 (g mg−1 min−1) is the rate constant ofpseudo-second-order adsorption. Taking into accountthe initial sorption rate v0 (mg g−1 min−1):

v0 = k2q2e (4)

Equation (3) can be rearranged to obtain:

t/qt = 1/v0 + t/qe (5)

The values of v0 and qe can be determinedexperimentally by plotting t/qt versus t.

The results are listed in Table 2. It was found thatwhatever the kinetic equation used, the descriptionof the adsorption kinetics was satisfactory. However,the pseudo-second-order model was better able todescribe the adsorption, and this suggests that theoverall rate of the dye adsorption process appeared tobe controlled by the chemical process.20

Effect of pH on ARB adsorptionFigure 3 presents the adsorption of ARB on MnFe2O4

as a function of solution pH, using 50 cm3 of100 mg dm−3 ARB solution. The quantity adsorbed,q, was strongly dependent on pH. At first, q (mg ARBg−1 adsorbent) increased with the increase in pH andreached the maximum adsorption of 21.5 mg g−1 at pH3.8; and then, it decreased with the increase in pH; atthe condition of pH > 7, the adsorption capacity wasactually very low. This trend is different from the resultof other researchers who investigated the adsorptionof natural organic materials and azo dyes on pure ironoxides.8,21

The relationship between q and pH perhapsresults from the combined effects of pH on surfacecomplexation reactions and electrostatic interactionsbetween ARB and the oxide surface. In the reactionof adsorption, the charges of adsorbent and adsorbateplayed an important role. Reactions (6) and (7) showthat ARB species (H3L, H2L−, HL2−, L3−) andsurface species of oxides (>M–OH2

+, >M–O−,

0

5

10

15

20

25

2 3 4 5 6 7 8 9

pH

q (m

g g-1

)

Figure 3. Adsorption capacity of ARB on MnFe2O4 versus final pH(the initial concentration of ARB: 100 mg dm−3, 50 cm3; MnFe2O4:0.200 g).

>M–OH) varied with [H+], which was due to thedifferent degrees of ionization (proton transfer) ofARB and oxides surface at different pH values.

H3L−H+−−−⇀↽−−−+H+

H2L− −H+−−−⇀↽−−−+H+

HL2− −H+−−−⇀↽−−−+H+

L3− (6)

> M–OH+2

−H+−−−⇀↽−−−+H+

> M–OH−H+−−−⇀↽−−−+H+

> M–O− (7)

In the adsorption reactions, the more H2L−, HL2−,L3− and M–OH2

+ species, the higher the value of qis. The distribution of ARB ionized species in solutiondepends on the pKa values of the dye and the pHof the solution. For example, at higher pH moreprotons tended to deviate from the ARB or metaloxides surface, causing them to be negatively charged.Orange II has a similar structure to ARB. From theliterature value pKa1 ≈ 1 for the –SO3H group andpKa2 value of 11.4 for the naphthalene –OH of OrangeII,8 it can be deduced that at lower pH, more of thedye exists as H3L and the adsorbent surface oxidesexist as >M–OH2

+; at higher pH, the dye exists moreas HL2− or L3− and the adsorbent surface oxidesexist as >M–OH or >M–O−. Neither conditionfavors the occurrence of surface complex reactions orelectrostatic interactions between ARB and the oxidesurface. Therefore, q increased with the increasing pHat first, and then reached its maximum adsorptionability. Finally, it decreased with increasing pH.

The adsorption reaction is often related to thesurface charge of adsorbent particles and the surfacecharge can be indicated by the zeta potential. Theeffect of pH on zeta potentials of MnFe2O4 adsorbent

Table 2. Kinetic parameters for ARB adsorption onto the MnFe2O4 adsorbent

Pseudo-first-order Pseudo-second-order

qe (mg g−1) k1 (min−1) R2 qe (mg g−1) k2 (g mg−1 min−1) R2

20.6 0.221 0.993 21.6 0.0196 0.995

J Chem Technol Biotechnol 80:20–27 (2005) 23

R Wu, J Qu

-20

-15

-10

-5

0

5

10

2 3 4 5 6 7

pH

Zet

a po

tent

ial (

mV

)

MnFe2O4 ARB/MnFe2O4

Figure 4. Zeta potential–pH profiles of MnFe2O4 and ARB/MnFe2O4.

particles in the absence and presence of ARB is shownin Fig 4. It is observed that zeta potentials of MnFe2O4

adsorbent particles decrease with increasing pH forthe whole range studied. At pH < 4.8, the MnFe2O4

adsorbent particles were positively charged; and atpH > 4.8, the MnFe2O4 adsorbent particles werenegatively charged. On the other hand, ARB existed asH2L−, HL2− or L3− at pH > 3.8 Thus, higher pH didnot favor adsorption. The adsorbent particles loadedwith ARB exhibited more negative zeta potentials,which is similar to the results of other researchers whoinvestigated the adsorption of azo dyes and oxalateon iron oxides.8 The more negative zeta potentials ofthe adsorbent particles loaded with ARB indicated thenegative charge of ARB adsorbed. The adsorption ofthe ARB shifted the point of zero change (PZC) ofthe adsorbent from pH 4.8 to pH 3.8, at which thehighest adsorption occurred. It should be noted thatthe adsorption also occurred even at pH>4.8 where theMnFe2O4 adsorbent particles together with ARB werenegatively charged. This indicated the existence ofchemisorption, as mentioned in the previous section.

Influence of anions on the removal of ARB by MnFe2O4

Cl− and SO42− are common co-existing anions which

occur as well as dyes in wastewater. The effect of Cl−and SO4

2− on ARB removal by MnFe2O4 adsorbentis shown in Fig 5. It is observed that there wasno effect on ARB removal in the presence of Cl−at the concentration range of 0–0.2 mol dm−3 Cl−.But the effect of SO4

2− on the removal of ARBwas very significant. In the presence of SO4

2−at 0.001 mol dm−3, the removal of ARB droppedfrom 86% to about 63%; at concentration of0.01 mol dm−3 SO4

2−, the removal of ARB droppedto 28%. It also can be seen that at concentration of0.1 mol dm−3 SO4

2−, the removal of ARB dropped toabout 12%, and no further effect appeared with theincreasing concentration of SO4

2−.The results of this test implied that the adsorption

of SO42− on MnFe2O4 may be due to strong

chemisorption, which does not exist between Cl−and MnFe2O4, So, the effect of SO4

2− on ARBadsorption could be attributed to competition with

0.00 0.05 0.10 0.15 0.200

10

20

30

40

50

60

70

80

90

Rem

oval

of A

RB

(%

)

Anion concentration (mol dm-3)

Cl-

SO42-

Figure 5. Effect of anions on ARB removal by MnFe2O4 (the initialconcentration of ARB: 50 mg dm−3, 50 cm3; MnFe2O4: 0.200 g;equilibrium pH 3.8).

ARB for active sites on MnFe2O4. Also, Cl− was notable to compete with ARB for sorption because ofthe weaker interaction between Cl− and adsorbent,therefore, there was no effect on ARB adsorption.

Adsorption capacity of initial and regenerated adsorbentsat different ARB concentrationsH2O2/Fe2+, known as Fenton’s reagent, is a powerfuloxidative reagent which has been applied to theoxidation of aromatic compounds.22 But, the directapplication of Fenton’s reagent to raw water hasbeen limited by the generation of slurry, in whichthe precipitate of ferric hydroxide requires additionalseparation and disposal, which is difficult,23 andsome toxic substances may be produced in thereaction and result in secondary pollution of thetreated water. To overcome the disadvantage, organiccompounds should be removed from raw water, andthen oxidized in another unit, which can be performedwith circulation.

Magnetic powder MnFe2O4 adsorbent can be usedfor adsorbing organic compounds from raw waterand then conveniently separated by the magneticseparation technique. After that, Fenton’s reagent(H2O2/Fe2+) can be used for the oxidation ofadsorbed organic pollutants and the regeneration ofthe adsorbent.

The adsorption capacities of initial and regeneratedadsorbents for ARB at pH 3.8 are shown in Fig 6. Itcan be seen that the adsorption capability of MnFe2O4

increased after the regeneration by H2O2/Fe2+, andthis is very important if the adsorbent is to be usedrepeatedly. The adsorption data shown in Fig 6 werefitted by a linear Langmuir equation and Table 3shows the results. It was found that the adsorptiondata fitted the Langmuir adsorption model very well.Furthermore, it was a better fit with increase in thenumber of adsorption–regeneration cycles. Table 3also indicates that the maximum capacity increasedfrom 36.7 mg g−1 to 51.5 mg g−1, 51.8 mg g−1 and then53.8 mg g−1 with the increasing number of cycles.

24 J Chem Technol Biotechnol 80:20–27 (2005)

Removal of water-soluble azo dye by MnFe2O4

0

10

20

30

40

50

0 50 100 150 200

Ce (mg dm-3)

q (m

g g-1

)

cycle1 cycle 2 cycle 3 cycle 4

Figure 6. Adsorption isotherms of ARB on MnFe2O4 for differentcycles. pH = 3.8.

Table 3. Comparative Langmuir model parameters for ARB

adsorption on MnFe2O4 after different cycles

Cycle no qm (mg g−1) K (mg−1) R2

1 36.7 0.011 0.9642 51.5 0.047 0.9953 51.8 0.071 0.9974 53.8 0.502 0.998

For comparison with another common magneticmaterial, Fe3O4, research on the adsorption ofFe3O4 (particle size 12.4 µm) towards ARB and theregeneration of loaded Fe3O4 has been carried out inour work since no related reference data are available.It was found that the maximum adsorption capacityof MnFe2O4 towards ARB (53.8 mg g−1) was muchhigher than that of Fe3O4 (32.4 mg g−1) at the sameadsorption and regeneration conditions.

This increase in adsorption capacity of MnFe2O4

is related to changes in the surface properties ofthe adsorbent, as presented in Table 4. Comparedwith initial states, the average pore diameter andBET surface area (m2 g−1) changed from 9.02 nmto 5.36 nm and from 68.6 m2 g−1 to 86.5 m2 g−1

respectively after regeneration. These changes may bein favor of adsorption. The EDAX spectrum analysisshowed that the ratios of O, Fe and Mn, as theprincipal elements on the surface of adsorbent, alsochanged from O:Fe:Mn = 2.87:1.91:1 to 4.50:2.38:1(mole ratio). The increase in the partition ratio of Feafter regeneration was attributed to the leaching of

Table 4. Characteristics of initial and regenerated MnFe2O4

Parameter Initial Regenerated

BET surface area (m2 g−1) 68.6 86.5Average pore diameter (nm) 9.02 5.36Pore volume (cm3 g−1) 0.15 0.12O (atom %) 49.72 57.04Fe (atom %) 32.98 30.24Mn (atom %) 17.30 12.71O: Fe: Mn 2.87:1.91:1 4.50:2.38:1

Mn and the adsorbing of ferric hydroxide generatedin Fenton’s reaction. The leaching of Mn intowater left cavities on the adsorbent surface, whichincreased the surface area and the ratio of Fe/Mn.Interestingly, it was found that the weight of adsorbentslightly increased after two cycles of regeneration.This was due to the adsorption of ferric hydroxideproduced in the regeneration reaction. Also, theferric hydroxides were usually ultrafine particles withhigh surface area and large adsorption capacity formany organic compounds, and when adsorbed ontothe adsorbent surface, this increased the adsorptioncapacity for ARB on the adsorbent. In a contrastingexperiment, when MnFe2O4 was firstly washed by0.1 mol dm−3 hydrochloric acid before adsorbingARB, the adsorption capacity increased only by about10% compared with the unwashed adsorbent. Thisindicated that the adsorption of ferric hydroxide onMnFe2O4 was more important for the increase inadsorption capacity after regeneration.

The TOC in the regeneration solution wasdetermined to be very low in the regeneration solution,indicating the mineralization of ARB by Fenton’sreagent. Iron oxide has been recently studied asa catalyst for oxidizing organic contaminants byhydrogen peroxide,24 and the catalytic property ofMnFe2O4 in the regeneration reaction needs to bestudied further.

FTIR spectra for ARB adsorption and adsorbentregenerationThe FTIR spectra of ARB, initial MnFe2O4, loadedand regenerated MnFe2O4 are shown in Fig 7. Thepeaks at 1602 and 1500 cm−1 are assigned to aro-matic C=C stretching vibration, while the peaks at1433 cm−1 and 1275 cm−1 correspond to –N=N–bond and –C–O– bond stretching vibration,8 respec-tively. The –SO3

− symmetric and asymmetric vibra-tions are shown at 1195 and 1365 cm−1. It canbe seen that peaks at 1433 cm−1 (–N=N–) and

11951275

1365

1433

1500

1602

a

1154

b

cd-0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

Abs

orba

nce

1200 1400 1600 1800

Wavenumbers (cm-1)

Figure 7. FTIR spectra for ARB adsorption and oxidation. a: ARB, b:ARB adsorbed on MnFe2O4, c: regenerated MnFe2O4, d: initialMnFe2O4.

J Chem Technol Biotechnol 80:20–27 (2005) 25

R Wu, J Qu

1365 cm−1 (–SO3−) almost disappeared for adsorbed

ARB on MnFe2O4, and the peak at 1195 cm−1 wasreplaced by a broad band at 1154 cm−1. This sug-gests the participation of both –N=N– and –SO3

−

groups in bond formation with the adsorbent surfaceand confirms the chemisorption of ARB and SO4

2−

on MnFe2O4 as mentioned in previous sections.It is noted that these peaks disappeared after regen-

eration and no evident absorption peaks appeared inthe range 1200–1600 cm−1. This indicates the com-plete oxidation of ARB by H2O2/Fe2+ and that thereis no residue of organic compounds on the surface ofthe adsorbent.

Metals dissolved in solutionFigure 8 presents the leaching of Fe and Mn duringthe adsorption studies. The Fe and Mn ions’concentrations in equilibrium adsorption solutionwere strongly dependent on pH with the greaterdissolution occurring at the lower pH. Besides, theMn was easier to dissolve than Fe and this was one ofthe reasons that the Mn/Fe atom ratio on the surfaceof regenerated adsorbent was less than that on theinitial surface. Also, it led to the increase in surfacearea and the decrease in average pore diameter afterregeneration.

Figure 8 confirms that the leaching of metals onlyoccurred in the first two or three adsorption cycles. In

(a) dissolved Fe

0

5

10

15

20

25

2 6 7

pH

Ce

(mg

dm-3

)

3 4 5 8

(b) dissolved Mn

0

10

20

30

40

50

2 4 5 6 8

pH

Ce

(mg

dm-3

)

cycle1 cycle2 cycle3 cycle4 cycle5

3 7

Figure 8. Amount of dissolved metals at different pH values.

the fourth cycle, the metal ion concentration was verylow (<0.5 mg dm−3) and had no effect on the effluent.

CONCLUSIONSIt has been proved in this study that powder MnFe2O4

was a very good adsorbent for the removal of ARB fromwater. The adsorption kinetic data can be describedby the second-order kinetic models. The adsorptioncapacity is related to the pH of the solution, andpH 3.8 is optimal. The loaded adsorbent can beefficiently regenerated with Fenton’s reagent. Afterregeneration of adsorbent, the adsorption capacityincreased significantly, which was the result of adecrease in average pore diameter and an increasein surface area of the adsorbent and the adsorption offerric hydroxide produced in the regeneration reaction.The leaching of metals only occurred in the first two orthree adsorption cycles; in the fourth cycle, the metalion concentration is very low.

The powder MnFe2O4 adsorbent consists ofmagnetic particles that could be recovered by themagnetic separation method. The adsorption capacityfor ARB was high, and the loaded adsorbent could beefficiently regenerated with Fenton’s reagent. Theseproperties allowed fast and efficient removal of ARBfrom a large volume of effluent by powder MnFe2O4

and pre-concentration on the magnetic powderadsorbent. After solid/liquid magnetic separation, thecontaminant on the adsorbent was mineralized byFenton’s reagent and the adsorbent was regeneratedat the same time. The quality of treated water was onlydependent on the adsorption efficiency.

ACKNOWLEDGEMENTThis research was supported by the National ScienceFund for Distinguished Young Scholars (Grant50225824).

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John Wiley and Sons Inc, New York (1996).2 Abbas K and Barry D, FTIR investigation of adsorption

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