Mn

SQa

b

h

••••

a

ARRAA

KFPHDA

1

gmdabia

0h

Journal of Hazardous Materials 270 (2014) 27–34

Contents lists available at ScienceDirect

Journal of Hazardous Materials

j o ur nal ho me pa ge: www.elsev ier .com/ locate / jhazmat

ussel-inspired polydopamine biopolymer decorated with magneticanoparticles for multiple pollutants removal

hengxiao Zhanga,∗, Yuanyuan Zhanga, Guoming Bib, Junshen Liua, Zhigang Wangb,iang Xua, Hui Xua, Xiaoyan Lia

School of Chemistry and Materials Science, Ludong University, Yantai 264025, Shandong Province, ChinaYantai Enironmental Monitoring Center, Yantai 264025, Shandong Province, China

i g h l i g h t s

The Fe3O4/PDA hybrid material was synthesized and characterized.The PDA polymer was firstly applied in environmental remediation.The Fe3O4/PDA exhibited high adsorption capacity for multiple pollutants.Removal efficiencies of pollutants with Fe3O4/PDA were pH dependent.

r t i c l e i n f o

rticle history:eceived 25 October 2013eceived in revised form 3 January 2014ccepted 16 January 2014vailable online 29 January 2014

eywords:e3O4 nanoparticleolydopamine polymereavy metal ion

a b s t r a c t

The polydopamine polymer decorated with magnetic nanoparticles (Fe3O4/PDA) was synthesized andapplied for removal of multiple pollutants. The resulted Fe3O4/PDA was characterized with elementalanalysis, thermo-gravimetric analyses, vibrating sample magnetometer, high resolution transmissionelectron microscope, Fourier transform infrared spectra, and X-ray photoelectron spectroscopy. The self-polymerization of dopamine could be completed within 8 h, and Fe3O4 nanoparticles were embedded intoPDA polymer. Superparamagnetism and large saturation magnetization facilitated collection of sorbentswith a magnet. Based on the catechol and amine groups, the PDA polymer provided multiple interactionsto combine with pollutants. To investigate the adsorption ability of Fe3O4/PDA, heavy metal ions and dyeswere selected as target pollutants. The adsorption of pollutants was pH dependent due to the variation

yedsorption

of surface charges at different solution pH. The removal efficiencies of cation pollutants enhanced withsolution pH increasing, and that of anion pollutant was just the opposite. Under the optimal solutionpH, the maximum adsorption capacity calculated from Langmuir adsorption isotherm for methyleneblue, tartrazine, Cu2+, Ag+, and Hg2+ were 204.1, 100.0, 112.9, 259.1, and 467.3 mg g−1, respectively. TheFe3O4/PDA shows great potential for multiple pollutants removal, and this study is the first applicationof PDA polymer in environmental remediation.

© 2014 Elsevier B.V. All rights reserved.

. Introduction

With developing of modern industry and agriculture, areat amount of pollutants have been released to environ-ent. Extensive attention has been focused on water pollution

ue to its close relationship to human health. Water pollut-nts, including heavy metal and toxic organic pollutants, can

e taken into human body through drinking, exposure, or

ntake of aquatic product, which will lead to various physicalnd mental diseases, for example, cancer, tumor, anemia and

∗ Corresponding author. Tel.: +86 535 6672176; fax: +86 535 6695905.E-mail addresses: beijingzsx@163.com, lduzsx@126.com (S. Zhang).

304-3894/$ – see front matter © 2014 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.jhazmat.2014.01.039

psychosis. To eliminate water pollution, a variety of methodshave been studied and put into practice, such as precipitation,adsorption, membrane filtration, coagulation and flocculation,flotation, catalysis, electrochemical, and biological treatment.Among them catalytic oxidation and adsorption are regardedas promising technologies due to high efficiency and lowcost.

Generally, the treatment of low concentration pollutant is costlyusing contemporary technologies. So nanomaterials with highsurface area and special properties have been developed as high-

efficiency catalyst and sorbent, but the difficulty of solid–liquidseparation limits their practical application. Magnetic nanoma-terials possessing paramagnetism, which can be separated fromsolution by using an external magnetic field, draw extensive

2 ardous Materials 270 (2014) 27–34

aibimvptnocmdpl

ssippafoasoactfiRciei[ap

naiAuiaitt

2

2

gac((stCR

S

N

N+N

methylene blue

S N N

N

N

OH

S

O

O

NaO

NaO

ONa

OO

O

8 S. Zhang et al. / Journal of Haz

ttentions in the field of environmental remediation [1,2]. Var-ous surface engineered or hybrid magnetic nanomaterials haveeen prepared for catalytic degradation of toxic dye and organ-

cs [3–6] or adsorption of organic pollutants [7–14] and heavyetals [15–22]. Effective though these methods are, many disad-

antages still remain to be overcome. For example, the preparationrocess of some nanomateral is complicated, and reaction condi-ion is required rigidly. Meanwhile, costly and toxic reagents areeeded sometimes, and the toxic byproduct should lead to sec-ndary pollution. In addition, multiple pollutants are commonlyoexisted in actual polluted water, but most reported magneticaterials are often applied for single pollutant. So it is urgent to

evelop magnetic material with simple and environmental friendlyreparation method and simultaneously effective for multiple pol-

utants.Mussels are promiscuous fouling organisms and have been

hown to attach to virtually all types of inorganic and organicurfaces. It has been revealed that mussels’ adhesive versatil-ty lies in the amino acid composition of proteins found nearlaque-substrate interface, which are rich in 3,4-dihydroxy-l-henylalanine (DOPA) and lysine amino acids [23]. Inspired by thedhesive proteins secreted by mussel for attachment to wet sur-aces, Lee et al. [24] identified dopamine as a small-molecule mimicf adhesive proteins which comprising both catechol (DOPA) andmine (lysine) groups. They have found simple immersion of sub-trates in an aqueous dopamine solution at pH 8.5 (a pH typicalf marine environments) will result in spontaneous deposition of

thin adherent polymer film, and the resultant surface catecholsan react further for broad applications. Spontaneous polymeriza-ion of dopamine onto surfaces has aroused great interest in theeld of surface chemistry, material and membrane science [25,26].ecently there are a few studies to report polydopamine (PDA)oated Fe3O4 nanoparticles for bio-separation [27,28] and chem-cal analysis [29,30]. Since the PDA polymer can stick to almostverything through covalent and noncovalent interactions includ-ng chelation, hydrogen bonds, Van der Waals force and �–� stack24,26,31], these interactions also exist between multiple pollut-nts in aqueous solution and PDA polymer, which suggests newathways to eliminate water pollution.

In this study, we prepared PDA polymer decorated with mag-etic nanoparticles (Fe3O4/PDA) for adsorption of pollutants fromqueous solution. The Fe3O4/PDA hybrid material was character-zed with several instruments. Three heavy metal ions (Hg2+, Cu2+,g+) and two organic dyes (methylene blue and tartrazine) weresed as model pollutants to investigate the adsorption behav-

ors of Fe3O4/PDA. Methylene blue and tartrazine were cationicnd anionic dye, respectively, and their structures were shownn Scheme 1. To the best of our knowledge, this is the firstime that PDA polymer is applied for environmental remedia-ion.

. Experimental

.1. Materials and chemicals

All reagents used in the experiment were analytical reagentrade and used without further purification. Anhydrous FeCl3,nhydrous CH3COONa, and diethylene glycol (DEG) were pur-hased from Sinopharm Chemical Reagent Beijing Cl., LtdBeijing, China). Trihydroxymethylaminomethane (Tris) and 2-3,4-dihydroxyphenyl)ethylamine hydrochloride (dopamine) were

upplied by J&K Scientific Ltd (Beijing, China). Methylene blue andartrazine were obtain form Aladdin Chemistry Co. Ltd (Shanghai,hina). HgCl2, CuCl2, and AgNO3 were supplied by Beijing Chemicaleagent Corporation (Beijing, China).

tartrazi ne

Scheme 1. Chemical structures of methylene blue and tartrazine.

2.2. Sorbents preparation

Fe3O4 nanoparticles were prepared according to a literaturewith a few modifications [32]. In detail, 0.972 g of anhydrous FeCl3and 1.5 g anhydrous CH3COONa were added into 60 mL of DEG, andthe mixture was stirred with a magnetic stirrer under heating toform a clear solution. Then, the solution was transferred into in aPTFE-lined autoclave, and maintained for 24 h at 200 ◦C. After reac-tion, the black Fe3O4 nanoparticles was separated with a strongRb–Fe–B magnet and washed thoroughly with ethanol to removeresidual reagents.

Fe3O4/PDA hybrid material was prepared as follows: 200 mgFe3O4, 400 mg tris and 240 mg dopamine were added into 200 mLpure water, and the mixture was sonicated for 5 min in ice waterbath. Then the mixture was mechanically stirred at a speed of600 rpm for 8 h at room temperature. The Fe3O4/PDA was isolatedwith a magnet and washed twice with pure water. The final productwas dispersed into 35 mL deionized water to get 10 mg mL−1 sus-pension. The PDA polymer was prepared through similar processexcept that no Fe3O4 nanoparticle was added. The obtained PDApolymer was filtered, rinsed with water, and dried at 40 ◦C.

2.3. Characterization of sorbents

The morphology and particle size analysis were carried out on ahigh resolution transmission electron microscope (HR-TEM) of Tec-nai G20 (FEI Corp., USA) with an acceleration voltage of 200 kV afterdropping the materials suspended in methanol onto copper grids.Thermogravimetric measurements were carried out with a SDTQ600 Thermo-gravimetric analyses (TGA) apparatus (TA Instru-ments, USA), and the samples were heated at a rate of 10 ◦C min−1

from room temperature to 800 ◦C at air atmosphere. Fourier trans-form infrared spectra (FT-IR) were taken in KBr pressed pellets ona NEXUS 670 FT-IR Spectrometer (Madison, WI, USA). Magneticproperty of the adsorbents was analyzed using a vibrating samplemagnetometer (VSM, LDJ9600). The point of zero charge (PZC) ofthe materials was determined with Zetasizer Nanoseries ZS instru-ment (Malvern Instruments, United Kingdom). The binding energyand atomic ratio on the sorbents surface were analyzed using X-ray

photoelectron spectroscopy (XPS) collected on a PHI-Quantera SXMsystem (Perkin–-Elmer Co., USA) with monochromatic Al K� radi-ation (1486.6 eV), and C1s peaks were used as an inner standardcalibration peak at 284.8 eV. Elemental analysis was performed

ardous Materials 270 (2014) 27–34 29

wG

2

pcaAbwhtit0t2si

asbaa

tbTamJ

3

3m

sFipttdiimc

tt

Table 1Elemental analysis of Fe3O4 and Fe3O4/PDA.

Sample N (%) C (%) H (%) C/N (w/w)

Fe3O4 nanoparticle 0.058 7.162 1.430 –Fe3O4/PDA—2 ha 1.961 14.79 1.392 7.542Fe3O4/PDA—4 h 2.826 20.28 1.712 7.176Fe3O4/PDA—6 h 3.264 23.30 1.922 7.138Fe3O4/PDA—8 h 3.660 26.02 2.213 7.109Fe3O4/PDA—10 h 3.746 26.19 2.165 6.991

a Reaction time: 2 h.

S. Zhang et al. / Journal of Haz

ith a Vario EL elemental analyzer (Elementar Analysen systemembH, Germany).

.4. Batch adsorption experiments

Pollutant adsorption experiments were performed in 100 mLolypropylene bottles containing 50 mL aqueous solution. The con-entration of sorbent was kept at 0.1 g L−1, and ionic strength wasdjusted to 50 mg L−1 with NaCl or NaNO3 (for Ag+) stock solution.fter solution pH was adjusted to 8, 3, 6.5, 5.5, and 6.5 for methylenelue, tartrazine, Cu2+, Ag+, and Hg2+, respectively, the suspensionas shaken at 30 ◦C for 4 h. Effect of solution pH on adsorption ofeavy metal was investigated in the range of 3–7, and for dye inhe range of 3–10. Adsorption isotherms were obtained by varyingnitial concentration of target pollutant. To investigate the adsorp-ion rate of pollutants, a 150 mL of aqueous solution containing.1 g L−1 sorbents was shaken at 30 ◦C, and the initial concentra-ions of methylene blue, tartrazine, Cu2+, Ag+, and Hg2+ were set at0, 20, 20, 30, and 60 mg L−1, respectively. Ten milliliter of suspen-ion was taken for determination of pollutant concentration at thenterval time of 5, 10, 20, 30, 60, 120, 240, 360 min.

The Fe3O4/PDA sorbent was separated with a magnet afterdsorption, and 10 mL of acid or base solution was added andhaken for 0.5 h to desorb the loaded pollutants. Then the sor-ent was separated and rinsed with 5 mL of desorption solutionnd 50 mL of pure water in sequence. The regenerated sorbent waspplied for next adsorption process.

After adsorption, the bottle was placed on a magnet for 5 mino separate the sorbents from aqueous solution. When the solutionecame limpid, a portion of supernatant was taken for analysis.he concentration of heavy metal was determined with a flametomic absorption spectrophotometer (AA240, Varian). Dyes wereeasured with an UV-2550 UV–vis spectrophotometer (Shimadzu,

apan).

. Results and discussion

.1. Preparation and characterization of PDA/Fe3O4 hybridaterial

The Fe3O4/PDA hybrid material was synthesized through two-tep route. First, using FeCl3 as an iron source, water-dispersiblee3O4 nanoparticles were prepared by a simple solution methodn the presence of anhydrous CH3COONa and DEG [32]. Then therepared Fe3O4 nanoparticles were dispersed into 10 mmol L−1

ris buffer solution (pH 8.5) containing dopamine. Under theypical marine environments, the spontaneous polymerization ofopamine occurred [24], and Fe3O4 nanoparticles were embedded

nto the PDA polymer to form Fe3O4/PDA hybrid material. Scheme 2llustrated the formation of Fe3O4/PDA, the structure of PDA poly-

er, and its interaction with metal ions through chelation and

oordination.

To investigate the effect of reaction time on polymerization,he Fe3O4/PDA hybrid materials with different polymerizationime were analyzed with an elemental analyzer, and the results

Scheme 2. The structure of Fe3O4/PDA a

Fig. 1. (a) TGA curves and (b) hysteresis loops of Fe3O4 and Fe3O4/PDA hybridmaterial with different polymerization time.

were shown in Table 1. For Fe3O4 nanoparticles, the content

of nitrogen was negligible, while a little carbon and hydrogenwas detected, which came from residual impurities and surfacehydroxyl groups, respectively. When Fe3O4 nanoparticles wereimmersed in dopamine solution for 2 h, the content of nitrogen

nd its interaction with metal ions.

3 ardou

aitgepeb1wair4

naTlawTtorrtPtwn

0 S. Zhang et al. / Journal of Haz

nd carbon increased dramatically to 1.961% and 14.79%, indicat-ng that PDA polymer was formed on Fe3O4 nanoparticles. Withhe reaction time increasing from 2 to 8 h, the content of nitro-en and carbon enhanced gradually to 3.660% and 26.02%. Furtherxtension of reaction time had little effect on the yield of PDAolymer since 10 h of reaction time only resulted in negligiblenhancement of nitrogen and carbon compared to 8 h. The car-on/nitrogen ratios (C/N, w/w) for reaction time of 2, 4, 6, 8, and0 h were 7.542, 7.176, 7.138, 7.109, and 6.991, respectively, whichere very close to the theoretical C/N ratio of dopamine (6.857),

nd the slight higher C/N ratio resulted from the carbon impurityn Fe3O4 nanoparticles. Calculated from the content of nitrogen, theatio of PDA polymer in the hybrid material with 8 h reaction was0.04%.

When these materials were heated, the weight loss of Fe3O4anoparticles and Fe3O4/PDA hybrid materials was analyzed with

TGA instrument, and the TGA curves were shown in Fig. 1(a).he TGA curve of Fe3O4 nanoparticles showed two steps of weightoss. About 3% of weight loss occurred from room temperature toround 130 ◦C, which was caused by the elimination of absorbedater. From 220 to 400 ◦C about 9.4% of weight loss was shown in

GA curve which should attribute to the decomposition of impuri-ies. Two steps of weight loss were also observed on the TGA curvesf Fe3O4/PDA with different reaction time. The first weight loss cor-esponding to elimination of absorbed water was about 5% wheneaction time was 2 h, and it enhanced to around 7.6% when reac-ion time was 8 and 10 h, indicating the hygroscopic character ofDA polymer. The second weight loss was caused by decomposi-

ion of PDA polymer, and it was 23.0, 32.3, 36.8, 42.8, and 42.6hen reaction time was 2, 4, 6, 8, and 10 h, respectively. There waso obvious difference for the second weight loss between 8 and

Fig. 2. TEM images of (a) Fe3O4 and (b and c

s Materials 270 (2014) 27–34

10 h, indicating the polymerization was almost completed within8 h. The second weight loss of 42.8% was corresponding to the con-tent of PDA polymer in the hybrid material, which was agreed wellwith the results of elemental analysis.

The hysteresis loops were measured with a VSM to investi-gate the magnetic property of these materials, and the resultswere shown in Fig. 1(b). There was no hysteresis in the hysteresisloops of Fe3O4 nanoparticles and Fe3O4/PDA hybrid materials, andthe remanence and coercivity were nearly zero, exhibiting typicalsuperparamagnetic behavior. The maximal saturation magnetiza-tion of Fe3O4 nanoparticles was 55.0 emu g−1, and it decreasedwith the introduction of nonmagnetic PDA polymer. When poly-merization time was2, 4, 6, 8, and 10 h, the maximal saturationmagnetization was 39.4, 34.6, 30.8, 28.7, and 29.1 emu g−1, respec-tively. The maximal saturation exhibited no obvious differencebetween 8 and 10 h, suggesting nonmagnetic PDA polymer was notproduced any more after 8 h of reaction. Due to the superparamag-netic property and large saturation magnetization of Fe3O4/PDAhybrid material, it could be separated from solution with a mag-net easily and redispersed rapidly as soon as the magnet was takenaway, which facilitated collection, regeneration, and reutilizationof the materials.

TEM image of Fe3O4 nanoparticles was shown in Fig. 2(a). Itrevealed that the Fe3O4 was mono-dispersed nanoparticles withquasi-spherical shape, and had nearly uniform distribution of par-ticle size with an average diameter of about 7 nm. Fig. 2(b) wasTEM image of Fe3O4/PDA hybrid material with polymerization timeof 8 h. It could be clearly observed that the dark Fe3O4 nanoparti-

cles were embedded in light PDA polymer uniformly. The Fe3O4nanoparticle showed a tendency to aggregate to large ones dur-ing polymerization of dopamine. The thickness of PDA polymer on

) Fe3O4/PDA, and their (d) IR spectra.

S. Zhang et al. / Journal of Hazardous Materials 270 (2014) 27–34 31

1000 800 600 400 200 0

0

1x104

2x104

3x104

4x104

5x104

6x104

7x104

8x104

9x104

1x105

Hg4f

Hg4p3

Hg4d

N1s O2sFe3p

Fe3sC1s

O1s

Fe2p

FeLMM

FeLMM1

FeLMM2

OKLL

Binding Energy

c/s

Fe3O

4

Fe3O

4/PDA

Fe3O

4/PDA-Hg

N1s

Fa

talpFeiw

tmhr1sapatppmbr

oatsAttwttaocat

3

tf

1098765432

0

20

40

60

80

100

Methylene blue

Tartrazine

Cu2+

Ag+

Hg2+

Re

mo

va

l e

ffic

ien

cy

(%

)

Solution pH

a

111098765432

-30

-20

-10

0

10

20

30

40

Fe3O

4

Fe3O

4/DPA

pH

Ze

ta P

ote

nti

al (m

V)

b

2+

ig. 3. Wide XPS scan of Fe3O4, Fe3O4/PDA, and Fe3O4/PDA after adsorption of Hg2+,nd the inset was narrow XPS spectrum for N1s of Fe3O4/PDA.

he edge of the hybrid material was about 25 nm. The catechol andmine groups on the PDA surface provided covalent and noncova-ent interactions to stick to almost everything [24,26], suggestingollutants could be adsorbed through combination with PDA.ig. 2(c) showed HR-TEM images of Fe3O4/PDA hybrid material. Itxhibited continuous parallel lattice fringes of Fe3O4 nanoparticles,ndicating their single crystalline nature, while the PDA polymer

as amorphous compound since no crystal structure was observed.FT-IR was employed to examine the surface functional groups of

he Fe3O4 nanoparticles, Fe3O4/PDA hybrid material, and PDA poly-er. As shown in Fig. 2(d), the spectra of Fe3O4 and Fe3O4/PDA

ybrid material showed strong peaks at 580 cm−1, which waselated to the vibration of the Fe–O bond [33], and the peak at630 cm−1 is corresponding to the surface-adsorbed water. In thepectrum of Fe3O4/PDA hybrid material new peaks at 1290, 1500,nd 1600 cm−1, which resulted from the aromatic rings in the PDAolymer [30]. Compared with that of Fe3O4 nanoparticles, the peakt around 3400 cm−1 of Fe3O4/PDA hybrid material exhibited aendency to become broad and divided, which attributed to theeaks overlapping of hydroxyls, adsorbed water, and amines of PDAolymer. The spectrum of PDA polymer also showed peaks of aro-atic rings as in Fe3O4/PDA hybrid material, while no peak of Fe–O

ond was observed. The FT-IR results demonstrated the successfulecombination of Fe3O4 and PDA polymer.

XPS was applied to investigate the chemical elements on surfacef the materials. The wide-scan XPS spectra for Fe3O4 nanoparticlesnd Fe3O4/PDA hybrid material were shown in Fig. 3. The charac-eristic peaks of Fe2s, Fe3s, Fe3p, O1s, and O2s appeared in thepectrum of Fe3O4, and the peak of C1s came from the impurities.fter the Fe3O4 nanoparticles were encapsulated into PDA polymer,

he peak of C1s enhanced, and that of O1s shrunk obviously due tohe high content of carbon and low content of oxygen in PDA. Mean-hile, new peak of N1s appeared, and it was not obvious because of

he low content of nitrogen. To confirm the existence of nitrogen,he narrow XPS spectrum for N1s was shown in the inset of Fig. 3,nd the peak of N1s could be observed clearly. In addition, peaksf iron disappeared almost, which could be expected since the XPSould penetrate less than 10 nm while the thick of PDA shell wasbout 25 nm from the TEM images. The XPS results demonstratedhat Fe3O4 nanoparticles were fully covered by PDA polymer.

.2. Effect of solution pH on removal efficiencies of pollutants

To investigate effect of pH on pollutants adsorption, the solu-ion pH was adjusted in the range of 3–10 for organic dyes, and 3–7or heavy metal ions to avoid hydrolytic precipitation. The initial

Fig. 4. (a) Effect of pH on removal efficiencies of methylene blue, tartrazine, Cu ,Ag+, and Hg2+ with Fe3O4/PDA sorbents, and (b) Zeta potential at varied pH of Fe3O4

and Fe3O4/PDA.

concentrations of methylene blue, tartrazine, Cu2+, Ag+, and Hg2+

were set at 20, 10, 10, 10, 60 mg L−1, respectively. The removalefficiencies of all pollutants at varied solution pH were shown inFig. 4(a). The removal efficiencies of heavy metal ions were lowat low pH, and even decreased to zero at pH 3 for Cu2+ and Hg2+.With the solution pH increasing to 7, the removal efficiencies ofheavy metal ions enhanced dramatically to over 90%. As for organicdyes, the removal efficiency of methylene blue exhibited the sametendency as heavy metal ions, while that of tartrazine showed theopposite tendency. Surface charge was an important factor to affectthe interaction between sorbents and pollutants. So zeta poten-tials of Fe3O4 and Fe3O4/PDA suspension under varying pH weredetermined, and the results were shown in Fig. 4(b). The PZC ofbare Fe3O4 was found to be around 7.4, and that of Fe3O4/PDAdecreased to about 3.7. The alteration of PZC not only further con-firmed the formation of PDA polymer on the surface of Fe3O4 butalso provided a reasonable interpretation for variation of removalefficiencies of pollutants. At pH below the PZC of Fe3O4/PDA, thesorbent surface was protonated to get positive charges. Since Cu2+,Ag+, Hg2+, and methylene blue existed as cations in the tested pHrange, the electrostatic repulsion was unfavorable for these cationscontacting with positively charged surface for further interaction.With solution pH increasing, the Fe3O4/PDA surface was depro-tonated, and negative charge enhanced sharply. The electrostaticattraction facilitated contact and adsorption of cation pollutants.

The PZC decrease of Fe3O4/PDA reduced the positive charge at lowpH, which was in favor of adsorption of cation pollutants. On thecontrary, tartrazine was anionic dye, so its removal efficiency washigh at low solution pH and decreased with solution pH increasing.

32 S. Zhang et al. / Journal of Hazardou

Fig. 5. (a) Adsorption isotherms of methylene blue, tartrazine, Cu2+, Ag+, and Hg2+

on Fe3O4/PDA sorbents, (b) fitting curves of Langmuir isotherm models, and (c)ps

3

f6ttacfipas

hotographs of Fe3O4/PDA hybrid material for methylene blue adsorption and sub-equent magnetic separation with a magnet.

.3. Adsorption isotherms

Adsorption isotherm was carried out at 30 ◦C, and solution pHor methylene blue, tartrazine, Cu2+, Ag+, and Hg2+ were set at 8, 3,.5, 5.5, 6.5, respectively. Initial concentrations of methylene blue,artrazine, Cu2+, and Ag+ were 5, 10, 15, 20, 25, 30, 40, 50 mg L−1, andhose of Hg2+ were 10, 20, 30, 40, 50, 60, 70 mg L−1. The equilibriumdsorption data were shown in Fig. 5(a). With the rise of pollutantsoncentrations, the adsorption capacity increased sharply in the

rst stage, and then attained a platform. The adsorbability of PDAolymer for various pollutants derived from their multiple inter-ctions with these compounds, just as the adhesive polymer filmecreted by mussel could stick to almost everything [26].

s Materials 270 (2014) 27–34

Two equilibrium isotherm models, namely Langmuir andFreundlich, were applied to analyze the adsorption data of thesepollutants on Fe3O4/PDA sorbents, and their linear equations weregiven below:

Ce

qe= 1

�b+ Ce

�(1)

log qe = log KF + 1n

log Ce (2)

where qe (mg g−1) and Ce (mg L−1) were the equilibrium adsorp-tion capacity and the equilibrium concentration; � (mg g−1) was themaximum adsorption capacity and b (L mg−1) was the equilibriumadsorption constant. The maximum adsorption capacity (�) couldbe calculated from the slope of the linear plot of Ce/qe versus Ce. KF

(mL1/n �g1−1/n) and n were the Freundlich constants. The value ofn and KF could be obtained from slope of linear plot of log qe versuslog Ce.

The fitting curves of Langmuir isotherm model were shown inFig. 5(b), and they exhibited good linearity. The fitting parame-ters of these two models were shown in Table 2. The correlationcoefficients (R2) of Langmuir model for all pollutants were over0.99, while those of Freundlich model were in the range of0.656–0.989. The adsorption data agreed well with Langmuirmodel, indicating that the adsorption of pollutants on Fe3O4/PDAsorbents occurred as a single monolayer [20]. Calculated from Lang-muir adsorption isotherm, the maximum adsorption capacity (�) ofmethylene blue, tartrazine, Cu2+, Ag+, and Hg2+ were 204.1, 100.0,112.9, 259.1, and 467.3 mg g−1, respectively. Compared with othermagnetic sorbents reported previously, the Fe3O4/PDA hybridmaterial not only showed availability for multiple pollutants butalso possessed high adsorption capacity. The Fe3O4/PDA hybridmaterial exhibited considerable potential for water purificationwith complex components.

Fig. 5(c) showed the photographs of methylene blue removal byFe3O4/PDA. A small amount of Fe3O4/PDA sorbents were dispersedin 20 mg L−1 of methylene blue solution, and then a hand-hold mag-net was place on the side of the bottle to separate the magneticsorbent. After a few minutes, the Fe3O4/PDA sorbents were iso-lated completely, and the solution became limpid as shown dueto the complete adsorption of dyes. After adsorption of Hg2+, thepeaks of Hg4p, Hg4d, and Hg4f were observed in the wide-scan XPSspectrum (Fig. 3), illustrating that Hg2+ was loaded on the sorbent.

3.4. Adsorption kinetics

The variation of adsorption capacity of methylene blue, tar-trazine, Cu2+, Ag+, and Hg2+ on Fe3O4/PDA with time increasing wasshown in >Fig. 6(a). The adsorption capacity was enhanced rapidlyin the initial time, and then slowed down, and reached equilibriumfinally. Two hours was enough to attain adsorption equilibrium,and the fast adsorption should attribute to the readily accessibilityof pollutant to the binding sites of PDA polymer which were coatedon the exterior surface of Fe3O4.

Pseudo-second order kinetic model was applied to describe theadsorption of these pollutants on Fe3O4/PDA, and its linear modelwas shown below:

t

qt= 1

kq2e

+ 1qe

t (3)

where k is the rate constant of adsorption (g mg−1 min−1), qt is theadsorption amount of pollutant at any time (mg g−1), qe is equilib-

rium adsorption capacity (mg g−1), and the initial adsorption rate,h (mg g−1 min−1) can be defined as

h = kq2e (t → 0) (4)

S. Zhang et al. / Journal of Hazardous Materials 270 (2014) 27–34 33

Table 2Langmuir and Freundlich isotherms parameters for pollutants adsorption on Fe3O4/PDA.

Pollutants Langmuir model Freundlich model

� (mg g−1) b (g mL−1) R2 KF (mL1/n �g1−1/n) n R2

Methylene blue 204.1 0.538 0.991 100.7 5.943 0.881Tartrazine 100.0 1.305 0.992 51.73 4.390 0.656Cu2+ 112.9 3.868 0.997 65.33 5.177 0.756Ag+ 259.1 0.896 0.995 125.0 4.183 0.989Hg2+ 467.3 0.107 0.994 106.7 2.936 0.977

S

acethF

3

a0t

Fo

Table 3Pseudo-second-order rate constants for pollutants adsorption on Fe3O4/PDA.

Pollutants k (g mg−1 min−1) h (mg g−1 min−1) R2

Methylene blue 3.802 × 10−3 60.31 0.9998Tartrazine 1.526 × 10−3 99.30 0.9996Cu2+ 1.892 × 10−4 26.54 0.9993Ag+ 7.454 × 10−4 17.53 0.9960Hg2+ 1.294 × 10−3 12.64 0.9980

olution pH: methylene blue, 8; tartrazine, 3; Cu2+, 6.5; Ag+, 5.5; Hg2+, 6.5.

Both k and h can be determined experimentally from the slopend intercept of plot of t/qt versus t. Fig. 6(b) showed the fittingurves of pseudo-second order kinetic model, exhibiting good lin-arity with R2 over 0.996. The constant k and h obtained fromhe slope and intercept of plots were presented in Table 3. Theigh h and k values revealed fast adsorption rate of pollutants one3O4/PDA sorbent.

.5. Desorption and reusability

Since >the removal efficiencies of cation pollutants were lowt low pH, methylene blue, Cu2+, and Hg2+ were desorbed with.01 mol L−1 HCl solution. To avoid the formation of AgCl precipi-ation, 0.01 mol L−1 HNO3 solution was used to desorb Ag+. Due to

0 100 20 0 300 40 0

0

100

200

300

400

t (min)

qt (m

g/g

)

Methylene blue

Tartrazi ne

Cu2+

Ag+

Hg2+ a

0 100 20 0 30 0 400

0

1

2

3

4

Methylene blue

Tartrazine

Cu2+

Ag+

Hg2+

t/q

t(m

in g

mg

-1)

t (min)

b

ig. 6. (a) Adsorption kinetics data of methylene blue, tartrazine, Cu2+, Ag+, and Hg2+

n Fe3O4/PDA sorbent, and (b) curves of pseudo-second-order kinetic fitting.

0

100

200

300

Tartrazine

qe (

mg

g-1)

cycle 1

cycle 2

cycle 3

Methylene Cu2+

Ag+

Hg2+

blue

Fig. 7. Reusability of Fe3O4/PDA for three cycles.

the low adsorption efficiency of tartrazine at high pH, it was des-orbed with 0.01 mol L−1 NaOH solution. The regenerated sorbentwas used in the next adsorption process. As shown in Fig. 7, thesorbent still remained good adsorption efficiency after three cycles.The adsorption capacities of methylene blue, tartrazine, Cu2+, andHg2+ showed a little decrease, which might be caused by the lossof sorbent or irreversible occupation of part adsorption sites. Therewas an obvious decrease of adsorption capacity of Ag+ during recy-cle, which should attribute to the reduction of part Ag+ to elementalsilver by PDA polymer and deposition on the sorbent [24]. All inall, the Fe3O4/PDA sorbent could efficiently remove metal ions anddyes from aqueous solution and showed good reusability.

4. Conclusions

The Fe3O4/PDA hybrid material were prepared and appliedfor pollutants removal. The dopamine polymerized spontaneouslyunder room temperature to encapsulate the Fe3O4 nanoparticles,and the synthesis method was simple and environmental friendly.The PDA polymer exhibited high adsorption capacity for multiple

pollutants based on the covalent and noncovalent interactions. Thesorbents loaded with pollutants could be isolated from solutionwith a magnet due to the superparamagnetism of Fe3O4 nanopar-ticles. This study not only provided an alternative sorbent for

3 ardou

pcfi

A

S2(oC

R

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[[

[

[

[

[

4 S. Zhang et al. / Journal of Haz

ollutants removal but also opened up new avenues for the appli-ation of PDA polymer in adsorption, separation, and environmenteld.

cknowledgments

This work was jointly supported by the National Naturalcience Foundation of China (21207059, 21171085, 21104030,1206066); the Natural Science Foundation of Shandong ProvinceZR2010BM027, ZR2011BQ012); the Foundation of State Key Lab-ratory of Environmental Chemistry and Ecotoxicology, Researchenter for Eco-Environmental Sciences (KF2011-27).

eferences

[1] C.T. Yavuz, J. Mayo, W.Y. William, A. Prakash, J.C. Falkner, S. Yean, L. Cong, H.J.Shipley, A. Kan, M. Tomson, Low-field magnetic separation of monodisperseFe3O4 nanocrystals, Science 314 (2006) 964–967.

[2] S.C.N. Tang, I.M.C. Lo, Magnetic nanoparticles: essential factorsfor sustainable environmental applications, Water Res. 47 (2013)2613–2632.

[3] H. Niu, Dizhang, Z. Meng, Y. Cai, Fast defluorination and removal of norfloxacinby alginate/Fe@Fe3O4 core/shell structured nanoparticles, J. Hazard. Mater.227–228 (2012) 195–203.

[4] H. Niu, D. Zhang, S. Zhang, X. Zhang, Z. Meng, Y. Cai, Humic acid coatedFe3O4 magnetic nanoparticles as highly efficient Fenton-like catalyst forcomplete mineralization of sulfathiazole, J. Hazard. Mater. 190 (2011)559–565.

[5] L. Xu, J. Wang, Fenton-like degradation of 2,4-dichlorophenol using Fe3O4 mag-netic nanoparticles, Appl. Catal., B 123–124 (2012) 117–126.

[6] H. Wang, Y. Huang, Prussian-blue-modified iron oxide magnetic nanoparticlesas effective peroxidase-like catalysts to degrade methylene blue with H2O2, J.Hazard. Mater. 191 (2011) 163–169.

[7] S. Qu, F. Huang, S. Yu, G. Chen, J. Kong, Magnetic removal of dyes from aqueoussolution using multi-walled carbon nanotubes filled with Fe2O3 particles, J.Hazard. Mater. 160 (2008) 643–647.

[8] H.Y. Zhu, Y.Q. Fu, R. Jiang, J. Yao, L. Xiao, G.M. Zeng, Novel magnetic chi-tosan/poly(vinyl alcohol) hydrogel beads: preparation, characterization andapplication for adsorption of dye from aqueous solution, Bioresour. Technol.105 (2012) 24–30.

[9] L. Fan, C. Luo, M. Sun, X. Li, F. Lu, H. Qiu, Preparation of novel magnetic chi-tosan/graphene oxide composite as effective adsorbents toward methyleneblue, Bioresour. Technol. 114 (2012) 703–706.

10] D. Zhang, H. Niu, X. Zhang, Z. Meng, Y. Cai, Strong adsorption ofchlorotetracycline on magnetite nanoparticles, J. Hazard. Mater. 192 (2011)1088–1093.

11] R. Kumar, B.S. Inbaraj, B.H. Chen, Surface modification of superparamagne-

tic iron nanoparticles with calcium salt of poly(�-glutamic acid) as coatingmaterial, Mater. Res. Bull. 45 (2010) 1603–1607.

12] F. Ge, H. Ye, M.M. Li, B.X. Zhao, Efficient removal of cationic dyes from aqueoussolution by polymer-modified magnetic nanoparticles, Chem. Eng. J. 198–199(2012) 11–17.

[

[

s Materials 270 (2014) 27–34

13] Y.R. Zhang, S.Q. Wang, S.L. Shen, B.X. Zhao, A novel water treatment magneticnanomaterial for removal of anionic and cationic dyes under severe condition,Chem. Eng. J. 233 (2013) 258–264.

14] D.P. Li, Y.R. Zhang, X.X. Zhao, B.X. Zhao, Magnetic nanoparticles coated byaminoguanidine for selective adsorption of acid dyes from aqueous solution,Chem. Eng. J. 232 (2013) 425–433.

15] J.F. Liu, Z.S. Zhao, G.B. Jiang, Coating Fe3O4 magnetic nanoparticles with humicacid for high efficient removal of heavy metals in water, Environ. Sci. Technol.42 (2008) 6949–6954.

16] F. Ge, M.-M. Li, H. Ye, B.-X. Zhao, Effective removal of heavy metal ionsCd2+, Zn2+, Pb2+, Cu2+ from aqueous solution by polymer-modified magneticnanoparticles, J. Hazard. Mater. 211–212 (2012) 366–372.

17] H. Dong, I.M.C. Lo, Influence of calcium ions on the colloidal stability of surface-modified nano zero-valent iron in the absence or presence of humic acid, WaterRes. 47 (2013) 2489–2496.

18] J. Hu, I. Lo, G. Chen, Performance and mechanism of chromate(VI) adsorptionby �-FeOOH-coated maghemite (�-Fe2O3) nanoparticles, Sep. Purif. Technol.58 (2007) 76–82.

19] J. Hu, G. Chen, I.M.C. Lo, Removal and recovery of Cr(VI) from wastewater bymaghemite nanoparticles, Water Res. 39 (2005) 4528–4536.

20] W. Yantasee, C.L. Warner, T. Sangvanich, R.S. Addleman, T.G. Carter, R.J. Wiacek,G.E. Fryxell, C. Timchalk, M.G. Warner, Removal of heavy metals from aqueoussystems with thiol functionalized superparamagnetic nanoparticles, Environ.Sci. Technol. 41 (2007) 5114–5119.

21] S. Singh, K.C. Barick, D. Bahadur, Surface engineered magnetic nanoparticlesfor removal of toxic metal ions and bacterial pathogens, J. Hazard. Mater. 192(2011) 1539–1547.

22] L. Feng, M. Cao, X. Ma, Y. Zhu, C. Hu, Superparamagnetic high-surface-areaFe3O4 nanoparticles as adsorbents for arsenic removal, J. Hazard. Mater.217–218 (2012) 439–446.

23] J.H. Waite, X. Qin, Polyphosphoprotein from the adhesive pads of Mytilus edulis,Biochemistry 40 (2001) 2887–2893.

24] H. Lee, S.M. Dellatore, W.M. Miller, P.B. Messersmith, Mussel-inspired surfacechemistry for multifunctional coatings, Science 318 (2007) 426–430.

25] P.B. Messersmith, Materials science: multitasking in tissues and materials, Sci-ence 319 (2008) 1767–1768.

26] J.H. Waite, Surface chemistry: Mussel power, Nat. Mater. 7 (2008) 8–9.27] F. Sahin, E. Turan, H. Tumturk, G. Demirel, Core–shell magnetic nanoparticles:

a comparative study based on silica and polydopamine coating for magneticbio-separation platforms, Analyst 137 (2012) 5654–5658.

28] M. Zhang, X. Zhang, X. He, L. Chen, Y. Zhang, A self-assembled polydopaminefilm on the surface of magnetic nanoparticles for specific capture of protein,Nanoscale 4 (2012) 3141–3147.

29] Q. Song, M. Li, L. Huang, Q. Wu, Y. Zhou, Y. Wang, Bifunctionalpolydopamine@Fe3O4 core–shell nanoparticles for electrochemical determi-nation of lead(II) and cadmium(II), Anal. Chim. Acta 787 (2013) 64–70.

30] Y. Wang, S. Wang, H. Niu, Y. Ma, T. Zeng, Y. Cai, Z. Meng, Preparation of poly-dopamine coated Fe3O4 nanoparticles and their application for enrichment ofpolycyclic aromatic hydrocarbons from environmental water samples, J. Chro-matogr. A 1283 (2013) 20–26.

31] H. Lee, Single-molecule mechanics of mussel adhesion, PNAS 103 (2006)12999–13003.

32] X. Jia, D. Chen, X. Jiao, S. Zhai, Environmentally-friendly preparation of water-dispersible magnetite nanoparticles, Chem. Commun. 96 (2009) 8–97, 0.

33] I.J. Bruce, T. Sen, Surface modification of magnetic nanoparticles with alkoxysi-lanes and their application in magnetic bioseparations, Langmuir 21 (2005)7029–7035.

本文献由“学霸图书馆-文献云下载”收集自网络，仅供学习交流使用。

学霸图书馆（www.xuebalib.com）是一个“整合众多图书馆数据库资源，

提供一站式文献检索和下载服务”的24 小时在线不限IP

图书馆。

图书馆致力于便利、促进学习与科研，提供最强文献下载服务。

图书馆导航：

图书馆首页 文献云下载 图书馆入口 外文数据库大全 疑难文献辅助工具