Magnetic chitosan nanoparticles for removal of Cr(VI) from aqueous solution

Magnetic chitosan nanoparticles for removal of CrVI from aqueous solutiona School of Chemical Engineering, Hanoi University of Science and Technology, 1, Dai Co Viet Road, Hanoi, Viet Na

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a

School of Chemical Engineering, Hanoi University of Science and Technology, 1, Dai Co Viet Road, Hanoi, Viet Nam

b

Institute of Chemistry, Vietnam Academy of Science and Technology, 18, Hoang Quoc Viet Road, Hanoi, Viet Nam

c

Institute of Marine Geology and Geophysics, Vietnam Academy of Science and Technology, 18, Hoang Quoc Viet Road, Hanoi, Viet Nam

d

Hanoi National University of Education,136 Xuan Thuy, Hanoi, Viet Nam

e

Institute of Materials Science, Vietnam Academy of Science and Technology, 18, Hoang Quoc Viet Road, Hanoi, Viet Nam

a b s t r a c t

a r t i c l e i n f o

Article history:

Received 28 April 2012

Received in revised form 29 August 2012

Accepted 3 December 2012

Available online 9 December 2012

Keywords:

Fe 3 O 4

Magnetic chitosan nanoparticles

Adsorption isotherm

Cr(VI)

A simple method was introduced to prepare magnetic chitosan nanoparticles by co-precipitation via epichlo-rohydrin cross-linking reaction The average size of magnetic chitosan nanoparticles is estimated at ca

30 nm It was found that the adsorption of Cr(VI) was highly pH-dependent and its kinetics follows the pseudo-second-order model Maximum adsorption capacity (at pH 3, room temperature) was calculated as 55.80 mg·g−1, according to Langmuir isotherm model The nanoparticles were thoroughly characterized be-fore and after Cr(VI) adsorption From this result, it can be suggested that magnetic chitosan nanoparticles could serve as a promising adsorbent for Cr(VI) in wastewater treatment technology

1 Introduction

Contamination of water by toxic heavy metals through the

dis-charge of industrial wastewater is a worldwide environmental

prob-lem Rapid industrialization has seriously contributed to the release

of toxic heavy metals to water streams Among the various heavy

metals, chromium (Cr) is one of the most toxic pollutants generated

by the electroplating, leather tanning, metalﬁnishing, steel

fabrica-tion, textile industries and chromate preparation The most common

oxidation states of Cr in nature are Cr(III) and Cr(VI) Chromium(VI)

is more hazardous than Cr(III) as it can diffuse as CrO4 −or HCrO4 −

through cell membranes and oxidize biological molecules[1] A wide

range of physical and chemical processes are available for the removal

of chromium from wastewater such asﬁltration[2], electrochemical

pre-cipitation[3], adsorption[4,5], electrodeposition[6]and membrane

sys-tems or ion exchange process[7–9] Among these methods, adsorption is

one of the most economically favorable while being technically easy

[10–12] Chitosan has excellent properties for the adsorption of heavy

metal ions, principally due to the presence of amino groups (–NH2) in

the polymer matrix, which interact with metal ions in solution by ion

ex-change and complexation reactions[13–15] Most of the chitosan-based

adsorbents are submicron to micron-sized and need large internal

poros-ities to ensure adequate surface area for adsorption However, the

diffusion limitation within the particles leads to the decrease in the

adsorption rate and available capacity Compared to the traditional micron-sized supports used in separation process, nano-sized adsorbents display better performance due to high speciﬁc surface area and the ab-sence of internal diffusion resistance The nano-adsorbents cannot be sep-arated easily from aqueous solution byﬁltration or centrifugation[16] The application of magnetic adsorbent technology to solve environmental problems has received considerable attention in recent years[16–18]

In this paper, magnetic chitosan nanoparticles were prepared, char-acterized and applied for the removal of Cr(VI) in the water solution Af-terwards, thermodynamic and kinetic aspects of the adsorption process were considered The improved magnetic and high adsorption uptake properties are two main features of the synthesized nanoparticles that can be advantageously used in water treatment

2 Experimental 2.1 Chemicals FeCl3·6H2O, FeSO4·7H2O, K2Cr2O7, CH3COOH, NaOH and NH3were

of analytical grade Chitosan (MW= 400,000, DA=70%) was purchased from Nha Trang Aquatic Institute (Vietnam) and re-characterized by viscometry and IR measurements at our laboratory[18] Double distilled water was used in the preparation of all solutions

2.2 Preparation of magnetic chitosan nanoparticles Magnetic chitosan nanoparticles were prepared by co-precipitation method[18–20], with several modiﬁcations Experiments were carried

⁎ Corresponding author Tel.: +84 4 37564129; fax: +84 438360705.

E-mail address: tdlam@vast.ac.vn (T.D Lam).

Contents lists available atSciVerse ScienceDirect

Materials Science and Engineering C

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / m s e c

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out in inert gas (N2) Fe3O4 nanoparticles were synthesized by

co-precipitation of ferric and ferrous salts The amount 6.4795 g of

FeCl3·6H2O and 3.3339 g of FeSO4·7H2O were dissolved into 150 mL

of deoxygenated distilled water After stirring for 30 min, chemical

pre-cipitation was achieved at 30 °C under vigorous stirring by adding

20 mL of NH3·H2O solution (28%, v/v) During the reaction process, pH

was maintained at about 10 The reaction system was kept at 70 °C for

1 h 1 g chitosanﬂake was dissolved in a 150 mL CH3COOH solution

2% (w/v) The chitosan solution was then dropped into the obtained

magnetic ﬂuid in the ﬂask through a dropper Afterwards, 2 mL of

pure epichlorohydrin was added into reaction ﬂask and stirred at

85 °C for 3 h, before theﬂask was cooled down to room temperature

The precipitate was washed with distilled water to remove all existing

in the efﬂuents Silver nitrate (AgNO3) was used to detect residue of

Cl− The precipitate was then washed with ethanol and dried at 50 °C,

in the vacuum oven

2.3 Adsorption experiments

The sorption experiments were performed by batch method

Sam-ples of 0.1 g of magnetic chitosan nanoparticles were equilibrated with

50 mL of solution containing various amount of Cr(VI) The pH value

of solutions was adjusted by using diluted solution of NaOH and HCl The temperature of the solutions (25 °C, 35 °C, 45 °C) was controlled with the thermostatic bath The adsorbed amount of Cr(VI) per unit weight of magnetic chitosan nanoparticles, qt(mg·g−1), was calculated from the mass balance equation as:

qt¼ðC0−CtÞ⋅V

where C0and Ct(mg/L) are the initial Cr(VI) concentration and the Cr(VI) concentrations at any time t, respectively; V (L) is the volume of the Cr(VI) solution; and m (g) is the mass of the magnetic chitosan nanoparticles Samples of the Cr(VI) solution were collected at pre-determined time intervals and analyzed using a UV–Vis Spectrophotom-eter (model UV-PC1600, Shimadzu), atλmax=540 nm, according to the 1,5-diphenyl-carbazide method[21] All measurements were conducted triplicate

2.4 Characterization methods X-ray diffraction (XRD) patterns were obtained at room tempera-ture by D8 Advance, Bruker ASX, using CuKα radiation (λ=1.5406 Å)

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in the range of 2θ=10°–60°, and a scanning rate of 0.02°·s−1

Mor-phology of magnetic chitosan nanoparticles was analyzed by Field

Emission Hitachi S-4500 Scanning Electron Microscope (FE-SEM) and

Transmission Electron Microscope (TEM, JEOL, Voltage: 80 kV)

Absor-bance measurements were carried out using Shimadzu UV-PC1600

spectrophotometer in the range of 400–800 nm

The magnetic properties were measured with home-made

vibrat-ing sample magnetometer (VSM) and evaluated in terms of

satura-tion magnetizasatura-tion and coercivity Chemical composisatura-tion of samples

was determined by JEOL Scanning Electron Microscope and Energy

Dispersive Spectroscopy (SEM/EDS) JSM-5410 Spectrometer

3 Results and discussion

3.1 Characterization of magnetic chitosan nanoparticles

TEM and SEM micrograph of magnetic chitosan particles provides

in-formation on their size and morphology It can be observed fromFig 1

that the magnetic particles have a spherical shape with a diameter of

about 30 nm

XRD pattern of magnetic chitosan nanoparticles shows six

charac-teristic peaks for Fe3O4corresponding to (220), (311), (400), (422),

(511) and (440) (JCPDSﬁle, PDF No 65-3107) (Fig 2) Quite weak

dif-fraction lines of composite indicated that Fe3O4particles have been

coat-ed by amorphous chitosan, which did not affect the phase and structure

of Fe3O4 Particle size of magnetic chitosan nanoparticles can be

estimat-ed approximately as 30 nm, via line broadening in the pattern, using

Debye–Scherrer equation (d=kλ/βcos θ)

Typical magnetization loops were recorded by VSM and shown on

Fig S1 (supporting information) From the plot of magnetization vs

magnetic ﬁeld and its enlargement near the origin, the saturation

magnetization, remanence magnetization, coercivity and squareness

could be calculated Because of no remanence and coercivity, it can be

suggested that the beads are superparamagnetic It can also be observed

from thisﬁgure that magnetization moment of Fe3O4 nanoparticles

decreases very little after chitosan surface coating, meaning that

chito-san does not affect magnetic properties of these magnetic chitochito-san

nanoparticles Therefore, maintaining such a high saturation

magnetiza-tion value (Ms) after coating these nanoparticles is advantageous and

susceptible to the external magneticﬁeld for magnetic separation

3.2 Effect of initial pH on the adsorption process

It is well known that some metals are preferentially adsorbed in

acid-ic media while chitosan can dissolve under this acidacid-ic condition In this

regard, the chemical modiﬁcation of chitosan by using crosslinking reac-tion offers an important pathway for producing chemically more stable chitosan derivatives, extending the potential applications of this bio-polymer In our study, the crosslinking approach with epichlorohydrin

to block/crosslink via hydroxyl (OH) group is expected to improve chemical stability, mechanical resistance and adsorption/desorption properties, compared to that with glutaraldehyde (to block amino (NH2) group respectively), when keeping reactive amino groups intact for complexing reaction with heavy metal ions[14,16,19]

Next, selecting an optimum pH is very important for the adsorp-tion process, since pH affects not only the surface charge of adsorbent, but also the degree of ionization and the speciation of the adsorbate during the reaction The effect of pH on the adsorption process was investigated over the range from 2 to 6 As indicated inFig 3, the maximum capacity of Cr(VI) absorption occurred at pH of 3 The ex-planation would be addressed as the pH of the aqueous solution af-fects to stability of chromium speciation and the surface charge of the adsorbent At pH 1, the chromium ions exists in the form of

H2CrO4, while in the pH range of 1–6, different forms of chromium such as Cr2O7 −, HCrO4 −, and Cr3O10 2−coexist while HCrO4 −predominates

As the pH increases, those form shifts to Cr2O4 −and Cr2O7 −[11] Cr(VI) exists predominantly as HCrO4 −in aqueous solution below pH 4 and the amino groups (–NH2) of magnetic chitosan nanoparticles would be in protonated cationic form (–NH3+) to a higher extent in acidic solution This results in the stronger attraction for negatively charged ions Electrostatic interaction between the sorbent and HCrO4 −ions also con-tributes to the high chromium removal However, at the pH lower than

3, decrease in uptake capacity is observed as the predomination of

H2CrO4 and the strong competition for adsorption sites between

H2CrO4and protons The decreasing of the adsorption capacity at higher

pH values may be explained by the dual competition of CrO4 −and OH− for adsorption[11] Thus, pH 3 was selected as the optimum pH value for the following adsorption experiment

3.3 Adsorption isotherms Equilibrium experimental data were successfully ﬁtted to the Langmuir isotherm whose equation can be expressed as

q¼qm⋅KL⋅Ce

where qm(mg·g−1) is the maximum sorption capacity (corresponding

to complete monolayer coverage), Ceis the equilibrium concentration

in the solution (mg/L), qeis the equilibrium Cr(VI) concentration in

(440)

(511)

(422) (400)

(311)

(220)

2θ

50 55 60 65 70 75 80 85

pH Fig 3 The inﬂuence of initial pH value on the adsorption of the Cr(VI) on magnetic

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the sorbent (mg·g−1), and KLis the sorption afﬁnity constant related to

the binding energy of sorption (L·mg−1) The experimental data

(Table 1)ﬁtted well with Langmuir model (R2> 0.99), conﬁrming that

the adsorption process is monolayer adsorption The results of

adsorp-tion studies by Langmuir model, indicating improved Cr(VI) uptake

properties of magnetic chitosan nanoparticles (55.80 mg·g−1, pH 3,

room temperature, compared to the other adsorbents (Table 2)),

proba-bly relates to the smaller loss of amine groups of chitosan, involved in

the cross-linking reaction when using epichlorohydrin as a cross-linker

Cr(VI) removal by adsorbent as a function of contact time with

different initial concentrations (40, 80 and 180 mg·L−1) of Cr(VI) is

shown in Fig 4, where the adsorption rate of metal uptake was

quite slow and the maximum uptake was observed within 100 min

3.4 Thermodynamic and kinetic studies

In this section, thermodynamic and kinetic aspects of the

adsorp-tion process will be considered The experimental data obtained at

different temperatures were used in calculating the thermodynamic parameters such as Gibbs free energy (ΔG), enthalpy (ΔH) and

entro-py (ΔS) according to the following equations:

Where K is the equilibrium constant, obtained from Langmuir iso-therms at different temperature and R is the universal gas constant

ΔH and ΔS were obtained from the slope and intercept of the plot log (qe/Ce) vs 1/T (Fig 5), namely:

ΔH ¼ −0:6853 kJ⋅mol −1 and ΔS ¼ −115:7366 J⋅mol −1⋅K−1:

Table 1

Adsorption equilibrium constants obtained from Langmuir isotherm in the adsorption

of Cr(VI) onto magnetic chitosan nanoparticles (volume: 50 mL; absorbent dose: 0.1 g;

initial concentrations: 60, 80 and 180 mg·L−1; pH value: 3.0; temperature: 298,308,

318 K).

Table 2

Comparison of adsorption capacities of Cr(VI) with other adsorbents.

(mg·g−1)

pH Ref.

Coconut shell activated carbon 20.00 2.0 [24]

Hazelnut shell activated carbon 17.70 2.0 [25]

Magnetic chitosan nanoparticles (this

study)

10

15

20

25

30

35

40

45

50

55

60

65

70

t(min)

40mg/L 80mg/L 180mg/L

Fig 4 Effect of contact time on Cr(VI) adsorption (volume: 50 mL; absorbent dose: 0.1 g;

−1

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

y=-13.92069+4.38115x,R=0.99017

C=80 mg/L

Fig 5 Thermodynamic plot of ln (q e /C e ) vs 1/T.

Table 3 Thermodynamic data of Cr(VI) adsorption process.

T (K) ΔG (kJ·mol −1 ) ΔH (kJ·mol −1 ) ΔS (J·mol −1 ·K −1 )

-7 -6 -5 -4 -3 -2 -1 0 1 2 3 4

t(min)

Fig 6 Kinetic pseudo-ﬁrst order sorption kinetics of Cr(VI) (volume: 50 mL; absorbent dose:

−1

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The negative value ofΔG obtained from Eq.(3)reﬂects a

spontane-ous (favorable) adsorption process of Cr(VI) (Table 3), while the

nega-tive value ofΔH indicates that the adsorption reaction is exothermic

and the adsorption of Cr(VI) is more effective at lower temperatures

Kinetically, in order to understand the behavior of the adsorbent and

to examine the controlling mechanism of the adsorption process, the

pseudo-ﬁrst-order and the pseudo-second-order were applied to the

ex-perimental data (Figs 6 and 7) The pseudo-ﬁrst-order rate expression of

Lagergren is given as: ln(qe−qt)=ln(qe)−k1⋅t where qeand qtare the

amounts of Cr(VI) (mg·g−1) adsorbed on the adsorbent at equilibrium

and at time t, respectively and k1is the rate constant ofﬁrst-order

adsorp-tion (min−1) The slopes and intercepts of plots of ln(qe−qt) vs t were

used to determine the ﬁrst-order rate constant k1 The

pseudo-second-order kinetic model is expressed as: t

qt¼ 1

k 2 ⋅q e þ1

qe t where k2

(g·mg−1·min−1) is the rate constant of second order adsorption The

slopes and intercepts of plots of t/qt vs t were used to calculate the

second-order rate constant k2and qe [22] Adsorption rate constants

were summarized inTable 4 The values of regression coefﬁcient for

pseudo-second-order model were close to 1 for all initial Cr(VI)

concen-trations The calculated values qe,calwere very close to obtained qe,exp

values Hence, the adsorption of Cr(VI) onto magnetic chitosan

nano-particles could obey the pseudo-second-order kinetic model

4 Conclusion

In this work, cross-linked with epichlorohydrin magnetic chitosan

nanoparticles were prepared and characterized The Cr(VI) adsorption

behavior on the prepared magnetic chitosan nanoparticles has been

studied under various conditions of different solution pH values and

ad-sorption contact times Optimal adad-sorption conditions of Cr(VI) were

found at pH 3, and contact time of 100 min, with maximum adsorption

capacity of 55.80 mg·g−1 The Langmuir model was found toﬁt

well with the experimental data (correlation coefﬁcient R2> 0.99),

indicating the occurrence of monolayer adsorption process Ther-modynamically, the adsorption of Cr(VI) is spontaneous (in term

ofΔG) and exothermic (in term of ΔH) process Kinetically, the adsorption of Cr(VI) onto magnetic chitosan nanoparticles obeyed the pseudo-second-order model Compared to the other adsor-bents, magnetic chitosan nanoparticles shows greatly improved uptake properties of Cr(VI), probably due to high concentration of remaining active sites on the surface of magnetic chitosan nano-particles The improved magnetic and adsorption uptake properties are two main features of the synthesized nanoparticles that can be advantageously used in water treatment

Acknowledgments Funding of this work was provided by Vietnam Ministry of Science and Technology (grant 08/2011/HÐ-NÐT)

Appendix A Supplementary data Supplementary data to this article can be found online athttp:// dx.doi.org/10.1016/j.msec.2012.12.013

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t(min) Fig 7 Kinetic pseudo-second order sorption kinetics of Cr(VI) (volume: 50 mL;

absor-bent dose: 0.1 g; initial concentrations: 40, 80 and 180 mg·L−1; pH value: 3.0;

temperature: 298 K).

Table 4

Comparison of the ﬁrst-order and second-order adsorption rate constants, calculated q e,cal and experimental q e,exp values for different initial Cr(VI) concentrations.

C 0 (mg·L −1 ) q e,exp (mg/g) First-order kinetic model Second-order kinetic model

k 1 (min−1) q e,cal (mg·g−1) R 2

k 2 (g·mg−1·min−1) q e,cal (mg·g−1) R 2

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