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

va ha Tr et R Roa , 18 oc V uce tion he l. M Lan tio adsorbent for Cr(VI) in wastewater treatment technology. heavy orldw usly co ms. Am Materials Science and Engineering C 33 (2013) 1214–1218 Contents lists available at SciVerse ScienceDirect Materials Science a .e ltion, 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 CrO42− 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 fromwastewater such asfiltration [2], electrochemical pre- cipitation [3], adsorption [4,5], electrodeposition [6] and membrane sys- tems or ion exchange process [7–9]. Among thesemethods, 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 polymermatrix, which interactwithmetal ions in solution by ion ex- acterized and applied for the removal of Cr(VI) in thewater 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 NH3 were of analytical grade. Chitosan (MW=400,000, DA=70%)was purchasedchange and complexation reactions [13–15]. M adsorbents are submicron tomicron-sized and ities to ensure adequate surface area for a diffusion limitation within the particles leads ⁎ Corresponding author. Tel.: +84 4 37564129; fax: + E-mail address: tdlam@vast.ac.vn (T.D. Lam). 0928-4931/$ – see front matter © 2012 Elsevier B.V. All the various heavy xic pollutants generated l finishing, steel fabrica- The application ofmagnetic adsorbent technology to solve environmental problems has received considerable attention in recent years [16–18]. In this paper, magnetic chitosan nanoparticles were prepared, char- metals, chromium (Cr) is one of the most to by the electroplating, leather tanning, metaAdsorption isotherm Cr(VI) 1. Introduction Contamination of water by toxic charge of industrial wastewater is a w lem. Rapid industrialization has serio of toxic heavy metals to water streametals through the dis- ide environmental prob- ntributed to the release 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 by filtration or centrifugation [16].Magnetic chitosan nanoparticles © 2012 Elsevier B.V. All rights reserved.Keywords: Fe3O4 could serve as a promisingMagnetic chitosan nanoparticles for remo Nguyen Ngoc Thinh a, Pham Thi Bich Hanh b, Le Thi T Vu Dinh Hoang a, Le Hai Dang d, Nguyen Van Khoi b, a School of Chemical Engineering, Hanoi University of Science and Technology, 1, Dai Co Vi b Institute of Chemistry, Vietnam Academy of Science and Technology, 18, Hoang Quoc Viet c Institute of Marine Geology and Geophysics, Vietnam Academy of Science and Technology 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 Qu a b s t r a c ta 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 A simple method was introd rohydrin cross-linking reac 30 nm. It was found that t pseudo-second-order mode 55.80 mg·g−1, according to fore and after Cr(VI) adsorp j ourna l homepage: wwwost of the chitosan-based need large internal poros- dsorption. However, the to the decrease in the 84 438360705. rights reserved.l of Cr(VI) from aqueous solution nh Ha a, Le Ngoc Anh c, Tran Vinh Hoang a, an Dai Lam e,⁎ oad, Hanoi, Viet Nam d, Hanoi, Viet Nam , Hoang Quoc Viet Road, Hanoi, Viet Nam iet Road, Hanoi, Viet Nam d to prepare magnetic chitosan nanoparticles by co-precipitation via epichlo- . The average size of magnetic chitosan nanoparticles is estimated at ca. adsorption of Cr(VI) was highly pH-dependent and its kinetics follows the aximum adsorption capacity (at pH 3, room temperature) was calculated as gmuir isotherm model. The nanoparticles were thoroughly characterized be- n. From this result, it can be suggested that magnetic chitosan nanoparticles nd Engineering C sev ie r .com/ locate /msecfrom Nha Trang Aquatic Institute (Vietnam) and re-characterized by viscometry and IRmeasurements 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 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 chitosan flake 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 the flask 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: qt ¼ C0−Ctð Þ⋅V m ð1Þ whereC0 and 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), at λmax=540 nm, according to the 1,5-diphenyl-carbazidemethod [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) 1215N.N. Thinh et al. / Materials Science and Engineering C 33 (2013) 1214–1218Fig. 1. Camera pictures (a, b); FE-SEM images (c, d) and TEM images (e, f) of magnetic chitosan nanoparticles. 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 magnetization and coercivity. Chemical composition 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 TEMandSEMmicrograph ofmagnetic chitosanparticles provides in- formation on their size and morphology. It can be observed from Fig. 1 that the magnetic particles have a spherical shape with a diameter of regard, the chemicalmodification 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 in Fig. 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 2− − 2− − 1216 N.N. Thinh et al. / Materials Science and Engineering C 33 (2013) 1214–1218about 30 nm. XRD pattern of magnetic chitosan nanoparticles shows six charac- teristic peaks for Fe3O4 corresponding to (220), (311), (400), (422), (511) and (440) (JCPDS file, PDF No. 65-3107) (Fig. 2). Quiteweakdif- fraction lines of composite indicated that Fe3O4 particles have been coat- ed by amorphous chitosan, which did not affect the phase and structure of Fe3O4. Particle size ofmagnetic 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 this figure 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 chitosan nanoparticles. Therefore, maintaining such a high saturationmagnetiza- tion value (Ms) after coating these nanoparticles is advantageous and susceptible to the external magnetic field for magnetic separation. 3.2. Effect of initial pH on the adsorption process It iswell known that somemetals are preferentially adsorbed in acid- ic media while chitosan can dissolve under this acidic condition. In this 20 30 40 50 60 70 (440) (511) (422) (400) (311) (220) In te ns ity 2θFig. 2. XRD pattern of magnetic chitosan nanoparticles.suchasCr2O7 , HCrO4 , andCr3O10 coexistwhileHCrO4 predominates. As the pH increases, those form shifts to Cr2O42− and Cr2O72− [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 H2CrO4 and protons. The decreasing of the adsorption capacity at higher pH values may be explained by the dual competition of CrO42− and OH− for adsorption [11]. Thus, pH 3was selected as the optimumpHvalue 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 q ¼ qm⋅KL⋅Ce 1þ KL⋅Ce ð2Þ where qm (mg·g−1) is the maximum sorption capacity (corresponding to complete monolayer coverage), Ce is the equilibrium concentration in the solution (mg/L), qe is the equilibrium Cr(VI) concentration in 2 3 4 5 6 50 55 60 65 70 75 80 85 R (% ) pH Fig. 3. The influence of initial pH value on the adsorption of the Cr(VI) on magnetic chitosan nanoparticles. the sorbent (mg·g−1), and KL is 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: ΔG ¼ ΔH−T  ΔS ð3Þ lnK ¼ ln qe=Ceð Þ ¼−ΔH=RT þ ΔS=R: ð4Þ Where K is the equilibrium constant, obtained from Langmuir iso- 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). Temperature (K) qmax (mg·g−1) KL R2 298 55.80 0.366 0.993 308 46.71 0.119 0.997 318 43.29 0.138 0.994 Table 2 Comparison of adsorption capacities of Cr(VI) with other adsorbents. Adsorbents Adsorption capacity (mg·g−1) pH Ref. Tires activated carbon 58.50 2.0 [23] Rubberwood activated carbon 44.05 2.0 [3] Coconut shell activated carbon 20.00 2.0 [24] Hazelnut shell activated carbon 17.70 2.0 [25] 3.20 3.22 3.24 3.26 3.28 3.30 3.32 3.34 3.36 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 y=- 13. 920 69+ 4.3 811 5x, R= 0.9 901 7 Ln (q e /C e ) 1/T*10-3 (1/K) C=80 mg/L Fig. 5. Thermodynamic plot of ln (qe/Ce) vs. 1/T. -3 -2 -1 0 1 2 3 4 40mg/L 80mg/L 180mg/L ln (q e - q t ) 1217N.N. Thinh et al. / Materials Science and Engineering C 33 (2013) 1214–1218Beech sawdust 16.10 1.0 [26] Sugarcane bagasse 13.40 2.0 [27] Coconut shell charcoal 10.88 4.0 [28] Coconut tree sawdust 3.60 3.0 [29] Chitosan 22.09 3.0 [30] Non-cross linked chitosan 78.00 5 [31] Cross linked chitosan 50.00 5 [31] Magnetic chitosan nanoparticles (this study) 55.80 3 35 40 45 50 55 60 65 70 q(m g/g ) 40mg/L 80mg/L 180mg/L0 50 100 150 200 250 300 350 400 10 15 20 25 30 t(min) Fig. 4. Effect of contact time on Cr(VI) adsorption (volume: 50 mL; absorbent dose: 0.1 g; initial concentrations: 40, 80 and 180 mg·L−1; pH value: 3.0; temperature: 298 K).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 3 Thermodynamic data of Cr(VI) adsorption process. T (K) ΔG (kJ·mol−1) ΔH (kJ·mol−1) ΔS (J·mol−1·K−1) 298 −35.164 −0.6853 −115.7366 303 −35.742 307 −36.205 312 −36.784-50 0 50 100 150 200 250 300 350 -7 -6 -5 -4 t(min) Fig. 6. Kinetic pseudo-first order sorptionkinetics of Cr(VI) (volume:50 mL; absorbent dose: 0.1 g; initial concentrations: 40, 80 and 180 mg·L−1; pH value: 3.0; temperature: 298 K). indicating the occurrence of monolayer adsorption process. Ther- modynamically, the adsorption of Cr(VI) is spontaneous (in term of ΔG) 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 10 12 14 16 18 40mg/L 80mg/L 180mg/L in. g/m g) 1218 N.N. Thinh et al. / Materials Science and Engineering C 33 (2013) 1214–1218The negative value of ΔG obtained from Eq. (3) reflects a spontane- ous (favorable) adsorption process of Cr(VI) (Table 3), while the nega- tive value of ΔH 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 qe and qt are the amounts of Cr(VI) (mg·g−1) adsorbed on the adsorbent at equilibrium and at time t, respectively and k1 is 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: tqt ¼ 1 k2⋅qe2 þ 1qe  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 k2 and qe [22]. Adsorption rate constants were summarized in Table 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,cal were very close to obtained qe,exp 0 50 100 150 200 250 300 350 0 2 4 6 8 t/q t(m 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).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 adsorption conditions of Cr(VI) were