J. Cent. South Univ. (2016) 23: 317-323

DOI: 10.1007/s11771-016-3076-2

Single-step synthesis of magnetic chitosan composites and application for chromate (Cr(VI)) removal

YANG Wei-chun(杨卫春)^{1, 2}, TANG Qiong-zhi(唐琼芝)¹, DONG Shu-yu(董舒宇)¹,

CHAI Li-yuan(柴立元)^{1, 2}, WANG Hai-ying(王海鹰)^{1, 2}

1. School of Metallurgy and Environment, Central South University, Changsha 410017, China;

2. Chinese National Engineering Research Center for Control & Treatment of Heavy Metal Pollution,Changsha 410017, China

 Central South University Press and Springer-Verlag Berlin Heidelberg 2016

Abstract: Magnetic chitosan composites (Fe₃O₄@chitosan) were synthesized in one single-step, characterized and applied in Cr(VI) removal from water. With the increase of loading proportion of chitosan, Cr(VI) adsorption capacity of Fe₃O₄@chitosan composites increased from 10.771 to 21.040 mg/g. The optimum adsorption capacities of Cr(VI) on Fe₃O₄@chitosan-3 were found in a pH range of 3.0-5.0. Kinetic study results show that the adsorption process follows pseudo-second-order model, indicating that the rate-limiting step in the adsorption of Cr(VI) involves chemisorptions. Moreover, FT-IR spectra analysis confirms that the amine and hydroxyl groups of chitosan are predominantly responsible for binding. Results from this work demonstrate that the prepared Fe₃O₄@chitosan composites possess great potential in Cr(VI) removal from contaminated water.

Key words: Cr(VI); magnetic chitosan; one single-step synthesis; adsorption

1 Introduction

Water contamination of chromium has become a serious pollution issue as it poses a threat to the environment and ecological health. The United States Environmental Protection Agency (EPA) set the maximum concentration limit (MCL) for chromium in drinking water of 0.1 mg/L [1] and the World Health Organization set a stricter threshold of 0.05 mg/L [2]. The most common forms of chromium are hexavalent chromium (Cr(VI) and trivalent chromium (Cr(Ⅲ)). Cr(VI) is highly soluble in aqueous media, acutely toxic, mutagenic, and carcinogenic [3]. Thus, the removal of Cr(VI) from natural waters has attracted considerable attention. Typical methods used for Cr(VI) removal from water include precipitation, reduction, electrolytic removal, ion exchange, reverse osmosis, and adsorption [4-7]. Adsorption has been considered one of the most promising technologies for Cr(VI) removal in terms of cost, simplicity of design and operation [8-9]. A wide range of adsorbents, such as carbonaceous material (biocharc [10], active carbon [11]), metal oxides [9, 12-13], kaolin clay [14], have been reported for Cr(VI) removal. However, there were several problems associated with their use, such as low adsorption efficiency, poor mechanical strength.

Chitosan is not only inexpensive and abundant in nature, but it also has been recognized as a potential bioadsorbent for the removal of metals ions since its hydroxyl and amino groups can act as coordination sites for metal ions [15]. Even though advanced results have been obtained, it can be difficult to separate chitosan- based adsorbents from the aqueous solution using traditional separation methods such as filtration and sedimentation [16]. Magnetic materials have a high potential to be applied in adsorption systems because they can easily be separated in magnetic field. Iron oxide-based materials are the commonly applied magnetic particles because of their high magnetization and other favorable characteristics in terms of cost, environmental impact and chemical stability (e.g. resistance to acids and bases, low solubility) [17-18]. In order to benefit from the advantages of both of these two kinds of adsorbents, a magnetic chitosan composite adsorbent was developed [16, 19-22]. However, there are few studies on application of magnetic chitosan for Cr(VI) removal, and the synthesis procedure for the magnetic chitosan composite is typically accomplished in two or three steps. The first step often involves the preparation of magnetic materials and the second step involves the coating or dispersion of the magnetic materials with chitosan [16]. The synthesis procedures for the modification of chitosan should be simplified. The preparation of MCCs in a single step should be encouraged.

In this work, a magnetic chitosan composite adsorbent for Cr(VI) removal was synthesized in one-single step via in situ synthetic route and  characterized. The adsorption behavior and mechanism of Cr(VI) on magnetic chitosan composite was investigated.

2 Materials and methods

2.1 Materials

All chemicals were of analytical reagent grade. Ferric chloride (FeCl₃·6H₂O), ferrous chloride (FeCl₂·4H₂O), chitosan and potassium dichromate were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). The molecular mass of chitosan, provided by the manufacturer, was 100 kg/mol.

The Cr(VI) stock solution(100 mg(Cr)/L) was prepared by dissolving proper amount of potassium dichromate in distilled water. Experimental solutions for adsorption and analysis were freshly prepared by diluting Cr(VI) stock solution with distilled water.

2.2 Preparation of bare Fe₃O₄ and chitosan-coated magnetite

The preparation of chitosan-coated magnetite in this study was prepared as follows. Different amounts of chitosan of 0, 1.78, 3.57, 10.71 and 14.28 g were dissolved in 150 ml of 10%-15% (v/v) acetic acid solution, stir it well until dissolved. Then, 8.39 g of FeCl₃·6H₂O and 3.08 g FeCl₂·4H₂O were dissolved into the above solution (the amount of Fe₃O₄ prepared was assumed to be 3.57 g). Afterwards, the resulting solution was added dropwise into 600 mL of 30%(v/v) NaOH solution over a period of 1 h under vigorous stirring. Then the suspension was left undisturbed for 4 h and then washed with deionized water several times and dried at 60 °C for 24 h to obtain the adsorbent (Fe₃O₄@chitosan). The adsorbents of Fe₃O₄@chitosan with initial mass ratios of Fe₃O₄ to chitosan varied as 1:0, 1:0.5, 1:1, 1:3 and 1:4 were denoted as Fe₃O₄, Fe₃O₄@chitosan-0.5, Fe₃O₄@chitosan-1, Fe₃O₄@ chitosan-3 and Fe₃O₄@chitosan-4, respectively.

2.3 Characterization

The morphology of the Fe₃O₄@chitosan was characterized by scanning electron microscopy (SEM, JSM-6360). The X-ray diffraction (XRD) patterns of the Fe₃O₄@chitosan were obtained using Rigaku D/Max-RB diffractometer with Cu K_α radiation (λ=0.15406 nm, 35 kV, 40 mA). The magnetic properties of the adsorbents were measured with a vibrating sample magnetometer (VSM, HH-15, Nanjing University Instrument Plant, China) at room temperature. Specific surface area of chitosan-coated magnetite was measured by adsorption-desorption of ultrapure N₂ on a quantachrome instruments system (Adsorb SI) via Brunauer-Emmett-Teller (BET) method. Fourier transformed infrared spectroscopy (FT-IR) spectra of the product was obtained using a Nicolet IS10 spectrometer (Thermo Fisher, USA) at 4 cm^-1 resolution.

2.4 Adsorption experiments

All the adsorption experiments were performed in 120 mL polyethylene vials filled with 100 ml aqueous Cr(VI) solution and 0.1 g adsorbent, and the vials were placed in thermostatic water bath and shaken for 24 h at 25 °C, afterwards, the samples were filtered using a 0.45-μm membrane filter (Millipore, Billerica, MA, USA) and the Cr(VI) concentration in solutions was determined by visible spectrophotometer (722 s, Shanghai Precision Science Instrument, Ltd.) using 1,5-diphenylcarbazide method[23]. While selected samples were used to determine the total chromium by potassium permanganate oxidation-diphenylcarbazide spectro- photometric [24]. The Cr(VI) concentration in solution was very closed to total chromium in solution. The amount of Cr(VI) adsorbed was calculated from the measured aqueous concentration based on mass balance. Meanwhile, blanks were analyzed to determine losses to reactor material and show minimal effect.

The adsorption isotherm was studied by varying initial Cr(VI) concentration from 2 to 45 mg/L at pH 4.0±0.1. Kinetic experiments were carried out at pH 4.0 with two initial Cr(VI) concentrations (2.13 and 9.81 mg/L), and at selected time intervals, samples were withdrawn and analyzed for residual Cr(VI) concentration. The effects of pH on Cr(VI) adsorption were investigated by adjusting initial solution pH from 3.0 to 9.0 using 0.1 mol/L HCl and 0.1 mol/L NaOH with initial Cr(VI) concentration of 10 mg/L. All the experimental data were the average of triplicate determinations with relative errors under 5%.

3 Results and discussion

3.1 Effect of Fe₃O₄ to chitosan mass ratio on Cr(VI) adsorption

To determine the optimal mass ratio of Fe₃O₄ to chitosan, a series of Fe₃O₄@chitosan composites with different mass ratios of Fe₃O₄ to chitosan from 1:0 to 1:4, were synthesized and tested for magnetic property and Cr(VI) adsorption. The saturation magnetization of Fe₃O₄, Fe₃O₄@chitosan-0.5, Fe₃O₄@chitosan-1, Fe₃O₄@chitosan-3 and Fe₃O₄@chitosan-4 were 47.64, 32.33, 27.13, 19.44 and 15.33 A·m²/kg, respectively. It is suggested that the coating of chitosan leads to a decrease in the magnetic properties of the composite. Ma et al [25] found that a saturation value of 16.3 A·m²/kg was sufficient for magnetic separation. Thus, the saturation magnetization value achieved with Fe₃O₄@chitosan-3 was high enough for magnetic separation.

Figure 1 shows the adsorption isotherms of Cr(VI) on Fe₃O₄@chitosan composites with various mass ratio of Fe₃O₄ to chitosan. It is suggested that Cr(VI) adsorption increases obviously with an increasing mass ratio of chitosan. When the mass ratio of Fe₃O₄ to chitosan reached 1:3, the adsorption amount of Cr(VI) did not change significantly.

Fig. 1 Adsorption isotherms of Cr(VI) on Fe₃O₄@chitosan composites with various of mass ration of Fe₃O₄ to chitosan

The isotherm models of Langmuir and Freundlich were used to fit the experimental adsorption equilibrium data of Cr(VI) on magnetic adsorbents. The Langmuir model and Freundlich model are represented as Eqs. (1) and (2), respectively.

                               (1)

                                  (2)

where q_e(mg/g) is the amount of Cr(VI) adsorbed per unit mass of adsorbent; C_e(mg/L) is the mass concentration of Cr(VI) at equilibrium; b is the equilibrium adsorption constant related to the affinity of binding sites (L/mg); q_max is the maximum amount of the arsenic per unit mass of adsorbent; K_F is roughly an indicator of the adsorption capacity; n is the heterogeneity factor which has a lower value for more heterogeneous surfaces. The adsorption parameters obtained from the isotherms are presented in Table 1. As listed in Table 1, higher regression coefficient suggests that the Freundlich model is more suitable for describing the adsorption behavior of Cr(VI) on Fe₃O₄@chitosan composites. The Langmuir model assumes that adsorption occurs on a homogeneous surface. A heterogeneous surface of the Fe₃O₄@chitosan composites may be formed because of the simultaneous presence of chitosan and iron oxide in the composite. The calculated Cr(VI) adsorption capacity of Fe₃O₄@chitosan by Langmuir model increases from 10.771 to 21.040 mg/g as the coating of chitosan increases. The Freundlich model describes sorption where the sorbent has a heterogeneous surface with sorption sites that have different sorption energies. Fe₃O₄@chitosan- 0.5 to Fe₃O₄@chitosan-4 for Cr(VI) adsorption exhibit decreased n value, indicating that the adsorption process becomes easier and more efficient [26].

Considering comprehensively adsorption effectiveness and magnetic property, the Fe₃O₄@chitosan-3 composites was chosen as feasible sorbent and was detailed examined in the following sections.

Table 1 Freundlich and Langmuir model parameters for Cr(VI) adsorption

3.2 Characterizations of Fe₃O₄@chitosan-3

The specific surface area of Fe₃O₄@chitosan-3 was determined to be 62.5 m²/g. The SEM images of Fe₃O₄@chitosan-3 and Fe₃O₄ are shown in Fig. 2. It is indicated that that the surface morphology of Fe₃O₄@chitosan-3 (Fig. 2(a)) appeared to have a fluffy and irregular porous texture. The XRD pattern (Fig. 3(a)) of the Fe₃O₄@chitosan-3 proved that in the composite, the main peaks at 2θ=30.4°, 35.6°, 43.3°, 57.3°, and 62.8° are characteristic peaks of Fe₃O₄. FT-IR spectra of the Fe₃O₄@chitosan-3 (Fig. 3(b)) shows that the characteristic absorption bands for chitosan appear at 3420 cm^-1 (O—H and N—H stretching vibrations), 1620^-1 (N—H bending vibrations) and 1069 cm^-1 (—C—OH stretching vibrations).

SEM, XRD and FT-IR results are important because they proved the successful coating of magnetite by chitosan with the aid of NaOH. Chitosan on the surface is available for coordinating with Cr(VI) ions. The synthetic procedure of Fe₃O₄@magnetite composites is schematically presented in Fig. 4.

3.3 Effect of initial pH value

The Cr(VI) adsorption behavior on Fe₃O₄@chitosan-3 is pH-dependent. The effect of pH onthe adsorption of Cr(VI) onto Fe₃O₄@chitosan-3 is shown in Fig. 5. The maximum adsorption is obtained at pH 3.0; the removal efficiency is almost 100%. The adsorption of Cr(VI) decreases with increasing pH. A strong pH-dependence is observed for Cr(VI) removal by the Fe₃O₄@chitosan-3, suggesting that adsorption or binding is mainly controlled by electrostatic interactions. Whereas at a lower pH solution the number of positively charged active sites increased due to the protonation of functional groups of Fe₃O₄@chitosan-3 composite, and Cr(VI) was mainly in the form of anion; this increases the electrostatic attraction between the negatively charged anions and positively charged surface of Fe₃O₄@chitosan-3 composite, resulting in an increase in anionic Cr(VI) adsorption. The optimum adsorption capacities of Cr(VI) were found in a pH range of 3.0-5.0.

Fig. 2 SEM images of Fe₃O₄@chitosan-3 (a) and Fe₃O₄(b)

Fig. 3 XRD pattern (a) and FT-IR spectra (b) of Fe₃O₄@chitosan-3

3.4 Adsorption kinetics

Figure 6 shows the time-dependence of Cr(VI) adsorption on the Fe₃O₄-chitosan-3 composites adsorbent at various initial concentrations. The process was obviously time-dependent, and the adsorption was rapid in the first 60 min (nearly 90% removal of Cr(VI) can be achieved), and thereafter it proceeded at a relatively slow rate.

The pseudo-first-order, pseudo-second-order and Elovich kinetic models were applied to describing the kinetics of Cr(VI) adsorption on the composite. The fits of the three kinetic models to the Cr(VI) adsorption kinetic data are shown in Figs. 6(b), (c) and (d), and the model parameters obtained by curve-fitting kinetic data are listed in Table 2. Pseudo-second-order model was considered more suitable in describing the adsorption kinetics of Cr(VI) on the adsorbent according to the related coefficient (R²) (Table 2). Pseudo-second-order model considers chemisorption process. This is an indication that the rate-limiting step in the adsorption of Cr(VI) onto Fe₃O₄-chitosan-3 composites adsorbent involves chemisorptions due to valence forces through the sharing or exchange of electrons between sorbent and sorbate, complexation, coordination, and/or chelation [27].

Fig. 4 Schematic formation process of Fe₃O₄@magenitite composites

Fig. 5 Effect of pH on chromium removal efficiency

Fig. 6 Effect of reaction time on Cr(VI) adsorption:

Table 2 Kinetic and statistical parameters of three kinetic models

3.5 FT-IR spectra analysis

The function groups and surface chemical properties are of importance to understand the adsorption mechanism of Cr(VI) onto the composites.

To confirm the adsorption mechanism of Cr(VI) onto Fe₃O₄@chitosan-3, the changes characteristic adsorption bands in the adsorbent before and after adsorption were investigated by FT-IR (Fig. 7). A slight shift from 3420 to 3400 cm^-1 occurs following chromium adsorption, suggesting an enhanced hydrogen bond. The relative intensities of N—H bending vibration bands (1620 cm^-1) and —C—OH stretching vibration bands (1069 cm^-1) increased, which provides an evidence for possible protonation reaction on amine and hydroxyl groups. It is suggested that the amine and hydroxyl groups of chitosan are predominantly responsible for binding. However, more studies and further evidence are needed to fully understand the adsorption mechanism of Cr(VI) onto the composites.

Fig. 7 FT-IR spectra of Fe₃O₄@chitosan-3 before and after chromium adsorption

4 Conclusions

1) The magnetic chitosan composites (Fe₃O₄@chitosan) can be synthesized in one single-step. The calculated Cr(VI) adsorption capacity of Fe₃O₄@chitosan by Langmuir model increases from 10.771 to 21.040 mg/g as the coating of chitosan increases.

2) The optimum adsorption capacities of Cr(VI) on Fe₃O₄@chitosan-3 are found in a pH range of 3.0-5.0. Kinetic study result of Cr(VI) adsorption on Fe₃O₄@chitosan-3 shows that the adsorption process followed pseudo-second-order model, indicating the rate-limiting step in the adsorption of Cr(VI) involves chemisorptions.

3) FT-IR spectra analysis confirms that the amine and hydroxyl groups of chitosan are predominantly responsible for binding. Results demonstrate the prepared Fe₃O₄@chitosan composites possess potential in Cr(VI) contaminated water.

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(Edited by YANG Hua)

Foundation item: Projects(51304252, 51374237) supported by the National Natural Science Foundation of China

Received date: 2014-12-15; Accepted date: 2015-05-20

Corresponding author: WANG Hai-ying, Associate professor; Tel: +86-731-88830875; E-mail: haiyw25@163.com