Fibrous mat of chitosan/polyvinyl alcohol/containing cerium(III) for the removal of chromium(VI) from aqueous solution

Abstract

In the present study, the fibrous mat of chitosan/poly (vinyl alcohol) containing (CS/PVOH/Ce) is prepared by electrospinning technique. The chemical, structural and morphology characteristics of the fibrous mat are determined by Fourier infrared spectrum (FTIR) and transmission electron microscope (SEM) method. The functional groups and porous structure of the CS/PVOH/Ce fibrous mat shown in the FTIR and SEM results imply its potential ability in the adsorption of Cr(VI). To verify the analysis above, the fibrous mat is experimentally used for the removal of Cr(VI) from aqueous solution in the optimized conditions. The adsorption data from the experiment can be fit well by Langmuir isotherm. Based on Langmuir model, is calculated to be 52.88 mg/g. In addition, adsorption kinetics of Cr(VI) ions is found to conform to pseudo-second order kinetics. The mechanism of adsorption can be interpreted as a combination of electrostatic adsorption coupled with reduction reaction between CS/PVA/Ce fibrous mat and Cr(VI) ions.

Keywords

electrospinning technique chitosan/poly (vinyl alcohol) containing fibrous mat adsorption kinetics isotherms mechanism

Introduction

With the rapid development of industrials, poisoning from heavy metal pollution has become one of the most serious and widespread consequences. Chromium (Cr(VI)) compounds are widely used in the dye, stainless steel, leather tanning and electroplating industries. Cr(VI) is one of the most poisonous substances that poses threats to plants and humankind when discharged arbitrarily.¹ It can cause chest tightness, nausea, vomiting, severe diarrhea, tarry stools, corrosion of skin, brachio spasms and lung carcinoma.^2–5 Hence, it is imperative to remove Cr(VI) contaminants from aqueous environmental systems.^6,7 There are many ways to remove Cr(VI) ions from water, including chemical precipitation,⁸ membrane separation, oxidation,⁹ ion exchange,¹⁰ reverse osmosis,¹¹ electrochemical treatment and solvent extraction.¹² Of these methods, adsorption is considered to be economic, simple and versatile.

Biopolymers like chitin, chitosan, lignin, etc., have been recognized as an effective new class of low-cost adsorbents for the removal of heavy metals and exhibit some particular merits such as biocompatibility, biodegradability and hydrophilicity.¹³ Among these biosorbents, chitosan is one of the most abundant natural polymeric materials and presents an affinity for the heavy metal ions because of its free reactive hydroxyl and amino groups. Literature has reported that chitosan and its various forms such as chitosan membrane,¹⁴ cross-linked chitosan beads,¹⁵ chitosan flakes¹⁶ and cross-linked chitosan with epichlorohydrin¹⁷ have been utilized and demonstrated good adsorption capability for the removal of heavy metal. Their applications, however, are constrained because of poor stability, weak mechanical properties and inefficiency. In addition, all reactive amino and hydroxyl groups are not involved in Cr(VI) sorption. Hence, various treatments, such as cross-linking, grafting of a new functional group and acetylation are carried out to overcome these disadvantages. A main way is to introduce functional groups into chitosan molecules to make them functionalized. Previous studies have shown that rare earth elements are safe and effective for the removal of Cr(VI) ions. For example, cerium oxide nanoparticles¹⁸ and flowerlike Y₂(OH)₅NO₃·1.5H₂O and Y₂O₃¹⁹ have been used for successful sorption of Cr(VI) ions. Arsenite and arsenate ions have been removed from aqueous solution using basic yttrium carbonate.²⁰ Moreover, chitosan can be modified by rare earth elements such as lanthanum and neodymium, which have shown very promising results in getting rid of fluorine from aqueous solutions.^4,21 The reasons for the choice of cerium(III) ions adulterating in fibrous mat can be summed up in three aspects. First, rare earth ions have smaller ion radii, higher electric charge and ion potential energy than other transitional ions. And rare earth ions can form complexes with polysaccharide, which is helpful in improving thermal and mechanical properties. Second, it is known that cerium is one of the cheapest and most abundant rare earth metals. More important, cerium(III) nitrate has the effect of sterilization disinfection and is non-toxic to humans.²²

The electrospinning technique has also been investigated in numerous studies; nanoﬁbers from electrospinning show many distinctive characteristics such as large surface area per unit mass, high porosity, excellent flexibility and superior mechanical performance.¹⁴ Nonetheless, chitosan and metal cation mixtures cannot be directly fabricated by electrospinning. In order to avoid this problem, polyvinyl alcohol (PVOH) is added to the blend solution containing chitosan and cerium(III) ions to improve its electrospinning ability. PVOH has an inherent ﬁber- and ﬁlm-forming ability, which easily facilitates ﬁber production.²³

In the present study, a fibrous mat of chitosan/PVOH containing cerium(III) (CS/PVOH/Ce) was prepared and applied to the adsorption of Cr(VI) ions from an aqueous solution. The microstructure of the electronspun CS/PVOH/Ce fibrous mat was examined by using scanning electron microscopy (SEM). The functional groups of electronspun mats of PVOH, PVOH/CS blends and CS/PVOH/Ce fibrous mat were characterized with Fourier transform infrared (FT-IR) spectroscopy. The CS/PVOH/Ce fibrous mat before and after adsorption were characterized by FT-IR and X-ray photoelectron spectroscopy (XPS). Adsorption of Cr(VI) onto the fibrous mat was tested as a function of solution pH value and contact time. To fully understand the adsorbent behavior of the CS/PVOH/Ce fibrous mat, the experimental data were fitted to Freundlich and Langmuir adsorption isotherm models. The results were also analyzed on the basis of pseudo-ﬁrst order and pseudo-second order kinetic equations.

Materials and methods

Materials

PVOH (molecular weight 89,000, degree of polymerization 1750), acetic acid (88 wt%), chitosan (85% deacetylated, molecular weight: 3 × 105), cerous nitrate hexahydrate (Ce(NO₃)₃ • 6H₂O), formic acid, caustic soda (NaOH), hydrochloric acid (HCl) solution, potassium dichromate (K₂Cr₂O₇) and other necessary chemicals were all supplied by Sinopharm Chemical Reagent Co. Ltd., China.

Electrospinning

First, chitosan powders were dissolved completely in a formic acid/water solution (2 wt%) at a concentration of 4 wt%. PVOH particles were dissolved in an acetic acid/water solution (2 wt%) at a concentration of 7 wt%. Subsequently, the chitosan solution was mixed with the PVOH solution in the volume ratio of 30:70 to form the chitosan/PVOH blend solution. Then, the cerium(III) nitrate particles were mixed with the chitosan/PVOH solution at various weight ratios. After that, the mixture solution was stirred for three hours and debubbled before removing the prepared solution into a plastic syringe for electrospinning.

Electrospinning was conducted at a voltage of 15 KV and the distance between the nozzle tip and collector was 10.5 cm. The feed rate of the solution was 0.3 mL/h. The prepared solution was stored in a 20 ml plastic syringe with a blunt needle tip (0.7 mm diameter) and the collected time was 24 hours. The fibrous mat was collected with roller collection devices. A high-supply voltage was connected to the hypodermic needle, which was used as a positive electrode.

Adsorption experiments

Batch experiments were conducted in conical flasks and 50 ml Cr(VI) solution at various concentrations was prepared with potassium dichromate (K₂Cr₂O₇) at room temperature (25°C). In a typical case, 20 mg of fibrous mat with a size around 1.5 cm × 2 cm was added to 50 ml Cr(VI) solution of initial concentration 10 mg/L. The conical flasks were shaken at 300 rpm using a thermostatic shaker (TS-100B, Tensuc, Shanghai). After adsorption, the solution was filtered out and the Cr(VI) concentration in the ﬁltrate was subsequently determined by 1,5-diphenylcarbazide spectrophotometric method.²⁴ The concentration of Cr(VI) was analyzed based on the reaction between Cr(VI) and diphenylcarbazide, i.e. forming a violet-red color in an acidic medium. A double beam UV-visible spectrophotometer (Shanghai Instruments Co., Ltd., UV-2802S) was used to measure the adsorption of chromium complex at a wavelength of 540 nm.

All experiments were conducted three times; the adsorption capacity q_e(mg/g) of adsorbent was calculated by the following equation:

q_{e} = \frac{(C_{0} - C_{e}) V}{M}

(1)

where C₀(mg/L) is the initial Cr(VI) concentration, C_e(mg/L) is the Cr(VI) concentration in equilibrium at any time t, V(L) is the volume of the Cr(VI) solution and M(g) is the mass of adsorbent used.

To evaluate the effect of pH on Cr(VI) adsorption, the initial pH values of Cr(VI) solutions (10 mg/L) were adjusted to 2.0–10.0 by adding 0.1 M HCl or 0.1 M NaOH solution. The solutions were then mixed with 20 mg of sorbent and agitated for eight hours. Only the optimum pH were used in the further study.

The effect of contact time on adsorption capability of the fibrous mat was studied at an initial Cr(VI) concentration of 30 mg/L with the adsorbent dosage of 20 mg at pH 4. During the experiments, kinetic study of adsorption of Cr(VI) ions was carried out to estimate the adsorption kinetics parameters. Equilibrium time was determined from the saturation point of the adsorption kinetics data. The study of the adsorption isotherm was conducted by adding 20 mg fibrous mat into various initial concentrations of Cr(VI) ions in the range of 10–50 mg/L at pH 4 and agitating for 60 minutes.

Characterization

The morphology and surface structure of the fibrous mat were investigated with a Hitachi JSM-5610 scanning electron microscope (SEM). Surface functional groups of the fibrous mat were characterized by FT-IR spectroscopy, which was recorded by a Nicolet Nexus 470 spectrophotometer. XPS can be used to measure the adsorbed Cr(VI) ions. XPS experiments were carried out on an RBD-upgraded PHI-5000C ESCA system (Perkin Elmer) with Mg Kα radiation (hν = 1253.6 eV) or Al Kα radiation (hν = 1486.6 eV). The X-ray anode was run at 250 W and the high voltage was kept at 14.0 kV with a detection angle at 54°. The data analysis was carried out by using the RBD Auger Scan 3.21 software provided by RBD Enterprises (NY, USA).

Computations were made by means of Microcal Origin (Version 7.0) software (Origin Lab, USA).

Results and discussion

Optimization of cerium(III) adulterating

An investigation of the different quantity of cerium(III) adulterated in the CS/PVOH/Ce blend solution focuses on the effect of electrospinning and adsorption capacities for the removal of Cr(VI). The adulteration rate of cerium(III) varied 1% to 5 wt%. The proper ratio of cerium(III) adulterated in CS/PVOH/Ce blend solution not only can help easier to spin but can enhance the adsorption capacities.

In the experiment, adding low-ratio cerium(III) ions into the CS/PVOH blend solution did not affect the solution viscosity and surface tension as much. When the cerous nitrate was added into the mixture solution, whose concentration is 1% ∼ 3 wt%, thickness uniformity and well-distributed fibers were obtained. When the concentration of cerous nitrate increased to 3.5% ∼ 5 wt%, the fibers of the CS/PVOH/Ce intersected with each other in different diameters and in a droplet shape. It can be concluded as following: the electric field stretching force increases with an increase of the blend solution surface state charge density, which leads to partial fiber mat fracture. To determine the optimum cerium(III) adulteration, adsorption experiments were conducted by changing the adulteration of cerium(III) from 1 to 3 wt%. The effect of cerium(III) adulterating on the removal of Cr(VI) from water is shown in Figure 1. It is observed that the cerium(III) adulteration rate of 2.5 wt% is the best option. Therefore, the CS/PVOH/Ce fibrous mat with 2.5 wt% cerium(III) adulteration is used for further experiments.

Figure 1.

Effect of cerium(III) adulterating in blend solution.

Surface morphology analysis

Chitosan is an oxygen-rich natural polysaccharide consisting of a large amount of amino polysaccharide, which has exhibited some particular chemical and physical properties; cerium(III) is highly electropositive and shows a powerful afﬁnity for electronegative Cr(VI) ions. As seen in Figure 2(a), the SEM micrograph of the surface morphology of CS/PVOH/Ce presents the porous structure before the sorption of Cr(VI). And the porous structure of the CS/PVOH/Ce fibrous mat facilitates the adsorption of Cr(VI). After the sorption of Cr(VI), there are no major morphology changes on the fibrous mat surface. The SEM of Cr(VI) sorbed on the CS/PVOH/Ce fibrous mat can be seen in Figure 2(b).

Figure 2.

Scanning electron microscope (SEM) micrograph chitosan/polyvinyl alcohol containing cerium(III) (CS/PVOH/Ce) fibrous mat (a) before adsorption; (b) after adsorption.

FT-IR spectroscopy

The characteristic functional groups were investigated through FT-IR spectra of the PVOH, CS/PVOH and CS/PVOH/Ce (as shown in Figure 3). The FT-IR spectrum of the pure PVOH shows absorption peaks at around 1716 cm⁻¹ and 1174 cm⁻¹ for the C–O group, and the next band at 1420 cm⁻¹ can be attributed to the OH group. The spectrum of CS/PVOH shows a broad band at around 3284 cm⁻¹, which is attributed to OH stretching vibrations, and this band overlaps with the NH bands of amine and amide. The bands observed at 1702 cm⁻¹ and at 1600 cm⁻¹ are due to the interaction between OH and NH bending vibrations of CS/PVOH. Compared to the spectra of the PVOH and CS/PVOH fibrous mat, the adsorption bands at 1309 cm⁻¹ and 1084 cm⁻¹ are attributed to the N-H group from chitosan coordinating with cerium(III).

Figure 3.

Fourier transform infrared spectroscopy (FT-IR) spectra of polyvinyl alcohol (PVOH), chitosan/PVOH (CS/PVOH) and chitosan/PVOH containing cerium(III) (CS/PVOH/Ce).

Effect of pH

To investigate the effect of pH, adsorption experiments were conducted at a temperature of 25°C in the pH range of 2–10. About 20 mg of CS/PVOH/Ce fibrous mat was added to 50 mL solution with a Cr(VI) initial concentration of 10 mg/L. As shown in Figure 4, the pH value of the medium is one of the important parameters that significantly affects the sorption capacity. It can be seen that the adsorption value takes the maximum at pH 4, whereas the removal of Cr(VI) is observed to decrease in alkaline solution. The decrease of Cr(VI) adsorption on the CS/PVOH/Ce fibrous mat in alkaline medium can be attributed to three main factors: (1) the competition between hydroxyl ions and Cr(VI) ions for adsorption sites; (2) a decrease of the surface charge on the adsorbent in the adsorption process; and (3) the appearance of chromium metal precipitation at pH higher than 6.0.²⁵ For these reasons, the further experiments were conducted in acidic solution at pH 4.

Figure 4.

pH effect on Cr(VI) sorption. Conditions: 10 mg/L Cr(VI) solution; temperature, 25°C; adsorbent, 20 mg; shaking time, eight hours.

Adsorption kinetics

Kinetic models can be helpful in understanding the mechanism of metal adsorption and evaluating the performance of the adsorbents for metal removal. The kinetics of Cr(VI) ion adsorption by fibrous mat is represented in Figure 5(a). The adsorption amount of metals increased sharply from five minutes to 40 minutes and then leveled off after 60 minutes. The equilibrium is reached in approximately 60 minutes and is constant up to 8 hours. A number of kinetic models have been developed to describe the kinetics of heavy metal removal. The kinetics of Cr(VI) adsorption on the fibrous mat fit well with the pseudo-second-order model (R²= 0.999).

Figure 5.

(a) Effect of time on Cr(VI) adsorption. Conditions: 30 mg/L Cr(VI) solution; temperature, 25°C; initial pH, 4. (b) Pseudo-second-order kinetic model for adsorption of Cr(VI). Conditions: 30 mg/L Cr (VI) solution; temperature, 25°C; initial pH, 4; adsorbent, 10 mg.

The pseudo-second-order model is shown as follows:

\frac{t}{q_{t}} = \frac{1}{h} + \frac{t}{q_{e}}

(2)

When t → 0, the initial sorption rate, h can be expressed as follows:

h = κ q_{e}^{2}

(3)

where q_e(mg/g) and q_t(mg/g) denote the amount of Cr(VI) adsorbed at equilibrium and adsorbed at time t, respectively. h (mg/g min) is the initial sorption rate and κ(g/mg min) is the rate constant of the adsorption process. The application of the pseudo-second-order by plotting

t / q_{t}

versus t is shown in Figure 5(b). The fitness of the data and the values of q_e, κ and hcalculated from the plot of

t / q_{t}

versus t are presented in Table 1. The calculated q_e(47.34 mg/g) shows a good agreement with the experimental value q_e,exp(46.99 mg/g), and the value of coefficient of determination (R²) for the pseudo-second-order kinetic model is 0.999. Therefore, the system under study is appropriately described by the pseudo-second-order model. These results suggest that the adsorption kinetics is controlled by chemisorptions.

Table 1.

Pseudo-second-order models for the adsorption of Cr(VI)

C₀ (mg/L)	q_e,exp (mg/g)	Pseudo-second-order
C₀ (mg/L)	q_e,exp (mg/g)	q_e (mg/g)	κ (g/mgmin)	h (mg/gmin)	R ²
30	46.99	47.34	0.004	7.867	0.999

Adsorption isotherms

An adsorption isotherm can be used to characterize the interaction of metal ions with adsorbents. The maximal adsorption of the fibrous mat in different concentrations of Cr(VI) are measured and shown in Figure 6(a). It can be seen that the relationship between the amounts of metal ions adsorbed onto the adsorbent surface q_e(mg/g) and the remaining metal ion concentration C_e(mg/L) is in the aqueous phase when both phases are in equilibrium.²⁶ It is showed that the adsorption capacity increased with the equilibrium concentration of the Cr(VI) ions in the solution, gradually reaching a saturation state of the adsorbent.

Figure 6.

(a) Inﬂuence of Cr(VI) equilibrium concentration on the adsorption on Cr(VI); (b) Langmuir isotherm for Cr(VI) adsorption. Condition: temperature, 25°C.

Various isotherms have been applied to demonstrate the adsorption equilibrium. In this work, the Langmuir isotherm fits well with the experimental data, based on the correlation coefficient (R²= 0.992).

The Langmuir isotherm²⁷ equation is shown as follows:

\frac{C_{e}}{q_{e}} = \frac{1}{K_{L} q_{m}} + \frac{C_{e}}{q_{m}}

(4)

where q_e(mg/g) is the amount of the sorbate sorbed at complete monolayer coverage, and C_e(mg/L) is the equilibrium concentration. The maximum amount of adsorption capacity q_m(mg/g) and the energy of adsorption K_L(L/mg) are determined from the slope and intercept. A linear plot is obtained for the sorbent by plotting

C_{e} / q_{e}

against C_e, as shown in Figure 6(b). The maximum adsorption capacity of Cr(VI) ions based the Langmuir isotherm is 52.88 mg/g. The energy of adsorption K_L is 2.148 L/mg. The basis of the Langmuir theory is that adsorption appears at specific homogeneous sites within the adsorbent,²⁸ and adsorption takes place on the same surface active position spot when the Cr(VI) ions bind to the active site. Hence, monolayer adsorption occurs on the CS/PVOH/Ce fibrous mat.

Comparison of adsorption capacity with other adsorbents

In the present study, the CS/PVOH/Ce fibrous mat is used as an adsorbent for the removal of Cr(VI) ions. It is found that the maximum adsorption capacity of Cr(VI) ions based on the Langmuir isotherm is 52.88 mg/g. For comparison, the Cr(VI)adsorption capacities of different adsorbents reported in the literature are given in Table 2. When compared with chitosan²⁹ and some chitosan-based adsorbents, such as modified chitosan beads,³⁰ chitosan-coated fly ash,³¹ alumina/chitosan biocomposite,³² La(III)-encapsulated silica gel/chitosan composite,³³ magnetic chitosan resin³⁴ and chitosan/zeolite conjugate 13X film,³⁵ the adsorption capacity of the CS/PVOH/Ce fibrous mat is higher. The cost-effective natural adsorbent activated carbon³⁶ is commonly used to remove Cr(VI) ions, but the sorption capacity is not satisfactory. Although rubberwood-activated carbon³⁷ obtains a high adsorption capacity, it is inferior to that of the CS/PVOH/Ce fibrous mat. In recent years, some metallic oxides with high sorption capacity for Cr(VI) ions have been developed. Recillas et al.³⁸ reported that the sorption capacity of cerium oxide reached 70.41 mg/g at pH 7. Liu et al.³⁹ reported that the sorption capacity of titanium oxide-Ag composite (TOAC) reached 25.7 mg/g at pH 2. However, the widespread use of these metals in preparation increased the cost of the adsorbents. It is notable that the CS/PVOH/Ce fibrous mat has a better sorption capacity among comparable adsorbents. The high adsorption capacity of the fibrous mat might be due to the large surface area per unit mass and the positive charge of cerium(III) at the surface.

Table 2.

Comparison of Cr(VI) adsorption capacities of various adsorbents

Adsorbents	Sorption capacity (mg/g)	Solution pH	Reference
Chitosan	22.09	4.0	29
Modiﬁed forms of chitosan beads	3.24	4.0	30
Chitosan-coated fly ash	33.27	5.0	31
Alumina/chitosan biocomposite	8.62	5.0	32
La(III)-encapsulated silica gel/chitosan composite	6.127	4.0	33
Magnetic chitosan resin	51.813	2.0	34
Chitosan/zeolite conjugate 13X film	17.28	4	35
Activated carbons	37	3.0	36
Rubberwood-activated carbon	44.05	2.0	37
Cerium oxide	70.41	7.0	38
Titanium oxide-Ag composite (TOAC)	25.7	2.0	39
Chitosan/polyvinyl alcohol containing cerium(III) fibrous mat	52.88	4.0	Present study

Mechanism of adsorption of Cr(VI) on fibrous mat

To verify the chemical reactions conducted on the surface of the fibrous mat, the samples are characterized by high-resolution XPS. Figure 7 shows the typical wide scan XPS spectra for the CS/PVOH/Ce fibrous mat before and after Cr(VI) adsorption. It is clear that a new peak at the banding energy (BE) about 580 eV appeared after Cr(VI) reduction. The presence of the band is designated to the photoelectron peak of Cr(VI) and indicates the uptake of Cr(VI) on the surface of the fibrous mat.

Figure 7.

Typical wide scan X-ray photoelectron spectroscopy (XPS) spectra for the fibrous mat before and after Cr(VI) adsorption: (a) before Cr(VI) adsorption; (b) after Cr(VI) adsorption. Conditions: 10 mg/L Cr(VI) solution; temperature, 25°C; initial pH, 4; adsorbent, 20 mg.

The removal mechanism of Cr(VI) by the fibrous mat can be described in two steps shown in Scheme 1: First, the electrostatic force attracts anionic hexavalent chromium ions ( ${HCrO}_{4}^{-}$ ) toward the fibrous mat. Second, there is a chemisorption for the removal of anionic chromium ions by means of complexation.

To better explain the mechanism of Cr(VI) anions adsorbed on the fibrous mat in a solution, it is necessary to analyze the Cr(VI) speciation and the surface charge with the variation of pH values. Cr(VI) exists primarily as salts of $H_{2} {CrO}_{4}$ , ${HCrO}_{4}^{-}$ and ${CrO}_{4}^{2 -}$ _, depending on the pH values. $H_{2} {CrO}_{4}$ predominates at pH less than 1.0, ${HCrO}_{4}^{-}$ at pH between 2.0 and 6.0, and ${CrO}_{4}^{2 -}$ pH above about 6.0.⁴⁰ In acidic solutions, more positively charged surface sites are developed by the protonation of amino groups on chitosan and the highly electropositive cerium (III), which will attract anionic hexavalent chromium ions ( ${HCrO}_{4}^{-}$ ). And the reactive amino groups in the fibrous mat are presumably responsible for selective chelation with anionic ${HCrO}_{4}^{-}$ .⁴¹ In addition, under acidic condition at pH between 2.0 and 6.0, the H⁺ ions in the solution are high and the reactive –OH and –NH₂ act as electron donor groups on the surface of the fibrous mat. Therefore, the following reaction also occurs:

{HCrO}_{4}^{-} + 7 H^{+} + 3 e^{-} \to {Cr}^{3 +} + 4 H_{2} O

(5)

Then the reduced Cr (III) ions may be chelated with the amine and hydroxyl groups of the fibrous mat to form complexations. To identify the interactions between fibrous mat and Cr(VI) ions, further analyses were carried out through the FT-IR spectra. Figure 8 presents the FT-IR spectra of the fibrous mat before and after adsorption of Cr(VI). And the FT-IR spectra of the Cr(VI)-loaded fibrous mat have a new band at 540 cm⁻¹, which confirms the formation of Cr (OH)₃ in the Cr(VI)- sorbed composite.³³ Figure 8(a) shows the characteristic peaks at 1690 and 3274 cm⁻¹, which contribute to the N–H and O–H bending vibration, respectively. After Cr(VI) adorption, the FT-IR spectrum for the fibrous mat in Figure 8(b) showed a shift of the peak at 1650 and 3280 cm⁻¹. The results prove that the functional groups of amide (II) and hydroxyl groups may participate in the Cr(VI) removal process.

Figure 8.

Fourier transform infrared spectroscopy (FT-IR) spectra for the fibrous mat before and after Cr(VI) adsorption: (a) before Cr(VI) adsorption; (b) after Cr(VI) adsorption.

Scheme 1.

The mechanism of Cr(VI) adsorption by chitosan/polyvinyl alcohol containing cerium(III) (CS/PVOH/Ce) fibrous mat.

At more acidic pH (below pH 4.0), the adsorption capacity of the fibrous mat for the removal of Cr(VI) decreases, which may be because the surface charge is not optimal. The results are identical to the pH adsorption experiments. Besides pH, another key factor that dominates the adsorption of Cr(VI) on the fibrous mat is the available surface area. The fibrous mat has porous structures that facilitate the reaction between the Cr(VI) and the fibrous mat.

Conclusion

In this study, a CS/PVOH/Ce fibrous mat was prepared and used as a biosorbent for the removal of Cr(VI) from aqueous solution. The CS/PVOH/Ce fibrous mat was characterized by FT-IR, SEM and XPS. Results showed that the fibrous mat presented a porous structure and the Cr(VI) were adsorbed onto the fibrous mat effectively. The adsorption kinetics studies of Cr(VI) on the CS/PVOH/Ce fibrous mat were performed based on pseudo-ﬁrst order and pseudo-second order. The data indicated that the adsorption kinetics of Cr(VI) on the CS/PVOH/Ce fibrous mat followed the pseudo-second order. The equilibrium data have been analyzed using the Langmuir isotherm. It was found that data followed by Langmuir isotherms and the equilibrium adsorption capacities for Cr(VI) are 52.88 mg/g. The adsorption of Cr(VI) on the surface of the CS/PVOH/Ce fibrous mat is found to depend mainly on the pH of the solution. The adsorption of Cr(VI) in acidic pH is higher as compared to alkaline pH. The mechanism of adsorption of Cr(VI) on CS/PVOH/Ce fibrous mat can be explained in terms of a combination of electrostatic adsorption coupled with reduction. Meanwhile, the experiment result proves that the CS/PVOH/Ce fibrous mat is more efficient than bare chitosan. Thus, this system is promising as a ﬁlter that can get rid of Cr(VI) ions.

Footnotes

Funding

The project was supported by the PhD Programs Foundation of the Ministry of Education of China (No. 20090093110006).

Conflict of interest statement

The authors declare that they have no conflict of interests.

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