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ferrous chloride composite nanofibers
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Removal of Cr(VI) using polyacrylonitrile/ferrous chloride composite nanofibers Fang Liu a, Xinhong Wang a, Bor-Yann Chen b, Shilin Zhou c, Chang-Tang Chang d,∗ a
Department of Environmental Science and Technology, College of the Environment and Ecology, and Key Laboratory of the Ministry of Education for Coastal and Wetland Ecosystem, Xiamen University, Xiang’an South Road, Xiang’an District, Xiamen 361102, China b Department of Chemical and Materials Engineering, National ILAN University, I-Lan 26047 Taiwan c College of Science, University of Shanghai for Science and Technology, Shanghai 200433, China d Department of Environmental Engineering, National ILAN University, I-Lan 26047, Taiwan
a r t i c l e
i n f o
Article history: Received 17 December 2015 Revised 18 October 2016 Accepted 23 October 2016 Available online xxx Keywords: Electrospinning PAN/FeCl2 composite nanofibers Cr(VI) Adsorption Ions effect
a b s t r a c t Considering economic feasibility and technical effectiveness, polyacrylonitrile/ferrous chloride composite nanofibers prepared by electrospinning was used to remove the Cr(VI). Maximum removal of Cr(VI) by composite nanofibers was observed in weakly acidic environment. The analysis of the reaction kinetics revealed that the adsorption behavior seemed to follow pseudo-second-order kinetic model. Through calculating thermodynamic parameter, the adsorption reaction was proved to be an exothermic and chemical process. The PAN…Fe1 −x Crx (OH)3 and PAN…Fe1 −x Crx (OH)x were proposed to be formed after the redox reaction was taken place. The interactive effects of coexisting ions of different metal ions like arsenic, lead and cadmium were also explored for practical consideration. The presence of these metal ions apparently decreased the efficiency of the Cr(VI) adsorption. In addition, the interactive inhibition of arsenic was more significant than other coexisting ions. It was suspected that the valence state (e.g., NO3 − , SO4 2− , Cl− , Ca2± , Na± and Mg2± ) may play a crucial role during the reaction. Due to the strong electrostatic attraction, anions had the more significant attenuating effect than cations. This study is helpful not only for doping design of iron oxide nanofibers but also for practical application during advanced wastewater treatment. © 2016 Published by Elsevier B.V. on behalf of Taiwan Institute of Chemical Engineers.
1. Introduction Chromium and its compounds are widely used in many industries, such as metal processing, electroplating, leather tanning, paint machining, pharmaceutical, mining, and textile industries. Apparently, these industries produce a significant amount of chromium-containing wastewater [1,2]. In fact, chromium is naturally present in the form of trivalent (III) and hexavalent (VI). In generally, trivalent chromium is considered an essential micronutrient for living organisms and is relatively less toxic than hexavalent chromium to be easily absorbed and accumulated in cells. However, due to high toxicity potency of Cr(VI), carcinogenic action, mobility in water, and ability to induce genetic mutations, Cr(VI) is evidently a threat to global lives [3,4]. Therefore, one important method to remove hexavalent chromium from the environment is to detoxify Cr(VI)–Cr(III) based on the redox reaction.
∗
Corresponding authors. E-mail addresses: [email protected] (X. Wang), [email protected] (C.-T. Chang).
In fact, several methods have been developed for the removal of Cr(VI) from aqueous solutions, including reverse osmosis [5], ion exchange [6,7], chemical precipitation, adsorption and flocculation [8]. Among these, adsorption has been extensively used because it is cost-effective and environmentally appropriate [9–11]. However, the properties of the adsorbent material strongly limited adsorption equilibrium time, selectivity, efficiency, and regeneration [12] for practicability. There are many types of nanoporous adsorbents, including nanocarbon materials, mesoporous carbon materials, mesoporous silica materials, metal oxide nanoparticles and nanofibers [12–14]. However, some of these materials required long periods of time to reach equilibrium as they had low adsorption capacity or difficulties to subsequent reuse. Recently, nanofibers provided a solution to these problems. They can satisfy the demand for materials used as adsorbents for a wide range of applications. Moreover, electrospinning is well known to be an effective and simple method for the preparation of nanofibers. In addition, the obtained polymer composite nanofibers owned a high surface area and good porosity and dimensional stability, making them more promising adsorbent materials.
http://dx.doi.org/10.1016/j.jtice.2016.10.043 1876-1070/© 2016 Published by Elsevier B.V. on behalf of Taiwan Institute of Chemical Engineers.
Please cite this article as: F. Liu et al., Removal of Cr(VI) using polyacrylonitrile/ferrous chloride composite nanofibers, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.10.043
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As our prior study revealed, polyacrylonitrile/ferrous chloride (PAN/FeCl2 ) composite nanofibers with electrospinning techniques were successfully prepared [15]. Here, the aim of this study tended to uncover the adsorption mechanisms at various experimental conditions with inspecting characterizations of X-ray powder diffraction (XRD), Fourier infrared spectroscopy (FTIR), Thermo gravimetric analyzer (TGA) and X-ray photoelectron spectroscopy (XPS). The effect of different ions and interactive competition between heavy metals on adsorption performance are also discussed herein. The surface of the composite nanofibers covered large quantities of PAN…Fe() (OH)+ complexes that strongly adsorbed Cr(VI) from the solution onto the composite nanofibers via redox reactions. That is, Cr(VI) could be detoxified to Cr(III) species at lower toxicity, as the ferrous ion was oxidized to be Fe(III). This study also provided model assessment upon Cr(VI) adsorption for conclusive remarks. 2. Experimental methods 2.1. Reagents and materials Polymer spinning solution polyacrylonitrile (PAN) powder was provided by the Sigma–Aldrich Inc. with a molecular weight of 150,0 0 0 g/mol. In addition, dimethylacetamide (DMAc) was used as spinning solvent and obtained from the Sigma–Aldrich Company. The modifying agent ferrous chloride (FeCl2 ·nH2 O) was purchased from NIHON SHIYAKU REAGENT. The pH of solutions was adjusted with HCl and NaOH solution (NIHON SHIYAKU REAGENT). The standard solution of hexavalent chromium (Cr(VI)) and the other ions were obtained from J. T. Baker Company for preparation of heavy metals and ions mixture solution. Deionized (DI) water was used throughout this work to prevent environmental interference. All the chemicals were used without further purification. 2.2. Preparation of PAN/FeCl2 composite nanofibers The PAN nanofibers was prepared by dissolving 1.05 g of PAN powder in 20 ml DMAC solution at 80 °C and then magnetically stirring for 10 h. Next, the FeCl2 ·nH2 O particles were then added to the abovementioned PAN solution and the reaction was carried out at 30 °C for 3 h. Different content of FeCl2 were used to prepare modifying agents were prepared. Comprehensive information regarding the synthesis methods is reported in our previous study [15]. The solution was separately sonicated for 3 h to obtain a homogeneous solution for spinning. Finally, the solutions were converted to composite nanofibers by electrospinning apparatus. The applied voltage between the tip to the needle and collector was 16 kV and the distance between the collector and the tip was 15 cm. The feed rate was 1.0 ml/h and the diameter size of needle was 0.52 mm. A rotating metal drum (diameter: 8 cm, rotating speed: 150 rpm) wrapped with aluminum foil was used to collect nanofibers. The same conditions were used to prepare the pure PAN nanofibers. 2.3. Characterization The morphological analysis of the composite nanofibers was characterized using a scanning electron microscopy (SEM, JHC S4700). First, a small sample was dispersed in the ethanol solution by sonication. After drying, the gold-plated metal stage was secured using carbon tapes to increase its electrical conductivity ˚ Finally, the surface morphology was (gold plating thickness: 150 A). observed. The content of the elements was observed by energy dispersive spectrometer (EDS, JHC H-7200). The impact of X-rays released by striking samples with an electron was used to analyze the elements as a form of qualitative and semi-quantitative-bases.
Because the hydrogen has only one electron, when the electron was energized that there was no more available electron remained for transferring. Therefore, there was no any signal appeared by Si(Li) detector, and thus the EDS spectra could not simply indicate the amount of hydrogen present in the sample. X-ray powder diffraction (XRD, MXP18) was used to analyze the crystal structure of the composite nanofibers with a light source Cu K as the radioactive source (λ = 0.1541 nm), an output power of 18 KW and a scanning range of 20 0–80 0 ° for 50 min−1 . Diffraction angle measures and strength pattern tests were implemented to analyze the crystal and polycrystalline microstructures. The functional groups of composite nanofibers were observed via Fourier infrared spectroscopy (FTIR, Spectrum 100). The surrounding of the instruments was purged with nitrogen to maintain dry conditions. By uniformly arranging the powder samples on the FITR detectors, the spindles and knobs on the samples were analyzed via infrared photo detectors. A thermo gravimetric analyzer (TGA, STA60 0 0) was used to observe thermal stability characteristics and components of the sample by changing the ambient temperature, where the sample was placed under nitrogen atmosphere. The x-ray photoelectron spectroscopy (XPS) analysis was carried out to determine the elements that presented on the surface of the nanofibers before and after adsorption. The concentration of iron ions and Cr(VI) was determined by an inductivity coupled plasma atomic emission spectrophotometer (ICP-AES, AU12440312). A pH meter (SP-701) was used for the measurement of the solution. 2.4. Adsorption capacity assessment The batch adsorption experiments were carried out using 500 ml flask containing 0.5 g dry PAN/FeCl2 composite nanofibers under different Cr(VI) concentrations at different temperatures (i.e., 298, 308, and 318 K). The flask was placed in a shaker to maintain well-mixed conditions. The pH level of the solutions was adjusted with 0.1 M HCl or 0.1 M NaOH solution. The effects of contact time, different pH values, and adsorption dosage on the performance of Cr(VI) removal were also examined. After completion of the experiment, the amount of metal ions was quantified by ICP-AES. After the adsorption reaction of the composite nanofibers, the deionized water was used to rinse off physically adsorbed ions. Subsequently, the composite nanofibers were treated with different concentrations of nitric acid (0.05−1 M) for 6 h to completely desorb Cr(III) and Cr(VI). After that, the deionized water was also used to rinse off the physically adsorbed ions. Finally, the used materials were dried in a vacuum for recycling (refer to Fig. 1 for experimental set-up). The amount of the metal ions adsorbed onto the composite nanofibers was calculated using the following Eq. (1):
q=
(Co − Ce ) · V M
,
(1)
where Co and Ce was the initial and the equilibrium concentration of the metal ions in the solution (mg/l), respectively. V, M and q denoted the volume of the solution (l), the weight of the ferrous chloride (g) and adsorption capacity (mg/g), respectively. To have better understanding of adsorption behavior, the adsorption kinetic analysis of Cr(VI) with the composite nanofibers was conducted with using several adsorption kinetics models (e.g., pseudo-first-order kinetic model (Eq. (2)), pseudo-second-order kinetic model (Eq. (3)) and the intraparticle diffusion model (Eq. (4))). These models were constructed to describe the adsorption mechanism in a solid-liquid system and could be represented by the following equation:
In (qe − qt ) = In qe −
Kt t 2.303
(2)
Please cite this article as: F. Liu et al., Removal of Cr(VI) using polyacrylonitrile/ferrous chloride composite nanofibers, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.10.043
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3
Fig. 1. The device of the experiment.
t 1 t = + 2 qe q K2 qe e
(3)
qt = K3 t 1/2 + C
(4)
where qt and qe is the reduction adsorption capacity of the composite nanofibers at arbitrary and equilibrium time t, respectively (mg/g). Parameters K1 , K2 , and K3 are kinetic constants of the pseudo-first-order kinetic model, the pseudo-second-order kinetic model, and the particle diffusion rate constant, respectively. C is a constant related to the thickness of the boundary layer of the intraparticle diffusion model. The thermodynamic parameters are related to the adsorption action. Generally, the standard of Gibbs free energy G0 , entropy, and enthalpy change S0 and H° can be expressed by the following (Eqs. (5)–(7)):
In Kd = Kd =
S o R
+
H o RT
,
(Co − Ce ) · V mCe
Go = −RTIn Kd ,
(5) (6) (7)
where m is the dosage of adsorbent (g); R is the gas constant (J/mol/K); T is the reaction solution temperature (K); and Kd is the distribution coefficient. 3. Results and discussion 3.1. Characteristics of adsorbent 3.1.1. SEM analysis The morphology of the PAN/FeCl2 composite nanofibers containing 1.4% FeCl2 was examined before and after adsorption reaction using SEM, as shown in Fig. 2. The composite nanofibers were continuous and smooth with diameters of about 153 nm prior to adsorption (Fig. 2(a)). After adsorption, the composite nanofibers still exhibited good nanofiber morphology. The average diameters (AD) of the composite nanofibers slightly increased to 206 nm after adsorption as shown in Fig. 2(b). In addition, after Cr(VI) ions were adsorbed, the composite nanofibers became more bent and the surface seemed to be more rough. The EDS analysis of Cr(VI) loaded nanofiber clearly showed that the Cr(VI) was adsorbed onto the composite nanofibers and confirmed with data of the element Cr identified, as shown in Fig. 2(c). Because spraying the Pt–Pd powder onto the surface of the
nanofibers enhance its conductivity, and the carrier for fixing the nanofibers was glass, there was still some Pt, Pd and Si detected as showed in Fig. 2(c). 3.1.2. FTIR analysis The FTIR spectra were obtained information on the structural changes of the pure PAN nanofibers and the 1.4% PAN/FeCl2 composite nanofibers before and after the Cr (VI) was adsorbed (Fig. 3). Apparently, the FTIR spectrum of the pure PAN nanofibers exhibited characteristic bands due to the stretching vibration of the nitrile group (2242 cm−1 ), carbonyl group (1731 cm−1 ), and the bending vibration of the carbon–oxygen single bond (1200– 1300 cm−1 ) as the characteristic peaks of the PAN nanofibers. The corresponding peaks may also be observed in the composite nanofibers [16]. However, these characteristic peaks obviously became gradually weaker and the stretching vibrations of the carbonyl group disappeared likely due to the chelation between the iron ions and the PAN. The spectra of the PAN/FeCl2 composite nanofibers, on the other hand, showed new adsorption bands at 1038 cm−1 (denoted as C–N/Fe), indicating a good loading of iron ions onto the composite nanofibers. Furthermore, the FTIR spectrum of the PAN/FeCl2 showed the composite nanofibers after Cr(VI) adsorption (Fig. 3). The bending vibrations of the N–C=N band in the 1650 cm−1 region disappeared. A new band around 1010 cm−1 was observed due to the interaction with Cr(VI). Moreover, the intensity of the bands at the 1440 cm−1 region also weakened. Thus, this demonstrates that Cr(VI) could be immobilized well onto the composite nanofibers due to the chemical reaction. 3.1.3. X-ray diffraction studies The crystal structure of the composite nanofibers could be presented in details via X-ray diffraction characterization. The spectra of the FeCl2 particles, pure PAN, and composite nanofibers of different iron content were shown in Fig. 4. The PAN nanofibers showed diffraction patterns with 2θ values of 16.9 corresponding to the orthorhombic PAN(110) refection and also revealed in other curves [17,18]. However, the peak became wider with increased content of iron. Further, the structure of the composite nanofibers is changed with an increase in iron content. It indicated that the PAN/FeCl2 composite nanofibers were successfully prepared. In addition, the PAN/FeCl2 composite nanofibers exhibited a rhombohedral crystal structure of iron oxide with 2θ values of 26, which tended to be widened with increasing the iron content [19]. There are no obvious peaks of the PAN/FeCl2 composite nanofibers,
Please cite this article as: F. Liu et al., Removal of Cr(VI) using polyacrylonitrile/ferrous chloride composite nanofibers, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.10.043
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Fig. 2. SEM images of nanofibers: (a) 0.7% Fe/PAN before adsorption reaction; (b) after adsorption reaction; (c) EDS spectrum after adsorption.
indicating an amorphous structure. In contrast, the FeCl2 powders have a crystalline structure, as reported by Lin et al. [20]. Formation of the porous and amorphous structure for the composite nanofibers was attributed to the rapid evaporation of solvent during the solution ejection from the needle. It could not provide sufficient time to align and blocks the crystallization of the macromolecular chains, to decrease the density of the nanofibers [21]. Therefore, the obtained composite nanofibers exhibit an amorphous mesoporous structure. However, this structure provides a convenient pathway for ion exchange between the nanofibers and the surrounding solution. 3.1.4. Thermal behavior by TGA–DTA analysis The thermal stability of the composite nanofibers was determined by thermogravimetric analysis in a temperature range of 25–10 0 0 °C at a rate of 5 °C/min under a nitrogen atmosphere. The figure in the attachment (Fig. 2) shows the TGA and DTA curves of the pure PAN and the modified nanofibers. In the graph, the weight of the nanofibers gradually decreased with increased temperature. The curve revealed a weight loss at three major regions during different temperature ranges. The initial weight loss region
between 50 and 150 °C was primarily resulted from the volatilization of physically adsorbed water and the solvent residues of the nanofibers. The second weight loss region between 200 and 350 °C could be ascribed to thermal decomposition of the C–C bond structure [22]. An abrupt increase in the weight loss at 400 °C might be attributed to degradation of the nitrile group of the PAN, breaking the structure of the nanofibers and leading to weight loss [23]. In addition, the results indicate that the peaks shift to lower temperature for the PAN/FeCl2 composite nanofibers. Therefore, the composite nanofibers become more unstable and the exothermic peak is shifted to the lower temperature regions. This simply suggested that the structure is more easily decomposed due to the change in the structure by the modified iron ions. Furthermore, the residual weight of the composite nanofibers is larger than that of pure PAN nanofibers. 3.2. Adsorption capacity assessment of the composite nanofibers 3.2.1. Effect of pH value The pH value has a significant effect on the capacity of the composite nanofibers to remove Cr(VI) in the solution. Fig. 5
Please cite this article as: F. Liu et al., Removal of Cr(VI) using polyacrylonitrile/ferrous chloride composite nanofibers, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.10.043
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Fig. 3. FTIR spectra of the composite nanofibers before and after adsorption.
Fig. 4. XRD pattern of different iron contents nanofibers, pure PAN nanofibers, and FeCl2 powder.
showed the efficiencies at various pH values. To study the effect of the pH value on the performance of the composite nanofibers, the initial pH of the solution was set at 3, 5, 7, and 9. The experiment was performed at an initial concentration of 10 mg/l, a 2.5 g/l adsorbent dosage, and a 1.4% iron content at room temperature. Clearly, all curves showed significant reductions within the initial stages of the experiment. As time went by, the adsorption capacity decreased slowly until equilibrium was gradually reached. However, at higher pH, the removal efficiency is lower than that in an acidic environment. The amount of Cr ions adsorbed onto the composite nanofibers is highest at a pH of 5. This result was consistent with those of Deng and Bai (2004) [24], who found that composite nanofibers should be used for Cr removal in a slightly acidic environment. Hence, too high or low pH value seemed to be not beneficial to Cr(VI) adsorption, primarily because:
(1) At a lower pH value (i.e., high concentration of H+ ions), the competition with metal ions was taken place for the same adsorption sites, resulting in a decrease in the adsorption capacity. Furthermore, the adsorbed hydrogen ions repel metal ions on the surface of the composite nanofibers. This is consistent with Mahapatra et al. (2013) [25] and Boudrahem et al. (2011) [26], both indicating that it was not advantageous to absorb metal ions at very low pHs. (2) The higher hydrolyzed products (e.g., Fe(II)(OH)+ and Fe(III)(OH)2 + ) will form on the surface of the composite nanofibers. At pH 5, it will be easier to interact with Cr(VI) ions [20]. In addition, the competition between hydrogen ion and Cr(VI) tended to be weakened, as there was not too much hydrogen in aqueous solution. Fortunately, actual industrial waste water is usually mild to weakly acidic. Therefore, it is feasible to using PAN/FeCl2 composite nanofibers.
Please cite this article as: F. Liu et al., Removal of Cr(VI) using polyacrylonitrile/ferrous chloride composite nanofibers, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.10.043
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pH=3 pH=5 pH=7 pH=9
Concentration, ppm
10
8
6
4
2
0 0
25
50
75
100
125
150
175
200
Time, min Fig. 5. Effect of the reaction solution pH value on the Cr(VI) reduction performances at initial concentration of 10 mg/l, iron content of 1.4%, adsorbent dosage of 2.5 g/l and temperature of 25 °C.
0.5 g L-1 1.5 g L-1 2.5 g L-1 3.5 g L-1
Concentration, mg L
-1
10
8
6
4
2
0 0
25
50
75
100
125
150
175
200
Time, min Fig. 6. Effect of the composite nanofibers dosage on the Cr(VI) reduction performances with at initial concentration of 10 mg/l, iron content of 1.4%, pH of 5 and temperature of 25 C.
(3) Fe(II) (OH)2 , Fe(III) (OH)3 , Fe(II) (OH)+ , and Fe(III) (OH)2+ will be generated when the pH value of the solution higher than 5. It is apparently disadvantageous for Cr(VI) removal. In this case, abundance of hydroxyl will easily form precipitates with the metal ions in the solution and be adsorbed onto the surface of the composite nanofibers via either suppressing Fe(II) leaching or blocking active reaction sites [17]. Moreover, the electrostatic repulsion between the solution and the composite fibers increases with increasing pH value, this may also introduce difficulties for Cr(VI) ions to be adsorbed onto composite nanofibers [24]. 3.2.2. Effect of adsorbent dosage The effect of the composite nanofiber dosage on the adsorption of Cr(VI) was investigated at 0.5, 1.5, 2.5 and 3.5 g/l of 1.4% Fe/PAN with an initial concentration of 10 mg/l, pH 5, and 25 °C. Fig. 6 showed that all curves exhibited a clear decreasing trend within
30 min from the start of the experiment. As time increased, the adsorption steady state is gradually reached. In addition, this indicates that the removal efficiency of Cr(VI) first increased with an increase in the composite naonofiber dosage, and then decreased. At a lower dosage, there are no sufficient iron ions and active reaction sites to interact with Cr(VI) ions, therefore the removal efficiency is relatively low. The efficiency of adsorption approached 93% when the dosage of the composite naonofibers was 2.5 g/l. However, the removal efficiency of the Cr(VI) increases slightly to 95% when the dosage is 3.5 g/l. This might be due to a gradual increasing in adsorption sites when the adsorbent dosage increased, and the content of iron ions was also increased. However, by increasing the dosage of the nanofibers sequentially, access to the residual sites for Cr(VI) adsorption seemed to be restricted. In addition, the effective surface area for adsorption is decreased likely due to the partial aggregation of the nanofibers at higher dosages [27]. More of the metals ions are adsorbed onto the nanofibers,
Please cite this article as: F. Liu et al., Removal of Cr(VI) using polyacrylonitrile/ferrous chloride composite nanofibers, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.10.043
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7
Fig. 7. The formation mechanism of the redox reaction.
100
5 mg L-1 -1 10 mg L 20 mg L-1 -1 40 mg L
t/qt, min· g mg-1
80
60
40
20
0 0
20
40
60
80
100
120
t, min Fig. 8. Pseudo-second-order kinetic of Cr(VI) onto the composite nanofibers.
reducing available sites for adsorption [17,24]. Therefore, the dosage of the adsorbent should be set at 2.5 g/l for economic feasibility, although the dosage of the nanofibers at 2.5 and 3.5 g/l is substantially identical. 3.3. Mechanism analysis According to prior study, 1.4% composite nanofibers were chosen as optimal materials based on their higher performance and lower operator costs. Iron ions play an important role in the redox reaction. Many studies have found that the reaction of ferrous ions and water can produce Fe(OH)+ , Fe(OH)2 , Fe(OH)2 + , Fe(OH)2+ , and Fe(OH)3 [28]. However, Lin et al. argued that a hydrolysis reaction will occur first in the composite nanofibers and form PAN…Fe(II) (OH)+ , PAN…Fe(II) (OH)2 , PAN…Fe(III) (OH)2+ , PAN…Fe(III) (OH)2 + , and PAN…Fe(III) (OH)3 , etc [20], as our study. In addition, they will preferentially adhere to the surface and pore walls, It will prevent ferrous ions escaping from the surface of the composite nanofibers to the solution, and thus reduce loss of ferrous ions. The group of PAN…Fe(II) (OH)+ will adsorb Cr(VI) and lead the redox reaction. The ferrous ions are oxidized to Fe(III) and the Cr(VI) is reduced to Cr(III) or forms products (e.g., PAN…Fe1 −x Crx (OH)3 and PAN…Fe1 −x Crx (OH)x [28-31]; Fig. 7). Moreover, some ferrous ions might dissolve into the solution, reacting with hexavalent chromium ions in the solution. This could be simply proposed as the following reaction (1):
Cr3+ +3Fe2+ +14H+ → 2Cr3+ +2Fe3+ +7H2 O
(1)
3.4. Adsorption kinetic analysis The adsorption kinetic mechanisms and rate constants of the composite nanofibers under different contact times and different initial Cr(VI) concentrations were evaluated via the pseudo-firstorder kinetic model, pseudo-second-order kinetic model, and intraparticle diffusion model as shown in Fig. 8 and the attachment (Figs. 4 and 5). The calculated kinetic parameters and the correlation coefficients are presented in Table 1. In the pseudo-first-order kinetic model, by setting the time t to the function of Ln(qe –qt ), qe and K1 can be obtained via calculation of the slope value and intercept of the line, as shown in the attachment (Fig. 4). Considering the relation between reaction time and the adsorption capacity of the composite nanofibers under the pseudo-first-order kinetics model, this represented a first-order relationship. Furthermore, the adsorption capacity of the composite nanofibers gradually increased over reaction time. The reaction rate constants are basically the same and conform to the requirements of the pseudo-first-order reaction model. In addition, with the increase of chromium ion concentration, the degree of curve fit improved, as confirmed by the correlation coefficient R2 values of the pseudo-first-order reaction model. In the pseudo-second-order kinetic model, by setting the time t to the function of t/qt , qe and K2 can be obtained by calculating the slope value and intercept of the line (as shown in Fig. 8). This second-order model considered the relation between the reaction time and the adsorption capacity of the composite nanofibers. Furthermore, with an increase in chromium ion concentration,
Please cite this article as: F. Liu et al., Removal of Cr(VI) using polyacrylonitrile/ferrous chloride composite nanofibers, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.10.043
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F. Liu et al. / Journal of the Taiwan Institute of Chemical Engineers 000 (2016) 1–10 Table 1 Kinetic parameters of Cr(VI) adsorption onto the composite nanofibers. C
Experiment
Pseudo-first-order model qe •cal (mg/g)
K1 (min−1 )
R2
qe •cal (mg/g)
K2 (min−1 )
R2
C
K3 (min−1 )
R2
(mg/l)
qe •exp (mg/g)
Pseudo-second-order model
Intraparticle diffusion model
5 10 20 40
1.95 3.97 4.34 6.42
1.01 1.50 1.67 2.14
0.04 0.03 0.03 0.04
0.9425 0.9152 0.9046 0.9693
2.01 4.10 4.42 6.43
0.09 0.02 0.02 0.01
0.9999 0.9945 0.9866 0.9990
1.14 1.78 1.93 2.19
0.073 0.169 0.172 0.372
0.6800 0.9101 0.9618 0.7832
Note: qe •exp ( mg/g) : equilibrium adsorption capacity evaluated from the experiment; qe •cal ( mg/g) : equilibrium adsorption capacity calculated from the experiment.
1.5
L n Kd
1.0
0.5
0.0
-0.5 0.00315
0.00320
0.00325
0.00330
0.00335
1/T Fig. 9. The thermodynamic parameters of the composite nanofibers.
the curve fitted better than that of the pseudo-first-order kinetic model. Table 1 showed that the correlation coefficient R2 values of the pseudo-second-order reaction model are also better than those of the pseudo-first-order kinetic model. In contrast, the adsorption capacity of the composite nanofibers is increased with an increase in the concentration of chromium ions in solution. However, the reaction rate is gradually decreased with increasing concentration and presents a significantly higher Cr(VI) uptake rate for the lowest initial concentration. Different rates of adsorption ware evaluated at different initial concentrations, demonstrating a variety of adsorption sites with different accessibilities and contributions on the composite nanofibers [32]. In the intraparticle diffusion model, the adsorption process of the experiment is under the control of other adsorption stages if all curves did not pass through the origin. If the plots exhibit a straight line, the adsorption process is simply diffusion-based [33]. The experimental data are shown in the attachment (Fig. 5). From the distribution point of the straight line, the dispersion seemed to be relatively high. At the beginning of the reaction, the plot may indicate a straight line passing through the origin that corresponds to the external adsorbate diffusion in the boundary layer. With the increase in the time, this occurs primarily on the surface of the composite nanofibers and rarely spreads into their interior. However, the linear equation fits better than the other conditions when the concentration of chromium ions is 10 or 20 mg/l. Therefore, intraparticle diffusion might be inhibited when the concentration is too low or too high. Thus, according to the R2 value of the correlation coefficient, the pseudo-second-order kinetic model may fit better than the pseudo-first-order kinetics and intraparticle diffusion model in this
system. Moreover, the qe .cal values as obtained from the pseudosecond-order kinetics model seemed to be more feasible to fit experimental data than pseudo-first-order kinetics model. Therefore, the adsorption process of Cr(VI) on the absorbent should follow the pseudo-second-order kinetic model. 3.5. Thermodynamic analysis The adsorption experiment was performed at temperatures of 298, 308, and 318 K, as shown in the attachment (Fig. 6). The results indicated that there was a decrease in the adsorption capacity with an increase in temperature. In fact, this phenomenon could be taken place due to an increase in temperature. The thermal motion of molecules accelerated, and enhanced the degree of disorder of the ions in the solution. By contrast, accelerating the speed at which Cr(VI) is absorbed by Fe2+ to the surface of the composite nanofibers, the relative speed of Cr(VI) desorbed from the surface of the composite nanofibers could also be promoted to slow the total adsorption rate. In fact, this phenomenon could be verified through thermodynamic analysis. The thermodynamic parameters are related of adsorption action (e.g., standard Gibbs free energy G0 , entropy and enthalpy change S0 and H0 ). By setting a time 1/T of LnKd as a function (Fig. 9), the value of H0 and S° could be obtained via the slope and intercept of the straight line (as summarized in Table 2). Apparently, Gibbs free energy G0 is negative at 298 and 308 K, indicating the feasibility and spontaneity of the adsorption reaction. Furthermore, the Gibbs free energy G0 is positive, and the reaction can be non-spontaneous when temperature reached 318 K. With increasing temperature, Gibbs free energy changed from
Please cite this article as: F. Liu et al., Removal of Cr(VI) using polyacrylonitrile/ferrous chloride composite nanofibers, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.10.043
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9
Adsorption efficiency, %
1.0
Mg2+ Ca2+ Na+ Cr6+ NO3-
0.8
0.6
ClSO42-
0.4
0.2
0.0
Ions
ly
ure mi xt Pb in Cr
−231.68 – –
re
−72.63 – –
mi xtu
−4.03 −0.64 0.78
Cd
298 308 318
in
S0 (J/mol/K)
Cr
H0 (KJ/mol)
As mi xtu re
G(KJ/mol)
Cr in
Temperature (K)
On
Table 2 Thermodynamic parameters for Cr(VI) adsorbed by the composite nanofibers.
Cr
Fig. 10. Influence of coexisting ions for the adsorption of Cr(VI).
1.0 A d so r p tio n e ffic ie n c y , %
negative to positive. It indicated that the adsorption of chromium ions tend to shift to non-spontaneous from spontaneous [34,35]. During the adsorption process, the enthalpy change value of H0 is negative, suggesting that this adsorption process is an exothermic reaction. In addition, the value is 20 KJ/mol, indicating that the adsorption is more favorable to chemical reaction. Finally, the entropy change value of S0 is negative. It suggests that the reaction system tended to be ordered.
0.8
0.6 0.4
3.6. The effect of coexisting ions
0.
2
nA
s mi
xtur
e n Cd i
re ixtu ture Cd m mix n Pb Pb i
0
As i
0.
Usually, there are significant numbers of different ions (e.g., NO3 − , SO4 2− , Cl− , Ca2+ , Na+ , Mg2+ ) in wastewater. Apparently, that could affect the adsorption efficiency [36]. Therefore, a certain amount of different ions were first added to 10 mg/l Cr(VI) solution, and then carried out adsorption experiments under predetermined conditions to observe the effect of the other ionic species (Fig. 10). From Fig. 11, the inhibitory effects of anions were more pronounced than that of cations for adsorption. Addition of SO4 2− markedly decreased the adsorption efficiency from about 95–25%, while addition of Mg2+ decreased to 80%. This might be due to significant amounts of Fe(II)(OH)+ and Fe(III)(OH)2+ will be formed on the surface of the composite nanofibers in a slightly acidic environment. This led to a strong electrostatic attraction of anions, making them adsorbed easier than cations by the active sites on the surface of the composite nanofibers. In addition, competitive interactions could also reduce the efficiency of adsorption Cr(VI). In contrast, the cations repelled each other due to the electrostatic repulsion that have less competitive with Cr(VI). However, there is still a few number of cations adsorbed to the surface of the composite nanofibers, revealing a fixed amount of the active reaction sites After being compared with anions, the effect was relatively negligible.
Fig. 11. Influence of different heavy metals for the adsorption of Cr(VI).
3.7. The competition between heavy metals The actual waste water often not only contain various ions, but also exist different species of metal ions. Therefore, the adsorption characteristics tended to be more complex. By mixing different kinds of metal ions in the solution the interactive effects of adsorbed chromium (based upon practical wastewater) could be analyzed. A series of heavy metals (e.g., Pb, As and Cd) were added into the 10 ppm Cr(VI) solution for adsorption (Fig. 11). Apparently, the efficiency of Cr(VI) adsorption significantly declined due to competition between heavy metal ions for available active sites. In particular, the influence of arsenic was obvious as its adsorption was reduced from ca. 98 to 70%. And the other heavy metals had different degrees of adsorption by composite nanofibers.
Please cite this article as: F. Liu et al., Removal of Cr(VI) using polyacrylonitrile/ferrous chloride composite nanofibers, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.10.043
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Regarding addition of three metals, the composite nanofibers adsorbed arsenic slightly larger than other two heavy metals. This may be due to the composite nanofibers strongly adsorbed Cr(VI) from the solution and antagonistically attenuated Cr(VI) to Cr(III) for removal. The oxidation valence state of arsenic was higher. Therefore, arsenic added into the mixed solution may be easier to proceed similar reduction compared with the other two heavy metal of lower valence state. As above mentioned, it implied that the FeCl2 /PAN composite nanofibers also affect removal performance of different kinds of heavy metals from wastewater. 4. Conclusions The FeCl2 /PAN composite nanofibers were successfully prepared by the electrospinning method as a novel material for adsorption of chromium ions in aqueous solutions. The composite nanofibers were analyzed by FTIR, TGA, SEM, and XRD. FTIR results showed that ferrous ions had been successfully modified onto the surface of the composite nanofibers. The XRD analysis showed an amorphous structure due to the rapid evaporation during spinning. XPS analysis also confirmed that the reduction was likely taken place on the composite nanofibers. The PAN…Fe () groups in the composite nanofibers played an important role in the adsorption without dispute. The pH level of the solution directly determined available capacities of adsorption of composite nanofibers. This study revealed that a slightly acidic environment is more appropriate to remove chromium ions. Furthermore, the adsorption characteristics seemed to follow the pseudo-second-order kinetic model. The intraparticle diffusion analysis demonstrated that diffusion primarily took place on the surface of the composite nanofibers. The thermodynamic analysis suggested that the adsorption process is exothermic. An increase in the temperature seemed to be more thermodynamically favorable for adsorption of chromium ions. The inhibitory effects of anions were more pronounced than that of cations for the adsorption. The presence of other heavy metals would decrease the efficiency of adsorption. The FeCl2 /PAN composite nanofibers exhibited good performance in removing chromium ions for promising wastewater treatment. Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jtice.2016.10.043. References [1] Afkhami A, Conway BE. Investigation of removal of Cr(Ⅵ), Mo(Ⅵ),W(Ⅵ),V(Ⅳ) and V (Ⅴ) oxy-ions from industrial waste-waters by adsorption and electrosorption at high-area carbon cloth. J Colloid Interface Sci 2002;251:248–55. [2] Cimino G, Passerini A, Toscano G. Removal of toxic cations and Cr(Ⅵ) from aqueous solution by hazelnut shell. Water Res 20 0 0;34:2955–62. [3] Owlad M, Aroua MK, Daud WAW, Baroutian S. Removal of hexavalent chromium-contaminated water and wastewater: a review. Water Air Soil Pollut 20 09;20 0:59–77. [4] Trunfio G, Crini G. The dechromatation step in wastewater treatment plants: fundamental role and optimization. Ind Eng Chem Res 2010;49:12217–23. [5] Lee ST, Mi FT, Shen YJ, Shyu SS. Equilibrium and kinetic studies of copper(II) ion uptake by chitosan-tripolyphosphate chelating resin. Polymer 2001;42:1879–92. [6] Ngah WS, Fatinathan W, Enuiron SJ. Pb(II) biosorption using chitosan and chitosan derivatives beads: equilibrium, ion exchange and mechanism studies. Sci (Beijing, China) 2010;22:338–46. [7] Yang T, Zall RR. Chitosan membranes for reverse osmosis application. J Food Sci 1984;49:91–3. [8] Assaad E, Azzouz A, Nistor D, Urus A, Sajin T, Miron DN, et al. Metal removal through synergic coagulation – flocculation using an optimized chitosan-montmorillonite system. Appl Clay Sci 2007;37:258–74.
[9] Li N, Bai R. Copper adsorption on chitosan – cellulose hydrogel beads: behaviors and mechanisms. Sep Purif Technol 2005;42:237–47. [10] Ng JCY, Cheung WH, Mckay G. Equilibrium studies of the sorption of Cu(II) ions onto chitosan. J Colloid Interface Sci 2002;255:64–74. [11] Justi KC, Favere VT, Larenjeira MC, Neves A, Peralta RA. Kinetics and equilibrium adsorption of Cu(II), Cd(II), and Ni(II) ions by chitosan functionalized with 2[-bis-(pyridyl-methyl)aminomethyl]-4-methyl-6-formylphenol. J Colloid Interface Sci 2005;291:369–74. [12] Yang H, Xu R, Xue XM, Li GT. Four novel three-dimensional pillared-layer metal-organic frameworks in the Zn/triazolate/carboxylate system: hydrothermal synthesis, crystal structure, and luminescence properties. J Hazard Mater 2008;152:690. [13] Mureseanu M, Reiss A, Stefanescu I, David E, Parvulescu V, Renard G, et al. Modified SBA-15 mesoporous silica for heavy metal ions remediation. Chemosphere 2008;73(14):99–1504. [14] Haider S, Park SY. Preparation of the electrospun chitosan nanofibers and their applications to the adsorption of Cu (II) and Pb (II) ions from an aqueous solution. J Memb Sci 2009;328:90–6. [15] Zhou SL, Liu F, Zhang Q, Chen BY, Lin CR, Chang CT. Preparation of polyacrylonitrile/ferrous chloride composite nanofibers by electrospinning for efficient reduction of Cr(VI). J Nanosci Nanotechnol 2015;15:5823–32. [16] Parvin KN, Mehdi R, Faramarz AT. Preparation of aminated-polyacrylonitrile nanofiber membranes for the adsorption of metal ions: comparison with microfibers. J Hazard Mater 2011;186:182–9. [17] Zhang CC, Li X, Bian XJ, Zheng T, Wang C. Polyacrylonitrile/manganese acetate composite nanofibers and their catalysis performance on chromium (Ⅵ) reduction by oxalic acid. J Hazard Mater 2012;229–230:439–45. [18] Chen M, Wang CJ, Fang W, Wang J, Zhang W, Jin G, et al. Electrospinning of calixarene-functionalized polyacrylonitrile nanofiber membranes and application as an adsorbent and catalyst support. Langmuir 2013;29:11858–67. [19] Mahapatra A, Mishra BG, Hota G. Electrospun Fe2 O3 -Al2 O3 nanocomopsite fibers as efficient adsorbent for removal of heavy metal ions from aqueous solution. J Hazard Mater 2013;258–259:116–23. [20] Lin YX, Cai WP, Tian XY, Liu XL, Wang GZ, Liang CH. Polyacrylonitrile/ferrous chloride composite porous nanofibers and their strong Cr-removal performance. J Mater Chem 2011;21:991–7. [21] Luoh R, Hahn HT. Electrospun nanocomposite fiber mats as gas sensors. Compos Sci Technol 2006;66:2436. [22] Oh GY, Ju YW, Jung HR, Lee WJ. Preparation of the novel manganese-embedded PAN-based activated carbon nanofibers by electrospinning and their toluene adsorption. J Anal Appl Pyrolysis 2008;81:211–17. [23] Kiani GR, Sheikloie H, Arsalani N. Heavy metal ion removal from aqueous solutions by functionalized polyacrylonitrile. Desalination 2011;269:266–70. [24] Deng SB, Bai RB. Removal of trivalent and hexavalent chromium with aminated polyacrylonitrile fibers: performance and mechanisms. Water Res 2004;38:2424–32. [25] Mahapatra A, Mishra BG, Hota G. Electrospun Fe2 O3 -Al2 O3 nanocomposite fibers as efficient adsorbent for removal of heavy metal ions form aqueous solution. J Hazard Mater 2013;258–259:116–23. [26] Boudrahem F, Benissad FA, Soualah A. Adsorption of lead() form aqueous solution by using leaves of date trees as an adsorbent. J Chem Eng Data 2011;56:1804–12. [27] Dastbaz A, Keshtkar AR. Adsorption of Th4+ , U6+ , Cd2+ , and Ni2+ from aqueous solution by a novel modified polyacrylonitrile composite nanofiber adsorbent prepared by electrospinning. Appl Surf Sci 2014;12:164–9. [28] Buerge IJ, Hug SJ. Kinetics and pH dependence of chromium (VI) reduction by iron (II). Environ Sci Technol 1997;31:1426–32. [29] Eary LE, Ral D. Chromate removal from aqueous wastes by reduction with ferrous ion. Environ Sci Technol 1988;22:972–7. [30] Fendorf SE, Laubscher HU. Kinetics of chromate reduction by ferrous iron. Environ Sci Technol 1996;30:1614–17. [31] Abdel-Samad H, Watson PR. An XPS study of the adsorption of chromate on goethite. Appl Surf Sci 1997;108:371–7. [32] Szlachta M, Chubar N. The application of Fe-Mn hydrous oxides based adsorbent for removing selenium species from water. Chem Eng J 2013;217:159–68. [33] Toor M, Jin B. Adsorption characteristics, isotherm, kinetic, and diffusion of modified natural bentonite for removing diazo dye. Chem Eng J 2012;187:79–88. [34] Wang YQ, Zhang ZB, Liu YH, Cao XH, Liu YT, Li Q. Adsorption of U(VI) from aqueous solution by the carboxyl-mesoporous carbon. Chem Eng J 2012;198:246–53. [35] Unlu N, Ersoz M. Removal of heavy metal ions by using dithiocarbamated-sporopollenin. Sep Purif Technol 2007;52:461–9. [36] Zhu H, Jiang R, Xiao L, Chang Y, Guan YJ, Li XD, et al. Photocatalytic decolorization and degradation of Congo Red on innovative crosslinked chitosan/nano-CdS composite catalyst under visible light irradiation. J Hazard Mater 2009;169:933–40.
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Journal of the Taiwan Institute of Chemical Engineers 0 0 0 (2016) 1–10
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Removal of Cr(VI) using polyacrylonitrile/ferrous chloride composite nanofibers Fang Liu a, Xinhong Wang a, Bor-Yann Chen b, Shilin Zhou c, Chang-Tang Chang d,∗ a
Department of Environmental Science and Technology, College of the Environment and Ecology, and Key Laboratory of the Ministry of Education for Coastal and Wetland Ecosystem, Xiamen University, Xiang’an South Road, Xiang’an District, Xiamen 361102, China b Department of Chemical and Materials Engineering, National ILAN University, I-Lan 26047 Taiwan c College of Science, University of Shanghai for Science and Technology, Shanghai 200433, China d Department of Environmental Engineering, National ILAN University, I-Lan 26047, Taiwan
a r t i c l e
i n f o
Article history: Received 17 December 2015 Revised 18 October 2016 Accepted 23 October 2016 Available online xxx Keywords: Electrospinning PAN/FeCl2 composite nanofibers Cr(VI) Adsorption Ions effect
a b s t r a c t Considering economic feasibility and technical effectiveness, polyacrylonitrile/ferrous chloride composite nanofibers prepared by electrospinning was used to remove the Cr(VI). Maximum removal of Cr(VI) by composite nanofibers was observed in weakly acidic environment. The analysis of the reaction kinetics revealed that the adsorption behavior seemed to follow pseudo-second-order kinetic model. Through calculating thermodynamic parameter, the adsorption reaction was proved to be an exothermic and chemical process. The PAN…Fe1 −x Crx (OH)3 and PAN…Fe1 −x Crx (OH)x were proposed to be formed after the redox reaction was taken place. The interactive effects of coexisting ions of different metal ions like arsenic, lead and cadmium were also explored for practical consideration. The presence of these metal ions apparently decreased the efficiency of the Cr(VI) adsorption. In addition, the interactive inhibition of arsenic was more significant than other coexisting ions. It was suspected that the valence state (e.g., NO3 − , SO4 2− , Cl− , Ca2± , Na± and Mg2± ) may play a crucial role during the reaction. Due to the strong electrostatic attraction, anions had the more significant attenuating effect than cations. This study is helpful not only for doping design of iron oxide nanofibers but also for practical application during advanced wastewater treatment. © 2016 Published by Elsevier B.V. on behalf of Taiwan Institute of Chemical Engineers.
1. Introduction Chromium and its compounds are widely used in many industries, such as metal processing, electroplating, leather tanning, paint machining, pharmaceutical, mining, and textile industries. Apparently, these industries produce a significant amount of chromium-containing wastewater [1,2]. In fact, chromium is naturally present in the form of trivalent (III) and hexavalent (VI). In generally, trivalent chromium is considered an essential micronutrient for living organisms and is relatively less toxic than hexavalent chromium to be easily absorbed and accumulated in cells. However, due to high toxicity potency of Cr(VI), carcinogenic action, mobility in water, and ability to induce genetic mutations, Cr(VI) is evidently a threat to global lives [3,4]. Therefore, one important method to remove hexavalent chromium from the environment is to detoxify Cr(VI)–Cr(III) based on the redox reaction.
∗
Corresponding authors. E-mail addresses: [email protected] (X. Wang), [email protected] (C.-T. Chang).
In fact, several methods have been developed for the removal of Cr(VI) from aqueous solutions, including reverse osmosis [5], ion exchange [6,7], chemical precipitation, adsorption and flocculation [8]. Among these, adsorption has been extensively used because it is cost-effective and environmentally appropriate [9–11]. However, the properties of the adsorbent material strongly limited adsorption equilibrium time, selectivity, efficiency, and regeneration [12] for practicability. There are many types of nanoporous adsorbents, including nanocarbon materials, mesoporous carbon materials, mesoporous silica materials, metal oxide nanoparticles and nanofibers [12–14]. However, some of these materials required long periods of time to reach equilibrium as they had low adsorption capacity or difficulties to subsequent reuse. Recently, nanofibers provided a solution to these problems. They can satisfy the demand for materials used as adsorbents for a wide range of applications. Moreover, electrospinning is well known to be an effective and simple method for the preparation of nanofibers. In addition, the obtained polymer composite nanofibers owned a high surface area and good porosity and dimensional stability, making them more promising adsorbent materials.
http://dx.doi.org/10.1016/j.jtice.2016.10.043 1876-1070/© 2016 Published by Elsevier B.V. on behalf of Taiwan Institute of Chemical Engineers.
Please cite this article as: F. Liu et al., Removal of Cr(VI) using polyacrylonitrile/ferrous chloride composite nanofibers, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.10.043
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As our prior study revealed, polyacrylonitrile/ferrous chloride (PAN/FeCl2 ) composite nanofibers with electrospinning techniques were successfully prepared [15]. Here, the aim of this study tended to uncover the adsorption mechanisms at various experimental conditions with inspecting characterizations of X-ray powder diffraction (XRD), Fourier infrared spectroscopy (FTIR), Thermo gravimetric analyzer (TGA) and X-ray photoelectron spectroscopy (XPS). The effect of different ions and interactive competition between heavy metals on adsorption performance are also discussed herein. The surface of the composite nanofibers covered large quantities of PAN…Fe() (OH)+ complexes that strongly adsorbed Cr(VI) from the solution onto the composite nanofibers via redox reactions. That is, Cr(VI) could be detoxified to Cr(III) species at lower toxicity, as the ferrous ion was oxidized to be Fe(III). This study also provided model assessment upon Cr(VI) adsorption for conclusive remarks. 2. Experimental methods 2.1. Reagents and materials Polymer spinning solution polyacrylonitrile (PAN) powder was provided by the Sigma–Aldrich Inc. with a molecular weight of 150,0 0 0 g/mol. In addition, dimethylacetamide (DMAc) was used as spinning solvent and obtained from the Sigma–Aldrich Company. The modifying agent ferrous chloride (FeCl2 ·nH2 O) was purchased from NIHON SHIYAKU REAGENT. The pH of solutions was adjusted with HCl and NaOH solution (NIHON SHIYAKU REAGENT). The standard solution of hexavalent chromium (Cr(VI)) and the other ions were obtained from J. T. Baker Company for preparation of heavy metals and ions mixture solution. Deionized (DI) water was used throughout this work to prevent environmental interference. All the chemicals were used without further purification. 2.2. Preparation of PAN/FeCl2 composite nanofibers The PAN nanofibers was prepared by dissolving 1.05 g of PAN powder in 20 ml DMAC solution at 80 °C and then magnetically stirring for 10 h. Next, the FeCl2 ·nH2 O particles were then added to the abovementioned PAN solution and the reaction was carried out at 30 °C for 3 h. Different content of FeCl2 were used to prepare modifying agents were prepared. Comprehensive information regarding the synthesis methods is reported in our previous study [15]. The solution was separately sonicated for 3 h to obtain a homogeneous solution for spinning. Finally, the solutions were converted to composite nanofibers by electrospinning apparatus. The applied voltage between the tip to the needle and collector was 16 kV and the distance between the collector and the tip was 15 cm. The feed rate was 1.0 ml/h and the diameter size of needle was 0.52 mm. A rotating metal drum (diameter: 8 cm, rotating speed: 150 rpm) wrapped with aluminum foil was used to collect nanofibers. The same conditions were used to prepare the pure PAN nanofibers. 2.3. Characterization The morphological analysis of the composite nanofibers was characterized using a scanning electron microscopy (SEM, JHC S4700). First, a small sample was dispersed in the ethanol solution by sonication. After drying, the gold-plated metal stage was secured using carbon tapes to increase its electrical conductivity ˚ Finally, the surface morphology was (gold plating thickness: 150 A). observed. The content of the elements was observed by energy dispersive spectrometer (EDS, JHC H-7200). The impact of X-rays released by striking samples with an electron was used to analyze the elements as a form of qualitative and semi-quantitative-bases.
Because the hydrogen has only one electron, when the electron was energized that there was no more available electron remained for transferring. Therefore, there was no any signal appeared by Si(Li) detector, and thus the EDS spectra could not simply indicate the amount of hydrogen present in the sample. X-ray powder diffraction (XRD, MXP18) was used to analyze the crystal structure of the composite nanofibers with a light source Cu K as the radioactive source (λ = 0.1541 nm), an output power of 18 KW and a scanning range of 20 0–80 0 ° for 50 min−1 . Diffraction angle measures and strength pattern tests were implemented to analyze the crystal and polycrystalline microstructures. The functional groups of composite nanofibers were observed via Fourier infrared spectroscopy (FTIR, Spectrum 100). The surrounding of the instruments was purged with nitrogen to maintain dry conditions. By uniformly arranging the powder samples on the FITR detectors, the spindles and knobs on the samples were analyzed via infrared photo detectors. A thermo gravimetric analyzer (TGA, STA60 0 0) was used to observe thermal stability characteristics and components of the sample by changing the ambient temperature, where the sample was placed under nitrogen atmosphere. The x-ray photoelectron spectroscopy (XPS) analysis was carried out to determine the elements that presented on the surface of the nanofibers before and after adsorption. The concentration of iron ions and Cr(VI) was determined by an inductivity coupled plasma atomic emission spectrophotometer (ICP-AES, AU12440312). A pH meter (SP-701) was used for the measurement of the solution. 2.4. Adsorption capacity assessment The batch adsorption experiments were carried out using 500 ml flask containing 0.5 g dry PAN/FeCl2 composite nanofibers under different Cr(VI) concentrations at different temperatures (i.e., 298, 308, and 318 K). The flask was placed in a shaker to maintain well-mixed conditions. The pH level of the solutions was adjusted with 0.1 M HCl or 0.1 M NaOH solution. The effects of contact time, different pH values, and adsorption dosage on the performance of Cr(VI) removal were also examined. After completion of the experiment, the amount of metal ions was quantified by ICP-AES. After the adsorption reaction of the composite nanofibers, the deionized water was used to rinse off physically adsorbed ions. Subsequently, the composite nanofibers were treated with different concentrations of nitric acid (0.05−1 M) for 6 h to completely desorb Cr(III) and Cr(VI). After that, the deionized water was also used to rinse off the physically adsorbed ions. Finally, the used materials were dried in a vacuum for recycling (refer to Fig. 1 for experimental set-up). The amount of the metal ions adsorbed onto the composite nanofibers was calculated using the following Eq. (1):
q=
(Co − Ce ) · V M
,
(1)
where Co and Ce was the initial and the equilibrium concentration of the metal ions in the solution (mg/l), respectively. V, M and q denoted the volume of the solution (l), the weight of the ferrous chloride (g) and adsorption capacity (mg/g), respectively. To have better understanding of adsorption behavior, the adsorption kinetic analysis of Cr(VI) with the composite nanofibers was conducted with using several adsorption kinetics models (e.g., pseudo-first-order kinetic model (Eq. (2)), pseudo-second-order kinetic model (Eq. (3)) and the intraparticle diffusion model (Eq. (4))). These models were constructed to describe the adsorption mechanism in a solid-liquid system and could be represented by the following equation:
In (qe − qt ) = In qe −
Kt t 2.303
(2)
Please cite this article as: F. Liu et al., Removal of Cr(VI) using polyacrylonitrile/ferrous chloride composite nanofibers, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.10.043
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3
Fig. 1. The device of the experiment.
t 1 t = + 2 qe q K2 qe e
(3)
qt = K3 t 1/2 + C
(4)
where qt and qe is the reduction adsorption capacity of the composite nanofibers at arbitrary and equilibrium time t, respectively (mg/g). Parameters K1 , K2 , and K3 are kinetic constants of the pseudo-first-order kinetic model, the pseudo-second-order kinetic model, and the particle diffusion rate constant, respectively. C is a constant related to the thickness of the boundary layer of the intraparticle diffusion model. The thermodynamic parameters are related to the adsorption action. Generally, the standard of Gibbs free energy G0 , entropy, and enthalpy change S0 and H° can be expressed by the following (Eqs. (5)–(7)):
In Kd = Kd =
S o R
+
H o RT
,
(Co − Ce ) · V mCe
Go = −RTIn Kd ,
(5) (6) (7)
where m is the dosage of adsorbent (g); R is the gas constant (J/mol/K); T is the reaction solution temperature (K); and Kd is the distribution coefficient. 3. Results and discussion 3.1. Characteristics of adsorbent 3.1.1. SEM analysis The morphology of the PAN/FeCl2 composite nanofibers containing 1.4% FeCl2 was examined before and after adsorption reaction using SEM, as shown in Fig. 2. The composite nanofibers were continuous and smooth with diameters of about 153 nm prior to adsorption (Fig. 2(a)). After adsorption, the composite nanofibers still exhibited good nanofiber morphology. The average diameters (AD) of the composite nanofibers slightly increased to 206 nm after adsorption as shown in Fig. 2(b). In addition, after Cr(VI) ions were adsorbed, the composite nanofibers became more bent and the surface seemed to be more rough. The EDS analysis of Cr(VI) loaded nanofiber clearly showed that the Cr(VI) was adsorbed onto the composite nanofibers and confirmed with data of the element Cr identified, as shown in Fig. 2(c). Because spraying the Pt–Pd powder onto the surface of the
nanofibers enhance its conductivity, and the carrier for fixing the nanofibers was glass, there was still some Pt, Pd and Si detected as showed in Fig. 2(c). 3.1.2. FTIR analysis The FTIR spectra were obtained information on the structural changes of the pure PAN nanofibers and the 1.4% PAN/FeCl2 composite nanofibers before and after the Cr (VI) was adsorbed (Fig. 3). Apparently, the FTIR spectrum of the pure PAN nanofibers exhibited characteristic bands due to the stretching vibration of the nitrile group (2242 cm−1 ), carbonyl group (1731 cm−1 ), and the bending vibration of the carbon–oxygen single bond (1200– 1300 cm−1 ) as the characteristic peaks of the PAN nanofibers. The corresponding peaks may also be observed in the composite nanofibers [16]. However, these characteristic peaks obviously became gradually weaker and the stretching vibrations of the carbonyl group disappeared likely due to the chelation between the iron ions and the PAN. The spectra of the PAN/FeCl2 composite nanofibers, on the other hand, showed new adsorption bands at 1038 cm−1 (denoted as C–N/Fe), indicating a good loading of iron ions onto the composite nanofibers. Furthermore, the FTIR spectrum of the PAN/FeCl2 showed the composite nanofibers after Cr(VI) adsorption (Fig. 3). The bending vibrations of the N–C=N band in the 1650 cm−1 region disappeared. A new band around 1010 cm−1 was observed due to the interaction with Cr(VI). Moreover, the intensity of the bands at the 1440 cm−1 region also weakened. Thus, this demonstrates that Cr(VI) could be immobilized well onto the composite nanofibers due to the chemical reaction. 3.1.3. X-ray diffraction studies The crystal structure of the composite nanofibers could be presented in details via X-ray diffraction characterization. The spectra of the FeCl2 particles, pure PAN, and composite nanofibers of different iron content were shown in Fig. 4. The PAN nanofibers showed diffraction patterns with 2θ values of 16.9 corresponding to the orthorhombic PAN(110) refection and also revealed in other curves [17,18]. However, the peak became wider with increased content of iron. Further, the structure of the composite nanofibers is changed with an increase in iron content. It indicated that the PAN/FeCl2 composite nanofibers were successfully prepared. In addition, the PAN/FeCl2 composite nanofibers exhibited a rhombohedral crystal structure of iron oxide with 2θ values of 26, which tended to be widened with increasing the iron content [19]. There are no obvious peaks of the PAN/FeCl2 composite nanofibers,
Please cite this article as: F. Liu et al., Removal of Cr(VI) using polyacrylonitrile/ferrous chloride composite nanofibers, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.10.043
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Fig. 2. SEM images of nanofibers: (a) 0.7% Fe/PAN before adsorption reaction; (b) after adsorption reaction; (c) EDS spectrum after adsorption.
indicating an amorphous structure. In contrast, the FeCl2 powders have a crystalline structure, as reported by Lin et al. [20]. Formation of the porous and amorphous structure for the composite nanofibers was attributed to the rapid evaporation of solvent during the solution ejection from the needle. It could not provide sufficient time to align and blocks the crystallization of the macromolecular chains, to decrease the density of the nanofibers [21]. Therefore, the obtained composite nanofibers exhibit an amorphous mesoporous structure. However, this structure provides a convenient pathway for ion exchange between the nanofibers and the surrounding solution. 3.1.4. Thermal behavior by TGA–DTA analysis The thermal stability of the composite nanofibers was determined by thermogravimetric analysis in a temperature range of 25–10 0 0 °C at a rate of 5 °C/min under a nitrogen atmosphere. The figure in the attachment (Fig. 2) shows the TGA and DTA curves of the pure PAN and the modified nanofibers. In the graph, the weight of the nanofibers gradually decreased with increased temperature. The curve revealed a weight loss at three major regions during different temperature ranges. The initial weight loss region
between 50 and 150 °C was primarily resulted from the volatilization of physically adsorbed water and the solvent residues of the nanofibers. The second weight loss region between 200 and 350 °C could be ascribed to thermal decomposition of the C–C bond structure [22]. An abrupt increase in the weight loss at 400 °C might be attributed to degradation of the nitrile group of the PAN, breaking the structure of the nanofibers and leading to weight loss [23]. In addition, the results indicate that the peaks shift to lower temperature for the PAN/FeCl2 composite nanofibers. Therefore, the composite nanofibers become more unstable and the exothermic peak is shifted to the lower temperature regions. This simply suggested that the structure is more easily decomposed due to the change in the structure by the modified iron ions. Furthermore, the residual weight of the composite nanofibers is larger than that of pure PAN nanofibers. 3.2. Adsorption capacity assessment of the composite nanofibers 3.2.1. Effect of pH value The pH value has a significant effect on the capacity of the composite nanofibers to remove Cr(VI) in the solution. Fig. 5
Please cite this article as: F. Liu et al., Removal of Cr(VI) using polyacrylonitrile/ferrous chloride composite nanofibers, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.10.043
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5
Fig. 3. FTIR spectra of the composite nanofibers before and after adsorption.
Fig. 4. XRD pattern of different iron contents nanofibers, pure PAN nanofibers, and FeCl2 powder.
showed the efficiencies at various pH values. To study the effect of the pH value on the performance of the composite nanofibers, the initial pH of the solution was set at 3, 5, 7, and 9. The experiment was performed at an initial concentration of 10 mg/l, a 2.5 g/l adsorbent dosage, and a 1.4% iron content at room temperature. Clearly, all curves showed significant reductions within the initial stages of the experiment. As time went by, the adsorption capacity decreased slowly until equilibrium was gradually reached. However, at higher pH, the removal efficiency is lower than that in an acidic environment. The amount of Cr ions adsorbed onto the composite nanofibers is highest at a pH of 5. This result was consistent with those of Deng and Bai (2004) [24], who found that composite nanofibers should be used for Cr removal in a slightly acidic environment. Hence, too high or low pH value seemed to be not beneficial to Cr(VI) adsorption, primarily because:
(1) At a lower pH value (i.e., high concentration of H+ ions), the competition with metal ions was taken place for the same adsorption sites, resulting in a decrease in the adsorption capacity. Furthermore, the adsorbed hydrogen ions repel metal ions on the surface of the composite nanofibers. This is consistent with Mahapatra et al. (2013) [25] and Boudrahem et al. (2011) [26], both indicating that it was not advantageous to absorb metal ions at very low pHs. (2) The higher hydrolyzed products (e.g., Fe(II)(OH)+ and Fe(III)(OH)2 + ) will form on the surface of the composite nanofibers. At pH 5, it will be easier to interact with Cr(VI) ions [20]. In addition, the competition between hydrogen ion and Cr(VI) tended to be weakened, as there was not too much hydrogen in aqueous solution. Fortunately, actual industrial waste water is usually mild to weakly acidic. Therefore, it is feasible to using PAN/FeCl2 composite nanofibers.
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pH=3 pH=5 pH=7 pH=9
Concentration, ppm
10
8
6
4
2
0 0
25
50
75
100
125
150
175
200
Time, min Fig. 5. Effect of the reaction solution pH value on the Cr(VI) reduction performances at initial concentration of 10 mg/l, iron content of 1.4%, adsorbent dosage of 2.5 g/l and temperature of 25 °C.
0.5 g L-1 1.5 g L-1 2.5 g L-1 3.5 g L-1
Concentration, mg L
-1
10
8
6
4
2
0 0
25
50
75
100
125
150
175
200
Time, min Fig. 6. Effect of the composite nanofibers dosage on the Cr(VI) reduction performances with at initial concentration of 10 mg/l, iron content of 1.4%, pH of 5 and temperature of 25 C.
(3) Fe(II) (OH)2 , Fe(III) (OH)3 , Fe(II) (OH)+ , and Fe(III) (OH)2+ will be generated when the pH value of the solution higher than 5. It is apparently disadvantageous for Cr(VI) removal. In this case, abundance of hydroxyl will easily form precipitates with the metal ions in the solution and be adsorbed onto the surface of the composite nanofibers via either suppressing Fe(II) leaching or blocking active reaction sites [17]. Moreover, the electrostatic repulsion between the solution and the composite fibers increases with increasing pH value, this may also introduce difficulties for Cr(VI) ions to be adsorbed onto composite nanofibers [24]. 3.2.2. Effect of adsorbent dosage The effect of the composite nanofiber dosage on the adsorption of Cr(VI) was investigated at 0.5, 1.5, 2.5 and 3.5 g/l of 1.4% Fe/PAN with an initial concentration of 10 mg/l, pH 5, and 25 °C. Fig. 6 showed that all curves exhibited a clear decreasing trend within
30 min from the start of the experiment. As time increased, the adsorption steady state is gradually reached. In addition, this indicates that the removal efficiency of Cr(VI) first increased with an increase in the composite naonofiber dosage, and then decreased. At a lower dosage, there are no sufficient iron ions and active reaction sites to interact with Cr(VI) ions, therefore the removal efficiency is relatively low. The efficiency of adsorption approached 93% when the dosage of the composite naonofibers was 2.5 g/l. However, the removal efficiency of the Cr(VI) increases slightly to 95% when the dosage is 3.5 g/l. This might be due to a gradual increasing in adsorption sites when the adsorbent dosage increased, and the content of iron ions was also increased. However, by increasing the dosage of the nanofibers sequentially, access to the residual sites for Cr(VI) adsorption seemed to be restricted. In addition, the effective surface area for adsorption is decreased likely due to the partial aggregation of the nanofibers at higher dosages [27]. More of the metals ions are adsorbed onto the nanofibers,
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7
Fig. 7. The formation mechanism of the redox reaction.
100
5 mg L-1 -1 10 mg L 20 mg L-1 -1 40 mg L
t/qt, min· g mg-1
80
60
40
20
0 0
20
40
60
80
100
120
t, min Fig. 8. Pseudo-second-order kinetic of Cr(VI) onto the composite nanofibers.
reducing available sites for adsorption [17,24]. Therefore, the dosage of the adsorbent should be set at 2.5 g/l for economic feasibility, although the dosage of the nanofibers at 2.5 and 3.5 g/l is substantially identical. 3.3. Mechanism analysis According to prior study, 1.4% composite nanofibers were chosen as optimal materials based on their higher performance and lower operator costs. Iron ions play an important role in the redox reaction. Many studies have found that the reaction of ferrous ions and water can produce Fe(OH)+ , Fe(OH)2 , Fe(OH)2 + , Fe(OH)2+ , and Fe(OH)3 [28]. However, Lin et al. argued that a hydrolysis reaction will occur first in the composite nanofibers and form PAN…Fe(II) (OH)+ , PAN…Fe(II) (OH)2 , PAN…Fe(III) (OH)2+ , PAN…Fe(III) (OH)2 + , and PAN…Fe(III) (OH)3 , etc [20], as our study. In addition, they will preferentially adhere to the surface and pore walls, It will prevent ferrous ions escaping from the surface of the composite nanofibers to the solution, and thus reduce loss of ferrous ions. The group of PAN…Fe(II) (OH)+ will adsorb Cr(VI) and lead the redox reaction. The ferrous ions are oxidized to Fe(III) and the Cr(VI) is reduced to Cr(III) or forms products (e.g., PAN…Fe1 −x Crx (OH)3 and PAN…Fe1 −x Crx (OH)x [28-31]; Fig. 7). Moreover, some ferrous ions might dissolve into the solution, reacting with hexavalent chromium ions in the solution. This could be simply proposed as the following reaction (1):
Cr3+ +3Fe2+ +14H+ → 2Cr3+ +2Fe3+ +7H2 O
(1)
3.4. Adsorption kinetic analysis The adsorption kinetic mechanisms and rate constants of the composite nanofibers under different contact times and different initial Cr(VI) concentrations were evaluated via the pseudo-firstorder kinetic model, pseudo-second-order kinetic model, and intraparticle diffusion model as shown in Fig. 8 and the attachment (Figs. 4 and 5). The calculated kinetic parameters and the correlation coefficients are presented in Table 1. In the pseudo-first-order kinetic model, by setting the time t to the function of Ln(qe –qt ), qe and K1 can be obtained via calculation of the slope value and intercept of the line, as shown in the attachment (Fig. 4). Considering the relation between reaction time and the adsorption capacity of the composite nanofibers under the pseudo-first-order kinetics model, this represented a first-order relationship. Furthermore, the adsorption capacity of the composite nanofibers gradually increased over reaction time. The reaction rate constants are basically the same and conform to the requirements of the pseudo-first-order reaction model. In addition, with the increase of chromium ion concentration, the degree of curve fit improved, as confirmed by the correlation coefficient R2 values of the pseudo-first-order reaction model. In the pseudo-second-order kinetic model, by setting the time t to the function of t/qt , qe and K2 can be obtained by calculating the slope value and intercept of the line (as shown in Fig. 8). This second-order model considered the relation between the reaction time and the adsorption capacity of the composite nanofibers. Furthermore, with an increase in chromium ion concentration,
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F. Liu et al. / Journal of the Taiwan Institute of Chemical Engineers 000 (2016) 1–10 Table 1 Kinetic parameters of Cr(VI) adsorption onto the composite nanofibers. C
Experiment
Pseudo-first-order model qe •cal (mg/g)
K1 (min−1 )
R2
qe •cal (mg/g)
K2 (min−1 )
R2
C
K3 (min−1 )
R2
(mg/l)
qe •exp (mg/g)
Pseudo-second-order model
Intraparticle diffusion model
5 10 20 40
1.95 3.97 4.34 6.42
1.01 1.50 1.67 2.14
0.04 0.03 0.03 0.04
0.9425 0.9152 0.9046 0.9693
2.01 4.10 4.42 6.43
0.09 0.02 0.02 0.01
0.9999 0.9945 0.9866 0.9990
1.14 1.78 1.93 2.19
0.073 0.169 0.172 0.372
0.6800 0.9101 0.9618 0.7832
Note: qe •exp ( mg/g) : equilibrium adsorption capacity evaluated from the experiment; qe •cal ( mg/g) : equilibrium adsorption capacity calculated from the experiment.
1.5
L n Kd
1.0
0.5
0.0
-0.5 0.00315
0.00320
0.00325
0.00330
0.00335
1/T Fig. 9. The thermodynamic parameters of the composite nanofibers.
the curve fitted better than that of the pseudo-first-order kinetic model. Table 1 showed that the correlation coefficient R2 values of the pseudo-second-order reaction model are also better than those of the pseudo-first-order kinetic model. In contrast, the adsorption capacity of the composite nanofibers is increased with an increase in the concentration of chromium ions in solution. However, the reaction rate is gradually decreased with increasing concentration and presents a significantly higher Cr(VI) uptake rate for the lowest initial concentration. Different rates of adsorption ware evaluated at different initial concentrations, demonstrating a variety of adsorption sites with different accessibilities and contributions on the composite nanofibers [32]. In the intraparticle diffusion model, the adsorption process of the experiment is under the control of other adsorption stages if all curves did not pass through the origin. If the plots exhibit a straight line, the adsorption process is simply diffusion-based [33]. The experimental data are shown in the attachment (Fig. 5). From the distribution point of the straight line, the dispersion seemed to be relatively high. At the beginning of the reaction, the plot may indicate a straight line passing through the origin that corresponds to the external adsorbate diffusion in the boundary layer. With the increase in the time, this occurs primarily on the surface of the composite nanofibers and rarely spreads into their interior. However, the linear equation fits better than the other conditions when the concentration of chromium ions is 10 or 20 mg/l. Therefore, intraparticle diffusion might be inhibited when the concentration is too low or too high. Thus, according to the R2 value of the correlation coefficient, the pseudo-second-order kinetic model may fit better than the pseudo-first-order kinetics and intraparticle diffusion model in this
system. Moreover, the qe .cal values as obtained from the pseudosecond-order kinetics model seemed to be more feasible to fit experimental data than pseudo-first-order kinetics model. Therefore, the adsorption process of Cr(VI) on the absorbent should follow the pseudo-second-order kinetic model. 3.5. Thermodynamic analysis The adsorption experiment was performed at temperatures of 298, 308, and 318 K, as shown in the attachment (Fig. 6). The results indicated that there was a decrease in the adsorption capacity with an increase in temperature. In fact, this phenomenon could be taken place due to an increase in temperature. The thermal motion of molecules accelerated, and enhanced the degree of disorder of the ions in the solution. By contrast, accelerating the speed at which Cr(VI) is absorbed by Fe2+ to the surface of the composite nanofibers, the relative speed of Cr(VI) desorbed from the surface of the composite nanofibers could also be promoted to slow the total adsorption rate. In fact, this phenomenon could be verified through thermodynamic analysis. The thermodynamic parameters are related of adsorption action (e.g., standard Gibbs free energy G0 , entropy and enthalpy change S0 and H0 ). By setting a time 1/T of LnKd as a function (Fig. 9), the value of H0 and S° could be obtained via the slope and intercept of the straight line (as summarized in Table 2). Apparently, Gibbs free energy G0 is negative at 298 and 308 K, indicating the feasibility and spontaneity of the adsorption reaction. Furthermore, the Gibbs free energy G0 is positive, and the reaction can be non-spontaneous when temperature reached 318 K. With increasing temperature, Gibbs free energy changed from
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Adsorption efficiency, %
1.0
Mg2+ Ca2+ Na+ Cr6+ NO3-
0.8
0.6
ClSO42-
0.4
0.2
0.0
Ions
ly
ure mi xt Pb in Cr
−231.68 – –
re
−72.63 – –
mi xtu
−4.03 −0.64 0.78
Cd
298 308 318
in
S0 (J/mol/K)
Cr
H0 (KJ/mol)
As mi xtu re
G(KJ/mol)
Cr in
Temperature (K)
On
Table 2 Thermodynamic parameters for Cr(VI) adsorbed by the composite nanofibers.
Cr
Fig. 10. Influence of coexisting ions for the adsorption of Cr(VI).
1.0 A d so r p tio n e ffic ie n c y , %
negative to positive. It indicated that the adsorption of chromium ions tend to shift to non-spontaneous from spontaneous [34,35]. During the adsorption process, the enthalpy change value of H0 is negative, suggesting that this adsorption process is an exothermic reaction. In addition, the value is 20 KJ/mol, indicating that the adsorption is more favorable to chemical reaction. Finally, the entropy change value of S0 is negative. It suggests that the reaction system tended to be ordered.
0.8
0.6 0.4
3.6. The effect of coexisting ions
0.
2
nA
s mi
xtur
e n Cd i
re ixtu ture Cd m mix n Pb Pb i
0
As i
0.
Usually, there are significant numbers of different ions (e.g., NO3 − , SO4 2− , Cl− , Ca2+ , Na+ , Mg2+ ) in wastewater. Apparently, that could affect the adsorption efficiency [36]. Therefore, a certain amount of different ions were first added to 10 mg/l Cr(VI) solution, and then carried out adsorption experiments under predetermined conditions to observe the effect of the other ionic species (Fig. 10). From Fig. 11, the inhibitory effects of anions were more pronounced than that of cations for adsorption. Addition of SO4 2− markedly decreased the adsorption efficiency from about 95–25%, while addition of Mg2+ decreased to 80%. This might be due to significant amounts of Fe(II)(OH)+ and Fe(III)(OH)2+ will be formed on the surface of the composite nanofibers in a slightly acidic environment. This led to a strong electrostatic attraction of anions, making them adsorbed easier than cations by the active sites on the surface of the composite nanofibers. In addition, competitive interactions could also reduce the efficiency of adsorption Cr(VI). In contrast, the cations repelled each other due to the electrostatic repulsion that have less competitive with Cr(VI). However, there is still a few number of cations adsorbed to the surface of the composite nanofibers, revealing a fixed amount of the active reaction sites After being compared with anions, the effect was relatively negligible.
Fig. 11. Influence of different heavy metals for the adsorption of Cr(VI).
3.7. The competition between heavy metals The actual waste water often not only contain various ions, but also exist different species of metal ions. Therefore, the adsorption characteristics tended to be more complex. By mixing different kinds of metal ions in the solution the interactive effects of adsorbed chromium (based upon practical wastewater) could be analyzed. A series of heavy metals (e.g., Pb, As and Cd) were added into the 10 ppm Cr(VI) solution for adsorption (Fig. 11). Apparently, the efficiency of Cr(VI) adsorption significantly declined due to competition between heavy metal ions for available active sites. In particular, the influence of arsenic was obvious as its adsorption was reduced from ca. 98 to 70%. And the other heavy metals had different degrees of adsorption by composite nanofibers.
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Regarding addition of three metals, the composite nanofibers adsorbed arsenic slightly larger than other two heavy metals. This may be due to the composite nanofibers strongly adsorbed Cr(VI) from the solution and antagonistically attenuated Cr(VI) to Cr(III) for removal. The oxidation valence state of arsenic was higher. Therefore, arsenic added into the mixed solution may be easier to proceed similar reduction compared with the other two heavy metal of lower valence state. As above mentioned, it implied that the FeCl2 /PAN composite nanofibers also affect removal performance of different kinds of heavy metals from wastewater. 4. Conclusions The FeCl2 /PAN composite nanofibers were successfully prepared by the electrospinning method as a novel material for adsorption of chromium ions in aqueous solutions. The composite nanofibers were analyzed by FTIR, TGA, SEM, and XRD. FTIR results showed that ferrous ions had been successfully modified onto the surface of the composite nanofibers. The XRD analysis showed an amorphous structure due to the rapid evaporation during spinning. XPS analysis also confirmed that the reduction was likely taken place on the composite nanofibers. The PAN…Fe () groups in the composite nanofibers played an important role in the adsorption without dispute. The pH level of the solution directly determined available capacities of adsorption of composite nanofibers. This study revealed that a slightly acidic environment is more appropriate to remove chromium ions. Furthermore, the adsorption characteristics seemed to follow the pseudo-second-order kinetic model. The intraparticle diffusion analysis demonstrated that diffusion primarily took place on the surface of the composite nanofibers. The thermodynamic analysis suggested that the adsorption process is exothermic. An increase in the temperature seemed to be more thermodynamically favorable for adsorption of chromium ions. The inhibitory effects of anions were more pronounced than that of cations for the adsorption. The presence of other heavy metals would decrease the efficiency of adsorption. The FeCl2 /PAN composite nanofibers exhibited good performance in removing chromium ions for promising wastewater treatment. Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jtice.2016.10.043. References [1] Afkhami A, Conway BE. Investigation of removal of Cr(Ⅵ), Mo(Ⅵ),W(Ⅵ),V(Ⅳ) and V (Ⅴ) oxy-ions from industrial waste-waters by adsorption and electrosorption at high-area carbon cloth. J Colloid Interface Sci 2002;251:248–55. [2] Cimino G, Passerini A, Toscano G. Removal of toxic cations and Cr(Ⅵ) from aqueous solution by hazelnut shell. Water Res 20 0 0;34:2955–62. [3] Owlad M, Aroua MK, Daud WAW, Baroutian S. Removal of hexavalent chromium-contaminated water and wastewater: a review. Water Air Soil Pollut 20 09;20 0:59–77. [4] Trunfio G, Crini G. The dechromatation step in wastewater treatment plants: fundamental role and optimization. Ind Eng Chem Res 2010;49:12217–23. [5] Lee ST, Mi FT, Shen YJ, Shyu SS. Equilibrium and kinetic studies of copper(II) ion uptake by chitosan-tripolyphosphate chelating resin. Polymer 2001;42:1879–92. [6] Ngah WS, Fatinathan W, Enuiron SJ. Pb(II) biosorption using chitosan and chitosan derivatives beads: equilibrium, ion exchange and mechanism studies. Sci (Beijing, China) 2010;22:338–46. [7] Yang T, Zall RR. Chitosan membranes for reverse osmosis application. J Food Sci 1984;49:91–3. [8] Assaad E, Azzouz A, Nistor D, Urus A, Sajin T, Miron DN, et al. Metal removal through synergic coagulation – flocculation using an optimized chitosan-montmorillonite system. Appl Clay Sci 2007;37:258–74.
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Please cite this article as: F. Liu et al., Removal of Cr(VI) using polyacrylonitrile/ferrous chloride composite nanofibers, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.10.043