- Email: [email protected]
“Flower-Like” PA6@Mg(OH)2 electrospun nanofibers with Cr (VI)-removal capacity
Chemical Engineering Journal 254 (2014) 98–105
Contents lists available at ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
‘‘Flower-Like’’ PA6@Mg(OH)2 electrospun nanofibers with Cr (VI)-removal capacity Bin-Bin Jia, Jiao-Na Wang, Jing Wu, Cong-Ju Li ⇑ College of Material Science and Engineering, Beijing Institute of Fashion Technology, Beijing 100029, PR China Beijing Key Laboratory of Clothing Materials R&D and Assessment, Beijing 100029, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
PA6@Mg(OH)2 nanofibrous
membrane was fabricated by electrospinning technique combined with hydrothermal strategy. The Cr (VI) adsorption capacity from water can be as high as 296.4 mg per gram of ‘‘flower-like’’ nanofibrous membrane. The nanofibrous membrane shows excellent cyclic utilization performance.
a r t i c l e
i n f o
Article history: Received 2 March 2014 Received in revised form 30 April 2014 Accepted 2 May 2014 Available online 29 May 2014 Keywords: Hexavalent chromium Nanofibrous membrane Magnesium hydroxide Flower-like structure
a b s t r a c t Hexavalent chromium (Cr (VI)) is a kind of high toxic pollutant which is harmful to marine organisms as well as humanity. Removal of Cr (VI) from water is a hot-spot in environmental remediation. In this work, PA6@Mg(OH)2 composite nanofibrous membrane was fabricated by electrospinning technique combined with hydrothermal strategy. The as-prepared nanofibrous membrane possesses a flower-like structure of which endows not only novel morphology but also increase the specific surface area of electrospun fibers. Owing to the 2D flower-like structure of which increase the specific surface area of the nanofibers, the Cr (VI) adsorption capacity from water can be as high as 296.4 mg per gram of nanofibrous membrane (when the concentration of Cr (VI) is 110 mg/g), which is much higher than the most ever reported research results. Meanwhile, the freestanding membrane can be easily separated from liquid solutions and shows excellent cyclic utilization performance. Therefore, such novel flower-like nanofibrous membrane can be potentially widely used in environmental remediation. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Currently, heavy metal ions pollution, which is seriously harmful to human resource, has become one of the most urgent environmental problems [1–6]. Among various kinds of heavy metal ions, hexavalent chromium (Cr (VI)) is extremely harmful because of its high solubility and toxicity in water [7]. Therefore, removal of Cr (VI) from source water has become an urgent need [8,9]. Conventional methods such as chemical precipitation [10], ion exchange ⇑ Corresponding author. Tel./fax.: +86 10 64288192. E-mail address: [email protected] (C.-J. Li). http://dx.doi.org/10.1016/j.cej.2014.05.005 1385-8947/Ó 2014 Elsevier B.V. All rights reserved.
[11], electrochemical removal [12], adsorption [13], membrane technology [14] and polymer enhanced ultrafiltration [15–17], have been widely used in Cr (VI) removal. Particularly, functional adsorption materials were found to be the most promising method because of its easy operation and high efficiency [18]. At present, various kinds adsorbents such as active carbon [19], titanium dioxide [20], magnesium hydroxideactive [21] and pseudo boehmite [22] have been reported. Compared with traditional adsorbents, some types of adsorbents with hierarchically structure have been applied in Cr (VI) removal, because its large specific surface area can enhance the adsorption capacity of Cr (VI) [23,24]. However, their applications have been hindered by some vital defects such
B.-B. Jia et al. / Chemical Engineering Journal 254 (2014) 98–105
as agglomeration and difficulty in separation. Therefore, novel kinds of adsorbents with excellent adsorption capacity as well as practicability are still needed to be prepared to solve these problems. In recent years, nanofibrous membranes have shown potential applications in heavy metal ions removal due to its large specific surface area, great separation property from liquid phase and excellent recycle property [25]. At the same time, electrospinning was considered to be a very effective method to prepare nanofibrous membranes [26–28]. Therefore, it supplies a good method for Cr (VI) removal by modifying electrospun nanofibrous membrane. Two main methods have been applied to Cr (VI) removal, one way is to modify the nanofibrous membrane by some functional groups [29,30]. Another approach is to fabricate the nanofibrous membrane by some inorganic materials which have adsorption properties for heavy metal ions [31,32]. According to above methods, if inorganic materials are adhered to nanofibrous membranes, the obtained composite nanofibrous membranes will possess both the advantages of nanofibers and inorganic materials. However, choosing a suitable inorganic material is a key point. As we all known, there are many kinds of inorganic materials applied in removal Cr (VI). Among all of them, magnesium hydroxide is widely used in heavy metals cations removal (e.g., Cu2+, Ni2+ and Cr (III)) due to their large specific surface area and high efficiency [33]. But there are little reports about removing Cr (VI). Magnesium hydroxide has potential applications in Cr (VI)-removal by research. So the growth of nanostructured magnesium hydroxide on the nanofibers is a great method in removing Cr (VI). However, nanofibers must have enough adhesive force for magnesium hydroxide to avoid separation. Therefore, choosing a proper nanofibrous membrane as the template to induce the growth of magnesium hydroxide is an important problem. It is known that some functional groups (>C@O, OCAN) of organic compounds can interact with magnesium hydroxide and form a weak complex. Therefore, if we fabricate the nanofibrous membrane by magnesium hydroxide combined with other strategies, the obtained composite nanofibrous membrane will be superior to other adsorbents in Cr (VI)-removal. In this work, polyamide 6 (PA6)@Mg(OH)2 composite fibers are fabricated by electrospinning combined with hydrothermal strategy. In general, the electrospun PA6 fibers are used as a template and nanostructured Mg(OH)2 crystals grow on the fiber’s surface. As far as all know, there are little reports about removing Cr (VI) by Mg(OH)2 crystals. However, in this work, the obtained composite nanofibrous membrane exhibits excellent Cr (VI) removal performance with high capacities, easy solid-liquid separation and a stable recyclable property. 2. Experiment
99
water. And then the obtained nanofibrous membrane was immersed into the solution. Secondly, 3 ml NH.3H2O (Beijing Tongguang Chemical Co., Ltd) is dissolved in the solution 10 min later. Thirdly, it was placed in the electric-heated thermostatic water bath at the temperature of 40 °C for 1–5 h. Finally, it was dried in the Electric Blast Drying Oven at the temperature of 60 °C for 2 h to obtain hierarchically PA6@Mg(OH)2 composite nanofibrous membrane. 2.3. Cr (VI) removal experiments A series of adsorption experiments were performed to investigate the efficiency of the obtained composite nanofibers for the removal of Cr (VI) from aqueous solutions in room temperature. The Cr (VI) solutions with different concentrations were prepared by dissolving K2Cr2O7 in deionized water. Firstly, to determine the influences of pH and hydrothermal time on removing Cr (VI), in the experiment, the pH values of the K2Cr2O7 solutions were adjusted to (1, 2, 3, 5, 7, 9) by 1 mol/L HCl and 1 mol/L NaOH solutions. 0.008 g of as-prepared adsorbents with different hydrothermal time (e.g., 1–5 h) were added into 40 ml of K2Cr2O7 solution (110 mg/L) and then measured concentrations of the Cr solutions at certain time intervals for 3 days. Secondly, to study the equilibrium adsorption kinetics, 0.008 g of as-prepared adsorbent (hydrothermal time is 2 h) was immersed into 40 ml of K2Cr2O7 solution (110.6 mg/L) at pH = 2, followed by sampling the solution at set intervals within 9 days for the analysis of Cr content in the solution. Thus, the adsorption kinetic model was determined. Finally, 8 mg of nanofibrous membrane was immersed into 40 ml K2Cr2O7 solutions with different initial Cr (VI) concentrations (26.8, 52.7, 80.7, 110.6, 134.3, 148.2, 165.8 mg/L) at room temperature. When the adsorption reached the equilibrium, the Cr (VI) content in the solution was measured. The adsorption capacity of the membrane for the Cr (VI) removal was calculated as follows:
qe ¼
ðC o C e Þ:V m
ð1Þ
where qe is the equilibrium adsorption quantity (mg/g), Co and Ce represent the initial and equilibrium Cr (VI) concentration (mg/L), respectively. V is the volume of the K2Cr2O7 solution (L), and m is the quality of membrane (g). To investigate the cyclic utilization performance of the obtained composite nanofibrous membrane, the nanofibrous membrane was firstly immersed into 100 ml of 0.1 mol/L NaOH solution for 2 h after the Cr (VI) removal experiment, and then was washed with deionized water four times and dried at 60 °C for 2 h.
2.1. Preparation of PA6 nanofibrous membrane
2.4. Characterization
1.6 g of PA6 was dissolved in 10 ml formic acid (Beijing Tongguang Fine Chemical Co., Ltd) to form homogeneous solution. Nearly 6 ml of precursor solution was placed in a 10 ml syringe. The electrospinning conditions were as follows: A syringe pump with a syringe containing electrospinning solution was used as the impetus supplier at the rate of 0.5 mlh1. The positive high voltage on the needle of the syringe was 20 kV and the distance between the needle tip and collector was 15 cm. By electrospinning the PA6 nanofibrous membrane was obtained after 10 h.
The morphology and structure of the nanofibrous membranes were examined by scanning electron microscopy (SEM, JEOL-7500F, Japan). The elemental analysis of the PA6@Mg(OH)2 nanofibrous membrane was performed with an energy dispersive X-ray spectrometry (EDX, JSM-6360LV EDX spectrometer). The chemical groups of membrane were characterized by the Fourier transform infrared spectrometry (FT-IR) on a Nicolet 8700 FT-IR spectrometry (USA) in the range of wave number from 600 to 4000 cm1. X-ray diffraction patterns (XRD) of samples were recorded on a D/MAX-IIIA diffractometer using Cu Ka radiation source (k = 1.54056 Å) at a scan rate of 5°min1 to determine the phase composition and the crystallinity. The accelerating voltage and beam current were 40 kV and 200 mA, respectively. The concentrations of the Cr solutions were measured by inductively coupled plasma atomic emission spectrophotometer (ICP-AES) (CIROS EOP, Germany).
2.2. Preparation of polyamide 6 (PA6)@Mg(OH)2 composite nanofibrous membrane The experiment process is stated as follows: Firstly, 1 g MgSO4 (Tianjin Damao Chemical Co., Ltd) is dissolved in 40 ml deionized
100
B.-B. Jia et al. / Chemical Engineering Journal 254 (2014) 98–105
3. Results and discussion 3.1. Morphology characterization In the experiment, PA6@Mg(OH)2 composite nanofibrous membrane was fabricated by electrospinning technique combined with hydrothermal strategy. To further confirm the morphology of the PA6 and PA6@Mg(OH)2 nanofibrous membranes, SEM images for the hierarchically structured nanofibrous membranes with different hydrothermal time were described in Fig. 1. It can be seen that the electrospun PA6 nanofibers are randomly arranged to form a net-like porous structure and the surface of PA6 nanofibers are smooth (Fig. 1a). PA6@Mg(OH)2 nanofibrous membranes with different morphologies were fabricated by controlling hydrothermal time (Fig. 1b–f). After heating for 1 h, compared with pure PA6 nanofibrous membrane visible changes can be observed on the surface of nanofibers (Fig. 1b). A lot of precipitates are observed and no hierarchical structure appears. Increasing the hydrothermal time, hierarchical structures can be obviously observed (Fig. 1c and d). As we can see in Fig. 1d and g, 3 h is the best hydrothermal time and flower-like PA6@Mg(OH)2 composite nanofibrous membrane was successfully obtained. Further prolonging hydrothermal time, the hierarchical structures were destroyed slowly (Fig. 1e and f), which may be due to the desquamation of Mg(OH)2 crystals from the surface of nanofibers.
A proposed mechanism for preparation of the PA6@Mg(OH)2 composite nanofibrous membrane is illustrated as follows: When PA6 nanofibrous membrane was immersed into MgSO4 solutions, PA6 has a special function group (>C@O) and the lone pair electrons of O can make the PA6 have negative electricity. Therefore, a lot of Mg2+ can be adhered to the surface of PA6 nanofibrous membrane. As a precipitator of Mg2+, NH3H2O is a kind of weak base, which can lower the degree of supersaturation in various ways. At the temperature of 40 °C, with increasing the concentration of NH3H2O, Mg2+ can react with NH3H2O on the surface of PA6 nanofibrous membrane and the process was illustrated as follow in (2).
Mg2þ þ 2NH3 H2 O ! MgðOHÞ2 þ 2NHþ4
ð2Þ
However, the crystallization process may need some time. Crystallization process involves two steps (nucleation and growth). Crystallization occurs, when the solution reached saturation. When the hydrothermal time reach to 3 h, flower-like PA6@Mg(OH)2 nanofibrous membrane was obtained. However, further prolonging hydrothermal time, the hierarchical structures were destroyed slowly due to the destruction of the force between PA6 and Mg(OH)2. In addition, based on the isothermal nitrogen sorption measurement and the Brunauer–Emmett–Teller (BET) equation, it is found that the growth of Mg(OH)2 on the surface of PA6 fibers can significantly increase the specific surface area from 10.8502 to 25.5240 m2/g.
Fig. 1. (a) SEM image for pure PA6 nanofibrous membrane. SEM images for the PA6@Mg(OH)2 nanofibrous membranes with different hydrothermal time: (b) 1 h; (c) 2 h; (d) 3 h; (e) 4 h; (f) 5 h. The inset (g) in d shows the high resolution SEM image.
B.-B. Jia et al. / Chemical Engineering Journal 254 (2014) 98–105
3.2. FTIR and EDS analyses FTIR analysis (Fig. 2a) is carried out to further confirm the coordination of Mg(OH)2 crystals with polymer chains of PA6 in the nanofibers. The peak at 1635 cm1 is ascribed to the typical C@O stretching model of carboxyl groups. The prominent peak at 1538 cm1 is assigned to the bending vibration of amino groups. The peak at 2858 cm1 and 2930 cm1 is ascribed to the symmetric CH2 stretching mode and asymmetric CH2 stretching mode, respectively. These peaks are the characteristic peaks of PA6 [34,35]. After hydrothermal strategy, besides near the above characteristic peaks, a new peak at 3691 cm1 appears, which is ascribed to the stretching vibration of free AOH bond. Since PA6 does not contain free AOH bond, which confirms the coordination chemistry reaction between PA6 and Mg(OH)2. Therefore, it is indicated that Mg(OH)2 crystals can effectively interact with PA6 and adhere to the surface of nanofibers. To further prove the formation of Mg(OH)2 on the PA6 nanofibers, the SEM-EDX for the PA6@Mg(OH)2 composite nanofibrous membrane is shown in Fig. 2b. The SEM-EDX spectra results show that the elements of C, O and Mg are the main composition of the nanofibrous membrane, and the Mg content is 4.32 wt%.
101
peaks of PA6 nanofibrous membrane are mainly centered around 15–28°. The peaks appear at 2h values of 20° and 24.8° (marked with ‘‘#’’), which indicated that PA6 is an a crystalline polymer. After the growth of Mg(OH)2, the XRD patterns changes obviously. As seen in Fig. 3B, except for the broad peak of PA6, new peaks located at 18.58°, 38.02°, 50.85°, 58.64°, 62.07°, 68.25° and 72.03° appeared. It indicates the successful growth of Mg(OH)2 nanoparticles (according to JCPDS 07-0239). Furthermore, it is found that the new peaks are stronger than the peaks of PA6. These results suggest that after the hydrothermal treatment, Mg(OH)2 nanoparticles with high crystallinity are successfully anchored on the nanofibers, providing PA6 nanofibers flower-like hierarchical structures. 3.4. Cr (VI) removal capacity The Cr (VI) removal capacity of the obtained PA6@Mg(OH)2 composite nanofibrous membrane was systematically investigated. Fig. 4 shows the Cr (VI) adsorption capacity of PA6@Mg(OH)2 composite nanofibrous membrane with different morphologies at
3.3. XRD analysis XRD analysis is applied to examine the crystal structure of PA6@Mg(OH)2 composite nanofibrous membrane. The XRD results of PA6 (Fig. 3A) and PA6@Mg(OH)2 (Fig. 3B) are shown in Fig. 3. The
Fig. 3. XRD patterns for PA6 (A) and PA6@Mg(OH)2 (B) nanofibrous membrane.
Fig. 2. (a) (A) FTIR spectra for PA6 nanofibrous membrane. (B) FTIR spectra for PA6@Mg(OH)2 nanofibrous membrane. (b) SEM-EDX (EDS) for the PA6@Mg(OH)2 nanofibrous membrane.
Fig. 4. Adsorption Cr (VI) capacity of PA6@Mg(OH)2 composite nanofibrous membrane with different hydrothermal time at different pH values in 3 days.
102
B.-B. Jia et al. / Chemical Engineering Journal 254 (2014) 98–105
different pH values in 3 days. In general, the adsorption capacity of PA6@Mg(OH)2 composite nanofibrous membrane for Cr (VI) ions increases first and then decreases with decreasing pH values. The highest Cr (VI) adsorption occurs at the pH value of 2. The adsorption capacity will reduce when increasing or decreasing pH values. Moreover, it shows that the acidic solution is more in favour of Cr (VI) removal and there is little Cr (VI) adsorption under the alkaline pH values. The main reasons are illustrated as following. In the K2Cr2O7 solution with an acidic pH value, the Mg(OH)2 nanoparticles are protonated with positive charge. In addition, due to the protonation of Cr2O2 7 , Cr (VI) ions mainly exist in the form of HCrO 4 . So the HCrO4 ions are easily adsorbed on the PA6@Mg(OH)2 composite nanofibrous membrane at low pH value. However, Mg(OH)2 nanoparticles can be partly dissolved under a lower pH (pH = 1). Conversely, in the alkaline pH value, the presences of a larger number of OH groups on Mg(OH)2 nanoparticles hinder the adsorption of Cr2O2 7 . Furthermore, as shown in Fig. 4 (pH = 2), the adsorption capacity also increases first and then decreases with increasing the hydrothermal time. The main reasons are illustrated as following. Increasing the hydrothermal time, more and more Mg(OH)2 crystals adhere to the surface of PA6 nanofibrous membrane, which is highly favorable to the adsorption of HCrO 4 . But further prolonging hydrothermal time, the hierarchical structures were destroyed slowly. Therefore, the Cr (VI) adsorption becomes lower. The Cr (VI) removal capacity of the obtained PA6@Mg(OH)2 composite nanofibrous membrane was investigated and the results are shown in Fig. 5. Fig. 5 shows the Cr (VI) removal experiment results by the PA6@Mg(OH)2 composite nanofibrous membrane in 9 days. The initial Cr (VI) concentration is 110.6 mg/L (pH = 2). The Cr (VI) concentration decreases from 110.6 to 48.4 mg/L in 7 days and then almost retains the same level. From the curve, the process of removing Cr (VI) can be divided into two stages: the first stage (0–7 days) and the second stage (7–9 days). In the first stage, The Cr (VI) concentration decreases slowly. Obviously, compared to the first stage, the second stage almost retains the same level. Therefore, it is considered to be the equilibration in the second stage and indicated that the Cr (VI) ions in the solution are almost removed in 7 days. To investigate the kinetics of Cr (VI) removal by the obtained PA6@Mg(OH)2 composite nanofibrous membrane, the experimental data were analyzed using the pseudo-first-order kinetic model. The fitting of the data was achieved with the pseudo-first-order equations expressed as:
Fig. 5. Change of the total Cr (VI) in the solution with time.
lnðqe qt Þ ¼ ln qe k1 t
ð3Þ
where qt is the amount of Cr (VI) adsorbed (mg/g) at time t; qe is the amount of Cr (VI) adsorbed (mg/g) at equilibrium time; k1 is the rate constant (d1) of adsorption. The value of k1 was determined by the slope of the graphs ‘‘log (qeqt) versus t’’. As shown in Fig. 6a, the value of k1 and R2 were calculated to be 0.39114 and 0.98857, respectively. The adsorption data were also analyzed by pseudo-secondorder kinetic model. This can be expressed by following equation:
t 1 t ¼ þ qt k2 q2e qe
ð4Þ
where k2 (gmg1d1) is the rate constant of pseudo-second-order reaction. The value of qe and k2 can be determined by the slope and intercept of the straight line of the plot of ‘‘t/qt versus t’’. As shown in Fig. 6b, the value of k2 and R2 were found to be 0.00254 and 0.99806, respectively. In addition, the adsorption isotherm is researched and the results are shown in Fig. 7. Fig. 7a shows the equilibrium Cr (VI) adsorption amounts of the obtained nanofibrous membranes with different initial Cr (VI) concentrations in 9 days. In general, the PA6@Mg(OH)2 nanofibrous membrane’s adsorption capacity generally improves with the increasing of initial Cr (VI) concentration. Obviously, the adsorption is not the Langmuir adsorption [36] (Langmuir isotherm assumes the adsorption onto the
Fig. 6. Pseudo-first-order plot (a) and Pseudo-second-order plot (b) kinetic model.
103
B.-B. Jia et al. / Chemical Engineering Journal 254 (2014) 98–105
The recyclable property of adsorbent is very important for its application. In this experiment, the recyclable property of PA6@Mg(OH)2 composite nanofibrous membrane was studied by repeating the Cr (VI) removal experiment 4 times using the same nanofibrous membrane after the regeneration using diluted NaOH solution (Fig. 8), which can effectively induce the desorption of Cr (VI) from inorganic materials. In the alkaline solution, the desorption was supposed to be attributed to the elimination of protonated Mg2+. Therefore, the surface of the HCr2O 7 will break away from PA6@Mg(OH)2 composite nanofibrous membrane. In Fig. 8, it is interesting to observe that the regenerated membrane of PA6@Mg(OH)2 also shows a high Cr (VI) removal ability. Several reasons can explain this phenomenon. First, Cr (VI) is absorbed onto the membrane in the form of anionic HCrO 4 , after the regeneration a lot of adsorbtion sites may still exist, which will favor the adsorption of Cr (VI). Secondly, the membrane consists of many layers, after regeneration, the inner lay is permeated more sufficiently, which will beneficial for the Cr (VI) removal. After 4 times recycling, the Cr removal efficiency still remains 52%, which indicates a stable recyclable property. Finally, as shown in Fig 9, there may be two possible mechanisms for Cr (VI) adsorption onto PA6@Mg(OH)2. When the pH values of the K2Cr2O7 solutions were adjusted to 2 by 1 mol/L HCl, a lot of Cl exits in solutions. The surface of Mg(OH)2 nanoparticles are protonated and have an acquired positive charge in acidic solutions.
MgðOHÞ2 þ H3 Oþ ! MgðOHÞ2 Hþ þ H2 O
ð6Þ
One mechanism about PA6@Mg(OH)2 composite nanofibrous membrane for Cr (VI) removal is anion exchange. First, the Cl ions in solutions that can associated with Mg(OH)2 to form Mg(OH)2H+/ Cl. Second, Mg(OH)2H+/Cl can exchange with the anionic Cr(VI) in solutions [40], which the reaction occurs as the following equation:
MgðOHÞ2 Hþ =Cl þ HCrO4 ! MgðOHÞ2 Hþ =HCrO4 þ Cl
Fig. 7. (a) Adsorption isotherm and (b) equilibrium adsorption isotherm of Cr (VI) fitted with Freundlich adsorption model.
homogeneous surface with specific number of equivalent sites), in which the adsorption will reach the maximum value. Thus, the data is fitted with Freundlich adsorption model [37] (Freundlich isotherm represents the sorption onto the heterogeneous surface). The mathematical expressions for Freundlich adsorption model is [38]
lnqe ¼
1 lnC e þ lnK F n
ð7Þ
Another mechanism may be surface complexation, which is suggested for anion adsorption onto Mg(OH)2 [33]. The association between the positively charged surface of Mg(OH)2 and the HCrO4 anions, which the reaction occurs as the following equation:
MgðOHÞ2 Hþ þ HCrO4 ! MgðOHÞ2 Hþ =HCrO4
ð8Þ
ð5Þ
where Ce and qe are the equilibrium concentration (mg/L) and equilibrium quantity (mg/g), KF and n are Freundlich constants related to adsorption capacity and adsorption intensity, respectively. KF and n can be obtained by a plot of ln (qe) against ln (Ce). Fig. 7b indicates that the adsorption of Cr (VI) onto the membrane is a multi-adsorption process, because Freundlich adsorption model assumes a multilayer adsorption and the stronger binding sites are occupied first by the adsorbates. The values of KF and n are listed in Fig 7b. It is usually accepted that a value of 2 6 n < 10 represents an easy adsorption, a value of 1 6 n < 2 represents a moderate adsorption, while a value of n < 1 represents a difficult adsorption. For the obtained membrane, the value of n is 1.5886, indicating that the adsorption is a moderate adsorption, which can be also identified by the high value of KF (13.895) [32,39].
Fig. 8. The recyclable properties of PA6@Mg(OH)2 composite nanofibrous membrane for Cr (VI) removal.
104
B.-B. Jia et al. / Chemical Engineering Journal 254 (2014) 98–105
Fig. 9. Mechanism for Cr (VI) removal by PA6@Mg(OH)2 composite nanofibrous membrane.
4. Conclusion In summary, the ‘‘Flower-Like’’ PA6@Mg(OH)2 composite nanofibrous membrane was successfully fabricated by electrospinning combined with hydrothermal method. The Cr (VI) removal experiment showed that the obtained nanofibrous membrane exhibited excellent performance for the removal of Cr (VI). The Cr (VI) adsorption capacity can reach 296.4 mg/g (when the concentration of Cr (VI) is 110 mg/g), which is much higher than the previously reported values, such as flowerlike a-Fe2O3 nanoparticles (5.4 mg Cr/g) [41], PAN@c-AlOOH fiber films (8 mg Cr/g) [42], PA6@FexOy nanofibrous membrane (150 mg Cr/g) [43]. The strong Cr (VI) adsorption ability is attributed to the high surface area and suitable pore-size distribution of the obtained nanofibrous membranes. Meanwhile, the freestanding membrane can be easily separated from liquid solutions and has excellent recyclable performance. Therefore, the obtained nanofibrous membrane can be an efficient materials for the removal of Cr (VI) from water and other environmental areas.
Acknowledgments This study was partly supported by the Beijing City Board of Education Upgrade Project (No. TJSHG201310012021), Natural Science Foundation of China (Grant No. 21274006), the 973 Project (Grant No. 2010CB933501), Research Project of Beijing Institute of Fashion Technology (Grant No. 2012A-09) and Beijing Outstanding Talents Cultivation (Grant No. 2013D005001000003).
References [1] T. Naimo, A review of the effects of heavy metals on freshwater mussels, Ecotoxicology 4 (1995) 341–362. [2] P.C. Nagajyoti, K.D. Lee, T.V.M. Sreekanth, Heavy metals, occurrence and toxicity for plants: a review, Environ. Chem. Lett. 8 (2010) 199–216. [3] F. Fu, Q. Wang, Removal of heavy metal ions from wastewaters: a review, J. Environ. Manage. 92 (2011) 407–418. [4] P. Xu, G.M. Zeng, D.L. Huang, C.L. Feng, S. Hu, M.H. Zhao, C. Lai, Z. Wei, C. Huang, G.X. Xie, Z.F. Liu, Use of iron oxide nanomaterials in wastewater treatment: a review, Sci. Total Environ. 424 (2012) 1–10. [5] Y. Yi, Z. Yang, S. Zhang, Ecological risk assessment of heavy metals in sediment and human health risk assessment of heavy metals in fishes in the middle and lower reaches of the Yangtze River basin, Environ. Pollut. 159 (2011) 2575– 2585. [6] A. Singh, R.K. Sharma, M. Agrawal, F.M. Marshall, Health risk assessment of heavy metals via dietary intake of foodstuffs from the wastewater irrigated site of a dry tropical area of India, Food Chem. Toxicol. 48 (2010) 611–619.
[7] B. Saha, C. Orvig, Biosorbents for hexavalent chromium elimination from industrial and municipal effluents, Coord. Chem. Rev. 254 (2010) 2959– 2972. [8] P. Miretzky, A.F. Cirelli, Cr (VI) and Cr (III) removal from aqueous solution by raw and modified lignocellulosic materials: a review, J. Hazard. Mater. 180 (2010) 1–19. [9] S. Gupta, B.V. Babu, Removal of toxic metal Cr (VI) from aqueous solutions using sawdust as adsorbent: equilibrium, kinetics and regeneration studies, Chem. Eng. J. 150 (2009) 352–365. [10] M. Gheju, I. Balcu, Removal of chromium from Cr (VI) polluted wastewaters by reduction with scrap iron and subsequent precipitation of resulted cations, J. Hazard. Mater. 196 (2011) 131–138. [11] S. Edebali, E. Pehlivan, Evaluation of Amberlite IRA96 and Dowex 18 ionexchange resins for the removal of Cr (VI) from aqueous solution, Chem. Eng. J. 161 (2010) 161–166. [12] G. Mouedhen, M. Feki, M. De Petris-Wery, H.F. Ayedi, Electrochemical removal of Cr (VI) from aqueous media using iron and aluminum as electrode materials: towards a better understanding of the involved phenomena, J. Hazard. Mater. 168 (2009) 983–991. [13] A.K. Bhattacharya, T.K. Naiya, S.N. Mandal, S.K. Das, Adsorption, kinetics and equilibrium studies on removal of Cr (VI) from aqueous solutions using different low-cost adsorbents, Chem. Eng. J. 137 (2008) 529–541. [14] X. Guo, F. Zhang, Q. Peng, S. Xu, X. Lei, D.G. Evans, X. Duan, Layered double hydroxide/eggshell membrane: an inorganic biocomposite membrane as an efficient adsorbent for Cr (VI) removal, Chem. Eng. J. 166 (2011) 81–87. [15] J. Müslehiddinog˘lu, Y. Uludag˘, et al., Effect of operating parameters on selective separation of heavy metals from binary mixtures via polymer enhanced ultrafiltration, J. Membr. Sci. 140 (1998) 251–266. [16] M.K. Aroua, F.M. Zuki, N.M. Sulaiman, Removal of chromium ions from aqueous solutions by polymer-enhanced ultrafiltration, J. Hazard. Mater. 147 (2007) 752–758. [17] M.A. Barakat, E. Schmidt, Polymer-enhanced ultrafiltration process for heavy metals removal from industrial wastewater, Desalination 256 (2010) 90–93. [18] V.K. Gupta, A. Rastogi, A. Nayak, Adsorption studies on the removal of hexavalent chromium from aqueous solution using a low cost fertilizer industry waste material, J. Colloid Interface Sci. 342 (2010) 135–141. [19] G. Huang, J.X. Shi, T.A.G. Langrish, Removal of Cr (VI) from aqueous solution using activated carbon modified with nitric acid, Chem. Eng. J. 152 (2009) 434–439. [20] N. Wu, H. Wei, L. Zhang, Efficient removal of heavy metal ions with biopolymer template synthesized mesoporous titania beads of hundreds of micrometers size, Environ. Sci. Technol. 46 (2011) 419–425. [21] R. Zhang, X. Pan, F. Li, L. Zhang, S. Zhai, Q. Mu, J. Liu, G. Qu, G. Jiang, B. Yan, Cell rescue by nanosequestration: reduced cytotoxicity of an environmental remediation residue, Mg(OH)2 nanoflake/Cr (VI) adduct, Environ. Sci. Technol. 48 (2014) 1984–1992. [22] Y.X. Zhang, Y. Jia, Z. Jin, X.Y. Yu, W.H. Xu, T. Luo, B.J. Zhu, J.H. Liu, X.J. Huang, Self-assembled, monodispersed, flower-like c-AlOOH hierarchical superstructures for efficient and fast removal of heavy metal ions from water, CrystEngComm 14 (2012) 3005–3007. [23] G. Aragay, J. Pons, A. Merkoçi, Recent trends in macro-, micro-, and nanomaterial-based tools and strategies for heavy-metal detection, Chem. Rev. 111 (2011) 3433–3458. [24] J.P. Ruparelia, S.P. Duttagupta, A.K. Chatterjee, S. Mukherji, Potential of carbon nanomaterials for removal of heavy metals from water, Desalination 232 (2008) 145–156. [25] S. Haider, S.Y. Park, Preparation of the electrospun chitosan nanofibers and their applications to the adsorption of Cu (II) and Pb (II) ions from an aqueous solution, J. Membr. Sci. 328 (2009) 90–96. [26] Z.M. Huang, Y.Z. Zhang, M. Kotaki, S. Ramakrishna, A review on polymer nanofibers by electrospinning and their applications in nanocomposites, Compos. Sci. Technol. 63 (2003) 2223–2253. [27] J.A. Matthews, G.E. Wnek, D.G. Simpson, G.L. Bowlin, Electrospinning of collagen nanofibers, Biomacromolecules 3 (2002) 232–238. [28] M. Bognitzki, W. Czado, T. Frese, A. Schaper, M. Hellwig, M. Steinhart, A. Greiner, J.H. Wendorff, Nanostructured fibers via electrospinning, Adv. Mater. 13 (2001) 70–72. [29] P. Kampalanonwat, P. Supaphol, Preparation and adsorption behavior of aminated electrospun polyacrylonitrile nanofiber mats for heavy metal ion removal, ACS Appl. Mater. Interfaces 2 (2010) 3619–3627. [30] S. Wu, F. Li, Y. Wu, R. Xu, G. Li, Preparation of novel poly(vinyl alcohol)/SiO2 composite nanofiber membranes with mesostructure and their application for removal of Cu2+ from waste water, Chem. Commun. 46 (2010) 1694–1696. [31] Y. Lin, W. Cai, X. Tian, X. Liu, G. Wang, C. Liang, Polyacrylonitrile/ferrous chloride composite porous nanofibers and their strong Cr-removal performance, J. Mater. Chem. 21 (2011) 991–997. [32] G.R. Xu, J.-N. Wang, C.-J. Li, Preparation of hierarchically nanofibrous membrane and its high adaptability in hexavalent chromium removal from water, Chem. Eng. J. 198–199 (2012) 310–317. [33] Y. Li, B. Gao, T. Wu, D. Sun, X. Li, B. Wang, F. Lu, Hexavalent chromium removal from aqueous solution by adsorption on aluminum magnesium mixed hydroxide, Water Res. 43 (2009) 3067–3075. [34] Q. Wu, X. Liu, L.A. Berglund, FT-IR spectroscopic study of hydrogen bonding in PA6/clay nanocomposites, Polymer 43 (2002) 2445–2449. [35] J. Wei, Y. Li, K.T. Lau, Preparation and characterization of a nano apatite/ polyamide6 bioactive composite, Compos. B Eng. 38 (2007) 301–305.
B.-B. Jia et al. / Chemical Engineering Journal 254 (2014) 98–105 [36] I. Langmuir, The adsorption of gases on plane surfaces of glass, mica and platinum, J. Am. Chem. Soc. 40 (1918) 1361–1403. [37] C. Ng, J.N. Losso, W.E. Marshall, R.M. Rao, Freundlich adsorption isotherms of agricultural by-product-based powdered activated carbons in a geosminwater system, Bioresour. Technol. 85 (2002) 131–135. [38] R.J. Umpleby Ii, S.C. Baxter, M. Bode, J.K. Berch Jr., R.N. Shah, K.D. Shimizu, Application of the Freundlich adsorption isotherm in the characterization of molecularly imprinted polymers, Anal. Chim. Acta 435 (2001) 35–42. [39] A. Aluigi, F. Rombaldoni, C. Tonetti, et al., Study of Methylene Blue adsorption on keratin nanofibrous membranes, J. Hazard. Mater. 268 (2014) 156–165. [40] D. Carriazo, M. del Arco, C. Martín, V. Rives, A comparative study between chloride and calcined carbonate hydrotalcites as adsorbents for Cr (VI), Appl. Clay Sci. 37 (2007) 231–239.
105
[41] N. Wu, H. Wei, L. Zhang, Efficient removal of heavy metal ions with biopolymer template synthesized mesoporous titania beads of hundreds of micrometers size, Environ. Sci. Technol. 46 (2011) 419–425. [42] Y. Lin, W. Cai, H. He, et al., Three-dimensional hierarchically structured PAN@c-AlOOH fiber films based on a fiber templated hydrothermal route and their recyclable strong Cr (VI)-removal performance, RSC Adv. 5 (2012) 1769– 1773. [43] C.J. Li, Y.J. Li, J.N. Wang, et al., PA6@FexOy nanofibrous membrane preparation and its strong Cr (VI)-removal performance, Chem. Eng. J. 220 (2013) 294–301.
Contents lists available at ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
‘‘Flower-Like’’ PA6@Mg(OH)2 electrospun nanofibers with Cr (VI)-removal capacity Bin-Bin Jia, Jiao-Na Wang, Jing Wu, Cong-Ju Li ⇑ College of Material Science and Engineering, Beijing Institute of Fashion Technology, Beijing 100029, PR China Beijing Key Laboratory of Clothing Materials R&D and Assessment, Beijing 100029, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
PA6@Mg(OH)2 nanofibrous
membrane was fabricated by electrospinning technique combined with hydrothermal strategy. The Cr (VI) adsorption capacity from water can be as high as 296.4 mg per gram of ‘‘flower-like’’ nanofibrous membrane. The nanofibrous membrane shows excellent cyclic utilization performance.
a r t i c l e
i n f o
Article history: Received 2 March 2014 Received in revised form 30 April 2014 Accepted 2 May 2014 Available online 29 May 2014 Keywords: Hexavalent chromium Nanofibrous membrane Magnesium hydroxide Flower-like structure
a b s t r a c t Hexavalent chromium (Cr (VI)) is a kind of high toxic pollutant which is harmful to marine organisms as well as humanity. Removal of Cr (VI) from water is a hot-spot in environmental remediation. In this work, PA6@Mg(OH)2 composite nanofibrous membrane was fabricated by electrospinning technique combined with hydrothermal strategy. The as-prepared nanofibrous membrane possesses a flower-like structure of which endows not only novel morphology but also increase the specific surface area of electrospun fibers. Owing to the 2D flower-like structure of which increase the specific surface area of the nanofibers, the Cr (VI) adsorption capacity from water can be as high as 296.4 mg per gram of nanofibrous membrane (when the concentration of Cr (VI) is 110 mg/g), which is much higher than the most ever reported research results. Meanwhile, the freestanding membrane can be easily separated from liquid solutions and shows excellent cyclic utilization performance. Therefore, such novel flower-like nanofibrous membrane can be potentially widely used in environmental remediation. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Currently, heavy metal ions pollution, which is seriously harmful to human resource, has become one of the most urgent environmental problems [1–6]. Among various kinds of heavy metal ions, hexavalent chromium (Cr (VI)) is extremely harmful because of its high solubility and toxicity in water [7]. Therefore, removal of Cr (VI) from source water has become an urgent need [8,9]. Conventional methods such as chemical precipitation [10], ion exchange ⇑ Corresponding author. Tel./fax.: +86 10 64288192. E-mail address: [email protected] (C.-J. Li). http://dx.doi.org/10.1016/j.cej.2014.05.005 1385-8947/Ó 2014 Elsevier B.V. All rights reserved.
[11], electrochemical removal [12], adsorption [13], membrane technology [14] and polymer enhanced ultrafiltration [15–17], have been widely used in Cr (VI) removal. Particularly, functional adsorption materials were found to be the most promising method because of its easy operation and high efficiency [18]. At present, various kinds adsorbents such as active carbon [19], titanium dioxide [20], magnesium hydroxideactive [21] and pseudo boehmite [22] have been reported. Compared with traditional adsorbents, some types of adsorbents with hierarchically structure have been applied in Cr (VI) removal, because its large specific surface area can enhance the adsorption capacity of Cr (VI) [23,24]. However, their applications have been hindered by some vital defects such
B.-B. Jia et al. / Chemical Engineering Journal 254 (2014) 98–105
as agglomeration and difficulty in separation. Therefore, novel kinds of adsorbents with excellent adsorption capacity as well as practicability are still needed to be prepared to solve these problems. In recent years, nanofibrous membranes have shown potential applications in heavy metal ions removal due to its large specific surface area, great separation property from liquid phase and excellent recycle property [25]. At the same time, electrospinning was considered to be a very effective method to prepare nanofibrous membranes [26–28]. Therefore, it supplies a good method for Cr (VI) removal by modifying electrospun nanofibrous membrane. Two main methods have been applied to Cr (VI) removal, one way is to modify the nanofibrous membrane by some functional groups [29,30]. Another approach is to fabricate the nanofibrous membrane by some inorganic materials which have adsorption properties for heavy metal ions [31,32]. According to above methods, if inorganic materials are adhered to nanofibrous membranes, the obtained composite nanofibrous membranes will possess both the advantages of nanofibers and inorganic materials. However, choosing a suitable inorganic material is a key point. As we all known, there are many kinds of inorganic materials applied in removal Cr (VI). Among all of them, magnesium hydroxide is widely used in heavy metals cations removal (e.g., Cu2+, Ni2+ and Cr (III)) due to their large specific surface area and high efficiency [33]. But there are little reports about removing Cr (VI). Magnesium hydroxide has potential applications in Cr (VI)-removal by research. So the growth of nanostructured magnesium hydroxide on the nanofibers is a great method in removing Cr (VI). However, nanofibers must have enough adhesive force for magnesium hydroxide to avoid separation. Therefore, choosing a proper nanofibrous membrane as the template to induce the growth of magnesium hydroxide is an important problem. It is known that some functional groups (>C@O, OCAN) of organic compounds can interact with magnesium hydroxide and form a weak complex. Therefore, if we fabricate the nanofibrous membrane by magnesium hydroxide combined with other strategies, the obtained composite nanofibrous membrane will be superior to other adsorbents in Cr (VI)-removal. In this work, polyamide 6 (PA6)@Mg(OH)2 composite fibers are fabricated by electrospinning combined with hydrothermal strategy. In general, the electrospun PA6 fibers are used as a template and nanostructured Mg(OH)2 crystals grow on the fiber’s surface. As far as all know, there are little reports about removing Cr (VI) by Mg(OH)2 crystals. However, in this work, the obtained composite nanofibrous membrane exhibits excellent Cr (VI) removal performance with high capacities, easy solid-liquid separation and a stable recyclable property. 2. Experiment
99
water. And then the obtained nanofibrous membrane was immersed into the solution. Secondly, 3 ml NH.3H2O (Beijing Tongguang Chemical Co., Ltd) is dissolved in the solution 10 min later. Thirdly, it was placed in the electric-heated thermostatic water bath at the temperature of 40 °C for 1–5 h. Finally, it was dried in the Electric Blast Drying Oven at the temperature of 60 °C for 2 h to obtain hierarchically PA6@Mg(OH)2 composite nanofibrous membrane. 2.3. Cr (VI) removal experiments A series of adsorption experiments were performed to investigate the efficiency of the obtained composite nanofibers for the removal of Cr (VI) from aqueous solutions in room temperature. The Cr (VI) solutions with different concentrations were prepared by dissolving K2Cr2O7 in deionized water. Firstly, to determine the influences of pH and hydrothermal time on removing Cr (VI), in the experiment, the pH values of the K2Cr2O7 solutions were adjusted to (1, 2, 3, 5, 7, 9) by 1 mol/L HCl and 1 mol/L NaOH solutions. 0.008 g of as-prepared adsorbents with different hydrothermal time (e.g., 1–5 h) were added into 40 ml of K2Cr2O7 solution (110 mg/L) and then measured concentrations of the Cr solutions at certain time intervals for 3 days. Secondly, to study the equilibrium adsorption kinetics, 0.008 g of as-prepared adsorbent (hydrothermal time is 2 h) was immersed into 40 ml of K2Cr2O7 solution (110.6 mg/L) at pH = 2, followed by sampling the solution at set intervals within 9 days for the analysis of Cr content in the solution. Thus, the adsorption kinetic model was determined. Finally, 8 mg of nanofibrous membrane was immersed into 40 ml K2Cr2O7 solutions with different initial Cr (VI) concentrations (26.8, 52.7, 80.7, 110.6, 134.3, 148.2, 165.8 mg/L) at room temperature. When the adsorption reached the equilibrium, the Cr (VI) content in the solution was measured. The adsorption capacity of the membrane for the Cr (VI) removal was calculated as follows:
qe ¼
ðC o C e Þ:V m
ð1Þ
where qe is the equilibrium adsorption quantity (mg/g), Co and Ce represent the initial and equilibrium Cr (VI) concentration (mg/L), respectively. V is the volume of the K2Cr2O7 solution (L), and m is the quality of membrane (g). To investigate the cyclic utilization performance of the obtained composite nanofibrous membrane, the nanofibrous membrane was firstly immersed into 100 ml of 0.1 mol/L NaOH solution for 2 h after the Cr (VI) removal experiment, and then was washed with deionized water four times and dried at 60 °C for 2 h.
2.1. Preparation of PA6 nanofibrous membrane
2.4. Characterization
1.6 g of PA6 was dissolved in 10 ml formic acid (Beijing Tongguang Fine Chemical Co., Ltd) to form homogeneous solution. Nearly 6 ml of precursor solution was placed in a 10 ml syringe. The electrospinning conditions were as follows: A syringe pump with a syringe containing electrospinning solution was used as the impetus supplier at the rate of 0.5 mlh1. The positive high voltage on the needle of the syringe was 20 kV and the distance between the needle tip and collector was 15 cm. By electrospinning the PA6 nanofibrous membrane was obtained after 10 h.
The morphology and structure of the nanofibrous membranes were examined by scanning electron microscopy (SEM, JEOL-7500F, Japan). The elemental analysis of the PA6@Mg(OH)2 nanofibrous membrane was performed with an energy dispersive X-ray spectrometry (EDX, JSM-6360LV EDX spectrometer). The chemical groups of membrane were characterized by the Fourier transform infrared spectrometry (FT-IR) on a Nicolet 8700 FT-IR spectrometry (USA) in the range of wave number from 600 to 4000 cm1. X-ray diffraction patterns (XRD) of samples were recorded on a D/MAX-IIIA diffractometer using Cu Ka radiation source (k = 1.54056 Å) at a scan rate of 5°min1 to determine the phase composition and the crystallinity. The accelerating voltage and beam current were 40 kV and 200 mA, respectively. The concentrations of the Cr solutions were measured by inductively coupled plasma atomic emission spectrophotometer (ICP-AES) (CIROS EOP, Germany).
2.2. Preparation of polyamide 6 (PA6)@Mg(OH)2 composite nanofibrous membrane The experiment process is stated as follows: Firstly, 1 g MgSO4 (Tianjin Damao Chemical Co., Ltd) is dissolved in 40 ml deionized
100
B.-B. Jia et al. / Chemical Engineering Journal 254 (2014) 98–105
3. Results and discussion 3.1. Morphology characterization In the experiment, PA6@Mg(OH)2 composite nanofibrous membrane was fabricated by electrospinning technique combined with hydrothermal strategy. To further confirm the morphology of the PA6 and PA6@Mg(OH)2 nanofibrous membranes, SEM images for the hierarchically structured nanofibrous membranes with different hydrothermal time were described in Fig. 1. It can be seen that the electrospun PA6 nanofibers are randomly arranged to form a net-like porous structure and the surface of PA6 nanofibers are smooth (Fig. 1a). PA6@Mg(OH)2 nanofibrous membranes with different morphologies were fabricated by controlling hydrothermal time (Fig. 1b–f). After heating for 1 h, compared with pure PA6 nanofibrous membrane visible changes can be observed on the surface of nanofibers (Fig. 1b). A lot of precipitates are observed and no hierarchical structure appears. Increasing the hydrothermal time, hierarchical structures can be obviously observed (Fig. 1c and d). As we can see in Fig. 1d and g, 3 h is the best hydrothermal time and flower-like PA6@Mg(OH)2 composite nanofibrous membrane was successfully obtained. Further prolonging hydrothermal time, the hierarchical structures were destroyed slowly (Fig. 1e and f), which may be due to the desquamation of Mg(OH)2 crystals from the surface of nanofibers.
A proposed mechanism for preparation of the PA6@Mg(OH)2 composite nanofibrous membrane is illustrated as follows: When PA6 nanofibrous membrane was immersed into MgSO4 solutions, PA6 has a special function group (>C@O) and the lone pair electrons of O can make the PA6 have negative electricity. Therefore, a lot of Mg2+ can be adhered to the surface of PA6 nanofibrous membrane. As a precipitator of Mg2+, NH3H2O is a kind of weak base, which can lower the degree of supersaturation in various ways. At the temperature of 40 °C, with increasing the concentration of NH3H2O, Mg2+ can react with NH3H2O on the surface of PA6 nanofibrous membrane and the process was illustrated as follow in (2).
Mg2þ þ 2NH3 H2 O ! MgðOHÞ2 þ 2NHþ4
ð2Þ
However, the crystallization process may need some time. Crystallization process involves two steps (nucleation and growth). Crystallization occurs, when the solution reached saturation. When the hydrothermal time reach to 3 h, flower-like PA6@Mg(OH)2 nanofibrous membrane was obtained. However, further prolonging hydrothermal time, the hierarchical structures were destroyed slowly due to the destruction of the force between PA6 and Mg(OH)2. In addition, based on the isothermal nitrogen sorption measurement and the Brunauer–Emmett–Teller (BET) equation, it is found that the growth of Mg(OH)2 on the surface of PA6 fibers can significantly increase the specific surface area from 10.8502 to 25.5240 m2/g.
Fig. 1. (a) SEM image for pure PA6 nanofibrous membrane. SEM images for the PA6@Mg(OH)2 nanofibrous membranes with different hydrothermal time: (b) 1 h; (c) 2 h; (d) 3 h; (e) 4 h; (f) 5 h. The inset (g) in d shows the high resolution SEM image.
B.-B. Jia et al. / Chemical Engineering Journal 254 (2014) 98–105
3.2. FTIR and EDS analyses FTIR analysis (Fig. 2a) is carried out to further confirm the coordination of Mg(OH)2 crystals with polymer chains of PA6 in the nanofibers. The peak at 1635 cm1 is ascribed to the typical C@O stretching model of carboxyl groups. The prominent peak at 1538 cm1 is assigned to the bending vibration of amino groups. The peak at 2858 cm1 and 2930 cm1 is ascribed to the symmetric CH2 stretching mode and asymmetric CH2 stretching mode, respectively. These peaks are the characteristic peaks of PA6 [34,35]. After hydrothermal strategy, besides near the above characteristic peaks, a new peak at 3691 cm1 appears, which is ascribed to the stretching vibration of free AOH bond. Since PA6 does not contain free AOH bond, which confirms the coordination chemistry reaction between PA6 and Mg(OH)2. Therefore, it is indicated that Mg(OH)2 crystals can effectively interact with PA6 and adhere to the surface of nanofibers. To further prove the formation of Mg(OH)2 on the PA6 nanofibers, the SEM-EDX for the PA6@Mg(OH)2 composite nanofibrous membrane is shown in Fig. 2b. The SEM-EDX spectra results show that the elements of C, O and Mg are the main composition of the nanofibrous membrane, and the Mg content is 4.32 wt%.
101
peaks of PA6 nanofibrous membrane are mainly centered around 15–28°. The peaks appear at 2h values of 20° and 24.8° (marked with ‘‘#’’), which indicated that PA6 is an a crystalline polymer. After the growth of Mg(OH)2, the XRD patterns changes obviously. As seen in Fig. 3B, except for the broad peak of PA6, new peaks located at 18.58°, 38.02°, 50.85°, 58.64°, 62.07°, 68.25° and 72.03° appeared. It indicates the successful growth of Mg(OH)2 nanoparticles (according to JCPDS 07-0239). Furthermore, it is found that the new peaks are stronger than the peaks of PA6. These results suggest that after the hydrothermal treatment, Mg(OH)2 nanoparticles with high crystallinity are successfully anchored on the nanofibers, providing PA6 nanofibers flower-like hierarchical structures. 3.4. Cr (VI) removal capacity The Cr (VI) removal capacity of the obtained PA6@Mg(OH)2 composite nanofibrous membrane was systematically investigated. Fig. 4 shows the Cr (VI) adsorption capacity of PA6@Mg(OH)2 composite nanofibrous membrane with different morphologies at
3.3. XRD analysis XRD analysis is applied to examine the crystal structure of PA6@Mg(OH)2 composite nanofibrous membrane. The XRD results of PA6 (Fig. 3A) and PA6@Mg(OH)2 (Fig. 3B) are shown in Fig. 3. The
Fig. 3. XRD patterns for PA6 (A) and PA6@Mg(OH)2 (B) nanofibrous membrane.
Fig. 2. (a) (A) FTIR spectra for PA6 nanofibrous membrane. (B) FTIR spectra for PA6@Mg(OH)2 nanofibrous membrane. (b) SEM-EDX (EDS) for the PA6@Mg(OH)2 nanofibrous membrane.
Fig. 4. Adsorption Cr (VI) capacity of PA6@Mg(OH)2 composite nanofibrous membrane with different hydrothermal time at different pH values in 3 days.
102
B.-B. Jia et al. / Chemical Engineering Journal 254 (2014) 98–105
different pH values in 3 days. In general, the adsorption capacity of PA6@Mg(OH)2 composite nanofibrous membrane for Cr (VI) ions increases first and then decreases with decreasing pH values. The highest Cr (VI) adsorption occurs at the pH value of 2. The adsorption capacity will reduce when increasing or decreasing pH values. Moreover, it shows that the acidic solution is more in favour of Cr (VI) removal and there is little Cr (VI) adsorption under the alkaline pH values. The main reasons are illustrated as following. In the K2Cr2O7 solution with an acidic pH value, the Mg(OH)2 nanoparticles are protonated with positive charge. In addition, due to the protonation of Cr2O2 7 , Cr (VI) ions mainly exist in the form of HCrO 4 . So the HCrO4 ions are easily adsorbed on the PA6@Mg(OH)2 composite nanofibrous membrane at low pH value. However, Mg(OH)2 nanoparticles can be partly dissolved under a lower pH (pH = 1). Conversely, in the alkaline pH value, the presences of a larger number of OH groups on Mg(OH)2 nanoparticles hinder the adsorption of Cr2O2 7 . Furthermore, as shown in Fig. 4 (pH = 2), the adsorption capacity also increases first and then decreases with increasing the hydrothermal time. The main reasons are illustrated as following. Increasing the hydrothermal time, more and more Mg(OH)2 crystals adhere to the surface of PA6 nanofibrous membrane, which is highly favorable to the adsorption of HCrO 4 . But further prolonging hydrothermal time, the hierarchical structures were destroyed slowly. Therefore, the Cr (VI) adsorption becomes lower. The Cr (VI) removal capacity of the obtained PA6@Mg(OH)2 composite nanofibrous membrane was investigated and the results are shown in Fig. 5. Fig. 5 shows the Cr (VI) removal experiment results by the PA6@Mg(OH)2 composite nanofibrous membrane in 9 days. The initial Cr (VI) concentration is 110.6 mg/L (pH = 2). The Cr (VI) concentration decreases from 110.6 to 48.4 mg/L in 7 days and then almost retains the same level. From the curve, the process of removing Cr (VI) can be divided into two stages: the first stage (0–7 days) and the second stage (7–9 days). In the first stage, The Cr (VI) concentration decreases slowly. Obviously, compared to the first stage, the second stage almost retains the same level. Therefore, it is considered to be the equilibration in the second stage and indicated that the Cr (VI) ions in the solution are almost removed in 7 days. To investigate the kinetics of Cr (VI) removal by the obtained PA6@Mg(OH)2 composite nanofibrous membrane, the experimental data were analyzed using the pseudo-first-order kinetic model. The fitting of the data was achieved with the pseudo-first-order equations expressed as:
Fig. 5. Change of the total Cr (VI) in the solution with time.
lnðqe qt Þ ¼ ln qe k1 t
ð3Þ
where qt is the amount of Cr (VI) adsorbed (mg/g) at time t; qe is the amount of Cr (VI) adsorbed (mg/g) at equilibrium time; k1 is the rate constant (d1) of adsorption. The value of k1 was determined by the slope of the graphs ‘‘log (qeqt) versus t’’. As shown in Fig. 6a, the value of k1 and R2 were calculated to be 0.39114 and 0.98857, respectively. The adsorption data were also analyzed by pseudo-secondorder kinetic model. This can be expressed by following equation:
t 1 t ¼ þ qt k2 q2e qe
ð4Þ
where k2 (gmg1d1) is the rate constant of pseudo-second-order reaction. The value of qe and k2 can be determined by the slope and intercept of the straight line of the plot of ‘‘t/qt versus t’’. As shown in Fig. 6b, the value of k2 and R2 were found to be 0.00254 and 0.99806, respectively. In addition, the adsorption isotherm is researched and the results are shown in Fig. 7. Fig. 7a shows the equilibrium Cr (VI) adsorption amounts of the obtained nanofibrous membranes with different initial Cr (VI) concentrations in 9 days. In general, the PA6@Mg(OH)2 nanofibrous membrane’s adsorption capacity generally improves with the increasing of initial Cr (VI) concentration. Obviously, the adsorption is not the Langmuir adsorption [36] (Langmuir isotherm assumes the adsorption onto the
Fig. 6. Pseudo-first-order plot (a) and Pseudo-second-order plot (b) kinetic model.
103
B.-B. Jia et al. / Chemical Engineering Journal 254 (2014) 98–105
The recyclable property of adsorbent is very important for its application. In this experiment, the recyclable property of PA6@Mg(OH)2 composite nanofibrous membrane was studied by repeating the Cr (VI) removal experiment 4 times using the same nanofibrous membrane after the regeneration using diluted NaOH solution (Fig. 8), which can effectively induce the desorption of Cr (VI) from inorganic materials. In the alkaline solution, the desorption was supposed to be attributed to the elimination of protonated Mg2+. Therefore, the surface of the HCr2O 7 will break away from PA6@Mg(OH)2 composite nanofibrous membrane. In Fig. 8, it is interesting to observe that the regenerated membrane of PA6@Mg(OH)2 also shows a high Cr (VI) removal ability. Several reasons can explain this phenomenon. First, Cr (VI) is absorbed onto the membrane in the form of anionic HCrO 4 , after the regeneration a lot of adsorbtion sites may still exist, which will favor the adsorption of Cr (VI). Secondly, the membrane consists of many layers, after regeneration, the inner lay is permeated more sufficiently, which will beneficial for the Cr (VI) removal. After 4 times recycling, the Cr removal efficiency still remains 52%, which indicates a stable recyclable property. Finally, as shown in Fig 9, there may be two possible mechanisms for Cr (VI) adsorption onto PA6@Mg(OH)2. When the pH values of the K2Cr2O7 solutions were adjusted to 2 by 1 mol/L HCl, a lot of Cl exits in solutions. The surface of Mg(OH)2 nanoparticles are protonated and have an acquired positive charge in acidic solutions.
MgðOHÞ2 þ H3 Oþ ! MgðOHÞ2 Hþ þ H2 O
ð6Þ
One mechanism about PA6@Mg(OH)2 composite nanofibrous membrane for Cr (VI) removal is anion exchange. First, the Cl ions in solutions that can associated with Mg(OH)2 to form Mg(OH)2H+/ Cl. Second, Mg(OH)2H+/Cl can exchange with the anionic Cr(VI) in solutions [40], which the reaction occurs as the following equation:
MgðOHÞ2 Hþ =Cl þ HCrO4 ! MgðOHÞ2 Hþ =HCrO4 þ Cl
Fig. 7. (a) Adsorption isotherm and (b) equilibrium adsorption isotherm of Cr (VI) fitted with Freundlich adsorption model.
homogeneous surface with specific number of equivalent sites), in which the adsorption will reach the maximum value. Thus, the data is fitted with Freundlich adsorption model [37] (Freundlich isotherm represents the sorption onto the heterogeneous surface). The mathematical expressions for Freundlich adsorption model is [38]
lnqe ¼
1 lnC e þ lnK F n
ð7Þ
Another mechanism may be surface complexation, which is suggested for anion adsorption onto Mg(OH)2 [33]. The association between the positively charged surface of Mg(OH)2 and the HCrO4 anions, which the reaction occurs as the following equation:
MgðOHÞ2 Hþ þ HCrO4 ! MgðOHÞ2 Hþ =HCrO4
ð8Þ
ð5Þ
where Ce and qe are the equilibrium concentration (mg/L) and equilibrium quantity (mg/g), KF and n are Freundlich constants related to adsorption capacity and adsorption intensity, respectively. KF and n can be obtained by a plot of ln (qe) against ln (Ce). Fig. 7b indicates that the adsorption of Cr (VI) onto the membrane is a multi-adsorption process, because Freundlich adsorption model assumes a multilayer adsorption and the stronger binding sites are occupied first by the adsorbates. The values of KF and n are listed in Fig 7b. It is usually accepted that a value of 2 6 n < 10 represents an easy adsorption, a value of 1 6 n < 2 represents a moderate adsorption, while a value of n < 1 represents a difficult adsorption. For the obtained membrane, the value of n is 1.5886, indicating that the adsorption is a moderate adsorption, which can be also identified by the high value of KF (13.895) [32,39].
Fig. 8. The recyclable properties of PA6@Mg(OH)2 composite nanofibrous membrane for Cr (VI) removal.
104
B.-B. Jia et al. / Chemical Engineering Journal 254 (2014) 98–105
Fig. 9. Mechanism for Cr (VI) removal by PA6@Mg(OH)2 composite nanofibrous membrane.
4. Conclusion In summary, the ‘‘Flower-Like’’ PA6@Mg(OH)2 composite nanofibrous membrane was successfully fabricated by electrospinning combined with hydrothermal method. The Cr (VI) removal experiment showed that the obtained nanofibrous membrane exhibited excellent performance for the removal of Cr (VI). The Cr (VI) adsorption capacity can reach 296.4 mg/g (when the concentration of Cr (VI) is 110 mg/g), which is much higher than the previously reported values, such as flowerlike a-Fe2O3 nanoparticles (5.4 mg Cr/g) [41], PAN@c-AlOOH fiber films (8 mg Cr/g) [42], PA6@FexOy nanofibrous membrane (150 mg Cr/g) [43]. The strong Cr (VI) adsorption ability is attributed to the high surface area and suitable pore-size distribution of the obtained nanofibrous membranes. Meanwhile, the freestanding membrane can be easily separated from liquid solutions and has excellent recyclable performance. Therefore, the obtained nanofibrous membrane can be an efficient materials for the removal of Cr (VI) from water and other environmental areas.
Acknowledgments This study was partly supported by the Beijing City Board of Education Upgrade Project (No. TJSHG201310012021), Natural Science Foundation of China (Grant No. 21274006), the 973 Project (Grant No. 2010CB933501), Research Project of Beijing Institute of Fashion Technology (Grant No. 2012A-09) and Beijing Outstanding Talents Cultivation (Grant No. 2013D005001000003).
References [1] T. Naimo, A review of the effects of heavy metals on freshwater mussels, Ecotoxicology 4 (1995) 341–362. [2] P.C. Nagajyoti, K.D. Lee, T.V.M. Sreekanth, Heavy metals, occurrence and toxicity for plants: a review, Environ. Chem. Lett. 8 (2010) 199–216. [3] F. Fu, Q. Wang, Removal of heavy metal ions from wastewaters: a review, J. Environ. Manage. 92 (2011) 407–418. [4] P. Xu, G.M. Zeng, D.L. Huang, C.L. Feng, S. Hu, M.H. Zhao, C. Lai, Z. Wei, C. Huang, G.X. Xie, Z.F. Liu, Use of iron oxide nanomaterials in wastewater treatment: a review, Sci. Total Environ. 424 (2012) 1–10. [5] Y. Yi, Z. Yang, S. Zhang, Ecological risk assessment of heavy metals in sediment and human health risk assessment of heavy metals in fishes in the middle and lower reaches of the Yangtze River basin, Environ. Pollut. 159 (2011) 2575– 2585. [6] A. Singh, R.K. Sharma, M. Agrawal, F.M. Marshall, Health risk assessment of heavy metals via dietary intake of foodstuffs from the wastewater irrigated site of a dry tropical area of India, Food Chem. Toxicol. 48 (2010) 611–619.
[7] B. Saha, C. Orvig, Biosorbents for hexavalent chromium elimination from industrial and municipal effluents, Coord. Chem. Rev. 254 (2010) 2959– 2972. [8] P. Miretzky, A.F. Cirelli, Cr (VI) and Cr (III) removal from aqueous solution by raw and modified lignocellulosic materials: a review, J. Hazard. Mater. 180 (2010) 1–19. [9] S. Gupta, B.V. Babu, Removal of toxic metal Cr (VI) from aqueous solutions using sawdust as adsorbent: equilibrium, kinetics and regeneration studies, Chem. Eng. J. 150 (2009) 352–365. [10] M. Gheju, I. Balcu, Removal of chromium from Cr (VI) polluted wastewaters by reduction with scrap iron and subsequent precipitation of resulted cations, J. Hazard. Mater. 196 (2011) 131–138. [11] S. Edebali, E. Pehlivan, Evaluation of Amberlite IRA96 and Dowex 18 ionexchange resins for the removal of Cr (VI) from aqueous solution, Chem. Eng. J. 161 (2010) 161–166. [12] G. Mouedhen, M. Feki, M. De Petris-Wery, H.F. Ayedi, Electrochemical removal of Cr (VI) from aqueous media using iron and aluminum as electrode materials: towards a better understanding of the involved phenomena, J. Hazard. Mater. 168 (2009) 983–991. [13] A.K. Bhattacharya, T.K. Naiya, S.N. Mandal, S.K. Das, Adsorption, kinetics and equilibrium studies on removal of Cr (VI) from aqueous solutions using different low-cost adsorbents, Chem. Eng. J. 137 (2008) 529–541. [14] X. Guo, F. Zhang, Q. Peng, S. Xu, X. Lei, D.G. Evans, X. Duan, Layered double hydroxide/eggshell membrane: an inorganic biocomposite membrane as an efficient adsorbent for Cr (VI) removal, Chem. Eng. J. 166 (2011) 81–87. [15] J. Müslehiddinog˘lu, Y. Uludag˘, et al., Effect of operating parameters on selective separation of heavy metals from binary mixtures via polymer enhanced ultrafiltration, J. Membr. Sci. 140 (1998) 251–266. [16] M.K. Aroua, F.M. Zuki, N.M. Sulaiman, Removal of chromium ions from aqueous solutions by polymer-enhanced ultrafiltration, J. Hazard. Mater. 147 (2007) 752–758. [17] M.A. Barakat, E. Schmidt, Polymer-enhanced ultrafiltration process for heavy metals removal from industrial wastewater, Desalination 256 (2010) 90–93. [18] V.K. Gupta, A. Rastogi, A. Nayak, Adsorption studies on the removal of hexavalent chromium from aqueous solution using a low cost fertilizer industry waste material, J. Colloid Interface Sci. 342 (2010) 135–141. [19] G. Huang, J.X. Shi, T.A.G. Langrish, Removal of Cr (VI) from aqueous solution using activated carbon modified with nitric acid, Chem. Eng. J. 152 (2009) 434–439. [20] N. Wu, H. Wei, L. Zhang, Efficient removal of heavy metal ions with biopolymer template synthesized mesoporous titania beads of hundreds of micrometers size, Environ. Sci. Technol. 46 (2011) 419–425. [21] R. Zhang, X. Pan, F. Li, L. Zhang, S. Zhai, Q. Mu, J. Liu, G. Qu, G. Jiang, B. Yan, Cell rescue by nanosequestration: reduced cytotoxicity of an environmental remediation residue, Mg(OH)2 nanoflake/Cr (VI) adduct, Environ. Sci. Technol. 48 (2014) 1984–1992. [22] Y.X. Zhang, Y. Jia, Z. Jin, X.Y. Yu, W.H. Xu, T. Luo, B.J. Zhu, J.H. Liu, X.J. Huang, Self-assembled, monodispersed, flower-like c-AlOOH hierarchical superstructures for efficient and fast removal of heavy metal ions from water, CrystEngComm 14 (2012) 3005–3007. [23] G. Aragay, J. Pons, A. Merkoçi, Recent trends in macro-, micro-, and nanomaterial-based tools and strategies for heavy-metal detection, Chem. Rev. 111 (2011) 3433–3458. [24] J.P. Ruparelia, S.P. Duttagupta, A.K. Chatterjee, S. Mukherji, Potential of carbon nanomaterials for removal of heavy metals from water, Desalination 232 (2008) 145–156. [25] S. Haider, S.Y. Park, Preparation of the electrospun chitosan nanofibers and their applications to the adsorption of Cu (II) and Pb (II) ions from an aqueous solution, J. Membr. Sci. 328 (2009) 90–96. [26] Z.M. Huang, Y.Z. Zhang, M. Kotaki, S. Ramakrishna, A review on polymer nanofibers by electrospinning and their applications in nanocomposites, Compos. Sci. Technol. 63 (2003) 2223–2253. [27] J.A. Matthews, G.E. Wnek, D.G. Simpson, G.L. Bowlin, Electrospinning of collagen nanofibers, Biomacromolecules 3 (2002) 232–238. [28] M. Bognitzki, W. Czado, T. Frese, A. Schaper, M. Hellwig, M. Steinhart, A. Greiner, J.H. Wendorff, Nanostructured fibers via electrospinning, Adv. Mater. 13 (2001) 70–72. [29] P. Kampalanonwat, P. Supaphol, Preparation and adsorption behavior of aminated electrospun polyacrylonitrile nanofiber mats for heavy metal ion removal, ACS Appl. Mater. Interfaces 2 (2010) 3619–3627. [30] S. Wu, F. Li, Y. Wu, R. Xu, G. Li, Preparation of novel poly(vinyl alcohol)/SiO2 composite nanofiber membranes with mesostructure and their application for removal of Cu2+ from waste water, Chem. Commun. 46 (2010) 1694–1696. [31] Y. Lin, W. Cai, X. Tian, X. Liu, G. Wang, C. Liang, Polyacrylonitrile/ferrous chloride composite porous nanofibers and their strong Cr-removal performance, J. Mater. Chem. 21 (2011) 991–997. [32] G.R. Xu, J.-N. Wang, C.-J. Li, Preparation of hierarchically nanofibrous membrane and its high adaptability in hexavalent chromium removal from water, Chem. Eng. J. 198–199 (2012) 310–317. [33] Y. Li, B. Gao, T. Wu, D. Sun, X. Li, B. Wang, F. Lu, Hexavalent chromium removal from aqueous solution by adsorption on aluminum magnesium mixed hydroxide, Water Res. 43 (2009) 3067–3075. [34] Q. Wu, X. Liu, L.A. Berglund, FT-IR spectroscopic study of hydrogen bonding in PA6/clay nanocomposites, Polymer 43 (2002) 2445–2449. [35] J. Wei, Y. Li, K.T. Lau, Preparation and characterization of a nano apatite/ polyamide6 bioactive composite, Compos. B Eng. 38 (2007) 301–305.
B.-B. Jia et al. / Chemical Engineering Journal 254 (2014) 98–105 [36] I. Langmuir, The adsorption of gases on plane surfaces of glass, mica and platinum, J. Am. Chem. Soc. 40 (1918) 1361–1403. [37] C. Ng, J.N. Losso, W.E. Marshall, R.M. Rao, Freundlich adsorption isotherms of agricultural by-product-based powdered activated carbons in a geosminwater system, Bioresour. Technol. 85 (2002) 131–135. [38] R.J. Umpleby Ii, S.C. Baxter, M. Bode, J.K. Berch Jr., R.N. Shah, K.D. Shimizu, Application of the Freundlich adsorption isotherm in the characterization of molecularly imprinted polymers, Anal. Chim. Acta 435 (2001) 35–42. [39] A. Aluigi, F. Rombaldoni, C. Tonetti, et al., Study of Methylene Blue adsorption on keratin nanofibrous membranes, J. Hazard. Mater. 268 (2014) 156–165. [40] D. Carriazo, M. del Arco, C. Martín, V. Rives, A comparative study between chloride and calcined carbonate hydrotalcites as adsorbents for Cr (VI), Appl. Clay Sci. 37 (2007) 231–239.
105
[41] N. Wu, H. Wei, L. Zhang, Efficient removal of heavy metal ions with biopolymer template synthesized mesoporous titania beads of hundreds of micrometers size, Environ. Sci. Technol. 46 (2011) 419–425. [42] Y. Lin, W. Cai, H. He, et al., Three-dimensional hierarchically structured PAN@c-AlOOH fiber films based on a fiber templated hydrothermal route and their recyclable strong Cr (VI)-removal performance, RSC Adv. 5 (2012) 1769– 1773. [43] C.J. Li, Y.J. Li, J.N. Wang, et al., PA6@FexOy nanofibrous membrane preparation and its strong Cr (VI)-removal performance, Chem. Eng. J. 220 (2013) 294–301.