Article pubs.acs.org/IECR
Poly(ethylene-co-vinyl alcohol) Functional Nanofiber Membranes for the Removal of Cr(VI) from Water Dandan Xu, Keying Zhu, Xiaoting Zheng, and Ru Xiao* State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai 201620, P. R. China S Supporting Information *
ABSTRACT: Poly(ethylene-co-vinyl alcohol) (EVOH) nanofibers with average diameter of 260 nm were initially fabricated through the melt-blending extrusion of immiscible blends. The resulting films obtained by layer deposition technology were then functionalized by in situ oxidative polymerization of pyrrole monomer in hydrochloric acid solution for hexavalent chromium [Cr(VI)] adsorption from water. Scanning electron microscopy and Fourier-transform infrared spectroscopy were used to characterize the morphology and structure of functionalized nanofiber membranes. Adsorption experiments were conducted to test the effects of solution pH, dose of adsorbents, water temperature, adsorption contact time, and initial concentration of Cr(VI) and to determine the Cr(VI) adsorption mechanism. The experimental results denoted that the adsorption process was endothermic, spontaneous, and highly pH dependent, and the kinetics data fitted well with a pseudo-second-order model. The adsorption equilibrium time was less than 100 min, and the maximum adsorption capacities were 90.74 mg/g from the adsorption kinetics study. The adsorption isotherm data followed the Langmuir isothermal model. Desorption results exhibited excellent reusability of the membrane for Cr(VI) adsorption.
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INTRODUCTION Toxic heavy-metal ions, such as Cr, Hg, Cd, Pb, and As, that exist in ecological water resources deteriorate water quality, which has attracted considerable attention today.1 The Cr ion, one of the common heavy-metal contaminants in wastewaters, originates from several industrial processes, such as leather tanning, metal finishing, electroplating, photography, steel manufacturing, and textile and dye industries. However, Cr ions exist in hexavalent [Cr(VI)] and trivalent [Cr(III)] oxidation states.2 Of these, Cr(III) is considered as an indispensable micronutrient for plants, animal metabolism, and humans and is far less toxic than Cr(VI), which may lead to epidermal, kidney, and gastric damage. The maximum capacity of Cr(VI) for inland surface water is 0.1 mg/L and for potable water sources is 0.05 mg/L, according to the US Environmental Protection Agency and the World Health Organization. Therefore, it is essential to develop effective methods and materials for Cr(VI) adsorption from aqueous solution to avoid the mischievous effects of Cr(VI) on human health. A variety of techniques, such as ion exchange, membrane separation, chemical reduction and precipitation, and adsorption have been developed for Cr(VI) adsorption from aqueous solution.3 Among these, adsorption has been regarded as a promising method due to its obvious advantages in cost and performance. Various types of adsorbents have been investigated for Cr(VI) adsorption, like activated carbon,4 metal oxide nanoparticles,5,6 chitosan,7 and lignocellulosic materials,8,9 among others. Reports have detailed that polypyrrole (PPy) appears to be an excellent prospect for removing Cr(VI) due to its ability to absorb aqueous cations via electrostatic or hydrogen bonding. Bhaumik reported a process for Cr(VI) uptake that involved ion exchange and reduction of Cr(VI) by PPy-polyaniline (PANI) nanofibers to form Cr(III).10 Some © XXXX American Chemical Society
reports have studied the adsorption of PPy coated on a variety of materials, such as Fe3O4,11 sepiolite,12 and graphene,13 for the removal of Cr(VI). Those materials show superior performance for the removal of Cr(VI). However, it is difficult to separate the load of adsorbents from the large volumes of water. In order to mediate this problem, polymer nanofiber membranes were used as the base material of the adsorbents because of its high gas permeability, high porosity, small interfibrous pore size, and, above all, high specific surface area per unit mass, which affords high adsorption capacity. It is relatively easy to modify these membranes with functional groups. For example, poly(vinyl alcohol-co-ethylene) (PVA-co-PE), prepared by melt extrusion and functionalized with iminodiacetic acid, was used for the removal Cu2+.14 The electrospun polyacrylonitrile(PAN) nanofibers were modified by diethylenetriamine to be used as the absorbent of Cu2+.15 Polyacrylonitrile (PAN) nanofibers functionalized with amine groups were prepared for Cr(VI) adsorption from aqueous solutions.16 Taking advantage of the PPy and nanofiber membranes concept, Wang prepared PPy/ nanofiber menbrane composites by an in situ chemical oxidative polymerization of Py in the presence of nanofiber menbranes.17 Because nanofiber membranes have wide application and good performance, several developed preparation methods were used to prepare micronanofibers, including electrospinning,18,19 polymerization, melt blowing,20,21 centrifugal force spinning,22,23 phase separation,24−26 and melt blend Received: March 15, 2015 Revised: June 19, 2015 Accepted: June 22, 2015
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DOI: 10.1021/acs.iecr.5b00995 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
25 °C with 100 rpm. At last, the EVOH/PPy nanofiber functionalized membrane was dried in a vacuum oven at 60 °C overnight after being washed with ethanol and deionized water three times to remove the residual pyrrole monomers. Apparatus and Instrumentation. The morphology of EVOH/PPy nanofiber functionalized membranes was characterized using scanning electron microscopy (SEM, S-3000N, Hitachi, Ltd.). (SEM). The functional groups on the surface of synthesized adsorbents were characterized using a Nicolet Nexus 8700 FT-IR. The Cr(VI) concentration was tested using inductively coupled plasma mass spectrometry (ICP-AES prodigy Leeman American) and ultraviolet−visible spectrophotometry (UV−vis Lambda 35). The surface chemical composition of functionalized membranes before and after adsorption was analyzed by X-ray photoelectron spectroscopy (XPS). Cr(VI) Removal Studies. Adsorption Studies. The adsorption behavior for Cr(VI) was evaluated using batch adsorption experiments. The various concentrations of Cr(VI) solution were prepared using potassium dichromate (K2Cr2O7). A 4 × 4 cm2 dry EVOH/PPy nanofiber functionalized membrane (about 100 mg) was added to a conical beaker containing 30 mL of a solution containing 100 ppm of Cr(VI) and incubated for 8 h. During the adsorption experiment, the conical beaker was shaken in the constant-temperature shaking bath with a constant rate. The Cr(VI) solution pH was adjusted by 0.1 M HCl or 0.1 M NaOH. The effects of solution pH and temperature and adsorbent dose on the Cr(VI) adsorption capacity of the membrane were explored. The removal percentage and equilibrium adsorption capacity (qe) of Cr(VI) were evaluated using the following equations
extrusion.27−35 Most polymer resins used for this purpose are thermoplastic polyolefins or polyesters, which can be melted and reprocessed. The melting blend extrusion method for preparing various kinds of thermoplastic polymer nanofibers is environment friendly, highly efficient, versatile, and continuous. The thermoplastic polymer nanofiber membranes can be obtained through the melt blending extrusion of immiscible blends with a dispersed phase stretched into nanofibers after the matrix phase is removed. In this reported study, cellulose acetate butyrate ester was used as the matrix to prepare poly(ethylene-co-vinyl alcohol) nanofibers by melting blend extrusion. Then nanofiber membranes were prepared by layer deposition technology to remove Cr(VI) from water. This was accomplished by functionalization using in situ oxidative polymerization of pyrrole monomer aqueous solution at room temperature, employing FeCl3 as a catalyst. Kinetic data for Cr(VI) removal at four different temperatures were analyzed by pseudo-firstand pseudo-second-order kinetic models, and the linearized Langmuir, Freundlich, and Tempkin models were used to describe the equilibrium isotherms for Cr(VI) adsorption systems. The thermodynamic parameters were also evaluated using the adsorption data.
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MATERIALS AND METHODS Materials. Poly(ethylene-co-vinyl alcohol) (EVOH) (model ET3803) with 38 mol % ethylene was supplied by Nippon Gohsei. Cellulose acetate butyrate ester (CAB) (model 381-20) was purchased from the Eastman Chemical Co. Pyrrole (Py) (98+%), iron(III) chloride hexahydrate (FeCl3·6H2O), and potassium dichromate (K2Cr2O7), were supplied by Sinopharm Chemical Reagent Co., Ltd. All reagents were used as received. Preparation of EVOH Nanofiber Membranes. The EVOH nanofiber membranes were prepared on the basis of a previously published procedure.36 The EVOH resin and CAB were dried for 24 h at 80 °C in a vacuum drying oven before the melt-blending extrusion process. EVOH and CAB with the weight ratio of 25/75 were fed into the corotating twin-screw extruder (D = 16 mm, L/D = 40; EUROLAB16, ThermoHaake Co.) with a screw speed of 80 rpm and a melting zone temperature ranging from 190 to 220 °C. The blends were extruded by a take-up device and water-cooled to ambient temperature. The extrudants were immersed in acetone via a Soxhlet extractor at ambient temperature for 15 h to remove CAB from the blends. Then the prepared nanofibers were dispersed by a mechanical disintegrator for 30 s to obtain a homogeneous fiber suspension, and a layer of the fibers was deposited onto a supporting polypropylene (PP) nonwoven mat. After naturally drying, the EVOH nanofiber membrane with a controlled thickness was released from the supporting PP nonwoven mat. The properties of EVOH nanofiber membranes, including the surface morphology, specific surface area, and pore size distribution, were investigated (shown in Figures S1−S3 and Table S1, Supporting Information). Preparation of EVOH/PPy Nanofiber Functionalized Membrane. The EVOH/PPy nanofiber functionalized membrane was fabricated via an in situ chemical oxidative polymerization technique described in detail elsewhere.17 A piece of EVOH nanofiber membrane was added to the FeCl3· 6H2O (0.0024 mol) solution with 10 mL of deionized water, and then the polymerization was started by mixing with pyrrole (0.024 mol) in 50 mL of ethanol. After that, the solution was shaken in a constant-temperature shaking bath, maintained at
%removal =
qe =
C0 − Ce × 100 C0
C0 − Ce V m
(1)
(2)
where C0 and Ce are the initial and equilibrium concentration of Cr(VI) in the aqueous solution (mg/L), qe is the equilibrium adsorption capacity (mg/g), m is the mass of adsorbent (g), and V is the volume of solution (L). The adsorption capacity qt at time t was calculated by the following equation: qt =
C0 − Ct V m
(3)
The kinetic adsorption performance of the Cr(VI) solutions was studied with various initial concentrations at 25 °C, and the initial Cr(VI) solution pH was set at 2.0. The adsorption isotherm was established at three different temperatures (25, 35, and 45 °C) by changing the initial concentration of Cr(VI) from 25 to 250 mg/L. Adsorption−Desorption Studies: Reusability. The reusability of adsorbents is one of the most important factors determined during the adsorption studies of heavy metal. To investigate the reusability of EVOH/PPy nanofiber functionalized membrane, the desorption experiments for Cr(VI) were carried out by batch adsorption experiments. These experiments were conducted as follows: 100 mg of dry EVOH/PPy nanofiber functionalized membrane was immersed into 30 mL of 100 mg/L Cr(VI) at 25 °C for 8 h. Then the adsorbent was put in 30 mL of 2 M NaOH or 0.2 M NaOH solution at 25 °C for 30 min. The adsorbent was then treated with 0.1 M HCl B
DOI: 10.1021/acs.iecr.5b00995 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research solution before the second Cr(VI) adsorption. The above procedure was repeated five times.
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RESULTS AND DISCUSSION Characterization of the Nanofiber Membranes. Morphology of the Nanofiber Membranes. Figure 1a shows
Figure 4. Effect of dosage of EVOH/PPy nanofiber functionalized membranes on the removal of Cr(VI). Figure 1. (a) SEM images of EVOH nanofiber film. (b) EVOH/PPy nanofiber functionalized membranes.
Figure 5. Effect of ionic strength on the removal of Cr(VI).
Figure 2. FTIR spectra of PPy, EVOH, and EVOH/PPy film.
Figure 6. Effect of contact time on the adsorption capacity of different initial Cr(VI) ion concentrations by EVOH/PPy nanofiber functionalized membranes (adsorbent dosage = 100 mg, V = 30 mL, T = 25 °C, pH 2).
Figure 3. Effect of initial solution pH on the removal of Cr(VI).
was distributed from 50 to 300 nm, which provided the specific surface area of the EVOH nanofiber membrane. The hydroxyl groups of the vinyl alcohol segments of EVOH enabled a variety of chemical modifications to be made on the EVOH nanofiber membrane. The PPy was grafted onto the EVOH membrane via an in situ chemical oxidative polymerization
an image of the EVOH nanofiber membrane, which was prepared by melt-blending technology of immiscible blends of EVOH and CAB with the weight ratio of 25/75, followed by removing the CAB matrix and then the layer deposition process. As shown in Figure1a, the diameter of EVOH fibers C
DOI: 10.1021/acs.iecr.5b00995 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
Figure 8. Equilibrium isotherms of adsorption of Cr(VI) on EVOH/ PPy nanofiber functionalized membranes with different solution temperatures.
Effect of Factors for Cr(VI) Adsorption. Effect of Initial Solution pH. The adsorption of metal ions from aqueous solution is highly pH dependent. As shown in Figure 3, the removal efficiency of Cr(VI) decreased from 86.8% to 55% with an increase of pH when the pH of the initial Cr(VI) solution is in the range of 2.0−6.0. The oxy anions of Cr are known to exist as described in the following equations:37 H 2CrO4 ↔ HCrO4 − + H+,
k1 = 1.21
Cr2O7 2 − + H 2O ↔ 2HCrO4 − ,
Figure 7. (a) Pseudo-first-order kinetic model and (b) pseudo-secondorder kinetic model for Cr(VI) adsorption on EVOH/PPy nanofiber functionalized membranes.
HCrO4 − ↔ CrO4 2 − + H+,
HCr2O7− ↔ Cr2O7 2 − + H+,
technique. Figure 1b shows that the PPy was well-distributed on the EVOH nanofiber membrane and the polymerization process occurred on the surface of the EVOH nanofibers. FTIR Study of the Nanofiber Membranes. The effects of pyrrole on the EVOH nanofiber membrane were investigated by FTIR, and the results are presented in Figure 2. In comparison to the original EVOH nanofiber membrane, it was found that several new absorption peaks that could be assigned to pyrrole ring stretching exist in the spectrum of EVOH/PPy nanofiber functionalized membrane. The peaks at 1545, 1164, and 1036 and 878 cm−1 can be attributed to C−N stretching vibrations, C−H stretching vibration, and C−H deformation, respectively. These peaks have a red-shift, which means that the corresponding peak has a shift toward smaller wavenumber due to the hydrogen bond interaction between hydroxyl on the surface of EVOH nanofiber and PPy. The results confirmed that PPy self-assembled on the EVOH nanofibers.
(4)
k 2 = 35.5
(5)
k 3 = 3 × 10−7
(6)
k4 = 0.85
(7)
An equilibrium is clearly revealed when the pH is in the range of 2.0−6.0, and the predominant Cr(VI) species are HCrO4− and Cr2O72−. At pH
