Research Article pubs.acs.org/journal/ascecg
Surface Activated Hydrothermal Carbon-Coated Electrospun PAN Fiber Membrane with Enhanced Adsorption Properties for Herbicide Rui Zhao, Yong Wang, Xiang Li,* Bolun Sun, Yumei Li, He Ji, Ju Qiu, and Ce Wang* Alan G. MacDiarmid Institute, Jilin University, Changchun 130012, People’s Republic of China S Supporting Information *
ABSTRACT: Recently, much attention has been focused on the hydrothermal carbonization processes that use biomass as a carbon source to produce functional carbonaceous materials. Despite much progress in their synthesis and applications, flexible and high mechanical strength hydrothermal carbonaceous materials are in demand in practical environmental applications. In this work, a flexible hydrothermal carbon-coated electrospun PAN fiber membrane was developed. The carboncoated PAN fibers showed an enhanced mechanical strength and were found to possess a good adsorption capacity toward paraquat. After the NaOH activation, the adsorption process became more efficient. The hydrothermal carbon coating and activation reactions were confirmed by SEM observations, TEM images, FT-IR spectra, Raman spectra, XPS analysis, and mechanical tests. The adsorption process followed the Langmuir isotherm and pseudo-second-order kinetic model; meanwhile, the activated adsorbent (AC-PAN) had a good recyclability and the removal efficiency remained at 83% after five cycles. The maximum adsorption capacity for AC-PAN was 437.64 mg/g, which was higher than those in classic paraquat adsorbents. Given the high adsorption and regenerability performance of the activated hydrothermal carbon-coated electrospun PAN fibers, they should have potential applications in water remediation. KEYWORDS: Hydrothermal carbonization, Electrospinning, Flexibility, Adsorption, Paraquat
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one-step hydrothermal method.16,17 The preparation process is easy and the size of the microspheres can be controlled. (ii) Various carbonaceous nanofibers were prepared by a templatedirected hydrothermal carbonization process using Te nanowires as templates. The free-standing fiber membranes could be fabricated using a simple casting process.12,18 The carbonaceous nanofibers showed excellent adsorption and filtration behaviors. (iii) Hydrothermal carbon was deposited onto the surface of inorganic hard templates, such as montmorillonite, attapulgite clay, and inorganic oxide, etc., to prepare nanocomposites.19,20 Composite materials showed enhanced catalytic, adsorption, and electricity performance. However, the carbon materials are all in the form of powder using either template-free or inorganic templates. For the casting carbonaceous nanofiber membranes, the membranes showed free-standing property, but the mechanical strength of the membranes was not high, restricting practical application. When the materials are used as adsorbents in water medium, the powder form is not conducive to recycling and low mechanical strength of the membranes cannot resist the impact of the water flow. Therefore, searching
INTRODUCTION With the fast development of nanotechnology during the past 2 decades, the syntheses of nanostructured carbon materials, such as carbon nanotube, carbon nanodot, and graphene, etc., have become a continuous hot topic due to their wide potential applications.1−3 They have found numerous applications in relevant fields of sensing, catalysis, bioimaging, water purification, energy storage, separation science, etc.4−7 There are several techniques to fabricate the carbon materials, including electric-arc discharge techniques,8 chemical vapor deposition,9 catalytic pyrolysis of organic compounds10 and similar techniques. However, these techniques need harsh conditions and high consumption. Recently, hydrothermal carbonaceous materials from natural biomass have attracted widespread attention.11−13 The precursors of the carbon in this way are carbohydrate-rich biomasses, such as cellulose,14 chitosan,13 cyclodextrin,15 and glucose,12 which are naturally replenished and yield rich. The preparation process is through the hydrothermal treatment (160−180 °C) in pure water reaction medium. Thus, the hydrothermal carbonization process is a low-cost, easy accessible, and environment-friendly method to fabricate carbon materials. Up to now, a series of hydrothermal carbonization-based materials have been reported. They can be divided into the following categories: (i) Template-free pure carbon microspheres were synthesized by a © XXXX American Chemical Society
Received: January 5, 2016 Revised: March 7, 2016
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DOI: 10.1021/acssuschemeng.6b00026 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering
Fabrication of Electrospun Fiber Template. Electrospinning. Briefly, the 8 wt % PAN solution was prepared by the as received PAN microfibers in DMF with stirring for 8 h at 65 °C. The above solution was loaded into a glass syringe and connected to high-voltage power supply. 18 kV was provided between the cathode and anode at a distance of 15 cm to prepare PAN nanofibers. Cross-Linking. The prepared PAN nanofibers (0.60 g), ethylenediamine (0.30 mL), and ethylene glycol (50 mL) were added to a 100 mL round-bottomed flask. The reaction was carried out at 135 °C in an oil bath for 3 h. Then the cross-linked PAN fibers were filtered and sequentially washed with deionized water to remove the residual reagent and dried in a vacuum oven at 60 °C overnight to obtain the electrospun fiber template. Hydrothermal Carbonization onto Fiber Template. In a typical procedure, 50 mg of the obtained fiber template was immersed in 40 mL of glucose solution (1 g glucose). Then, the mixed solution was put into a Teflon-lined stainless steel autoclave with a volume capacity of 50 mL. The autoclave was sealed and heated at 160 °C for 18 h. After cooling down to room temperature, the as-prepared brown fiber membrane was washed several times with distilled water and dried in vacuum oven at 60 °C for 12 h. NaOH Solution Activation of the Hydrothermal Carbon Functional PAN Fibers. To activate the carbon on the surface of the functional PAN fibers, 40 mg of dry carbon functional PAN fibers was immersed in 30 mL of 1 M NaOH aqueous solution and the solution was shaken in a model BETS-M1 shaker (Kylin-Bell Lab Instruments Co., Ltd., China) with a shaking speed of 80 rpm for 2 h. The final product was washed several times with distilled water to remove the residual NaOH and dried in a vacuum oven at 60 °C for 12 h. Characterization. The morphology of the fibers was characterized by a field-emission scanning electron microscopy (SEM, FEI Nova NanoSEM). The mean diameter and diameter distribution of the fibers were calculated from measuring the different parts of the fibers at 100 different fibers using the commercial software package, Image-Pro Plus. Transmission electron microscopy (TEM) images were recorded on a JEOL-2010 electron microscope operating at 200 kV. The N2 adsorption−desorption isotherms were recorded on an ASAP 2020 instrument at 77 K. Specific surface areas were calculated according to the Brunauer−Emmett−Teller (BET) method. Pore size and total pore volume were calculated on the basis of the Brrett−Joyner− Halenda (BJH) method. FT-IR spectra were obtained on a Fourier transform infrared spectrometer (FT-IR, BRUKER VECTOR 22) from 4000 to 400 cm−1. Raman spectra were obtained by using a HORIBA Jobin Yvon-LabRAM ARAMIS at an excitation wavelength of 514.5 nm. Analysis of the X-ray photoelectron spectra (XPS) was performed on Thermo ESCALAB 250 spectrometer with a Mg K (1253.6 eV) achromatic X-ray source. The mechanical properties of the fiber membranes were performed by assembling the membranes (dimensions: length = 20 mm, width = 5 mm) between two stainless steel clamps with a tensile speed of 20 mm·min−1 on a mechanical strength microtest device (410R250, Test Resources, Shakopee, MN, USA). Batch Adsorption Experiments. To acquire a solution of the pollutant, paraquat was dissolved in deionized water and was then diluted to the required concentration before use. The pH values were adjusted with 0.1 M HCl or 0.1 M NaOH solution, and the pH values of the adsorption solutions were measured using a pH meter (Starter 2100, Ohaus Instruments Co., Ltd.). The herbicide concentration was measured by using a Shimadzu UV-2501 UV−vis spectrophotometer at 257 nm based on an appropriate calibration curve (Figure S1). Batch adsorption experiments were performed on a model BETS-M1 shaker (Kylin-Bell Lab Instruments Co., Ltd., China) with a shaking speed of 120 rpm. The adsorption capacity (q) of paraquat adsorbed onto the adsorbents was calculated on the basis of the following equation:
for a flexible and high strength polymer membrane template for loading the hydrothermal carbon is necessary. Electrospinning is a versatile, simple, and effective technique to prepare one-dimensional continuous polymer fibers with diameters in the range of nanometers to a few micrometers.21,22 The electrospun fibers possess high porosity, large surface-tovolume ratio, easy preparation, diverse in composition, and facile modification properties, which have found wide applications.23,24 Because of the easy preparation, flexibility, and facile chemical/physical surface functionalization, electrospun polymer fibers are often used as templates or carriers to load functional materials, such as catalysts,25 zeolites,26 and medicine.27 The functional fiber membranes show flexibility and satisfying mechanical strength, which can be easily recycled. Therefore, the disadvantages of the difficulty to recycle and easy aggregation for hydrothermal carbon would be resolved when we combine hydrothermal carbonization with electrospinning technique. Nowadays, contamination in water with the development of industry and agriculture has become a serious problem. As a class of water pollutants, herbicides discharged from agricultural irrigation systems have received considerable concern.28,29 As a most widely used cationic herbicide, paraquat is famous for its high toxicity. Although forbidden in many countries and areas such as the United States, New Zealand, and Europe, paraquat is still largely used worldwide as a herbicide. Its residues in water resources would markedly increase a serious threat to humans and the ecosystem.30,31 Thus, removing paraquat from the contaminated water is necessary. Many processes, including Fenton’s reagent,32 photo-oxidation,33 adsorption,31 and biological degradation,34 have been used to solve this contamination in water. Among these processes, adsorption seems to be an effective way due to its low cost, easy regeneration, and versatile property. Hydrothermal carbonaceous materials have attracted much attention for adsorption removal of organic dyes and toxic metal ions due to abundant functional oxygencontaining groups on the surface.12,19 We consider that the hydrothermal carbonaceous materials can be effective adsorbents to remove paraquat pollution. Herein, we report a carbonaceous material coated electrospun fiber membrane by hydrothermal carbonization. The precursor of the carbons is glucose, and polyacrylonitrile (PAN), a commonly used polymer, acts as the electrospun fiber template. To improve the resistance to hydrothermal conditions, PAN electrospun fibers were cross-linked by ethylenediamine. The composite hydrothermal carbon functional electrospun fibers showed good adsorption ability toward paraquat. For hydrothermal carbonaceous materials, it has been reported that the adsorption capacities toward cationic contaminants (dyes and metal ions) would increase by chemical activation with bases.35,36 The obtained carbon functional electrospun fibers were also activated by NaOH solution, and the surface activated composite fibers had a remarkable improvement for adsorption of paraquat. To the best of our knowledge, it is the first time to combine hydrothermal carbonization with an electrospinning technique to prepare flexible hydrothermal carbon functional electrospun fiber membrane with certain mechanical strength for removing herbicide paraquat.
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MATERIALS AND METHODS
Materials. Please see the Supporting Information. All chemicals were used as received without further purification.
q (mg/g) = B
(C0 − Ce)V W
(1) DOI: 10.1021/acssuschemeng.6b00026 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering where C0 and Ce are the initial and the equilibrium concentrations of paraquat in the test solution (mg/L), V is the volume of the testing solution (L), and W is the weight of the adsorbent (g). The initial pH of the paraquat solution was adjusted to values in the range of 3−10 to investigate the influence of pH and lasted for 12 h. To study the adsorption kinetic, 10.0 mg of adsorbent was added to 100 mL of 40 mg/L paraquat solution at pH 7.0. Adsorption isotherms were conducted with initial concentrations ranging from 20 to 350 mg/L (initial pH 7) for 12 h. The effect of coexisting cations (urea, MB, Cd2+, and Na+) on paraquat adsorption capacities was evaluated, with an initial concentration of 40 mg/L for each cation contaminant and lasted for 12 h. For the desorption-readsorption experiment, the saturated paraquat-adsorbed fibers were washed thoroughly with deionized water. Then the fibers were put into 1 M HCl aqueous solution (1 mL HCl aqueous solution can regenerate 4 mg paraquatadsorbed AC-PAN fibers) for 2 h. After desorption equilibrium, the fibers were washed several times with deionized water and were reused in adsorption experiments (adsorbent, 40 mg; paraquat, 40 mg/L; initial pH, 7; time, 12 h) and the process was repeated five times. All experiments were performed at 20 ± 2 °C. Each adsorption experiment above was conducted twice to obtain reproductive results with error 75%) on the surface of CPAN and AC-PAN suggests that the hydrothermal treatment resulted in the formation of carbon structures. High-resolution O1s spectra of C-PAN and AC-PAN are shown in Figure 2b,c. The O1s peak of as-prepared C-PAN fibers can be deconvoluted into four components, 530.88, 531.90, 532.78, and 533.90 eV, corresponding to OC, −CO, COH, and −COOR groups, respectively.42,43 After the activation, the peak area ratios of COH and −COOR groups increase and the peak area ratios of OC decrease, which is consistent with the results of FT-IR measurement, indicating that the activation reaction has happened. The XPS results further confirm that the carbon structures are formed on the surface of the fibers through hydrothermal treatment and the oxygen-containing groups, which benefit the adsorption, increase after the activation. Tensile strength tests were also performed to investigate the mechanical property, which is important for adsorbents in practical application. The stress−strain curves of the obtained fiber membranes are shown in Figure 2d. As the results show,
The infrared results confirmed the successful happening of the hydrothermal carbonaceous and activation processes. The formation of the hydrothermal carbon materials was further examined by Raman spectroscopic measurement, which was one of the powerful techniques for characterizing carbon materials (see the Supporting Information, Figure S6). In D
DOI: 10.1021/acssuschemeng.6b00026 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering
Figure 3. (a) Effects of pH values on the adsorption capacities of C-PAN fibers and AC-PAN fibers toward paraquat. (b) Adsorption ability of various adsorbents at saturated state. (c) Adsorption kinetics of paraquat on C-PAN and AC-PAN fibers. (d) Pseudo-second-order fit to the experiment data.
solution at the initial concentration of 40 mg/L at pH 7 are shown in Figure 3b. Paraquat (1,1′-dimenthyl-4,4′-bipyridyl dichloride) is a cationic herbicide and its molecular structure is shown in Figure 3b. N-PAN fibers, obtained by ethylenediamine cross-linking, have almost no adsorption capacity for paraquat; and the adsorption amount is 0.62 mg/g. After the hydrothermal process, the adsorption amount reaches to 35.51 mg/g, suggesting that the coated carbons play a role in the herbicide removal. The activation reaction makes the adsorption amount increase significantly to 93.56 mg/g. The oxygen-containing groups, which benefit the adsorption, increase after the activation, thus the adsorption amount increases (the detailed adsorption mechanism is shown in the adsorption mechanism section). One can conclude the effects of the hydrothermal and activation through the comparison. Adsorption Kinetics. To understand better the adsorption properties, the adsorption of paraquat onto C-PAN and ACPAN fiber absorbents in the initial concentration of 40 mg/L as functions of time is shown in Figure 3c. In the first 2 h, both the adsorbents show initial rapid adsorption, followed by a slower increase until equilibrium within 12 h. Two kinetic models (the pseudo-first-order model and pseudo-second-order model) are applied to evaluate kinetics data. The lineal forms of the models can be expressed as follows:44 Pseudo-first-order model:
the tensile strength increases from 8.07 to 15.61 MPa and the elongation at break increases from 25.71% to 42.12% after the cross-linking. This is because the cross-linking reaction makes the polymer bond to each other. So, both the tensile strength and elongation at break increase. After the hydrothermal process, the coating carbons make the fibers connect each other more closely. Thus, the tensile strength noticeably increases to 29.76 MPa and the elongation at break decreases to 13.61% due to the fragility of inorganic carbons. After the NaOH activation, the tensile strength has a slight decrease and the elongation at break has a slight increase. However, the carbon-coated fibers before and after activation both still have good flexibility (Figure S3). Such enhanced mechanical strength by hydrothermal carbonization is beneficial for their applications in wastewater treatment field. Effect of pH and the Comparison of the Adsorption Amounts. Figure 3a shows the adsorption results for 40 mg/L paraquat at various initial pH values (from 3 to 10). At low pH value (
