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Heteroatoms doped metal iron–polyvinylidene fluoride (PVDF) membrane for enhancing oxidation of organic contaminants
Accepted Manuscript Title: Heteroatoms doped metal iron–polyvinylidene fluoride (PVDF) membrane for enhancing oxidation of organic contaminants Authors: Yunjin Yao, Chao Lian, Yi Hu, Jie Zhang, Mengxue Gao, Yu Zhang, Shaobin Wang PII: DOI: Reference:
S0304-3894(17)30376-X http://dx.doi.org/doi:10.1016/j.jhazmat.2017.05.026 HAZMAT 18585
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
3-2-2017 27-4-2017 14-5-2017
Please cite this article as: Yunjin Yao, Chao Lian, Yi Hu, Jie Zhang, Mengxue Gao, Yu Zhang, Shaobin Wang, Heteroatoms doped metal iron–polyvinylidene fluoride (PVDF) membrane for enhancing oxidation of organic contaminants, Journal of Hazardous Materialshttp://dx.doi.org/10.1016/j.jhazmat.2017.05.026 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Heteroatoms Doped Metal Iron–Polyvinylidene Fluoride (PVDF) Membrane for Enhancing Oxidation of Organic Contaminants Yunjin Yao a,b,*, Chao Lian a, Yi Hu a, Jie Zhang a, Mengxue Gao a, Yu Zhang a, Shaobin Wang c,* a
Anhui Key Lab of Controllable Chemical Reaction & Material Chemical Engineering, School of Chemistry and Chemical Engineering, Hefei University of Technology, Tunxi Road 193, Hefei 230009, China b School of Chemistry and Material Science, University of Science and Technology of China, Hefei 230026, P. R. China c Department of Chemical Engineering, Curtin University, G.P.O. Box U1987, Perth, Western Australia 6845, Australia AUTHOR INFORMATION * To whom correspondence should be addressed: Name: Yunjin Yao; Phone: +86 551 62901548; Fax: +86 551 62901450; E-mail:[email protected]. Name: Shaobin Wang; Phone: +61 8 9266 3776; Fax: +61 8 9266 2681; E-mail: [email protected].
1
Highlights
▸NSC-Fe-X@PVDF catalytic membranes were fabricated by phase inversion technique. ▸Hierarchical structures are formed by anchoring NSC-Fe-X NPs on PVDF membranes. ▸Catalytic performance of NSC-Fe-X@PVDF was affected by several key parameters. ▸Sulfate and hydroxyl radicals are responsible for this persulfate-driven oxidation. ▸Morphological and structural features of membrane enhance catalytic activity.
ABSTRACT: Iron nanoparticles (NPs) embedded in S, N-codoped carbon was prepared by one-step pyrolysis of a homogeneous mixture consisting of Fe, S, N, C precursors, and then immobilized in poly (vinylidene fluoride) membranes as a multifunctional catalytic system (NSC-Fe@PVDF) to effectively activate peroxymonosulfate (PMS) and oxidize organic compounds in water. The NSC-Fe@PVDF membranes effectively decolorized organic pollutants at a wide pH range (2.05-10.85), due to the synergistic effects between the S, N-doped carbon and iron NPs. The efficiency depended on the doping type, amount of metal, PMS dosages, reaction temperature, solution pH, and organic substrates. In-situ electron spin resonance (ESR) spectroscopy and sacrificial-reagent incorporated catalysis indicate radical intermediates such as sulfate and hydroxyl radicals are mainly responsible for this persulfate-driven oxidation of organic compounds. Membrane’s porous structure and high internal surface area not only minimize the NPs agglomeration, but also allow the facile transport of catalytic reactants to the active surface of metal catalysts. The results demonstrate the morphological and structural features of a catalytic membrane enhance the overall catalytic activity. Keywords: PVDF membrane; Organic pollutant; Catalytic degradation; Sulfate radical.
1. Introduction Increasingly serious organic chemical pollution is one of the most pervasive problems afflicting human health throughout the world. Persulfate-based advanced oxidation processes have ever been recognized as a viable, alternative oxidation process for realizing environmental remediation [1-3]. Generally, the relatively stable peroxides such as peroxymonosulfate ( HSO5 , PMS) and peroxydisulfate ( S2O28 , PDS) are
activated
on-site
to
generate
reactive
species
such
as
sulfate
radical
0 2 0 (SO 4 , E (SO4 /SO4 ))=2.43 VNHE ) and hydroxyl radical ( OH, E ( OH / H2 O) 2.81 VNHE ) [1, 4]. By
various strategies through energy or electron transfer reactions, peroxides have effectively produced reactive species. For instance, thermolysis, UV photolysis, and transition metal mediated activations initiate intramolecular electron transfer and the associated homolytic cleavage of peroxide bonds within 2
the persulfate molecules [5]. Comparing with PDS, PMS is easily activated by transition metals due to the asymmetric molecular structure, which is considered as the most simple and sustainable method without requiring external energy [6-8]. Consequently, the development of efficient catalysts for PMS is the key objective in the chemical research [9]. To date, various types of catalysts with highly efficient intrinsic active sites, including heteroatomdoped carbon materials, transition-metal-coordinating macrocyclic compounds, metal/nitrogen/carbon (M-Nx/C, M=Fe, Co, Ni) compounds and chalcogenides supported on carbon materials [10, 11], have been explored. For example, we previously fabricated novel M-Nx/C as an efficient Fenton-like catalyst for removal of organic pollutants [6, 7]. However, most of the researchers ignored the separation of the utilized nanomaterials from the treated water, which created a secondary pollution problem [12, 13]. Besides, many of those catalysts aggregate easily, reducing their activity, and thus are undesirable for industrial applications. One of strategies to address this problem is to assemble these nanoparticles (NPs) on substrates or membranes. Huang et al. [14] recently reported a catalytic membrane reactor, which contains a membrane matrix and a catalytic film of alloy NP-loaded β-lactoglobulin fibrils for the reduction of 4-nitrophenol. In these regards, polymer substrates play an important role in immobilizing catalyst because of the good film formation, toughness, and separation, etc. In this study, we attempt to develop an economical and effective NP-loaded catalytic membrane to achieve the removal of contaminants. Within a range of materials, poly (vinylidene fluoride) (PVDF) membranes have attracted much attention in wastewater purification, due to its excellent thermal stability, chemical resistance, and high mechanical strength [15, 16]. The previous researches on NPs incorporation into the membrane structure are mainly improved physical properties, such as the hydrophilicity, fouling resistance or antibacterial performance [17]. It is likewise of interest for designing catalytic PVDF membranes for the removal of organic pollutants. For instance, Wang et al. [18] found that Pd/PVDF membrane displayed excellent performance for the reduction of p-nitrophenol to p-aminophenol. Alpatova et al. [19] developed PVDF membranes with inclusion of carbon nanotubes and Fe2O3 NPs for removal of organic contaminants by 3
H2O2. Besides, metal NPs can be long used and are very stable [18]. However, the pores of such porous membranes are usually so small that reactants cannot rapidly diffuse through the interface to internal NPs for catalytic reactions. In addition, most procedures to prepare such catalytic membranes with an excellent activity are time-consuming, complicated, and need special instruments. In this work, we report a novel PVDF membrane decorated with heteroatom-doped iron NPs in membrane pores for the first time and demonstrate their high performance for water treatment. Morphological and structural characterizations were performed to confirm the successful formation of NPs on the PVDF membrane using several microscopic and spectroscopic techniques. The influences of several critical factors, such as initial PMS dosages, reaction temperature, dye types, initial pH, and inorganic anions on oxidation of organic compounds were further investigated in detail. In-situ electron spin resonance (ESR) spectroscopy and sacrificial-reagent incorporated catalysis were employed to address the primary activation mechanism. 2. Materials and Methods 2.1 Preparation of iron NPs embedded in S, N-codoped carbon The method for the synthesis of iron NPs embedded in S, N-codoped carbon catalysts was developed by our group and described previously [6, 20]. In a typical synthesis, dicyandiamide (5 g) and thiourea (2 g) as C, S, N precursors were firstly dissolved in 200 mL of MeOH solution at 50 °C. Secondly, Fe(NO3)3·9H2O with the designed loadings (1.0, 1.5, 2.0, and 2.5 g) as an iron precursor was slowly added to the above mixture via stirring for 60 min. Then, the solvent was removed using an evaporator operated at 75 °C. The prepared gel-like mixture was dried at 70 °C overnight. After grinding, the powder was then pyrolyzed at 700 °C for 2 h under 0.2 mL/min of N2 atmosphere with a heating rate of 5 °C /min. The pyrolyzed composites were filtered, washed with deionized (DI) water, and then dried in an oven. The asprepared catalysts were denoted as NSC-Fe-X (where X represents the initial mass of iron salts). The Sdoped and N-doped carbons were synthesized using the similar preparation method to NSC-Fe-X, but with the addition of S or N precursors. 4
2.2. Preparation of NSC-Fe-X@PVDF Membranes. NSC-Fe-X blended PVDF catalytic membranes were fabricated by the phase inversion technique [21]. Briefly, the synthesized NSC@Fe-X composites (0.05 g), polyvinylpyrrolidone (PVP, 0.25 g) and PVDF (0.5 g) were dissolved to a N, N-dimethylformamide (DMF, 5 mL) solution under vigorous agitation for 10 h at 60 °C. Afterwards, the homogeneous solution was transferred into a drying oven for 12 h to remove air bubbles. After degassing, the solutions were poured into a Petri dish (24 cm2) with a controlled casting rate, and then immersed in a coagulation bath (ethanol aqueous solution at 25 °C). The generated membranes labeled as NSC-Fe-X@PVDF were rinsed to remove the residual solvent, and kept in DI water before further use. Meanwhile, Fe2O3, NC-Fe-2.0, and SC-Fe-2.0 blended PVDF membranes (denoted as Fe2O3@PVDF, NC-Fe-2.0@PVDF, and SC-Fe-2.0@PVDF) were also synthesized in the same way. The synthetic procedure of NSC-Fe-X@PVDF membrane is illustrated in Figure 1. [Figure 1]. 2.3 Characterization Powder X-ray diffraction (XRD) patterns were obtained with a Philips X’Pert Pro MPD diffractometer (40 kV, 40 mA; Cu Kα radiation). Fourier transform infrared (FT-IR) spectroscopy with an attenuated total reflectance (ATR) accessory (Vector 22, Bruker) was used to characterize the fabricated membranes. The morphologies of the samples were examined using a JSM-6700F Field Emission Scanning Electron Microscope (FE-SEM, SU8020, Hitachi, Japan) with an accelerating voltage of 5 kV. Field-emission transmission electron microscopy (FE-TEM) images were collected by a JEOL JEM-2100 instrument at an accelerating voltage of 200 kV. Energy dispersive X-ray spectra (EDS) and the elemental mapping were collected from three randomly selected areas of each sample. X-ray photoelectron spectroscopy (XPS) measurements were performed on an ESCALAB250 spectrometer with a focused monochromatic Al Kα radiation at 12 kV and 20 mA. The surface roughness was measured by the atomic force microscopy (AFM, Dimension Icon, Bruker). 2.4 Catalytic Activity Test 5
Decolorization of various target organic compounds was conducted in 250 mL glass bottles in water bath under magnetic stirring. The catalytic membrane was firstly added to the bottles, followed by PMS with an aliquot stock solution to initiate the reaction. Sample aliquots were withdrawn at predetermined time intervals, filtered using a 5 mL glass syringe and a 0.25 μm nylon syringe filter, and injected into a 2 mL amber glass vial containing excess MeOH (0.5 M) to quench any residual radicals. Select experiments were performed to evaluate pH effect, and the initial pH of the suspensions was adjusted using 0.1 M H2SO4 or NaOH solution. The influences of several critical factors (including PMS dosage, reaction temperature, dye types, and inorganic anions) on oxidation of organic compounds were also investigated. To ascertain whether decolorization of organic compounds was caused by reactive oxygen species (ROS), MeOH and tert-butyl alcohol (TBA) were employed to distinguish OH and SO 4 radicals. The catalytic durability of NSC-Fe-X@PVDF was tested for 4 cycles, in which the catalysts were recycled by filtration and washing with ultrapure water after each cycle. The concentrations of organic compounds were determined by a UV-vis spectrophotometer. The total iron leaching concentrations was determined by atomic absorption spectroscopy (AAS, Perkin-Elmer AA800). The total amount of dissolved carbon (TOC) was determined by a TOC analyzer (Vario TOC select, Elementar). Electron spin resonance (ESR) spectrum was recorded to further verify reactive oxygen species using 5, 5dimethyl-1-pyrroline-N-oxide (DMPO) as the spin trapping agent. The chemical solutions of Orange II, catalyst and PMS were mixed, taken, and immediately injected into DMPO. The obtained solution was separated, transferred, and then inserted into the cavity of the equipment. The settings of the ESR instrument (JES-FA200, electron spin resonance spectrometer system, JEOL, Japan) were as follows: center field, 324.3 mT; sweep width, 10.0 mT; microwave frequency, 9.072 GHz; microwave attenuator, 30 dB; microwave power, 2.0 mW; temperature, 25 °C; sweep time, 30 seconds.
3. Results and Discussion 3.1 Catalyst Characterization Figure 2a represents XRD patterns of pure PVDF, NC-Fe-2.0@PVDF, SC-Fe-2.0@PVDF, and NSCFe-2.0@PVDF membranes. For PVDF, the broad reflections at about 26.4° was observed, which could be attributed to the polar β-phase of amorphous PVDF substances [22]. The diffraction peaks at 30.1, 35.4, 48.2, 56.9, and 65.2° corresponded to Fe3O4 (JCPDS No. 89-0691), whereas the peaks at 44.7 and 65.2° can be assigned to the metallic Fe0 (JCPDS No.06-0696). This suggests that the transformed metal oxides were in situ reduced to Fe0 by carbon [23, 24]. NSC-Fe-X@PVDF prepared at different 6
Fe(NO3)3·9H2O dosages (X=1.0, 1.5, 2.0 g) also showed Fe3O4, Fe0 and PVDF phase (Figure 2b). At X = 2.5 g, the peaks of Fe0 are not observed in the composites. At high dosages of Fe(NO3)3·9H2O, the carbothermal reduction reaction is not complete, without the formation of the highly-crystallized Fe0 NPs [25]. The Fe(NO3)3·9H2O dosage is the key factor affecting the structure of materials. The chemical structure was also detected by FT-IR (Figure 2c, d). The peak at 1070 cm-1 was attributed to the C-F bond stretching vibration mode. The strong absorption band at 1280 cm-1 is the characteristic of CF2 found in PVDF. The peaks at 1403 and 1600 cm-1 are corresponding to -CH2 and C=C, respectively. The bands observed at 840 and 878 cm−1 indicate the presence of polar β phase of PVDF [19]. No trace of the nonpolar α-phase by the FT-IR band at 766 cm−1 is found in the composites, suggesting PVDF hybrid membranes crystallizes in the β-phase regardless of solvent evaporation rates [26]. The FT-IR spectra certified that hybrid membranes were composed of PVDF and iron oxide phase. As will be shown later, catalytic decolorization indicated that the NSC-Fe-2.0@PVDF membrane had the greatest reactivity, hence, this membrane was further analyzed by FE-SEM, FE-TEM, and XPS. [Figure 2]. The microstructures of the NSC-Fe-2.0 NPs were examined by TEM and SEM measurements. Figure 3a, j indicates irregular-shaped crystallites with slight agglomeration. High-resolution TEM image suggests that all iron NPs are embedded in the carbon matrix (Figure 3b, c). The enlarged TEM image (Figure 3b) of the region marked with a rectangle shows the lattice spacings of 0.25 and 0.21 nm were consistent with the (331) and (440) crystalline planes of Fe3O4, respectively. The clear lattice fringes with an interplanar distance of 0.20 nm can be ascribed to the (110) planes of Fe0 (Figure 3c). In addition, the d-spacing was d002=0.34 nm, corresponding to the characteristic of graphitic carbon. EDS analysis clearly shows the compositions of NSC-Fe-2.0 comprising of C, S, N, O, and Fe elements (Figure S1). The element mapping also demonstrated that iron, nitrogen, sulfur species are distributed in the carbon matrix uniformly, indicating the nitrogen and sulfur atoms were successfully doped into the carbon lattice (Figure 3d-i).
7
The successful codoping of carbon with N and S atoms was further investigated using XPS. The C 1s XPS spectrum (Figure 3k) can be deconvoluted into three spectral components, C-C (284.8 eV), hydroxyl C-OH (285.8 eV), and carbonyl C=O (288.4 eV), respectively [27, 28]. The XPS spectra of N 1s (Figure 3l) were fitted with three individual peaks, corresponding to pyridine-like (398.4 eV), pyrrolic (399.4 eV), and graphitic (400.9 eV) N species, indicating that the N heteroatom has been doped into the carbon backbones [29]. Pyridinic N and graphitic N are generally regarded as the species responsible for the catalytic activity [30, 31]. In S 2p XPS spectra, the peak at 163.4 eV corresponds to C-Sn-C (n = 1 or 2) bonds, indicating efficient S-doping within the carbon network[29]. In addition, two peaks at 162.1 and 168.7 eV correspond to the reduced (–SH) sulfur moieties and the oxidized S (-SOn-), respectively (Figure 3n) [32]. Furthermore, Figure 3p shows the high-resolution scan of Fe 2p electrons, where deconvolution yielded two pairs of peaks for Fe2+ (710.7 and 724.2 eV) and Fe3+(712.6 and 726.1 eV), with a satellite peak at 720.0 eV [33]. Besides these, a characteristic peak at 707.4 eV was assigned to the metallic Fe0, confirming the regeneration of metallic Fe during the synthesis process, which is in accordance with XRD analyses (Figure 3m) [24]. [Figure 3]. NSC-Fe-2.0 NPs immobilized on a PVDF membrane is shown in Figure 4a. While the pure PVDF membrane sample is white in color, the NSC-Fe-2.0@PVDF membrane is uniformly black in color, indicating a uniform distribution of the NSC-Fe-2.0 NPs on the PVDF scaffold. Figure 4b, c presents top layer and cross-section pore structure of membranes casted by NSC-Fe-2.0 NPs. Visible NSC-Fe-2.0 aggregates appeared in the field of view of all membranes, which contained NSC-Fe-2.0 in their structure as well as within the pore openings. It enables facile transport and penetration of catalytic reactants to the active surface without suffering high mass-transfer resistance. During the membrane formation process, NSC-Fe-2.0 NPs acted as pore forming centers, precipitated more slowly, and thus remained presence in the surface layer. Incorporation of NPs is recognized to improve pore formation and increase membrane porosity. EDS mapping analysis revealed that iron was uniformly distributed on the membrane surface (Figure 4e, f). Through AFM analysis (Figure 4d), the average surface roughness (Ra) and root-mean8
square roughness (Rq) were determined to be 104 nm and 132 nm, respectively. High surface area and roughness could increase the efficient contact area, which led to the enhancement of catalytic performance. Totally, NSC-Fe-2.0 NPs could improve the microstructure and influence the hydrophilicity of PVDF membranes. [Figure 4]. 3.2 Catalytic evaluation The catalytic performance of the various membranes was evaluated for color removal of Orange II by activation of PMS (Figure 5a, c). The catalytic data were fit to pseudo-first-order kinetics and the obtained rate constants (kobs) are given in Figure 5b, d. Less than 2% of Orange II could be decolorized by PMS alone indicates that thermal activation of PMS without metal catalysts was negligible. A control test using only membranes shows that less than 6% of Orange II could be adsorbed in 150 min. Moreover, kinetic curves of NSC-Fe-X@PVDF prepared at different Fe(NO3)3·9H2O dosages (X=1.0, 1.5, 2.0, and 2.5 g) showed color removal was increased with the increased Fe(NO3)3·9H2O dosage up to 2.0 g and decreased thereafter. One main explanation for the negative effect was that the dosage of additives would also induce greater NPs aggregation and the total active surface area would decrease in membranes [34]. To demonstrate the roles of Fe, N, S, C sources in the Fenton-like reaction, PVDF, Fe-2.0@PVDF, NCFe-2.0@PVDF, SC-Fe-2.0@PVDF, and NSC-Fe-2.0@PVDF were prepared using the same method. The values of kobs were 2.26×10-3 min−1 for PVDF, 1.68×10-3 min−1 for Fe-2.0@PVDF, 8.72×10-3 min−1 for NC-Fe-2.0@PVDF, 0.011 min-1 for SC-Fe-2.0@PVDF, and 0.023 min-1 for NSC-Fe-2.0@PVDF. After incorporating Fe, N, C, S sources, an obvious increase in kobs was observed for NSC-Fe-2.0@PVDF, indicating the heteroatoms have a synergetic effect in enhancing catalytic performance. It was also evidenced by the photograph of reaction solution (Figure 5) and the absorption peaks in UV–vis absorption spectra of Orange II at 485 nm dramatically decrease (Figure S2). Meanwhile, the new peak at 255 nm increases, indicating that smaller molecules derived from Orange II are formed during the chemical process. In contrast, TOC values nearly unchanged, indicating that the mineralization of organic compounds is very limited. The reason is that intermediates derived from the organic dye are enriched in 9
a variety of small-chain organic acids, which is consistent with the previous results [35, 36]. The best catalytic performances of NSC-Fe-2.0@PVDF membrane was selected as a typical NSC-Fe@PVDF for the following experiments. 3.3. Stability and reusability of NSC-Fe-2.0@PVDF membrane The stability and reusability of the multifunctional membrane are crucial to the actual application that must be considered. The recycling capability of NSC-Fe-2.0@PVDF sample was successfully evaluated by color removal under the same experimental condition (Figure 5e). The membrane still showed comparatively good catalytic activity after four consecutive runs, which proved to be a potent reusable catalyst. No significant leaching of iron ions (0.037 mg/L) was detected by AAS, suggesting Fe NPs are stably embedded in the carbon structure. In addition, NSC-Fe-2.0@PVDF membrane still remained in the crystalline form after the reaction (Figure S3), suggesting the heterogeneous Fenton-like reaction, even under acidic conditions. We did not observe remarkable changes in the FT-IR spectra of the fresh and used NSC-Fe-2.0@PVDF membrane (Figure 5e), suggesting a negligible adsorption of these species on the surface of the catalyst and/or complete color removal. In the practical application, nanocatalysts need not to be separated and recovered from reaction solution. Therefore, the catalytic membrane used for the oxidative decolorization of various organics is convenient, environmentally friendly, and economical. All in all, nanocatalysts immobilized in PVDF membrane suggested a few interesting features that prevent the loss and improve the usability of the catalysts [18]. [Figure 5]. 3.4. Effects of reaction parameters on organic decolorization in NSC-Fe-2.0@PVDF/PMS system The optimal sample of NSC-Fe-2.0@PVDF was selected to study the influences of several critical factors, such as initial PMS dosage, reaction temperature, different dyes, initial pH, and inorganic anions on oxidation of organic compounds. An increase in PMS dosage (0.08-0.49 mM) had an obvious positive effect on color removal, ascribing to the formation of more amount of reactive species during the reaction (Figure S4a). When its dosage increased from 0.49 to 0.65 mM, no obvious improvement in decolorization performance occurred, due to self-scavenging during the oxidation reaction. Hence, optimized PMS 10
concentration was taken at 0.49 mM. Figure S4b shows the decolorization rate increases quickly with the increase of reaction temperature, which demonstrates that the increasing temperature has the benefit to the catalytic activity. The improvement of oxidation capacity is mainly due to that the elevated temperature would promote more energy for the reactant molecules to lower the barrier of activation energy [37]. The efficacy of PMS activated by NSC-Fe-2.0@PVDF to decolorize various organic compounds was further evaluated (Figure S4c). The use of catalytic membrane yielded a removal efficiency of 90.1% for Rhodamin B, 97.7% for Orange II, 85.7% for Congo red, 81.2% for Methylene blue, 93.9% for Neutral red, 97.8% for Methyl violet, and 96.4% for Methyl orange. These trends clearly reveal that molecular structures influence the removal rates [38], and NSC-Fe-2.0@PVDF is effective for decolorization of a wide range of organic compounds. Figure S4d presents the effect of initial pH on PMS activation efficiency in the NSC-Fe-2.0@PVDF /PMS process. An obvious pHo decrease after the reaction was observed, due to the formation of acid intermediates, suggesting that H+ was produced during the process [39]. Color removal was not significantly affected by the pH change in solution between 2.05 to 10.85. When the experimental suspensions were initially adjusted to an acidic or a basic pH value, the pH change in solution was marginal over the course of PMS activation. This pH independence is one notable advantage, compared to other peroxide-based oxidation processes such as Fe2+/H2O2 which is highly pH sensitive [1]. The variation in removal can be explained that pH changes the surface charge of the membrane, the electrostatic interaction between organic molecules and membrane, and the number of charged radical species generated during the oxidation process [40].
Furthermore, some coexisting anions such as HCO3 , Cl , SO24 , CH3COO , NO3 , NO2 , and HPO24 are generally present in water, and can interfere the removal process. As indicated in Figure S4e, The color removal rates decreased with the increase of HCO3 concentration up to 1.6 mM. In addition, it is observed that all the added inorganic anions have an inhibitive effect on the oxidation process (Figure S4 f). The reasons were that coexisting anions would compete with the organic substance for the adsorption sites, resulting in the blockage of the active sites on the membrane surface [41]. 3.5. Contribution of radicals in NSC-Fe-2.0@PVDF/PMS system The results above suggest that radical species is produced in the reaction between PMS and NSC-Fe 2.0@PVDF. Generally, SO 4 or OH is considered to be the radical species in oxidation processes. Alcohols with or without α-hydrogen have different reactivities with radical species. MeOH (with a-hydrogen) has a similar
11
9 −1 −1 6 −1 −1 reactivity to OH and SO 4 ( k OH : 1.2-2.8 × 10 M s , kSO : 1.6-7.8 × 10 M s ), while alcohols without α4
hydrogen, such as TBA, are known to react much slower with sulfate radicals, which has 1000-fold higher rate 5 −1 −1 constant with OH ( k OH : 3.8-7.6 × 108 M−1s−1) than with SO 4 ( kSO : 4-9.1 × 10 M s ) [42]. Therefore, 4
MeOH was used to scavenge both radicals, and TBA was used to selectively quench OH [43]. The effects of MeOH and TBA on NSC-Fe-2.0@PVDF/PMS oxidation showed that TBA had a much higher inhibition effect on the color removal rate compared with MeOH (Figure 6a, b). For instance, Color removal decreased by ∼49.4% in the presence of 3 M TBA in comparison with the control experiment, in which no scavenger was present, whereas only around 19.9% inhibition was observed in the presence of the same amount of MeOH. This result may indicate that a significant role in the generation of SO 4 was played by OH . By scavenging OH , the added TBA affects the yield of SO 4 (eq 1).
2HSO5 +2 OH SO 4 +2H 2O+O2
(1)
Furthermore, the addition of 6 M MeOH only scavenged about 10% of color removal, indicating alcohols fail
to capture surface-adsorbed radicals completely [44]. Note, SO4 radicals adsorbed on the surface of catalysts are beneficial to accelerate the organics oxidation. In addition, both alcohols are oxidized directly or via OH and other reactive oxygen species, which will limit their reactivity with SO 4 and OH radicals. Therefore, the addition of specific alcohol quenchers could not elucidate between sulfate and hydroxyl radical contributions to the oxidation of Orange II. Nevertheless, the observed decrease in the presence of alcohol quenchers indicates a significant contribution of the radical species. To provide direct evidence regarding the free radical species that might be involved in the NSC-Fe2.0@PVDF/PMS process, the ESR technique was used. As illustrated in Figure 6c, the ESR signals of DMPO- HO (four lines, 1:2:2:1) and DMPO- SO 4 (six lines, 1:1:1:1:1:1) from their hyperfine splitting constants ((DMPO
OH : H = N = 14.7 G; DMPO− SO 4 : N = 13.3 G, H = 9.5 G, H = 1.46 G, and H = 0.77 G) were detected
in our reaction systems [45]. The results further confirm that Orange II oxidation was induced by SO 4 and
OH reactive species generated in NSC-Fe-2.0@PVDF/PMS process. The peak intensities of DMPO- SO 4 and
DMPO− OH first increased and then decreased rapidly (Figure 6d). The drop of HO and SO 4 amount was due to the consumption by Orange II oxidation. Moreover, the intensity of DMPO- SO 4 adducts signals was much weaker than that of DMPO- OH adducts signals, which could be interpreted with the fast transformation from DMPO- SO 4 adducts to DMPO- OH adducts [46].
[Figure 6]. 3.6. Reaction Mechanism According to all above-mentioned results and discussions, a mechanism is proposed and shown in Figure 9. Firstly, organic molecules and PMS in the solution were transferred and adsorbed on NSC-Fe-2.0 NPs loaded on the porous PVDF membranes. Secondly, the surface adsorbed PMS can achieve persulfate driven oxidation by NSC-Fe-2.0 NPs to produce ROS, interacted with the adsorbed organic molecules for in-situ reaction, and then released the preoccupied sites. Thirdly, the released free sites on NSC-Fe-2.0@PVDF accelerated aqueous reactants transfer, and then an adsorption-decolorization cycle occurred again. In this way, the abovementioned cycles underwent continuously until all organic compounds in water were removed.
In the present system, a series of oxidation reactions occur via surface catalysis. In the initial step, Fe0 is oxidized to FeII via a two-electron transfer. The surface FeII then reacts with PMS to create surface12
II III bound SO can be achieved because of the 4 radicals. Simultaneously, the regeneration of Fe from Fe
high potential difference (1.21 V) between Fe2+/Fe0 and Fe3+/Fe2+. The deposition of carbon materials serves as excellent electron transfer supports, and can also activate PMS and oxidize organic compounds via non-radical mechanisms [47-49]. Therefore, given the roles of the Fe2+/Fe0 and Fe3+/Fe2+ pairs, electron transfer is enhanced over the metallic surface, eventually enhancing the removal efficiency. As previously described, Fe NPs embedded in carbon could lower the local work function of the carbon surface because of the facile electron transfer from the iron to the carbon [6, 7]. The donor-acceptor complexes at surface sites were promoted by the embedded Fe NPs and the S, N dopants would improve the interfacial electron transfer. In addition, the carbon shell builds up a reservoir that effectively retards the dissolving of iron into solution. Therefore, owing to the synergistic effects between S, N-doped carbon and iron NPs, NSC-Fe-2.0@PVDF catalyst intrinsically displays an enhanced catalytic performance. The exhibited good catalytic property of NSC-Fe-2.0@PVDF membranes may be attributed to two aspects as follows: First, the small nanocatalysts usually lead to an increase of the catalytic efficiencies because of the high surface energy and great surface area-to-volume ratio; Hence, more active atoms on the membrane surface are expected to be available for the catalysis. Second, the presence of porous and interconnected skeleton structure of NSC-Fe-2.0@PVDF membranes enables facile transport and penetration of catalytic reactants to the active surface without suffering high mass-transfer resistance. Therefore, both the nanosize effect of metal NPs and a relatively low diffusion resistance of organics on the surface of catalytic membrane were beneficial for the sufficient contact of organics with the active sites and facilitates the good catalytic performance of membranes [50]. [Figure 7].
4. Conclusions In summary, we demonstrated novel PVDF membrane decorated with iron NPs embedded in S, Ncodoped carbon in membrane pores to effectively activate PMS and oxidize organic compounds in water. Hierarchical nanostructures are formed by uniformly anchoring NSC-Fe-X NPs on the PVDF membranes by the phase inversion technique. The synergistic effects between the S, N-doped carbon and iron NPs 13
improve the oxidative ability of the optimized NSC-Fe-2.0@PVDF materials. The catalytic efficiency depended on several reaction conditions including the doping type, amount of metal, PMS dosages, reaction temperature, solution pH, and organic substrates. ESR spectroscopy and sacrificial-reagent incorporated catalysis indicate SO and OH radicals are mainly responsible for this persulfate-driven 4 oxidation process. Comparative analysis demonstrates the NSC-Fe-X@PVDF membranes have clear advantages over NSC-Fe-X NPs because of their flexible and macroporous architecture. These results indicate that the NSC-Fe-2.0@PVDF membranes are promising for pollution control, and might be expected to contribute to other important areas of environmental application. The development of this type of catalytic membrane for continuous-flow reactions and separation will be evaluated in the future.
ACKNOWLEDGEMENTS The financial supports by the Anhui Provincial Natural Science Foundation (NO. 1708085MB41), the China Postdoctoral Science Foundation (NO. 2015M570547, 2016T90585), National Natural Science Foundation of China (Grant 51372062), and Undergraduate Training Programs for Innovation and Entrepreneurship (NO. 2017CXCY049) are acknowledged. The partial support from the Australian Research Council for DP 150103026 is also acknowledged. Special thank is given to Prof. T. Xu for the useful discussion in the initial preparation of this manuscript.
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Figure;1; Captions Figure 1. Synthetic procedure of the NSC-Fe-X@PVDF membranes. Figure 2. (a, b) XRD and (c, d) FT-IR of pure PVDF, NC-Fe-2.0@PVDF, SC-Fe-2.0@PVDF, and NSCFe-X@PVDF membranes prepared at different Fe(NO3)3·9H2O dosages (X=1.0, 1.5, 2.0 g). Figure 3. Morphology and compositions of NSC-Fe-2.0 NPs. (a) Representative TEM image. (b-c) highresolution TEM images. (d) TEM image and its corresponding elements mapping images: (e) Fe, (f) O, (g) N, (h) C, and (i) S element. (j) Representative SEM image. (m) XRD pattern. High-resolution scans of (k) C 1s, (l) N 1s, (n) S 2p and (p) Fe 2p electrons. Figure 4. Characterization of NSC-Fe-2.0@PVDF membrane: (a) digital photo; (b) SEM top view; (c) SEM cross-sectional view; (d) AFM 3D surface image;(e-f) EDS mapping from SEM top and crosssectional view. Figure 5. (a, c) Comparison of the catalytic activity of pure PVDF, NC-Fe-2.0@PVDF, SC-Fe2.0@PVDF, and NSC-Fe-X@PVDF membranes prepared at different Fe(NO3)3·9H2O dosages (X=1.0, 1.5, 2.0 g). (b, d) The rate constant in different reaction systems. (e) Recycle experiments for Orange II removal using NSC-Fe-2.0@PVDF sample. (f) Color changes of Orange II with the reaction times increasing. Unless otherwise stated, the reaction conditions are: [Orange II] = 20 mg/L, T =25 °C, [PMS] =0.60 g/L, without pH adjustment. Figure 6. Effects of TBA (a) and MeOH (b) scavengers on the Orange II removal by the NSC-Fe2.0@PVDF/PMS process. (c) DMPO-trapped ESR spectra. (d) Comparison of ESR signal intensity of DMPO- OH and DMPO- SO 4 adducts; Unless otherwise stated, the reaction conditions are: [Orange II] = 20 mg/L, T =25 °C, [PMS] =0.60 g/L, without pH adjustment. Figure 7. Schematic illustration of the reaction mechanism for organics removal by the NSC-Fe2.0@PVDF/PMS system.
21
Figure 1. Synthetic procedure of the NSC-Fe-X@PVDF membranes.
22
(a)
(b)
(c)
(d)
Figure 2. (a, b) XRD and (c, d) FT-IR of pure PVDF, NC-Fe-2.0@PVDF, SC-Fe-2.0@PVDF, and NSC-Fe-X@PVDF membranes prepared at different Fe(NO3)3·9H2O dosages (X=1.0, 1.5, 2.0 g).
23
(a)
(b)
Fe3O4
3.4 Å
(c)
Carbon shell
Carbon shell
2.1 Å
2.5 Å Fe
Fe3O4
2.0Å
Fe0 (d)
(e)
(f)
(j)
(k)
(m)
(n)
(h)
(g)
(i)
(l)
(p)
Figure 3. Morphology and compositions of NSC-Fe-2.0 NPs. (a) Representative TEM image. (b-c) high-resolution TEM images. (d) TEM image and its corresponding elements mapping images: (e) Fe, (f) O, (g) N, (h) C, and (i) S element. (j) Representative SEM image. (m) XRD pattern. XPS spectra of (k) C 1s, (l) N 1s, (n) S 2p and (p) Fe 2p electrons. 24
(a) (c)
(b)
PVDF membrane
(d)
NSC-Fe-2.0@PVDF membrane
(e)
Fe Ka1
(f)
Fe Ka1
Figure 4. Characterization of NSC-Fe-2.0@PVDF membrane: (a) digital photo; (b) SEM top view; (c) SEM cross-sectional view; (d) AFM 3D surface image;(e-f) EDS mapping from SEM top and crosssectional view.
25
(a)
(c)
(e)
(b)
(d)
(f)
Figure 5. (a, c) Comparison of the catalytic activity of pure PVDF, NC-Fe-2.0@PVDF, SC-Fe2.0@PVDF, and NSC-Fe-X@PVDF membranes prepared at different Fe(NO3)3·9H2O dosages (X=1.0, 1.5, 2.0 g). (b, d) the corresponding apparent degradation rate constants. (e) Recycle experiments for Orange II removal using NSC-Fe-2.0@PVDF sample. (f) Color changes of Orange II with the reaction
26
times increasing. Unless otherwise stated, the reaction conditions are: [Orange II] = 20 mg/L, T =25 °C, [PMS] =0.60 g/L, without pH adjustment. (a)
(c)
(b)
(d)
Figure 6. Effects of TBA (a) and MeOH (b) scavengers on the Orange II removal by the NSC-Fe2.0@PVDF/PMS process. (c) DMPO-trapped ESR spectra at different reaction time. (d) Comparison of ESR signal intensity of DMPO- OH and DMPO- SO adducts. Unless otherwise stated, the reaction 4 conditions are: [Orange II] = 20 mg/L, T =25 °C, [PMS] =0.60 g/L, without pH adjustment.
27
Figure 7. Schematic illustration of the reaction mechanism for organics removal by the NSC-Fe-2.0@PVDF/PMS system.
28
S0304-3894(17)30376-X http://dx.doi.org/doi:10.1016/j.jhazmat.2017.05.026 HAZMAT 18585
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
3-2-2017 27-4-2017 14-5-2017
Please cite this article as: Yunjin Yao, Chao Lian, Yi Hu, Jie Zhang, Mengxue Gao, Yu Zhang, Shaobin Wang, Heteroatoms doped metal iron–polyvinylidene fluoride (PVDF) membrane for enhancing oxidation of organic contaminants, Journal of Hazardous Materialshttp://dx.doi.org/10.1016/j.jhazmat.2017.05.026 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Heteroatoms Doped Metal Iron–Polyvinylidene Fluoride (PVDF) Membrane for Enhancing Oxidation of Organic Contaminants Yunjin Yao a,b,*, Chao Lian a, Yi Hu a, Jie Zhang a, Mengxue Gao a, Yu Zhang a, Shaobin Wang c,* a
Anhui Key Lab of Controllable Chemical Reaction & Material Chemical Engineering, School of Chemistry and Chemical Engineering, Hefei University of Technology, Tunxi Road 193, Hefei 230009, China b School of Chemistry and Material Science, University of Science and Technology of China, Hefei 230026, P. R. China c Department of Chemical Engineering, Curtin University, G.P.O. Box U1987, Perth, Western Australia 6845, Australia AUTHOR INFORMATION * To whom correspondence should be addressed: Name: Yunjin Yao; Phone: +86 551 62901548; Fax: +86 551 62901450; E-mail:[email protected]. Name: Shaobin Wang; Phone: +61 8 9266 3776; Fax: +61 8 9266 2681; E-mail: [email protected].
1
Highlights
▸NSC-Fe-X@PVDF catalytic membranes were fabricated by phase inversion technique. ▸Hierarchical structures are formed by anchoring NSC-Fe-X NPs on PVDF membranes. ▸Catalytic performance of NSC-Fe-X@PVDF was affected by several key parameters. ▸Sulfate and hydroxyl radicals are responsible for this persulfate-driven oxidation. ▸Morphological and structural features of membrane enhance catalytic activity.
ABSTRACT: Iron nanoparticles (NPs) embedded in S, N-codoped carbon was prepared by one-step pyrolysis of a homogeneous mixture consisting of Fe, S, N, C precursors, and then immobilized in poly (vinylidene fluoride) membranes as a multifunctional catalytic system (NSC-Fe@PVDF) to effectively activate peroxymonosulfate (PMS) and oxidize organic compounds in water. The NSC-Fe@PVDF membranes effectively decolorized organic pollutants at a wide pH range (2.05-10.85), due to the synergistic effects between the S, N-doped carbon and iron NPs. The efficiency depended on the doping type, amount of metal, PMS dosages, reaction temperature, solution pH, and organic substrates. In-situ electron spin resonance (ESR) spectroscopy and sacrificial-reagent incorporated catalysis indicate radical intermediates such as sulfate and hydroxyl radicals are mainly responsible for this persulfate-driven oxidation of organic compounds. Membrane’s porous structure and high internal surface area not only minimize the NPs agglomeration, but also allow the facile transport of catalytic reactants to the active surface of metal catalysts. The results demonstrate the morphological and structural features of a catalytic membrane enhance the overall catalytic activity. Keywords: PVDF membrane; Organic pollutant; Catalytic degradation; Sulfate radical.
1. Introduction Increasingly serious organic chemical pollution is one of the most pervasive problems afflicting human health throughout the world. Persulfate-based advanced oxidation processes have ever been recognized as a viable, alternative oxidation process for realizing environmental remediation [1-3]. Generally, the relatively stable peroxides such as peroxymonosulfate ( HSO5 , PMS) and peroxydisulfate ( S2O28 , PDS) are
activated
on-site
to
generate
reactive
species
such
as
sulfate
radical
0 2 0 (SO 4 , E (SO4 /SO4 ))=2.43 VNHE ) and hydroxyl radical ( OH, E ( OH / H2 O) 2.81 VNHE ) [1, 4]. By
various strategies through energy or electron transfer reactions, peroxides have effectively produced reactive species. For instance, thermolysis, UV photolysis, and transition metal mediated activations initiate intramolecular electron transfer and the associated homolytic cleavage of peroxide bonds within 2
the persulfate molecules [5]. Comparing with PDS, PMS is easily activated by transition metals due to the asymmetric molecular structure, which is considered as the most simple and sustainable method without requiring external energy [6-8]. Consequently, the development of efficient catalysts for PMS is the key objective in the chemical research [9]. To date, various types of catalysts with highly efficient intrinsic active sites, including heteroatomdoped carbon materials, transition-metal-coordinating macrocyclic compounds, metal/nitrogen/carbon (M-Nx/C, M=Fe, Co, Ni) compounds and chalcogenides supported on carbon materials [10, 11], have been explored. For example, we previously fabricated novel M-Nx/C as an efficient Fenton-like catalyst for removal of organic pollutants [6, 7]. However, most of the researchers ignored the separation of the utilized nanomaterials from the treated water, which created a secondary pollution problem [12, 13]. Besides, many of those catalysts aggregate easily, reducing their activity, and thus are undesirable for industrial applications. One of strategies to address this problem is to assemble these nanoparticles (NPs) on substrates or membranes. Huang et al. [14] recently reported a catalytic membrane reactor, which contains a membrane matrix and a catalytic film of alloy NP-loaded β-lactoglobulin fibrils for the reduction of 4-nitrophenol. In these regards, polymer substrates play an important role in immobilizing catalyst because of the good film formation, toughness, and separation, etc. In this study, we attempt to develop an economical and effective NP-loaded catalytic membrane to achieve the removal of contaminants. Within a range of materials, poly (vinylidene fluoride) (PVDF) membranes have attracted much attention in wastewater purification, due to its excellent thermal stability, chemical resistance, and high mechanical strength [15, 16]. The previous researches on NPs incorporation into the membrane structure are mainly improved physical properties, such as the hydrophilicity, fouling resistance or antibacterial performance [17]. It is likewise of interest for designing catalytic PVDF membranes for the removal of organic pollutants. For instance, Wang et al. [18] found that Pd/PVDF membrane displayed excellent performance for the reduction of p-nitrophenol to p-aminophenol. Alpatova et al. [19] developed PVDF membranes with inclusion of carbon nanotubes and Fe2O3 NPs for removal of organic contaminants by 3
H2O2. Besides, metal NPs can be long used and are very stable [18]. However, the pores of such porous membranes are usually so small that reactants cannot rapidly diffuse through the interface to internal NPs for catalytic reactions. In addition, most procedures to prepare such catalytic membranes with an excellent activity are time-consuming, complicated, and need special instruments. In this work, we report a novel PVDF membrane decorated with heteroatom-doped iron NPs in membrane pores for the first time and demonstrate their high performance for water treatment. Morphological and structural characterizations were performed to confirm the successful formation of NPs on the PVDF membrane using several microscopic and spectroscopic techniques. The influences of several critical factors, such as initial PMS dosages, reaction temperature, dye types, initial pH, and inorganic anions on oxidation of organic compounds were further investigated in detail. In-situ electron spin resonance (ESR) spectroscopy and sacrificial-reagent incorporated catalysis were employed to address the primary activation mechanism. 2. Materials and Methods 2.1 Preparation of iron NPs embedded in S, N-codoped carbon The method for the synthesis of iron NPs embedded in S, N-codoped carbon catalysts was developed by our group and described previously [6, 20]. In a typical synthesis, dicyandiamide (5 g) and thiourea (2 g) as C, S, N precursors were firstly dissolved in 200 mL of MeOH solution at 50 °C. Secondly, Fe(NO3)3·9H2O with the designed loadings (1.0, 1.5, 2.0, and 2.5 g) as an iron precursor was slowly added to the above mixture via stirring for 60 min. Then, the solvent was removed using an evaporator operated at 75 °C. The prepared gel-like mixture was dried at 70 °C overnight. After grinding, the powder was then pyrolyzed at 700 °C for 2 h under 0.2 mL/min of N2 atmosphere with a heating rate of 5 °C /min. The pyrolyzed composites were filtered, washed with deionized (DI) water, and then dried in an oven. The asprepared catalysts were denoted as NSC-Fe-X (where X represents the initial mass of iron salts). The Sdoped and N-doped carbons were synthesized using the similar preparation method to NSC-Fe-X, but with the addition of S or N precursors. 4
2.2. Preparation of NSC-Fe-X@PVDF Membranes. NSC-Fe-X blended PVDF catalytic membranes were fabricated by the phase inversion technique [21]. Briefly, the synthesized NSC@Fe-X composites (0.05 g), polyvinylpyrrolidone (PVP, 0.25 g) and PVDF (0.5 g) were dissolved to a N, N-dimethylformamide (DMF, 5 mL) solution under vigorous agitation for 10 h at 60 °C. Afterwards, the homogeneous solution was transferred into a drying oven for 12 h to remove air bubbles. After degassing, the solutions were poured into a Petri dish (24 cm2) with a controlled casting rate, and then immersed in a coagulation bath (ethanol aqueous solution at 25 °C). The generated membranes labeled as NSC-Fe-X@PVDF were rinsed to remove the residual solvent, and kept in DI water before further use. Meanwhile, Fe2O3, NC-Fe-2.0, and SC-Fe-2.0 blended PVDF membranes (denoted as Fe2O3@PVDF, NC-Fe-2.0@PVDF, and SC-Fe-2.0@PVDF) were also synthesized in the same way. The synthetic procedure of NSC-Fe-X@PVDF membrane is illustrated in Figure 1. [Figure 1]. 2.3 Characterization Powder X-ray diffraction (XRD) patterns were obtained with a Philips X’Pert Pro MPD diffractometer (40 kV, 40 mA; Cu Kα radiation). Fourier transform infrared (FT-IR) spectroscopy with an attenuated total reflectance (ATR) accessory (Vector 22, Bruker) was used to characterize the fabricated membranes. The morphologies of the samples were examined using a JSM-6700F Field Emission Scanning Electron Microscope (FE-SEM, SU8020, Hitachi, Japan) with an accelerating voltage of 5 kV. Field-emission transmission electron microscopy (FE-TEM) images were collected by a JEOL JEM-2100 instrument at an accelerating voltage of 200 kV. Energy dispersive X-ray spectra (EDS) and the elemental mapping were collected from three randomly selected areas of each sample. X-ray photoelectron spectroscopy (XPS) measurements were performed on an ESCALAB250 spectrometer with a focused monochromatic Al Kα radiation at 12 kV and 20 mA. The surface roughness was measured by the atomic force microscopy (AFM, Dimension Icon, Bruker). 2.4 Catalytic Activity Test 5
Decolorization of various target organic compounds was conducted in 250 mL glass bottles in water bath under magnetic stirring. The catalytic membrane was firstly added to the bottles, followed by PMS with an aliquot stock solution to initiate the reaction. Sample aliquots were withdrawn at predetermined time intervals, filtered using a 5 mL glass syringe and a 0.25 μm nylon syringe filter, and injected into a 2 mL amber glass vial containing excess MeOH (0.5 M) to quench any residual radicals. Select experiments were performed to evaluate pH effect, and the initial pH of the suspensions was adjusted using 0.1 M H2SO4 or NaOH solution. The influences of several critical factors (including PMS dosage, reaction temperature, dye types, and inorganic anions) on oxidation of organic compounds were also investigated. To ascertain whether decolorization of organic compounds was caused by reactive oxygen species (ROS), MeOH and tert-butyl alcohol (TBA) were employed to distinguish OH and SO 4 radicals. The catalytic durability of NSC-Fe-X@PVDF was tested for 4 cycles, in which the catalysts were recycled by filtration and washing with ultrapure water after each cycle. The concentrations of organic compounds were determined by a UV-vis spectrophotometer. The total iron leaching concentrations was determined by atomic absorption spectroscopy (AAS, Perkin-Elmer AA800). The total amount of dissolved carbon (TOC) was determined by a TOC analyzer (Vario TOC select, Elementar). Electron spin resonance (ESR) spectrum was recorded to further verify reactive oxygen species using 5, 5dimethyl-1-pyrroline-N-oxide (DMPO) as the spin trapping agent. The chemical solutions of Orange II, catalyst and PMS were mixed, taken, and immediately injected into DMPO. The obtained solution was separated, transferred, and then inserted into the cavity of the equipment. The settings of the ESR instrument (JES-FA200, electron spin resonance spectrometer system, JEOL, Japan) were as follows: center field, 324.3 mT; sweep width, 10.0 mT; microwave frequency, 9.072 GHz; microwave attenuator, 30 dB; microwave power, 2.0 mW; temperature, 25 °C; sweep time, 30 seconds.
3. Results and Discussion 3.1 Catalyst Characterization Figure 2a represents XRD patterns of pure PVDF, NC-Fe-2.0@PVDF, SC-Fe-2.0@PVDF, and NSCFe-2.0@PVDF membranes. For PVDF, the broad reflections at about 26.4° was observed, which could be attributed to the polar β-phase of amorphous PVDF substances [22]. The diffraction peaks at 30.1, 35.4, 48.2, 56.9, and 65.2° corresponded to Fe3O4 (JCPDS No. 89-0691), whereas the peaks at 44.7 and 65.2° can be assigned to the metallic Fe0 (JCPDS No.06-0696). This suggests that the transformed metal oxides were in situ reduced to Fe0 by carbon [23, 24]. NSC-Fe-X@PVDF prepared at different 6
Fe(NO3)3·9H2O dosages (X=1.0, 1.5, 2.0 g) also showed Fe3O4, Fe0 and PVDF phase (Figure 2b). At X = 2.5 g, the peaks of Fe0 are not observed in the composites. At high dosages of Fe(NO3)3·9H2O, the carbothermal reduction reaction is not complete, without the formation of the highly-crystallized Fe0 NPs [25]. The Fe(NO3)3·9H2O dosage is the key factor affecting the structure of materials. The chemical structure was also detected by FT-IR (Figure 2c, d). The peak at 1070 cm-1 was attributed to the C-F bond stretching vibration mode. The strong absorption band at 1280 cm-1 is the characteristic of CF2 found in PVDF. The peaks at 1403 and 1600 cm-1 are corresponding to -CH2 and C=C, respectively. The bands observed at 840 and 878 cm−1 indicate the presence of polar β phase of PVDF [19]. No trace of the nonpolar α-phase by the FT-IR band at 766 cm−1 is found in the composites, suggesting PVDF hybrid membranes crystallizes in the β-phase regardless of solvent evaporation rates [26]. The FT-IR spectra certified that hybrid membranes were composed of PVDF and iron oxide phase. As will be shown later, catalytic decolorization indicated that the NSC-Fe-2.0@PVDF membrane had the greatest reactivity, hence, this membrane was further analyzed by FE-SEM, FE-TEM, and XPS. [Figure 2]. The microstructures of the NSC-Fe-2.0 NPs were examined by TEM and SEM measurements. Figure 3a, j indicates irregular-shaped crystallites with slight agglomeration. High-resolution TEM image suggests that all iron NPs are embedded in the carbon matrix (Figure 3b, c). The enlarged TEM image (Figure 3b) of the region marked with a rectangle shows the lattice spacings of 0.25 and 0.21 nm were consistent with the (331) and (440) crystalline planes of Fe3O4, respectively. The clear lattice fringes with an interplanar distance of 0.20 nm can be ascribed to the (110) planes of Fe0 (Figure 3c). In addition, the d-spacing was d002=0.34 nm, corresponding to the characteristic of graphitic carbon. EDS analysis clearly shows the compositions of NSC-Fe-2.0 comprising of C, S, N, O, and Fe elements (Figure S1). The element mapping also demonstrated that iron, nitrogen, sulfur species are distributed in the carbon matrix uniformly, indicating the nitrogen and sulfur atoms were successfully doped into the carbon lattice (Figure 3d-i).
7
The successful codoping of carbon with N and S atoms was further investigated using XPS. The C 1s XPS spectrum (Figure 3k) can be deconvoluted into three spectral components, C-C (284.8 eV), hydroxyl C-OH (285.8 eV), and carbonyl C=O (288.4 eV), respectively [27, 28]. The XPS spectra of N 1s (Figure 3l) were fitted with three individual peaks, corresponding to pyridine-like (398.4 eV), pyrrolic (399.4 eV), and graphitic (400.9 eV) N species, indicating that the N heteroatom has been doped into the carbon backbones [29]. Pyridinic N and graphitic N are generally regarded as the species responsible for the catalytic activity [30, 31]. In S 2p XPS spectra, the peak at 163.4 eV corresponds to C-Sn-C (n = 1 or 2) bonds, indicating efficient S-doping within the carbon network[29]. In addition, two peaks at 162.1 and 168.7 eV correspond to the reduced (–SH) sulfur moieties and the oxidized S (-SOn-), respectively (Figure 3n) [32]. Furthermore, Figure 3p shows the high-resolution scan of Fe 2p electrons, where deconvolution yielded two pairs of peaks for Fe2+ (710.7 and 724.2 eV) and Fe3+(712.6 and 726.1 eV), with a satellite peak at 720.0 eV [33]. Besides these, a characteristic peak at 707.4 eV was assigned to the metallic Fe0, confirming the regeneration of metallic Fe during the synthesis process, which is in accordance with XRD analyses (Figure 3m) [24]. [Figure 3]. NSC-Fe-2.0 NPs immobilized on a PVDF membrane is shown in Figure 4a. While the pure PVDF membrane sample is white in color, the NSC-Fe-2.0@PVDF membrane is uniformly black in color, indicating a uniform distribution of the NSC-Fe-2.0 NPs on the PVDF scaffold. Figure 4b, c presents top layer and cross-section pore structure of membranes casted by NSC-Fe-2.0 NPs. Visible NSC-Fe-2.0 aggregates appeared in the field of view of all membranes, which contained NSC-Fe-2.0 in their structure as well as within the pore openings. It enables facile transport and penetration of catalytic reactants to the active surface without suffering high mass-transfer resistance. During the membrane formation process, NSC-Fe-2.0 NPs acted as pore forming centers, precipitated more slowly, and thus remained presence in the surface layer. Incorporation of NPs is recognized to improve pore formation and increase membrane porosity. EDS mapping analysis revealed that iron was uniformly distributed on the membrane surface (Figure 4e, f). Through AFM analysis (Figure 4d), the average surface roughness (Ra) and root-mean8
square roughness (Rq) were determined to be 104 nm and 132 nm, respectively. High surface area and roughness could increase the efficient contact area, which led to the enhancement of catalytic performance. Totally, NSC-Fe-2.0 NPs could improve the microstructure and influence the hydrophilicity of PVDF membranes. [Figure 4]. 3.2 Catalytic evaluation The catalytic performance of the various membranes was evaluated for color removal of Orange II by activation of PMS (Figure 5a, c). The catalytic data were fit to pseudo-first-order kinetics and the obtained rate constants (kobs) are given in Figure 5b, d. Less than 2% of Orange II could be decolorized by PMS alone indicates that thermal activation of PMS without metal catalysts was negligible. A control test using only membranes shows that less than 6% of Orange II could be adsorbed in 150 min. Moreover, kinetic curves of NSC-Fe-X@PVDF prepared at different Fe(NO3)3·9H2O dosages (X=1.0, 1.5, 2.0, and 2.5 g) showed color removal was increased with the increased Fe(NO3)3·9H2O dosage up to 2.0 g and decreased thereafter. One main explanation for the negative effect was that the dosage of additives would also induce greater NPs aggregation and the total active surface area would decrease in membranes [34]. To demonstrate the roles of Fe, N, S, C sources in the Fenton-like reaction, PVDF, Fe-2.0@PVDF, NCFe-2.0@PVDF, SC-Fe-2.0@PVDF, and NSC-Fe-2.0@PVDF were prepared using the same method. The values of kobs were 2.26×10-3 min−1 for PVDF, 1.68×10-3 min−1 for Fe-2.0@PVDF, 8.72×10-3 min−1 for NC-Fe-2.0@PVDF, 0.011 min-1 for SC-Fe-2.0@PVDF, and 0.023 min-1 for NSC-Fe-2.0@PVDF. After incorporating Fe, N, C, S sources, an obvious increase in kobs was observed for NSC-Fe-2.0@PVDF, indicating the heteroatoms have a synergetic effect in enhancing catalytic performance. It was also evidenced by the photograph of reaction solution (Figure 5) and the absorption peaks in UV–vis absorption spectra of Orange II at 485 nm dramatically decrease (Figure S2). Meanwhile, the new peak at 255 nm increases, indicating that smaller molecules derived from Orange II are formed during the chemical process. In contrast, TOC values nearly unchanged, indicating that the mineralization of organic compounds is very limited. The reason is that intermediates derived from the organic dye are enriched in 9
a variety of small-chain organic acids, which is consistent with the previous results [35, 36]. The best catalytic performances of NSC-Fe-2.0@PVDF membrane was selected as a typical NSC-Fe@PVDF for the following experiments. 3.3. Stability and reusability of NSC-Fe-2.0@PVDF membrane The stability and reusability of the multifunctional membrane are crucial to the actual application that must be considered. The recycling capability of NSC-Fe-2.0@PVDF sample was successfully evaluated by color removal under the same experimental condition (Figure 5e). The membrane still showed comparatively good catalytic activity after four consecutive runs, which proved to be a potent reusable catalyst. No significant leaching of iron ions (0.037 mg/L) was detected by AAS, suggesting Fe NPs are stably embedded in the carbon structure. In addition, NSC-Fe-2.0@PVDF membrane still remained in the crystalline form after the reaction (Figure S3), suggesting the heterogeneous Fenton-like reaction, even under acidic conditions. We did not observe remarkable changes in the FT-IR spectra of the fresh and used NSC-Fe-2.0@PVDF membrane (Figure 5e), suggesting a negligible adsorption of these species on the surface of the catalyst and/or complete color removal. In the practical application, nanocatalysts need not to be separated and recovered from reaction solution. Therefore, the catalytic membrane used for the oxidative decolorization of various organics is convenient, environmentally friendly, and economical. All in all, nanocatalysts immobilized in PVDF membrane suggested a few interesting features that prevent the loss and improve the usability of the catalysts [18]. [Figure 5]. 3.4. Effects of reaction parameters on organic decolorization in NSC-Fe-2.0@PVDF/PMS system The optimal sample of NSC-Fe-2.0@PVDF was selected to study the influences of several critical factors, such as initial PMS dosage, reaction temperature, different dyes, initial pH, and inorganic anions on oxidation of organic compounds. An increase in PMS dosage (0.08-0.49 mM) had an obvious positive effect on color removal, ascribing to the formation of more amount of reactive species during the reaction (Figure S4a). When its dosage increased from 0.49 to 0.65 mM, no obvious improvement in decolorization performance occurred, due to self-scavenging during the oxidation reaction. Hence, optimized PMS 10
concentration was taken at 0.49 mM. Figure S4b shows the decolorization rate increases quickly with the increase of reaction temperature, which demonstrates that the increasing temperature has the benefit to the catalytic activity. The improvement of oxidation capacity is mainly due to that the elevated temperature would promote more energy for the reactant molecules to lower the barrier of activation energy [37]. The efficacy of PMS activated by NSC-Fe-2.0@PVDF to decolorize various organic compounds was further evaluated (Figure S4c). The use of catalytic membrane yielded a removal efficiency of 90.1% for Rhodamin B, 97.7% for Orange II, 85.7% for Congo red, 81.2% for Methylene blue, 93.9% for Neutral red, 97.8% for Methyl violet, and 96.4% for Methyl orange. These trends clearly reveal that molecular structures influence the removal rates [38], and NSC-Fe-2.0@PVDF is effective for decolorization of a wide range of organic compounds. Figure S4d presents the effect of initial pH on PMS activation efficiency in the NSC-Fe-2.0@PVDF /PMS process. An obvious pHo decrease after the reaction was observed, due to the formation of acid intermediates, suggesting that H+ was produced during the process [39]. Color removal was not significantly affected by the pH change in solution between 2.05 to 10.85. When the experimental suspensions were initially adjusted to an acidic or a basic pH value, the pH change in solution was marginal over the course of PMS activation. This pH independence is one notable advantage, compared to other peroxide-based oxidation processes such as Fe2+/H2O2 which is highly pH sensitive [1]. The variation in removal can be explained that pH changes the surface charge of the membrane, the electrostatic interaction between organic molecules and membrane, and the number of charged radical species generated during the oxidation process [40].
Furthermore, some coexisting anions such as HCO3 , Cl , SO24 , CH3COO , NO3 , NO2 , and HPO24 are generally present in water, and can interfere the removal process. As indicated in Figure S4e, The color removal rates decreased with the increase of HCO3 concentration up to 1.6 mM. In addition, it is observed that all the added inorganic anions have an inhibitive effect on the oxidation process (Figure S4 f). The reasons were that coexisting anions would compete with the organic substance for the adsorption sites, resulting in the blockage of the active sites on the membrane surface [41]. 3.5. Contribution of radicals in NSC-Fe-2.0@PVDF/PMS system The results above suggest that radical species is produced in the reaction between PMS and NSC-Fe 2.0@PVDF. Generally, SO 4 or OH is considered to be the radical species in oxidation processes. Alcohols with or without α-hydrogen have different reactivities with radical species. MeOH (with a-hydrogen) has a similar
11
9 −1 −1 6 −1 −1 reactivity to OH and SO 4 ( k OH : 1.2-2.8 × 10 M s , kSO : 1.6-7.8 × 10 M s ), while alcohols without α4
hydrogen, such as TBA, are known to react much slower with sulfate radicals, which has 1000-fold higher rate 5 −1 −1 constant with OH ( k OH : 3.8-7.6 × 108 M−1s−1) than with SO 4 ( kSO : 4-9.1 × 10 M s ) [42]. Therefore, 4
MeOH was used to scavenge both radicals, and TBA was used to selectively quench OH [43]. The effects of MeOH and TBA on NSC-Fe-2.0@PVDF/PMS oxidation showed that TBA had a much higher inhibition effect on the color removal rate compared with MeOH (Figure 6a, b). For instance, Color removal decreased by ∼49.4% in the presence of 3 M TBA in comparison with the control experiment, in which no scavenger was present, whereas only around 19.9% inhibition was observed in the presence of the same amount of MeOH. This result may indicate that a significant role in the generation of SO 4 was played by OH . By scavenging OH , the added TBA affects the yield of SO 4 (eq 1).
2HSO5 +2 OH SO 4 +2H 2O+O2
(1)
Furthermore, the addition of 6 M MeOH only scavenged about 10% of color removal, indicating alcohols fail
to capture surface-adsorbed radicals completely [44]. Note, SO4 radicals adsorbed on the surface of catalysts are beneficial to accelerate the organics oxidation. In addition, both alcohols are oxidized directly or via OH and other reactive oxygen species, which will limit their reactivity with SO 4 and OH radicals. Therefore, the addition of specific alcohol quenchers could not elucidate between sulfate and hydroxyl radical contributions to the oxidation of Orange II. Nevertheless, the observed decrease in the presence of alcohol quenchers indicates a significant contribution of the radical species. To provide direct evidence regarding the free radical species that might be involved in the NSC-Fe2.0@PVDF/PMS process, the ESR technique was used. As illustrated in Figure 6c, the ESR signals of DMPO- HO (four lines, 1:2:2:1) and DMPO- SO 4 (six lines, 1:1:1:1:1:1) from their hyperfine splitting constants ((DMPO
OH : H = N = 14.7 G; DMPO− SO 4 : N = 13.3 G, H = 9.5 G, H = 1.46 G, and H = 0.77 G) were detected
in our reaction systems [45]. The results further confirm that Orange II oxidation was induced by SO 4 and
OH reactive species generated in NSC-Fe-2.0@PVDF/PMS process. The peak intensities of DMPO- SO 4 and
DMPO− OH first increased and then decreased rapidly (Figure 6d). The drop of HO and SO 4 amount was due to the consumption by Orange II oxidation. Moreover, the intensity of DMPO- SO 4 adducts signals was much weaker than that of DMPO- OH adducts signals, which could be interpreted with the fast transformation from DMPO- SO 4 adducts to DMPO- OH adducts [46].
[Figure 6]. 3.6. Reaction Mechanism According to all above-mentioned results and discussions, a mechanism is proposed and shown in Figure 9. Firstly, organic molecules and PMS in the solution were transferred and adsorbed on NSC-Fe-2.0 NPs loaded on the porous PVDF membranes. Secondly, the surface adsorbed PMS can achieve persulfate driven oxidation by NSC-Fe-2.0 NPs to produce ROS, interacted with the adsorbed organic molecules for in-situ reaction, and then released the preoccupied sites. Thirdly, the released free sites on NSC-Fe-2.0@PVDF accelerated aqueous reactants transfer, and then an adsorption-decolorization cycle occurred again. In this way, the abovementioned cycles underwent continuously until all organic compounds in water were removed.
In the present system, a series of oxidation reactions occur via surface catalysis. In the initial step, Fe0 is oxidized to FeII via a two-electron transfer. The surface FeII then reacts with PMS to create surface12
II III bound SO can be achieved because of the 4 radicals. Simultaneously, the regeneration of Fe from Fe
high potential difference (1.21 V) between Fe2+/Fe0 and Fe3+/Fe2+. The deposition of carbon materials serves as excellent electron transfer supports, and can also activate PMS and oxidize organic compounds via non-radical mechanisms [47-49]. Therefore, given the roles of the Fe2+/Fe0 and Fe3+/Fe2+ pairs, electron transfer is enhanced over the metallic surface, eventually enhancing the removal efficiency. As previously described, Fe NPs embedded in carbon could lower the local work function of the carbon surface because of the facile electron transfer from the iron to the carbon [6, 7]. The donor-acceptor complexes at surface sites were promoted by the embedded Fe NPs and the S, N dopants would improve the interfacial electron transfer. In addition, the carbon shell builds up a reservoir that effectively retards the dissolving of iron into solution. Therefore, owing to the synergistic effects between S, N-doped carbon and iron NPs, NSC-Fe-2.0@PVDF catalyst intrinsically displays an enhanced catalytic performance. The exhibited good catalytic property of NSC-Fe-2.0@PVDF membranes may be attributed to two aspects as follows: First, the small nanocatalysts usually lead to an increase of the catalytic efficiencies because of the high surface energy and great surface area-to-volume ratio; Hence, more active atoms on the membrane surface are expected to be available for the catalysis. Second, the presence of porous and interconnected skeleton structure of NSC-Fe-2.0@PVDF membranes enables facile transport and penetration of catalytic reactants to the active surface without suffering high mass-transfer resistance. Therefore, both the nanosize effect of metal NPs and a relatively low diffusion resistance of organics on the surface of catalytic membrane were beneficial for the sufficient contact of organics with the active sites and facilitates the good catalytic performance of membranes [50]. [Figure 7].
4. Conclusions In summary, we demonstrated novel PVDF membrane decorated with iron NPs embedded in S, Ncodoped carbon in membrane pores to effectively activate PMS and oxidize organic compounds in water. Hierarchical nanostructures are formed by uniformly anchoring NSC-Fe-X NPs on the PVDF membranes by the phase inversion technique. The synergistic effects between the S, N-doped carbon and iron NPs 13
improve the oxidative ability of the optimized NSC-Fe-2.0@PVDF materials. The catalytic efficiency depended on several reaction conditions including the doping type, amount of metal, PMS dosages, reaction temperature, solution pH, and organic substrates. ESR spectroscopy and sacrificial-reagent incorporated catalysis indicate SO and OH radicals are mainly responsible for this persulfate-driven 4 oxidation process. Comparative analysis demonstrates the NSC-Fe-X@PVDF membranes have clear advantages over NSC-Fe-X NPs because of their flexible and macroporous architecture. These results indicate that the NSC-Fe-2.0@PVDF membranes are promising for pollution control, and might be expected to contribute to other important areas of environmental application. The development of this type of catalytic membrane for continuous-flow reactions and separation will be evaluated in the future.
ACKNOWLEDGEMENTS The financial supports by the Anhui Provincial Natural Science Foundation (NO. 1708085MB41), the China Postdoctoral Science Foundation (NO. 2015M570547, 2016T90585), National Natural Science Foundation of China (Grant 51372062), and Undergraduate Training Programs for Innovation and Entrepreneurship (NO. 2017CXCY049) are acknowledged. The partial support from the Australian Research Council for DP 150103026 is also acknowledged. Special thank is given to Prof. T. Xu for the useful discussion in the initial preparation of this manuscript.
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Figure;1; Captions Figure 1. Synthetic procedure of the NSC-Fe-X@PVDF membranes. Figure 2. (a, b) XRD and (c, d) FT-IR of pure PVDF, NC-Fe-2.0@PVDF, SC-Fe-2.0@PVDF, and NSCFe-X@PVDF membranes prepared at different Fe(NO3)3·9H2O dosages (X=1.0, 1.5, 2.0 g). Figure 3. Morphology and compositions of NSC-Fe-2.0 NPs. (a) Representative TEM image. (b-c) highresolution TEM images. (d) TEM image and its corresponding elements mapping images: (e) Fe, (f) O, (g) N, (h) C, and (i) S element. (j) Representative SEM image. (m) XRD pattern. High-resolution scans of (k) C 1s, (l) N 1s, (n) S 2p and (p) Fe 2p electrons. Figure 4. Characterization of NSC-Fe-2.0@PVDF membrane: (a) digital photo; (b) SEM top view; (c) SEM cross-sectional view; (d) AFM 3D surface image;(e-f) EDS mapping from SEM top and crosssectional view. Figure 5. (a, c) Comparison of the catalytic activity of pure PVDF, NC-Fe-2.0@PVDF, SC-Fe2.0@PVDF, and NSC-Fe-X@PVDF membranes prepared at different Fe(NO3)3·9H2O dosages (X=1.0, 1.5, 2.0 g). (b, d) The rate constant in different reaction systems. (e) Recycle experiments for Orange II removal using NSC-Fe-2.0@PVDF sample. (f) Color changes of Orange II with the reaction times increasing. Unless otherwise stated, the reaction conditions are: [Orange II] = 20 mg/L, T =25 °C, [PMS] =0.60 g/L, without pH adjustment. Figure 6. Effects of TBA (a) and MeOH (b) scavengers on the Orange II removal by the NSC-Fe2.0@PVDF/PMS process. (c) DMPO-trapped ESR spectra. (d) Comparison of ESR signal intensity of DMPO- OH and DMPO- SO 4 adducts; Unless otherwise stated, the reaction conditions are: [Orange II] = 20 mg/L, T =25 °C, [PMS] =0.60 g/L, without pH adjustment. Figure 7. Schematic illustration of the reaction mechanism for organics removal by the NSC-Fe2.0@PVDF/PMS system.
21
Figure 1. Synthetic procedure of the NSC-Fe-X@PVDF membranes.
22
(a)
(b)
(c)
(d)
Figure 2. (a, b) XRD and (c, d) FT-IR of pure PVDF, NC-Fe-2.0@PVDF, SC-Fe-2.0@PVDF, and NSC-Fe-X@PVDF membranes prepared at different Fe(NO3)3·9H2O dosages (X=1.0, 1.5, 2.0 g).
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(a)
(b)
Fe3O4
3.4 Å
(c)
Carbon shell
Carbon shell
2.1 Å
2.5 Å Fe
Fe3O4
2.0Å
Fe0 (d)
(e)
(f)
(j)
(k)
(m)
(n)
(h)
(g)
(i)
(l)
(p)
Figure 3. Morphology and compositions of NSC-Fe-2.0 NPs. (a) Representative TEM image. (b-c) high-resolution TEM images. (d) TEM image and its corresponding elements mapping images: (e) Fe, (f) O, (g) N, (h) C, and (i) S element. (j) Representative SEM image. (m) XRD pattern. XPS spectra of (k) C 1s, (l) N 1s, (n) S 2p and (p) Fe 2p electrons. 24
(a) (c)
(b)
PVDF membrane
(d)
NSC-Fe-2.0@PVDF membrane
(e)
Fe Ka1
(f)
Fe Ka1
Figure 4. Characterization of NSC-Fe-2.0@PVDF membrane: (a) digital photo; (b) SEM top view; (c) SEM cross-sectional view; (d) AFM 3D surface image;(e-f) EDS mapping from SEM top and crosssectional view.
25
(a)
(c)
(e)
(b)
(d)
(f)
Figure 5. (a, c) Comparison of the catalytic activity of pure PVDF, NC-Fe-2.0@PVDF, SC-Fe2.0@PVDF, and NSC-Fe-X@PVDF membranes prepared at different Fe(NO3)3·9H2O dosages (X=1.0, 1.5, 2.0 g). (b, d) the corresponding apparent degradation rate constants. (e) Recycle experiments for Orange II removal using NSC-Fe-2.0@PVDF sample. (f) Color changes of Orange II with the reaction
26
times increasing. Unless otherwise stated, the reaction conditions are: [Orange II] = 20 mg/L, T =25 °C, [PMS] =0.60 g/L, without pH adjustment. (a)
(c)
(b)
(d)
Figure 6. Effects of TBA (a) and MeOH (b) scavengers on the Orange II removal by the NSC-Fe2.0@PVDF/PMS process. (c) DMPO-trapped ESR spectra at different reaction time. (d) Comparison of ESR signal intensity of DMPO- OH and DMPO- SO adducts. Unless otherwise stated, the reaction 4 conditions are: [Orange II] = 20 mg/L, T =25 °C, [PMS] =0.60 g/L, without pH adjustment.
27
Figure 7. Schematic illustration of the reaction mechanism for organics removal by the NSC-Fe-2.0@PVDF/PMS system.
28