manganese acetate composite nanofibers and their catalysis performance on chromium (VI) reduction by oxalic acid

Journal of Hazardous Materials 229–230 (2012) 439–445

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Polyacrylonitrile/manganese acetate composite nanofibers and their catalysis performance on chromium (VI) reduction by oxalic acid Chengcheng Zhang, Xiang Li ∗ , Xiujie Bian, Tian Zheng, Ce Wang ∗ Jilin University Alan G MacDiarmid Institute, Changchun 130012, China

h i g h l i g h t s  We have successfully prepared PAN/Mn(CH3 COO)2 composite nanofibers.  The nanofibers exhibit excellent catalysis performance for Cr(VI) reduction.  The nanofibers are effective and environment-friendly materials to remove Cr(VI).

a r t i c l e

i n f o

Article history: Received 26 November 2011 Received in revised form 23 May 2012 Accepted 24 May 2012 Available online 1 June 2012 Keywords: Cr(VI) reduction Oxalic acid Electrospinning Photocatalytic activities

a b s t r a c t Polyacrylonitrile(PAN)/manganese acetate(Mn(CH3 COO)2 ) composite nanofibers have been fabricated by electrospinning, a simple and effective technology. The obtained composite nanofibers were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and Fourier transform infrared spectrometer (FT-IR). The composite nanofibers are amorphous in structure, continuous, even and smooth. At the same time, the reduction performance of Cr(VI) by oxalic acid in the presence of the composite nanofibers is also investigated. The results indicate that the composite nanofibers have exhibited excellent catalysis performance for Cr(VI) reduction from a Cr2 O7 2− -containing solution by oxalic acid. And the critical parameters, such as the catalyst dosage, oxalic acid content, chromium concentration, the pH value of the reaction solution and light have important impact on the reduction process. Under the simulated solar light irradiation, after only 60 min, 1.2 mM initial Cr(VI) solution was reduced absolutely in the presence of PAN/Mn(CH3 COO)2 composite nanofibers containing 17.5 wt.% Mn(CH3 COO)2 by 0.3 mL 0.5 M oxalic acid. In light, the reduction of Cr(VI) by oxalic acid is markedly accelerated. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Chromium (Cr) and chromium compounds are widely used in many industrial processes such as printing inks, leather tanning, textile dyeing, chromium plating, electroplating, paints and pigments as critical industry materials [1,2]. However, with the development of industries, chromium becomes one of the most hazardous heavy metal pollutants in industrial waste water due to improper disposal of wastes and accidental releases. Usually, Cr exists mainly in hexavalent [Cr(VI)] and trivalent [Cr(III)] forms in the natural environment [2,3]. Cr(VI), which is highly soluble and mobile in aquatic systems, is not only highly toxic but also carcinogenic to humans, animals and plants. In contrast, Cr(III) is less toxic and normally precipitates as Cr(OH)3 or Fex Crl−x (OH)3 in soil and water [4], and easily adsorbs on the mineral surface. Therefore, the key strategy to remove chromium pollution is to transform

∗ Corresponding authors. Tel.: +86 431 85168292; fax: +86 431 85168292. E-mail addresses: [email protected] (X. Li), [email protected] (C. Wang). 0304-3894/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhazmat.2012.05.085

the pollutants from Cr(VI) to Cr(III) based on Cr(VI) reduction. As a result, considerable attention has been focused on looking for effective reduction methods for the treatment of chromium pollution. Thus, many reductants such as polyaniline [2], zero-valent iron [5], divalent iron [6] and sulfide [7] have been utilized. Soil organic matter can effectively reduce Cr(VI) to Cr(III). The reduction of Cr(VI) by various organic matters (citric acid [8], salicylic acid [9], tartaric acid [3] and humic acid [10]) has been the investigation subject of a great many researchers. However, the reaction between Cr(VI) and soluble soil organic substances is slow. Dissolved and surface-bound metal ions could effectively catalyze organic matters to reduce Cr(VI) to Cr(III). Li et al. [4] found that the externally added Mn(II) strongly catalyzed the reduction of Cr(VI) by citrate acid, with the higher initial Mn(II) concentration having higher Cr(VI) reduction rate. Lan et al. pointed out that the role of clay minerals in accelerating the reduction reaction of Cr(VI) by citric acid directly correlated with the amount of adsorbed Mn(II) ions on the surfaces. And the reduction of Cr(VI) by citric acid is increased markedly in illumination in the presence of Fe(III) [11]. However, in the Cr(VI) reduction process, great quantities of

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metal ions are consumed. Besides, these metal ions also generate secondary waste. Therefore, exploiting an effective, environmentfriendly and economical material to remove Cr(VI) fast for practical application is very necessary. Electrospinning technology is known to be a simple and versatile method to generate nano- to sub-micrometre fibers [12]. The electrospinning technique was first developed for the synthesis of nanofibers since 1934 [13]. It is a process which uses a strong electrostatic force by a high static voltage applied to a polymer solution placed into a container that has a millimetre diameter nozzle. Under the applied electrical force, the polymer solution is ejected from the nozzle. After the solvents evaporate during the course of jet spraying, the nanofibers are collected on a grounded collector [14]. The obtained polymer composite nanofiber films have good orientation, large specific surface area, large aspect ratio, and dimensional stability, which are widely applied in wastewater treatment, such as heavy metal ion or dye adsorption [15,16] and Cr(VI) reduction [17]. It is known that some functional groups ( C O, CN, OC N, etc.) of organic compounds can interact with metal ions. Especially, the nitrile group of polyacrylonitrile (PAN), a common, cheap and nontoxic polymer material used for electrospinning, can form a weak complex with cations, or CH2 CH(CN)− · · ·M(M = Zn2+ , Fe3+ , Fe2+ , Li+ , Na+ , K+ ) [17–19]. For instance, Lin et al. has fabricated polyacrylonitrile/ferrous chloride composite porous nanofibers and found that the composite nanofibers exhibited excellent performance for Cr-removal from a Cr2 O7 2− -containing solution [17]. In the present paper, we prepared PAN/Mn(CH3 COO)2 composite nanofibers and demonstrated that the composite nanofiber membrane can catalyze the reduction of Cr(VI) into Cr(III) rapidly and effectively from a Cr2 O7 2− -containing solution by oxalic acid. The Cr-reduction capability is about 187.2 mg Cr/g nanofibers, which is much higher than the previously reported value for other polyacrylonitrilebased materials as PAN/FeCl2 porous nanofibers that give a removal capacity of 11.7 mg Cr/g nanofibers [17], and it is also easier to separate from the solution.

2. Experimental 2.1. Materials Potassium dichromate (K2 Cr2 O7 ) and oxalic acid were purchased from Beijing Chemical Factory. N,N-dimethylformamide (DMF) was supplied by Tianjin Tiantai Chemical Factory. Manganese acetate tetrahydrate [Mn(CH3 COO)2 ·4H2 O] was purchased from Tianjin Guangfu Chemical Factory. All the above chemicals were analytical grade and used as received without further purification. Polyacrylonitrile (PAN, Mw ≈ 80,000) was purchased from Jilin Carbon Group.

2.2. Preparation of PAN/Mn(CH3 COO)2 composite nanofibers films In a typical procedure, 10 g DMF solution of PAN (9 wt.%) containing 0.27 g Mn(CH3 COO)2 ·4H2 O was delivered to a plastic syringe (inner diameter: about 1 mm). As a high voltage of 15 kV was applied, the solution jet accelerated towards the cathode, which was placed 15 cm from the needle tip, leading to the formation of nanofibers arrays containing 17.5 wt.% Mn(CH3 COO)2 onto the substrate accompanied by partial solvent evaporation. Similarly, pure PAN nanofibers without Mn(CH3 COO)2 and PAN/Mn(CH3 COO)2 composite nanofibers containing 22.0 wt.% Mn(CH3 COO)2 were also prepared under the same conditions.

2.3. Characterization The morphology of the composite nanofibers was observed by scanning electron microscopy (SEM, SHIMADZU SSX-550). The microstructure of the samples was characterized by X-ray diffraction (XRD) equipment (Siemens D5005 diffractometer using Cu K␣ radiation) in the scan range 2 between 5◦ and 70◦ . IR spectra were obtained on a Fourier transform infrared spectrometer (FT-IR, BRUKER VECTOR 22). The concentration of Cr(VI) was analyzed by using a UV-2501 PC Spectrometer. The amounts of Mn(II) ion and the total Cr were determined using an inductive coupled plasma emission spectrometer (ICP, PerkinElmer OPTIMA 3300DV). The pH values of the reaction solutions were measured by using a pH meter (Orion 410A+, Thermo). 2.4. Cr(VI) reduction measurements The aqueous solutions with different Cr(VI) concentrations were prepared by dissolving analytical grade potassium dichromate (K2 Cr2 O7 ) in deionized water. Oxalic acid solution was prepared by dissolving appropriate quantity of the solids in deionized water. 30 mL Cr(VI) solution of desired concentration was taken into the 50 mL flask and the pH value of solution was adjusted by 1 M HCl or 1 M NaOH to get the pH value at 3, 6, 9 and 11. The required content of oxalic acid with the concentration of 0.5 M was added. The addition of oxalic acid would reduce the solution pH. After addition of 0.3 mL 0.5 M oxalic acid in the 30 mL 1.2 mM Cr(VI) solution of pH 3, 6, 9 and 11, the solution pH was reduced down to 2.3, 2.5, 2.7, 3.7. After stirring for a moment, the zero time reading was obtained from the mixing solution. Fixed qualities of the composite nanofibers were added to the mixing solutions, and then aliquots of the reaction mixture were removed at definite time intervals. The progress of the reaction was followed at 349 nm by monitoring the changes in absorbance of K2 Cr2 O7 solution on a UV–vis spectrophotometer [20]. The reduction of Cr(VI) could be calculated by D = C/C0 = A/A0 , in which C0 and A0 are the initial concentration and absorbency of K2 Cr2 O7 before reaction, while C and A are the equilibrium concentration and absorbency of K2 Cr2 O7 at homologous time. To investigate the influence of the light on the Cr(VI) reduction reaction, we used a 500 W Xenon lamp (CHFXQ500W, 14200 LX) to provide the simulated solar light and visible light with a UV filter to isolate the UV light of wavelengths shorter than 420 nm. 3. Results and discussion 3.1. Morphology and structure The morphology of the electrospun nanofibers containing different Mn(CH3 COO)2 contents before and after catalytic reaction was examined using SEM. It was found that the Mn(CH3 COO)2 content had a marked influence on the morphology and fiber diameter of the nanofibers. From Fig. 1a, the nanofibers containing 17.5 wt.% Mn(CH3 COO)2 were randomly distributed on the substrate, continuous, uniform and even with diameters between 200 and 350 nm. By contrast, the diameters of the nanofibers containing 22.0 wt.% Mn(CH3 COO)2 were much larger, about 1.1–1.3 ␮m (Fig. 1b). As shown in Fig. 1c and d, after the catalytic reaction, a well-defined fiber texture kept unchanged for the nanofibers with different Mn(CH3 COO)2 contents. Furthermore, we also investigated the microstructure of the composite nanofibers containing 22.0 wt.% Mn(CH3 COO)2 in more detail. The XRD spectra of Mn(CH3 COO)2 ·4H2 O powders, pure PAN nanofibers and the composite nanofibers containing 22.0 wt.% Mn(CH3 COO)2 were shown in Fig. 2A. A crystalline peak (17◦ ) corresponding to the orthorhombic PAN (1 1 0) refection [21], a broad

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Fig. 1. SEM images of as-spun PAN/Mn(CH3 COO)2 composite nanofibers containing 17.5 wt.% (a) (c)and 22.0 wt.% Mn(CH3 COO)2 (b) (d)before (a) (b) and after (c) (d) the catalytic Cr(VI) reduction reaction.

noncrystalline peak (20–30◦ ) and two sharp peaks (9◦ and 13◦ ) of the substrate were detected from the spectrum of the pure PAN nanofibers (Fig. 2A(b)). And, it was found that the crystallizability of the composite nanofibers was greatly reduced in contrast to Mn(CH3 COO)2 ·4H2 O powders and pure PAN nanofibers with high crystalline structures which can be attributed to the rapid evaporation of solvent during electrospinning, thus blocking the crystallization of macromolecular chains [17]. The FT-IR spectra shown in Fig. 2B were measured to get information for the structural changes related to the pure PAN nanofibers and the PAN/Mn(CH3 COO)2 composite nanofibers before and after the catalytic Cr(VI) reduction reaction. The FT-IR spectrum (Fig. 2B(d)) of the pure PAN nanofibers exhibited characteristic bands due to the stretching vibration of nitrile (2246 cm−1 ), carbonyl (1734 cm−1 ), and ether (1260–1050 cm−1 ) groups [22]. For the composite nanofibers (Fig. 2B(e)), new absorbance peaks at 1449, 1578 and 644 cm−1 due to C O (sym) and C O (asym) of acetate anion combined with the vibration band of Mn O appeared [23]. Moreover, the addition of Mn(CH3 COO)2 into the nanofibers led to the red shift of the band at 1664 cm−1 for pure PAN nanofibers, which indicated a large amount of the weak coordination bonds, PAN. . .Mn(II), were formed in the composite nanofibers [18]. After the catalytic reaction, as shown in Fig. 2B(f) the bands at 1578 and 644 cm−1 became weaker, which demonstrated part of Mn(CH3 COO)2 consumed. 3.2. Catalytic performance of PAN/Mn(CH3 COO)2 composite nanofibers 3.2.1. High catalytic capacity The plots of experimentally derived from the values of Cr(VI) reduction versus time under different reaction conditions were described in Fig. 3. Direct reduction of Cr(VI) by oxalic acid, in the absence of the catalyst within 180 min, was minimal (Fig. 3a). In order to eliminate the effect of the adsorption on the catalytic reduction of Cr(VI) to Cr(III) by the PAN/Mn(CH3 COO)2

composite nanofibers, we also tested the reduction of Cr(VI) without oxalic acid in the presence of the composite nanofibers and found that the reduction of Cr(VI) was little and could be negligible comparing with that catalyzed by the PAN/Mn(CH3 COO)2 composite nanofibers with oxalic acid (Fig. 3b). These experiments demonstrated that both oxalic acid and the catalyst PAN/Mn(CH3 COO)2 composite nanofibers were of the essence for the effective reduction of Cr(VI) solution. At the same time, it was obviously observed from the figure that the reduction of Cr(VI) was significantly influenced by Mn(CH3 COO)2 content and the reduction rate was accelerated along with the increase of Mn(CH3 COO)2 content within the range studied. Within 180 min, 1.2 mM initial Cr(VI) solution was reduced by 10% and 90% in the presence of PAN/Mn(CH3 COO)2 composite nanofibers containing 17.5 wt.% and 22.0 wt.% Mn(CH3 COO)2 , respectively (Fig. 3c and d). Comparatively, the Mn(II) solution (16.9 ppm), with the same amount of Mn(II) as the PAN/Mn(CH3 COO)2 composite nanofibers containing 22.0 wt.% Mn(CH3 COO)2 in aqueous solution during the catalytic reaction, removed less amount of Cr(VI) (Fig. 3e). As for the pure PAN, the electron-rich group ( CN) in the nanofibers presents a negatively charged surface, which cannot adsorb Cr2 O7 2− radicals [17] and hence cannot remove Cr(VI) (Fig. 3f). Only the PAN/Mn(CH3 COO)2 composite nanofibers exhibited good Cr-removal performance for the reduction of Cr(VI) into Cr(III) from Cr2 O7 2− solution. The inset in Fig. 3 showed that 0.01 g of PAN/Mn(CH3 COO)2 composite nanofibers containing 22.0 wt.% Mn(CH3 COO)2 could completely decolorize 30 mL of 1.2 mM Cr(VI) aqueous solution in 180 min. Through the catalytic reaction of PAN/Mn(CH3 COO)2 composite nanofibers, Mn(II) was really believed to act as a catalyst. Huber and Haight proposed that when oxalate was the reductant, the catalysis mechanism for Cr(VI) reduction was through by Cr(VI)oxalate-Mn(II) activated complexes: HCrO4 − + H2 C2 O4 + Mn(C2 O4 )2 2− → Cr(VI)-oxalate-Mn(II)

(1)

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The subsequent reactions can be expressed as shown in the formulae below. From the reactions, it was easily understood that the pH values increased due to the consumption of hydrogen ions. On the basis of reactions (2)–(4), it can be seen that Cr(VI)oxalate-Mn(II) rapidly broke down to Mn(C2 O4 )3 3− and H3 CrO4 , and then, similarly the H3 CrO4 can rapidly form an octahedral OCr(OH)3 (C2 O4 )2− which was rapidly reduced by the abundant Mn(C2 O4 )2 2− via a transition state like the one above for the Cr(VI). The labile Cr(IV) produced can acquire oxalate ligands from solution before being reduced to produce the observed distribution of Cr(III) products among hexaaquo- and mono-, bis-, and trisoxalato species [24]. Cr(VI)-oxalate-Mn(II) → Mn(C2 O4 )3 3− + H3 CrO4 2−

H3 CrO4 + H2 C2 O4 → OCr(OH)3 (C2 O4 )

+ 2H

(2)

+

(3)

3H+ + H2 C2 O4 + OCr(OH)3 (C2 O4 )2− + 2Mn(C2 O4 )2 2− → Cr3+ + 2Mn(C2 O4 )3 3− + 4H2 O

(4)

2Mn(C2 O4 )3 3− → 2Mn(C2 O4 )2 2− + C2 O4 2− + 2CO2 3−

(5) 2−

At last, Mn(C2 O4 )3 can decompose to Mn(C2 O4 )2 which continued to participate in the reaction. Therefore, the composite nanofibers display advantages used for Cr(VI) catalytic reduction.

Fig. 2. (A) XRD patterns. Curves (a–c) show the phase structures of the Mn(CH3 COO)2 ·4H2 O powders, pure PAN nanofibers and the PAN/Mn(CH3 COO)2 composite nanofibers containing 22.0 wt.% Mn(CH3 COO)2 . (B) FTIR spectra. Curves (d–f) show the structures of the pure PAN nanofibers, the PAN/Mn(CH3 COO)2 composite nanofibers containing 22.0 wt.% Mn(CH3 COO)2 before and after the catalytic Cr(VI) reduction reaction.

Fig. 3. The Cr(VI) reduction measurements in the Cr2 O7 2− -containing solutions with the Cr(VI) concentration 1.2 mM under different reaction conditions. Curve (a) with only 0.3 mL 0.5 M oxalic acid without the catalyst, (b) with only the nanofibers containing 17.5 wt.% Mn(CH3 COO)2 without oxalic acid, (c) with the nanofibers containing 17.5 wt.% Mn(CH3 COO)2 , (d) with the nanofibers containing 22.0 wt.% Mn(CH3 COO)2 , (e) with the Mn(II) solution (16.9 ppm) with similar concentration to the concentration of Mn(II) in aqueous solution during the catalytic reaction by PAN/Mn(CH3 COO)2 composite nanofibers containing 22.0 wt.% Mn(CH3 COO)2 and (f) with pure PAN nanofibers. (Inset) The photograph before and after the Cr(VI) reduction reaction with the nanofibers containing 22.0 wt.% Mn(CH3 COO)2 in 180 min.

3.2.2. Influences of the catalyst dosage, oxalic acid content and chromium concentration Due to the higher catalytic activity of the composite nanofibers containing 22.0 wt.% Mn(CH3 COO)2 , further experiments to investigate the influences of the catalyst dosage, oxalic acid content and chromium concentration on Cr(VI) reduction capacity were performed using the composite nanofibers containing 22.0 wt.% Mn(CH3 COO)2 . Fig. 4a showed the influence of the catalyst dosage on the Cr(VI) reduction efficiency. As shown in Fig. 4a, among the three different dosages, the largest catalyst dosage (0.02 g) showed the highest reduction efficiency of ∼92% after 180 min. However, during the same reaction period, about 89% of initial Cr(VI) concentration was reduced when the catalyst dosage was 0.01 g. This result may be due to the fact that at larger catalyst dosage, oxalic acid concentration became the main factor to affect the reaction rate. Additionally, from Fig. 4b, an increase in the initial content of oxalic acid also accelerated Cr(VI) reduction in the presence of 1.2 mM Cr(VI) solution, and the impact on the reaction rate was similar to that of the catalyst dosage. The effect of the initial concentration of Cr(VI) varying from 0.6 to 1.2 mM on the catalytic reduction process was also investigated using the composite nanofibers containing 22.0% Mn(CH3 COO)2 at pH 6 and the obtained results were presented in Fig. 4c. It was found out from the figure that the reduction increased with the decrease of the Cr(VI) initial concentration within the range studied. The decrease in reduction process at higher Cr(VI) concentration may be due to the fact that the higher Cr(VI) concentration required larger catalyst dosage, but the availability of the catalyst dosage remained constant in this experiment. 3.2.3. Influence of the pH value The pH value in the solution had significant influence on the catalytic Cr(VI) reduction capacity of the composite nanofibers. The catalytic Cr(VI) reduction efficiencies at varied pH values were presented in Fig. 5. We can see that decreasing pH led to the increases of catalytic activities from 34% (at a pH 3.7) to 98% (at a pH 2.3) (Fig. 5b). The reaction rate increased greatly when the pH value changed little. Thus, the acidic solution was beneficial to Cr(VI) reduction. The mechanism of pH dependent Cr(VI) reduction may include:

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Fig. 5. Influence of the reaction solution pH value on the Cr(VI) reduction performances in the Cr2 O7 2− -containing solutions, where the initial Cr(VI) concentration is 1.2 mM and the oxalic acid content is 0.3 mL.

Fig. 4. Influences of the catalyst dosage, oxalic acid content and chromium concentration on the Cr(VI) reduction performances in the Cr2 O7 2− -containing solutions. Figure (a) displays the influence of the catalyst dosage with 1.2 mM Cr(VI) solution and 0.3 mL oxalic acid, where curves (a-c) show that the dosage of the composite nanofibers containing 22.0 wt.% Mn(CH3 COO)2 is 0.005, 0.01 and 0.02 g, respectively; figure (b) displays the influence of the oxalic acid content with 0.01 g catalyst and 1.2 mM Cr(VI) solution, where curves (a-c) show that the oxalic acid content is 0.3, 0.6 and 0.9 mL, respectively; figure (c) displays the influence of the Cr(VI) concentration with 0.01 g catalyst and 0.3 mL oxalic acid, where curves (a-c) show that the Cr(VI) concentration is 0.6, 1.0 and 1.2 mM, respectively.

(1) Since the predominant Mn(II) species present in these catalytic reactions are MnC2 O4 and Mn(C2 O4 )2 2− , this suggests an activated state formed by reactions of HCrO4 − , Mn(C2 O4 )2 2− , C2 O4 2− , and two H+ [24]. So amounts of hydrogen ions were needed. (2) The reduction potential of Cr(VI) to Cr(III) is pH dependent and the thermodynamic driving force decreases with increasing pH, the reduction being favored at low pH [25]. (3) During the reaction process, the pH value of the reaction solution will increase gradually. When the Cr(VI) reaction solution pH value is 2.7 or 3.7, after the reaction with oxalic acid, the pH value of the reaction solution gets to 6. At pH values above 5–7, aqueous uncompledxed Cr(III) hydrolyzes to sparsely soluble chromium(III) hydroxides, and adsorbs to the nanofiber surface [26]. So at high pH value, the catalytic reaction was markedly slowed down by the Cr(III) precipitation on nanofiber surface via either suppressing Mn(II) leaching or blocking catalytic active sites. At the same time, Mn(II) can also hydrolyze to manganese(II) hydroxides to reduce its catalytic activity. To investigate the kinetics of Cr(VI) catalytic reduction under different pH values, the experimental data were analyzed using the pseudo-first order [27] kinetic model: log(qe − qt ) = log qe −

k1 t 2.303

(1)

where qe and qt are the reduction capacity at equilibrium and at the time t, respectively (mg g−1 ). k1 is the rate constant of the pseudofirst order reaction (min−1 ). The value of log(qe − qt ) were linearly correlated with t. The plots of log(qe − qt ) versus t should give a

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Fig. 6. The pseudo-first order kinetics plots for Cr(VI) catalytic reduction under different pH values.

linear relationship from which k1 and qe can be determined from the slope and intercept of the plot, respectively, as shown in Fig. 6. All the kinetic parameters are summarized in Table 1. 3.2.4. Influence of the light It should be mentioned that a few Mn(II) ions in the composite nanofibers will be released during soaking in the Cr(VI) containing solution and such release depends on the Mn(CH3 COO)2 content in the composite nanofibers. From the ICP data, the concentration of manganese ions in the final solution rises to 16.3 ppm from 2.19 ppm with the Mn(CH3 COO)2 content increasing from 17.5 wt.% to 22.0 wt.% with 1.2 mM Cr(VI) solution and 0.3 mL oxalic acid. According to the previously reported, the reduction of Cr(VI) to Cr(III) in the presence of organic compounds is able to be promoted by a photocatalytic process because of the synergistic effect between the photocatalytic reduction of Cr(VI) and the photocatalytic oxidation of organic compounds [28–30]. In order to obtain high catalytic activity and low manganese ions transudation concentration in the final solution, we investigated the influence of the light on the Cr(VI) reduction process in the presence of oxalic acid using the composite nanofibers containing 17.5 wt.% Mn(CH3 COO)2 . Before studying and comparing the photocatalytic activities of the composite nanofibers, the reduction of Cr(VI) with only the composite nanofibers in the dark in the presence of oxalic acid was first investigated. As shown in Fig. 7a, in the absence of light irradiation, only a small amount of Cr(VI) about 10% was reduced due to the catalytic activity of PAN/Mn(CH3 COO)2 composite nanofibers in the presence of oxalic acid. Next, the direct photolysis reduction of Cr(VI) without any catalysts under the simulated solar light irradiation in the presence of oxalic acid was also measured. The results demonstrated Cr(VI) was reduced only 22% after 180 min due to the reduction effect of CO2 •− and HO2 /O2 •− [26] decomposed from oxalic acid under simulated solar light irradiation (Fig. 7b). This meant that both chemical reduction of Cr(VI) with the composite nanofibers and the direct photolysis reduction of Cr(VI) with only oxalic acid were negligible compared with the reduction reaction in the presence of PAN/Mn(CH3 COO)2 composite nanofibers Table 1 Kinetic parameters for the catalytic reduction of Cr(VI) under different pH values. pH value

2.3 2.5 2.7 3.7

Pseudo-first order model qe,exp (mg g−1 )

k1 (min−1 )

qe,cal (mg g−1 )

R2

157.96 132.02 97.45 56.15

0.00818 0.00873 0.00864 0.01338

168.34 141.78 107.09 66.51

0.99457 0.98928 0.98961 0.98906

Fig. 7. Influence of the light on the Cr(VI) reduction performances in the Cr2 O7 2− containing solutions with the Cr(VI) concentration 1.2 mM under different reactions. Curve (a) with only the composite nanofibers containing 17.5 wt.% Mn(CH3 COO)2 and oxalic acid in the dark, (b) with only the oxalic acid and the simulated solar light without the composite nanofibers (c) the reaction under the visible light with the composite nanofibers and (d) the reaction under the simulated solar light with the composite nanofibers.

and oxalic acid under the light irradiation. Under the light irradiation, the Cr(VI) reduction reaction was greatly accelerated and increased with increasing the light intensity, which was dramatic. After 90 min of visible light irradiation, Cr(VI) solution concentration cannot be detected by the UV–vis spectrophotometer and was thought to be reduced to Cr(III) completely. At the moment, Cr(III) concentration (the total Cr concentration) in the solution (56.5 mg/L) was almost equal to the initial Cr(VI) solution concentration (57.1 mg/L) measured by ICP, which demonstrated that in this reduction process, Cr abided by the mass balance in the range of permitted errors. In contrast, in the presence of the simulated solar light, reaction time was less than 60 min. In addition, the composite nanofibers have excellent stability and durability. The composite nanofibers are stable up to 240 ◦ C. The composite nanofibers in air are found to be constant over long time. The exposure to the air condition even during 10 months does not cause any changes in catalytic activity. 30 mL 1.2 mM initial Cr(VI) solution could be reduced absolutely for only 60 min under the simulated solar light. The composite nanofibers also show good regeneration and reuse. For the first time, the catalytic reduction reaction can be completed absolutely in 60 min. After vacuum desiccating at room temperature for 12 h, the second catalytic reduction reaction can be completed 95% after 150 min and absolutely after 180 min. These results have demonstrated that the as-prepared composite nanofiber membranes can be used as a kind of effective pollutant elimination material to easily realize one-step reduction of Cr(VI) from solution and be facilely integrated into devices. 4. Conclusions In summary, we have successfully prepared PAN/Mn(CH3 COO)2 composite nanofibers by a simple electrospinning technique. The composite nanofibers have performed excellent capability for catalyzing Cr(VI) reduction by oxalic acid from a Cr2 O7 2− containing solution and the larger the content of Mn(CH3 COO)2 was, the faster the reduction reaction became. After 180 min, 1.2 mM initial Cr(VI) solution was reduced by 90% in the presence of PAN/Mn(CH3 COO)2 composite nanofibers containing 22.0 wt.% Mn(CH3 COO)2 by 0.3 mL 0.5 M oxalic acid, which was much higher than the previously reported values of the other reduction methods [17,31–33]. The catalyst dosage, oxalic acid content, chromium concentration and the pH value of the reaction solution had

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significant impact on the reduction process. Under the light irradiation, the catalytic performance of the PAN/Mn(CH3 COO)2 composite nanofibers on Cr(VI) reduction by oxalic acid was markedly enhanced through the experiments. After 90 min of visible light irradiation, Cr(VI) solution concentration could not be detected by the UV–vis spectrophotometer and was thought to be reduced completely, which had met the Synthetical Draining Standard of Sewage (0.5 mg/L, the Standard of PR China). Moreover, the composite nanofibers have excellent stability and durability. So, we propose that the PAN/Mn(CH3 COO)2 composite nanofibers be suitable for the waste water treatment for Cr(VI). Acknowledgements

[15]

[16]

[17]

[18]

[19]

[20]

This work was supported by the research grants from the National 973 Project (S2009061009), the National Natural Science Foundation of China (50973038).

[21]

References

[22]

[1] L. Khezami, R. Capart, Removal of chromium(VI) from aqueous solution by activated carbons: kinetic and equilibrium studies, J. Hazard. Mater. 123 (2005) 223–231. [2] X. Guo, G.T. Fei, H. Su, L.D. Zhang, High-performance and reproducible polyaniline nanowire/tubes for removal of Cr(VI) in aqueous solution, J. Phys. Chem. C 115 (2011) 1608–1613. [3] J. Sun, J.D. Mao, H. Gong, Y.Q. Lan, Fe(III) photocatalytic reduction of Cr(VI) by low-molecular-weight organic acids with ␣-OH, J. Hazard. Mater. 168 (2009) 1569–1574. [4] C. Li, Y.Q. Lan, B.L. Deng, Catalysis of manganese(II) on chromium(VI) reduction by citrate, Pedosphere 17 (2007) 318–323. [5] I.H. Yoon, S. Bang, J.S. Chang, M.G. Kim, K.W. Kim, Effects of pH and dissolved oxygen on Cr(VI) removal in Fe(0)/H2 O systems, J. Hazard. Mater. 186 (2011) 855–862. [6] B. Mukhopadhyay, J. Sundquist, R.J. Schmitz, Removal of Cr(VI) from Crcontaminated groundwater through electrochemical addition of Fe(II), J. Environ. Manage. 82 (2007) 66–76. [7] M. Pettine, I. Barra, L. Campanella, F.J. Millero, Effect of metals on the reduction of chromium (VI) with hydrogen sulfide, Water Res. 32 (1998) 2807–2813. [8] J.W. Yang, Z.S. Tang, R.F. Guo, S.Q. Chen, Soil surface catalysis of Cr(VI) reduction by citric acid, Environ. Prog. 27 (2008) 302–307. [9] N. Wang, Y. Xu, L. Zhu, X. Shen, H. Tang, Reconsideration to the deactivation of TiO2 catalyst during simultaneous photocatalytic reduction of Cr(VI) and oxidation of salicylic acid, J. Photochem. Photobiol. A 201 (2009) 121–127. [10] Q. Wang, N. Cissoko, M. Zhou, X. Xu, Effects and mechanism of humic acid on chromium(VI) removal by zero-valent iron (Fe0 ) nanoparticles, Phys. Chem. Earth 36 (2011) 442–446. [11] Y.Q. Lan, C. Li, J.D. Mao, J. Sun, Influence of clay minerals on the reduction of Cr6+ by citric acid, Chemosphere 71 (2008) 781–787. [12] A. Greiner, J.H. Wendorff, Electrospinning: a fascinating method for the preparation of ultrathin fibers, Angew. Chem. Int. Ed. 46 (2007) 5670–5703. [13] A. Formhals, U.S. Patent specification, 1975504, 1934. [14] W. Wang, Q.B. Yang, L. Sun, H.G. Wang, C.Q. Zhang, X.L. Fei, M.D. Sun, Y.X. Li, Preparation of fluorescent nanofibrous film as a sensing material and

[23]

[24]

[25]

[26]

[27]

[28]

[29]

[30]

[31]

[32]

[33]

445

adsorbent for Cu2+ in aqueous solution via copolymerization and electrospinning, J. Hazard. Mater. 194 (2011) 185–192. S.J. Wu, F.T. Li, Y.N. Wu, R. Xu, G.T. Li, Preparation of novel poly(vinyl alcohol)/SiO2 composite nanofiber membranes with mesostructure and their application for removal of Cu2+ from waste water, Chem. Commun. 46 (2010) 1694–1696. S.L. Chen, X.J. Huang, Z.K. Xu, Functionalization of cellulose nanofiber mats with phthalocyanine for decoloration of reactive dye wastewater, Cellulose 18 (2011) 1295–1303. Y.X. Lin, W.P. Cai, X.Y. Tian, X.L. Liu, G.Z. Wang, C.H. Liang, Polyacrylonitrile/ferrous chloride composite porous nanofibers and their strong Cr-removal performance, J. Mater. Chem. 21 (2011) 991–997. J.M. Du, X.W. Zhang, Role of polymer–salt–solvent interactions in the electrospinning of polyacrylonitrile/iron acetylacetonate, J. Appl. Polym. Sci. 109 (2008) 2935–2941. M.A. Phadke, D.A. Musale, S.S. Kulkarni, S.K. Karode, Poly(acrylonitrile) ultrafiltration membranes. I. Polymer-salt-solvent interactions, J. Polym. Sci. B 43 (2005) 2061–2073. J.J. Testa, M.A. Grela, M.I. Litter, Heterogeneous photocatalytic reduction of chromium(VI) over TiO2 particles in the presence of oxalate: involvement of Cr(V) species, Environ. Sci. Technol. 38 (2004) 1589–1594. Z.Y. Li, H.M. Huang, T.C. Shang, F. Yang, W. Zheng, C. Wang, S.K. Manohar, Facile synthesis of single-crystal and controllable sized silver nanoparticles on the surfaces of polyacrylonitrile nanofibres, Nanotechnology 17 (2006) 917–920. K. Saeed, S.-Y. Park, T.-J. Oh, Preparation of hydrazine-modified polyacrylonitrile nanofibers for the extraction of metal ions from aqueous media, J. Appl. Polym. Sci. 121 (2011) 869–873. T. Rohani Bastami, M.H. Entezari, A novel approach for the synthesis of superparamagnetic Mn3 O4 nanocrystals by ultrasonic bath, Ultrason. Sonochem. 19 (2012) 560–569. Charles F. Huber, C.P. Haight Jr., The oxidation of manganese(II) by chromium(VI) in the presence of oxalate ion, J. Am. Chem. Soc. 98 (1976) 4128–4131. G. Cappelletti, C.L. Bianchi, S. Ardizzone, Nano-titania assisted photoreduction of Cr(VI) The role of the different TiO2 polymorphs, Appl. Catal. B 78 (2008) 193–201. S.J. Hug, H.-U. Laubscher, Iron(III) catalyzed photochemical reduction of chromium(VI) by oxalate and citrate in aqueous solutions, Environ. Sci. Technol. 31 (1997) 160–170. D. Vu, Z.Y. Li, H.N. Zhang, W. Wang, Z.J. Wang, X.R. Xu, B. Dong, C. Wang, Adsorption of Cu(II) from aqueous solution by anatase mesoporous TiO2 nanofibers prepared via electrospinning, J. Colloid Interface Sci. 367 (2012) 429–435. L.M. Wang, N. Wang, L.H. Zhu, H.W. Yu, H.Q. Tang, Photocatalytic reduction of Cr(VI) over different TiO2 photocatalysts and the effects of dissolved organic species, J. Hazard. Mater. 152 (2008) 93–99. L. Yang, Y. Xiao, S. Liu, Y. Li, Q. Cai, S. Luo, G. Zeng, Photocatalytic reduction of Cr(VI) on WO3 doped long TiO2 nanotube arrays in the presence of citric acid, Appl. Catal. B 94 (2010) 142–149. G. Colon, M.C. Hidalgo, J.A. Navio, Influence of carboxylic acid on the photocatalytic reduction of Cr(VI) using commercial TiO2 , Langmuir 17 (2001) 7174–7177. X.H. Guan, H.R. Dong, J. Ma, Influence of phosphate, humic acid and silicate on the transformation of chromate by Fe(II) under suboxic conditions, Sep. Purif. Technol. 78 (2011) 253–260. L. Sun, L.D. Zhang, C.H. Liang, Z.G. Yuan, Y. Zhang, W. Xu, J.X. Zhang, Y.Z. Chen, Chitosan modified Fe0 nanowires in porous anodic alumina and their application for the removal of hexavalent chromium from water, J. Mater. Chem. 21 (2011) 5877–5880. S.T. Earley, B.E. Alcock, J.P. Lowry, C.B. Breslin, Remediation of chromium(VI) at polypyrrole-coated titanium, J. Appl. Electrochem. 39 (2009) 1251–1257.