visible-driven photocatalysis bifunctional roles

Applied Surface Science 404 (2017) 206–215

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Enhanced removal of azo dye using modified PAN nanofibrous membrane Fe complexes with adsorption/visible-driven photocatalysis bifunctional roles Fu Li a , Yongchun Dong a,b,∗ , Weimin Kang a,b , Bowen Cheng a,b , Guixin Cui a,c a

School of Textiles, Tianjin Polytechnic University, Tianjin 300387, China State Key Laboratory of Separation Membranes and Membrane Process, Tianjin Polytechnic University, Tianjin 300387, China c Jiangnan Branch, China Textile Academy, Shaoxing 312071, China b

a r t i c l e

i n f o

Article history: Received 12 December 2016 Received in revised form 17 January 2017 Accepted 25 January 2017 Available online 28 January 2017 Keywords: PAN nanofibrous membrane Surface modification Fenton catalyst Adsorption Photocatalysis

a b s t r a c t A series of polyacrylonitrile (PAN) nanofibrous membrane Fe complexes as the Fenton heterogeneous catalysts were fabricated through surface modification with different ratio of hydrazine hydrate (HH) and hydroxylamine (HA) and subsequent coordination with Fe3+ ions for the synergistic removal of a typical azo dye, Reactive Red 195 (RR 195) via adsorption and visible-driven photocatalytic oxidation. Effect of molar ratio of HH and HA on surface structure characteristics of the resulting complexes were examined. Their adsorptive or photocatalytic activity was also compared by changing molar ratio of HH and HA. The results indicated that three PAN nanofibrous membrane Fe complexes prepared with simultaneous modification of HA and HH exhibited much higher adsorption and visible photocatalytic activities than the complex modified solely with HA or HH due to their distinctive surface structures containing more active sites. Their adsorption and visible photocatalytic kinetics of RR 195 followed pseudo-second-order model equation. Their high photocatalytic rate constant and large amount of dye adsorption were regarded as the main reasons for better dye removal efficiency and durability in cyclic reuse by means of the synergistic adsorption-photocatalysis process. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Organic dyes have been widely used in many modern industries such as textiles, plastics and paper process. The wastewater containing these dyes has posed severe environmental challenges and may be carcinogenic and mutagenic if discharged without any treatment because most of the dyes are known to be based on largely non-biodegradable aromatic structures and azo groups [1,2]. Although several methods are used to remove dyes from industrial wastewater, adsorption and catalytic oxidation are still considered as the most attractive technologies. Nano-scale materials are usually employed for improving the performance of absorbents or catalysts because of their small size or larger specific surface area which offers much enhanced and even new functions that cannot be achieved by bulk materials [2–8]. More importantly, some new nanomaterials including highly porous Co-doped

∗ Corresponding author at: Division of Textile Chemistry & Ecology, School of Textiles, Tianjin Polytechnic University, 399 Binshui West Road, Xiqing District, Tianjin 300387, China. E-mail addresses: [email protected], [email protected] (Y. Dong). http://dx.doi.org/10.1016/j.apsusc.2017.01.268 0169-4332/© 2017 Elsevier B.V. All rights reserved.

TiO2 nanotubes [9], TiO2 /active carbon composite [10], nanostructured potassium polytitanate [11], nano-size carbon tungsten composite [12] and Fe-zeolites [13] have been used to prepare adsorption/catalysis bifunctional materials for water and wastewater purification since the combination of increased substrate adsorption and catalytic oxidation can significantly promote the removal of organic pollutants from the environment [10,13]. However, it is difficult to recover these nanoparticle materials at the end of the reaction due to its small size and dilute concentration, thus re-contaminating the treated water [14–16]. To avoid the limitation, nanofibrous material with large aspect ratio (lengthto-diameter) has been used to enhance contact with pollutants because it is superior to particles as far as the recycling and aggregation are concerned. Electrospinning is a simple and versatile method for producing nanofibrous membrane materials [17–19]. They have potential advantages to remove pollutants from wastewater due to their high surface area and porosity, hence which have excellent adsorption capacities [20–25]. Compared with other nanofibrous materials, electrospun polyacrylonitrile (PAN) nanofibrous membranes have been of particular interests due to extraordinary properties such as small fiber diam-

F. Li et al. / Applied Surface Science 404 (2017) 206–215

eters, extremely large specific surface areas, remarkably high porosity, better capability to control fiber diameter and interfibrous pore sizes among nanofibers as well as cost effectiveness [26]. Moreover, the cyano-groups of PAN fiber can be transformed into various functional groups to improve their adsorption capacity [2,27–29] or catalytic performance [30–32] for obtaining high pollutant removal efficiency. In this present study, we demonstrated an integrated process synergistically combining physical adsorption and photocatalytic decomposition by using different simultaneous modified PAN nanofibrous membrane Fe complexes as the adsorption/catalysis bifunctional materials. In case of dye adsorption from the solution, the complex provides increased number of adsorption sites for dye owing to the presence of large surface area, high porosity and basic nitrogen functional groups. The dye molecules concentrated on its surface can be photocatalytically decomposed by supported Fe3+ ions. It should be noticed that hydrazine hydrate (HH) and hydroxylamine (HA) were selected to modify PAN nanofibrous membrane owing to two main advantages: one is that Fe complex prepared with HA and HH simultaneous modified PAN fiber showed a higher photocatalytic activity for dye degradation with H2 O2 , and the other is that HA was added to the modifying solution to increase the degree of Fe3+ ions coordinated with PAN fiber [33]. Besides, HH as a crosslinking agent could enhance the mechanical strength and oxidation resistance of PAN fibrous membrane for the durability and regeneration [34,35]. By optimizing the ratio of HA and HH in modifying solution, adsorption/photocatalysis hybrid capability of the modified PAN fibrous membrane Fe complex was successfully developed for dye removal through two pathways. Thus, the adsorption and photocatalytic degradation kinetics of dye on the complex were investigated and equilibrium parameters were devoted. In addition, the effect of fiber diameter on these parameters is also discussed. This work could give a better insight into exploring more efficient adsorbentbased heterogeneous Fenton catalysts for water purification. 2. Experimental 2.1. Materials and reagents

207

water and dried under vacuum and weighed. 1.0 g n-PAN sample was treated with 150 mL aqueous modifying solution containing different concentration of HA and HH at pH = 9.5 and 95 ◦ C for 2 h. It was noticed that appropriate amounts of NaOH were added to obtain the required pH level of the modifying solution. And then modifying solution was separated from resulted modified n-PAN when the reaction stopped. The modified n-PAN was thoroughly washed with distilled water until neutral, and dried under vacuum at 60 ◦ C. Finally dried modified n-PAN was weighed, and weight gain rate (Wg %) of which was also calculated. 2.4. Preparation of modified n-PAN Fe complex 1.0 g of dried modified n-PAN was stirred in a 100 mL FeCl3 aqueous solution at pH = 2.0 and 50 ◦ C for a given time. The resulting modified n-PAN Fe complex was then filtered, washed with deionized water and dried under vacuum at 60 ◦ C. The residual concentration of Fe3+ ions in solution after reaction was determined using a VISTA MPX inductively coupled plasma optical emission spectrometer (ICP-OES, Varian Corp., USA) for calculating the Fe content (QFe ) of the obtained complex. 2.5. Characterization of modified n-PAN and its Fe complex Fiber average diameter (Dm ) of modified n-PAN or its Fe complex was characterized quantitatively by images analysis of scanning electron microscope (SEM) images. A field emission scanning electron microscope (Hitachi S-4800, Japan) was used to obtain their SEM images, which were then imported into an image analyzer (Image-Pro Plus, Media Cybernetics Inc., USA) for measuring these fiber diameters and their distribution. Specific surface areas (SBET ) of the samples were determined by N2 adsorption using Micromeritics ASAP2010 equipment. The composition of the samples was verified by using a Nicolet Magna-560 Fourier transform spectrometer (Nicolet Inc. USA) with 4 cm−1 resolution. In order to measure their porosity, the samples were cut into squares (3 cm × 3 cm), the thickness of which was determined using a thickness gauge (Shanghai Chuanlu measure instruments Co., China). Its porosity (P) is expressed by the following equation [37].

Commercial PAN yarns consisting of conventional fibers were used in this study. N,N-dimethylformamide (DMF), hydroxylamine (HA), hydrazine hydrate (HH), FeCl3 . 6H2 O and H2 O2 (30%, w/w) were of analytical grade and used as received. An azo dye, Reactive Red 195 (abbr. RR 195) was used without purification, and its chemical structure was presented in our previous work [31]. Double distilled and deionized water was used throughout the study.

P = 1 − W/Z

2.2. Fabrication of PAN nanofibrous membranes

The adsorption experiments of RR 195 on the complex were carried out in the dark before photocatalysis. 0.10 g of modified n-PAN Fe complex was immersed into a Pyrex glass beaker containing 50 mL of 0.10 mmol L−1 RR 195 aqueous solution. The solution pH was adjusted to 6.0 with 0.10 mol L−1 NaOH or HNO3 aqueous solution, and measured using DHS-25C digital pH meter (Shanghai Jingmi Instrumental Co., China). And then the beaker was put in a constant-temperature shaker water bath with a revolution speed of 100 rpm. The adsorption temperature and period employed here were 25 ◦ C and 10 h, respectively, in order to reach the adsorption equilibrium. After the adsorption, H2 O2 was added to the RR 195-adsorbed test solution (6.0 mmol L−1 ) and a 400 W high-pressure mercury lamp (Foshan Osram Illumination Co., Foshan, China) was used as the light source for the dye degradation to evaluate the photocatalytic activity under visible irradiation. Meanwhile, a cut-off filter was used to ensure irradiation only by visible light (␭ > 420 nm). The visible light intensity over the test solution surface was measured to be 8.42 mW cm−2 using an FZA radiometer (Beijing BNU Light and Electronic Instrumental Co.,

According to the references [6,36], PAN solution (16 wt.%) was prepared by dissolving appropriate weight of PAN yarns in DMF under constant stirring at room temperature. The electrospinning of the resulting PAN solution was conducted under an electrical field with optimized voltage (20 kV), PAN solution feeding rate (0.60 mL h−1 ) and the distance between the needle tip and the collector (15 cm). The grounded collection disk covered with aluminum foil was used to collect the nanofiber as the form of membrane. PAN nano-fibrous membrane (denoted as n-PAN) was obtained after removing the residual solvent at 60 ◦ C for 24 h. 2.3. Modification of n-PAN with HA and HH The modification of n-PAN was carried out using a method reported in our previous work [35]. Before modification, n-PAN was first treated with a solution containing 1.0 g L−1 NaCO3 and 2 g L−1 of soap at about 50 ◦ C for 30 min, then thoroughly washed with cold

(1)

Where W is the basic weight of modified PAN nanofibrous membrane (g cm−2 ),  is the density of PAN with a value of 1.184 g cm−3 and Z is the thickness (cm). 2.6. Dye adsorption and photocatalytic degradation

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Table 1 Modified n-PANs with different concentrations of HA and HH. Modified n-PAN Samples

[HA]0 (mol L−1 )

[HH]0 (mol L−1 )

Wg (%)

Dm (nm)

SBET (m2 g−1 )

P (%)

HA-n-PAN M-n-PAN(i) M-n-PAN(ii) M-n-PAN(iii) HH-n-PAN

1.60 1.20 0.80 0.40 0.00

0.00 0.40 0.80 1.20 1.60

54 45 40 26 3

533 393 347 302 260

5.16 5.56 5.63 5.68 5.76

73.69 74.65 74.97 75.28 75.57

Scheme 1. Modification of n-PAN with HA and/or HH in the alkali medium.

China). The beaker was exposed to the visible irradiation at 25 ◦ C. At different time intervals, 1–2 mL of the test solution was sampled and characterized immediately using a UV-2401 Shimadzu spectrophotometer (Shimadzu Co., Japan) at the maximum absorption wavelength (522 nm) of RR 195 for measure of the dye concentration during the experiment. The amount of dye adsorption or photocatalytic oxidation was calculated as follows: qt = (C 0 –C t )VM/m

(2)

Where qt is the amount of dye adsorption or photocatalytic oxidation at time t (mg g−1 ). C0 and Ct are the initial and residual concentrations of the dye (mmol L−1 ) at time t, respectively. V is the volume of the solution (L). M is the molecular mass of RR 195 with a value of 1136.30 g mol−1 and m is the mass of the dry modified n-PAN Fe complex used (g).

due to the stretching of CH2 , C N, C O, bending deformation of CH2 , rocking of CH2 , and CH and/or C O vibration, respectively [32,41]. Little absorption peak of C N group (2240 cm−1 ) was found in presence of HA (Curves a-d). Moreover, four modified n-PAN, especially HA-n-PAN showed the typical bands including 3500-3000, 1632 and 923 cm−1 of the amidoxime group [41]. The intensities of these typical bands decreased as [HH]0 level increased. Meanwhile, 1560–1530 cm−1 due to carboxy and amido groups of HH modified PAN fiber were observed in the spectrum of HH-n-PAN (Curves e) [38]. The absorption intensity of C N groups was also reduced. These suggested the occurrence of crosslinking reaction between HH and CN group in the PAN fibrous membrane and led to the formation of the hydrazide groups [34]. 3.2. Preparation and characterization of modified n-PAN Fe complexes

3. Results and discussion 3.1. Modification of n-PAN with HA and HH The modification of n-PAN with Dm = 232 nm was conducted by controlling the initial concentrations of HA and HH in aqueous solution ([HA]0 and [HH]0 ) to obtain a series of modified n-PANs. They were characterized by FE-SEM and FTIR, and the results were shown in Figs. 1 and 2. And then their Wg %, Dm , SBET and P values were also measured and listed in Table 1. As shown in Table 1, Wg % value significantly decreased with [HH]0 level increasing, suggesting that HH has a lower reactivity with n-PAN than HA at the same conditions [38]. Besides, HH could crosslink with PAN membrane or/and HA [33,34], generally promotes resistance to chemical attacks [39,40] and reduces the mobility of the polymer chains, thus possibly limiting the reaction of HA with n-PAN. The main reactions between HA, HH and n-PAN were provided in Scheme 1. Furthermore, increasing [HH]0 level also led to a gradual reduction of Dm value, while SBET and P values exhibited the insignificant reverse trends. These may be mainly dependent upon the degree of Wg %. It should be noticed in Fig. 1 that the reaction of HA with n-PAN caused an swelling up of nanofibers and adjoining between them, thus increasing their diameters and reducing SBET and P values of the obtained modified n-PANs. Fig. 2 showed that the characteristic absorption peaks of original n-PAN were at 2928, 2240, 1738, 1454, 1360, 1232 and 1033 cm−1 ,

All the above resulting modified n-PANs were used to coordinate with Fe3+ ions (0.10 mol L−1 ) at 50 ◦ C for a given time, respectively for obtaining different modified n-PAN Fe complexes. Their Fe contents (QFe ) were determined and presented in Fig. 3. The Fe content at equilibrium (QFe,eq ), Dm , SBET and P values were also measured and provided in Table 2. Fig. 3 showed that the QFe value of n-PAN was relatively low for lack of coordinating groups such as hydroxyl and amino groups on its surface. Moreover, all the QFe values of the modified nPAN significantly increased with the prolongation of reaction time, and were kept to be constant over 400 min, proposing that Fe3+ ions were gradually fixed on the surface of the modified n-PAN by the coordination of its O- or N-containing groups with Fe3+ ions in solution. The coordination reached the equilibrium when reaction duration was longer than 400 min. It could be observed from Table 2 that QFe,eq values of the resulting modified n-PAN Fe complexes reduced with [HH]0 used increasing. And QFe,eq value of Fe-HA-n-PAN was about seven times higher than that of FeHH-n-PAN. This was mainly attributed to big difference in surface molecular structure between the modified n-PANs. As shown in Scheme 1, HA-n-PAN is an amidoximated groups-containing bidentate ligand, which has a stronger reactivity with transition metal ions. On the other hand, M-n-PANs have the relatively complicated surface molecular structures with the crosslink formed with the amidoxime and amidrazone groups between both PAN nanofiber chains due to incorporation of HH [34,35]. Therefore, it is believed

F. Li et al. / Applied Surface Science 404 (2017) 206–215

Fig. 1. FE-SEM images of modified n-PANs including (a) HA-n-PAN, (b) M-n-PAN(i), (c) M-n-PAN(ii), (d) M-n-PAN(iii), (e) HH-n-PAN and (f) Original n-PAN.

Fig. 2. FTIR spectra of modified n-PAN including (a) HA-n-PAN, (b) M-n-PAN(i), (c) M-n-PAN(ii), (d) M-n-PAN(iii) and (e) HH-n-PAN.

209

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Fig. 3. Coordination of Fe3+ ions with different modified n-PAN.

that higher [HH]0 increases the surface crosslink degree of the resulting complexes, thus blocking the contact of their coordinating groups with Fe3+ ions in solution. In addition, although Dm values of the modified n-PAN Fe complexes correspondingly increased, little change in their SBET and P values was found after coordination with Fe3+ ions. These modified n-PAN Fe complexes were then characterized through FE-SEM and FTIR, respectively, and the results were shown in Figs. 4 and 5. Fig. 4(a)–(d) showed that when being coordinated with Fe3+ ions, the surface of modified n-PAN Fe complexes, especially FeHA-n-PAN and Fe-M-n-PAN(i) was covered by a mud-like layer, thus causing significant adjoining between nanofibers. Besides, there were many particles on the surface of nanofibers for Fe-Mn-PAN(iii). It was noticed from Fig. 4(e)–(f) that surface layer of Fe-HH-n-PAN was much smoother than that of the other four complexes. However, some uneven bulges were still observed on the surface of nanofiber. This may be due to lower QFe,eq value of Fe-HHn-PAN. Accordingly, the morphological change suggested that the varied amounts of Fe species were located on the modified n-PANs. Comparing with spectra of five modified n-PAN and their corresponding Fe complexes in Figs. 2 and 5, it was found that after coordination with Fe3+ ions, the characteristic bands including 1626–1650 cm−1 , 1030–1034 cm−1 and 918–934 cm−1 of the amidoxime group were less intensive and shifted to higher frequencies. 1560–1530 cm−1 due to carboxy and amido groups also showed the relatively insignificant changes. More importantly, the shapes, positions and intensities of all the adsorption bands mentioned above varied with the molar ratio of HH/HA used for the preparation of the complexes. These finding revealed that these coordinating groups had participated in the reaction with Fe3+ ions.

3.3. Adsorption and photocatalytic performance Previous study [42] reported that although a good heterogeneous catalyst should efficiently produce oxidizing species, it is

necessary to adsorb the reactant molecules strongly enough for them to react. Moreover, the organic compounds that have better adsorption on the surface are more likely to be degraded in the photocatalytic process [43–45]. In this work, to examine the combination effect of adsorption and photocatalysis of the modified n-PAN Fe complexes with different fiber diameter and porosity, a series of the complexes containing similar QFe value (approx. 1.20 mmol g−1 ) were fabricated with different modified n-PANs mentioned above through controlling coordination duration or Fe3+ ion initial concentration. A typical anionic water soluble azo dye, RR 195 was then used at pH = 6.0 and room temperature to investigate the adsorption and photocatalytic degradation performance of the complex samples. In order to distinguish adsorption from photocatalysis, the dye adsorption was firstly conducted using five complexes in the dark, followed by its photocatalytic degradation with the addition of H2 O2 under visible exposure, and the results were given in Fig. 6. Fig. 6 showed that the adsorption of RR 195 on the three FeM-n-PANs reached equilibrium within 10 h, which was longer than the adsorption equilibrium time (3 h) for the two other complexes. However, their amounts of dye adsorption at equilibrium were much greater than those of the later, and ranked in this order: FeM-n-PAN(i) (13.84 mg g−1 ) > Fe-M-n-PAN(ii) (12.11 mg g−1 ) > FeM-n-PAN(iii) (10.70 mg g−1 ) > Fe-HH-n-PAN (4.56 mg g−1 ) > Fe-HAn-PAN (0.376 mg g−1 ). It is worth noticing that the adsorption capacities of three Fe-M-n-PANs are higher than those of metaldoped TiO2 nanotubes (8.10–10.6 mg g−1 ) reported by Chien and co-workers [9]. The main reason for this order was that three Fe-M-n-PANs have much more amino and imino groups on surface hydrazine-containing structure (see Scheme 1). These N-based groups were cationized in acidic solution to easily react with anionic dye molecules with sulphonate groups through electrostatic attraction, thus resulting in a significant adsorption between them. Besides, SBET and P values of the five complexes were measured to be 5.17–5.66 m2 g−1 and 73.45–75.21%, respectively.

Table 2 Characters of different modified n-PAN Fe complexes. Modified n-PAN Fe complexes

QFe,eq (mmol g−1 )

Dm (nm)

SBET (m2 g−1 )

P (%)

Fe-HA-n-PAN Fe-M-n-PAN(i) Fe-M-n-PAN(ii) Fe-M-n-PAN(iii) Fe-HH-n-PAN n-PAN

8.15 7.14 6.57 6.02 0.98 0.21

623 452 364 340 313 232

5.06 5.51 5.56 5.60 5.66 5.81

73.07 74.70 74.85 75.02 75.21 75.75

F. Li et al. / Applied Surface Science 404 (2017) 206–215

211

Fig. 4. SEM images of the modified n-PAN Fe complexes including (a) Fe-HA-n-PAN, (b) Fe-M-n-PAN(i), (c) Fe-M-n-PAN(ii), (d) Fe-M-n-PAN(iii), (e) Fe-HH-n-PAN and (f) high magnification of Fe-HH-n-PAN.

Fig. 5. FTIR spectra of different modified n-PAN Fe complexes including (a) Fe-HA-n-PAN, (b) Fe-M-n-PAN(i), (c) Fe-M-n-PAN(ii), (d) Fe-M-n-PAN(iii) and (e) Fe-HH-n-PAN.

9.65 × 10 2.53 × 10−2 1.43 × 10−2 1.12 × 10−2 4.35 × 10−2 8.01 45.90 45.92 45.93 4.968 0.9974 0.9971 0.9984 0.9987 0.9990

R

−3

kp (g min−1 mg−1 ) qe (mg g−1 )

Photocatalytic degradation

2

0.8938 0.9997 0.9997 0.9990 0.7413

F. Li et al. / Applied Surface Science 404 (2017) 206–215

R2

212

Therefore it is not believed that slight change in the specific surface area or porosity is the main reason for affecting their adsorption capacities. But, surface crosslinking structure of Fe-HH-n-PAN or adjoining between nanofibers of Fe-HA-n-PAN may lead to a fine and close texture of their surface, thus inducing the inaccessibility for RR 195 into the entrance or channel of the complexes. On the other hand, five modified n-PAN Fe complexes also showed the different photocatalytic function as the heterogeneous Fenton catalysts for RR 195 degradation after 30 min visible irradiation. Three Fe-M-n-PANs were found to present much better photocatalytic degradation capacities of RR 195 (44.22–45.02 mg g−1 ) than the other two complexes (4.75–5.70 mg g−1 ) at the same conditions, which was identical with their adsorption performances. This confirmed that simultaneous modification of n-PAN with HA and HH is an efficient way to enhance not only adsorption but also photocatalysis ability of the resulting Fe complexes. Accordingly, it is concluded that the photocatalytic degradation of RR 195 belongs to a surface-catalyzed reaction. And the ratio of HA and HH during the preparation of the complexes plays a key role in dye removal from aqueous solution. In order to understand dye adsorption and photocatalytic decomposition kinetics and correlation between them, the pseudosecond-order kinetic model was used to evaluate the data in Fig. 6. The linearised form of pseudo-second-order kinetic model was expressed as: (3)

ka (g min )

2.93 × 10 2.31 × 10−3 2.60 × 10−3 3.56 × 10−3 9.63 × 10−3

mg

−1 −1

0.381 14.55 12.63 11.01 4.773 73.45 74.52 74.94 75.13 75.21 5.17 5.46 5.58 5.63 5.66 568 413 352 324 313 Fe-HA-n-PAN Fe-M-n-PAN(i) Fe-M-n-PAN(ii) Fe-M-n-PAN(iii) Fe-HH-n-PAN

qe (mg g

−1

Dark adsorption P (%) SBET (m2 g−1 ) Dm (nm) Complex samples

Table 3 Adsorption and visible-light driven photocatalytic degradation kinetic model parameters on different modified n-PAN Fe complexes.

)

t/qt = 1/kqe 2 + t/qe

Where k is the rate constant of the pseudo-second-order model for adsorption (ka ) or photocatalytic degradation (kp ) of RR 195 on the complex. qe and qt are the amounts of RR 195 adsorbed or visible-light photocatalyzed per unit mass of the complex at steady state and any time t, respectively. The slope and intercept of the linear plot of t q/ t against t yielded the values of qe and k values. The regression results of Fig. 6 by Eq. (3) for the adsorption and photocatalytic degradation of RR 195 on the complexes were listed in Table 3. It is observed from Table 3 that the correlation coefficients (R2 ) values for the second-order kinetic models at dark adsorption and photocatalytic degradation were >0.99 in case of three Fe-M-n-PANs. Moreover, the theoretical amounts of dye adsorption or degradation at equilibrium were only slightly different from the experimental values (Fig. 6) in the case of pseudo-second order kinetics. These results demonstrated that the entire reaction process followed a pseudo-second-order model for three Fe-Mn-PANs. However, Eq. (3) provided better fits for only the dye adsorption on the other two complexes. This confirmed that dye photocatalytic degradation mechanism was mainly determined by the surface molecular structure characteristics of the complexes modified with different ratio of HA and HH. Reasonably, this effect revealed that there were more active sites over Fe-M-n-PANs, which favored the dye adsorption and photocatalytic degradation rates. In addition, it is noticed that the kp value had one order higher than the ka value for the Fe-M-n-PANs, indicating that the dye molecule adsorbed on their surface sites was rapidly photocatalyzed through the heterogeneous Fenton reaction. This may vacate the adsorptive site for next dye molecule, thus accelerating its adsorption/photocatalytic degradation process. On the basis of all the above experimental results, we proposed a possible mechanism for the synergistic effect of adsorption and photocatalytic degradation in Fe-M-n-PANs/H2 O2 system in the dye removal illustrated in Fig. 7. 3.4. Reusability of Fe-M-n-PANs in cyclic experiments Cyclic experiments of the catalyst are crucial in determining its practical potential because the catalyst may be poisoned by reac-

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Fig. 6. Dark adsorption and visible-driven photocatalytic degradation curves for different modified n-PAN Fe complexes at room temperature.

Fig. 7. A schematic removal model of azo dyes from water through combination of adsorption and photocatalytic oxidation of Fe-M-n-PAN in the presence of H2 O2.

tion intermediates and other species during the reaction process [8]. The modified PAN fiber metal complexes could be reused as heterogeneous Fenton catalyst due to their good resistance to common oxidizing agents and sunlight [32]. Hence, reuse stability of three Fe-M-n-PANs (QFe = 1.20 mmol g−1 ) as the catalysts were examined by the additional degradation process of fresh RR 195 solution with the used catalysts from the previous runs through the same conditions in recycling experiments. The used catalyst was thoroughly washed with a deionized water-ethanol (v:v = 1:1) mixed solution after each run several times and then oven dried at 50 ◦ C for 6 h, and RR 195 and H2 O2 were added before the next run. The catalyst was used up to five times, and the results were shown in Fig. 8. As can be seen from Fig. 8, RR 195 concentration sharply decreased with Fe-M-n-PAN and H2 O2 within 3 h, indicating that three Fe-M-n-PANs showed the stronger removal efficiency of RR 195 from water. More importantly, no obvious deactivation of FeM-n-PANs in three successive runs was found when compared with the first run, and its catalytic activity slightly declined in the fifth run. This proposed that dyes can be quickly removed with Fe-M-nPAN through a combined adsorption and photocatalytic oxidation process. The intermediates generated during the catalytic decomposition of the adsorbed dyes can also be easily replaced by dyes during adsorption in the next cycle. The small loss in their catalytic capacity may be owing to the intermediate residue on their surface, which inhibited the interaction of complex and dye, agreeable with the previous works by other researchers [8,46,47]. In addition, one problem with the heterogeneous photocatalysts is metal ions

213

Fig. 8. Changes in RR 195 concentration in the presence of Fe-M-n-PAN and H2 O2 at initial pH 6.0 and 25 ◦ C under visible light irradiation.

Fig. 9. Concentration of Fe3+ ions leaching of recycled Fe-M-n-PANs.

leaching from the catalyst that causes secondary contamination to wastewater in pilot-scale application. Therefore, the concentration of Fe3+ ions leaching from 0.20 g Fe-M-n-PAN in 100 mL solution at pH 6.0 under visible irradiation was measured by ICP-OES method, and presented in Fig. 9. Fig. 9 showed that Fe3+ ions slowly leached out from the complexes after the reaction had started. However, the final measured concentrations of Fe3+ ions in the solution within 24 h were lower than the amount of 2 ppm in water for European standard [8]. These findings demonstrated that three Fe-M-n-PANs could act as stable and efficient Fenton heterogeneous catalysts for the organic dye removal by means of the synergistic effect of adsorption and photocatalytic degradation in the presence of H2 O2 . 4. Conclusions A series of PAN nanofibrous membrane Fe complexes were prepared through surface modification with different ratio of HA and HH and subsequent coordination with Fe3+ ions, and then used as the Fenton heterogeneous catalysts for significantly enhancing the adsorption and visible photocatalytic performance of azo dye in water. The higher dye removal efficiency of the complexes was due mainly to a novel synergistic effect of adsorption and photocatalytic degradation in the presence of H2 O2 , which was determined by the ratio of HA and HH during their preparation. Three Fe-M-n-PANs produced with simultaneous modification of HA and HH exhibited much higher adsorption and visible photocatalytic activities than the complex modified solely with HA or HH because of their

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distinctive surface molecular structures containing more active sites, thus accelerating the removal of dyes from aqueous solution. Their adsorption and visible photocatalytic kinetics of azo dye could be described using pseudo-second-order model with a better correlation. Higher visible photocatalytic rate of Fe-M-n-PANs led to a fast degradation of dye molecule adsorbed on their surface sites, which vacates the adsorptive site for next dye molecule, thus remarkably promoting adsorption/photocatalytic degradation cycle process. Additionally, excellent reuse stabilities were also found in the cycle experiments of Fe-M-n-PANs. They displayed better chemical stability after five consecutive dye removal cycles. The present work may not only obtain a proper surface modification technique for PAN nanofibrous membrane materials with bifunctional roles as the adsorbent-based heterogeneous Fenton photocatalyst, but also has promising potentials to treat industrial wastewater pollution.

Acknowledgements This work was kindly supported by a grant from the Tianjin Municipal Science and Technology Committee for a Research Program of Application Foundation and Advanced Technology [grant number 11JCZDJ24600]; the National Natural Science Foundation of China [grant number 51102178]; National Key Technology Support Program [grant number 2015BAE01B03]; Innovation & Pioneering Talents Plan of Jiangsu Province [grant number 2015-26]; and Shaoxing Public-benefit Project [grant number 2014B70006].

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