Ferrous-oxalate-decorated polyphenylene sulfide fenton catalytic microfiber for methylene blue degradation

Composites Part B 176 (2019) 107220

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Ferrous-oxalate-decorated polyphenylene sulfide fenton catalytic microfiber for methylene blue degradation Lingquan Hu a, 1, Zhixiao Liu a, 1, Chenchen He a, Pei Wang a, Shaohua Chen a, *, Jing Xu a, Jing Wu a, Luoxin Wang a, **, Hua Wang b a

College of Materials Science and Engineering, State Key Laboratory of New Textile Materials & Advanced Processing Technology, Wuhan Textile University, Wuhan, 430073, China High-Tech Organic Fibers Key Laboratory of Sichuan Province, Sichuan Textile Science Research Institute, Chengdu, 610072, China

b

A R T I C L E I N F O

A B S T R A C T

Keywords: Heterogeneous fenton catalyst Ferrous oxalate Polyphenylene sulfide Catalytic microfiber Methylene blue

In this study, ferrous-oxalate(FeC2O4)-decorated polyphenylene sulfide(PPS) catalytic microfiber(PPS/FeC2O4) was synthesized as an efficient heterogeneous Fenton catalyst through a chemical precipitation method. More­ over, an underlying mechanism of synthetic processes was investigated and verified by monitoring intermediate products. Regarding the mechanism, the catalytic microfiber was found to solidly anchor FeC2O4 crystal grains on the surface due to sulfonated PPS interface bonding, remarkably improving the operational stability and reusability of the PPS/FeC2O4 catalyst. Additionally, the separation process of PPS/FeC2O4 was simplified without time-consuming filtration, attributable to the tangled structure of non-woven fabric that prevented the fibers from dispersing. In the typical process, improvement in the PPS/FeC2O4 catalyst revealed methylene blue degradation efficiency of the catalytic fiber above 90% at 6 min and 99% at 15 min, superior to that of the FeC2O4 catalyst. Therefore, PPS/FeC2O4 can be utilized as a potential heterogeneous Fenton catalyst for organic contaminant degradation in wastewater.

1. Introduction Advanced oxidation processes(AOPs) represent an efficient tech­ nique for degrading organic contaminants in wastewater. Hydroxyl radicals(⋅OH) with efficient reactivity can be activated in AOPs to degrade organic residues [1–3]. To decrease costs associated with AOPs, ferrous iron(Fe2þ) can be used to activate hydrogen peroxide(H2O2) into ⋅OH under mild conditions [4–6]. In the Fenton reaction shown in Eq. (1), ⋅OH is generated from H2O2 while Fe2þ is oxidized into ferric iron (Fe3þ). At a pH around 3, Fe3þ can be reduced by H2O2 to complete circulation of iron catalystsfor subsequent activation [6–9] as shown in Eqs. (2) and(3). H2O2 þ Fe2þ→Fe3þ þ ⋅OH þ OH Fe

3þ

Fe

3þ

þ H2O2→ Fe

2þ

2þ

þ HO2⋅→ Fe

þ HO2⋅ þ H þ O2 þ H

þ

þ

(1) (2) (3)

However, traditional homogeneous Fenton processes are difficult to perform efficiently due to the time-consuming processes of pH adjust­ ment, precipitation and filtering to separate the dissolved Fe2þ catalyst [10–19]. Therefore, heterogeneous Fenton processes have been reported in recent years [20–23], while some catalytic fibers of that were re­ ported for efficient decontamination of the organic wastewater by simplifying the process [24–30], as shown in Fig. 1. The efficiency of these catalytic fibers can be further improved via decoration with high-performance catalysts [31]. Therefore, a novel catalytic fiber decorated with efficient heterogeneous Fenton catalysts need to be designed. To improve the decoration stability, ion-exchange fibers were selected as a skeleton material due to their functional groups which can anchore the catalyst on the fiber surface. Furthermore, the most signif­ icant factors influencing catalytic performance should be investigated during synthetic processes. Polyphenylene sulfide(PPS) non-woven fabric can be maintained under ⋅OH-containing conditions due to its excellent chemical

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (S. Chen), [email protected] (L. Wang). 1 Lingquan Hu and Zhixiao Liu as co-first author contributed equally to this work. https://doi.org/10.1016/j.compositesb.2019.107220 Received 17 April 2019; Received in revised form 18 June 2019; Accepted 23 July 2019 Available online 24 July 2019 1359-8368/© 2019 Elsevier Ltd. All rights reserved.

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sulfate heptahydrate(FeSO4⋅7H2O, analytical grade); and potassium oxalate monohydrate(K2C2O4⋅H2O, analytical grade). All were dissolved in deionized water. Hydrogen peroxide(H2O2, 30 wt%) was diluted with deionized water to 10 wt%, and methanol(CH3OH) was diluted with deionized water to 50 wt%. 2.2. Preparation and characterization of PPS/FeC2O4 catalyst The PPS non-woven fabric was sulfonated by ClSO3H; 1.5 g of PPS fabric was immersed in 150 ml of 1,2-dichloroethane at 60 � C for 30 min. After cooling to below 0 � C, 7.5 ml of 1,2-dichloroethane diluent containing 2.0 ml of ClSO3H was added, and the mixture was mechan­ ically oscillated at 120 r/min for 30 min. Sulfonated PPS fabric was obtained after being washed with absolute ethanol and dried at 60 � C overnight. The PPS/FeC2O4 catalyst was synthesized using a simple, inexpen­ sive chemical precipitation method. First, 1.5 g of sulfonated PPS fabric was immersedin the FeSO4 solution(0.1 M, 150 ml) for 2 h. Then, theK2C2O4 solution(0.1 M, 150 ml) was added and the mixture was mechanically oscillated at 120 r/min for 2 h. The PPS/FeC2O4 catalyst was obtained after being washed with deionized water and dried at 60� Covernight. The morphology and elemental composition of samples were analyzed using SEM-EDS. Imaging experiments were conducted on a JSM-IT300A analytical scanning electron microscope with an EDS(JEOL Ltd., Tokyo, Japan). The crystal structure of samples was characterized by XRD analysis using a Rigaku Miniflex 600 X-ray diffractometer (Rigaku Corp., Tokyo, Japan), employing Cu Kα as the source of radia­ tion at λ ¼ 1.54056 Å over an angular range of 10� –80� . The surface functionalization and bonding configuration of samples were charac­ terized by FTIR spectroscopy analyzed with an FTIR-650 spectrometer (Tianjin Gangdong Sci.&Tech. Development Co. Ltd., Tianjin, China). The thermal analysis(TGA) was carried out by TG-209-F1(Bruker Corp., Karlsruhe, Germany). The Electron spin resonance(ESR) spectrum of the radical spin trapped by 5,5-dimethyl pyridine N-oxide(DMPO) was characterized by ESP 300E spectrometer(Bruker, Karlsruhe, Germany).

Fig. 1. Separation process of heterogenous Fenton reaction for waster­ water treatment.

durability. Moreover, the tangled structure of non-woven fabric can render separation more efficiently. Microfibers provide a high specific surface area for chemical modification and catalyst decoration. Addi­ tionally, the PPS fabric can be modified during the sulfonation process, which can functionalize the fiber with ion-exchange to investigate fac­ tors that influence performance. These properties make sulfonated PPS fabric a potential carrier that can improve performance for the hetero­ geneous Fenton catalyst. The selected heterogeneous Fenton catalyst should be synthesized under mild conditions, which can decorate a sufficient amount of cata­ lyst on the surface without damaging the organic microfiber. The mildest method for ion-exchange fibers is chemical precipitation for catalytic functionalization. However, the synthetic conditions of many reported heterogeneous Fenton catalysts are harsh for organic fibers [32–38]. During our investigation, ferrous oxalate(FeC2O4) as a novel heterogeneous Fenton catalyst was synthesized using a simple chemical precipitation method. Therefore, FeC2O4 was selected for catalytic functionalization due to its mild synthetic conditions. In this study, the catalytic fiber PPS/FeC2O4 was synthesized as a heterogeneous Fenton catalyst using a simple and inexpensive chemical precipitation method. To investigate and verify the mechanism of syn­ thetic processes and performance improvement of PPS/FeC2O4, samples were characterized using scanning electron microscopy(SEM), energy dispersive spectrometry(EDS), X-ray diffraction(XRD), and Fourier transform infrared(FTIR) analysis after processing. As a typical model of cationic and non-biodegradable dye, methylene blue(MB) was selected given its prevalence as an organic contaminant in textile dyeing wastewater and simple detection method as an indicator. Catalyst per­ formance was studied by evaluating the efficiency of MB degradation based on different experimental variables, such as H2O2 concentration, catalyst dosage, initial MB concentration, various structural dyes, and initial pH(pH0). An optimal parameter set was provided for industrial applications. The operational stability and reusability of PPS/FeC2O4 were analyzed during recycling periods, and compared to the catalytic fiber without sulfonation to investigate factor that influenced performance.

2.3. Experimental procedure The typical degradation process was carried out with 100 mg of the catalyst, which was added into a 250-ml glass beaker with 200 ml of 10 mg/L MB and 30 mg/L H2O2 solution at a near-neutral pH at 20 � C. The pH was then adjusted with 0.1 mol/L sulfuric acid and 0.1 mol/L sodium hydroxide. During this process, the solution pH remained nearly unchanged due to the low concentration of the MB solution. Solutions in all experiments were stirred at 1000 rpm with a magnetic stirrer for homogenization. After certain time intervals, a stock of the solution was diluted to 4 ml by 50 wt% methanol solution for concentration analysis. The solution absorbance was obtained using a 756S UV–Vis spectro­ photometer(Xian Yima Optoelec Co., Ltd., Shaanxi, China) at 664 nm, corresponding to the maximum absorbance of MB under typical conditions. 3. Results and discussion 3.1. Catalyst characterization

2. Experimental

The crystalline structure of the sulfonated PPSnon-woven fabric, assynthesized FeC2O4 catalyst, and as-synthesized PPS/FeC2O4 catalyst was analyzed by XRD analysis as shown in Fig. 2. High peaks at the 2θ values of 18.3� , 23.0� , 28.5� , 28.6� , 34.3� , 42.6� , 45.3� , and 48.2� can be ascribed to the ð 2 0 2 Þ, ð 0 0 4 Þ, ð 1 1 4 Þ, ð 4 0 0 Þ, ð 0 2 2 Þ, ð 2 2 4 Þ, ð 6 0 2 Þ, and ð 0 2 6 Þcrystal planes of β-FeC2O4(JCPDS No. 22-0635). The wide peak at 20.7� can be attributed to the ð 1 1 1 Þ crystal plane of PPS(JCPDS No. 37-1912). These results confirm that all diffraction peaks could be attributed toβ-FeC2O4, the main crystalline composition

2.1. Reagents and materials PPS non-woven fabric was produced via melt blowing spinning in the State Key Laboratory of New Textile Materials & Advanced Processing Technology. The following chemicals were purchased from Sinopharm Chemical Reagent Co., Ltd. and used without further purification: chlorosulfonic acid(ClSO3H, chemical pure); 1,2-dichloroethane (C2H4Cl2, analytical grade); MB(C16H18ClN3S, biological stain); ferrous 2

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Fig. 2. XRD analysis of crystalline structure of sulfonated PPS, FeC2O4, and PPS/FeC2O4.

of the catalyst in subsequent experiments. The diffraction pattern of the PPS/FeC2O4 catalyst was similar to that of the FeC2O4 catalyst, and the peaks demonstrated no significant deviation at 2θ values. In addition, the peak of the PPS/FeC2O4catalyst at 21.1� can be ascribed to the combined peak of PPS ð 1 1 1 Þ and FeC2O4 ð 2 2 4 Þ. Additionally, peaks of the PPS/FeC2O4 catalyst were broader than that of FeC2O4, demonstrating that smaller crystal grains were anchored on the micro­ fibers. Therefore, the PPS/FeC2O4 catalyst was composed of PPS and β-FeC2O4, and active sites of catalytic microfibers were exposed to improve the performance. The surface morphology and elemental composition of the sulfo­ nated PPS microfiber, Fe-anchored PPS(Fe-PPS) microfiber, and PPS/ FeC2O4 catalyst were examined using SEM-EDS analysis as shown in Fig. 3. The porous surface of sulfonated PPS microfiber displayed in Fig. 3(a) was prepared via a sulfonation process. The porous structure supported a high specific surface area for anchoring Fe2þ. After the ionexchange process, Fe2þ was anchored on the microfiber surface, whereas characteristic Fe peaks were observed in the EDS spectrum indicated in Fig. 3(f). Further, the FeC2O4 crystalline grains coated the microfiber surface (Fig. 3(c)) while PPS/FeC2O4 was obtained. The elemental composition of sulfonated PPS demonstrates that the atom ratio of C to S, which was close to 6, agrees with (C6H4S)n, the chemical formula of PPS. Combining the low atom rate of Fe in Fe-PPS, a small amount of -SO3H was modified in the PPS surface during the sulfonation process. These groups could anchor a mass of FeC2O4 grains on the microfiber surface and increase the operational stability and reusability (Fig. 3(h)). Meanwhile, the decorated grains were smaller than those of assynthesized FeC2O4, as shown in Fig. 3(d), corresponding to the XRD analysis results. Therefore, PPS/FeC2O4, with smaller grains growing on the microfibers, was successfully synthesized by the chemical precipi­ tation method. To obtain detailed analysis of surface functionalization and bonding configuration during the preparation process, FTIR spectra of samples during this process are presented in Fig. 4. The characteristic absorption bands of PPS non-woven fabric are shown in Fig. 4(b and c). The bands at 1571 cm 1 and 1470 cm 1can be attributed to C-C stretching vibra­ tion of the benzene ring. The bands at 1008 cm 1, 806 cm 1, and 741 cm 1can be assigned to the C-H in-plane deformation of ring hy­ drogens and the C-H deformation vibration of ring hydrogens of the disubstituted benzene ring and monosubstituted benzene ring, respec­ tively. The bands at 1092 cm 1 and 1072 cm 1 were characteristic Ar-Sbands, and the band of C-S stretching vibration was observed at 704 cm 1. The bands of -SO2- asymmetric and symmetric stretching vibration of sulfones at 1387 cm 1 and 1179 cm 1 were due to the low content of mixed sulfones for melt blowing spinning. These results

Fig. 3. SEM and EDS images of the morphology and elemental composition of (a,e)sulfonated PPS, (b,f)Fe-PPS, (c,g)PPS/FeC2O4, (d)FeC2O4, and (h)used PPS/FeC2O4.

confirm that the microfiber was aggregated by linear PPS, and the de­ gree of polymerization was low enough to make the monosubstituted benzene ring hydrogens of end groups easily distinguishable. After the sulfonation process, new bands were observed in the curve of sulfonated PPS shown in Fig. 4(b and c). The bands at 1227 cm 1 and 1058 cm 1 can be attributed to S¼O asymmetric and symmetric stretching vibration of -SO2-OH. The bands at 687 cm 1 and 620 cm 1 were derived from C-S stretching vibration and O-H out-of-plane deformation vibration of C-SO2-OH. A band of C-C stretching vibration of the trisubstituted benzene ring manifested at 1445 cm 1, and the intensity of the C-H wagging band of the disubstituted benzene ring reduced accordingly. These results indicate that the surface of the PPS microfiber was modified by sulfonic acid groups, with sulfonation occurring mainly on the disubstituted benzene ring. After the ion-exchange process, a band of H-O-H bending vibration of crystal water appeared at1636 cm 1, and the intensity of the band at 620 cm 1 of O-H deformation vibration of C-SO2-OH declined accord­ ingly as shown in Fig. 4(b and c). The reduction in intensity suggests that the Fe2þ was anchored by exchanging Hþ of -SO2-OH to form [–SO3]Fe2þ anchor sites. When decorated with FeC2O4, PPS/FeC2O4 presented a similar spectrum as FeC2O4 synthesized under the same conditions, as illus­ trated in Fig. 4(d). The bands at 1649 cm 1, 1621 cm 1, and 818 cm 1 3

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Fig. 4. FTIR spectrum of surface functionalization and bonding configuration of (I) PPS; (II) sulfonated PPS; (III) Fe-PPS; (IV) PPS/FeC2O4; and (V) FeC2O4.

can be attributed to the H-O-H bending vibration of crystal water, O-C-O asymmetric stretching vibration, and O¼C-O bending vibration. The bands at 1361 cm 1 and 1317 cm 1 can be assigned to O-C-O symmetric stretching vibration [39–41]. Characteristic bands at 1092 cm 1, 1072 cm 1, and 1008 cm 1 of PPS were observed in the PPS/FeC2O4 spectrum. These results indicate successful synthesis of PPS/FeC2O4. In nitrogen gas atmosphere(40 ml/min), the thermal decomposition of samples was illustrated in Fig. 5, heating at 10 � C/min. The mass loss of as-prepared FeC2O4 set at 150 � C and was almost completed at 400 � C, finally decreasing 57.5% of the total. During this temperature interval, PPS exhibited excellent thermostability with 0.5% mass loss at 400 � C. Meanwhile, the total mass loss of PPS/FeC2O4 at 400 � C was 42.3%. Thus, the content of FeC2O4 in PPS/FeC2O4 catalytic fiber was calcu­ lated at 73.3%. Generally, PPS/FeC2O4 was successfully synthesized via the chemi­ cal precipitation method. In this process, stable anchoring sites formed on the surface of sulfonated PPS. These sites can anchor the FeC2O4 grain coating on the surface of the microfiber to improve operational stability and reusability throughout the process.

3.2. Catalytic activity of PPS/FeC2O4 The degradation efficiency of MB was analyzed under different conditions, as shown in Fig. 6. Degradation was not observed in the process under the presence of either the catalyst or H2O2 only. The process involving the catalyst and H2O2 appeared highly efficient degrading of MB under dark conditions. In the typical process with the FeC2O4 catalyst, degradation efficiency of over 90% was observed at 10 min and 96.6% at 15 min. Moreover, this performance improved when the process was conducted with the PPS/FeC2O4 catalyst, and degradation efficiency exceeding 90% was observed at 6 min and 99.1% at 15 min and the catalytic activity analysis, Therefore, the catalyst performance of FeC2O4 was remarkably improved by adhering on the surface of sulfonated PPS microfiber, which was demonstrated by this improvement of the Fenton experiments with the lower content(73.3%) of FeC2O4 in PPS/FeC2O4 according to the results of TGA.

Fig. 5. Thermal analysis of FeC2O4, PPS/FeC2O4 and PPS in nitrogen.

Fig. 6. Catalytic activity of the catalyst under typical condition. 4

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3.3. Influence of experimental conditions on catalyst performance

radical activation, the catalyst performance for decontamination was assigned to decomposing H2O2 into O2, leading to unfavorable con­ sumption and compromised degradation efficiency. Therefore, the initial H2O2 concentration needed to be controlled at a favorable value to ensure efficient degradation of organic residue. Based on these find­ ings, 40 mg/L of H2O2 was chosenas the optimal concentration in sub­ sequent experiments.

3.3.1. Effect of H2O2 concentration The effect of initial H2O2 concentration was investigated by varying the concentration between 10 and 50 mg/L to obtain the optimal con­ centration, as shown in Fig. 7(a). With an increase in concentration from 10 to 40 mg/L, the degradation efficiency increased because more rad­ icals formed for degradation. However, the degradation efficiency clearly declined with continued increases in H2O2 concentration. This reduction phenomenon could be explained by the hydroxyl radical scavenging effect, depicted in Eq. (4). As the concentration of H2O2 increased excessively, radical traps formed to catch ⋅OH and generated ⋅HO2. Furthermore, generated ⋅HO2 caught the ⋅OH, produced oxygen (O2), and consumed the radicals as shown in Eq. (5). Combined with

⋅OH þ H2O2→⋅HO2 þ H2O

(4)

⋅HO2 þ⋅OH→O2 þH2O

(5)

Fig. 7. Influence of experimental conditions on catalyst performance([H2O2]0 ¼ 40 mg/L, [catalyst] ¼ 500 mg/L, [MB] ¼ 10 mg/L, pH0 ¼ 7,T ¼ 20 � C): (a) H2O2 concentration; (b) catalyst dosage; (c) MB concentration; (d) various dyes; (e) pH0; (f) five periods of catalyst recycling, PPS/FeC2O4 without sulfonation process (black curves) and PPS/FeC2O4(red curves). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 5

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3.3.2. Effect of catalyst dosage The effect of initial catalyst dosage was investigated by varying the dosage between 200 and 1000 mg/L, as shown in Fig. 7(b). With an increase in the dosage from 200 to 1000 mg/L, the degradation effi­ ciency improved due to an increase in the number of active sites for ⋅OH activation. However, this improvement was insufficient between 500 mg/L and further doubling of the dosage. Under the same condition, overdosage of the catalyst with excess active sites activated H2O2 and formed extra ⋅OH. The extra ⋅OH thus increased the rate of unfavorable reactions, as displayed in Eqs. (4) and (5). As reported by other scholars [32], excess active sites can capture extra ⋅OH and inactivate it, as shown in Eq. (6). Therefore, undesirable catalyst consumption increased alongside lower improvement of degradation efficiency. Based on these results and cost considerations, 500 mg/L of PPS/FeC2O4 was used as the optimal dosage in subsequent experiments. Fe2þ þ ⋅OH → Fe3þ þ OH

and adsorption behavior of dye molecules on the catalyst surface [36]. The degradation efficiency of pH0 at 3 was not improved with any further decrease in pH0. As reported in other studies [32,42,43], catalyst performance declines at a low pH because Fe3þ is attacked by H2O2 and, according to Eq. (7), inactivated into Fe4þ. When the pH0 declined to 3, the growth rate of degradation efficiency from ⋅OH activation was balanced by a lower number of active sites. Therefore, the pH0 of the process with the PPS/FeC2O4 catalyst was determined to have an optimal value of 5 for MB degradation. Fe3þ þ H2O2 þ Hþ→ Fe4þ þ ⋅OH þ H2O

(7)

3.3.6. Stability and reusability of PPS/FeC2O4 The stability and reusability of PPS/FeC2O4 were investigated by recycling the catalyst for five periods, as shown in Fig. 7(f). The catalyst was simply separated without filtration and then washed and dried in the same way as in preparation of the fresh catalyst for the next typical process. In the second recycling period, the degradation efficiency of the process declined due to inactivation of the number of active sites. Organic residues were absorbed on the surface of active sites and inac­ tivated them; however, the rate constant increased abnormally with the recycling process after the second recycling period due to self-activation of FeC2O4 [44]. During the fifth recycling period, the efficiency clearly declined because the mass of FeC2O4 fell under the limit in which the catalyst could support enough active sites to activate ⋅OH. The catalyst displayed effective performance in the four periods of the recycling process, and the catalytic activity fell markedly after the fourth period. In addition, the microfiber could be effectively reused to synthesize fresh PPS/FeC2O4 due to the reticular structure of the fabric. Comparing with the catalytic fiber without sulfonation, the operational stability and reusability of PPS/FeC2O4 were remarkably improved, which main­ tained catalyst performance during five recycling periods. This improvement can be attributed to the -SO3H groups of sulfonated PPS modified on the fiber surface, which transformed to the interface after the decoration process and steadily anchored the FeC2O4 grains coating the fibers. Therefore, PPS/FeC2O4 demonstrated high operational sta­ bility and reusability in the recycling process throughout the five periods because the sulfonated PPS layer anchored the FeC2O4.

(6)

3.3.3. Effect of initial MB concentration The effect of initial MB concentration was investigated by varying the concentration between 10 and 50 mg/L, as shown in Fig. 7(c). With an increase in the concentration from 10 to 50 mg/L, the rate constant declined. Combined with the initial MB concentration, the final degraded concentration increased at 15 min. Because the growth of degradation efficiency lagged behind the increase in the initial con­ centration, the rate constant presented a declining tendency. Due to a high MB concentration, active sites were surrounded by adequate MB molecules, and activated ⋅OH had more opportunities to attack MB and immediately degrade it. Simultaneously, the occurrence rate of unfa­ vorable reaction shown in Eqs. (4)–(6) decreased due to an increase in degradation probability. Therefore, the degradation efficiency improved with an increase in MB concentration. Additionally, the pH0 would reduce to 6.3 when the MB concentration increased to 50 mg/L. To maintain the stability of the solution near a neutral pH, 10 mg/L of MB (pH0 ¼ 6.8, 20 � C) was selected in subsequent experiments. 3.3.4. Effectiveness in degradation of various structural dyes Effectiveness in the degradation of various structural dyes was evaluated using model organic dyes in the typical process, as shown in Fig. 7(d). Based on differences in surface charges, two types of dyes were selected: cationic dyes (MB, Malachite green [MG], and Rhodamine B [RhB]) and anionic dyes (Orange II [OII], and Orange G [OG]). In the typical process, the degradation efficiency was not significantly different between various structural dyes. The degradation efficiency of the selected anionic dyes firstly increased and finally decreased compared with cationic dyes. This phenomenon could be explained by positive charge on the surface of catalytic fibers. After the ion-exchange process, the generated –[SO3]-Fe2þ supported the positive charge on the fibers, which would attract the anionic dye molecules and efficiently degrade them. It would accelerate the degradation of anionic dye at first. How­ ever, their degradation efficiency decreased comparing with that of cationic dyes due to the changes of the charge on the fiber surface during the process. Therefore, the process demonstrated efficient degradation of various structural dyes. As a typical model cationic and nonbiodegradable dye, MB was selected for degradation experiments to evaluate catalyst performance.

3.4. Mineralization with reaction time The mineralization in the typical process was evaluated by TOC removal efficiency, illustrating in Fig. 8. Before the TOC analysis, solu­ tion samples were diluted into 10 ml by 60 mg/L Na2S2O3 solution to terminate the reaction. The TOC removal efficiency of MB rose rapidly to

3.3.5. Effect of initial pH (pH0) The effect of pH0 was investigated by varying the pH0 between 3 and 9, as shown in Fig. 7(e). In the experiments, pH0 ¼ 7 was approximated by the initial MB solution without adjustment due to the low MB con­ centration. With a reduction in pH0 from 9 to 5, the degradation effi­ ciency improved because⋅OH was effectively activated under acidic conditions according to Eq. (1). Moreover, pH influenced the charges

Fig. 8. The mineralization of the typical process analysing by TOC removal efficiency. 6

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72.4% at 10 min. After 10 min, it increased slightly to 78.5% at 60 min. Therefore, the typical process could efficiently mineralize the MB molecules. 3.5. Possible mechanism 3.5.1. Synthetic process of PPS/FeC2O4 After characterization of a sample under different processes, the mechanism of the synthetic process was illustrated in Fig. 9(a–c). In the sulfonation process, ClSO3H attacked the benzene ring of the polymer backbone and substituted the hydrogens of the disubstituted benzene ring while porous sulfonated PPS microfiber was synthesized with formed hydrogen chloride(HCl), as illustrated in Fig. 9(a). Then, the porous microfiber was immersed in the FeSO4 solution. Fe2þ exchanged the Hþ of -SO2-OH to form [–SO3]- Fe2þ anchor sites on the microfiber surface, as shown in Fig. 9(b). When the K2C2O4 solution was added, the ferrous oxalate complex formed immediately without precipitation. The microfiber with anchor sites could concentrate the complex, while FeC2O4 crystal grains were grown from anchor sites as the crystal nuclei, as displayed in Fig. 9(c). When the FeC2O4 catalyst coated the surface, the layer of sulfonated PPS transformed into the interface. The interface bonded the FeC2O4 and PPS microfiber to improve anchoring strength and support operational stability for the catalyst. After water shearing, the unstable crystal layer was stripped, while the anchored layer solidly coated the microfiber surface and PPS/FeC2O4 was synthesized after drying. Meanwhile, the photo of catalytic fabric was shown in Fig. 9(c). Moreover, the results of the recycling process demonstrate that the PPS/ FeC2O4 catalyst had high operational stability and reusability. There­ fore, the sulfonated PPS microfiber could solidly anchor FeC2O4 crystal grains on the surface, and the sulfonated PPS interface could improve the operational stability and reusability of the PPS/FeC2O4 catalyst.

Fig. 10. The ESR signal of the DMPO-⋅OH at 5min in the typical process with 1 g/L DMPO solution.

organic molecules. At the same time, Fe2þ can be oxidized into Fe3þ, and generated Fe3þ can be reduced by C2O24 for reactivation of active sites [14–23], as shown in Eq. (8). Remarkably, the catalyst performance of FeC2O4 was improved by adhering on the surface of sulfonated PPS microfiber, demonstrating by the improvement of the Fenton experi­ ments with the lower content of FeC2O4 in PPS/FeC2O4. The anchor sites of the microfiber were supported to anchor the FeC2O4 grains and pre­ vent the grains from agglomerating while maintaining a high specific surface area of PPS/FeC2O4, thereby improving catalyst performance. Simultaneously, the sulfonated PPS microfiber can also concentrate the MB molecules to surround the active sites, as shown in Fig. 9(d). As a result of this concentration, molecules were more susceptible to be attacked and degraded by generated ⋅OH. Therefore, the PPS microfiber was found to improve the catalyst performance of PPS/FeC2O4 for MB degradation.

3.5.2. Improvement of MB degradation PPS/FeC2O4/H2O2 is a novel heterogeneous Fenton process in which ⋅OH can be activated at active sites on the surface of PPS/FeC2O4 [32–38] shown in Fig. 9(d), which can be determinated in the ESR signal of the DMPO-⋅OH [44–46]. The activated ⋅OH can efficiently degrade

2Fe3þ þ C2O24 þ 2H2O → 2Fe2þ þ O2 þ 2CO2 þ 2Hþ

(8)

Fig. 9. Possible underlying mechanism of synthetic process and performance improvement: (a) PPS microfiber sulfonation process; (b) sulfonated PPS microfiber ion-exchange process; (c) Fe-PPS microfiber decoration process; and (d) degradation process. 7

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4. Conclusion The PPS/FeC2O4 catalyst was synthesized by a simple and inexpen­ sive chemical precipitation method in this study. Due to sulfonated PPS interface bonding, the microfiber was found to solidly anchor FeC2O4 crystal grains on the surface to improve the operational stability and reusability of the PPS/FeC2O4 catalyst. Moreover, the catalyst can effectively activate ⋅OH for MB degradation under mild conditions. In the typical process, degradation efficiency exceeding 90% was observed at 6 min and 99% at 15 min. Compared with the FeC2O4 catalyst, the sulfonated PPS microfiber effectively improved the performance of the FeC2O4 catalyst by supporting a high specific surface area and concen­ trating MB molecules. In addition, PPS/FeC2O4 can be simply separated for the next recycling process to decrease processing costs. It can resist the current of the continuous Fenton processes. Therefore, PPS/FeC2O4 represents a potential heterogeneous Fenton catalyst for degradation of organic contaminants in wastewater, which can be utilized in novel continuous heterogeneous Fenton processes. Acknowledgement This work was supported by the Hubei Provincial Department of Education [No. 20171605] and the High-tech Organic Fibers Key Lab­ oratory of Sichuan Province [No. 201603]. Natural Science Foundation of Hubei Province [No. 2018CFB685]. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.compositesb.2019.107220. References [1] Pignatello JJ. Dark and photoassisted Fe3þ-catalyzed degradation of chlorophenoxy herbicides by hydrogen peroxide. Environ Sci Technol 1992;26: 944. ~ ez P, Maldonado MI, Blanco J, Gernjak W. [2] Malato S, Fern� andez-Ib� an Decontamination and disinfection of water by solar photocatalysis: recent overview and trends. Catal Today 2009;147:1–59. ~ ez P, Blanco J, Malato S. Solar photo[3] Gernjak W, Fuerhacker M, Fern� andez-Ib� an Fenton treatment-process parameters and process control. Appl Catal B Environ 2006;64:121. [4] Huston PL, Pignatello JJ. Degradation of selected pesticide active ingredients and commercial formulations in water by the Photo-assisted Fenton reaction. Water Res 1999;33:1238–46. [5] Pignatello JJ, Sun Y. Complete oxidation of metolachlor and methyl parathion in water by the photo-assisted Fenton reaction. Water Res 1995;29:1837–44. [6] Liu M, Yu Y, Xiong S, Lin P, Hu L, Chen S, Wang H, Wang L. A flexible and efficient electro-Fenton cathode film with aeration function based on polyphenylene sulfide ultra-fine fiber. React Funct Polym 2019;139:42–9. [7] Monteagudo JM, Dur� an A, Lopez-Almodovar C. Homogeneous ferrioxalate-assisted solar photo-Fenton degradation of orange II aqueous solutions. Appl Catal B Environ 2008;83:46–55. [8] Evgenidou E, Konstantinou I, Fytianos K, Poulios I. Oxidation of two organophosphorus insecticides by the photo-assisted Fenton reaction. Water Res 2007;41:2015–27. [9] Alalm MG, Tawfik A, Ookawara S. Degradation of four pharmaceuticals by solar photo-Fenton process: kinetics and costs estimation. J Environ Chem Eng 2015;3: 46–51. [10] Zepp RG, Faust BC, Hoign� e J. Hydroxyl radical formation in aqueous reactions (pH 3–8) of iron(II) with hydrogen peroxide: the photo-Fenton reaction. Environ Sci Technol 1992;26:313–9. [11] Ruppert G, Bauer R, Heisler GJ. The photo-Fenton reaction–an effective photochemical wastewater treatment process. Photochem Photobiol 1993;73:75–8. [12] Pignatello JJ, Chapa G. Degradation of PCBs by ferric ion, hydrogen peroxide and UV light. Environ Toxicol Chem 1994;13:423–7. [13] Dong Y, Chen J, Li C, Zhu H. Decoloration of three azo dyes in water by photocatalysis of Fe(III)-oxalate complexes/H2O2 in the presence of inorganic salts. Dyes Pigments 2007;73:261–8. [14] Monteagudo JM, Dur� an A, Martín IS, Aguirre M. Effect of continuous addition of H2O2 and air injection on ferrioxalate-assisted solar photo-Fenton degradation of orange II. Appl Catal B Environ 2009;89:510–8.

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