WO3 membrane for the photoelectrocatalytic removal of Reactive Red 120 dye applied in a flow reactor

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Sandwich Nylon/stainless-steel/WO3 membrane for the photoelectrocatalytic removal of Reactive Red 120 dye applied in a flow reactor Alysson Stefan Martinsa, , Abdou Lachgarb, Maria Valnice Boldrin Zanonia, ⁎

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a

National Institute of Alternative Technologies for Detection, Toxicological Evaluation and Removal of Micropollutants and Radioactive Substances (INCT-DATREM), Institute of Chemistry, São Paulo State University, Araraquara 14800-900, São Paulo, Brazil b Department of Chemistry and Center for Energy, Environment, and Sustainability, Wake Forest University, NC, USA

ABSTRACT

Coupling photocatalysis and filtration has emerged as an attractive alternative to improve the performance and minimize effects inherent to current technologies used for wastewater treatment. Herein, we report the preparation of a novel sandwich membrane constituted of a Nylon fiber and a stainless steel mesh with electrodeposited WO3 integrated to photoelectrocatalysis/filtration. The sandwich membrane showed high performance in the photoelectrocatalysis (PEC) treatment of waste water due to synergistic effect of filtration, adsorption and photoelectric processes in a flow reactor. Under optimized conditions, the membrane filtration integrated with photoelectrocatalysis achieved high oxidation rate and adsorption/filtration of the textile dye RR-120. The dye was completely removed after 150 min when an aqueous solution of RR-120 (1 × 10−5 mmolL−1) at pH = 4.0 was flown through the membrane at 160 mL min−1 with an applied potential of 1.00 V.

1. Introduction Membrane filtration is widely used for wastewater decontamination due to its low energy consumption, no-use of reagents and easy handling process that ensures sustainable clean water production. The technology is based in the utilization of fibrous and/or porous materials capable of separating and retaining organic, inorganic and biological contaminants [1,2]. Although membrane filtration technology is widely employed, it suffers from two critical obstacles. The most important issue is membrane-fouling which decreases permeation flux through the concentration polarization, and ultimately results in pore blocking [3–5]. The filtration efficiency is thus limited by the saturation of the material that constitute the membrane. The other issue is difficulty to retain and remove contaminants with sizes smaller than membrane pores/fibers, as well as highly soluble compounds [6–8]. Thus, it is important to design, fabricate, and test membranes that can maintain high performance under most stringent conditions. The integration of filtration systems with other treatment technologies can mitigate the aforementioned limitations of membrane filtration processes. In this context, semiconductor photocatalysis (SPC) represents an attractive technology to integrate into membrane filtration systems, because a wide range of organic pollutants can be photocatalytically oxidized by photogenerated radicals such as hydroxyl (OH%) [9–12]. Thus, photocatalytic processes can greatly reduce pore blocking,

⁎

therefore, control membrane fouling [13]. However, semiconductor photocatalytic processes suffer from low efficiency due to high recombination rate of photogenerated charge careers (i.e. electrons and holes). Consequently, photocatalytic oxidation of organic pollutants though attractive has rather low efficiency, and does not effectively solve the stability issues of membrane filtration [14,15]. The photocatalytic efficiency can be enhanced by applying a potential to the semiconductor photocatalyst. The process referred to as photoelectrocatalysis leads to better charge careers separation, which leads to improved photo-oxidation of organics [14,15]. However, the literature reports few works dealing with the integration of membrane filtration and photoelectrocatalysis [16,17]. The use of TiO2/carbon/ Al2O3 composite as photoelectrocatalytic membrane with voltage supply showed improvement of removal of natural organic matters (NOMs) 1.2 or 1.7 times higher than only filtration with UV irradiation or filtration alone [16]. Similarly, the photoeletrocatalytic performance of the N-doped TiO2/NaY zeolite membrane used as electrode in the phenol degradation showed great potentiality with high recycling performance [17]. Fabrication of a device in which both, filtration and photoelectrocatalysis can proceed simultaneously is rather challenging, because of the need to create a junction between the catalyst and a conducting substrate that has efficient filtration properties. In general, a good

Corresponding authors. E-mail addresses: [email protected] (A.S. Martins), [email protected], [email protected] (M.V. Boldrin Zanoni).

https://doi.org/10.1016/j.seppur.2019.116338 Received 22 July 2019; Received in revised form 8 November 2019; Accepted 20 November 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved.

Please cite this article as: Alysson Stefan Martins, Abdou Lachgar and Maria Valnice Boldrin Zanoni, Separation and Purification Technology, https://doi.org/10.1016/j.seppur.2019.116338

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Fig. 1. Schematic representation of photoelectrocatalytic flow reactor in: dismantled system (A); cross section (B); and front view (C). Contact to the photoelectrocatalytic membrane (1); counter electrode (platinun network) (2); reference electrode (3); inlet (4); quartz disk (5); photoelectrocatalytic membrane (6); outlet (7); and setup (D).

membrane must have: (a) stable junction between its components; (b) high and stable photocatalytic oxidation power (c) sufficient resistant to fluid flow; and (d) able to retain/remove the contaminants. Considering such characteristics, the construction of a photoelectrocatalytic membrane joining two different modules, called sandwich, can afford a new robust material combining the filtration with the PEC system. To address these issues, the present work propose the fabrication of a new PEC sandwich membrane designed by junction of two components: (i) a filtration part that consists of Nylon fibers, made of

synthetic polymer based on polyamides structures, that can be easily processed, and have excellent thermal and mechanical properties, malleability and good compatibility with other materials [18]; (ii) a PEC part that consists of a steel mesh coated by a semiconductor was selected as the electroconductive substrate due to its high stability and availability, economic feasibility, and easy handling. Among semiconductor photocatalysts, WO3 was chosen due to its low bandgap energy (Eg = 2.7–2.8 eV) and appropriate band positions relative to oxidation processes [12,19]. Furthermore, WO3 can be easily

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Fig. 2. Molecular structure of textile dye reactive red -120 (RR-120).

deposited on different surfaces (such as steel mesh) via electrodeposition, producing a stable coating that has been shown to results into high photoactivity [20–22]. Although, numerous studies on PC membranes have been reported [23–26], reactors involving PEC membranes are more complex, because the electrodes arrangement of the counter and reference electrode can compromise the flow as well as interfere with the irradiation of the working electrode. The proposed lab-scale reactor for simultaneous PEC and filtration is shown in Fig. 1. The reactor is a standard three-electrode system, which consists of the membrane as working electrode, platinum mesh as the counter electrode and Ag/AgCl reference electrode. Herein, we report the design, characterization, and performance assessment of a novel Nylon/Steel-WO3 membrane used in a continuous flow reactor for simultaneous filtration/PEC process. The textile dye Reactive Red -120 (RR-120) (Fig. 2) was chosen as a model of organic contaminant to be photoelectrocatalytically oxidized in the reactor. RR120 is an azo dye widely used in textile finishing processes [27–29]. It belongs to the largest group of carcinogenic and mutagenic xenobiotic pollutants ubiquitously discharged from textile industries and, therefore, presents serious health risks. Furthermore, RR-120 is resistant to degradation by traditional treatment methods [27–29].

mechanical pressure between the two compartments in the flow reactor. The circular geometric area exposed to irradiation was 2.3 cm2 (Fig. 1a).

2. Experimental

The photoelectrocatalytic activity of the steel mesh-WO3 electrodes were assessed using linear sweep voltammetry in the dark and under visible-light irradiation, at scan rate of 50 mV s−1, in the potential range of −0.50 to +1.50 V using a 0.1 M solution of Na2SO4 as electrolyte. The arrangement of three-electrode in the reactor flow, is shown Fig. 1b and c. The solar illuminator employed was a Xe arc lamp (ORIEL, 300 W) equipped with a set of lenses for light collection and focus, externally located to the reactor with a quartz window facing the membrane electrode.

2.2. Structural characterization The crystallinity and purity of the electrodeposited WO3 was confirmed by powder X-ray diffraction using a D500 (Siemens) diffractometer (Cu Kα radiation, λ = 1.5406 Å). The electrodes’ surface morphology and elemental compositions were examined by field emission gun scanning electron microscopy (FEG-SEM), using a JEOL 7500 F microscope equipped with energy dispersive X-ray (EDX) detector. The optical properties of the membranes were determined by the diffuse reflectance spectroscopy (DRS) using a Cary 60 UV–vis spectrometer equipped with an external module of diffuse reflectance probe (Barrelino™, Harrick Scientific, Pleasantville, NY, USA). The calibration was performed using a Spectralon standard (Labsphere USRS-99-020, 99% reflectance) and the reflectance and absorbance were measured between 250 and 800 nm. 2.3. Electrochemical characterization

2.1. Preparation of anodic sandwich membrane of Nylon/Steel-WO3 The anodic sandwich membrane was fabricated by junction of two components. In the first component, the semiconductor, WO3 was electrochemically deposited on steel mesh using a three-electrodes electrochemical cell (200 mL capacity), where the steel mesh was the working electrode (geometric area of 3.0 × 4.0 cm2), the counter electrode was platinum foil and a saturated calomel was used as reference electrode (SCE). To test the effect of mesh size, the electrodeposition was performed on steel with mesh sizes (30; 100; 200; 300; 400 and 500-mesh). WO3 was electro-deposited from an electrolyte solution (70 mL) containing Na2WO4 5 mmol L−1, H2O2 0.075% (v/v) and pH = 1.4 adjusted by HNO3 concentrated. A potential of −0.45 V (vs. SCE) was applied for 20 min using a Metrohm Autolab (Utrechet, Netherlands) model PGSTAT 302 potentiostat/galvanostat [30]. The steel mesh-WO3 electrodes were washed with ultrapure water, dried under nitrogen, then sintered at 450 °C for 2 h (ramp of 2 °C min−1). The other component of the membrane was a Nylon fiber, commercially produced by Unifil (0.45 µm porosity), which was circularly cut (diameter 3 cm) and attached to the steel-WO3 electrode by

2.4. Membranes performance in the flow reactor Fig. 1 shows the lab-scale flow reactor used to evaluate membranes’ performance. The cell with 30 mL capacity consists of two compartments separated by the membrane; the three electrodes arrangement are disposed in the first compartment consisting of the working electrode (membrane - 2.3 cm2 circular geometric area), the counter electrode (platinum network) and the reference Ag/AgCl which is allocated laterally to the membrane. The cell was connected to Metrohm Autolab potentiostat/galvanostat. An aqueous solution of RR-120 (1 × 10−5 mmolL−1) used as model of organic contaminant (SigmaAldrich 60% m/m, C.I. 61951-82-4), and Na2SO4 (0.1 mol L−1) used as

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oxygen evolution which could compete to the hydroxyl radicals generation. To monitor the decomposition of RR-120, samples were collected at various time intervals and analyzed by high performance liquid chromatography using a Shimadzu (Kyoto, Japan) equipped with a PFP Phenomenex Kinitex (150 × 4.60 mm, 5 µm, 100 Å) at 30 °C and coupled to a UV–vis detector set at 293 nm. The flow rate was 1 mL min−1 conducted with acetonitrile/ammonium acetate (10 mM) 12/88 increase to 30/70 within 20 min of run. The samples’ UV–vis absorption spectra were also recorded using a Varian Cary 1 spectrophotometer with quartz sample cell with 1 cm path length. The rate of organic carbon mineralization was evaluated using the optimized condition of photoelectrocatalytic degradation established by measuring TOC using a Shimadzu TOC-VCPN analyzer.

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3. Results and discussion

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To select the best Anodic Steel Mesh (ASM) to integrate into the photoelectrocatalytic sandwich membrane, the photoactivity of different ASMs with electrodeposited WO3 were measured in the dark and under irradiation. In the dark (Fig. 3a), all modified ASM show negligible photocurrent under an applied potential of −0.50 to 1.25 V. At more positive potentials high current is observed due to oxygen evolution from electrochemical water oxidation. The current profile show that ASM can be used as conductive substrate in the membranes in a wide potential range without interference with WO3 photoactivity. Under irradiation, Fig. 3b, the current increases substantially for all WO3-modified ASMs. The charges of the WO3 are photoexcited creating an effective charge separation; the holes migrate towards the surface and the electrons move to the bottom of the semiconductor [21,31]. Surprisingly, the photocurrent decreases progressively as ASMs’ mesh number increases (ASM30 to ASM500). The highest photocurrent is observed for ASM30. This could be due to structural deformation of the ASM substrate after thermal treatment. This structural deformation, which becomes more pronounced as the porosity increases affects the stability of the conductor substrate, and results in low photoactivity of WO3-modified ASMs as porosity increases. The structural stability might be associated with reduction of steel wires thickness from ASM30 to ASM500, which become less robust after WO3 sintering. To optimize the membrane, i.e. obtain the highest photocurrent, WO3 was electrodeposited on ASM30 at interval times of 10 (ASM3010); 20 (ASM30-20) and 30 min (ASM30-30). The photocurrent increased as deposition time increased (Fig. 3c), which is attributed to more WO3 content deposition, as previously reported [19,32]. It is presumed that the number of charge careers (electron and holes) increases with greater semiconductor content leading to substantially higher photocurrent. ASM30-30 was thus chosen as photoectrocatalytic component to integrate into the membrane.

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Fig. 3. Photocurrent vs potential of the steel mesh unmodified (a); ASM30; ASM100; ASM200; ASM300; ASM400 and ASM500 electrodes (b) and ASM3010, ASM30-20 and ASM30-30 electrodes (c); evaluated under UV–Vis irradiation and under dark. Linear sweep voltammetry was conducted at 50 mV s−1 in 0.1 mol L−1 Na2SO4 using UV–Vis irradiation with a Xe arc lamp.

3.2. Morphological and structural characterization Scanning electron microscopy (SEM) images of ASM30-30 are shown in Fig. 4. The steel-mesh (Fig. 4a) shows linear grooves with some surface deformation characteristic of the steel substrate. After electrodeposition (Fig. 4b and c, the surface exhibits homogeneous distribution of WO3, indicating that the electrode was successfully modified. Fig. 4f shows the elemental mapping profile data of ASM3030, the dashed white line shows the interface between the steel-mesh and the semiconductor, and confirms the high WO3 content. The

electrolyte was flown through the membrane using a peristaltic pump (100 mL min−1). The solution (50 mL) was recirculated through a jacketed glass reservoir (50 mL capacity) maintained at 20 °C. The PEC experiments were initially performed at +1.00 V vs Ag/AgCl (pH = 6), such potential is higher than the flat band potential and does not show

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Fig. 4. Field emission gun-scanning electron microscopy (FEG-SEM) showing the components of the photoelectrocatalytic sandwich membrane ASM30-30: steel mesh substrate (a); WO3 deposited on the steel mesh in low (b) and high (c) magnification; nylon fiber in low (d) and high (e) magnification; and (f) elemental colour mapping after WO3 deposition.

mapping also confirms the elemental distribution of iron, chromium, nickel and carbon that constitute the steel mesh. The total amount of WO3 was also estimated on the electrode surface applying the first Faraday law from the chronoamperometry curve (Fig. 1S), where the mass electrodeposited was calculated to be 4.764 × 10−3 g. Fig. 2S demonstrates that a thin layer of WO3 was electrodeposited onto the stainless steel (the WO3 layer was detached by emerging the electrode in liquid nitrogen). The deposition was very thin compared to the stainless steel and therefore there was not significantly affect the porosity. The Nylon fiber to be coupled with the ASM30-30 to form the phoelectrocatalytic membrane, was also characterized by SEM (Fig. 4d and e). The fiber exhibits a porous sheet with pore size 100–900 nm, uniformly distributed on the surface of the electrode. Since the fiber is allocated after the steel-mesh modified and with flow in its direction, it act as a filter to remove organic pollutants not photo-mineralized by the WO3-modified steel mesh.

The powder X-ray diffractograms (PXRD) of the ASM30 and ASM3030 electrode after annealing are shown in Fig. 5. Prior to WO3 modification (Fig. 5a), the PXRD shows only peaks of metal oxides on stainless steel 304, the peaks mainly correspond to FeCr2O4 (JCPDS 340140) and Fe3O4 (JCPDS 19-0629) [33,34]. After WO3 deposition, the PXRD (Fig. 5b and c) matches the monoclinic phase of WO3 (JCPDS 43–1035) [19]. 3.3. Diffuse reflectance spectroscopy (DRS) characterization Diffuse reflectance UV–vis spectrum of the ASM30-30 electrode is shown in Fig. 5d. The steel mesh substrate has no significant absorption. After modification, an intense absorption band appears in the visible range between 400 nm and 600 nm, which is attributed to the monoclinic phase of WO3 [35,36].

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Configuration

Designation

Filtration (FT) Photolysis (PT) Filtration + Photolysis (FT) + (PT) Photocatalysis (PC) Filtration + Photocatalysis (FT) + (PC) Photoelectrocatalysis (PEC) Filtration + Photoeletrocatalysis (FT) + (PEC)

Nylon fiber UV–Vis Irradiation Nylon fiber + UV–Vis Irradiation

FT (NY) PT FT/PT (NY)

Steel Mesh/WO3 + UV–Vis Irradiation Nylon fiber + Steel Mesh/WO3 + UV–Vis Irradiation

PC (Steel Mesh-WO3) FT/PC (NY/Steel Mesh-WO3)

Steel Mesh/WO3 + UV–Vis Irradiation + Potential Applied Nylon fiber + Steel Mesh/WO3 + UV–Vis Irradiation + Potential Applied

PEC (Steel Mesh-WO3) FT/PEC (NY/Steel Mesh-WO3)

3.4. Removal of the dye RR-120 in a flow reactor

concentration was reduced by only 3% after 90 min photolysis - PT. In contrast, using NY alone, i.e. filtration, a significant removal of RR-120, up to 24% was achieved. The observed enhanced removal of RR-120 is more pronounced in the first 10 min and remains practically constant until the end of the experiment. However, when NY filtration is carried out under irradiation (PT) 33% of RR-120 is removed, the efficiency is higher when considers the total percentage of methods FT and PT alone (27% in total). The enhanced removal of RR-120 when using a combination of FT and PT may be due to combination of photooxidation of RR-120 retained in the fibers [37,38]. Moreover, the filtration process

Given the complexity of the membrane and the processes involved, different configurations needed to be studied (Table 1). The performance of the Nylon fiber (NY) and WO3-modified steel-mesh (ASM3030), and that of the combined membrane (NY/ASM30-30), were evaluated using filtration (FT), photolysis (PT), photocatalysis (PC) and photoelectrocatalysis (PEC) processes. Under only UV–vis irradiation (photolysis), no significant change of the concentration of RR-120 was observed (Fig. 6a). The initial RR-120

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Fig. 6. RR-120 removal using different configurations in the flow reactor: (a) PT and FT using NY fiber alone; (b) PT, PC and PEC using ASM30-30 alone; (c) PT, PC, PEC and FT using NY/ASM30-30 sandwich membrane and (d) comparison between PC and PEC using ASM30-30 alone and NY/ASM30-30 sandwich membrane. Conditions: 1 × 10−5 mol L−1 RR-120 dye in Na2SO4 0.1 mol L−1, pH 6, flow of 100 mL min−1, Xe arc lamp.

can pre-concentrate the dye RR-120 retained in membrane where the photolytic reaction could be more efficient. The residence time of the RR-120 is affected and consequently the photolysis is improved; such effect is common in photolytic process using heterogeneous process. Thus, the combination FT/PT can improve the removal of RR-120 comparing to the filtration using only NY in the dark. Subsequently, the removal of RR-120 by photocatalysis (PC) and photoelectrocatalysis (PEC) using ASM30-30 membrane alone was studied (Fig. 6b). In the PT process the concentration of RR-120 gradually decreases during the experiment. A maximum 10% degradation of RR-120 is observed by the end of the experiment. Using PEC process, significantly higher degradation efficiencies are observed, with 30 and 40% of RR-120 removed when applying potentials of +0.50 and +1.00 V, respectively. The higher efficiency observed when using PEC process can be attributed to better separation of photo-generated charge careers leading to lower recombination rate. In the PEC process, electrons are driven from the catalyst surface towards the cathode via the external circuit, while holes remain at the anode [39]. In the first 30 min, using PEC process, ASM30-30 has lower removal efficiency compared to NY under PT. However, its performance increased progressively over time. Hence, FT is able to quickly remove RR-120 but its efficiency is limited by saturation of the fibers, in contrast removal of RR-120 by ASM30-30 membrane using PEC process is continuously

removed due to oxidation. Once each component’s performance under all processes used was performed, the complete sandwich membrane that consists of NY and ASM30-30 was assembled and its performance was assessed in a continuous flow reactor (Fig. 6c). Under irradiation the sandwich membrane removed 37% of RR-120 after 90 min. This removal rate far exceeds the 10% removed when ASM30-30 alone was irradiated (Fig. 6b). When a potential is applied to the membrane, 42 and 50% of RR-120 are removed at +0.50 and +1.00 V, respectively. It is presumed that RR-120 is promptly adsorbed by the NY via FT, and consequently oxidized by the ASM30-30 via PEC process. These results demonstrate that combination of FT and PEC work synergistically to more efficiently remove RR-120 via a tandem adsorption-oxidation process. The removal capacity of the sandwich membrane (NY/ASM30-30) and steel mesh alone (ASM30-30) used under PC and PEC methods are better compared in Fig. 6d. In the first 45 min, the sandwich membrane shows higher performance for both PC and PEC when combined with filtration. After 60 min, the removal efficiency of RR-120 using ASM3030 alone in a PEC process gradually increases becoming higher than that of the sandwich membrane (NY-ASM30-30) under PC conditions. The results suggest that even though the Nylon fiber initially has a significantly better performance in the removal of RR-120 compared to both PC and PEC, its performance capacity remains constant after

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the hydrodynamic pressure can compromise its structure, generating preferential pathways and/or increased pore size, which may affect the retention of the organic compounds. The 160 mL min−1 flow rate was thus adopted for further studies. The pH of the solutions was then investigated. This parameter plays an important role in the PEC performance since it affects the reactions at the membrane/electrolyte interface due to (i) pH dependent ionization of RR-120, (ii) adsorption of electroactive species and (iii) oxidation of H2O/OH− to OH% or other radicals [41]. The performance of the membrane was evaluated in acidic (pH = 4.0), neutral (pH = 6.0) and alkaline (pH = 9.0) conditions. The removal of RR-120 increased as pH decreased, 50% at pH = 9, 55% at pH = 6, and 80% at pH = 4. (Fig. 7b). The excellent performance observed in acidic conditions can be explained by greater oxidizing power of the OH% radicals generated at the surface of the membrane. Indeed, the Nernst equation (E (OH%aq / H2O) = 2.59–0.059xpH) predicts higher reduction potential of OH% as the pH decreases. Furthermore, in acidic medium the dye is adsorbed on the electrode surface via electrostatic interactions. Since the isoelectric point of WO3 is at around 0.5 [42] and the pKa for RR-12- is ~12.5 [43], higher adsorption occurs in acid medium when WO3 is positively charged, maximizing photooxidation. Another important factor that may explain lower RR-120 removal as the pH increases is the generation of (HCO3)− and (CO3)2− in basic conditions during the oxidation of RR-120. These compounds are radical scavengers and, consequently, reduce the degradation efficiency at pH 9 [44]. Finally, the time needed to reach maximum removal/oxidation of RR-120 was determined. No RR-120 was not found in the solution after 150 min (Fig. 8a). The chromatograms (Fig. 8b) recorded during the experiment show intermediates observed at 6 and 9 min retention times. These intermediates gradually disappear as they are further oxidized. The mineralization rate of organic compounds was also evaluated since byproducts of the degradation process may remain in solution. In three hours of experiment it was achieved about 91% of TOC removal (inset of Fig. 8a). Thus, the optimized condition is not only required for the efficient removal of RR-120 but particularly important to degradation of intermediates/byproducts generated during the degradation process. The UV–vis absorption spectra of RR-120 (Fig. 8c) display two absorption bands, one in the UV region at ≈ 290 nm due to the π-conjugation in the naphthalene ring and a doublet peak in the visible region at ≈ 512–535 nm characteristic of its conjugation through hydrazone unit [45]. Both absorptions decrease during the experimental time, as the solution became completely colorless and transparent after 150 min (insert in Fig. 8c). The membrane’s electrochemical stability was studied by monitoring the photocurrent during RR-120 removal/degradation experiments (Fig. 8d). Initially, the photocurrent was found to be 2.0 mA, and decreased to ≈ 1.2 mA after 50 min of degradation. Such reduction may be related to loss of poorly attached WO3 or to the adsorption of RR-120 to the surface of the electrode. However, over the remaining 130 min of degradation the photocurrent is lightly increased achieving values greater than 1.3 mA, which indicates an increase of the number of redox reactions in the surface and a good stability for the stainless steel substrate even over 3 h of experiment. Moreover, this behavior may also be related to increased transparency in the solution due to RR120 removal, which improves the intensity of incident radiation at the electrode surface. After the experiment, the photocurrent obtained for the modified membrane device was compared before and after photoelectrolysis and the values was maintained constant indicating good stability of the material.

FLOW -1 100 mL min -1 160 mL min -1 230 mL min

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60 min while the performance of the PEC process become better even when using ASM30-30 alone. Therefore, we can conclude that removal of RR-120 in a continuous flow reactor is maximized when coupling PEC and FT. These results, led us to optimize the NY/ASM30-30 sandwich membrane capacity in the removal/degradation of dye textile RR-120 combining PEC and FT methods. 3.5. Optimization of the operation parameters of the sandwich membrane NY/ASM30-30 To optimize the removal of RR-120 using the sandwich membrane under combined PEC/FT processes, several parameters were studied. First, the flow rate, which can have significant influence on the membrane’s performance was investigated [40]. To attain better performance, organic pollutants must easily reach the membrane surface, where oxidation of the pollutants occur. Three flow rates were investigated; 100; 160 and 230 mL min−1 (Fig. 7a). Removal of RR-120 improved by 10% when the flow rate was increased from 100 to 160 mL min−1, indicating better mass transport in the system. However, no significant improvement was observed when the flow rate was increased from 160 to 230 mL min−1. Under high flow conditions, the stability of the Nylon fiber is an important factor to be considered since

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Fig. 8. Photoelectrocatalytic degradation of RR-120 using the NY/ASM30-30 photoelectrocatalytic sandwich membrane in optimized conditions: 1 × 10−5 mol L−1 RR-120 dye in Na2SO4 0.1 mol L−1, pH 4, experimental time 3 h, flow rate 160 mL min−1, at applied potential of +1.0 V vs Ag/AgCl and Xe arc lamp. RR-120 removal, the inset shows the TOC removal of RR-120 (a); chromatograms (b); the UV–vis absorption spectra (c) and photocurrents (d) evaluated during the experiments.

4. Conclusion

Declaration of Competing Interest

This study described the design, fabrication, and performance of a sandwich membrane for the removal of an organic contaminant in aqueous solution via simultaneous adsorption, filtration and photoelectrocatalytic oxidation processes. Membrane’s saturation, which negatively affect its performance was significantly minimized due to the synergy between adsorption/filtration and subsequent electrophotoelectric oxidation. The coupling of Nylon fiber and WO3-modified steelmesh provided prompt retention of RR-120 dye in the first minutes followed by continuous PEC oxidation over the time of the experiment. Under optimized conditions, an excellent removal rate of the textile dye RR-120 was achieved. The dye was completely removed after 150 min when an aqueous solution of RR-120 (1 × 10−5 mmolL−1) at pH = 4.0 was flown through the membrane at 160 mL min−1 with an applied potential of 1.00 V. The findings show that the new generation of sandwich membranes and simultaneous use of FT and PEC processes is a good alternative for the development of efficient water treatment processes. Further studies are underway to evaluate the effect of pore size of the Nylon fiber and that of dissolved and undissolved organic compounds to gain a more comprehensive understanding of the synergetic effects between the two methods in real samples which generally contain more than one pollutant.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors would like to express their gratitude for the financial support provided by the Brazilian funding agencies, namely, FAPESP (processes #2017/13123-4 and 2014/50945-4), CAPES and CNPq (process #465571/2014-0). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.seppur.2019.116338. References [1] H. Song, J. Shao, Y. He, B. Liu, X. Zhong, Natural organic matter removal and flux decline with PEG-TiO2-doped PVDF membranes by integration of ultrafiltration with photocatalysis, J. Memb. Sci. 405–406 (2012) 48–56. [2] L. Ao, W. Liu, L. Zhao, X. Wang, Membrane fouling in ultrafiltration of natural water after pretreatment to different extents, J. Environ. Sci. (China) 43 (2016) 234–243. [3] L. Fan, J.L. Harris, F.A. Roddick, N.I.C.A. Booker, Influence of the characteristics of natural organic matter on the fouling of microfiltration membranes, Wat. Res. 35

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