Highly Efﬁcient Visible Light Photodegradation of Cr(VI) Using Electrospun MWCNTs-Fe 3 O 4 @PES Nanoﬁbers

: The development of highly efﬁcient photocatalysis has been prepared by two different methods for the photodegradation of Cr(VI) from an aqueous solution under visible light. The electrospun polyethersulfone (PES)/iron oxide (Fe 3 O 4 ) and multi-wall carbon (MWCNTs) composite nanoﬁbers have been prepared using the electrospinning technique. The prepared materials were characterized by SEM and XRD analysis. The result reveals the successful fabrication of the composite nanoﬁber with uniformly and smooth nanoﬁbers. The effect of numerous parameters were explored to investigate the effects of pH value, contact time, concentration of Cr(VI), and reusability. The MWCNTs-Fe 3 O 4 @PES composite nanoﬁbers exhibited excellent photodegradation of Cr(VI) at pH 2 in 80 min. The photocatalysis materials are highly stable without signiﬁcant reduction of the photocatalytic efﬁciency of Cr(VI) after ﬁve cycles. Therefore, due to its easy separation and reuse without loss of photocatalytic efﬁciency, the photocatalysis membrane has tremendous potential for the removal of heavy metals from aqueous solutions.


Introduction
Heavy metal pollution is one of the most serious environmental and public health issues. Chromium is one of the most dangerous heavy metals, and it has been used in a variety of industries, such as dyes, textiles, leather tanning, electroplating, and alloying [1,2]. Cr(VI) and Cr(III) are the two oxidation states of chromium, where Cr(VI) is more toxic to humans than Cr(III) with high stability, toxicity, and carcinogenic properties [3,4]. Therefore, a lot of methods have been used to remove Cr(VI) from wastewater such as ion exchange [5,6], adsorption [7,8], photocatalyst [9,10], membrane separation [11,12], coagulation and precipitation [13,14], and solvent extraction [15]. Photocatalysts are commonly considered as the most promising and frequently used feasible strategy, owing to its easy operation, eco-friendly, low cost, regeneration, and high performance when compared to other approaches [16,17]. Various photocatalyst for the removal of Cr (VI) have been investigated, including active carbon [18], metal oxide nanoparticles [19,20], synthesized polymer beads [21] and agriculture waste [22]. Among these, iron oxide (Fe 3 O 4 ) is one of the most widely studied due to its high efficiency, chemical stability, low cost, and availability [23,24]. However, after photodegradation, it is extremely difficult to separate the catalyst from the solution, which would raise the operational costs [25,26]. In order to avoid this problem, some researchers used nanofibers to remove a variety of pollutants [27,28].
Electrospinning is a simple and efficient technique for fabricating polymer-inorganic nanocomposite fibers due to the higher mesoporosity and specific surface area [29,30]. Various nanomaterials can be introduced into the polymer matrix through the electrospinning process to improve the physical, chemical, and catalytic properties of the polymer fibers [31]. Due to its excellent characteristics, such as industrial availability, favorable chemical resistance, thermal resilience, strong mechanical properties, vast possibilities for surface functionalization, biocompatible and non-toxic composition, polyethersulfone (PES) has been used to manufacture nanofibers [32]. It has recently shown that incorporating two different types of nanomaterials into a polymer matrix has significant possible benefits in terms of improving property enhancement [33]. CNTs have excellent mechanical, thermal, electrical, and optical properties, as well as high aspect ratios [29,34]. Therefore, mixing iron oxide NPs with electrospun polymeric nanofibers can overcome the agglomeration and recycling problem of the nanoparticles. In addition, the mixture can be used as an efficient photocatalyst for the removal of heavy metal ions and development the magnetic nanofibers [35,36]. Recently, Hota et al. have reported the electrospinning fabrication of PAN/iron oxide for the removal performance of CR dye from aqueous solution [32]. In addition, Li et al. announced the electrospinning fabrication of a nylon 6/iron oxide composite nanofibers membrane for the removal of Cr(VI) ions from the aqueous solution [37]. Moreover, Mallon et al. fabricated the electrospun composite nanofibers containing carbon nanotubes and iron oxide (MWCNT-Fe 3 O 4 ) [38].
In the present work, novel composite nanofibers were fabricated for the photodegradation of Cr(VI) from aqueous solutions, which can be easily separated from the aqueous solution. Photodegradation experiments were used to investigate the effects of pH, contact time, and initial concentration on Cr(VI). The electrospinning technique was also used to examine the incorporation of the respective MWCNT-Fe 3 O 4 as filler nanomaterials into PES nanofibers. Furthermore, the preparation process and the photocatalytic mechanism of composite nanofibers for Cr(VI) were investigated. The morphology and structure of the composite nanofibers were characterized by scanning electronic microscope (SEM) and X-ray diffraction (XRD). The composite nanofiber showed promising potential for wastewater treatment. Figure 1 shows the SEM images of PES, PES/MWCNTs-Fe 3 O 4 , MWCNTs-Fe 3 O 4 @PES composite nanofiber before and after photodegradation. The surface morphology of the PES nanofiber is uniform without beads with an average size of 105 ± 15 nm. The incorporation of MWCNTs and Fe 3 O 4 NPs in the PES nanofiber can be seen in Figure 1b. The NPs can be seen inside the nanofiber with an average size of 95 ± 10 nm, due to the electrically conductive nature of MWCNT, which enhances solution conductivity, resulting in smaller nanofiber diameters. The successful fabrication of MWCNTs-Fe 3 O 4 @PES nanofibers before and after photodegradation is depicted in Figure 1c,d. It was observed that the NPs are agglomerated to each other and crosslinked well to the nanofiber even after photodegradation of Cr(VI).

Results and Discussion
In order to confirm the presence of NPs in the synthesized composite nanofiber, XRD analysis has been done as shown in Figure 2.  In order to confirm the presence of NPs in the synthesized composite nanofiber, XRD analysis has been done as shown in Figure 2. The peaks at 30°, 35°, 44°, 57°, and 63° correspond to the presence of Fe3O4. The broad peaks of PES nanofiber at 2θ of 13.5°, 30° and 42.3°, indicating the amorphous nature of PES [39]. For the MWCNTs-Fe3O4@PES composite nanofiber, peaks at 2θ = 25.5° correlate to the presence of MWCNTs, whereas the peak at 13.5° related to the PES nanofiber and the peaks at 30°, 35°, 44°, 57°, and 63° associated to the Fe3O4 NPs. In addition, after photodegradation the same diffraction peaks are observed indicated the stability of the composite nanofiber.

Photocatalytic Performance
Catalysts dose plays a crucial role in the photocatalysis process. The results depicted in Figure 3

Photocatalytic Performance
Catalysts dose plays a crucial role in the photocatalysis process. The results depicted in Figure 3 exhibited that as the amount of MWCNTs increased, the photocatalytic efficiency of Cr(VI) increased with a fixed amount of Fe 3 O 4 (1 wt.%) at pH 2, 80 min and 20 mgL −1 of Cr(VI). It is clear that the 3 and 5 wt.% of MWCNTs have the same photocatalytic efficiency, as the increase in the catalyst dose offers more binding sites, resulting in improving the photodegradation.

Photocatalytic Performance
Catalysts dose plays a crucial role in the photocatalysis process. The results d in Figure 3 exhibited that as the amount of MWCNTs increased, the photocataly ciency of Cr(VI) increased with a fixed amount of Fe3O4 (1 wt.%) at pH 2, 80 min mgL −1 of Cr(VI). It is clear that the 3 and 5 wt.% of MWCNTs have the same photoc efficiency, as the increase in the catalyst dose offers more binding sites, resultin proving the photodegradation.  During photocatalysis experiments, the contact time between photocatalysis and contaminants is critical for determining photocatalytic efficiency. The photocatalytic efficiency of Cr(VI) as a function of time using the PES nanofiber, PES/MWCNTs-Fe 3 O 4 CNM, and MWCNTs-Fe 3 O 4 @PES CNM were determined as shown in Figure 4. All the experiments were conducted at 20 mgL −1 of Cr(VI) and pH 2. The results reveal that the maximum photocatalytic efficiency of Cr(VI) occurs at 80 min using the MWCNTs-Fe 3 O 4 @PES CNM under visible light, whereas for PES/MWCNTs-Fe 3 O 4 CNM only 75% photocatalytic efficiency of Cr(VI) was achieved at the same condition. These results attributed to the large surface area and the large number of active sites in the case of MWCNTs-Fe 3 O 4 @PES CNM. In addition, both the composite nanofiber had higher photocatalytic efficiency than PES nanofiber.
The pH of a solution has a significant impact on the behaviour of photocatalyst against pollutants due to the surface charge of the photocatalyst, as well as the speciation of the metal ions in the solution [40]. The influence of pH on the photocatalytic efficiency of Cr(VI) using the MWCNTs-Fe 3 O 4 @PES CNM was established at various pH solutions in the range of 2-10 at 20 mgL −1 and 80 min as shown in Figure 5a. The results demonstrated that the highest photocatalytic efficiency of Cr(VI) occurs at a lower pH and decreased until 83% at pH 10. This result attributed to the electrostatic interactions between the positively charged iron oxide and the negatively charged of Cr(VI) that exists as an oxyanion (CrO 4 −2 , Cr 2 O 7 −2 , or HCrO 4 − ) in acidic media [40,41]. In addition, the electrostatic repulsions between the photocatalyst and the dichromate ions, resulting in a decline in photocatalytic efficiency of Cr(VI) in the basic medium. Furthermore, various concentrations (20, 40, 60, 80, and 100 mgL −1 ) of Cr(VI) at pH 2 using the MWCNTs-Fe 3 O 4 @PES CNM were used at a contact time of 80 min, to determine the initial concentration effect, as shown in Figure 5b. The photocatalytic efficiency of Cr(VI) decreased from 99 to 66 % when the concentration increased from 20-100 mgL −1 , due to the binding sites being occupied by Cr(VI), leaving little available binding sites for the photocatalyst process, resulting in a decrease in photocatalytic efficiency [42][43][44].
Catalysts 2021, 11, x FOR PEER REVIEW large surface area and the large number of active sites in the case of MWCNTs-Fe3 CNM. In addition, both the composite nanofiber had higher photocatalytic efficie PES nanofiber. The pH of a solution has a significant impact on the behaviour of photo against pollutants due to the surface charge of the photocatalyst, as well as the sp of the metal ions in the solution [40]. The influence of pH on the photocatalytic e of Cr(VI) using the MWCNTs-Fe3O4@PES CNM was established at various pH s in the range of 2-10 at 20 mgL −1 and 80 min as shown in Figure 5a. The results strated that the highest photocatalytic efficiency of Cr(VI) occurs at a lower pH creased until 83% at pH 10. This result attributed to the electrostatic interactions the positively charged iron oxide and the negatively charged of Cr(VI) that exi oxyanion (CrO4 −2 , Cr2O7 −2 , or HCrO4 − ) in acidic media [40,41]. In addition, the elec repulsions between the photocatalyst and the dichromate ions, resulting in a d photocatalytic efficiency of Cr(VI) in the basic medium. Furthermore, various co tions (20, 40, 60, 80, and 100 mgL −1 ) of Cr(VI) at pH 2 using the MWCNTs-Fe3O4@P were used at a contact time of 80 min, to determine the initial concentration e shown in Figure 5b. The photocatalytic efficiency of Cr(VI) decreased from 99 when the concentration increased from 20-100 mgL −1 , due to the binding sites bei pied by Cr(VI), leaving little available binding sites for the photocatalyst process, r in a decrease in photocatalytic efficiency [42][43][44].   The pH of a solution has a significant impact on the behaviour of photocatalyst against pollutants due to the surface charge of the photocatalyst, as well as the speciation of the metal ions in the solution [40]. The influence of pH on the photocatalytic efficiency of Cr(VI) using the MWCNTs-Fe3O4@PES CNM was established at various pH solutions in the range of 2-10 at 20 mgL −1 and 80 min as shown in Figure 5a. The results demonstrated that the highest photocatalytic efficiency of Cr(VI) occurs at a lower pH and decreased until 83% at pH 10. This result attributed to the electrostatic interactions between the positively charged iron oxide and the negatively charged of Cr(VI) that exists as an oxyanion (CrO4 −2 , Cr2O7 −2 , or HCrO4 − ) in acidic media [40,41]. In addition, the electrostatic repulsions between the photocatalyst and the dichromate ions, resulting in a decline in photocatalytic efficiency of Cr(VI) in the basic medium. Furthermore, various concentrations (20, 40, 60, 80, and 100 mgL −1 ) of Cr(VI) at pH 2 using the MWCNTs-Fe3O4@PES CNM were used at a contact time of 80 min, to determine the initial concentration effect, as shown in Figure 5b. The photocatalytic efficiency of Cr(VI) decreased from 99 to 66 % when the concentration increased from 20-100 mgL −1 , due to the binding sites being occupied by Cr(VI), leaving little available binding sites for the photocatalyst process, resulting in a decrease in photocatalytic efficiency [42][43][44].

Reusability of the Photocatalysis
For cost-effectiveness, the reusability and stability of catalyst materials is a critical factor. The regeneration of CNM was performed at pH 2, 20 mg L −1 and 80 min, as shown in Figure 6. After each cycle, the composite nanofiber treated with HCl solution and then washed with purified water. The results show that the composite nanofibers were effectively reused without significant reduction in the photocatalytic efficiency of Cr(VI) after 5 cycles, reaching 99%. In addition, Table 1 shows the photodegradation of Cr(VI) using different photocatalysis composite nanofiber, to compare the catalytic activity of different photocatalysis for Cr(VI) photodegradation under visible light irradiation.
Catalysts 2021, 11, 868 6 of 10 in Figure 6. After each cycle, the composite nanofiber treated with HCl solution and then washed with purified water. The results show that the composite nanofibers were effec tively reused without significant reduction in the photocatalytic efficiency of Cr(VI) afte 5 cycles, reaching 99%. In addition, Table 1 shows the photodegradation of Cr(VI) using different photocatalysis composite nanofiber, to compare the catalytic activity of differen photocatalysis for Cr(VI) photodegradation under visible light irradiation.

Preparation of Iron Oxide Nanoparticles by Hydrothermal Process
First, 1.7 g of iron chloride was dissolved in 10 mL distilled water, and then 1.9 mL of Acetylacetone was added to the solution after 15 min. After that, 6.25 g of sodium ace tate was added to the solution with slow stirring. The solution was cooled to a tempera

Preparation of Iron Oxide Nanoparticles by Hydrothermal Process
First, 1.7 g of iron chloride was dissolved in 10 mL distilled water, and then 1.9 mL of Acetylacetone was added to the solution after 15 min. After that, 6.25 g of sodium acetate was added to the solution with slow stirring. The solution was cooled to a temperature rang about 0-5 • C, which tends to result red crystalline. After that, iron (III) acetylacetonate was recrystallized from absolute ethanol. The prepared material was dissolved in 70 mL deionized water, and ethylene diamine was added into the solution under stirring condition to keep the pH between 10 and 11. After that, the resulting suspension transferred into a Teflon lined autoclave, where the hydrothermal reaction was carried out for 12 h at 150 • C. After the completion of the reaction, the mixture was washed several times with distilled Catalysts 2021, 11, 868 7 of 10 water and finally with alcohol and then isolated by centrifugation. The precipitate was then dried in an electric oven at 60 • C for further use.

Synthesis of Electrospun Composite Nanofiber
The PES nanofibers were fabricated via the electrospinning technique. Briefly, 1 g of PES was added into 4 mL of DMF and mixed by magnetic stirrer until the solution was homogeneous. The prepared PES solution was poured into a 6 mL plastic syringe. The electrospinning was performed at 15 kV, 1 mL·h −1 of solution flow rate and 20 cm distance from needle to the collector. The obtained nanofibers were taken off from the aluminium foil for further use. The PES nanofibers were immersed in a crosslinking medium containing 100 mL purified water with 2.5 wt% GA for 24 h [52]. Then, the GA was separated, and the composite nanofiber washed and dried. After that, 2 mL of the prepared suspension mixture of MWCNTs and Fe 3 O 4 was applied to the nanofibers for 24 h. Finally, the CNM was washed out with deionized water and ethanol, and then dried for further use.

Characterization
The morphology of the synthesized composite nanofibers membrane (CNM) was examined using a scanning electron microscope (SEM, Gemini Zeiss-Ultra 55, ZEISS, Jena, Germany). The average diameter of the CNMs were calculated using Image J software and calculated by selecting the fiber diameter observed on the SEM image. The X-ray diffraction (XRD) patterns were conducted using (D8-Advance, Bruker, Billerica, MA, USA) varying from 3 • to 30 • with Cu-Kα radiation to establish the composition and crystallinity of Fe 3 O 4 and the composite nanofiber membrane. UV-Vis Absorbance (LAMBDA 750, Perkin Elmer, Solingen, Germany) was used to determine the concentration of Cr(VI) in aqueous solutions.

Photocatalysis Experiments
The photocatalysis experiments were carried out to investigate the effect of contact time, initial Cr(VI) concentrations and pH values (2-10) under visible light (125 W xenon lamp) using the composite nanofiber membrane for the photodegradation of Cr(VI). Photocatalysis experiments were conducted in petri dish, with50 mL of Cr(VI) concentration (20-100 mgL −1 ) and 20 mg of composite nanofiber. The fabricated nanofibers was placed into the petri dish and mixed with Cr(VI) solution at room temperature in the dark for 30 min to assure that the adsorption equilibrium of Cr(VI) was reached. After equilibration, the solution was irradiated with visible light, where the distance between the light source and the CNM is 20 cm. After that, 3 mL of the suspension was taken at scheduled intervals for the analysis. Potassium dichromate (K 2 Cr 2 O7) was used to prepare different concentration of Cr (VI). The Cr(VI) concentration in the solution was determined by recording the absorbance at 350 nm on a UV-Vis Absorbance (LAMBDA 750, Perkin Elmer).The photocatalytic performance of Cr(VI) is given as follows: Photocatalytic efficiency (%) = ( where C 0 and C t is the initial and final Cr(VI) concentration (mg/L).

Conclusions
The PES/MWCNTs-Fe 3 O 4 and MWCNTs-Fe 3 O 4 @PES composite nanofibers were successfully prepared by the electrospinning technique. The NPs were blending to the PES solution and/or loaded to the surface of the nanofiber to prepare the composite nanofibers. The prepared MWCNTs-Fe 3 O 4 @PES composite nanofibers have a higher photodegradation efficiency of Cr(VI) than the PES/MWCNTs-Fe 3 O 4 and PES alone. The maximum photodegradation of Cr(VI) was 99% after 80 min. The photodegradation efficiency was highest at acidic pH 2 (99%) whereas 83% was achieved at pH 10 due to the electrostatic interactions between active sites on the surface of the catalyst with positive charges of HCrO 4 − . In addition, the photocatalytic efficiency of Cr(VI) decreased from 99 to 66% when the concentration increased from 20-100 mgL −1 . Moreover, after 5 cycles of photodegradation of Cr(VI), the composite nanofibers demonstrated high performance, stability, and reusability. The results demonstrated that the catalysts materials have a potential photodegradation of Cr(VI) from industrial wastewater.