Integrated adsorption and photocatalytic removal of methylene blue dye from aqueous solution by hierarchical Nb2O5@PAN/PVDF/ANO composite nanofibers

Abstract

This work presents the development of hierarchical niobium pentoxide (Nb20s)-based composite nanofiber membranes for integrated adsorption and photocatalytic degradation of methylene blue (MB) pollutants from aqueous solutions. The Nb205 nanorods were vertically grown using a hydrothermal process on a base electrospun nanofibrous membrane made of polyacrylonitrile/polyvinylidene fluoride/ammonium niobáte (V) oxalate hydrate (Nb2O5@PAN/PVDF/ANO). They were characterized using field-emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD) analysis, and Fourier transform infrared (FTIR) spectroscopy. These composite nanofibers possessed a narrow optical bandgap energy of 3.31 eV and demonstrated an MB degradation efficiency of 96 % after 480 min contact time. The pseudo-first-order kinetic study was also conducted, in which Nb2O5@PAN/PVDF/ANO nanofibers have kinetic constant values of 1.29 x 10-2 min-1 and 0.30 x 10-2 min-1 for adsorption and photocatalytic degradation of MB aqueous solutions, respectively. These values are 17.7 and 7.8 times greater than those of PAN/PVDF/ANO nanofibers without Nb2û5 nanostructures. Besides their outstanding photocatalytic performance, the developed membrane materials exhibit advantageous characteristics in recycling, which subsequently widen their practical use in environmental remediation applications.

Headnote

ARTICLE INFO

Keywords:

Hierarchical nanostructure

Composite nanofiber

Niobium pentoxide

Dye degradation

Synergetic adsorption and photocatalysis

ABSTRACT

This work presents the development of hierarchical niobium pentoxide (Nb20s)-based composite nanofiber membranes for integrated adsorption and photocatalytic degradation of methylene blue (MB) pollutants from aqueous solutions. The Nb205 nanorods were vertically grown using a hydrothermal process on a base electrospun nanofibrous membrane made of polyacrylonitrile/polyvinylidene fluoride/ammonium niobáte (V) oxalate hydrate (Nb2O5@PAN/PVDF/ANO). They were characterized using field-emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD) analysis, and Fourier transform infrared (FTIR) spectroscopy. These composite nanofibers possessed a narrow optical bandgap energy of 3.31 eV and demonstrated an MB degradation efficiency of 96 % after 480 min contact time. The pseudo-first-order kinetic study was also conducted, in which Nb2O5@PAN/PVDF/ANO nanofibers have kinetic constant values of 1.29 x 10-2 min-1 and 0.30 x 10-2 min-1 for adsorption and photocatalytic degradation of MB aqueous solutions, respectively. These values are 17.7 and 7.8 times greater than those of PAN/PVDF/ANO nanofibers without Nb2û5 nanostructures. Besides their outstanding photocatalytic performance, the developed membrane materials exhibit advantageous characteristics in recycling, which subsequently widen their practical use in environmental remediation applications.

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1. Introduction

Wastewater treatment is a pressing issue in our rapidly evolving society, especially when it comes to removing organic pollutants like dyes [1,2]. These dyes are a ubiquitous concern in industrial and domestic wastewater due to their high toxicity, mutagenicity, and resistance to biodegradation [3,4]. Their widespread use across multiple industries, including textiles, paper, and cosmetics, leads to substantial environmental impact, particularly affecting aquatic ecosystems and drinking water quality. Hence, it is crucial to develop effective strategies and technologies to mitigate these adverse effects. Conventional wastewater treatment methods (e.g., adsorption, precipitation, coagulation, and ultra-filtration) have proven effective against a range of pollutants [5,6]. Among them, adsorption stands out for its versatility, ease of operation, and cost-effectiveness [7,8]. However, it often produces undesirable by-products that require additional treatments [9]. To overcome these limitations, advanced oxidation processes (AOPs) such as photocatalytic degradation techniques have been developed [10-12].

Semiconductor nanomaterials, such as titanium dioxide (T1O2) [13], zinc oxide (ZnO) [14-16], iron oxide (РегОз) [17], copper oxide (CuO) [18], cerium oxide (СеОз) [19], and niobium pentoxide (Nb2Os) [20,21], are commonly used in photocatalytic treatments. НЬзОз is an n-type semiconductor with a band gap energy ranging from 3.1 to 4.0 eV, depending on its crystalline phase [22,23]. It is also typically a non-toxic solid oxide and exhibits strong redox ability and unique Lewis acid sites (LASs) [24]. It has gained attention for its application in adsorption and as a catalyst material [25]. Our previous studies show that №205 with a deformed orthorhombic structure was successfully synthesized using facile hydrothermal methods and has excellent synergetic adsorption/photocatalytic properties for cationic dye removals [26-28]. Nevertheless, Nb2O5 faces practical challenges; its synthesis predominantly results in nano-powder form, which is susceptible to agglomeration and difficult to separate after use [29]. To tackle this, efforts have been made to grow metal oxide nanomaterials directly on nanofibrous membranes, forming the so-called hierarchical composite structures, which can enhance their surface area, porosity, and structural stability [30-34]. Moreover, among the various methods for constructing nanofibers, electrospinning has been favorably used and gained widespread applications in energy [35, 36], electronics [37], sensors [38,39], healthcare [40,41], water [42,43], and environmental remediation [30,44].

In this study, we developed hierarchical Nb2O5-integrated polyacrylonitrile/polyvinylidene fluoride (PAN/PVDF) composite nanofibers for integrated adsorption and photocatalytic degradation of methylene blue (MB) pollutants. Hybrid PAN/PVDF nanofibers were chosen because of their beneficial properties (e.g., high mechanical strength and waterinsolubility [45,46]). Firstly, these nanofibers were produced using electrospinning, while №26)5 nanostructures were grown on the nanofibers via hydrothermal processes, which are effective for materials unstable at high temperatures [47-50]. Ammonium niobáte (V) oxalate hydrate (ANO) was added to create a nanofibrous composite system of Nb2O5@PAN/PVDF/ANO. The integrated efficient removal of MB dyes via adsorption and photocatalytic activity of the developed composite nanofibers was demonstrated, in which unmodified (only PAN/PVDF) nanofibers were used as a control. Lastly, besides discussion on the physical phenomena during adsorption and photocatalysis, the performance comparison of the developed Nb2O5@PAN/PVDF/ANO nanofibers to the state-of-the-art hierarchical metal oxide nanofiber photocatalysts is provided.

2. Materials and methods

2.1. Materials

The polyacrylonitrile (PAN, average Mw -150,000 g/mol (typical)), polyvinylidene fluoride (PVDF, average Mw -534,000 g/mol by GPC, powder), and ammonium niobáte (V) oxalate hydrate (ANO, 99.99 % trace metal basis) were purchased from Sigma-Aldrich, Singapore. The N,N-dimethyl formamide (Merck, Darmstadt, Germany) and deionized water were used as solvents. The methylene blue (MB) organic pollutants were purchased from Merck, Darmstadt, Germany. For the free radicals trapping experiments, we employed several scavenger materials including isopropyl alcohol (IPA, Merck), p-Benzoquinone (BQ, SigmaAldrich), and ethylenediaminetetraacetic acid (EDTA, Merck). All samples were used as received without any further purifications.

2.2. Preparation of PAN/PVDF nanofibers

The polyacrylonitrile/polyvinylidene fluoride (PAN/PVDF) solution was fabricated by dissolving 1.00 g of PVDF and 0.25 g of PAN, simultaneously, into 10 mL of DMF. The solution was stirred at a speed of 600 rpm and a temperature of 60 °C for 3 h until it had reached high homogeneity. Two types of PAN/PVDF solutions were fabricated (i.e., with and without niobium seed). For the fabrication of PAN/PVDF with niobium seed (hereafter refers to as PAN/PVDF/ANO), a small amount (0.05 g) of ammonium niobáte (V) oxalate hydrate was added to the prepared PAN/PVDF solution. The mixture was stirred using similar parameters previously. After 1 h, the solution was then sonicated utilizing an ultrasonic cell disruptor (BIOBASE, model UCD-PO1-250W) for 20 min before the electrospinning process.

Both PAN/PVDF and PAN/PVDF/ANO solutions were then transferred to 10-mL plastic syringes for the electrospinning process using a digital electrospinner (PT Nanosense Instrument Indonesia, Indonesia). The solutions were electrospun using 10 kV voltage difference with a tipto-collector distance of 15 cm for -4 h. Both PAN/PVDF and PAN/PVDF/ ANO nanofibrous membranes were dried inside a digital dry cabinet overnight before use.

2.3. Preparation of Nb2O5@PAN/PVDF/ANO nanofibers

The nanostructured niobium pentoxide (НЬзОз) was prepared using a hydrothermal method as reported in previous studies [26,51]. In brief, 3.4 g of ammonium niobáte (V) oxalate hydrate was dissolved in 70 mL distilled water and stirred at a speed of 500 rpm for 10 min. Thereafter, 0.1 g of PAN/PVDF nanofiber membrane was added, and the solution was then transferred to 100-mL of a Teflon-lined autoclave, followed by heating process at a temperature of 175 °C for 72 h. The Nb2O5 growth mechanism in the hydrothermal method involves three main processes. First, when the ANO powder was dissolved in deionized (DI) water to create a clear solution, the complex niobáte ions, oxalate ions, and ammonium ions were formed (i). Second, the release niobium ions (Nb5+), then underwent hydrolysis, forming hydroxide species or other intermediate niobium species (ii). Third, the hydrolyzed niobium species (NbOH4+) could further undergo condensation and polymerization reactions, gradually forming a niobium oxide network (iii). At elevated temperature (175 °C) and high pressure, these colloidal niobium species-initiated crystallization into НЬзОз structures. The presence of ammonium ions (NH4 ) could act as a buffering agent, possibly affecting the kinetics and mechanisms of №205 formation.

... (i)

... (ii)

... (iii)

After the autoclave temperature was naturally cooled down to room temperature, the nanofiber membrane was then washed using DI water and dried in an oven with a temperature of 80 °C for 6 h. The Nb2O5@PAN/PVDF nanofibrous membrane was ready to be characterized. The Nb2O5@PAN/PVDF/ANO nanofibrous membrane was fabricated using a similar step, in which the only difference was the used base membrane matrix. The detailed fabrication processes of the Nb2û5@-PAN/PVDF/ANO membranes are depicted in Fig. 1. Meanwhile, the used fabrication parameters of four different composite membrane samples (i.e., PAN/PVDF, PAN/PVDF/ANO, Nb2O5@ PAN/PVDF, and Nb2O5@-PAN/PVDF/ANO) are listed in Table 1.

2.4. Material characterizations

The composite nanofiber membranes were characterized in the field- emission scanning electron microscopy (FE-SEM) using FE-SEM JEOL-JIB-4610F to investigate their morphologies. Meanwhile, for the elemental analysis, we used FESEM equipped with energy-dispersive X-ray spectroscopy (EDS) using JEOL-JSM-IT700HR. To inspect the crystal quality and phase of the composite nanofiber membranes, X-ray diffraction (XRD) measurement and analysis were carried out using an XRD Bruker with Co radiation, acceleration voltage of 40 kV, current of 25 mA, and Bragg-Brentano (0-20) configuration. The Fourier transform infrared (FTIR) spectroscopy characterization of the nanofibers was conducted employing an FTIR-Thermo Scientific Nicolet iS-Ю. The optical characteristics of the samples were analyzed using UV-Vis spectrophotometer Maya2000 Pro Series Spectrometer from Ocean Insight. All these material characterizations were carried out at room temperature.

2.5. Adsorption and photocatalytic degradation measurements

The adsorption-photocatalytic performance of the synthesized composite nanofibers was evaluated for the decomposition of methylene blue (MB). In a typical adsorption-photodegradation experiment, -0.15 g of the synthesized nanofibers was added to 250 mb of MB solution with an initial concentration of 10 mg/L. The mixture was stirred at 500 rpm in the dark for 180 min (3h) to reach adsorption-desorption equilibrium and investigate its adsorption performance. After equilibrium was achieved, the mixture was subjected to UV light irradiation (12 W, X = 253.7 nm) and the UV lamp was placed beside the solution, initiating the photocatalytic process. The concentration of MB in the solution was monitored every 60 min for a total duration of 480 min (8 h) by withdrawing 4 mb of samples and measuring the absorbance of the supernatant using a UV-Vis spectrophotometer (Thermo Scientific Genesys 50). The removal efficiency and adsorption-photodegradation performance of the synthesized composite nanofibers were then evaluated based on the reduction in MB concentration.

3. Results and discussion

The morphologies of four different composite nanofiber samples are shown in Fig. 2a - 21. A smooth and continuous nanofiber morphology was observed in the PAN/PVDF (Fig. 2a, e, and i) and PAN/PVDF/ANO (Fig. 2b, f, and j) samples, confirming the successful electrospinning process. Both samples show relatively uniform nanofiber diameters. However, the PAN/PVDF nanofiber shows a slightly higher average nanofiber diameter (i.e., (710 ± 90) nm) compared to the PAN/PVDF/ ANO nanofiber (i.e., (450 ± 80) nm), which might be attributed to changes in the solution parameters as a result of the addition of ANO powder into the PAN/PVDF solution. Adding ANO into the PAN/PVDF solution may enhance the solution conductivity due to the releasing of niobium(V) ions, oxalate ions, and ammonium ions. The increase in solution conductivity can result in nanofibers with lower average diameter [52]. Moreover, some beads (see yellow circle lines in Fig. 2b) were found in the PAN/PVDF/ANO nanofiber sample.

The hydrothermal growth of the №205 on the nanofiber surfaces was also characterized using the FE-SEM images shown in Fig. 2c-d. For the PAN/PVDF without niobium seed (Nb2O5@PAN/PVDF), the №205 layer was observed with a coral-like form (see Fig. 2c) similar to that reported in literature [53]. Here, the nanofiber structure was preserved below the №205 layer (see red circle lines in Fig. 2c), where its coral-like form comprises vertical nanorods (i.e., urchin-like №205 nanostructures) [54, 55]. The distinct layers of the nanofiber and the №26)5 confirm that the niobium seed needs to be added in the PAN/PVDF nanofiber before the hydrothermal process. For the Nb2O5@PAN/PVDF/ANO composite structure, the nanofiber form was still observed (see Fig. 2d), contrasting with the Nb2Os@PAN/PVDF sample. It implies that the growth of the Nb2O5 nanorods was vertical from the PAN/PVDF/ANO, which is similar to that in previous studies [56-58]. From the FE-SEM image in Fig. 21, the Nb205 nanorods were observed to conformally envelop the PAN/-PVDF/ANO nanofiber. This confirms that the hierarchical Nb2û5@-PAN/PVDF/ANO composite nanofiber structure was successfully fabricated using combined electrospinning and hydrothermal methods.

We also analyzed the energy dispersive X-ray spectroscopy (EDS) of the Nb2O5@PAN/PVDF/ANO composite nanofiber based on the FE-SEM images shown in the Fig. 3a. Here, urchin-like Nb2O5 nanorod structures were observed, where their diameters and heights were estimated in the ranges of -(10 - 20) nm and -(70 -100) nm, respectively. Therefore, an aspect ratio of up to -10 can be demonstrated by our Nb2O5 nanorods, which is similar to those of other architectures reported in literature [55, 59]. This high aspect ratio has led to a larger active area for the adsorption of MB molecules. From the previous studies, the photo-catalytic degradation performance of nanorods was found to increase linearly with the rising of nanorod aspect ratio [60,61]. The elemental EDS mapping of the Nb2O5@PAN/PVDF/ANO composite nanofiber is depicted in Fig. 3b, which shows distributions of C, O, F, and Nb elements. Here, the major components of the composite surfaces were owned by the Nb2O5 (i.e., Nb and О elements) with a small number of C and F, corresponding to the PAN/PVDF nanofiber matrix. Fig. 3b also shows the mass and atomic weight percentages of surface elemental composition of the Nb2O5@PAN/PVDF/ANO composite nanofiber. The Nb and О elements dominate the elemental compositions with 33.5 and 47.4 atomic wt%, respectively. These results again confirm that the Nb2O5 urchin-like nanorod structures conformally envelop the PAN/PVDF nanofibers.

The XRD patterns of all samples (i.e., PAN/PVDF, PAN/PVDF/ANO, Nb2O5@PAN/PVDF, and Nb2O5@PAN/PVDF/ANO) are shown in Fig. 3c. The PAN/PVDF nanofiber sample has a strong similarity to the pure PVDF sample (PDF #38-1638). The characteristic peak of 29 = 21.5° (020) corresponds to the ot-phase, while the characteristic peak of 29 = 23.5° (110) is believed to be owned by the ß-phase of crystalline PVDF [46,62,63]. The a- and ß-PVDF characteristics are also observed for the PAN/PVDF/ANO and Nb2O5@PAN/PVDF composite nanofiber samples. It confirms that the PAN/PVDF part remains intact except in the Nb2O5@PAN/PVDF/ANO composite nanofiber. For the Nb2<)5@PAN/PVDF/ANO composite nanofiber, a characteristic peak of hexagonal niobium pentoxide (TT-Nb2Os, PDF # 28-0317) occurred mainly at 29 = 26.3°, which corresponds to the (001) plane [64]. The next characteristic peaks of Nb2û5 are observed at 20 = 54° (also observed for Nb2û5@PAN/PVDF sample), 65°, and 66°, which are related to the (002), (102), and (111) planes of hexagonal Nb205, respectively [65-67]. These XRD results confirm the finding shown in the FE-SEM images (see Fig. 3a), where the Nb205 nanorods have indeed conformally enveloped the PAN/PVDF/ANO structure.

The FTIR spectra of all four different nanofiber samples are displayed in Fig. 3d. From the spectra of the PAN/PVDF nanofiber sample, peak characteristics of both PAN and PVDF compounds are found, which are similar to the previously reported studies [62,68,69]. The characteristic peak of C = N bond stretching vibration (2244 cm-1) of PAN is observed in the PAN/PVDF and PAN/PVDF/ANO samples, while it is not seen in the Nb2O5@PAN/PVDF and Nb2O5@PAN/PVDF/ANO composite nanofibers. In the fingerprint region (1500-700 cm-1), the characteristic peak of PVDF was dominant (see Fig. 3d right). Both characteristic peaks of a-PVDF (769 cm-1) and ß-PVDF (1275 and 836 cm-1) are observed [62]. The existence of the C-F stretching vibration (1176 cm-1) is also observed for all samples, except for the Nb2O5@PAN/PVDF/ANO composite nanofiber. The results indicate that PAN and PVDF structure are reserved in all samples but the Nb2O5@PAN/PVDF/ANO nanofiber. This finding further confirms that the Nb2Os nanorods were indeed enveloping the nanofiber as previously confirmed by the FE-SEM and XRD analysis. The FTIR spectrum of the Nb2O5@PAN/PVDF/ANO has shown some characteristic peaks of Nb2O5 [64,79]. The peak observed at 1409 cm-1 corresponds to the C=O bond and is probably due to the residual of the oxalate precursor during the hydrothermal process [71]. Moreover, the peak at -600 cm-1 is assigned to the stretching vibration of the inorganic Nb-0 [72], which is observed for all samples except the PAN/PVDF nanofiber. These results confirm the existence of niobium at PAN/PVDF/ANO and prove the Nb205 chemical structure in Nb2û5@PAN/PVDF and Nb2O5@PAN/PVDF/ANO composite nanofibers. Interestingly, from the FTIR spectrum and XRD pattern of the Nb2O5@PAN/PVDF/ANO composite nanofibers, the main characteristics or peak signatures of PAN/PVDF nanofibers were missing. In contrast, these signatures were still clearly shown in Nb2O5@PAN/PVDF samples. It may indicate that the structural alteration had occurred in PAN/PVDF/ANO samples during the hydrothermal process.

Fig. 4a shows the UV-Vis reflectance spectra of the as-prepared samples. The PAN/PVDF and PAN/PVDF/ANO nanofiber samples show no light adsorption along the measured wavelength. This behavior is due to the insulating properties of the PAN/PVDF and PAN/PVDF/ANO nanofibers. In contrast, the hierarchical Nb2O5@PAN/PVDF and Nb2O5@PAN/PVDF/ANO composite nanofibers start to show light absorption at a wavelength below 499 nm. It indicates the optical semiconducting behavior of both samples after the growth of Nb2Os nanorods on the nanofibers. The Nb2O5@PAN/PVDF/ANO composite nanofiber can absorb light at longer wavelengths implying the narrower band gap energy compared to the Nb2O5@PAN/PVDF composite nanofiber. The optical band gap energy of the sample can be calculated using Equation (1) based on the reflectance data in Fig. 4a.

... (1)

where F(Roq) is the Kubelka-Munk function, which is the ratio of the absorption coefficient (k = (1 - R^)2) to the scattering coefficient (s = 2Roo). The ho, A, and Eg are defined as photon energy, proportionality constant, and band gap energy, respectively [73]. The calculated indirect band gap energy (Fig. 4b) of the Nb2O5@PAN/PVDF and Nb2û5@PAN/PVDF/ANO composite nanofibers were found to be 3.45 and 3.31 eV, respectively. These values are similar to that of the previously reported Nb2O5 nanostructures (3.4 eV) [22,24]. The calculation of band gap energy using the UV-Vis spectroscopy in nanocomposites has some caveats, where the complexity of the materials may lead to overlapping adsorption peaks. Nonetheless, it can still be used in qualitative property comparison of the tested catalyst materials, especially in the case of organic/inorganic composites as previously reported in other studies [74,75]. The Nb2O5@PAN/PVDF/ANO nanofiber has slightly narrower band gap energy compared to its Nb2O5@PAN/PVDF counterpart. This slightly observed difference is due to the smaller organic part of the Nb2O5@PAN/PVDF/ANO compared to the Nb2O5@PAN/PVDF composite nanofibers, where it has been explained previously by the XRD and FTIR spectroscopy results.

The adsorption and photocatalysis performance of the as-prepared samples for the degradation of methylene blue (MB) were evaluated under room temperature using 254 nm UV irradiation. Fig. 5a-d show the UV-Vis absorption spectra of MB solution concentration changes over time after being in contact with the nanofiber materials. The PAN/PVDF and PAN/PVDF/ANO samples clearly were not able to degrade the MB solution either using adsorption or employing photocatalytic degradation (see Fig. 5a and b). The changes in MB concentration with increase of the reaction time were observed for the Nb2O5@PAN/PVDF and Nb2û5@PAN/PVDF/ANO composite nanofibers (see Fig. 5c and d). The absorption intensities of the UV-Vis spectra were clearly decreased with increasing reaction times indicating the decrease in the MB concentrations. The MB solution color changes were also observed for Nb2Os@PAN/PVDF and Nb2O5@PAN/PVDF/ANO composite nanofibers in contrast to the unchanged color of the MB when being contact with the PAN/PVDF and PAN/PVDF/ANO samples (see insets in Fig. 5a-d). The growth of the Nb2û5 nanostructures is clearly beneficial for both adsorption and photocatalytic behaviors of the nanofiber samples.

Fig. 5e shows the variation in the MB concentration with time interval, where the Co is the initial MB concentration, and the C is the MB concentration at any time. The concentration of the MB solution is calculated using the maximum absorbance value of MB solution (664 nm) and the MB solution calibration coefficient. The MB concentration of the PAN/PVDF sample clearly changed over time, while the PAN/PVDF/ANO was only able to degrade a tiny amount of MB (9%) after total reaction time of 480 min, indicating the inability of the PAN/PVDF and PAN/PVDF/ANO to degrade organic pollutant. A faster degradation process was observed for the Nb2O5@PAN/PVDF composite nanofiber, which was able to degrade 45% of the MB molecules even only after 120 min of contact time. Moreover, when the reaction time was increased to 480 min (8 h), the Nb2O5@PAN/PVDF had a degradation efficiency of 55%.

The highest efficiency of MB degradation was achieved by the hierarchical Nb2O5@PAN/PVDF/ANO composite nanofibers, where it could already degrade 54% of MB molecules even after only 30 min contact time. The composite nanofiber reached a degradation efficiency of 90% after 240 min (6 h) of contact with the MB solution. Finally, the hierarchical Nb2O5@PAN/PVDF/ANO composite nanofibers after 8 h of contact time obtained a degradation efficiency of 96%. These results have proven that the hierarchical Nb2O5@PAN/PVDF/ANO composite nanofibers possess a superior degradation capability compared to other samples, which makes them a promising candidate for wastewater treatment and environmental remediation applications.

The pseudo-first-order kinetic modelling was carried out to further comprehend the MB degradation of the Nb2O5@PAN/PVDF and Nb2O5@PAN/PVDF/ANO composite nanofibers, which is based on the following Equation (2):

... (2)

where Co, C, k, and t are the MB initial concentration (mg/L), MB concentration at given time (mg/L), the pseudo-first-order constant (min-1), and the contact time (min), respectively. The pseudo-first-order kinetics of the PAN/PVDF, PAN/PVDF/ANO, Nb2O5@PAN/PVDF and Nb2O5@PAN/PVDF/ANO composite nanofibers for the degradation of MB pollutants are shown in Fig. 5f. The detailed corresponding kinetic rate constant (k) and the correlation coefficient (R2) values of all prepared samples are listed in Table 2. The samples without №205 (i.e., both PAN/ PVDF and PAN/PVDF/ANO nanofibers) clearly do not exhibit adsorption and photocatalytic activity toward MB molecules as confirmed previously. Meanwhile, different phenomena are found in two varied №205integrated composite variants (i.e., Nb2O5@PAN/PVDF and Nb2O5@PAN/PVDF/ANO nanofibers). Here, the data fit well with the linear fitting shown by the calculated coefficient (R2) values of 0.942 and 0.966 for the adsorption and photocatalysis of Nb2O5@PAN/PVDF nanofiber, respectively. Moreover, the R2 values of the Nb2O5@PAN/PVDF/ANO samples are even higher (i.e., 0.998 and 0.999) for the adsorption and photocatalysis processes, respectively. These results show that the occurring processes are in accordance with the pseudo-first-order kinetic model.

The dark condition (t = 0-180 min) in Fig. 5f was used to characterize the kinetics of the adsorption process, while the UV exposure (t = 180-480 min) in Fig. 5f was employed to investigate the photocatalytic degradation rate of the materials. Here, except the PAN/PVDF sample that almost does not show any reaction, the other three composite materials (i.e., PAN/PVDF/ANO, Nb2O5@PAN/PVDF, and Nb2O5@PAN/ PVDF/ANO nanofibers) have better adsorption behavior than photocatalytic activity, as indicated by the higher k^ value compared to the ^pho value (see Table 2). The PAN/PVDF/ANO sample has low к values of 0.073 x 10-2 min-1 and 0.039 x 10-2 min-1 for adsorption and photocatalysis process, respectively. Nonetheless, when the №205 has been incorporated to the composite system resulting in Nb2O5@PAN/PVDF/ ANO nanofibers, these к values have been drastically increased up to 1.29 x 10-2 min-1 and 0.30 x 10-2 min-1 for the adsorption and photocatalytic degradation of MB molecules, respectively. These values are 3.1 and 5.1 times higher than the kads and kph0 values of the Nb2O5@PAN/PVDF nanofibers without ANO, respectively. In summary, the kinetic analysis reveals that the Nb2O5@PAN/PVDF/ANO composite nanofibers have been superior compared to other composite samples, where the addition of №205 and ANO to the composite system has a positive impact on the overall performance of the nanofibers in removing MB from the wastewater.

To further evaluate the reactive oxygen species (ROS) that is responsible for attacking the MB structure during photocatalysis, free radical trapping experiments were performed (Fig. 5g). The tests were performed by using 25 mg of Nb2O5@PAN/PVDF/ANO composite nanofibers and 10 mg/L MB solutions (50 mL) with addition of free radical trapping agents. The experiments were carried out fully under UV irradiation. Three different free radical trapping agents (scavengers) were used, i.e., isopropyl alcohol (IPA), p-benzoquinone (BQ), and ethylenediaminetetraacetic acid (EDTA). They were used for trapping the hydroxyl radical (·OH), superoxide radical CO2 ), and hole (h+) that were possibly generated during photocatalysis, respectively. These experimental results can be evaluated by calculating the degradation percentage using Equation (3), where C and Co are the remaining concentrations after 180 and 0 min of reaction time, respectively.

... (3)

The degradation percentage of the Nb2O5@PAN/PVDF/ANO composite nanofibers under influence of IPA did not change significantly compared to that of the sample without scavenger (blank). It shows that the hydroxyl radical was not the main reactive species during the photocatalysis. The MB degradation percentages of the Nb2O5@PAN/PVDF/ ANO composite nanofibers under BQ and EDTA influences are lower than that of the blank sample. The degration percentage decreases from 97.7% (blank) to 82.3% and 69.9% when BQ and EDTA scavengers were present, respectively. It shows that both superoxide radical (·02) and hole were responsible for the photocatalytic degradations of MB under UV irradiation. These reactive oxygen species then degraded the MB molecules into smaller inorganic molecules (i.e., H2O, Cl-, CO2, SO2and NO3) as proposed previously [76,77].

We also evaluated the performance of our Nb2O5@PAN/PVDF/ANO composite nanofibers in comparison to those of other existing state-ofthe-art hierarchical metal oxide nanofiber catalysts in Table 3. In terms of the base nanofiber matrix, other reported studies only employed single polymer material (i.e., polyvinylpyrrolidone (PVP), PAN, or PVDF). Meanwhile, ours used hybrid structures of PAN/PVDF/ANO. As for the metal oxide options, most of other catalyst materials involved either bismuth oxyiodide (BiOI) [78-80] or zinc oxide (ZnO) [57,58] when they targeted to degrade rhodamine B or methylene blue, respectively. Thus, it is obvious that our study has demonstrated, for the first time, a combination of Nb2û5 nanorods with hybrid PAN/PVDF/ANO nanofiber membrane. Moreover, our samples exhibit either comparable or superior performance compared to the others, where a high degradation efficiency of 96 % has been achieved, even when tested with a significantly larger pollutant volume (i.e., 250 mb as opposed to 10-50 mb in other studies). Besides having high pollutant degradation efficiency, our samples can be synthesized using a relatively simple and straightforward method. Given these findings, our work serves as a valuable addition to the existing photocatalyst options in literature for wastewater treatment through the use of hierarchical composite nanofiber materials.

4. Conclusions

We successfully fabricated hierarchical Nb2O5@PAN/PVDF/ANO composite nanofibers through a combination of electrospinning and hydrothermal methods. Material analyses, including FE-SEM, XRD, FTIR spectroscopy, and optical band gap energy assessments, confirmed the vertical growth of №205 nanorods on the nanofiber surfaces. The incorporation of an oxide precursor into the polymer solution prior to electrospinning emerged as a critical step for achieving this hierarchical structure. Remarkably, these composite nanofibers were capable of degrading 250 mb of 10 mg/L aqueous MB dye within an 8-h reaction time. The unique structure considerably enhanced both adsorption and photocatalytic degradation of MB pollutants. Specifically, the kinetic coefficients for these two processes were 1.29 x 10-2 min-1 and 0.30 x 10-2 min-1, respectively, which are considerably higher than those achieved with Nb2O5@PAN/PVDF composite nanofibers. Free radical trapping experiments revealed that superoxide radicals (·02) and hole (h+) were responsible for the photocatalytic degradations of MB under UV irradiation. This development provides a new strategy for designing novel semiconductor catalysts loaded on nanofiber membranes for effective and efficient degradation of dye pollutants in the industrial wastewater treatment applications. For future research, optimization of hydrothermal parameters is essential to further increase the surface area and lower the band gap energy, thereby enhancing both the adsorption capacity and the photocatalytic activity of the Nb2O5 composite nanofibers.

Data availability statement

The datasets generated during and/or analyzed during the current study are available from the corresponding author.

Author contributions

AR: Conceptualization, Methodology, Investigation, Writing - Original Draft, Writing - Review & Editing, Visualization, Funding acquisition. KDPM: Investigation. EKAM: Investigation. RA: Resources. YGW: Methodology. IPM: Funding acquisition. NY: Resources, Formal analysis. JW: Resources, Writing - Review & Editing. KT: Resources. HSW: Writing - Original Draft, Writing - Review & Editing. TT: Conceptualization, Methodology, Writing - Original Draft, Writing - Review & Editing, Supervision, Visualization, Funding acquisition. All authors approved the final manuscript.

Declaration of competing interest

The authors declare that there are no conflicts of interest.

Acknowledgement

This work was fully funded by the Minister of Education, Culture, Research, and Technology of Indonesia through a research scheme of "Penelitian Fundamental - Reguler (PFR) 2023" under a contract number of 1115C/IT9.2.1/PT.01.03/2023.

Sidebar

Received 8 August 2023; Accepted 26 October 2023

Available online 11 November 2023

Footnote

* Corresponding author. Department of Materials Engineering, Institut Teknologi Sumatera, Terusan Ryacudu, Way Hui, Jati Agung, Lampung Selatan, 35365, Indonesia.

* · Corresponding author. PT Nanosense Instrument Indonesia, Umbulharjo, Yogyakarta, 55167, Indonesia.

* ·· Corresponding author. Center for Green and Sustainable Materials, Institut Teknologi Sumatera, Terusan Ryacudu, Way Hui, Jati Agung, Lampung Selatan, 35365, Indonesia.

E-mail addresses: [email protected] (A. Rianjanu), [email protected] (H.S. Wasisto), [email protected] (T. Taher).

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