Preparation and characterisation of Preyssler heteropolyacid-cellulose acetate hybrid nanofibers: a new, green and recyclable nanocatalyst for photodegradation of methyl orange as the model dye

ABSTRACT For the first time, sodium 30-tungstopentaphosphate, a green and efficient solid acid, was used for the isolation of cellulose from rice husk, resulting in the generation of Preyssler heteropolyacid-cellulose acetate hybrid nanofibers. These modified fibres were characterised by Fourier transform infrared spectroscopy spectrum, scanning electron microscopic images and energy-dispersive spectrometry. It was also showed that Preyssler heteropolyacid was evenly dispersed on or into the nanofibers and the distribution became denser as the concentration of Preyssler HPA increased. The electrospuned nanofibers were employed successfully for the photodegradation of methyl orange, a common azo dye pollutant in the environment.


Introduction
Due to the unique properties of nanofibers, their fabrication has recently gained increased attention. They have a very large surface area to mass ratio, light weight, thinness, fibre diameter at nanoscale, porous structure, etc. These qualities make them good candidates to be used for a wide variety of applications [1][2][3]. Nanofibers can be fabricated by a wide range of strategies [4][5][6][7]. Most but not all are synthetically provided by electrospinning process, which fabricates fibres from polymer solution utilising a fruitful interaction among fluid dynamics electrically charged liquids and electrically charged surfaces [8]. Nevertheless, process capability, biocompatibility and biodegradability of the provided composites are restricted by comparison with those of naturally occurring polymers.
Thus, there is a growing interest in the fabrication of nanofibers using natural polymers. This attraction of attention is partly due to merits such as the introduction of CONTACT Fatemeh F. Bamoharram fbamoharram@mshdiau.ac.ir additional functionalities such as biodegradability, biocompatibility, recyclability and readily availability [9]. In this line, there are various naturally occurring nanofibers, such as deoxyribonucleic acid (DNA) with double helix, etc. [10]. Undoubtedly, cellulose fibrils are the most abundant naturally occurring fibre [11]. However, electrospinning of some biopolymers such as cellulose is usually problematic [12], mostly due to their insolubility in common organic solvents and their desirability to form gels via aggregation [13]. Even fruitful attempts to electrospin cellulose ended to usual low efficiency and yielding even in relatively long time [14]. Cellulose is soluble in few solvents, thus practically can be processed by electrospinning. However, it requires elevated temperatures for totally being dissolved, making the process even more challenging. It has also been reported that residual ions are cumbersome to remove from the provided fibres [15,16]. Thus, to extend the applications of cellulose nanofibers, functionalisation of the cellulose nanofiber surface is still in much demand. Functionalised nanofibers can be well defined as nanofibers with specific extraneous materials for adding particular functionalities and abilities to nanofibers. In this way, they can be placed to a numerous of applications. Functionalising of nanofibers provides scope for both basic and applied research. There are a few materials that can provide functionality to nanofibers, which can be selected based on their final applications [17]. They can be metals and metal oxides at nanoscale, biological supplies, i. e. enzymes, natural products or even carbon nanotubes [17]. Among them, the addition of metal oxides to the base fibre matrix has stirred up much interest. Polyoxometalates (POMs) are a class of metal oxides that exhibit a wide range of molecular clusters and can be easily employed in the generation of new materials [18].
A literature survey showed no reports regarding the preparation of Preyssler heteropolyacid-cellulose acetate hybrid nanofibers. Thus we were motivated to the present study.
In continuation of our investigation on the applications of Preyssler acid from different points of view [20 and cited references therein], particularly its key role in catalysis, and with our ongoing interests in the expansion of the applications of Preyssler, and to comply with the green principle of bio-synthesis of nanofibers, herein, we wish to disclose the results for the application of Preyssler acid as an metal oxide cluster in the functionalisation of cellulose via the electrospinning method. In this line we report the preparation of novel Preyssler heteropolyacid-cellulose acetate hybrid nanofibers.
Due to the importance of natural biopolymers in nanotechnology, medicine and industry, we extracted cellulose from rice husk (RH). RH is one of the most easily accessible agricultural wastes in many rice producing countries worldwide. It contains 75%-90% of organic materials [21,22] and the abundant constituent of RH is cellulose. It is a thermodynamically stable compound with crystalline structure capable of making plethora of hydrogen bonds. Then we attempted to prepare cellulose acetate (CA) nanofiber hybrid based on Preyssler acid via electrospinning technique. The prepared hybrid nanofibers were employed for the removal of the hazardous methyl orange (MO) as a common model of azo dyes under UV light. At first, inexpensive, commercially available, ecofriendly, reusable and readily accessible Preyssler anion was used as an efficient solid acid for an easy isolation of cellulose from RH. Then Preyssler anion was used to create functional inorganic-cellulose hybrid modified fibres. We attempted to develop a novel functionalised CA/heteropolyacid composite nanofiber (CA/HPA) with catalytic properties for operational treatment of wastewater. The aforementioned modified fibres were prepared by sol-gel combined with electrospinning technique.

Chemicals and instruments
RH was supplied by a local rice milling industry (northern part of Iran). All reagents were of analytical grade and used as received without further purification. Scanning electron microscopy (SEM) (VEGA\\TESCAN-XMU, Czech Republic), energy-dispersive spectrometry (EDS) (Mira 3-XMU, Czech Republic), Fourier transform infrared spectroscopy (FTIR) (Avatar.370 FTIR Thermo Nicolet spectrophotometer, England), UV-visible spectrophotometer (Optizen UV3220, Germany) and image analysis program (ImageJ) were used for the characterisation of compounds. The specific surface area (S BET ) of Preyssler was determined by nitrogen adsorption/desorption techniques.
Preyssler acid was prepared by the passage of a solution of potassium salt in water through a column (50 cm £ 1 cm) of Dowex 50W £ 8 in the H C form and evaporation of the elute to dryness under vacuum (S BET D 0.78 m 2 /g). Cellulose powder was extracted from RH and then CA was synthesised by acetylation reaction of cellulose [23].

Preparation of electrospinning biocomposite nanofibers
CA solution was prepared by dissolving 10 wt% CA in acetone/DMAc and stirred for 3 h. HPA particles at different contents (0, 1, 2, 3 and 4 wt%) were added into the CA solution. Then, the solutions were placed under sonication for 4 h. Electrospinning was carried out at an applied voltage of 25 kV, tip-to-collector distance of 15 cm and a solution feed rate of 0.2 mL h ¡1 at room temperature. The ambient temperature and humidity were controlled around 25 C and 22%, respectively.

Photodegradation experiments
Photodegradation experiments were carried out to establish the effect of contact time on the photodegradation process and to identify the photodegradation rate. The photo reactor was designed in our laboratory. In a typical reaction, a 250-mL Pyrex glass was equipped with a magnetic stirrer, MO solution and catalyst. The mixture was stirred and purged with nitrogen for 1 h, and then it was irradiated under the high-pressure mercury lamp (Philips, 125 W, wavelength 254 nm) as UV light source. The temperature in the glass reactor was set to 25 § 2 C by the circulating water.
The experimental procedure is described as follows: A series of 10 mL solutions containing 30 mg L ¡1 of MO solution were prepared and the pH of the solution was adjusted to 4.0. The nanofiber films with different amounts of the Preyssler acid as catalyst (0, 1, 2, 3 and 4 wt%) were placed into an MO aqueous solution and was shaken for a period of time. The sample was placed under light irradiation and the MO concentration was determined using a UV-vis spectrophotometer.

Isolation of cellulose from RH
Alkali pre-treatment was carried out to remove lignin and hemicelluloses from RH. In the first step of alkaline and acid treatment, the lignin and hemicellulose part was dissolved and in the second step, bleaching for whitening the RH was performed.
All strategies so far reported in the first step utilised liquid acids such as H 2 SO 4 . Generally, the substitution of the known toxic, corrosive and polluting liquid acid by solid acid is one of the severe demands of all societies. Green technology could be provided by the use of benign chemicals and catalysts involving the replacement of liquid acids with solid acids. In this line, we successfully replaced Preyssler solid acid catalyst with sulphuric acid used in the first step. Our findings revealed that this catalyst is highly active rendering efficient acidification comparable with sulphuric acid. The extracted CA from RH, and the SEM image were shown in Figure 1(a) and (b), respectively.

Acetylation of cellulose
One of the most conventional modifications of cellulose nanofibers is the chemical modification of the cellulose nanofiber surface. Among the chemical modifications, acetylation is considered a simplest, most popular, and inexpensive strategy to improve the surface property. Thus, CA was prepared from RH by employing acetic anhydride under solventless system, according to earlier works [23].

Synthesis of CA/heteropolyacid composite nanofibers
Inorganic metal oxide-biopolymer nanofibers are rather new materials. They show similar qualities of nanofibers, for instance porosity, high surface, diameters in the nanoscale, etc., and the functional properties similar to those doping materials. Typically, the addition of POMs as metal oxide clusters to the base fibre matrix is a good example. This inorganic metal oxide-biopolymer nanofiber was developed via the combination of instrumentation, nanoscale science and material fabrication. In this research, we employed the very recent developments achieved for the design of functional systems. Functionalised CA/heteropolyacid composite nanofibers were synthesised to practically treat wastewater. The synthesised modified fibres was provided via combination of sol-gel and electrospinning methods.
SEM images of the nanofibers with different HPA contents (1%-4%) were illustrated in Figure 2(a)-(e). Those vividly show that beadless nanofibers have been incorporated with HPA particles.
As it can be clearly observed in Figure 2(a), pure CA nanofibers exhibited the uniform, smooth and beadles nanofibers. When Preyssler acid is added, HPA particles deposited on/in the nanofibers evenly could be clearly observed and their presence could be denser if their concentration is increased (Figure 2(b)-(e)). Nevertheless, HPA particles were agglomerated to some extent as the concentration increased due to different parameters such as the charge density/potential of the powder surfaces and van der Waals forces. In order to examine higher concentration of Preyssler, several percentages (5%-7%) of Peryssler were evaluated. Our studies revealed that under these conditions, the reaction rate was clearly influenced by the increase of HPA concentration. The rate and yield of the obtained nanofibers increase along with the concentration of HPA up to 4% and any further increase in the concentration decreases both the rate and yield. In other words, any further increase in the amount of HPA does not show any appreciable effect on the yields of reaction. This is perhaps due to blocking of needles by HPA. As it can be seen in Figure 2, pure CA nanofibers had an average diameter of 121.11 nm. As the concentration of Preyssler increased from 0 to 4 wt%, the average fibre diameter enhanced gradually from 121.11 nm to 156.68 nm. This increase in fibre diameter could be attributed to the increase in the concentration of the Preyssler which can be proven by Figure 2.
The results for EDS analysis are shown in Figure 3(a)-(e). The quantitative results are shown in Tables 1 and 2. These tables show that the atomic percentage of W increased from 2.67 wt% to 20.98 wt%. Besides, compared with EDS spectrum for pure CA in Figure 3(a), Figure 3(b)-(e) shows that Preyssler HPA were well introduced and characteristic W La at 8.380 keV were stronger with the increasing of Preyssler. This effect can also be seen for P Ka at 2.010 keV from 0.18 to 0.26. Figure 4 shows biocomposite nanofiber diameter distribution using 4 wt% HPA contents for the measurement of 150 fibres by using image analysis program.
FTIR spectra of Preyssler, CA and Preyssler/CA nanofibers are shown in Figure 5 on the 400-4000 cm ¡1 region. Preyssler structure gives rise to four types of oxygen that are responsible for the fingerprints bands of Preyssler anion between 1200 and 600 cm ¡1 . The characteristic bands of Preyssler structure, H 14 (Figure 5(a)). These bands can shift, weaken, strengthen or mask in different conditions. The characteristic peaks of CA at 3400 cm ¡1 for -OH, 1750 cm ¡1 for CDO and 1367 cm ¡1 and 1050 cm ¡1 for C-O can be observed in Figure 5(b). Figure 5(c) shows the characteristic bands of Preyssler and CA that many of them have blue-shifted and many of them have red-shifted, indicating that many of the bonds were strengthened and the others were weakened. Some of the characteristic bands are overlapped together.
The W-O-W band placed at 948 cm ¡1 is masked by that of CA. This figure shows, after the incorporation of Preyssler HPA, the intensity of peak for -OH at 3400 cm ¡1 is   increased, which revealed that the introduction of HPA can affect the hydrophilic property of CA, so obviously can increase the hydrophobicity.

Catalytic activity
Catalytic activity of the CA biocomposite nanofibers with different Preyssler acid contents was studied by the photocatalytic degradation of MO dye under the irradiation of UV light (Figure 6(a)-(d)). This figure shows that MO solution can be degraded under UV light in different time period. It is inferred from this study that the photodegradation increases with an increase in the amount of Preyssler, obviously due to the higher number of photocatalytically active sites. Also, since photocatalytic reactions mainly take place on the surface of the catalyst, a high surface-to-volume ratio is of great significance for increasing the photodegradation rate. After 25 minutes, the total absorption of MO disappeared and 97.8% decolorisation was obtained with 4% Preyssler. The photocatalytic decolorisation is shown in Figure 7.
Regarding Figure 6(e), it is clear that insignificant degradation of MO occurred when neat CA was irradiated with UV light, whereas the efficiency of the HPA/CA composite nanofibers increased variously by the deposition of different Preyssler contents. For CA  nanofibers with 0, 1, 2, 3 and 4 wt% HPA contents, concentration of MO reduced by about 15%, 53%, 93.1% and 97.8%, respectively, after 25 min. Evidently, a higher reaction constant of the photocatalytic degradation of MO can be obtained by CA nanofibers containing higher Preyssler acid contents. R 2 was calculated for both zero-and first-order reactions and regarding to R 2 values, and good linearity for zero-order reaction 'C D f(t)', we propose zero-order reaction. Among a wide variety of chemicals in textile wastewaters, organic dyes are important pollutants and hazard to the environment. Therefore, different separation techniques have been used in the removal of dyes from aqueous solutions, including biological (aerobic and anaerobic degradation), chemical (Fenton reagent, ozonation and photocatalyst) and physicochemical (adsorption, ion exchange, membrane filtration and coagulation) methods. All these methods have been successful in the decolourisation of textile effluents, but their applications are limited by large sludge production, high operational cost, disposal problems, production of toxic by-products, slow process, need to further treatment, low surface area and low volume treatment ( Table 3).
The prepared modified nanofibers in this research showed excellent photodegradation activity (97.8%) in the decolourisation of MO under UV irradiation, through a conventional chemical method (entry 2, Table 3). As a green technology, photocatalytic treatment of polluted water has been considered as one of the most attractive approaches [25]. In comparison with the reported methods (Table 3), and with respect to the large surface area of nanofibers, along with advantages of Preyssler such as optical stability, adjustable oxidisability, more stable chemical structure, low toxicity and inexpensiveness, we suggest that advantages of this synthesised modified nanofibers are high surface area, no toxic by-products due to degradation of  photocatalyst, no sludge production, the high level exposure of the photocatalysts due to their large surface areas, the easy separation, low cost, greenness, optical activity, decolourisation in a short time with high efficiency and reusability.
The several time recoveries had only slightly decreased the catalytic activity, pointing to the stability and retention capability of this useful nanocatalyst.

Conclusions
The Preyssler heteropolyacid-CA nanofibers containing different amounts of Preyssler HPA were successfully prepared by electrospinning process. Catalytic activity of the synthesised nano-biocomposite with different Preyssler contents was examined by the photodegradation of MO. SEM images indicated that Preyssler HPA had been embedded on or into the CA nanofibers. Moreover, as the concentration of Preyssler increased, the average fibre diameter of the nanofibers increased gradually. EDS spectrum confirmed the presence of Preyssler HPA in the synthesised modified fibres. FTIR analysis confirmed that the introduction of Preyssler HPA affects the hydrophilic property of CA. In addition, all these analyses revealed that the incorporation percentage of Preyssler HPA on or into the nanofibers increased as its concentration increased from 1 to 4%. Photocatalytic degradation of MO showed that a higher photodegradation in shorter time can be obtained by higher Preyssler HPA contents.
We have found that this catalyst can be reused several times without loss of activity. The remarkable photodegradation of MO in the presence of these modified fibres, excellent recyclability and stability led to a promising, economical and environmentally friendly novel material for potential applications in dyeing water treatment. Regarding to cellulose is a green biopolymer, environmental friendly, and biocompatible [26], and heteropolyacids are versatile green catalysts in a variety of reaction media [27]; this research can be lie in the line of green chemistry. Degrade of some photocatalyst into toxic by-products.

Ozonation
No production of sludge.
Half-life is very short (20 min) and high operational cost. 4 Fenton reagent Effective process and cheap reagent.
Sludge production and disposal problems.
Aerobic degradation Efficient in the removal of azo dyes and low operational cost.
Very slow process and provide suitable environment for growth of microorganisms. 7 Anaerobic degradation By-products can be used as energy sources.
Need further treatment under aerobic conditions and yield of methane and hydrogen sulfide. 8 Physiochemical 9 Adsorption High adsorption capacity for all dyes.
High cost of adsorbents. Need to dispose of adsorbents. Low surface area for some adsorbents. 10 Ion exchange No loss of sorbents. Not effective for disperse dyes. 11 Membrane filtration Effective for all dyes with high quality effluent.
Suitable for treating low volume and production of sludge. 12 Electrokinetic coagulation Economically feasible. Need further treatments by flocculation and filtration and production of sludge.