Photocatalytic Degradation of Bisphenol A with α-Fe2O3 Fibers and Particles

Two-step synthesis of Fe2O3 particles, performed by precipitation from iron(III) chloride (FeCl3•6H2O) using ammonium hydroxide in first step and calcination at 400 and 700 C during 4 h, provided Fe2O3 400 C and Fe2O3 700 C photocatalysts, respectively. The electrospinning process was used to prepare iron oxide fibers, named Fe2O3 fiber. Morphological and structural properties of samples were determined by Scanning Electron Microscopy (SEM), X-ray diffraction (XRD), Fourier Transform Infra-Red (FT-IR) and BET/BJH analysis. It was found that the α-Fe2O3 phase (hematite) has shown the compaction of the structure at 700 C, i.e. lower textural properties. Hematite particles and fibers are used for bisphenol A (BPA) removal by photocatalytic decomposition and the enhanced catalytic performance was found with the use of Fe2O3 400 C particles.


Introduction
Iron oxide plays an important role in many geological and biological processes and represents a very important material due to its non-toxicity, low cost, catalytic activity, and biocompatibility. The most used iron oxides are FeO, Fe 2 O 3 , Fe 3 O 4 . Iron(III) oxide has four crystallographic phases : hematite (α-Fe 2 O 3 ), β-Fe 2 O 3 , maghemit (γ-Fe 2 O 3 ) and ε-Fe 2 O 3 [1]. Among them, α-Fe 2 O 3 is the most stable under ambient conditions and has a wide range of applications in catalysis, pigments, biomedical materials, lithium batteries, adsorbents, solar cells, etc. [2][3][4][5][6]. Different techniques may be used in the preparation of powdered hematite: hydrothermal technique [7], sol-gel method [8] and co-precipitation method [9,10]. Among these techniques, for the preparation of powdered hematite, chemical precipitation is attractive due to low costs, high purity, short preparation time, homogeneous particle distribution and the ability to produce excellent polycrystalline samples.
Electrospinning is a simple method for producing nanofibers for various applications [11,12]. α-Fe 2 O 3 nanofibers may be obtained via a simple hydrothermal route or electrospinning process followed by calcination that may be used in the removal and recovery of noxious Cr(VI) from wastewater [13,14]. Interconnected 1D hollow structure of α-Fe 2 O 3 nanofibers was made by annealing of electrospun poly(vinyl alcohol) (PVA)-Fe 3 O 4 composite fibers and used in dye removal [15,16].
Bisphenol A (BPA) has become a significant contaminant since three million tons of this compound are produced every year worldwide. It is used as a raw material for the preparation of epoxy and polycarbonate resins (eg coatings for water containers, baby bottles and medical devices). Due to its massive use, BPA contamination is found in all environmental protection sections, including air, water and soil [17]. Many recent research efforts have focused on the photocatalytic treatments for BPA removal [Error! Bookmark not defined.8, 8,9]. In the context of environmental protection, iron-enriched clay catalysts even at lower activity have an advantage over homogeneous iron catalysts which can be further improved by increasing the iron content [18,19]. In addition, the photocatalysts should be easily separable from the water, in order to improve the ease of recycling and reusing the photocatalysts [20].
The aim of this study was: (i) to explore the synthesis of hematite particles and fibers, and (ii) to test their catalytic activity as heterogeneous catalysts for Fenton degradation of BPA in aqueous solutions. Two different methods were used: precipitation followed by calcination at two different temperatures for the synthesis of α-Fe 2 O 3 particles, i.e. Fe 2 O 3 400 o C and Fe 2 O 3 700 o C, and electrospinning for the synthesis of hematite fibers, i.e. Fe 2 O 3 fiber. These heterogeneous catalysts were meant to enable not only academic significance but also industrial due to good photo-catalytic activity and enhanced feasibility of the Fenton process for the treatment of wastewaters.

Materials
Iron(III) chloride (FeCl 3 ·6H 2 O) was purchased in the crystallized state from the Clariant company. Poly(vinyl alcohol) (PVA), trademark Mowiol 18 -88 with M w = 130.000 g/mol was purchased from Sigma Aldrich. Ammonium hydroxide (NH 4 OH) was purchased from NRK INŽENjERING doo, Serbia. Hydrogen peroxide (H 2 O 2 ) and Bisphenol A (BPA) was supplied from Sigma Aldrich. All the chemicals were used without further purification. Deionized water was used in all experiments.

Preparation of electrospun hematite fibers
The electrospinning procedure was used for the preparation of iron oxide ceramic fibers. The water solution of PVA/FeCl 3 ·6H 2 O was made with a mass ratio of 1:1. The solution was stirred on the laboratory mixer for 3h, and subjected to electrospinning using Electrospinner CH -01 (Linari Engineering, Italy). The solution was placed into the 20 mL plastic syringe having a needle of 0.8 cm orifice. The high voltage supply (Spelmann PCM50P120, USA) capable of producing the 30 kV was used in experiments. The precursor solutions were used in supplied to the nozzles using syringe pumps of the R100E type (Razel Scientific Instruments, USA). The voltage applied in experiments was ~28 kV. The mass flow varied around 0.05 mL/h, while the distance between the needle and the alumina foil collector was 15 cm. Obtained fibers were dried and then heat-treated at 700 o C during 2 h in air, and denoted as Fe 2 O 3 fiber.

Preparation of Fe 2 O 3 particles
Hematite particles were synthesized by the chemical precipitation method. An aqueous solution was prepared by dissolving 5 g FeCl 3 ·6H 2 O in 100 mL of deionized water under magnetic stirring for 30 min. An aqueous solution of 2M of NH 4 OH (50 mL), used as the precipitating agent, was added gradually to maintain a pH value of 11. The resulting precipitations were collected and centrifuged at 6000 rpm, washed with distilled water and ethanol for several times and dried in air. Particles calcined at 400 o C and 700 o C for 4 h were denoted Fe 2 O 3 400 o C and Fe 2 O 3 700 o C, respectively.

Photocatalytic experiments
The photocatalytic performance of the synthesized catalyst samples was evaluated according to the rate of BPA degradation. A 300 W xenon lamp (PE300BF) was used as a light source. In a typical procedure, 10 mg of catalyst was suspended in 50 mL of BPA aqueous solution (C 0 = 20 mgL -1 , pH = 5.0), and the mixture was stirred for 30 min to reach an adsorption equilibrium. Aqueous solution H 2 O 2 (1.0 mL, 1 %) was added to the reaction solution at the beginning of the irradiation. Then, the suspension was stirred in the dark for 30 min before the lamp was turned on. The concentration of BPA in the system was followed according to the peak at 276 nm by UV-Vis spectroscopy. It was found that the solution pH had no significant impact on photocatalysis, while the acidic conditions slightly reduced sorption capacity and hydrogen bonding between Fe 2 O 3 and BPA [21].

Characterization methods
The morphology of the fibers and particles was examined using the FE-SEM, MIRA 3 TESCAN electron microscope operated at 20 kV. The image analysis tool (Image-Pro Plus 4.0, Media Cybernetics) was used to obtain the fiber and particles diameter distribution.
X-ray diffraction (XRD) patterns were recorded with an Ital Structure APD2000 Xray diffractometer in a Bragg-Brentano geometry using CuKα radiation (λ = 1.5418 Å) and step-scan mode (range: 20−90° 2θ, step-time: 0.5 s, step-width: 0.02°). The specific surface area and pore volume/size were measured using Micromeritics ASAP 2020 surface area and porosity analyzer. The samples were previously degassed under vacuum at 150 o C for 11 h.
The structural analysis of calcined powders synthesized at different temperatures was performed by single-beam Fourier-Transfer Infrared Spectroscopy (FTIR) using a Nicolet iS10 spectrometer (Thermo Scientific) in the attenuated total reflectance (ATR) mode with a single bounce 45 °F Golden Gate ATR accessory with a diamond crystal, and DTGS detector. FTIR spectra were obtained at 4 cm -1 resolution with ATR correction. The FTIR spectrometer was equipped with OMNIC software and recorded the spectra in the wavelength range from 2.5 μm to 20 μm (i.e., 4000-500 cm -1 ). The concentrations of the bisphenol A were determined using UV-Vis spectroscopy (Shimadzu UV-1800 spectrophotometer).
A laboratory pH meter, InoLab Cond 730 precision conductivity meter (WTW GmbH), with an accuracy of ± 0.01 pH units, was used for the pH measurements. The pH values at the point of zero charges (pH PZC ) were measured using the pH drift method [22].

The microstructure of the ferrous oxide based fibers and particles
In order to explain properly the photocatalytic activity of the crystalline materials, it is important to study morphology evolution (surface area, porosity, etc.). Morphology evolution is of particular importance because it may significantly modify physical properties and surface reactivity. The SEM micrographs of particles and electrospun fibers are shown in  (Fig. 1a) particles show somewhat higher surface roughness than Fe 2 O 3 700 o C (Fig. 1b). Higher temperatures lead to the compaction of particles resulting in smooth microstructure and the occurrence of very fine particles that can be caused by the breaking off of larger particles due to compaction. A smooth surface of raw Fe 2 O 3 fibers is noticed due to the presence of PVA in its structure. PVA degradation at 700 o C caused the formation of fibers porosity that can be of interest for the application in photocatalytic experiments. These images were used to measure the diameters of α -Fe 2 O 3 spherical particles and fibers obtained by the electrospinning method. The image analysis software (Image Pro Plus 6.0) was used to obtain the fiber and particles diameter distribution. Mean diameter distribution of α-Fe 2 O 3 fibers and α-Fe 2 O 3 particles calcined at 400 and 700 o C is shown in Fig. 2.
The mean diameters of the spherical α-Fe 2 O 3 particles calcined at 400 o C and the α-Fe 2 O 3 particles calcined at 700 o C were found to be 296.6 nm and 253.1 nm, respectively, according to results presented in Fig.s 2a and 2b. Accordingly, the calcination temperature increase affects the reduction of the particle diameter. The mean diameter of the raw fibers after calcination decreases from 1255 nm to 878 nm, Fig. 2c and 2d. Transformation to the well-defined crystal structure of particles leads to the collapse of the pores of the lowtemperature generated phase and the consequent decrease of the particle specific surface area [19]. Compaction of α-Fe 2 O 3 particles at higher temperatures leads to a decreased particles size. In order to obtain high adsorption of BPA and good photocatalytic activity, it is necessary to provide high availability of surface active sites at the particle/solution interfaces. Such behavior depends on the morphology of the catalyst, i.e., particle size and surface area, as well as on particle interaction with BPA. Determined textural characteristics are presented in Table I. Larger pore volume, diameter and S BET of α-Fe 2 O 3 400 o C particles, in comparison to particles calcined at 700 o C, i.e. α-Fe 2 O 3 700 o C, was found. These phenomena could be explained as the consequence of the structure densification as noticed by the decrease of particle diameter at the higher temperature of calcination. The electrospinning process enabled obtaining slightly higher S BET than the α-Fe 2 O 3 700 o C particles produced at the same calcination temperature. The significantly increased porosity of the α-Fe 2 O 3 fiber was obtained as well.
Determined pH PZC values for calcined particles at 400 and 700 o C was 4.9 and 5.1, respectively, while the pH PZC of α-Fe 2 O 3 fiber was found to be 5.0. Small differences in pH PZC values indicate low influence of production technology/parameters on the surface properties of obtained materials. The pH of the point of zero charges, pH PZC , is the pH value at which the surface concentrations of positive and negative charges/groups are equivalent. In aqueous systems, the surface of synthesized samples is covered with functional groups that can be protonated at pH values below pH PZC or deprotonated at higher pH. Electrostatic and hydrogen bonding attractive/repulsive forces between BPA and surface charges are responsible for adsorption and photocatalytic activity. Such results indicate that irrespectively to used photocatalysts similar pH-dependent adsorption behavior could be expected, considering electrostatic interactions. Effective ionization of BPA could be expected at pH>8. In general, adsorption efficiency at operational pH significantly depends on hydrogen bonding of BPA hydroxyl groups with particles surface functionalities capable to participate in protondonating/accepting interactions.

X-ray diffraction
The X-ray diffraction is used to identify the crystalline structure of obtained iron oxide structure. By the X-ray powder diffraction (XRPD) technique, the Fe 2 O 3 was identified as a stable α-Fe 2 O 3 (hematite) form (ICSD 161294 card) in all samples. The program PowderCell [23] was used for an approximate phase analysis. The mean crystallite size of α-Fe 2 O 3 phase was estimated from the most intensive diffraction peaks by the program PowderCell. nm. Also, noticed a slight decrease in lattice parameters proved the presumption about the structure densification at higher temperatures.

Fourier-Transfer Infrared Spectroscopy (FTIR)
The hematite composition was observed using Fourier Transform Infra-Red (FTIR) spectroscopy. The spectra of the title compounds were performed at room temperature between 4000 and 450 cm -1 . The FTIR spectral bands are shown in Fig. 4.   Fig. 4. FTIR spectra of raw fiber α-Fe 2 O 3 before calcination, and α-Fe 2 O 3 particles calcined at 400 and 700 o C.
A spectrum of raw Fe 2 O 3 fibers shows broad characteristic peaks at 3162 to 3333 cm −1 and peak at 1632 cm -1 attributed to the -OH stretching and bending vibration of PVA, respectively [8,24]. In the same wavelength region, Fe 2 O 3 400 o C particles show the absorption of at 3420 cm −1 that originates from Fe 2 O 3 surface OH groups. The peak at 2925 and 2956 cm −1 corresponds to the C-H stretching of -CH 2 groups [25]. There is a peak at 1456 cm -1 assigned to the deformation of CH 3 group [2]. For the calcined hematite synthesized by the precipitation method, the strong absorption peaks at 480, 530 and 565 cm -1 can be attributed to the Fe-O band vibrations [26,27].

BPA removal
An index of the propensity of a metal oxide surface to ionize as a function of pH is the value of required pH to give zero net surface charge, denoted as pH PZC . A variation of pH PZC was observed in relation to applied calcination temperature (Table I). It was found that the nature and the surface area of metal oxides determine the pH PZC value [26]. Low difference between the Fe 2 O 3 400 o C and Fe 2 O 3 700 o C may be a consequence of similar crystal structure and a difference in surface area, i.e. 13.050 and 2.788 m 2 /g, respectively. Similar stands for Fe 2 O 3 fibers. Applied thermal treatment for α-Fe 2 O 3 700 o C cause contraction in particle diameter and a decrease in pore diameter, which in turn cause a slight increase in pH PZC . Change in pH PZC reflects a change in a ration of ionizable surface groups but not their quantitative value. This aspect of photocatalyst surface properties significantly influences BPA absorptivity as one of a prerequisite for the achievement of effective photocatalytic reaction with generated active oxygen species at particles surface.
On the other hand, photocatalytic efficiency is also determined by an effective hole/electron separation induced by photon absorption. Their subsequent transport to photocatalyst surface and participation in the reaction of holes and surface-bound water and/or hydroxyl groups, i.e. time-dependent production of the most active oxidative species (the hydroxyl radical -OH • ) determine photocatalytic efficiency. If photo-excited electrons and holes are trapped at crystal facets, and subsequent surface transfer of electrons/holes (initiation of surface reaction) occurs efficiently, such processes would minimize possible recombination, which causes a decrease of photocatalytic efficiency. Fig. 5 represents the efficiency of α-Fe 2 O 3 particles and fibers for BPA removal used as catalysts under UV light. Presented results suggest that the dense surface morphology of α-Fe 2 O 3 700 o C particles contribute to the decrease of catalytic activity. Calcination at lower temperatures produces particles with the looser structure which gives α-Fe 2 O 3 400 o C particles 40 % higher efficiency for BPA removal than α-Fe 2 O 3 700 o C particles. An interesting phenomenon is noticed for α-Fe 2 O 3 fibers. Their higher efficiency was superior for 34.4 % compared to α-Fe 2 O 3 700 o C particles, this may be prescribed to the porous surface morphology (Fig. 1) due to the PVA removal. These results suggest that the simple method at lower temperatures for obtaining α-Fe 2 O 3 particles may be used for BPA removal whose efficiency is almost identical to α-Fe 2 O 3 fibers obtained by more demanding process. BPA degradation has been found to be predominantly achieved via direct oxidation as well due to OH • oxidation which are the major oxidants that degrade BPA [20,1]. The most possible pathways are presented in Fig. 6.  Fig. 6. Proposed mechanism of OH • radical mediated photocatalytic degradation of BPA [20].
Complete mineralization of BPA, as the main goal of the photocatalytic study, could be achieved using Fenton or Photo-Fenton processes, and fragmentation paths lead through mostly acidic species. As an evidence of suggested processes pH decreases from initial pH of 5 to 4.4 which clearly evidenced generation of acidic fragments/structure.

Comparative analysis of the catalytic performance of α-Fe 2 O 3 and other catalysts
The efficacy of studied materials is compared with the efficacy of other catalysts used for BPA removal and it is presented as comparative results in Table II. There is a lack of information in the literature on photocatalytic properties of Fe 2 O 3 particles and fibers. Table II compares the photodegradation efficiency of BPA using the iron oxide prepared in this work with the iron oxide based catalyst and others similar ones. In the literature, the researcher sometimes used longer times for photocatalysis (i.e. 360, 390 min). Since the most efficient time of catalysis in this work was 60 min, Table 2 also provides the extracted values of photocatalytic efficiency at 60 min from literature charts. According to the presented values for 60 min of catalysis, Fe 2 O 3 400 o C particles showed higher efficiency than other iron oxide based catalysts even at much lower dosage. The best photocatalytic performances of TiO 2 property for the removal of BPA are obvious according to Table II.

Conclusion
In this study, iron oxide (α-Fe 2 O 3 ) particles were prepared with precipitation method. The particles were calcined at 400 and 700 o C and average diameter for particles were 296.6 nm and 253.1 nm, respectively. Iron(III) chloride/PVA fibers were obtained using the electrospinning method. The calcination was done at 700 o C and the fibers had 878 nm average diameter. XRD, BET, SEM, FT-IR and UV-Vis techniques were used to characterize the synthesized samples. The synthesized samples are used for the removal of Bisphenol A by photocatalytic decomposition. The removal efficiency of α-Fe 2 O 3 400 o C particles is significantly higher (for 40 %) than the efficiency of α-Fe 2 O 3 700 o C particles due to more loosely surface morphology obtained at lower calcination temperatures. In addition, a high catalytic activity may be obtained at higher temperatures (700 o C) using the electrospinning process whereby α-Fe 2 O 3 fibers showed 34.4 % higher catalytic activity then α-Fe 2 O 3 700 o C.