The penetration probability of different chemical pollutants to the water resources and finally environmental cycle always has been the main concern of environmental experts. Some of these pollutants because of their consistent molecular structure may remain constant many years in the environment and left their destructive effects on various creatures including human being . Elimination of these chemical pollutants is not an easy work and needs using different new and effective methods . The organic dyes are one of the most important categories of chemical pollutants with almost large molecular structures that easily penetrate environments, especially water resources and their elimination needs using advanced methods. The MR considered as the most widely used organic dyes in different industries especially textile dying and paper printing industries . This red dye is harmful and causing irritation in skin, eye and digestive tract if inhaled or swallowed. MR is a mitotic poison, mutagen and a suspected carcinogen [4-5].
Many different chemical and physical methods like chemical degradation (oxidation and molecular fractures) and or adsorption have been used for the elimination of MR up to this time [6-7]. Among chemical methods, complex of advanced oxidation processes (AOPs) is one of the most important, effective and high applied methods for the elimination of chemical pollutants [8-9]. Photocatalytic methods are part of AOPs in which one or some semiconductor catalysts activated or its activation increased through absorbing the energy of light. Through impacting the light to the catalyst, the huge amount of free radicals like hydroxyl radicals (OH•) was produced. The hydroxyl radicals can attack the pollutant molecules and convert them to mineral species such as H2O and CO2 . ZnO, SiO2, Fe2+, and Ag+ doped TiO2 are examples of the photocatalysts that used for degradation of pollutants [11-13]. In this regard, Iron oxide can also be used.
Hematite (α-Fe2O3) with rhombohedral structure is the most common system of iron oxide that because of its various applications in several fields like magnetic materials, gas sensors, data storages, pigments and catalyst, has drawn attention of researchers and industries in recent years . One of the most important applications of hematite is its application as a functional and effective support for environmental catalyst . Studies show that hematite particles display more activity in nano scales, thus regarding their weight fewer amount would be needed to reach the desired goal.
So far, various techniques have been used for the synthesis of α-Fe2O3 nanoparticles such as co-precipitation, sol-gel, sonochemical synthesis, Micelle synthesis, hydrothermal synthesis, thermal decomposition and FHRC methods [16-22].
Undoubtedly, the most important application of photocatalytic processes is water treatment; this claim is evident from the different published scientific reports in this field. The α-Fe2O3 can be used as a fine powder or crystals dispersed in water, but after catalytic reaction, the filtering step is very difficult. In order to solve this problem, researchers have examined methods for supporting α-Fe2O3on the surface of organic, inorganic or organic-inorganic materials .
In the pollution removal process using the photocatalytic method, some materials with photocatalytic properties can be used as a catalyst support . In such situations, catalyst support in addition to holding the catalyst helps to the degradation process. Different methods can be used in order to put the catalyst on the surface of the support. One of the easiest, high profits and the most cost-efficient methods is SSD in which catalyst precursor and catalyst support are mixed together with solid state. Then, the catalyst is formed during calcination and thermally positioned on the surface of catalyst support .
In this article, iron oxide nanoparticles are synthesized through FHRC method and then thermally stabilized on the surface of silica sand by SSD technique. The prepared composite (α-Fe2O3/SiO2) used as a nanophotocatalyst for degradation and removal of MR in aqueous solution. For achieving the highest percentage of degradation, all the factors affecting the oxidation and degradation process were optimized through single variable method.
All the chemicals used in this study were purchased from Merck Company and used without further purification. These materials included Iron (III) chloride hexahydrate, urea, MR, sulfuric acid 96%, sodium hydroxide and ethanol. Also, silica sand (particle size < 0.04 μm) was purchased from an Iranian company of Kimia Sadr (Yazd, Iran). Deionized water used in order for the solution preparation and washing the laboratory tools. The Molecular structure of MR is depicted in Fig. 1.
The FTIR spectra, Surface morphology, BET surface area and X-ray diffraction patterns of silica and α-Fe2O3/SiO2 were performed using a Perkin-Elmer spectrophotometer (Spectrum Two, model), a Philips XL-30 SEM, a BELSORP-mini II BET instrument and a DX27-mini diffractometer, respectively. Also, all UV/Vis absorption spectra were obtained using an Agilent 8453 spectrophotometer.
Synthesis of α-Fe2O3
According to Bharathi et al. , about 100 ml of urea 1 M was added to 100 ml iron (III) chloride 0.25 M slowly and drop by drop while stirring with magnetic stirrer. Then the resulting solution was placed under the reflux condition for 12 h at 90-95°C. The reflux sediment washed with deionized water to remove all unreactive materials. The washed sediment (iron hydroxide) was dried at 70°C and placed at 300°C for 1 h. The resulted solid with dark brown color was iron oxide.
Preparation of α-Fe2O3/SiO2 catalyst (SSD method)
According to Nikazar et al. , the produced iron hydroxide was mixed with SiO2 in the proportion of 1:3 for preparation of α-Fe2O3/SiO2 catalyst using an agate pestle and mortar for 1 h. For easier mixing, a little quantity of ethanol was added to the mixture to obtain clay like state. After the full mixing process, the resulting mixture was dried and then it was positioned in the furnace for 1 h at 300 oC for calcination and conversion of supported iron hydroxide to iron oxide.
The schematic of photocatalytic reactor used in this study showed in Fig. 2. The two mercury lamps, UV-C, 15 W from Philips Company of Holland were used as UV light sources and positioned on the upper section of the box. The radiation is generated almost exclusively at 254 nm. In order to perform any test, at first 1000 ml of MR contaminated water (with a definite concentration was poured inside the reactor. The pH was adjusted by using NaOH and H2SO4. A was used. A specific quantity of catalyst was added to the reactor center while the lamps are turned on. The reaction time was calculated from the moment that catalyst was added and the lamps were illuminated. Sampling was done by a 5 ml syringe, every 20 min. In order to fully separate the catalyst from solution, the samples were centrifuged for 4 min with 3500 rpm speed. The concentration of MR solutions was determined at λmax=520 nm. The photo degradation efficiency (x %) as a function of time is given as the following (Eq. 1) :
Where C0 and C are the concentration of MR (mg/l) at t=0 and t, respectively. In the experiments, the initial concentration of MR varied from 10 to 50 mg/l (10, 20, 30, 40 and 50), pH varied from 3 to 11 (3, 5, 7, 9 and 11) and catalyst mass varied from 0.5 to 2.0 grams (0, 0.5, 1.0, 1.5 and 2.0 g) at five levels.
RESULT AND DISCUSSION
The three different SEM images of catalyst support (silica) with various magnitudes (×150, ×850 and ×46000) were depicted in Fig. 3. Also, three SEM images of α-Fe2O3/SiO2 catalyst with 850, 14000 and 46000 times magnification were presented in Fig. 4. At depicted images in Fig. 4, especially the middle one seems that particles of iron oxide spherically and uniformly dispersed on the surface of SiO2.
In Figs. 5a and 5b, the FTIR spectra of silica and α-Fe2O3/SiO2 have been shown, respectively. In spectrum 5a specific peak related to Fe-O vibration was labeled that because of the iron oxide existence in silica structure. The reason for the appearance of this peak is a natural existence of iron oxide in the mineral silica structure. This characteristic peak in 5b spectrum has more intensity that shows the prosperous supporting of iron oxide on the surface of silica. Characteristic peaks of α-Fe2O3 have well appeared and are in agreement with Bharati et al., . These characteristic peaks which are related to stretching and bending modes of OH and Fe–O binding in FeOOH, in some cases overlapped with absorption peaks of SiO2.
In Fig. 6, XRD patterns related to the SiO2 (lower pattern) and α-Fe2O3/SiO2 (upper pattern) were depicted. As can be seen, characteristic peaks related to the SiO2 in the lower pattern, without any changing in their position appear in the upper pattern. It shows that the structure of catalyst support (SiO2) during the catalyst stabilization process was stable and did not change. In the upper pattern, α-Fe2O3-specified peaks (which also marked by vertical lines) appeared and it is in agreement with results of Bharati et al. . Taking account of device errors, the crystallite size of spherical α-Fe2O3 supported on the surface of silica was calculated by using XRD. The average crystallite sizes of the α-Fe2O3/SiO2 were calculated using the Debye–Scherer Eq. (2) from the major diffraction peaks .
Where K is equal to 0.9 and β is the Perfect Width at Half- Maximum (FWHM) of the peak, λ is the wavelength of the used X-ray and θ is the Bragg angle. The average size of the crystalline was 27.5 nm for the α-Fe2O3 based on SiO2. The nanoparticle size was obtained from the interpretation of XRD analysis but not from SEM images.
The surface area of materials was measured using BET device and N2 adsorption-desorption method at 77 K. before the BET measurement, the materials were degassed under vacuum at 473 K for 12 h. The BET surface area of silica and α-Fe2O3/SiO2 were determined 18.33 and 61.45 (m2/g), respectively. The BET results show that by supporting the iron oxide particles on the surface of silica, the catalyst surface area increased up to three times. It improves the catalyst performance because of increasing the surface area, the number of available active sites for chemical reactions increased.
UV/Vis test of MR
The absorbance of MR solutions during photo degradation process at initial and after 20, 40, 60, 80, 100, 120, 140, 160 and 180 min of irradiation versus wavelength were shown in Fig. 7. Maximum wavelength (λmax) in the absorption spectrum of MR was equal to 520 nm; Hence absorption of all samples after sampling read at λmax =520 nm.
When semiconductor catalyst absorbing energy, its electrons are transmitted from valence band(VB) to conduction band(CB). Indeed the initiation of photocatalytic process is through the giving energy to catalyst. When the UV light emitted on the supported α-Fe2O3, the light’s energy caused many electron-hole pairs in the catalyst. The VB potential (hVB) is positive enough to generate hydroxyl radicals at the surface, and the CB potential (eCB) is negative enough to reduce molecular oxygen. By generating the hydroxyl radicals, which are the strong oxidizing agent, the degradation of pollutants happens. The hydroxyl radicals attack to the molecular structure of pollutant on the surface or near the surface of catalysts and through its degradation gradually the contaminant eliminates .
According to this description, the photocatalytic degradation procedure of MR by UV light and α-Fe2O3/SiO2 catalyst can be written as the following (Eqs.3-12):
α-Fe2O3 + hν → α-Fe2O3 (e–CB + h+VB) (3)
h+VB + H2O (ads) → H+ + •OH (ads) (4)
h+VB + OH‾(ads) → •OH (ads) (5)
e–CB + O2(ads) → •O‾2(ads) (6)
H2O ↔ H+ + OH– (7)
•O‾2(ads) + H+ → •HO2 (8)
2 •HO2 → H2O2 + O2 (9)
H2O2 + α-Fe2O3 (e–CB) → •OH + OH‾ + α-Fe2O3 (10)
•OH (ads) + MR → Degradation of MR (11)
h+VB + MR → Oxidation of MR (12)
Fig. 8 shows a schematic diagram of the MR degradation process which represents the above mentioned reactions. A process continuously initiates by absorbing energy of light and terminate by degrading the molecular structure of MR using hydroxyl radicals and it converted to CO2 and H2O.
Effect of medium pH
The effect of pH on the removal percent of MR(X%) is presented in Fig. 9. As can be seen,the highest degradation percentage is obtained at pH=5. Regarding this diagram, generally,can be said that between pH=5-7 the highest degradation percentage is attained. The Point of zero charge (PZC) of the catalyst α-Fe2O3/SiO2 was tested and it was 5. At this pH, the surface charge of the catalyst was zero, above this pH the surface charge of the catalyst was negative and below this pH, the surface charge of the catalyst was positive.
Also, the results showed that at alkali mediums (pH>7) degradation amount sharply decreased. Because in alkali mediums the surface area of Fe2O3 particles negatively charged, attracts more positively charged specimens. On the other hand, MR is an anionic dye and attracted by Fe2O3 particles less in alkali mediums so its degradation was low. This is the reason that why degradation percentage decreased as pH increased. At pH values below than 5.3, MR dye has the cationic form ; on the other hand, in acidic medium surface of Fe2O3 particles is positively charged, for these reasons at pH of 5, the molecules of MR dye attracted by particles of catalyst and the x% partly increased. Decreasing of x% by reducing pH from 5 to 3 has happened because of leaching of α-Fe2O3 from the support of catalyst. Therefore, at pH=5 the highest degradation amount (75.40 %) was obtained.
Effect of the initial Concentration of MR
The effect of initial concentration of MR on x% in optimum pH (pH=5) is shown in Fig. 10. As can be seen, the amount of degradation was decreased with increasing in the concentration of MR. By increasing the number of MR molecules in the reactor, catalyst and light need more time to degrade the MR fully, therefore by increasing the concentration of MR degradation amount was decreased. Also, the UV light can be made to go more easily through the solution to irradiate the catalyst when the initial concentration of MR is lower, hence, photocatalytic efficiency increased by decreasing the initial concentration of MR.
Effect of catalyst mass
Fig. 11 showed a diagram related to the effect of catalyst mass on x% under optimum conditions: pH=5 and initial concentration of MB=10 mg/l. Results showed that as the catalyst mass increased degradation of MR increased up to the point that by using 2 g of catalyst after 120 min of reaction, 98.46 % of MR eliminated. By devoting more attention to the diagram represented in Fig. 11, it could be seen that by using 0 g of catalyst, 7.86% of MR was removed. This amount of degradation occurred because of the exposure of UV light to the solution and partial degradation of pollutant. By increasing the catalyst mass, the number of free electrons in CB was increased and these free electrons produce more free radicals. Therefore, increasing the mass of catalyst increases the x% .
In this study iron hydroxide particles as catalyst precursor by FHRC method were prepared and used. The iron hydroxide was prepared by conventional and low-cost method of SSD and then supported on the surface of mineral silica at 300 oC. The average crystallite sizes of the α-Fe2O3/SiO2 were obtained at 27.5 nm through the XRD analysis and the Debye–Scherer equation. The properties of catalyst and its support was explored by FT-IR spectroscopy, SEM images, XRD patterns and BET surface area. The FT-IR technique shows that the structure of SiO2 during inserting of α-Fe2O3 on it was stable and did not change. The α-Fe2O3/SiO2 was employed as a nanophotocatalyst for degradation of MR dye. The efficient parameters on degradation such as pH, the initial concentration of MR and catalyst mass were optimized by single-variable method. Results showed that under the optimum conditions, including pH at 5, initial concentration of MR at 10 mg/l, and 2 g of catalyst, about 98.46% of MR were degraded after 120 min of reaction.
The authors of this paper thanks and appreciate the Maghsoud porcelain group (Mashhad, IRAN) because of their material and spiritual supports.
CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.