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Photocatalytic degradation of tetracycline aqueous solutions by nanospherical α-Fe2O3 supported on 12-tungstosilicic acid as catalyst: using full factorial experimental design

Photocatalytic degradation of tetracycline aqueous solutions by nanospherical α-Fe2O3 supported... Int J Ind Chem (2017) 8:297–313 DOI 10.1007/s40090-016-0108-6 RESEARCH Photocatalytic degradation of tetracycline aqueous solutions by nanospherical a-Fe O supported on 12-tungstosilicic acid 2 3 as catalyst: using full factorial experimental design 1 1 Majid Saghi Kazem Mahanpoor Received: 21 June 2016 / Accepted: 7 November 2016 / Published online: 16 November 2016 The Author(s) 2016. This article is published with open access at Springerlink.com Abstract In this paper, spherical a-Fe O nanoparticles and catalyst concentration variables are at maximum levels 2 3 (NPs) were supported on the surface of 12-tungstosilicic and the initial concentration of TC and H O concentration 2 2 acid (12-TSA7H O) using two different solid-state dis- variables are at minimum levels (pH 8, catalyst concen- persion (SSD) and forced hydrolysis and reflux condensa- tration = 150 ppm, initial concentration of TC = 30 ppm, tion (FHRC) methods. Photocatalytic activity of supported H O concentration = 0.1 ppm). A first order reaction with 2 2 -1 a-Fe O NPs (a-Fe O /12-TSA7H O) for tetracycline k = 0.0098 min was observed for the photocatalytic 2 3 2 3 2 (TC) degradation in aqueous solution was investigated degradation reaction. using UV/H O process and the results were compared 2 2 with that of pure a-Fe O NPs. a-Fe O and 12-TSA7H O Keywords Photocatalytic degradation  Tetracycline 2 3 2 3 2 were synthesized according to previous reports and all a-Fe O  12-Tungstosilicic acid  a-Keggin 2 3 products were characterized by using FTIR, SEM, EDX and XRD. Design of experiments (DoEs) was utilized and photocatalytic degradation process was optimized using Introduction full factorial design. The experiments were designed con- sidering four variables including pH, the initial concen- From the perspective of green chemistry, degradation of tration of TC, catalyst concentration and H O chemical pollutants in wastewater has attracted a lot of 2 2 concentration at three levels. TC concentration reduction in attention. Antibiotics are one of the larger groups of these the medium was measured using UV/Vis spectroscopy at pollutants in wastewater released from pharmaceutical k = 357 nm. The results of experiments indicated that industries [1]. Besides, TC is one broad spectrum of max supporting a-Fe O NPs on the surface of 12-TSA7H O antibiotics repeatedly detected in urban and industrial 2 3 2 through SSD and FHRC methods caused to improve the wastewaters, drinking water, surface water and ground- filtration, recovery and photocatalytic activity of NPs. water [2–6]. The molecular structure of TC is shown in Also, it was indicated that those NPs supported through Fig. 1. Various techniques are used to degrade TC; one of SSD method, have better photocatalytic performance than these techniques is photocatalytic degradation [7]. NPs those supported through FHRC method. The statistical play an important role in heterogeneous photocatalysis. analyses revealed that the maximum TC degradation Metal oxide NPs, i.e., iron oxides, have a special position (97.39%) is obtained under those conditions in which pH in the science and technologies because of having wide applications and unique properties [8]. a-Fe O (hematite) 2 3 which is the most common form of iron oxides, has the & Kazem Mahanpoor rhombohedral structure and it is an attractive compound k-mahanpoor@iau-arak.ac.ir because of its applications in data storage, gas sensor, Majid Saghi magnets materials, pigment, catalysis and photocatalysis m-saghi@iau-arak.ac.ir [9–14]. Various techniques including co-precipitation, sol– gel, thermal decomposition, Micelle synthesis, sonochem- Department of Chemistry, Islamic Azad University, ical synthesis, hydrothermal synthesis and FHRC have Arak Branch, Ara¯k, Iran 123 298 Int J Ind Chem (2017) 8:297–313 OH O OH O O d- and e-Keggin, Wells–Dawson, Preysler, Stromberg and Anderson–Evans are served as critical types. OH 12-tungstosilicic acid (hereafter, 12-TSA) is a HPA with formula H SiW O and a-Keggin crystal structure (see NH 2 4 12 40 Fig. 2). The central Si heteroatom is surrounded by a tetrahedron whose oxygen vertices are each linked to one OH of the four W O sets. Each W O set consists of three 3 13 3 13 H H W O octahedrals linked in a triangular arrangement by 3 6 H C OH N(CH ) sharing edges and the four W O are linked together by 3 13 3 2 sharing corners [34]. So far, numerous experimental studies Fig. 1 Molecular structure of TC have been done about supporting HPAs on the surface of various organic and inorganic catalyst supports, but HPAs been utilized to synthesize monodisperse a-Fe O NPs. 2 3 have rarely been used as catalyst support [35–38]. [15–21]. Among various photocatalytic processes, water 12-TSA has suitable physical and chemical properties to and wastewater treatments are of the most important a- be used as a catalyst support. The pores existed on the Fe O NPs applications. In these processes, a-Fe O NPs 2 3 2 3 crystalline surface of 12-TSA provide a suitable condition could be used in the form of a fine powder or crystals to support NPs [39]. To optimize a process like the pho- dispersed in water, but it is vital to know that filtering these tocatalytic degradation process, it is essential to study all NPs following reaction is difficult and costly. To solve this factors influencing the process. But studying the effects of problem, researchers have examined methods for support- individual factors on the process is difficult and time- ing a-Fe O NPs on the surface of organic, inorganic or 2 3 consuming, especially if these factors are not independent organic/inorganic catalyst supports [22, 23]. Various and they affect each other. Employing experimental design methods have been applied for supporting a-Fe O NPs on 2 3 could eliminate these problems because the interaction the surface of catalyst support. Utilizing any of these effects of different factors could be attained using DoEs methods depends on the chemical and physical properties only. Full factorial is an appropriate method for DoEs of catalyst and catalyst support as well as the purpose of the because it could reduce the total number of experiments as process. One of these methods is SSD method in which well as optimize the process by optimizing all the affecting catalyst precursor and catalyst support are separately syn- factors collectively, at a time [40]. The design could thesized and then are mixed with specific weight ratio determine the effect of each factor on the response as well using an appropriate solvent [24]. Then, during calcination, as how this effect varies with the change in level of other the catalyst is both formed and thermally supported on the factors. surface of catalyst support. In another technique such as Various crystal structures of a-Fe O NPs including rod- 2 3 FHRC, the catalyst support is added to the precursor shape [21], spherical and elliptical forms [41] have been solution(s) during catalyst preparation (if it was stable in synthesized and identified until now. In this work, spherical reaction medium) and the catalyst is supported on the a-Fe O NPs are supported through two different SSD and 2 3 surface of catalyst support while it is simultaneously FHRC methods on the surface of 12-TSA7H O(a-Fe O / 2 2 3 formed. In FHRC method, all steps related to the synthesis 12-TSA7H O). Then, the performance of pure and sup- of NPs were done on the surface of catalyst support and ported a-Fe O NPs on the TC photocatalytic degradation 2 3 ‘‘NP/catalyst support’’ was obtained after nucleation and was investigated using full factorial experimental design. growth of NPs. Polyoxometalates (POMs) are a great class of inorganic compounds as multi-core metal–oxygen clusters [25]. If an atom named heteroatom (such as Si, P, Experimental As, B, etc.) enters the molecular structure of POM in addition to metal and oxygen, then heteropoly acids Material and apparatuses (HPAs) will be obtained [26]. Thermodynamically, HPAs have stable arrangements and maintain their crystal struc- All chemicals used in this work including sodium tungstate ture in aqueous and non-aqueous solutions. This class of dihydrate, sodium silicate, diethyl ether, iron (III) chloride materials has various applications in catalysis [27], ana- hexahydrate, urea, hydrogen peroxide (30% pure), lytical chemistry [28], medicinal chemistry (anti-tumor, hydrochloric acid (37% pure), sulfuric acid (96% pure), anti-cancer, anti-bacteria, anti-microbial and anti-clotting) sodium hydroxide and ethanol were purchased from Merck [29–31], radioactive materials [32] and gas absorbents [33] and were used without further purification. The required owing to their structural diversity and unique properties. TC was purchased from Razak pharmaceutical laboratory HPAs have different crystal structures of which a-, b-, c-, (Tehran, Iran). Also, deionized water was used throughout 123 Int J Ind Chem (2017) 8:297–313 299 Fig. 2 a-Keggin structure of 4- [SiW O ] 12 40 the experiments. The Fourier transform infra-red (FTIR) transform to iron oxide. Consequently, a dark brown solid spectra of products were recorded on a Perkin-Elmer of a-Fe O was obtained. 2 3 spectrophotometer (Spectrum Two, model) in the range of -1 450–4000 cm . The shape, size and surface morphology Synthesis of 12-TSA7H O of the synthesized 12-TSA7H O and a-Fe O /12- 2 2 3 TSA7H O were examined using the obtained images of a 12-TSA7H O was synthesized according to literature 2 2 Philips XL-30 scanning electron microscope (SEM). The procedure [42]. Firstly, 15 g sodium tungstate dihydrate X-ray diffraction (XRD) analysis of the samples was done was dissolved in 30 ml deionized water and then 1.16 g using a DX27-mini diffractometer. BET surface area of sodium silicate solution (density 1.375 g/ml) was added to materials was determined by N adsorption–desorption it. The resulted mixture was heated up to about boiling method at 77 K, measured using a BELSORP-mini II point, and while it was stirred, 10 ml concentrated HCl was instrument. The samples were degassed under vacuum at added to it during 30 min, smoothly. Then, the solution 473 K for 12 h before the BET measurement. All ultravi- was naturally cooled down to RT and slight precipitate olet/visible (UV/Vis) absorption spectra were obtained formed (silicic acid) in it was filtered. Again, 5 ml con- using an Agilent 8453 spectrophotometer and the pH val- centrated HCl was added to the solution and was trans- ues were determined by a Metrohm pH meter model 827. ferred to separatory funnel after cooling it again down to Likewise, to separate the catalyst from samples, an ALC RT. Then, 12 ml diethyl ether was added to it and well 4232 centrifuge was employed. shaken. Therefore, three layers were formed inside sepa- ratory funnel, middle layer of which was yellow-colored. Synthesis of a-Fe O NPs Bottom layer which was oily ether was separated and 2 3 transferred into a beaker. To further extract, separatory The synthesis of a-Fe O NPs was carried out according to funnel was further shaken again and the bottom layer was 2 3 Bharathi et al. [21]. Firstly, 100 ml iron (III) chloride once more separated and transferred into the beaker. This hexahydrate 0.25 M which was considered as a source of extraction process was done so much that the yellow color 3? Fe , was poured into a flat-bottom flask. When Iron of middle layer was fully faded. The extracted ether solution was agitated by stirrer, it was added drop by drop complex which was inside the beaker was transferred to to it 100 ml urea 1 M (as a supplying agent of hydroxyl another separatory funnel and then 16 ml HCl 25% (v/v) ions). The more gentle and regular adding urea, the smaller was added to it. Next, 4 ml diethyl ether was added to it, and more uniform-sized formed a-Fe O particles will be. subsequently. The contents inside separatory funnel were 2 3 The obtained mixture was stirred for 30 min and then shaken and bottom layer (ether) was transferred to the placed under the reflex at 90–95 C for 12 h. Then, the evaporating dish after separating. Evaporating dish was precipitate after separation was washed with 100 ml exposed to air and remained motionless to evaporate the deionized water because unreacted ions will be completely solvent and form the 12-TSA7H O crystals. Finally, 12- removed. The washed precipitate was dried at 70 C for TSA7H O formed crystals were placed at 70 C for 2 h 2 h. Having fully dried, one light brown solid (iron until it was completely dried. The chemical reaction hydroxide) was yielded. Finally, this solid remained at occurred in the process of 12-TSA7H O synthesis has 300 C for 1 h; hence the iron hydroxide particles will been shown in (1)[42]. 123 300 Int J Ind Chem (2017) 8:297–313 Table 1 Experimental range and levels of the variables 12 Na WO þ Na SiO þ 26 HCl 2 4 2 3 ð1Þ H SiW O  xH O þ 26 NaCl þ 11 H O Variables Range and levels 4 12 40 2 2 –1 0 ?1 Preparation of a-Fe O /12-TSA7H O 2 3 2 pH 4 6 8 Initial con. of TC (ppm) 30 50 70 SSD method Catalyst con. (ppm) 50 100 150 H O con. (ppm) 0.1 0.3 0.5 2 2 Firstly, the synthesized iron hydroxide (light brown solid) and 12-TSA7H O catalyst support were mixed with three levels (50, 100 and 150 ppm) and H O concentration weight ratio of 1:3 iron hydroxide/12-TSA7H O (weight 2 2 from 0.1 to 0.5 ppm at three levels (0.1, 0.3 and 0.5 ppm). In of catalyst support is three times of catalyst weight) using Table 2, 19 experiments related to this factorial design and an agate pestle and mortar for 1 h. To have better mixture, their experimental conditions have been listed. The removal ethanol was sprayed on the mixture until it becomes dough- efficiency of TC was a dependent response. In order to do form. During mixing, in the vaporization phase, ethanol is DoEs, Minitab 16 version 16.2.0 statistical software was again added in order to keep the dough-form of the mix- utilized. Also, analysis of variance (ANOVA) was run to ture. The resulted mixture was dried under air for 1 h and analyze the results. then was kept at 80 C for 2 h. To do calcination and transform iron hydroxide particles fixed on the surface of General procedure for photocatalytic degradation 12-TSA7H O into iron oxide (a-Fe O ), the obtained solid 2 2 3 of TC was kept at 300 C for 1 h. Figure 3 shows one schematic diagram of photocatalytic FHRC method reactor used in the work. An MDF box was designed inside which a circular Pyrex reactor with 300 ml capacity was Firstly, 50 ml iron (III) chloride hexahydrate 0.25 M was placed. On the upper section of the box, three mercury poured into a beaker. While it was agitated by stirrer, 3.5 g lamps (Philips 15 W) were built-in as UV light sources. 12-TSA7H O was gently added to it. The obtained mix- The radiation is generated almost exclusively at 254 nm. ture was stirred for 4–5 h. Then, stirring was stopped for These lamps were set up with the same intervals, so light 2 h until the solid within mixture was deposited. The solid was evenly radiated on the whole liquid surface inside the accumulated at bottom of beaker was separated and reactor. The liquid inside the reactor was agitated by transferred into one flat-bottom flask and the same 10 ml magnetic stirrer and the air inside the box was conditioned solution inside beaker was added to it. When mixture by a fan (built-in at back of box). In order to carry out each inside flat-bottom flask was being stirred, 50 ml urea 1 M experiment (according to Table 2), firstly 250 ml TC was gradually added to it. The mixture was placed under solution was made as specified concentration and poured reflux at 90–95 C for 12 h. Then, the precipitate resulted inside the reactor. Then, at related pH, the specified amount after separation was washed with 100 ml ethanol/deionized of photocatalyst and H O were added to the solution water 1:1 solution because unreacted ions were completely 2 2 inside the reactor. In all experiments, pH adjustment was removed. The washed precipitate was dried in the air for done via minimum use of H SO and NaOH. Then, stirrer 2 h and then was kept at 80 C for 2 h. In order to calci- 2 4 and UV lamps were immediately turned on to initiate the nation, the obtained solid was kept at 300 C for 1 h. process. Sampling was done by a 5 ml syringe, every 10 min. To fully separate the catalyst from solution, the Full factorial experimental design samples were centrifuged for 3 min with 3500 rpm speed. The TC concentration of the samples was determined using The photocatalytic efficiency of pure a-Fe O NPs and a- 2 3 a UV/Vis spectrophotometer at k = 357 nm. The per- Fe O /12-TSA7H O prepared by SSD and FHRC methods max 2 3 2 centage of initial concentration of pollutant decomposed by on the TC degradation were investigated using DoE. The the photocatalytic process or the percent of photodegra- experiments were designed considering four variables dation efficiency (x%) as a function of time is given by including pH, the initial concentration of TC, catalyst con- centration and H O concentration at three levels. Experi- 2 2 C  C x% ¼  100 ð2Þ mental range and levels of variables are shown in Table 1. pH varied from 4 to 8 at three levels (4, 6 and 8), the initial where C and C are the concentration of TC (ppm) at t = 0 concentration of TC from 30 to 70 ppm at three levels (30, 50 and t, respectively. and 70 ppm), catalyst concentration from 50 to 150 ppm at 123 Int J Ind Chem (2017) 8:297–313 301 Table 2 Experimental Exp. no. Variables conditions for photocatalytic process pH Initial con. Catalyst H O con. 2 2 of TC (ppm) con. (ppm) (ppm) 1–1 –1 –1 –1 2 ?1–1 ?1 ?1 3–1 –1 ?1–1 4–1 ?1 ?1–1 5 ?1 ?1 ?1–1 60 0 0 0 7 ?1–1 ?1–1 8–1 ?1–1 ?1 9 ?1–1 –1 –1 10 ?1 ?1 ?1 ?1 11 –1 ?1–1 –1 12 ?1–1 –1 ?1 13 ?1 ?1–1 –1 14 –1 ?1 ?1 ?1 15 ?1 ?1–1 ?1 16 0 0 0 0 17 –1 –1 ?1 ?1 18 0 0 0 0 19 –1 –1 –1 ?1 Fig. 3 Schematic diagram of photocatalytic reactor. 1 MDF box, 50 9 50 9 50 cm; 2 Mercury lamps, Philips 15 W; 3 The distance between surface of TC solution and lamps, 5 cm; 4 Reactor, 300 ml capacity; 5 TC solution, 250 ml; 6 Magnet; 7 Magnetic stirrer; 8 Fig. 4 SEM image of the synthesized 12-TSA7H O Sampling port existed on the surface of this catalyst support provide a Results and discussion suitable condition to support a-Fe O NPs. IR is a suit- 2 3 able method for the structural characterization of HPAs Characterization [26]. FTIR spectrum of the synthesized 12-TSA7H O has been shown in Fig. 5a. There are four kinds of oxygen The synthesized 12-TSA7H O atoms in 12-TSA7H O structure, 4 Si–O in which one 2 a oxygen atom connects to Si, 12 W–O –W oxygen bridges SEM image of the synthesized 12-TSA7H O is shown in 2 (corner-sharing oxygen-bridge between different W O 3 13 Fig. 4. Surface morphology of 12-TSA7H O shows that 2 groups), 12 W–O –W oxygen bridges (edge-sharing oxy- this product has suitable structural properties and can be gen-bridge within W O groups) and 12 W=O terminal 3 13 d regarded as a catalyst support. In other words, the pores oxygen atoms. The symmetric and asymmetric stretching 123 302 Int J Ind Chem (2017) 8:297–313 of the different kinds of W–O bonds are observed in the The prepared a-Fe O /12-TSA7H O 2 3 2 -1 following spectral regions: Si–O bonds (1020 cm ), -1 W = O bonds (1000–960 cm ), W–O –W bridges Figures 7 and 8 show SEM/EDX images of a-Fe O /12- d b 2 3 -1 -1 (890–850 cm ), W–O –W bridges (800–760 cm )[43]. TSA7H O prepared by SSD and FHRC methods, respec- c 2 tively. These images indicate that in both methods, a- In Table 3, vibrational frequencies of the synthesized 12-TSA7H O and equivalent values reported in previous Fe O particles were spherically supported on the surface 2 2 3 of 12-TSA7H O. The spheres in SSD method are bigger studies [43, 44] have been listed. Comparing the vibra- tional frequencies reveals that 12-TSA7H O has been well and have covered more area of 12-TSA7H O than that of FHRC method. Possibly in SSD method, spherical a-Fe O synthesized. XRD is one of the most important character- 2 3 ization tools used in solid state chemistry and materials particles are adhered to each other and bigger spheres have science. Figure 6a shows the XRD pattern of the synthe- formed while it did not occur in FHRC method and a- sized 12-TSA7H O. This pattern indicates that the char- Fe O particles were separately supported. It is assumed 2 3 acteristic peaks corresponded to the 12-TSA were well that the causes of this phenomena are as follows: (1) pos- appeared and it means that the synthesized 12-TSA7H O sibly, a-Fe O synthesized particles by SSD method are 2 2 3 crystals were well formed [44]. smaller than that of FHRC method and this contributed to their adherence, (2) Supporting through SSD method is done in solid state and this increases the possibility of particles adhering to each other and forming bigger spheres and (3) supporting through FHRC method is done in liquid phase, so the particles could freely move and be separately fixed on the 12-TSA7H O surface. In Fig. 5b, c, FTIR spectra of a-Fe O /12-TSA7H O prepared by SSD and 2 3 2 FHRC methods have been shown, respectively. It is clear that absorption peaks of 12-TSA7H O have appeared without considerable change in the wavenumbers (only their intensities have been slightly changed). It means that in both methods, 12-TSA7H O was stable and it had not been changed chemically during preparing a-Fe O /12- 2 3 TSA7H O. Also, absorption peaks of a-Fe O have well 2 2 3 appeared and are in agreement with results of Bharati et al. [21]. These absorption 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 12-TSA7H O. Comparing FTIR spectra reveals that absorption peaks of a-Fe O related to SSD method are 2 3 more intense than that of FHRC method. This partly con- firms the results of SEM images. Hence in SSD method, surface of 12-TSA7H O has been covered by more a- Fe O particles. In Fig. 6b, c, XRD patterns of a-Fe O /12- 2 3 2 3 TSA7H O prepared by SSD and FHRC methods have been illustrated, respectively. In both of these patterns, characteristic peaks of 12-TSA7H O have well appeared which indicates that 12-TSA7H O was stable during the supporting process in both SSD and FHRC methods. In these patterns, the characteristic peaks of a-Fe O which 2 3 have also been marked have appeared and it is in agree- ment with results of Bharati et al. [21]. In XRD related to SSD method, intensity of 12-TSA7H O and a-Fe O 2 2 3 characteristic peaks is lower and higher than that of FHRC method, respectively. This issue confirms the results of SEM and FTIR, so during supporting through SSD method, 12-TSA7H O surface has been covered by the greater Fig. 5 FTIR spectra of the synthesized 12-TSA7H O(a) and a- amount of a-Fe O particles. The size of spherical a-Fe O Fe O /12-TSA7H O prepared by SSD (b) and FHRC (c) methods 2 3 2 2 3 2 3 123 Int J Ind Chem (2017) 8:297–313 303 Table 3 Vibrational Number The synthesized 12-TSA7HO[43, 44] frequencies of the synthesized -1 12-TSA7H O and equivalent 2 Wavenumber (cm ) Transmittance % values reported in previous reports 1 1019.04 13.29 1020 (weak) 2 980.68 8.81 981 (sharp) 3 924.31 5.92 928 (very sharp) 4 882.63 11.52 880 (medium) 5 780.28 5.77 785 (very sharp) 6 537.41 13.35 540 (medium) Fig. 6 X-ray diffractogram of the synthesized 12-TSA7H O (a), a-Fe O /12-TSA7H O 2 3 2 prepared by SSD (b) and FHRC (c) methods Fig. 7 SEM image and EDX results of a-Fe O /12-TSA7H O prepared by SSD method 2 3 2 particles supported on the surface of 12-TSA7H O were FHRC methods were determined 57.53 and 39.84 (m /g), calculated using XRD and Warren–Averbach method respectively. It seems that the high amount of iron oxide (taking account of device errors) whose averages for SSD formed on the base has been increase the BET surface area and FHRC methods were 50.5 and 70.82 nm, respectively. of catalyst prepared with SSD method. The BET surface area of catalyst prepared by SSD and 123 304 Int J Ind Chem (2017) 8:297–313 Fig. 8 SEM image and EDX results of a-Fe O /12-TSA7H O prepared by FHRC method 2 3 2 UV/Vis spectra FHRC methods. This means that supporting a-Fe O NPs 2 3 leads to increase their photocatalytic activity. Also, com- The absorbance of TC solutions during photocatalytic paring the results of SSD and FHRC methods indicates that process (using a-Fe O /12-TSA7H O prepared by SSD a-Fe O /12-TSA7H O prepared through SSD method was 2 3 2 2 3 2 method and according to exp. no. 8) at initial and after 10, effective from the aspect of TC photocatalytic degradation 20, 30, 40 and 50 min irradiation time verses wavelength and has yielded more x% values. Comparing x% values in are depicted in Fig. 9. In all experiments, x% was calcu- one series of experiments (1 through 19) shows that the lated at k = 357 nm. The wavelength of maximum highest degradation percentage has been obtained in exp. max absorbance (in 357 nm) did not change with time, then this no. 7. To better compare the results, x% histogram versus wavelength for measuring the concentration of pollutants experiment number for pure a-Fe O and a-Fe O /12- 2 3 2 3 was chosen. Furthermore, absorbance changes in 357 nm TSA7H O prepared through SSD and FHRC methods has were completely regular and measurable. been shown in Fig. 10. The histogram clearly indicates that in all experiments a-Fe O NPs supported on the surface of 2 3 Performance of photocatalysts 12-TSA7H O (particularly through SSD method) had more photocatalytic efficacy and has degraded more TC. Having carried out all experiments based on Table 2, x% values were calculated at k = 357 nm following 50 min Photocatalytic mechanism max after reaction which have been reported in Table 4.In general, comparing x% values reveals that the degree of TC According to exp. no. 7, the effects of UV irradiation, pure a- photocatalytic degradation by pure a-Fe O is lower than Fe O NPs and a-Fe O /12-TSA7H O prepared by two 2 3 2 3 2 3 2 that of a-Fe O /12-TSA7H O prepared through SSD and different SSD and FHRC methods on the photodegradation 2 3 2 Fig. 9 UV/Vis spectral absorption changes of TC solution photodegraded by a- Fe O /12-TSA7H O prepared 2 3 2 through SSD method (pH 4, Initial concentration of TC = 70 ppm, catalyst concentration = 50 ppm, H O 2 2 concentration = 0.5 ppm) 123 Int J Ind Chem (2017) 8:297–313 305 Table 4 x% values after 50 min photodegradation process at of TC are presented in Fig. 11. This Figure designates that in k = 357 nm max the presence of a-Fe O /12-TSA7H O prepared by SSD 2 3 2 method and UV irradiation 97.39% of TC was degraded at Exp. no. x% the reaction time of 50 min while it was 88.44, 82.17 and Pure a-Fe O /12-TSA a-Fe O /12-TSA 2 3 2 3 10.2% for a-Fe O /12-TSA7H O prepared by FHRC 2 3 2 a-Fe O 7H O prepared 7H O prepared 2 3 2 2 method, pure a-Fe O NPs and only UV, respectively. When NPs by SSD method by FHRC method 2 3 a-Fe O is illuminated by the light, electrons are promoted 2 3 1 64.11 78.59 66.32 from the valence band (VB) to the conduction band (CB) of 2 75.39 93.61 85.95 the semi conducting oxide to give electron–hole pairs. The 3 67.56 85.35 73.29 VB potential (h ) is positive enough to generate hydroxyl VB 4 48.83 62.74 53.22 radicals at the surface, and the CB potential (e ) is negative CB 5 37.14 60.51 45.07 enough to reduce molecular oxygen. The hydroxyl radical is 6 36.97 69.61 60.38 a powerful oxidizing agent and attacks TC molecules present 7 82.17 97.39 88.44 at or near the surface of a-Fe O . It causes the photo-oxi- 2 3 8 37.95 52.51 42.66 dation of TC according to the following reactions [45–50]: 9 65.46 84.34 74.61 a-Fe O þ hm ! a-Fe O ðe þ h Þ 2 3 2 3 CB VB 10 44.37 58.94 47.92 þ þ h þ H O ! H þ OH 2 ðadsÞ 11 32.84 47.91 38.91 VB ðÞ ads 12 62.00 91.48 74.28 h þ OH ! OH ðÞ ads VB ðÞ ads 13 29.84 47.06 37.71 e þ O  O 2ðadsÞ CB 2ðÞ ads 14 40.69 62.45 48.72 15 39.31 55.34 45.62 H O  H þ OH 16 37.02 69.75 59.79 O þ H ! HO 2ðÞ ads 17 66.83 82.21 77.13 2 HO ! H O þ O 2 2 2 2 18 36.83 69.44 60.13 H O þ a-Fe O ðe Þ! OH þ OH þ a-Fe O 2 2 2 3 2 3 CB 19 65.83 87.17 81.84 OH þ TC ! degradation of TC a ðÞ ads Maximum value of x% þ þ h þ TC TC ! oxidation of TC: VB Fig. 10 x% values versus experiment number 123 306 Int J Ind Chem (2017) 8:297–313 view of the TC photocatalytic degradation, then in this The mechanism is summarized in Fig. 12. The main role section we carry out the statistical results analysis of the of the foundation is creating the perfect conditions for photocatalytic process in which a-Fe O /12-TSA7H O 2 3 2 putting the TC and hydroxyl radical beside each other. prepared by SSD method has been utilized. Analysis of Photocatalytic activity increased after stabilizing iron oxide variance (ANOVA) is a set consists of a number of sta- on 12-TSA7H O. To comment on this result, we propose tistical methods used to analyze the differences among that the hydroxyl radicals, on the surface of iron oxide, are group means and their associated procedures. ANOVAs are easily transferred onto the surface of 12-TSA7H O. That useful for testing three or more means variables for sta- means the organic pollutants such as TC, which have tistical significance. ANOVA was used for graphical already been adsorbed on the nonphotoactive analyses of the data to obtain the interaction between the 12-TSA7H O, have chances to be degraded due to the process variables and the responses. The quality of the fit appearance of hydroxyl radicals, resulting in the enhance- polynomial model was expressed by the coefficient of ment of the photodegradation performance of a-Fe O /12- 2 3 determination R , and its statistical significance was TSA7H O (as shown in Fig. 12b). checked by the Fisher’s F test in the same program. Model terms were evaluated by the P value. In Table 5, the esti- Kinetics of photocatalytic degradation of TC mated effects and coefficients for x% have been listed. In this table, standard deviation (S), correlation coefficient, Figure 13 displays the plot of ln(C /C) versus reaction time 2 2 pried R and adjusted R values were also reported. The for TC. The linearity of the plot suggests that the pho- square of the correlation coefficient for each response was todegradation reaction approximately follows the pseudo- computed as the coefficient of determination (R ). The first order kinetics with a rate coefficient -1 accuracy and variability of the model can be evaluated by k = 0.0098 min . 2 2 R .The R value is always between 0 and 1. The closer the R value to 1, the stronger the model is and the better the The statistical analysis (optimum conditions) model predicts the response (x%). R value was reported to be 0.9915 in this paper. The ‘‘pried R ’’ of 0.9662 is in Since a-Fe O NPs supported through SSD method have 2 3 reasonable agreement with the ‘‘adj R ’’ of 0.9848, shown more effective than other photocatalysts from the Fig. 11 Effect of UV light, pure a-Fe O NPs and a-Fe O /12-TSA7H O prepared by FHRC and SSD methods on TC degradation (pH 8, 2 3 2 3 2 Initial concentration of TC = 30 ppm, catalyst concentration = 150 ppm, H O concentration = 0.1 ppm) 2 2 123 Int J Ind Chem (2017) 8:297–313 307 Fig. 12 General mechanism of the photocatalysis (a) and photocatalytic activity of a-Fe O /12-TSA7H O(b) 2 3 2 confirming good predictability of the model. Due to both negative and roughly the same effects on the x% value Table 5 and the significant variables effects on the (-4.67 and -4.66, respectively). In Table 5, the coeffi- response, affect magnitudes of the initial concentration of cients of each term have been reported which are the same TC, pH, H O concentration and catalyst concentration term coefficients in response function which they will be 2 2 equal to 31.59, 3.72, 2.48 and 7.35, respectively. Thus, the given in the following. It is vital to note that P values have significant reaction parameters were (the most to the least been assessed considering Alpha (a) = 0.05. Table 6 significant): initial concentration of TC [ catalyst con- depicts the results of ANOVA. The effect on the response centration [ pH [ and H O concentration. Of course, it is was increased by increasing the value of F parameter and 2 2 necessary to note that despite other three variables, the decreasing P parameter. For main effects (with 4 degrees variable of the initial concentration of TC has a negative of freedom) including the initial concentration of TC, pH, effect on the response (-31.59). This means that increasing H O concentration and catalyst concentration, F and 2 2 the initial concentration of TC leads to decrease x% and P values have obtained as 278.34 and \0.0001, respec- conversely. In this way, the effects about the variables tively. Besides, these values were 18.41 and \0.0001 for interaction were reported in Table 5. As can be seen from 2-way interactions (with 3 freedom degree), respectively. these results, it is the only interaction of variables, namely In Table 7, complementary results have been listed which the initial concentration of TC and the catalyst concentra- have been used for drawing residual plots. Residual values tion which have positive effects (3.10). The interaction of were calculated from subtracting experimental x% values the initial concentration of TC with pH and the interaction and fitted values. of H O concentration with catalyst concentration have In order to compare the variables effect (from the 2 2 viewpoint of magnitude) on the response, the Fig. 14a could be investigated which is one Pareto chart of the standardized effects. In this Figure, those variables whose effects on response is negative (-) or positive (?) have been marked. The results revealed that the effect of the initial concentration of TC on the x% is greater than other variables effect (at least three times) but the effect of this variable is negative i.e. increasing or decreasing the initial concentration of TC leads to decrease and increase x%, respectively. In order to better investigate the residual values, residual plot versus exp. no. has been illustrated in Fig. 14b. As it is seen, eight points (residuals) are located under zero line (negative), nine points above zero line Fig. 13 Plot of reciprocal of pseudo-first order rate constant against (positive) and two points roughly on the zero line. Due to initial concentration of TC = 70 ppm, catalyst concentra- this and comparing distance of points from zero line, it tion = 50 ppm, H O concentration = 0.5 ppm and pH 4 2 2 123 308 Int J Ind Chem (2017) 8:297–313 Table 5 Estimated effects and coefficients for x% Term Effect Coef SE Coef T (Coef/SE Coef) P value Result Constant – 71.73 0.4905 146.22 \0.0001 Significant Initial con. of TC –31.59 –15.79 0.4905 –32.19 \0.0001 Significant pH 3.72 1.86 0.4905 3.79 0.004 Significant H O con. 2.48 1.24 0.4905 2.52 0.03 2 2 Catalyst con. 7.35 3.68 0.4905 7.49 \0.0001 Significant Initial con. of TC 9 pH –4.66 –2.33 0.4905 –4.75 0.001 Significant Initial con. of TC 9 catalyst con. 3.10 1.55 0.4905 3.16 0.010 H O con. 9 catalyst con. –4.67 –2.34 0.4905 –4.76 0.001 Significant 2 2 Center point – –2.13 1.2345 –1.72 0.116 2 2 2 S = 1.96219, R = 99.15%, Pred R = 96.62%, Adj R = 98.48% Table 6 ANOVA results Source Degree of Seq SS Adj SS Adj MS F value freedom Initial con. of TC 1 3990.68 3990.68 3990.68 1036.49 pH 1 55.25 55.25 55.25 14.35 H O con. 1 24.54 24.54 24.54 6.37 2 2 Catalyst con. 1 216.12 216.12 216.12 56.13 Initial con. of TC 9 pH 1 86.73 86.73 86.73 22.53 Initial con. of TC 9 catalyst con. 1 38.55 38.55 38.55 10.01 H O con. 9 catalyst con. 1 87.36 87.36 87.36 22.69 2 2 could be said that residual distribution is normal. An Table 7 Residual values extremely useful procedure is to construct a normal prob- Exp. no. x% Fit SE fit Residual (x%–fit) St resid ability plot of the residuals. If the underlying error distri- bution is normal, this plot will resemble a straight line. 1 78.59 77.6338 1.3875 0.9562 0.69 Figure 14c shows normal probability plot. In this plot, it is 2 93.61 92.7296 1.3875 0.8804 0.63 fully clear that residuals distribution is normal because 3 85.35 86.5531 1.3875 –1.1977 –0.86 points (especially central points) are close to straight line. 4 62.74 62.7282 1.3875 0.0118 0.01 If the model is correct and if the assumptions are satisfied, 5 60.51 61.7882 1.3875 –1.2782 –0.92 the residuals should be structureless; in particular, they 6 69.61 69.6000 1.1329 0.0100 0.01 should be unrelated to any other variable including the 7 97.39 94.9260 1.3875 2.4640 1.78 predicted response. A simple check is to plot the residuals 8 52.51 54.7502 1.3875 –2.2402 –1.61 versus the fitted values. Figure 14d displays plot of resid- 9 84.34 86.0067 1.3875 –1.6667 –1.20 uals versus fitted values. Mathematical model representing 10 58.94 59.5918 1.3875 –0.6518 –0.47 TC photocatalytic degradation in the range studied can be 11 47.91 47.5998 1.3875 0.3102 0.22 expressed by the following equation: 12 91.48 93.1572 1.3875 –1.6778 –1.21 13 47.06 46.6598 1.3875 0.4002 0.29 Response ¼ x% ¼ 71:7315:79A þ 1:86B þ 1:24C 14 62.45 60.5318 1.3875 1.9182 1.38 þ 3:68D2:33AB þ 1:55AD2:34CD 15 55.34 53.8102 1.3875 1.5298 1.10 16 69.75 69.6000 1.1329 0.1500 0.09 where A, B, C and D are the initial concentration of TC, 17 82.21 84.3567 1.3875 –2.1467 –1.55 pH, H O concentration and catalyst concentration, 2 2 18 69.44 69.6000 1.1329 –0.1600 –0.10 respectively. 19 87.17 84.7843 1.3875 2.3882 1.72 In Fig. 15, the plots of main effects have been shown. These plots indicate that of four main effects, only the 123 Int J Ind Chem (2017) 8:297–313 309 Fig. 14 a Pareto chart of the standardized effects, b plot of residuals versus exp. no., c Normal probability plot and d plot of residuals versus fitted values variable of the initial concentration of TC has a negative in order to reach maximum degradation (x% = 97.39) the effect on response (x%); effects of other variables on variables of pH, the initial concentration of TC, catalyst response were positive. In effect, increasing the initial concentration and H O concentration should be at levels 2 2 concentration of TC and decreasing pH, H O concen- of ?1(8), -1(30 ppm), ?1(150 ppm) and -1(0.1 ppm), 2 2 tration and catalyst concentration will be caused to respectively. Generally, considering the interaction effects decrease and increase x%, respectively (if the interaction is very important because it may place the unpre- effect of variable is ignored). The slope of line in main dictable effects on the response. For example, based on the effect plots is one indicator of magnitude related to the results of Fig. 15 even though H O concentration had 2 2 variable effect on the response. Therefore, the order of simply a positive effect on x%, the maximum x% was affecting variables from magnitude viewpoint is as achieved in those conditions where H O concentration 2 2 initial concentration of TC [ catalyst concentra- was at its minimum level (see exp. no 7 in Table 4). For the tion [ pH [ H O concentration which confirm the same reason, the interaction effect of variables should not 2 2 results of Fig. 14a. be ignored in studying variables for reaching optimal In Fig. 16, interaction plots for x% have been presented. conditions. Generally, in such plots the more parallel the lines, the Finally, to determine the stability of the catalyst after 5 lower the interaction effect would be and the more inter- steps photocatalytic decomposition process, catalyst sepa- secting the lines, the higher the interaction effect would be. ration and then drying it, the FTIR spectrum of the sample As it is observed, there is a significant interaction effect showed that the catalyst structure have not changed. To among catalyst concentration and H O concentration determine the reusability of catalyst, 5 times was repeated 2 2 variables. This effect is slightly found at interaction among experiment in the optimal conditions. Results, respectively, pH and catalyst concentration variables. Figure 17 shows a are as follows: X1 = 97.39, X2 = 97.32, X3 = 97.24, cube plot for x%. Using this plot, one could easily identify X4 = 97.20, X5 = 97.21. These results show that the conditions for reaching the desirable x%. For example, reusability of catalyst is appropriate. 123 310 Int J Ind Chem (2017) 8:297–313 Fig. 15 Main effects plot for x% Fig. 16 Interaction plot for x% NPs photocatalytic efficiency and chemical change of Conclusions 12-TSA7H O which are indicative of being effective The results revealed that: these supporting methods. 2. While supporting a-Fe O NPs on the surface of 2 3 1. Spherical a-Fe O NPs had been successfully synthe- 2 3 12-TSA7H O help to recover them from the medium sized and supported on the surface of 12-TSA7H O and reusing them, it causes to enhance their photocat- through SSD and FHRC methods with no decrease of alytic activities. 123 Int J Ind Chem (2017) 8:297–313 311 Fig. 17 Cube plot (data means) for x% 3. Nanophotocatalytic effect of a-Fe O /12-TSA7H O concentration = 0.1 ppm so that they cause to reach 2 3 2 prepared through SSD and FHRC methods on the TC maximum degradation (97.39%). degradation is greater than pure a-Fe O NPs. 9. The kinetics of photocatalytic degradation of TC is of 2 3 -1 4. As shown in the analysis of EDX, amount of iron oxide the pseudo-first order with k = 0.0098 min . supported on the 12-TSA7H O using SSD method is greater than the FHRC method. Then photo-activity of Acknowledgements The authors wish to thank the Islamic Azad catalyst that prepared with SSD method is higher than University of Arak, Iran, for financial support. FHRC method. Therefore, SSD method is more Open Access This article is distributed under the terms of the suitable. Creative Commons Attribution 4.0 International License (http://crea 5. The statistical analysis results indicated that the model tivecommons.org/licenses/by/4.0/), which permits unrestricted use, used in this paper is significantly reliable and valid. distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a 6. 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Photocatalytic degradation of tetracycline aqueous solutions by nanospherical α-Fe2O3 supported on 12-tungstosilicic acid as catalyst: using full factorial experimental design

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Chemistry; Industrial Chemistry/Chemical Engineering; Polymer Sciences; Nanochemistry; Environmental Chemistry
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Abstract

Int J Ind Chem (2017) 8:297–313 DOI 10.1007/s40090-016-0108-6 RESEARCH Photocatalytic degradation of tetracycline aqueous solutions by nanospherical a-Fe O supported on 12-tungstosilicic acid 2 3 as catalyst: using full factorial experimental design 1 1 Majid Saghi Kazem Mahanpoor Received: 21 June 2016 / Accepted: 7 November 2016 / Published online: 16 November 2016 The Author(s) 2016. This article is published with open access at Springerlink.com Abstract In this paper, spherical a-Fe O nanoparticles and catalyst concentration variables are at maximum levels 2 3 (NPs) were supported on the surface of 12-tungstosilicic and the initial concentration of TC and H O concentration 2 2 acid (12-TSA7H O) using two different solid-state dis- variables are at minimum levels (pH 8, catalyst concen- persion (SSD) and forced hydrolysis and reflux condensa- tration = 150 ppm, initial concentration of TC = 30 ppm, tion (FHRC) methods. Photocatalytic activity of supported H O concentration = 0.1 ppm). A first order reaction with 2 2 -1 a-Fe O NPs (a-Fe O /12-TSA7H O) for tetracycline k = 0.0098 min was observed for the photocatalytic 2 3 2 3 2 (TC) degradation in aqueous solution was investigated degradation reaction. using UV/H O process and the results were compared 2 2 with that of pure a-Fe O NPs. a-Fe O and 12-TSA7H O Keywords Photocatalytic degradation  Tetracycline 2 3 2 3 2 were synthesized according to previous reports and all a-Fe O  12-Tungstosilicic acid  a-Keggin 2 3 products were characterized by using FTIR, SEM, EDX and XRD. Design of experiments (DoEs) was utilized and photocatalytic degradation process was optimized using Introduction full factorial design. The experiments were designed con- sidering four variables including pH, the initial concen- From the perspective of green chemistry, degradation of tration of TC, catalyst concentration and H O chemical pollutants in wastewater has attracted a lot of 2 2 concentration at three levels. TC concentration reduction in attention. Antibiotics are one of the larger groups of these the medium was measured using UV/Vis spectroscopy at pollutants in wastewater released from pharmaceutical k = 357 nm. The results of experiments indicated that industries [1]. Besides, TC is one broad spectrum of max supporting a-Fe O NPs on the surface of 12-TSA7H O antibiotics repeatedly detected in urban and industrial 2 3 2 through SSD and FHRC methods caused to improve the wastewaters, drinking water, surface water and ground- filtration, recovery and photocatalytic activity of NPs. water [2–6]. The molecular structure of TC is shown in Also, it was indicated that those NPs supported through Fig. 1. Various techniques are used to degrade TC; one of SSD method, have better photocatalytic performance than these techniques is photocatalytic degradation [7]. NPs those supported through FHRC method. The statistical play an important role in heterogeneous photocatalysis. analyses revealed that the maximum TC degradation Metal oxide NPs, i.e., iron oxides, have a special position (97.39%) is obtained under those conditions in which pH in the science and technologies because of having wide applications and unique properties [8]. a-Fe O (hematite) 2 3 which is the most common form of iron oxides, has the & Kazem Mahanpoor rhombohedral structure and it is an attractive compound k-mahanpoor@iau-arak.ac.ir because of its applications in data storage, gas sensor, Majid Saghi magnets materials, pigment, catalysis and photocatalysis m-saghi@iau-arak.ac.ir [9–14]. Various techniques including co-precipitation, sol– gel, thermal decomposition, Micelle synthesis, sonochem- Department of Chemistry, Islamic Azad University, ical synthesis, hydrothermal synthesis and FHRC have Arak Branch, Ara¯k, Iran 123 298 Int J Ind Chem (2017) 8:297–313 OH O OH O O d- and e-Keggin, Wells–Dawson, Preysler, Stromberg and Anderson–Evans are served as critical types. OH 12-tungstosilicic acid (hereafter, 12-TSA) is a HPA with formula H SiW O and a-Keggin crystal structure (see NH 2 4 12 40 Fig. 2). The central Si heteroatom is surrounded by a tetrahedron whose oxygen vertices are each linked to one OH of the four W O sets. Each W O set consists of three 3 13 3 13 H H W O octahedrals linked in a triangular arrangement by 3 6 H C OH N(CH ) sharing edges and the four W O are linked together by 3 13 3 2 sharing corners [34]. So far, numerous experimental studies Fig. 1 Molecular structure of TC have been done about supporting HPAs on the surface of various organic and inorganic catalyst supports, but HPAs been utilized to synthesize monodisperse a-Fe O NPs. 2 3 have rarely been used as catalyst support [35–38]. [15–21]. Among various photocatalytic processes, water 12-TSA has suitable physical and chemical properties to and wastewater treatments are of the most important a- be used as a catalyst support. The pores existed on the Fe O NPs applications. In these processes, a-Fe O NPs 2 3 2 3 crystalline surface of 12-TSA provide a suitable condition could be used in the form of a fine powder or crystals to support NPs [39]. To optimize a process like the pho- dispersed in water, but it is vital to know that filtering these tocatalytic degradation process, it is essential to study all NPs following reaction is difficult and costly. To solve this factors influencing the process. But studying the effects of problem, researchers have examined methods for support- individual factors on the process is difficult and time- ing a-Fe O NPs on the surface of organic, inorganic or 2 3 consuming, especially if these factors are not independent organic/inorganic catalyst supports [22, 23]. Various and they affect each other. Employing experimental design methods have been applied for supporting a-Fe O NPs on 2 3 could eliminate these problems because the interaction the surface of catalyst support. Utilizing any of these effects of different factors could be attained using DoEs methods depends on the chemical and physical properties only. Full factorial is an appropriate method for DoEs of catalyst and catalyst support as well as the purpose of the because it could reduce the total number of experiments as process. One of these methods is SSD method in which well as optimize the process by optimizing all the affecting catalyst precursor and catalyst support are separately syn- factors collectively, at a time [40]. The design could thesized and then are mixed with specific weight ratio determine the effect of each factor on the response as well using an appropriate solvent [24]. Then, during calcination, as how this effect varies with the change in level of other the catalyst is both formed and thermally supported on the factors. surface of catalyst support. In another technique such as Various crystal structures of a-Fe O NPs including rod- 2 3 FHRC, the catalyst support is added to the precursor shape [21], spherical and elliptical forms [41] have been solution(s) during catalyst preparation (if it was stable in synthesized and identified until now. In this work, spherical reaction medium) and the catalyst is supported on the a-Fe O NPs are supported through two different SSD and 2 3 surface of catalyst support while it is simultaneously FHRC methods on the surface of 12-TSA7H O(a-Fe O / 2 2 3 formed. In FHRC method, all steps related to the synthesis 12-TSA7H O). Then, the performance of pure and sup- of NPs were done on the surface of catalyst support and ported a-Fe O NPs on the TC photocatalytic degradation 2 3 ‘‘NP/catalyst support’’ was obtained after nucleation and was investigated using full factorial experimental design. growth of NPs. Polyoxometalates (POMs) are a great class of inorganic compounds as multi-core metal–oxygen clusters [25]. If an atom named heteroatom (such as Si, P, Experimental As, B, etc.) enters the molecular structure of POM in addition to metal and oxygen, then heteropoly acids Material and apparatuses (HPAs) will be obtained [26]. Thermodynamically, HPAs have stable arrangements and maintain their crystal struc- All chemicals used in this work including sodium tungstate ture in aqueous and non-aqueous solutions. This class of dihydrate, sodium silicate, diethyl ether, iron (III) chloride materials has various applications in catalysis [27], ana- hexahydrate, urea, hydrogen peroxide (30% pure), lytical chemistry [28], medicinal chemistry (anti-tumor, hydrochloric acid (37% pure), sulfuric acid (96% pure), anti-cancer, anti-bacteria, anti-microbial and anti-clotting) sodium hydroxide and ethanol were purchased from Merck [29–31], radioactive materials [32] and gas absorbents [33] and were used without further purification. The required owing to their structural diversity and unique properties. TC was purchased from Razak pharmaceutical laboratory HPAs have different crystal structures of which a-, b-, c-, (Tehran, Iran). Also, deionized water was used throughout 123 Int J Ind Chem (2017) 8:297–313 299 Fig. 2 a-Keggin structure of 4- [SiW O ] 12 40 the experiments. The Fourier transform infra-red (FTIR) transform to iron oxide. Consequently, a dark brown solid spectra of products were recorded on a Perkin-Elmer of a-Fe O was obtained. 2 3 spectrophotometer (Spectrum Two, model) in the range of -1 450–4000 cm . The shape, size and surface morphology Synthesis of 12-TSA7H O of the synthesized 12-TSA7H O and a-Fe O /12- 2 2 3 TSA7H O were examined using the obtained images of a 12-TSA7H O was synthesized according to literature 2 2 Philips XL-30 scanning electron microscope (SEM). The procedure [42]. Firstly, 15 g sodium tungstate dihydrate X-ray diffraction (XRD) analysis of the samples was done was dissolved in 30 ml deionized water and then 1.16 g using a DX27-mini diffractometer. BET surface area of sodium silicate solution (density 1.375 g/ml) was added to materials was determined by N adsorption–desorption it. The resulted mixture was heated up to about boiling method at 77 K, measured using a BELSORP-mini II point, and while it was stirred, 10 ml concentrated HCl was instrument. The samples were degassed under vacuum at added to it during 30 min, smoothly. Then, the solution 473 K for 12 h before the BET measurement. All ultravi- was naturally cooled down to RT and slight precipitate olet/visible (UV/Vis) absorption spectra were obtained formed (silicic acid) in it was filtered. Again, 5 ml con- using an Agilent 8453 spectrophotometer and the pH val- centrated HCl was added to the solution and was trans- ues were determined by a Metrohm pH meter model 827. ferred to separatory funnel after cooling it again down to Likewise, to separate the catalyst from samples, an ALC RT. Then, 12 ml diethyl ether was added to it and well 4232 centrifuge was employed. shaken. Therefore, three layers were formed inside sepa- ratory funnel, middle layer of which was yellow-colored. Synthesis of a-Fe O NPs Bottom layer which was oily ether was separated and 2 3 transferred into a beaker. To further extract, separatory The synthesis of a-Fe O NPs was carried out according to funnel was further shaken again and the bottom layer was 2 3 Bharathi et al. [21]. Firstly, 100 ml iron (III) chloride once more separated and transferred into the beaker. This hexahydrate 0.25 M which was considered as a source of extraction process was done so much that the yellow color 3? Fe , was poured into a flat-bottom flask. When Iron of middle layer was fully faded. The extracted ether solution was agitated by stirrer, it was added drop by drop complex which was inside the beaker was transferred to to it 100 ml urea 1 M (as a supplying agent of hydroxyl another separatory funnel and then 16 ml HCl 25% (v/v) ions). The more gentle and regular adding urea, the smaller was added to it. Next, 4 ml diethyl ether was added to it, and more uniform-sized formed a-Fe O particles will be. subsequently. The contents inside separatory funnel were 2 3 The obtained mixture was stirred for 30 min and then shaken and bottom layer (ether) was transferred to the placed under the reflex at 90–95 C for 12 h. Then, the evaporating dish after separating. Evaporating dish was precipitate after separation was washed with 100 ml exposed to air and remained motionless to evaporate the deionized water because unreacted ions will be completely solvent and form the 12-TSA7H O crystals. Finally, 12- removed. The washed precipitate was dried at 70 C for TSA7H O formed crystals were placed at 70 C for 2 h 2 h. Having fully dried, one light brown solid (iron until it was completely dried. The chemical reaction hydroxide) was yielded. Finally, this solid remained at occurred in the process of 12-TSA7H O synthesis has 300 C for 1 h; hence the iron hydroxide particles will been shown in (1)[42]. 123 300 Int J Ind Chem (2017) 8:297–313 Table 1 Experimental range and levels of the variables 12 Na WO þ Na SiO þ 26 HCl 2 4 2 3 ð1Þ H SiW O  xH O þ 26 NaCl þ 11 H O Variables Range and levels 4 12 40 2 2 –1 0 ?1 Preparation of a-Fe O /12-TSA7H O 2 3 2 pH 4 6 8 Initial con. of TC (ppm) 30 50 70 SSD method Catalyst con. (ppm) 50 100 150 H O con. (ppm) 0.1 0.3 0.5 2 2 Firstly, the synthesized iron hydroxide (light brown solid) and 12-TSA7H O catalyst support were mixed with three levels (50, 100 and 150 ppm) and H O concentration weight ratio of 1:3 iron hydroxide/12-TSA7H O (weight 2 2 from 0.1 to 0.5 ppm at three levels (0.1, 0.3 and 0.5 ppm). In of catalyst support is three times of catalyst weight) using Table 2, 19 experiments related to this factorial design and an agate pestle and mortar for 1 h. To have better mixture, their experimental conditions have been listed. The removal ethanol was sprayed on the mixture until it becomes dough- efficiency of TC was a dependent response. In order to do form. During mixing, in the vaporization phase, ethanol is DoEs, Minitab 16 version 16.2.0 statistical software was again added in order to keep the dough-form of the mix- utilized. Also, analysis of variance (ANOVA) was run to ture. The resulted mixture was dried under air for 1 h and analyze the results. then was kept at 80 C for 2 h. To do calcination and transform iron hydroxide particles fixed on the surface of General procedure for photocatalytic degradation 12-TSA7H O into iron oxide (a-Fe O ), the obtained solid 2 2 3 of TC was kept at 300 C for 1 h. Figure 3 shows one schematic diagram of photocatalytic FHRC method reactor used in the work. An MDF box was designed inside which a circular Pyrex reactor with 300 ml capacity was Firstly, 50 ml iron (III) chloride hexahydrate 0.25 M was placed. On the upper section of the box, three mercury poured into a beaker. While it was agitated by stirrer, 3.5 g lamps (Philips 15 W) were built-in as UV light sources. 12-TSA7H O was gently added to it. The obtained mix- The radiation is generated almost exclusively at 254 nm. ture was stirred for 4–5 h. Then, stirring was stopped for These lamps were set up with the same intervals, so light 2 h until the solid within mixture was deposited. The solid was evenly radiated on the whole liquid surface inside the accumulated at bottom of beaker was separated and reactor. The liquid inside the reactor was agitated by transferred into one flat-bottom flask and the same 10 ml magnetic stirrer and the air inside the box was conditioned solution inside beaker was added to it. When mixture by a fan (built-in at back of box). In order to carry out each inside flat-bottom flask was being stirred, 50 ml urea 1 M experiment (according to Table 2), firstly 250 ml TC was gradually added to it. The mixture was placed under solution was made as specified concentration and poured reflux at 90–95 C for 12 h. Then, the precipitate resulted inside the reactor. Then, at related pH, the specified amount after separation was washed with 100 ml ethanol/deionized of photocatalyst and H O were added to the solution water 1:1 solution because unreacted ions were completely 2 2 inside the reactor. In all experiments, pH adjustment was removed. The washed precipitate was dried in the air for done via minimum use of H SO and NaOH. Then, stirrer 2 h and then was kept at 80 C for 2 h. In order to calci- 2 4 and UV lamps were immediately turned on to initiate the nation, the obtained solid was kept at 300 C for 1 h. process. Sampling was done by a 5 ml syringe, every 10 min. To fully separate the catalyst from solution, the Full factorial experimental design samples were centrifuged for 3 min with 3500 rpm speed. The TC concentration of the samples was determined using The photocatalytic efficiency of pure a-Fe O NPs and a- 2 3 a UV/Vis spectrophotometer at k = 357 nm. The per- Fe O /12-TSA7H O prepared by SSD and FHRC methods max 2 3 2 centage of initial concentration of pollutant decomposed by on the TC degradation were investigated using DoE. The the photocatalytic process or the percent of photodegra- experiments were designed considering four variables dation efficiency (x%) as a function of time is given by including pH, the initial concentration of TC, catalyst con- centration and H O concentration at three levels. Experi- 2 2 C  C x% ¼  100 ð2Þ mental range and levels of variables are shown in Table 1. pH varied from 4 to 8 at three levels (4, 6 and 8), the initial where C and C are the concentration of TC (ppm) at t = 0 concentration of TC from 30 to 70 ppm at three levels (30, 50 and t, respectively. and 70 ppm), catalyst concentration from 50 to 150 ppm at 123 Int J Ind Chem (2017) 8:297–313 301 Table 2 Experimental Exp. no. Variables conditions for photocatalytic process pH Initial con. Catalyst H O con. 2 2 of TC (ppm) con. (ppm) (ppm) 1–1 –1 –1 –1 2 ?1–1 ?1 ?1 3–1 –1 ?1–1 4–1 ?1 ?1–1 5 ?1 ?1 ?1–1 60 0 0 0 7 ?1–1 ?1–1 8–1 ?1–1 ?1 9 ?1–1 –1 –1 10 ?1 ?1 ?1 ?1 11 –1 ?1–1 –1 12 ?1–1 –1 ?1 13 ?1 ?1–1 –1 14 –1 ?1 ?1 ?1 15 ?1 ?1–1 ?1 16 0 0 0 0 17 –1 –1 ?1 ?1 18 0 0 0 0 19 –1 –1 –1 ?1 Fig. 3 Schematic diagram of photocatalytic reactor. 1 MDF box, 50 9 50 9 50 cm; 2 Mercury lamps, Philips 15 W; 3 The distance between surface of TC solution and lamps, 5 cm; 4 Reactor, 300 ml capacity; 5 TC solution, 250 ml; 6 Magnet; 7 Magnetic stirrer; 8 Fig. 4 SEM image of the synthesized 12-TSA7H O Sampling port existed on the surface of this catalyst support provide a Results and discussion suitable condition to support a-Fe O NPs. IR is a suit- 2 3 able method for the structural characterization of HPAs Characterization [26]. FTIR spectrum of the synthesized 12-TSA7H O has been shown in Fig. 5a. There are four kinds of oxygen The synthesized 12-TSA7H O atoms in 12-TSA7H O structure, 4 Si–O in which one 2 a oxygen atom connects to Si, 12 W–O –W oxygen bridges SEM image of the synthesized 12-TSA7H O is shown in 2 (corner-sharing oxygen-bridge between different W O 3 13 Fig. 4. Surface morphology of 12-TSA7H O shows that 2 groups), 12 W–O –W oxygen bridges (edge-sharing oxy- this product has suitable structural properties and can be gen-bridge within W O groups) and 12 W=O terminal 3 13 d regarded as a catalyst support. In other words, the pores oxygen atoms. The symmetric and asymmetric stretching 123 302 Int J Ind Chem (2017) 8:297–313 of the different kinds of W–O bonds are observed in the The prepared a-Fe O /12-TSA7H O 2 3 2 -1 following spectral regions: Si–O bonds (1020 cm ), -1 W = O bonds (1000–960 cm ), W–O –W bridges Figures 7 and 8 show SEM/EDX images of a-Fe O /12- d b 2 3 -1 -1 (890–850 cm ), W–O –W bridges (800–760 cm )[43]. TSA7H O prepared by SSD and FHRC methods, respec- c 2 tively. These images indicate that in both methods, a- In Table 3, vibrational frequencies of the synthesized 12-TSA7H O and equivalent values reported in previous Fe O particles were spherically supported on the surface 2 2 3 of 12-TSA7H O. The spheres in SSD method are bigger studies [43, 44] have been listed. Comparing the vibra- tional frequencies reveals that 12-TSA7H O has been well and have covered more area of 12-TSA7H O than that of FHRC method. Possibly in SSD method, spherical a-Fe O synthesized. XRD is one of the most important character- 2 3 ization tools used in solid state chemistry and materials particles are adhered to each other and bigger spheres have science. Figure 6a shows the XRD pattern of the synthe- formed while it did not occur in FHRC method and a- sized 12-TSA7H O. This pattern indicates that the char- Fe O particles were separately supported. It is assumed 2 3 acteristic peaks corresponded to the 12-TSA were well that the causes of this phenomena are as follows: (1) pos- appeared and it means that the synthesized 12-TSA7H O sibly, a-Fe O synthesized particles by SSD method are 2 2 3 crystals were well formed [44]. smaller than that of FHRC method and this contributed to their adherence, (2) Supporting through SSD method is done in solid state and this increases the possibility of particles adhering to each other and forming bigger spheres and (3) supporting through FHRC method is done in liquid phase, so the particles could freely move and be separately fixed on the 12-TSA7H O surface. In Fig. 5b, c, FTIR spectra of a-Fe O /12-TSA7H O prepared by SSD and 2 3 2 FHRC methods have been shown, respectively. It is clear that absorption peaks of 12-TSA7H O have appeared without considerable change in the wavenumbers (only their intensities have been slightly changed). It means that in both methods, 12-TSA7H O was stable and it had not been changed chemically during preparing a-Fe O /12- 2 3 TSA7H O. Also, absorption peaks of a-Fe O have well 2 2 3 appeared and are in agreement with results of Bharati et al. [21]. These absorption 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 12-TSA7H O. Comparing FTIR spectra reveals that absorption peaks of a-Fe O related to SSD method are 2 3 more intense than that of FHRC method. This partly con- firms the results of SEM images. Hence in SSD method, surface of 12-TSA7H O has been covered by more a- Fe O particles. In Fig. 6b, c, XRD patterns of a-Fe O /12- 2 3 2 3 TSA7H O prepared by SSD and FHRC methods have been illustrated, respectively. In both of these patterns, characteristic peaks of 12-TSA7H O have well appeared which indicates that 12-TSA7H O was stable during the supporting process in both SSD and FHRC methods. In these patterns, the characteristic peaks of a-Fe O which 2 3 have also been marked have appeared and it is in agree- ment with results of Bharati et al. [21]. In XRD related to SSD method, intensity of 12-TSA7H O and a-Fe O 2 2 3 characteristic peaks is lower and higher than that of FHRC method, respectively. This issue confirms the results of SEM and FTIR, so during supporting through SSD method, 12-TSA7H O surface has been covered by the greater Fig. 5 FTIR spectra of the synthesized 12-TSA7H O(a) and a- amount of a-Fe O particles. The size of spherical a-Fe O Fe O /12-TSA7H O prepared by SSD (b) and FHRC (c) methods 2 3 2 2 3 2 3 123 Int J Ind Chem (2017) 8:297–313 303 Table 3 Vibrational Number The synthesized 12-TSA7HO[43, 44] frequencies of the synthesized -1 12-TSA7H O and equivalent 2 Wavenumber (cm ) Transmittance % values reported in previous reports 1 1019.04 13.29 1020 (weak) 2 980.68 8.81 981 (sharp) 3 924.31 5.92 928 (very sharp) 4 882.63 11.52 880 (medium) 5 780.28 5.77 785 (very sharp) 6 537.41 13.35 540 (medium) Fig. 6 X-ray diffractogram of the synthesized 12-TSA7H O (a), a-Fe O /12-TSA7H O 2 3 2 prepared by SSD (b) and FHRC (c) methods Fig. 7 SEM image and EDX results of a-Fe O /12-TSA7H O prepared by SSD method 2 3 2 particles supported on the surface of 12-TSA7H O were FHRC methods were determined 57.53 and 39.84 (m /g), calculated using XRD and Warren–Averbach method respectively. It seems that the high amount of iron oxide (taking account of device errors) whose averages for SSD formed on the base has been increase the BET surface area and FHRC methods were 50.5 and 70.82 nm, respectively. of catalyst prepared with SSD method. The BET surface area of catalyst prepared by SSD and 123 304 Int J Ind Chem (2017) 8:297–313 Fig. 8 SEM image and EDX results of a-Fe O /12-TSA7H O prepared by FHRC method 2 3 2 UV/Vis spectra FHRC methods. This means that supporting a-Fe O NPs 2 3 leads to increase their photocatalytic activity. Also, com- The absorbance of TC solutions during photocatalytic paring the results of SSD and FHRC methods indicates that process (using a-Fe O /12-TSA7H O prepared by SSD a-Fe O /12-TSA7H O prepared through SSD method was 2 3 2 2 3 2 method and according to exp. no. 8) at initial and after 10, effective from the aspect of TC photocatalytic degradation 20, 30, 40 and 50 min irradiation time verses wavelength and has yielded more x% values. Comparing x% values in are depicted in Fig. 9. In all experiments, x% was calcu- one series of experiments (1 through 19) shows that the lated at k = 357 nm. The wavelength of maximum highest degradation percentage has been obtained in exp. max absorbance (in 357 nm) did not change with time, then this no. 7. To better compare the results, x% histogram versus wavelength for measuring the concentration of pollutants experiment number for pure a-Fe O and a-Fe O /12- 2 3 2 3 was chosen. Furthermore, absorbance changes in 357 nm TSA7H O prepared through SSD and FHRC methods has were completely regular and measurable. been shown in Fig. 10. The histogram clearly indicates that in all experiments a-Fe O NPs supported on the surface of 2 3 Performance of photocatalysts 12-TSA7H O (particularly through SSD method) had more photocatalytic efficacy and has degraded more TC. Having carried out all experiments based on Table 2, x% values were calculated at k = 357 nm following 50 min Photocatalytic mechanism max after reaction which have been reported in Table 4.In general, comparing x% values reveals that the degree of TC According to exp. no. 7, the effects of UV irradiation, pure a- photocatalytic degradation by pure a-Fe O is lower than Fe O NPs and a-Fe O /12-TSA7H O prepared by two 2 3 2 3 2 3 2 that of a-Fe O /12-TSA7H O prepared through SSD and different SSD and FHRC methods on the photodegradation 2 3 2 Fig. 9 UV/Vis spectral absorption changes of TC solution photodegraded by a- Fe O /12-TSA7H O prepared 2 3 2 through SSD method (pH 4, Initial concentration of TC = 70 ppm, catalyst concentration = 50 ppm, H O 2 2 concentration = 0.5 ppm) 123 Int J Ind Chem (2017) 8:297–313 305 Table 4 x% values after 50 min photodegradation process at of TC are presented in Fig. 11. This Figure designates that in k = 357 nm max the presence of a-Fe O /12-TSA7H O prepared by SSD 2 3 2 method and UV irradiation 97.39% of TC was degraded at Exp. no. x% the reaction time of 50 min while it was 88.44, 82.17 and Pure a-Fe O /12-TSA a-Fe O /12-TSA 2 3 2 3 10.2% for a-Fe O /12-TSA7H O prepared by FHRC 2 3 2 a-Fe O 7H O prepared 7H O prepared 2 3 2 2 method, pure a-Fe O NPs and only UV, respectively. When NPs by SSD method by FHRC method 2 3 a-Fe O is illuminated by the light, electrons are promoted 2 3 1 64.11 78.59 66.32 from the valence band (VB) to the conduction band (CB) of 2 75.39 93.61 85.95 the semi conducting oxide to give electron–hole pairs. The 3 67.56 85.35 73.29 VB potential (h ) is positive enough to generate hydroxyl VB 4 48.83 62.74 53.22 radicals at the surface, and the CB potential (e ) is negative CB 5 37.14 60.51 45.07 enough to reduce molecular oxygen. The hydroxyl radical is 6 36.97 69.61 60.38 a powerful oxidizing agent and attacks TC molecules present 7 82.17 97.39 88.44 at or near the surface of a-Fe O . It causes the photo-oxi- 2 3 8 37.95 52.51 42.66 dation of TC according to the following reactions [45–50]: 9 65.46 84.34 74.61 a-Fe O þ hm ! a-Fe O ðe þ h Þ 2 3 2 3 CB VB 10 44.37 58.94 47.92 þ þ h þ H O ! H þ OH 2 ðadsÞ 11 32.84 47.91 38.91 VB ðÞ ads 12 62.00 91.48 74.28 h þ OH ! OH ðÞ ads VB ðÞ ads 13 29.84 47.06 37.71 e þ O  O 2ðadsÞ CB 2ðÞ ads 14 40.69 62.45 48.72 15 39.31 55.34 45.62 H O  H þ OH 16 37.02 69.75 59.79 O þ H ! HO 2ðÞ ads 17 66.83 82.21 77.13 2 HO ! H O þ O 2 2 2 2 18 36.83 69.44 60.13 H O þ a-Fe O ðe Þ! OH þ OH þ a-Fe O 2 2 2 3 2 3 CB 19 65.83 87.17 81.84 OH þ TC ! degradation of TC a ðÞ ads Maximum value of x% þ þ h þ TC TC ! oxidation of TC: VB Fig. 10 x% values versus experiment number 123 306 Int J Ind Chem (2017) 8:297–313 view of the TC photocatalytic degradation, then in this The mechanism is summarized in Fig. 12. The main role section we carry out the statistical results analysis of the of the foundation is creating the perfect conditions for photocatalytic process in which a-Fe O /12-TSA7H O 2 3 2 putting the TC and hydroxyl radical beside each other. prepared by SSD method has been utilized. Analysis of Photocatalytic activity increased after stabilizing iron oxide variance (ANOVA) is a set consists of a number of sta- on 12-TSA7H O. To comment on this result, we propose tistical methods used to analyze the differences among that the hydroxyl radicals, on the surface of iron oxide, are group means and their associated procedures. ANOVAs are easily transferred onto the surface of 12-TSA7H O. That useful for testing three or more means variables for sta- means the organic pollutants such as TC, which have tistical significance. ANOVA was used for graphical already been adsorbed on the nonphotoactive analyses of the data to obtain the interaction between the 12-TSA7H O, have chances to be degraded due to the process variables and the responses. The quality of the fit appearance of hydroxyl radicals, resulting in the enhance- polynomial model was expressed by the coefficient of ment of the photodegradation performance of a-Fe O /12- 2 3 determination R , and its statistical significance was TSA7H O (as shown in Fig. 12b). checked by the Fisher’s F test in the same program. Model terms were evaluated by the P value. In Table 5, the esti- Kinetics of photocatalytic degradation of TC mated effects and coefficients for x% have been listed. In this table, standard deviation (S), correlation coefficient, Figure 13 displays the plot of ln(C /C) versus reaction time 2 2 pried R and adjusted R values were also reported. The for TC. The linearity of the plot suggests that the pho- square of the correlation coefficient for each response was todegradation reaction approximately follows the pseudo- computed as the coefficient of determination (R ). The first order kinetics with a rate coefficient -1 accuracy and variability of the model can be evaluated by k = 0.0098 min . 2 2 R .The R value is always between 0 and 1. The closer the R value to 1, the stronger the model is and the better the The statistical analysis (optimum conditions) model predicts the response (x%). R value was reported to be 0.9915 in this paper. The ‘‘pried R ’’ of 0.9662 is in Since a-Fe O NPs supported through SSD method have 2 3 reasonable agreement with the ‘‘adj R ’’ of 0.9848, shown more effective than other photocatalysts from the Fig. 11 Effect of UV light, pure a-Fe O NPs and a-Fe O /12-TSA7H O prepared by FHRC and SSD methods on TC degradation (pH 8, 2 3 2 3 2 Initial concentration of TC = 30 ppm, catalyst concentration = 150 ppm, H O concentration = 0.1 ppm) 2 2 123 Int J Ind Chem (2017) 8:297–313 307 Fig. 12 General mechanism of the photocatalysis (a) and photocatalytic activity of a-Fe O /12-TSA7H O(b) 2 3 2 confirming good predictability of the model. Due to both negative and roughly the same effects on the x% value Table 5 and the significant variables effects on the (-4.67 and -4.66, respectively). In Table 5, the coeffi- response, affect magnitudes of the initial concentration of cients of each term have been reported which are the same TC, pH, H O concentration and catalyst concentration term coefficients in response function which they will be 2 2 equal to 31.59, 3.72, 2.48 and 7.35, respectively. Thus, the given in the following. It is vital to note that P values have significant reaction parameters were (the most to the least been assessed considering Alpha (a) = 0.05. Table 6 significant): initial concentration of TC [ catalyst con- depicts the results of ANOVA. The effect on the response centration [ pH [ and H O concentration. Of course, it is was increased by increasing the value of F parameter and 2 2 necessary to note that despite other three variables, the decreasing P parameter. For main effects (with 4 degrees variable of the initial concentration of TC has a negative of freedom) including the initial concentration of TC, pH, effect on the response (-31.59). This means that increasing H O concentration and catalyst concentration, F and 2 2 the initial concentration of TC leads to decrease x% and P values have obtained as 278.34 and \0.0001, respec- conversely. In this way, the effects about the variables tively. Besides, these values were 18.41 and \0.0001 for interaction were reported in Table 5. As can be seen from 2-way interactions (with 3 freedom degree), respectively. these results, it is the only interaction of variables, namely In Table 7, complementary results have been listed which the initial concentration of TC and the catalyst concentra- have been used for drawing residual plots. Residual values tion which have positive effects (3.10). The interaction of were calculated from subtracting experimental x% values the initial concentration of TC with pH and the interaction and fitted values. of H O concentration with catalyst concentration have In order to compare the variables effect (from the 2 2 viewpoint of magnitude) on the response, the Fig. 14a could be investigated which is one Pareto chart of the standardized effects. In this Figure, those variables whose effects on response is negative (-) or positive (?) have been marked. The results revealed that the effect of the initial concentration of TC on the x% is greater than other variables effect (at least three times) but the effect of this variable is negative i.e. increasing or decreasing the initial concentration of TC leads to decrease and increase x%, respectively. In order to better investigate the residual values, residual plot versus exp. no. has been illustrated in Fig. 14b. As it is seen, eight points (residuals) are located under zero line (negative), nine points above zero line Fig. 13 Plot of reciprocal of pseudo-first order rate constant against (positive) and two points roughly on the zero line. Due to initial concentration of TC = 70 ppm, catalyst concentra- this and comparing distance of points from zero line, it tion = 50 ppm, H O concentration = 0.5 ppm and pH 4 2 2 123 308 Int J Ind Chem (2017) 8:297–313 Table 5 Estimated effects and coefficients for x% Term Effect Coef SE Coef T (Coef/SE Coef) P value Result Constant – 71.73 0.4905 146.22 \0.0001 Significant Initial con. of TC –31.59 –15.79 0.4905 –32.19 \0.0001 Significant pH 3.72 1.86 0.4905 3.79 0.004 Significant H O con. 2.48 1.24 0.4905 2.52 0.03 2 2 Catalyst con. 7.35 3.68 0.4905 7.49 \0.0001 Significant Initial con. of TC 9 pH –4.66 –2.33 0.4905 –4.75 0.001 Significant Initial con. of TC 9 catalyst con. 3.10 1.55 0.4905 3.16 0.010 H O con. 9 catalyst con. –4.67 –2.34 0.4905 –4.76 0.001 Significant 2 2 Center point – –2.13 1.2345 –1.72 0.116 2 2 2 S = 1.96219, R = 99.15%, Pred R = 96.62%, Adj R = 98.48% Table 6 ANOVA results Source Degree of Seq SS Adj SS Adj MS F value freedom Initial con. of TC 1 3990.68 3990.68 3990.68 1036.49 pH 1 55.25 55.25 55.25 14.35 H O con. 1 24.54 24.54 24.54 6.37 2 2 Catalyst con. 1 216.12 216.12 216.12 56.13 Initial con. of TC 9 pH 1 86.73 86.73 86.73 22.53 Initial con. of TC 9 catalyst con. 1 38.55 38.55 38.55 10.01 H O con. 9 catalyst con. 1 87.36 87.36 87.36 22.69 2 2 could be said that residual distribution is normal. An Table 7 Residual values extremely useful procedure is to construct a normal prob- Exp. no. x% Fit SE fit Residual (x%–fit) St resid ability plot of the residuals. If the underlying error distri- bution is normal, this plot will resemble a straight line. 1 78.59 77.6338 1.3875 0.9562 0.69 Figure 14c shows normal probability plot. In this plot, it is 2 93.61 92.7296 1.3875 0.8804 0.63 fully clear that residuals distribution is normal because 3 85.35 86.5531 1.3875 –1.1977 –0.86 points (especially central points) are close to straight line. 4 62.74 62.7282 1.3875 0.0118 0.01 If the model is correct and if the assumptions are satisfied, 5 60.51 61.7882 1.3875 –1.2782 –0.92 the residuals should be structureless; in particular, they 6 69.61 69.6000 1.1329 0.0100 0.01 should be unrelated to any other variable including the 7 97.39 94.9260 1.3875 2.4640 1.78 predicted response. A simple check is to plot the residuals 8 52.51 54.7502 1.3875 –2.2402 –1.61 versus the fitted values. Figure 14d displays plot of resid- 9 84.34 86.0067 1.3875 –1.6667 –1.20 uals versus fitted values. Mathematical model representing 10 58.94 59.5918 1.3875 –0.6518 –0.47 TC photocatalytic degradation in the range studied can be 11 47.91 47.5998 1.3875 0.3102 0.22 expressed by the following equation: 12 91.48 93.1572 1.3875 –1.6778 –1.21 13 47.06 46.6598 1.3875 0.4002 0.29 Response ¼ x% ¼ 71:7315:79A þ 1:86B þ 1:24C 14 62.45 60.5318 1.3875 1.9182 1.38 þ 3:68D2:33AB þ 1:55AD2:34CD 15 55.34 53.8102 1.3875 1.5298 1.10 16 69.75 69.6000 1.1329 0.1500 0.09 where A, B, C and D are the initial concentration of TC, 17 82.21 84.3567 1.3875 –2.1467 –1.55 pH, H O concentration and catalyst concentration, 2 2 18 69.44 69.6000 1.1329 –0.1600 –0.10 respectively. 19 87.17 84.7843 1.3875 2.3882 1.72 In Fig. 15, the plots of main effects have been shown. These plots indicate that of four main effects, only the 123 Int J Ind Chem (2017) 8:297–313 309 Fig. 14 a Pareto chart of the standardized effects, b plot of residuals versus exp. no., c Normal probability plot and d plot of residuals versus fitted values variable of the initial concentration of TC has a negative in order to reach maximum degradation (x% = 97.39) the effect on response (x%); effects of other variables on variables of pH, the initial concentration of TC, catalyst response were positive. In effect, increasing the initial concentration and H O concentration should be at levels 2 2 concentration of TC and decreasing pH, H O concen- of ?1(8), -1(30 ppm), ?1(150 ppm) and -1(0.1 ppm), 2 2 tration and catalyst concentration will be caused to respectively. Generally, considering the interaction effects decrease and increase x%, respectively (if the interaction is very important because it may place the unpre- effect of variable is ignored). The slope of line in main dictable effects on the response. For example, based on the effect plots is one indicator of magnitude related to the results of Fig. 15 even though H O concentration had 2 2 variable effect on the response. Therefore, the order of simply a positive effect on x%, the maximum x% was affecting variables from magnitude viewpoint is as achieved in those conditions where H O concentration 2 2 initial concentration of TC [ catalyst concentra- was at its minimum level (see exp. no 7 in Table 4). For the tion [ pH [ H O concentration which confirm the same reason, the interaction effect of variables should not 2 2 results of Fig. 14a. be ignored in studying variables for reaching optimal In Fig. 16, interaction plots for x% have been presented. conditions. Generally, in such plots the more parallel the lines, the Finally, to determine the stability of the catalyst after 5 lower the interaction effect would be and the more inter- steps photocatalytic decomposition process, catalyst sepa- secting the lines, the higher the interaction effect would be. ration and then drying it, the FTIR spectrum of the sample As it is observed, there is a significant interaction effect showed that the catalyst structure have not changed. To among catalyst concentration and H O concentration determine the reusability of catalyst, 5 times was repeated 2 2 variables. This effect is slightly found at interaction among experiment in the optimal conditions. Results, respectively, pH and catalyst concentration variables. Figure 17 shows a are as follows: X1 = 97.39, X2 = 97.32, X3 = 97.24, cube plot for x%. Using this plot, one could easily identify X4 = 97.20, X5 = 97.21. These results show that the conditions for reaching the desirable x%. For example, reusability of catalyst is appropriate. 123 310 Int J Ind Chem (2017) 8:297–313 Fig. 15 Main effects plot for x% Fig. 16 Interaction plot for x% NPs photocatalytic efficiency and chemical change of Conclusions 12-TSA7H O which are indicative of being effective The results revealed that: these supporting methods. 2. While supporting a-Fe O NPs on the surface of 2 3 1. Spherical a-Fe O NPs had been successfully synthe- 2 3 12-TSA7H O help to recover them from the medium sized and supported on the surface of 12-TSA7H O and reusing them, it causes to enhance their photocat- through SSD and FHRC methods with no decrease of alytic activities. 123 Int J Ind Chem (2017) 8:297–313 311 Fig. 17 Cube plot (data means) for x% 3. Nanophotocatalytic effect of a-Fe O /12-TSA7H O concentration = 0.1 ppm so that they cause to reach 2 3 2 prepared through SSD and FHRC methods on the TC maximum degradation (97.39%). degradation is greater than pure a-Fe O NPs. 9. The kinetics of photocatalytic degradation of TC is of 2 3 -1 4. As shown in the analysis of EDX, amount of iron oxide the pseudo-first order with k = 0.0098 min . supported on the 12-TSA7H O using SSD method is greater than the FHRC method. Then photo-activity of Acknowledgements The authors wish to thank the Islamic Azad catalyst that prepared with SSD method is higher than University of Arak, Iran, for financial support. FHRC method. Therefore, SSD method is more Open Access This article is distributed under the terms of the suitable. Creative Commons Attribution 4.0 International License (http://crea 5. The statistical analysis results indicated that the model tivecommons.org/licenses/by/4.0/), which permits unrestricted use, used in this paper is significantly reliable and valid. distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a 6. 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International Journal of Industrial ChemistrySpringer Journals

Published: Nov 16, 2016

References