Magnetic photocatalysts from industrial residues and TiO2 for the degradation of organic contaminants

Accepted Manuscript Title: Magnetic photocatalysts from industrial residues and TiO2 for the degradation of organic contaminants Authors: Leydiane de O. Pereira, St´efany G. de Moura, Gesiane C.M. Coelho, Luiz C.A. Oliveira, Eduardo T. de Almeida, Fabiano Magalh˜aes PII: DOI: Reference:

S2213-3437(18)30749-8 https://doi.org/10.1016/j.jece.2018.102826 JECE 102826

To appear in: Received date: Revised date: Accepted date:

18 September 2018 16 November 2018 7 December 2018

Please cite this article as: de O. Pereira L, de Moura SG, Coelho GCM, Oliveira LCA, de Almeida ET, Magalh˜aes F, Magnetic photocatalysts from industrial residues and TiO2 for the degradation of organic contaminants, Journal of Environmental Chemical Engineering (2018), https://doi.org/10.1016/j.jece.2018.102826 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Magnetic photocatalysts from industrial residues and TiO2 for the degradation of organic contaminants Leydiane de O. Pereiraa*, Stéfany G. de Mouraa, Gesiane C. M. Coelhoa, Luiz C. A. Oliveirab, Eduardo T. de Almeidac and Fabiano Magalhãesa,* a

Departamento de Química, Universidade Federal de Lavras, 37200-000 Lavras, MG, Brazil Departamento de Química, ICEx, Universidade Federal de Minas Gerais, 31270-901 Belo Horizonte, MG, Brazil c Instituto de Química, Universidade Federal de Alfenas – MG, 37130-000 Alfenas, MG Brazil b

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Corresponding authors. E-mail: [email protected]; [email protected]; phone: (+55) 35 2142 2131; Fax: (+55) 35 3829 1812

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HIGHLIGHTS Photocatalysts, TiO2/C/RM, were prepared from TiO2, tar pitch and red mud (RM). RM and Fe3O4 are in the bulk and on the surface of the carbonaceous matrix. TiO2 particles were agglomerated on the photocatalysts surface. The photocatalysts showed high efficiency to discolor the remazol black dye. The photocatalysts can be separated from the reaction by a magnetic field.

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Graphical abstract

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Abstract In this study, magnetic photocatalysts were prepared with different levels of TiO 2 supported on tar pitch and red mud (RM) (40, 60, 80Ti/C/RM), which were used as carbon (C) and iron sources, respectively. The characterization by TEM, SEM and energy dispersive X-ray spectrometry showed that the RM particles are distributed in the bulk and on the surface of the carbonaceous matrix, forming the C/RM composite. These characterizations also confirmed the presence of TiO2 particles agglomerated on the photocatalysts surface. XDR results showed that the goethite and hematite in the RM was reduced to Fe3O4 and elemental analysis and Raman spectroscopy confirmed the carbon matrix. The obtained photocatalysts showed high efficiency to discolor the remazol black B dye (RB5). The 60 and 80Ti/C/RM photocatalysts decolorized 99% of the RB5 dye in reaction with solar radiation, while sample 40Ti/C/RM discolored 83%. The reactions performed with UV light showed that the 40, 60 and 80Ti/C/UV photocatalysts, decolorized 36, 60, and 71% of the RB5 and reduce 13, 34 and 52% of total organic carbon (TOC), respectively. Magnetic separation was preceded and the characterizations of the magnetic (MF) and non-magnetic fraction (NMF) confirmed that about 25% of TiO2 did not fix in the 60Ti/C/RM photocatalyst. MF and NMF decolorized 70 and 80% of the RB5, respectively, at the photocatalytic reaction (UV light). Sedimentation kinetics showed that photocatalysts are separated faster from aqueous environment than pure TiO2. Keywords: Photocatalysis; Red mud; Tar Pitch; Titanium Oxide; Degradation.

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Introduction Advanced oxidation processes (AOPs) are based on different processes (Fenton, photo-Fenton, ozonolysis, photocatalysis and photolysis) that produce radicals derived from oxygen, especially hydroxyl radicals [1]. The high potential of radical reduction OH (Eo = 2.8 V) [1], allows this species to oxidize organic compounds present in aqueous medium. In this way, AOPs have been extensively studied in reactions for mineralization of persistent organic pollutants (POPs), forming CO2, H2O and inorganic ions [1,2]. Heterogeneous photocatalysis using titanium dioxide has been well studied since Fujishima and Honda discovered, in 1972, the photocatalytic capacity of this semiconductor in water separation [3,4]. Among the several semiconductors that can be applied in photocatalytic reactions, TiO2 is the most used, since it shows physical, chemical, and biological stabilities, high oxidative capacity, non-toxicity, and low cost [5,6]. However, one of the limitations of using TiO2 in photocatalytic reactions is the difficulty of separating it from the effluent after the treatment steps [7]. Titanium dioxide immobilized on substrates such as zeolite, silica, sand, activated carbon, magnetite, alumina, and glass spheres can improve the efficiency in the separation step of the photocatalyst from the aqueous medium [8]. The obtention of magnetic photocatalysts that can be easily recovered by the action of a magnetic field has been broadly reported in the literature [7,9,10]. These materials are obtained by supporting the photocatalyst, usually TiO2, in magnetic compounds, such as -FeOOH, Fe3O4, and -Fe2O3 [11]. Beydoun et al. [12] carried out a study that shows a negative influence on TiO2 photocatalytic activity when it is supported on the ferrite surface (Fe 3O4) due to the electronic interactions between these two oxides [12]. In order to avoid this negative effect on photocatalytic activity, several authors prepared magnetic photocatalysts supporting TiO2 on SiO2/Fe3O4 with core-shell configuration [13-15]. The inert SiO2 film that recovers the magnetic phase avoids the electronic interactions, increasing the surface area of the magnetic photocatalyst and its photocatalytic activity [16]. Another alternative to obtain magnetic photocatalysts is to support TiO 2 on carbon/magnetic iron oxide composites. The carbon used as support for TiO2 can produce a synergistic effect and improve photocatalytic activity [1719]. Thus, in this study, magnetic photocatalysts were prepared with TiO2 P25 supported on the core-shell configuration of carbon/magnetic iron oxide. Carbon and magnetic iron oxide were obtained from industrial waste, tar and red mud (rich in Fe2O3 and FeOOH), respectively. Red mud (RM) is a residue generated in the Bayer process for obtaining alumina. It is composed of hematite, goethite, calcium oxide, aluminum oxide, silicon dioxide, titanium dioxide, and sodium oxide (main chemical composition). RM is a non-noble residue available in large amounts and therefore can be used as raw material for the production of magnetic iron oxides [10]. Tar is a carbon-rich industrial waste that can be used as a source of carbon for obtaining the carbon/iron oxide composite [20]. When heated under inert atmosphere, tar undergoes carbonization, forming the carbon, which can reduce the Fe2O3 present in the red mud, forming the magnetic phase, Fe 3O4 [21,22]. The obtention of magnetic photocatalysts from TiO2, tar and red mud (Ti/C/RM) will be obtained from the thermal decomposition of tar, followed by the carbothermic reduction of the hematite and goethite (present in the RM), and finally the TiO2 will be supported in C/RM composite. The magnetic photocatalysts will be obtained with different

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TiO2 contents (40, 60, and 80% w/w), characterized by XRD, SEM, MET, Raman spectroscopy, thermal analysis (TG), CHN elemental analysis, and sedimentation kinetics. The photocatalytic activity of the obtained materials will be evaluated in reactions for discoloration of the model contaminant, remazol black B (anionic textile dye) in the presence of UV radiation (Hg lamp) and visible (sunlight).

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Experimental Materials and Reagents

Catalysts preparation

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The tar, which is produced from eucalyptus, was provided by a company from the southeastern region of Minas Gerais, Brazil and was used as a carbon source. Red mud (RM) was supplied by the company Alumínio Alcoa Brasil. The following reagents were used: Triton X-100 (99% PA, Synth), acetone (99.5% PA, Synth), TiO2 P25 (Evonik), acetylacetone (99.5% PA, Neon), and remazol black B dye (Sigma-Aldrich).

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2.2.1 Preparation of C/RM magnetic support The hematite reduction (Fe2O3) present in the RM was performed under a hydrogen flow (H 2, 100 mL min-1) for 2 h at 400 °C, using a tubular furnace [21]. In order to obtain the RM support coated with coal (C/RM), the tar residue was used as a source of carbon. The impregnation of tar on RM was performed with a ratio of 4:1 w/w (tar/RM) using acetone as solvent. After evaporating the solvent (60 °C), the material was pyrolyzed at 600 °C in an inert atmosphere (flow rate of 100 ml min-1 N2) for 1 h.

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2.2.2 Preparation of Ti/C/RM magnetic photocatalysts The procedure performed to support TiO2 on the C/RM composite was based on the methodology of Arabatzis et al. [22]. To obtain 1 g of photocatalyst with 40% TiO2, 0.4 g of this oxide was added in 1 mL of water containing 0.1 mL of acetylacetone. Water was slowly added (1.7 mL) to reduce viscosity of the formed mixture. Then, one drop of Triton X-100 was added and mixed with 0.6 g of the C/RM magnetic support. Subsequently, the resulting mixture was dried at 100 °C for 20 min and then thermally treated in a tubular furnace at 350 °C for 30 min, in air atmosphere. The photocatalysts were obtained with 40, 60, and 80% TiO2 and were named 40Ti/C/RM, 60Ti/C/RM, and 80Ti/C/RM, respectively. Characterization of the prepared materials

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Raman scattering analysis was performed using a Senterra (Bruker) equipment, with excitation laser with wavelength of 532 nm. The structure of the sample phase was identified through a Rigaku, Ultima IV model, with Cu K α = 1.54051 Å. Scans were performed between the angles 15 <2θ <80° with velocity of 4° min -1. Specific superficial area were analyzed by adsorption and desorption from N 2 at 77 K using the Autosorb1-MP Quantachrome equipment. Samples were degassed at 200 °C for 24 hours before the analyses. Transmission electron microscopy analyses (TEM) were performed on an equipment of the brand TECNAL - G200. For the morphology study of the materials, scanning electron microscopy (SEM) analyses and energy-dispersive X-ray spectroscopy (EDS) were performed using a LEO EVO 40XVP (Carl Zeiss SMT) equipment, at a voltage of 25 kV. The powder samples were fixed with double-sided carbon tape. TG curves were obtained using a thermobalance - T.A. Instruments - SDTQ600 and alumina crucible. The employed heating rate was 20 °C min-1 under a synthetic air atmosphere (100 mL min-1). The elemental analysis of the prepared materials was performed on an elemental analyzer from Leco Instrumentos LTDA - TruSpec CHNS-O model using a tin sample port. 2.4

Photocatalytic reactions

Before initiating the photocatalytic reactions, 200 mL of remazol black B dye solution 40 mg L-1 (RB5) was kept in the dark in contact with 200 mg of catalyst for 1 h for adsorption. Photocatalytic reactions were performed in a reactor equipped with magnetic stirring and low-pressure mercury lamp with emission spectrum at 254 nm (15W - Philips, I =

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0.871 mW cm-2). The germicidal lamp was placed in the reactor’s upper part at a distance of approximately 20 cm from the dye solution, and the radiation incidence area was 13 cm of diameter. The solar photocatalytic reactions were carried out on a sunny day (25 ± 2 oC). The average intensity of solar radiation was 1.420 mWcm-2. This value was obtained using a radiometer (Model SDL470) and a UVB/UVA sensor (390 to 280 nm). The UV-vis spectra in the UV-vis region and the discoloration kinetics of the RB5 dye was monitored using a UV-visible spectrophotometer (Micronal AJX-3000PC) at wavelength of 300-800 and 598 nm, respectively. Total organic carbon (TOC) (Shimadzu TOC – VCPH) measurements were obtained to evaluate the photocatalysts efficiency to degrade the RB5. To perform these analyzes, the supernatant was separated from the photocatalysts using a magnetic field (magnet) and subsequent centrifugation (when necessary). Separation of the magnetic fraction (MF) and non-magnetic fraction (NMF) of the Ti/C/RM photocatalyst

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In order to verify the presence of NMF in the prepared photocatalyst, it was added in distilled water, stirred and then a magnet was placed on the bottom of the glass to attract the MF. The dark supernatant was transferred to a beaker. This procedure was performed several times until the supernatant was clear, thus remaining only the MF. Then, the supernatant was separated from the aqueous fraction by centrifugation and the solids (MF and NMF) were oven dried (100 °C for 4h). Sedimentation kinetics

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Results and discussion

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The sedimentation kinetics of the photocatalysts was investigated through turbidity measurements of the photocatalyst suspension in distilled water. In the sample holder of the turbidimeter (HANNA INSTRUMENTS HI 98703), 5 mg of photocatalyst and 50 mL of distilled water were added and the turbidity variation was monitored at different time intervals in the presence of a magnetic field (magnet).

Characterization

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3.1.1 Raman spectroscopy The Raman spectrum of the C/RM (Fig.1) shows the D band at 1342 cm-1 due the more disorganized amorphous structure, while the G band at 1590 cm-1 indicates a more organized crystalline structure [23]. The presence of these two bands in the Raman spectrum of the C/RM support shows that the tar’s thermal decomposition formed amorphous and graphitic carbon on RM. Fig. 1

3.1.2 X-ray diffraction (XRD) The composition of the studied materials was investigated by XRD (Fig. 2).

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It is noted in Fig. 2A that the red mud consists of different minerals, such as calcite (CaCO3), muscovite (KAl2Si3AlO10(OH)2), sodalite (Na8Al6Si6O24Cl2), gibbsite (Al(OH)3), goethite (FeOOH), chantalite - CaAl2SiO4(OH)4) and hematite (Fe2O3), which were also observed by Borra et al. [24]. After the reduction of RM using H2, obtaining C/RM, it can be observed that the intensity of the diffraction lines of hematite (4) and goethite (5) decreases, and signals of magnetite (Fe3O4) are increased (6), indicating that part of Fe3+ present in RM was reduced to Fe2+. An enlarged signal present in the range between 17 and 33° is also observed in the diffractogram of C/RM support, indicating amorphous carbon formation, as shown in the Raman spectrum (Fig. 1). The signals of anatase and rutile phases are observed in the TiO2 P25 diffractogram. The highest intensity of anatase phase signals is due to its higher proportion in relation to rutile [25,26].

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The magnetic photocatalysts diffractograms are similar to TiO2 P25, which confirms the presence of this oxide. It is noted that the intensity of diffraction lines of anatase and rutile phases decreased with the TiO 2 content in the magnetic photocatalysts. The presence of iron oxides and RM constituents in these materials was not identified due to the low content of the prepared material. The diffractograms obtained for MF and NMF separated from the 60Ti/C/RM photocatalyst are shown in Fig. 3. Fig. 3

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The diffractograms obtained for NMF and MF have diffraction lines related to TiO 2 phases, indicating that it is present in both samples. These results show that a fraction of TiO2 used in the synthesis of the magnetic photocatalyst was not fixed to the C/RM support. For this reason, it was possible to separate the NMF from the Ti/C/RM photocatalyst using a magnet.

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3.1.3 Specific surface area In Table 1 are presented the values of BET specific surface area, volume and average pore diameter obtained for the magnetic photocatalysts.

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The surface area, volume and average pore diameter values obtained for the photocatalysts, 40, 60 and 80Ti/C/RM, increased with the TiO2 content. These increases are related to the TiO2 P25 used to obtain the magnetic photocatalysts. The low surface area obtained for these materials, especially for the 40Ti/C/RM photocatalyst, is certainly related to the low surface area of red mud (ca 14 to 20 m2 g-1) [28-30] and of the coal, which was obtained by thermal decomposition of the tar and was not activated. These results showed that these values are strongly dependent on the TiO2 content in the photocatalysts. Scanning Electron Microscopy and energy-dispersive X-ray spectroscopy TEM images from the C/RM support (Fig. 4) show particles smaller than 0.5 μm. In Fig. 4A, a lighter shade covering a dark area of the sample can be observed, which are constituted by carbon and RM, respectively. In Fig. 4B, it can be clearly seen that RM particles are distributed in the bulk and the surface of the carbonaceous matrix, forming the C/RM composite.

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The morphology of the studied materials was investigated by SEM. The images obtained for RM, C/RM, 60Ti/C/RM, NMF, and MF are shown in Fig. 5. Fig. 5

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The results obtained for samples C/RM, 60Ti/C/RM, MF and NMF show great difference to the RM morphology. In the RM images, it can be observed that red mud has agglomerated particles with irregular size, shape and distribution, which corroborates with the results from other authors [24,31]. On the other hand, in the image of C/RM support, the presence of larger particles is noted with indefinite forms and with more regular surface, suggesting the carbon formation. Image D shows a larger magnification of C/RM, in which a carbon block can be observed with several smaller particles on its surface. These particles are possibly from RM not completely covered by the carbon matrix that remained on its surface. The micrograph of the photocatalyst also differs from the RM. The presence of larger particles can be observed with more regular surface, which indicates carbon of the C/RM composite. Smaller particles of rounded shape distributed on the surface, which is certainly TiO2 supported in C/RM, are also observed (Fig. 5F). In Fig. 5G, the presence of smaller particles of rounded form is observed, which are certainly TiO 2 unsupported on the surface of the C/RM composite. These particles were separated from the 60Ti/C/RM photocatalyst and called NMF, being analyzed separately from the MF (60Ti/C/RM without excess TiO2). Differences in morphology are observed when comparing the NMF and MF images. In the images G and H, NMF is composed of rounded particles and larger agglomerates with irregular shape, which indicates that NMF is composed mainly by TiO2 that did not bind or

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leach from the 60Ti/C/RM photocatalyst. On the other hand, MF images show larger particles with a more regular surface, indicating C/RM with supported TiO2 agglomerates. These results strongly suggest that part of TiO2 did not bind to the surface of C/RM support during the magnetic photocatalyst, and it was magnetically separated from the 60Ti/C/RM. Fig. 6 shows EDS images with point and mapping analysis for the RM, C/RM, 60Ti/C/Fe, NMF, and MF samples. Fig. 6 The atomic percentages of elements that compose the materials are show in Table 2.

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Table 2

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Thermal (TG) and elemental analysis (CHN) analysis Fig. 7 shows the TG curves obtained for materials prepared from RM.

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According to the results obtained for the atomic percentage of elements, the surface of these materials is relatively homogeneous, with high content of iron and carbon. These results are confirmed by the image of mapping B and D, which shows a homogeneous distribution rich in iron and coal in RM and C/RM, respectively. EDS results and mapping obtained for the 60Ti/C/RM photocatalyst show that this material consists of coal blocks containing TiO2 agglomerates (red dots) distributed on its nonhomogeneous surface. On the other hand, on RM, iron is distributed over the entire surface of the material (yellow dots). EDS results from Table 2 confirm these results, where points 1 and 2 of image E are constituted mainly of TiO 2 (84%) and coal (87%), respectively. EDS results for NMF show a high percentage of titanium at points 1 and 2, confirming being TiO2 separated from the 60Ti/C/RM photocatalyst. This result can be clearly observed in image H, where it is noted that most of the sample consists of titanium with some small points containing iron and coal. On the other hand, the results obtained for MF are similar to those obtained for 60Ti/C/RM photocatalyst, where it can be observed sample regions rich in titanium and others in coal (points 1 and 2, Fig. 6I, respectively).

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The three mass losses observed in the TG curve of RM (Fig.7A) are related to the dehydration, decomposition and dehydroxylation processes of the minerals (gibbsite - Al(OH)3, goethite - FeOOH, calcite - CaCO3, and chantalite CaAl2SiO4(OH)4) present in the sample, respectively [32]. The first event in c.a. 100 °C is observed for all materials and is related to water loss. The second event observed between 220 and 320 °C is due to the decomposition of gibbsite and dehydroxylation of goethite (Equations. 1 and 2), [32]. The third mass loss observed in c.a. 650 ºC is due to the decomposition of calcite and dehydroxylation of chantalite (Equations. 3 and 4), [32]. 2Al(OH)3  Al2O3 + 3H2O (1) 2FeOOH  Fe2O3 + H2O (2) CaCO3  CaO + CO2 (3) CaAl2SiO4(OH)4  CaOAl2O3SiO2 + 2H2O (4) The thermal analysis results of the support C/RM of the magnetic photocatalysts (Ti/C/RM) and the magnetic (MF) and non-magnetic (MFN) fractions show a mass loss between 360 and 560 ° C, which is related to the oxidation of the deposited carbon on the RM surface, as shown in Eq. 5. Cdeposited + O2 → CO2 (5) In Table 3 are presented the percentages of carbon and residue obtained by thermogravimetry, multipoint energy-dispersive, X-ray spectroscopy and elemental analysis for the studied samples. These results show small differences among the amounts of carbon obtained by CHN, TG, and EDS (qualitative) analyses. It can be observed that the prepared C/RM support shows on average 57% of carbon, higher than that obtained by Magalhães and Lago [20], who prepared C/Fe3O4 composites using tar and obtained about 31% of carbon. Table 3 As expected, it is noted that the carbon content in the magnetic photocatalysts decreases as TiO 2 content increases. The results obtained for NMF are also in agreement with the expected results, since the percentage of carbon present in

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the NMF is low (4.5 to 6%) and the residue content is high (92%), confirming that this sample is rich in TiO2 separated from 60Ti/C/RM. On the other hand, MF shows percentage of carbon (30.8 to 35%) higher than NMF. These results are in agreement with those obtained by SEM and mapping, confirming that after the magnetic separation of the 60Ti/C/RM sample, NMF consists of high TiO2 content and MF by TiO2, carbon and RM. TiO2 contents present in the 40, 60, and 80 Ti/C/RM and MF photocatalysts calculated by the thermal analysis results are 39, 60, 76, and 35%, respectively. 3.2

Photocatalytic reactions performed with magnetic catalysts

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Fig. 8 shows the structural formula of the dye molecule RB5 (Fig. 8A), the UV-vis spectra of the results of photocatalytic reactions for discoloration of RB5 using 80Ti/C/ RM (Fig. 8B) and after 240 minutes of reaction using 20, 40 and 80Ti/C/RM (Fig. 8C). Fig. 8

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The UV-Vis spectrum of the RB5 dye presents four molecular absorption bands at 310, 400, 500 and 598 nm, with the latter being the most intense one (Fig. 8B). Therefore, the absorbance at 598 nm was used to monitor the discoloration of RB5 (Fig. 8A) during the photocatalytic reactions and adsorption test. According to Puentes-Cardenas et al. [33], the maximum in the 310 nm region represents the presence of naphthalene groups and the N = N bond of the azo group is manifested in the 598 nm region. The discoloration of the solution is due to the breakdown of the N = N and C-N bonds by the radicals formed during photocatalysis [33]. It was also observed that the absorbance of RB5 decreases with the reaction time, indicating that the dye is being oxidized by the hydroxyl radicals causing its discoloration and later its mineralization. The absence of new absorption bands in Figs. 8B and 8C indicates that the reaction intermediates do not significantly absorb radiation in the region between 300 and 800 nm. In Fig. 8C, it is clear that the magnetic photocatalysts showed good efficiency to discolor the dye and efficiency increased with the TiO 2 content in the photocatalyst. Fig. 9 shows the adsorption and kinetics results of RB5 discoloration during photocatalytic reactions using the photocatalysts 40, 60 and 80Ti/C/RM (Fig. 9A), as well as the values of the rate constants (k), the percentage of discoloration of RB5 and removal of TOC (Fig. 9B).

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Before the photocatalytic reactions, an adsorption study was performed, where the catalysts were kept in contact with the dye solution for 1 h to eliminate the effect caused by adsorption. The materials 40, 60, and 80 Ti/C/RM adsorbed about 0, 1 and 1.7% of RB5, respectively (Fig 9A). This low adsorption is certainly related to the low surface area of the photocatalysts. When UV light was switched on, color removal of RB5 dye was faster and the photocatalytic efficiency of catalysts increased with the TiO2 content (Fig. 9A), where the 40, 60, and 80Ti/C/RM photocatalysts discolored about 36, 60, and 71% of RB5 in 240 min of reaction, respectively. On the other hand, there was no discoloration in the white reaction (dye + UV). The linearity of these results indicates zero pseudo-order kinetics and the values of the discoloration rate constant (k) are shown in Fig. 9B. It is observed that the k values and dye discoloration increase similarly with the TiO2 content of the photocatalysts. It is also noted that the reactions performed with 80Ti/C/RM and 60Ti/C/RM were 1.83 and 1.64 times faster than the reaction with the 40Ti/C/RM photocatalyst, respectively. The efficiency of the magnetic photocatalysts to mineralize the RB5 dye was evaluated by the TOC reduction (Fig. 9B). It is noted that the efficiency increased with increasing TiO 2 contents present in the photocatalysts. The composite 80Ti/C/RM reduced 52% of the TOC and 71% of the color. In Fig. 9B it is clear that the TOC removal values are lower than those of discoloration. This result is observed because when •OH radicals react with the dye molecule, other chemical species (oxidation intermediates) are formed, which may be colorless and do not absorb the radiation at visible wavelengths (Fig. 8 and Eq. 6). After several steps, the formed intermediates are completely converted into CO 2, H2O and inorganic ions (Eq. 7), resulting in organic matter mineralization. Thus, the discoloration step is faster than the mineralization. Discoloration: Organic dye + •OH → intermediates (6) Mineralization: Intermediates + •OH →→ + CO2 + H2O + inorganic ions (7) These results show that the magnetic photocatalysts presented good efficiency not only to discolor the RB5 solution but also to mineralize it.

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Photocatalytic reactions performed with MF and NMF separated from the 60Ti/C/RM photocatalyst

Fig. 10 presents the results of photocatalytic reactions performed for discoloration of RB5 dye (A), and NMF (B) and MF (C) images in the presence of magnetic field. Fig. 10

Reactions performed using solar radiation

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These results indicate that NMF and MF samples showed high photocatalytic efficiency (80 and 70%, respectively) in the reactions for the RB5 discoloration. The linearity of the points shown in the graph (Fig. 10A) allows assuming that these reactions have pseudo-zero order kinetics. The values of the rate constants obtained for these reactions (Fig. 10A) are very close and confirm that the TiO2 present in MF (from 60Ti/C/RM) has photocatalytic activity. According to Beydoun et al. and Gad-Allah et al. [12,14], the direct contact of titanium dioxide and iron oxide (TiO2/Fe3O4) results in a material with low photocatalytic activity. This was not observed for the photocatalysts prepared in this work. The similar efficiency presented by MF (from 60Ti/C/RM) and NMF (TiO2) for the RB5 discoloration show that the carbon in the magnetic photocatalyst plays an important role to preserve or improve the TiO2 photocatalytic activity. The amorphous carbon in the magnetic photocatalysts acts as an inert phase and avoids the contact and electronic interactions between TiO2 and Fe3O4. Furthermore, graphite and TiO2 combination can increase the photocatalytic efficiency due to the electronic transfer of the electron from the TiO 2 conduction band to the graphitic phase (electronic conductor, electron trap), which results in an increase in the life time of the e-/h+ pair [18]. As observed in the Raman spectroscopy results, the amorphous and graphitic carbon phases present in the C/RM precursor contribute to the high photocatalytic activity of the obtained magnetic photocatalysts.

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Fig. 11 shows the UV-vis spectra obtained after 240 min of reaction (Fig. 11A) and the kinetics results (Fig. 11B) of RB5 dye discoloration obtained during the photocatalytic reactions performed with magnetic and sunlight photocatalysts. Fig. 11

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After 240 minutes of reaction, there was a large reduction in the absorbance of the UV-vis spectra obtained for the RB5 solutions (Fig. 11A). As observed in the spectra of Fig. 8, these results did not present new absorbance bands between 300 and 800 nm, indicating that there was no formation of intermediates that absorbed the radiation significantly in this interval. The results of the RB5 discoloration kinetics (Fig.11B) showed that there was no discoloration of the dye in the white reaction (dye + sunlight). However, the reactions using the photocatalysts showed excellent efficiency, where the photocatalysts 60Ti/C/RM and 80Ti/C/RM showed similar and better results than the 40Ti/C/RM. With only 90 min of reaction, the 60Ti/C/RM and 80Ti/C/RM photocatalysts discolored 99% of RB5 dye, while the 40Ti/C/RM photocatalyst discolored only 83%. These results show that the reactions performed with the magnetic photocatalysts for RB5 discoloration showed different discoloration kinetics of reactions performed with artificial UV light (pseudo-zero order). The reactions performed with sunlight showed that the discoloration of RB5 was faster in the first minutes (c.a. 30 min), which suggests pseudo-first order kinetics in relation to the dye concentration. The graph of Figure 12A showed linear results after the kinetic treatment (pseudo-first order), allowing to calculate the rate constant of degradation of the RB5 dye, which was obtained by the slope of the straight line. Figure 12B shows the graph with the k values as a function of the TiO2 content. Fig. 12 It can be observed from the rate constants that the photocatalytic activity of the materials in the presence of sunlight increased with the TiO2 content. There was no increase in the reaction speed above 60% TiO2 in the photocatalyst, which suggests that this content would be ideal for the synthesis of the magnetic photocatalyst. Figure 13 shows the percentages of RB5 discoloration in reactions performed with sunlight and artificial light (15 W Hg lamp). Fig. 13

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It is observed that the photocatalysis in the presence of sunlight showed catalytic activity better than the photocatalysis in the presence of artificial light, discoloring a higher percentage of dye in less reaction time. This can be explained by the greater intensity of solar radiation (UVA and UVB), which was 1.420 mWcm-2 (average value), while the average intensity of radiation emitted by UV lamp of 15 W is 0.871 mWcm-2. Although it is reported in the literature [34] that only 5% of the solar spectrum that reaches the surface of the Earth comes from UVA and UVB light, this radiation was sufficient to activate the TiO2 semiconductor and to discolor the RB5 dye with high efficiency. These results show that the use of solar radiation is interesting, since it reduces the reaction time and the energy cost for the treatment when compared to the use of an artificial light bulb. In Table 4 are presented the experimental conditions and photocatalytic efficiency of photocatalysts similar to those prepared in this work with different experimental conditions, used to degrade different types of contaminants.

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Table 4

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The photocatalysts presented in the above table show photocatalytic efficiency ranging from 14.5 to 95.6% to degrade/discolor different contaminants. In the studies of Zhang et al. [36] and Aghamali et al. [35], the authors used coal to coat Fe3O4, as performed in this work. Therefore, when comparing the efficiency of the photocatalyst Fe 3O4/C/TiO2 /N-CQDs with those obtained in this work, a great similarity can be observed when the reactions were carried out in the presence of solar radiation. When reactions were carried out in the presence of UV radiation (15 W Hg lamp), the efficiency of the photocatalysts 60 and 80Ti/C/RM was c.a. 4.3 and 5 times higher, respectively, than the efficiency of the Fe3O4/C/TiO2/N-CQDs photocatalyst. On the other hand, the Fe3 O4@C@TiO2 photocatalyst prepared by Zhang et al. [36] showed higher efficiency than Ti/C/RM photocatalysts, discoloring 95.6% of the methylene blue dye solution in only 8 minutes of reaction. This high efficiency obtained in a few minutes is certainly related to the high power of the Hg lamp (500W), volume of methylene blue solution and mass of catalyst used (20 mL and 10 mg) in the experiments. It is important to note that the experimental conditions used by Aghamali et al. are the ones that are closest to those used in this work, but the photocatalyst Fe3O4/C/TiO2/N-CQDs was used in a very different photoreactor (cylindrical reactor provided with aeration with oxygen source). Thus, taking into consideration the procedures used to obtain the photocatalysts described in the literature, it can be said that those obtained from red mud and tar presented in this work, have high photocatalytic efficiency in the presence of sunlight and UV (Hg lamp). Moreover, they were obtained from industrial waste, which adds value and contributes to green chemistry efforts. Sedimentation kinetics

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Fig. 14 shows the sedimentation kinetics of TiO2, 40, 60, and 80TiC/RM photocatalysts and MF in the presence of a magnetic field. Fig. 14

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It is observed in Fig. 14 that the pure TiO2 did not deposit during the 60 min. On the other hand, it is noted that the 40, 60, and 80Ti/C/RM photocatalysts deposited about 74, 70, and 60%, respectively. The results follow the expected trend because the higher the percentage of TiO2 in the photocatalyst, the slower and less efficient the sedimentation, since the magnetism decreases as TiO2 increases. After 40 min, there was no significant reduction of turbidity, indicating that part of the TiO2 did not attach to the C/RM support, remaining in suspension. On the other hand, the MF sedimentation test showed that the sedimentation of almost all the material (99%) occurred in only 1 min. This result corroborates with those previously obtained and confirms that a fraction of TiO 2 did not attach to the C/RM support. These results confirm the efficiency of the incorporation of TiO 2 in the C/RM magnetic support in order to obtain a material with excellent photocatalytic activity and magnetic properties that allow its rapid separation from the aqueous medium in the presence of a magnetic field. This implies a cost reduction, since it is possible to reuse the photocatalyst and recover it in a simpler way.

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Conclusion The results obtained for the characterization of Ti/C/RM magnetic catalysts showed that the procedure used for its synthesis was efficient. The XRD results showed hematite and goethite phases in RM were reduced to magnetite after

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the heat treatment in the presence of H2 and later with tar. The results obtained by TEM, SEM, energy dispersive X-ray spectrometry showed that the RM particles are distributed in the bulk and the surface of the carbonaceous matrix. The reactions performed with the 40, 60 and 80Ti/C/RM magnetic photocatalysts showed excellent results to discolor the reactive black dye 5 and for the removal of TOC, proving the efficiency of the prepared materials. However, the removal of a non-magnetic fraction (NMF) present in the 60Ti/C/RM photocatalyst showed that part of the TiO 2 did not attach to the C/RM support. However, the magnetic fraction (60Ti/C/RM without unsupported TiO2) did not lose its photocatalytic activity. The magnetic properties of these photocatalysts allowed them being easily separated from the reaction medium by the simple application of a magnetic field. These results evidence that the synthesis of photocatalysts from the industrial residues, red mud and tar, was viable in order to obtain magnetic materials with high photocatalytic activity. Thus, it can be concluded that these materials have high potential to be applied in reactions for environmental remediation.

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Conflict of interest The authors declare no conflict of interest.

Acknowledgment The authors are grateful to CNPq, FAPEMIG and UFLA for the financial support and to the Scanning Eletronic Microscopy Laboratory of UFLA, to the Electronic Microscopy Center of the UFMG and to the Cristalography and Elementary Analysis Laboratories of UNIFAL-MG for the characterization analyzes.

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References

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[40] Y. Fan, C. Ma, W. Li, Y. Yin, Synthesis and properties of Fe 3O4/SiO2/TiO2 nanocomposites by hydrothermalsyntheticmethod. Mater. Sci. Semicond. Process. 15 (2012) 582-585. http://dx.doi.org/10.1016/j.mssp.2012.04.013.

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[42] A. Kumar, M. Khan, X. Zeng, I. M.C. Lo, Development of g-C3N4/TiO2/Fe3O4@SiO2 heterojunction via sol-gel route: A magnetically recyclable direct contact Z-scheme nanophotocatalyst for enhanced photocatalytic removal of ibuprofen from real sewage efluente under visible light. Chem. Eng. J. 353 (2018) 645–656. https://doi.org/10.1016/j.cej.2018.07.153

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Figure captions

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Fig. 1. Raman spectrum of the C/RM support.

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Fig. 2. Diffractograms obtained for the materials: A) C/RM and RM and B) TiO 2 P25 and 40, 60, and 80Ti/C/RM photocatalysts. (1= Sodalite, 2= Calcite, 3= Muscovite, 4= Hematite, 5= Goethite, 6= Magnetite, 7= Gibbsite, 8= Chantalite, 9= Anatase, 10= Rutile).

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Fig. 3. Diffractograms obtained for the 60Ti/C/RM, MF and NMF materials. (1= Anatase, 2= Rutile).

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Fig. 4. Images obtained by TEM from C/RM support.

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Fig. 5. Images obtained by SEM for RM, C/RM, 60Ti/C/RM, NMF, and MF with magnification of 500 and 1500x.

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Fig. 8. A) Structure of the dye molecule RB5. B) UV-vis spectra during photocatalytic reactions for discoloration of RB5 using 80Ti/C/RM and C) 20, 40 and 80Ti/C/RM after 240 min of reaction.

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Fig.9. A) Adsorption and photocatalysis reactions for discoloration of RB5 (50 mg L -1) dye using 40, 60, and 80 Ti/C/RM photocatalysts. B) Values of the discoloration rate constant of RB5 (k), percentage of color and TOC removal as a function of the TiO2 content in the catalyst.

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Fig. 10. Photocatalytic reaction using MF and NMF to discolor RB5 dye (A), and illustrative figure of NMF (B) and MF (C) in the presence of magnetic field (magnet).

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Fig. 11. A) Espectros de varredura no UV-vis após as reações fotocatalíticas com radiação solar para descoloração do RB5 utilizando os fotocatalisadores 20, 40 e 80Ti/C/RM. B) Cinética de descoloração do RB5 utilizando todos os fotocatalisadores e a reação branco.

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Fig. 12. A) Graph of the kinetic treatment performed for solar photocatalysis reactions of RB5 dye using 40, 60, and 80Ti/C/RM photocatalysts. B) Values of k as a function of TiO2 content in the photocatalyst.

Fig. 13. Percentage of RB5 dye degradation using sunlight and artificial light (15 W Hg lamp).

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Fig. 14. Sedimentation kinetics of TiO 2 and 40, 60, and 80Ti/C/RM photocatalysts and MF in the presence of a magnet.

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Tables captions

Table 1. BET surface area, pore volume and average pore diameter of the photocatalysts. Table 2. Qualitative chemical analysis by ESD of materials at points 1 and 2 according to Fig. 6. Table 3. Carbon contents in the studied materials - values obtained by CHN, TG, and EDS.

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Table 4. Experimental conditions and photocatalytic efficiency of photocatalysts of articles found in the literature and the materials presented in this work.

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Table 4. BET surface area, pore volume and average pore diameter of the photocatalysts. BET Average Pore Sample surface area pore diameter 3 -1 volume (cm g ) (m2g-1) (Å) 40Ti/C/RM 18 0.290 328.0 60Ti/C/RM 39 0.310 509.3 80Ti/C/RM 58 0.402 501.9 TiO2 P25* 57 0.570 402.0 * Phongamwong et al. [27]

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Table 5. Qualitative chemical analysis by ESD of materials at points 1 and 2 according to Fig. 6. Mate rial

Point C

RM

C/R

1

11

2

0

1

82

2

79

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Elements (%Atomic) F T O e i thers* 4 2 46 1 5 3 40 7 1 0 8 0 8 1 12

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Table 6. Carbon contents in the studied materials - values obtained by CHN, TG, and EDS. Amount of carbon in samples Residue amount (%) (%) Sample s CHN E T TG DS G RM 1.6 5 -85 C/RM 55.4 58 59 38 40Ti/C/ 33.8 33 62 RM 60Ti/C/ 23.6 18 20 77 RM 80Ti/C/ 11.2 12 84 RM MF 30.8 30 35 59 NMF 4.5 6 5 92

27

Fe3O4/SiO2/TiO2 Fe3O4/SiO2/TiO2 Fe3O4/SiO2/TiO2 Fe3O4/SiO2/TiO2

Methyl orange Methyl orange (50 mgL-1)

Ibuprofen from real sewage effluent

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80 [ 35] 80

Hg/500W

95.6

8

UV/12 W

88.8

30

UV/6 W

70

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95.5

Hg /45 W

50

Visible light

15

Xenon/300 W

66

UV/15 W Visible light source, consisting of eight compact fluorescent lamps (8 W)

93.5 92

[36] [37]

14 00

[15] 12

0

[38] 32

0

[39] 30

[40]

12 0

[41] 18

0

[42]

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C3N4/TiO2/Fe3O4 @SiO2

Methylene blue (6,4 mgL-1) Olive mill wastewater Methylene blue (10 mgL-1)

14.1

Sunlight

N

Fe3O4/SiO2/TiO2

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Fe3O4@SiO2@Ti O2/rGO

Methylene blue (10 mgL-1) 2,4-Dinitrophenol (40 mgL-1)

M

Fe3O4@C@TiO2

Hg/15 W Methylene blue (20 mgL-1)

ED

Fe3O4/C/TiO2/NCQDs

IP T

Table 7. Experimental conditions and photocatalytic efficiency of photocatalysts of articles found in the literature and the materials presented in this work.” Re Photocata action R Catalyst Contaminat Light/power lyst Eficiency time ef. (%) (mi n) 24 36 0 40Ti/C/RM 83 90 24 T 60 Remazol Black B Hg/15 W 0 60Ti/C/RM his -1 (40mgL ) Sunlight work 99 90 24 71 0 80Ti/C/RM 99 90

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