TiO 2 Immobilized on Fibrous Clay as Strategies to Photocatalytic Activity

2020 TiO 2 immobilized in Sepiolite (TiO 2 /Sep) was successfully prepared by the sol-gel technique, with titanium isopropoxide as the precursor for the formation of TiO 2 in the anatase phase calcined at 400 °C. The prepared samples were characterized by X-ray diffraction, Fourier Transform Infrared spectroscopy, Scanning Electron Microscopy coupled to energy dispersive spectroscopy, and thermogravimetric analysis. The results showed that TiO 2 /Sep structure was identified in all characterizations, showing the specific peaks, bands, mass loss, and morphology after the impregnation process. Photocatalytic experiments were performed under UV irradiation with various photocatalyst concentrations and pH effects in the reaction. The prepared samples presented 72% photocatalytic efficiency for eosin (EA) dye discoloration after 150 min under UV light. This efficiency was attributed to the radicals generated from the TiO 2 and the high specific surface area, showing that TiO 2 /Sep is promising candidate in the degradation of organic


Introduction
Among the anthropogenic activities that have contributed to the degradation of water quality, industrial activity is undoubtedly one of the main sources of pollution 1,2 . Synthetic dyes are extensively used in various textures, in the production of leather, cosmetics, paper, food, and pharmaceuticals, among others 3 .
Among them, eosin (EA) is an anionic dye (C 20 H 6 O 5 Br 4 Na 2 , 2,4,5,7-tetrabromofluorescein) widely used in dyeing, printing, leather, and fluorescent pigments to offer reddish coloration on wool fabrics [4][5][6] . Toxicological information shows that it may cause irritation to the skin and eyes, and ingestion may have several adverse effects on vital organs such as liver and kidneys. Dye inhalation reduces gaseous exchange in the lungs, and its metabolites are highly toxic and carcinogenic 7,8 .
Ecologically correct and economically viable technologies have guaranteed the quality of the water resources through the degradation or immobilization of these compounds in effluents 9,10 . Advanced oxidative processes (AOPs) have been especially highlighted for their ability to degrade a large number of recalcitrant substances through procedures that require only simple and low-cost operations 11,12 . Among photochemical AOPs, photocatalysis is an emerging technology for the degradation of organic pollutants 13 . There are several advantages over competing processes, such as complete mineralization, ambient temperature and pressure conditions, and, above all, a reduction in environmental impact for solid waste disposal 3,14,15 .
Based on the photocatalytic scope, the application of clay-based materials as carriers and/or heterostructures with semiconducting oxides [16][17][18][19] , for example, TiO 2 (titania) has attracted growing interest due to its properties of low toxicity, low cost and high photocatalytic efficiency, are being used as reference and/or standard photocatalysts because they combine attractive properties like chemical stability, high oxidizing power and nontoxicity [20][21][22][23] . However, despite the positive aspects, there are some limitations that hinder its practical application as the low surface area of TiO 2 and extremely thin thickness causing difficulty in the separation, recovery and reuse of particles and/or nanoparticles in aqueous solution 15,[24][25][26] . Thus, TiO 2 clusters become unfavorable to photocatalytic process 27 .
Modified clay materials have recently been used in the treatment of various polluting compounds 3,14,18,[28][29][30] to allow alterations in the semiconductor structure that favor charge separation mobility and, consequently, the yield in a photocatalytic oxidation process 18 . In addition, clay minerals are promising candidates for support due to their strong mechanical-chemical stability, cost-effectiveness, good adsorption capacity, which minimizes TiO 2 nanoparticle aggregation, inducing enhanced photocatalytic performance 15,31,32 .
Researchers have been preparing photocatalysts supported from various types of mineral clay 15 percentages were 99% and 78%, respectively, after only 90 min of irradiation under UV light and sunlight, indicating the efficiency of TiO 2 /clay due to the stability of the anatase phase that simultaneously attacked the RB19 dye.
In particular, sepiolite is a naturally occurring phyllosilicate with a fibrous morphology found in a wide variety of geological environments and has been extracted for centuries because of its useful properties, such as good adsorption capacity and non-toxicity 35 . In addition, sepiolite is prominent in the photocatalytic field due to its high surface area, around 210 m 2 /g, which can be used as photocatalytic support and its morphology 19,36 , composed of molecular-sized structural channels growing in the direction of the fibers 15,19 . Another relevant characteristic is the acidic centers [SiO 4 ] in their structure, capable of structurally modifying the compounds and easily to the degradation process 15,19,37 .
In relation to photocatalysis and the use of sepiolite, Liu et al. 21 , synthesized a new composite called AgTiS (Ag+TiO 2 +sepiolite) and systematically studied, under UV and visible light, methyl orange degradation. According to the results, TiO 2 , Sep, TiO 2 /Sep, and AgTiS showed higher photocatalytic activity due to several factors, such as the band gap energy, charge separation, pore distribution, and higher adsorption capacity. In this same sense, Zhou et al. 15 evaluated the degradation of the orange G dye using nanocomposites of the TiO 2 and Sep.
In this sense, this work uses a kind of derived-clay material to evaluate the photocatalytic oxidation process of eosin y anion dye under UV irradiation. TiO 2 was immobilized onto fibrous support and has been used for synthesize TiO 2 /Sep functional material for application as a photocatalyst. In the synthesis, sepiolite (Mg 8 Si 12 O 30 (OH) 4 (OH 2 ) 4 . nH 2 O; n ≤ 8) was deagglomerated and underwent treatment for removing impurities and organic matter, as described by Almeida et al. 38 . The immobilization of titanium dioxide in the clay was carried out by the sol-gel method. For the synthesis, 6.0 mL (0.02 mol) of titanium isopropoxide Ti[OCH(CH 3 ) 2 ] 4 and 6.0 mL (0.33 mol) of H 2 O were mixed and then diluted in 100.0 mL (1.72 mol) of ethyl alcohol (C 2 H 6 O), to obtain the sol phase. Then the mixture was magnetically stirred for 1 h. In another beaker, 5.0 g sepiolite and 20.0 mL of ethanol were mixed and magnetically stirred for 30 min. The two solutions were then mixed in a single vessel and allowed to stand for 24 h. After this process, the obtained material was dried at 75°C for 12 h in an oven, deagglomerated and calcined in a muffle furnace at 400°C for 2 h with a heating rate of 10 °C min -1 . The obtained material was stored and represented by TiO 2 /Sep. The TiO 2 /Sep ratio was 1:5 (m/m), considering the amount used in the synthesis.

Photocatalytic activity
The photocatalytic activity tests were performed in a photochemical reactor containing 180.0 mL of dye solution under magnetic stirring at 700 rpm. This reactor was coupled to a thermostatic bath (TECNAL-TE-2005) at 25.0 ± 1.0 °C. The radiation source used was a 125 W mercury without bulb 39 , which intensity was monitored using a radiometer (HANNA -HI 97500, Luxmeter), obtaining a value of 10.00 ± 1.00 μW cm -1 . Samples were withdrawn at time intervals of 0, 5, 10, 15, 30, 45, 60, 90, 120 and 150 minutes of irradiation. The discoloration activity was monitored using a CARY 300 model UV-Vis spectrophotometer in the 200-800 nm range. The photocatalytic tests were performed with 0.5, 1.0, and 1.5 g L -1 of the catalyst. Changes in the EA concentration were monitored at 516 nm, the maximum wavelength region of the dye 7 . Also, the effect of pH on the degradation of the dye at 2.0x10 -5 mol/L was tested at three distinct pH values (4.0, 7.0 and 10.0) by the addition of NaOH and/or HCl.
The tests were done with an angular range of 3-75° (2θ) and a sweep rate of 2° min -1 in continuous mode. Fourier transform infrared spectroscopy (FTIR) (Bruker VERTEX 70) in transmittance mode, in the 4000-400 cm -1 region, 64 scans and 4 cm -1 resolution was carried out on the powders. The textural properties of the solids were investigated by adsorption-desorption analysis of N 2 at 77 K using Quantachrome Autosorb-iQ Instruments. Thus, before each analysis, about 200 mg of the sample was degassed for 4 hours at 200 °C. The surface area was calculated by the Brunauer-Emmett-Teller (BET) method and to estimate the volume and pore diameter the Barrett-Joyner-Halenda (BJH) method was used from the N 2 adsorption curves. Thermal stability analysis of the clay and compound was performed on a TA Instruments model SDT Q600. Between 5.0 and 7.0 mg of sample were used for the tests. The materials were heated at 10 °C min -1 to 900 °C under a 50 mL min -1 argon flow. Morphological analysis was performed using coupled scanning electron microscopy-dispersive energy spectroscopy (SEM-EDS); the tests were carried out on a field-emitting electron-emitting device (SEM-FEI, FEG-250).

Characterization
The XRD diffractograms of sepiolite (a) and TiO 2 /Sep (b) are presented in Figure 1. The sepiolite showed reflections on 2θ = 7.44º, 12.06º, 13 [42][43][44] . The most intense and characteristic diffraction peak of TiO 2 in the anatase phase was detected at 25.30º and was attributed to the (101) plane. As can be seen from the plane overlays, the crystallinity of clay decreased compared to the material with TiO 2 immobilized, although it maintains the crystallographic profile of the sepiolite, ensuring incorporation of the oxide into the clay structure, as identified by the XRD technique. Figure 2 presents the FTIR of sepiolite and TiO 2 /Sep. The bands between 3700 and 3400 cm -1 are attributed to the presence of different types of hydroxyl presented in the water molecules of mineral structure (adsorbed and zeolite water) and to the OH group octahedral coordinated with Mg and the surface. The bands between 3700 and 3570 cm -1 correspond to the vibration elongation of hydroxyl groups (OH) attached to the octahedral sheet with Mg ions located in the channels of the sepiolite and to elongation vibrations at the clay surface. A characteristic band at 3574 cm -1 is attributed to water-coordinated Mg (Mg-OH stretching vibration) 36,40,45 . The OH group at around 1669 cm -1 refers to the presence of zeolite water in the nanostructured channels of the clay. The bands at 690 cm -1 and 440 cm -1 are respectively attributed to the presence of hydroxyl (OH) group elongation vibrations bound to the octahedral sheet with magnesium ions and the vibrations of Si-O-Mg bonds. Bands between 1031 and 1400 cm -1 are characteristic of silicates and appear due to the vibration of Si-O-Si bonds 40,45 .
When comparing the sepiolite and TiO 2 /Sep spectra, bands appear at 3421 and 1639 cm -1 , promoted by stretching vibrations through the interaction of water molecules on the surface of the oxide. The disappearance of the bands at 3618 and 3557 cm -1 can be explained by the immobilization of the TiO 2 particles in the clay by condensation of the hydroxyl groups of the titanium precursor with the hydroxyls of the clay occurring during thermal treatment at 400°C, corroborating with Zhang et al. 36 .
The N 2 gas adsorption and desorption isotherms of the Sep and TiO 2 /Sep samples at 77K are shown in Figure 3, where P/P o represents the relative vapor pressure of N 2 and V is the adsorbed/desorbed N 2 volume (Figure 3).
It is observed that the isotherms for both materials exhibit similar behavior. According to the classification determined by the International Union of Pure and Applied Chemistry (IUPAC), the materials presented type IV isotherms, which are defined as mesoporous materials (corroborating the pore distribution, with pore diameter from 2 to 50 nm). A small amount of macropores can also be attributed to the porosity generated by intraparticle grains 46 . Besides, it was observed a deviation in the desorption curves, which is called hysteresis, a phenomenon caused by capillary condensation. This hysteresis consists of the phase shift of N2 (from gas to liquid phase) within the pore at a pressure lower than its saturation pressure, caused by the formation of concave shaped meniscus in a group of pores of specific size and shape 47 . According to the IUPAC classification, the hysteresis demonstrated by the materials is represented by type H 3 48 . This type of hysteresis can be attributed to materials having interleaved layer aggregates that give rise to slit-shaped pores.
The surface area, the pore volume and their diameter were calculated from the obtained isotherms and the application of the BET and BJH methods as shown in Table 1.
After incorporation of the oxide into the clay structure, the average pore diameter decreased and the specific area decreased 49 , probably due to a significant decrease in the mesoporous area due to the introduction of the oxide and a slight increase in the amount of pore micropores in the region near 2 nm due to the TiO 2 formed. Figure 4 shows the thermogravimetric (TG) and derivative (DTG) curves, which show the mass loss events for sepiolite and TiO 2 /Sep nanocomposite during four different events 45 . For pristine sepiolite, the first event, at approximately 63 °C, comes from the dehydration of surface water and the zeolitic water in the channels, with 5.4% mass loss. At approximately 264 °C there is a second event corresponding to the departure of water molecules coordinated to the Mg atoms of the  octahedral sheet, with a 3.0% mass loss. The third event occurs between 400 and 600 °C is attributed to the departure of the second coordinating water molecule, with a mass loss of around 2.8%. At 800 ºC, a fourth event is observed, with a 2.2% mass loss that is attributed to dehydroxylation of the octahedrally coordinated hydroxyl groups. As a result of dehydroxylation, the so-called anhydrous sepiolite is transformed into an amorphous phase called meta-sepiolite. Mass losses above 800°C are due to total dehydroxylation of the clay structure and formation of the enstatite phase (MgSiO 3 ) 50 . According to Perraki (4) Despite the similarity between the thermogravimetric curves of sepiolite and TiO 2 /Sep, less significant mass losses were observed in the nanocomposite, thus evidencing alterations in the composition of the material after incorporation of the oxide. The main differences between the compounds occur in the first two events since, after the incorporation of TiO 2 , the hydroxyl groups were reduced due to the condensation of these groups in the immobilization process 44 . Zeolite water loss was observed at 52 °C, with a 2.8% mass loss. Co-ordinated water removal from TiO 2 was observed at approximately 245 °C with approximately 1.2% mass loss. The species chemically adsorbed in the sepiolite structure had a maximum degradation temperature of approximately 500 ºC and a 2.1% loss of mass. Finally, dehydroxylation of the nanocomposite was observed with a maximum temperature of 800 ºC and a 1.7% mass loss.

SEM analysis
The SEM image for sepiolite shows the aggregation of microfibers that can have sizes up to 100 nm and have a large specific surface area 51 . Analyzing the SEM image present in Figure 5a, the sepiolite presents a fibrous morphology. The immobilized material (Figure 5b, c) is observed along with fibrous morphology and the formation of agglomerates in a rounded form, which reinforces the presence of TiO 2 in the anatase form incorporated into the surface of the clay 51 . In addition, the presence of the constituent elements of sepiolite, silicon, oxygen and, magnesium, were observed along with small amounts of aluminum and iron in the SEM-EDS analyses ( Figure 6). This observation was also made by Bahabadi et al. 52 , showing the main constituent of sepiolite and the secondary elements highlighted in this work. The EDS spectrum for the TiO 2 /Sep material identified the presence of all the mentioned elements, in addition to titanium after chemical modification 41 .
For a better understanding of the immobilization of the oxide in the clay network, the formation of the TiO 2 oxide can be expressed using small colloidal Ti(OH) 4 particles that precipitate on the surface of the sepiolite and interact with the surface hydroxyls of fibers, thus generating the incorporated TiO 2 particles. A possible formation mechanism for this particle process is suggested schematically in Figure 7

Photocatalytic discoloration of eosin y
The ideal concentration of photocatalyst is one of the main factors that affect the efficacy and rate of a photocatalytic reaction since it directly interferes in the rate of degradation reaching a critical yield value [53][54][55][56][57] . Preliminarily, photocatalytic tests were performed against EA discoloration (2.0x10 -5 mol L -1 ) in aqueous solution, in order to effectively verify whether the textile compound is degraded. Photolytic and photocatalytic quantifications were monitored by the decrease of the characteristic peak at 516 nm, the maximum wavelength of the chromophore groups. Figure 8 shows the UV-Vis spectra for TiO 2 /Sep catalyzed dye discoloration at concentrations of 0.5, 1.0 and 1.5 g L -1 and the respective degradation percentages. According to the discoloration process, EA was photocatalytically oxidized at all concentrations, although the 1.0 g L -1 (0.18 g to 180 mL) showed a higher discoloration rate. It is known that excess photocatalyst may cause a reduction in radiation passage due to particle aggregation and/or solution turbidity 27,58-60 compromising the passage of light and reducing discoloration. As this condition was compromised, the maximum concentration of photocatalyst guaranteed an oxidation yield lower than the intermediate variation studied. In addition, it is possible to observe, from the economic point of view, that when incorporating TiO 2 in the clay structure to obtain TiO 2 /Sep, the conversion reaches about 72% in 150 min of reaction, thus obtaning a 2.5 times increase than Sep (Figure 9). Specifically, clays of this type have a shielding effect; that is, they can absorb    or disperse incoming UV light, causing a reduction of the photocatalytic process. Thus, Sep behaves as a blocker in the dye degradation process 61 .
The main reason for the high activity of TiO 2 /Sep is probably due to the structural modifications caused by the oxide that act to favor charges separation mobility in the interstitial interior to favor the oxidation process and avoid recombination between the oxide loads and clay 14,18 . Another important factor in efficient activity may be the high surface area of the composite, resulting from the synergistic effect between TiO 2 and clay 36 . Besides, the pristine TiO 2 showed a 68% discoloration of the dye in the same condition, showing that the composite is better than pristine materials (sepiolite and TiO 2 ). It is also believed that the ratio of TiO 2 :Sep (w/w) used to obtain the desired phase was essentially important for generating highly oxidizing radicals (OH, O 2 -• ) since TiO 2 is able to oxidize organic compounds due to its electronic properties and bandgap value suitable to the region of UV light 14 . This efficiency was  also observed by Zhang et al. 36 using TiO 2 /Sep composites, prepared at low temperature by the sol-gel technique, in the degradation of red acid G (ARG) and 4-nitrophenol (4-NP). The results obtained showed that 99.2% of ARG was degraded after 120 min, whereas 4-NP (hardly degradable pollutant) reached 79% conversion after 240 min. The results suggested that the TiO 2 /Sep composite is a promising candidate for degrading pollutants under UV illumination.
In general, during the photocatalytic process, species formed by the action of the photocatalyst favor the degradation process represented by the following Equations 5-12 60 Another important factor is the pH of the solution 62 . The concentration variations and discoloration percentages ( Figure 10) showed that the higher photocatalytic performance of the TiO 2 /Sep material was at neutral pH. The pH variation changes the surface charge of the TiO 2 particles, in other words, under acid or alkaline conditions the TiO 2 surface can be protonated or deprotonated, according to Equations 13 and 14, respectively 60 Thus, the surface of TiO 2 present in the material will remain positively charged in acid media and negatively charged in basic media. According to the literature 54 , TiO 2 has higher oxidizing activity at acidic pH, although excess H + ions can decrease the reaction rate. The fact that the pH test has the best efficiency at neutral pH can be explained by the fact that hydroxyl radicals are considered the predominant species at neutral and basic pH. These • OH radicals are easier to generate by oxidation with more hydroxyl ions available on the TiO 2 surface, causing process efficiency to increase. In contrast, excess OHions hinder the formation of hydroxyl radicals at basic pH. It is also worth noting that in alkaline solutions there will be coulombic repulsion between the negatively charged surface of the photocatalyst and the hydroxide anions, which could prevent the formation of • OH and thus decrease the photo-oxidation to promote competition between the anionic charges of the dye and the surface of the photocatalyst 60,63 . With excess • OH there will also be competition between molecules of the anionic dye to adsorb on the surface of the photocatalyst 63 . Therefore, the nature of the dye (anionic or cationic), surface, and pH are factors that directly influence the activity of the photocatalyst.

Conclusions
In the present work, TiO 2 /Sep samples were successfully obtained by the sol-gel method at 400°C. The presence of TiO 2 nanoparticles in the clay structure decreased the crystallinity of Sepiolite. The infrared results showed the characteristic spectra of the sepiolite clay and some alterations upon the immobilization and interaction of the TiO 2 with the clay, such as the disappearance of the band at 3700 cm -1 and the appearance of a new band around 1669 cm -1 . Through scanning electron microscopy, the presence of clay fibers and agglomerated spherical particles of TiO 2 was observed in the nanocomposite as well as the presence of the constituent elements of clay and TiO 2 . The photocatalytic results showed that at a 1.0 g L -1 concentration had a higher EA discoloration rate (72% in 150 min). This improved performance for TiO 2 /Sep system can be attributed to the synergy of the isolated systems (TiO 2 +clay) associated with the formation of intermediate energy bands that suppress electron/hole recombination and consequently lead to the generation of oxidative species and final dye conversion. Another important factor in efficient activity may be the composite's high surface area resulting from the synergistic effect between TiO 2 and clay. Finally, the importance of optimizing operational parameters to guarantee success and cost/benefit for future applications in photocatalytic processes for the degradation of emerging pollutants.