CHEMICAL ENGINEERING TRANSACTIONS VOL. 70, 2018 A publication of The Italian Association of Chemical Engineering Online at www.aidic.it/cet Guest Editors: Timothy G. Walmsley, Petar S. Varbanov, Rongxin Su, Jiří J. Klemeš Copyright © 2018, AIDIC Servizi S.r.l. ISBN 978-88-95608-67-9; ISSN 2283-9216 Visible Light Active Fe-doped TiO2 for the Oxidation of Arsenite to Arsenate in Drinking Water Giuseppina Iervolinoa, Vincenzo Vaianoa,*, Luigi Rizzob aDepartment of Industrial Engineering, University of Salerno, Via Giovanni Paolo II, 142, 84084 Fisciano (SA), Italy bDepartment of Civil Engineering, University of Salerno, Via Giovanni Paolo II, 142, 84084 Fisciano (SA) Italy vvaiano@unisa.it The aim of this work was to study photocatalysts based on Titania doped with Fe, active in the presence of visible light and effective in the oxidation of As(III) to As(V). Fe-doped TiO2 photocatatysts were prepared using sol-gel method and characterized by different techniques, such as XRD, UV-Vis DRS and Raman spectroscopy. Photocatalytic experiments were carried out using a cylindrical photoreactor irradiated with a strip of visible- LEDs (nominal power: 10W) with emission in the range 400 - 600 nm. Aqueous samples containing 6 mg L-1 of As(III) were used in photocatalytic experiments. Chemical-physical characterization results evidenced that the doping of TiO2 with Fe induced a decrease of band gap value from 3.2 eV to 2.9 eV, thus making the photocatalyst active under visible light. Consistently with the characterization results, the semiconductor Fe- doped TiO2 exhibited good photoactivity in the visible light driven tests: the complete oxidation of As(III) to As(V) took place after only 30 min of irradiation in distilled water and after 60 min in drinking water. 1. Introduction Arsenic is a toxic element that can be found in many forms in the environment due to the numerous anthropogenic and natural sources. Its presence at concentrations exceeding the limit (10 g L-1) set by WHO (2001) has been documented in waters and soils in many areas of the world. It is estimated that the arsenic contamination of drinking water affects over 144 million people around the world (Shankar et al., 2014). Arsenic (As) contamination in groundwater and soil is widely recognized as a global health problem (Qin et al., 2016). In particular, arsenic typically occurs in the environment in different oxidation states (−III, 0, +III and +V). The distribution of As(III) and As(V) in natural water depends on the redox potential and pH of water (Smedley and Kinniburgh, 2002). Compared with As(V), As(III) is generally reported to have a low affinity to the surface of various minerals, because it mainly exists as non-ionic H3AsO3 in natural water when pH < 9 (Qin et al., 2016). Nevertheless, As(V) adsorbs easily to solid surfaces, so it is easier to be removed. Since As(III) is more toxic and more difficult to remove than As(V), a pre-oxidation process transforming As(III) to As(V) is highly desirable to remove arsenic from water (Zhang et al., 2014). Different methods have been reported in literature about the oxidation of As(III) to As(V), including biological oxidation, chemical oxidation with conventional oxidants, such as chlorine, chlorine dioxide (ClO2), chloramine (NH2Cl), permanganate (MnO4 −), manganese oxides, and hydrogen peroxide (Oennby et al., 2014), and photo-oxidation (Xu and Meng, 2009). Interesting results have already been published in the literature concerning the oxidation of As(III) in As(V) using the photocatalytic process (Vaiano et al., 2016a). In particular, in literature it is reported the effectiveness of this process even in a short time and in presence of a real drinking water matrix (Iervolino et al., 2016). Due to its chemical and biological stability, availability, non-toxicity and high efficiency in water treatment TiO2 is the most investigated semiconductor (Xu et al., 2010). However, TiO2 has high band gap energy (3.2 eV) and it can only be activated by the UV fraction of the solar light. For efficient photocatalytic activity, it is necessary to extend the photoresponse of the TiO2 from the UV to the visible region by modification of its optical properties. Recently, the scientific community is working on studying visible active photocatalysts (Vaiano et al., 2016b), which can also be applied in the photocatalytic oxidation of arsenic present in drinking water (Qin et al., 2017). The most popular transition metals dopants used for the modification of the TiO2 optical properties are Cr, Fe, Ni, V, Mn, and Cu (Hou et al., 2009). Fe doping has found to enhance the photocatalytic activity of the TiO2 (Sacco et al., DOI: 10.3303/CET1870263 Please cite this article as: Iervolino G., Vaiano V., Rizzo L., 2018, Visible light active fe-doped tio2 for the oxidation of arsenite to arsenate in drinking water , Chemical Engineering Transactions, 70, 1573-1578 DOI:10.3303/CET1870263 1573 2015) for the degradation of dye under visible light (Moradi et al., 2016). Still scarce is the literature about the photocatalytic oxidation of As(III) to As(V) under visible light. Accordingly, the aim of this work was to investigate the photocatalytic oxidation of As(III) into As(V) through a Fe-doped TiO2 photocatalyst, synthesized by a facile method, active under visible light irradiation. 2. Experimental 2.1 Photocatalysts synthesis and characterization The photocatalysts were prepared by sol gel method. In particular 50 mg of iron acetylacetonate Fe(C₅H₇O₂)₃ (Sigma-Aldrich) was dissolved in 25 mL of titanium isopropoxide (Sigma-Aldrich). This solution was sonicated until complete solubilization of iron acetylacetonate. Then, 100 mL of distilled water were added to obtain a precipitate which was separated from the liquid phase by centrifugation for 5 min at 5,000 rpm. The sample was washed with distilled water to remove any impurities and it was calcined at 450°C for 30 min. In addition, a sample of undoped Titania was prepared following the same procedure. All the samples were characterized using different techniques. The Raman spectra of the samples were recorded with a Dispersive MicroRaman system (Invia, Renishaw), equipped with 514 nm laser, in the range 100 - 2,000 cm-1 Raman shift. X-ray diffraction patterns were obtained with an X-ray diffractometer (Assing), using Cu-Kα radiation. UV–vis reflectance spectra of powder catalysts were recorded by a Perkin Elmer spectrometer Lambda 35 using an RSA-PE-20 reflectance spectroscopy accessory (Labsphere Inc., North Sutton, NH). All spectra were obtained using an 8° sample positioning holder, giving total reflectance relative to a calibrated standard SRS-010-99 (Labsphere Inc., North Sut-ton, NH). Equivalent band gap determinations of the photocatalysts were obtained from Kubelka–Munk function F(R∞) by plotting [F(R∞) x h]2 vs. h(Iervolino et al., 2018). 2.2 Photocatalytic activity tests Photocatalytic tests were carried out in a pyrex cylindrical reactor (ID = 2.5 cm; height = 18 cm) equipped with an air distributor device (Qair = 150 cm3 min−1 under standard temperature and pressure conditions), a magnetic stirrer (to maintain the photocatalyst suspended in the aqueous solution), and a temperature controller. The photoreactor was irradiated by a strip of visible LEDs with wavelength emission range 400-600 nm positioned around the external surface of the reactor so that the light source uniformly irradiated the reaction volume. The suspension was left in dark conditions for 1 hour and then the reaction was started under visible light up to 3 hours. Different catalysts dosage (in the range 1.5 - 4.5 g L-1) were tested in 100 mL of aqueous solution containing 6 mg L-1 of As(III). Photocatalytic tests were carried out in both distilled and real drinking water. During the photocatalytic test, the liquid samples were collected at fixed time and analyzed. In particular, the As(V) concentration was analyzed by a spectrophotometric method based on the formation of molybdenum blue permitting the evaluation of the As(V) concentration at λ = 880 nm trough a Perkin Elmer UV-Vis spectrophotometer (Iervolino et al., 2016). The concentration of the total arsenic was analyzed by preliminary oxidizing As(III) fraction through the addition of KMnO4 aqueous solution (0.01 M). In this way As(III) was totally oxidized to As(V) and then analyzed with the molybdenum blue method in both untreated and treated solutions (Vaiano et al., 2016a). 3. Results 3.1 Photocatalysts characterization The crystalline structure of the photocatalysts characterized by X-ray diffraction analysis (Figure 1) showed only patterns related to anatase Titania for both samples (Fe-TiO2 and TiO2) (Vaiano et al., 2016a). No signals due to iron oxides appear in the figure. The average size of photocatalysts crystallites was calculated using the Scherrer equation on diffraction plane (1 0 1) (Dai et al., 2010). The results in terms of crystallite size for undoped TiO2 and Fe-doped TiO2 are quite similar, being about 8 nm for both samples (Table 1). Table 1: Photocatalysts and their characteristics Photocatalysts Crystallite size (nm) Equivalent band gap energy (eV) TiO2 8.4 3.2 Fe-doped TiO2 7.9 2.9 1574 Figure 1: XRD spectra of the photocatalysts Figure 2: Raman spectra The Raman spectra of photocatalysts show the common peaks at 144, 396, 514 637 cm−1 and a weak shoulder at 195 cm−1 due to the anatase form of TiO2 (Gao et al., 2013) (Figure 2). The signals reported for all photocatalysts are only due to Titania in the anatase crystalline form and there are no signals due to iron oxides. Therefore, these results confirm that TiO2 doping process was correctly finalized. The data obtained from UV–Vis reflectance spectra were used for evaluating the band-gap energy of TiO2 and of Fe-doped TiO2 photocatalysts, respectively. The doping of TiO2 with iron significantly affected the band-gap energy, decreasing from 3.2 eV (for undoped TiO2) to 2.9 eV for Fe-doped TiO2 (Table 1). 3.2 Photocatalytic activity results The efficiency of the prepared and characterized photocatalysts was evaluated in the photocatalytic oxidation of As(III) in As(V) under visible light. In particular, control tests were carried out in the presence of As(III) and irradiating the photoreactor with visible LEDs in the absence of photocatalyst (photolysis reaction), respectively. The behaviour of the As(III) relative concentration (a) and As(V) produced (b) as a function of irradiation time are shown in Figure 3. The photolysis control test did not result in any oxidation activity. An important decrease of the As(III) concentration (Figure 3a) was obtained in presence of Fe-doped photocatalyst. In particular, in the case of undoped TiO2 photocatalyst, the concentration of As(III) decreased up to 30 min of irradiation, reaching a As(III) conversion as low as 2%. After 30 min, As(III)/As(III)0 remained almost constant indicating the TiO2 is 1575 not able to oxidize the As(III) present in the solution. On the contrary, in presence of Fe-doped TiO2 photocatalyst, the complete oxidation of As(III) to As(V) was achieved after only 30 min of visible light irradiation. This result clearly shows that iron doping induces a photocatalytic activity in the presence of visible irradiation. In order to confirm this result, in addition to the analysis of the As(III), the analysis of the As(V) released in solution during the run time was also performed (Figure 3b). After 180 min of irradiation time, As(V) concentration in solution was equal to 6 mg L-1 (i.e. equal to the As(III) initial value). This last results is very important because clearly shows that As(V) produced by the photo-oxidation of As(III) was completely released into the solution, thus not occupying the active sites of the catalyst, avoiding its deactivation and enhancing its stability (Vaiano et al., 2014). Moreover, the As(III) conversion, achieved after 30 min of irradiation time, using Fe-doped TiO2 photocatalyst at different dosages (in the range 1.5 - 4.5 g L-1), increased as catalyst loading was increased to 3 g L-1. But, a further increase of catalyst dosage to 4.5 g L-1, resulted in a decrease of As(III) oxidation. This result may be explained by the increased opacity of the water solution due to the increase of the catalyst loading, which possibly made light penetration through the solution more difficult (Vaiano et al., 2014). Accordingly, 3 g L-1 of photocatalyst loading was considered the optimum dosage. Figure 3: Behaviour of As(III) relative concentration in solution (a) and behaviour of As(V)concentration in solution (b) as a function of visible light irradiation time for both undoped TiO2 and Fe-doped TiO2 photocatalysts. Figure 4: Influence of Fe-doped TiO2 dosage on the As(III) conversion after 30 min of visible light of irradiation time. 1576 Figure 5: Comparison between the photocatalytic test performed on the solution of As(III) in bi-distilled water and on the solution of As(III) in real drinking water under visible light irradiation with Fe-doped TiO2. Once the optimal photocatalyst dosage has been established (3 g L-1), its effectiveness has also been tested on a drinking water matrix. Figure 5 shows the comparison between the photocatalytic test performed on the solution of As(III) in bi-distilled water with an initial concentration of As(III) of 6 mg L-1 and the photocatalytic test carried out on the solution of As(III) in real drinking water with the same As(III) initial concentration. It is possible to observe that the efficiency of the photocatalytic oxidation of As(III) in real drinking water was lower than that one observed in bi-distilled water. As matter of fact, after 30 minutes of irradiation, the percentage of As(III) oxidized to As(V) was as low as 27 % compared to 100 % in bi-distilled water. It is well known that ionic species like bicarbonates, sulfates and chlorides, generally present in drinking water, act as radical scavengers in photocatalytic processes (Iervolino et al., 2016), worsening the photocatalytic activity. However, it is worth noting that, the Fe-doped TiO2 photocatalyst was able to completely oxidize As(III) after 60 minutes of visible light irradiation in real drinking water. 4. Conclusions Photocatalytic oxidation of As(III) into As(V) in mild conditions and under visible light was investigated in the presence of Fe-doped TiO2 catalyst. Characterization results evidenced that the doping of TiO2 with Fe induced a decrease of band gap value from 3.2 eV to 2.9 eV. The Raman and XRD analysis evidenced the presence of only signals related to the Titania in the anatase crystalline form and the absence of signals due to iron oxides. Photocatalytic experiments were carried out under visible-LEDs irradiation with emission in the range 400 - 600 nm. The obtained results showed that Fe-doped TiO2 exhibited good performances in presence of visible light, achieving the complete oxidation of As(III) to As(V) after only 30 min of irradiation time. Moreover, As(V) produced by the photo-oxidation of As(III) was completely released into the solution, thus not occupying the active sites of the catalyst, avoiding its deactivation and enhancing its stability. Finally, the synthetized visible light active photocatalyst was also able to completely oxidize As(III) into As(V) in drinking water matrix in 60 min of irradiation light. The Fe-doped TiO2 catalyst can be considered an interesting semiconductor to be used in the photocatalytic oxidation of As(III) into As(V) in presence of the direct sunlight, making this process ecofriendly and with low energy consumption. In fact, the photocatalytic tests have shown its effectiveness in a short time in the presence of visible light. The formulated photocatalyst makes the oxidation phase very fast, allowing to reach the complete arsenic removal by means the use of an adsorption process after the photocatalytic oxidation. However, further studies should be focused on the formulation of visible light active structured photocatalysts in order to avoid the recovery of catalyst particles after the photocatalytic process. References Dai, S., Wu, Y., Sakai, T., Du, Z., Sakai, H., Abe M., 2010. Preparation of Highly Crystalline TiO2 Nanostructures by Acid-assisted Hydrothermal Treatment of Hexagonalstructured Nanocrystalline Titania/Cetyltrimethyammonium Bromide Nanoskeleton, Nanoscale Research Letters, 5, 1829-1835. Gao, Y., Luan, T., LÜ, T., Cheng, K., Xu, H., 2013. Performance of V2O5-WO3-MoO3/TiO2 catalyst for selective catalytic reduction of NOx by NH3, Chinese Journal of Chemical Engineering, 21, 1-7. 1577 Hou, X.G., Huang, M.D., Wu, X.L., Liu, A.D., 2009. First-principles calculations on implanted TiO2 by 3D transition metal ions, Sci. China, Ser. G: Phys., Mech. Astron., 52, 838-842. Iervolino, G., Vaiano, V., Rizzo, L., Sarno, G., Farina, A., Sannino, D., 2016. Removal of arsenic from drinking water by photo-catalytic oxidation on MoOx/TiO2 and adsorption on γ-Al2O3, Journal of Chemical Technology and Biotechnology, 91, 88-95. Iervolino, G., Vaiano, V., Sannino, D., Rizzo, L., Galluzzi, A., Polichetti, M., Pepe, G., Campiglia, P., 2018. Hydrogen production from glucose degradation in water and wastewater treated by Ru-LaFeO3/Fe2O3 magnetic particles photocatalysis and heterogeneous photo-Fenton, International Journal of Hydrogen Energy, 43, 2184-2196. Moradi, H., Eshaghi, A., Hosseini, S. R., Ghani, K., 2016. Fabrication of Fe-doped TiO2 nanoparticles and investigation of photocatalytic decolorization of reactive red 198 under visible light irradiation, Ultrason. Sonochem., 32, 314-319. Oennby, L., Kumar, P. S., Sigfridsson, K. G. V., Wendt, O. F., Carlson, S., Kirsebom, H., 2014. Improved arsenic(III) adsorption by Al2O3 nanoparticles and H2O2: Evidence of oxidation to arsenic(V) from X-ray absorption spectroscopy, Chemosphere, 113, 151-157. Qin, Y., Cui, Y., Tian, Z., Wu, Y., Li, Y., 2017. Synthesis of AG@AgCl Core–Shell structure nanowires and its photocatalytic oxidation of Arsenic (III) under visible light, Nanoscale Research Letters, 12. Qin, Y., Li, Y., Tian, Z., Wu, Y., Cui, Y., 2016. Efficiently visible-light driven photoelectrocatalytic oxidation of As(III) at low positive biasing using Pt/TiO2 nanotube electrode, Nanoscale Res. Lett., 11, 1-13. Sacco, O., Vaiano, V., Han, C., Sannino, D., Dionysiou, D.D. Ciambelli, P., 2015. Long afterglow green phosphors functionalized with Fe-N doped TiO2 for the photocatalytic removal of emerging contaminants. Chemical Engineering Transactions, 43, 2107-2112. Shankar, S., Shanker, U., Shikha, 2014. Arsenic contamination of groundwater: a review of sources, prevalence, health risks, and strategies for mitigation, The Scientific World Journal, 2014. Smedley, P.L., Kinniburgh, D.G., 2002. A review of the source, behaviour and distribution of arsenic in natural waters. Appl. Geochem., 17, 517-568. Vaiano, V., Iervolino, G., Sannino, D., Rizzo, L., Sarno, G. 2016a. MoOx/TiO2 immobilized on quartz support as structured catalyst for the photocatalytic oxidation of As(III) to As(V) in aqueous solutions, Chemical Engineering Research and Design, 109, 190-199. Vaiano, V., Iervolino, G., Sannino, D., 2016b. Photocatalytic removal of tartrazine dye from aqueous samples on LaFeO3/ZnO Photocatalysts, Chemical Engineering Transactions, 52, 847-852. Vaiano, V., Iervolino, G., Sannino, D., Rizzo, L., Sarno, G., Farina, A., 2014. Enhanced photocatalytic oxidation of arsenite to arsenate in water solutions by a new catalyst based on MoOx supported on TiO2, Applied Catalysis B: Environmental, 160-161, 247-253. Xu, J., Ao, Y., Chen, M., Fu, D., Yuan, C., 2010. Photocatalytic activity of vanadium-doped titania-activated carbon composite film under visible light, Thin Solid Films, 518, 4170-4174. Xu, Z., Meng, X., 2009. Size effects of nanocrystalline TiO2 on As(V) and As(III) adsorption and As(III) photooxidation, J. Hazard. Mater., 168, 747-752. Zhang, P., Tong, M., Yuan, S., Liao, P., 2014. Transformation and removal of arsenic in groundwater by sequential anodic oxidation and electrocoagulation, J. Contam. Hydrol., 164, 299-307. 1578