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Ilmenite: Properties and photodegradation kinetic on Reactive Black 5 dye
Accepted Manuscript Title: Ilmenite: Properties and photodegradation kinetic on Reactive Black 5 dye Authors: Ru-Bin Lee, Joon-Ching Juan, Chin-Wai Lai, Kian-Mun Lee PII: DOI: Reference:
S1001-8417(17)30084-0 http://dx.doi.org/doi:10.1016/j.cclet.2017.03.006 CCLET 4001
To appear in:
Chinese Chemical Letters
Received date: Revised date: Accepted date:
16-11-2016 18-1-2017 6-3-2017
Please cite this article as: Ru-Bin Lee, Joon-Ching Juan, Chin-Wai Lai, Kian-Mun Lee, Ilmenite: Properties and photodegradation kinetic on Reactive Black 5 dye, Chinese Chemical Letters http://dx.doi.org/10.1016/j.cclet.2017.03.006 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.
Original article Ilmenite: Properties and photodegradation kinetic on Reactive Black 5 dye Ru-Bin Lee, Joon-Ching Juan, Chin-Wai Lai*, Kian-Mun Lee Nanotechnology & Catalysis Research Centre (NANOCAT), Institute of Graduate Studies (IPS), University of Malaya, 50603 Kuala Lumpur, Malaysia
Corresponding authors. E-mail address: [email protected], [email protected]
Graphical Abstract
Natural ilmenite has good photo-catalytic activity under acidic conditions when irradiated under artificial sunlight.
ABSTRACT Ilmenite is natural mineral ore made up with titanium and iron mineral; including small portion of magnesium and manganese. To the best of our knowledge, photo-degradation of Reactive Black 5 dye (RB 5) using ilmenite under solar irradiation is still lacking. In the present study, the physicochemical properties of ilmenite were characterized by using X-ray diffraction (XRD), Scanning electron microscope (SEM), BET and Raman Spectroscopy. Based on our results obtained, 73% solar-driven photo-degradation of RB 5 was successfully obtained when the catalyst loading increased up to 2.0 g/L for 20 min. In general, the photo-degradation of RB 5 by ilmenite followed first-order kinetics. The pH had a significant effect, with the most rapid degradation occurring at pH less than 7.
Keywords: Ilmenite Photo-degradation Reactive Black 5 Solar irradiation
Photo-catalytic 1. Introduction Titanium dioxide (TiO2) is a rising heterogeneous photo-catalyst in advance oxidation process (AOPs) due to its excellent efficiency in mineralization and remains inert under mild temperature, pressure and wide pH range conditions when it is irradiated by ultraviolet light (UV light) [1-4]. Therefore, researches have been done to improve the potency of TiO2 into visible light region by reducing the band gap energy. Several works suggest foreign element, ions and compound able to reduce the band gap of TiO2 [5, 6]. Synthetic ilmenite (FeTiO3) has high potential in the fields of materials science and engineering, especially in heterogeneous photocatalysis, solar cells, electronic circuits and gas sensors [7-10]. FeTiO3 can be synthesized via hydrothermal emulsion, solid-state
———
reaction and sol gel methods [6, 11-14]. Kim works showed hetero-junction of FeTiO3 nanodisc and TiO2 nanoparticle demonstrated great photo-catalytic activity in mineralization of 2-propanol under visible light irradiation [15]. Truong et al. demonstrated CO2 reduction to CH3OH by using FeTiO3/TiO2 composite under both visible and UV-vis light irradiation [16]. Natural ilmenite formed originally in magma, with moderate titanium content, usually around 45%–60%. The structure of natural FeTiO3 is rhombohedral crystal in space group R-3H with hexagonal packing. The oxygen atoms occupying 2/3 of the octahedral positions; Fe and Ti occupy alternating layers [17-19]. Ilmenite is a semi-conductor with a wide band gap (2.5-2.7 eV) and antiferromagnetic properties [7]. This mineral is a feed stock for bulk TiO2 production and it has abundant of deposit in most of continents on Earth with a current estimation of reserve exceed 680 million tons [20]. Current price in the market of ilmenite are in the range of 100 – 120 USD/ton [21]. Although synthetic FeTiO3 has been studied in a long period, only handful of researches have been reported on the potential capability of natural ilmenite. Tao et al. works showed that natural ilmenite nanoflower has distinct and stable pseudo-capacitance, hence it showed functionality as an electrode material for super-capacitors [22]. Moctezuma et al. reported photodegradation of phenol to carboxylic acid by using ilmenite as catalyst [23]. This research paper presents the photo-catalytic activity of natural ilmenite and the relative controlling factors were investigated in the photo-degradation of Reactive Black 5 dye. The physicochemical characteristic of natural ilmenite was characterized by several analytical techniques in order to understand the effect on the photo-catalytic activity of natural ilmenite. 2. Results and discussion 2.1 Qualitative analysis of the catalyst Fig. 1 shows XRD profile of the quality and crystalline phase of ilmenite. The existence of ilmenite in the XRD pattern was clearly shown from the presence of the (104) peak at 2θ = 32.65ᵒ (JCPDS: 29-0733); whereas in the titanium dioxide (rutile) spectrum, the (110), (101), (111), and (211) had been noticed. The intensity at (101), (111) and (211) had higher intensity compared with pure rutile TiO2 [24]. The presence of iron in the lattice structure has enhanced non active site of rutile TiO2 as several works suggested the (110) peak is rutile active site, thus reduces photo-catalytic activity of ilmenite [25, 26]. Rutile has the characteristic of thermodynamically stable polymorph at all temperatures and pressure, therefore ilmenite normally has high content of rutile TiO2 as the minerals sustains extreme temperature and pressure during geological formation [27-29]. Based on the Scherrer equation, the crystalline size of ilmenite and rutile were 6.39 nm and 2.96 nm respectively. Fig. 2 shows the zeta potential plot of ilmenite. The point of zero charge (PZC) of ilmenite is located at pH 4.5. Thus, ilmenite exhibit better adsorption and better photo-catalytic activity when the solution are acidic (pH < 4.5). Mehdilo et al. suggested impurities in ilmenite lattice have influence on zeta potential of ilmenite, thus the range of PZC of ilmenite could be between pH 4 to pH 6.5 [30]. The surface area and the pore structure of the natural ilmenite were determined by N2 physical adsorption-desorption studies. Fig. 3 shows the BET isotherm and the relative Barret-Joyner-Halender (BJH) pore size distributions obtained from the desorption branch of the isotherms of the catalyst. It was observed that the natural ilmenite has mesoporous surface as the isotherms are of Type IV. Based on the adsorption data in relative pressure (P/P0) range (0.44-0.98), the BET specific surface area for ilmenite was found to be 16.171 m2/g. From the BJH pore size distributions, it was observed that the sample showed a narrow pore size distribution with pore width between 1.0-1.5 nm. Micro morphological studies of ilmenite using FESEM reveals ilmenite has irregular shape with rough surface. Tiny irregular shape of pores can be found (Fig. 4b) in ilmenite, thus increase surface area of ilmenite. Existence of pores is beneficial for the catalytic activity of ilmenite as the Reactive Black 5 reaction takes place on the surface of the catalyst, thus more Reactive Black 5 dye can be adsorbed on the surface of ilmenite. The band gap energy of ilmenite is 2.5 eV (Table 1) which are lower than pure rutile TiO2 (3.0 eV) [31-33]. Low band gap of ilmenite is because occurrence of Fe provides new dopant levels, thus lower the charge transfer transition between valence band and conduction band. From Table 1, the presence of I phase (680 cm-1), Fe2O3 (424 cm-1, 1320 cm-1) and FeO (154 cm-1, 298 cm-1, 546 cm1 ) are in accordance with the XRD patterns. The Raman peak of Fe2O3 and FeO are corresponding with Faria et al. [34]. Hence, Fe is located inside ilmenite lattice structure by formation of Fe-O-Ti bond. Photoluminescence (PL) has been carried out to exanimate the recombination rate of photo-induced charge carries of ilmenite. The peaks/shoulders (Table 1) suggest the oxygen vacancies are in/on the crystal lattice of ilmenite. Similar pattern of PL peaks is also reported by synthesized rutile TiO2 doped with iron ions [35]. Other researchers suggest high energy peaks can be designated as band edge luminescence of the TiO2 particle and oxygen vacancies cause lower energy peaks/shoulders [36, 37]. As ilmenite formed naturally, it has natural defect in the lattice structure, leading low photocatalytic activity of ilmenite as process of recombination of photo-generated charge carriers through oxygen vacancy-cascade occurred. 2.2 Impact of pH The results of control experiments with the absence of ilmenite at different pH values were shown in Fig. 5a. The removal of Reactive Black 5 (RB 5) dye was negligible when the experiments were conducted at pH 4, 6 and 9, respectively, while only 10% of RB 5 was degraded at pH 3. The removal increased slightly at pH 2 (25%). On the other hand, the dye degradation (Ct/C0) of RB 5 showed higher degradation rate (except for pH 9) when the reaction medium were irradiated under a Xenon light for 20 min (Fig. 5b). The highest dye degradation was observed at pH 2 (Ct/C0 = 0.4415) and decreased gradually when the pH value increased. A complete
photo-catalytic activity inhibition of ilmenite occurred at pH 9. The pseudo-first order rate constants, determined using Eq. (1); where k’ is the pseudo-first order rate constant, C0 the initial concentration of RB 5 and C is the concentration at time “t”. The kinetic constant rate of ilmenite reduced from 0.04047 min-1 at pH 2 to 0 at pH 9.
ln
(1)
The pH of solution is a significant parameter in photo-catalytic degradation because pH affects the ionization state of the photocatalyst surface as well as pollutants. The changes of pH can influence the adsorption of pollutants onto the surface of photo-catalyst; hence increase the photo-degradation rate of pollutants. As the PZC of ilmenite is 4.5 (Fig. 2), thus the surface of ilmenite is positively charged at pH < 4.5 and negatively charged at pH > 4.5. RB 5 is an anionic dye as the sodium ion react with water molecules and leaves the dye molecule with negative charged, hence lower pH could increase the electrostatic attraction between RB 5 dye molecules and surfaces of ilmenite and favor the adsorption reaction; the increase of adsorption led to higher degradation. It is necessary to highlight that as the lower of pH in the solution, the amount of Fe could leached out from ilmenite (Table 2), thus increase the production of radical hydroxyl group through homogenous Fenton reaction, thus dye degradation (Ct/C0) increased drastically from pH 3 to pH 2. Interestingly, Al, Mg and Si were detected as well, probably due to the impurities in Ilmenite sand, whereas Na was from Reactive Black 5 dye.
2.3 Impact of inorganic ions Several works have shown the presence of inorganic ions can affect the efficiency of photo-catalytic degradation of organic pollutants because of the competition between pollutants and inorganic ions. 0.04047, 0.01958, 0.00553, and 0.00299 of the kinetic rate of dye degradation (min-1) were observed at optimum pH condition (pH 2) with the presence of nitric acid, hydrochloric acid, sulfuric acid and phosphorus acid in the RB 5 solution respectively, the presence of nitrate ion has the highest kinetic rate. The order of inhibition of these anions are PO43- > SO42- > Cl- > NO3-. On the whole, the presence of NO3- has less effect on RB 5 dye photo-degradation than PO43-, SO42- and Cl- due to NO3- will not react with hydroxyl radicals, thus the photo-degradation rate of RB 5 dye is not inhibited [38-40]. On the other side, sulfate and phosphate ion form bond with Fe on the surface of ilmenite, reduce surface area of ilmenite for RB 5 adsorbed on it. In addition, chloride, sulfate and phosphate ions are hydroxyl radicals scavengers, which reduced the amount of hydroxyl radical generated in the photo-degradation. Cl•, SO4•- and PO4•2- are less reactive than hydroxyl radicals, thus they cannot degrade RB5 dye. Below are reactions between hydroxyl radicals, chloride, sulfate and phosphate ions [41-44]: OH• + Cl- → Cl• + OHOH• + SO42- → SO4•- + OHOH• + PO43- → PO4•2- + OH-
2.4 Impact of catalyst loading Experiment was conducted with different loadings of ilmenite from 0.5 g/L to 2.0 g/L. The result (Table 3) shows the increment of catalyst loading is directly proportional to the overall dye degradation ratio. A significant increment in degradation efficiency is shown when the catalyst loading increased from 0.5 g/L (Ct/C0 = 0.4858) to 2.0 g/L (Ct/C0 = 0.27). All employed different concentration of ilmenite obeyed pseudo-first order kinetic rate constant and the kinetic rate constant of ilmenite increased from 0.03621 min-1 (0.5 g/L) to 0.07088 min-1 (2.0 g/L). This is because of the additional catalyst in the solution provided more interface area for pollutant to be adsorbed on the surface of catalyst before further reaction begin. Besides that, more Fe could leached out from ilmenite, thus increase Fe ions in the RB 5 solution, leading higher production of hydroxyl group for photo-degradation. Complete degradation of dye happened after 120 minutes irradiation of artificial light with 2.0 g/L catalyst loading. 3. Conclusion Natural ilmenite found from magma contains rutile TiO2 and Fe. Existence of Fe in the lattice of ilmenite has significant impact on physiochemical characteristic of ilmenite. Ilmenite showed capability in the RB 5 degradation under acidic condition when irradiated with artificial light. Complete degradation of RB 5 was achieved by 2.0 g/L of catalyst loading of ilmenite after 2 h irradiation. 4. Experimental Chemical and Materials: Natural ilmenite mineral (Tor Minerals Co., Ltd., Malaysia) was used as the starting material. Ilmenite was sieved to ensure equal particle size (<45 μm) before experiment. The chemical composition of ilmenite contains TiO2 (chemical composition of FeO (12%-16%), Al2O3 (12%-16%), SiO2 (12%-16%), SiOAl2O3 (12%-16%)), Reactive Black 5 (RB 5) was purchased from Sigma Aldrich. Analytical grade reagents sulfuric acid (H2SO4), hydrochloric acid (HCl), nitric acid (HNO3), phosphoric acid (H3PO4) and sodium hydroxide (NaOH) were used. The deionized water was used to prepare experiment solutions. The pH of the RB 5 dye solution was adjusted by HNO3 and NaOH.
Characterization: Ilmenite was characterized by X-ray diffraction (D8 Discover Bruker, diffractometer Cu Kα radiation λ = 0.15406 nm, 40 kV, 30 mA) with a scanning range from 2θ = 20ᵒ-70ᵒ at step size of 0.02ᵒ/s, FESEM (JEOL JSM-7600 F), Zeta potential (Malvern Zetasizer Nano Series ZS), Brunauer-Emmett-Teller (BET) (MicroActive ASAP 2020), photoluminescence Raman spectrometer (Renishaw in Via Raman Microscope, 514 nm, 5 mW, 1 μm focus spot) and DR-UV-vis (Varian Cary 100). The elemental analysis of RB 5 was examined by ICP-OES (DV 5300, Perkin Elmer). Analysis: The photocatalytic measurement of ilmenite was conducted as batch photocatalytic reaction system. A photo-reactor with a 150 W Xenon Arc lamp (200-2500nm, Newport), magnetic stirrer and air pump was installed (Fig. 6). A 1.0 g/L of ilmenite powder was dispersed in of 5 mg/L of RB 5 dye. The reaction was set for 30 minutes in the dark followed by 20 minutes light irradiation and the measurement was initiated by switching on the Xenon lamp. The quartz reactor tube filled with solution was removed at the fixed time interval. The solution was then filtered and measured by UV-vis spectrophotometer (Hach 4000U Spectrophotometer) to determine the remaining absorbance of RB 5 at wavelength of 597 nm. In factor of pH study, the RB 5 solution was controlled initially in pH 2, 3, 4, 6 (original pH of RB 5 solution) and 9 respectively, which were set in this experiment. After obtained the optimum photocatalytic activity condition from different pH values, hydrochloric acid (HCl), nitric acid (HNO3), sulfuric acid (H2SO4) and phosphate acid (H3PO4) were used to determine the effect of photocatalytic activity in the presence of inorganic ion. Lastly, 0.5, 1.0, 1.5, and 2.0 g/L of catalyst loading was used to determine the optimum catalyst loading for RB 5 removal.
Acknowledgment This work is supported by Fundamental Research Grant Scheme (No. FRGS: FP008-2015A), Postgraduate Research Grant (No. PPP: PG050-2015A), Research Officer Grant Scheme (No. BR006-2015), Science Fund (No. MOSTI: 03-01-03-SF1032), Trans Disciplinary Research Grant Scheme (No. TR002-2015A) and Prototype Research Grant Scheme (No. PR002-2016). References [1] M.N. Chong, B. Jin, C.W.K. Chow, C. Saint, Recent developments in photocatalytic water treatment technology: a review, Water Res. 44 (2010) 2997-3027. [2] S. Esplugas, J. Giménez, S. Contreras, E. Pascual, M. Rodrı́guez, Comparison of different advanced oxidation processes for phenol degradation, Water Res. 36 (2002) 1034-1042. [3] U.I. Gaya, A.H. Abdullah, Heterogeneous photocatalytic degradation of organic contaminants over titanium dioxide: a review of fundamentals, progress and problems, J. Photochem. Photobiol. C: Photochem. Rev. 9 (2008) 1-12. [4] P.V.L. Reddy, K.H. 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Fig. 1. XRD pattern of ilmenite (R: rutile TiO2; I: ilmenite).
Fig. 2. Zeta potential of ilmenite, PZC of ilmenite from the plot is at pH 4.5.
Fig. 3. N2 adsorption-desorption BET isotherm for ilmenite. The insets show the BJH pore size distribution (from the desorption branch of the isotherms).
(a)
(c)
(c)
Fig. 4. FESEM images of the ilmenite: a) ilmenite powder; b,c) a close up on the surface of ilmenite powder.
(a)
(b)
Fig. 5. Effect of dye degradation at various pH values in the absence of ilmenite (a) and under a Xenon light (b). Dye concentration 5 ppm, temperature 25 ᵒC).
Fig. 6. Schematic diagram of the photo-reactor set up.
Table 1 Summary of physicochemical characteristic of ilmenite. Surface area (m2/g)
BJH pore size distribution (nm) 1.0-1.5
16.171
Band gap (eV)
Raman spectroscopy peak (cm-1)
PL peak/shoulder* (nm)
2.5
154, 298, 424, 546, 680, 1320
380, 410, 445, 465*, 490, 532, 580
Table 2 Elementary analysis of RB 5 solution at pH 2 with catalyst loading 1.0 g/L under solar irradiation for 20 min. Element Fe Al Na Mg Si
Concentration (mg/L) 0.088 0.009 0.773 0.034 0.294
Standard deviation 0.0035 0.0022 0.0089 0.0019 0.0101
RSD (%) 3.92 24.27 1.15 8.70 3.44
Table 2 Kinetic rate of different catalyst loading, coefficient of determination (R2) and efficiency of photo-degradation. Catalyst (g/L) 0.5 1.0 1.5 2.0
loading
Kinetic rate (min-1) 0.03621 0.04047 0.05048 0.07088
R2
Ct/C0 (t = 20 min)
0.9919 0.9957 0.9111 0.9535
0.4858 0.4415 0.3901 0.27
S1001-8417(17)30084-0 http://dx.doi.org/doi:10.1016/j.cclet.2017.03.006 CCLET 4001
To appear in:
Chinese Chemical Letters
Received date: Revised date: Accepted date:
16-11-2016 18-1-2017 6-3-2017
Please cite this article as: Ru-Bin Lee, Joon-Ching Juan, Chin-Wai Lai, Kian-Mun Lee, Ilmenite: Properties and photodegradation kinetic on Reactive Black 5 dye, Chinese Chemical Letters http://dx.doi.org/10.1016/j.cclet.2017.03.006 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.
Original article Ilmenite: Properties and photodegradation kinetic on Reactive Black 5 dye Ru-Bin Lee, Joon-Ching Juan, Chin-Wai Lai*, Kian-Mun Lee Nanotechnology & Catalysis Research Centre (NANOCAT), Institute of Graduate Studies (IPS), University of Malaya, 50603 Kuala Lumpur, Malaysia
Corresponding authors. E-mail address: [email protected], [email protected]
Graphical Abstract
Natural ilmenite has good photo-catalytic activity under acidic conditions when irradiated under artificial sunlight.
ABSTRACT Ilmenite is natural mineral ore made up with titanium and iron mineral; including small portion of magnesium and manganese. To the best of our knowledge, photo-degradation of Reactive Black 5 dye (RB 5) using ilmenite under solar irradiation is still lacking. In the present study, the physicochemical properties of ilmenite were characterized by using X-ray diffraction (XRD), Scanning electron microscope (SEM), BET and Raman Spectroscopy. Based on our results obtained, 73% solar-driven photo-degradation of RB 5 was successfully obtained when the catalyst loading increased up to 2.0 g/L for 20 min. In general, the photo-degradation of RB 5 by ilmenite followed first-order kinetics. The pH had a significant effect, with the most rapid degradation occurring at pH less than 7.
Keywords: Ilmenite Photo-degradation Reactive Black 5 Solar irradiation
Photo-catalytic 1. Introduction Titanium dioxide (TiO2) is a rising heterogeneous photo-catalyst in advance oxidation process (AOPs) due to its excellent efficiency in mineralization and remains inert under mild temperature, pressure and wide pH range conditions when it is irradiated by ultraviolet light (UV light) [1-4]. Therefore, researches have been done to improve the potency of TiO2 into visible light region by reducing the band gap energy. Several works suggest foreign element, ions and compound able to reduce the band gap of TiO2 [5, 6]. Synthetic ilmenite (FeTiO3) has high potential in the fields of materials science and engineering, especially in heterogeneous photocatalysis, solar cells, electronic circuits and gas sensors [7-10]. FeTiO3 can be synthesized via hydrothermal emulsion, solid-state
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reaction and sol gel methods [6, 11-14]. Kim works showed hetero-junction of FeTiO3 nanodisc and TiO2 nanoparticle demonstrated great photo-catalytic activity in mineralization of 2-propanol under visible light irradiation [15]. Truong et al. demonstrated CO2 reduction to CH3OH by using FeTiO3/TiO2 composite under both visible and UV-vis light irradiation [16]. Natural ilmenite formed originally in magma, with moderate titanium content, usually around 45%–60%. The structure of natural FeTiO3 is rhombohedral crystal in space group R-3H with hexagonal packing. The oxygen atoms occupying 2/3 of the octahedral positions; Fe and Ti occupy alternating layers [17-19]. Ilmenite is a semi-conductor with a wide band gap (2.5-2.7 eV) and antiferromagnetic properties [7]. This mineral is a feed stock for bulk TiO2 production and it has abundant of deposit in most of continents on Earth with a current estimation of reserve exceed 680 million tons [20]. Current price in the market of ilmenite are in the range of 100 – 120 USD/ton [21]. Although synthetic FeTiO3 has been studied in a long period, only handful of researches have been reported on the potential capability of natural ilmenite. Tao et al. works showed that natural ilmenite nanoflower has distinct and stable pseudo-capacitance, hence it showed functionality as an electrode material for super-capacitors [22]. Moctezuma et al. reported photodegradation of phenol to carboxylic acid by using ilmenite as catalyst [23]. This research paper presents the photo-catalytic activity of natural ilmenite and the relative controlling factors were investigated in the photo-degradation of Reactive Black 5 dye. The physicochemical characteristic of natural ilmenite was characterized by several analytical techniques in order to understand the effect on the photo-catalytic activity of natural ilmenite. 2. Results and discussion 2.1 Qualitative analysis of the catalyst Fig. 1 shows XRD profile of the quality and crystalline phase of ilmenite. The existence of ilmenite in the XRD pattern was clearly shown from the presence of the (104) peak at 2θ = 32.65ᵒ (JCPDS: 29-0733); whereas in the titanium dioxide (rutile) spectrum, the (110), (101), (111), and (211) had been noticed. The intensity at (101), (111) and (211) had higher intensity compared with pure rutile TiO2 [24]. The presence of iron in the lattice structure has enhanced non active site of rutile TiO2 as several works suggested the (110) peak is rutile active site, thus reduces photo-catalytic activity of ilmenite [25, 26]. Rutile has the characteristic of thermodynamically stable polymorph at all temperatures and pressure, therefore ilmenite normally has high content of rutile TiO2 as the minerals sustains extreme temperature and pressure during geological formation [27-29]. Based on the Scherrer equation, the crystalline size of ilmenite and rutile were 6.39 nm and 2.96 nm respectively. Fig. 2 shows the zeta potential plot of ilmenite. The point of zero charge (PZC) of ilmenite is located at pH 4.5. Thus, ilmenite exhibit better adsorption and better photo-catalytic activity when the solution are acidic (pH < 4.5). Mehdilo et al. suggested impurities in ilmenite lattice have influence on zeta potential of ilmenite, thus the range of PZC of ilmenite could be between pH 4 to pH 6.5 [30]. The surface area and the pore structure of the natural ilmenite were determined by N2 physical adsorption-desorption studies. Fig. 3 shows the BET isotherm and the relative Barret-Joyner-Halender (BJH) pore size distributions obtained from the desorption branch of the isotherms of the catalyst. It was observed that the natural ilmenite has mesoporous surface as the isotherms are of Type IV. Based on the adsorption data in relative pressure (P/P0) range (0.44-0.98), the BET specific surface area for ilmenite was found to be 16.171 m2/g. From the BJH pore size distributions, it was observed that the sample showed a narrow pore size distribution with pore width between 1.0-1.5 nm. Micro morphological studies of ilmenite using FESEM reveals ilmenite has irregular shape with rough surface. Tiny irregular shape of pores can be found (Fig. 4b) in ilmenite, thus increase surface area of ilmenite. Existence of pores is beneficial for the catalytic activity of ilmenite as the Reactive Black 5 reaction takes place on the surface of the catalyst, thus more Reactive Black 5 dye can be adsorbed on the surface of ilmenite. The band gap energy of ilmenite is 2.5 eV (Table 1) which are lower than pure rutile TiO2 (3.0 eV) [31-33]. Low band gap of ilmenite is because occurrence of Fe provides new dopant levels, thus lower the charge transfer transition between valence band and conduction band. From Table 1, the presence of I phase (680 cm-1), Fe2O3 (424 cm-1, 1320 cm-1) and FeO (154 cm-1, 298 cm-1, 546 cm1 ) are in accordance with the XRD patterns. The Raman peak of Fe2O3 and FeO are corresponding with Faria et al. [34]. Hence, Fe is located inside ilmenite lattice structure by formation of Fe-O-Ti bond. Photoluminescence (PL) has been carried out to exanimate the recombination rate of photo-induced charge carries of ilmenite. The peaks/shoulders (Table 1) suggest the oxygen vacancies are in/on the crystal lattice of ilmenite. Similar pattern of PL peaks is also reported by synthesized rutile TiO2 doped with iron ions [35]. Other researchers suggest high energy peaks can be designated as band edge luminescence of the TiO2 particle and oxygen vacancies cause lower energy peaks/shoulders [36, 37]. As ilmenite formed naturally, it has natural defect in the lattice structure, leading low photocatalytic activity of ilmenite as process of recombination of photo-generated charge carriers through oxygen vacancy-cascade occurred. 2.2 Impact of pH The results of control experiments with the absence of ilmenite at different pH values were shown in Fig. 5a. The removal of Reactive Black 5 (RB 5) dye was negligible when the experiments were conducted at pH 4, 6 and 9, respectively, while only 10% of RB 5 was degraded at pH 3. The removal increased slightly at pH 2 (25%). On the other hand, the dye degradation (Ct/C0) of RB 5 showed higher degradation rate (except for pH 9) when the reaction medium were irradiated under a Xenon light for 20 min (Fig. 5b). The highest dye degradation was observed at pH 2 (Ct/C0 = 0.4415) and decreased gradually when the pH value increased. A complete
photo-catalytic activity inhibition of ilmenite occurred at pH 9. The pseudo-first order rate constants, determined using Eq. (1); where k’ is the pseudo-first order rate constant, C0 the initial concentration of RB 5 and C is the concentration at time “t”. The kinetic constant rate of ilmenite reduced from 0.04047 min-1 at pH 2 to 0 at pH 9.
ln
(1)
The pH of solution is a significant parameter in photo-catalytic degradation because pH affects the ionization state of the photocatalyst surface as well as pollutants. The changes of pH can influence the adsorption of pollutants onto the surface of photo-catalyst; hence increase the photo-degradation rate of pollutants. As the PZC of ilmenite is 4.5 (Fig. 2), thus the surface of ilmenite is positively charged at pH < 4.5 and negatively charged at pH > 4.5. RB 5 is an anionic dye as the sodium ion react with water molecules and leaves the dye molecule with negative charged, hence lower pH could increase the electrostatic attraction between RB 5 dye molecules and surfaces of ilmenite and favor the adsorption reaction; the increase of adsorption led to higher degradation. It is necessary to highlight that as the lower of pH in the solution, the amount of Fe could leached out from ilmenite (Table 2), thus increase the production of radical hydroxyl group through homogenous Fenton reaction, thus dye degradation (Ct/C0) increased drastically from pH 3 to pH 2. Interestingly, Al, Mg and Si were detected as well, probably due to the impurities in Ilmenite sand, whereas Na was from Reactive Black 5 dye.
2.3 Impact of inorganic ions Several works have shown the presence of inorganic ions can affect the efficiency of photo-catalytic degradation of organic pollutants because of the competition between pollutants and inorganic ions. 0.04047, 0.01958, 0.00553, and 0.00299 of the kinetic rate of dye degradation (min-1) were observed at optimum pH condition (pH 2) with the presence of nitric acid, hydrochloric acid, sulfuric acid and phosphorus acid in the RB 5 solution respectively, the presence of nitrate ion has the highest kinetic rate. The order of inhibition of these anions are PO43- > SO42- > Cl- > NO3-. On the whole, the presence of NO3- has less effect on RB 5 dye photo-degradation than PO43-, SO42- and Cl- due to NO3- will not react with hydroxyl radicals, thus the photo-degradation rate of RB 5 dye is not inhibited [38-40]. On the other side, sulfate and phosphate ion form bond with Fe on the surface of ilmenite, reduce surface area of ilmenite for RB 5 adsorbed on it. In addition, chloride, sulfate and phosphate ions are hydroxyl radicals scavengers, which reduced the amount of hydroxyl radical generated in the photo-degradation. Cl•, SO4•- and PO4•2- are less reactive than hydroxyl radicals, thus they cannot degrade RB5 dye. Below are reactions between hydroxyl radicals, chloride, sulfate and phosphate ions [41-44]: OH• + Cl- → Cl• + OHOH• + SO42- → SO4•- + OHOH• + PO43- → PO4•2- + OH-
2.4 Impact of catalyst loading Experiment was conducted with different loadings of ilmenite from 0.5 g/L to 2.0 g/L. The result (Table 3) shows the increment of catalyst loading is directly proportional to the overall dye degradation ratio. A significant increment in degradation efficiency is shown when the catalyst loading increased from 0.5 g/L (Ct/C0 = 0.4858) to 2.0 g/L (Ct/C0 = 0.27). All employed different concentration of ilmenite obeyed pseudo-first order kinetic rate constant and the kinetic rate constant of ilmenite increased from 0.03621 min-1 (0.5 g/L) to 0.07088 min-1 (2.0 g/L). This is because of the additional catalyst in the solution provided more interface area for pollutant to be adsorbed on the surface of catalyst before further reaction begin. Besides that, more Fe could leached out from ilmenite, thus increase Fe ions in the RB 5 solution, leading higher production of hydroxyl group for photo-degradation. Complete degradation of dye happened after 120 minutes irradiation of artificial light with 2.0 g/L catalyst loading. 3. Conclusion Natural ilmenite found from magma contains rutile TiO2 and Fe. Existence of Fe in the lattice of ilmenite has significant impact on physiochemical characteristic of ilmenite. Ilmenite showed capability in the RB 5 degradation under acidic condition when irradiated with artificial light. Complete degradation of RB 5 was achieved by 2.0 g/L of catalyst loading of ilmenite after 2 h irradiation. 4. Experimental Chemical and Materials: Natural ilmenite mineral (Tor Minerals Co., Ltd., Malaysia) was used as the starting material. Ilmenite was sieved to ensure equal particle size (<45 μm) before experiment. The chemical composition of ilmenite contains TiO2 (chemical composition of FeO (12%-16%), Al2O3 (12%-16%), SiO2 (12%-16%), SiOAl2O3 (12%-16%)), Reactive Black 5 (RB 5) was purchased from Sigma Aldrich. Analytical grade reagents sulfuric acid (H2SO4), hydrochloric acid (HCl), nitric acid (HNO3), phosphoric acid (H3PO4) and sodium hydroxide (NaOH) were used. The deionized water was used to prepare experiment solutions. The pH of the RB 5 dye solution was adjusted by HNO3 and NaOH.
Characterization: Ilmenite was characterized by X-ray diffraction (D8 Discover Bruker, diffractometer Cu Kα radiation λ = 0.15406 nm, 40 kV, 30 mA) with a scanning range from 2θ = 20ᵒ-70ᵒ at step size of 0.02ᵒ/s, FESEM (JEOL JSM-7600 F), Zeta potential (Malvern Zetasizer Nano Series ZS), Brunauer-Emmett-Teller (BET) (MicroActive ASAP 2020), photoluminescence Raman spectrometer (Renishaw in Via Raman Microscope, 514 nm, 5 mW, 1 μm focus spot) and DR-UV-vis (Varian Cary 100). The elemental analysis of RB 5 was examined by ICP-OES (DV 5300, Perkin Elmer). Analysis: The photocatalytic measurement of ilmenite was conducted as batch photocatalytic reaction system. A photo-reactor with a 150 W Xenon Arc lamp (200-2500nm, Newport), magnetic stirrer and air pump was installed (Fig. 6). A 1.0 g/L of ilmenite powder was dispersed in of 5 mg/L of RB 5 dye. The reaction was set for 30 minutes in the dark followed by 20 minutes light irradiation and the measurement was initiated by switching on the Xenon lamp. The quartz reactor tube filled with solution was removed at the fixed time interval. The solution was then filtered and measured by UV-vis spectrophotometer (Hach 4000U Spectrophotometer) to determine the remaining absorbance of RB 5 at wavelength of 597 nm. In factor of pH study, the RB 5 solution was controlled initially in pH 2, 3, 4, 6 (original pH of RB 5 solution) and 9 respectively, which were set in this experiment. After obtained the optimum photocatalytic activity condition from different pH values, hydrochloric acid (HCl), nitric acid (HNO3), sulfuric acid (H2SO4) and phosphate acid (H3PO4) were used to determine the effect of photocatalytic activity in the presence of inorganic ion. Lastly, 0.5, 1.0, 1.5, and 2.0 g/L of catalyst loading was used to determine the optimum catalyst loading for RB 5 removal.
Acknowledgment This work is supported by Fundamental Research Grant Scheme (No. FRGS: FP008-2015A), Postgraduate Research Grant (No. PPP: PG050-2015A), Research Officer Grant Scheme (No. BR006-2015), Science Fund (No. MOSTI: 03-01-03-SF1032), Trans Disciplinary Research Grant Scheme (No. TR002-2015A) and Prototype Research Grant Scheme (No. PR002-2016). References [1] M.N. Chong, B. Jin, C.W.K. Chow, C. Saint, Recent developments in photocatalytic water treatment technology: a review, Water Res. 44 (2010) 2997-3027. [2] S. Esplugas, J. Giménez, S. Contreras, E. Pascual, M. Rodrı́guez, Comparison of different advanced oxidation processes for phenol degradation, Water Res. 36 (2002) 1034-1042. [3] U.I. Gaya, A.H. Abdullah, Heterogeneous photocatalytic degradation of organic contaminants over titanium dioxide: a review of fundamentals, progress and problems, J. Photochem. Photobiol. C: Photochem. Rev. 9 (2008) 1-12. [4] P.V.L. Reddy, K.H. 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Fig. 1. XRD pattern of ilmenite (R: rutile TiO2; I: ilmenite).
Fig. 2. Zeta potential of ilmenite, PZC of ilmenite from the plot is at pH 4.5.
Fig. 3. N2 adsorption-desorption BET isotherm for ilmenite. The insets show the BJH pore size distribution (from the desorption branch of the isotherms).
(a)
(c)
(c)
Fig. 4. FESEM images of the ilmenite: a) ilmenite powder; b,c) a close up on the surface of ilmenite powder.
(a)
(b)
Fig. 5. Effect of dye degradation at various pH values in the absence of ilmenite (a) and under a Xenon light (b). Dye concentration 5 ppm, temperature 25 ᵒC).
Fig. 6. Schematic diagram of the photo-reactor set up.
Table 1 Summary of physicochemical characteristic of ilmenite. Surface area (m2/g)
BJH pore size distribution (nm) 1.0-1.5
16.171
Band gap (eV)
Raman spectroscopy peak (cm-1)
PL peak/shoulder* (nm)
2.5
154, 298, 424, 546, 680, 1320
380, 410, 445, 465*, 490, 532, 580
Table 2 Elementary analysis of RB 5 solution at pH 2 with catalyst loading 1.0 g/L under solar irradiation for 20 min. Element Fe Al Na Mg Si
Concentration (mg/L) 0.088 0.009 0.773 0.034 0.294
Standard deviation 0.0035 0.0022 0.0089 0.0019 0.0101
RSD (%) 3.92 24.27 1.15 8.70 3.44
Table 2 Kinetic rate of different catalyst loading, coefficient of determination (R2) and efficiency of photo-degradation. Catalyst (g/L) 0.5 1.0 1.5 2.0
loading
Kinetic rate (min-1) 0.03621 0.04047 0.05048 0.07088
R2
Ct/C0 (t = 20 min)
0.9919 0.9957 0.9111 0.9535
0.4858 0.4415 0.3901 0.27