Titanium Dioxide Supported on Different Porous Materials as Photocatalyst for the Degradation of Methyl Green in Wastewaters

Bel Hadjltaief, Haithem; Omri, Abdessalem; Ben Zina, Mourad; Da Costa, Patrick; Galvez, Maria Elena

doi:https://doi.org/10.1155/2015/759853

Advances in Materials Science and Engineering

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Research Article | Open Access

Volume 2015 | Article ID 759853 | https://doi.org/10.1155/2015/759853

Titanium Dioxide Supported on Different Porous Materials as Photocatalyst for the Degradation of Methyl Green in Wastewaters

Haithem Bel Hadjltaief,¹Abdessalem Omri,¹Mourad Ben Zina,¹Patrick Da Costa,^2,3and Maria Elena Galvez^2,3

Academic Editor: Charles C. Sorrell

Received14 Apr 2015

Revised03 Aug 2015

Accepted03 Aug 2015

Published13 Oct 2015

Abstract

TiO₂ nanoparticles were immobilized on two porous materials used as catalyst supports, namely, activated carbon (AC) and natural clay (NC), through an impregnation process using TiO₂ (P25) as precursor. The so-prepared composite materials were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transition electron microscopy (TEM), and nitrogen physisorption, that is, Brunauer-Emmett-Teller (BET) surface area determination. SEM and TEM observation evidenced that TiO₂ was deposited on AC and NC surface. XRD results confirmed that TiO₂ existed in a mixture of anatase and rutile phases. The specific surface area of photocatalysts decreased drastically in comparison with the original materials. The photocatalytic activity of these materials was assayed in the oxidation of Methyl Green (MG) dye in aqueous medium under UV irradiation. TiO₂/AC exhibited higher photocatalytic oxidation activity than TiO₂ at neutral pH. Total mineralization of MG was confirmed by means of COD analysis, pointing to these materials as an efficient, cost-effective, and environment friendly alternative for water treatment.

1. Introduction

Water pollution has become a subject of global environmental concern in recent years. Dyes from different industries such as dye synthesis, paper, printing, electroplating, food, and cosmetics are one of the major sources of water pollution [1]. Various types of dyes exist such as azo, anthraquinone, reactive, acidic, basic, neutral, disperse, and direct dyes. Azo and anthraquinone dyes are however the most commonly used ones [1–3]. Methyl Green (MG) is a basic triphenylmethane and a dicationic type dye usually used for staining of solutions in medicine and biology [4]. It is also used as a photochromophore to sensitize gelatinous film [5].

Advanced oxidation processes (AOPs) involve the generation of highly reactive radical species by several ways [6–9]. These techniques are based on the production of reactive species such as hydroxyl radicals (HO^•), which are powerful oxidizing agents and are capable of attacking a wide variety of organic molecules. AOPs photocatalysis systems include combination of semiconductors and light and semiconductor and oxidants. Heterogeneous photocatalysis is the most promising method of AOPs for toxic components in wastewater, such as phenols, herbicides, pesticides, dyes, and surfactants [7, 8]. Titanium dioxide (TiO₂) is the most widely used photocatalyst owing to its efficiency, low cost, and chemical stability [6, 10]. TiO₂ powders such as Degussa P25 have been commercially available for several years. Nevertheless, after degradation tests, photocatalysts in the powder form must be removed from the suspension and this step is an important factor that may cause important problems in industrial applications [6, 11]. To solve these problems, photocatalysts (e.g., TiO₂) have been immobilized on various supports such as silica [12], silicon carbide [13], perlite [14], fly ash [15], zeolites [16], clay [17], and activated carbon [10, 18]. Also, several preparation methods for TiO₂ on these supports, such as hydrothermal method, sol-gel method, boil deposition, dip coating, metal organic chemical vapor deposition (MOCVD), and impregnation, have been reported [19, 20].

The advantages of using TiO₂ supported photocatalysts were summarized as follows: (i) The support adsorbs a high amount of pollutants around the loaded TiO₂. Then, the rate of photooxidation is improved [20]. (ii) The adsorbed pollutants (organic substances) are oxidized at the surface of the photocatalyst, and the resulting intermediates are also adsorbed and then further oxidized. Toxic intermediates, if formed, are not released in the air atmosphere and/or in solution phase, thus preventing secondary pollution. (iii) Since the pollutants are finally oxidized into CO₂, the lifetime of the hybrid photocatalysts is long [15].

The purpose of the present work is to load TiO₂ on the surface of both porous materials, that is, activated carbon (AC) and natural clay (NC), in order to prepare active and stable catalysts for the photocatalytic degradation of Methyl Green dye in wastewaters.

2. Experimental

2.1. Materials

The clay used in this study was sampled in Jebel Tejera-Esghira deposits located in the southeast of Tunisia from the area of Medenine. The natural clay was first purified by dispersion in water, decantation, and extraction of the fraction with a particle size smaller than 2 μm. Then, the natural clay was modified by sodium exchange described in our previous work [3, 21]. Activated carbon (AC) was prepared from the physical activation of Lawsonia inermis wood, which was obtained from the field in Gabès (Tunisia). The preparation process has been described in detail elsewhere [22]. Commercial TiO₂ (Degussa P25, Degussa Chemical) was used as titanium source for the preparation of photocatalysts. Methyl Green (MG, dicationic dye, chemical formula C₂₇H₃₅Cl₂N₃·ZnCl₂, g/mol, Sigma-Aldrich) was used to evaluate the photocatalytic efficiency of synthesized photocatalysts.

2.2. Synthesis of Photocatalytic Materials

TiO₂/AC and TiO₂/NC photocatalytic materials were prepared through an impregnation method. First, different weights of AC or NC were added under stirring to a TiO₂ (P25) aqueous suspension (mixing 30 mg of TiO₂ (P25) and 100 mL of distilled water). After the impregnation process, the samples obtained were washed by double-distilled water, filtered, and then dried at 110°C for 24 h. Finally, the dried material was calcined to 500°C for 3 h in a nitrogen-purged reactor tube with a heating rate of 5°C/min. The steps of the photocatalysts preparation are presented in Figure 1.

2.3. Characterization of Photocatalysts

Several techniques were employed for the characterization of the samples. Samples were also examined under a scanning electron microscope (SEM, Hitachi SU-70). The structural feature of these samples was observed at the accelerated voltage of 1.0 kV. High resolution transmission electron microscopy (HRTEM) images were thus acquired (JEOL JEM 2011) equipped with LaB6 filament and operating at 200 kV. The images were collected with a pixels’ CCD camera (Gatan Orius SC1000) coupled with the Digital Micrograph software. Chemical analyses were obtained by an EDX microanalyzer (PGT IMIX PC) mounted on the microscope. The catalysts were dispersed in ethanol and sonicated. A drop of the dispersion was deposited on a carbon-coated copper grid for the TEM observations.

Nitrogen adsorption-desorption isotherms were measured at −196°C (Fisons Sorptomatic 1990) after outgassing (10⁻⁵ Pa) for 24 h at ambient temperature. The specific surface area, , was determined according to the Brunauer-Emmett-Teller (BET) method at the relative pressure, , in the range of 0.05–0.1.

The porosity of the photocatalysts was measured by pycnometry. In order to determine the real density (), water was chosen since it can penetrate pores [23], whereas, for the apparent density (), mercury was chosen since it does not penetrate into the porous network. The porosity () can be calculated by the following equation:

For the crystal phase composition and the crystallite size of the photocatalysts, X-ray diffraction measurements were carried out at room temperature using an X-ray diffractometer (Philips PW 1710 diffractometer) (Cu Kα, 40 kV/40 mA, scanning rate of per min). The crystallite size was calculated by X-ray line-broadening analysis using Scherrer equation [17, 19].

2.4. Photoreactor and Photodegradation Procedure

Photocatalytic oxidation experiments were performed in an open Pyrex-glass cell with 250 mL capacity (of 5 cm inside diameter and 11 cm height). The design description of photocatalytic reactor was reported in our earlier study [3]. Irradiation was carried out using an UV-lamp (Black-Ray B 100 W UV-lamp, V-100AP series) with a wavelength of 365 nm.

A suspension was prepared by adding appropriate amount of support TiO₂/AC or TiO₂/NC to 200 mL of MG aqueous solution (75 mg/L) at pH 7. The pH of suspension was measured using PH-035 digital pH meter from Changlilai Technology Co., Ltd. (Shenzhen, China), by adding H₂SO₄ (0.1 mol/L) or NaOH (0.1 mol/L). All the experiments were performed at an ambient temperature (25 ± 1°C) and atmospheric pressure. During the experiment, air was continuously bubbled into the catalyst suspension. Prior to irradiation, the suspension was magnetically stirred in the dark for at least 60 min to ensure the establishment of an adsorption/desorption equilibrium. The experimental work of this study involved several parameters such as the pH, catalyst loading, initial dye concentration, and H₂O₂ concentration. Firstly, for examining the effect of pH, the pH of MG solution was adjusted using 0.1 HCl and 0.1 NaOH aqueous solutions to be 3 to 11, while fixing the MG concentration at 75 mg/L and the dosages at 1 g/L of the photocatalysts. Catalyst concentration was chosen from 0.25 to 1.75 g/L, for 75 mg/L of MG solution, at . The influence of the MG dye concentration was also evaluated by varying its concentration range from 25 mg/L to 150 mg/L, at a fixed pH of 7 in the presence of 0.75 g/L. Finally, the influence of the H₂O₂ amount added to the MG solution was evaluated by both dosages 8 mL and 16 mL (1000 mg L⁻¹ of H₂O₂ solution, prepared from H₂O₂ (30%) Merck reagent), at a fixed pH of 7 and MG concentration of 75 mg/L, in the presence of 0.75 g/L.

During reaction, about 4 mL of aliquots was sampled and separated by filtration using PTFE filters (0.45 μm) to determine the residual MG concentration using UV-Vis spectrophotometer (Shimadzu 2450, Japan) at the maximal adsorption wavelength of MG nm. Chemical Oxygen Demand (COD) was determined using the reactor digestion method based on the method of acidic oxidation by bichromate [24].

Discoloration efficiency and mineralization efficiency were calculated as follows: where and denote the time-dependent concentration/COD and the initial concentration/COD, respectively.

3. Results and Discussion

3.1. Characterization of the Prepared Photocatalysts

The structure and morphology of the catalyst are very important parameters since they strongly determine its photocatalytic behavior. SEM images of NC and AC before and after deposition of TiO₂ are shown in Figure 2. From the micrograph, it is clear that the surface morphologies of NC (Figure 2(a)) and AC (Figure 2(b)) have a porous nature with uniform structures. After impregnation process, the TiO₂ particles are found to be well-dispersed. However, the immobilization of TiO₂ in both supports partially blocks the porosity. TEM images also confirm that TiO₂ was successfully loaded onto the surface of NC and AC at an average particle size of about 10 nm. The presence of TiO₂ in the prepared materials was confirmed by EDX analysis (Table 1). TiO₂/AC evidences the highest TiO₂ concentration of 23.7 wt.%.

(a)

(b)

(c)

(d)

(e)

(f)

The BET surface and pore volume of the prepared samples are presented in Table 1. The large decrease in the surface area, total pore volume, and porosity of TiO₂/NC and TiO₂/AC indicates that the pores of AC or NC are remarkably blocked by the loaded TiO₂. The presence of an anatase phase for TiO₂ can further contribute to this reduction in the surface area and pore volumes due to its presence within the micro-meso texture. Thus, the percentage of anatase can be considered to be proportional to the reduction in surface area and pore volume. Thus, this parameter may be used as an inverse indicator of the loss of surface area and porosity in these materials.

The X-ray diffraction (XRD) pattern of pure TiO₂ (P25), TiO₂/AC, and TiO₂/NC is shown in Figure 3. The XRD diffraction patterns for the natural clay evidenced quartz, kaolinite, and illite coexisting as the main crystalline phases [17, 21]. The XRD for the activated carbon evidences two wide diffraction peaks typical of graphitic structures in activated carbon [22]. Upon TiO₂ loading, the XRD patterns of TiO₂/NC and TiO₂/AC photocatalysts show the appearance of peaks at 2θ values of 25.281°, 37.934°, 48.376°, 55.296°, and 62.728° corresponding to the (101), (103), (200), (105), and (213) planes of anatase TiO₂ [18], JCPDS number 21-1272. The peaks at 2θ values of 27.4° and 36.1° are characteristic of rutile TiO₂, JCPDS number 21-1276. The crystallization of amorphous TiO₂ into anatase occurs at about 380°C [25]. The transformation of anatase into rutile takes place between 550°C and 650°C [25]. This is the reason why, upon calcination at 500°C for 3 h, the rutile phase appears in TiO₂/AC and TiO₂/NC materials. As listed in Table 1, the relative contents of anatase and rutile are estimated using the Spurr-Myers equation [26]:where is the relative content of anatase and and are the intensities of the anatase (101) peak at and the rutile (101) peak at . The anatase content of materials is 94% and 89% for TiO₂/AC and TiO₂/NC, respectively. These values show that the anatase phase is highly predominant in all cases. The main active crystal phases of TiO₂ are anatase and rutile. Out of the two TiO₂ phases, anatase is typically more active in photocatalytic reactions [18, 19]. According to Ambrus et al. [27], photocatalysts containing the anatase phase are more active than rutile-only catalyst.

The TiO₂ crystal sizes in TiO₂/NC and TiO₂/AC materials were calculated using Scherrer’s equation through the XRD line-broadening method:In this equation, is the crystallite size of the photocatalyst, is dimensionless constant (0.9), is the wavelength of X-ray (Cu Kα radiation Å), is the full width at half-maximum (FWHM) of the diffraction peak, and is the diffraction angle. The average crystallite size for TiO₂/NC was found to be 18.5 nm, whereas a crystallite size of 21.8 nm was estimated for TiO₂/AC.

3.2. Photocatalytic Degradation of Methyl Green

Figure 4 shows the results of the photocatalytic experiments in the presence of the different materials prepared and under different reaction conditions. The photolysis of MG under UV irradiation, without the presence of photocatalysts, results in negligible degradation of the dye, at least during the 180 min duration of the test. Regarding the direct adsorption of MG on the surface of the photocatalysts (tests in the absence of UV irradiation), the amount of MG adsorbed onto TiO₂/AC irradiation was found to be higher than for TiO₂/NC. This fact can be assigned to the larger surface area of this material and to the well-known good adsorption properties of activated carbon. For the sake of comparison, the nonimpregnated supports were tested in the photocatalytic degradation of MG: 53.7% and 39.5% of the dye were, respectively, removed by the activated carbon and natural clay from 100 mg/L MG solutions after 60 minutes of reaction. These percentages of MG degradation can be only related to enhanced absorption of the organic dye on the support surface. The incorporation of TiO₂ results in extensive pore blockage and in a considerable reduction of the adsorption capacity. TiO₂ supported on activated carbon or on natural clay is more active than pure TiO₂ (P25) powder. It can be seen that the removal of MG using pure TiO₂ (P25) reached 52.8% within 60 min irradiation time. In the presence of TiO₂/AC and TiO₂/NC, MG degradation reached 98.6% and 90.2%, within the same irradiation time. Note that the photocatalytic activity of TiO₂/AC is higher than that of TiO₂/NC. This result is in correlation with the physicochemical properties of materials; the anatase content and the specific surface area of TiO₂/AC were higher than those of TiO₂/NC (Table 1). Moreover, better light absorption properties of a “black” material such as activated carbon can be expected.

It has been established that photocatalyzed degradation of organic pollutants in aqueous solution is initiated by the photoexcitation of TiO₂, followed by the formation of an electron-hole pair on the surface of catalyst [7, 18, 19]:The high oxidation potential of the hole () allows the oxidation of organic pollutant (dye) to the degradation products:However, very reactive hydroxyl radicals (^•OH) can also be formed either by the decomposition of water or by the reaction of the hole with OH⁻:^•OH is an exclusively strong, nonselective oxidant, which conducts the partial or complete mineralization of organic pollutants [7, 8].

Electrons () in the conduction bond on the TiO₂ surface can reduce molecular oxygen to superoxide anions. These superoxide anions are responsible for the generation of ^•OH that has been indicated as the primary source of pollutant degradation [19]:

3.3. Effect of pH

Wastewaters from textile industries usually have a wide range of pH. Thus, pH plays an important role in photodegradation processes [4, 8, 28]. The effect of the initial solution pH on the photocatalytic degradation in the presence of TiO₂ supported on AC and NC is presented in Figure 5. MG removal increases with increasing initial pH of the dye solution. The influence of pH on photocatalytic activity has been related to the surface charge properties of the photocatalysts and could be explained based on the point of zero charge (pzc). The point of zero charge (pzc) of TiO₂ is 6.8 [28]. In acidic media (pH < 6.8), the surface of TiO₂ is positively charged, whereas it is negatively charged under alkaline conditions (pH > 6.8) according to Methyl Green, a cationic dye, is positivity charged due to the ammonium groups which are ionized in water; their electrostatic attraction to the catalyst surface is favorable in basic solution and hindered in acidic media, due to the columbic repulsion between the positively charged photocatalyst surface and the positively charged dye molecules [4, 28, 29]. Thus, interaction and thus reaction might be favored at high pH. Moreover, the generation of HO^• radicals by the effect of UV irradiation may also be responsible for increasing reaction rate in a basic environment. In acidic media (pH < 7), such radical species are rapidly scavenged and therefore the reaction rate decreases, thus resulting in favored reaction at high pH.

3.4. Effect of Catalytic Concentration

In order to avoid the use of excess catalyst, it is necessary to find out the optimum loading for efficient removal of the organic compounds in wastewaters. Several authors have investigated the reaction rate as a function of catalyst loading in photocatalytic degradation process [10, 29–31]. The effect of catalyst concentration on degradation of the MG dye over TiO₂, TiO₂/NC, and TiO₂/AC is shown in Figure 6. The catalyst loading changed from 0.25 to 1.75 g/L. The figure shows that the percentage of removal of MG is enhanced when the amount of catalyst in the reactor increases up to 1 g/L over TiO₂ (P25) and up to 0.75 g/L over both TiO₂/NC and TiO₂/AC and then decreases for higher catalyst loads. This increase in the degradation efficiency in the presence of an increased amount of catalyst in the solution is obviously due to the increase of the active material and thus TiO₂ active sites, resulting in enhanced free hydroxyl radical generation [29–31]. However, for catalyst loads higher than 1 g/L, a steady state is reached in terms of degradation since a plateau is reached considering the experimental errors.

3.5. Effect of Initial Dye Concentration

Dye concentration was varied between 25 and 150 mg/L with constant catalyst loading at pH 7. The influence of initial dye concentration is shown in Figures 7(a), 7(b), and 7(c), in the presence of TiO₂ (P25), TiO₂/NC, and TiO₂/AC, respectively, where a plot of versus time of irradiation is represented. From the results, the photocatalytic degradation of MG followed pseudo-first-order kinetics (Table 2). A linear relation between dye concentration and irradiation time has been observed. It was found that the increase in the dye concentration decreases the removal rate. The results are listed in Table 2. One can conclude that TiO₂/AC catalyst is more efficient than TiO₂/NC, with the ratio of apparent rate varying from 1.2 to 1.6 depending on the conditions.

(a)

(b)

(c)

Similar results have been reported for the photocatalytic oxidation of other dyes [1, 17, 30]. The rate of degradation relates to the rate of ^•OH radicals formation and to the probability of ^•OH radicals to react with the dye molecules. Since the generation of radicals remains constant for each set of reaction conditions, the probability of dye molecule to react with hydroxyl radical decreases as dye concentration decreases, resulting in lower reaction rate. However, at relatively high initial dye concentrations, the path length of the photons entering the solution also decreases. Thus, the photocatalytic degradation efficiency decreases within the range of very high dye concentrations [16, 30, 32].

3.6. Effect of Addition of H₂O₂

Addition of Hydrogen Peroxide (HP) to TiO₂ suspensions is a well-known strategy towards increasing the degradation efficiency of organic pollutants in wastewaters [7, 8, 32, 33]. H₂O₂ is considered to have two functions in the photocatalytic oxidation. It accepts a photogenerated electron from the conduction band of the semiconductor to form ^•OH radical (reaction (14)), and, in addition, it forms ^•OH radicals according to reaction (15) [8]:Figure 8 shows the results of the photodegradation experiments carried out at both H₂O₂ dosages 8 mL and 16 mL, at pH = 7, in the presence of 0.75 g/L photocatalysis and UV irradiation. Results point out an optimal oxidant dosage. Excess of H₂O₂ can act in fact as ^•OH scavenger resulting in the generation of perhydroxyl radical () (see (16)–(18)), a less strong oxidant as compared to hydroxyl radicals [3, 8]:

3.7. UV-Visible Absorption and COD Analysis

Mineralization of MG (complete oxidation) in the presence of the different photocatalytic materials has been evaluated by means of following the evolution of UV-Vis absorption and by means of COD measurements. Degradation experiments were performed at pH of 6.51, 75 mg/L dye concentration, 100 mg of photocatalyst, and 8 mL of H₂O₂. Solutions were then analyzed every 30 minutes. Figure 9 shows the evolution of the UV-Vis spectra. Four absorption peaks can be observed in the spectrum corresponding to the initial MG solution: the peak observed in the visible region at 631 nm is due to the green color of the chromophore of MG, and three other peaks at 420 nm, 315 nm, and 256 nm correspond to the benzoic rings in the dye structure. Upon 30 min irradiation, in the presence of the TiO₂/AC catalyst, the peak corresponding to the dye chromophore completely disappeared. COD analyses were performed to confirm the complete mineralization of MG. The percentage of COD elimination is presented in Table 3. After 60 min irradiation in the presence of TiO₂/AC and TiO₂/NC, 98.80% and 95.11% of COD elimination are obtained, respectively. These results confirmed that TiO₂/AC are more efficient for MG degradation, as already observed from kinetic study (Figure 7).

3.8. Reusing of the Photocatalytic Materials

In order to study the reusability of the proposed photocatalysts, repeated degradation experiments were performed under optimized conditions. At the end of the degradation process, the suspension was centrifuged and the catalyst washed with water and dried at 120°C for future uses (Figure 10). The decrease in degradation efficiency is not significant for the supported TiO₂ materials, TiO₂/AC and TiO₂/NC, whereas, for the nonsupported titania powder, TiO₂ (P25), the decrease in activity is really noticeable. These results prove the positive effect of using a porous support for stabilizing TiO₂ particles, resulting not only in higher photocatalytic activity but also in enhancing the stability of the proposed materials.

4. Conclusions

TiO₂ nanoparticles were immobilized on the surface of two different supports, natural clay and activated carbon. The resulting photocatalysts TiO₂/AC and TiO₂/NC were characterized by means of XRD, SEM, TEM, and N₂ physisorption. TiO₂/AC was found to be more efficient towards UV-photocatalytic degradation of Methyl Green in aqueous solution than TiO₂/NC. This can be due to the particular physicochemical properties of the AC support together with increased light absorption related to its optical properties. Both TiO₂ supported catalysts were considerably more efficient than the nonsupported TiO₂ (P25) powder, pointing to a positive effect of TiO₂ nanoparticle immobilization on both supports. Degradation was found to be slightly more efficient at high pH. The optimum catalyst loading for efficient removal was found to be 0.75 g/L, for both supported catalysts. The evolution of UV-Vis spectra together with COD measurements confirmed the complete mineralization of the organic dye. After 60 min irradiation in the presence of TiO₂/AC and TiO₂/NC, 98.80% and 95.11% of COD elimination were, respectively, measured. Moreover, using AC and NC as supports for immobilizing TiO₂ nanoparticles resulted in enhancement of the stability of the supported photocatalysts, in comparison to nonsupported TiO₂ powders.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

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Copyright © 2015 Haithem Bel Hadjltaief et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Advances in Materials Science and Engineering

Titanium Dioxide Supported on Different Porous Materials as Photocatalyst for the Degradation of Methyl Green in Wastewaters

Abstract

1. Introduction

2. Experimental

2.1. Materials

2.2. Synthesis of Photocatalytic Materials

2.3. Characterization of Photocatalysts

2.4. Photoreactor and Photodegradation Procedure

3. Results and Discussion

3.1. Characterization of the Prepared Photocatalysts

3.2. Photocatalytic Degradation of Methyl Green

3.3. Effect of pH

3.4. Effect of Catalytic Concentration

3.5. Effect of Initial Dye Concentration

3.6. Effect of Addition of H2O2

3.7. UV-Visible Absorption and COD Analysis

3.8. Reusing of the Photocatalytic Materials

4. Conclusions

Conflict of Interests

References

Copyright

3.6. Effect of Addition of H₂O₂