The main objective of this study was to investigate the photodegradation of azo dye Cibacron Brilliant Yellow 3G-P using Anatase, Degussa-P25 and ZnO. These semi-conductors were characterized using XRD, BET and TEM-EDX. The variation of the amount of semi-conductors significantly affect the rate of color removal. The decolorization rate increased as the catalyst dosage was increased. Other parameters were also studied, such as stirring speed, pH, and initial dye concentration. It was found that the rate of decolorization increases with the increase of stirring speed. Decolorization of about 30, 60 and 80% was respectively achieved in the case of Anatase, Degussa-P25 and ZnO at low stirring speed (50rpm). At pH = 3, the degradation rate was found to be higher than the alkaline pH, about 95.58 and 85.71% of color has been decolorized with Anatase and Degussa-P25 respectively. While using ZnO, the color removal reached maximum in acidic and alkaline solutions, more than 95% of dye was decolorized. The concentrations dye solutions less than 80ppm led to the removal rate of about 95% in the case of ZnO, while it was only about 8–15% in the case of TiO2 with the concentration more than 20 ppm.

  • Decolourization and photocatalytic degradation of Cibacron Brilliant Yellow 3G-P under UV light radiation.

  • Anatase displayed higher photocatalytic efficiency than Degussa-P25 and ZnO.

  • The photocatalytic degradation efficiency of Anatase and Degussa-P25 was observed to be higher at pH = 3.

Graphical Abstract

Graphical Abstract
Graphical Abstract
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Graphical Abstract
Graphical Abstract
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Anatase, Degussa-P25, dyes removal, heterogeneous photocatalysis, wastewater treatment, ZnO

There are several varieties of dyes generated from the textiles industries and used such as reactive dyes, direct dyes, disperse dyes, acid dyes, basic dyes, and vat dyes. Almost 45% of textiles products in the world belong to the reactive dyes (Jalali Sarvestani & Doroudi 2020). The presence of too small amounts of dyeing components from textiles industries in water is one of the main sources of severe pollution, which may cause higher visibility and has a significant effect on the photosynthetic activity, and this is a matter of concern for the effects on aquatic ecosystem and is likely to cause economic and ecological harm or harm to human health such as liver, brain and dysfunction of the kidneys (Salleh et al. 2011; Berradi et al. 2019). Reactive dyes are one of the most commonly used in the textiles industries due to their wide range of shaède gamut, their flexibility in meeting the various applications and they are thus characterized by their excellent fastness properties they may offer when they have been dyed on wool, silk, cotton and regenerated cellulose fibers (Yaseen & Scholz 2019). The presence of these substances in the environment remains a major concern. Reactive dyes are highly resistant to environmental degradation due to their fused aromatic structures, low biodegradability and thus they remain colored in wastewater for an extended period. Therefore, the increasing use of reactive dyes has highlighted the need to use a process that allows not only the possibility of dye removal and to know how about the operational conditions can be affected the but also the products that can be generated. There are different physical, chemical, and biological methods currently in use which treat the polluted water from textiles colored with dyes (Dajic et al. 2019), the choice of adequate techniques of wastewater treatment primarily depends on a variety of parameters such as type of pollutant and concentration. Advanced oxidation processes (AOP) are alternative methods developed to treat wastewaters based on the use of hydroxyl radicals (OH•) which present high oxidation potential when they are in the contact with organic pollutant to achieve complete elimination. Several researchers have investigated the use of these processes for decolorization of colored waste effluents (Ghribi et al. 2020; Dong et al. 2022; Rayaroth et al. 2022). Heterogeneous photocatalysis belongs to AOP and it has been proved to be a promising method for the treatment of wastewater contaminated with organic and inorganic pollutants. It produces more compounds which are readily biodegradable and have less toxic substances (Aoudjit et al. 2020; Olatunde et al. 2020). In photocatalytic processes, destroying organic recalcitrant compounds is governed by the combined actions of a photocatalyst comprising of an inorganic semi-conductor, a source of radiation and an oxidizing agent. Most of these investigations have used aqueous suspensions of semi-conductors (e.g., TiO2, ZnO), illuminated by UV light to photodegrade the pollutants. Cibacron Brilliant Yellow 3G-P (CBY 3G-P) are widely used in dyeing processes due to their characteristics: bright color, effective application techniques, low energy consumption for the dyeing process and high water solubility. In terms of history, photocatalysis has attracted several researchers and Fujishima & Honda in (1972) showed that when TiO2 is excited by light, the water is able to decompose into hydrogen and oxygen.

The use of semi-conductor oxides as catalysts in the heterogeneous photocatalysis have been able to deal with the environmental problems related to the removal of toxic and hazardous compounds of the wastewater. They have favorable band gap energy on account of having a filled valence band and an empty conduction band. TiO2 and ZnO have been widely used as photocatalysts due to their high activity, desirable physical and chemical properties, low cost, high specific surface area and availability (Ghribi et al. 2020; Aoudjit et al. 2021; Shathy et al. 2022). The mechanism that has been suggested regarding the heterogeneous photocatalytic degradation of dyes can be expressed by the fact that when a catalyst is exposed to UV radiation, electrons are promoted from the valence band to the conduction band and, as a consequence, an electron–hole pair is produced (Touahra et al. 2022):
formula
(1)
where and are the electrons in the conduction band and the electron vacancy in the valence band, respectively. These entities can migrate to the catalyst surface, where they can involve in a redox reaction with other species available on the surface. In most cases can react easily with surface bound H2O to generate OH• radicals, whereas can react with O2 to produce superoxide radical anion of oxygen. Then, the OH• and produced can react with the dye to generate other species which are responsible for the discoloration of the dye (Shathy et al. 2022):
formula
(2)
formula
(3)
formula
(4)
formula
(5)
formula
(6)
formula
(7)

This study investigates the removal of anionic Cibracorn Brilliant Yellow 3GP reactive dye from synthetic aqueous solutions by using a heterogeneous photocatalysis process. Three various semi-conductor oxides of pure Anatase, Degussa-P25 TiO2 and ZnO were characterized to investigate their influence for removal of colors and dye by photocatalytic degradation. The various parameters influencing photodegradation such as stirring speed, initial dye concentration and pH were studied.

Chemicals and reagents

CBY 3G-P is a synthetic azo dye and was supplied by Sigma-Aldrich®. Table 1 shows some of the dye's general characteristics. Chemical compounds, NaOH and HCl were of analytical grade. The photocatalysts TiO2 and ZnO were also supplied by Sigma-Aldrich®. The electrical conductivity of the demineralized water used throughout the preparation of all aqueous solutions must not exceed the value of 23.2 μS/cm.

Table 1

Characteristics of CBY 3G-P

Chemical formula C25H19Cll3N9NaO10S3 
Molar weight 872.97 g·mol−1 
Chemical structure  
Chemical formula C25H19Cll3N9NaO10S3 
Molar weight 872.97 g·mol−1 
Chemical structure  
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Characterizing methods

XRD patterns of all semi-conductors oxides were collected in the region of 2θ = 10°− 80° using a Siemens D500. The average particle size (p) of semi-conductor oxides was estimated using the Debye-Scherrer formula (Equation (8)):
formula
(8)
where λ = 0.15406 nm is the Cukα line, θ is the Braggs' XRD diffraction angle and β is the full width at half maximum in radians. The specific surface area was determined following BET measurements under nitrogen adsorption–desorption isotherms in a Micrometrics Tristar 3000. The surface morphology of TiO2 Anatase, TiO2-P25 Degussa and ZnO was examined by Transmission Electron Microscopy images using JEOL-JEM-1200EX at 100 kv.

Photocatalytic activity

Determination of the maximum absorption wavelength of CBY 3G-P

The UV-Vis absorption spectrum of the aqueous solution of dyestuff at room temperature measured using UV visible spectrometry LAMBDA™ 25 from PERKIN-ELMER is given in Figure 1. The maximum absorption wavelength of CBY 3G-P was located at 405 nm.
Figure 1
UV/vis spectrum of CBY 3GP 3G-P.
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UV/vis spectrum of CBY 3GP 3G-P.

Figure 1
UV/vis spectrum of CBY 3GP 3G-P.
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UV/vis spectrum of CBY 3GP 3G-P.

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Determination of the point of zero charge

The pH of solution is one of the important parameters on the photocatalytic activity of semiconductors; therefore, it is interesting to know the point zero charge (pHpzc) of Anatase, Degussa P25 and ZnO. The pHpzc of different catalysts was measured using the potentiometric titration method (Barišić et al. 2021). The pHpzc (Figure 2) of Anatase, Degussa P25 and ZnO was around 7.0, 6.6 and 8.7 respectively.
Figure 2
Determination of point zero charge (pHpzc) for different semi-conductor oxides.
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Determination of point zero charge (pHpzc) for different semi-conductor oxides.

Figure 2
Determination of point zero charge (pHpzc) for different semi-conductor oxides.
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Determination of point zero charge (pHpzc) for different semi-conductor oxides.

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Photoreactor

Photocatalytic degradation of CBY 3G-P was performed in solution using pyrex beakers of 250 mL capacity as batch reactor. All the experiments were carried out at room temperature and pressure. The reactor was shaken mechanically using Jar Tests (stuart floculateur SW6) which has stirrers with variable stirring speeds to meet the same operating conditions and to obtain accurate data and reproducible results. The reactor was illuminated from the outside with a 24W medium pressure mercury vapor lamp (for UV radiation); this light source was assembled on a support so as to be parallel to the plan of reactor.

Procedure and analysis

CBY 3G-P solutions were prepared by stirring a weighed amount of CBY 3G-P in distilled water at room temperature and pressure. Two hundred and fifty mL of CBY 3G-P solution was put into the photoreactor. Firstly, photolysis was investigated; an experiment was performed without catalysts under UV irradiation only. Secondly, the photocatalysis was studied; semi-conductors were added to the dye solutions. The reaction system was continuously stirred under controlled speed to achieve a homogeneous suspension and increase the oxygen transfer to the solution. The suspensions were stirred for at least 30 min in the dark to allow adsorption equilibrium of the system prior to irradiation. After UV light radiation, a Millipore 0.45-μm membrane filter was used to separate semi-conductors from suspension. Cibacron Brilliant Yellow concentrations during irradiation at selected time intervals were analyzed by UV visible spectrometry LAMBDA™ 25 from PERKIN-ELMER. To adjust the pH of different solutions, 0.1 M HCl or 0.1 M NaOH was added; pH was measured by STARTER 3100 pH meter. Similar experiments were carried out by varying the stirring rate (50–150 rpm), dye concentration (10–100 ppm), catalyst amount (0.1–1.5 g/L) and pH of solution (3–11).

The decolorization efficiency (%) was calculated as follows:
formula
(9)
where C0 corresponds to the initial concentration of dye before photo-irradiation; Ce corresponds to the final concentration of dye after photo-irradiation time.

The XRD analysis

Figure 3 shows the XRD pattern of TiO2 Anatase, TiO2-P25 Degussa and ZnO. The diffraction peaks of TiO2 Anatase at 25.43, 37.92, 48.03, 53.97, 55.05, 62.70, 68.80, 70.39 and 75.05 of TiO2 (Figure 3(a)) are identical with the standard pattern of Anatase reported in JCPDS card N°. 01-078-2486C. Figure 3(a) also shows the XRD pattern of TiO2-P25 Degussa. The peaks obtained for TiO2-P25 Degussa show both Anatase and rutile lines (01-089-0553C). The diffraction peaks of ZnO (Figure 3(b)) found at 2θ = 31.7, 34.4, 36.2, 47.5, 56.6, 62.8, 66.3, 67.9, and 69.1° are attributed to the hexagonal phase of ZnO. The average crystalline size of TiO2 Anatase, TiO2-P25 Degussa and ZnO, calculated from XRD result by Scherrer formula, were obtained to be 18, 18 and 25 nm respectively.
Figure 3
XRD patterns from (a) TiO2 and TiO2-P25 Degussa and (b) ZnO.
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XRD patterns from (a) TiO2 and TiO2-P25 Degussa and (b) ZnO.

Figure 3
XRD patterns from (a) TiO2 and TiO2-P25 Degussa and (b) ZnO.
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XRD patterns from (a) TiO2 and TiO2-P25 Degussa and (b) ZnO.

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Surface area analysis

BET surface area analysis reveals that TiO2 Anatase has a surface area of 54 m2 g−1 which is higher than those of TiO2-P25 Degussa (49 m2 g−1) and ZnO (40 m2 g−1).

Transmission electron microscopy analysis

TEM images of TiO2 Anatase, TiO2-P25 Degussa and ZnO are shown in Figure 4. It can be seen that the TiO2 Anatase (Figure 4(a)) and TiO2-P25 Degussa (Figure 4(b)) particles are quite uniform and monodispersed, with an average diameter of about 20 nm. The TEM image of ZnO (Figure 4(c)) depicts that the particles in the surface of the ZnO is non-uniform and the size of the ZnO particles (24 nm) is higher when compared to TiO2 Anatase, TiO2-P25 Degussa.
Figure 4
TEM image of (a) TiO2, (b) TiO2-P25 Degussa and (c) ZnO.
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TEM image of (a) TiO2, (b) TiO2-P25 Degussa and (c) ZnO.

Figure 4
TEM image of (a) TiO2, (b) TiO2-P25 Degussa and (c) ZnO.
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TEM image of (a) TiO2, (b) TiO2-P25 Degussa and (c) ZnO.

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Photolysis experiment

The influence of UV irradiation on the color removal of dye was investigated. The experiment was performed in the absence of any catalyst and with a dye concentration of 10 ppm under UV irradiation. From Figure 5, it was observed that there was no significant decolorization of dye, less than 12% of dye was decolorized after 120 min irradiation. Dye can be considered photo-stable.
Figure 5
Color removal of CBY 3G-P with UV only (C0 = 10 ppm).
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Color removal of CBY 3G-P with UV only (C0 = 10 ppm).

Figure 5
Color removal of CBY 3G-P with UV only (C0 = 10 ppm).
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Color removal of CBY 3G-P with UV only (C0 = 10 ppm).

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Effect of catalyst loading

The effect of catalyst loading on decolorization of CBY 3G-P was investigated using two forms of TiO2 (Anatase, Degussa P25) and ZnO at different concentrations, keeping all other parameters identical. The results of decolorization and percentage removal of CBY 3G-P are shown in Figure 6. CBY 3G-P at small concentrations (10 ppm) in aqueous solution was decolorized upon illumination in presence of suspended TiO2 (Anatase, Degussa P25) or ZnO. For all catalysts, a steady increase in the color removal of CBY 3G-P was observed with increasing load catalysts, followed by a slow decrease at higher loadings. In the case of ZnO and Anatase, the maximum decolorization was achieved at 0.5 g/L of catalysts, while higher decolorization was obtained at 1.0 g/L with Degussa P25. The higher decolorization with increase in catalyst loading can be attributed to the increase in catalyst surface area, which allows the increase of light absorption and consequently the creation of higher numbers of active species. However, at higher loadings beyond the optimum, part of the catalyst are in the dark and there will be a decrease in the light penetration. Comparing the efficiencies of the catalysts, ZnO is a superior catalyst, even though TiO2 also is highly efficient for the CBY 3G-P removal. The observed variation in the photocatalytique activity may be due to the impurities and density of hydroxyl groups on the catalysts. The efficiency of photocatalysts was shown to follow the order: ZnO > Anatase > Degussa P25.
Figure 6
Effect of catalyst concentration in the range of 0.1–1.5 g/L on the decolorization of CBY 3G-P: (a) TiO2 Anatase; (b) TiO2 Degussa P25; (c) ZnO; (d) efficiency under different catalyst concentration (C0 = 10 ppm, natural pH 6.42).
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Effect of catalyst concentration in the range of 0.1–1.5 g/L on the decolorization of CBY 3G-P: (a) TiO2 Anatase; (b) TiO2 Degussa P25; (c) ZnO; (d) efficiency under different catalyst concentration (C0 = 10 ppm, natural pH 6.42).

Figure 6
Effect of catalyst concentration in the range of 0.1–1.5 g/L on the decolorization of CBY 3G-P: (a) TiO2 Anatase; (b) TiO2 Degussa P25; (c) ZnO; (d) efficiency under different catalyst concentration (C0 = 10 ppm, natural pH 6.42).
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Effect of catalyst concentration in the range of 0.1–1.5 g/L on the decolorization of CBY 3G-P: (a) TiO2 Anatase; (b) TiO2 Degussa P25; (c) ZnO; (d) efficiency under different catalyst concentration (C0 = 10 ppm, natural pH 6.42).

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Effect of stirring speed

In the photocatalysis process, the mass transport limitation is an important problems, the creation of sufficient turbulence in the reactor was overcome to avoid this limitation. The effect of stirring speed on decolorization rate is shown in Figure 7. As shown, the decolorization rate of CBY 3G-P increases with increasing stirring speed. At low stirring speeds (50 rpm), the rate of decolorization of CBY 3G-P is lower because the catalyst particles are weakly mixed with the solution, whereas at higher stirring speed the mixture is improved. In the case of ZnO, no significant change was observed above 100 rpm. So, 150 rpm rotation has been chosen as the experimental stirring speed. According to Eskandari et al. (2018), photodegradation of dye without or at slow stirring resulted in sedimentation of the catalyst and photodegradation was not achieved, whereas with higher speed, no sedimentation was observed.
Figure 7
Effect of stirring speed on the color removal of CBY 3G-P (C0 = 10 mg/L, load of catalysts 0.5 g/L, pH free).
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Effect of stirring speed on the color removal of CBY 3G-P (C0 = 10 mg/L, load of catalysts 0.5 g/L, pH free).

Figure 7
Effect of stirring speed on the color removal of CBY 3G-P (C0 = 10 mg/L, load of catalysts 0.5 g/L, pH free).
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Effect of stirring speed on the color removal of CBY 3G-P (C0 = 10 mg/L, load of catalysts 0.5 g/L, pH free).

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Effect of initial pH on the photocatalysis of CBY 3G-P

pH is considered as a major factor of determining the efficiency of photocatalytic degradation process, because the pH of solution influences the charge on the photocatalyst surface (positive or negative charge), thereby affecting the interfacial electron transfer, the photo redox process and the release of oxidizing species. The experiments were performed with initial pH adjustment using HCl (1 M) or NaOH (1 M) from 3.0 to 11.0. The comparison of decolorization rate of dye at C0 = 10 ppm, in the presence of ZnO or TiO2 at the same concentration (0.5 g/L), is given in Figure 8. As seen from Figure 8(a)–8(c), for both TiO2, a higher color removal rate of CBY 3G-P was obtained in acidic medium than that under neutral and/or alkaline conditions. The maximum color rate removal efficiency of CBY 3G-P dye for the concentration of 10 ppm was achieved at 95.58 and 85.71% respectively for Anatase and Degussa P25 at pH 3. In contrast, the amount of dye removal percentage decreased with increasing pH of solution. The observed maximum decolorization under acidic conditions may result in a greater adsorption of CBY 3G-P and hence increase the colorless rate. The results could be explained by the variation of the surface charge of TiO2 with the variation of pH values, for both TiO2, the point of zero charge (pHpzc) as shown in Figure 2 were about 7.0 and 6.6 for Anatase and Degussa P25 respectively. At acidic medium, the surface of Anatase and Degussa P25 was positively charged which (pH < pHpzc) induces a strong adsorption of CBY 3G-P onto the TiO2 surface due to the electrostatic attraction of the positive charge on TiO2 surface with the dye. At alkaline pH (pH > pHpzc), the dye molecules were negatively charged and the density of TiO− groups on the semiconductor surface increase induces a repulsion of dye molecules onto the TiO2 surface. Hence, this adsorption onto the surface of semiconductor was scarce. In the case of methyl orange (Abbas & Trarib 2021) and methyl violet (Touahra et al. 2022), using TiO2 as a semiconductor, higher decolorization efficiency was reported under acidic conditions. In the case of ZnO, decolorization was reached an initial maximum at pH 4, then it decreased around the neutral region and picked up again to reach a maximum at pH 11. The color removal rate at higher pH was better which may be due to the enhanced formation of hydroxyl radicals, which are strong oxidizing species. In alkaline solution water is adsorbed on the exited surface of ZnO semiconductor, which present the amphotric propriety, water adsorbed is decomposed producing OH−. Hence, more OH• radicals are reduced by the oxidation of the OH− by the hole (Miao et al. 2014). Several works have studied the photocatalytic degradation of dye using ZnO and different results were observed. Jia et al. (2016) reported that the degradation rate of Cibacron Brilliant Yellow decreased on acidic or alkaline medium, while Miao et al. (2014) observed that color removal of mordant black 11 decreased with a decrease in pH value except at pH ∼ pHpzc of ZnO.
Figure 8
Effect of pH value on the decolorization of CBY 3G-P: (a) TiO2 Anatase; (b) TiO2 Degussa P25; (c) ZnO; (C0 = 10 ppm, catalysts concentration 0.5 g/L).
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Effect of pH value on the decolorization of CBY 3G-P: (a) TiO2 Anatase; (b) TiO2 Degussa P25; (c) ZnO; (C0 = 10 ppm, catalysts concentration 0.5 g/L).

Figure 8
Effect of pH value on the decolorization of CBY 3G-P: (a) TiO2 Anatase; (b) TiO2 Degussa P25; (c) ZnO; (C0 = 10 ppm, catalysts concentration 0.5 g/L).
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Effect of pH value on the decolorization of CBY 3G-P: (a) TiO2 Anatase; (b) TiO2 Degussa P25; (c) ZnO; (C0 = 10 ppm, catalysts concentration 0.5 g/L).

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Effect of initial concentrations of CBY on the photocatalytic degradation rate

Figure 9 shows the results of the decolorization of dye at different initial concentrations in the range between 10 and 100 ppm vs time of irradiation. For a constant Anatase dosage (0.5 mg·L−1), at the initial dye concentration in the range of 50–100 ppm, no significant difference in decolorization efficiency was observed; however, increasing in the concentration of dye from 10 to 20 ppm, the decolorization efficiency of CBY 3G-P decreased from 89 to 34% in 120 min of irradiation. In the case of ZnO, the percentage of dye removal decreased as the dye concentration increased, however, efficient dye removal was observed when the initial dye concentration was lower than 80 ppm. Therefore, for both photocatalysts, the color removal efficiency increased as initial dye concentration decreases. This may be explained that at higher concentrations of dye many dye molecules were adsorbed on the catalysts surface thus resulting in less numbers of photons reaching the catalysts surface and then the OH• will be reduced (Rauf et al. 2011). Moreover, as the amount of dye increased more inorganic ions were formed, including sulfate and nitrate ions, in aqueous solution, which compete with the dye molecules and the intermediates organic products for the reaction with oxidative species (Bukallah et al. 2007).
Figure 9
Effect of initial concentration of CBY 3G-P on the decolorization: (a) TiO2 Anatase, (b) ZnO (free pH, catalysts concentration 0.5 g/L).
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Effect of initial concentration of CBY 3G-P on the decolorization: (a) TiO2 Anatase, (b) ZnO (free pH, catalysts concentration 0.5 g/L).

Figure 9
Effect of initial concentration of CBY 3G-P on the decolorization: (a) TiO2 Anatase, (b) ZnO (free pH, catalysts concentration 0.5 g/L).
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Effect of initial concentration of CBY 3G-P on the decolorization: (a) TiO2 Anatase, (b) ZnO (free pH, catalysts concentration 0.5 g/L).

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Kinetic study of photocatalytic degradation of CBY

Experimental results showed that the photocatalytic removal of the dye have exponential curves and fit entirely a pseudo-first-order photocatalytic degradation kinetic reaction that is expressed as follows:
formula
(10)
The integration of Equation (10) (C = C0 at t = 0) will lead to the expected relation:
formula
(11)
with C0 initial concentration of dye after dark adsorption at t = 0, C is the dye concentration at irradiation time t and kapp the apparent rate constant. As illustrated in Figure 10, for the initial concentration range, the variation in ln (C0/C) related to irradiation time gave a straight line (0.96 < R2 < 0.99) proving that all reactions follow a pseudo-first-order. The slopes give the apparent rate constants kapp listed in Table 2, the apparent first-order rate constant kapp decreases with the increasing of the initial concentration of CBY 3G-P. Similar results have been obtained by photodegradation of other dyes (Kansal et al. 2007; Liu et al. 2010). This was due to the fact that as the initial concentration of the dye increased, so did the concentration of unabsorbed dye in the solution, resulting in less penetration of light through the solution onto the surface of the catalyst, thereby decreasing the concentration of radicals on the surface and thus decreasing the degradation rate. However, at lower dye concentrations, where the light intensity and time of irradiation remain constant but photon interception to the catalyst surface increases, results in the formation of more radicals and thus an increase in the rate of degradation (Subramani et al. 2007).
Table 2

Variation of apparent rate constants kapp versus initial concentration of CBY 3G-P

Anatase
ZnO
[CBY 3G-P]0 (mg/L)kapp (min−1)R2kapp (min−1)R2
10 0.0193 0.980 0.0383 0.960 
20 0.0047 0.971 0.0310 0.986 
50 0.0011 0.980 0.0262 0.990 
80 0.0013 0.985 0.0097 0.999 
100 0.0013 0.968 0.0055 0.970 
Anatase
ZnO
[CBY 3G-P]0 (mg/L)kapp (min−1)R2kapp (min−1)R2
10 0.0193 0.980 0.0383 0.960 
20 0.0047 0.971 0.0310 0.986 
50 0.0011 0.980 0.0262 0.990 
80 0.0013 0.985 0.0097 0.999 
100 0.0013 0.968 0.0055 0.970 
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Figure 10
Effect of initial concentration of CBY 3G-P on the pseudo-first-order apparent rate: (a) TiO2 Anatase, (b) ZnO (free pH, catalysts concentration 0.5 g/L).
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Effect of initial concentration of CBY 3G-P on the pseudo-first-order apparent rate: (a) TiO2 Anatase, (b) ZnO (free pH, catalysts concentration 0.5 g/L).

Figure 10
Effect of initial concentration of CBY 3G-P on the pseudo-first-order apparent rate: (a) TiO2 Anatase, (b) ZnO (free pH, catalysts concentration 0.5 g/L).
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Effect of initial concentration of CBY 3G-P on the pseudo-first-order apparent rate: (a) TiO2 Anatase, (b) ZnO (free pH, catalysts concentration 0.5 g/L).

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Suggested mechanism

Once UV light is absorbed by semi-conductors (Anatase, Degussa P25 and ZnO), peroxide species are generated which can later dissociate to give the desired hydroxyl radicals as per the equations shown below (Equations (12)–(14)) (Jia et al. 2016):
formula
(12)
formula
(13)
formula
(14)
From the chemical structure of CBY-3G-P, the diazo group is selectively oxidized to form nitrocompounds which are further decomposed, always under the influence of the hydroxyl radicals, to result in other advanced intermediates and ultimately form CO2, HNO3, (NH2)2, HCO2H, NH3, H2SO4 and NaOH (Figure 11).
Figure 11
Suggested mechanism of photo degradation of CBY 3G-P.
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Suggested mechanism of photo degradation of CBY 3G-P.

Figure 11
Suggested mechanism of photo degradation of CBY 3G-P.
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Suggested mechanism of photo degradation of CBY 3G-P.

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Decolorization of CBY 3G-P in aqueous solution has been studied using two different processes: photolysis (without semi-conductor) and heterogeneous photocatalysis in the presence of three semi-conductors (Anatase, Degussa P25 and ZnO). The results show that the presence of catalyst is more essential for efficient dye removal. Operational parameters, namely stirring speed, catalysts dosage, dye concentration, pH of solution and initial dye concentration, affect the rate of dye decolorization. Decolorization of dye is very effective at high stirring speed, the results indicate that there exists an optimum catalysts loading (0.5 g/L for both catalysts: Anatase, ZnO and 1 g/L for Degussa P25) where the color removal rates reaches a maximal value. From an efficiency catalytic standpoint, ZnO showed a better activity for color removal by heterogeneous photocatalysis compared to Anatase and Degussa P25. The decolorization rate increases with a decrease in the initial dye concentration. The experimental results showed that the kinetics of photocatalysis color removal follows a pseudo-first-order kinetic and depends on the initial dye concentration. The apparent rate constant increased when decreasing the concentration of initial dye concentration.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.

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