TiO₂/clay as a heterogeneous catalyst in photocatalytic/photochemical oxidation of anionic reactive blue 19

2.1 Materials and chemicals

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 a sodium exchange described in our previous work (Bel Hadjltaief et al., 2014).

Titanium(IV)-isopropoxide, (Ti(OPrⁱ)₄, TTIP, 97%) purchased from Aldrich and obtained from Degussa Chemical was used as titanium source for the preparation of TiO₂−Clay photocatalyst. Anionic reactive blue 19 (RB19) was purchased from Aldrich chemical company. Hydrogen peroxide (H₂O₂(35%)), potassium peroxodisulfate (K₂S₂O₈ (99%)), sodium carbonate (97%), acetonitrile, sodium hydroxide, hydrochloric acid and phosphoric acid, high-purity analytical grade reagents were supplied by Sigma–Aldrich.

The TiO₂−clay photocatalyst was prepared by a typical impregnation method. First, a fixed amount (5.0 g) of Na⁺−clay (particle size < 63 μm) was added under stirring into the 20 mL of TTIP solution. The temperature of the mixing has been fixed at 150 °C in order to activate the reaction of titanium deposition during 5 h. After the impregnation process, the samples were washed with Milli-Q water to remove any free TiO₂ particles that were not physisorbed or chemisorbed to the Na⁺−clay support. The TiO₂−clay samples have been filtered, and dried at 125 °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.

2.2 Raw clay and photocatalyst physico-chemical characterization

Several techniques were employed for the characterization of the samples. In order to determine the crystal phase composition and the crystallite size of the photocatalysts, X-ray diffraction measurements were carried out at room temperature using a X-ray diffractometer and Philips® PW 1710 diffractometer (Cu Kα, 40 kV/40 mA, scanning rate of 2θ per min). The crystallite size was calculated by X-ray line-broadening analysis using Scherrer equation. S_BET surface areas of the solids were measured by nitrogen adsorption at −196 °C using a Micrometitics ASAP 2010 analyzer. The samples with particles of a size lower than 100 m were firstly degassed at 60 °C for 72 h. The surface functional groups were studied by using Fourier transform infrared (FT-IR) spectroscopy (IR, Digilab Excalibur FTS 3000 spectrometer). The morphology of the samples was studied using scanning electronic microscopy (SEM, Hitachi SU-70). Thermal analysis measurements were performed using a Set Soft 2000 apparatus. The dried samples were treated under nitrogen; the temperature was raised at a rate of 100 °C min⁻¹ from room temperature to 900 °C.

2.3 Photo-catalytic reactor and activity tests

Photocatalytic performances of TiO₂–Clay catalysts were evaluated in the photodegradation of anionic reactive blue 19 (RB19) under either artificial solar light or UV-lamp (UV-A (365 nm), Black-Ray B 100 W UV-lamp, V-100AP series). In a typical process, aqueous solution of RB19 (75 mg/L, 200 mL) and photocatalysts (100 mg) were put into a 250 mL Pyrex reactor equipped with a magnetic stirrer (150 rpm) at a room temperature of 25 °C. In all runs, the distance between the aqueous dye solution and the UV source was kept constant at 15 cm. The reaction solution was firstly stirred for 30 min in the dark to achieve adsorption equilibrium. The photocatalytic reaction was then started by solar light or UVA-lamp. In a photochemical reaction, oxidants 8 mL of the H₂O₂, K₂S₂O₈ or Na₂CO₃ (prepared from 250 mg/L of H₂O₂ (35%), K₂S₂O₈ (98%) or Na₂CO₃ (98%)) were added into the reaction solution just before switching on the light.

2.4 Analytical methods

The disappearance of RB19 was analyzed either by UV–Vis spectrophotometer chemical oxygen demand (COD) or by HPLC. UV–vis spectrophotometer from Shimadzu was used to analyze the concentration of Rb19 dye solution as the reaction proceeds. The analysis was carried out with a 5 mL glass cuvette (1 cm square path length) and measured at the established maximum absorbance wavelength (λ_max = 591 nm) which was determined from absorption spectra. Chemical oxygen demand (COD) was determined using the reactor digestion method based on the method of acidic oxidation by bichromate (Emna et al., 2009).

The extent of discoloration efficiency and mineralization efficiency is measured according to Eqs. (1) and (2), respectively.

The objectif of HPLC detection is to determinate the full degradation intermediaries of Rb19. The quantitative analyses of RB19 and its oxidation products were made by high liquid chromatography using a Perkin Elmer 200 series HPLC apparatus. The products formed are separated on a C₁₈ column and then analyzed quantitatively using a UV-V detector measuring the optical density at 592 nm during 15 min. The mobile phase was a mixture of 70% acetonitrile and 30% sulfuric acid (5 × 10⁻² mol L⁻¹). The mobile flow rate was fixed at 0.5 mL min⁻¹.

3.1 Raw clay and catalyst characterization

Fig. 1 shows the nitrogen adsorption–desorption isotherms of the raw and modified clay samples. The adsorption isotherm of raw clay is type III (BDDT classification (Sing et al., 1985)), which is characteristic of nitrogen adsorption on macroporous adsorbents whereas the adsorption isotherms of the TiO₂–clay sample is type IV, corresponding to mesoporous adsorbents. This suggests that most of the total porosity consists of mesoporosity. The hysteresis loops of these isotherms seem to be of type H3 (IUPAC classification (Sing and Williams, 2004)), corresponding to slit-shaped pores in layered materials. As shown in Table 1, the specific surface area (S_BET) increases from 36.6 to 116.7 m²/g and the total pore volume was increased from 0.13 to 0.26 cm³/g. This large increase in the surface area and the total pore volume of TiO₂–Clay indicates the successful pillaring of the TiO₂ species into the silicate layers of the clay.

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Figure 1

Nitrogen adsorption–desorption isotherms of the raw clay and TiO₂–clay photocatalyst.

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The X-ray diffraction (XRD) patterns of raw and modified clay material catalyst are shown in Fig. 2. The crystalline structure of the raw clay identified by XRD measurements (Fig. 2) showed that clay is composed mainly of quartz (26.7°), kaolinite (22.8°) and illite (12.6°) as already described elsewhere (Bel Hadjltaief et al., 2014). The XRD patterns of TiO₂–clay catalyst show the appearance of peaks at 2θ values of 25.281°, 37.934°, 48.376°, 55.296° and 62.728° corresponding to the (1 0 1), (1 0 3), (2 0 0), (1 0 5) and (2 1 3) planes of anatase type (Ao et al., 2008 Binitha and Sugunan, 2006) matching with (JCPDS No. 21-1272). Other crystal phase corresponding to peak at and 27.234° were assigned the diffraction peak of (1 0 1) of rutile (JCPDS No. 21-1272) (Lin et al., 2011).

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Figure 2

XRD patterns of the raw clay and TiO₂–clay.

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The relative content of anatase and rutile, as listed in Table 1, was estimated using Spurr–Myers equation (Eq. (6)) (Spurr and Myers, 1957): $$A(\%)=\frac{I_A}{I_A+1.265\times I_R}\times 100$$ where A (%) is the relative content of anatase, I_A and I_R are the intensities of the anatase (1 0 1) peak at 2θ = 25.281° and the rutile (1 0 1) peak at 2θ = 27.234°.

The anatase content of TiO₂–clay was then 96.168. This value shows that the anatase is highly predominant in the sample. The main active crystal phases of TiO₂ are anatase and rutile. According to Ambrus et al. (2008), photocatalyst containing the anatase phase is more efficient than the rutile one. This also suggests that the catalyst (TiO₂–clay) with more than 96% TiO₂ in anatase form will improve the photocatalytic activity (Ambrus et al., 2008).

Based on the XRD results, the average crystal size of TiO₂ was calculated using the Debye–Scherrer equation as follows (Eq. (7)): $$D=\frac{K\lambda }{\beta \mathit{Cos}(\theta )}$$ where D is the average crystallite size (nm), λ the wavelength of the X-ray radiation (λ = 0.1540 nm), K the Scherrer constant (K = 0.9), β the full-width at half maximum (FWHM) of the (1 0 1) plane and θ is the Bragg angle. The average sizes of TiO₂ crystallite in the catalyst are listed in Table 1.

The infrared spectra of raw clay and TiO₂/clay are shown in Fig. 3 (curves a and b, respectively). The FTIR spectrum of raw clay shows bands at 3630 and 3440 cm⁻¹ in the –OH stretching region; these two bands are assigned to the –OH stretching vibration of the structural hydroxyl groups in the clay and the water molecules present in the interlayer, respectively (Mishra and Rao, 2004; Xu et al., 2004; Binitha and Sugunan, 2006; Bineesh et al., 2010). These bands at 3630 and 3440 cm⁻¹ broaden due to the introduction of single bond OH groups: this is interpreted as impregnating effect (Binitha and Sugunan, 2006; Kurian and Sugunan, 2003). Moreover, the band at around 1600 cm⁻¹ is assigned to the bending vibrations of water, and the one at around 1060 cm⁻¹ corresponds to the asymmetric stretching vibrations of SiO₂ in tetrahedric form (Carriazo et al., 2010). The spectrum of the TiO₂–clay (Fig. 1b) exhibits the characteristic transmittance peaks of raw clay (Fig. 3a), the appearance of broad absorption band was observed in the range of 1825–3736 cm⁻¹ (Fig. 3b) and was ascribed to the introduction of single bond OH groups of the pillar, described as a impregnation effect in previous studies (Kurian and Sugunan, 2003) dealing with the insertion of TiO₂ in clay.

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Figure 3

FTIR spectra of the raw clay (a) and TiO₂–clay (b).

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Coupled diagrams of differential thermal analysis (DTA) and thermogravimetric analysis (TG) of the raw (a) and TiO₂–clay (b) materials are presented in Fig. 4. The raw clay presented a total loss of mass about 14% between 29 °C and 1000 °C. The first mass loss (11%) obtained until 200 °C was due to the removal of the moisture and zeolitic water (Caillère et al., 1982; Brown, 1988). The second mass loss (3%) observed at 420 °C is due to the elimination of constitutional water resulting from the dehydroxylation of the clay minerals (Caillère et al., 1982; Brown, 1988). The third peak of mass loss (2%) occurring at 680 °C corresponds to the decomposition of calcite (CaCO₃). In addition to the peak observed for the raw clay, the TiO₂–clay presents intense endothermic peaks located generally in the range of 200–500 °C. From the TGA data of the as-prepared powders, one can presume that TiO₂ is formed.

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Figure 4

TGA/DTA diagram of the raw clay and TiO₂–clay.

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The SEM micrographs of natural clay and TiO₂ clay are presented in Fig. 5 which are quite helpful to clarify the change in the morphological features upon an impregnation process. Clearly, the surface morphology of natural clay (Fig. 5a) is different from that of TiO₂–clay (Fig. 5b). The raw clay presents larger particle aggregates with smooth surfaces whereas after treatment with Ti isopropoxide, uniform TiO₂ grains of about 10–30 nm were observed on the surface clay. Thus we can conclude on a homogeneous distribution of TiO₂ grains on the raw material.

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Figure 5

SEM image of (a) the raw clay and (b) TiO₂–clay.

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3.2 Photocatalytic degradation of reactive blue 19

Fig. 6 shows that pure photolysis by UV solar and UVA irradiation without a catalyst was unable to degrade RB19. On the contrary, in the presence of the TiO₂–clay catalyst, and in absence of visible light and UVA, a maximal RB19 conversion was found around 20% which becomes stable after 180 min reaction time. This RB19 removal efficiency can be ascribed to the adsorption of RB19 on the catalytic surface of TiO₂. For the sake of comparison (figure not shown), a non-impregnated clay has been tested and only 5% removal efficiency had been obtained after 180 min of time on stream. The experimental data show that the TiO₂–clay photocatalyst was highly photoactive in RB19 degradation under UV light. After 90 min of irradiation, 99% of RB19 was degraded. The degradation efficiency in the presence of solar light was lower than UV irradiation, since TiO₂–clay photocatalyst is mainly photoactive in UV_A. Same time of irradiation resulted in 78% degradation of RB19 in exposed solar light.

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Figure 6

Photocatalytic oxidation of RB19 at various conditions: (a) UV-Solar; (b) UV-A; (c) TiO₂−clay; (d) UV-Solar/TiO₂−clayand (e) UV-A/TiO₂−clay.

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The photocatalytic degradation of RB19 under the two sources of irradiation was described by first order reaction kinetics and the rate constants for TiO₂–clay + solar and TiO₂–clay + UVA were calculated to be 0.009 and 0.016 s⁻¹, respectively. Under adsorption, the reactions would be zeroth order kinetics. The rate constant for TiO₂–clay during the first 30 min was calculated to be 0.071 s⁻¹. Several authors reported that degradation of RB19 by catalysts modified by TiO₂ under visible light or UVB also followed first order kinetics. Thus, for example, Zhang et al., 2011 reported that degradation of RB19 by TiO₂-FAU-type zeolites under visible light also followed first order kinetics with a rate constant of 0,00,015 s⁻¹.

3.3 Photochemical oxidation

3.3.1

3.3.1 Effect of oxidating agents

From the results presented above, TiO₂–clay has a better photocatalytic activity in UVA than UV solar. Thus, TiO₂–clay was further investigated for degradation of RB19 under UVA in photochemical reaction with different oxidating agents. The effect of these agents on the efficiency of RB19 degradation is presented in Fig. 7. The chosen agents such as hydrogen peroxide (H₂O₂), potassium peroxodisulfate (K₂S₂O₈) and sodium carbonate (Na₂CO₃) behaved differently in the photochemical degradation of RB 19. The results show that the degradation was most effective with the photolysis of H₂O₂, followed by the photolysis of K₂S₂O₈ and photolysis of Na₂CO₃ systems (Fig. 7). These later results can be attributed to the formation of both HO^• and SO₄^•− radicals during the reaction leading to a higher conversion. The UV/Na₂CO₃ system also had the lowest RB 19 degradation rate constant, because CO₃^•−/HCO₃^• has a lower radical reaction rate constant, and oxidizing power (E° = 1.78 V) in comparison with the oxidizing power of HO^• (E° = 2.80 V) and SO₄^•−(E° = 2.05 V).

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Figure 7

Photochemical oxidation of RB19 at various conditions: (a) UV_A/Na₂CO₃; (b) UV_A/K₂S₂O₈; (c) UV_A/H₂O₂; (d) UV_A/TiO₂−clay; (e) TiO₂−clay + Na₂CO₃/UV_A; (f) TiO₂−clay + K₂S₂O₈/UV_A and (j) TiO₂−clay + H₂O₂/UV_A.

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Various studies have noted that K₂S₂O₈ and H₂O₂ can be decomposed by direct UV radiation (Manuel et al., 2013; Muruganandham and Swaminathan, 2006; Saien et al., 2011). However, no data are available on the generation of HCO₃^−• and CO₃^−• radicals by direct UV radiation in the presence of CO₃²⁻ or HCO₃⁻. Thus, the generation of CO₃^•− radicals in aqueous solution, including one-electron oxidation of bicarbonate or carbonate by hydroxyl radical (Canonica et al., 2005).

The generation of oxidizing agents, such as HO^•, S₂O₈^2−•, HCO₃^−• and CO₃^−• is given by the following chain reaction (Eqs. (3)–(6)) (Manuel et al. 2013; Muruganandham and Swaminathan, 2006; Saien et al., 2011a,b; Kumar and Mathur 2006).

(3)

$${\text{H}}_2{\text{O}}_2+\text{h}\nu \to 2\text{H}{\text{O}}^{\text{•}}$$

(4)

$${\text{S}}_2{\text{O}}_8^{2-}+\text{h}\nu \to 2{\text{S}}_2{\text{O}}_4^{\text{•}-}$$

(5)

$$\text{H}{\text{O}}^{\text{•}}+{\text{CO}}_3^{2-}\to {\text{HCO}}_3^{\text{•}-}+{\text{OH}}^{-}$$

(6)

$${\text{HO}}^{\text{•}}+{\text{HCO}}_3^{-}\to {\text{CO}}_3^{\text{•}-}+{\text{H}}_2\text{O}$$ Fig. 7 also shows the degradation is more efficient when reaction is performed in the presence of the TiO₂–clay catalyst, upon H₂O₂, K₂S₂O_8, and Na₂CO₃ addition and UV irradiation, the reduction for 20 min of treatment was 99.8% with the H₂O₂ + TiO₂–clay/UV, 87.9% with K₂S₂O₈ + TiO₂–clay/UV, and 80.1% with Na₂CO₃ + TiO₂–clay/UV. However, addition of the H₂O₂, K₂S₂O_8, and Na₂CO₃ agents to photocatalytic process showed an impressive performance for RB19 decolorization. Thus, the combination of photocatalyst and oxidizing agent is found to be the most efficient for the decolorizing RB19 under high UV photon flux (Muruganandham and Swaminathan, 2006).

On the other hand, the high photodegradation of RB 19 by TiO₂–clay is due to the anatase phase stability of the used photocatalyst. The anatase nanocrystals generate various oxidizing species by UV irradiation, which simultaneously attack the pre-enriched RB19 dye from many different directions. As a result, the RB 19 dye is in no time cleaved into small molecule intermediates. Subsequently, the resultants diffuse out of the pores into the solution. In this process, the hydroxyl radical, the sulfate and carbonate radicals are efficiently used because of the synchronous role of adsorption and photocatalytic oxidation, generating the excellent activity (Muruganandham and Swaminathan, 2006; Manuel et al., 2013).

The possible reaction scheme for the photocatalytic degradation of RB 19 from aqueous solution is given in Eqs. (6)–(12).

1. photo-excitation with light of an energy greater than the TiO₂ band gap, which promotes the transfer of an electron, e⁻, from the valence band to the conduction band, leaves an electronic vacancy or hole, h⁺, in the valence band, and thus generates an electron–hole pair (Eq. (6)):

(6)

$$\text{Clay}\approx \text{Ti}{\text{O}}_2+h\nu (\text{UV})\to \text{clay}\approx \text{Ti}{\text{O}}_2(h^{+})+\mathit{clay}\approx \text{Ti}{\text{O}}_2(e^{-})$$

1. Generation of hydroxyl, OH^•,CO₃^• SO₄^•−,radicals: (Eq. (6)).

(7)

$$h^{+}+{\text{H}}_2\text{O}\to {\text{HO}}^{\text{•}}+{\text{H}}^{+}$$

(8)

$$\text{Clay}\approx \text{Ti}{\text{O}}_2(h^{+})+\text{C}{\text{O}}_3^{2-}\to \text{C}{\text{O}}_3^{{-}^{\text{•}}}+\text{O}{\text{H}}^{-}$$

(9)

$$\text{clay}\approx \text{Ti}{\text{O}}_2(h^{-})+{\text{H}}_2{\text{O}}_2\to \text{Clay}\approx \text{Ti}{\text{O}}_2+\text{H}{\text{O}}^{\text{•}}+\text{O}{\text{H}}^{-}$$

(10)

$$\text{clay}\approx \text{Ti}{\text{O}}_2(h^{-})+{\text{S}}_2{\text{O}}_8^{2-}\to \text{Clay}\approx \text{Ti}{\text{O}}_2+{\text{S}}_2{\text{O}}_4^{{-}^{\text{•}}}+{\text{S}}_2{\text{O}}_4^{2^{-}}$$

1. Attack of hydroxyl and sulfate radicals to adsorbed dye and their mineralization to CO₂ and H₂O:

(11)

$$\text{clay}\approx \text{Ti}{\text{O}}_2+\text{RB}19+\text{Oxidizing species}\to \text{Reactions intermediates}$$

(12)

$$\text{Reactions intermediates}+\text{Oxidizing species}\to \text{C}{\text{O}}_2+{\text{H}}_2\text{O}$$

3.3.2

3.3.2 Effect of dye concentration and subsequent kinetic study

Fig. 8 shows the kinetics of the photocatalytic degradation of RB 19 solution using different initial concentrations. It is evident that the photodegradation rate depends on the initial concentration of this organic pollutant. At 20 min of irradiation time, the increase of dye concentration from 75 to 200 mg/L leads to a decrease of the degradation from 99.6% to 39.8%. The rate of degradation is related to the OH radical formation on catalyst surface and probability of OH radical reacting with dye molecule. For all initial dye concentrations, the catalyst weight and light intensity are the same. Since the generation of hydroxyl radical remains constant, the probability of dye molecule to react with hydroxyl radical decreases. At high initial dye concentration, the path length of photon entering into the solution also decreases. Thus, the photocatalytic degradation efficiency decreases, but at low concentration the reverse effect is observed thereby increasing photon absorption by the catalyst (Krishnakumar et al., 2010). The large amount of adsorbed dye may also have a competing effect on the adsorption of oxygen and OH onto the surface of catalyst.

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Figure 8

Effect of the initial dye concentration on the discoloration efficiency of RB 19: (■) 75 mg/L; (▴) 150 mg/L and (♦) 200 mg/L. (a) Discoloration efficiency versus irradiation time. (b) Rate constants as a function of the initial dye concentration.

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Generally, first-order kinetics is appropriate for photocatalytic reactions (Bel Hadjltaief et al., 2013). The formation of reactive intermediate products of RB 19 and their reaction with hydroxyl radicals could explain the change of the degradation rates.

Kinetics model as follows:

(13)

$$-r_A=-\frac{\mathit{dC}}{\mathit{dt}}=\mathit{kt}$$ After integration of Eq. (6), the following equation is obtained:

(14)

$$\ln \left(\frac{C_0}{C}\right)=\mathit{kt}$$ where r_A is the oxidation rate of the RB 19 (mg (min⁻¹)), k the apparent constant of the reaction rate (the constant of first order reaction), C the concentration of the RB 19 (mg), C₀ the initial concentration of RB 19, t the time required for the initial concentration of RB 19, C₀ to become C(min).

The plot of $\text{ln}\left(\frac{{\text{C}}_0}{\text{C}}\right)$ versus t with different initial concentrations of RB 19 is shown in the insert in Fig. 8. All the curves showed a good linear correlation (R² > 0.99), suggesting that the degradation of RB 19 by H₂O₂ + TiO₂–clay/UV followed a first-order kinetic. Under the same experimental conditions, the kinetic constant of RB 19 decolorization was 0.162, 0.042 and 0.024 min⁻¹, while the initial concentration was 75, 150 and 200 mg/L, respectively.

Experimental results showed that the destruction rates of RB 19 decolorization by H₂O₂ + TiO₂–clay/UV fitted the Langmuir–Hinshelwood (L–H) kinetic model as follows:

(15)

$$-\frac{\mathit{dC}}{\mathit{dt}}=\frac{K_r{K}_{e}C}{1+K_{e}C}$$ where K_r is the reaction rate constant (mg/L min) and K_e is the adsorption coefficient of the MTBE (l/mg). With regard to Eq. (9), the constant k can be defined as follows:

(16)

$$\frac{1}{k}=\frac{1}{K_e{K}_r}+\frac{C_0}{K_e}$$ K_r was calculated from the slope of the 1/k versus C₀ plot, and K_e was then calculated from intercept of this plot (Fig. 9). The reaction constant and the adsorption coefficient of the reactant as well as correlation coefficient (R²) values for RB 19 degradation at different initial RB 19 concentrations are presented in Table 2.

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Figure 9

Evolution of UV–Visible spectra of RB19.

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Table 2 The values of reaction constant and the adsorption coefficient of the reactant.

Catalyst	Kr (mg(min−1))	Ke (mg−1)	R2
TiO2–clay	0.2835	0.0621	0.990

3.4 Mineralization of RB19 photochemical process

The mineralization/degradation of the process was monitored by UV–Vis absorption spectra to investigate the structural change of R19, HPLC and DCO. Degradation experiments were performed at pH 5.51, 75 mg/L dye concentration, 100 mg of the TiO₂–clay catalyst, and 250 mg/L of H₂O₂. Solution samples were analyzed in time intervals 0–35 min. Thus, one can note that the corresponding degradation efficiency curve in terms of percent efficiency (%) is the filled-square symbol plots in Fig. 9.

Based on the analysis of UV–Vis presented in Fig. 9a, the dye reached the peak of maximum absorption at three wavelengths (λ) namely: 227, 325, and 592 nm (Guimarães et al., 2012; Siddique et al., 2011). The peak observed in the visible region at 592 nm was due to the blue color of the chromophore (Guimarães et al., 2012; Siddique et al., 2011). This figure shows a high decrease of RB19 absorption at the wavelength of 591 nm observed during the first few 20 min. Fig. 9b reports a picture of color change in the reaction solution during the degradation of RB 19 which indicated some highly colored intermediate compounds. Moreover, these colored intermediates are more refractory and difficult to oxidize (Violet and yellow color). After treatment (30 min of reaction), the solution becomes colorless, this result shows the full discoloration of RB 19 in the solution.

A COD removal efficiency was calculated and plotted as a function of the irradiation time (Fig. 10). As expected COD decreases, and therefore the COD removal efficiency increases, with irradiation time. After 45 min of reaction, the maximal reaction time, the COD removal efficiency reached 98%. In the same condition and the same processing time, the RB 19 peak with the RT of 3.112 completely disappears from the HPLC chromatogram (Fig. 10b), which indicates that TiO₂–clay exhibits excellent photocatalytic activity in the degradation of RB 19.

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Figure 10

Evaluation of COD and HPLC chromatograms with treatment time.

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