ZnO/spiral-shaped glass for solar photocatalytic oxidation of Reactive Red 120

2.1 Materials used

The commercial azo dye C.I. Reactive Red 120 (C₄₄Cl₂H₂₄N₁₄Na₆O₂₀S₆;) obtained from Ciba, (Switzerland) was used throughout the present investigation as received and without further purification. All other chemicals used in this study were obtained from Merck (Germany) and used without further purification; also, H₂O₂ solution (35% w/w), H₂SO₄ and NaOH, HCl, and zinc metal were purchased from Fluka (Germany).

2.2 Preparation of ZnO and coated glass spirals with ZnO

Zinc oxide was prepared by dissolution of high grade zinc metal in hydrochloric acid. The resulted solution was cooled to below 15 °C and left at room temperature for 24 h and then filtered. Precipitation of zinc hydroxide was induced with ammonia solution. The resultant mixture was stirred for 4 hours; zinc hydroxide was separated by filtration, and thoroughly washed with de-ionized water to remove remaining chlorides. Then the precipitate was dried at 60 °C in oven. The dried material was subsequently calcined at 500 °C for 2 h. Then, the prepared ZnO powder was immobilized on cylindrical amorphous borosilicate glass spirals (Wilson spirals, 12 mm diameter, weight 0.8 g) as follows. The glass spirals were etched with dilute hydrofluoric acid (2.5 mM) for 24 h in order to fabricate a rough surface for better adherence of ZnO to the glass surface. 2 g of prepared ZnO was suspended in 100 mL distilled water and stirred overnight. The slurry and roughened glass spirals were added to a rotating flask immersed in water bath, which was connected to a vacuum pump. ZnO/GS was obtained after evaporation of the water. The coated spirals were then dried at 120 °C in oven for 24 h, after that they were thoroughly washed with distilled water to remove weakly adhered ZnO particles. Finally, the ZnO/GSs were calcined at 200 °C in a muffle furnace for 4 h.

2.3 Material characterization

Characterization of prepared ZnO/GS was investigated by X-ray diffraction (XRD), and scanning electron microscope (SEM). Powder XRD measurements were performed on a Bruker (USA) D8 diffractometer employing Cu Kα radiation, Ni filters and operated at 40 kV and 40 mA with angular scanning range 2 $ϴ$  = 4–90^o. The morphology of ZnO particles was determined by SEM employing a JEOL (USA) microscope equipped with XL-30 ESEM NEW XL-30 135-10 microanalysis system. The specimens were mounted on 15 mm diameter stubs coated with a thin layer of carbon paste.

2.4 Configurations of the solar collector

Fig. 1.a shows a schematic diagram of the set-up of the unit of the parabolic solar reactor and a photograph of the parabolic collector. The solar reactor composed of a UV transparent glass tube (1.5 m long and 40 mm in diameter) connected to a feeding tank in a closed recirculation loop with a constant flow (50 L/h), which was maintained by a peristaltic pump. The glass tube was mounted in the focal point of the parabolic reflector. An irradiated surface area of 2.1 m² was fabricated from aluminium sheet placed on fixed metal support as a parabolic concentrating collector that is to be oriented about a horizontal east–west axis so as to constantly minimize the angle of incidence and thus maximize the incident solar intensity.

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Figure 1 (a) Schematic diagram of the solar photocatalytic reactor and photograph of the solar parabolic collector with UV transparent glass tube reactor positioned in the focal point. (b) Average direct solar radiation per month at Greater Cairo.

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2.5 Solar irradiation experiments

All solar photocatalytic experiments were carried out under the same conditions during period from January to March 2011. The intensity (J) was found to be approximately constant during the experiments as shown in Fig. 1b. Each experimental run was carried out using the following procedure: (1) the solar reactor tube was filled with ZnO/GS, (2) the dye solution was circulated through the solar reactor tube at a constant flow (1 L/min) for 30 min prior to irradiation, (3) the dye solution was subsequently irradiated by sunlight for 180 min, and (4) small samples (5 cm³) of the dye solution were withdrawn at regular time intervals every 20 min through the sample valve (Fig. 1a). Several series of experiments were conducted with aeration to determine: (i) photolytic dye removal in the absence of photocatalyst in, (ii) adsorption of dye over photocatalyst, (iii) photocatalytic dye removal over ZnO/GS under sunlight; that depends on dye concentration, solution pH, hydrogen peroxide amount and presence of sodium carbonate. The initial pH value was adjusted with H₂SO₄ or NaOH. In the experiments of addition hydrogen peroxide to the dye solution, hydrogen peroxide was injected before the start of the solar catalytic oxidation. The stability of ZnO/GS was assessed by analysis of amount of zinc in dye solution after 180 min of sunlight irradiation using atomic absorption spectroscopy (AAS Varian 220). The reusability of catalyst was evaluated by reclaiming the catalyst of the reaction in the batch mode, washing, drying in air at 110 °C and using it for dye degradation under similar experimental conditions.

2.6 Analysis

Prior to analysis, 0.5 mL NaOH was added to the withdrawn samples in order to stop further reactions in the dye solution. The optical absorbance was determined by a spectrophotometer (PD-303 UV spectrophotometer APEL, Japan). The absorbance at 510 nm is used to examine the decolourization of the dye. The absorbance at 254 nm represents degradation of dye due to decomposition of the aromatic part of the dye. The pH of the solution was determined by using a HANNA pH meter. ZnO content of the coated glass spirals as Zn was analysed by atomic Absorption Spectroscopy. The percents of decolourization and degradation were calculated using the following formula: $$\text{Decolourization percent}=((A_{510}\text{of initial dye solution}-A_{510}\text{at}t)/A_{510}\text{of initial dye solution})\times 100\%$$ $$\text{Degradation percent}=((A_{254}\text{of initial dye solution}-A_{254}\text{at}t)/A_{254}\text{of initial dye solution})\times 100\%$$

3.1 Characterization of ZnO supported on GS

Fig. 2a shows XRD pattern of the prepared ZnO supported on GS surface, that displays the dominant diffraction lines at 2 $ϴ$ values of 36.65°, 39.44° and 41.49 ° corresponding to (2 0 0) and (2 1 0) planes that traduced to cubic phase of ZnO. Also, Fig. 2a revealed that there is no distinct additional diffraction peaks for glass spiral. The broadness of the peaks of XRD lines indicates the formation of nanoparticle’s polycrystalline structure. In consequence, the well-prepared ZnO crystallinate size is calculated from the maximum diffraction peak (Fig. 2a) by the Scherrer equation as follows (Azaoff, 1968):

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Figure 2 Characterization of ZnO supported on GS (a) XRD diffraction patterns. (b) SEM image.

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3.2 Solar decolourization and degradation of RR-120 dye over ZnO/GS

Prior to evaluation of solar decolourization and degradation of the dye over ZnO/GS under sunlight, control experiment was carried out under sunlight in the absence of ZnO/GS. All experiments were conducted with dye concentration of 5 × 10⁻⁵ mol/L and pH of 7. It was observed that the colour of dye was reduced by 12% after 180 min. It can be concluded that the dye is resistant to photolysis. In the presence of ZnO/GS in dark, the dye concentration was decreased by 26% after 60 min of reaction time as shown in Fig. 3. This is attributed to adsorption of dye on the surface of ZnO/GS. The decolourization and degradation rates of RR-120 against irradiation time are shown in Fig. 3. It was interesting that under simultaneous irradiation and aeration complete decolourization 70% of degradation of the dye were achieved over ZnO/GS after 100 min of reaction time. Thus, it is worth noting that the degradation of RR-120 took place over ZnO/GS under sunlight. These findings were comparable to previous studies (Marto et al., 2009).

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Figure 3 Removal rate of CI Reactive Red 120 as a function of time of solar irradiation over ZnO/GS. (1) photodecolourization of dye, (2) photodegradation of dye, and (3) Dye treated with ZnO/GS in dark [dye concentration of 5 × 10⁻⁵ mol/L and pH of 7].

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According to the principle of photocatalysis, electron (e⁻) – hole (h⁺) pairs are generated, when ZnO photocatalyst is irradiated with sunlight. Separated electrons (e⁻) and holes (h⁺) diffuse to the surface of semiconductor and react with water, hydroxyl group and molecular oxygen absorbed on semiconductor producing reactive radicals, such as O₂^−•, ^•OH, and H₂O₂. These reactive radicals react with adsorbed dye on ZnO/GS and degrade it. Furthermore, the photocatalytic activity of ZnO/GS mainly depends on two factors: (1) the electron–hole generation capacity i.e. high utilization of incident light (visible); (2) the separation efficiency of the photo-generated charge pair. In ZnO/GS, nanocrystalline and rod-like ZnO supported GS surface could form defects which could provide a shallow trap for photo-generated hole to inhibit the recombination of electron–hole pair and extend the lifetime of charge separation. Therefore, the photogeneration rate of hydroxyl radicals could be enhanced and dye photodegradation was enhanced. It was found that decolourization of the dye is faster than the degradation. Decolourization took place via attack on chromophoric groups leading to cleavage of N⚌N, while degradation occurred via the cleavage of aromatic ring to form smaller byproducts such as aliphatic acids. Photocatalytic degradation of triazine containing azo dyes proceeds via three steps. In the first step, the more active bonds were hydroxylated, which included the C—N bond linked to the benzene ring and the naphthalene ring to form organic acids with or without the hydroxyl group that leads to dye decolourization. Then, the groups linked to the triazine ring were replaced by the hydroxyl group to yield the well-known cyanuric acid. At the same time, the aromatic acids produced from the first step were subsequently hydroxylated and led to the cleavage of aromatic ring to form aliphatic acids (Konstantinou and Albanis, 2004). Thus, immobilized ZnO on GS surface is a promising photocatalyst for wastewater purification without needing to recovery from solution.

3.3 Effect of pH

Solution pH is an important parameter that influences on the photocatalytic degradation. To investigate the effect of pH experiments on photodegradation of RR-120, solar photocatalytic experiment was performed in the range of pH 3–11 in the presence of ZnO/GS (data not shown for brievity) with dye concentration of 5 × 10⁻⁵ mol/L. The obtained results indicated that the increase of pH of the dye solution up to pH 9 results in an increased decolourization of dye from 70% to 100%, while the degradation increases from 58% to 93% after 60 min of solar irradiation. At low pH, corrosion of ZnO occurred and photocatalytic reaction is prohibited due to dissolution of Zn²⁺ in the solution as well as decreasing adsorption of dye over ZnO/GS. This leads to lowering of degradation rate (Lakshmi et al., 1995; Shourong et al., 1997). But, RR-120 dye contains the sulphate group in its structure, which is negatively charged in alkaline conditions (pH > 9). Therefore, effective dye adsorption is hindered from electrostatic reasons. Hence, at intermediate pH, i.e. between 7 and 9 optimum conditions for dye adsorption and photogeneration of hydroxyl radicals occur and produce a maximum in the dye removal rate vs. pH. Furthermore, it is evident that photocatalytic decolourization and degradation rates increased with raising pH of RR-120 wastewater to 9.

3.4 Effect of dye concentration

The effect of dye concentrations, on the solar photocatalytic decolourization and degradation was investigated in the range of 1 × 10⁻⁵ to 7 × 10⁻⁵ mol/L. It was found that the increase in dye concentration decreased the removal rate of dye. When the dye concentration is increased from 1 × 10⁻⁵ to 7 × 10⁻⁵ mol/L, the decrease of the decolourization from ca 100% to 71% and decrease of the degradation from 65% to 27% were achieved within 40 min of irradiation. As initial concentration increases, more and more dye is adsorbed on the surface of ZnO. This reduces the generation of hydroxyl radicals, since there are only a fewer active sites for the adsorption of hydroxyl ions and the generation of hydroxyl radicals. Further, as the concentration of dye solution increases, incident photons were adsorbed by dye before they can reach the catalyst surface. Hence, the absorption of photons by the catalyst decreases, and consequently the degradation rate is reduced (Byrappa et al., 2006; Sobana and Swaminathan, 2007).

3.5 Kinetic analysis

Non-competitive Langmuir–Hinshelwood (L–H) kinetic model was used to describe photocatalytic degradation of organic pollutants i.e. dye in the solid–liquid phase reaction. Since, the dye degradation process involves a relatively rapid adsorption step to achieve equilibrium and slow (rate determining) reaction step (Ollis, 1985; Galindo et al., 1999). It was assumed that the rate of oxidation of dye at photocatalyst surface is proportional to the surface coverage with dye that can be expressed in the following Eq. (2):

The effect of dye concentration on the coverage surface with dye is then given by Eqs. (3) and (4):

At low initial dye concentrations, K C₀ << 1, the rate Eq. (4) can be expressed in (5):

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Figure 4 Kinetics for (a) dye decolourization and (b) dye degradation at pH 7 and different initial concentrations (1) 1 × 10⁻⁵ mol/L, (2) 2 × 10⁻⁵ mol/L, (3) 3 × 10⁻⁵ mol/L, (4) 4 × 10⁻⁵ mol/L, (5) 5 × 10⁻⁵ mol/L, (6) 6 × 10⁻⁵ mol/L and (7) 7 × 10⁻⁵ mol/L.

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The L–H model (Eq. (4)) of the decolourization and degradation rate reactions is confirmed by the linear plot obtained by plotting reciprocal of the initial rate (1/r) against reciprocal of the initial concentration (1/C₀) shown in Fig. 5a. The values K and k₁ are 1.37 × 10⁻⁵ mol⁻¹ L and 0.22 × 10⁻⁵ mol/L min for decolourization and 1.37 × 10⁻⁵ mol⁻¹ L and 0.054 × 10⁻⁵ mol/L min for degradation, respectively from the linear plot. The product of k₁ was in agreement well with the apparent rate constant values obtained at low initial concentrations. There is additional support to confirm applicability of L–H model. Eq. (4) can be rewritten as follows:

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Figure 5 Plot of L–H model for decolourization and degradation of C.I. Reactive Red 120 dye (a) 1/r versus 1/C₀, (b) t_0.5 versus 0.5 C₀, initial pH of 7.

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3.6 Effect of hydrogen peroxide as electron acceptor

The recombination of photo-generated hole–electron pairs was the limiting factor for the photocatalytic degradation of organics. The rates and efficiencies of photo-assisted degradation of organic substrates are significantly improved by addition of electron acceptor i.e. hydrogen peroxide, which is attributed to an increased concentration of produced hydroxyl radical (Daneshvar et al., 2002), as shown in following equations.

3.7 Effect of Na₂CO₃ as hydroxyl scavenger

From a practical viewpoint, it is necessary to assess the effects of the other chemicals used in the textile industry that may affect photocatalytic reaction. Moreover, sodium carbonate (Na₂CO₃) is added to adjust the pH of the dyeing bath (Nansheng et al., 1996). Therefore, the textile industry wastewater could contain a considerable amount of carbonate ions. So it needed to study their influence on the solar photocatalytic degradation of dye. The effect of additions of Na₂CO₃ on the solar photocatalytic oxidation of dye is shown in Table 3 at pH 7, dye concentration of 5 × 10⁻⁵ mol/L. It was observed that small amounts of carbonate ions suppressed the solar photocatalytic decolourization and degradation reactions. Increasing the Na₂CO₃ concentration decreases the degradation rate. Since carbonate ions are efficient hydroxyl scavengers as follows:

3.8 Reusability of photocatalyst

Reusability of ZnO/GS photocatalyst for the degradation of RR-120 under sunlight was evaluated. The used ZnO/GS after photocatalytic degradation of dye was washed with water and dried. The dried ZnO/GS catalyst was used for the degradation of RR-120 under similar conditions (dye concentration was 5 × 10⁻⁵ mol/L_; pH of dye solution was 9). The treated dye solution was subjected to AAS analysis to assess the leaching of Zn²⁺ ions to solutions as a result of leaching of ZnO from GS support. It was worth noting that the dissolution of photocatalyst was found to be negligible (0.2 mg/L of zinc was observed during 180 min of reaction time). Table 4 shows the efficiency of ZnO/GS for photocatalytic degradation of RR-120 under sunlight after successive five runs. Slight decrease of catalytic activity of used photocatalyst was noticed. It can be concluded that ZnO supported on glass spiral had high stability and reusability after several usages in photocatalytic degradation of persistent pollutants.