Immobilization of ZnO Suspension on Glass Substrate to Remove Filtration During the Removal of Remazol Red R from Aqueous Solution

Photodegradation of textile dyes in the presence of an aqueous suspension of semiconductor oxides has been of growing interest. Although this method of destruction of dyes is efficient, the main obstacle of applying this technique in the industry is the time and cost involving separation of oxides from an aqueous suspension. In this research, an attempted was made to develop ZnO films on a glass substrate by simple immobilization method for the adsorption and photodegradation of a typical dye, Remazol Red R (RRR) from aqueous solution. Adsorption and photodegradation of RRR were performed in the presence of glass supported ZnO film. Photodegradation of the dye was carried out by varying different parameters such as the catalyst dosage, initial concentrations of RRR, and light sources. The percentage of adsorption as well as photodegradation increased with the amount of ZnO, reaches a maximum and then decreased. Maximum degradation has been found under solar light irradiation as compared to UV-light irradiation. Removal efficiency was also found to be influenced by the pre-sonication of ZnO suspension. suspension taken on the bare substrate was tried to distribute uniformly with the help of a spatula and kept overnight to dry. The film was heated for 3 hours at 110°C temperature in an oven to remove water. The amount of ZnO required for the preparation of film was calculated from the weight of the bare substrate and the weight of ZnO film after heating. About 200 mL of 1 × 10 -4 M RRR solution was taken in a 500 mL beaker and placed into photolysis chamber. A film of ZnO was immersed into the solution with the help of Aluminium wire. A magnetic stirrer was used for the continuous rotation of the solution at a speed of 500 rpm. Adsorption study was carried out in the absence of light until equilibrium time for adsorption was determined. The UV heating light was then switched on and the degradation time was monitored using a stopwatch. A particular portion of the irradiated solution was taken out after a definite time interval and the absorbance of the solution were measured using Shimadzu 1800 UV-Visible spectrophotometer. the surface area would remain constant. Accordingly, the percentage removal would show a steady value after that amount of ZnO but in actual case was found to decrease, which might be due to retarded separation of the electron-hole pair with higher coating thickness. These results are consistency with literature results,


INTRODUCTION
Dyes have become a part and parcel of human life from the commencement of modern civilization. Dye producers and users are very much concern about the stability and fastness, especially the quality of the product. As a result, it becomes harder to degrade the dyes after use [1]. Direct dyes are water soluble for containing ionic groups such as sulfonic or amino groups. These dyes are not biodegradable and cause allergy, skin irritation, cancer and mutagenic diseases to living organisms [2]. The release of dyestuffs not only produce toxic amines by reductive cleavage of azo linkages which may cause severe effects on human beings through damaging their vital organs such as the brain, liver, kidneys, central nervous and reproductive systems [3,4] in an aquatic environment but damage the ecosystem also. Therefore, their removal becomes a prime environmental anxiety in industrialized countries and is subjected to many types of scientific research. The effluent of dyes produced from different industries causes a severe destruction of the environment. Drainage of dyestuff from industry to canals, rivers, and seas destroy the ecosystem of water as well as the whole environment. The abundance of non-biodegradable compounds in the wastewater is a new challenge for the future world. Some methods like biosorption [5], conventional activated sludge treatment process [6], electrochemical technologies [7,8], and reverse osmosis [9], had been applied for the treatment of industrial wastewater. Photocatalysis using a semiconductor as a photocatalyst is an alternative to conventional methods [10][11][12]. Complete mineralization of the dyestuff takes place by this method without the formation of sludge at atmospheric pressure and near ambient temperature [13]. When light is illuminated on semiconductor (i.e. ZnO, TiO2, CdO, CdS, etc.) with photon energy greater or equal to the band gap energy of the semiconductor, valence band electrons are excited to the conduction band creating a hole behind. ZnO has broad band gap energy of 3.37 eV at room temperature and exciton energy 60 meV [14]. It has no adverse effect on the environment and also readily available at low cost. Superoxide anion radicals (•O2 -) and hydroxyl radicals (•OH) are being generated in the aqueous medium of ZnO, and these species are responsible for accelerating the oxidation of pollutants [15,16]. ZnO absorbs a significant fraction of the solar spectrum and more light quanta than TiO2 [17]. Some researchers highlighted the better degradation efficiency of ZnO than TiO2 [18,19]. Incorporation of other metals to semiconductor also enhanced the photocatalytic degradation [20,21]. The use of an aqueous suspension of photocatalyst for the treatment of industrial wastewater is common one [22]. The major obstacle of this procedure is the filtration, which is time-consuming and cost-intensive too. To overcome these disadvantages, coating of active materials on the solid support can be used as a photocatalyst or adsorbent instead of suspension, where filtration of the suspension would not be required. Preparation of films on a solid substrate is very common and efficient in different industries and laboratories, which depends on the available technology. Spin coating of a solution [23] and direct deposition during chemical reactions [24] on solid substrates had already been reported. Expensive plant requirements or operational costs discourage the industrialists to depollute or detoxify wastewater. In this respect, films of ZnO was prepared by simple immobilization method on the glass substrate without high-cost instrumental set up for the purpose of removing filtration. The experimental results demonstrated the coating of ZnO on the solid support effectively worked as a photocatalysis as well as an adsorbent for removing dyes from the aqueous solution under both in UV and sunlight irradiation. EXPERIMENTAL RRR (supplied by Dystar, Germany) and ZnO (purchased from Merck, Germany) were used without further purification. About 4 g of ZnO was taken in a beaker with 50 mL deionized water. Then the aqueous suspension was sonicated for 2 hours and taken on a previously weighed glass plate (Area: 25.4 × 76.2 mm 2 & 1-1.2 mm thick, China) with a dropper. The suspension taken on the bare substrate was tried to distribute uniformly with the help of a spatula and kept overnight to dry. The film was heated for 3 hours at 110°C temperature in an oven to remove water. The amount of ZnO required for the preparation of film was calculated from the weight of the bare substrate and the weight of ZnO film after heating. About 200 mL of 1 × 10 -4 M RRR solution was taken in a 500 mL beaker and placed into photolysis chamber. A film of ZnO was immersed into the solution with the help of Aluminium wire. A magnetic stirrer was used for the continuous rotation of the solution at a speed of 500 rpm. Adsorption study was carried out in the absence of light until equilibrium time for adsorption was determined. The UV heating light was then switched on and the degradation time was monitored using a stopwatch. A particular portion of the irradiated solution was taken out after a definite time interval and the absorbance of the solution were measured using Shimadzu 1800 UV-Visible spectrophotometer.

Effect of Catalyst Dosage
The removal efficiency was monitored by UV-visible analysis. Maximum peaks of RRR were found at 523, 383 and 285 nm. Almost same type of spectra was found for a reactive red dye [25]. Beer-Lambert law (A=εcl) was used to determine the value of molar extinction coefficient (ε), which were 1.333 × 10 4 , 3.646 × 10 3 and 1.296 × 10 4 Lmol -1 cm -1 for the corresponding wavelengths, respectively. A decrease in the absorbance of RRR solution indicates the degradation of the dye. An overlay spectra of RRR at different time intervals in the presence of ZnO films under UV irradiation is shown in supporting information (Fig. S1). As the amount of ZnO increases adsorption also increases up to a maximum of about 0.15 g and then decreases for both adsorption (blue line) and photodegradation (red line) (Fig. 1). The degradation efficiency might be attributed to the high surface areas including an internal surface and an appropriate thickness of the film that assists more efficient separation of electron-hole pairs [26][27][28]. It is expected that, after the immobilization of a certain amount of ZnO, the surface area would remain constant. Accordingly, the percentage removal would show a steady value after that amount of ZnO but in actual case was found to decrease, which might be due to retarded separation of the electron-hole pair with higher coating thickness. These results are consistency with literature results, where TiO2 nanoparticles were deposited on a glass substrate and used as a photocatalyst for removing methylene blue from aqueous solution [26].

Effect of Initial Dye Concentration
Adsorption experiments were carried out with varying the concentration of RRR solution from (0.5 -2.5) × 10 -4 M to find the intrinsic kinetic parameters. The amount of ZnO, immobilized on glass substrate were 0.15 (± 5%) g. Under these conditions, the equilibrium time for the adsorption of RRR was found to be 40 minutes. The amount of adsorbate adsorbed (qe) per unit mass of adsorbent for 1 × 10 -4 M RRR solution was found 5.26 mg/g. The pseudo-first and second order kinetic equations [29,30] were compared to determine the order of adsorption. Sigma plot software was used to determine the value of all parameters, which are given in Table 1. All the correlation coefficient (R 2 ) values for the pseudo-first order kinetic model are closer to one than the pseudo-second order model suggests that the adsorption of RRR on ZnO follows the pseudo-first order kinetics.  The plot of (log qe vs log Ce) and (1/qe vs 1/Ce) is shown in Fig. 2 (a) and (b) respectively. Freundlich and Langmuir's parameters are enlisted in Table 2. The value of 1/n (>1) indicated a cooperative adsorption [30]. But the regression value suggested that the Freundlich equation could explain the adsorption isotherm more precisely. The effect of initial dye concentration on photodegradation of RRR on the ZnO-coated glass substrate under UV-light irradiation is given in Fig.3. The concentration of RRR solution was varied from (0.5 -2.5) × 10 -4 M and the amount of ZnO immobilized on glass substrate was 0.15 (± 5%) g. The photodegradation efficiency was found to be inversely affected by the dye concentration. With the increase of dye concentration, the equilibrium adsorption of dye increases on the catalyst surface active sites; hence competitive adsorption of OH on the same sites decreases, meaning a lower formation rate of ·OH radical, which is the principal oxidant necessary for high degradation efficiency [31]. On the other hand, considering the Beer-Lambert law, as the initial dye concentration increases, the path length of photons entering the solution decreases, resulting in lower photon absorption on catalyst particles and consequently a lower photodegradation rate.

Effect of Light Source
The Efficiency of photodegradation on light irradiation is shown in Fig. 4. Three different light sources (UV, Solar, and Visible light) were applied for photodegradation experiments. The measured intensity of UV and the solar light was 0.14 mWcm -2 and 700-750 Wm -2 respectively. After 90 min of light irradiation, the photocatalytic activity under sunlight was found to 24.05 % and 31.41 % higher than the artificial UV light and visible light, respectively. ZnO is a semiconductor having band gap energy of 3.37 eV corresponds to 368 nm of the radiation. Thus, any light having a wavelength equal or less than 368 nm would be capable of producing electron-hole pairs and enabled photodegradation of dye. This mechanism of photodegradation involving semiconductor oxide as a photocatalyst under UV light is now well established [31,32].

Fig. 4: Percentage of photodegradation under the illumination of different light sources.
In a photosensitization reaction, the dye molecules become excited by absorbing the photon. These excited molecules then transfer electrons to the conduction band of ZnO and proceed the photodegradation process. Photosensitization reaction occurred in the case of artificial visible light and solar light, the mechanism of which is already reported by Gary A. Epling [31]. However, a higher rate of degradation by the sunlight than the artificial visible light should be related to the intensity of light.

Effect of Sonication
Pre-sonication of ZnO suspension was carried out by using a Sonicator (Eyela, Tokyo Rikakikai Co.Ltd). Adsorption as well as photodegradation both was increased with sonication time of ZnO suspension (Fig. 5). The amount of adsorption was about 4.64 % for the film of ZnO prepared without sonication. A slight increase up to 4.99% had been found with 5 hours sonication of ZnO suspension. Under the same conditions, the photodegradation after 90 min UV irradiation were 8.15 % and 10.33 %, respectively. There might be two possibilities for the enhanced removal efficiency with sonication; one would be the decrease in the particle size of the ZnO, which would increase the surface area of the ZnO and other would be the increase dispersion ability of these particles. SEM images of the ZnO suspensions were observed by Hitachi S-4500 to clarify the exact reason for the enhancement of removal efficiency (shown in Fig. 6). The SEM images clearly predicted that there was almost no change in the size and the shape of the ZnO particles. Therefore, it could be concluded that the better dispersion of the ZnO particles made a rather smooth surface for the glass supported film and increased its effectiveness.

CONCLUSION
In the present research, we have shown a simple and facile route for immobilization of ZnO on a glass substrate, which enables to eliminate the problem of filtration during the traditional dye removal process. Since filtration is not required in this process and photodegradation follows adsorption, the removal efficiency will be significant rather than individual adsorption and photodegradation process. We used the ZnO-coated glass substrate for the adsorption of textile dye RRR untill the equilibrium was achieved. The same substrate was then used for photodegradation. Sunlight has been found to be more effective than the artificial UV & visible light due to the different mechanism of photodegradation and intensity of light. Adsorption on semiconducting materials containing film enhances the photodegradation under solar light rather than UV light is a new finding of this study which increases the feasibility of the process. We expect the successive adsorption and photodegradation process will be cost effective for long term application to depollute and detoxify industrial wastewater.