Photodegradation Study of Toluidine Blue Dye in Aqueous Solution using Magnesium Oxide as a Photocatalyst

The photocatalytic degradation of Toluidine Blue dye (TB) in aqueous solution was investigated under UV light in the presence of magnesium oxide (MgO) as a photocatalyst at different operating parameters. The operating conditions were photocatalyst dose, initial dye concentration and the pH of the solution. Increasing of photocatalyst dose from 10 to 70 mg enhanced the degradation rate of TB dye. However, the increasing of TB dye concentration from 2 to 8 mg/L negatively affected the degradation rate. It was found that the percent of dye removal reached the maximum value at high acidic medium. In dark condition, 15 % of dye was adsorbed by MgO. Furthermore, the kinetics involved in the degradation of TB dye was examined and the degradation was found to follow pseudo first order kinetic model.

Recent interest in advanced oxidation processes (AOPs) has considerably increased for the complete degradation of dyes.These processes are based on generation of hydroxyl radicals that react with a broad range of organic contaminants rapidly and non-selectively (Das, Kamat, Padmaja, Au, & Madison, 1999;Yang, Wyatt Ii, & Bahorsky, 1998).AOPs include photocatalytic systems such as combination of light and semiconductors, and oxidants with semiconductor.Heterogeneous photocatalytic has emerged as a significant destructive technology leading to the total mineralization of most of the organic contaminants including organic dyes (Galindo, Jacques, & Kalt, 2001;Khodja, Sehili, Pilichowski, & Boule, 2001;Kusvuran, Samil, Atanur, & Erbatur, 2005;Neppolian, Choi, Sakthivel, Arabindoo, & Murugesan, 2002).Titanium dioxide (TiO 2 ) is characterized by non-toxicity, cheap production cost and chemical stability, therefore it represents one of the most important oxides that used in various fields of photochemistry, for instance, in photoelectrolysis of water, environmental remediation, and dye-sensitized solar cells (Anpo, 2004;Cappelletti et al., 2009;Chen & Mao, 2007;Gratzel, 2001).Furthermore, the Fenton process is being increasingly used in the treatment of textile industries (Bautista, Mohedano, Gilarranz, Casas, & Rodriguez, 2007;Lofrano, Meriç, Belgiorno, Nikolaou, & Napoli, 2007;Meriç, Lofrano, & Belgiorno, 2005).Among AOPs, homogeneous photocatalysis using UV with hydrogen peroxide, H 2 O 2 , have received a great attention in degrading or reducing organic pollutant by generation of two molecules of hydroxyl radicals (Al-Ekabi, Safarzadeh-Amiri, Sifton, & Story, 1991;Chu, Lau, & Fung, 2006;Saien, Ojaghloo, Soleymani, & Rasoulifard, 2011).In some cases, a combination of various treatment processes is needed to improve the overall efficiency of the water treatment systems and to optimize economic requirements (Oller, Malato, & Sánchez-Pérez, 2011;Parrino, Camera-Roda, www watt) e the ecific t (10, It is made from PYREX glass and fitted with a sample port.The reactor was equipped with a plunging tube in which a SEMTEC 12 watt lamp was placed horizontally.A glass syringe with 5 mL volume was used, at a specific schedule, to collect samples.The pH values, from 2 to 12, of these solutions were adjusted using different molarities of NaOH and HNO 3 .

Effect of Photocatalyst Dose
The catalyst dose, in photocatalytic process, is an important parameter since increasing in the amount of photocatlyst increased the active sites number on the photocatalyst causing generation more OH • radicals (Nishio, Tokumura, Znad, & Kawase, 2006).The degradation of TB was studied in the semi-batch reactor.A 500 mL volume of TB solution with the concentration of 4 mg/L were charged in the reactor with different amount of MgO (10, 30, 50 and 70 mg).As shown in Figure 2, the concentration of TB reduced with time.It also shows that the increasing of MgO amount increases the removal efficiency.This is due to the formation of highest amount of hydroxyl radical when more amount of MgO was used.The degradation of TB reaches maximum value when 50 mg of MgO was used in 70 min of reaction time.A further increase in the catalyst dose does not affect the removal efficiency of TB dye.This is due to the blocking of UV light penetration with increasing photocatalyst amount (Saquib, Abu Tariq, Haque, & Muneer, 2008).

Effect of Initial TB Concentration
The influence of the initial concentration of TB dye on its photocatalytic degradation rate was studied for (2, 4, 6 and 8 mg/L) using 50 mg of MgO.The results show that increasing the initial concentration of TB reduced the degradation rate (see Figure 3).As shown in Figure 3, when TB concentration increased from 10 to 70 mg/L, the removal percentage of TB at 70 min of reaction time decreased from 95 % to 81 %, respectively.The results presented in this study are in agreement with the previous study for other dyes such as Reactive Orange 5, methylene blue and Mercurochrome dyes when MgO used as a photocatalyst (Bandara & Ranasinghe, 2007;Kamel, Mashaly, & Abdelghaffar, 2013;Xiang, Xie, Li, & Li, 2013).The deeper colour of TB solution, at high concentration, would not be high transparent to UV light as in low concentration of dye and a significant amount of UV light would be absorbed by the TB molecules causing less light to reach MgO particle.Hence, the formation of OH • radicals reduced.The rate constant, k', was found to decrease linearly with increased initial TB concentration.The photodegradation of TB dye on the surface of MgO catalyst follows pseudo first order kinetic law and, expressed as follows: Where C o and C are the dye concentration at t=0 and t=t, respectively, k' is the rate constant.The plot of the experimental data (ln C/C o against t) yielding to a straight line, as shown in Figure 4, with relatively high regression coefficients (R 2 ).The kinetic parameters of photocatalytic degradation of TB by MgO in aqueous solution at various initial TB concentrations were recorded in Table 2.

Effect of pH
A series of experiments were performed at various pH values to test the pH effect on photocatalytic degradation of TB dye.

Figure 4 .
Figure 4. Pseudo first order kinetics of photocatalytic degradation of TB as a function of initial TB concentration (MgO= 50 mg/L, Volume = 500 mL, pH = 4).
Figure ption of TB on adsorption on nt was perform % of TB dye w

Table 2 .
Kinetic parameters of photocatalytic degradation of TB as a function of initial TB concentration