2.2.2. Effect of the Initial SDB Concentration

The influence of the initial SDB concentration was investigated with four different concentrations (30, 50, 70 and 90 ppm) as shown in Figure 12. The amount of dye degraded diminishes with an increase in SDB concentration, displaying a higher degradation rate for a low initial concentration. As the initial SDB concentration increases, the degradation rate also decreases, as more SDB molecules are adsorbed on the surface of Qds, but degradation is not successful. Since intensity and catalyst dose are constant, higher SDB concentration results in a smaller light penetration depth. Additionally, a maximum number of active sites may be available at that concentration. On further increasing the concentration, the active sites present get blocked-up, thus the dye degradation rate decreases [27,31].

**Figure 12.** Effect of variation of the initial concentration of SDB on the sonophotocatalytic degradation under optimum conditions (15 mL of SDB, pH 6, 75 min irradiation, 40 mg Qds).
