2.2.4. Effect of Initial pH

The pH of the solution is one of the significant parameters that affect the sonochemical and photocatalytic oxidation of various organic compounds. To study the pH effect on the SDB degradation, the experiments were done at various pH values (2.0–12.0), with 15 mL of 70 ppm initial SDB concentration and 40 mg Qds, for 75 min.

The sonocatalytic and photocatalytic degradation of SDB was higher at pH 6 (Figure 14) than at higher values of pH, as also observed in the previous results obtained with SDB [55,78]. The sonophotocatalytic degradation improved with pH increase, but maximum adsorption and degradation were obtained at pH 5–6. Higher values of pH caused lower levels of adsorption and decolorization of the dye.

**Figure 14.** Influence of varying pH values on sonophotocatalytic degradation of SDB in the presence of Mn2+:ZnS Qds at optimum conditions (15 mL of SDB 70 ppm, 75 min irradiation, 40 mg Qds).

The zero-point charge (pHzpc) of ZnS is 7–7.5, therefore, the surface of Qds is negatively charged above a pH of 7, whereas it is positively charged below pH 7 [100]. As an anionic dye, SDB is absorbed by Qds without any trouble at acidic range. However, at pH values higher than the isoelectric point, the quantity of positively charged surface adsorbent sites decreases, while the negative site increases, which does not favor the adsorption of negatively charged dye anions [27]. Therefore, pH 6 was chosen as the optimum pH value for the subsequent experiments.
