*2.2. Efficiencies of Various Methods for SDB Degradation*

Various types of nanostructures [46,47,78–85], carbon nanotubes [54], nanocubes [55], semiconductors [56], microorganisms [86,87], electric discharge methods [88], as well as conventional sorbents [57,89–91] have been utilized for the removal of SDB dye molecules from aqueous solutions. These findings are summarized in Table 1, together with our data. When comparing the contact time for degradation, and the initial concentration of dye and dosage of catalyst, the results indicate that the results obtained with Mn2+:ZnS Qds and sonophotocatalysis are better than those reported in other works for different nanostructures and adsorbents, as shown in Table 1.

**Table 1.** Comparison of the results obtained in the present work with literature studies of other researchers for the removal of Solochrome dark blue dye.



**Table 1.** *Cont.*

The efficiency of the Qds catalyst, UV irradiation and ultrasonication in the SDB degradation process was studied by performing some preliminary tests. A very small dye removal rate was observed with photolysis (4%), sonolysis (5%) and ultrasound with UV (7%) without any catalysts after 75 min as shown in Figure S2. For the sonophotocatalytic experiments, the Qds solution was stirred in the dark for 20 min, to reach the adsorption–desorption equilibrium in the presence of catalysts with different Mn doping concentrations.

With UV light and Qds catalyst, the degradation was higher (47%) compared to the sole photolysis. An improvement in the photodegradation rate was found up to 3% doping with Mn and a maximum degradation of 55.2% was accomplished within 75 min of irradiation. As shown in Figure 8, the photocatalytic activity of Mn2+:ZnS Qds was larger than that of pure ZnS. The photocatalytic absorption under UV light increased from 0 to 3% doping as the energy-gap of Qds increased due to a decrease of the particle sizes (from 1.32 to 1.13 nm, for pure ZnS and Mn2+:ZnS Qds, respectively, as confirmed by XRD).

**Figure 8.** (**a**) SDB absorbance changes at the maximum wavelength (λmax = 560 nm) and (**b**) the kinetic model for the photodegradation in the presence of pure and Mn2+:ZnS Qds under optimum conditions (15 mL of 70 ppm SDB, pH 6, 75 min irradiation, 40 mg Qds).

During sonocatalytic degradation of pollutants, the ultrasonic frequency is a significant factor which greatly influences size and duration of cavitation bubbles, affecting the production of HO• [92]. Sonication produces a kind of turbulence causing a mass transfer from the solution to the surface of the catalyst [93].

The SDB degradation in aqueous solutions was investigated at two different frequencies (40 and 80 kHz). Figure 9 shows the decline in SDB concentration with irradiation time. The degradation efficiency is directly related to the H2O2 generation, depending on the applied ultrasound frequencies [94]. From Figure 9, it was found that the differences in the degradation rates at two different frequencies were minor; still, the decrease in SDB concentration was larger at 40 kHz (62.9%) compared to 80 kHz (53%) due to the higher production of H2O2, which caused more HO• generation, leading to more degradation at this frequency [95]. Thus, 40 kHz was chosen for further study.

**Figure 9.** Effect of ultrasound frequency on degradation of SDB at optimum conditions (15 mL of 70 ppm SDB, pH 6, 75 min irradiation, 40 mg Qds).

With ultrasound and catalyst, the degradation obtained was 62.3%, which was higher, compared to sonolysis. The SDB removal rate increased with Mn concentration, accomplishing maximum degradation of 69.7%, after 75 min of ultrasonication. Figure 10a,b show the higher sonocatalytic removal rate of Mn2+:ZnS Qds compared to that of pure ZnS.

**Figure 10.** (**a**) SDB absorbance changes at maximum wavelength (λmax = 560 nm), (**b**) kinetic model for the ultrasound assisted degradation in the presence of pure and Mn2+:ZnS Qds, under optimum conditions (15 mL 70 ppm of SDB, pH 6, 75 min ultrasound irradiation, 40 mg Qds, 40 kHz), (**c**) SDB absorbance changes at absorption maximum (λmax = 560 nm) and (**d**) kinetic model for the ultrasound and UV light assisted degradation in the presence of pure and Mn2+:ZnS Qds at optimum conditions (15 mL of 70 ppm SDB, pH 6, 75 min ultrasound with UV light irradiation, 40 mg Qds, 40 kHz).

To enhance the degradation efficiency of the system, UV radiation assisted photocatalysis was coupled with ultrasound; with the combined process, sonophotocatalysis significantly enhanced the degradation rate of the system. The UV light source enhanced the process of photoexcitation, consequently after 60 min ultrasonicating, 80% degradation occurred in the presence of UV light. On further irradiation, 89% degradation was achieved after 75 min, as shown in Figure 10c,d. Thus, the sonophotocatalysis experiments established the semiconducting Mn2+:ZnS Qds to be a prominent sonophotocatalyst for the

degradation of SDB through the combined effect of ultrasound and UV light. Therefore, it was used for further studies.

### 2.2.1. Kinetic Study

Kinetics can provide information about the efficiency and mechanism of a photocatalytic process. These experiments were performed at optimum experimental conditions for the SDB dye solution. The pseudo second order kinetic model given in Equation (2) [27,61] was relevant, applying a linear fitting of *qt* versus *t*, where *k* is the rate constant (g mg−<sup>1</sup> min<sup>−</sup>1), and *qe* and *qt* are the equilibrium adsorption capacity and adsorption capacity at time *t*, respectively [96].

$$t/q\_l = 1/\left(k\,q\_\varepsilon^{\,2}\right) + \,\,t/q\_\varepsilon\tag{2}$$

The SDB degradation rates under sonophotocatalysis, sonocatalysis and photocatalysis were calculated from the slopes of Figure 11a plots. Figure 11b shows UV–Vis absorption spectra of the SDB suspension under sonophotocatalytic conditions, with Mn2+:ZnS Qds nanocatalyst, for 75 min. As the time for sonophotocatalysis progresses, the absorption band steadily decreased, showing the decomposition of the SDB chromophoric structure. The values of *R*<sup>2</sup> indicate that pseudo second order model fits the experimental data. The values for the kinetic data, rate constants (*k*), correlation coefficients (*R*2) and decolorization efficiency (DE) for pure and Mn2+:ZnS Qds under different conditions of sono and/or photocatalysis are depicted in Table S1.

**Figure 11.** (**a**) Second order kinetic model fitting to the Qds-based degradation of SDB data with pure and Mn2+:ZnS Qds and (**b**) absorption spectra of the SDB aqueous solution during sonophotocatalysis in the presence of Mn2+:ZnS Qds at optimum conditions (15 mL of SDB 70 ppm, pH 6, 75 min irradiation, 40 mg Qds).
