2.2.5. Effect of Mn2+ Dopant Concentration

The doping of ZnS with Mn significantly influenced the photodegradation efficiency of the Qds. To study the doping effect, experiments were done under optimum conditions by varying the amount of dopant (1–5.0% *w*/*w*). On evaluating the SDB degradation efficiencies, with pure and doped ZnS Qds, it was observed that doping enhanced the efficiency of the catalyst through sonophotocatalysis, even though a negative effect was found at high concentrations. Figure 15 shows that the sonophotocatalytic degradation efficacies of the Qds increase from 0 to 3% and decrease above 3%, indicating potential activity at 3% Mn2+ doping. The variations in the degradation efficiency may be attributed to differences in the size of the nanoparticles, as well as the recombination rates of the e− and h+ upon doping, which affects their catalytic behavior [101]. Also, at high dopant concentration, Mn2+ entraps both the charge carriers, which consequently recombine by quantum tunneling as the distance between the trapping sites reduces. At low dopant concentration, only the h+ are trapped, which move to the surface and combine with the hydroxide ions present there, generating hydroxyl radicals (HO•), which are the primary oxidizing radicals for the dyes [102–104].

### 2.2.6. Effect of Ultrasonic Power on Degradation of SDB

To study the influence of power dissipation, experiments were carried out at two different power values (60 and 120 W). Figure 16 shows that an enhancement in the degradation rate was observed with an increase in power from 60 to 120 W. With the increase in power dissipation, the cavitational effects also increase, in turn producing additional turbulence and greater generation of free radicals, causing an enhancement in the degradation rate of SDB from 79 to 88%. Though the difference is minor, this may possibly be due to the cushioning effects, resulting from combination of a huge quantity of bubbles ensuing unproductive collapse action [105]. Thus, smaller energy gets used up for the free radical generation despite higher power dissipation [106,107]. Thus, 120 W was selected as the optimum power supply. The cavitational yield obtained for the system is 3.74 × <sup>10</sup>−<sup>12</sup> mol/J.

**Figure 16.** Effect of power on degradation of SDB at optimum conditions (15 mL of SDB, pH 6, 75 min irradiation, 40 mg Qds).

### *2.3. Re-Usability and Stability of Photocatalysts*

Sustainability and reusability are important parameters of photocatalysts. In order to study the stability and durability of the as-prepared ZnS Qds, recycling experiments were performed for the removal of SDB. In the end of each cycle, the photocatalyst was removed, washed, dried and re-used [100]. During the washing process, loss of some catalyst amounts occurred, causing reduction in activity after the consecutive cycles [5,108]. Figure 17 shows that there is no significant loss of degradation efficiency after 5 consecutive cycles. Therefore, the doped ZnS Qds may be considered as a re-usable and photostable nanocatalyst during the degradation process.

**Figure 17.** Recycling performance of Mn2+:ZnS Qds over multiple cycles on sonophotocatalytic degradation of SDB.

The stability of the photocatalyst was also ascertained by XRD analysis. The XRD analysis of Mn2+:ZnS Qds was performed before and after degradation of the SDB dye molecule (Figure 17). After sonophotocatalytic degradation, small transformations occurred and the peaks shifted to lower 2θ values, as observed in Figure 18. This is attributed to the ultrahigh strain rates generated by sonication [109].

### *2.4. Mechanism of the Sonophotocatalytic Degradation*

Sonication of water is known to generate active radicals such as OH• and H• by cavitation, which degrades the organic compounds present in water [110]. The presence of the nanocatalyst augment this phenomenon since the small bubbles present in water have a tendency to break into smaller ones, causing an increase in the total area of high pressure and temperature [111]. The oxygen molecules present in water act as a source for nucleus cavitation, while the HO• radicals degrade the SDB dye species [112].

Moreover, the agglomerated molecules get dispersed by sonication. This deagglomeration enhances the surface area of the nanocatalyst, increasing the active sites for adsorption of the dye molecules as well as for the absorption of light producing more reactive species. Sonication also avoids catalyst deactivation, attributed to the upsurge of microstreaming and microbubbles which eliminates the molecules adsorbed at the surface of the nanocatalyst [113]. This causes cleaning of the catalyst surface, further enhancing the reaction.

**Figure 18.** XRD patterns of Mn2+:ZnS Qds before and after sonophotocatalytic degradation of SDB.

During photocatalysis, the irradiation of ZnS with UV light photons, results in HO• radical generation, due to oxidation of water by valence band holes. The active species electrons (e−), holes (h+), hydroxyl radicals (HO•) and superoxide radicals (O2 •−) are generally produced subsequent to UV irradiation. The h<sup>+</sup> with high oxidative potential allow direct oxidation of pollutants to highly reactive intermediates; also they could react with chemisorbed H2O, generating reactive species, such as the hydroxyl (HO•) radicals [114].

$$\text{Qds} + \text{hv} \to \text{Qds} \left(\text{e}^- + \text{h}^+\right) \tag{3}$$

$$\text{hh}^+ + \text{SDB} \rightarrow \text{oxidation of SDB} \tag{4}$$

$$\rm{h}^{+} + \rm{OH}^{-} \rightarrow \rm{HO}^{\bullet} \tag{5}$$

$$\rm{H}^+ + \rm{H}\_2\rm{O} \rightarrow \rm{H}^+ + \rm{HO}^\bullet \tag{6}$$

The generated electrons react with the dissolved oxygen molecules, originating several radicals [101]. Transition metals at the surface of ZnS and oxygen atoms work as an electron sink and increase the electron hole separation. The electrons of the conduction band reduce the molecular oxygen, originating a superoxide anion at the catalyst surface, which then reacts with H2O, forming H2O2, which originates HO• radicals [115]. The dye molecules can be degraded or oxidized by the hydroxyl radicals (HO•), causing the dye to dissociate into smaller and not so toxic species.

$$\text{Fe}^- + \text{O}\_2 \rightarrow \text{O}\_2\text{}^{\bullet-} \tag{7}$$

$$\rm O\_2^{\bullet-} + H\_2O \to H\_2O\_2 \tag{8}$$

$$2\text{ H}\_2\text{O}\_2 + \text{e}^- \rightarrow 2\text{HO}^\bullet \tag{9}$$

The application of ultrasound in water causes acoustic cavitation. This comprises the formation, growth and collapse of cavity bubbles, entrapped gases or vapors surrounding water. During the sonolysis of water, it is well known that acoustic cavitation generates highly reactive primary radicals such as OH• and H•, due to the thermal decomposition of water, as shown in reaction (10) [116,117]. A number of recombinations and other reactions (namely, reactions (11)–(14)) occur within the bubble following primary radical generation [118].

From a thermodynamic view, bubble collapse is significant, as it causes a large change in bubble volume. As the bubble collapse occurs quickly (<1 μs), the associated "work done" (PdV) leads to "near" adiabatic heating of the bubble contents, which results in the generation of very high temperatures and pressures within the bubble. As a result, numerous local hot spots with extremely high temperature and pressure are generated, consequently inducing the dissociation of water [116,118].

Thus, primary radical generation takes place due to various recombination and other reactions within the bubble. Among these radicals, HO• is a powerful nonselective oxidant that has a high redox potential value (2.8 V) and can oxidize most of the organic pollutants.

$$\text{H}\_2\text{O} \overset{()()()()()}{\rightarrow} \text{HO}^\bullet + \text{H}^\bullet \tag{10}$$

$$\rm H^{\bullet} + H\_{2}O \to HO^{\bullet} + H\_{2} \tag{11}$$

$$\text{H}^\bullet + \text{O}\_2 \rightarrow \text{HO}\_2^\bullet\tag{12}$$

$$\rm{HO}^{\bullet} + \rm{H}^{\bullet} \to \rm{H}\_{2}\rm{O} \tag{13}$$

$$\rm{HO}^{\bullet} + \rm{HO}^{\bullet} \to \rm{H}\_2\rm{O}\_2 \tag{14}$$

$$\text{H}\_2\text{HO}\_2^\bullet \rightarrow \text{H}\_2\text{O}\_2 + \text{O}\_2\tag{15}$$

$$\text{H}\_2\text{O}\_2 + \text{e}^- \rightarrow \text{OH}^- + \text{HO}^\bullet \tag{16}$$

where )))))) refers to sonication.

Active HO• radicals+ SDB molecules → Degradation of SDB

Thus, in both methods, the HO• acts as a primary oxidizing radical, but other degradation processes can occur, since solutes have varying capacities to adsorb on the catalyst surface, as compared to the bubbles surface. Volatile solutes can be thermally degraded by entering the core of a collapsing bubble, but direct oxidation by the hole is also possible on the photocatalyst surface. Those extra processes can have improved effects when combined treatments are used, especially when the intermediates of the degradation process have different chemical properties than the parent molecules.

## *2.5. Role of Radical Trapping Agents*

To elucidate the main contributors in the photodegradation reaction, the degradation rates of SDB in the presence of different scavengers were obtained. For this, the reactive species capture studies were carried out similar to the approach used for photocatalytic experiments. The experiments were performed by adding 0.01 M of different scavengers, for example, sodium azide (NaN3), potassium iodide (KI), sodium chloride (NaCl) and formic acid (HCOOH). The scavengers were added prior to the addition of the photocatalyst into the dye solution.

Figure 19 shows various control experiments for the photodegradation of SDB. HCOOH was added as the HO• scavenger, NaCl as the h+ scavenger and NaN3 for scavenging 1O2 and HO• [119]. Additionally, KI works for scavenging h+ and HO•s at the catalyst surface [119,120]. The experiments were carried out by adding 0.01 M of different scavengers prior to the addition of photocatalyst into the dye solution. The maximum degradation (89%) of SDB was found without any scavenger. A small change in SDB photodegradation was found with the addition of NaCl, indicating that the photoexcited h+ also contributes in photodegradation as a minor factor. With the addition of NaCl the rate of photodegradation of SDB slightly declined to 66%, signifying that h+ were not the main active species. Further, the inhibition effect in photocatalytic efficiency was observed with a degradation of 19%, when HCOOH was added as the quencher, confirming the role of HO• and H• in the photocatalytic process [121–123]. Meanwhile, the addition of NaN3 resulted in a significant decrease, with a degradation of 33%, indicating the important roles of 1O2 and

HO• in the photocatalytic process. The formation of O2 •− is directly influenced by the reduction of O2, as it determines the production of HO• by its multistep reduction. Also, the photodegradation activity of SDB declined to 45% after the addition of KI, indicating the important roles of h+ and HO•s in the photodegradation process.

**Figure 19.** Mn2+:ZnS Qds based sonophotocatalytic degradation of SDB with different scavengers.

Furthermore, to confirm the formation of HO• radicals, a terephthalic acid test was conducted. The highest intensity peak in the fluorescence spectra of the terephthalic acid test represents the larger generation of HO• radicals. Figure 20 confirms the generation of a higher amount of HO• radicals during the sonophotocatalytic degradation of SDB with highest fluorescence intensity, as compared to the sonocatalytic and photocatalytic processes [124,125].

**Figure 20.** Trapping experiment: terephthalic acid tests for sonophotocatalytic, sonocatalytic and photocatalytic degradations of SDB.
