**4. Conclusions**

In the present study, Mn2+:ZnS Qds were synthesized by a simple and fast precipitation method. The photocatalytic, sonocatalytic and sonophotocatalytic degradation of SDB was investigated. Optimal parameters involved in the degradation such as the initial dye concentration, catalyst loading, pH, power dissipation and ultrasonic frequency were determined. The prepared nanocatalysts showed an improved efficiency for the sonophotocatalytic degradation of SDB in comparison to sonocatalysis or photocatalysis alone. An explanation can be found in the enhanced electron-holes separation at the heterointerface, by generation of highly reactive radicals and improved active surface area. The sonophotocatalytic dye removal process follows pseudo second-order kinetics. Therefore, Mn2+:ZnS Qds provided effective removal of SDB, the process being straightforward and potentially useful for removal of organic pollutants from wastewater.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/10 .3390/catal11091025/s1, Figure S1: (a) N2 adsorption-desorption isotherms of Mn2+:ZnS Qds. (b) Pore size distribution obtained by BJH-adsorption, Figure S2: SDB absorbance changes at absorption maximum (λmax = 560 nm) for the ultrasound and UV light assisted degradation in the absence of pure and doped Mn2+:ZnS Qds at optimum conditions (15 mL of 70 ppm SDB, pH 6, 75 min ultrasound and/or UV light irradiation), Figure S3: LC–MS of Mn2+ doped ZnS assisted photodegraded SDB solution. Inset: SDB solution before and after degradation (after 75 mins), Figure S4: Degradation products of SDB photodegradation catalyzed byMn2+:ZnS Qds, Figure S5: Schematic diagram of sonophotocatalytic experimental setup, Table S1: Kinetic parameters: rate constants (*k*), correlation coefficients (*R*2) and decolorization efficiency (DE) for the removal of Solochrome dark blue dye by Mn2+:ZnS Qds, Table S2: Data for Solochrome dark blue dye.

**Author Contributions:** Conceptualization: J.P. and B.J.; methodology: J.P., B.J. and S.Y.; software: J.P., A.K.S., S.Y. and S.A.C.C.; validation: S.A.C.C. and A.K.S.; formal analysis: J.P.; investigation: J.P., B.J., S.Y. and A.K.S.; resources: A.K.S. and M.A.B.H.S.; data curation: J.P. and B.J.; writing—original draft preparation: J.P.; writing—review and editing: M.A.B.H.S. and S.A.C.C.; visualization: A.K.S.; supervision: A.K.S.; project administration: J.P., A.K.S., M.A.B.H.S. and S.A.C.C.; funding acquisition: J.P., A.K.S., M.A.B.H.S. and S.A.C.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** J.P. is thankful to DST, New Delhi, India for Research fellowship under Women Scientist Scheme (SR/WOS-A/CS-82/2018). This work has also been supported by FCT—Fundação para a Ciência e a Tecnologia, I.P., under the Scientific Employment Stimulus-Institutional Call (CEECINST/00102/2018) and the Associate Laboratory for Green Chemistry-LAQV which is financed by national funds from FCT/MCTES (UIDB/50006/2020 and UIDP/50006/2020).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data will be made available upon request.

**Acknowledgments:** Authors are also thankful to SAIF centers: STIC Kochi, IIT Madras, MNIT Jaipur, NIT Raipur and Center for Basic Sciences, Pt. RSSU Raipur for providing instrumental analysis facilities, and to the Department of Chemistry, Govt. V.Y.T.PG. Autonomous College, Durg (C.G.) for providing basic instruments for the kinetic study.

**Conflicts of Interest:** Authors declare no conflict of interest.
