**1. Introduction**

From the viewpoint of environmental sustainability, textile processing and production release a huge quantity of polluted wastewaters, which enter the environment with unknown effects on health. Those wastewaters contain various synthetic dyes used in coloring of textile fabrics [1]. These textile dyes are a matter of concern, due to their high stability and low degradation capacity in wastewater treatment plants. A variety of techniques can be used for the degradation of such dyes [2].

Solochrome dark blue (SDB) is an essential azo dye used for dyeing nylon, wool, silk and other fibers. Solochrome dark blue (SDB) is mainly used as an indicator in complexometric titrations for the determination of total hardness of water due to elements such as calcium, zinc, magnesium, and to a lesser extent for other metal ions, including manganese. These metal ions readily undergo oxidation in alkaline media to form products of uncertain stoichiometry. It is a hazardous dye, and its degradation intermediates may be carcinogenic. Therefore, it is highly desirable to develop an effective method for removal of such dye pollutants from wastewater effluents, even at trace levels.

In recent years, quantum dots (Qds) gained substantial interest due to their exceptional properties [3]. Their dimension is comparable to the excitonic Bohr radius and they

**Citation:** Patel, J.; Singh, A.K.; Jain, B.; Yadav, S.; Carabineiro, S.A.C.; Susan, M.A.B.H. Solochrome Dark Blue Azo Dye Removal by Sonophotocatalysis Using Mn2+ Doped ZnS Quantum Dots. *Catalysts* **2021**, *11*, 1025. https:// doi.org/10.3390/catal11091025

Academic Editors: Ioan Balint and Monica Pavel

Received: 27 July 2021 Accepted: 20 August 2021 Published: 24 August 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

are well-known for their tunable size, broad absorption and narrow emission spectra, high photo-stability and strong signal intensity [4]. In recent years, much interest was shown towards semiconductor-based photocatalytic degradation of hazardous organic pollutants [5,6]. To enlarge the light absorption of these nanomaterials, modifications were made using semiconductor coupling, dye sensitization, impurity doping, and metal deposition, using coordination metal complexes, etc. [7,8]. Efforts to improve the efficiency of photocatalysis by combination with other methods were also made [9]. Among these, sonochemical oxidation was successfully applied for the degradation of several recalcitrant pollutants in water treatment methodologies [10,11]. By introducing ultrasonic waves of defined frequencies (20–1000 kHz), bubbles can be generated, followed by subsequent collapse action [12], generating very high temperatures up to ~4700 ◦C inside the core of bubbles [13,14], producing free radicals, such as H• and HO•, due to homolytic cleavage of H2O. The generated free radicals and the pyrolysis in cavitation bubbles lead to the degradation of organic compounds into a range of short-chain by-products.

In sonophotocatalysis, a photocatalyst is used, together with ultrasonic waves and UV irradiation. It is considered a useful method for enhancing the efficiency of photocatalysis, as hydrophilic components are degraded by photocatalysis and hydrophobic components by sonolysis [15]. It is also considered as a more effective procedure than photocatalysis or sonolysis alone [16–20]. Due to the extended processes taking place during the adsorption of pollutants on the catalyst surface, the photocatalytic efficiency is reduced, thus blocking the active sites. The use of ultrasound avoids the build-up of contaminants and intermediates formed during degradation, by cleaning the catalyst surface and generating effective radicals for degradation. Moreover, there is an improvement in the mass transportation of contaminants over the surface of the catalyst [21] and the H2O2 sonochemically produced is more stable, at low pollutant levels [22], being cleaved to HO• radicals during photolysis.

A wide variety of semiconductor materials can act as photocatalysts [23–25]. Among these, zinc sulfide (ZnS) Qds, having a low dimension, i.e., 1–10 nm, are well known wide band gap II-VI semiconductor materials, extensively used in photocatalytic decomposition of organic dyes and water purification, due to the high photocatalytic activity, high photochemical stability, non-toxicity and low cost [26]. Moreover, zinc based Qds have no toxic elements, show higher surface area than their bulk counterparts, have wider gap energy, and are excellent hosts for the doping of a huge variety of metals [27].

Doping or using intentional impurity atoms in Qds gives rise to further discrete energy levels in the intrinsic dots controlling the behavior of materials and enhancing the energy dynamics of excitons. It is an efficient technique to tune the energy levels and surface states, in addition to tuning optical, structural, electrical, and the magnetic behavior of the semiconducting nanocrystals [28–30]. This leads to extensive applications of Qds to light emitting devices [31–33], spintronics [34,35], solar cells [36,37], bioimaging [38–40] and sensing [41,42]. Doping allows an efficient transfer of energy from absorbed photons to the impurity, rapidly confining the excitation by restraining unwanted reactions at the crystal surface [43]. ZnS incorporated with transition elements such as chromium, manganese, iron, nickel and copper show a positive impact on their structural, magnetic and optical property [44]. ZnS nanoparticles doped with nickel, manganese, cobalt [45–47], copper [45] and iron [47–49], can be successfully prepared by simple and efficient methods, even at room temperature.

For Qds to become useful for clinical purposes, it is essential to obtain them without toxic elements. Thus, it is necessary to reduce the cytotoxicity of Qds [50,51]. Pyridine is a very good capping agent with significant photoluminescence properties. In the present work, we used nicotinic acid, which belongs to the group of the pyridine carboxylic acids, as a capping agent. It is an organic compound and a form of vitamin B3. These Qds offer a good candidate system for evading toxic elements in traditional ones. Thus, good quality and noxious-element free aqueous Qds will generate materials able to be used for in vivo bio-applications.

The aim of this study was to prepare Mn2+:ZnS Qds and to explore their sonophotocatalytic activity in SDB degradation. The goal was to improve the efficiency of the ultrasonic-based process for low-cost degradation of organic pollutants. This combination of ultrasound with a Mn2+:ZnS Qds photocatalyst was rarely investigated. In fact, this work is the first report on sonophotocatalysis of Mn2+:ZnS Qds for SDB dye removal from wastewater. The results show that the photocatalytic activity of Qds remarkably improved with the use of ultrasound, compared to another study dealing with sonophotocatalysis, as well as the other conventional methods for the removal of SDB molecules from an aqueous solution [52–57].
