ReviewA review on hybrid techniques for the degradation of organic pollutants in aqueous environment
Introduction
Urbanization and industrialization are necessary for the modern world. The consumption of various forms of energy for industrialization results in the disposal of a substantial amount of waste into the environment. This, in turn, leads to greenhouse gas emissions, water pollution, and other public health problems [1], [2]. Of which, water pollution has become a severe problem due to large amounts of domestic sewage and industrial effluent discharged into the water body. Wastewater coming from industries like textile, pharmacy, pesticide, and petrochemical processing contains large amounts of organic compounds such as textile dyes, aromatic compounds, chlorinated hydrocarbons, and phenolic compounds. These compounds are toxic to the microorganisms and thus, conventional biochemical processes are not able to completely degrade them. If these wastewaters are discharged to the environment without any treatment, these dyes can remain in the environment for an extended period due to their high stability to light and temperature. To protect the aqueous environment from such toxic pollutants, Advanced Oxidation Processes (AOP) [3], [4], [5] have been developed. Various AOP techniques such as Fenton, Fenton-like, photo-assisted Fenton, photocatalysis, etc. have recently been exploited (Table 1) for the degradation of the organic pollutants [6], [7], [8], [9]. These processes are cost-prohibitive and complex at the present level of their development. Additional impediment exists in the treatment of dye wastewater with relatively higher concentration of dyes, as AOPs are only effective for wastewater with very low concentrations of organic dyes.
All AOPs are characterized by the utilization of highly reactive hydroxyl radicals (OH) with a redox potential of 2.80 eV while in some cases sulfate free radicals (SO4−) with a redox potential of 2.5–3.1 eV as oxidizing agents [10], [11], [12]. A rapid evolution of such reactive radicals need to be increase for efficient pollutant removal. To attain such criterion, researchers utilized ultrasound irradiation because the cavitation effect generated by ultrasound cuases the production of heat, promotion of mixing or mass transfer, promotion of contact between materials, dispersion of contaminated layers of chemicals and production of free radicals [13], [14], [15], [16], [17], [18]. Besides, the physical effects of ultrasound accelerate the reaction due to the proper mixing of reagents and enhanced surface area of the catalyst. However, the limitation is volatile organics formed during sonication may evaporate and escape into the atmosphere making it advisable to use a closed reactor for treating these contaminants. Besides, CO2 produced during the reaction will redissolve and reduce the pH of the solution.
Many review articles [19], [20], [21], [22], [23], [24], [25] are available on the sonochemical degradation of pollutants in aqueous environment. In this review, attention is given to ultrasound combined with heterogeneous AOPs (ultrasound/metal ions, ultrasound/metal oxides, and ultrasound/photocatalysis) and homogeneous AOPs (ultrasound/ozone, ultrasound/H2O2, and ultrasound/persulfate).
Section snippets
Sonolysis
In a sonochemical process, ultrasound waves are transmitted through an aqueous solution to generate acoustic cavitation. Highly reactive radicals are generated during acoustic cavitation (Reaction 1) [26], [27], [28], [29], [6], [8], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40]. The highly reactive hydrogen (H) turns into hydroperoxyl (HO2) radicals (Reaction 2) in the presence of oxygen atmosphere, and later forms hydrogen peroxide (H2O2) (Reactions 3 and 4). The oxidizing
Sonolysis coupled with ozonation
Sonolysis combined with ozonation has been investigated for the removal of under ground water treatment plant (UWTPs) sewages [81], [82], [83], [84]. The combined sonication with ozonation experiments was performed using the schematic experimental setup by Naddeo et al. [85] (Fig. 16). Naddeo et al [85] performed sonolysis coupled ozonation experiments for the degradation of DCF. The removal rate increased significantly upon compared to ozonation alone and such synergic enhancement is
Summary
From the observed results, it is clear that the ultrasonic-assisted procedure gained interest in the removal of pollutants found in wastewater. Several publications available in the literature show that cavitation induced by ultrasound accelerates the destruction of various contaminants significantly present in the wastewater. Nevertheless, the ultrasound alone is not a sufficient tool for the faster mineralization as all the cavitation energy cannot be converting into physical/chemical
Future scope
In the past few decades, ultrasound treatment of wastewater has become a very popular technique as it converts the pollutant to the end products i.e., carbon dioxide and water. Nevertheless, ultrasound alone is not a sufficient tool for faster mineralization as all the cavitation energy cannot be transferred into physical and chemical effects. The combination of ultrasound with other advanced oxidation processes (such as ozonation system) and/or oxidizing agents (such as H2O2 or persulfate)
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The research described herein was supported by the Department of Science and Technology (DST), India, under the Water Technology Initiative scheme (DST/TM/WTI/2k16/258(G)). The authors SA & MA thank MHRD, New Delhi for sanctioning them a joint Scheme for Promotion of Academic and Research Collaboration project (SPARC/2018-2019/P236/SL). The authors are also thankful to the Research Center for Environmental Medicine, Kaohsiung Medical University, Kaohsiung City, Taiwan for “The Featured Areas
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