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Review

Removal of Azo Dyes from Wastewater through Heterogeneous Photocatalysis and Supercritical Water Oxidation

by
Vincenzo Vaiano
1 and
Iolanda De Marco
1,2,*
1
Department of Industrial Engineering, University of Salerno, Via Giovanni Paolo II, 132, 84084 Fisciano (Salerno), Italy
2
Research Centre for Biomaterials BIONAM, University of Salerno, Via Giovanni Paolo II, 132, 84084 Fisciano (Salerno), Italy
*
Author to whom correspondence should be addressed.
Separations 2023, 10(4), 230; https://doi.org/10.3390/separations10040230
Submission received: 24 February 2023 / Revised: 15 March 2023 / Accepted: 24 March 2023 / Published: 27 March 2023

Abstract

Azo dyes are synthetic organic dyes used in the textile, leather, and paper industries. They pose environmental problems due to their toxic and persistent nature. The toxicity is due to the presence of azo groups in the dye molecule that can break down into aromatic amines, which are highly toxic to aquatic organisms and humans. Various treatment methods have been developed to remove azo dyes from wastewater. Conventional wastewater treatments have some drawbacks, such as high operating costs, long processing times, generation of sludge, and the formation of toxic by-products. For these reasons, a valid alternative is constituted by advanced oxidation processes. Good results have been obtained using heterogeneous photocatalysis and supercritical water oxidation. In the former method, a photocatalyst is in contact with wastewater, a suitable light activates the catalyst, and generated reactive oxygen species that react with pollutants through oxidative reactions to their complete mineralization; the latter involves pressurizing and heating wastewater to supercritical conditions in a reactor vessel, adding an oxidizing agent to the supercritical water, and allowing the mixture to react. In this review paper, works in the literature that deal with processing wastewater containing azo dyes through photocatalysts immobilized on macroscopic supports (structured photocatalysts) and the supercritical water oxidation technique have been critically analyzed. In particular, advancement in the formulation of structured photocatalysts for the degradation of azo dyes has been shown, underlying different important features, such as the type of support for the photoactive phase, reactor configuration, and photocatalytic efficiency in terms of dye degradation and photocatalyst stability. In the case of supercritical water oxidation, the main results regarding COD and TOC removal from wastewater containing azo dyes have been reported, taking into account the reactor type, operating pressure, and temperature, as well as the reaction time.
Keywords: heterogeneous photocatalysis; supercritical fluids; textile wastewater; organic pollutants heterogeneous photocatalysis; supercritical fluids; textile wastewater; organic pollutants

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MDPI and ACS Style

Vaiano, V.; De Marco, I. Removal of Azo Dyes from Wastewater through Heterogeneous Photocatalysis and Supercritical Water Oxidation. Separations 2023, 10, 230. https://doi.org/10.3390/separations10040230

AMA Style

Vaiano V, De Marco I. Removal of Azo Dyes from Wastewater through Heterogeneous Photocatalysis and Supercritical Water Oxidation. Separations. 2023; 10(4):230. https://doi.org/10.3390/separations10040230

Chicago/Turabian Style

Vaiano, Vincenzo, and Iolanda De Marco. 2023. "Removal of Azo Dyes from Wastewater through Heterogeneous Photocatalysis and Supercritical Water Oxidation" Separations 10, no. 4: 230. https://doi.org/10.3390/separations10040230

APA Style

Vaiano, V., & De Marco, I. (2023). Removal of Azo Dyes from Wastewater through Heterogeneous Photocatalysis and Supercritical Water Oxidation. Separations, 10(4), 230. https://doi.org/10.3390/separations10040230

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