The photocatalytic degradation of azo dyes containing different functionalities has been reviewed using TiO2 as photocatalyst in aqueous solution under solar and UV irradiation. The mechanism of the photodegradation depends on the radiation used. Charge injection mechanism takes place under visible radiation whereas charge separation occurred under UV light radiation. The process is monitored by following either the decolorization rate and the formation of its end-products. Kinetic analyses indicate that the photodegradation rates of azo dyes can usually be approximated as pseudo-first-order kinetics for both degradation mechanisms, according to the Langmuir–Hinshelwood model. The degradation of dyes depend on several parameters such as pH, catalyst concentration, substrate concentration and the presence of electron acceptors such as hydrogen peroxide and ammonium persulphate besides molecular oxygen. The presence of other substances such as inorganic ions, humic acids and solvents commonly found in textile effluents is also discussed. The photocatalyzed degradation of pesticides does not occur instantaneously to form carbon dioxide, but through the formation of long-lived intermediate species. Thus, the study focuses also on the determination of the nature of the principal organic intermediates and the evolution of the mineralization as well as on the degradation pathways followed during the process. Major identified intermediates are hydroxylated derivatives, aromatic amines, naphthoquinone, phenolic compounds and several organic acids. By-products evaluation and toxicity measurements are the key-actions in order to assess the overall process.

TiO2-assisted photocatalytic degradation of azo dyes in aqueous solution: kinetic and mechanistic investigations A review

Removal of synthetic dyes from wastewaters: a review.

Solar photocatalytic degradation of azo dye: comparison of photocatalytic efficiency of ZnO and TiO2

New ecolabels for textile products and tighter restrictions on waste- water discharges are forcing textile wet processors to reuse process water and chemicals. This challenge has prompted intensive research in new advanced treatment technologies, some of which currently making their way to full-scale installations. These comprise polishing treatments such as -ltration, chemical oxidation and specialized Nocculation techniques and pre-treatment steps includ- ing anaerobic digestion, -xed--lm bioreactors, FentonIs reagent oxidation, elec- trolysis, or foam Notation. Though several of these new technologies are promising in terms of cost and performance, they all su†er limitations which require further research and/or need broader validation. A segment of the research deals with the separate handling of speci-c sub-streams such as dyebath effluents to which membrane -ltration is sometimes applied. The main limitation of this approach is the treatment of the concentrate stream. The spectrum of available technologies may, in the future, be further broadened to include oxidation, specialized bio-sorptive processes, solvent extrac- fungi/H 2 O 2 -driven tion, or photocatalysis. 1998 SCI ( J. Chem. T echnol. Biotechnol. 72, 289E302 (1998)

Treatment and Reuse of Wastewater from the Textile Wet-Processing Industry : Review of Emerging Technologies

The public demand for color-free waste discharge to receiving waters and tougher color standards have made decolorization of a variety of industrial wastes a top priority. Unfortunately, with the complicated color-causing compounds, the decolorization of these wastes is a difficult and challenging task. This article first describes the background information of dye molecules and dye waste characteristics. The methods for color measurements and standards are then discussed. Different techniques including almost all the known physical, chemical and biological techniques are described for decolorization. Each process alone may not be able to meet the requirements. A combination of these processes, for example, chemical-biological, biological-chemical, chemical-physical, chemical-chemical, etc. is often used. The formation of intermediates during the decolorization process is emphasized. These byproducts may be more toxic than the parent compounds. Thus, the extent of the mineralization in waste decolorizatio...

Decolorization of Wastewater

The photocatalytic decolorization of municipal wastewater contaminated with textile dyes was studied using a batch reactor. Degussa P25 titanium dioxide was used as the photocatalyst and proved to be effective for dye degradation when irradiated with UV light in the presence of air. In addition to removing the color from the wastewater, the photocatalytic reaction simultaneously reduced the COD which suggests that the dissolved organics had been oxidized. The activation energy for the photocatalytic decolorization reaction was only about 3 to 6 kJ/mole indicating a weak temperature dependence of the rate. These results suggest that the photocatalytic degradation of textile dyes may be a viable method for decolorizing and oxidizing organics in wastewater

https://www.jstor.org/stable/info/25044407

John L. Gainer

Papers

Photocatalytic decolorization of wastewater dyes