Principles and mechanisms of photocatalytic dye degradation on TiO2 based photocatalysts: a comparative overview

Abstract

Pictorial representation of all possible dye degradation reaction in UV light initiated indirect dye degradation mechanism. This mechanism is practically more important over visible light initiated direct mechanism.

The total annual production of synthetic dye is more than 7 × 10 ⁵ tons. Annually, through only textile waste effluents, around one thousand tons of non-biodegradable textile dyes are discharged into natural streams and water bodies. Therefore, with growing environmental concerns and environmental awareness there is a need for the removal of dyes from local and industrial water effluents with a cost effective technology. In general, these dyes have been found to be resistant to biological as well as physical treatment technologies. In this regard, heterogeneous advanced oxidation processes (AOPs), involving photo-catalyzed degradation of dyes using semiconductor nanoparticles is considered as an efficient cure for dye pollution. In the last two decades TiO ₂ has received considerable interest because of its high potential as a photocatalyst to degrade a wide range of organic material including dyes. This review starts with (i) a brief overview on dye pollution, dye classification and dye decolourization/degradation strategies; (ii) focuses on the mechanisms involved in comparatively well understood TiO ₂ photocatalysts and (iii) discusses recent advancements to enhance TiO ₂ photocatalytic efficiency by (a) doping with metals, non-metals, transition metals, noble metals and lanthanide ions, (b) structural modifications of TiO ₂ and (c) immobilization of TiO ₂ by using various supports to make it a flexible and cost-effective commercial dye treatment technology.