Applied Chemical Engineering (2018) Volume 1 doi: 10.24294/ace.v1i4.615 1 An overview to photo-catalytic degradation of dyes in waste water Mohd. Hanief Najar 1 , Ishtiyaq Ahmed Najar 2 1 Department of Chemistry, Govt. College of Engineering and Technology Safapora, Ganderbal-191201, J&K India 2 Department of Environmental science, Govt. Degree College Ganderbal-191201, J&K India ABSTRACT Organic dyes used in textile and food industries are the important sources of environmental contaminations due to their non-bio degradability and high toxicity to aquatic creatures and carcinogenic effects on humans. This demands environmental remediation by the use of techniques which are environmentally benign. For this purpose, a general overview of dye degradation by light in the presence of materials as photo-catalysts has been given. The mechanism of action has also been described. Importantly, the materials involved in dye degradation usually involve nano-composites of either conducting polymers or metal-oxidesor graphene based systems which are insoluble in aqueous solutions, hence will be environmentally benign and can therefore be recovered after use. Keywords: Nano-materials; Organic dyes; Photo-catalysis 1. Introduction and criteria for materials to be best photo-catalysts for dye degradation The use of dyes in textile industries has become a serious environmental issue because of the unacceptable color of dye effluents, high chemical oxygen demand and resistance to bio-degradation on account of having com- plex aromatic structures. For this purpose, dyes in waste water are either removed by adsorption or degraded by photo-catalysis. In this area, semiconductor materials have attracted much attention. Among semiconductors, TiO 2 is considered a bench mark photo-catalyst for dye degradation. This is because of having high efficiency, low cost, chemical corrosion inertness, and long-term stability against photo-corrosion and chemical corrosion. Moreover, it is a photosensitive material, possesses high photo-catalytic activity, is stable in aqueous systems and has low environmental toxicity [1] .But the major short- coming to the use of TiO 2 photo-catalysts lies in having low quantum efficiency and the confined utilization of sunlight. For a material to exhibit high photo-catalytic activity, it must be porous and possess high surface to volume ratio so as to lead increased adsorption of dye. The high surface to volume ratio (surface area) can be attained provided the material to be either highly porous or nano-sized. This is the reason that benchmark TiO 2 photo-catalyst has been found to be less efficient than nano-sized TiO 2 .The material must exhibit slower recombination rate of photo-generated electron-hole pairs. This will help to cause facile degradation of dyes. ZnO was found to have almost same band gap energy as that of TiO 2 , hence it was anticipated to be a better an alternative for TiO 2 photo-catalysts, but the fast recom- bination rate of photo-generated charge carriers limits its applicability. Reduction in the recombination rate can be attained by forming hetero-junctions between different components. This is indicative of making a composite material. However, this would demand the proximal in- terface contact. Moreover, materials with wider absorp- tion range lead to enhanced efficiency in the degradation of dyes. For such a factor, TiO 2 again has limited ap- plicability as it absorbs only a small portion of UV light. Thus to widen its absorption band spectrum, doping, composite science and the formation of hetero-junctions is essential. All the factors discussed above are comple- mentary to one another. This we mean to say that a mate- rial with high surface to volume ratio does not suffice to be a good photo-catalyst unless other factors are more or less favoring. This demands the balance in the proper- ties of materials desirable for photo-catalysis [2-5] . Copyright © 2018 Mohd. Hanief Najar et al. doi: 10.24294/ace.v1i4.615 EnPress Publisher LLC.This work is licensed under the Creative Commons Attribution-NonCommercial 4.0 International License (CC BY-NC 4.0). http://creativecommons.org/licenses/ by/4.0/