Novel photocatalytic techniques for organic dye degradation in water

Abstract Around 11 million tons of water is polluted per year and dye stuff industries contribute ∼20% of the total industrial water pollution. Dyes are the recalcitrant pollutants in water because of their complex aromatic structure, high molecular weight, stability, degradation resistance, carcinogenic and mutagenic nature, and tendency to screen sunlight inhibiting photosynthetic reactions. Adsorption and filtration (reverse osmosis using membranes) have always been considered as efficient and economic dye removal methods that can be effortlessly scaled-up in wastewater treatment plants. However, due to high stability and recalcitrant nature of organic dyes these general treatment methods often suffer from low level of efficiency. Given these complexities, in past few decades photocatalytic degradation has grabbed much attention for organic dye removal reflecting its ability for complete mineralization. Due to excellent stability, low toxicity as well as low cost, TiO2 has become one of the most widely used photocatalysts. However, its application only in the UV region and fast recombination of e− and h+ pair makes them unsuitable for complete dye removal. Novel photocatalysts based on combination of suitable metals or metal oxides have been developed to facilitate photogeneration of electrons over a wider spectral range for complete mineralization. Charge separation (e− and h+) caused by such unique combination promotes the generation of reactive oxygen species (•O2− and •OH radicals) that photodecompose the organic dyes. These novel photocatalysts and their applicability thus hold great promise to tackle current and future industrial dye removal challenges. This chapter focuses on progressive increase in the production of dyes, their environmental impacts, and removal from water/wastewater systems using novel photocatalytic materials. More specifically, merits and demerits of existing and prospective photocatalytic materials in terms of their dye degradation efficiencies, different mechanism of actions for photocatalytic dye degradation, and influence of operational conditions are discussed.