An ideal photocatalyst should exhibit the following

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the bulk of the material. An ideal photocatalyst should exhibit the following properties: high photoactivity biological and chemical inertness photostability non-toxicity cost-effectiveness. Some examples of typically used nanostructured semiconductor photocatalysts are zinc oxide (ZnO), titanium dioxide (TiO 2 ), zinc sulphide (ZnS), ferric oxide (Fe 2 O 3 ) and cadmium sulphide (CdS). The wide band gap semiconductors absorb light in the UV region of the solar spectrum. However, using high-energy UV light sources to excite the photocata- lysts may not be a cost-effective solution in all cases. Therefore, research is cur- rently focusing on using visible portion of the solar spectrum for conducting photocatalysis experiments. The advantages of using solar light for photocatalysis are that solar energy is free and abundantly available. Some of the key areas in which photocatalysis can be used for the treatment of textile wastewater include removal of: Organic contaminants. Inorganic contaminants. Heavy metals. 7.1.1 Removal of Organic Contaminants Photocatalysis is widely used for the decomposition of harmful organic contami- nants present in wastewater into harmless by-products, mostly carbon dioxide and 80 P. Senthil Kumar et al.
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water. The organic contaminants that can be removed from wastewater using photocatalysis include carboxylic acids, alcohols, phenolic derivatives and chlori- nated aromatic compounds (Bhatkhande et al. 2002 ; Mills et al. 1993 ). Dyes being the major pollutants released into the ef fl uent stream have to be treated and the nanophotocatalysts shown to be successful in this regard are semiconductor metal oxides such as TiO 2 and ZnO. Additionally, photocatalysis has been employed to degrade natural organic matter also called as humic substances. Humic substances are naturally occurring yellow brown organic materials with high molecular weight. TiO 2 nanoparticles have made it possible to achieve almost 50 % reduction in humic acid concentration in drinking water, observed under irradiation from a mercury lamp (Eggins et al. 1997 ). 7.1.2 Removal of Inorganic Contaminants Inorganic contaminants include chemicals such as halide ions, cyanide, thiocyanate, ammonia, nitrates and nitrites that can be effectively decomposed by photocatalytic reaction (Hoffmann et al. 1995 ; Mills et al. 1996 ). The photocatalytic removal of toxic Hg(II) and CH3Hg(II) chlorides using TiO 2 nanoparticles under simulated solar light (AM1.5) (Serpone et al. 1987 ) and the removal of toxic potassium cyanide and Cr(VI) ions from water using visible light and ZnO nanoparticles have been achieved. Recently it has been found that CdS/titanate nanotubes are useful in photocatalytic oxidation of ammonia in water (Lee et al. 2002 ). 7.1.3 Removal of Heavy Metals Heavy metals present in wastewater streams is another area of concern as it directly affects human health and presents a challenge for treatment plants, since the amount of can vary, depending upon the type of industry. However, due to the rare availability and high cost of some metals, recovery of the metals is mostly preferred over removal of the metals itself. Photocatalysis has been shown to successfully
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  • Fall '19
  • N. Gokarneshan

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