**4. Conclusions**

The optimised reactions parameters for the degradation of TAT, RY-17 and RB-5 were found to be:

The initial dye concentration (TAZ = 1 × 10<sup>−</sup><sup>4</sup> M and for both RY-17 and RB-5 = 1 × 10<sup>−</sup><sup>5</sup> M), catalyst weight (1.5 g/L), pH (7) and the time taken for complete decolourisation (TAZ: 5 ½ h (UV) 6 ½ h (visible), RY-17: 5 h (UV) 6 h (visible), RB-5: 6 h (UV) 7 h (visible). Decolourisation and degradation were comparatively faster under UV than under visible. Inhibitors such as sodium chloride, sodium carbonate and ethanol decreased the degradation rate and electron acceptors such as H2O2 and potassium per sulphate showed beneficial effect. As far as the photocatalytic efficiency of the catalysts are concerned, the following order was observed under both UV and visible irradiations,

Au/TiO2 > Ag/TiO2~Pt/TiO2 > Synthesised TiO2 > TiO2 (P − 25 Degussa)

Among the noble metals, Au/TiO2 catalyst was found to be the most active catalyst. This may be attributed to the small band gap value of Au/TiO2 leading to better visible light absorption.

All the M/TiO2 catalysts were found to be more active towards the decolourisation of all the dyes even under visible irradiation. The enhanced photocatalytic activity of the M/TiO2 under visible light irradiation may be due to the presence of impregnated metals, which act as electron traps and by this way they prevent electron hole recombination. The synthesised M/TiO2 catalysts were found to be very active towards the decolourisation and degradation of textile dye effluent collected from an industry as well.
