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INTRODUCTIONThere has been a growing interest in the remediation of organic pollutants from industrial waste water exhausts based on semiconductor mediated heterogeneous photocatalysis. Interest in this technique is sustained because several organic pollutants can be oxidized non-selectively at ambient temperature without formation of any hazardous intermediates. Titanium dioxide (TiO 2 ) is the widely studied and extensively used photocatalyst because it is inexpensive, easy to synthesize, chemically inert and non-photodegradable. Nevertheless, the wide band gap associated with TiO 2 allows photo absorption only in the UV region which constitutes < 5 % in solar radiation, rendering the process energy constrictive. Though TiO 2 can be transformed into a visible light active photocatalyst by means of suitable doping with transition metal atoms, anions and cations, the enhanced photocatalytic efficiency is limited in terms of an optimal concentration of dopants because excess dopant concentration causes dopants as recombination centres for photo generated eand h + and becomes detrimental. Alternately, a number of binary metal oxides Fe 2 O 3 , WO 3 , Cu 2 O and ternary metal oxides Bi2WO6 a visible light photocatalyst is synthesized by solid-state metathesis reaction between BiOCl and Na2WO4 as precursors. X-ray diffraction pattern of ground homogenized mixture of precursors heat treated at 600 °C for 4 h and washed free from byproduct NaCl indicated formation of phase pure Bi2WO6. Photocatalytic activity of Bi2WO6 powder is evaluated in terms of photocatalytic degradation of methyl orange and congo red under visible light irradiation, presence of external oxidant is found enhance the rate of degradation in both cases. Formation of • OH free radicals during irradiation is ascertained through scavenger D-manitol.

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