Visible light assisted photo catalytic degradation of 2, 4- dinitrophenol and 2, 4, 6- trinitrophenol using H2O2 sensitized BiVO4

Heterogeneous photocatalysis has been widely investigated for the abatement of several toxic organic pollutants because of its ability to completely mineralize hazardous organic contaminants into harmless products at ambient temperature [1]. TiO2 is used extensively as a photocatalyst because it is inexpensive, easy to synthesize, non-toxic, chemically inert, and highly photostable. But, the wide band gap of TiO2 restricts its absorption to U.V region with  ˂ 380nm which require substantial electrical input. Terrestrial solar radiation is composed of 4% U.V radiation, 52% visible light and 44% near infrared light. In order to exploit the large component of solar radiation, it is essential that the photocatalyst needs to be visible light responsive. Different strategies have been demonstrated [2] to extend photo response of TiO2 into visible light through (i) doping of metal atoms/anions and/or cations, (ii) surface sensitization with dyes, pthalocyanins, porphyrins and (iii) fabrication of nano composites with higher surface to volume ratio. Simultaneous research on ternary metal oxides as potential photocatalysts led to the development of ZnWO4 [3], CaBi2O4 [4], BaBiO3 [5], BiVO4 [6], Bi2WO6 [7], Bi2MoO6 [8], Bi2Mo2O9 [9], Bi2Mo3O12 [10], Fe2Mo3O12 [11], NaBiO3 [12], FeV3O8 [13] etc. as potential photocatalysts for degradation of dyes and other aromatic pollutants. Majority of these visible light active photocatalysts contain Bi as one of the components. Binary metal oxide systems Bi2O3-V2O5, Bi2O3-MoO3 and Bi2O3-WO3 form a large group of visible light responsive photocatalysts, probably due to the valance band formed from Bi (6s) and O (2p) orbitals resulting in a smaller band gap [14]. Remediation of several dyes using Bi2Mo3O12. xMoO3 [15-16] and aromatic pollutants such as nitrobenzene [17], 2, and 4- nitrophenols [18], acetophenone [19], and Brilliant green [20] using BiVO4 have been recently reported from this laboratory.

Nitro aromatic compounds are identified as environmentally hazardous due to their toxicity. Nitrophenols are widely distributed in waste waters, rivers and soil because of their extensive use in manufacturing industries of pesticides, herbicides, dyes, explosives, pharmaceuticals, wood preservatives and photographic developers. 2, 4- dinitrophenol (2, 4- DNP) is a toxic compound used in petrochemical industry as polymerization inhibiter for vinyl aromatics and in agriculture as pesticide. 2, 4-DNP may also be formed as a result of photochemical reaction between benzene and nitrogen monoxide in polluted air. It reacts with alkyl-peroxy radicals in water and has the potential to photolyze. Remediation of 2, 4- DNP has been reported in terms of adsorption [21], biodegradation [22], cavitation [23, 24], anodic oxidation [25], electro chemical [26], Fenton [27], photocatalytic degradation over TiO2 [28-33], carbon nano tubes/TiO2 composite [34], Bi2O3/Bi2MoO6 [35], V2O5-ZnO [36], Fe3O4/Al2O3/TiO2 nano composite [37], Nitrogen and Indium co-doped mesoporous TiO2 nano composites [38], Ag+ doped BiVO4 [39], photo and photo-Fenton processes [40] solar Fenton catalytic oxidation using BiFeO3 [41]. Present paper describes visible light heterogeneous photocatalytic degradation of 2, 4- DNP and 2, 4, 6- trinitrophenol using H2O2 sensitised monoclinic BiVO4.