Novel catalytic photochemical and hydrothermal treatment processes for acid dye wastewater

The purpose of the present study was to evaluate the catalytic effect of a heteropoly acid, namely H₄SiW₁₂O₄₀, on aqueous Acid Orange 7, a well-known, representative textile monoazo dye applied to nylon fabrics, in sub-critical water (T=160°C-290°C; P=0.6-3.0 MPa, pH=2.6) and under near-UV-VIS light irradiation (isopropanol: water ratio=1:10; pH=2.8). SiW₁₂O₄₀^4--catalyzed photochemical Acid Orange 7 bleaching was also explored in the presence of two dye assisting chemicals as well as in simulated acid dye wastewater at its natural, acidic pH under otherwise identical reaction conditions. Though less explored than TiO₂-mediated, heterogeneous photocatalytic treatment, heteropolyacid-catalyzed thermal and/or photoatalytic re-mediation could be an attractive alternative for the degradation of refractory, colored compounds using relatively mild wet oxidation operating conditions or even natural solar light.