Photocatalytic transformation of acid orange 20 and Cr(VI) in aqueous TiO2 suspensions

The ultraviolet (UVA)-induced photocatalytic treatment of aqueous solutions containing the azodye acid orange 20 alone, hexavalent chromium alone as well as their mixture over TiO2 anatase suspensions was investigated. Experiments were conducted at dye concentrations varying between 25 and 100 mg/L, catalyst loadings between 50 and 500 mg/L, Cr(VI) concentrations between 5 and 20 mg/L, solution pH values between 2.5 and 10 and in the presence of several water matrix components, while the photocatalytic activity was assessed in terms of color removal, chemical oxygen demand (COD) decrease and Cr(VI) reduction. Treatment efficiency for the binary dye/TiO2 system generally increased with increasing catalyst loading, decreasing dye concentration and decreasing solution pH and it was also promoted in the presence of dissolved oxygen. The presence of sodium azide, sodium chloride or sodium sulfate in the reaction mixture at concentrations up to 5000 mg/L decreased decolorization due to the scavenging of radicals and other reactive moieties. Complete color and COD removal could be achieved after 120 or 240 min of treatment at 500 or 250 mg/L catalyst loading, respectively, at near-neutral pH. Dark redox reactions between the dye and Cr(VI) occurred readily at acidic conditions but not at near-neutral conditions in the dye/Cr(Vl)/TiO2 ternary system. However, UVA irradiation accelerated significantly both dye and metal conversion regardless the working solution pH. Interestingly, the rates of dye degradation and metal reduction for the ternary system were generally lower than those for the respective binary systems and this was more pronounced at near-neutral conditions; this could, to some extent, be due to partial catalyst deactivation. (c) 2006 Elsevier B.V. All rights reserved.