Application of polyoxotungstates as environmental catalysts: Wet air oxidation of acid dye Orange II

The catalytic effect of two Keggin-type polyoxotungstates, namely H₄SiW₁₂O₄₀ and Na₂HPW₁₂O₄₀, on the wet air oxidation of azo dye Acid Orange 7 (AO7; C_dye = 248 μM) at varying reaction temperatures (T = 160-290 °C at P_H2O=0.6-3.0 MPa, respectively) was examined. Arrhenius parameters for catalytic and non-catalytic wet air oxidation were obtained in terms of temperature dependent AO7 and TOC abatement kinetics. To get a further insight into the dominant reaction pathway of the catalytic and non-catalytic wet air oxidation processes, isopropyl alcohol and bromide (300 μM) were added as OH scavengers. Obtained results revealed an important homogenous contribution for WAO of AO7 (Arrhenius parameters E_a = 84 kJ mol^-1 and A = 5.6×10⁷ min^-1). However, mineralization (TOC removal) was noticeably influenced by the investigated polyoxotungstates (C_catalyst = 140 μM) and the activation energy decreased from 40 kJ mol^-1 in the absence of catalyst to only 28 kJ mol^-1 (PW₁₂^3-) and 22 kJ mol^-1 (SiW₁₂^4-). The catalytic processes were appreciably less sensitive to the presence of OH scavengers than uncatalyzed wet air oxidation implying that polyoxotungstate-catalyzed wet air oxidation did not solely depend upon a free radical type reaction mechanism. The obtained results tend to support the free radical chain mechanism to account for Orange dye degradation only for uncatalyzed wet air oxidation.