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

Abstract

Abstract The catalytic effect of two Keggin-type polyoxotungstates, namely H 4 SiW 12 O 40 and Na 2 HPW 12 O 40 , 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 H 2 O= 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 =5.6×10 7 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 (PW 12 3− ) and 22 kJ mol −1 (SiW 12 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.