Microwave electrodeless lamp assisted catalytic degradation of X-GRL with manganese dioxides: Adsorption and manganese(IV) reductive dissolution effects

Abstract

This study focuses on the adsorption and microwave assisted-oxidation of Cationic Blue (X-GRL) on hydrous manganese oxides (δMnO 2) and commercial MnO 2 (βMnO 2). The effects of X-GRL adsorption and Mn(IV) reductive dissolution are investigated. δMnO 2 exhibits higher adsorbing potential for X-GRL than βMnO 2 does, with dissolved organic carbon (DOC) removal potential (removed DOC at per gram MnO 2) of 4.92 and 0.31 mg/g, respectively. The ratio of absorption at different wavelength ( A 202 nm / A 300 nm / A 610 nm ) and ratio of DOC to A 610 nm ( R DOC/ A 610 nm ) varies after adsorption. This implies the electron transfer between manganese oxides surfaces and X-GRL, which is confirmed by dissolved Mn(II) concentration increase. δMnO 2 exhibits more remarkable oxidizing effect than βMnO 2 does. X-GRL adsorption onto δMnO 2 involves in hydrogen bonding and electrostatic interactions, which is inhibited by Ca 2+ presence and favored at higher pH. Manganese species (δMnO 2, βMnO 2 and Mn 2+) facilitates X-GRL degradation and DOC removal in O 2/MWL systems, and more significant effects are observed for δMnO 2 due to its combined effects of adsorption and catalysis. The k app (apparent reaction constant) in the initial 10 min with δMnO 2 presence is about 2.5 and 4.2 times higher than that with βMnO 2 and Mn 2+ presence, respectively. However, with reactions continuing, DOC mineralization is inhibited in heterogeneous systems due to the adverse effect of high dissolved Mn 2+ concentration.