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

This study focuses on the adsorption and microwave assisted-oxidation of Cationic Blue (X-GRL) on hydrous manganese oxides (δMnO2) and commercial MnO2 (βMnO2). The effects of X-GRL adsorption and Mn(IV) reductive dissolution are investigated. δMnO2 exhibits higher adsorbing potential for X-GRL than βMnO2 does, with dissolved organic carbon (DOC) removal potential (removed DOC at per gram MnO2) of 4.92 and 0.31 mg/g, respectively. The ratio of absorption at different wavelength (A202 nm/A300 nm/A610 nm) and ratio of DOC to A610 nm (RDOC/A610 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. δMnO2 exhibits more remarkable oxidizing effect than βMnO2 does. X-GRL adsorption onto δMnO2 involves in hydrogen bonding and electrostatic interactions, which is inhibited by Ca2+ presence and favored at higher pH. Manganese species (δMnO2, βMnO2 and Mn2+) facilitates X-GRL degradation and DOC removal in O2/MWL systems, and more significant effects are observed for δMnO2 due to its combined effects of adsorption and catalysis. The kapp (apparent reaction constant) in the initial 10 min with δMnO2 presence is about 2.5 and 4.2 times higher than that with βMnO2 and Mn2+ presence, respectively. However, with reactions continuing, DOC mineralization is inhibited in heterogeneous systems due to the adverse effect of high dissolved Mn2+ concentration.