Microwave-Assisted Rapid Photocatalytic Degradation of Malachite Green in TiO<sub>2</sub> Suspensions: Mechanism and Pathways

Microwave-assisted photocatalytic (MPC) degradation of malachite green (MG) in aqueous TiO<sub>2</sub> suspensions was investigated. A 20 mg/L sample of MG was rapidly and completely decomposed in 3 min with the corresponding TOC removal efficiency of about 85%. To gain insight into the degradation mechanism, both GC-MS and LC-ESI-MS/MS techniques were employed to identify the major intermediates of MG degradation, including <i>N</i>-demethylation intermediates [(<i>p</i>-dimethylaminophenyl)(<i>p</i>-methylaminophenyl)phenylmethylium (DM-PM), (<i>p</i>-methylaminophenyl)(<i>p</i>-methylaminophenyl)phenylmethylium (MM-PM), (<i>p</i>-methylaminophenyl)(<i>p</i>-aminophenyl)phenylmethylium (M-PM)]; a decomposition compound of the conjugated structure (4-dimethylaminobenzophenone (DLBP)); products resulting from the adduct reaction of hydroxyl radical; products of benzene removal; and other open-ring intermediates such as phenol, terephthalic acid, adipic acid, benzoic acid, etc. The possible degradation mechanism of MG included five processes: the <i>N</i>-demethylation process, adduct products of the hydroxyl radical, the breakdown of chromophores such as destruction of the conjugated structure intermediate, removal of benzene, and an open-ring reaction. To the best of our knowledge, it is the first time the whole MG photodegradation processes have been reported.