Microwave-Assisted Rapid Photocatalytic Degradation of Malachite Green in TiO₂ Suspensions: Mechanism and Pathways

Abstract

Microwave-assisted photocatalytic (MPC) degradation of malachite green (MG) in aqueous TiO2 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 N-demethylation intermediates [(p-dimethylaminophenyl)(p-methylaminophenyl)phenylmethylium (DM-PM), (p-methylaminophenyl)(p-methylaminophenyl)phenylmethylium (MM-PM), (p-methylaminophenyl)(p-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 N-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.

Photo-degradation of malachite green in mudfish tissues — investigation of UV-induced photo-degradation

“…2b). These hypsochromic shifts of the absorption band might be due to the formation of a series of MG intermediates, which is almost consistent with the MG degradation tendencies described in a previous report (13). MG intermediates were further identified with LC-ESI-MS (Fig.…”

“…MG photo-degraded intermediates in aqueous solution MG intermediates which were produced by photo-degradation were reported in previous studies (11,13,16,17). The kinds of MG intermediate might be related to the operation conditions such as the photocatalyst and UV intensity.…”

“…Most reports have focused on new photocatalysts, the experimental conditions, and the possibility of environmental applications. Five possible photo-degradation pathways of MG were introduced: the N-de-methylation process, adduct products of the hydroxyl radical, the breakdown of chromophores such as destruction of the conjugated structure intermediates, the removal of benzene rings, and open-ring reactions (13). Thus, it is of great interest to identify MG photo-degradation pathways and confirm their intermediates.…”

Malachite green “a cationic dye” and its removal from aqueous solution by adsorption

“…Many treatment technologies have been applied to decolorize MG from aqueous medium, such as biodegradation and decolorization Levin et al 2004;Ren et al 2006;Eichlerová et al 2006;Daneshvar et al 2007a, b;Ayed et al 2008;Ali et al 2009;Wu et al 2009;Cheriaa and Bakhrouf 2009;Ayed et al 2010;Du et al 2011;Khataee et al 2011a, b;Shedbalkar and Jadhav 2011;Hu et al 2011;Abd-El-Kareem and Taha 2012); photocatalytic degradation (Hasnat et al 2003;Kominami et al 2003;Lv et al 2005;Yang et al 2005;Chen et al 2007;Sayılkan et al 2007aSayılkan et al , b, 2008Tayade et al 2007;Wang et al 2008a;Ju et al 2008;Asiltürk et al 2009;Prado and Costa 2009;Bansal et al 2009;Kaneva et al 2010;Liu et al 2010;Bojinova and Dushkin 2011; Maximum wavelength 618 nm Table 2 Applications and toxicity of Malachite green dye…”

TiO2/WO3 hybrid structures produced through a sacrificial polymer layer technique for pollutant photo- and photoelectrooxidation under ultraviolet and visible light illumination

“…The absorbance of the visible band at k = 617 nm decreases and thereby the concentration of the dye decreases with increasing time of exposure. The absorbance peaks at 425 and 315 nm have also declined, which indicates that the entire conjugated chromophore structure of MG was destroyed [31]. The relative change in the dye concentration (Fig.…”