Photooxidative N-de-methylation of crystal violet dye in aqueous nano-TiO2 dispersions under visible light irradiation

The photodegradation of dye pollutants under visible light irradiation in TiO2 dispersions continues to draw considerable attention because of the improving effective utilization of cost free solar energy and its potential application to treating wastewater from textile and photographic industries. To get a better understanding of the mechanistic details of this TiO2-assisted photodegradation of crystal violet dye (CV, N,N,N′,N′,N″,N″-hexamethylpararosaniline) with visible light irradiation, the intermediates of the process were identified and examined by HPLC–ESI-MS techniques in this research. The analytical results indicated nine intermediates, namely: N,N-dimethyl-N′,N′-dimethyl-N″-methylpararosaniline (DDMPR), N,N-dimethyl-N′-methyl-N″-methylpararosaniline (DMMPR), N,N-dimethyl-N′,N′-dimethylpararosaniline (DDPR), N-methyl-N′-methyl-N″-methylpararosaniline (MMMPR), N,N-dimethyl-N′-methylpararosaniline (DMPR), N-methyl-N′-methylpararosaniline (MMPR), N,N-dimethylpararosaniline (DPR), N-methylpararosaniline (MPR), and pararosaniline (PR). The experimental results indicated that the N-de-methylation degradation of CV dye took place in a stepwise manner to yield mono-, di-, tri-, tetra-, penta-, and hexa-de-methylated CV species. The possible degradation pathways were proposed and discussed in this research. The reaction mechanism of TiO2/V proposed in this research would be useful for future application of the technology to the decoloration of dyes using solar energy.