Microwave induced catalytic degradation of crystal violet in nano-nickel dioxide suspensions

Nickel oxide catalyst was obtained by precipitation-oxidation method with the assistance of microwave irradiation. The samples were characterized by X-ray diffraction, Raman spectrophotometer, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, surface area and porosity analyzer. On the basis of the results, the as-prepared product was nano-NiO2 with OH group and active oxygen. The catalytic activity of the as-prepared product might be attributed to its microwave absorbing property and the role of active oxygen, OH group under microwave irradiation. The microwave induced catalytic degradation process (MICD) with as-prepared product was further applied to degrade triphenylmethane dye crystal violet (CV). 97% of a 100 mg L−1 sample of CV was rapidly degraded in 5 min with the corresponding 81% TOC removal. The main intermediates were separated and identified by LC-ESI-MS and GC-MS techniques. The LC-ESI-MS analytical results demonstrated that a series of N-de-methylation products were obtained in a stepwise manner, namely mono-, di-, tri-, tetra-, penta-, and hexa-de-methylated CV species. Nine organic acids with benzene ring and four low molecular acids were yielded with the assistance of GC-MS. The proposed degradation pathways were discussed in this study. The degradation processes might include N-de-methylation, destruction of conjugated structure and opening-benzene ring. MICD, as a potential technique with wide application perspective, can be used to purify triphenylmethane dye wastewater with nanosized nickel dioxide.