The decolorization of Amaranth aqueous solution by irradiation with an atmospheric-pressure plasma jet was examined by absorbance spectrum measurement and nuclear magnetic resonance (NMR) and mass spectrometry (MS) analyzes. An atmospheric-pressure argon (Ar) plasma jet was generated at a frequency of 10 kHz with an applied voltage of 10 kV and an Ar gas flow rate of 10 l min−1. When 10 ml of 100 mg l−1 Amaranth aqueous solution was irradiated with this plasma jet, the red solution gradually became lighter with increasing irradiation time and was almost completely decolorized after 70 min of irradiation. The results of NMR and MS analyzes indicated that the decolorization was caused by the destruction of the azo bond in the Amaranth molecules.
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