Scavenging effects of ascorbic acid and mannitol on hydroxyl radicals generated inside water by an atmospheric pressure plasma jet

In this study, we have studied the scavenging effects of radical scavengers (ascorbic acid and mannitol) on •OH radicals generated inside water by an atmospheric pressure plasma jet. Plasma was generated by using a sinusoidal power supply (applied voltage: 2.4 kV, frequency: 40 kHz) at a constant argon gas flow rate of 400 sccm and it was characterized by electrical and optical measurements. The technique of ultra-violet (UV) absorption spectroscopy was applied to investigate the effects of scavenging at 3, 6 and 9 mm depths below the water surface by using terephthalic acid as a •OH radical sensitive probe. Also, the effects of scavenging were studied by changing the concentration of radical scavengers and plasma exposure time. For both radical scavengers, the reduced concentrations of •OH radicals were found to decrease with increasing depths as their production would also be reduced by decreasing intensities of plasma-initiated UVs with increasing depths. The effectiveness of ascorbic acid on the scavenging of •OH radicals appeared much stronger than mannitol at all depths inside water since the deprotonated form of ascorbic acid consumes two •OH radicals whereas mannitol consumes only one. Also, the reduced concentration of OH radicals with addition of scavengers was found to be increased under longer plasma exposure time even though the generation of OH radicals were also increased. The detailed mechanism on the formation of •OH radicals inside water has been discussed along with the causes of scavenging. These results can be an important milestone in the applications of non-thermal plasma sources requiring the reduction of •OH radicals.