Structure-Degradation efficiency studies in the remediation of aqueous solutions of dyes using nanosecond-pulsed DBD plasma

Abstract

• The NSP plasma system was effective to degrade five structurally different dyes and their mixtures. • Degradation kinetic order MV > MO > MB > OII > SY and TOC removal order OII > SY > MO > MV > MB. • MV, MO and MB produce intermediates that resist full degradation. • Structural features and chemical reactivity of dyes towards RONS drive the process. • All dyes were completely degraded with an outstanding energy yield (1.5–2.0 g/kWh) Cold atmospheric plasma (CAP) is regarded as a highly competitive advanced oxidation process towards the removal of organic pollutants from wastewater. Nevertheless, the differences in plasma-induced degradation mechanisms and kinetics are poorly understood and affect their efficient remediation. In this work, nanosecond pulsed dielectric barrier discharge plasma (NSP-DBD) was employed to investigate the degradation/mineralization kinetics of five popular dyes (Orange II, Methylene Blue, Methyl Violet, Methyl Orange and Sunset Yellow). We found that the degradation kinetic order is MV > MO > MB > OII > SY nevertheless the faster degraded dyes MV, MO and MB produced intermediates that resist full degradation whereas the degradation of OII and SY may be slow to start but generated intermediates that were faster and fully degraded. Their difference with respect to total degradation was linked to structural features and their chemical reactivity primarily towards ·OH, 1 O 2 and electron transfer which was confirmed by experiments with appropriate scavengers. Consistent degradation maps were also generated. Under the optimized operating window, all dyes were completely degraded at treatment times ranging from 15 to 20 min with the corresponding energy yield being 1.5–2.0 g/kWh. The effect of critical factors such as water inherent properties and complex dye mixture were examined revealing the potential effectiveness of the system in real conditions. Following a comparison with equivalent systems it appears that nanosecond pulsed discharges is one of the most efficient among those reported for dye degradation in air–liquid plasma reactors.