Actions of nitrogen plasma in the 4-chrolophenol degradation by pulsed high-voltage discharge with bubbling gas

Air is often used as bubbling gas with the purpose of improving the liquid discharge performance in some pulsed high-voltage discharge systems. Nitrogen in air could dissociate into active species that might participate in a series of liquid chemical reactions. In the discharge system with 4-chlorophenol as target contaminant, some more toxic intermediates were formed and the energy efficiency was decreased by bubbling air and nitrogen. The G50 (energy cost at 50% conversion) is about 0.91 g (KW h)−1, 0.70 g (KW h)−1 and 0.49 g (KW h)−1 respectively by bubbling oxygen, air and nitrogen. Nitro- compounds such as 4-chloro-2-nitrophenol and 4-nitrocatechol were identified, providing evidence for the formation of nitro group. Chloride ion (Cl−) dropped in the degradation process could be activated and react with non-degradable 4-chlorophenol to form 2,6-dichlorophenol. Phenol was also identified for higher yield of reductive radicals. Comparing 4-chlorophenol degradation by bubbling air or nitrogen with that by oxygen, the degradation rates of original contaminants and intermediate products were lower under same energy input. By discharging for 60 min with applied input energy density of 175 W L−1, almost all of aromatic substances were removed with bubbling oxygen but they still remained at about 8.83% with bubbling air and 12.79% with nitrogen. Furthermore, nitrogen in the bubbling gas was transformed into inorganic nitrogen such as NO2- and NO3- that increased linearly with discharge time and achieved a formation rate twice as the degradation rate of original target contaminant.

► Nitrogen is dissociated to plasma to join in chemical process induced by discharge.
► This may decrease the efficiency of 4-chlorophenol degradation in discharge system.
► By bubbling O2, air and N2, the G50 is 0.91, 0.70 and 0.49 g (KW h)−1 respectively.
► Nitrophenols and mineral nitrogenous such as HNO2 and HNO3 could be produced.
► The yield of HNO3 increased linearly at a rate about twice to the degradation rate.