Applications Of Atmospheric Pressure Non-equilibrium Plasma In Wastewater Treatment

When water or aqueous solution is in contact with an air plasma,RONS are usually produced from gaseous plasma and the plasma-liquid interactions,because of the existence of N2,O2 and H2O.When these RONS transport towards the liquid phase from the gaseous plasma,they either directly become aqueous species or produce new RONS by a series reactions in bulk liquid as well as in the plasma-liquid interface.These dissolved aqueous RONS as well as derivative species in the plasma-liquid interface can take part in the reactions in the liquid surface and in bulk liquid,which are responsible for the plasma-based applications especially in waste water treatment.In order to clarify the mechanism of plasma-liquid interaction and improve the efficiency of plasma-induced waste water treatment,we investigate the yield and transfer of the main reactive species generated by plasma-liquid interactions and also their effect on the waste water treatment.A derivative absorption spectroscopic method is used to in situ simultaneously trace and quantify the H2O2,NO3-,and NO2-generated during the plasma-liquid interactions.The results indicate that the time evolutions of H2O2,NO3-and NO2-generated from the plasma-liquid interactions strongly depend on the solution's pH value which varies with the plasma treatment.The concentrations of aqueous H2O2,NO3-,and NO2-increase independently from each other during the plasma treatment when the solution's pH value is higher than 3.0.However,when the solution's pH value is less than 3.0,the aqueous NO3-is mainly formed from the reaction between H2O2 and N02-as well as the decomposition of molecular HNO2.H2O2 is usually considered to be an important production of plasma-water interactions.Early studies show that direct synthesis of H2O2 by plasma-water interactions is possible.Therefore,we studied the H2O2 formation by plasma-water interactions.The results indicate that the most important factors for the H2O2 production are the processes taking place at the plasma-water interface,including sputtering,electric field induced hydrated ion emission,and evaporation.The H2O2 production rate reaches?1200 ?mol/h when the liquid cathode is purified water or an aqueous solution of NaCl with an initial conductivity of 10500 ?S cm-1.Generally,the pH value of solution is very important for the liquid chemistry.It is well known that the pH value of air plasma treated solution will decrease because of the production of HNO2 and HNO3 in solution.Thus,the control of pH value is of great importance for an air plasma-water system.Here,we found the NaHCO can be used as a pH buffer to maintain the pH value during the plasma treatment,but attention should be paid to the experimental conditions such as discharge polarity,and liquid temperature in order to achieve better resistance to the pH change.The results imply that higher concentration of NaHCO and suitable solution temperature can improve the buffer capacity.Moreover,the discharge polarity can influence the pH value resistance of NaHCO3 by affecting the liquid temperature.To study the wastewater application,we explored the plasma treatment of a MO synthetic wastewater.We found that the short-lived reactants generated by the plasma-liquid interactions such as OH radicals predominantly contribute to the MO degradation,and the plasma-induced long-lived reactants such as H2O2 almost have no contribution to the MO degradation without other additives.The results also indicate that the pH value of the treated MO solution can strongly affect the plasma treatment efficiency.Lower pH value can induce the production of high reactive NO+ and ONOOH in the solution through the reaction of H2O2 and NO2-which can contribute to the MO decomposition.When an appropriate amount of Fe2+ ions is added in the solution,the MO decomposition can be promoted according to the Fenton's reaction(Fe2+ reacting with the plasma-liquid interaction produced H2O2).It is also found that lower pH value is beneficial for the Fenton's reaction process by avoiding the appearance of Fe(OIH)3 precipitate which can reduce the treatment efficiency.Moreover,the discharge distance can affect the MO decomposition rate by tuning the change speed of the pH value of the treated solution.