Diagnostic And Application Of Reactive Species Produced In The Plasma-liquid Systems

Plasma and liquid interactions lead to a complex series of physical and chemical processes that produce large amounts of highly reactive species.Because of these physical and chemical processes and highly reactive species,plasma-liquid systems can be used in a wide range of applications.In this thesis,we explored the methods of detecting highly reactive oxidizing species in plasma-liquid systems and studied their applications in the decomposition of organic compounds.Hydroxyl radical is one of the most important highly reactive oxidizing species produced by the plasma-liquid interactions and many applications of plasma-liquid systems are based on the hydroxyl radicals.Therefore,qualitative and quantitative analysis of hydroxyl radicals in plasma-liquid systems is very important.Molecular probe method is a commonly used method for detecting reactive species in solutions and it has been used in many literatures for the quantification of hydroxyl radicals.In this thesis,we used dimethyl sulfoxide as the molecular probe,the results indicated that the measured concentration of the hydroxyl radical in the plasma-liquid system is lower than its actual value.In addition to molecular probe method,colorimetric method is often used to diagnose reactive species in solutions.The highly reactive oxidizing species produced in the plasma-liquid system can oxidize Fe2+to Fe3+,and Fe3+ can combine with the thiocyanate ion to produce a colored complex of FeSCN2+.By analyzing the peak of the FeSCN2+ absorbance at 450 nm,qualitative and quantitative analysis of highly active oxidizing species can be indirectly achieved.In this thesis,the highly reactive oxidizing species with a redox potential higher than 0.771 eV are detected using this colorimetric method.However,this method needs to be optimized since FeSCN2+can also be oxidized by the strong oxidizing species such as a hydroxyl radical.The ability of decomposing organic matter by plasma-liquid systems is an important basis for plasma-based wastewater treatment.In this thesis,methylene blue was used as a model organic matter to verify the feasibility of decomposing organic matter by the plasma-liquid system.The effects of electrolytes,current,temperature and other factors on the decomposition efficiency were discussed.The decomposition principle was also explored.We found that there are differences in the degradation process when the solution act as cathode and anode,and the oxidation of methylene blue by the plasma-liquid interactions is partially recoverable,while the reduction is unrecoverable.