Degradation of Aqueous Paraquat by Surface Air Plasma: A Kinetic Study

In this work, the kinetics of aqueous paraquat (PQ) degradation in a surface air plasma reactor was studied under a variety of experimental conditions. Additionally, stable reactive oxygen species (ROS) and reactive nitrogen species (RNS) were determined. PQ degradation followed pseudo-second order kinetics, increasing both observed rate constant (kobs) and removal efficiency by increasing the temperature. Increasing PQ concentration decreased kobs, as a constant amount of reactive species is generated at the same conditions. Both the decrease of the entropy of activation (–157.1 J K-1 mol-1) and the low value of the enthalpy of activation (+9.9 kJ mol-1) supported a bimolecular associative mechanism for producing the transition state. From the ionic strength effect, the transition state is formed by two ions of opposite charges (zAzB= –3.6). Produced RNS (nitrous and nitric acid) followed zero-order kinetics. Gaseous ozone was the only ROS identified, as hydrogen peroxide concentration was below the limit of detection.