I. Introduction
Advanced oxidation processes (AOPs) using O3, H2O2, UV, or Fenton reactions have shown potential in treating the hard-degradable and toxic organic compounds in wastewater [1], [2]. Hydroxyl radicals (OH) produced in the AOPs have high oxidation ability, and the reactions do not produce carcinogenic by-products [3]; they can react with substances in a nonselective manner and convert organic compounds into carbon dioxide and water. The general hydroxyl radical reaction constant is more than [4]. Hydroxyl radicals are also an important source of H2O2 formation. As an alternative to the traditional AOPs, the nonthermal plasma-induced AOPs have been extensively investigated over the last 30 years. The reactions occurring at the plasma–water interface lead to chemical activations by producing active species in situ, including ions, reactive radicals, excited molecules, and atoms, without the requirement for additional chemicals [5]. Hydrogen peroxide has been considered as a useful, but not perfect, indicator of OH radicals in plasma systems [6]; it is believed to be the major product of OH radical dimerization reactions [6], [7].