Modeling Kinetics of Illuminated and Dark Advanced Oxidation Processes

Advanced oxidation processes (AOPs) have great potential for the complete destruction of hazardous contaminants; however, the complex kinetics and mechanisms of the free-radical reactions involved often render process optimization difficult. In this paper, a kinetic model and associated rate expressions for AOPs are developed to facilitate evaluation and optimization of treatment performance. The kinetic model has been formulated based on key reactions known to occur in both illuminated and dark AOP systems in the aqueous phase. As a result of fast free-radical chain reactions, the hydroxyl radical (OH$\dot{s}$), which is mostly responsible for decontamination, is expected to be at low steady-state concentrations, depending on treatment conditions. Thus, the OH$\dot{s}$ concentration must be optimized with respect to treatment conditions to promote fast decontamination reactions and reduce wasteful scavenging reactions. A final rate expression is derived for the steady-state concentration of OH$\dot{s}$ as a function of light intensity, quantum yield, illuminated surface, reaction volume, total absorption coefficient of the water, extinction coefficients of O ₃ and H ₂ O ₂, concentrations of oxidants (i.e., O ₃ and H ₂ O ₂ ) and OH$\dot{s}$ scavenger, and pH.