The optimal dose of oxidants in UV-based advanced oxidation processes with respect to primary radical concentrations

UV-based advanced oxidation processes (AOPs) via photolysis of precursor chemical oxidants have been of interest to numerous researchers over the past several decades due to their capacity to generate highly active radical species and interesting radical chemistry. However, applications of UV-based AOPs have been commonly optimized case by case, due to the lack of theoretical investigations on process optimization, especially on oxidant doses. In this study, a simple equation for UV/H₂O₂ (•OH as the sole primary reactive species (PRS)) to obtain the theoretical optimal concentration (C_{opt-theoretical}) for H₂O₂ was derived (C_{opt-theoretical}=A_b·S_cε·k). The equation was then validated for its accuracy in the calculation of C_{opt-theoretical} for H₂O₂ in the UV/H₂O₂ AOP using a well-established comprehensive kinetic model. A competition kinetics method for the measurement of scavenging capacity (S_c, the unknown parameter for the simple equation) was designed, for which nitrobenzene was employed as the probe compound and tert‑butyl alcohol was introduced as the standard compound. Based on this simple equation, we calculated the C_{opt-theoretical} of 77 environmental water samples and introduced the concept of a practical optimal oxidants dose for the UV/H₂O₂ AOP, while minimizing the operation costs in engineering applications. Moreover, this study mathematically proved that the simple equation obtained from UV/H₂O₂ could be successfully extended to other UV-based AOPs, including UV/chlorine, UV/NH₂Cl, UV/S₂O₈^2-, and UV/peracetic acid. The simple equation of C_{opt-theoretical} derived in this study may not only help to provide instructions for engineering applications, but also point out the ultimate treatment capability of each UV-based AOPs.